Spontaneous Redox Approach to the Self-Assembly Synthesis of Au

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Energy, Environmental, and Catalysis Applications

A Spontaneous Redox Approach to Self-Assembly Synthesis of Au/CeO2 Plasmonic Photocatalysts with Rich Oxygen Vacancies for Selective photocatalytic Conversion of Alcohols Zhiqing Cui, Weikang Wang, Cuijiao Zhao, Chun Chen, Miaomiao Han, Guozhong Wang, Yunxia Zhang, Haimin Zhang, and Huijun Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10705 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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ACS Applied Materials & Interfaces

A Spontaneous Redox Approach to Self-Assembly Synthesis of Au/CeO2 Plasmonic Photocatalysts with Rich Oxygen Vacancies for Selective photocatalytic Conversion of Alcohols Zhiqing Cui,†,‡ Weikang Wang,†,‡ Cuijiao Zhao,†,‡ Chun Chen,† Miaomiao Han,† Guozhong Wang,† Yunxia Zhang,† Haimin Zhang,†,* and Huijun Zhao†,ξ †

Key Laboratory of Materials Physics, Centre for Environmental and Energy

Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China ‡

University of Science and Technology of China, Hefei 230026, China

ξ

Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus,

QLD 4222, Australia * Corresponding author at: Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China E-mail address: [email protected]

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ABSTRACT: We present self-assemble synthesis of core-shell structure Au/CeO2 composites with different Au loadings through a spontaneous chemical redox approach at ambient temperature utilizing HAuCl4 and Ce(NO3)3 as reaction substrates in alkaline environment. The results demonstrate that the as-synthesized Au/CeO2 composites exhibit spherical-shape morphologies with porous structure, composed of Au nanoparticles (~10 nm) core and CeO2 nanoparticles shell with abundant oxygen vacancies. The introduction of Au nanoparticles in CeO2 not only effectively improves the visible light utilization efficiency, but also provides rich surface catalytic active sites for highly efficient visible light photocatalysis. As visible light photocatalysts (λ>400 nm), the as-synthesized Au/CeO2 composites with Au loading amount ≥ 4.0 wt.% exhibit high conversion and selectivity (~100%) of benzyl alcohol to benzaldehyde under the given experimental conditions. Moreover, the Au/CeO2 also shows a general applicability as visible light photocatalyst for selective oxidation of other alcohols to corresponding aldehydes or ketones. The photocatalytic mechanism studies indicate that the photoelectrons/holes produced from the photoexcited Au and the formed superoxide radicals in the oxygen vacancies of CeO2 synergistically contribute high performance of selective photocatalytic oxidation of alcohols to aldehydes or ketones. KEYWORDS: spontaneous redox approach, self-assembly synthesis, Au/CeO2, selective oxidation of alcohols, visible light activity


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1. INTRODUCTION It is well known that the production of aldehydes or ketones from selective oxidation of aromatic alcohols is critically important, owing to their wide applications as intermediates in organic synthesis process.1-6 Traditionally, some strong oxidizing agents, such as CrO3, KMnO4 and MnO2 etc., are chosen to selectively oxidize aromatic alcohols to corresponding aldehydes or ketones7,8 However, their excess utilization inevitably gives rise to some environmental issues and enhanced production costs. Therefore, it is highly needed to explore more environmentally friendly methods enabling selective oxidation of aromatic alcohols to corresponding aldehydes or ketones without the overoxidized products such as carboxylic acids and CO2. In recent years, photocatalytic synthesis of high value-added chemicals has attracted great research interests in the area of organic synthesis, owing to its several advantages, such as the utilization of renewable solar energy, high product selectivity, easy modification of photocatalyst, and mild reaction conditions.9-12 Even so, an overarching concern in this field is to develop high-efficiency photocatalysts with visible light activity and high light utilization efficiency.13 Among various synthetic strategies of visible light photocatalysts, the integration of plasmonic Au or Ag nanoparticles with wide-bandgap semiconducting photocatalysts (e.g., TiO2, CeO2) has been the most efficient means to boost photocatalytic performance of visible light photocatalysts.14-18 The introduction of plasmonic noble metals can provide several advantages:16,17 (i) unique surface plasmon resonance caused from Au or Ag nanoparticles contributes the visible light response of photocatalyst; (ii) noble metal nanoparticles possess superior surface catalytic activity, synergistically enhancing the photocatalytic performance; (iii) the coupling of noble metal nanoparticles and semiconducting photocatalyst is favourable for the separation of photoelectrons and



holes, thus effectively improving the photocatalytic efficiency. These benefits make plasmonic noble metal modified semiconducting photocatalysts promising for selective oxidation of alcohols to corresponding aldehydes or ketones under visible light irradiation. ceria (CeO2) with a bandgap of ~3.2 eV has been widely investigated as photocatalyst for various applications, due to its superior properties, for instance, easy Ce3+/Ce4+ redox cycle, abundant oxygen vacancies, and high chemical/photochemical stability.19,20 To afford visible light catalytic activity of CeO2, several approaches such as doping and composing plasmonic noble metals or narrow-bandgap semiconductors have been intensively explored, and some achievements have been made in recently reported works.20-22 In particular, gold (Au) nanoparticles modified on various structured CeO2 by photochemical deposition, chemical deposition, electrospun, and aerosol-spray etc. have demonstrated great potential as visible light photocatalysts for organic decomposition and synthesis.23-26 In these reported studies, it is found that the sizes of Au nanoparticles modified on CeO2 have important influence on their visible light utilization efficiency and photocatalytic performance.24-26 Moreover, the presence of oxygen vacancies in CeO2 has been proven to be synergistically effective on high-efficient and selective oxidation of organics.23-25 On the basis of the reported studies, the development of effective methods to fabricate Au nanoparticles modified CeO2 with controllable Au nanoparticle sizes and abundant oxygen vacancies is still highly desired for enhancing visible light utilization and photocatalytic conversion efficiencies. In the previous works reported by our and other groups,27-30 a spontaneous chemical redox approach utilizing the reaction between noble metal ion and Ce3+ has been employed to fabricate noble metal nanocrystals modified CeO2 nanoparticles composites (e.g., Pd/CeO2), as catalysts exhibiting superior catalytic


