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Article

Enhanced Hydrogen Production from Ethanol Photoreforming by Site-Specific Deposition of Au on Cu2O/TiO2 p-n Junction

by
Lan Luo
1,†,
Tingting Zhang
1,†,
Xin Zhang
1,
Rongping Yun
1,
Yanjun Lin
1,
Bing Zhang
2 and
Xu Xiang
1,*
1
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 15 Beisanhuan Donglu, Beijing 100029, China
2
School of Chemical Engineering, Zhengzhou University, 100 Science Avenue, Zhengzhou 450001, China
*
Author to whom correspondence should be addressed.
†
These authors contribute equally.
Catalysts 2020, 10(5), 539; https://doi.org/10.3390/catal10050539
Submission received: 15 April 2020 / Revised: 6 May 2020 / Accepted: 8 May 2020 / Published: 13 May 2020
(This article belongs to the Special Issue Towards Green, Enhanced Photocatalysts for Hydrogen Evolution)
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Abstract

:
Hydrogen production by photoreforming of biomass-derived ethanol is a renewable way of obtaining clean fuel. We developed a site-specific deposition strategy to construct supported Au catalysts by rationally constructing Ti3+ defects inTiO2 nanorods and Cu2O-TiO2 p-n junction across the interface of two components. The Au nanoparticles (~2.5 nm) were selectively anchored onto either TiO2 nanorods (Au@TiO2/Cu2O) or Cu2O nanocubes (Au@Cu2O/TiO2) or both TiO2 and Cu2O (Au@TiO2/Cu2O@Au) with the same Au loading. The electronic structure of supported Au species was changed by forming Au@TiO2 interface due to the adjacent Ti3+ defects and the associated oxygen vacancies while unchanged in Au@Cu2O/TiO2 catalyst. The p-n junction of TiO2/Cu2O promoted charge separation and transfer across the junction. During ethanol photoreforming, Au@TiO2/Cu2O catalyst possessing both the Au@TiO2 interface and the p-n junction showed the highest H2 production rate of 8548 μmol gcat−1 h−1 under simulated solar light, apparently superior to both Au@TiO2 and Au@Cu2O/TiO2 catalyst. The acetaldehyde was produced in liquid phase at an almost stoichiometric rate, and C−C cleavage of ethanol molecules to form CH4 or CO2 was greatly inhibited. Extensive spectroscopic results support the claim that Au adjacent to surface Ti3+ defects could be active sites for H2 production and p-n junction of TiO2/Cu2O facilitates photo-generated charge transfer and further dehydrogenation of ethanol to acetaldehyde during the photoreforming.

Graphical Abstract

1. Introduction

Hydrogen is extensively used in various industrial processes, e.g., in the petrochemical industry, metallurgy, fine chemical engineering, etc. [1]. Hydrogen, as a clean and renewable fuel, has aroused tremendous attention from both academic and industrial perspectives in the past decades because its energy-extraction process produces only water as a byproduct and emits no greenhouse gases, e.g., CO2 or any pollutants [2,3]. At present, industrial production of hydrogen depends predominantly on steam reforming of CH4, an energy-consuming process accompanied by CO2 emissions. Therefore, it is highly desirable to develop efficient, economical and energy-neutral processes for sustainable H2 production.
Ethanol could be produced in a sustainable way from huge-amount, low-grade biomass, e.g., lignocelluloses and agriculture waste besides from the conventional petrochemical route [4]. In addition, over several tens of million tons biomass-derived ethanol (bio-ethanol) is produced per year. Therefore, ethanol reforming provides an alternative and promising means of H2 production from abundant and low-value biomass resources [5,6]. Photoreforming could be one of the most promising potential reforming ways because H2 production is activated on catalysts by sunlight and is achieved conveniently at room temperature and ambient pressure [7,8,9,10]. Another advantage of ethanol photoreforming is that hydrogen in the gas phase and acetaldehyde in the liquid phase can be formed at a stoichiometric rate without CO2 emission. Acetaldehyde can be used directly in many situations, or can be further converted into other chemicals. Exploiting high-efficiency catalysts is of great importance for achieving high activity in ethanol photoreforming under mild conditions.
Metal-supported TiO2 photocatalysts have been extensively studied in alcohol photoreforming. Au-supported TiO2 catalysts have been tuned with respect to metal particle size, annealing conditions in different gas atmospheres, and the phase structure of TiO2 (anatase and rutile) [11]. The AuPd alloy was further studied for adjusting the electronic structure of metal component, resulting in enhanced activity in this reaction [12]. The films of metal/TiO2 supported on glass were tested under UV light irradiation of ethanol/water mixtures, revealing that Pt was slightly more active than Au under those conditions [13]. It is recognized that the longitudinal SPR mode of Au is the main channel for transferring the hot electrons from Au toTiO2 [14]. The selective deposition of TiO2 or other electron acceptors at the tip of Au nanorods (NRs) or at the edge of Au nanodisks (NDs) results in highly active plasmonic photocatalysts [15]. Yang and co-workers studied the gold nanodisks and TiO2 nanophases sandwich between zeolite nanosheets for hydrogen production by plasmonic photocatalytic reforming of methanol [16]. It has been reported that anatase TiO2 is more active than the rutile one, and the higher surface availability of the former for Au is beneficial to H2 production from ethanol under UV light irradiation. However, the significant amounts of other gaseous products (mostly CO, CO2 and CH4) were also released, probably by further photo-induced decomposition of ethanol [17].
It is highly desirable to explore alternative strategies of catalyst design for enhancing hydrogen production from bio-ethanol photoreforming and simultaneously inhibiting C–C cleavage of ethanol. Herein, we propose a strategy for constructing both p-n junctions between two kinds of semiconducting oxides and Au@TiO2 interface by site-specific Au deposition for enhancing hydrogen production from ethanol reforming. The results indicate that the interaction of Au and the adjacent Ti3+ defects on Au@TiO2/Cu2O catalyst affects the electronic structure of Au. In addition, the p-n junction between Cu2O and TiO2 facilitates the charge separation and transfer across the interface. The simultaneous construction of the p-n junction and the metal/oxide interface on the supported catalyst results in a record-level H2 production rate of 8548 μmol gcat−1 h−1 under simulated solar light and excellent recycling stability. Accompanied by hydrogen production, acetaldehyde at a stoichiometric rate is solely produced in the liquid phase, indicative of efficient inhibition of C–C cleavage during ethanol reforming.

