Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

: The development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under speciﬁc reaction conditions and may not be stable either in air or under high vacuum conditions. The identiﬁcation of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox properties. activation complete of the results suggested a synergetic e ﬀ ect both oxides the nanocomposite syngas These results indicated that a fully oxidised Ni / CeO 2 (111) surface is catalytically inert for methanol reaction. The same measurements were performed over a reduced Ni / CeO 1.8 (111) surface, which was found to behave differently. O 1s spectra showed an enhancement in the relative intensity of hydroxyl / methoxy peak (–OH / CH 3 O–) at 300 K, resulting from the higher amount of oxygen vacancies on the oxide substrate for dissociative adsorption of methanol, but again it attenuated and finally disappeared as temperature increased to 700 K. Nevertheless, at 500 K they detected a new contribution, which was attributed to a Ni carbide (Ni 3 C) species. Finally, another experiment was performed by heating the pre-dosed methoxy covered surface in a background of 2.66 × 10 –5 bar of water, so as to check the effect of surface hydroxyl and H 2 O. Similarly, surface C / Ni 3 C species were at 500 K, no carbon was detected on the surface upon heating to 700 K, fact the significance of hydroxyls / water the selectivity electrochemical cell, with 300 nm thick Ni / GDC WE deposited onto 1 mm-thick polycrystalline YSZ substrate and a porous Pt CE on the opposing side of the support. NAP-XPS measurements of CO / CO 2 surface electrochemistry on Ni / GDC were performed on the cell at 893 K and maintaining the total pressure around 0.5 mbar, with P CO / P CO2 ratios at 1:1 and 1:9. Analysis demonstrated that dry CO oxidation and carbon dioxide dissociation are substantially slower than their H 2 and H 2 O analogs on Ni / GDC with current densities for CO / CO 2 roughly one order of magnitude lower than H 2 / H 2 O. Operando XPS studies allowed obtaining local overpotentials and surface species factions across the Ni / GDC interface.

the ceria nanocubes impregnated with Pt. The study of Sohn et al. was also focused on the surface oxygen mobility and oxygen vacancy formation of ceria nanoparticles, in this case depending on the particle dimensions as well as on the presence of an active metallic phase [101]. Nevertheless, ethanol steam reforming reaction conditions were used for their study instead of H 2 . Briefly, oxidation states of small ceria nanoparticles (NPs,~4 nm) and larger particles (MPs,~120 nm) were compared to those of cobalt-loaded NPs and MPs during a reductive pre-treatment at 673 K under 0.26 mbar H 2 and the successive ethanol steam reforming under 0.13 mbar of ethanol and 1.3 mbar of water at 623, 673 and 723 K. Although both bare ceria NPs and MPs supports were active for the ethanol steam reforming, Co/CeO 2 -NP was found to be the most active one and the one with the highest extent of reduced CeO 2 .
A well-known method to notably enhance cerium oxide redox activity is the addition of 3d transition metals to its unit cell to form solid solutions with an ordered atomic arrangement [3,102]. For instance, it has been demonstrated that Ce 1−x M x O 2−y mixed oxides (where M = Cr [103,104], Mn [104][105][106], Fe [104,106], Co [104][105][106], Ni [105,106], Cu [107,108], Zr [109][110][111], La [112][113][114]) exhibit lower reduction temperatures than pure ceria. The incorporation of noble metals to ceria to generate Ce 1−x M' x O 2−y mixed oxides (M' = Ru [115], Rh [116,117], Pd [117,118]) has also demonstrated to efficiently promote reducibility of cerium ions and greatly decrease the reduction temperatures too [119]. Ikemoto et al. investigated the reversible redox activity of Cr 0. 19 Rh 0.06 CeO x by means of NAP-XPS, while its catalytic properties were tested with CO and 1-octanol oxidation reactions [120]. NAP-XPS measurements were performed under an atmosphere of 1.3 mbar H 2 , heating the sample from room temperature to 385 K and then cooling it again. Reduction was followed by an oxidation treatment at 573 K under 2 mbar O 2 . Combined with in situ XAFS analysis, the results revealed the transformation of dispersed Rh δ+ species and small CrO x nanoparticles supported on CeO 2 to Rh nanoclusters, Cr(OH) 3 species and CeO 2−x when treated with H 2 . Thus, they demonstrated a remarkable and reversible low-temperature redox activity of Cr 0. 19 Rh 0.06 CeO x due to the combined contribution of the three metal species, which do not reduce below 373 K when they exist separately.
Della Mea et al. studied the redox properties of differently prepared ceria nanoparticles exposed to 1 mbar CO reducing atmosphere and high temperature [121]. Their aim was to tune the oxygen vacancy population in order to design a rational and optimal ceria catalyst for CO oxidation. More than ten different CeO 2 nanoparticle types were prepared by modifying synthesis parameters and two of them were studied with NAP-XPS, as well as standard CeO 2 . They found that small size, high initial Ce 3+ content, high surface area and low pore volume decreased the ceria reduction temperature. Similar investigations were performed by Pereira-Hernández and co-workers [122], who compared the low-temperature (< 423 K) CO oxidation performance of Pt/CeO 2 -based catalysts prepared via conventional wet chemical synthesis (strong electrostatic adsorption) or high-temperature gas phase synthesis (atom trapping). The samples prepared by atom trapping caused Pt to covalently bond with the surface oxygen atoms as well as ceria restructuration. As synthesised, both catalysts were inactive for low-temperature CO oxidation, but their reactivity improved after a treatment under 2 mbar CO at 548 K. The combination of NAP-XPS and CO temperature-programmed reduction (CO-TPR) showed that the catalyst prepared by atom trapping achieved higher activity for CO oxidation after reduction in CO at 548 K, step that led to the partial transformation of Pt single atoms into Pt clusters.
Sayle et al. claimed on the basis of non-equilibrium Molecular Dynamics simulations that the catalytic activity of cerium oxide in the catalytic CO oxidation reaction could be predicted by knowing the oxygen vacancy population at its surface [123]. A catalytic activity map for ceria was presented in their report, calculated as a function of size, shape, architecture and defect content. Their simulations were supported by NAP-XPS analyses of two samples of ceria nanoparticles with different initial surface Ce 3+ /Ce 4+ ratios.
Another approach for the study of ceria catalytic properties by NAP-XPS was reported by Gopal et al., who quantified the effect that large biaxial strain generated on ultrathin ceria films had on the surface redox behaviour of CeO 2−x [124]. They prepared ultrathin cerium oxide films under biaxial compression on single-crystalline (001) yttria-stabilized zirconia (YSZ) and under biaxial tension on Catalysts 2020, 10, 286 7 of 48 (001) SrTiO 3 ( STO), taking advantage of the high lattice mismatch between ceria and both substrates, which modifies the interatomic distances of ceria lattice and causes significant changes in its electronic structure and redox capacity. Samples were studied by different characterization techniques, but surface Ce 3+ and oxygen vacancy concentrations were directly quantified by means of NAP-XPS at 723 and 823 K in both O 2 and H 2 /H 2 O atmospheres and compared with those of fully relaxed ceria films. The results revealed a significant enhancement of Ce 3+ and oxygen vacancy concentrations near the surface for both compressive and tensile strained oxide films.

Gas-Solid Catalysis
Since NAP-XPS was first developed, investigations of gas-solid interfaces have been the frontrunner of the technique due to the many relevant applications they have in the fields of catalysis, corrosion, energy materials and atmospheric science [62,125]. Additionally, the preparation and measurement of gas-solid interfaces with NAP-XPS are relatively simple compared to, e.g., measurements of liquid interfaces. In particular, there has been recently great interest and success in the study of real and model catalytic systems, as well as electrochemical devices, with one or multiple reactant gases. These studies included adsorption, reaction induced restructuring and catalytic performance of systems ranging from highly-ordered single crystals to supported nanoparticles. Therefore, in this second block we have collected and organised all the reports of gas-solid catalytic reactions with ceria-based systems found in the literature, which emphasize the advantages and deficiencies of the NAP-XPS technique in these gas-solid interfaces. Table 1 provides a list of reports of NAP-XPS studies of ceria-based catalysts in the presence of gases published so far, which are described in the following paragraphs.   [158] a NP = nanoparticles; b MP = microparticles; * L = Langmuir.

CO Oxidation and Preferential CO Oxidation (PROX)
The preferential oxidation of carbon monoxide (PROX) is considered an essential reaction to produce high-pure hydrogen for proton-exchange membrane fuel cells (PEMFCs), which must be CO-free for their proper operation [159]. In fact, the concentration of CO in the hydrogen feed must be kept as low as possible (below 1-100 ppm as a rule) [160]. This can be achieved by using sequential water-gas shift (WGS) and PROX units; the former one reduces the amount of CO to 0.5%−1% [159] and then PROX reaction reduces this concentration to < 100 ppm by selectively oxidising CO, avoiding the oxidation of hydrogen. An appropriate catalyst for PROX reaction should not only adsorb carbon monoxide but also supply activated oxygen, as long as hydrogen adsorption is suppressed. Many catalytic formulations have already been applied for PROX reaction, among them supported noble metals such as Au [161][162][163][164], Pt [162,[164][165][166][167], Rh [165] and Ru [165,168], which are responsible for CO adsorption. However, oxygen does not adsorb on these metals because their surface is fully covered by CO molecules, so different activation sites are required for O 2 . Here is where cerium oxide becomes important as a component of these catalysts, because of its role as a reducible support for metallic nanoparticles, able to promote oxidation even under oxygen-poor conditions [76]. In order to gain further insight into the reaction mechanism of the PROX reaction on ceria-based catalysts, several research groups examined changes in the oxidation state of ceria and the supported metal, as well as the surface adsorbed species, under PROX conditions by means of the NAP-XPS technique. Pozdnyakova et al. examined the Pt/ceria [126] and Pd/ceria [127] catalytic systems in separate papers using the same techniques, so as to obtain a better understanding of the PROX reaction mechanism when comparing both systems, although it is well-known that palladium is far less active in this reaction. Both systems were performed with 5 wt % metal loading and studied under approximately 0.5 mbar PROX mixture (CO + O 2 + H 2 ) after being first activated in an oxygen environment (0.5 mbar, 573 K), but also were studied under a CO and O 2 mixture, pumping the hydrogen pressure out of the analysis chamber. The combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which aided the identification of surface species, and NAP-XPS allowed the clarification of the main question of the work: why both Pt and Pd catalysts are active in CO oxidation but Pd is not active and selective in the presence of H 2 (PROX reaction). Researchers concluded that at PROX conditions and low temperatures (T = 350−380 K) CO oxidation was suppressed due to the easy formation of Pd β-hydride, identified by NAP-XPS. Adsorbed H rapidly reacted with oxygen from both gas-phase and support sites (also from PdO x phase) and formed water, which desorbed easily. However, CO adsorption was not completely inhibited, and instead of being oxidised it partially became surface formate and formyl (-CHO) species. The hydride phase decomposed at higher temperatures, which led to an increase of the selectivity toward CO oxidation. Nevertheless, it was still lower on Pd/CeO 2 than on Pt/CeO 2 , likely due to the preference of metallic Pd for H adsorption rather than CO, and also because PdO 2 surface preferentially oxidises H with respect to CO. Teschner and co-workers [128] also examined Pt/CeO 2 catalysts (with nominal platinum loading of 5 wt %) with operando XPS but under different PROX mixtures, by varying oxygen pressure in the reactant feed of a first set of measurements and CO pressure in a second set, always maintaining the same H 2 pressure value for both series so that the total chamber pressure was approximately 1 mbar. The prepared catalyst removed 99 % of CO from a model feed, and the authors suggested the "classic" non-competitive mechanism of low-temperature CO oxidation on ceria-supported Pt [169] (e.g., CO adsorption on platinum and oxygen activation on the ceria support), and CO oxidation on the metal/oxide interface. Additionally, they found that oxygen vacancy formation, detected by operando XPS, was directly related to an enhancement of CO oxidation activity, because higher vacancy densities hindered water desorption, so that water molecules blocked H ads oxidation sites and, therefore, more oxygen atoms were freely available for the oxidation of carbon monoxide. Ceria-supported Pt [170], Rh [171] and Pd [172] are also notably active in the low-temperature CO oxidation, a catalytic reaction important to lower automotive emissions [34,173]. For instance, Artiglia et al. [132] characterised the surface of a 3 wt % Pt/CeO 2 powder catalyst under 1 mbar of a reacting mixture of CO and O 2 , employing time-resolved techniques to study the short-lived Ce 3+ species, difficult to detect under steady-state conditions [174]. This new approach, based on NAP-XPS, consisted on the acquisition of a Ce 3d core level fast scan while switching the oxygen on and off (and replacing it with nitrogen) in a carbon monoxide/oxygen reaction mixture. The results obtained with the combination of these pulsed experiments and analyses of the depth profile, achieved by modifying the excitation energy, indicated that active Ce 3+ sites were formed transiently at the most superficial layers of ceria and at the Pt/CeO 2 interface when oxygen was switched off [175]. Active sites were immediately reoxidised to Ce 4+ upon dosing oxygen. Furthermore, when performing the same experiment on bare ceria they found insignificant reduction, which revealed the role of platinum as a promoter of the formation of Ce 3+ at the interface. Recently, Pereira and co-workers have also investigated the performance of Pt/CeO 2 -based catalysts for the low-temperature (< 423 K) CO oxidation [122], as mentioned earlier. The use of copper for ceria based catalyst is an interesting and promising alternative to noble metals for processes involving also CO oxidation, such as the water-gas shift reaction or the PROX [176,177]. Precisely, the work presented by Monte et al. consisted of the comparison of two morphologically different ceria supports (nanospheres and nanocubes) impregnated with a copper metallic phase subjected to interaction with carbon monoxide at different temperatures [130]. Their results provided direct evidence of interfacial Cu + sites, which are suggested to be the most active ones for CO oxidation [177,178], observed during near-ambient XPS experiments performed over both catalysts subjected to thermal reduction under a flow of diluted CO. These experimental results were supported by density functional theoretical calculations, supplemented with a Coulombic interaction parameter (DFT+U), and they demonstrated lower reducibility of the CuO nanoparticles supported on ceria nanocubes, which led to a higher barrier to oxidise H 2 and thus to an enhancement of the CO PROX selectivity, as observed in previous investigations for ceria support with (100) faces [179]. Another type of design that has been used for CO oxidation reaction, and it has also been studied with operando spectroscopic techniques such as NAP-XPS, consists of bimetallic ceria-based catalysts. The work of Holgado et al. is focused on Au-Ni/CeO 2 bimetallic catalysts and their analogous monometallic samples [129]. In particular, they studied the effect of Ni incorporation into gold nanoparticles on their catalytic activity, as well as the distribution of Ni and Au atoms in bimetallic nanoparticles using various techniques. The development of gold bimetallic catalysts receives economic interest if the second metal is cheaper than Au, such as copper or nickel. Their investigation demonstrated that Au-Ni/CeO 2 bimetallic catalysts presented higher reactivity towards CO oxidation than monometallic Au/CeO 2 and Ni/CeO 2 . The different operando characterization techniques used, including NAP-XPS, allowed them to establish a core-shell Au@NiO distribution, in which Ni surface atoms experienced an electronic effect on the local density of Ni d states from Au atoms in the core, modifying, in turn, their chemical and reactivity properties. It is worth mentioning that the bimetallic catalyst was studied by NAP-XPS under 1 mbar H 2 reducing atmosphere instead of CO (also a reducing agent) due to the possible formation of volatile Ni carbonyl species that could contaminate the analysis chamber. As already mentioned above, Della Mea and co-workers [121] also studied differently prepared ceria catalysts, altering their oxygen vacancy population, in order to design a rational ceria catalyst for CO oxidation. In this case, NAP-XPS conditions were 1 mbar of CO gas phase and temperatures from 373 to 773 K. Finally, another alternative to non-noble metals for PROX catalysts is the use of transition metal oxides, such as copper and cobalt-based catalysts, which show potential for this reaction [179][180][181][182][183][184]. Cobalt oxide catalysts have been extensively studied and, in order to improve their catalytic performance, reducible metal oxides (e.g., MnO 2 , CeO 2 ) have been incorporated [183,184]. Although Co 3 O 4 is a promising candidate as a CO-PROX catalyst, detailed knowledge of its active sites under reaction conditions is still missing. Therefore, in order to improve the understanding of PROX over cobalt oxide-based catalysts, Lukashuk et al. studied them employing advanced operando methods such as NAP-XPS at low photon energies, which allowed higher surface sensitivity to monitor changes in the surface composition [131]. In this case, the cerium oxide role was not as a support for Co nanoparticles but was instead loaded (10 wt %) on as received Co 3 O 4 nanoparticles via wet impregnation. Both bare Co 3 O 4 and CeO 2 /Co 3 O 4 catalysts were studied under pure CO (0.15 mbar), pure H 2 (0.4 mbar) or under PROX mixture (CO/O 2 /H 2 ratio of 1/1/12, at 0.5 mbar), and the ability of Co 3 O 4 to reduce in pure CO and easily reoxidise in O 2 suggested that adsorbed CO molecules react with lattice oxygen, which is refilled upon dosing O 2 . Moreover, the addition of CeO 2 to Co 3 O 4 promoted the PROX activity and increased the reduction temperatures under CO and H 2 , although being a less active material.

