Increasing the Activity and Selectivity of TiO2-Supported Au Catalysts

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Increasing the Activity and Selectivity of TiO2-Supported Au Catalysts for Renewable Hydrogen Generation from Ethanol Photoreforming by Engineering Ti3+ Defects Xin Zhang, Lan Luo, Rongping Yun, Min Pu, Bing Zhang, and Xu Xiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b02008 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Sustainable Chemistry & Engineering

Increasing the Activity and Selectivity of TiO2-Supported Au Catalysts for Renewable Hydrogen Generation from Ethanol Photoreforming by Engineering Ti3+ Defects

Xin Zhang,a‡ Lan Luo,a‡ Rongping Yun, a Min Pu, a Bing Zhang,b Xu Xiang*a a

State Key Laboratory of Chemical Resource Engineering, Beijing University of

Chemical Technology, 15 Beisanhuan Donglu, Beijing 100029, People’s Republic of China b

School of Chemical Engineering, Zhengzhou University, 100 Science Avenue,

Zhengzhou 450001, PR China *

Email: [email protected]

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ABSTRACT: We examine in detail the roles of Ti3+ defects and the associated oxygen vacancies in the activity and selectivity of TiO2-supported Au catalysts (Au@TiO2) for renewable hydrogen production by photoreforming of ethanol.

A

series of Au@TiO2 catalysts was synthesized using varied exposure to reducing agent NaBH4. The electronic structure of the series of Au@TiO2 catalysts was examined spectroscopically and showed that increased exposure to NaBH4 increased the concentration of Ti3+ defects and the associated oxygen vacancies in TiO2, and increased the amount of electron-rich Au. The activity and selectivity of the catalysts increased as the concentration of defect sites increased. During ethanol photoreforming, the Au@TiO2 catalyst with the highest concentration of defects produced high-purity H2 at a record rate of ≈ 7093 μmol gcat−1 h−1 and the carbon-carbon bond (C-C) cleavage of ethanol to form CH4 and CO2 was significantly inhibited. Extensive spectroscopic data support the conclusion that TiO2 surface oxygen vacancies adjacent to Au may be active catalytic sites that assist the adsorption and activation of ethanol as well as the delivery of photogenerated charge carriers to the activated species during the photoreforming of ethanol.

KEYWORDS: supported catalysts, photocatalysis, biomass hydrogen production, defect engineering, oxygen vacancy


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INTRODUCTION Hydrogen is a promising candidate for use as a renewable fuel underpinning a carbon-neutral energy infrastructure.1,2 Hydrogen can be combusted with high efficiency without emitting any greenhouse gases or pollutants, is already used widely in industrial and manufacturing processes, and is used as fuel in emerging fuel-cell passenger vehicles.3 Currently, large-scale production of H2 relies primarily on steam reforming of methane, a process that emits CO2.4 The development of efficient, low-cost ways to produce hydrogen without emitting CO2 or other pollutants is critical to realizing the promise of a green-energy infrastructure based on hydrogen. Alcohol photoreforming is a promising alternative to conventional steam reforming of methane, and offers advantages over other approaches to renewable hydrogen production:5-8 1) Ethanol can be sustainably produced from high-volume low-value biomass such as lignocellulose and agriculture residues;9 2) Photocatalysts allow H2 to be produced at a high rate at room temperature under ambient pressure using only sunlight as an energy input; and, 3) H2 production by ethanol photoreforming is not accompanied by CO2 emission, because the ethanol is oxidized only to acetaldehyde in an ideal way (Equations 1-2). CH3CH2OH + 2h+ →CH3CHO +2H+ (1) 2H+ + 2e- → H2 (2) TiO2 is a prototypical photocatalyst for hydrogen evolution, either from water splitting or alcohol photoreforming, because TiO2 is widely available, stable, and non-toxic.10 However, TiO2 only absorbs wavelengths in the ultraviolet region, and