2. Results and Discussion

The procedure for catalyst preparation is shown in Scheme 1. The experimental details are stated in Section 3 Materials and Methods.
High-resolution transmission-electron microscope (HRTEM) photographs are shown in Figure 1. Cu2O nanocubes with an average size of ~40 nm were grown on TiO2 nanorods with a lateral size of 80~120 nm. The close contact between them can be clearly observed and the lattice d-spacing of 0.243 nm and 0.31 nm is assigned to the plane (103) of anatase phase TiO2 [18] and (110) of cubic phase Cu2O [19], respectively (Figure 1a). The junction can be formed between two oxides having intimate contact [20]. The Au nanoparticles are site-specific deposited on either TiO2 or Cu2O or both, showing the same average size of 2.5 nm and high dispersion (Figure 1b–d). The Au loading is determined to be 0.85 wt% in the three catalysts Au@Cu2O/TiO2, Au@TiO2/Cu2O andAu@TiO2/Cu2O@Au by ICP analyses. The lattice distance of 0.235 nm corresponds to the plane (111) of cubic phase Au [21]. The Au nanoparticles display a well-defined spherical morphology and high crystallinity (Figure 1b–d). The size and loading of Au nanoparticles change a little, whatever Au is deposited on any support. EDS elemental mappings show the homogeneous distribution of Ti and O in the nanorods and Cu and O in the nanocubes (Figure 1).
X-ray diffraction patterns of the samples indicate that TiO2 exist in the form of anatase phase (JCPDS No. 21-1272). The second phase in the samples with p-n junction can be indexed to cubic Cu2O (Figure S1). The diffraction related to Au phase is not observed in the Au-supported samples because of the low loading of Au (~0.85 wt%) and/or high dispersion of small-sized Au particles (~2.5 nm) on the surface of the support. X-ray photoelectron spectra (XPS) were used to analyze the chemical valence and electronic structure of each element. Two peaks were fitted at binding energy (B.E.) of 457.9 and 458.3 eV in the Ti 2p3/2 region of Cu2O/TiO2 and Au-supported catalysts, corresponding to Ti3+ and Ti4+ species, respectively (Figure 2A) [22,23,24]. The B.E. values hardly shift among these samples. However, the peak area ratio of Ti3+ to Ti4+ is lower when Au is deposited on TiO2 i.e., Au@TiO2/Cu2O and Au@TiO2/Cu2O@Au (0.96 and 1.03), compared to that of Au-free TiO2/Cu2O and Au deposition on Cu2O i.e., Au@Cu2O/TiO2 (1.17 and 1.11) (Table 1). This indicates that the site-specific deposition of Au differs at defect-rich TiO2 and Cu2O, leading to a variation to Ti3+ content when Au is deposited to TiO2. In our previous studies, the ratio of Ti3+:Ti4+ increased as the amount of NaBH4 increased. When the ratio reaches around 1, even when increasing the amount of NaBH4, the ratio will not increase any more, which suggests that it is approaching saturation of Ti3+:Ti4+ ratio [25].
The O 1s spectra were deconvoluted to three peaks at 530.0 eV, 532.0 eV and 533.5 eV (Table 2), respectively, which are assigned to lattice oxygen (OL), adsorbed oxygen adjacent to surface oxygen vacancy (OV) and surface chemisorbed or dissociated oxygen species (OC) (Figure 2B) [25]. The B.E. values of each oxygen species shift little among different samples. Nevertheless, the OV/OL ratio associated with the peak area is decreased when Au is deposited on Au@TiO2/Cu2O and Au@TiO2/Cu2O@Au (0.71 and 0.74) compared with that of TiO2/Cu2O (0.86). The OV/OL ratio in Au@Cu2O/TiO2 (0.81) is close to that of TiO2/Cu2O (0.86). This tendency is consistent with that of the Ti3+/Ti4+ ratio. The amount of oxygen vacancy (OV) decreases with the decreasing Ti3+ content because the Ti3+ defects are closely associated with OV in defect-rich TiO2 support [26].
Au 4f spectra of each Au-supported catalyst were shown and compared in Figure 2C. The main Au 4f peaks are located at 87.8 eV (4f 5/2) and 84.0 eV (4f 7/2). The fitted peak appears at B.E. of 84.0 eV in the Au 4f7/2 region of Au@Cu2O/TiO2, which is assigned to Au0 species [27]. In contrast, when Au is deposited on both TiO2 and Cu2O, the B.E. shifts to a lower value of 83.7 eV, with a negative shift of 0.3 eV (Table 3). The negative shift of B.E. hints at the increased electron density of Au0 species on Au@TiO2/Cu2O@Au compared to that on Au@Cu2O/TiO2. The B.E. continually shifts to a lower value of 83.4 eV when Au is only deposited on TiO2. The more negative shift (−0.6 eV) indicates higher electron density of Au because of strong metal-support interaction (SMSI) between Au nanoparticles and defect-rich TiO2 in Au@TiO2/Cu2O [28,29]. The negatively charged Au adjacent to Ti3+ sites associated with oxygen vacancies could be active sites for ethanol reforming [25]. The higher electron density of Au on Au@TiO2/Cu2O facilitates proton reduction to hydrogen during ethanol photoreforming.
