Zirconia-Supported Silver Nanoparticles for the Catalytic Combustion of Pollutants Originating from Mobile Sources

This work presents the physicochemical characterization and activity of zirconia-supported silver catalysts for the oxidation of pollutants present in diesel engine exhaust (propane, propene, naphthalene and soot). A series of silver-supported catalysts AgxZ (x = 1, 5 and 10 wt.%, Z = zirconia) were prepared, which were studied by various characterization techniques. The results show that silver is mainly found under the form of small metal nanoparticles (<10 nm) dispersed over the support. The metallic phase coexists with the AgOx oxidic phases. Silver is introduced onto the zirconia, generating Ag–ZrO2 catalysts with high activity for the oxidation of propene and naphthalene. These catalysts also show some activity for soot combustion. Silver species can contribute with zirconia in the catalytic redox cycle, through a synergistic effect, providing sites that facilitate the migration and availability of oxygen, which is favored by the presence of structural defects. This is a novel application of the AgOx–Ag/ZrO2 system in the combustion reaction of propene and naphthalene. The results are highly promising, given that the T50 values found for both model molecules are quite low.


Introduction
Mobile emission sources generate a large amount of pollutants, which are emitted into the atmosphere and cause serious environmental problems, both historically and in human health. Among these pollutants are nitrogen oxides (NO x ), carbon oxides (CO x ), volatile organic compounds (VOCs), sulfur oxides (SO x ) and particulate matter, usually called soot or diesel soot [1].
Among the VOCs, different light hydrocarbons (C2-C10), known as NMHCs (non-methane hydrocarbons), are found in very low concentrations. The NMHCs that are the most difficult to eliminate are C3-C4, and polyaromatic compounds (PAHs), which present a level of high toxicity [2,3]. On the other hand, diesel soot consists of solid particles of small size, whose main component is be regenerated [59][60][61][62][63]. It is clear that surface species must have some redox capacity, and also the capacity to form oxygen vacancies [56]. In this context, zirconia is presented as a suitable support, due to its redox capacity [26]. In addition, when zirconia are added to a host cation in its crystalline network, the bulk and sub-surface defects are generated. This facilitates the migration of oxygen species from the bulk to surface. The presence of oxygen vacancies has been correlated with the catalytic activity in oxidation reactions; for example, the oxidation of CO and soot [8,56,58].
Various works concerning oxidation mechanisms indicate that not only the presence of oxygen vacancies is necessary, but also the presence of electrophilic species called "active oxygen O x − " (x = 1 or 2). Some authors even indicate that even the presence of an excessive number of surface defects can generate the deactivation of these O x − species, due to the generation of O 2 − species, which are nucleophilic. In a recent work, Wang et al. have studied the roles of oxygen vacancies, hydroxyls and O x − in different oxidation reactions, using CeO 2 and Ag/CeO 2 as catalysts [56,58]. Moreover, they have shown that the formation of O x − species can be promoted with the presence of Ag to obtain higher NO and soot oxidation activities. Another desirable characteristic is that the support possesses certain levels of acidity to favor the adsorption of the hydrocarbon. In this sense, there are some reports on the activity of pure zirconia for oxidation reactions [26].
Zirconia is a support that has a certain redox capacity [64]; it presents polymorphs with controllable acid-base properties and good thermal stability. Besides, in its amorphous hydrous state, it presents a high surface area that can favor the dispersion of the supported precursors, and thus avoid agglomeration during the thermal treatments [64][65][66].
In addition to the different reaction mechanisms that can be proposed, there are several parameters influencing the behavior of the active species in these systems. Thus, a fundamental factor to consider is the size of the metal particles (an optimum value is <10 nm) [67]. Also, the catalytic efficiency of the Ag catalysts can be related to the adsorption of some hydrocarbons on this metal [68].
In this work, it is proposed that the preparation of zirconia-supported Ag catalysts, obtained by the impregnation of ZrO 2 ·nH 2 O (hydrogel hydrous zirconium oxide) with AgNO 3 solutions. The focus will be on the formation of supported Ag nanoparticles, and in the study of this catalytic system in oxidation reactions of pollutants generated in emission sources. The activity of these catalysts is analyzed in the oxidation reactions of model molecules of different hydrocarbons at very low concentrations: propane, propene, and naphthalene. It is worth mentioning that the Ag/ZrO 2 system has not yet been used for propene and naphthalene elimination. Also, their activities are analyzed in the combustion of diesel soot/particulate matter reaction. The prepared catalysts are characterized using several physicochemical techniques: N 2 adsorption-desorption, scanning electron microscopy, and energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS-UV-vis), transmission electron microscopy (TEM), and temperature-programmed reduction (TPR). X-ray photoelectron spectroscopy (XPS) was employed to obtain information about the species at the surface level. Table 1 presents the characterization results for zirconia and silver-zirconia-supported catalysts. The BET surface area values corresponding to the original hydrogel and the prepared catalysts are presented in Table 1. The starting hydrogel exhibits a high specific surface area value. Then, the addition of Ag and the subsequent thermal process decreases the surface area, due to the loss of the water molecules of the hydrogel, and also the increase of the metal-host-metal interactions. An increase in the Ag content decreases the specific surface area without decreasing the pore volume. The crystallization process of pure zirconia generates a system with a slightly larger area than that of the Ag-catalysts. This indicates that the thermal treatment is the predominant factor in textural changes.  Figure 2 shows the XRD patterns of the support and the AgxZr catalysts. The XRD pattern of the undoped ZrO2 support, calcined for 2 h at 600 °C presents diffraction lines that are typical of the monoclinic phase of ZrO2, with peaks being located at 28.2, 31.5, 34.2, and 50.2° (PDF N° 03-065-1025). The diffraction lines of the meta-stable tetragonal phase, located at 30.0°, 35.0°, and 50.0° (PDF N° 01-089-7710) were identified for all AgxZ catalysts. The addition of Ag strongly influences the crystallization of amorphous hydrated zirconium oxide, and promotes the stabilization of the metastable tetragonal crystalline phase. This specific phase has been proposed as the active phase on certain reactions [69]. The presence of the meta-stable phase, even in the catalyst with the lowest Ag content, indicates a strong Ag-support interaction, generated in the crystallization and sintering stages.

