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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 2

*

Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy; [email protected] (R.F.); [email protected] (L.D.); [email protected] (G.C.) Dipartimento di Energia, Ingegneria dell’Informazione e modelli Matematici (DEIM), Università di Palermo, Viale delle Scienze 90128, Palermo, Italy; [email protected] (M.B.); [email protected] (L.P.) Correspondence: [email protected]; 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 [2–8]. The good quantum yield and stability, high oxidative power, low cost and easy production [9–11] 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 H2 O. Catalysts 2016, 6, 121; doi:10.3390/catal6080121

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Catalysts 2016, 6, 121

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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 [22,27–33]. 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 (H2 O Ñ H2 + 1{2 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.

Catalysts 2016, 6, 121  Catalysts 2016, 6, 121  Catalysts 2016, 6, 121  Catalysts 2016, 6, 121  Catalysts 2016, 6, 121 Catalysts 2016, 6, 121 

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HH evolution 2 H evolution(mmol) (mmol) (mmol) 22 2 evolution 2 H 2 evolution (mmol)

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˝°C  Figure  1.  Photocatalytic  H production  at  30  under  ultraviolet  (UV)  irradiation  over:  TiO (),  Figure  1.  Photocatalytic H H  production  production at  at 30  30 °C  °C under  under ultraviolet  ultraviolet (UV)  (UV) irradiation  irradiation over:  over: TiO TiO (),  Figure  1.  Photocatalytic  22 2 2 production  22 2 2 (),  Figure  1.  Photocatalytic  H at  30  °C  under  ultraviolet  (UV)  irradiation  over:  TiO Figure 1. Photocatalytic production at 30 C under ultraviolet (UV) irradiation over: ( (),  ), Figure  1.  Photocatalytic  H22  production  at  30  °C  under  ultraviolet  (UV)  irradiation  over:  TiO22  (),  CeO CeO 2 (), TiO 2‐10%CeO ‐10%CeO 2 (), Au/TiO 2 (), Au/CeO 2 () and Au/TiO  () and Au/TiO ‐10%CeO  ().  22 (), Au/TiO 22 ().  CeO 2 (), TiO 2‐10%CeO 2 (), Au/TiO 2 (), Au/CeO 2 () and Au/TiO 2‐10%CeO 2 ().  CeO ( (), TiO ), TiO 22‐10%CeO -10%CeO ( (), Au/TiO ), Au/TiO22 (), Au/CeO ( (), Au/CeO ), Au/CeO22 () and Au/TiO ( ) and Au/TiO22‐10%CeO -10%CeO ( ). CeO 2222 (), TiO 22‐10%CeO 22 ().  CeO  (), TiO22‐10%CeO22 (), Au/TiO22 (), Au/CeO22 () and Au/TiO2‐10%CeO 2 (). 

For all samples it is possible to note that, after a short induction period (around 10 min), due to  For all samples it is possible to note that, after a short induction period (around 10 min), due to  For all samples it is possible to note that, after a short induction period (around 10 min), due to  For all samples it is possible to note that, after a short induction period (around 10 min), due to  For all samples it is possible to note that, after a short induction period (around 10 min), due to the 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  the  stabilization  of lamp  lamp irradiation  irradiation  and/or  water saturation  saturation  with evolved  evolved  gases [36],  [36], hydrogen  hydrogen  the  stabilization  of  water  with  the  stabilization  of  lamp  irradiation  and/or  water  saturation  with  evolved  gases  [36],  hydrogen  stabilization of lamp and/orand/or  water saturation with evolved [36],gases  hydrogen the  stabilization  of  irradiation lamp  irradiation  and/or  water  saturation  with gases evolved  gases  [36], production hydrogen  production firstly undergoes an almost linear increment for up to 40 min, followed by a moderate  production firstly undergoes an almost linear increment for up to 40 min, followed by a moderate  production firstly undergoes an almost linear increment for up to 40 min, followed by a moderate  production firstly undergoes an almost linear increment for up to 40 min, followed by a moderate  firstly undergoes an almost linear increment for up to 40 min, followed by a moderate decrease of the 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  decrease of  of the  the production  production  rate.  According  to the  the literature  literature  [37,38]  this  can be  be the  the result  result of  of two  two  decrease  rate.  to  [37,38]  this  decrease  of  the  production  rate.  According  to  the  literature  [37,38]  this  can  be  the  result  of  two  production According to the According  literature [37,38] can be the result of can  two fundamental decrease  of rate. the  production  rate.  According  to  the this literature  [37,38]  this  can  be  the  result  effects: of  two  fundamental effects: (1) a recombination of charge carriers, namely the photo‐generated electron‐hole  fundamental effects: (1) a recombination of charge carriers, namely the photo‐generated electron‐hole  fundamental effects: (1) a recombination of charge carriers, namely the photo‐generated electron‐hole  fundamental effects: (1) a recombination of charge carriers, namely the photo‐generated electron‐hole  (1) a recombination of charge carriers, namely the photo-generated electron-hole pairs, as electrons of 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  pairs, as electrons of the conduction band can quickly recombine with holes of the valence band, thus  pairs, as electrons of the conduction band can quickly recombine with holes of the valence band, thus  pairs, as electrons of the conduction band can quickly recombine with holes of the valence band, thus  the conduction band can quickly recombine with holes of the valence band, thus releasing energy as 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  releasing energy  energy  as unproductive  unproductive  heat  or photons;  photons;  (2) a  fast backward  backward  reaction, namely  namely  the  releasing  heat  or  (2)  reaction,  the  releasing  energy  as  unproductive  heat  or  photons;  (2)  a a fast  fast  backward  reaction,  namely  the  unproductive heatas  or photons; (2) a fast backward reaction, namely the recombination hydrogen releasing  energy  as  unproductive  heat  or  photons;  (2)  a  fast  backward  reaction, of namely  the  recombination  of  hydrogen  and  oxygen  into  water.  It  is  noteworthy  that  repetitive  photocatalytic  recombination  of  hydrogen  and  oxygen  into  water.  It  is  noteworthy  that  repetitive  photocatalytic  recombination  of  hydrogen  and  oxygen  into  water.  It  is  noteworthy  that  repetitive  photocatalytic  recombination  of  hydrogen  and  oxygen  into  water.  It  is  noteworthy  that  repetitive  photocatalytic  and oxygen intoof water. It is noteworthy photocatalytic tests,repetitive  using thephotocatalytic  same sample recombination  hydrogen  and  oxygen that into repetitive water.  It  is  noteworthy  that  tests, using the same sample three times in succession, gave the same catalytic profile, with good data  tests, using the same sample three times in succession, gave the same catalytic profile, with good data  tests, using the same sample three times in succession, gave the same catalytic profile, with good data  tests, using the same sample three times in succession, gave the same catalytic profile, with good data  three times in succession, gave the same catalytic profile, with good data reproducibility, thus ruling 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  reproducibility,  thus  ruling  out that  that  hydrogen  might  partially  arise from  from the  the presence  presence  of organic  organic  reproducibility,  thus  ruling  out  hydrogen  partially  arise  reproducibility,  thus  ruling  out  that  hydrogen  might  partially  arise  from  the  presence  of  organic  out that hydrogen might partially arise from themight  presence of organic to the of  synthesis, reproducibility,  thus  ruling  out  that  hydrogen  might  partially  arise residues from  the due presence  of  organic  residues due to the synthesis, acting as sacrificial agents.  residues due to the synthesis, acting as sacrificial agents.  residues due to the synthesis, acting as sacrificial agents.  residues due to the synthesis, acting as sacrificial agents.  acting as sacrificial agents. residues due to the synthesis, acting as sacrificial agents.  Interestingly, both bare TiO  (black line) and CeO Interestingly, both bare TiO 2 (black line) and CeO  (black line) and CeO 2 (brown line)  (brown line)  samples showed some activity  Interestingly, both bare TiO Interestingly, both bare TiO (brown line)  samples showed some activity  samples showed some activity  Interestingly, both bare TiO22222 (black line) and CeO (black line) and CeO22222   (brown line) (brown line)   samples showed some activity  samples showed some activity Interestingly, both bare TiO  (black line) and CeO (brown line) samples showed some activity  in the production of hydrogen which was found to increase in the presence of gold. The coupling of  in the production of hydrogen which was found to increase in the presence of gold. The coupling of  in the production of hydrogen which was found to increase in the presence of gold. The coupling of  in the production of hydrogen which was found to increase in the presence of gold. The coupling of  in the production of hydrogen which was found to increase in the presence of gold. The coupling in the production of hydrogen which was found to increase in the presence of gold. The coupling of  CeO  with TiO  positively affected the photocatalytic activity with a further increase obtained by the  CeO 2 with TiO  with TiO 2 positively affected the photocatalytic activity with a further increase obtained by the   positively affected the photocatalytic activity with a further increase obtained by the  CeO 222 with TiO 222 positively affected the photocatalytic activity with a further increase obtained by the  CeO of CeO CeO 2 with TiO 2 positively affected the photocatalytic activity with a further increase obtained by the  2 with TiO 2 positively affected the photocatalytic activity with a further increase obtained deposition of gold particles in the binary system of TiO 22‐CeO ‐CeO . In fact, both TiO ‐10%CeO  (green  deposition of gold particles in the binary system of TiO 2‐CeO ‐CeO 2. In fact, both TiO . In fact, both TiO 2‐10%CeO ‐10%CeO 2 (green   (green  deposition of gold particles in the binary system of TiO 222‐10%CeO 222 (green  deposition of gold particles in the binary system of TiO by the deposition of gold particles in the binary system 2of TiO22222. In fact, both TiO -CeO TiO2 -10%CeO deposition of gold particles in the binary system of TiO 2‐CeO . In fact, both TiO 2‐10%CeO 2 (green  2 . In fact, both 2 line)  and  Au/TiO ‐10%CeO (blue  line)  catalysts  showed  better  performance  than  bare  TiO (H line)  and  Au/TiO 2‐10%CeO ‐10%CeO   (blue  line)  catalysts  showed  better  performance  than  bare  TiO  (H (H line)  and  Au/TiO 222‐10%CeO 222  2 (blue  line)  catalysts  showed  better  performance  than  bare  TiO 222  2 (H 22 2 2   line)  and  Au/TiO (blue  line)  catalysts  showed  better  performance  than  bare  TiO (green line)Au/TiO and Au/TiO (bluecatalysts  line) catalysts showed performance line)  and  2‐10%CeO 2  (blue 2line)  showed  better better performance  than  than bare bare TiO2TiO   (H22  2 -10%CeO evolution around two times higher) and Au/TiO  (H  evolution around 25% higher). As reported in  evolution around two times higher) and Au/TiO 2 (H  (H 2 evolution around 25% higher). As reported in   evolution around 25% higher). As reported in  evolution around two times higher) and Au/TiO 222 (H 222 evolution around 25% higher). As reported in  evolution around two times higher) and Au/TiO (H (H evolution around two times higher) and Au/TiO 2 (H22 evolution around 25% higher). As reported in  2 evolution around two times higher) and Au/TiO 2 evolution around 25% higher). As reported the literature, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  the literature, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  the literature, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  the literature, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  in the literature, despite the bulk ceria and titania not having a similar crystal structure, cerium the literature, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  3+ 4+ 4+ 3+ 4+ 4+ 3+ 4+ 4+ 3+ 4+ 4+ (Ce  and Ce ) can replace the Ti  ions, modifying the physicochemical properties of TiO . Such an  (Ce  and Ce ) can replace the Ti  ions, modifying the physicochemical properties of TiO 2. Such an  . Such an  (Ce  and Ce ) can replace the Ti  ions, modifying the physicochemical properties of TiO 222. Such an  (Ce  and Ce ) can replace the Ti  ions, modifying the physicochemical properties of TiO 3+ 4+ 4+ 3+ 4+ 4+ ions and) can replace the Ti Ce ) can replace the Ti ions, modifying the physicochemical properties of TiO2 . (Ce (Ce  and Ce  ions, modifying the physicochemical properties of TiO 2. Such an  interaction  between  the  CeO and  TiO frameworks  could  be  the  key  factor  explaining  the  interaction  between  the  CeO and  TiO frameworks  could  be  the  key  factor  explaining  the  interaction  between  the  CeO 222  2  and  TiO 222  2  frameworks  could  be  the  key  factor  explaining  the  interaction  between  the  CeO and  TiO frameworks  could  be  the  key  factor  explaining  the  Such an interaction between the and2  TiO frameworks could be the key factor explaining the interaction  between  the  CeO 2  CeO and  2TiO frameworks  could  be  the  key  factor  explaining  the  2 enhancement of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].  enhancement of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].  enhancement of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].  enhancement of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].  enhancement of the photocatalytic activity of this mixed oxide system towards water splitting [39,40]. 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 The metal atoms, instead, pile up the electrons from the TiO 2 conduction band and transfer them to   conduction band and transfer them to  The metal atoms, instead, pile up the electrons from the TiO The metal atoms, instead, pile up the electrons from the TiO 2 conduction band and transfer them to  The metal atoms, instead, pile up the electrons from the TiO2222 conduction band and transfer them to  conduction band and transfer them to The metal atoms, instead, pile up the electrons from the TiO  conduction band and transfer them to  hydrogen protons, acting as H  evolution centers.    hydrogen protons, acting as H 2 evolution centers.   evolution centers.    hydrogen protons, acting as H 222 evolution centers.    hydrogen protons, acting as H   hydrogen protons, acting as H22 evolution centers.  evolution centers.   hydrogen protons, acting as H 2.2. Photocatalytic Oxidation of 2‐Propanol  2.2. Photocatalytic Oxidation of 2‐Propanol  2.2. Photocatalytic Oxidation of 2‐Propanol  2.2. Photocatalytic Oxidation of 2‐Propanol  2.2. Photocatalytic Oxidation of 2-Propanol 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  Figure 2 shows the activity data at 25 °C of the 2‐propanol photocatalytic oxidation on all tested  Figure 2 shows the activity data at 25 °C of the 2‐propanol photocatalytic oxidation on all tested  Figure 2 shows the activity data at 25 °C of the 2‐propanol photocatalytic oxidation on all tested  Figure 2 shows the activity data at 25 ˝ C of the 2-propanol photocatalytic oxidation on all tested 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  catalysts in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to  catalysts in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to  catalysts in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to  catalysts in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity catalysts in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to  CO 2  (Figure 2c), and yield to CO  (Figure 2d). It must be noted that the first point was taken after 20  CO 2  (Figure 2c), and yield to CO 2 (Figure 2d). It must be noted that the first point was taken after 20   (Figure 2d). It must be noted that the first point was taken after 20  CO 2  (Figure 2c), and yield to CO 222 (Figure 2d). It must be noted that the first point was taken after 20  CO 2  (Figure 2c), and yield to CO to CO 2 (Figure 2c), and yield to2 (Figure 2d). It must be noted that the first point was taken after 20  2 (Figure 2d). It must be noted that the first point was taken COCO 2 (Figure 2c), and yield to CO min to allow the lamp to reach a stable energy status.  min to allow the lamp to reach a stable energy status.  min to allow the lamp to reach a stable energy status.  min to allow the lamp to reach a stable energy status.  after 20 min to allow the lamp to reach a stable energy status. min to allow the lamp to reach a stable energy status. 

