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Catalysts 2016, 6(8), 121; doi:10.3390/catal6080121

Article
Au/TiO2-CeO2 Catalysts for Photocatalytic Water Splitting and VOCs Oxidation Reactions
Roberto Fiorenza 1, Marianna Bellardita 2, Luisa D’Urso 1, Giuseppe Compagnini 1, Leonardo Palmisano 2 and Salvatore Scirè 1,*
1
Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy
2
Dipartimento di Energia, Ingegneria dell’Informazione e modelli Matematici (DEIM), Università di Palermo, Viale delle Scienze 90128, Palermo, Italy
*
Correspondence: Tel.: +39-095-738-5112; Fax: +39-095-580-138
Academic Editor: Leonarda F. Liotta
Received: 1 July 2016 / Accepted: 4 August 2016 / Published: 10 August 2016

Abstract

:
Photocatalytic water splitting for H2 production and photocatalytic oxidation of 2-propanol, an example of volatile organic compounds, were investigated over TiO2 catalysts loaded with gold and/or ceria. In the water splitting reaction the presence of gold only slightly affected the performance of TiO2 whereas the presence of CeO2 had a more remarkable positive effect. In the 2-propanol oxidation Au/TiO2 was the most active sample in terms of alcohol conversion whereas Au/TiO2-CeO2 exhibited the highest CO2 yield. On the basis of characterization experiments (X-Ray Diffraction (XRD), Energy Dispersive X-ray Analysis EDX, surface area measurements, Diffuse Reflectance Spectroscopy (DRS) and Raman spectroscopy), it was suggested that the interaction of Au with TiO2 causes an increase in the charge separation between the photo-excited electron/hole pairs, leading to an enhanced photocatalytic activity (to acetone over Au/TiO2 and to CO2 over Au/TiO2-CeO2), whereas the presence of ceria, acting as a hole trap, positively mainly affects the formation of hydrogen by water splitting.
Keywords:
photocatalysis; gold; titanium dioxide; cerium oxide; H2 production