Results and Discussion
As it has been widely reported, some ions, called "host ions" or dopants, can be incorporated into the crystal lattice of zirconia by replacing the central ion, Zr (IV). In this sense, the effective ionic radius of Ag (I) is similar to those of the most common cations used in doping zirconia, such as Y (III) and Ca (II). Furthermore, if the dopants have a lower oxidation state than the central ion in the crystalline structure, defects are generated at the bulk and surface levels, with the consequent formation of oxygen vacancies [70].  Figure 2 shows the XRD patterns of the support and the AgxZr catalysts. The XRD pattern of the undoped ZrO 2 support, calcined for 2 h at 600 • C presents diffraction lines that are typical of the monoclinic phase of ZrO 2 , with peaks being located at 28.2, 31.5, 34.2, and 50.2 • (PDF N • 03-065-1025). The diffraction lines of the meta-stable tetragonal phase, located at 30.0 • , 35.0 • , and 50.0 • (PDF N • 01-089-7710) were identified for all AgxZ catalysts. The addition of Ag strongly influences the crystallization of amorphous hydrated zirconium oxide, and promotes the stabilization of the meta-stable tetragonal crystalline phase. This specific phase has been proposed as the active phase on certain reactions [69]. The presence of the meta-stable phase, even in the catalyst with the lowest Ag content, indicates a strong Ag-support interaction, generated in the crystallization and sintering stages.
As it has been widely reported, some ions, called "host ions" or dopants, can be incorporated into the crystal lattice of zirconia by replacing the central ion, Zr (IV). In this sense, the effective ionic radius of Ag (I) is similar to those of the most common cations used in doping zirconia, such as Y (III) and Ca (II). Furthermore, if the dopants have a lower oxidation state than the central ion in the crystalline structure, defects are generated at the bulk and surface levels, with the consequent formation of oxygen vacancies [70]. The zirconia crystallite size, obtained by Debye-Scherrer´s equation, is shown in Table 1.
The results indicate that AgxZ catalysts contains nanometric zirconia crystals, with a size slightly larger than those present in pure zirconia.
On the other hand, the patterns do not exhibit diffraction lines of Ag 0 crystallites, located at 38.15 • , 44.34 • , and 64.51 • , (PDF N • 03-065-2871), or lines of Ag 2 O, located at 32.88 • and 38.15 • (PDF N • 03-065-6811). This indicates that, if these species exist, they are present at a size that cannot be detected by XRD. It should be noted that there are no marked differences between the XRD profiles of AgxZ catalysts. Only a very slight increase in the intensity of the line at 30.0 • , belonging to the support, is observed. This indicates the presence of good stabilization of the metastable tetragonal phase, and a good distribution of the supported species on the surface, since no segregated crystalline phases are observed. The zirconia crystallite size, obtained by Debye-Scherrer´s equation, is shown in Table 1. The results indicate that AgxZ catalysts contains nanometric zirconia crystals, with a size slightly larger than those present in pure zirconia.
On the other hand, the patterns do not exhibit diffraction lines of Ag 0 crystallites, located at 38.15°, 44.34°, and 64.51°, (PDF N° 03-065-2871), or lines of Ag2O, located at 32.88° and 38.15° (PDF N°03-065-6811). This indicates that, if these species exist, they are present at a size that cannot be detected by XRD. It should be noted that there are no marked differences between the XRD profiles of AgxZ catalysts. Only a very slight increase in the intensity of the line at 30.0°, belonging to the support, is observed. This indicates the presence of good stabilization of the metastable tetragonal phase, and a good distribution of the supported species on the surface, since no segregated crystalline phases are observed.  Figure 3 presents the SEM images and elemental maps (MAP) of the prepared catalysts. These analyses were carried out in order to determine the distribution of silver on the catalyst surface. The overlapping of zirconium and Ag distribution in the MAP images showed that the structures containing Ag are formed on the particles of the catalysts. Moreover, the Ag species are widely distributed over the Ag5Z and Ag10Z catalysts ( Figure 3). Some EDS analyses were carried out for the very small areas, where the highest intensity was observed in the mapping of silver. It was found that the experimental Ag/Zr atomic ratio was greater than the nominal one.  Figure 3 presents the SEM images and elemental maps (MAP) of the prepared catalysts. These analyses were carried out in order to determine the distribution of silver on the catalyst surface. The overlapping of zirconium and Ag distribution in the MAP images showed that the structures containing Ag are formed on the particles of the catalysts. Moreover, the Ag species are widely distributed over the Ag5Z and Ag10Z catalysts ( Figure 3). Some EDS analyses were carried out for the very small areas, where the highest intensity was observed in the mapping of silver. It was found that the experimental Ag/Zr atomic ratio was greater than the nominal one.  In order to analyze the presence of reducible phases in the catalysts, TPR analyses were carried out, and the results are depicted in Figure 4. The TPR diagram of the pure ZrO2 (not shown) does not present any signal, which indicates the absence of reducible phases at the studied temperature range, although it is known that it can be partially reduced at high temperature (about 800 °C). The TPR diagrams of the catalysts containing 5 and 10 wt.% of Ag presented a signal at very low temperature (approximately 90 °C). This can be assigned to the reduction of oxidic or cationic Ag species, in low concentrations, which are well-dispersed on the support, or presenting low interactions with it. This signal also can be assigned to superficial oxygen atoms coming from its migration from zirconia [58]. The integrated area of the signal increases with the increase of Ag content. However, it is noteworthy that all the AgxZ catalysts had an extremely low level of H2 consumption (Table 1), which is lower than the expected if the supported phase corresponds to the stoichiometric reduction of the Ag2O oxide (Equation (1)).