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activities over thermally catalytic conversion of biomass platform molecules.27,31,32 The prepared noble metal/CeO2 composites by such spontaneous chemical redox approach could possess following advantages: (i) integrating plasmonic effective noble metal nanocrystals (e.g., Au) into CeO2 nanoparticles, combined with superior surface catalytic activity of noble metal nanocrystals, for visible light driven photocatalysis; (ii) owing to the limited redox capability between noble metal ions and Ce3+ under room temperature conditions, the formed noble metal nanocrystals (e.g., Au) on CeO2 nanoparticles having small particle sizes (~10 nm), beneficial for the exposure of more catalytic active sites; (iii) small-sized noble metal nanoparticles (e.g., Au) anchored on CeO2 nanoparticles constructed frameworks, difficult of aggregating to form large-sized particles during photocatalysis, thus maintaining highly catalytic active surface and good applicable stability; (iv) the formed CeO2 nanoparticles with porous structure and rich oxygen vacancies, to facilitate mass transport and synergetic catalysis with plasmonic noble metals, thus enhancing visible light photocatalytic performance. These advantages could make noble metal (e.g., Au) modified CeO2 composites fabricated by this spontaneous redox approach promising as visible light photocatalysts for selectively photocatalytic oxidation of alcohols. However, related studies are seldom in literatures. Herein, we employed a spontaneous chemical redox approach at ambient temperature to self-assembly synthesize core-shell structure Au/CeO2 composites with different Au loadings using HAuCl4 and Ce(NO3)3 as reactants in alkaline environment. The results demonstrated that the as-synthesized Au/CeO2 composites with different Au loadings exhibit similar spherical-shape structures (~80 nm in diameter) and porous characteristics, composed of small-sized Au nanoparticles (~10 nm) core and CeO2 nanoparticles shell with abundant oxygen vacancies. Owing to the



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introduction of Au nanoparticles in CeO2, the resultant composites with Au loading amount ≥ 4.0 wt.% as photocatalysts displayed superior photocatalytic performances of alcohols oxidation to aldehydes or ketones (conversion and selectivity all approximate to ~100%) after 4 h of visible light photocatalytic reaction (λ > 400 nm). Furthermore, the selective photocatalytic oxidation mechanism has been discussed and proposed according to the experimental and characterization results. 2. EXPERIMENTAL SECTION Chemicals. All reagents used in the experiments are analytic grade. Cerium nitrate

hexahydrate

(Ce(NO3)3·6H2O),

chloroauric

acid

(HAuCl4·4H2O),

benzotrifluoride (BTF, C7H5F3), 1-phenylethyl alcohol (C8H10O), benzyl alcohol (C7H8O), 2-pentanol (C5H12O), phenylpropanol (C9H12O), cinnamic alcohol (C9H10O), 4-methoxybenzyl alcohol (C8H10O2), and 5,5-dimethyl-l-pyrroline-N-oxide (DMPO, C6H11NO) were purchased from Aladdin Industrial Corporation. Sodium hydroxide (NaOH), benzoquinone (BQ), ammonium oxalate (AO), and tert-butyl alcohol (TBA) were commercially obtained from Shanghai Sinopharm Chemical Reagent Co., Ltd. All solutions were prepared by using MilliQ water. Preparation of Au/CeO2-X Composites. In a typical synthetic process, deionized water (300 mL) was first bubbled by nitrogen (N2) for 30 min under stirring to adequately remove O2 at room temperature, then 2.17 g (5.0 mmoL) of Ce(NO3)3·6H2O was completely dissolved into the above deionized water. Subsequently, 5.0 mL of NaOH (2.0 M) aqueous solution was rapidly dropped to the above solution, followed by quick addition of 1.0 mL HAuCl4 aqueous solution with different concentrations of 0.02, 0.05, 0.075, 0.1, 0.125, 0.15, and 0.175 M. After stirring for 1 h at ambient conditions, the formed Au nanocrystals modified CeO2 product was centrifugally collected from the solution and washed adequately with


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deionized water for several times. Finally, the as-synthesized product was dried at 60 °C in vacuum for 12 h for further use. In this work, the obtained Au nanocrystals modified CeO2 sample with different Au loading amounts was denoted as Au/CeO2-X, in where X presents the weight percent (wt.%) of Au loading calculated from the inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements. For comparison, CeO2 nanoparticles were also prepared as the same procedure as Au/CeO2-X samples without the addition of HAuCl4 and utilizing N2 to remove O2 in reaction solution during synthesis. Characterizations. Powder X-ray difraction (XRD) patterns of the samples were collected on a Philips X-Pert Pro X-ray diffractometer with Cu-Kα radiation (Kα=0.15418 nm). The morphologies and structures of the samples were characterized by scanning electron microscope (SEM, FEI Sirion 200) and transmission electron microscopy (TEM, JEOL JEM-2010 and Tecnai TF20 TMP). High-angle annular dark-field scanning transmission electron microscopy-scanning transmission electron microscopy (HAADF-STEM) images and elemental mapping images of the Au-CeO2-X samples were recorded by a high-resolution TEM (Tecnai TF20 TMP). X-ray photoelectron spectroscopy (XPS) measurements of the samples were conducted on an ESCA LAB250 X-ray photoelectron spectrometer (Thermo, America) equipped with A1 Kα as the X-ray source. The UV-vis diffuse reflectance spectroscopy

measurements

were

performed

by

an

ultraviolet

visible

spectrophotometer (UV-2700 Shimadzu) with BaSO4 as the internal reflectance standard. Au loading amount in Au/CeO2-X composites was determined by the ICPOES 6300, (Thermo Fisher Scientific) after microwave digestion of the samples. Electron spin-resonance (ESR) spectroscopy of the radicals spin-trapped by DMPO was recorded on a JEOL JES-FA200 ESR spectrometer (300 K, 9.063 GHz, X-band).