1. Introduction

Since the publication of Fujishima and Honda [1], TiO2 has extensively been used as a photocatalyst with growing interest both from an academic and industrial point of view. During this time, photocatalysis with TiO2 was applied with various success to several reactions, among them H2 production by water splitting or abatement of undesired and harmful organic compounds in air or water [2,3,4,5,6,7,8]. The good quantum yield and stability, high oxidative power, low cost and easy production [9,10,11] are the key reasons for the success of TiO2.
By increasing the environmental concern, the removal of organic contaminants from air and water has become a key issue. Among eco-friendly methods of destroying recalcitrant organic pollutants, the advanced oxidation processes (AOPs) represent a valid alternative to conventional chemical methods. AOPs are based on in situ generation of reactive radical species, mainly OH, by means of solar, chemical or other forms of energy [12,13]. In this field the photocatalytic oxidation (PCO) in the presence of TiO2 to give total or partial oxidation of liquid or gaseous contaminants to benign substances is one of the most promising environmentally friendly techniques for the abatement of volatile organic compounds (VOCs) [14,15]. In fact, the formation of electron-hole pairs on TiO2 by light irradiation with a suitable light source plays a key role in the mineralization of VOCs into CO2 and H2O.
Another important application of heterogeneous photocatalysis is the production of hydrogen by water splitting. Hydrogen, in fact, is regarded as an ideal fuel in sustainable clean energy production, being suitable for fuel cell technology. Unfortunately, at present, H2 is mainly obtained from fossil fuels, such as natural gas, through the steam reforming process. Photochemical hydrogen generation via splitting of water by ultraviolet (UV) or visible light represents a total green alternative to its production.
TiO2, however, presents some drawbacks: its wide band-gap energy (ca. 3.2 eV for anatase and 3.0 eV for rutile) makes it possible to use only about 5% of the solar spectrum and the high electron-hole recombination rate limits its photo-activity. In this regard, doping with metals or metal oxides could provide two positive effects: firstly, it could cause a decrease in the band gap energy, thus shifting the absorption band towards the visible region; secondly, the electron-hole recombination rate could be reduced by metal nanoparticles (NPs) acting as electron traps. In fact, several examples of titania doping with metals such as Fe [16,17], Pd [18], Pt [19,20], Cu [21] or other oxides as CeO2 [22,23], ZnO [24,25] or SiO2 [26] were reported in the literature.
Recently gold nanoparticles were used as an efficient doping system of TiO2 [22,27,28,29,30,31,32,33]. Au-TiO2 NPs showed, in fact, a strong absorption of the visible light due to the surface resonance plasmon (SPR) of their free electrons [27,28]. Therefore, the Au-TiO2 plasmonic photocatalyst exhibited high efficiency in UV or visible light photo-activated reactions such as 2-propanol degradation [22,29], chemo-selective oxidation of alcohols [30], CO2 reduction [31] and water splitting for H2 and O2 generation [32,33].
The enhancement of the performance under UV irradiation was ascribed to the more efficient interfacial charge transfer in the presence of metallic NPs whereas the emergence of high activity under visible irradiation was attributed to the occurrence of the SPR effect, which allows the absorption of visible light. To explain the above effects, two different roles of Au nanoparticles have been claimed in the literature: on the one hand, the photo-excited electrons of the gold surface plasmon can be injected into the TiO2 conduction band, thus creating separated electron holes and then increasing their lifetime by hindering the recombination process [34]; on the other hand, Au NPs can favor electron transfer from the TiO2 surface to the adsorbed molecular oxygen. The SPR phenomenon has been reported to be affected by the size, the shape, the content and the neighboring environment of gold NPs [29,35]. The above features of gold are particularly useful for photocatalytic water splitting; in fact, using excitation wavelengths matching the gold plasmon band, Au NPs absorb photons and inject electrons into the conduction band of the TiO2. This latter effect is not common for a metal, but the nanometer size of Au particles and the occurrence of quantum size effects could be responsible for this mechanism and can explain the good activity of the Au/TiO2 system for this reaction [32,33].
This work aims to evaluate how the presence of gold and/or ceria affects the chemico-physical properties and the photocatalytic activity of TiO2 in the production of hydrogen by overall water splitting and in the oxidation of 2-propanol (chosen as the VOCs model).

2. Results and Discussion

2.1. H2 Generation by Photocatalytic Water Splitting

The photo-activity of all investigated catalysts in the water splitting reaction (H2O → H2+ ½ O2), evaluated in terms of hydrogen evolution versus reaction time, is compared in Figure 1. For all samples, we observed the formation of O2 in an almost stoichiometric amount (half moles than H2), with only a slight defect of oxygen. By taking into account that the experiments were carried out in pure water without sacrificial agents, this confirms the occurrence of the water splitting reaction.
For all samples it is possible to note that, after a short induction period (around 10 min), due to the stabilization of lamp irradiation and/or water saturation with evolved gases [36], hydrogen production firstly undergoes an almost linear increment for up to 40 min, followed by a moderate decrease of the production rate. According to the literature [37,38] this can be the result of two fundamental effects: (1) a recombination of charge carriers, namely the photo-generated electron-hole pairs, as electrons of the conduction band can quickly recombine with holes of the valence band, thus releasing energy as unproductive heat or photons; (2) a fast backward reaction, namely the recombination of hydrogen and oxygen into water. It is noteworthy that repetitive photocatalytic tests, using the same sample three times in succession, gave the same catalytic profile, with good data reproducibility, thus ruling out that hydrogen might partially arise from the presence of organic residues due to the synthesis, acting as sacrificial agents.
Interestingly, both bare TiO2 (black line) and CeO2 (brown line) samples showed some activity in the production of hydrogen which was found to increase in the presence of gold. The coupling of CeO2 with TiO2 positively affected the photocatalytic activity with a further increase obtained by the deposition of gold particles in the binary system of TiO2-CeO2. In fact, both TiO2-10%CeO2 (green line) and Au/TiO2-10%CeO2 (blue line) catalysts showed better performance than bare TiO2 (H2 evolution around two times higher) and Au/TiO2 (H2 evolution around 25% higher). As reported in the literature, despite the bulk ceria and titania not having a similar crystal structure, cerium ions (Ce3+ and Ce4+) can replace the Ti4+ ions, modifying the physicochemical properties of TiO2. Such an interaction between the CeO2 and TiO2 frameworks could be the key factor explaining the enhancement of the photocatalytic activity of this mixed oxide system towards water splitting [39,40]. The metal atoms, instead, pile up the electrons from the TiO2 conduction band and transfer them to hydrogen protons, acting as H2 evolution centers.