XPS spectra were carried out to determine the chemical states of Cu species in the catalysts. Figure 3A shows the Cu 2p core level spectra. The binding energies at 932.1 eV and 951.9 eV, respectively, in Cu 2p3/2 and Cu 2p1/2 region were assigned to Cu+ species [30]. The binding energies hardly shift among these catalysts. There is no shake-up satellite peak in the Cu 2p spectra. The absence of the satellite peak excludes the existence of Cu2+ species [31,32]. However, the Cu+ and Cu0 species cannot be distinguished in Cu XPS spectra because the binding energies assigned to Cu+ and Cu0 are very close to each other. The difference in binding energies is merely around 0.1–0.2 eV [33,34]. Consequently, Cu LMM Auger spectra were analyzed to distinguish between Cu0 and Cu+ species (Figure 3B). The spectra were fitted to two peaks at 917.0 eV and 919.0 eV, respectively, which is assigned to Cu+ and Cu0 species [35]. The Cu+ is the dominating species on the basis of Auger spectra. The coexistence of Cu+ and Cu0 species in Cu2O nanocubes or nanoparticles is also observed in the previous literatures [36]. There is no shift in the kinetic energies regardless of Au deposition on either Cu2O or TiO2. This is an indication that the chemical states of Cu2O changes little in these catalysts.
The electron paramagnetic resonance (EPR) spectra are shown in Figure 4A. An intense EPR signal appears at a g value of 2.003 for Cu2O/TiO2. This signal is usually assigned to surface O-species, formed via the interaction of dioxygen and Ti3+ defects in TiO2 [24,37,38,39]. The EPR signal appears at the same g value for Au@Cu2O/TiO2, Au@TiO2/Cu2O@Au and Au@TiO2/Cu2O. The intensity of the signal in Au-supported catalysts is lower than that in un-supported Cu2O/TiO2. The intensity of O-related EPR signal (g = 2.003) could be associated with the Ti3+/Ti4+ ratio in TiO2. The lower signal intensity in Au-supported catalysts indicates the smaller Ti3+/Ti4+ ratio. The Au@TiO2/Cu2O has the smallest Ti3+/Ti4+ ratio of 0.96 among these samples, showing the lowest signal intensity of g value. This is consistent with the results presented in XPS Ti 2p analyses (Table 1).
The digital photos of the catalysts display that all are in black color (Figure 4B). The UV-Vis absorption spectra were compared in Figure 4C. Compared to the spectrum of TiO2, the Cu2O/TiO2 junction shows stronger absorption in the visible light region besides in the UV regime. The Au-supported catalysts possess more intensive absorption throughout the visible light wavelength. The absorption band (~560 nm) associated with Au surface plasmon resonance (SPR) is not clearly observed within the UV-vis absorption spectra of Au-supported catalysts because of the intense visible absorption of Cu2O/TiO2 itself [40]. The fluorescence emission spectra of TiO2 and Cu2O/TiO2 showed a broad band peaked at 423 nm under the excitation of 350 nm (Figure 4D). The latter has lower emission intensity, indicating that the carrier recombination is inhibited due to the existence of Cu2O/TiO2 junction. After Au is deposited on either TiO2 or Cu2O, the emission intensity of supported catalysts further decreases. It indicates that forming the interface of Au and semiconducting oxides facilitates the separation of photogenerated electrons and photogenerated holes by charge transfer across the metal-oxide interface [22,41].