therefore does not make efficient use of the solar spectrum. Heteroatom doping, defect doping,11,12 and dye sensitization can extend the absorption spectrum of TiO2 into the visible range.13 The efficiency of unmodified TiO2 photocatalysts is also limited by short charge-carrier lifetimes. Loading TiO2 with noble metal hydrogen-evolution catalysts, or creating junctions between TiO2 and other semiconductors can inhibit charge-carrier recombination by accelerating carrier extraction, thus boosting photocatalytic activity.14,15 For example, Haruta et al found that Pt- and Au-loaded TiO2 showed excellent activity towards H2 production from aqueous ethanol under irradiation from a Hg lamp.16 Montini and Dal Santo et al demonstrated that reduction of bimetallic Pt-Au/TiO2 under H2 activated the catalysts for H2 production from aqueous ethanol under either UV light from Hg lamp or simulated sunlight;17 however, the Pt-Au/TiO2 catalyst showed lower activity (H2 production rate ≈ 1800 μmol gcat−1 h−1) under simulated sunlight than under UV light. In this case of the Pt-Au/TiO2 catalyst, the yield of gaseous byproducts, such as CH4 or CO2, which are most likely from further photo-induced C-C bond-cleavage reactions, was not negligible and affected the purity of the H2. Corma and coworkers prepared Au-supported anatase-rich TiO2 using a deposition-precipitation method.18 This Au/TiO2 catalyst showed high activity for H2 production from ethanol photoreforming, and the activity under CO2 was compared with the activity under Ar. The morphology of the TiO2 support affects its H2-production activity. Recent studies by Murray et al showed that the rate of H2 production by TiO2 nanorods was changed when the length of the nanorods was changed, with appropriately longer TiO2


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nanorods leading to a higher H2 production rate.19 Although noble-metal-supported TiO2 photocatalysts for alcohol photoreforming have been studied extensively, an understanding of the electronic structure and defects sufficient to allow tuning of the activity and inhibition of C-C cleavage due to over-oxidation, remains to be developed. Inducing defects in TiO2, e.g. Ti3+ states and the associated oxygen vacancies, is routinely used to increase the range of absorption relative to inherently UV-responsive TiO2, and improves the photocatalytic activity by introducing defect energy levels and abundant surface adsorption sites.20 On the other hand, tuning the metal-support interaction can affect the electronic structure and electron transfer across the metal/support interface.21 Herein, we explore the construction of Au-loaded TiO2 nanorods (TiO2-NR) with defect levels tuned by controlled reduction via NaBH4. We intensively examine the effects of Ti3+ defects on the electronic structure of the catalyst, and on the activity and selectivity of the catalyst for ethanol reforming, to develop a mechanistic understanding of structure-function relationships in these systems.

RESULTS AND DISCUSSION TiO2 nanorods (TiO2-NR) were synthesized by hydrothermal method using a modified procedure from the literature.22 The defect-rich TiO2 nanorods were synthesized by one-step reduction method, in which the reducing agent was NaBH4. The as-prepared samples were denoted TiO2-NR-X, where X referred to the molar ratio of TiO2 to NaBH4 during the synthesis. The Au@TiO2-NR-X catalysts were



synthesized by a deposition-precipitation method by using Au(en)2Cl3 as a precursor. 1wt% gold loading relative to the weight of TiO2 was intended in the preparation stage. Detailed experimental procedures are provided in Supporting Information. Figure S1 shows X-ray diffraction spectra for TiO2-NR and TiO2-NR-X samples (where X=8, 6, 4, 2) before and after deposition of Au nanoparticles (Au NP) onto the TiO2 nanorods. The spectra for all samples were consistent with TiO2 in the anatase phase (JCPDS No. 21-1272). No change in the positions of the diffraction peaks was observed when the reduction conditions were varied, and the diffraction peaks for all samples were essentially unchanged after deposition of Au nanoparticlesonto the nanorods (Au@TiO2-NR and Au@TiO2-NR-X). No diffraction peaks associated with Au were detected, possibly due to the high dispersion of the small Au nanoparticles, the low loading (< 1 wt%) of the Au nanoparticles on the TiO2 support, or both. Figure 1 shows high-resolution transmission-electron microscope (HRETM) images of interfaces between Au NPs and TiO2-NR supports. Figure 1A shows a sample of Au NPs on TiO2 nanoparticles (TiO2-NP). The Au NPs had an average size of 2.2 nm, and the lattice images showed d-spacings of 0.352 nm and 0.235 nm, corresponding to the (101) facet of TiO2 in the anatase phase and the (111) facet of Au in the cubic phase, respectively.23,24 For Au@TiO2-NR sample, Au nanoparticles were highly dispersed on the support and had a diameter of ~3.0 nm (Figure 1B). The sizes and the actual loading (0.8 wt%, as determined by ICP) of the Au NP were approximately the same across all of the Au@TiO2-NR-X samples (Figure 1C-F, Table S1). As the amount of NaBH4 used to reduce the samples was increased, an