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Specific surface area and porosity of the samples were determined from nitrogen adsorption data at 77 K using the Barrett-Emmett-Teller (BET) technique on a Tristar3020M. Photocatalytic Oxidation Measurements. The photocatalytic performances of Au-CeO2-X composites were evaluated by photocatalytic conversion of a series of alcohols (e.g., 2-pentanol, 1-phenylethyl alcohol, benzyl alcohol, 2-phenylethyl alcohol, cinnamic alcohol, and 4-methoxybenzyl alcohol) under λ > 400 nm visible light. The photocatalytic measurements were conducted in a transparent quartz tube, and the catalyst suspension was prepared by dispersing 20 mg Au-CeO2-X photocatalyst into 4.0 mL of BTF solution under ultrasonication for 20 min. The above mixture after the addition of 40 µmoL alcohol was first kept in the dark environment for 30 min under stirring to ensure sufficient adsorption of reactant molecules on photocatalyst. O2 was continuously bubbled into the above solution in the reaction process under normal pressure to provide an oxygen atmosphere. After that, a Xe lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd.) with a 400 nm long-pass filter (CEL-UVIRCUT400) was turned on as the light source. All photocatalytic tests were carried out at 30 °C with a visible light power of 0.7 W cm-2. After the photocatalytic reaction with a desired time period, the reaction solution was centrifugated to separate the solid catalyst and then the photocatalytic products in reaction solution were analyzed by gas chromatography (GC-7920, Beijing China Education Au-light Co., Ltd.) with a flame ionization detector. The conversion and selectivity of photocatalytic conversion of alcohols were calculated according to the following equations: (1)


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(2) where C0 is the initial concentration of alcohol, C1 and C2 are the concentrations of the substrate alcohol and the corresponding oxidation product respectively, after the photocatalytic oxidation reaction with a desired time period. 3. RESULTS AND DISCUSSION Structure and Composition of Au/CeO2-X. In this work, Au/CeO2-X composites with different Au loadings were synthesized through a spontaneous chemical redox approach in the presence of Ce(NO3)3 and HAuCl4 at ambient temperature. The ICP-OES measurements display that with different HAuCl4 amounts during synthesis, the obtained Au/CeO2-X composites can be indicated as Au/CeO20.7wt.%,

Au/CeO2-1.6wt.%,

4.0wt.%,

Au/CeO2-4.8wt.% and Au/CeO2-4.9wt.%,

Au/CeO2-2.5wt.%,

Au/CeO2-3.3wt.%,

Au/CeO2-

respectively (Table S1,

Supporting Information). Interestingly, it is found that when Ce(NO3)3 amount in reaction precursor is fixed at 5.0 mmoL, Au loading amount in composite is first increased with increasing HAuCl4 amount in reaction precursor, and then reaches a stable status with further increasing HAuCl4 (Figure S1, Supporting Information). This is due to the limited reduction capability provided by Ce3+ with a constant amount in reaction precursor under the ambient temperature conditions. Figure 1a shows the XRD patterns of Au/CeO2-X composites. As shown, all composites exhibit similar XRD patterns composed of CeO2 with cubic fluorite structure (JCPDS, No. 78-0694) and cubic phase metallic Au (JCPDS, No. 99-0056). Moreover, an increase in XRD diffraction peak intensity of metallic Au can be observed for Au/CeO2-X composites with increasing Au loading amount. For comparison, pure CeO2 without Au loading was also characterized by XRD technique, only exhibiting cubic fluorite structure CeO2 (JCPDS, No. 78-0694) (Figure S2a, Supporting Information). The



formation of CeO2 with cubic fluorite structure in the absence of HAuCl4 at ambient temperature can be due to the transformation of Ce3+ to Ce4+ in O2-containing alkaline environment, indicating certain reducing capability of Ce3+.28 The results mentioned above confirm that the existence of Au in CeO2 does not change its crystalline structure. Figure 1b shows the SEM image of the Au/CeO2-4.0wt.%, exhibiting spherical-shape structures with an average size of ~80 nm. In the presence of HAuCl4 with different concentrations, the synthesized other Au/CeO2-X samples also display similar spherical-shape structures, as shown in Figuer S3 (Supporting Information). In a strong contrast, CeO2 without Au loading is composed of ultrafine nanoparticles with a lattice spacing of 0.31 nm corresponding to the CeO2 (111) plane (Figure S2b and inset, Supporting Information). The above results demonstrate that the addition of HAuCl4 in reaction precursor obvisouly changes resultant CeO2 morphology by means of this spontaneous chemical redox approach, and the formed Au/CeO2 composites possess larger particle sizes compared to the prepared CeO2 without Au loading, possibly favourable for improving the light utilization efficiency of Au/CeO2 composite photocatalysts resulted from their higher light scattering capability.33 The inset in Figure 1b presents the TEM image (low-magnification) of Au/CeO2-4.0wt.%. Interestingly, it is found that the formed Au/CeO2 composite is analogous to a coreshell structure, which has been further confirmed by HAADF- STEM characterization. As shown in Figure 1c, specific core-shell structures can be observed for Au/CeO24.0wt.%. Moreover, the high-resolution TEM (HRTEM) image exhibits that the shell layer is consisted of cubic fluorite structure CeO2 nanoparticles with a lattice spacing of 0.31 nm. This means that the core structure in Au/CeO2-4.0wt.% should be from the formed Au nanoparticles. The above results can be further validated by the elemental mapping information. Figure 1d shows the HAADF-STEM image of


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Au/CeO2-4.0wt.%, and the elemental mapping images indicate the co-existence of Ce, O and Au in sample. Moreover, Ce and O are uniformly distributed over the Au/CeO2 particles, while Au element is mainly concentrated on the core structures of Au/CeO2 particles. These results further reveal that the formed Au/CeO2-4.0wt.% is a core-shell structure consisted of Au nanoparticles core and CeO2 nanoparticles shell. Similar results can be also achieved for other Au/CeO2-X samples, as shown in Figure S4 (Supporting Information). For all cases, it can be seen that the particle sizes of Au core are in the range of ~10 nm, which is significantly differed from the synthesized Au nanoparticles with larger sizes (few tens of nanometers) on CeO2 nanostructures at high Au-containing precursor concentration by other reported methods.14 This can be ascribed to the unique advantage of this spontaneous redox approach.28 Moreover, when the Au loading content in Au/CeO2 composites is below 4.0 wt.%, single Au nanoparticle core formed in composite is dominant (Figure S4a-d, Supporting Information), while more Au nanoparticles can be observed with further increasing Au loading content to 4.8 wt.% and 4.9 wt.% (Figure S4e, f, Supporting Information). The obtained results demonstrate that core-shell structure Au/CeO2 composites can be readily synthesized by a spontaneous redox self-assemble approach used in this work. Based on the obtained results, the production of core-shell structure Au/CeO2 composites synthesized by this spontaneous redox self-assemble approach could mainly defer to the following two steps:28 (i) in an alkaline reaction environment, Ce3+ ions first react with OH− to form Ce(OH)3; (ii) subsequently, AuCl4- ions adsorbed on Ce(OH)3 further react with Ce(OH)3 through spontaneous redox to form metallic Au0 and CeO2. Moreover, the formed Au nanoparticles are surrounded by CeO2 nanoparticles to construct a core-shell structure, and the redox capability of Au3+ and