2.2. Photocatalytic Oxidation of 2-Propanol

Figure 2 shows the activity data at 25 °C of the 2-propanol photocatalytic oxidation on all tested catalysts in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to CO2 (Figure 2c), and yield to CO2 (Figure 2d). It must be noted that the first point was taken after 20 min to allow the lamp to reach a stable energy status.
The partial oxidation of 2-propanol can proceed with two reaction pathways [41,42]: the oxidative dehydrogenation to acetone and water or the dehydration to propene. Over Au/CeO2 and CeO2 catalysts no propene was formed during the photocatalytic tests, whereas over TiO2-based samples (TiO2, TiO2-10%CeO2, Au/TiO2 and Au/TiO2-10%CeO2) the formation of propene occurred to a very low extent (1%–3% selectivity). In this latter case, the formation of propene can be ascribed to the more acidic character of TiO2 with respect to CeO2 [43,44].
Considering the conversion of 2-propanol (Figure 2a), the best results were found over the Au/TiO2 catalyst (violet line), which was more active than, in order, TiO2 (black line), Au/TiO2-10%CeO2 (blue line) and TiO2-10%CeO2 samples (green line). The increase of TiO2 photocatalytic activity in the presence of Au particles can be ascribed to the different Fermi levels of the two species leading to an increased charge separation between the excited electron (e) and the hole (h+) [45,46,47]. The high activity of the Degussa P25 TiO2 used in this work can be due to the occurrence of an interaction between the two phases of TiO2 (80% anatase, 20% rutile) that increases both the charge carrier (electron-hole) separation and the total photo-efficiency [48,49]. The bare CeO2 (dark red curve) and the Au/CeO2 (orange curve) samples exhibited a low activity for the conversion of 2-propanol (maximum conversion of around 30%), while the presence of ceria negatively affected the performance of TiO2, the maximum conversion being, in fact, lower on TiO2-10%CeO2 (50%) compared to TiO2 (70%). Differently from TiO2, the presence of gold did not affect the CeO2 performances, with the 2-propanol conversion being almost the same for CeO2 and Au/CeO2 samples.
The selectivity to acetone (Figure 2b) generally showed a slight decrease over time, with a corresponding increase in the selectivity to CO2. The bare TiO2 displayed the highest selectivity according to data reported in the literature for this reaction [18,49], whereas the presence of CeO2 or Au had a negative effect on the acetone selectivity, causing a decrease of the maximum value from 95% over TiO2 to 70% over Au/TiO2, 50% over TiO2-10%CeO2 and 40% over Au/TiO2-10%CeO2. Consequently, the selectivity for CO2 (Figure 2c) and the yield of CO2 (Figure 2d), defined as the product of the 2-propanol conversion and the CO2 selectivity, had a reverse trend, with the Au/TiO2-10%CeO2 system exhibiting the highest values of CO2 selectivity. These results suggest that the presence of gold and/or CeO2 improved the total oxidation of 2-propanol to CO2 more than its selective oxidation to acetone.