Water-Gas Shift Reaction (WGS)
The water-gas shift (WGS) reaction (see Equation (1)) is widely used in industry to tune the CO/H 2 ratio in several chemical processes and to increase the yield of hydrogen in reforming processes. Again, ceria-based catalysts are promising candidates for such application [86,185], and their combination with noble metals (Au, Pt, Pd, Ni, Co) have received widespread attention due to the enhancement of their activity [76,86,169,186].
Wen and colleagues [133] used NAP-XPS to track the surface chemistry of two types of prepared ceria-based catalysts under WGS reaction conditions. They synthesised metal nanoclusters (Au, Pt, Pd, Cu) and supported them in channels of mesoporous ceria (abbreviated M@mp-CeO 2 ), as well as on ceria nanorods, and compared their performance for the WGS reaction. NAP-XPS measurements allowed the identification of the metallic state for Au, Pt, Pd and Cu nanoclusters, and revealed a higher concentration of oxygen vacancies under WGS reaction conditions on the internal concave surface of mp-CeO 2 pores than on ceria nanorods surface. They associated the lower density of oxygen vacancies in ceria nanorods with stronger adsorption of OHgroups because the limited space of the concave internal surface of mp-CeO 2 increases repulsion between neighbouring hydroxyl groups. These results correlated well with the low calculated activation energy of WGS reaction on M@mp-CeO 2 catalysts in contrast to those of M/CeO 2 nanorods.
Ceria-based catalysts, most of them composed by ceria nanoparticles loaded with a metallic phase by incipient wet impregnation, have been widely used so far. Some cerium oxide films have also been assessed and quoted, yet few inverse configurations of catalysts (i.e., those in which ceria is supported on a metallic phase, exchanging its role as catalyst support) have been studied albeit in some cases they could apparently display enhanced activity compared to the conventional direct configuration [187,188]. From an economical point of view, catalysts formulations that combine copper and cerium oxide become more interesting than those based on noble metals. Copper-ceria catalysts have been proposed to operate at a relatively high temperatures above 573 K for WGS reaction, where CO and H 2 O act as a reducing and oxidising agent of ceria, respectively, in the presence of active metallic Cu [188]. In this context, López Cámara et al. prepared an inverse CeO 2 /CuO catalyst by a microemulsion-based method and examined it separately by operando XPS and DRIFTS spectroscopies under reactant mixtures relevant to the low temperature WGS reaction [134]. Their experiments demonstrated that water adsorption promoted the reduction of the catalyst to achieve its most active state since H 2 O molecules favoured the decomposition of surface carbonate species which hamper the reduction process. Another attempt to shed light on the role of metal-oxide interfaces for the WGS reaction mechanism is the work of Mudiyanselage et al., who combined NAP-XPS, infrared reflection absorption spectroscopy (IRRAS) and DFT calculations to investigate the WGS reaction on CeO x nanoparticles deposited on Cu (111) and Au (111) substrates [39]. Cu(111) constitutes a typical benchmark for water-gas shift reaction studies, whereas Au(111) is inactive for such reaction. Nevertheless, it can be activated in the presence of ceria nanoparticles [187,189]. Under mild WGS conditions, adsorbed CO 2 δ− species were detected over both CeO x /Cu (111) and CeO x /Au(111) systems, as well as a high degree of reduction of ceria. Both NAP-XPS and DFT analyses showed that CO 2 δ− species, originated from a carboxy (HOCO) intermediate, are stabilized at the metal-oxide interface of the catalysts, and the simultaneous contribution of atoms present on the metal and the oxide allow the formation of such species, favouring a hydrogen production reaction mechanism which is not efficient on bare copper or bare ceria.

Soot Oxidation
Soot particles consist of an amorphous carbon core of few nanometers surrounded by a graphitic shell, often carrying many toxic compounds [190][191][192], and are one of the main pollutants emitted by diesel engines. Since thermal combustion of soot requires temperatures above 873 K with oxygen, and the temperature of diesel exhaust gases typically lies between 473 and 673 K, a suitable catalyst is needed to decrease the ignition temperature [193,194]. Among all the reported catalysts, ceria-based ones appear to be exceptional candidates for soot oxidation [195][196][197]. Concerning this reaction, it is generally assumed that it conforms to the Mars-van Krevelen mechanism, in which lattice oxygen atoms of the outmost layers of ceria are transferred onto soot, and exposure to gaseous O 2 subsequently fills up the vacancies created on the oxide [198,199]. Therefore, it is claimed that the formation of paramagnetic O 2 − superoxide and diamagnetic O 2 2− peroxide species takes place when reduced CeO 2-x is exposed to molecular O 2 [26,200,201], which spillover onto soot surface [202], and that these active species are actually the main cause of soot oxidation [203,204]. By means of ambient pressure XPS, Soler et al. provided direct evidence of the redox chemistry and the influence of the reaction conditions in the oxidation of carbon soot over ceria-based catalysts [135]. With this aim, they investigated a sample of conventional ceria and a sample of Ce 0.8 Zr 0.2 O 2 mixed with soot, which were subjected to increasing temperatures (from 300 to 823 K) under 1 mbar argon atmosphere in a first set of experiments (with a final replacement of Ar with O 2 at 823 K), and under 1 mbar oxygen atmosphere in a second set of measurements. It is well-known that heating ceria under an inert atmosphere such as Ar results in the formation of oxygen vacancies and the concurrent reduction of Ce 4+ to Ce 3+ [205], and the results showed indeed a Ce 3+ increase with the temperature. However, when samples were treated with O 2 , the amount of Ce 3+ species remained low over the entire temperature range, since molecular oxygen rapidly reacted with the oxygen vacancies created at the ceria-soot interface. Interesting results were obtained when comparing the behaviour of CeO 2 -soot and Ce 0.8 Zr 0.2 O 2 -soot systems, since the amount of Ce 3+ species upon heating under an Ar atmosphere was higher for the Zr-doped sample in all cases. Figure 1 shows Ce 3d (a, b) and O 1s spectra (c, d) of the Ce 0.8 Zr 0.2 O 2 -soot sample recorded at 823 K under an argon environment (a, c) and after replacing Ar with O 2 at the same temperature (b, d). As expected, the replacement of the atmosphere caused an immediate and huge decrease of the Ce 3+ amount (Figure 1a,b), but this phenomenon was also detected through O 1s spectra, which showed three different components. The bands at 528.7 and 530.2 eV of binding energy (Figure 1c,d) were attributed to ceria lattice oxygen and surface oxygen atoms with low coordination due to vacancy formation, respectively, and the peak at higher binding energy (532.2 eV) corresponded to superoxide species. When the argon gas phase was replaced by O2, cerium oxide reoxidised and the O 1s band of the surface oxygen associated to Ce 3+ , located at 530.2 eV, decreased its intensity ( Figure 1d). Concurrently, the signal at 532.2 eV attributed to superoxide species appeared noticeably intense, demonstrating that these active oxygen species resulted from the reaction between molecular O2 and oxygen vacancies. Therefore, the NAP-XPS technique allowed Soler and co-workers to demonstrate that soot oxidation over ceria-based materials involved a two-way cooperative mechanism: on one side, the formation of oxygen vacancies and Ce 3+ species at the ceria-soot interface and, on the other side, the oxidation of the surface of soot by active superoxide species, generated from the reaction between the oxygen vacancies and gaseous O2. Both routes occurred simultaneously and mutually strengthen each other.

CO2 Hydrogenation
Nowadays, the synthesis of methanol from CO2 is receiving strong attention [206][207][208][209][210] not only as a strategy to abate this greenhouse pollutant but also due to the potential use of CO2 as an alternative source of carbon. The conversion of CO2 to a valuable commodity such as synthesis gas The bands at 528.7 and 530.2 eV of binding energy (Figure 1c,d) were attributed to ceria lattice oxygen and surface oxygen atoms with low coordination due to vacancy formation, respectively, and the peak at higher binding energy (532.2 eV) corresponded to superoxide species. When the argon gas phase was replaced by O 2 , cerium oxide reoxidised and the O 1s band of the surface oxygen associated to Ce 3+ , located at 530.2 eV, decreased its intensity ( Figure 1d). Concurrently, the signal at 532.2 eV attributed to superoxide species appeared noticeably intense, demonstrating that these active oxygen species resulted from the reaction between molecular O 2 and oxygen vacancies. Therefore, the NAP-XPS technique allowed Soler and co-workers to demonstrate that soot oxidation over ceria-based materials involved a two-way cooperative mechanism: on one side, the formation of oxygen vacancies and Ce 3+ species at the ceria-soot interface and, on the other side, the oxidation of the surface of soot by active superoxide species, generated from the reaction between the oxygen vacancies and gaseous O 2 . Both routes occurred simultaneously and mutually strengthen each other.