Ag O + H ( ) ↔ H O ( ) + 2Ag
(1) These results would indicate that silver is mainly in a reduced state. These species could have been formed during the calcination process, probably due to the reducing environment of the zirconia hydrogel-containing surface OH groups, and the low stability of the Ag oxides at high temperature. Corro et al. propose that the Ag 0 formation is associated with the low enthalpy of formation of Ag2O at high temperature [71]. In previous studies, it has been reported that the reducibility of transition metal species increases on ZrO2 [26,72]. It has also been proposed that the formation of oxygen vacancies affects the redox properties of the supported species. A mode of behavior that similar to that observed in this work was reported by Aneggi et al., who studied the Ag/ZrO2 catalytic system [53]. These authors show that during the calcination process, AgNO3 decomposes almost completely into metallic Ag, although they observed by HRTEM the presence of Ag2O on the surface of the catalysts. In order to analyze the presence of reducible phases in the catalysts, TPR analyses were carried out, and the results are depicted in Figure 4. The TPR diagram of the pure ZrO 2 (not shown) does not present any signal, which indicates the absence of reducible phases at the studied temperature range, although it is known that it can be partially reduced at high temperature (about 800 • C). The TPR diagrams of the catalysts containing 5 and 10 wt.% of Ag presented a signal at very low temperature (approximately 90 • C). This can be assigned to the reduction of oxidic or cationic Ag species, in low concentrations, which are well-dispersed on the support, or presenting low interactions with it. This signal also can be assigned to superficial oxygen atoms coming from its migration from zirconia [58]. The integrated area of the signal increases with the increase of Ag content. However, it is noteworthy that all the AgxZ catalysts had an extremely low level of H 2 consumption (Table 1), which is lower than the expected if the supported phase corresponds to the stoichiometric reduction of the Ag 2 O oxide (Equation (1)).
These results would indicate that silver is mainly in a reduced state. These species could have been formed during the calcination process, probably due to the reducing environment of the zirconia hydrogel-containing surface OH groups, and the low stability of the Ag oxides at high temperature. Corro et al. propose that the Ag 0 formation is associated with the low enthalpy of formation of Ag 2 O at high temperature [71]. In previous studies, it has been reported that the reducibility of transition metal species increases on ZrO 2 [26,72]. It has also been proposed that the formation of oxygen vacancies affects the redox properties of the supported species. A mode of behavior that similar to that observed in this work was reported by Aneggi et al., who studied the Ag/ZrO 2 catalytic system [53]. These authors show that during the calcination process, AgNO 3 decomposes almost completely into metallic Ag, although they observed by HRTEM the presence of Ag 2 O on the surface of the catalysts.  In order to obtain more information about the nature of the supported Ag phases, DRS UV-Vis studies were performed. The spectra obtained for AgxZ catalysts are shown in Figure 5. An energy absorption band located between 330 nm and 390 nm can be clearly observed. In this region of the spectrum, the absorption is assigned to Ag clusters Agn δ+ (n = 2-7) or particles of metallic silver of small size (3-5 nm). As the size of the particles grows, this band can suffer a bathochromic shift, towards longer wavelengths. This is probably the reason for the slight shift of this band in the Ag10Z catalyst spectrum. The bands located at 220 and 265nm are associated with the presence of the Ag +1 species, and the charge transfer band of the Agn δ+ clusters respectively [67]. Although there is absorbance in the region of the plasmon resonance, 400-450 nm, these bands are not well-resolved or defined, due to the presence of energy absorption of the support, ZrO2. The characterization techniques performed, which provide information at a mass level, suggesting that a part of Ag is present in a metallic state, Ag 0 . In order to corroborate the presence of this type of species, transmission electron microscopy (TEM) analysis were carried out ( Figure 6). It is noticeable that the micrographs corresponding to Ag5Z and Ag10Z catalysts show zirconia In order to obtain more information about the nature of the supported Ag phases, DRS UV-Vis studies were performed. The spectra obtained for AgxZ catalysts are shown in Figure 5. An energy absorption band located between 330 nm and 390 nm can be clearly observed. In this region of the spectrum, the absorption is assigned to Ag clusters Ag n δ+ (n = 2-7) or particles of metallic silver of small size (3-5 nm). As the size of the particles grows, this band can suffer a bathochromic shift, towards longer wavelengths. This is probably the reason for the slight shift of this band in the Ag10Z catalyst spectrum. The bands located at 220 and 265nm are associated with the presence of the Ag +1 species, and the charge transfer band of the Ag n δ+ clusters respectively [67]. Although there is absorbance in the region of the plasmon resonance, 400-450 nm, these bands are not well-resolved or defined, due to the presence of energy absorption of the support, ZrO 2 .  In order to obtain more information about the nature of the supported Ag phases, DRS UV-Vis studies were performed. The spectra obtained for AgxZ catalysts are shown in Figure 5. An energy absorption band located between 330 nm and 390 nm can be clearly observed. In this region of the spectrum, the absorption is assigned to Ag clusters Agn δ+ (n = 2-7) or particles of metallic silver of small size (3-5 nm). As the size of the particles grows, this band can suffer a bathochromic shift, towards longer wavelengths. This is probably the reason for the slight shift of this band in the Ag10Z catalyst spectrum. The bands located at 220 and 265nm are associated with the presence of the Ag +1 species, and the charge transfer band of the Agn δ+ clusters respectively [67]. Although there is absorbance in the region of the plasmon resonance, 400-450 nm, these bands are not well-resolved or defined, due to the presence of energy absorption of the support, ZrO2. The characterization techniques performed, which provide information at a mass level, suggesting that a part of Ag is present in a metallic state, Ag 0 . In order to corroborate the presence of this type of species, transmission electron microscopy (TEM) analysis were carried out ( Figure 6). It is noticeable that the micrographs corresponding to Ag5Z and Ag10Z catalysts show zirconia The characterization techniques performed, which provide information at a mass level, suggesting that a part of Ag is present in a metallic state, Ag 0 . In order to corroborate the presence of this type of species, transmission electron microscopy (TEM) analysis were carried out ( Figure 6). It is noticeable that the micrographs corresponding to Ag5Z and Ag10Z catalysts show zirconia structures with highly dispersed silver nanoparticles. In the micrographs of the Ag1Z catalyst, although some Ag particles were observed, the resolution of the technique does not allow for the visualization of a significant number of them. Ag particle size distribution was analyzed from some selected regions of the TEM micrographs; the results obtained for the mean particle size are listed in Table 1.