2.3. Discussion

The catalytic activity data reported in the preceding section clearly pointed out that gold and/or CeO2 affected the photocatalytic performance of TiO2 differently, depending on the reaction taken into consideration.
In particular, in the photocatalytic water splitting (Figure 1), the presence of gold produced an increase of the hydrogen production both on TiO2 and on CeO2. The rate of H2 production was further enhanced by using ternary Au/TiO2-CeO2 systems, the co-presence of gold and ceria leading to the highest hydrogen evolution.
Also in the photocatalytic 2-propanol oxidation (Figure 2), the presence of gold was necessary to obtain a good performance, with Au/TiO2 being the most active sample for the alcohol conversion and Au/TiO2-10%CeO2 being the catalyst showing the best mineralization yield. The effect of CeO2 addition to TiO2 was instead detrimental for the 2-propanol conversion, resulting, however, in a considerable increase in the CO2 yield.
The chemico-physical characterization of the investigated Au/TiO2-CeO2 catalysts helped us to rationalize the above results. The main properties of the catalysts are displayed in Table 1. As revealed by XRD measurements, and reported by some of us in a previous paper [22], TiO2 anatase was the main crystal phase for all samples, and the presence of CeO2 and/or Au caused a slight decrease in the crystallites’ size. The Raman spectra (Figure 3a), exhibiting bands at around 150 cm−1, 403 cm−1, 524 cm−1 and 647 cm−1, confirmed that anatase was the main TiO2 polymorphic phase in these samples [50,51]. The Au/TiO2 sample (red line) showed the same bands of bare TiO2 (black line). In the TiO2-10%CeO2 sample (green line) the signal at 466 cm−1 was associated with the cubic phase of the CeO2 fluorite [52,53,54] and the small component at 600 cm−1 was assignable to intrinsic O vacancies in ceria as a result of its non-stoichiometric composition due to the presence of Ce3+ in the lattice [54,55].
Interestingly, over the Au/TiO2-10%CeO2 sample, the peak associated with cubic CeO2 was less intense, broader and shifted to lower frequencies compared to over the TiO2-10%CeO2 sample (Figure 3b, orange and green lines, respectively). This could be due to a less crystalline and more defective structure of ceria in the presence of gold. In fact, Raman has been reported to be sensitive to the degree of crystallinity of samples, with broader, less intense Raman peaks in the case of less crystalline material [56].
Figure 3c shows the position of the main vibrational mode of anatase Eg, pointing out that there was only a slight red shift (about 2 cm−1) on the TiO2-10%CeO2 and Au/TiO2-10%CeO2 samples. By considering that the Eg anatase mode at 148 cm−1 is associated with the O-Ti-O vibration, the presence of CeO2 in the TiO2 lattice could probably cause a bond distortion resulting in the observed shift of the vibration band.
To analyze the distribution of cerium oxide on TiO2 a Raman mapping analysis was performed. This technique allows non-destructive and non-invasive analysis of features such as the separation of chemical species in multi-component samples. Chemical maps of TiO2 and CeO2 nanostructures performed on the TiO2-10%CeO2 sample, based on the detailed Raman image (over a 15 µm × 15 µm image scan, with 150 points per line and 150 lines per image), are presented in Figure 4.
Representative spectra of two different regions are reported in red and violet color scales. The spectra show almost identical features but with very different intensities. The CeO2 characteristic band (466 cm−1) of higher intensity was recorded in the red region, while in the violet region TiO2 peaks (150 cm−1, 403 cm−1, 524 cm−1 and 647 cm−1) are very visible with the ceria band of decreased intensity (according to the 10%), pointing out that the ceria was not homogenously dispersed on the TiO2 bulk.