Catalysts 2016, 6, 121 Catalysts 2016, 6, 121 

4 of 13 4 of 13  100 Selectivity to acetone (%)

2-propanol conversion (%)

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60

40

20

80

60

40

20 Catalysts 2016, 6, 121  Catalysts 2016, 6, 121 

6, 121  016, 6, 121  16, 6, 121  16, 6, 121 

3 of 13  3 of 13  3 of 13  3 of 13  0

0 40 60 Time (min)

80

20

0.6 0.6 0.6 0.6 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2

11

80

 

(b)

0.8 0.8

50

80

40

60

CO2 Yield (%)

0.8 0.8 0.8 0.8 Selectivity to CO2 (%)

H 2 evolution (mmol)

H 2 evolution (mmol) H H 22 evolution evolution (mmol) (mmol)

(a) 

40 60 Time (min)

H 2 evolution (mmol) H 2 evolution (mmol)

20

1 111

40

20

0.6 0.6

30

0.4 0.4

20

0.2 0.2 10

0

0 000 20 40 60 80 0 000 20 20 60 60 20 40Time 40(min) 60 80 80 80 20 40 60 80 40 Time (min) Time (min) Time (min) Time (min)  

(c) 

00 00

0 20

    

40 60 Time (min)

80

 

2020

4040 Time (m Time (mi

(d)

Figure  Photocatalytic  2  production  30  °C  unde Figure  1. 1.  Photocatalytic  HH 2  production  at at  30  °C  under  Figure 2. Photocatalytic 2‐propanol oxidation over: TiO 2 (), CeO2 (), TiO2‐10%CeO2 (), Au/TiO2  1.  H2 H production  at  °C  under  ultraviolet  (UV)  irradiation  over:  TiOTiO 2TiO   (),  ure  1. Photocatalytic  Photocatalytic    production  at 30  30 °C  °C under  under  ultraviolet  (UV)  irradiation  over:  2(),    (),  re Photocatalytic  1.  Photocatalytic  HH production  at  30  °C  under  ultraviolet  (UV)  irradiation  over:  (),  CeO 2‐10%CeO 2 (), Au/CeO 2 ( CeO 2 (), Au/TiO 2 (), Au/CeO 2 ( Figure 2. 30  Photocatalytic 2-propanol oxidation over: TiO ( 22  ), CeO (2 (), TiO ), TiO22‐10%CeO -10%CeO ( ), re  1.  22  2production  at  ultraviolet  (UV)  irradiation  over:  2TiO 22 (), TiO 22 (), Au/TiO  () and Au/TiO 2‐10%CeO2 (): (a) 2‐propanol conversion; (b) Selectivity to acetone; (c)  (), Au/CeO ), TiO 2‐10%CeO 2 (), Au/TiO 2 (), Au/CeO 2 () and Au/TiO 2‐10%CeO 2  ().  O2 (), TiO 2 (), TiO 2‐10%CeO 2 (), Au/TiO 2( (), Au/CeO 2( () and Au/TiO 2 ‐10%CeO 2  ().   (), TiO ‐10%CeO  (), Au/TiO  (), Au/CeO 2 () and Au/TiO  () and Au/TiO ‐10%CeO 2  ().  Au/TiO ), 2Au/CeO ) and Au/TiO -10%CeO ( ): (a) 2-propanol conversion; (b) Selectivity to 22‐10%CeO 22 (), Au/TiO 222 (), Au/CeO 22 22‐10%CeO 2  ().  2 2 2; (d) Yield to CO2.  Selectivity to CO For all samples it is possible to note that, after a s For all samples it is possible to note that, after a sh acetone; (c) Selectivity to CO2 ; (d) Yield to CO2 .