The photocurrent tests were carried out to evaluate the charge separation within the catalysts. The photocurrent−time traces of photoelectrodes were obtained in the photoelectrochemical cell at a bias of 1.23 V vs. RHE under chopped AM 1.5G light illumination (Figure 5). The Au loading on Cu2O/TiO2 enhances the photocurrent compared to Cu2O/TiO2 itself, which indicates the improved charge separation across the metal/oxide interface. The Au@TiO2 electrode shows a higher photocurrent than Au@Cu2O/TiO2. The interface of Au and TiO2 is more favorable to charge separation than that of Au and Cu2O due to the lower conduction band potential of TiO2 than that of Cu2O [42]. In addition, the existence of Ti3+ defects associated with adjacent oxygen vacancies intensifies the interaction of Au and TiO2 [25]. The Au@TiO2/Cu2O electrode exhibits the highest photocurrent among all electrodes, 2.5 times higher than Au@TiO2. This hints at the contribution of TiO2/Cu2O p-n junction combined with the interface of Au and TiO2 to charge separation and transfer. The photocurrent of Au@TiO2/Cu2O is also higher than Au@TiO2/Cu2O/Au. This is an indication that the site-specific deposition of Au on TiO2 leads to charge separation more efficiently under light illumination than the random deposition of Au on both TiO2 and Cu2O.
The photoreforming of ethanol was achieved using pure ethanol in an Ar atmosphere (1.4 bar) under simulated solar irradiation. The H2 production rate is contrasted on each catalyst (Figure 6A). The rate of H2 production reaches 6932 μmol gcat−1 h−1 on Au@Cu2O/TiO2, 8.35 times higher than that on Cu2O/TiO2 (830 μmol gcat−1 h−1, Table 4). It is apparent that proton reduction is greatly accelerated on the Au-supported catalyst for H2 production. Additionaly, the charge separation is improved by loading Au on the semiconductor, evidenced by photocurrent measurements (Figure 5). The Au@TiO2 catalyst exhibits higher activity (7143 μmol gcat−1 h−1) towards H2 production than Au@Cu2O/TiO2. This suggests that constructing the interface of Au and TiO2 is more favorable than that of Au and Cu2O, which could direct the structural design of catalysts for ethanol photoreforming. The rate of H2 production on Au@TiO2/Cu2O (8548 μmol gcat−1 h−1) is apparently higher than that on either Au@TiO2 (7143 μmol gcat−1 h−1) or Au@TiO2/Cu2O@Au (7348 μmol gcat−1 h−1). Table S4 compares the H2 yield rates reported for TiO2-supported noble-metal catalysts with the result in this work. The p-n junction between TiO2 and Cu2O contributes the enhanced H2 production. However, the selective deposition of Au on defect-rich TiO2 is essential for higher activity because the Au-Ov-Ti3+ sites are more active in ethanol photoreforming [25]. The quantity of Au-Ov-Ti3+ sites is relatively less in randomly deposited Au@TiO2/Cu2O@Au than in site-specifically deposited Au@TiO2/Cu2O. As a consequence, the latter shows higher activity than the former. In addition, the lower H2 production rate of Au@Cu2O/TiO2may be due to the alloying of Au-Cu, leading to a smaller number of active sites on gold according to the previous report [43].
The products except H2 in the gas phase, e.g., CH4, CO, CO2, are below 0.3% on all catalysts (Table 4), which clearly indicates the efficient inhibition of C–C cleavage during the photoreforming. Also, high-purity H2 can be produced on these catalysts. For instance, the purity of H2 is beyond 99.9% on Au@TiO2/Cu2O catalyst. Acetaldehyde is the only detectable product in the liquid phase, evidenced by GC-MS analysis (Figure S2). The high selectivity of acetaldehyde is due to the absence of O2 activation to peroxide intermediates and hydroxyl species on the surface of Au NPs under Ar ambient [43,44]. In addition, acetaldehyde is produced in almost stoichiometric yield with H2, which is consistent with the reforming pathway of ethanol via a redox process [11]. The photo-generated holes in the valence band of semiconductor oxidize ethanol to acetaldehyde and the electrons on the active sites adjacent to Au reduce protons to H2 during the process.
The optimal Au@TiO2/Cu2O catalyst was recycled five times and the H2 yield was hardly decreased (Figure 6B). The structural stability of catalysts was further evidenced by HRTEM and Ti 2p, O 1s, Au 4f XPS and Cu LMM Auger analyses. The size of Au particles remains 2.5 nm, the same as that before the reactions (Figure S3). In addition, the Au loading after reaction is not changed by ICP measurement (0.84 wt%). The Ti3+/Ti4+ ratio and OV/OL ratio in the catalyst have no change before and after reactions (Figure S4, Tables S1 and S2). The peak in the Au 4f region appears at 83.5 eV and has little shift compared to that before reactions (Figure S4, Table S3). Additionally, the peaks in Cu 2p XPS and Cu LMM Auger spectra keep unchanged after photoreforming (Figure S5). These findings verify that the catalyst is structurally stable and keeps activity after repeated use.