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increased presence of short-range disordered or amorphous regions was observed in the near-surface regions of the TiO2-NR-X (Figure 1C-F), where the Au NP were partially embedded in the TiO2 support, consistent with a strong metal-support interaction (SMSI).25 The reduction of TiO2 by NaBH4 is expected to induce Ti3+ defects and associated oxygen vacancies,26 and the defect-rich TiO2 may strongly affect the electronic structure of supported metals.27

Figure 1. HRTEM images and Au particle size distribution diagrams of (A) Au@TiO2-NP, (B) Au@TiO2-NR, (C) Au@TiO2-NR-8, (D) Au@TiO2-NR-6, (E) Au@TiO2-NR-4, (F) Au@TiO2-NR-2. Figure 2 shows X-ray photoelectron spectra (XPS) in the Ti 2p3/2, O 1s, and Au 4f7/2 spectral regions for the various Au@TiO2-NR-X samples. The low-energy peak in the Ti 2p3/2 region of Au@TiO2-NR sample was fitted to one peak at binding energy of 458.3 eV, corresponding to the Ti4+ species (Figure 2A-a).26 For Au@TiO2-NR-X samples, a new fitted peak appears at ~457.9 eV, which is assigned to Ti3+ species (Figure 2A b-e).28,29 The positions of these peaks did not shift significantly among



samples, however, the ratio of the integrated areas of the two peaks varied considerably. The Ti3+: Ti4+ ratio gradually increased from 0.32 for the unreduced Au@TiO2-NR to 0.99 in the highly reduced Au@TiO2-NR-2 (Table 1). The increased Ti3+:Ti4+ ratio indicates that increased amounts of NaBH4 used during synthesis resulted in an increased presence of Ti3+ species at the surface. When the TiO2-NR:NaBH4 ratio was further increased to 1:1 to yield Au@TiO2-NR-1, the Ti3+:Ti4+ ratio did not increase beyond that for Au@TiO2-NR-2 (Figure S2 and Table S2), suggesting that the Ti3+ concentration accessible using NaBH4 approaches saturation near a Ti3+:Ti4+ ratio of 1.

Figure 2. XPS analyses of (A) Ti 2p, (B) O 1s, (C) Au 4f core level spectra: (a) Au@TiO2-NR, (b) Au@TiO2-NR-8, (c) Au@TiO2-NR-6, (d) Au@TiO2-NR-4, (e) Au@TiO2-NR-2.


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Table 1 Ti 2p XPS analyses of Au@TiO2-NR-X catalysts. 2p1/2

2p3/2 Samples

Ti3+/Ti4+ ratio

Ti

Ti

Ti

Ti

(eV)

(eV)

(eV)

(eV)

Au@TiO2-NR

-

458.3

-

464.3

-

Au@TiO2-NR-8

457.9

458.3

463.3

464.2

0.63(0.39)

Au@TiO2-NR-6

457.9

458.3

463.4

464.3

0.75(0.43)

Au@TiO2-NR-4

457.8

458.2

463.4

464.2

0.87(0.47)

