Au/TiO2-CeO2 Catalysts for Photocatalytic Water Splitting and VOCs Oxidation Reactions

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.


Introduction
Since the publication of Fujishima and Honda [1], TiO 2 has extensively been used as a photocatalyst with growing interest both from an academic and industrial point of view.During this time, photocatalysis with TiO 2 was applied with various success to several reactions, among them H 2 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 TiO 2 .
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 TiO 2 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 TiO 2 by light irradiation with a suitable light source plays a key role in the mineralization of VOCs into CO 2 and H 2 O.
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, H 2 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.TiO 2 , 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 CeO 2 [22,23], ZnO [24,25] or SiO 2 [26] were reported in the literature.
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 TiO 2 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 TiO 2 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 TiO 2 .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/TiO 2 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 TiO 2 in the production of hydrogen by overall water splitting and in the oxidation of 2-propanol (chosen as the VOCs model).

H 2 Generation by Photocatalytic Water Splitting
The photo-activity of all investigated catalysts in the water splitting reaction (H 2 O Ñ H 2 + 1 {2 O 2 ), evaluated in terms of hydrogen evolution versus reaction time, is compared in Figure 1.For all samples, we observed the formation of O 2 in an almost stoichiometric amount (half moles than H 2 ), 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 (Ce 3+ and Ce 4+ ) can replace the Ti 4+ 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.

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.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 (Ce 3+ and Ce 4+ ) can replace the Ti 4+ 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.

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.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 (Ce 3+ and Ce 4+ ) can replace the Ti 4+ 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.

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.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 (Ce 3+ and Ce 4+ ) can replace the Ti 4+ 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.

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.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 (Ce 3+ and Ce 4+ ) can replace the Ti 4+ 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.

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.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 (Ce 3+ and Ce 4+ ) can replace the Ti 4+ 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.

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.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 (Ce 3+ and Ce 4+ ) can replace the Ti 4+ 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.

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.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 TiO 2 (black line) and CeO 2 (brown line) samples showed some activity in the production of hydrogen which was found to increase in the presence of gold.The coupling of CeO 2 with TiO 2 positively affected the photocatalytic activity with a further increase obtained by the deposition of gold particles in the binary system of TiO 2 -CeO 2 .In fact, both TiO 2 -10%CeO 2 (green line) and Au/TiO 2 -10%CeO 2 (blue line) catalysts showed better performance than bare TiO 2 (H 2 evolution around two times higher) and Au/TiO 2 (H 2 evolution around 25% higher).As reported in the literature, despite the bulk ceria and titania not having a similar crystal structure, cerium ions (Ce 3+ and Ce 4+ ) can replace the Ti 4+ ions, modifying the physicochemical properties of TiO 2 .Such an interaction between the CeO 2 and TiO 2 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 TiO 2 conduction band and transfer them to hydrogen protons, acting as H 2 evolution centers.