Ce3+ determines the formed Au nanoparticles with smaller sizes under the given experimental conditions. To further obtain the information on compositions and valence states of Au/CeO2 composites, XPS analysis was conducted. Compared to pure CeO2 sample, the surface survey XPS spectrum of the Au/CeO2-4.0wt.% shows the presence of Ce, Au and O elements (Figure 2a). The composition information on all samples characterized by XPS technique has been summarized in Table S2 (Supporting Information). Obviously, the total Ce content is slightly increased with Au loading, possibly due to the oxidation contribution of Au3+ introduction. The Au content on sample surface is first increased from 0.09 at.% to 0.41at.% with Au loading increase from 0.7 wt.% to 4.0 wt.%, and then almost reaches a stable status when further increasing Au loading to 4.8 wt.% and 4.9 wt.%. Moreover, the change in Au content on sample surface measured by XPS is almost proportional to the variation of Au loading on CeO2 determined by ICP-OES. The high-resolution Ce 3d XPS spectra (Figure 2b) of Au/CeO2-4.0wt.% and pure CeO2 can be divided into eight peaks denoted as V and U, ascribed to the spin-orbit coupling of Ce 3d5/2 and Ce 3d3/2 states, respectively.23,34 Figure 2b displays two peaks at 885.3 eV (VI) and 903.7 eV (UI) , which can be due to Ce3+, while other six fitting peaks at binding energies of 882.6 eV (V), 889.9 eV (VII), 898.8 eV (VIII), 901.1 eV (U), 908.1 eV (UII) and 917.1 eV (UIII) can be indexed to Ce4+.23,34 Similar results can be also obtained for other Au/CeO2 composites (not given for other cases). The above results indicate the co-existence of Ce3+ and Ce4+ species in Au/CeO2-4.0wt.% and pure CeO2 samples. Moreover, the Ce3+ content in CeO2 is 16.3%, obviously higher than that (15.1%) in Au/CeO2-4.0wt.% (Table S2, Supporting Information), mainly attributed to the reduction role of Ce3+ over Au3+. Similarly, it is also found that the Ce3+ content gradually decreases from 16.1% to


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15.1% with Au loading from 0.7 wt.% to 4.0 wt.%, and then the Ce3+ content is almost unchanged with further increasing Au loading to 4.8 wt.% and 4.9 wt.% (Table S2, Supporting Information). The above results reveal that Ce3+ species in CeO2 and Au/CeO2 composites is abundantly existent, meaning the formation of rich oxygen vacancies defects on samples.19,22-25 The existence of rich oxygen vacancies defects is favourable for enhancing photocatalysis performance.22-25 The Au 4f spectra (Figure 2c) of Au/CeO2-4.0wt.% show two peaks appeared at binding energies of 83.8 eV and 87.5 eV, corresponding to Au 4f7/2 and Au 4f5/2, respectively.28 Compared to the values of Au at 84.0 eV (Au 4f7/2) and 87.7 eV (Au 4f5/2), a ~0.2 eV shift toward low binding energy can be obtained for Au/CeO2-4.0wt.%, mainly resulted from the electronic interaction between CeO2 and Au nanoparticles.

28

Similar results can be

also achieved for other Au/CeO2 samples (not given for other cases). As we know, the chemically adsorbed oxygen is generally regarded as the source of active oxygen species for high-efficiency catalytic oxidation reactions.22,25 Figure 2d presents the O 1s spectra of CeO2 and Au/CeO2-4.0wt.%. The peak at 529.5 eV is attributed to the lattice oxygen (Olatt) in CeO2, while the peak located at 531.6 eV can be owing to the adsorption oxygen (Oads) on sample surface.22,34,35 Other Au/CeO2 composites also give similar XPS results. Also, this can be further verified by the following chemisorbed characterization technique. Physiochemical Properties of Au/CeO2-X. To obtain the surface area and pore size distribution information on the synthesized samples, we performed the N2 adsorption-desorption isotherm measurements of CeO2, Au/CeO2-0.7wt.%, Au/CeO24.0wt.% and Au/CeO2-4.9wt.% as representative cases. All investigated samples exhibit a typical IV isotherm with a hysteresis loop (Figure 3a), implying their porous structures. The calculated specific surface area is 57.3, 39.5, 41.6, 40.1 m2 g-1 for



CeO2, Au/CeO2-0.7wt.%, Au/CeO2-4.0wt.% and Au/CeO2-4.9wt.%, respectively. Compared to the dispersed CeO2 nanoparticles, a decrease in specific surface area can be observed for Au/CeO2 composites, possibly resulted from the formation of spherical-shape structures with larger sizes. The pore size distribution curves shown in Figure 3b reveal that all investigated samples exhibit microporous and mesoporous structures. The pore sizes of CeO2 sample are mainly distributed in the range of 1.3−6.5 nm, while Au/CeO2 composites show similar pore size distribution characteristics with pore sizes mainly centered at 1.4, 2.8, 3.9, 6.4 and 11.3 nm. The presence of porous structures in Au/CeO2 composites is favourable for mass transport and light utilization when these composites are used as photocatalysts. The optical absoption characters of CeO2 and Au/CeO2 composites were characterized by UV-Vis diffuse reflectance spectroscopy (DRS). Figure 3c shows the DRS spectra of CeO2 and Au/CeO2-4.0wt.%. Apparently, only absorption band below ~455 nm can be observed for CeO2 attributed to a bandgap of ~2.73 eV, while Au/CeO2-4.0wt.% exhibits two absorption bands. One absorption band below ~398 nm corresponding to a bandgap of ~3.12 eV is owing to the intrinsic bandgap absorption of CeO2, whereas a broad Au-aroused SPR absorption appears in 450~750 nm with its center at ~585 nm. Obvisouly, CeO2 with a bandgap of ~2.73 eV possibly possesss certain visible light activity when used as photocatalyst, due to the doping role of high-proportioned Ce3+ in CeO2.36 Interestingly, a blue-shift (~0.39 eV) of the absorption band edge of CeO2 for Au/CeO2-4.0wt.% compared to CeO2 can be clearly observed when Au nanoparticles are introduced into CeO2, possibly resulted from the decrease of Ce3+ content in composite (Table S2, Supporting Information). Similar results can be also obtained for other Au/CeO2 composites (Figure 3d derived from Figure S5, Supporting Information). Furthermore, the absorption intensity of Au/CeO2