Au@TiO2-NR-2

457.8

458.3

463.4

464.2

0.99(0.50)

a

3+

4+

3+

4+

a

The ratio was calculated from the integrated area of corresponding peaks. The value

in parentheses is the fractional concentration of Ti in the Ti3+ state. The spectra for the O 1s region were fitted to three peaks: one at 530.0 eV corresponding to lattice oxygen, Ti-O-Ti (OL); a second at 532.0 eV corresponding to oxygen species adsorbed on the surface adjacent to an oxygen vacancy (OV); and a third at 533.5 eV, corresponding to chemisorbed or dissociated oxygen species from H2O molecules (OC) (Figure 2B). The binding energies (B.E.) for these peaks did not shift significantly among samples (Table 2); however, the OV:OL peak ratio, which was used to evaluate the relative amount of surface oxygen vacancies, increased from 0.2 for Au@TiO2-NR to 0.9 in Au@TiO2-NR-4, and further increased greatly to 2.3 for Au@TiO2-NR-2, indicating an abundance of surface oxygen vacancies. These results show that surface oxygen species near oxygen vacancies increased as the concentration of Ti3+ defects was increased in reduced samples.30



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Table 2 O 1s XPS analyses of Au@TiO2-NR-X catalysts Samples

OV

OL (eV)

a

(eV)

OC

OV/OL

(eV)

ratio a

Au@TiO2-NR

530.0

532.1

533.6

0.2

Au@TiO2-NR-8

530.0

532.0

533.4

0.4

Au@TiO2-NR-6

530.0

532.0

533.4

0.7

Au@TiO2-NR-4

530.0

532.0

533.5

0.9

Au@TiO2-NR-2

530.0

532.1

533.5

2.3

The ratio was calculated from the integrated area of corresponding peaks. Figure 2C shows the XPS Au 4f spectra. The spectra in the Au 4f7/2 region of all

samples were fitted to one peak at the B.E. value of 84.0~83.4 eV, which is assigned to Au0 species.31 The B.E. values corresponding to Au0 peak shift towards a lower energy with the increasing Ti3+ content in the samples (Table 3). For instance, the Au0 peak in Au@TiO2-NR-2 (at 83.4 eV) shifts 0.6 eV compared to Au@TiO2-NR. The low-energy shift of Au0 peak indicates the increased electron density of Au0 species. This could be caused by the electron transfer across the metal-support interface. The XPS results suggest that Au-OV-Ti3+ sites are formed by strong electronic metal-support interactions (EMSI) in Au@TiO2-NR-X catalysts.27 Table 3 Au 4f XPS analyses of Au@TiO2-NR-X catalysts Au 4f7/2 (Au0)

Samples B.E. (eV)

△B.E. (eV) a

Au@TiO2-NR

84.0

-

Au@TiO2-NR-8

83.8

0.2

Au@TiO2-NR-6

83.7

0.3

Au@TiO2-NR-4

83.6

0.4

Au@TiO2-NR-2

83.4

0.6


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a △B.E. refers to the B.E. shift towards a lower energy relative to that of Au0 in Au@TiO2-NR. Figure 3A shows the Raman spectra for TiO2-NR-X samples. An intense peak at 145 cm−1, was assigned to the external vibration of Ti-O bond (Eg mode) of anatase phase TiO2. The other small peaks at 197, 395, 514, and 638 cm−1 were also assigned to Raman-active vibrations of anatase TiO2.32 The position of the intense Eg peak shifted from 145 cm−1 to 150 cm−1 with the increased Ti3+:Ti4+ ratio in TiO2-NR-X samples (enlarged image in Figure 3A), clearly indicating the presence of Ti3+ states and the associated oxygen vacancies in the lattice of TiO2-NR-X.33,34 These Raman results are consistent with the Ti 2p and O 1s core-level XPS. Furthermore, the colors of the TiO2-NR-X samples changed from white to gray to dark blue as more reductant was used in the synthesis, consistent with NaBH4 reduction inducing defects in the TiO2 nanorods (Figure S3), which can cause a blue-shift of the Raman-active Eg mode.35 Figure 3B shows the electron paramagnetic resonance (EPR) spectra for unreduced and reduced TiO2 nanorods. The room-temperature EPR measurements showed no distinct absorption peak for TiO2-NR; the EPR for TiO2-NR-8 was similar to that for TiO2-NR. However, an intense EPR signal appeared at g ≈ 2.003 for TiO2-NR-6, and the intensity of the signal gradually increased for samples with increased reductant, indicating an increased Ti3+:Ti4+ ratio. The signal at g ≈ 2.003 is commonly attributed to surface O- species, which could be produced from the interaction of oxygen molecules with the Ti3+ defects associated with surface oxygen vacancies.29,32,36,37