samples it is possible to note that, after a short induction period (around 10 min), due to   all samples it is possible to note that, after a short induction period (around 10 min), due to  all samples it is possible to note that, after a short induction period (around 10 min), due to  the stabilization  stabilization of of lamp  lamp irradiation  irradiation and/or  and/or water  water sat s the  all samples it is possible to note that, after a short induction period (around 10 min), due to  The and/or  partial  oxidation  of  2‐propanol  can  proceed  with  two  reaction  pathways  [41,42]:  the  ation  of  of  lamp  irradiation  water  saturation  with  evolved  gases  [36],  hydrogen  ilization  of lamp  lamp  irradiation  and/or  water  saturation  with  evolved  gases  [36],  hydrogen  lization  of  lamp  irradiation  and/or  water  saturation  with  evolved  gases  [36],  hydrogen  production firstly undergoes an almost linear increm production firstly undergoes an almost linear increme ization  irradiation  and/or  water  saturation  with  evolved  gases  [36],  hydrogen  The partial oxidation of 2-propanol can proceed with two reaction pathways [41,42]: the oxidative oxidative dehydrogenation to acetone and water or the dehydration to propene. Over Au/CeO 2 and  firstly undergoes an almost linear increment for up to 40 min, followed by a moderate  ion firstly undergoes an almost linear increment for up to 40 min, followed by a moderate  on firstly undergoes an almost linear increment for up to 40 min, followed by a moderate  decrease of  of the  the production  production  rate.  According to to the  the lite li rate.  According  on firstly undergoes an almost linear increment for up to 40 min, followed by a moderate  dehydrogenation to acetone and water or the dehydration todecrease  propene. Over Au/CeO 2 and CeO2 2  catalysts  no  propene  was  formed  during  the  photocatalytic  tests,  whereas  over  TiO2‐based  CeO the  production  rate.  According  to  the  literature  [37,38]  this  can  be  the  result  of  two  e of  of the  the production  production  rate. According  According  to was the literature  literature  [37,38]  this  can be  be the  the result  result  of two  two over TiO -based samples of  the  production  rate.  According  to  the  literature  [37,38]  this  can  be  the  result  of  two  fundamental effects: (1) a recombination of charge carr fundamental effects: (1) a recombination of charge carrie rate.  to  the  [37,38]  this  can  of  catalysts no propene formed during the photocatalytic tests, whereas 2 samples (TiO2, TiO2‐10%CeO2, Au/TiO2 and Au/TiO2‐10%CeO2) the formation of propene occurred to  l effects: (1) a recombination of charge carriers, namely the photo‐generated electron‐hole  ental effects: (1) a recombination of charge carriers, namely the photo‐generated electron‐hole  ntal effects: (1) a recombination of charge carriers, namely the photo‐generated electron‐hole  pairs, as electrons of the conduction band can quickly  pairs, as electrons of the conduction band can quickly re ntal effects: (1) a recombination of charge carriers, namely the photo‐generated electron‐hole  (TiO2 , TiO2 -10%CeO2 , Au/TiO2 and Au/TiO2 -10%CeO2 ) the formation of propene occurred to a very a very low extent (1%–3% selectivity). In this latter case, the formation of propene can be ascribed to  ctrons of the conduction band can quickly recombine with holes of the valence band, thus   electrons of the conduction band can quickly recombine with holes of the valence band, thus  electrons of the conduction band can quickly recombine with holes of the valence band, thus  releasing  energy  as unproductive  unproductive  heat or  or photons;  photons releasing  as  electrons of the conduction band can quickly recombine with holes of the valence band, thus  low extent (1%–3% selectivity). In this latter case, the formation of propeneenergy  can be ascribed to the moreheat  the more acidic character of TiO 2 with respect to CeO2 [43,44].    nergy  as as  unproductive  heat  or or  photons;  (2) (2)  a  backward  reaction,  namely  the the  g  energy  as unproductive  unproductive  heat  or TiO photons;  (2) fast  fast  backward  reaction,  namely  the  of of    energy  energy  as  unproductive  heat  or  photons;  (2)  a a fast  fast  backward  reaction,  namely  the  recombination  hydrogen  and  oxygen  into  water.  recombination  hydrogen  and  oxygen  into  water.  It heat  photons;  a  backward  reaction,  namely  acidic character of 2 with respect to CeO2 [43,44]. Considering  the  conversion  of  2‐propanol  (Figure  2a),  the  best  results  were  found  over  the  on  of  of  hydrogen  and  oxygen  into  water.  It  is  noteworthy  that  repetitive  photocatalytic  nation  of hydrogen  hydrogen  and  oxygen  into  water.  It  is  noteworthy  that  repetitive  photocatalytic  nation  of  hydrogen  and  oxygen  into  water.  It  is  noteworthy  that  repetitive  photocatalytic  tests, using the same sample three times in succession, tests, using the same sample three times in succession, g ation  and  oxygen  into  water.  It  is  noteworthy  that  repetitive  photocatalytic  Considering the conversion of 2-propanol (Figure 2a), the best results were found over Au/TiO2  catalyst  (violet  line),  which  was  more  active  than,  in  order,  TiO2  (black  line),  Au/TiO 2‐ he same sample three times in succession, gave the same catalytic profile, with good data  ing the same sample three times in succession, gave the same catalytic profile, with good data  ng the same sample three times in succession, gave the same catalytic profile, with good data  reproducibility,  thus  ruling  out  that  hydrogen  migh thus  ruling  out  that  hydrogen  might  ng the same sample three times in succession, gave the same catalytic profile, with good data  the Au/TiO2 catalyst (violet line), which was more activereproducibility,  than, in order, TiO 2 (black line), 10%CeO 2  (blue  line)  and  TiO2‐10%CeO2  samples  (green  line).  The  increase  of  TiO2  photocatalytic  lity,  thus  ruling  out  that  hydrogen  might  partially  arise  from  the  presence  of  organic  cibility,  thus  ruling  out  that  hydrogen  might  partially  arise  from  the  presence  of  organic  cibility,  thus  ruling  out  that  hydrogen  might  partially  arise  from  the  presence  of  organic  residues due to the synthesis, acting as sacrificial agen residues due to the synthesis, acting as sacrificial agents ibility,  thus  ruling  out  that  hydrogen  might  partially  arise  from  the  presence  of  organic  Au/TiO2 -10%CeO2 (blue line) and TiO2 -10%CeO2 samples (green line). The increase of TiO2 activity in the presence of Au particles can be ascribed to the different Fermi levels of the two species  e to the synthesis, acting as sacrificial agents.   due to the synthesis, acting as sacrificial agents.  due to the synthesis, acting as sacrificial agents.  2 (black line) and CeO 2 (black line) and CeO 2 due to the synthesis, acting as sacrificial agents.  photocatalytic activity in the presence of Au particles can be ascribed Interestingly, both bare TiO toInterestingly, both bare TiO the different Fermi levels of the −) and the hole (h+) [45–47].  leading to an increased charge separation between the excited electron (e ingly, both bare TiO 2  (black line) and CeO 2   (brown line)   samples showed some activity  erestingly, both bare TiO 2 (black line) and CeO  (brown line)  separation samples showed some activity  restingly, both bare TiO  (black line) and CeO (brown line) samples showed some activity  in the production of hydrogen which was found to inc estingly, both bare TiO 22 (black line) and CeO 22  2(brown line) two species leading to an increased charge  samples showed some activity  betweenin the production of hydrogen which was found to incr the excited electron (e´ ) and the hole The  high  activity  of  the  Degussa  P25  TiO2  used  in  this  work  can  be  due  to  the  occurrence  of  an  uction of hydrogen which was found to increase in the presence of gold. The coupling of  roduction of hydrogen which was found to increase in the presence of gold. The coupling of  oduction of hydrogen which was found to increase in the presence of gold. The coupling of  + CeO 2 with TiO 2 positively affected the photocatalytic a CeO 2 with TiO 2 positively affected the photocatalytic ac oduction of hydrogen which was found to increase in the presence of gold. The coupling of  (h ) [45–47]. The high activity of the Degussa P25 TiO2 used in this work can be due to the occurrence interaction between the two phases of TiO 2 (80% anatase, 20% rutile) that increases both the charge  iO2 positively affected the photocatalytic activity with a further increase obtained by the  th TiO 2 positively affected the photocatalytic activity with a further increase obtained by the  h TiO  positively affected the photocatalytic activity with a further increase obtained by the  deposition of gold particles in the binary system of T deposition of gold particles in the binary system of Ti h TiO 22 positively affected the photocatalytic activity with a further increase obtained by the  of an interaction between the two phases of TiO2 (80% anatase, 20% rutile) that increases both the carrier (electron‐hole) separation and the total photo‐efficiency [48,49]. The bare CeO 2 (dark red curve)  of gold particles in the binary system of TiO 2‐CeO 2. In fact, both TiO 2‐10%CeO 2line)   (green  on of gold particles in the binary system of TiO 2‐CeO 2. In fact, both TiO 2‐10%CeO 2 (green  on of gold particles in the binary system of TiO ‐CeO . In fact, both TiO ‐10%CeO  (green  line)  and  Au/TiO 2‐10%CeO  2(blue  line) catalysts  catalysts show sho and  Au/TiO 2‐10%CeO 2  2(blue  line)  n of gold particles in the binary system of TiO 22‐CeO 22. In fact, both TiO 22‐10%CeO 22 (green  charge carrier (electron-hole) separation and the total photo-efficiency [48,49]. The bare CeO (dark and the Au/CeO 2 (orange curve) samples exhibited a low activity for the conversion of 2‐propanol  u/TiO 2‐10%CeO 2  (blue  line)  catalysts  showed  better  performance  than  bare  TiO 2TiO   (H   d  Au/TiO 2‐10%CeO   (blue  line)  catalysts  showed  better  performance  than  bare    (H d  Au/TiO ‐10%CeO (blue  line)  catalysts  showed  better  performance  than  bare  TiO (H evolution around two times higher) and Au/TiO 2 (H 2  evolution around two times higher) and Au/TiO 2 (H 2 ev   Au/TiO 22‐10%CeO 22  2(blue  line)  catalysts  showed  better  performance  than  bare  TiO 22  22(H 22  2  red curve) and the Au/CeO2 (orange curve) samples exhibited a low activity for the conversion of (maximum  conversion  of  around  30%),  while  the  presence  of  ceria  negatively  affected  the  ound two times higher) and Au/TiO 2  (H 2  evolution around 25% higher). As reported in  n around two times higher) and Au/TiO  (H 2  evolution around 25% higher). As reported in  n 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 h the literature, despite the bulk ceria and titania not ha  around two times higher) and Au/TiO 2 evolution around 25% higher). As reported in  2-propanol (maximum2 (H conversion of around 30%), while the presence of ceria negatively affected the performance  of  TiO2,  the  maximum  conversion  being,  in  (Ce fact,  lower  on  TiO2‐10%CeO2  (50%)  3+ 4+) can replace the Ti 4+ ions, modifying the 3+ and Ce 4+ 4+ ions, modifying the  e, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  ature, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  ture, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  (Ce  and Ce ) can replace the Ti ture, despite the bulk ceria and titania not having a similar crystal structure, cerium ions  performance of TiO2 , the maximum conversion being, in fact, lower on TiO2 -10%CeO 2 (50%) compared compared  to  TiO 2   (70%).  Differently  from  TiO 2 ,  the  presence  of  gold  did  not  affect  the  CeO 2  4+ 4+ 4+4+ 4+4+ 4+ 4+ ed Ce ) can replace the Ti  ions, modifying the physicochemical properties of TiO 2. Such an  ) can replace the Ti  ions, modifying the physicochemical properties of TiO 2interaction  . Such an  between  d Ce ) can replace the Ti  ions, modifying the physicochemical properties of TiO . Such an  between the  the CeO CeO   and  TiO   framewor interaction  2  2and  TiO 2  2framework  Ce ) can replace the Ti  ions, modifying the physicochemical properties of TiO 22. Such an  to TiO 2 (70%). Differently from TiO2 , the presence of gold did not affect the CeO2 performances, with performances, with the 2‐propanol conversion being almost the same for CeO 2 and Au/CeO2 samples.  between  the  CeO 2   and  TiO 2   frameworks  could  be  the  key  factor  explaining  the  on  between the  the CeO CeO and TiO TiO frameworks  could  be  the  key  factor  explaining  the  on  between  the  CeO and  TiO frameworks  could  be  the  key  factor  explaining  the  enhancement of the photocatalytic activity of this mixe enhancement of the photocatalytic activity of this mixed n  between  22  2 and  22  2 frameworks  could  be  the  key  factor  the  the 2-propanol conversion being almost the same for CeOexplaining  2 and Au/CeO2 samples. The  selectivity  to  acetone  (Figure  2b)  generally  showed  a  slight  decrease  over  time,  with  a  nt of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].  ment of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].  ment of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].  The metal atoms, instead, pile up the electrons from th The metal atoms, instead, pile up the electrons from the ment of the photocatalytic activity of this mixed oxide system towards water splitting [39,40].  The selectivity to acetone (Figure 2b) generally showed a slight decrease over time, with a corresponding  increase  in  the  selectivity  to  CO2.  The  bare  TiO 2  displayed  the  highest  selectivity  toms, instead, pile up the electrons from the TiO al atoms, instead, pile up the electrons from the TiO 2 conduction band and transfer them to  al atoms, instead, pile up the electrons from the TiO  conduction band and transfer them to  hydrogen protons, acting as H 2 evolution centers.  hydrogen protons, acting as H 2 evolution centers.      l atoms, instead, pile up the electrons from the TiO 22 conduction band and transfer them to  corresponding increase in the2 conduction band and transfer them to  selectivity to CO2 . 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  rotons, acting as H2 evolution centers.         en protons, acting as H 2 evolution centers.  n protons, acting as H  evolution centers.  n protons, acting as H 22 evolution centers.  Au had a negative effect on the acetone selectivity, causing a decrease of the maximum value from  2.2. Photocatalytic Oxidation of 2‐Propanol  2.2. Photocatalytic Oxidation of 2‐Propanol  95%  over  TiO2  to  70%  over  Au/TiO2,  50%  over  TiO2‐10%CeO2  and  40%  over  Au/TiO2‐10%CeO2.  alytic Oxidation of 2‐Propanol  ocatalytic Oxidation of 2‐Propanol  ocatalytic Oxidation of 2‐Propanol  catalytic Oxidation of 2‐Propanol  Figure 2 shows the activity data at 25 °C of the 2‐ Figure 2 shows the activity data at 25 °C of the 2‐pr 2 shows the activity data at 25 °C of the 2‐propanol photocatalytic oxidation on all tested  ure 2 shows the activity data at 25 °C of the 2‐propanol photocatalytic oxidation on all tested  ure 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), s catalysts in terms of alcohol conversion (Figure 2a), se re 2 shows the activity data at 25 °C of the 2‐propanol photocatalytic oxidation on all tested  terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to  s in terms of alcohol conversion (Figure 2a), selectivity to acetone (Figure 2b), selectivity to  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  CO 2 (Figure 2c), and yield to CO 2 (Figure 2d). It must b in terms 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  gure 2c), and yield to CO 2 (Figure 2d). It must be noted that the first point was taken after 20  ure 2c), and yield to CO  (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.  min to allow the lamp to reach a stable energy status.  ure 2c), and yield to CO 22 (Figure 2d). It must be noted that the first point was taken after 20 