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composites in visible light region is dramatically enhanced with increasing Au content in CeO2, and their aqueous solution color gradually changes from lacte of CeO2 to dark violet of Au/CeO2-4.9wt.% (inset in Figure S5, Supporting Information). The above results indicate that the enhanced Au content in CeO2 is very effective to increase the visible light utilization of the resultant Au/CeO2 composite, combined with superior surface catalytic properties of Au nanoparticles, favourable for improving the photocatalytic efficiency of Au/CeO2 photocatalyst. Photocatalytic Selective Conversion of Alcohols. Selective conversion of alcohols to aldehydes or ketones is vital in organic synthesis.1,14 The development of effective synthetic approach to improve their conversion and selectivity is still highly needed. In recent years, Au/CeO2 composites have been fabricated by photochemical deposition, chemical deposition, electrospun, and aerosol-spray etc, exhibiting superior performance as visible light photocatalysts.23-26 In our study, selective photocatalytic conversion of alcohols (e.g., 2-pentanol, 1-phenylethyl alcohol, benzyl alcohol, phenylpropanol, cinnamic alcohol and 4-methoxybenzyl alcohol) were evaluated using Au/CeO2 composites with different Au loadings as photocatalysts with visible light irradiation (λ > 400 nm, light intensity of 0.7 W cm-2). Figure 4a shows the conversion and selectivity of benzyl alcohol (as a representative case) using different Au/CeO2 composites as photocatalysts with a reaction time of 4 h in O2saturated reaction environment. For all investigated cases, the product of benzyl alcohol oxidation was detected to be benzaldehyde with a high selectivity of ~100% (Figure 4a and inset). Furthermore, it is found that CeO2 with a bandgap of ~2.73 eV exhibits certain photocatalytic activity toward oxidation of benzyl alcohol with a conversion yield of 19.7%. With Au loading, the conversion from benzyl alcohol to benzaldehyde is obviously enhanced for Au/CeO2 photocatalysts. In detail, the



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conversion is dramatically increased from 29.3% for Au/CeO2-0.7wt.% to ~100% for Au/CeO2-4.0wt.%, and then reaches a stable status with further increasing Au content to 4.8 wt.% and 4.9 wt.% in CeO2. The above results demonstrate that high Au loading in CeO2 is critically important for high-efficiency conversion of benzyl alcohol under the given experimental conditions. Therefore, we choose Au/CeO24.0wt.% as a representative photocatalyst for the following experiments. Figure 4b shows the dependence of the conversion of benzyl alcohol on photocatalytic reaction time using Au/CeO2-4.0wt.% as photocatalyst under visible light irradiation. With increasing photocatalytic time, the conversion of benzyl alcohol to benzaldehyde is obviously increased, and the conversion reaches almost ~100% after 4 h of photocatalytic reaction. Additionally, the recyclability of Au/CeO2-4.0wt.% was also tested. The results (Figure 4c) of five times repeated experiments show that no significant decay of photocatalytic activity toward benzyl alcohol oxidation can be observed, indicating Au/CeO2-4.0wt.% as photocatalyst with good applicable stability and durability in the photocatalytic oxidation reaction. Moreover, the XRD and HADDF-STEM results (Figure S6, Supporting Information) also demonstrate that the crystalline phase and structure of the Au/CeO2-4.0wt.% after five times repeated photocatalytic experiments are unchanged, furthering indicating its high applicable stability. To illustrate the general applicability of the synthesized photocatalyst, we also examined the photocatalytic activity of Au/CeO2-4.0wt.% toward selective oxidation

of

2-pentanol,

1-phenylethyl

alcohol,

4-methoxybenzyl

alcohol,

phenylpropanol and cinnamic alcohol. As shown in Figure 4d, after reaction of 4 h, the chosen alcohol reactants can be selectively converted into the ketones and aldehydes products with high selectivity approximate to ~100%. In detail, the conversion yield is ~100%, 47.9%, ~100%, 20.4% and 14.7% for 2-pentanol, 1-


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phenylethyl alcohol, 4-methoxybenzyl alcohol, phenylpropanol and cinnamic alcohol, respectively. The detailed information has been summarized in Table S3 (Supporting Information). For the control experiments, we also performed the photocatalytic tests under the conditions of without light irradiation (including Au/CeO2-4.0wt.%) and in the absence of Au/CeO2-4.0wt.% with light irradiation. The results (Figure S7, Supporting Information) show that ignorable conversion of benzyl alcohol can be detected for all experiments, indicating the significant role of Au/CeO2-4.0wt.% in photocatalytic conversion of benzyl alcohol. The above results demonstrate the general applicability of using Au/CeO2-4.0wt.% as photocatalyst for photocatalytic conversion of alcohols into ketones or aldehydes products with high selectivity. Photocatalytic Selective Oxidation Mechanism. Recently, several studies have demonstrated that a synergistic effect of Au nanoparticles and CeO2 in Au/CeO2 composites contributes their high performance toward selective photocatalytic oxidation of organics.14,21,23,24 In our study, the formed Au nanoparticles in Au/CeO2 composites possess smaller particle sizes of ~10 nm through this spontaneous redox synthesis approach at ambient temperature, possibly affording higher surface catalytic activity toward photocatalytic oxidation of alcohols resulted from their higher fraction of coordinatively unsaturated surface atoms. In addition, this spontaneous redox approach synthesized CeO2 in Au/CeO2 composites contains abundant oxygen vacancies that have been proven to be active for oxygen molecules’ adsorption and activation.19,22,25,37 Therefore, the high photocatalytic performance toward selective oxidation alcohols using Au/CeO2 composites in this study could be collectively originated from a synergetic influence of Au nanoparticles and CeO2 support, which deserves a further investigation. Figure 5a presents the photocurrent responses of Au/CeO2-4.0wt.% and CeO2 photoanodes obtained at a potential of 0.3 V in 0.2 M