The in situ FTIR spectra of ethanol adsorption on the catalysts were first collected in the dark after adsorption of ethanol for 40 min, and then recorded every hour during photoreforming. The spectra of the three supported catalysts showed intensive absorption bands at 1000~1200 cm−1, 1200~1500 cm−1, 2700~3100 cm−1, and 3600~3800 cm−1, which are respectively assigned to the vibrations of ν(C–O), δ(CHx), ν(CHx), and ν(O–H) of ethanol (Figure 7) [45].
The band at 1755 cm−1 gradually increases with the extension of irradiation time, which corresponds to the vibration related with acetaldehyde molecule [46]. This is consistent with the increased acetaldehyde production in liquid phase with the reaction. The band shifts little among the three catalysts. The band at around 2350 cm−1, assigned to the vibration of CO2 [47], does not appear in the FTIR spectra. This verifies that the C–C cleavage of ethanol is greatly inhibited on the three Au-supported catalysts during the photoreforming. Consequently, the dehydrogenation of ethanol to acetaldehyde is the dominating step, accompanied with high-purity H2 production on these catalysts.
The energy levels of semiconductors play a critical role in the activity and selectivity of ethanol photoreforming. In a catalyst with a p-n junction, the charge separation and transfer are strongly dependent on the relationships of energy levels [48]. The flat band potentials (Efb) of the TiO2 nanorods and Cu2O nanocubes were determined by electrochemical Mott-Schottky measurements at varied frequencies and room temperature. The Mott-Schottky plots of TiO2 exhibit positive slopes, indicative of their characteristic of n-type semiconductor (Figure 8A). The Efb values are estimated by extrapolating the linear portion of the plots measured at varied frequencies to the intercept of x-axis. It is generally considered that the bottom of the conduction band (CB) in n-type semiconductors is approximately equal to its flat band potential. [49,50] Consequently, the conduction band of TiO2 nanorod is −0.63 V vs. Ag/AgCl. This potential was converted to −0.02 V vs. RHE according to the following formula [51].
ERHE = EAg/AgCl + 0.197 + 0.059pH,
The bandgap of TiO2 nanorod was estimated to be 2.76 eV using the Kubelka−Munk equation, i.e., F(R) = (1 − R)2/2R, where R is the reflectance (Figure S6). This value is smaller than that reported for bulk anatase TiO2 of ~3.2 eV [52]. It is documented that theTi3+ defect-rich anatase TiO2 exhibits narrower bandgap owing to the involvement of defect energy levels [53]. The valence band TiO2 nanorod is calculated to be 2.74 eV from the bandgap and the conduction band level.
The Mott-Schottky plots of Cu2O show negative slope, an indication of p-type semiconductor (Figure 8B). The flat band potential of 0.65 V vs. Ag/AgCl was converted to 1.26 V vs. RHE according to Formula (1). It is usually considered that the bottom of valence band (VB) in p-type semiconductors was 0.30 V more positive than the flat band potential [54,55]. The valence band level of Cu2O is 1.56 V vs. RHE. Based on the bandgap of Cu2O (2.2 eV) [56,57], the conduction band (CB) level of Cu2O is estimated to be −0.64 V.
Based on the findings above, we proposed possible charge transfer pathways on the Au@TiO2/Cu2O catalyst during ethanol photoreforming. The p-n junction is formed when p-type Cu2O and n-typeTiO2 come into contact with each other (Scheme 2). The photo-generated holes are transferred from the valence band of TiO2 to that of Cu2O owing to the more positive valence band level of TiO2. The electrons generated on Cu2O under visible light irradiation are transferred to the conduction band of TiO2. In addition, the hot electrons of Au under visible light irradiation are transferred to the conduction band of TiO2 due to the SPR effect of Au. The strong interaction between Au and defect-rich TiO2 enhances the hot electron transfer from Au to the interface of Au/TiO2. As a consequence, the interface of Au and TiO2 is rich in the electrons, leading to high electron density on the Au-Ov-Ti3+ interfacial sites. The proton reduction is promoted on the electron-rich interfacial sites, resulting in higher H2 production rate than the Au@Cu2O/TiO2 catalyst without the interfacial sites. Consequently, the dual effects of Au-Ov-Ti3+ active sites and p-n junction endow Au@TiO2/Cu2O catalyst with higher activity towards ethanol photoreforming by the site-specific deposition of Au on defect-rich TiO2 nanorods.