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 CO 2 (Figure 2c), and yield to CO 2 (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.
(a) ( 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.
amples it is possible to note that, after a short induction period (around 10 min), due to tion of lamp irradiation and/or water saturation with evolved gases [36], hydrogen irstly undergoes an almost linear increment for up to 40 min, followed by a moderate the production rate.According to the literature [37,38] this can be the result of two effects: (1) a recombination of charge carriers, namely the photo-generated electron-hole trons of the conduction band can quickly recombine with holes of the valence band, thus ergy as unproductive heat or photons; (2) a fast backward reaction, namely the n of hydrogen and oxygen into water.It is noteworthy that repetitive photocatalytic e same sample three times in succession, gave the same catalytic profile, with good data ity, thus ruling out that hydrogen might partially arise from the presence of organic to the synthesis, acting as sacrificial agents.ngly, both bare TiO2 (black line) and CeO2 (brown line) samples showed some activity ction of hydrogen which was found to increase in the presence of gold.The coupling of O2 positively affected the photocatalytic activity with a further increase obtained by the f gold particles in the binary system of TiO2-CeO2.In fact, both TiO2-10%CeO2 (green /TiO2-10%CeO2 (blue line) catalysts showed better performance than bare TiO2 (H2 und two times higher) and Au/TiO2 (H2 evolution around 25% higher).As reported in , despite the bulk ceria and titania not having a similar crystal structure, cerium ions 4+ ) can replace the Ti 4+ ions, modifying the physicochemical properties of TiO2.Such an etween the CeO2 and TiO2 frameworks could be the key factor explaining the t of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].oms, instead, pile up the electrons from the TiO2 conduction band and transfer them to otons, acting as H2 evolution centers.lytic Oxidation of 2-Propanol shows the activity data at 25 °C of the 2-propanol photocatalytic oxidation on all tested erms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to 2c), and yield to CO2 (Figure 2d).It must be noted that the first point was taken after 20 For all samples it is possible to note that, after a sh the stabilization of lamp irradiation and/or water sat production firstly undergoes an almost linear increme decrease of the production rate.According to the lite fundamental effects: (1) a recombination of charge carrie pairs, as electrons of the conduction band can quickly re releasing energy as unproductive heat or photons; recombination of hydrogen and oxygen into water.It tests, using the same sample three times in succession, g reproducibility, thus ruling out that hydrogen might residues due to the synthesis, acting as sacrificial agents Interestingly, both bare TiO2 (black line) and CeO2 in the production of hydrogen which was found to incr CeO2 with TiO2 positively affected the photocatalytic ac deposition of gold particles in the binary system of Ti line) and Au/TiO2-10%CeO2 (blue line) catalysts show evolution around two times higher) and Au/TiO2 (H2 ev the literature, despite the bulk ceria and titania not ha (Ce 3+ and Ce 4+ ) can replace the Ti 4+ ions, modifying the interaction between the CeO2 and TiO2 framework enhancement of the photocatalytic activity of this mixed The metal atoms, instead, pile up the electrons from the hydrogen protons, acting as H2 evolution centers.

Photocatalytic Oxidation of 2-Propanol
Figure 2 shows the activity data at 25 °C of the 2-pr catalysts in terms of alcohol conversion (Figure 2a), se CO2 (Figure 2c), and yield to CO2 (Figure 2d).It must b min to allow the lamp to reach a stable energy status.For all samples it is possible to note that, after a s the stabilization of lamp irradiation and/or water s production firstly undergoes an almost linear increm decrease of the production rate.According to the li fundamental effects: (1) a recombination of charge carr pairs, as electrons of the conduction band can quickly releasing energy as unproductive heat or photons recombination of hydrogen and oxygen into water.tests, using the same sample three times in succession, reproducibility, thus ruling out that hydrogen migh residues due to the synthesis, acting as sacrificial agen Interestingly, both bare TiO2 (black line) and CeO in the production of hydrogen which was found to inc CeO2 with TiO2 positively affected the photocatalytic a deposition of gold particles in the binary system of T line) and Au/TiO2-10%CeO2 (blue line) catalysts sho evolution around two times higher) and Au/TiO2 (H2 the literature, despite the bulk ceria and titania not h (Ce 3+ and Ce 4+ ) can replace the Ti 4+ ions, modifying the interaction between the CeO2 and TiO2 framewor enhancement of the photocatalytic activity of this mixe The metal atoms, instead, pile up the electrons from th hydrogen protons, acting as H2 evolution centers.