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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 [52–54] 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]. Table 1. Chemico-physical properties of Au/TiO2 -CeO2 catalysts. Sample

Surface Area (m2 /g)

Eg (eV)

Crystallite Size (nm) a

Crystal Phase a

TiO2 CeO2

44.8 110.2

2.98 2.90

24 10

TiO2 -10%CeO2

47.5

2.93

22

Au/TiO2 Au/CeO2

46.4 112.7

2.97 2.95

21 11

50.5

2.96

19

TiO2 Anatase-Rutile CeO2 Fluorite TiO2 Anatase-Rutile CeO2 Fluorite TiO2 Anatase-Rutile CeO2 Fluorite TiO2 Anatase-Rutile, CeO2 Fluorite

Au/TiO2 -10%CeO2

a

Estimated by XRD measurements.

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

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467

Catalysts 2016, 6, 121  461 461 461 461 6 of 13  6 of 13  461 461

6 of 13  6 of 13  146 146 146 146 146 146

Intensity (a.u.)

(b)

148 148

148 148 148 148 120 140 160 180 200x 5 -1 148 Raman shift (cm ) 148

Intensity (a.u.)

Intensity (a.u.) (a.u.) Intensity Intensity (a.u.)

Intensity (a.u.)

 

xx 55

146 430 440 146 450 460 470 480 490 100 466 467 467 467 Raman shift (cm-1) 467 647 403 524 467 467

Intensity (a.u.)

(a) 

1000

Intensity (a.u.) (a.u.) Intensity Intensity (a.u.)

647 647 647 647 403 466 466 524 524 524 403524 403 403 400 800 461 600 461 647 524 524 403 (cm-1647 Raman 403 shift ) Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.) (a.u.) Intensity Intensity (a.u.)