CO 2 Hydrogenation
Nowadays, the synthesis of methanol from CO 2 is receiving strong attention [206][207][208][209][210] not only as a strategy to abate this greenhouse pollutant but also due to the potential use of CO 2 as an alternative source of carbon. The conversion of CO 2 to a valuable commodity such as synthesis gas (CO + H 2 ), hydrocarbon compounds (CH 4 , olefins) and oxygenates (alcohols, ethers, acids) is, thus, highly desirable and numerous reports have appeared recently [206,[211][212][213][214][215]. However, the activation of CO 2 is a challenging task due to the inconveniences related to the chemical inertness of CO 2 . [207][208][209][210]. Currently, there is a particular resurgence in the study of CO 2 hydrogenation to C1 and higher alcohols (xCO 2 + yH 2 → C x H 3 OH + xH 2 O) [216][217][218][219], a process which is mainly associated with supported Cu-based catalysts with a Cu/ZnO/Al 2 O 3 formulation [217,220,221]. Many research groups have investigated the optimization of the configuration of metal-oxide catalysts to activate CO 2 and convert it into valuable chemicals, and they found an improvement in the catalytic activity of Cu (which interacts very poorly with CO 2 on its own) [207,209,222] upon dispersing this metal on a ZnO substrate. Moreover, recent studies have identified the presence of ZnO x aggregates on top of Cu particles of a conventional Cu/ZnO catalyst active for methanol synthesis [220,223,224]. Graciani and co-workers presented a completely different catalyst formulation for CO 2 activation: a copper-ceria interface, which was highly active for methanol synthesis [136]. Approximately 0.2 ML of CeO x nanoparticles were deposited onto Cu (111), and the combination of NAP-XPS and ambient pressure infrared reflection absorption spectroscopy (AP-IRRAS) allowed the identification of adsorbed CO 2 δspecies on the surface of the CeO x /Cu(111) catalyst under a CO 2 /H 2 mixture (0.39 total mbar). C 1s spectrum presented a weak feature at~284 eV, corresponding to a very small amount of C deposited on the surface of the catalyst because of the complete decomposition of CO 2 . However, the spectrum was dominated by a main feature, which could be fitted with two bands at 289.2 and 288.4 eV, attributed to formate and carboxylate species, respectively. The former band was more intense than the band associated to CO 2 δ− species, denoting the high stability of formates, which may not be efficient as intermediate species in the CO 2 to CH 3 OH conversion. With this work, Graciani et al. demonstrated that the metal-oxide interface generated by combining CeO x nanoparticles with Cu(111) provided adsorption/reaction sites for the synthesis of methanol, which would be very difficult to create on a pure metal or alloy surface. Following this work, Senanayake and colleagues compared the catalytic activity of ZnO/Cu (111) and CeO x /Cu(111) systems when modifying the coverage of the oxides on the metallic substrate [137]. Their results indicated that the catalytic activity was strongly influenced not only by the oxide coverage, but also by the nature of the oxide. Specifically, CeO x /Cu(111) was the most active catalyst among the inverse catalysts with an oxide/metal configuration, which have higher catalytic properties than conventional Cu/ZnO and Cu/CeO 2 catalytic systems. Again, by combining NAP-XPS and AP-IRRAS techniques, researchers found that Ce 3+ oxidation state prevailed at low coverages and provided an efficient reaction pathway to adsorb and hydrogenate CO 2 through a CO 2 δ− intermediate, clearly indicating that the ceria-copper interface was essential for a high catalytic activity in the methanol synthesis. A completely different catalyst configuration was investigated during CO 2 hydrogenation by Lin et al. [97], who prepared copper-ceria catalysts using different nanostructured ceria supports: nanorods (Cu/CeO 2 -NR) and nanospheres (Cu/CeO 2 -NS). By means of NAP-XPS and other techniques, they found that copper-ceria catalysts produced primarily CO at atmospheric pressures through the reverse water-gas shift (RWGS) reaction and a negligible amount of methanol. Cu/CeO 2 -NR catalyst displayed a higher activity, which provides direct evidence of the morphological effect of ceria support on catalytic performance. Time-resolved X-ray diffraction (TR-XRD) and NAP-XPS measurements showed important oxidation state changes of the catalysts under reaction conditions, being metallic Cu and partially reduced ceria the active phase for the reaction. Studies with NAP-XPS also revealed a more effective CO 2 dissociative activation at high temperature and a preferential formation of bidentate carbonate and formate intermediates over ceria nanorods, which mainly expose (110) terminations. Finally, Winter and colleagues [138] reported a study of CO 2 hydrogenation over CeO 2 -supported Ni catalysts with different metal loading. Their NAP-XPS measurements revealed that the oxidation state of Ni remained metallic under reaction conditions for all metal loadings (0.5-5 wt % Ni), which implies that the Ni chemical state at the surface of the catalysts does not explain the selectivity differences. For all tested catalysts, ceria support appeared partially reduced under reducing conditions and also upon exposure to CO 2 and H 2 mixture, suggesting that oxygen from carbon dioxide did not reoxidise the CeO x and consistent with the assumption that a mixture of Ce 3+ and Ce 4+ oxidation states plays a role in CO 2 activation.

Hydrocarbons
Natural gas, mainly composed of methane, is abundantly found in many large deposits around the word. However, most of it is located in remote zones and needs to be transported across wide distances before engaging in trade. Methane conversion to more useful and readily transferable chemicals has therefore become a primary issue for the chemistry society and has been extensively investigated during the last 30 years, and synthesis gas is the most economically available route known to date [225,226]. Depending on the purpose of industrial application, several syngas production routes from methane are available, some of them described below.

Methane Partial and Complete Oxidation
The catalytic partial oxidation of methane (MPO) to syngas (CO + H 2 ) is a slightly exothermic reaction first suggested in 1929 by Liander [227], who investigated it for the ammonia process. As high methane conversion and syngas selectivity are achieved at short contact times, this reaction can be used to transform methane, the main constituent of natural gas, into syngas in small-medium scale plants, e.g., to produce it for distribution [228][229][230][231][232][233]. In the past decades, many groups have studied the production of syngas via MPO using Ni, Co and noble metals supported on reducible oxides such as Al 2 O 3 , MgAl 2 O 4 , CeO 2 and ZrO 2 [234][235][236][237][238]. Focusing on ceria-based catalysts, Zhu et al. [139] examined the surface chemistries of CeO 2 doped with Pd, Pt and Rh during methane partial oxidation at different temperatures by means of NAP-XPS. Under the same catalytic conditions (2.6 and 1.3 mbar of CH 4 and O 2 , respectively) Pt and Rh appeared completely oxidised on the ceria surface, whereas metallic Pd nanoparticles were detected, which induced quite different catalytic activities in MPO. Among all catalysts, Rh-doped ceria exhibited the highest catalytic performance and selectivity for methane partial oxidation.
The catalytic complete oxidation of methane constitutes a decisive process to remove the unburned methane emitted from natural gas power plants or engines of vehicles using natural gas or liquefied petroleum gas as fuels. Unlike gasoline-fuelled engines, engines using natural gas or liquefied petroleum gas combust at relatively low temperatures, typically lower than 873 K [239]. Therefore, a catalyst which is highly active below 873 K for complete transformation of methane into CO 2 is required to remove the unburned methane or short-chain hydrocarbons in the exhaust line before releasing it into the environment, since CH 4 is a much stronger greenhouse pollutant than CO 2 [240][241][242][243]. Noble metal-based catalysts such as supported Pd and Pt have been extensively investigated, owing to their high catalytic activity for complete oxidation of methane at relatively low temperature [244][245][246][247]. However, they are limited to any use at large scale due to their restrictively high cost, extremely low abundance in earth and prompt deactivation of catalysts caused by sintering of supported noble metal nanoparticles. To improve the lifetime of noble metal-based catalysts and decrease their price, dispersion of nanoparticles on high surface-area oxide is used. In this sense, ceria is an important support in oxidative catalysis owing to its high activity in molecular O 2 activation [3]. With this aim, Dou et al. [140] prepared a Co 3 O 4 /CeO 2 nanocomposite catalyst that consisted of Co 3 O 4 nanoparticles supported on ceria nanorods, and investigated its surface during complete oxidation of CH 4 (flowing mixture of 0.26 mbar CH 4 and 1.33 mbar O 2 ) in the temperature range of 423-773 K with near-ambient XPS. Kinetic studies of pure CeO 2 , pure Co 3 O 4 and Co 3 O 4 /CeO 2 catalysts were first performed to calculate the different activation energies for complete oxidation of methane, and the results suggested a synergetic effect between both oxides since the nanocomposite catalyst exhibited lower activation energy than that of pure ceria and cobalt oxide. As for NAP-XPS studies, C 1s spectra allowed the identification of a stable intermediate methyl species formed on the surface of Co 3 O 4 /CeO 2 during catalysis, which could be generated through dissociation of C−H of methane on Ce 4+ , Co 3+ or Co 2+ sites, since no oxygen vacancies were formed and detected on the cerium oxide support. Very similar results were obtained by Zhang and co-workers [141], who prepared CeO 2 nanorods with supported NiO nanoclusters of 10-12 nm average size and studied them under the same methane complete oxidation conditions as the work of Dou and colleagues [140]. Tracking the evolution of C 1s NAP-XPS features with increasing temperature under reaction conditions, they deduced the formation of a stable CH 3 -like intermediate bound to the Ni cation at 423−473 K. The generated surface CH 3 could be further activated to form CH 2 or even CH species, which in turn could combine with surface lattice oxygen atoms to form CO 2 and H 2 .