3. Materials and Methods

3.1. Materials

C2H5OH (anhydrous ethanol), CH3OH (methanol), CuSO4 (anhydrous copper sulfate), PVP (polyvinyl pyrrolidone), TBOT (tetrabutyltitanate), HNO3 (nitric acid), NaBH4 (sodium borohydride), AA (ascorbic acid), HAuCl4·3H2O, EDTA (Ethylenediamine) were obtained from Shanghai Aladdin Biochemical Technology Co. Ltd. (Beijing, China). H2 (99.9%), N2 (99.9%) and Ar (99.9%) were supplied by Beijing Haipu Gas Co. Ltd. (Beijing, China). All reagents were of analytical grade and used as received. Deionized water was used throughout the experiments.

3.2. Synthesis of Defect-Rich TiO2 Nanorods

The defect-rich TiO2 nanorods were synthesized by a NaBH4 reduction method according to a previous literature [22]. In a typical procedure, the mixture (TiO2/NaBH4 = 2:1 molar ratio) was continuously stirred for 30 min and treated at 350 °C for 1 h in a flow of N2. The sample was washed with diluted HCl solution and water thoroughly. The sample was named as TiO2.

3.3. Synthesis of Cu2O Nanocubes

The Cu2O nanocubes were prepared by a chemical precipitation method, modified from a literature’s one [58]. In a typical run, 155 mg of CuSO4 and 400 mg of PVP were dissolved in 160 mL of deionized water to obtain a light blue solution. The pH was adjusted to 10 by dropwise addition of NaOH (1 M). An amount of 176 mg of ascorbic acid (AA) was dispersed in the solution to reduce Cu2+ to Cu2O. The suspension was stirred for 20 min. Subsequently, the suspension was centrifuged and washed thoroughly with deionized water and ethanol to remove PVP. The product was collected after the color of solution changed from light blue to brownish yellow. The solid product was finally dried under vacuum at 60 °C.

3.4. Synthesis of Cu2O/TiO2

2 g of TiO2 nanorod sample was added into the CuSO4 and PVP solution, and the pH of the solution was adjusted to 10 with an aqueous solution of NaOH (1 M). Subsequently, AA was added to the solution and stirred for 20 min. The solid product was collected by centrifugation and washed with ethanol and water. The product was finally dried under vacuum at 60 °C.

3.5. Synthesis of Au@TiO2 and Au@TiO2/Cu2O

2 g of TiO2 nanorod sample was dispersed in 200 mL of H2O containing 2 mL of HAuCl4 solution (0.051 M). The pH of suspension was adjusted to 10 with an aqueous solution of NaOH (1 M). 10 mL of methanol was added and stirred for 3 h. The Au@TiO2 sample was obtained by a photo-reduction method under UV irradiation. After the photo-reduction, the precipitation was centrifuged, washed with water thoroughly to neutral and dried in vacuum at 60 °C. The Au@TiO2 product was suspended in 160 mL of aqueous solution containing 155 mg of CuSO4 and 400 mg of PVP. The pH was adjusted to 10 with an aqueous solution of NaOH (1 M). 176 mg of AA was added and stirred for 20 min. The solid product was collected by centrifugation and washed with by ethanol and water and finally dried under vacuum at 60 °C. The product was denoted as Au@TiO2/Cu2O.

3.6. Synthesis of Au@Cu2O/TiO2

The as-synthesized Cu2O and PVP was suspended in 100 mL of H2O. 2 mL of HAuCl4 solution (0.051 M) was added and the pH of suspension was adjusted to 10 with an aqueous solution of NaOH (1 M). 10 mL of methanol was added and stirred for 3 h. The Au@Cu2O sample was obtained by a photo-reduction method under visible light irradiation. The as-synthesized TiO2 nanorod was suspended in the solution and stirred for 1 h. The solid product was collected by centrifugation and washed with by ethanol and water and finally dried under vacuum at 60 °C. The product was denoted as Au@Cu2O/TiO2.