The histograms presented in Figure 6 show that for both Ag5Z and Ag10Z catalysts, the Ag particles had sizes of between 2 and 10 nm. The histogram of the Ag5Z catalyst exhibits a relatively narrow particle size distribution, mainly constituted of particles having diameters of between 4 and 5 nm. The increase in the silver loading of the catalysts leads to a greater contribution of the larger particles, and therefore, the average particle size increases. This increase in the silver particle size leads to a loss of metal dispersion (D%), which decreases from 23% to 17% for Ag5Z and Ag10Z, respectively. structures with highly dispersed silver nanoparticles. In the micrographs of the Ag1Z catalyst, although some Ag particles were observed, the resolution of the technique does not allow for the visualization of a significant number of them. Ag particle size distribution was analyzed from some selected regions of the TEM micrographs; the results obtained for the mean particle size are listed in Table 1.
The histograms presented in Figure 6 show that for both Ag5Z and Ag10Z catalysts, the Ag particles had sizes of between 2 and 10 nm. The histogram of the Ag5Z catalyst exhibits a relatively narrow particle size distribution, mainly constituted of particles having diameters of between 4 and 5 nm. The increase in the silver loading of the catalysts leads to a greater contribution of the larger particles, and therefore, the average particle size increases. This increase in the silver particle size leads to a loss of metal dispersion (D%), which decreases from 23% to 17% for Ag5Z and Ag10Z, respectively. The TEM results agree with those obtained by H2-TPR and UV-Vis spectroscopy. Both techniques suggested that part of the Ag was in a metallic state under the form of nano-sized particles, which cannot be observed by XRD. However, there is evidence for the existence of oxidic species in low concentration, due to the presence of signals at low temperature in the TPR analysis, and some absorption bands in UV-Vis, associated with this species.
The oxidation states of the supported species of Ag surface were studied by XPS, although several reports indicate that it is difficult to distinguish between chemical states of Ag, since the binding energies values of Ag3d5/2 (BE) for Ag(0) and their oxides are very similar [73]. Thus, the AgMNN Auger signals were also analyzed, in order to obtain more precise information, since the chemical displacements of the Auger peaks were generally greater than the displacements of the photoelectronic peaks, therefore being more appropriate for considering the use of the modified Auger parameter (AP*) to identify the Ag oxidation states. Therefore, in this work, the XPS spectra The TEM results agree with those obtained by H 2 -TPR and UV-Vis spectroscopy. Both techniques suggested that part of the Ag was in a metallic state under the form of nano-sized particles, which cannot be observed by XRD. However, there is evidence for the existence of oxidic species in low concentration, due to the presence of signals at low temperature in the TPR analysis, and some absorption bands in UV-Vis, associated with this species.
The oxidation states of the supported species of Ag surface were studied by XPS, although several reports indicate that it is difficult to distinguish between chemical states of Ag, since the binding energies values of Ag3d 5/2 (BE) for Ag(0) and their oxides are very similar [73]. Thus, the AgMNN Auger signals were also analyzed, in order to obtain more precise information, since the chemical displacements of the Auger peaks were generally greater than the displacements of the photoelectronic peaks, therefore being more appropriate for considering the use of the modified Auger parameter (AP*) to identify the Ag oxidation states. Therefore, in this work, the XPS spectra of the catalysts in the Ag3d region and in the Auger region, AgMNN ( Figure 7A,B), were studied. Table 2 presents the binding energies (BE) of the Ag3d 5/2 peak, the kinetic energy of the AgMNN Auger transition (KE), and the calculated AP* (AP* = KE + BE) for the different components of the AgMNN signals.
BE values for the Ag3d 5/2 peaks are very close to each other, for the three studied catalysts. The values are between 368.2 eV and 367.7 eV, decreasing slightly when the metallic content decreases. In the AgMNN region, the signals are wide, complex, and have several contributions (Table 2). Those whose AP* are at values of ca. 725 eV and 720 eV, can be associated with the presence of Ag in the metallic state. On the other hand, signals that are located at lower values of KE and AP* (around 717.5 eV), are associated with the presence of oxidic phases of Ag. The metallic particles are probably oxidized on their surfaces [73].
Catalysts 2019, 9, x FOR PEER REVIEW 9 of 19 of the catalysts in the Ag3d region and in the Auger region, AgMNN ( Figure 7A and 7), were studied. Table 2 presents the binding energies (BE) of the Ag3d5/2 peak, the kinetic energy of the AgMNN Auger transition (KE), and the calculated AP* (AP* = KE + BE) for the different components of the AgMNN signals. BE values for the Ag3d5/2 peaks are very close to each other, for the three studied catalysts. The values are between 368.2 eV and 367.7 eV, decreasing slightly when the metallic content decreases. In the AgMNN region, the signals are wide, complex, and have several contributions (Table 2). Those whose AP* are at values of ca. 725 eV and 720 eV, can be associated with the presence of Ag in the metallic state. On the other hand, signals that are located at lower values of KE and AP* (around 717.5 eV), are associated with the presence of oxidic phases of Ag. The metallic particles are probably oxidized on their surfaces [73].   Table 2 also presents the Zr 3d5/2 and O 1s binding energy values (in eV) from the core-level spectra signals. The spectra of the Zr 3d core level spectrum shows two binding energies about 182 and 184 eV, which are assigned to the doublet Zr 3d5/2 and Zr 3d3/2, respectively. These peaks are  Table 2. XPS BE (eV) values for the Ag3d, O1s, and Zr3d, Auger parameters, Ag/Zr atomic ratios, and oxygen species ratio for the studied catalysts.