Na2SO4 solution with λ > 400 nm visible light irradiation. It can be seen that there is no photocurrent response for these two photoanodes without light irradiation. Under visible light irradiation, CeO2 photoanode only exhibits very small photocurrents, while significantly enhanced photocurrents can be observed for Au/CeO2-4.0wt.% photoanode, indicating that the Au introduction can dramatically boost the photocatalytic activity of photocatalyst. The enhanced photocatalytic performance of Au/CeO2-4.0wt.% is mainly due to the excited plasmonic Au nanoparticles to generate more photoelectrons compared to CeO2 photoanode with visible light irradiation. To illustrate the role of different active species in selective photocatalytic conversion of alcohols (taking benzyl alcohol as an example) using Au/CeO2-4.0wt.%, several control experiments have been performed under visible light irradiation with photocatalytic reaction time of 1 h (Figure 5b). In the presence of O2 without the addition of any quencher in reaction solution, 39.2% conversion from benzyl alcohol to benzaldehyde can be obtained, while the conversion decreases to 13.2% in Ar atmosphere. The above results demonstrate that molecular oxygen (O2) in reaction solution may be active species source for high photocatalytic oxidation performance. It is well known that O2 is a superior acceptor of photoelectrons to produce ·

superoxide radical species (O2 -), thus capable of effectively decreasing the photoelectrons/holes recombination to improve the photocatalytic performance.38-40 In ·

this work, the presence of superoxide radical species (O2 -) in the reaction solution has been determined by the ESR technique. Figure 5c reveals that no ESR signals can be observed without light irradiation for the photocatalytic reaction system using Au/CeO2-4.0wt.% in the existence of O2, whereas strong ESR signals of superoxide radical species in the reaction solution were detected with visible light irradiation. ·

Furthermore, the ESR signal intensity of O2 - for Au/CeO2-4.0wt.% introduced


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photocatalytic reaction system is obviously stronger than that obtained in CeO2 photocatalytic reaction system (Figure S8, Supporting Information). The ESR analysis indicates that the existence of Au nanoparticles in CeO2 can dramatically enhance the ·

generation efficiency of O2 - in photocatalytic reaction system, favourable for photocatalytic oxidation of alcohols.24, 38 This can be further validated by the addition ·

of BQ as a scavenger for O2 - radicals in the reaction system. Figure 5b exhibits that the conversion of benzyl alcohol decreases to 19.1% from initial 39.2% obtained in the photocatalytic reaction system without the addition of BQ, indicating the ·

significant role of O2 - radicals for high photocatalytic performance toward benzyl alcohol oxidation. As a radical scavenger for photogenerated holes, AO was also added to the photocatalytic reaction system with an obviously decreased conversion (15.4%) of benzyl alcohol (Figure 5b), suggesting the important contribution of photogenerated holes on photocatalytic conversion of benzyl alcohol. Interestingly, it is found that the addition of TBA (as the scavenger of hydroxyl radical species, ·OH) into the reaction system only gives rise to insignificant change in benzyl alcohol conversion (Figure 5b), indicating no generation of hydroxyl radicals in the photocatalytic oxidation of benzyl alcohol to benzaldehyde. Also, the ESR analysis does not detect the presence of ·OH radicals in the photocatalytic reaction solution, further demonstrating this point. The above experimental results demonstrate that the ·

superoxide radical species (O2 -) and photogenerated holes are assuredly involved in the photocatalytic oxidation of benzyl alcohol to benzaldehyde. Moreover, molecular ·

oxygen (O2) in reaction solution is believed to be the primary source for O2 ·

formation. At the same time, the formation of O2 - radicals also implies the significant role of photogenerated electrons in the photocatalytic oxidation of benzyl alcohol. In



this study, under visible light (λ > 400 nm) irradiation, CeO2 substrate only exhibits low visible light absorption capability owing to its wide bandgap of ~2.73 eV, while the introduction of plasmonic Au nanoparticles results in high visible light utilization efficiency of Au/CeO2 composites. Therefore, the photogenerated electrons and holes produced on the light excited Au nanoparticles in Au/CeO2 composites should significantly contribute the photocatalytic oxidation activity toward benzyl alcohol. In this work, CeO2 as Au nanoparticles loading substrate has at least two significant roles: one is capable of accepting the photogenerated electrons from the excited Au nanoparticles under visible light irradiation, thus beneficial for the effective separation of photogenerated electrons and holes to improve the photocatalytic performance; the other is that the abundant oxygen vacancies in CeO2 can effectively adsorb and activate molecular oxygen (O2), followed by accepting the ·

photogenerated electrons to form O2 - radicals.41,42 For the latter, we further performed the hydrogen temperature-programmed reduction (H2-TPR) and oxygen temperatureprogrammed desorption (O2-TPD) measurements to confirm the role of the oxygen vacancies in CeO2. Figure S9 (Supporting Information) shows the H2-TPR profiles of CeO2 and Au/CeO2-4.0wt.%. In the investigated temperature range, the CeO2 sample exhibits two reduction peaks at 293 °C and 465 °C, mainly ascribed to the reduction of the surface capping oxygen of CeO2.43,44 When the Au nanoparticles are introduced into CeO2, the Au/CeO2-4.0wt.% also shows two reduction peaks at 123 °C and 465 °C, corresponding to the reduction of the surface capping oxygen of CeO2. Comparatively, the reduction peak temperature (123 °C) of Au/CeO2-4.0wt.% is significantly lower than that (293 °C) obtained from CeO2, indicating that the Au nanoparticles in CeO2 can effectively facilitate the reduction of the surface capping oxygen of CeO2. The exact information on the surface reduced oxygen species of


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CeO2 can be further achieved by the O2-TPD measurement. As shown in Figure 5d, the CeO2 sample mainly shows two O2 desorption peaks at 89 °C and 365 °C, which ·

·

can be attributed to the adsorbed superoxide (O2 -) and peroxide (O -) radicals, respectively.25,45 The presence of the abundant oxygen vacancies in CeO2 is believed to significantly contribute the chemical adsorption of molecular O2 on the CeO2 surface.45 Compared to the CeO2, the Au/CeO2-4.0wt.% sample presents two stronger ·

