catalysts Article
Au/TiO2-CeO2 Catalysts for Photocatalytic Water Splitting and VOCs Oxidation Reactions Roberto Fiorenza 1 , Marianna Bellardita 2 , Luisa D’Urso 1 , Giuseppe Compagnini 1 , Leonardo Palmisano 2 and Salvatore Scirè 1, * 1 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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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.
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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.
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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
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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
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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
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(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
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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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