The band-gap energy values (Table 1), estimated by reporting the modified Kubelka-Munk function, [F(R)hν]1/2 against the exciting light energy [57], showed that the TiO2-10%CeO2 sample had a lower Eg (2.93 eV) compared to the bare TiO2 (2.98 eV). This can be related to the replacement of Ti4+ cations by Ce4+ or Ce3+ cations in the TiO2 network [51,54,58]. Moreover, looking at the DRS spectra of the investigated samples in the visible region (Figure 5), it can be seen that all gold-loaded samples (Au/TiO2, Au/CeO2 and Au/TiO2-10%CeO2) exhibit a clear absorbance band at around 550 nm, attributed to the plasmon resonance of gold nanoparticles [59]. As reported in the literature, the photo-excited electrons of the gold surface plasmon can be injected to the TiO2 conduction band, thus creating separated electrons and holes and then increasing their lifetime by hindering the recombination process [29,32,33,34,35]. It must be underlined that no significant variation in the surface area (Brunauer-Emmett-Teller (BET) analysis) of investigated samples was detected in the presence of gold and/or CeO2. It must also be noted that the bare TiO2 showed a surface area of 44.8 m2g−1, lower than the values found in the literature for P25 TiO2 (50−54 m2g−1), reasonably due to the thermal pretreatment of TiO2 (calcination at 350 °C) [60].
The surface EDX analysis of investigated samples is reported in Figure 6. It is possible to note that the signals related to cerium (at 4.8 and 5.3 KeV, the first one overlapped with the signal of Ti) were more intense for the Au/TiO2-10%CeO2 system. Moreover, the elemental composition of these catalysts showed that on Au/TiO2-10%CeO2, the Ce atomic percentage is about four times greater than that found on TiO2-10%CeO2 (5.1% and 1.2%, respectively), indicating that the presence of gold on TiO2-CeO2 oxide leads to a remarkable cerium surface enrichment.
The different role of gold and CeO2 in affecting the properties and the catalytic behavior of TiO2 in the photo-oxidation and the photoreduction reactions investigated here could be rationalized by taking into account the surface active species involved in these reactions. As for the photocatalytic water/air oxidative purification, the photocatalytic hydrogen production essentially requires photo-generation of hole/electron pairs. Nevertheless, the role of holes/electrons, as well as the surface reactions involved, are different. In fact, in the photocatalytic oxidation, valence band (VB) holes are the key elements involved in the removal of contaminants, whereas in H2 production via photocatalytic water splitting reducing Conduction Band (CB) electrons becomes crucial as their role is mainly that of reducing protons to hydrogen molecules.
The addition of gold to TiO2 results in an enhancement of the photocatalytic activity towards 2-propanol oxidation, due to an increase in the charge separation between the excited electron and the hole of the titania [45,46,61]. The proposed scheme of the electron transfer phenomena taking place in the Au/TiO2-CeO2 system is illustrated in Figure 7. We must underline that under the irradiation conditions used in this work (medium pressure Hg lamp, providing UV and to a lesser extent visible photons), the SPR effect of Au nanoparticles, involving an inverse transfer of electrons from Au to the CB of TiO2, should play a minor role, becoming important only when visible light is used as the irradiation source. Interestingly, when the cerium oxide was also present, photo-generated active species (superoxide oxygen and hydroxyl radicals) could allow an easier re-oxidation of ceria, thus speeding up its redox process [22,23]. These processes were beneficial for the complete oxidation to CO2. Furthermore, the basic and redox characteristics of CeO2 sites with respect to the more acid TiO2 sites could facilitate the direct combustion of 2-propanol to CO2 [42,62], resulting in the highest CO2 yield over the Au/TiO2-10%CeO2 system.
The surface mechanisms induced by gold were less efficient for the photocatalytic water splitting. In fact, even if the photo-generated electrons and holes have potentials which are thermodynamically adequate for the water splitting, they tend to recombine with each other if the number of surface active sites for the redox reaction is not sufficient. In this case, the substitution of cerium ions (Ce3+ and Ce4+) in the TiO2 framework, as suggested by DRS measurements, was the key factor for having a good performance. The cerium defects act, in fact, as hole traps [39,40,63], avoiding the recombination of active electrons and holes and thus favoring the reduction of water. In this case, gold positively affects the photocatalytic performance, both increasing the defective structure of ceria, as shown by Raman, and favoring the enrichment of ceria on the surface of TiO2, as shown by EDX, thus explaining the highest H2 production rate of the Au/TiO2-10%CeO2 system.