Dry Reforming of Hydrocarbons
Natural gas and biogas, composed mainly of methane, have become cheap and abundant alternatives to traditional fossil fuels such as petroleum and coal [248]. They can be combusted with oxygen for the production of electricity or heat, but they can also be used as a carbon source in the manufacture of commodity chemicals [249]. This can be achieved by means of reforming methane to synthesis gas (CO/H 2 ) and subsequently using syngas in industrial processes such as methanol synthesis, ammonia synthesis, hydrogenations or Fischer-Tropsch (FT) reactions [250,251]. There are three oxidative routes to generate syngas from methane: partial oxidation (Equation (2)), steam reforming (Equation (3)) and dry reforming (Equation (4)).
Although dry reforming of methane (DRM) is the most difficult of these processes, it is attractive as an initial step in the Fischer-Tropsch reaction and methanol synthesis, due to its CO:H 2 = 1:1 production ratio [252,253]. Additionally, dry reforming of methane is a desirable process since it converts two active greenhouse gases (CO 2 and methane) into syngas, representing a promising approach to mitigate problems caused by global emissions [254,255]. The high stability of C−H bonds of methane makes its activation difficult [256], and the extraction of oxygen from CO 2 entails a large activation barrier [249]. Therefore, enabling low-temperature activation of methane is a major technological goal, and a detailed investigation of the DRM through in situ techniques will help to obtain a better understanding of the reaction mechanism for the selective CH 4 activation by evading pathways to complete oxidation. Typically, pure noble metals (e.g., Pt, Pd, Rh, Ru, and Ir) are highly active for this reaction, but at elevated temperatures suffer from rapid deactivation by particle sintering and chemical poisoning (carbon deposition) [257][258][259]. Late transition metals such as Ni, Co and Fe represent an alternative option owing to its low cost and higher abundance [260,261]. Again, the dispersion of nanoparticles of these metals on oxide surfaces is an attractive option as a catalyst for the DRM process.
Among all the formulations, the inexpensive Ni/Al 2 O 3 is the most widely used catalyst for this reaction, although research is still required in order to diminish its deactivation by carbon formation and improve the whole process. Ceria is also an excellent candidate as a support to improve the catalytic performance of nickel in these reforming reactions, particularly due to the SMSI between the oxide and the deposited metals [262]. Precisely, Gonzalez-Delacruz et al. prepared a Ni/CeO 2 catalyst in order to study its behaviour in the dry reforming of methane with the use of various in situ techniques, such as X-ray absorption spectroscopy (XAS) and NAP-XPS [142]. The sample was subjected to different treatments at 923 K, the highest achievable temperature with their near-ambient XPS apparatus. Under 1.33 mbar DRM conditions (CH 4 /CO 2 mixture), Ce 4d spectra of the catalyst presented no changes with respect to the fresh sample, but when it was submitted to a pure methane treatment at the same temperature, important changes appeared in the profile of Ce 4d spectra, revealing an almost complete reduction of the ceria surface to Ce 3+ species. The original signal was not entirely recovered after subsequent treatment with pure CO 2 , indicating the presence of a Ce 3+ and Ce 4+ mixture on the surface of the support. This opposition to be reoxidised was most probably caused by the deposition of carbon on the catalyst, which could be observed by scanning electron microscopy (SEM) after the DRM reaction. The same catalyst formulation was studied by Liu and co-workers [143], although they prepared a model Ni/CeO 2 catalyst which contained~2 nm size NiO nanoparticles dispersed on a CeO 2−x (111) substrate. After annealing the sample at 500 K, some Ni particles sintered and a large fraction of Ni migrated into the ceria support, forming a Ce 1−x Ni x O 2−y solid solution. In this case, they investigated the interaction of CH 4 , CO 2 and a CH 4 /CO 2 mixture with model CeO 2 (111) and Ni/CeO 2 (111) surfaces (in a temperature range of 300−700 K) with NAP-XPS to better understand the chemistry of the DRM process on this type of catalyst. Under a pressure of 0.13 mbar of methane, the bands that appeared in the C 1s region at 300 K indicate that methane dissociates on the Ni/CeO 2 surface at room temperature. When the first hydrogen atom is removed from methane molecules, they quickly dissociate and a CH 3 → CH 2 → CH → C transformation occurs on the catalyst surface, generating C atoms that ultimately react with ceria lattice oxygen atoms to result in gaseous CO and adsorbed CO x species, the latter ones were evidenced by a strong band near 290.2 eV [39]. Weak features at 285-286 eV were probably originated by CH x species on the surface. However, at higher temperatures, CO x and CH x features no longer appeared in the C 1s region. The corresponding spectrum for the Ce 4d and Ni 3p regions did not show evident signs of modifications of Ce 4+ and Ni 2+ oxidation states between 300−500 K, but at 700 K their line shape clearly changed, denoting the reduction of both phases to Ce 3+ and Ni 0 . The amount of Ce 3+ species formed on a plain CeO 2 (111) surface at temperatures above 600 K was much smaller than that on a Ni/CeO 2 (111) surface under similar conditions. Under 0.13 mbar of CO 2 , a strong feature for adsorbed CO x species at 290.2 eV dominated the C 1s XPS spectrum for both CeO 2 (111) and Ni/CeO 2 (111) surfaces, but disappeared upon heating the samples to 500 or 700 K. Additionally, no change in the oxidation state of Ce 4+ and Ni 2+ species was observed at any temperature. Therefore, the reactivity of the system towards CO 2 was not affected by the addition of nickel to CeO 2 (111).
Under an atmosphere of 0.26 mbar CH 4 and CO 2 at 300 K, the C 1s region of the Ni/CeO 2 (111) catalyst exhibited a CO x band near 290.2 eV with enhanced intensity, due to the presence of both gaseous reactants, as well as peaks of adsorbed CH x species (285−286 eV) and CH 4 and CO 2 gas phases (see Figure 2). The signals for adsorbed CO x and CH x species disappeared when the sample was heated from 300 to 500 K. When heating to 700 K, the spectrum showed features for gaseous CO and surface CO x species in addition to the signals for the reactants, as illustrated in Figure 2. Below 700 K, the surface of the catalyst was mainly composed of Ni 2+ and Ce 4+ species, and no catalytic activity was observed. Nevertheless, at 700 K the system became catalytically active when the decomposition products of methane generated Ni 0 and partially reduced Ce 4+ to Ce 3+ . Under a CH 4 /CO 2 mixture, the Ce 3+ fraction generated was again much smaller than under pure methane due to the presence of CO 2 in the gas phase, which reacts with the ceria surface and recovers a significant amount of the oxygen vacancies, CO 2 (g) + Vac → CO(a) + O-Vac. Moreover, the C formed by total dissociation of methane did not react with ceria lattice oxygen atoms but with the O adatoms generated by the decomposition of CO 2 instead.
Following this work, Lustemberg et al. published an investigation related to the effect of Ni coverage on the performance of the same Ni/CeO 2 (111) catalyst for dry reforming of methane [144]. Although they only performed NAP-XPS experiments for a system with low Ni content (Θ Ni~0 .1 ML) to avoid the formation of NiC x , Ni/CeO 2 (111) surfaces with Ni coverages from 0.15 ML to 0.5 ML were studied by multiple characterization techniques. The obtained results revealed that ceria surfaces with a small coverage of Ni are able to activate methane at room temperature, generating adsorbed CO x and CH x species. In other words, Ni coverage on ceria substrate extremely determines metal-support interactions, which are in turn responsible for the low barrier for C−H bond cleavage of methane on this metal/oxide system. To complete the investigation, Liu and co-workers studied the metal-oxide interactions of a series of metal/CeO 2 (111) (metal = Co, Ni and Cu) under DRM conditions at relatively low temperatures (600−700 K) [146]. The behaviour of Co, Ni and Cu on a CeO 2 (111) substrate (with a coverage of 0.2 ML) was first compared using conventional XPS and kinetic testing, as well as theoretical calculations based on DFT. Among the systems examined in a temperature range of 300-700 K, Co/CeO 2 (111) exhibited the best catalytic performance for dry reforming of methane, whereas Cu/CeO 2 (111) had negligible activity. Catalytic tests were in agreement with in situ XPS measurements performed to study their ability to activate pure CH 4 , which showed that the surface with cobalt reduced the most and, consequently, reacted better with methane than Ni/CeO 2 (111) or Cu/CeO 2 (111) catalysts. Indeed, the partial reduction of ceria support is essential for the activation of both reactants. Operando XPS was then employed to study the chemical changes in the best Co/CeO 2 (111) catalyst under reaction conditions (CH 4 /CO 2 mixture), and experiments indicated that methane dissociates on the ceria support with 0.2 ML of Co at temperatures as low as 300 K, generating CH x and CO x species on the surface. Both ceria and Co 2+ appeared partially reduced at 500-700 K, but at 700 K and under DRM conditions, CO 2 dissociates on the oxide surface and slightly reoxidises Co and Ce 3+ , establishing a catalytic cycle without coke deposition. Additionally, catalytic activity for C2 production was also observed at 650 K, since a significant part of the adsorbed CH x species recombined to yield ethane and ethylene. Catalytic activity of the Co/CeO 2 (111) catalyst for DRM and ethane/ethylene production was also examined as a function of cobalt coverage, and they observed a maximum at a coverage of approximately 0.15 ML and 0.1 ML for syngas and ethane/ethylene production, respectively. Therefore, not only the nature of the metal is crucial for the catalyst DRM activity and stability, but also the preservation of a low metal loading (below 0.2 ML), since at higher loadings carbon is formed on the surface leading to catalyst deactivation.
Catalysts 2020, 10, x FOR PEER REVIEW 17 of 48 hydrogen atom is removed from methane molecules, they quickly dissociate and a CH3 → CH2 → CH → C transformation occurs on the catalyst surface, generating C atoms that ultimately react with ceria lattice oxygen atoms to result in gaseous CO and adsorbed COx species, the latter ones were evidenced by a strong band near 290.2 eV [39]. Weak features at 285-286 eV were probably originated by CHx species on the surface. However, at higher temperatures, COx and CHx features no longer appeared in the C 1s region. The corresponding spectrum for the Ce 4d and Ni 3p regions did not show evident signs of modifications of Ce 4+ and Ni 2+ oxidation states between 300−500 K, but at 700 K their line shape clearly changed, denoting the reduction of both phases to Ce 3+ and Ni 0 . The amount of Ce 3+ species formed on a plain CeO2(111) surface at temperatures above 600 K was much smaller than that on a Ni/CeO2 (111) surface under similar conditions. Under 0.13 mbar of CO2, a strong feature for adsorbed COx species at 290.2 eV dominated the C 1s XPS spectrum for both CeO2 (111) and Ni/CeO2 (111) surfaces, but disappeared upon heating the samples to 500 or 700 K. Additionally, no change in the oxidation state of Ce 4+ and Ni 2+ species was observed at any temperature. Therefore, the reactivity of the system towards CO2 was not affected by the addition of nickel to CeO2 (111).