Ag1Z
Ag5Z Ag10Z Ag 3d Table 2 also presents the Zr 3d 5/2 and O 1s binding energy values (in eV) from the core-level spectra signals. The spectra of the Zr 3d core level spectrum shows two binding energies about 182 and 184 eV, which are assigned to the doublet Zr 3d 5/2 and Zr 3d 3/2 , respectively. These peaks are typical of Zr 4+ in ZrO 2 . The O 1s signal was decomposed in three components, with binding energies of about 530, 531, and 532-533 eV ( Figure 7C), being related to surface oxygen species with different chemical environments. The species with binding energy between 529-530 eV, were attributed to the O α , which is characteristic of lattice O in ZrO 2 , [15,24] and those in the 531-533 eV region, were related to O β , which are associated with surface oxygen vacancies, oxygen-adsorbed species, surface hydroxyl, adsorbed water molecules, or carbonate species [74]. In our case, however, the presence of carbonates was not evident in the spectra of the C1s region (not shown). The Ag5Z and Ag10Z catalysts presented a higher proportion of O β species (Table 2), which are proposed as possible active sites.
The Ag/Zr surface atomic ratios have been calculated from XPS analysis, and then compared with the theoretical composition of the bulk ( Table 2). As expected, the surface presents an enrichment of Ag, due to the predominant location of Ag on the external surface of the support, both for the Ag5Z and Ag10Z catalysts.
The AgxZ catalysts, and also the pure zirconia were used in the catalytic oxidation of three model molecules of hydrocarbons (HC): propane, as a saturated linear hydrocarbon molecule; propene, as an unsaturated linear hydrocarbon molecule; and naphthalene, as a polyaromatic hydrocarbon molecule. In addition, the prepared catalysts were tested in the catalytic oxidation of particulate matter (diesel soot). Table 3 summarizes the catalytic results obtained in all of the studied reactions. The combustion reactions of hydrocarbons were carried out in fixed-bed micro-reactors. The obtained T50 values in each experiment were presented, with T50 being the temperature where 50% HC conversion is reached. The results corresponding to Tmax for the soot combustion reactions, carried out in a thermogravimetric reactor, are also presented in Table 3. Tmax is defined as being the temperature at which the derivative of the TGA curve shows a minimum. This derivative curve represents the loss of mass by combustion, which is associated with the maximum burning rate.
As can be seen, all of the studied catalysts presented a level of activity for the total oxidation of all of the selected model molecules. In addition, in all the reactions the support contributed to the activity, with its redox characteristics. The influence of Ag and its metallic load depended on the nature of the molecule to be oxidized.  490  110  520  110  280  150  612  38  Ag1Z  485  115  470  160  260  170  547  97  Ag5Z  480  120  280  350  172  258  525  125  Ag10Z  450  150  210  420  210  220  425  In the case of propane combustion, the catalysts exhibited low activity, and the contribution of Ag was not significant (Table 3). Using pure zirconia, T50 was 490 • C and the addition of Ag at different concentrations slightly increased the activity. Within the AgxZ series, the most active series the propane combustion resulted was the Ag10Z catalyst. When this catalyst is employed, it is possible to reduce T50 by 150 • C, with respect to the uncatalyzed reaction (600 • C). A series of materials with greater activity than those studied here for propane combustion, are presented in the bibliography [25,27,30,31,75]. Different reports present catalytic systems where the T50 values are lower than 300 • C, even when using high-space velocities. For example, in a recent paper by X. Li et al. [30], the activities of the CoCeO x catalysts was reported and compared with other similar systems, which presented very low values of T50. The poor performance of the AgxZ catalysts can be attributed to the low capacity of the Ag species to promote the activation of propane.
In contrast, the AgxZ catalysts had very good level of activity for the oxidation of propene. The results for propene oxidation reaction, expressed as propene conversion to CO 2 % vs temperature, are plotted for all the studied catalysts in Figure 8A. No other by-products or CO were observed as reaction products; CO 2 selectivity was higher than 99%. In the absence of a catalyst, T50 was 630 • C, and for pure ZrO 2 , 50% propene conversion was reached at 520 • C. The addition of Ag to the system substantially increased the catalytic activity: Ag5Z and Ag10Z catalysts presented great performance, reaching 50% of CO 2 conversion at 280 • C and at 210 • C, respectively. Both catalysts exhibited high levels of conversion (> 90%) at temperatures less than or equal to 330 • C. It is evident that the catalysts are more effective for the elimination of propene than propane. The results here presented are original, since there are no reports on the activity of the Ag/zirconia system for the elimination of propone, a molecule that is classified as a VOC.