3. Materials and Methods

3.1. Catalyst Preparation

TiO2 used in this work was commercial P25 Degussa (Degussa, Frankfurt, Germany). CeO2 was instead prepared by precipitation with KOH 0.1 M (Fluka, Buchs, Switzerland) from Ce(NO3)3·6H2O (Fluka, Buchs, Switzerland) water solution, filtration and treatment in air at 450 °C for 4 h of the obtained powder. Mixed TiO2-CeO2 composites were prepared with 10 wt. percent of CeO2, according to the following procedure: aliquots of TiO2 were impregnated with an appropriate amount of Ce(NO3)3·6H2O solution, the obtained slurries were stirred for 4 h, dried at 120 °C and finally treated in air at 350 °C for 4 h. Calcination in air at 350 °C for 4 h was also carried out on the bare TiO2.
Gold (1 wt %) was loaded on the bare or mixed TiO2-CeO2 oxides by deposition-precipitation. After the pH of the aqueous solution of the Au precursor (HAuCl4, Sigma-Aldrich, Buchs, Switzerland) was adjusted to the value of 8 by 0.1 M aqueous solution of KOH, the support was added under stirring (500 rpm), keeping the slurry at 70 °C for 3 h, then digesting for 24 h, filtering and washing several times (until chlorides disappearance), drying at 110 °C and finally grounding before use.

3.2. Catalyst Characterization Experiments

X-ray diffraction (XRD) was performed with a Bruker AXSD5005 XRD (Bruker, Karlsruhe, Germany) instrument using Cu Kα radiation. Peaks of crystalline phases were compared with those of standard compounds of the JCPDS Data File (Bruker, Diffrac. Suite™ Software package Karlsruhe, Germany).
Energy-dispersive X-ray spectroscopy (EDX) was carried with an INCA Energy Oxford solid state detector (Oxford Instruments plc, Tubney Woods, Abingdon, Oxfordshire, United Kingdom). A field emission gun scanning electron microscopy (FE-SEM, (Carl Zeiss SMT AG Company, Oberkochen, Germany ) equipped with a ZEISS SUPRA 55 VP microscope was used to determine the morphologies of the investigated samples.
Raman spectra were recorded with a WITec alpha 300 confocal Raman system (WITec Wissenschaftliche Instrumente und Technologie GmbH Ulm, Germany). The excitation source for Raman measurement was a 532 nm laser line of a Coherent Compass Sapphire Laser. All measurements were performed at low irradiation power to avoid laser induced heating. A 100× objective lens with a NA = 0.90 was used.
UV-VIS reflectance spectra were performed by diffuse reflectance spectroscopy (DRS, with a Shimadzu UV-2401 PC instrument (Shimadzu Corporation, Kyoto, Japan), recording spectra in the range 200–800 nm and using BaSO4 as reference sample.
Surface area measurements were carried out with a Sorptomatic series 1990 (Thermo Quest, Milano, Italy) instrument using the BET nitrogen adsorption method. All samples were previously outgassed (10−3 Torr) at 120 °C.