Under an atmosphere of 0.26 mbar CH4 and CO2 at 300 K, the C 1s region of the Ni/CeO2(111) catalyst exhibited a COx band near 290.2 eV with enhanced intensity, due to the presence of both gaseous reactants, as well as peaks of adsorbed CHx species (285−286 eV) and CH4 and CO2 gas phases (see Figure 2). The signals for adsorbed COx and CHx species disappeared when the sample was heated from 300 to 500 K. When heating to 700 K, the spectrum showed features for gaseous CO and surface COx species in addition to the signals for the reactants, as illustrated in Figure 2. Below 700 K, the surface of the catalyst was mainly composed of Ni 2+ and Ce 4+ species, and no catalytic activity was observed. Nevertheless, at 700 K the system became catalytically active when the decomposition products of methane generated Ni 0 and partially reduced Ce 4+ to Ce 3+ . Under a CH4/CO2 mixture, the Ce 3+ fraction generated was again much smaller than under pure methane due to the presence of CO2 in the gas phase, which reacts with the ceria surface and recovers a significant amount of the oxygen vacancies, CO2(g) + Vac → CO(a) + O-Vac. Moreover, the C formed by total dissociation of methane did not react with ceria lattice oxygen atoms but with the O adatoms generated by the decomposition of CO2 instead.  In another report, Zhang et al. also investigated the dry reforming of methane over a series of ceria-supported powder catalysts with different cobalt loadings (2−30 wt %) in order to elucidate the interaction between Co and CeO 2 during the catalytic process, and thus optimise the design of a catalyst with improved activity [147]. Various in situ techniques were used to achieve so, such as in situ time-resolved XRD (TR-XRD). Their results showed a huge reduction of the CoO x -CeO 2 system at temperatures between 473 and 623 K as a consequence of the hydrogen produced by the dissociation of C−H bonds in methane, which fully converted the Co 3 O 4 oxide to metallic Co and partially reduced Ce 4+ to Ce 3+ . A catalytic cycle for DRM was achieved on the catalysts upon dosing CO 2 , at temperatures below 773 K. Among the different Co loaded catalysts, the 10 wt % Co/CeO 2 catalyst appeared to have the highest catalytic performance, exhibiting desirable stability for the DRM process with the lowest effect of coke accumulation. For this reason, and since TR-XRD is a primarily bulk-sensitive technique, the surface of the 10 wt % Co/CeO 2 sample was also examined under reaction conditions with NAP-XPS. As depicted in Figure 3, results showed a dynamic evolution in the oxidation state of the catalyst under reaction conditions. The partial reoxidation of ceria upon switching the H 2 reducing atmosphere to a CH 4 /CO 2 mixture is evident at room temperature since Ce 3+ features at the Ce 3d region attenuate and the bands of Ce 4+ species become more intense. A temperature increase led to a further reduction of ceria, as seen in Figure 3, although CO 2 weakened the reducing effects of methane. As expected, the initial Co 3 O 4 phase was completely reduced to metallic Co after the H 2 pretreatment. When the atmosphere was changed to DRM conditions, a small amount of cobalt reoxidised, but it remained mainly Co 0 as the reaction advanced to 773 K. Analyses of the O 1s region allowed the detection of CO x species (e.g., carbonate, carboxyl, bicarbonate) adsorbed on the catalyst surface, ascribing CO x as a possible reaction intermediate. Xie and colleagues [148] also investigated the activity for DRM of a cobalt-based catalyst; they prepared a PtCo/CeO2 bimetallic catalyst and compared it with the corresponding monometallic ones to explore the Pt-Co synergy for DRM. As in situ XRD, DRIFTS and XAFS analyses provided bulk-averaged structural information, NAP-XPS experiments were performed for the bimetallic sample to obtain surface information during reaction conditions. Again, the results revealed a dynamic evolution in the chemical composition of the catalyst surface: upon exposure to the reactant stream, both Pt and ceria slightly oxidised due to the presence of CO2 in the environment. With the combination of multiple techniques such as in situ XRD, TPR, in situ XAFS and DRIFTS, PtCo/CeO2 sample was found to have the highest catalytic activity, and both Pt-Co alloy and isolated Co particles co-existed in its structure. Nevertheless, the Pt-Co alloy is the dominant active structure, which remained nearly metallic during the reaction.
Very recently, Liu and co-workers proposed another ceria-based catalyst formulation for dry reforming of methane by changing the nature of the supported metal, a key component in such catalytic process [149]. In their work, they reported a highly active and stable ceria-supported Ru-nanocluster (< 1 nm) catalyst (denoted as Ru(NC)−CeO2) for the DRM, since Ru-based catalysts have exhibited high activity and stability against the deactivation by carbon accumulation [257,263]. In situ XRD and XAFS were used to elucidate the structure-reactivity relationship that caused the remarkable catalytic performance, whereas the surface chemistry and possible surface-active intermediates were monitored by NAP-XPS and DRIFTS. As the activation of methane is a crucial step in the DRM process, the catalyst was subjected to an atmosphere of pure methane before studying it under DRM conditions with operando XPS. Ce 3d NAP-XPS spectra evidenced a gradual Xie and colleagues [148] also investigated the activity for DRM of a cobalt-based catalyst; they prepared a PtCo/CeO 2 bimetallic catalyst and compared it with the corresponding monometallic ones to explore the Pt-Co synergy for DRM. As in situ XRD, DRIFTS and XAFS analyses provided bulk-averaged structural information, NAP-XPS experiments were performed for the bimetallic sample to obtain surface information during reaction conditions. Again, the results revealed a dynamic evolution in the chemical composition of the catalyst surface: upon exposure to the reactant stream, both Pt and ceria slightly oxidised due to the presence of CO 2 in the environment. With the combination of multiple techniques such as in situ XRD, TPR, in situ XAFS and DRIFTS, PtCo/CeO 2 sample was found to have the highest catalytic activity, and both Pt-Co alloy and isolated Co particles co-existed in its structure. Nevertheless, the Pt-Co alloy is the dominant active structure, which remained nearly metallic during the reaction.
Very recently, Liu and co-workers proposed another ceria-based catalyst formulation for dry reforming of methane by changing the nature of the supported metal, a key component in such catalytic process [149]. In their work, they reported a highly active and stable ceria-supported Ru-nanocluster (< 1 nm) catalyst (denoted as Ru(NC)−CeO 2 ) for the DRM, since Ru-based catalysts have exhibited high activity and stability against the deactivation by carbon accumulation [257,263]. In situ XRD and XAFS were used to elucidate the structure-reactivity relationship that caused the remarkable catalytic performance, whereas the surface chemistry and possible surface-active intermediates were monitored by NAP-XPS and DRIFTS. As the activation of methane is a crucial step in the DRM process, the catalyst was subjected to an atmosphere of pure methane before studying it under DRM conditions with operando XPS. Ce 3d NAP-XPS spectra evidenced a gradual reduction of ceria surface, that is, the formation of oxygen vacancies and Ce 3+ species, after exposing the sample to pure CH 4 . Ceria reduction, which occurred at temperatures as low as 423 K, was accompanied by reduction of RuO 2 , as expected. Under a mixture of CH 4 /CO 2 at 423 K, the ceria surface underwent an initial reduction, comparable to the case of methane alone, suggesting an effective activation of methane. Figure 4 illustrates the NAP-XPS spectra for the Ce 3d and C 1s + Ru 3d regions of the 0.5 wt % Ru(NC)-CeO 2 catalyst under DRM conditions. At temperatures greater than 573 K, a relevant degree of ceria reoxidation was observed, which involved a significant dissociation of CO 2 on either ceria or Ru sites, generating O adatoms and subsequently refilling the oxygen vacancies of the surface. This dual-site mechanism not only allows the direct activation of CO 2 on the metal sites, but also allows the O adsorbed on ceria sites to assist the oxidation of surface carbon on Ru sites.  As depicted in Figure 4b, carbon deposited on the catalyst surface as the reaction developed. At 573 K or below, methane activation led to the formation of surface carbon on the catalyst, and at temperatures higher than 573 K, the O adatoms generated from CO2 dissociation reoxidise the surface carbon due to the intimate Ru−O−Ce interaction, completing the catalytic cycle. This evolution is evidenced by a clear drop of surface carbon peak intensity, at ~ 284.6 eV. Figure 4b also revealed that under reaction conditions, the active structure of the Ru(NC)-CeO2 catalyst was partially oxidised Ru clusters stabilized by reduced ceria (Ru δ+ −CeO2−x).
We have seen that DRM is an attractive process since it transforms two greenhouse gases (CO2 and methane) into syngas, which can be subsequently used to produce value-added chemicals and fuels. Nevertheless, DRM is a highly endothermic reaction and demands high temperatures to achieve significant conversion. Therefore, an alternative approach to convert CO2 to syngas is the use of ethane and other light alkanes (e.g., butane) present in shale gas [264,265]. Dry reforming of ethane (DRE, Equation (5)) and dry reforming of butane (DRB, Equation (6)) generate synthesis gas through: C2H6 + 2CO2 → 3H2 + 4CO; ΔH298 = 428.1 kJ/mol, Temperatures of approximately 760 and 720 K are required to attain the 50% conversion of CO2 in DRE and DRB, respectively, under equilibrium conditions of stoichiometric ratio. Due to the decrease of the reaction temperatures with respect to the DRM process, catalyst deactivation occurs As depicted in Figure 4b, carbon deposited on the catalyst surface as the reaction developed. At 573 K or below, methane activation led to the formation of surface carbon on the catalyst, and at temperatures higher than 573 K, the O adatoms generated from CO 2 dissociation reoxidise the surface carbon due to the intimate Ru−O−Ce interaction, completing the catalytic cycle. This evolution is evidenced by a clear drop of surface carbon peak intensity, at~284.6 eV. Figure 4b also revealed that under reaction conditions, the active structure of the Ru(NC)-CeO 2 catalyst was partially oxidised Ru clusters stabilized by reduced ceria (Ru δ+ −CeO 2−x ).
We have seen that DRM is an attractive process since it transforms two greenhouse gases (CO 2 and methane) into syngas, which can be subsequently used to produce value-added chemicals and fuels. Nevertheless, DRM is a highly endothermic reaction and demands high temperatures to achieve significant conversion. Therefore, an alternative approach to convert CO 2 to syngas is the use of ethane and other light alkanes (e.g., butane) present in shale gas [264,265]. Dry reforming of ethane (DRE, Equation (5)) and dry reforming of butane (DRB, Equation (6)) generate synthesis gas through: Temperatures of approximately 760 and 720 K are required to attain the 50% conversion of CO 2 in DRE and DRB, respectively, under equilibrium conditions of stoichiometric ratio. Due to the decrease of the reaction temperatures with respect to the DRM process, catalyst deactivation occurs to a lower extent and, consequently, there are more options in the design of a stable catalyst.
Yan et al. prepared model and conventional PtNi/CeO 2 catalysts, as well as their corresponding monometallic catalysts, and compared their catalytic performance for DRE and DRB [145]. In spite of the elevated cost of noble metal loaded catalysts (such as Pt) for large-scale processes, they commonly show higher resistance to coke accumulation and can be used to promote Ni-based catalysts so as to reduce C deposition and, hence, increase its lifetime. Results from both model thin films and supported powder catalysts revealed that the PtNi/CeO 2 bimetallic catalyst shows higher activity than the corresponding monometallic samples. In their study, multiple spectroscopic techniques, including NAP-XPS, in situ XRD, in situ XAFS and DRIFTS were employed to probe catalyst structures and surface intermediate species under reaction conditions. NAP-XPS measurements were performed only on model catalysts and, due to the stronger interaction of butane in comparison to ethane, only DRB was investigated with this technique. Model catalysts were prepared by depositing small coverages of PtNi, Pt or Ni (0.1 ML) onto a CeO 2 film (3 ML) over a TiO 2 (110) substrate, while supported catalysts were loaded with 1.7 wt % of Pt and 1.5 wt % of Ni, corresponding to an atomic ratio Pt:Ni of 1:3. Under reaction conditions (0.13 mbar CO 2 and 0.26 mbar butane), ceria substrate and metals, except for the Ni of Ni/CeO 2 , were partially reduced. Interestingly, Ni of PtNi/CeO 2 catalyst was reduced to metallic nickel, which indicates that the presence of Pt enhances Ni reduction. In the C 1s region, a band attributed to carbonate and carboxyl (CO 2 δ− ) was observed for all catalysts, indicating the effective activation of CO 2 for all three surfaces.
Additionally, two more peaks appeared at 284.5 and 289.8 eV, which were assigned to the adsorbed O−C x H y species (generated from butane decomposition) and adsorbed formate, respectively. The latter was generated upon hydrogenation of adsorbed carbonate/carboxylate species. It is worth noting that each surface presented different band intensity of the adsorbed O−C x H y species, following the trend PtNi/CeO 2 > Ni/CeO 2 > Pt/CeO 2 , which denoted that Ni is more active than Pt, although PtNi bimetallic surface further improves butane activation.