Figure 3 Room-temperature Raman and X-band EPR spectra for (a) TiO2-NR, (b) TiO2-NR-8, (c) TiO2-NR-6, (d) TiO2-NR-4, (e) TiO2-NR-2. We did not observe the EPR signals at g=1.96~1.99, that have been assigned to Ti3+ and that were observed for hydrogenated TiO2.32,38 For example, Kim et al observed a broad peak near g=1.96~1.99 in the EPR spectra for anatase TiO2 hydrogenated at 600~700 oC.32 However, the signal related to Ti3+ was not observed in some literatures.30,39 In our case, TiO2 nanorods were reduced by varied amounts of NaBH4 at 350oC while under an N2 atmosphere, and oxygen vacancies are formed preferentially on the surface at lower temperature.40 The surface oxygen vacancies and neighboring Ti3+ species, when exposed to the ambient air atmosphere, can react with molecular oxygen (O2), to generate O2- which can react further, producing O- species at sites of surface oxygen vacancies.32 The increased intensity of the O--related EPR peak (g=2.003) with increased Ti3+:Ti4+ ratio suggests that Ti3+ defects in the bulk can diffuse to the surface to generate surface oxygen vacancies, which can then react with O2 to form O- species on the surface of TiO2-NR-X samples. These EPR results agree with the XPS core-level spectra for Ti 2p and O 1s. Figure 4 compares the UV-Vis absorption spectra for the catalyst supports with


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the spectra for the Au@TiO2-NR catalysts. The unreduced TiO2-NR only showed absorption in the UV region at wavelengths below 370 nm, consistent with its white color (Figure S3). The reduced TiO2-NR-X samples exhibited absorption spectra that were red-shifted relative to the pristine TiO2-NR (Figure 4A). Also, the red-shifted absorption increased step-wise with increased concentration of Ti3+ in the TiO2 support, and TiO2-NR-2 showed the most intense absorption in the visible region, corresponding to the darkest color, i.e. dark blue, observed among the samples. To estimate the bandgap of the samples, UV-Vis diffuse reflectance spectra were converted using the Kubelka-Munk equation, i.e. F(R) = (1–R)2/2R, where R is the reflectance (Figure 4B).41 The TiO2-NR-X samples showed a reduced bandgap relative to that of TiO2-NR (3.25 eV), with TiO2-NR-2 showing a remarkably narrow bandgap of 2.75 eV. The introduction of Ti3+ sites into anatase TiO2 has been reported to narrow the bandgap due to involvement of defect energy levels.42

Figure 4. (A) UV-Vis absorption spectraand (B) Bandgap estimation of TiO2-NR and



TiO2-NR-X samples (X: 8, 6, 4, 2), (C) UV-Vis absorption spectra of Au@TiO2-NR, Au@TiO2-NR-8, Au@TiO2-NR-6, Au@TiO2-NR-4, Au@TiO2-NR-2, and (D) Fluorescence emission spectra of TiO2-NP, TiO2-NR, TiO2-NR-X samples (X: 8, 6, 4, 2). The excitation wavelength was 350 nm. . When Au NPs were deposited onto the TiO2, a new absorption band centered at around 560 nm appeared in the spectra (Figure 4C). This band corresponds to absorption by the Au surface plasmon resonance (SPR).43,44 The SPR effect and photo-excited hot carriers in Au have been hypothesized to facilitate charge separation and transfer in photocatalysts.45,46 The Au SPR peak was observed for less-reduced samples, but was not observable in the spectra for Au@TiO2-NR-4 and Au@TiO2-NR-2 samples, due to the intense visible absorption of the highly reduced support. The colors of Au@TiO2-NR-X are darker than those un-supported samples (Figure S4). Fluorescence emission spectra for TiO2-NR and TiO2-NR-X samples showed a broad emission band with a peak at 421 nm (Figure 4D). The intensity of the emission band gradually decreased as the concentration of Ti3+ in the samples increased. Figure S5 shows the fluorescence lifetime decay for the supports and catalysts; the lifetime increased for samples with increased Ti3+ concentration (Table S5 and S6), suggesting that defect doping decreased the rate of recombination of photo-generated charge carriers due to hole trapping by the oxygen vacancy.26 The catalytic activity of Au@TiO2-NR-X samples for photoreforming of