3.3. Photocatalytic Activity Experiments

The 2-propanol photo-oxidation was performed in a home-made Pyrex cylindrical shape photo-reactor) (diameter 58 mm, height 100 mm) at 25 °C and atmospheric pressure in gas–solid regime. Then 300 mg of catalyst were placed inside the reactor with a porous inlet glass septum allowing homogeneous distribution of the inlet gas mixture. This mixture consisted of 2-propanol (0.1 mM) and air (flow rate of 20 cm3min−1). An UV Helios-Italquartz 125 W medium pressure Hg lamp (Helios Italquartz Srl, Milano, Italy) was used as the irradiation source. Water flowed through a jacket surrounding the lamp to cut-off the infrared radiation and to maintain constant the temperature inside the reactor. The lamp radiant power in the range 300−400 nm, measured by a radiometer Delta Ohm DO9721 (Delta Ohm Srl, Padova, Italy), was equal to 1.5 mW cm−2. The evolution of the species formed during the runs was followed by withdrawing 500 µl of gas from the reactor by a gas-tight syringe. The 2-propanol and acetone were detected by a Shimadzu GC 2010 (Shimadzu Corporation, Kyoto, Japan) equipped with a Phenomenex Zebron Wax-Plus (30 μm × 0.32 μm × 0.53 μm) column and a FID detector while CO2 was measured by a HP 6890 Series GC (Agilent Technologies, Santa Clara, CA, US) System equipped with a Supelco packed column GC 60/80 Carboxen™-1000 and a thermal conductivity detector using Helium as carrier gas. The carbon balance was always higher than 95%.
Hydrogen generation by photocatalytic water splitting was performed in a home-made Pyrex jacketed reactor thermostated at 30 °C. The reactor headspace was linked to an inverted buret, filled with water at atmospheric pressure. This allows the quantification of the evolved gas. The evolution of H2 was confirmed by analyzing the effluent gases with an online gas chromatograph (HP 6890 Series GC System, Agilent Technologies, Santa Clara, CA, US) equipped with a packed column (Carboxen 1000) and thermal conductivity detector. Specifically, the catalyst (50 mg) was placed inside the photo-reactor, with 100 mL of deionized water under stirring. The suspension was purged with a nitrogen flow for at least 30 min before irradiation in order to remove dissolved air. The suspension was then irradiated for 80 minutes using a 100 W mercury lamp.

4. Conclusions

The photocatalytic performance of the Au/TiO2-CeO2 system was studied both in the oxidation of 2-propanol and in the water splitting reaction. Characterization experiments (XRD, EDX, surface area measurements, DRS and Raman spectroscopy) allowed us to suggest that the interaction of gold with TiO2 causes an increase in the photocatalytic oxidation activity, due to a charge separation enhancement between the excited electron and the hole of TiO2. The co-existence of Au and both TiO2 and CeO2 oxides favors the mineralization of the alcohol. In the water splitting reaction, the presence of ceria, acting as a hole trap, is essential to have a high hydrogen production rate, while Au conveys the electron transfer from TiO2 to the H+ ions. Notably, the SPR effect of Au nanoparticles could induce electron transfer to the CB of TiO2, but the SPR, described as relevant when visible light is used, would play a minor role in our case (both UV and visible light irradiation).

Acknowledgments

Authors would like to acknowledge the Bio-nanotech Research and Innovation Tower (BRIT) project that supports this research activity.