Intensity (a.u.)

xx55

6 of 13  6 of 13 

xxx55x5 5

466 466 466 466 200

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

defective structure of ceria in the presence of gold. xIn5 fact, Raman has been reported to be sensitive 466 to the degree of 524 crystallinity of samples, with broader, less intense Raman peaks in the case of less 647 403 148 crystalline material [56]. Catalysts 2016, 6, 121  6 of 13  Catalysts 2016, 6, 121  6 of 13 

461

(c)  467

Figure 3. (a) Vibrational modes of Raman spectra of TiO 2 (─), Au/TiO2 (─), TiO2‐10%CeO2 (─), and  467 467 Au/TiO2‐10%CeO2 (─) samples; (b) Raman shift of the signal of cubic CeO2 in TiO2‐10%CeO2(─), and  2‐10%CeO2 (─) samples; (c) Raman shift of the main Eg vibrational mode of TiO2 anatase in  Au/TiO 200 430 100 200 400 400 600 600 800 800 1000 1000 430 440 440 450 450 460 460 470 470 480 480 490 490 200 400 600 800 1000 430 440 450 460 470 480 490 200 400 600 800 1000 430 440 450 460 470 480 490 100 120 120 140 140 160 160 180 180 200 200 100 120 140 160 180 200 100 120 140 160 180 200 -1 -1 -1 -1 -1 -1 -1  (─), TiO 2‐10%CeO2 (─), and Au/TiO 2cm ‐10%CeO 2 (─) samples.  Raman shift (cm-1 TiO2 (─), Au/TiO -1 -1 -1 Raman shift ((2cm ))-1) Raman shift ( cm ) ) Raman shift (cm Raman shift ( cm ) Raman shift cm Raman shift ( ) Raman shift cm Raman shift cm Raman shift ( cm ) Raman shift ( cm ) Raman shift cm   200 200