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ethanol was tested under an Ar atmosphere (1.4 bar) at room temperature under simulated solar irradiation. Table 4 presents the rates of formation for gaseous and liquid products; H2 accounted for ~99% (in moles) of the gaseous products, with CH4, CO2 and CO making up the balance of gas-phase products. CO2 can be generated from ethanol via a deep oxidative process involving photo-generated holes.47 CH4 and the trace amount of CO result from cleavage of C-C bonds in ethanol.48,49 GC-MS results show that acetaldehyde (CH3CHO) was the only liquid product detected (Figure S6). Acetic acid (CH3COOH) is also a possible liquid product of ethanol photoreforming, but was not detected. The data show that H2 and acetaldehyde were yielded in approximately equal stoichiometric amounts, consistent with the reforming of ethanol to H2 and acetaldehyde via a redox process,18 as shown in Equations 1-2. For the case of Au@TiO2-NR-X-catalyzed ethanol reforming, holes photogenerated in the valance band of TiO2 transfer to the surface, and electrons from the TiO2 conduction band are injected into Au, forming Auδ-. The holes on the surface of TiO2 oxidize ethanol to acetaldehyde, releasing protons that are reduced by Auδ- to H2 molecules. The data in Table 4 clearly show an increased rate of H2 production with increased Ti3+ concentration in the catalyst. Specifically, the rate of H2 production increased monotonically from 5152 µmol gcat−1 h−1 for Au@TiO2-NR to 7093 µmol gcat−1 h−1 for Au@TiO2-NR-2, the catalyst with the highest Ti3+ concentration. We also measured the activity of Au@TiO2-NR-1, which had a Ti3+ concentration close to Au@TiO2-NR-2 (Figure S2); for Au@TiO2-NR-1, the rate of H2 production was 7038 µmol gcat−1 h−1 overwhelming negligible other gaseous product, comparable to the rate



for Au@TiO2-NR-2 (Table S3). Furthermore, the yields of CH4 and CO2 decreased as the Ti3+ concentration increased, consistent with inhibition of C-C cleavage and deep oxidation of ethanol by catalysts with Au-NPs supported on defect-rich TiO2-NRs. The H2 yield is negligible on TiO2-NP (5 µmol gcat−1 h−1) and TiO2-NR (13 µmol gcat−1 h−1). In addition, the H2 production is very limited on TiO2-NR-2 with a yield of 125 µmol gcat−1 h−1 (Table S4). This indicates that the supported Au nanoparticles play a critical role in producing H2 via photoreforming of ethanol. The Au@TiO2-NR-2 was recycled after reactions through centrifugation, washed with ethanol and water, and dried at 60 oC. Figure 5 shows the H2 production on Au@TiO2-NR-2 catalyst with 5 cycles. The H2 yield keeps at the same level, indicating the excellent recycled performance of the catalyst. XPS and HRTEM analyses show that the Au@TiO2-NR-2 catalyst is structurally stable after reactions (Figure S7 and S8). Figure S9 shows a plot of the H2 yield versus the fraction of Ti atoms in the catalyst in the Ti3+ state. A sharp linear increase in H2 yield was observed when the Ti3+ concentration varied from 0.40 to 0.50. This is a hint that Ti3+ sites play a key role in promoting H2 production.