Photocatalytic Oxidation of 2-Propanol
Figure 2 shows the activity data at 25 °C of the 2catalysts in terms of alcohol conversion (Figure 2a), s CO2 (Figure 2c), and yield to CO2 (Figure 2d).It must min to allow the lamp to reach a stable energy status.ll samples it is possible to note that, after a short induction period (around 10 min), due to zation of lamp irradiation and/or water saturation with evolved gases [36], hydrogen n firstly undergoes an almost linear increment for up to 40 min, followed by a moderate f the production rate.According to the literature [37,38] this can be the result of two tal effects: (1) a recombination of charge carriers, namely the photo-generated electron-hole lectrons of the conduction band can quickly recombine with holes of the valence band, thus energy as unproductive heat or photons; (2) a fast backward reaction, namely the tion of hydrogen and oxygen into water.It is noteworthy that repetitive photocatalytic g the same sample three times in succession, gave the same catalytic profile, with good data bility, thus ruling out that hydrogen might partially arise from the presence of organic ue to the synthesis, acting as sacrificial agents.stingly, both bare TiO2 (black line) and CeO2 (brown line) samples showed some activity duction of hydrogen which was found to increase in the presence of gold.The coupling of TiO2 positively affected the photocatalytic activity with a further increase obtained by the of gold particles in the binary system of TiO2-CeO2.In fact, both TiO2-10%CeO2 (green Au/TiO2-10%CeO2 (blue line) catalysts showed better performance than bare TiO2 (H2 around two times higher) and Au/TiO2 (H2 evolution around 25% higher).As reported in ure, despite the bulk ceria and titania not having a similar crystal structure, cerium ions Ce 4+ ) can replace the Ti 4+ ions, modifying the physicochemical properties of TiO2.Such an between the CeO2 and TiO2 frameworks could be the key factor explaining the ent of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].atoms, instead, pile up the electrons from the TiO2 conduction band and transfer them to protons, acting as H2 evolution centers.
atalytic Oxidation of 2-Propanol e 2 shows the activity data at 25 °C of the 2-propanol photocatalytic oxidation on all tested n terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to re 2c), and yield to CO2 (Figure 2d).It must be noted that the first point was taken after 20 ll samples it is possible to note that, after a short induction period (around 10 min), due to ization of lamp irradiation and/or water saturation with evolved gases [36], hydrogen n firstly undergoes an almost linear increment for up to 40 min, followed by a moderate of the production rate.According to the literature [37,38] this can be the result of two tal effects: (1) a recombination of charge carriers, namely the photo-generated electron-hole lectrons of the conduction band can quickly recombine with holes of the valence band, thus energy as unproductive heat or photons; (2) a fast backward reaction, namely the ation of hydrogen and oxygen into water.It is noteworthy that repetitive photocatalytic g the same sample three times in succession, gave the same catalytic profile, with good data ibility, thus ruling out that hydrogen might partially arise from the presence of organic ue to the synthesis, acting as sacrificial agents.estingly, both bare TiO2 (black line) and CeO2 (brown line) samples showed some activity duction of hydrogen which was found to increase in the presence of gold.The coupling of TiO2 positively affected the photocatalytic activity with a further increase obtained by the n of gold particles in the binary system of TiO2-CeO2.In fact, both TiO2-10%CeO2 (green Au/TiO2-10%CeO2 (blue line) catalysts showed better performance than bare TiO2 (H2 around two times higher) and Au/TiO2 (H2 evolution around 25% higher).As reported in ure, despite the bulk ceria and titania not having a similar crystal structure, cerium ions Ce 4+ ) can replace the Ti 4+ ions, modifying the physicochemical properties of TiO2.Such an n between the CeO2 and TiO2 frameworks could be the key factor explaining the ent of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].l atoms, instead, pile up the electrons from the TiO2 conduction band and transfer them to protons, acting as H2 evolution centers.
catalytic Oxidation of 2-Propanol re 2 shows the activity data at 25 °C of the 2-propanol photocatalytic oxidation on all tested in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to re 2c), and yield to CO2 (Figure 2d).It must be noted that the first point was taken after 20 all samples it is possible to note that, after a short induction period (around 10 min), due to lization of lamp irradiation and/or water saturation with evolved gases [36], hydrogen on firstly undergoes an almost linear increment for up to 40 min, followed by a moderate of the production rate.According to the literature [37,38] this can be the result of two ntal effects: (1) a recombination of charge carriers, namely the photo-generated electron-hole electrons of the conduction band can quickly recombine with holes of the valence band, thus energy as unproductive heat or photons; (2) a fast backward reaction, namely the ation of hydrogen and oxygen into water.It is noteworthy that repetitive photocatalytic ng the same sample three times in succession, gave the same catalytic profile, with good data ibility, thus ruling out that hydrogen might partially arise from the presence of organic due to the synthesis, acting as sacrificial agents.restingly, both bare TiO2 (black line) and CeO2 (brown line) samples showed some activity oduction of hydrogen which was found to increase in the presence of gold.The coupling of h TiO2 positively affected the photocatalytic activity with a further increase obtained by the n of gold particles in the binary system of TiO2-CeO2.In fact, both TiO2-10%CeO2 (green Au/TiO2-10%CeO2 (blue line) catalysts showed better performance than bare TiO2 (H2 around two times higher) and Au/TiO2 (H2 evolution around 25% higher).As reported in ture, despite the bulk ceria and titania not having a similar crystal structure, cerium ions Ce 4+ ) can replace the Ti 4+ ions, modifying the physicochemical properties of TiO2.Such an n between the CeO2 and TiO2 frameworks could be the key factor explaining the ent of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].l atoms, instead, pile up the electrons from the TiO2 conduction band and transfer them to n protons, acting as H2 evolution centers.
catalytic Oxidation of 2-Propanol re 2 shows the activity data at 25 °C of the 2-propanol photocatalytic oxidation on all tested in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to ure 2c), and yield to CO2 (Figure 2d).It must be noted that the first point was taken after 20 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/CeO 2 and CeO 2 catalysts no propene was formed during the photocatalytic tests, whereas over TiO 2 -based samples (TiO 2 , TiO 2 -10%CeO 2 , Au/TiO 2 and Au/TiO 2 -10%CeO 2 ) 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 TiO 2 with respect to CeO 2 [43,44].
Considering the conversion of 2-propanol (Figure 2a), the best results were found over the Au/TiO 2 catalyst (violet line), which was more active than, in order, TiO 2 (black line), Au/TiO 2 -10%CeO 2 (blue line) and TiO 2 -10%CeO 2 samples (green line).The increase of TiO 2 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 TiO 2 used in this work can be due to the occurrence of an interaction between the two phases of TiO 2 (80% anatase, 20% rutile) that increases both the charge carrier (electron-hole) separation and the total photo-efficiency [48,49].The bare CeO 2 (dark red curve) and the Au/CeO 2 (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 TiO 2 , the maximum conversion being, in fact, lower on TiO 2 -10%CeO 2 (50%) compared to TiO 2 (70%).Differently from TiO 2 , the presence of gold did not affect the CeO 2 performances, with the 2-propanol conversion being almost the same for CeO 2 and Au/CeO 2 samples.
The selectivity to acetone (Figure 2b) generally showed a slight decrease over time, with a corresponding increase in the selectivity to CO 2 .The bare TiO 2 displayed the highest selectivity according to data reported in the literature for this reaction [18,49], whereas the presence of CeO 2 or Au had a negative effect on the acetone selectivity, causing a decrease of the maximum value from 95% over TiO 2 to 70% over Au/TiO 2 , 50% over TiO 2 -10%CeO 2 and 40% over Au/TiO 2 -10%CeO 2 .Consequently, the selectivity for CO 2 (Figure 2c) and the yield of CO 2 (Figure 2d), defined as the product of the 2-propanol conversion and the CO 2 selectivity, had a reverse trend, with the Au/TiO 2 -10%CeO 2 system exhibiting the highest values of CO 2 selectivity.These results suggest that the presence of gold and/or CeO 2 improved the total oxidation of 2-propanol to CO 2 more than its selective oxidation to acetone.