400 600 800   1000 1000 430 440 440 450 450 460 460 470 470 480 480 490 490 400 600 800 430 100 120 120 140 140 160 160 180 180 200 200   100 -1 -1 -1 -1 (a)  (b) Raman shift ((cm cm-1-1)) Raman shift ((cm cm Raman Raman shift Raman shift cm Raman (a)  shift (b) 200 (c)  (a)  (b) (c)  400)) 600 800 1000 (c)  430 shift 440((cm 450)) 460 470 480 490    Interestingly, over the Au/TiO 2‐10%CeO2 sample, the peak associated with cubic CeO Raman shift (cm-1) Raman shift (cm-1) (a)  100 (b)2 (─), Au/TiO (c)  2 was less  (a)  (b) (c)  1000 430 440 440 450 450 460 460 470 470 480 480 490 490 1000 430   100 120 120 140 140 160 160 180 180 200 200 Figure 3. (a) Vibrational modes of Raman spectra of TiO 222 (─), TiO 222‐10%CeO 2  Figure 3. (a) Vibrational modes of Raman spectra of TiO 2 (─), Au/TiO 2 (─), TiO 2‐10%CeO 2 (─), and  Figure 3. (a) Vibrational modes of Raman spectra of TiO  (─), Au/TiO  (─), TiO ‐10%CeO 2 (─), and  (─), and  Figure 3. (a) Vibrational modes of Raman spectra of TiO 22 (─), Au/TiO  (─), TiO ‐10%CeO 2  (─), and  -1 -1 Figure 3. (a) Vibrational modes of Raman spectra of TiO (—), Au/TiO , TiO -10%CeO , and intense, broader and shifted to lower frequencies compared to over the TiO 2 ‐10%CeO 2  sample (Figure  -1 -1 2 Raman shift shift ((cm cm )) Raman Ramanshift shift ((cm cm )) Raman (a)  2 2 in TiO22‐10%CeO22(─), and  (b)    222‐10%CeO 222 (─) samples; (b) Raman shift of the signal of cubic CeO Au/TiO Figure 3. (a) Vibrational modes of Raman spectra of TiO  (─), Au/TiO 2 (─), TiO  (─), TiO ‐10%CeO (─), and  Figure 3. (a) Vibrational modes of Raman spectra of TiO 22 (─), Au/TiO 22‐10%CeO 2 2 (─), and  2‐10%CeO 2 (─) samples; (b) Raman shift of the signal of cubic CeO 22 in TiO 2‐10%CeO 2(─), and  Au/TiO ‐10%CeO  (─) samples; (b) Raman shift of the signal of cubic CeO  in TiO ‐10%CeO (─), and  Au/TiO ‐10%CeO  (─) samples; (b) Raman shift of the signal of cubic CeO 22 in TiO 22‐10%CeO 22(─), and  Au/TiO Au/TiO samples; (b)(c)  Raman shift of the signal of cubic CeO , 3b, orange and green lines, respectively). This could be due to a less crystalline and more defective  2 -10%CeO2 2 in TiO2 -10%CeO2 (b) (c)  (b) 222‐10%CeO 222 (─) samples; (c) Raman shift of the main E ggg vibrational mode of TiO 222 anatase in  Au/TiO Figure 3. (a) Vibrational modes of Raman spectra of TiO 2 ‐10%CeO 2  (─) samples; (b) Raman shift of the signal of cubic CeO 2  in TiO 2 ‐10%CeO 2 (─), and  Au/TiO 2 ‐10%CeO 2  (─) samples; (b) Raman shift of the signal of cubic CeO 2  in TiO 2 ‐10%CeO 2 (─), and  Au/TiO 2 ‐10%CeO 2  (─) samples; (c) Raman shift of the main E g  vibrational mode of TiO 2  anatase in  Au/TiO ‐10%CeO  (─) samples; (c) Raman shift of the main E  vibrational mode of TiO  anatase in  Au/TiO ‐10%CeO  (─) samples; (c) Raman shift of the main E  vibrational mode of TiO  anatase in  Au/TiO and Au/TiO samples; (c) Raman shiftRaman  of the main mode of TiO g vibrational 2 -10%CeO 2 anatase structure  of  ceria  in  the  2presence  of  gold.  In  fact,  has Ebeen  reported  to  be  sensitive  to  the  2 (─), Au/TiO2 ( 222 (─), Au/TiO 222 (─), TiO 222‐10%CeO 2  (─), and Au/TiO 222‐10%CeO 222 (─) samples.  TiO  modes of Raman spectra of TiO 2  (─), Au/TiO 2  (─), TiO 2 ‐10%CeO 2  (─), and  2 ‐10%CeO 2  (─) samples; (b) Raman shift of the signal of cubic CeO Au/TiO modes of Raman spectra of TiO 2  (─), Au/TiO 2  (─), TiO 2 ‐10%CeO 2  (─), and  2 ‐10%CeO 2  (─) samples; (c) Raman shift of the main E g  vibrational mode of TiO 2  anatase in  Au/TiO 2 ‐10%CeO 2  (─) samples; (c) Raman shift of the main E g  vibrational mode of TiO 2  anatase in  Au/TiO 2  (─), Au/TiO 2  (─), TiO 2 ‐10%CeO 2  (─), and Au/TiO 2 ‐10%CeO 2  (─) samples.  TiO  (─), Au/TiO  (─), TiO ‐10%CeO 2  (─), and Au/TiO ‐10%CeO  (─) samples.  TiO  (─), Au/TiO  (─), TiO ‐10%CeO 2  (─), and Au/TiO ‐10%CeO  (─) samples.  TiO in TiO (—), Au/TiO2of  samples,  , TiO2 -10%CeO , and Au/TiO samples. 2 -10%CeO 2 degree  of 2 crystallinity  with 2 broader,  less  intense  Raman  peaks  in  the  case  of  less  amples; (b) Raman shift of the signal of cubic CeO  in TiO22‐10%CeO (─), and  amples; (b) Raman shift of the signal of cubic CeO 22 in TiO 22(─), and  g vibration Au/TiO222‐10%CeO  (─), Au/TiO22 (─), TiO  (─), TiO 2‐10%CeO ‐10%CeO 2 (─), and Au/TiO (─), and Au/TiO ‐10%CeO22 (─) samples; (c) Raman shift of the main E  (─) samples.  TiO22 (─), Au/TiO 2‐10%CeO 2   (─) samples.  TiO crystalline material [56].  Interestingly, over the Au/TiO 222‐10%CeO 222 sample, the peak associated with cubic CeO 222 was less  Interestingly, over the Au/TiO 2‐10%CeO 2 sample, the peak associated with cubic CeO 2 was less  2‐10%CeO2 (─) s Interestingly, over the Au/TiO ‐10%CeO  sample, the peak associated with cubic CeO  was less  samples; (c) Raman shift of the main E  vibrational mode of TiO  anatase in  amples; (c) Raman shift of the main E gg vibrational mode of TiO 22 anatase in  TiO2 (─), Au/TiO2 (─), TiO2‐10%CeO2 (─), and Au/TiO Figure 3c shows the position of the main vibrational mode of anatase Egg,, pointing out that there  pointing out that there Figure 3c shows the position of the main vibrational mode of anatase E intense, broader and shifted to lower frequencies compared to over the TiO 222‐10%CeO 222 sample (Figure  Interestingly, over the Au/TiO ‐10%CeO  sample, the peak associated with cubic CeO  was less  Interestingly, over the Au/TiO 22‐10%CeO 22 sample, the peak associated with cubic CeO 22 was less  TiO22‐10%CeO ‐10%CeO2 2 (─), and Au/TiO (─), and Au/TiO ‐10%CeO22 (─) samples.   (─) samples.  TiO 22‐10%CeO intense, broader and shifted to lower frequencies compared to over the TiO 2‐10%CeO 2 sample (Figure  intense, broader and shifted to lower frequencies compared to over the TiO ‐10%CeO  sample (Figure  ´ 1 −1 was only a slight red shift (about 2 cm ) on the TiO ) on the TiO was only a slight red shift (about 2 cm 2‐10%CeO 2 and Au/TiO 2‐10%CeO 2 samples. By  2 -10%CeO 2 and Au/TiO 2 -10%CeO 2 samples. 3b, orange and green lines, respectively). This could be due to a less crystalline and more defective  Interestingly, over the Au/TiO 2‐10%CeO 2 sample, the peak associ intense, broader and shifted to lower frequencies compared to over the TiO 2 ‐10%CeO 2  sample (Figure  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  3b, orange and green lines, respectively). This could be due to a less crystalline and more defective  −1 is associated with the O‐Ti‐O vibration, the presence  By considering that the Eg anatase mode at 148 cm´1 is associated with the O-Ti-O vibration, the considering that the E g anatase mode at 148 cm e Au/TiO ‐10%CeO  sample, the peak associated with cubic CeO 2fact,   was less   Au/TiO22‐10%CeO 22structure   sample, the peak associated with cubic CeO 2fact,   was less  structure  of  ceria  in  the  presence  of  gold.  In  Raman  has  been  reported  to  be  sensitive  to  the  intense, broader and shifted to lower frequencies compared to over the  3b, orange and green lines, respectively). This could be due to a less crystalline and more defective  3b, orange and green lines, respectively). This could be due to a less crystalline and more defective  of  ceria  in  the  presence  of  gold.  In  fact,  Raman  has  been  reported  to  be  sensitive  to  the  structure  of  ceria  in  the  presence  of  gold.  In  Raman  has  been  reported  to  be  sensitive  to  the  presence of CeO2 in2 lattice could probably cause a bond distortion resulting in the observed shift of  the TiO2 lattice could probably cause a bond distortion resulting in the observed of CeO2 in the TiO d to lower frequencies compared to over the TiO 2‐10%CeO ‐10%CeO  sample (Figure  d to lower frequencies compared to over the TiO 2presence  22 sample (Figure  degree  of  of  samples,  with  broader,  less  intense  Raman  the  case  of  3b, orange and green lines, respectively). This could be due to a less c structure  of ceria  ceria in  in  the  presence  of  gold.  In fact,  fact,  Raman  has  been peaks  reported  to  be  sensitive  to the  the  structure  of  the  of  gold.  In  Raman  has  been  reported  to  be  sensitive  to  degree  of crystallinity  crystallinity  of  samples,  with  broader,  less  intense  Raman  peaks in  in  the  case  of less  less  degree  of  crystallinity  of  samples,  with  broader,  less  intense  Raman  peaks  in  the  case  of  less  shift of the vibration band. the vibration band.  , respectively). This could be due to a less crystalline and more defective  , respectively). This could be due to a less crystalline and more defective  crystalline material [56].  structure  of  ceria  in  the  presence  of  gold.  In  fact,  Raman  has  been  r degree  of  crystallinity  of  samples,  with  broader,  less  intense  Raman  peaks  in  the  case  of  less  degree  of  crystallinity  of  samples,  with  broader,  less  intense  Raman  peaks  in  the  case  of  less  crystalline material [56].  crystalline material [56].  To analyze the distribution of cerium oxide on TiO22 a Raman mapping analysis was performed.  a Raman mapping analysis was performed. To analyze the distribution of cerium oxide on TiO resence of  of gold.  gold. In  In fact,  fact,  Raman  has  been  reported  to  be  sensitive  to  the  resence  Raman  has  been  reported  to  be  sensitive  to  the  Figure 3c shows the position of the main vibrational mode of anatase E ggg, pointing out that there  degree  of  crystallinity  of  samples,  with  broader,  less  intense  Ram crystalline material [56].  crystalline material [56].  Figure 3c shows the position of the main vibrational mode of anatase E g, pointing out that there  Figure 3c shows the position of the main vibrational mode of anatase E , pointing out that there  This technique allows non-destructive −1and non-invasive analysis of features such as the separation This technique allows non‐destructive and non‐invasive analysis of features such as the separation  −1 −1 samples,  with  with  broader,  broader,  less  intense  Raman  Raman  peaks  peaks  in  in  −1 the  case crystalline material [56].  of  less  less  samples,  less  intense  the  case  of  was only a slight red shift (about 2 cm ) on the TiO 222‐10%CeO 222 and Au/TiO 222‐10%CeO 222 samples. By  Figure 3c shows the position of the main vibrational mode of anatase E , pointing out that there  Figure 3c shows the position of the main vibrational mode of anatase E gg, pointing out that there  was only a slight red shift (about 2 cm ) on the TiO 2‐10%CeO 2 and Au/TiO 2‐10%CeO 2 samples. By  was only a slight red shift (about 2 cm ) on the TiO ‐10%CeO  and Au/TiO ‐10%CeO  samples. By  of chemical species in multi‐component samples. Chemical maps of TiO of chemical species in multi-component samples. Chemical maps of TiO22 and CeO and CeO22 nanostructures  nanostructures −1 −1 −1 −1 −1 −1 considering that the E g  anatase mode at 148 cm  is associated with the O‐Ti‐O vibration, the presence  Figure 3c shows the position of the main vibrational mode of anat was only a slight red shift (about 2 cm ) on the TiO 2 ‐10%CeO 2  and Au/TiO 2 ‐10%CeO  samples. By  was only a slight red shift (about 2 cm ) on the TiO 2‐10%CeO2 and Au/TiO2‐10%CeO22 samples. By  considering that the E g anatase mode at 148 cm  is associated with the O‐Ti‐O vibration, the presence  considering that the E  is associated with the O‐Ti‐O vibration, the presence  gg anatase mode at 148 cm ‐10%CeO22 sample, based on the detailed Raman image (over a 15 μm × 15 μm  performed on the TiO performed on the TiO22-10%CeO sample, based on −1 the detailed Raman image (over a 15−1µm ˆ 15 µm −1 osition of the main vibrational mode of anatase E , pointing out that there  osition of the main vibrational mode of anatase E gg, pointing out that there  of CeO 222 in the TiO 222 lattice could probably cause a bond distortion resulting in the observed shift of  was only a slight red shift (about 2 cm ) on the TiO2‐10%CeO2 and A considering that the E  anatase mode at 148 cm  is associated with the O‐Ti‐O vibration, the presence  considering that the E gg anatase mode at 148 cm  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  of CeO  in the TiO  lattice could probably cause a bond distortion resulting in the observed shift of  image scan, with 150 points per line and 150 lines per image), are presented in Figure 4.  image scan, with 150 points per line and 150 lines per image), are presented in Figure 4.    (about 2 cm ) on the TiO ‐10%CeO  and Au/TiO ‐10%CeO22 samples. By   samples. By  (about 2 cm−1−1) on the TiO 22‐10%CeO 22 and Au/TiO 22‐10%CeO the vibration band.  considering that the Eg anatase mode at 148 cm−1 is associated with the O of CeO  in the TiO  lattice could probably cause a bond distortion resulting in the observed shift of  of CeO 22 in the TiO 22 lattice could probably cause a bond distortion resulting in the observed shift of  the vibration band.  the vibration band.  tase mode at 148 cm−1−1 is associated with the O‐Ti‐O vibration, the presence   is associated with the O‐Ti‐O vibration, the presence  tase mode at 148 cm To analyze the distribution of cerium oxide on TiO of CeO22 in the TiO 2 lattice could probably cause a bond distortion res the vibration band.  the vibration band.  To analyze the distribution of cerium oxide on TiO 2 a Raman mapping analysis was performed.  To analyze the distribution of cerium oxide on TiO  a Raman mapping analysis was performed.  22 a Raman mapping analysis was performed.  could probably cause a bond distortion resulting in the observed shift of  could probably cause a bond distortion resulting in the observed shift of  This technique allows non‐destructive and non‐invasive analysis of features such as the separation  the vibration band.  To analyze the distribution of cerium oxide on TiO  a Raman mapping analysis was performed.  To analyze the distribution of cerium oxide on TiO 22 a Raman mapping analysis was performed.  This technique allows non‐destructive and non‐invasive analysis of features such as the separation  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 222 and CeO 222 nanostructures  To analyze the distribution of cerium oxide on TiO This technique allows non‐destructive and non‐invasive analysis of features such as the separation  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  2 a Raman map of chemical species in multi‐component samples. Chemical maps of TiO  and CeO  nanostructures  ution of cerium oxide on TiO 2  a Raman mapping analysis was performed.  ution of cerium oxide on TiO 2  a Raman mapping analysis was performed.  2 ‐10%CeO 2  sample, based on the detailed Raman image (over a 15 μm × 15 μm  performed on the TiO This technique allows non‐destructive and non‐invasive analysis of fe of chemical species in multi‐component samples. Chemical maps of TiO  and CeO22 nanostructures   nanostructures  of chemical species in multi‐component samples. Chemical maps of TiO 22 and CeO 2‐10%CeO 2 sample, based on the detailed Raman image (over a 15 μm × 15 μm  performed on the TiO performed on the TiO 22‐10%CeO 22 sample, based on the detailed Raman image (over a 15 μm × 15 μm  ‐destructive and non‐invasive analysis of features such as the separation  ‐destructive and non‐invasive analysis of features such as the separation  image scan, with 150 points per line and 150 lines per image), are presented in Figure 4.       of chemical species in multi‐component samples. Chemical maps of T ‐10%CeO22 sample, based on the detailed Raman image (over a 15 μm × 15 μm   sample, based on the detailed Raman image (over a 15 μm × 15 μm  performed on the TiO22‐10%CeO performed on the TiO image scan, with 150 points per line and 150 lines per image), are presented in Figure 4.  image scan, with 150 points per line and 150 lines per image), are presented in Figure 4.  ti‐component samples. Chemical maps of TiO  and CeO22 nanostructures   nanostructures  ti‐component samples. Chemical maps of TiO 22 and CeO 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 presented in Figure 4.  image scan, with 150 points per line and 150 lines per image), are presented in Figure 4.     %CeO22 sample, based on the detailed Raman image (over a 15 μm × 15 μm   sample, based on the detailed Raman image (over a 15 μm × 15 μm  %CeO image scan, with 150 points per line and 150 lines per image), are prese s per line and 150 lines per image), are presented in Figure 4.     s per line and 150 lines per image), are presented in Figure 4. 