Figure 5. Recycled use of Au@TiO2-NR-2 for hydrogen production from ethanol photoreforming


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Table 4 H2 production via photoreforming of ethanol under simulated solar light Production rate a Photocatalysts

(µmol gcat−1 h−1) Gas products

Liquid products

H2

CH4

CO

CO2

CH3CHO CH3COOH

5152

52

3

69

5494

-

Au@TiO2-NR-8 5341

40

1

29

5766

-

Au@TiO2-NR-6 5812

19

-

9

6138

-

Au@TiO2-NR-4 6305

14

-

8

6639

-

Au@TiO2-NR-2 7093

8

-

6

7348

-

Au@TiO2-NR

(a)

The photocatalyst (1g/L) was suspended in ethanol (25 mL) under stirring and irradiated with simulated solar light (power 100 mW cm−2). The reactor was sealed and kept under an Ar gas atmosphere (1.4 bar) at 25°C for 6 h. The gas phase products were monitored online by a GC equipped with a TCD. The liquid-phase products were measured using a GC equipped with a FID and methane reformer.

The H2 yield rate for the best catalyst in this study, Au@TiO2-NR-2, was greater than that previously reported for TiO2-supported noble-metal catalysts (Au, Pt, Pd) under comparable conditions,13,17,18,50,51 and represents a new state-of-the-art level. Table 5 compares the H2 yield rates published for TiO2-supported noble-metal catalysts with the rate measured for Au@TiO2-NR-2 in this work. We hypothesize that the excellent activity of Au@TiO2-NR-2 can be attributed to a synergy between Auδand the abundant Ti3+ defects associated with oxygen vacancies, and that Auδ--OV-Ti3+ sites could be active sites for ethanol reforming. Table 5 Comparisons of photocatalysts for ethanol photoreforming Photocatalysts

Light source

1.0% Au/TiO2

simulated solar light

H2

CH3CHO

(µmol gcat−1 h−1)

(µmol gcat−1 h−1)

6151

6522

(100 mW cm−2)


Ref. 18


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MWCNT/Pd@TiO2

Xe lamp (150W)

1500

-

50

dye/Pt/TiO2

visible light

4359

-

13

(λ>420 nm) Si/Au/TiO2

Xe lamp (300W)

5143

-

51

Pt0.5–Au0.5/TiO2

Xe lamp (150W)

1800

-

17

Au@TiO2-NR-2

simulated solar light

7093

7348

This

(100 mW cm−2)

work

The adsorption of ethanol is a key step in the catalytic cycle. Figure 6 shows the FTIR spectra in the low-frequency range of 1200–950 cm−1 for Au@TiO2-NR-X catalysts exposed to ethanol. Six bands, at 1101, 1078, 1066, 1054, 1039, and 1027 cm−1, were apparent in the spectra, and are assignable to stretching vibrations of ν(C-O) in -OCH2CH3 and ν(C-C) in -CH2CH3.52,53 The band at 1101 cm−1 is attributed to the on-top adsorption of ethoxide on a single Ti site (Type I). The bands at 1054 (Type II) and 1039 cm−1 (Type II′) are associated with ethoxide adsorption bridging Ti-O-Ti and Ti-OV-Ti sites, respectively; the relative intensity of the Type II′ band was enhanced for samples with increased Ti3+ concentration, consistent with defect-rich TiO2 enabling

stronger

adsorption

of

ethanol

on

Ti-OV-Ti

sites

with

a

coordination-unsaturated oxygen vacancy. The C-O bond in the ethoxide species can be weakened by Ti cations having an adjacent oxygen vacancy.54 A very weak band associated with triple-bridging adsorption of ethoxide on Ti sites was observed (1027 cm−1, Type III). The band at 1078 cm-1 is assigned to the C-C stretching of -CH2CH3.55 The FTIR results support the conclusion that Ti3+ and the associated oxygen vacancy accelerate the activation of ethanol to ethoxide and the concomitant release of protons.