Discussion
The catalytic activity data reported in the preceding section clearly pointed out that gold and/or CeO 2 affected the photocatalytic performance of TiO 2 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 TiO 2 and on CeO 2 .The rate of H 2 production was further enhanced by using ternary Au/TiO 2 -CeO 2 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/TiO 2 being the most active sample for the alcohol conversion and Au/TiO 2 -10%CeO 2 being the catalyst showing the best mineralization yield.The effect of CeO 2 addition to TiO 2 was instead detrimental for the 2-propanol conversion, resulting, however, in a considerable increase in the CO 2 yield.
The chemico-physical characterization of the investigated Au/TiO 2 -CeO 2 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], TiO 2 anatase was the main crystal phase for all samples, and the presence of CeO 2 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 TiO 2 polymorphic phase in these samples [50,51].The Au/TiO 2 sample (red line) showed the same bands of bare TiO 2 (black line).In the TiO 2 -10%CeO 2 sample (green line) the signal at 466 cm ´1 was associated with the cubic phase of the CeO 2 fluorite [52-54] 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 Ce 3+ in the lattice [54,55].Interestingly, over the Au/TiO 2 -10%CeO 2 sample, the peak associated with cubic CeO 2 was less intense, broader and shifted to lower frequencies compared to over the TiO 2 -10%CeO 2 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].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.  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.  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.  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.  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.  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.  Au/TiO2-10%CeO2 sample, the peak associated with cubic CeO2 was less to lower frequencies compared to over the TiO2-10%CeO2 sample (Figure respectively).This could be due to a less crystalline and more defective esence of gold.In fact, Raman has been reported to be sensitive to the samples, with broader, less intense Raman peaks in the case of less sition of the main vibrational mode of anatase Eg, pointing out that there about 2 cm −1 ) on the TiO2-10%CeO2 and Au/TiO2-10%CeO2 samples.By ase mode at 148 cm −1 is associated with the O-Ti-O vibration, the presence ould probably cause a bond distortion resulting in the observed shift of tion of cerium oxide on TiO2 a Raman mapping analysis was performed.Au/TiO2-10%CeO2 sample, the peak associated with cubic CeO2 was less to lower frequencies compared to over the TiO2-10%CeO2 sample (Figure respectively).This could be due to a less crystalline and more defective esence of gold.In fact, Raman has been reported to be sensitive to the samples, with broader, less intense Raman peaks in the case of less sition of the main vibrational mode of anatase Eg, pointing out that there (about 2 cm −1 ) on the TiO2-10%CeO2 and Au/TiO2-10%CeO2 samples.By ase mode at 148 cm −1 is associated with the O-Ti-O vibration, the presence ould probably cause a bond distortion resulting in the observed shift of tion of cerium oxide on TiO2 a Raman mapping analysis was performed.destructive and non-invasive analysis of features such as the separation i-component samples.Chemical maps of TiO2 and CeO2 nanostructures CeO2 sample, based on the detailed Raman image (over a 15 μm × 15 μm per line and 150 lines per image), are presented in Figure 4. Interestingly, over the Au/TiO2-10%CeO2 sample, the peak associ intense, broader and shifted to lower frequencies compared to over the 3b, orange and green lines, respectively).This could be due to a less c structure of ceria in the presence of gold.In fact, Raman has been r degree of crystallinity of samples, with broader, less intense Ram crystalline material [56].
Figure 3c shows the position of the main vibrational mode of anat was only a slight red shift (about 2 cm −1 ) on the TiO2-10%CeO2 and A considering that the Eg anatase mode at 148 cm Raman shift (cm -1 ) x 5 samples.
Figure 3c shows the position of the main vibrational mode of anatase E g , pointing out that there was only a slight red shift (about 2 cm ´1) on the TiO 2 -10%CeO 2 and Au/TiO 2 -10%CeO 2 samples.By considering that the E g anatase mode at 148 cm ´1 is associated with the O-Ti-O vibration, the presence of CeO 2 in the TiO 2 lattice could probably cause a bond distortion resulting in the observed shift of the vibration band.
To analyze the distribution of cerium oxide on TiO 2 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 TiO 2 and CeO 2 nanostructures performed on the TiO 2 -10%CeO 2 sample, based on the detailed Raman image (over a 15 µm ˆ15 µm image scan, with 150 points per line and 150 lines per image), are presented in Figure 4. 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 CeO 2 characteristic band (466 cm ´1) of higher intensity was recorded in the red region, while in the violet region TiO 2 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 TiO 2 bulk.