O2 desorption peaks at 95 °C and 325 °C, due to the adsorbed superoxide (O2 -) and ·

peroxide (O -) species, respectively.25,45 These results suggest that the introduction of Au nanoparticles, combined with effective adsorption and activation of molecular O2 in the oxygen vacancies of CeO2, can significantly enhance the molecular oxygen reduction efficiency. This is very favourable for photocatalytic oxidation of alcohols to aldehydes or ketones under visible light irradiation. On the basis of the above experimental and characterization results, the selective photocatalytic oxidation of alcohols to aldehydes or ketones using Au/CeO2 photocatalyst under visible light irradiation can be summarized as the following several processes (Figure 6): (1) owing to the wide bandgap (~2.73 eV) of CeO2, the plasmonic Au nanoparticles in CeO2 are mainly excited under the visible light (λ > 400 nm) irradiation to form photogenerated electrons (e-) and holes (h+); (2) subsequently, the generated plasmonic hot electrons take place rapid transfer from the excited Au nanoparticles to CeO2 through their interfaces, and react with the molecular O2 adsorbed at the oxygen vacancies of CeO2 to form the superoxide ·

radicals (O2 -); (3) at the same time, the small-sized Au nanoparticles (~10 nm) with rich surface catalytic active sites adsorb and activate alcohol reactants to form active intermediates via the oxidation of photogenerated holes (h+) on Au; (4) at last, the ·

formed O2 - radicals further react with the alcohol derived active intermediates to ACS Paragon Plus Environment


form aldehydes or ketones products. In this work, the self-assembly synthesized Au/CeO2 composites by the spontaneous redox approach at ambient room have been verified to possess superior plasmonic effect, small Au nanoparticle sizes (~10 nm) and abundant oxygen vacancies in CeO2. These attributes of Au/CeO2 composites make them promising as high-performance photocatalysts for photocatalytic organic synthesis applications under visible light irradiation. 4. CONCLUSIONS In summary, core-shell structure Au/CeO2 composites with spherical-shape morphologies and porous structures have been successfully fabricated through a spontaneous chemical redox approach at ambient temperature. The as-synthesized Au/CeO2 composites as photocatalysts exhibit a general applicability of selective photocatalytic oxidation of alcohols to aldehydes or ketones with high conversion, selectivity and stability under visible light irradiation (λ > 400 nm). This can be ascribed to the plasmonic Au nanoparticles with high visible light activity and rich surface catalytic active sites and CeO2 nanoparticles with abundant oxygen vacancies in Au/CeO2 composite, which synergistically enhance the photocatalytic performance toward selective oxidation of alcohols. This spontaneous chemical redox approach can be also extended to fabricate other desired visible light active photocatalysts for high-efficiency organic synthesis applications. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Supported tables, supported images, DRS, ESR and H2-TPR spectra of the samples AUTHOR INFORMATION


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Corresponding Author *Address correspondence to [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Key R&D Program of China (2017YFA0207202), the Natural Science Foundation of China (Grant No. 51672277, 51432009), the CAS Pioneer Hundred Talents Program, and the CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, China. REFERENCES (1) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Solvent-Free Oxidation of Primary Alcohols to Aldehydes using Au-Pd/TiO2 Catalysts. Science 2006, 311, 362-365. (2) Guo, Z.; Liu, B.; Zhang, Q.; Deng, W.; Wang, Y.; Yang, Y. Recent Advances in Heterogeneous Selective Oxidation Catalysis for Sustainable Chemistry. Chem. Rev. 2014, 43, 3480-3524. (3) Karimi, B.; Khorasani, M.; Vali, H.; Vargas, C.; Luque, R. Palladium Nanoparticles Supported in the Nanospaces of Imidazolium-Based Bifunctional PMOs: The Role of Plugs in Selectivity Changeover in Aerobic Oxidation of Alcohols. ACS Catal. 2015, 5, 4189-4200. (4) Li, B.; Shao, L.; Wang, R.; Dong, X.; Zhao, F.; Gao, P.; Li, Z. Interfacial Synergism of PdDecorated BiOCl Ultrathin Nanosheets for the Selective oOxidation of Aromatic Alcohols. J. Mater. Chem. A. 2018, 6, 6344-6355. (5) Zhang N.; Yang, M. Q.; Tang, Z. Y.; Xu, Y.-J. Toward Improving the GrapheneSemiconductor Composite Photoactivity via the Addition of Metal Ions as Generic Interfacial Mediator. ACS Nano 2014, 8, 623-633. (6) Zhang, N.; Liu, S.; Fu, X.; Xu, Y.-J. Fabrication of Coenocytic Pd@CdS Nanocomposite as a Visible Light Photocatalyst for Selective Transformation under Mild Conditions. J. Mater. Chem. 2012, 22, 5042-5052. (7) Tojo, G.; Fernández, M. Oxidation of Alcohols to Aldehydes and Ketones; Tojo, G., Ed.; Springer: New York, 2010. (8) Tojo, G.; Fernández, M. Oxidation of Primary Alcohols to Carboxylic Acids; Tojo, G., Ed.; Springer: New York, 2010.



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(111)

Figures and Captions

30

(c)

40

50

2θ (degrees)

60

70

(331) (311) (420)

(400)

(220)

(311) (222)

(220)

Au JCPDS 99-0056

(200)

(200) g f e d c b a

20

(b)

CeO2 JCPDS 78-0694

(111)

(a) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

(d)

20 nm

O

Ce

Au

Figure 1. (a) XRD patterns of Au/CeO2-X composites. a-Au/CeO2-0.7wt.%, bAu/CeO2-1.6wt.%, c-Au/CeO2-2.5wt.%, d-Au/CeO2-3.3wt.%, e-Au/CeO2-4.0wt.%, fAu/CeO2-4.8wt.% and g-Au/CeO2-4.9wt.%. (b) SEM and TEM images of Au/CeO24.0wt.%. (c) HAADF-STEM and HR-TEM images of Au/CeO2-4.0wt.%. (d) HAADF-STEM image and corresponding elemental mapping images of Ce, O and Au of Au/CeO2-4.0wt.%.



(a)

(b)

Ce 3d

C 1s

Intensity (a.u.)

Intensity (a.u.)