Catalysts 2019, 9, x FOR PEER REVIEW 11 of 19 which presented very low values of T50. The poor performance of the AgxZ catalysts can be attributed to the low capacity of the Ag species to promote the activation of propane. In contrast, the AgxZ catalysts had very good level of activity for the oxidation of propene. The results for propene oxidation reaction, expressed as propene conversion to CO2% vs temperature, are plotted for all the studied catalysts in Figure 8A. No other by-products or CO were observed as reaction products; CO2 selectivity was higher than 99%. In the absence of a catalyst, T50 was 630 °C, and for pure ZrO2, 50% propene conversion was reached at 520 °C. The addition of Ag to the system substantially increased the catalytic activity: Ag5Z and Ag10Z catalysts presented great performance, reaching 50% of CO2 conversion at 280 °C and at 210 °C, respectively. Both catalysts exhibited high levels of conversion (> 90%) at temperatures less than or equal to 330 °C. It is evident that the catalysts are more effective for the elimination of propene than propane. The results here presented are original, since there are no reports on the activity of the Ag/zirconia system for the elimination of propone, a molecule that is classified as a VOC. On the other hand, the prepared catalysts presented noticeable activity the naphthalene oxidation. Figure 8B shows the results obtained, expressed as naphthalene conversion to CO2% vs temperature. In the absence of a catalyst, naphthalene is oxidized at high temperatures, reaching T50 at 430 °C. Pure zirconia presented good activity, and attained 50% conversion at 280 °C. The addition of Ag significantly promoted its activity. Thus, the Ag5Z catalyst was the most active of the series, achieving a high level of conversion at low temperatures (250 °C). Using this catalyst, T50 decreased by more than 250 °C compared to the temperature that was reached in the absence of the catalyst. On the other hand, the prepared catalysts presented noticeable activity the naphthalene oxidation. Figure 8B shows the results obtained, expressed as naphthalene conversion to CO 2 % vs temperature. In the absence of a catalyst, naphthalene is oxidized at high temperatures, reaching T50 at 430 • C. Pure zirconia presented good activity, and attained 50% conversion at 280 • C. The addition of Ag significantly promoted its activity. Thus, the Ag5Z catalyst was the most active of the series, achieving a high level of conversion at low temperatures (250 • C). Using this catalyst, T50 decreased by more than 250 • C compared to the temperature that was reached in the absence of the catalyst. When increasing the Ag content from 5 to 10%, a slight decrease in activity was observed. Probably, an increase in the size of Ag particles, and the consequent decrease in metal dispersion, generated a lower availability of active sites for the activation of naphthalene.
These results are an unprecedented contribution, taking into account that the silver-zirconia system has not been studied before, for the total oxidation of naphthalene. Besides, these results are promising, given that the T50 reached with the Ag5Z catalyst is 172 • C. This value is very low, and also comparable to previously obtained results using transition metal oxide catalysts supported on zirconia [26] and with results recently reported by M. Liu et al. [58] with the Ag/CeO 2 catalysts. Even more, as the Ag5Z catalyst was the most active, it was reused three times in the reaction cycles, keeping its activity.
Next, the activity results for diesel soot oxidation are presented. Table 3 shows the maximum oxidation rate temperature (Tmax) obtained for diesel soot combustion in the presence of air, using loose contact conditions, and carried out in a thermogravimetric reactor. Table 3 includes the differences between Tmax in a reaction with and without a catalyst (∆Tmax). The obtained values for ∆Tmax indicate that both the support and the catalysts presented levels of activity. While pure zirconia exhibits poor activity, the activity of the AgxZ catalysts depends on the Ag content (Table 3, Supplementary Material). It was found that with the increase in Ag content, the Tmax decreased. In this reaction, Ag10Z catalyst was the most active of the series, reaching a ∆Tmax of 225 • C. These results indicate that the presence of the Ag species generates a promoting effect.
As stated previously, although the catalysts presented activity, they depended on the Ag content and the nature of the molecule to be oxidized. The systems did not have high efficiency for propane oxidation, as it was being evident that it is not possible to activate this saturated molecule. On the other hand, they showed activity for the diesel soot oxidation, reaching combustion temperatures that were comparable with other reported catalysts containing silver, and other oxidic systems, when measurements were made in "loose contact" mode [53,76]. However, there are numerous systems in the literature that show evidence of higher activity; therefore, the presented formulations could be optimized to achieve better results. Even so the obtained value of Tmax with the Ag10Z catalyst can be considered to be acceptable, since it is in the range of temperatures at which the exhaust pipe operates.
In the oxidation of molecules containing π-π bonds, such as propene and naphthalene, the catalysts exhibited very good performance. It was achieved a significant reduction at T50, especially when using the Ag5Z and Ag10Z catalysts. As previously mentioned, these results are original, since the silver-zirconia system has not been applied to for the elimination of these unsaturated molecules. Moreover, a crystal-size effect of silver nanoparticles has been found in the catalytic performance of the system in the naphthalene combustion, which was not observed for the combustion of propene. As expected, zirconia provided some activity in all the reactions studied, and the addition of high Ag content (5 and 10 wt.%) generated an Ag-zirconia system with a greater availability of superficial oxygen species. This fact may be responsible for the acceleration of the reaction and the regeneration of the active sites, through successive redox cycles. M. Skaf et al. have demonstrated the ability for Ag species (Ag 0 and Ag + ) supported on ceria to adsorb oxygen molecules through redox cycles [77]. The Ag-zirconia synergic effect has been observed from the characterization techniques, and the formation of a metastable tetragonal phase is observed by XRD, leading to the generation of oxygen vacancies.
The coexistence of both types of species, AgO x and Ag 0 , may favor the redox capacity of the surface, with the oxidic Ag species being responsible for oxidizing the adsorbed substrate onto the catalytic surface, and hence adopting a lower oxidation state. This situation allows for an explanation of the reaction mechanism, employing Mars-van Krevelen formalism. Oxidic sites must be regenerated, and this step is favored through the capacity of the system, which contains oxygen vacancies and a high availability of adsorbed O x − species, as evidenced by XPS. According to this technique, the catalysts with the highest metallic loading, Ag5Z and Ag10Z, which are the most active, contain a greater proportion of oxidic species of high availability (O β ).
According to the results of TEM, DRS UV-vis, and XPS techniques, Ag is preferably found on the surface. Ag 0 nanoparticles co-exist with oxidic Ag species (Ag δ+ and Ag + ). According to TPR results, the presence of these oxidic phases is conditioned by the presence of reducible phases in a very low proportion. Supported Ag nanoparticles found are small (<10 nm), and well-dispersed over zirconia.