3.7. Synthesis of Au@TiO2/Cu2O@Au

2 g Cu2O/TiO2 sample was suspended in 200 mL of H2O containing 2 mL of HAuCl4 solution (0.051 M). The pH of suspension was adjusted to 10 with an aqueous solution of NaOH (1 M). 10 mL of methanol was added and stirred for 3 h. The Au@TiO2/Cu2O@Au sample was obtained by a photo-reduction method under UV-visible light irradiation. The solid product was collected by centrifugation and washed with ethanol and water and finally dried under vacuum at 60 °C.

3.8. Characterization

Powder X-ray diffraction (XRD) patterns of the samples were obtained by a Shimadzu XRD-6000 diffractormeter using graphite-filtered CuKα radiation (40 kV, 30 mA, λ = 0.15418 nm) in a 2theta range of 10−70°.
Transmission electron microscopy (TEM) measurements were carried out on a JEOL JEM-3010 high-resolution transmission electron microscope.
Elemental analysis was performed on a Shimadzu ICPS-7500 inductively coupled plasma atomic emission spectrometer (ICP-AES).
The XPS spectra were recorded on a Thermo VG ESCALAB MK II X-ray photoelectron spectrometer at a pressure of 2 × 10−9 Pa using Al Kα X-ray as the excitation source (1486.6 eV). The positions of all binding energies were calibrated using theC1s line at 284.6 eV.
UV-visible diffuse reflectance spectra were performed on a Shimadzu UV-3000 spectrometer equipped with an integrating sphere attachment with BaSO4 (10 mg) as reference.
Photoluminescence (PL) emission spectra were recorded by a Hitachi F-7000 spectrofluorometer using laser excitation at a wavelength of 350 nm.
Electron paramagnetic resonance (EPR) measurements were performed on a Bruker E500 spectrometer with a 9.53 GHz X-band. The sample mass was 50 mg. The spectra were recorded in the magnet field range of 318–328 mT.
The liquid phase products were determined by GC-MS measurements using a Thermo Fisher ISQ Trace1300 and the sample volume was 1 μL.
Electrochemical impedance spectroscopy (EIS) was performed on an electrochemical workstation at an open-circuit voltage of 0.3 V vs. RHE under illumination with 10 mV amplitude of perturbation and a frequency between 1.0 kHz, 1.5 kHz and 2.0 kHz. Mott-Schottky plots were measured at room temperature in the dark.
In situ Fourier Transform Infrared Spectroscopy (FTIR) was conducted in an in situ reaction cell on a Bruker Tensor II spectrometer installed with MCT narrow-band detector. The sample was pretreated in a flow of high-purity N2 at 100 °C for 1 h. After an initial scan as the background spectrum, ethanol was induced into the cell through a flow of N2 for 30 min. After flowing N2 to remove the residual ethanol vapor, the FTIR spectra were collected in the range of 4000~950 cm−1 at room temperature.

3.9. Photoreforming of Ethanol

In a typical run, 25 mg of catalyst was added to 25 mL of ethanol and ultrasonically dispersed for 15 min. The suspension was transferred into a high-pressure stainless steel reactor (volume: 50 mL) equipped with a sapphire crystal window. A flow of Ar was purged into the reactor for 30 min and the reactor was evacuated. The Ar was re-purged and the pressure in the reactor was maintained at 1.4 bar. The suspension was irradiated by a 300 W Xenon lamp equipped with an AM 1.5G filter (100 mW·cm−2) under magnetic stirring for 6 h. After the reaction, the catalyst was removed from the solution by filtration. The gaseous products were detected by an online gas chromatograph (GC-2014C, Shimadzu, Japan) equipped with a high-sensitivity thermal conductivity detector (TCD) and Ar was used as the carrier gas. The liquid products were analyzed by a gas chromatograph (GC-2014C, Shimadzu) equipped with a flame ionization detector (FID). The reaction rate of H2 followed the formula below:
Reaction   rate   of   production = n production ( μ mol ) m cat . ( g )   ×   time   ( h )

3.10. Recycled Use

The catalyst was separated from the solution after the reaction, washed with deionized water, and finally dried under vacuum at 60 °C for 6 h. The dried catalyst was reused in a next catalysis run under the same reaction conditions.