The band-gap energy values (Table 1), estimated by reporting the modified Kubelka-Munk function, [F(R 8 1 )hν] 1/2 against the exciting light energy [57], showed that the TiO 2 -10%CeO 2 sample had a lower E g (2.93 eV) compared to the bare TiO 2 (2.98 eV).This can be related to the replacement of Ti 4+ cations by Ce 4+ or Ce 3+ cations in the TiO 2 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/TiO 2 , Au/CeO 2 and Au/TiO 2 -10%CeO 2 ) 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 TiO 2 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 CeO 2 .It must also be noted that the bare TiO 2 showed a surface area of 44.8 m 2 g ´1, lower than the values found in the literature for P25 TiO 2 (50´54 m 2 g ´1), reasonably due to the thermal pretreatment of TiO 2 (calcination at 350 ˝C) [60].
Catalysts 2016, 6, 121 7 of 13 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 Ti 4+ cations by Ce 4+ or Ce 3+ 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 [29,32−35].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 m 2 g −1 , lower than the values found in the literature for P25 TiO2 (50−54 m 2 g −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.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 Ti 4+ cations by Ce 4+ or Ce 3+ 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 [29,32−35].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 m 2 g −1 , lower than the values found in the literature for P25 TiO2 (50−54 m 2 g −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.er the Au/TiO2-10%CeO2 sample, the peak associated with cubic CeO2 was less shifted to lower frequencies compared to over the TiO2-10%CeO2 sample (Figure lines, respectively).This could be due to a less crystalline and more defective the presence of gold.In fact, Raman has been reported to be sensitive to the ty of samples, with broader, less intense Raman peaks in the case of less 56]. the position of the main vibrational mode of anatase Eg, pointing out that there shift (about 2 cm −1 ) on the TiO2-10%CeO2 and Au/TiO2-10%CeO2 samples.By 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 Ti 4+ cations by Ce 4+ or Ce 3+ 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 [29,32−35].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 m 2 g −1 , lower than the values found in the literature for P25 TiO2 (50−54 m 2 g −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.Interestingly, over the Au/TiO2-10%CeO2 sample, the peak asso intense, broader and shifted to lower frequencies compared to over th 3b, orange and green lines, respectively).This could be due to a less structure of ceria in the presence of gold.In fact, Raman has been degree of crystallinity of samples, with broader, less intense Ram crystalline material [56].
Figure 3c shows the position of the main vibrational mode of an was only a slight red shift (about 2 cm −1 ) on the TiO2-10%CeO2 and considering that the Eg anatase mode at 148 cm −1 is associated with th of CeO2 in the TiO2 lattice could probably cause a bond distortion re the vibration band.
To analyze the distribution of cerium oxide on TiO2 a Raman ma This technique allows non-destructive and non-invasive analysis of of chemical species in multi-component samples.Chemical maps of 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/TiO 2 -10%CeO 2 system.Moreover, the elemental composition of these catalysts showed that on Au/TiO 2 -10%CeO 2 , the Ce atomic percentage is about four times greater than that found on TiO 2 -10%CeO 2 (5.1% and 1.2%, respectively), indicating that the presence of gold on TiO 2 -CeO 2 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 photogeneration 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 2propanol 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 different role of gold and CeO 2 in affecting the properties and the catalytic behavior of TiO 2 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 H 2 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 TiO 2 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/TiO 2 -CeO 2 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 TiO 2 , 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 CO 2 .Furthermore, the basic and redox characteristics of CeO 2 sites with respect to the more acid TiO 2 sites could facilitate the direct combustion of 2-propanol to CO 2 [42,62], resulting in the highest CO 2 yield over the Au/TiO 2 -10%CeO 2 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 (Ce 3+ and Ce 4+ ) in the TiO 2 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 TiO 2 , as shown by EDX, thus explaining the highest H 2 production rate of the Au/TiO 2 -10%CeO 2 system.
destructive and non-invasive analysis of features such as the separation i-component samples.Chemical maps of TiO2 and CeO2 nanostructures CeO2 sample, based on the detailed Raman image (over a 15 μm × 15 μm per line and 150 lines per image), are presented in Figure 4.