As previously mentioned, in the literature, various mechanisms that are related to this kind of systems have been proposed for oxidation reactions. Therefore, it is not possible to rule out the dissociative adsorption of molecular oxygen on Ag nanoparticles, or that a combination of both mechanisms occurs. When the activated oxygen (O n x− ) is consumed during the oxidation of the substrate, it is restored by a new round of adsorption of the O 2 present in the gas phase, or by the migration of oxygen species through structural defects. These defects are present in the Ag nanoparticles and in the support, which also can chemisorb oxygen and provide mobility. Oxygen vacancies exist not only at the surface level, but also at the bulk level. The behavior observed in naphthalene oxidation, where the most active catalyst is Ag5Z, and a later increase in the Ag content, generates a less active catalyst. Probably, this can be associated with an increase in the size of the Ag particles, with a resultant decrease of dispersion, and a lower capacity towards the adsorption of naphthalene. This point must be studied more deeply in the future, in order to correlate the catalytic results with experimental evidence. In addition, a possible geometric effect by the Ag ensembles, which cause a decrease in the capacity of naphthalene adsorption, has not yet been ruled out [68]. Probably, given the geometry of the naphthalene molecule, with two conjugated aromatic rings, its adsorption is sensitive to the structure of the catalyst surface. There is also evidence that propene combustion does not follow the same behavior, in comparison to the size of the silver crystallite, marking a difference between both molecules, which must be analyzed with greater depth in the future. All of the studied catalysts were active in the soot combustion reaction, with the Ag10Z catalyst being the most active one. Again, the ability of these catalysts to generate a synergism between the Ag species and zirconia for the activation and regeneration of oxygen is evidenced. This leads to an oxidation on the catalytic interphase, which in this case involves a gas phase and two solid phases.

Material Preparation
Hydrous zirconium oxide, ZrO 2 ·nH 2 O, was prepared by precipitation from a ZrOCl 2 ·6H 2 O (Fluka, Buchs, Switzerland) solution with ammonium hydroxide (pH 10). The process was carried out at room temperature with constant stirring for 6 h. The pH was maintained close to 10. After filtration, the solid was washed (Cl − negative test in the solid) and dried at 100 • C for 6 h. Portions of ZrO 2 ·nH 2 O were impregnated with an aqueous solution of AgNO 3 , to obtain solids with a silver concentration of 1, 5, and 10 wt.% (grams of Ag per 100 g of catalyst). After drying, the samples were thermally treated at 600 • C for 2 h. The materials so obtained were named Ag1Z, Ag5Z and Ag10Z, where Z = zirconia.

Catalyst Characterization
The textural characterization of the support and catalysts was determined by using the BET method, using a Micromeritics Accusorb 2100 E apparatus (Micromeritics, Norcross, GA, USA). Samples were degassed at 100 • C prior to analysis.
Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) analyses were performed using a SEM Philips 505 equipment (Philips Co, Amsterdam, The Netherlands). The energy dispersive X-ray analysis of the samples was performed using an EDAX DX PRIME 10 analyzer (EDAX, New Jersey, NJ, USA) at a working potential of 15 kV.
Temperature programmed reduction (TPR) analysis was performed with a home-made equipment. In a typical run, the sample (30.0 mg) was placed in an electrically heated fixed-bed quartz micro-reactor, and heated from 50 to 800 • C, with a heating rate of 10 • C min −1 . In the experiments, a feed of 10% hydrogen in nitrogen (flow rate of 20 mL min −1 ) was used. Hydrogen consumption was detected by a thermal conductivity detector.
Diffuse reflectance UV-visible spectra (DRS-UV-vis) of AgxZ catalysts were obtained with a Perkin Elmer Lambda 35 UV-VIS spectrometer (Perkin Elmer Inc., Waltham, MA, USA). The distribution of metal particle sizes was determined by transmission electron microscopy (TEM) using a JEOL 100 CX instrument (Jeol Ltd., München, Germany), having a resolution of 6 A and an accelerating voltage of ca. 100 kV. The samples were ground and ultrasonically dispersed in distilled water. To estimate the mean particle size, the particles were considered to be spherical, and the second moment of the distribution was employed. The expression used for the calculation was: where n i is the number of particles of d i size. Over 200 particles were measured. The metal dispersion (D) was estimated from the Ag particle size distribution obtained from the TEM measurements by the following equation [24], assuming spherical particles: where M Ag and ρ Ag are the molar mass (107.87 g·mol −1 ) and density of Ag (10.5 g·cm −3 ), respectively, and σ is the area that is occupied by 1 mol of Ag at the surface (8.75 × 10 −16 cm 2 ·mol −1 ). XPS measurements were carried out, using a Physical Electronics spectrometer (PHI Versa Probe II Scanning XPS Microprobe, Physical Electronics, Chanhassen, MN, USA) with monochromatic X-ray Al Kα radiation (100 µm, 100 W, 20 kV, 1486.6 eV), and a dual beam charge neutralizer. The energy scale of the spectrometer was calibrated, using Cu 2p 3/2 , Ag 3d 5/2 and Au 4f 7/2 photoelectron lines at 932.7, 368.2, and 84.0 eV, respectively. Under a constant pass energy mode under 23.5 eV conditions, the Au 4f 7/2 line was recorded with 0.73 eV FWHM at a binding energy (BE) of 84.0 eV. The collected XPS spectra were analyzed using PHI SmartSoft software and processed using MultiPak 9.3 package. The binding energy values were referenced to the adventitious carbon C 1s signal (284.8 eV). The recorded spectra were always fitted using Gauss-Lorentz curves. The atomic concentration percentages of the characteristic elements of the surfaces were determined, taking into account the corresponding area sensitivity factor for the differently measured spectral regions.