O 1s

Au 4f

Au/CeO2-4.0wt.%

800

600

VII VI

CeO2

400

200

0

920

Binding Energy (eV)

(c)

V

Au/CeO2-4.0wt.%

CeO2 1000

Ce 3d

U VIII

UIII UII UI

4f7/2

910

900

890

880

Binding Energy (eV)

Au 4f

(d)

O 1s

Olatt

4f5/2

Oads

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

Au/CeO2-4.0wt.%

CeO2

92

88

84

Binding Energy (eV)

80

534

532

530

528

526

Binding Energy (eV)

Figure 2. (a) Surface survey XPS spectra of CeO2 and Au/CeO2-4.0wt.%. (b) Highresolution Ce 3d XPS spectra of CeO2 and Au/CeO2-4.0wt.%. (c) High-resolution Au 4f XPS spectra of Au/CeO2-4.0wt.%. (d) High-resolution O 1s XPS spectra of CeO2 and Au/CeO2-4.0wt.%.


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(b)

(a)

CeO2 Au/CeO2-4.0wt.%

dV/dr (cc/nm/g)

Volume (cc/g)

Au/CeO2-0.7wt.%

Au/CeO2-4.9wt.% Au/CeO2-4.0wt.% Au/CeO2-0.7wt.%

Au/CeO2-4.9wt.%

CeO2

0.0

0.2

0.4

0.6

0.8

0

1.0

10

Relative Pressure (P/P0)

20

30

40

Pore Diameter (nm)

(d)

(c)

300

400

500

600

700

Au/CeO2-4.9wt.%

Au/CeO2-4.8wt.%

Au/CeO2-4.0wt.%

Au/CeO2-3.3wt.%

Au/CeO2-2.5wt.%

1

CeO2

Au/CeO2-1.6wt.%

2

Au/CeO2-0.7wt.%

Au/CeO2-4.0wt.%

CeO2

Bandgap (eV)

3

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

Wavelength (nm)

Figure 3. (a) N2 adsorption-desorption isotherms of CeO2, Au/CeO2-0.7wt.%, Au/CeO2-4.0wt.% and Au/CeO2-4.9wt.%. (b) Pore size distribution curves of CeO2, Au/CeO2-0.7wt.%, Au/CeO2-4.0wt.% and Au/CeO2-4.9wt.%. (c) UV-Vis diffuse reflection spectra (DRS) of CeO2 and Au/CeO2-4.0wt.%. (d) Bandgap of CeO2, Au/CeO2-0.7wt.%,

Au/CeO2-1.6wt.%,

Au/CeO2-2.5wt.%,

Au/CeO2-3.3wt.%,

Au/CeO2-4.8wt.% and Au/CeO2-4.9wt.%, derived from their UV-Vis diffuse reflectance spectra.



th

5 Run

(d)120

0

4

5

6

100

60

30

3

Selectivity Conversion

90

0.2 0.0

2

Reaction Time (h)

80 60 40 20

Selectivity (%)

Au/CeO2-4.9wt.%

Au/CeO2-4.8wt.%

Au/CeO2-4.0wt.%

Au/CeO2-2.5wt.%

Au/CeO2-3.3wt.% th

4 Run

1

cinnamic alcohol

0.4

rd

0

phenylpropanol

0.6

nd

0

0

2 Run 3 Run

30

4-methoxybenzyl alcohol

0.8

st

1 Run

30

60

1-phenylethyl alcohol

(c)1.0

60

Conversion (%)

0

Au/CeO2-1.6wt.%

30

Au/CeO2-0.7wt.%

Selectivity Conversion

60

90

2-pentanol

90

Selectivity (%)

90

Conversion (%)

120 (b)120

CeO2

Conversion (%)

(a) 120

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

0

Figure 4. (a) The conversion and selectivity of photocatalytic oxidation of benzyl alcohol using different CeO2 and Au/CeO2 composites as photocatalysts in the presence of O2 under visible light irradiation for 4 h. (b) The dependence of the conversion of benzyl alcohol on photocatalytic reaction time using Au/CeO2-4.0wt.% as photocatalyst in the presence of O2 under visible light irradiation. (c) Recyclability of the Au/CeO2-4.0wt.% photocatalyst for photocatalytic oxidation of benzyl alcohol in five cycles under visible light irradiation. (d) The conversion and selectivity of photocatalytic oxidation of other chosen alcohols using Au/CeO2-4.0wt.% photocatalyst in the presence of O2 under visible light irradiation for 4 h.


Page 31 of 33

(a)

(b)50

CeO2

0

50

100

150

200

Conversion (%)

light on light on

light off

light off

light on

light off

light on

light off

light off

Photocurrent (µA)

Au/CeO2-4.0wt.% light on

250

40

No quencher

TBA

30 BQ

20

AO

Ar

10

300

0

350

Time (s)

Light on

TCD Signal (a.u.)

(d)

(c) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Au/CeO2-4.0wt.%

CeO2

Light off 3180

3200

3220

3240

3260

3280

Magnetic Field (G)

100

200

300

Temperature (°C)

400

500

Figure 5. (a) Photocurrent-time (i-t) curves of Au/CeO2-4.0wt.% and CeO2 photoanodes in 0.2 M Na2SO4 solution under visible light (λ > 400 nm) irradiation; the applied potential of 0.3 V. (b) Control experiments under Ar-saturated reaction system and using different radical scavengers over the Au/CeO2-4.0wt.% photocatalyst with visible light irradiation for 1 h; BQ as superoxide radical scavenger, AO as photogenerated hole scavenger, and TBA as hydroxyl radical scavenger. (c) ESR spectra of superoxide radical species trapped by DMPO over the Au/CeO24.0wt.% photocatalyst dispersed in the benzotrifluoride solvent. (d) O2-TPD profiles of the CeO2 and Au/CeO2-4.0wt.% samples.



Figure 6. Schematic mechanism of the selective photocatalytic oxidation of alcohols over the Au/CeO2 photocatalyst under the visible light (λ > 400 nm) irradiation.


Page 32 of 33

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hλ (1) Au/CeO2

hλ

e- + h+

eO2(ad) eEg=~2.73 eV O2(ad) e

+ h+ h+ h

CeO2 O2·- +

(2) O2 + e-

Au R-CH2OH or R1-CHOH-R2

*R-CH2OH+ or *R1-CHOH+-R2

O2·-

(3) R-CH2OH + h+ or R1-CHOH-R2

*R-CH2OH+ or *R1-CHOH+-R2

(4) *R-CH2OH+ + O2·or + *R1-CHOH -R2

R-CHO or R1-CO-R2

R-CHO or R1-CO-R2


Spontaneous Redox Approach to the Self-Assembly Synthesis of Au

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