  Figure 4. Confocal Raman mapping image of TiO ‐10%CeO22 sample.  Figure 4. Confocal Raman mapping image of TiO22-10%CeO sample.

     Figure 4. Confocal Raman mapping image of TiO 222‐10%CeO 222 sample.  Figure 4. Confocal Raman mapping image of TiO 2‐10%CeO 2 sample.  Figure 4. Confocal Raman mapping image of TiO ‐10%CeO  sample.  Figure 4. Confocal Raman mapping image of TiO ‐10%CeO  sample.  Figure 4. Confocal Raman mapping image of TiO22‐10%CeO ‐10%CeO22 sample.   sample.  Figure 4. Confocal Raman mapping image of TiO

  

Catalysts 2016, 6, 121 Catalysts 2016, 6, 121  Catalysts 2016, 6, 121  Catalysts 2016, 6, 121 

Representative spectra of two different regions are reported in red and violet color scales. The  Representative spectra of two different regions are reported in red and violet color scales. The  Representative spectra of two different regions are reported in red and violet color scales. The Representative spectra of two different regions are reported in red and violet color scales. The  spectra  show  identical  features  but  with  very different  different  intensities.  The  CeO   characteristic  spectra  show  almost  identical  features  but  with  very  intensities.  The  2  2characteristic  spectra show almost identical features but very different intensities. The characteristic spectra  show  almost almost  identical  features  but with with  very  different  intensities.  The CeO CeO characteristic  22 CeO ´ 1 −1 −1 −1 band (466 cm ) of higher intensity was recorded in the red region, while in the violet region TiO ) of higher intensity was recorded in the red region, while in the violet region TiO 2  2  bandband (466 cm (466 cm ) of higher intensity was recorded in the red region, while in the violet region TiO ) of higher intensity was recorded in the red region, while in the violet region TiO22  band (466 cm ´1 , 524 1 and 1 ) are −1 and 647 cm −1) are very visible with the ceria band of decreased  −1−1 −1−1 −1 and 647 cm −1) are very visible with the ceria band of decreased  −1 −1´ −1´ peaks (150 cm , 403 cm , 524 cm , 403 cm , 524 cm peakspeaks (150 cm (150 cm´−11, 403 cm , 403 cm cm 647 cm very visible with the ceria band of decreased peaks (150 cm , 524 cm  and 647 cm ) 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  intensity (according to the 10%), pointing out that the ceria was not homogenously dispersed on the  intensity (according to the 10%), pointing out that the ceria was not homogenously dispersed on the intensity (according to the 10%), pointing out that the ceria was not homogenously dispersed on the  TiO 2 bulk.  TiO 2 bulk.  bulk. TiO2 bulk.  The band‐gap  band‐gap  energy  values  (Table  1), estimated  estimated  by reporting  reporting  the modified  modified  Kubelka‐Munk  The  energy  values  1),  by  the  Kubelka‐Munk  The band‐gap  band-gap energy values (Table(Table  1), by the Kubelka-Munk The  energy  values  (Table  1),  estimated estimated  by  reporting reporting  the  modified modified  Kubelka‐Munk  1 1/2 1/2 ′ 1/2 ′ 1/2 ′ function, [F(R ∞ against the exciting light energy [57], showed that the TiO )hν]  against the exciting light energy [57], showed that the TiO 2‐10%CeO 2 sample  function, [F(R ∞ )hν]  against the exciting light energy [57], showed that the TiO 2‐10%CeO 2 sample  function, [F(R8 )hν] against the exciting light energy [57], showed that the TiO22‐10%CeO -10%CeO sample function, [F(R ∞ )hν] 22 sample  had a lower E geV)  (2.93 eV) compared to the bare TiO 2eV).  (2.98 eV). This can be related to the replacement  g (2.93 eV) compared to the bare TiO had ahad a lower E lower Egg (2.93 eV) compared to the bare TiO (2.93 compared to the bare TiO22 (2.98 eV). This can be related to the replacement  (2.982 (2.98 eV). This can be related to the replacement  This can be related to the replacement had a lower E 4+ 3+ cations 4+ cations by Ce 4+ 4+ cations by Ce 3+3+ 4+4+ 3+ of Ti  or Ce  cations in the TiO 2 network [51,54,58]. Moreover, looking at the DRS  of Ti  cations in the TiO 2 network [51,54,58]. Moreover, looking at the DRS  of Ti4+ cations by Ce or4+ or Ce Ce in the TiO [51,54,58]. Moreover, looking at the of Ti  cations by Ce  or Ce  cations in the TiO 2 network [51,54,58]. Moreover, looking at the DRS  2 network spectra of the investigated samples in the visible region (Figure 5), it can be seen that all gold‐loaded  DRS spectra of the investigated samples in the visible region (Figure 5), it can be seen that all gold‐loaded  spectra of the investigated samples in the visible region (Figure 5), it can be seen that all 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  samples (Au/TiO 2, Au/CeO 2 and Au/TiO ‐10%CeO 2) exhibit a clear absorbance band at around 550  gold-loaded samples (Au/TiO and 2Au/TiO samples (Au/TiO 2, Au/CeO 2 and Au/TiO 2) exhibit a clear absorbance band at around 550  2 , Au/CeO 22‐10%CeO 2 -10%CeO 2 ) exhibit a clear absorbance band at nm, attributed to the plasmon resonance of gold nanoparticles [59]. As reported in the literature, the  nm, attributed to the plasmon resonance of gold nanoparticles [59]. As reported in the literature, the  around 550 nm, attributed to the plasmon resonance of gold nanoparticles [59]. As reported in 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  photo‐excited electrons of the gold surface plasmon can be injected to the TiO 2 conduction band, thus  the literature, the photo-excited electrons of the gold surface plasmon can 2be injected to the TiO2 photo‐excited electrons of the gold surface plasmon can be injected to the TiO  conduction band, thus  creating  separated  electrons  and holes  holes  and  then  increasing  their lifetime  lifetime  by  hindering  the  creating  separated  electrons  and  then  increasing  by  hindering  conduction band, thus creating separated electrons and holes andtheir  thentheir  increasing lifetime by the  creating  separated  electrons  and  holes  and  and  then  increasing  lifetime  by their hindering  the  recombination process [29,32−35]. It must be underlined that no significant variation in the surface  recombination process [29,32−35]. It must be underlined that no significant variation in the surface  hindering the recombination process [29,32–35]. It must be underlined that no significant variation 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  in thearea (Brunauer‐Emmett‐Teller (BET) analysis) of investigated samples was detected in the presence  surface area (Brunauer-Emmett-Teller (BET) analysis) of investigated samples was detected in area (Brunauer‐Emmett‐Teller (BET) analysis) of investigated samples was detected in the presence  −1, lower  2g2−1 2g−1, lower  of gold and/or CeO 2. It must also be noted that the bare TiO 2 showed a surface area of 44.8 m g, lower  of gold and/or CeO 2. It must also be noted that the bare TiO 2 showed a surface area of 44.8 m the presence of gold and/or CeO2 . It must also be noted that the bare TiO2 showed a surface area 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 ´ 1 2 ´ 1 2 −1 2 −1 2 −1 than  the  values  found  in  the literature  literature  for  P25  TiO  for (50−54  m ), reasonably  reasonably  due  the thermal  thermal  than  found  in  the  for  P25  2  2(50−54  m g g2), (50´54 due  to to  the  of 44.8 m values  gthe  , values  lower than values found the literature TiO m g to  ),the  reasonably than  the  found  in  the the  literature  for in P25  TiO 2  TiO (50−54  mP25 g ),  reasonably  due  thermal  pretreatment of TiO 2 (calcination at 350 °C) [60].  pretreatment of TiO 2 (calcination at 350 °C) [60].  due to the thermal pretreatment of TiO2 (calcination at 350 ˝ C) [60]. pretreatment of TiO 2 (calcination at 350 °C) [60]. 