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Figure 6. FTIR spectra of ethanol adsorption on the catalysts: Au@TiO2-NR, Au@TiO2-NR-8, Au@TiO2-NR-6, Au@TiO2-NR-4, Au@TiO2-NR-2. The hollow square (□) refers to oxygen vacancy. Figure 7 shows FTIR spectra collected in situ during photoreforming of ethanol on unreduced and reduced Au@TiO2 catalysts. The spectra of each sample were first collected in the dark after adsorption of ethanol for 40 min, and spectra were then recorded every hour during photoreforming under irradiation. The spectra of all samples showed distinct bands at 1000–1200 cm-1, 1200–1500 cm-1, 2700–3100 cm-1 and 3600–3800 cm-1, corresponding to the characteristic vibrations of ν(C−O), δ(CHx), ν(CHx) and ν(O−H) in ethanol, respectively (Figure 7A).56 In addition, the peak at 2356 cm−1 is assigned to the vibration of CO2 owing to the over-oxidation of ethanol,57 and the band at 1755 cm−1 is attributable to the vibration associated with acetaldehyde.58,59 The intensity of the CO2 peak (2356 cm−1) gradually decreased as the Ti3+ concentration in the catalyst increased, at the same time, the intensity of the acetaldehyde peak (1755 cm−1) increased as the Ti3+ concentration in the catalyst increased (Figure 7A to 7E). The increased intensity of acetaldehyde peak on



Au@TiO2-NR-2 hints its good adsorption capability towards ethanol and rapid transformation to acetaldehyde by dehydrogenation of ethanol (Figure 7E).59,60 These results show that the amount of acetaldehyde increases and the CO2 concentration decreases as Ti3+ concentration in the Au@TiO2-NR-X catalysts increases, consistent with the products measured (Table 4). The in situ FTIR analyses further supports the critical role of Ti3+ and the associated oxygen vacancy in boosting the rate of H2 production during ethanol photoreforming.

Figure 7. In situ FTIR spectra of ethanol adsorption on the catalysts during the photoreforming: (A) Au@TiO2-NR, (B) Au@TiO2-NR-8, (C) Au@TiO2-NR-6, (D) Au@TiO2-NR-4, (E) Au@TiO2-NR-2.

CONCLUSIONS In summary, we have synthesized a series of catalysts for ethanol photoreforming consisting of Au-NPs on TiO2-NR supports, and have shown conclusively that the


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concentration of Ti3+ defect sites, and the associated oxygen vacancies, can be systematically varied by varying exposure to the reducing agent NaBH4 during synthesis. The activity and selectivity of the catalysts increased as the concentration of defect sites increased.

Extensive spectroscopic data support the conclusion that

oxygen vacancies on the surface of defect-rich TiO2 assist adsorption of ethanol molecules and the transfer of photogenerated holes to ethanol, while photogenerated electrons are transferred to the Au-NPs. The emerging mechanism for ethanol photoreforming by Au@TiO2 is that higher concentrations of defects in TiO2 not only increase the rate of charge-carrier generation by increasing absorption of visible light, but also increase the proximity of electron-rich Au sites to oxygen-vacancy sites where protons are released by the adsorption and activation of ethanol, and that this proximity accelerates the rate of reduction of protons to H2.

ASSOCIATED CONTENT Supporting Information. Au content, Additional XPS data, Fluorescence decay lifetime, XRD patterns, Digital photographs, Bandgap, Fluorescence emission spectra, Time-resolved fluorescence decay, GC-MS spectra, Recycled use of catalyst (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]



Author Contributions ‡

These authors contribute equally.

ǁ

The manuscript was written through contributions of all authors. All authors

have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21576016, 21521005), the National Key R&D Program of China (Grant 2017YFA0206804), and the Fundamental Research Funds for the Central Universities. Abbreviations NP = nanoparticles NR = nanorods SMSI = strong metal-support interaction OL = lattice oxygen OV = oxygen vacancy OC = chemisorbed or dissociated oxygen species from H2O molecules B.E. = binding energy EMSI = electronic metal-support interactions


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SPR = surface plasmon resonance

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TOC graphic and synopsis

Engineering the defects in Au-supported TiO2 nanorods greatly promotes renewable production of high-purity hydrogen from ethanol photoreforming.


Increasing the Activity and Selectivity of TiO2-Supported Au Catalysts

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