Author Contributions

R.F. and M.B. conceived and performed the photocatalytic experiments and XRD, EDX, DRS and BET characterization, L.D. e G.C. performed and discussed Raman analysis, S.S. and L.P. conceived the idea of writing the paper, supervised the work and edited the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photocatalytic H2 production at 30 °C under ultraviolet (UV) irradiation over: TiO2 (⃟), CeO2 ( ), TiO2-10%CeO2 ( ), Au/TiO2 ( × ), Au/CeO2 ( ) and Au/TiO2-10%CeO2 ( ).
Figure 1. Photocatalytic H2 production at 30 °C under ultraviolet (UV) irradiation over: TiO2 (⃟), CeO2 ( ), TiO2-10%CeO2 ( ), Au/TiO2 ( × ), Au/CeO2 ( ) and Au/TiO2-10%CeO2 ( ).
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Figure 2. Photocatalytic 2-propanol oxidation over: TiO2 (⃟), CeO2 ( ), TiO2-10%CeO2 ( ), Au/TiO2 ( × ), Au/CeO2 ( ) and Au/TiO2-10%CeO2 ( ): (a) 2-propanol conversion; (b) Selectivity to acetone; (c) Selectivity to CO2; (d) Yield to CO2.
Figure 2. Photocatalytic 2-propanol oxidation over: TiO2 (⃟), CeO2 ( ), TiO2-10%CeO2 ( ), Au/TiO2 ( × ), Au/CeO2 ( ) and Au/TiO2-10%CeO2 ( ): (a) 2-propanol conversion; (b) Selectivity to acetone; (c) Selectivity to CO2; (d) Yield to CO2.
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Figure 3. (a) Vibrational modes of Raman spectra of TiO2 (), Au/TiO2 ( ), TiO2-10%CeO2 ( ), and Au/TiO2-10%CeO2 ( ) samples; (b) Raman shift of the signal of cubic CeO2 in TiO2-10%CeO2( ), and Au/TiO2-10%CeO2 ( ) samples; (c) Raman shift of the main Eg vibrational mode of TiO2 anatase in TiO2 (), Au/TiO2 ( ), TiO2-10%CeO2 ( ), and Au/TiO2-10%CeO2 ( ) samples.
Figure 3. (a) Vibrational modes of Raman spectra of TiO2 (), Au/TiO2 ( ), TiO2-10%CeO2 ( ), and Au/TiO2-10%CeO2 ( ) samples; (b) Raman shift of the signal of cubic CeO2 in TiO2-10%CeO2( ), and Au/TiO2-10%CeO2 ( ) samples; (c) Raman shift of the main Eg vibrational mode of TiO2 anatase in TiO2 (), Au/TiO2 ( ), TiO2-10%CeO2 ( ), and Au/TiO2-10%CeO2 ( ) samples.
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Figure 4. Confocal Raman mapping image of TiO2-10%CeO2 sample.
Figure 4. Confocal Raman mapping image of TiO2-10%CeO2 sample.
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Figure 5. Diffuse Reflectance Spectroscopy (DRS) spectra (visible region) of CeO2 ( ), TiO2 (), TiO2-10%CeO2 ( ), Au/TiO2 ( ), Au/CeO2 ( ) and Au/TiO2-10%CeO2 ( ) samples.
Figure 5. Diffuse Reflectance Spectroscopy (DRS) spectra (visible region) of CeO2 ( ), TiO2 (), TiO2-10%CeO2 ( ), Au/TiO2 ( ), Au/CeO2 ( ) and Au/TiO2-10%CeO2 ( ) samples.
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Figure 6. EDX (Energy-dispersive X-ray spectroscopy) spectra of (a) Au/TiO2, (b) TiO2-10%CeO2 and (c) Au/TiO2-10%CeO2 samples.
Figure 6. EDX (Energy-dispersive X-ray spectroscopy) spectra of (a) Au/TiO2, (b) TiO2-10%CeO2 and (c) Au/TiO2-10%CeO2 samples.
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Figure 7. Scheme of the electron transfer phenomena taking place in the Au/TiO2-CeO2 system by irradiation with UV light.
Figure 7. Scheme of the electron transfer phenomena taking place in the Au/TiO2-CeO2 system by irradiation with UV light.
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Table 1. Chemico-physical properties of Au/TiO2-CeO2 catalysts.
Table 1. Chemico-physical properties of Au/TiO2-CeO2 catalysts.
SampleSurface Area (m2/g)Eg (eV)Crystallite Size (nm) aCrystal Phase a
TiO244.82.9824TiO2 Anatase-Rutile
CeO2110.22.9010CeO2 Fluorite
TiO2-10%CeO247.52.9322TiO2 Anatase-Rutile
CeO2 Fluorite
Au/TiO246.42.9721TiO2 Anatase-Rutile
Au/CeO2112.72.9511CeO2 Fluorite
Au/TiO2-10%CeO250.52.9619TiO2 Anatase-Rutile, CeO2 Fluorite
a Estimated by XRD measurements.
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