Figure 3 .
Figure 3. (a) Vibrational modes of Raman spectra of TiO2 (─), Au/TiO2 ( Au/TiO2-10%CeO2 (─) samples; (b) Raman shift of the signal of cubic CeO Au/TiO2-10%CeO2 (─) samples; (c) Raman shift of the main Eg vibration TiO2 (─), Au/TiO2 (─), TiO2-10%CeO2 (─), and Au/TiO2-10%CeO2 (─) s −1 is associated with the O of CeO2 in the TiO2 lattice could probably cause a bond distortion res the vibration band.To analyze the distribution of cerium oxide on TiO2 a Raman map This technique allows non-destructive and non-invasive analysis of fe of chemical species in multi-component samples.Chemical maps of T performed on the TiO2-10%CeO2 sample, based on the detailed Raman image scan, with 150 points per line and 150 lines per image), are prese

g
anatase mode at 148 cm −1 is associated with the O-Ti-O vibration, the presence attice could probably cause a bond distortion resulting in the observed shift of istribution of cerium oxide on TiO2 a Raman mapping analysis was performed.s non-destructive and non-invasive analysis of features such as the separation

Figure 7 .
Figure 7. Scheme of the electron transfer phenomena taking place in the Au/TiO2-CeO2 system by irradiation with UV light.
a Estimated by XRD measurements.