Alcohols
The high environmental impact of fossil fuels has urged the need for alternative energy sources over the last decades. Hydrogen is being considered as the future clean and affordable fuel to be used in fuel cells or in large-scale processes such as ammonia synthesis, considering the abundant availability of H 2 -containing substances in nature, its high energy content (120.7 kJ/g) and its non-polluting combustion. For this reason, fuel cell research and development have received a huge amount of funding in the last years. Among the multiple chemical carriers of hydrogen, light alcohol methanol and ethanol constitute important candidates to produce H 2 via different catalytic pathways.

Steam Reforming of Alcohols
Hydrogen production from ethanol has been extensively studied, since ethanol can be obtained by the fermentation of agricultural wastes (biomass) and, therefore, constitutes a carbon-neutral renewable precursor [266,267]. Currently, catalytic research is focusing on steam reforming of ethanol (SRE), partial oxidation of ethanol (POX) and the oxidative steam reforming (OSR) as potential process candidates to produce H 2 [267,268]. SRE (see Equation (7)) is the most efficient pathway of renewable hydrogen production, but it is an endothermic reaction that requires an active catalyst and a sufficient energy input to obtain a high H 2 yield and ethanol conversion with a reasonable reaction rate.
Generally, noble metals supported on inorganic oxides have proved to exhibit higher catalytic activity for SRE than non-noble metal-based ones [269][270][271][272][273][274]. As an alternative to expensive metals such as Rh, Ru, Pd and Pt, non-noble metal-based catalysts (Ni, Co and Cu) have also been investigated at higher metal loadings, and they have become promising catalysts for the reaction [275][276][277][278][279][280]. The catalytic performance can also be influenced by the oxide support. Among the various metal oxides studied, ceria has exhibited a key role in reducing coke accumulation on the surface of the catalyst as well as modifying the reaction kinetics [281,282]. As already stated, the excellent performance of ceria as a support or even as a catalyst is associated to its high OSC and its rapid change between Ce 4+ and Ce 3+ oxidation states. Carbon deposition is mitigated on account of the transportation of oxygen species from ceria to the supported metal, which confers significantly improved catalytic activities under SRE conditions. Bimetallic catalysts have also been studied in the SRE reaction. Divins et al. used the NAP-XPS technique to demonstrate that the presence of a reducible CeO 2 support greatly influenced the surface rearrangement of bimetallic Rh-Pd nanoparticles under SRE conditions on real catalysts [68].
They performed the reaction between ethanol and water (EtOH:H 2 O = 1:6) at 823 K and a sample pressure of 0.05 mbar over (a) unsupported model Rh 0.5 Pd 0.5 nanoparticles (NPs) and (b) Rh 0.5 Pd 0.5 NPs supported on ceria powder. Among noble metals, Rh is highly active for both C-C and C-H bond cleavage, induces hydrogenation reaction and causes very low carbon deposition [283]. Both systems were subjected to reducing, oxidising and SRE conditions to produce H 2 , and three different photon energies (670, 875 and 1150 eV) were used to obtain a depth-profile study of both samples and deduce the rearrangement and the development of a core-shell structure induced by the environment. Results showed that unsupported model NPs were more strongly reduced for all the atmospheres tested. Upon reduction with H 2 at 573 K, Pd segregated towards the surface for both unsupported and supported NPs. Nevertheless, under SRE conditions at 823 K, unsupported NPs suffered from a restructuration as Rh atoms migrated toward the surface and became reduced due to the reducing effect of the hydrogen produced during the ethanol steam reforming reaction. No migration of Rh or Pd was found for ceria supported NPs under ESR and, most importantly, both metals became more oxidised, as illustrated in Figure 5. Moreover, palladium developed a core-shell structure of oxidation states: as seen in Figure 5, the outer shell (670 eV photon energy spectrum) of the Rh 0.5 Pd 0.5 /CeO 2 catalyst exhibited a high amount of oxidised Pd species. This oxidation of the outermost layers of the supported NPs is likely due to the creation of -OH groups at the catalyst surface upon activation of water by ceria. Therefore, the interaction of the metal NPs with the support plays a crucial role in reactions catalysed by the RhPd NPs since it limits the dynamic reorganization of the metals under operating conditions ("quenching effect") and supplies active oxygen atoms to the metals at the surface of the NPs. In a deeper analysis of the same Rh 0.5 Pd 0.5 /CeO 2 system, Soler et al. reported the influence of the support morphology in the reorganization of bimetallic nanoparticles, which has important consequences for catalytic performance, also by means of NAP-XPS [69]. With this aim, they monitored the surface composition and chemical states of preformed Rh 0.5 Pd 0.5 model NPs of 4 nm size supported on two different types of nanoshaped ceria: nanocubes (CeO 2 -c) and nanorods (CeO 2 -r), during the catalytic SRE at 823 K. Both systems were also exposed to reducing, SRE and final reducing conditions, and again three different photon energies were used to acquire the corresponding spectra and perform a depth-profile study of the bimetallic NPs under operando experiments, as previously described [68]. Under initial H 2 conditions at 573 K, both catalysts exhibited the same amount of oxidised and reduced Rh, whereas Rh 0.5 Pd 0.5 /CeO 2 -c contained a larger fraction of metallic Pd than Rh 0.5 Pd 0.5 /CeO 2 -r, which also exhibited a minor fraction of Pd IV species, almost inexistent in CeO 2 -c. Furthermore, a core-shell structure of oxidation states of Pd was observed for the bimetallic NPs supported on CeO 2 -r, Pd being more reduced in the inner region of the bimetallic NPs. However, the two catalysts showed completely different behaviours under SRE conditions at 823 K: bimetallic NPs supported on CeO 2 -c appeared dramatically oxidised, with predominant Pd 4+ and Rh + /Rh 3+ species, whereas Pd of bimetallic NPs supported on nanorods became more reduced with respect to the activation treatment under H 2 , with minor fractions of Pd 2+ and Pd 4+ species. This difference in the catalytic performance between both systems can be explained by the ability of the different exposed crystallographic planes to release oxygen atoms: indeed, oxygen vacancy formation on {100} planes of ceria nanocubes is thermodynamically more favourable than on {110} and {111} planes usually found in CeO 2 -r and polycrystalline ceria [4]. For this reason, ceria nanocubes easily transferred oxygen atoms to the supported NPs, which resulted in their oxidation. Moreover, SRE reaction did not progress on Rh 0.5 Pd 0.5 /CeO 2 -c because, although ethanol can be effectively dehydrogenated into acetaldehyde and H 2 over metal oxides [284], the metallic function is required for methane steam reforming, which constitutes the last step in the SRE. Additionally, NPs of both catalysts rearranged under SRE conditions at 823 K: Pd segregated toward the surface of the NPs supported on CeO 2 -r, while the exact opposite (Rh segregation to the surface) was observed for the Rh 0.5 Pd 0.5 /CeO 2 -c system. Finally, the reduction step at 823 K caused the reduction of the metals for both systems, but did not result in a modification of the relative distribution of Pd and Rh. As for Ce 3d spectra, illustrated in Figure 6, they indicated a sharp increase of Ce 3+ species for both catalysts upon exposure to H 2 at 823 K, as expected. Although the Ce 3+ /Ce 4+ ratio of the Rh 0.5 Pd 0.5 /CeO 2 -c catalyst exceeded that of Rh 0.5 Pd 0.5 /CeO 2 -r during the last reduction step, the latter exhibited higher Ce 3+ /Ce 4+ ratio under SRE conditions with respect to Rh 0.5 Pd 0.5 /CeO 2 -c, which was attributed to its higher hydrogen production.     Among the alternatives to noble metal-based catalysts for SRE, cobalt supported catalysts are considered promising candidates for the reaction, since they have comparable activity with noble metals for C-C bond scission in adsorbed ethanol at moderate temperatures, but considerably low prices [276,279,284]. Óvári et al. used NAP-XPS to study the interaction of ethanol with a well-ordered CeO 2 (111) film evaporated over Cu (111) and with Co/CeO 2 (111)/Cu(111) model catalyst [150]. In this case, researchers did not investigate their systems under ethanol steam reforming conditions, but simply studied the interaction of ethanol with their catalysts to determine not only the chemical nature of transient intermediates but also the oxidation states of the surface-active components. After characterizing the evaporated CeO 2 (111) film on the Cu(111) surface, Óvári et al. investigated the adsorption and decomposition of ethanol on the oxide at 300 K and different pressures (10 −6 , 10 -4 , 0.01, 0.1 and 1 mbar). At this temperature, an increase in ethanol pressure resulted in a gradual reduction of ceria, probably via H 2 O desorption involving lattice oxygen atoms. However, the reduction was hindered at pressures of 10 -4 mbar or higher due to a reduced mobility of either oxygen or Ce 3+ centres. Ethanol adsorption at 300 K was also detected through the recorded O 1s spectra, which at low pressures showed a shoulder at 531.5 eV usually assigned to formation of -OH groups, but also possibly originated by ethoxide and acetaldehyde surface species, expected at this binding energy too [267,285]. At higher pressures, an additional and weak contribution at 534.3 eV was detected, attributed to weakly held, molecularly adsorbed ethanol. Researchers also investigated the reaction of 0.1 mbar ethanol on bare CeO 2 (111) film at different temperatures (from 320 up to 600 K) before cobalt deposition. During this temperature increase, the reduction of Ce 4+ to Ce 3+ species raised significantly, observable through Ce 3d and O 1s spectra. The primary intermediate, ethoxide, was again detected at all temperatures (285.7 eV) and formed by dissociative adsorption of ethanol, and no coke deposition was observed up to 600 K on ceria. Upon deposition of 0.7 ML of cobalt at 300 K, partial reduction of ceria was observed, which also led to the formation of Co 2+ sites but still leaving metallic Co in metal particles, suggested by the large width of the main peak at Co 2p region. Since the reaction of cobalt and ceria is expected to develop mainly at the support/metal interface, the small amounts of unreacted Co 0 species are likely to be found primarily on top of Co clusters, available for interaction with ethanol. When exposing the Co/CeO 2 (111) model catalyst to 0.1 mbar of ethanol, Co 2+ species decreased drastically with increasing temperature, and Co was mostly metallic at 600 K. This process was accompanied by further reduction of ceria and the formation of surface carbon deposits, which was not observed on pure ceria and contributed to the severe intensity loss in the Co 2p spectra, together with Co nanoparticles sintering.
Different cobalt-based catalyst configurations were reported by Turczyniak and co-workers [152], who investigated the effect of SRE conditions over various forms of Co (single crystal, nanoparticles, and supported on CeO 2 and ZnO) by means of ambient pressure XP and absorption spectroscopies. Systems were exposed to oxidising, reducing and SRE atmospheres, and results showed that under 0.2 mbar O 2 at 523 K, Co 2p 3/2 signal and the Co L 3 -edge spectra were characteristic for the Co 3 O 4 spinel oxide for each cobalt configuration, while Ce 3d XP spectra exhibited typical features for bulk CeO 2 . Nevertheless, under 0.2 mbar H 2 at 693 K, ceria-supported Co 3 O 4 was not entirely reduced to metallic Co compared to the single crystal (Co-sc) and the nanopowdered sample, but in fact a significant amount of unreduced CoO remained on the surface. As expected, ceria partially reduced under H 2 environment, but it is worth noting that Co presence promoted the reduction of the ceria support, as compared to pure ceria. Interestingly, a gaseous mixture of ethanol and H 2 O (C 2 H 5 OH:H 2 O ratio of 1:3) caused a higher reducing effect for cobalt oxides than molecular H 2 , since supported Co got fully reduced to the metallic state under 0.3 mbar SRE conditions. Thus, the milder reduction conditions for Co-sc and Co nanoparticles suggested that the ceria support had a stabilization effect over supported CoO. As for the CeO 2 support, both NAP-XPS and NEXAFS results indicated slight oxidation of the oxide. Apart from the features related to the contribution of gaseous H 2 O and C 2 H 5 OH, O 1s spectra of the systems under SRE showed additional components: the band at 531.5 eV was attributed to adsorbed -OH groups, whereas the peak located at 533.4 was correlated with molecularly adsorbed H 2 O species and/or oxygenated carbon impurities. Finally, an asymmetric peak at 284.4 eV prevailed in all C 1s XP spectra, characteristic of hydrocarbon or graphitic carbon species. However, a severe difference in the amount of deposited C under SRE conditions was detected between unsupported and supported cobalt-based samples, revealing the role of mobile ceria lattice oxygen atoms in the prevention of catalyst coking. Following their work, Turczyniak et al. examined the impact of the Co/CeO 2 catalyst composition and surface oxidation state on the SRE reaction performance by combining operando and ex situ XPS in a wide pressure range (0.2-20 mbar) [151]. The catalyst was subjected to oxidative (O 2 ) or reductive (H 2 or ethanol vapour) gaseous environments before exposing it to SRE conditions. Under 0.2 mbar O 2 and 523 K, Co 2p and Ce 3d signals indicated the presence of Co 3 O 4 spinel phase and bulk CeO 2 , respectively. Contrarily, reducing pretreatment conditions (0.2 mbar H 2 or gaseous ethanol at 693 K) induced the complete reduction of Co to the metallic state and a partial reduction of ceria, leading to a mixture of Ce 3+ and Ce 4+ species with a higher Ce 3+ /Ce 4+ ratio under ethanol atmosphere than under hydrogen. Independent of the prior surface oxidation state under the pretreatment atmospheres, metallic Co 0 bands dominated the Co 2p 3/2 photoemission spectrum during SRE conditions (EtOH:H 2 O = 1:3 at 693 K), while the mixture of Ce 3+ and Ce 4+ species present under reaction conditions was influenced by the pretreatment, as indicated by the small but notable differences in the Ce 3d spectra. CO and H 2 production were favoured with this surface state, indicating that C-C bond scission is the key route in this pressure regime. The population of adsorbed hydroxyl groups increased with the degree of ceria reduction, but they surprisingly inhibited the SRE activity and the C-C bond yield. Therefore, oxidised ceria supports promoted the cleavage of C-C ethanol bond with Co keeping its metallic state.
Another example of a cobalt-based catalyst for SRE reaction is reported in the work of Sohn et al. [153], who investigated the effect of supported cobalt nanoparticles on the catalytic performance of nano-ceria (CeO 2 -NP) and micro-ceria (CeO 2 -MP) under ethanol steam reforming conditions, by using NAP-XPS and X-ray absorption near edge structure (XANES) techniques. Those characterization methods allowed the study of both surface and bulk properties of the samples, respectively. CeO 2 -NP size varied from 2 to 10 nm, with an average size of 4 nm, while CeO 2 -MP exhibited much larger particle sizes, which varied significantly from 40 to 200 nm with a mean particle size of 120 nm. Cobalt particle sizes were also different between the two Co/CeO 2 samples, with bigger Co particles observed over CeO 2 -MP. Researchers found that surface reducibility was altered by the particle size of bare ceria particles, smaller ones leading to a higher surface reduction degree. The presence of completely oxidised Co nanoparticles on CeO 2 -NP and CeO 2 -MP hindered surface reducibility of ceria. This effect could be explained since reducing agents (such as ethanol and produced hydrogen) may be principally consumed to reduce the cobalt oxide species (Co 3 O 4 and CoO), which are fully oxidised at the initial steps (He, 300 K). It could also be caused by the dissociation of H 2 O molecules on the Co surface, which may spillover to the ceria support and subsequently partially oxidise its surface. Under SRE conditions and increasing temperature (623-723 K with 0.13 and 1.33 mbar of C 2 H 5 OH and H 2 O, respectively, so as to achieve an ethanol and water mixture of 1:10 molar ratio), researchers observed an increasing degree of ceria reduction. NAP-XPS measurements indicated the presence of both metallic Co and CoO x at the surface of cobalt nanoparticles, which actually consisted of a metallic Co-based shell and CoO x -based core. Eventually, they found much larger differences between Co/CeO 2 -NP and Co/CeO 2 -MP than between bare ceria supports, which reflected the importance of metallic Co in SRE catalysis. In another work, Sohn et al. [101] examined the same catalysts (Co loaded CeO 2 -NP and MP) and the focus of their study was the surface oxygen mobility and oxygen vacancy formation on their samples, which has been already reviewed in Section 3.1.
Ni-oxide based materials have also emerged as promising catalysts for the SRE reaction owing to their ability to activate C-C and C-H bonds in hydrocarbon oxygenates [280] and their activity comparable to that of expensive noble metals such as Rh, Pt and Pd [269,271]. The combination of Ni and ceria in a catalyst confers the ability to activate both ethanol (C-C and C-H bonds) and H 2 O (O-H), and selectively extract hydrogen avoiding the production of CH 4 or other C-O by-products (aldehydes or olefins). Liu and colleagues [154] combined NAP-XPS and AP-IRRAS techniques to elucidate the catalytic chemistry (active phases and surface species) and the elementary steps related to the SRE reaction over Ni/CeO 2 (111) model catalysts. Ceria was evaporated onto a Ru single crystal (0001) and was estimated to be ca. 4 nm thick. Ni was then deposited on the as-prepared ceria film by physical evaporation, and its coverage was estimated to be 0.15 ML. Although ceria alone is not catalytically active for the SRE process, researchers studied the chemistry of ethanol and H 2 O at elevated pressures over bare CeO 2 (111) before investigating the SRE reaction over Ni/CeO 2 (111) catalyst. The exposure of~0.05 mbar of ethanol at 300 K led to the formation of ethoxy species (CH 3 CH 2 O-) on the ceria surface, as reflected in both C 1s and O 1s regions, formed upon binding of deprotonated H to ceria lattice oxygen due to the dissociative adsorption of ethanol. After adding~0.26 mbar of water into the reaction to achieve a~5:1 (H 2 O:EtOH) vapour mixture, an additional band appeared in the C 1s region, indicating the formation of small amounts of dioxyethylene species (CH 3 CHO 2 -). In this atmosphere, the sample was then heated from 300 to 700 K and, according to the recorded C 1s and O 1s spectra, most of the ethoxy species recombined with surface hydroxyls and gradually desorbed from the surface up to 700 K. Two additional features appeared, attributed to dioxyethylene (CH 3 CHO 2 -) and acetate (C 2 H 3 OO-) species, generated through the oxidation of ethoxy species, but no evidence of C-C bond scission was observed. As expected, bare ceria is not likely to perform the key step of the SRE reaction. Nevertheless, ceria strongly became reduced by ethanol from 500 to 700 K generating Ce 3+ species, as well as oxygen vacancies, which in turn dissociated H 2 O into hydroxyl groups and even led to the formation of cerium hydroxide compounds. The same experimental procedure was followed for the Ni/CeO 2 (111) catalyst, and NAP-XPS results (see Figure 7) revealed that under SRE conditions small supported Ni nanoparticles were present as Ni 0 /Ni x C, the active phase that leads to both C-C and C-H scission of ethanol and also carbon accumulation. Concurrently, the ceria surface appeared highly reduced and hydroxylated and played an important role in the deprotonation of ethanol and water with subsequent generation of hydroxyls, which are essential intermediates to react and remove CH x or surface carbon. Indeed, the active phase of CeO x was a Ce 3+ (OH) x compound resulted from the reduction by ethanol and the efficient dissociation of H 2 O. reduced by ethanol from 500 to 700 K generating Ce species, as well as oxygen vacancies, which in turn dissociated H2O into hydroxyl groups and even led to the formation of cerium hydroxide compounds. The same experimental procedure was followed for the Ni/CeO2(111) catalyst, and NAP-XPS results (see Figure 7) revealed that under SRE conditions small supported Ni nanoparticles were present as Ni 0 /NixC, the active phase that leads to both C-C and C-H scission of ethanol and also carbon accumulation. Concurrently, the ceria surface appeared highly reduced and hydroxylated and played an important role in the deprotonation of ethanol and water with subsequent generation of hydroxyls, which are essential intermediates to react and remove CHx or surface carbon. Indeed, the active phase of CeOx was a Ce 3+ (OH)x compound resulted from the reduction by ethanol and the efficient dissociation of H2O. Despite the multiple advantages of bio-derived ethanol, as its low sulphur content and low toxicity as compared to methanol, steam reforming of methanol (SRM) has a lower activation temperature and better selectivity (less CO or coke formation) than other heavier alcohols owing to the absence of a C-C bond [286]. The most common studied catalysts for SRM are Cu-based metal-oxides because of copper's well-known capability of catalysing methanol synthesis in the industry [287][288][289]. Nevertheless, Cu-based catalysts possess multiple drawbacks such as their pyrophoricity and catalytic deactivation due to metal sintering. Nickel has recently been reported as a non-expensive alternative metal with favourable catalytic activity and selectivity for the SRM Despite the multiple advantages of bio-derived ethanol, as its low sulphur content and low toxicity as compared to methanol, steam reforming of methanol (SRM) has a lower activation temperature and better selectivity (less CO or coke formation) than other heavier alcohols owing to the absence of a C-C bond [286]. The most common studied catalysts for SRM are Cu-based metal-oxides because of copper's well-known capability of catalysing methanol synthesis in the industry [287][288][289]. Nevertheless, Cu-based catalysts possess multiple drawbacks such as their pyrophoricity and catalytic deactivation due to metal sintering. Nickel has recently been reported as a non-expensive alternative metal with favourable catalytic activity and selectivity for the SRM reaction [290,291]. In spite of being prone to deactivation due to the coke formation, Liu et al. showed that the combination of Ni with Ce is able to significantly mitigate the accumulation of surface carbon. [154]. For this reason, they recently investigated both powder (Ni/CeO 2 ) and model (Ni/CeO 2 (111)) catalysts for SRM reaction in order to elucidate structure-reactivity correlations under reaction conditions [155]. In situ XRD and DRIFTS were used to study the active phase evolution and surface species transformation on powder catalysts, and they observed phase transitions of NiO → NiC → Ni and CeO 2 → CeO 2-x during the reaction. By means of NAP-XPS, researchers first examined the interaction of methanol with an oxidised Ni/CeO 2 (111) surface in the temperature range of 300-700 K. The formation of methoxy and hydroxyls groups at room temperature, as a result of dissociative adsorption of methanol, was detected through both O 1s and C 1s spectra. However, these contributions disappeared upon heating the surface up to 700 K, likely due to methanol desorption. With a moderate dose of methanol (2 L, L = Langmuir), no Ni reduction was detected and only a minor reduction of ceria was observed. These results indicated that a fully oxidised Ni/CeO 2 (111) surface is catalytically inert for methanol reaction. The same measurements were performed over a reduced Ni/CeO 1.8 (111) surface, which was found to behave differently. O 1s spectra showed an enhancement in the relative intensity of hydroxyl/methoxy peak (-OH/CH 3 O-) at 300 K, resulting from the higher amount of oxygen vacancies on the oxide substrate for dissociative adsorption of methanol, but again it attenuated and finally disappeared as temperature increased to 700 K. Nevertheless, at 500 K they detected a new contribution, which was attributed to a Ni carbide (Ni 3 C) species. Finally, another experiment was performed by heating the pre-dosed methoxy covered surface in a background of 2.66 × 10 -5 bar of water, so as to check the effect of surface hydroxyl and H 2 O. Similarly, surface C/Ni 3 C species were formed at 500 K, but no carbon was detected on the surface upon heating to 700 K, fact that emphasized the significance of hydroxyls/water on the selectivity of SRM and the role of metal-support interactions, that links the metal and the oxide to complete the reaction cycle and contributes to the high selectivity of the catalysts.