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Figure 5. Diffuse Reflectance Spectroscopy (DRS) spectra (visible region) of CeO 2 (─), TiO 2 (─), TiO Figure 3. (a) Vibrational modes of Raman spectra of TiO 2 (─), Au/TiO 2‐ 2‐ Figure 5. Diffuse Reflectance Spectroscopy (DRS) spectra (visible region) of CeO 2 (─), TiO 2‐ FigureFigure 5. Diffuse Reflectance Spectroscopy (DRS) spectra (visible region) of CeO 5. Diffuse Reflectance Spectroscopy (DRS) spectra (visible region) of2 (─), TiO CeO22 (─), TiO TiO22 (─), TiO (—), 2  (─), Au/TiO 2  (─), Au/CeO 2  (─) and Au/TiO 2 ‐10%CeO 2  (─) samples.  10%CeO f TiO 2 (─), Au/TiO2 (─), TiO 2 ‐10%CeO 2  (─), and  2 ‐10%CeO 2  (─) samples; (b) Raman shift of the signal of cubic C Au/TiO ational modes of Raman spectra of TiO 2  (─), Au/TiO 2  (─), TiO 2 ‐10%CeO 2  (─), and  (─), Au/TiO 2  (─), Au/CeO 2  (─) and Au/TiO 2 ‐10%CeO 2  (─) samples.  10%CeO 10%CeO 2  (─), Au/TiO 2  (─), Au/CeO 2  (─) and Au/TiO 2 ‐10%CeO 2  (─) samples.  TiO2 -10%CeO2 , Au/TiO2 , Au/CeO2 and Au/TiO2 -10%CeO2 samples. 2 in TiO2‐10%CeO2(─), and  Ohe signal of cubic CeO 2 (─) samples; (b) Raman shift of the signal of cubic CeO 2 in TiO2‐10%CeO2(─), and  Au/TiO2‐10%CeO2 (─) samples; (c) Raman shift of the main Eg vibrati The surface EDX analysis of investigated samples is reported in Figure 6. It is possible to note  The surface EDX analysis of investigated samples is reported in Figure 6. It is possible to note  g vibrational mode of TiO 2 anatase in  O the main E 2 (─) samples; (c) Raman shift of the main E g vibrational mode of TiO2 anatase in  TiO2 (─), Au/TiO2 (─), TiO2‐10%CeO2 (─), and Au/TiO2‐10%CeO2 (─) The surface EDX analysis of investigated samples is reported in Figure 6. It is possible to note  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)  that the signals related to cerium (at 4.8 and 5.3 KeV, the first one overlapped with the signal of Ti)  2‐10%CeO 2 (─) samples.  O/TiO 2 (─), TiO 2‐10%CeO 2 (─), and Au/TiO 2‐10%CeO2 (─) samples.  that the signals related to cerium (at 4.8 and 5.3 KeV, the first one overlapped with the signal of Ti) 

that the signals related to cerium (at 4.8 and 5.3 KeV, the first one overlapped with the signal of Ti) Interestingly, over the Au/TiO2‐10%CeO2 sample, the peak asso were more intense for the Au/TiO 2‐10%CeO 2 system. Moreover, the elemental composition of these  were more intense for the Au/TiO 2‐10%CeO 2 system. Moreover, the elemental composition of these  were more intense for the Au/TiO were more intense for the Au/TiO22‐10%CeO -10%CeO22 system. Moreover, the elemental composition of these  system. Moreover, the elemental composition of these ple, the peak associated with cubic CeO 2  was less  ver the Au/TiO2‐10%CeO 2  sample, the peak associated with cubic CeO 2  was less  intense, broader and shifted to lower frequencies compared to over th catalysts  showed  that  on  Au/TiO 2 ‐10%CeO 2 ,  the  Ce atomic  atomic  percentage  about  four times  times greater  greater  catalysts  showed  that  on  Au/TiO 2 ‐10%CeO 2 ,  the  Ce  percentage  is is about  four  catalysts  showed  that  on  Au/TiO2‐10%CeO2,  the  Ce  atomic  percentage  is  about  four  times  greater  catalysts showed that on Au/TiO2 -10%CeO2 , the Ce atomic percentage is about four times greater mpared to over the TiO 2 ‐10%CeO 2  sample (Figure   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 than that found on TiO 2‐10%CeO 2 (5.1% and 1.2%, respectively), indicating that the presence of gold  than that found on TiO 2‐10%CeO 2 (5.1% and 1.2%, respectively), indicating that the presence of gold  than that found on TiO 2‐10%CeO 2 (5.1% and 1.2%, respectively), indicating that the presence of gold  than that found on TiO2 -10%CeO2 (5.1% and 1.2%, respectively), indicating that the presence of gold d be due to a less crystalline and more defective  n 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 on TiO 2 ‐CeO 2  oxide leads to a remarkable cerium surface enrichment.  on TiO 2 ‐CeO 2  oxide leads to a remarkable cerium surface enrichment.  on TiO2‐CeO2 oxide leads to a remarkable cerium surface enrichment.  on TiO2 -CeO2 oxide leads to a remarkable cerium surface enrichment. has  been  reported  to  be  sensitive  the reported  to  be  sensitive   Raman  the  presence  of  gold.  In  fact,  Raman  has to  been  degree to  of the  crystallinity  of  samples,  with  broader,  less  intense  Ram   less  Raman  in  less  the  case  of  Raman  less  ity  of intense  samples,  with  peaks  broader,  intense  peaks  in  the crystalline material [56].  case  of  less  [56].  Figure 3c shows the position of the main vibrational mode of an tional mode of anatase Eg, pointing out that there  s 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 TiO2‐10%CeO2 and  −1) on the TiO O2‐10%CeO2 and Au/TiO 2‐10%CeO 2 samples. By  d shift (about 2 cm 2‐10%CeO 2 and Au/TiO2‐10%CeO2considering that the E  samples. By  g anatase mode at 148 cm−1 is associated with th −1 associated with the O‐Ti‐O vibration, the presence  Eg anatase mode at 148 cm  is associated with the O‐Ti‐O vibration, the presence  of CeO2 in the TiO2 lattice could probably cause a bond distortion re bond distortion resulting in the observed shift of  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 ma  TiO2 a Raman mapping analysis was performed.  distribution of cerium oxide on TiO 2 a Raman mapping analysis was performed.  This technique allows non‐destructive and non‐invasive analysis of vasive analysis of features such as the separation  ws non‐destructive and non‐invasive analysis of features such as the separation  of chemical species in multi‐component samples. Chemical maps of

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Figure 6. EDX (Energy‐dispersive X‐ray spectroscopy) spectra of (a) Au/TiO Figure 6. EDX (Energy-dispersive X-ray spectroscopy) spectra of (a) Au/TiO22, (b) TiO , (b) TiO22‐10%CeO -10%CeO22 and  and 2‐10%CeO2 samples.  (c) Au/TiO (c) Au/TiO -10%CeO samples. 2

2

The different role of gold and CeO2 in affecting the properties and the catalytic behavior of TiO2  The different role of gold and CeO2 in affecting the properties and the catalytic behavior of TiO2 in in the photo‐oxidation and the photoreduction reactions investigated here could be rationalized by  the photo-oxidation and the photoreduction reactions investigated here could be rationalized by taking taking into account the surface active species involved in these reactions. As for the photocatalytic  into account the surface active species involved in these reactions. As for the photocatalytic water/air water/air oxidative purification, the photocatalytic hydrogen production essentially requires photo‐ oxidative purification, the photocatalytic hydrogen production essentially requires photo-generation of generation  of  hole/electron  pairs.  Nevertheless,  the  role  of  holes/electrons,  as  well  as  the  surface  hole/electron pairs. Nevertheless, the role of holes/electrons, as well as the surface reactions involved, reactions involved, are different. In fact, in the photocatalytic oxidation, valence band (VB) holes are  are different. In fact, in the photocatalytic oxidation, valence band (VB) holes are the key elements the  key  elements  involved  in  the  removal  of  contaminants,  whereas  in  H2  production  via  involved in the removal of contaminants, whereas in H2 production via photocatalytic water splitting photocatalytic water splitting reducing Conduction Band (CB) electrons becomes crucial as their role  reducing Conduction Band (CB) electrons becomes crucial as their role is mainly that of reducing is mainly that of reducing protons to hydrogen molecules.  protons to hydrogen molecules. The addition of gold to TiO2 results in an enhancement of the photocatalytic activity towards 2‐ The addition of gold to TiO2 results in an enhancement of the photocatalytic activity towards propanol oxidation, due to an increase in the charge separation between the excited electron and the  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  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  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  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  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  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  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  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  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  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 CO 2 [42,62], resulting in the highest CO2  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.  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.

  Figure  7.  Scheme  of  the  electron  transfer  phenomena  taking  place  in  the  Au/TiO2‐CeO2  system  by  irradiation with UV light. 

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  Catalysts 2016, 6, 121 9 of 13 yield over the Au/TiO2‐10%CeO2 system. 

  Figure 7.  7. Scheme  Scheme of  of the -CeO22  system  system by  by Figure  the  electron electron transfer transfer phenomena phenomena taking taking place place in in the the Au/TiO Au/TiO22‐CeO irradiation with UV light. irradiation with UV light. 

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 ¨6H2 O (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 ¨6H2 O 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 100ˆ 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.

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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 cm3 min´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 ˆ 0.32 µm ˆ 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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