Catalytic Activity for Propane and Propene Oxidation
The catalytic activity for propane and propene oxidation was measured by using a fixed-bed quartz reactor that was electrically heated. Figure 9 shows a schematic diagram of the reaction system. and an accelerating voltage of ca. 100 kV. The samples were ground and ultrasonically dispersed in distilled water. To estimate the mean particle size, the particles were considered to be spherical, and the second moment of the distribution was employed. The expression used for the calculation was: where ni is the number of particles of di size. Over 200 particles were measured. The metal dispersion (D) was estimated from the Ag particle size distribution obtained from the TEM measurements by the following equation [24], assuming spherical particles: where MAg and ρAg are the molar mass (107.87 g·mol −1 ) and density of Ag (10.5 g·cm −3 ), respectively, and σ is the area that is occupied by 1 mol of Ag at the surface (8.75 × 10 −16 cm 2 ·mol −1 ). XPS measurements were carried out, using a Physical Electronics spectrometer (PHI Versa Probe II Scanning XPS Microprobe, Physical Electronics, Chanhassen, MN, USA) with monochromatic Xray Al Kα radiation (100 μm, 100 W, 20 kV, 1486.6 eV), and a dual beam charge neutralizer. The energy scale of the spectrometer was calibrated, using Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 photoelectron lines at 932.7, 368.2, and 84.0 eV, respectively. Under a constant pass energy mode under 23.5 eV conditions, the Au 4f7/2 line was recorded with 0.73 eV FWHM at a binding energy (BE) of 84.0 eV. The collected XPS spectra were analyzed using PHI SmartSoft software and processed using MultiPak 9.3 package. The binding energy values were referenced to the adventitious carbon C 1s signal (284.8 eV). The recorded spectra were always fitted using Gauss-Lorentz curves. The atomic concentration percentages of the characteristic elements of the surfaces were determined, taking into account the corresponding area sensitivity factor for the differently measured spectral regions.

Catalytic Activity for Propane and Propene Oxidation
The catalytic activity for propane and propene oxidation was measured by using a fixed-bed quartz reactor that was electrically heated. Figure 9 shows a schematic diagram of the reaction system.
The feed employed for the propane oxidation consisted of a mixture of 1000 ppm of C3H8, 8% of O2 and He to close the balance. Propene combustion was carried out using a gas mixture, having a composition of 1000 ppm of C3H6, 6% of O2 and He to close the balance. For each experiment, the mass of the catalyst used was 100 mg, and the reaction temperature varied from 150 to 600 °C. A total flow rate of 50 mL·min −1 was used in both processes. The products were analyzed by a Shimadzu GC 2014 (Shimadzu Corporation, Kyoto, Japan) chromatograph with a thermal conductivity detector. Figure 9. Flow diagram of the reaction system. Components: unit (1) gas cylinders (He, O 2 , C 3 H 8 ,C 3 H 6 ); unit (2) On-off valves; unit (3) mass flow controllers; unit (4) Tee valves; unit (5) four-way valve; unit (6) quartz reactor; unit (7) furnace; unit (8) thermocouple type k; unit (9) thermostat; unit (10) gas chromatograph; unit (11) injection valve; unit (12) vent.
The feed employed for the propane oxidation consisted of a mixture of 1000 ppm of C 3 H 8 , 8% of O 2 and He to close the balance. Propene combustion was carried out using a gas mixture, having a composition of 1000 ppm of C 3 H 6 , 6% of O 2 and He to close the balance. For each experiment, the mass of the catalyst used was 100 mg, and the reaction temperature varied from 150 to 600 • C. A total flow rate of 50 mL·min −1 was used in both processes. The products were analyzed by a Shimadzu GC 2014 (Shimadzu Corporation, Kyoto, Japan) chromatograph with a thermal conductivity detector.

Catalytic Activity for Naphthalene Oxidation
The catalysts were tested for the oxidation of naphthalene, using an electrically heated fixed-bed quartz reactor and 100 mg of catalyst. The reaction feed consisted of 10% O 2 and 90% He, and 150 ppm naphthalene. A total flow rate of 30 mL·min −1 was used. Catalytic activity was measured over a temperature range from 150 to 500 • C. Data were obtained at each temperature after a certain time, in order to obtain a stable concentration of naphthalene in the gas phase. The feed flow passes through a thermostated saturator containing naphthalene. The combustion was initiated as soon as a stable vapor pressure was reached. The products were analyzed using a Shimadzu GC 2014 chromatograph with a thermal conductivity detector.

Catalytic Activity for Diesel Soot Oxidation
Diesel soot oxidation tests were performed in a thermogravimetric reactor (TA-50 Shimadzu, Shimadzu Corporation, Kyoto, Japan) over a range of 200 to 650 • C, with a heating rate of 10 • C·min −1 . The reaction mixture (8 vol.% O 2 ) was obtained from two feed lines, air and He respectively, individually controlled to close the balance. Printex-U was used as the model diesel soot. Before the reaction, the Printex-U was mixed with the catalyst at a 1/10 ratio, with a spatula (loose contact). The mass loss and the reaction temperature were recorded as a function of time. From the mass loss information as a function of time, the derivative curve (DTGA) was obtained, and from it, the temperature at which the combustion rate is maximum (Tmax).

Conclusions
This paper reported the preparation of a series of Ag-ZrO 2 catalysts having three different silver concentrations (1, 5, and 10 wt.%). The prepared catalysts were examined by using various analytical techniques (SEM-EDS, XRD, DRS, TEM, TPR, and XPS). It was observed that part of the silver was in a metallic state, in the form of nano-sized particles, and evidence was also found for the existence of oxidic species at low concentration. The catalysts were tested for the catalytic combustion of propane, propene, naphthalene, and particulate matter. Their activities depended on the Ag content and the nature of the molecule to be oxidized. Particularly, the Ag5Z and Ag10Z catalysts exhibited very good performance in the oxidation of molecules containing π-π bonds (propene and naphthalene). Particularly, these catalysts are innovative and display promising results, given that the resulting silver/zirconia system is very active for the elimination of unsaturated molecules, and can be considered a non-toxic catalyst. All of the studied catalysts were also active in the soot combustion reaction, with the Ag10Z catalyst being the most active one. These results could likely be attributed to an Ag-ZrO 2 synergic effect.