Methanol Oxidation
The oxidation of methanol has been proposed as a probe reaction to characterise the catalytic performance of metal oxides [292]. Depending on the products generated, catalysts have been classified as acidic, basic or redox. Acidic materials yield coupling products such as dimethyl ether, basic materials cause dehydrogenation with CO and CO 2 as products, and redox catalysts produce formaldehyde. Very recently, Mullins [156] reported the reaction of methanol with and without O 2 on a flat and highly crystalline CeO 2 (100) surface as a function of temperature and pressure, via ambient pressure XPS. The results indicated that, in the absence of O 2 , methoxy (CH 3 O-) is the prevailing surface species in both low-pressure (≤ 1.3 × 10 -5 mbar) and high-pressure regimes (≥ 0.13 mbar). Moreover, methanol decomposition considerably reduced the ceria thin film and C x accumulated on its surface. Upon dosing oxygen, C x surface depositions were not observed, and the nature of the dominant surface species was dependent on the pressure. Methoxy still prevailed in the low-pressure regime, and its coverage decreased with temperature and was smaller than the coverage in the absence of O 2 , which evidences the reaction between methoxy groups and the gaseous oxygen. In the high-pressure regime, surface formate (HCOO) − dominated. Therefore, the nature of surface species appeared to be related to the oxygen ability to maintain a completely oxidised ceria surface during the methanol reaction.

Hydrogenation Reactions
Recently, ceria has received intense interest in reactions such as hydrogenation of alkynes to alkenes [293,294]. Indeed, the interactions of H 2 with CeO 2 have long been experimentally and theoretically studied to understand the redox properties of ceria-based catalysts, as ceria oxygen vacancies are commonly formed through hydrogen reduction [295,296]. Nevertheless, there is still no direct experimental evidence for the presence of cerium hydride (Ce−H) upon hydrogen dissociation over ceria, and the mechanism of the hydrogenation reaction still remains elusive. In their work, Wu et al. [157] reported for the first time direct experimental evidence for the formation of both surface and bulk Ce−H species upon H 2 interaction with ceria nanorods by using in situ inelastic neutron scattering spectroscopy (INS). Combined with other in situ spectroscopic techniques such as IR, Raman and NAP-XPS, as well as DFT calculations, their results confirmed that hydrogen dissociates over ceria creating homolytic products (OHs) on a close-to-stoichiometric ceria surface, while heterolytic products (Ce−H and OH) result with the presence of induced oxygen vacancies in the oxide surface. NAP-XPS measurements were performed over ceria nanorods during an H 2 treatment (0.65 mbar) at different temperatures (533, 623 and 673 K) in order to monitor ceria oxidation states upon hydrogen interaction. Another investigation of ceria's hydrogenation ability, specifically C=O bond hydrogenation, was carried out on a series of CeO 2 /Pt catalysts by Mueanngern and co-workers [158]. The goal of their work was to examine the scaling of catalytic activity for a support-mediated reaction pathway with respect to distance from the active metal-support interface. With this aim, researchers prepared an inverse catalyst system based on CeO 2 nanocubes supported on a Pt thin film. Langmuir-Blodgett deposition was employed to deposit the ceria nanocubes at well-controlled coverages onto the metallic film, and surface pressure was followed as a function of film compression during nanoparticle deposition. They showed that deposition of CeO 2 nanoparticles leads to two types of Pt/CeO 2 interfaces that extend over multiple length scales: firstly, a nanoscale interface defined by the contact of individual ceria nanoparticles with the Pt surface and, secondly, a larger, mesoscale interface defined by the limit between domains of self-assembled nanoparticles and the surrounding Pt support. Results indicated that almost no C=O bond hydrogenation occurred within a domain of closely spaced ceria nanoparticles. Instead, the rate of C=O bond hydrogenation was quite high at the boundary between a domain of ceria nanoparticles and the surrounding Pt substrate. Three different catalysts with CeO 2 nanoparticles at 450, 150 and 75 cm 2 compression areas were analysed by means of NAP-XPS under approximately 0.4 mbar H 2 and a temperature range of 300−473 K, and measurements confirmed that the observed kinetics are not caused by variations in the ceria oxidation state due to H spillover or Pt decoration by the migration of reduced Ce atoms during reaction. Alternatively, they assumed that reaction kinetics were rate-limited by the surface displacement of crotyl-oxy intermediates as they form on ceria nanoparticles and consequently diffuse and react on Pt.

Gas-Solid Electrocatalysis
Nowadays, research and development of electrochemical devices such as batteries, fuel cells and supercapacitors have surged due to the demand for clean, secure and sustainable energy sources. Solid oxide electrochemical cells (SOCs) are among the most promising technologies for efficient fuel generation and electric power production. SOCs are a general class of solid-state electrochemical devices that comprise solid oxide fuel cells (SOFCs), which convert fuels and oxygen to electric power, and solid oxide electrolysis cells (SOECs) for fuel generation from electricity. Although SOCs have enormous potential for future mass H 2 production [297] and offer several attractive advantages, including high efficiency and tolerance to catalyst poisoning, reformation of hydrocarbon fuels and the possibility of burning hydrocarbon fuels directly, these devices have not yet found extensive use in quotidian applications [298]. The understanding of fundamental processes in the bulk and at the interfaces of electrochemical devices is decisive in order to develop new technologies with improved efficiency and performance.
Common electrochemical evaluations of electrode overpotential usually employ voltammetry and electrochemical impedance spectroscopy. Nevertheless, there is still a lack of direct knowledge regarding the surface chemistry and electrochemical processes that guide these systems, owing to the inherently convoluted nature of electrochemical processes and the need for suitable in situ techniques that explore these issues at relevant pressures and temperatures. Surface analytical techniques such as atomic force microscopy (AFM), scanning tunnelling microscopy (STM) and conventional UHV XPS cannot be used due to the gaseous reactant environment, high operating temperatures (> 873 K) and far-from-equilibrium conditions related with the operating devices. For this reason, new in situ and operando tools are being developed and already began to provide fundamental insight into SOC processes. Among them, NAP-XPS allows the resolution of local surface potentials, electrochemically active regions and shifts in surface oxidation states in operating SOCs [299][300][301][302].
Solid oxide electrochemical cells are complex devices composed of three basic components: a porous anode, an electrolyte membrane and a porous cathode. Figure 8 shows a schematic representation of the most common design of an electrolyte supported SOFC [303], in which the dense electrolyte membrane supports both electrodes. The materials typically employed in SOECs are basically similar to those used for SOFCs [304][305][306]. The electrolyte, usually a dense oxide-ion conductor such as yttria-stabilized zirconia (YSZ), electronically isolates the air and fuel compartments and enhances pure oxygen ion transport between the anode and the cathode. Other materials, including scandia-stabilized zirconia (ScSZ), ceria-based electrolytes (fluorite structure) or the lanthanum gallate (LSGM, perovskite structure) are also considered [298], but the high temperatures (> 923 K) required to transport oxide through the solid-state electrolyte restrict the materials that can be used as SOC components [307]. Electrodes must combine oxide-ion conduction with catalytically active electronically conducting materials, that is, exhibit mixed ionic-electronic conductivity (MIEC). For this reason, the most commonly used MIEC material for the cathode (which catalyses the oxidation of fuel in electrolysis mode) is a metal-oxide composite composed of YSZ and metallic nickel (Ni/YSZ), but alternative materials include samaria doped ceria (SDC) with dispersed Ni nanoparticles, titanate/ceria composites or the perovskite material lanthanum strontium chromium manganite (LSCM). As for the anode, lanthanum strontium manganite (LSM)/YSZ composite is the most habitually used material to date, although different materials have also been proposed.
Doped cerium oxide, Ce 1−x M x O 2−δ (M: rare-earth or alkaline-earth cations), has received considerable interest for potential use in SOCs because of its higher ionic conductivity with respect to yttria-stabilized zirconia and a lower cost compared with lanthanum gallate-based phases. Solid electrolytes based on doped ceria materials allow a decrease in the SOFC operation temperature due to its high oxide ion conduction, which consequently simplifies various technological issues. SOFC anode can also be based on doped ceria materials, and Ce 1−x M x O 2−δ solid solutions (where M = Gd or Sm, and x = 0.10-0.20) exhibit the highest level of oxide ion conductivity among ceria-based ionic conductors [80,308]. Nevertheless, relatively few studies have used doped ceria materials as solid electrolyte under electrolysis mode, likely due to the partial reduction of Ce 4+ to Ce 3+ under operation caused by the high voltages applied, which results in electronic conduction and thus a short-circuit of the cell [298].
are basically similar to those used for SOFCs [304][305][306]. The electrolyte, usually a dense oxide-ion conductor such as yttria-stabilized zirconia (YSZ), electronically isolates the air and fuel compartments and enhances pure oxygen ion transport between the anode and the cathode. Other materials, including scandia-stabilized zirconia (ScSZ), ceria-based electrolytes (fluorite structure) or the lanthanum gallate (LSGM, perovskite structure) are also considered [298], but the high temperatures (> 923 K) required to transport oxide through the solid-state electrolyte restrict the materials that can be used as SOC components [307]. Electrodes must combine oxide-ion conduction with catalytically active electronically conducting materials, that is, exhibit mixed ionic-electronic conductivity (MIEC). For this reason, the most commonly used MIEC material for the cathode (which catalyses the oxidation of fuel in electrolysis mode) is a metal-oxide composite composed of YSZ and metallic nickel (Ni/YSZ), but alternative materials include samaria doped ceria (SDC) with dispersed Ni nanoparticles, titanate/ceria composites or the perovskite material lanthanum strontium chromium manganite (LSCM). As for the anode, lanthanum strontium manganite (LSM)/YSZ composite is the most habitually used material to date, although different materials have also been proposed. Doped cerium oxide, Ce1−xMxO2−δ (M: rare-earth or alkaline-earth cations), has received considerable interest for potential use in SOCs because of its higher ionic conductivity with respect to yttria-stabilized zirconia and a lower cost compared with lanthanum gallate-based phases. Solid electrolytes based on doped ceria materials allow a decrease in the SOFC operation temperature due to its high oxide ion conduction, which consequently simplifies various technological issues. SOFC anode can also be based on doped ceria materials, and Ce1−xMxO2−δ solid solutions (where M = Gd or Sm, and x = 0.10-0.20) exhibit the highest level of oxide ion conductivity among ceria-based ionic conductors [80,308]. Nevertheless, relatively few studies have used doped ceria materials as solid electrolyte under electrolysis mode, likely due to the partial reduction of Ce 4+ to Ce 3+ under operation caused by the high voltages applied, which results in electronic conduction and thus a short-circuit of the cell [298].
Thus, in this third and last block we have gathered all reported gas-solid electrocatalytic reactions examined with the NAP-XPS technique, based on both SOFC and SOEC systems with Thus, in this third and last block we have gathered all reported gas-solid electrocatalytic reactions examined with the NAP-XPS technique, based on both SOFC and SOEC systems with cerium oxide as a component. Four different types of electrocatalytic reactions performed through these systems are found in literature, and Table 2 provides the list of the reports published so far.  The first experiments performed with NAP-XPS in order to probe oxidation states of all exposed surfaces and local electric potentials of ceria thin film electrodes operating in SOCs are associated to DeCaluwe and co-workers [75]. The SOC cell design consisted on a 300 nm thick CeO 2−x thin film with a patterned Au current collector as working electrode (WE) and a porous Pt counter electrode (CE), both supported on the same side of a 1 mm thick single-crystal YSZ electrolyte to enable NAP-XPS to access the electrode-electrolyte interfaces. XPS measurements were performed during electrochemical oxidation of H 2 and electrolysis of H 2 O (see Equations (8) and (9), respectively) at 973 K, and results showed the extent of near-surface Ce 4+ reduction to Ce 3+ upon increasing cell voltage. Large negative biases caused an increase of Ce 4+ species with respect to equilibrium values, as well as a resistance drop due to H 2 oxidation activity. Contrarily, the application of positive voltages drove H 2 O electrolysis on ceria and a further increase of Ce 3+ species. Similar findings were reported by DeCaluwe et al. [300] in another study of the same SOC cell geometry. Additional tests indicated that highly reduced ceria surface, caused by an increased H 2 -to-H 2 O ratio, was more active for electrolysis.
Further studies of the same SOC system (with variable ceria electrode thickness) were performed by Zhang and researchers [309,310,312] by using NAP-XPS, among other characterization tools. Operando spectroscopic analyses, carried out under a~1 mbar pressure gaseous mixture of H 2 and H 2 O (1:1) and at temperatures above 973 K, allowed the measurement of local surface ceria oxidation states and the detection of electrochemically active regions of ceria thin films. Results revealed that the electrochemically active regions extend 150−200 µm away from the current collectors on ceria electrodes, and the persistence of the Ce 3+ /Ce 4+ shifts in these regions suggested that the surface reaction kinetics and lateral electron transport on the ceria electrodes are co-limiting processes. Using a different experimental setup, Chueh and co-workers [311,313] combined ambient pressure XPS and electrochemical impedance spectroscopy to simultaneously quantify the concentration of active Ce 3+ species on the surface and in the bulk of a Sm-doped CeO 2 (111) film under catalytically relevant temperatures and H 2 -H 2 O gas mixtures. On both works, SDC thin films were grown on YSZ single crystal substrates and interdigitated Pt electrical contacts were deposited and patterned on top of the thin films. The results revealed a highly reduced and stable surface even under relatively oxidising conditions, whereas the bulk doped ceria was almost entirely Ce 4+ . Measurements of the chemical potential of surface oxygen indicated a large deviation from the bulk values, and this entropic difference played a key role in surface Ce 3+ stabilization. Upon comparing the surface chemical capacitance of CeO 2−x and SDC, researchers found that Sm leads to a slight decrease of surface Ce 3+ concentration, but a 10-fold increase of the surface capacitance. Therefore, cation substitution represents an approach to tune the surface capacitance of MIECs. Finally, Papaefthimiou et al. investigated the surface composition and oxidation state of a Ni/GDC electrode in a SOEC cell during water electrolysis by means of ambient pressure XPS and NEXAFS spectroscopies [315]. Papaefthimiou and colleagues [314] had previously reported on the effect of a steam environment on the oxidation state and composition of Ni/GDC cermets and found that, in the mbar pressure regime and at intermediate temperature conditions, water acted as an oxidant for Ni but had a dual oxidant/reducing function for doped ceria. In their succeeding work, they showed that the oxidation state and composition of the electrode during steam electrolysis are dynamic and determined by a complex interplay between the thermo-chemical oxidation caused by water vapour and the electrochemical reduction of Ni.
Finally, Nurk et al. [316] monitored changes in the surface chemistry of a Ni/GDC WE by combining NAP-XPS and impedance spectroscopy (IS). Researchers used a dual chamber NAP-XPS measurement cell with a three-electrode configuration, which provided the possibility of measuring the WE potential against a reference Pt electrode in a well-known atmosphere and monitoring the oxygen partial pressure on the studied electrode. Changes in the surface chemistry of the Ni/GDC electrode (i.e., the evolution of Ni 3p, Ce 4d and O 1s regions) during its reduction at 923 K were monitored simultaneously with the electrochemical impedance properties of the electrode and results indicated that reduction of Ce 4+ to Ce 3+ species and NiO to metallic Ni occurred concurrently.

CH 4 Electro-Oxidation
Direct feeding of the SOFCs anode with light hydrocarbons from fossil or renewable sources appears more attractive than the use of hydrogen as a fuel. Moreover, there is a significant interest in methane-fuelled SOFCs because H 2 is mainly generated by the reforming of natural gas, mostly composed of CH 4 . It is known that direct feeding with methane can lead to the accumulation of carbon onto YSZ, but incorporation of ceria (in the form of GDC) limits this deposition and also improves the cell performance. Employing the same cell configuration, which consisted on an 80 µm thick NiO/GDC thin film anode on a YSZ electrolyte and a platinum film on the reverse side that acted as the cathode electrode, Papaefthimiou et al. [318] provided experimental evidence of the active surface oxidation state and composition of the anode during methane electro-oxidation (0.1 mbar pressure) at intermediate working temperatures (973 K). A mixture of reduced Ni 0 and Ce 3+ species in the anode, with an optimum Ni/Ce surface ratio close to 0.4, was found to be the most favourable design to achieve maximum cell currents.

CO 2 Electrolysis/CO Electro-Oxidation
Carbon dioxide can be used as a chemical feedstock in electrochemical systems, and this approach has become an interesting methodology for the production of hydrocarbon compounds, e.g., methane, methanol and carbon monoxide. Nevertheless, it is well-known that the activation and reduction of CO 2 to CO and oxygen is a demanding task owing to the large positive enthalpy (∆H = 283.0 kJ/mol) [319]. Recent investigations suggest that high-temperature electrolysis of CO 2 using molten carbonates and solid oxide electrolysers of CO 2 is one of the most promising and practical ways of CO 2 reduction, exhibiting faster kinetics and higher selectivity. This approach was first pursued by NASA to generate O 2 from the Martian atmosphere, rich in CO 2 [320]. Two reports of ceria-based solid oxide CO 2 electrolysis cells found in the literature study the carbon oxide chemistry (CO 2 electrolysis and CO electro-oxidation reactions) by means of ambient pressure XPS. On one hand, Yu et al. [303] designed a planar architecture SOC with both the ceria film WE and the 300 nm Pt CE patterned on the same side of the electrolyte, a polycrystalline YSZ substrate, and exposed to 0.65 mbar CO-CO 2 gas mixture at 873 K (see Figure 8). Researchers identified carbonate species (CO 3 2− ) on the ceria surface as reaction intermediates. When CO 2 electrolysis is promoted on ceria electrodes at positively applied biases, a higher carbonate concentration over a 400 mm-wide active region on the ceria surface is observed, while CO 3 2concentration appeared to decrease during CO electro-oxidation (see Figure 9). hand, Yu et al. [303] designed a planar architecture SOC with both the ceria film WE and the 300 nm Pt CE patterned on the same side of the electrolyte, a polycrystalline YSZ substrate, and exposed to 0.65 mbar CO-CO2 gas mixture at 873 K (see Figure 8). Researchers identified carbonate species (CO3 2− ) on the ceria surface as reaction intermediates. When CO2 electrolysis is promoted on ceria electrodes at positively applied biases, a higher carbonate concentration over a 400 mm-wide active region on the ceria surface is observed, while CO3 2-concentration appeared to decrease during CO electro-oxidation (see Figure 9).  These results suggest that CO 2 electrolysis reaction requires a pre-coordination of CO 2 to the ceria surface to form a CO 3 2− intermediate, and this reaction step precedes a rate-limiting electron transfer process involving carbonate reduction to give CO and oxide ions. This electron transfer step is also rate-limiting in the reverse direction.
On the other hand, Wang and Jackson [317] designed a two-sided electrochemical cell, with 300 nm thick Ni/GDC WE deposited onto 1 mm-thick polycrystalline YSZ substrate and a porous Pt CE on the opposing side of the support. NAP-XPS measurements of CO/CO 2 surface electrochemistry on Ni/GDC were performed on the cell at 893 K and maintaining the total pressure around 0.5 mbar, with P CO /P CO2 ratios at 1:1 and 1:9. Analysis demonstrated that dry CO oxidation and carbon dioxide dissociation are substantially slower than their H 2 and H 2 O analogs on Ni/GDC with current densities for CO/CO 2 roughly one order of magnitude lower than H 2 /H 2 O. Operando XPS studies allowed obtaining local overpotentials and surface species factions across the Ni/GDC interface.

Summary and Outlook
Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) has been demonstrated to be a powerful tool to study the surface and subsurface of ceria-based catalysts under working conditions. The excellent oxygen storage capacity and redox properties of ceria due to the ability to accommodate a large number of oxygen vacancies in its structure can be studied in detail to ultimately obtain structure-activity relationships. As a support, ceria can strongly modify the reactivity of metal nanoparticles. The unique properties of ceria to provide surface oxygen species to the metal nanoparticles and their dynamic behaviour under reaction can be conveniently studied by NAP-XPS using different photon energies in synchrotron facilities to obtain information from different depths. This is particularly attractive for following segregation phenomena under reaction conditions in bimetallic catalysts. In turn, the presence of metal nanoparticles on ceria strongly modifies the reducibility of the support. This synergy originates complex systems and the use of NAP-XPS turns to be invaluable and necessary to get information about the nature of the metal-ceria interface. Due to its capabilities, NAP-XPS is one of the most demanded tools for the study of ceria-based catalysts, both for solid-liquid and solid-gas interfaces. NAP-XPS can provide not only information on the electronic state and surface composition of ceria and metal nanoparticles supported on it under a wide range of environmental conditions, but also information about adsorbed/desorbed molecules in the vicinity of the surface.
Author Contributions: X.G. drafted and wrote the original manuscript. All authors contributed to the final manuscript. C.E. and J.L. supervised the project. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.