4. Conclusions

The TiO2/Cu2O-supported Au catalysts were delicately constructed by the site-specific deposition of Au on either defect-rich TiO2 nanorods or Cu2O nanocubes. The selective anchoring of Au nanoparticles on TiO2 nanorods combined with the p-n junction of TiO2/Cu2O leads to the highest activity towards ethanol photoreforming. The H2 production rate reaches a record level of 8548 μmol gcat−1 h−1 under simulated solar light. The acetaldehyde in liquid phase was generated at an almost stoichiometric rate, which indicates the effective inhibition of C−C cleavage of ethanol to CH4 or CO2. Extensive spectroscopic studies verified that Au species adjacent to Ti3+ defects and the associated oxygen vacancies on TiO2 nanorods activate the proton reduction to H2. The p-n junction between TiO2 and Cu2O facilitates charge separation and transfer owing to the matching of energy levels, which accelerates photo-generated hole transfer and the dehydrogenation of ethanol to acetaldehyde on the junction. This site-specific deposition strategy could be applicable in the enhancement in other biomass hydrogen production by rationally designing the delicate structure of catalysts and maximizing the catalytic capability.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/5/539/s1, Figure S1: XRD patterns; Figure S2: GC-MS spectra; Figure S3: HRTEM photographs; Figure S4: Ti 2p, O 1s and Au 4f XPS spectra; Figure S5: Cu 2p XPS and Cu LMM Auger spectra; Figure S6: Bandgap estimation; Table S1: Ti 2p XPS analyses; Table S2: O 1s XPS analyses; Table S3: Au 4f XPS analyses; Table S4: Comparisons of photocatalysts for ethanol photoreforming.

Author Contributions

Conceptualization, X.X.; methodology, T.Z. and X.Z.; formal analysis, L.L., T.Z., X.X.; investigation, X.Z., L.L.; data curation, L.L., T.Z., R.Y.; writing—original draft preparation, L.L., T.Z., X.X.; writing—review and editing, T.Z., R.Y., Y.L., B.Z., X.X.; supervision, X.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant 21978021, 21521005), the National Key R&D Program of China (grant 2017YFA0206804), and the Fundamental Research Funds for the Central Universities.

Conflicts of Interest

The authors declare no competing financial interest.

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Catalysts 10 00539 sch001 550
Scheme 1. The procedure for catalyst preparation by site-specific deposition of Au on the support.
Scheme 1. The procedure for catalyst preparation by site-specific deposition of Au on the support.
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Catalysts 10 00539 g001 550
Figure 1. HRTEM images and EDS mapping of (a) Cu2O/TiO2 (b) Au@Cu2O/TiO2, (c) Au@TiO2/Cu2O, (d) Au@TiO2/Cu2O@Au, with histogram of Au NPs size distribution of each sample (100 particles are counted).
Figure 1. HRTEM images and EDS mapping of (a) Cu2O/TiO2 (b) Au@Cu2O/TiO2, (c) Au@TiO2/Cu2O, (d) Au@TiO2/Cu2O@Au, with histogram of Au NPs size distribution of each sample (100 particles are counted).
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Figure 2. XPS spectra of (A) Ti 2p, (B) O 1s: (a) Cu2O/TiO2, (b) Au@TiO2/Cu2O, (c) Au@TiO2/Cu2O@Au, (d) Au@Cu2O/TiO2, and (C) Au 4f core level spectra: (a) Au@TiO2/Cu2O, (b) Au@TiO2/Cu2O@Au, (c) Au@Cu2O/TiO2.
Figure 2. XPS spectra of (A) Ti 2p, (B) O 1s: (a) Cu2O/TiO2, (b) Au@TiO2/Cu2O, (c) Au@TiO2/Cu2O@Au, (d) Au@Cu2O/TiO2, and (C) Au 4f core level spectra: (a) Au@TiO2/Cu2O, (b) Au@TiO2/Cu2O@Au, (c) Au@Cu2O/TiO2.
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Catalysts 10 00539 g003 550
Figure 3. (A) Cu 2p core level spectra: (a) Cu2O/TiO2, (b) Au@TiO2/Cu2O, (c) Au@Cu2O/TiO2, (d) Au@TiO2/Cu2O@Au, and (B) Cu LMM Auger spectra: (a) Cu2O/TiO2, (b) Au@TiO2/Cu2O, (c) Au@Cu2O/TiO2, (d) Au@TiO2/Cu2O@Au.
Figure 3. (A) Cu 2p core level spectra: (a) Cu2O/TiO2, (b) Au@TiO2/Cu2O, (c) Au@Cu2O/TiO2, (d) Au@TiO2/Cu2O@Au, and (B) Cu LMM Auger spectra: (a) Cu2O/TiO2, (b) Au@TiO2/Cu2O, (c) Au@Cu2O/TiO2, (d) Au@TiO2/Cu2O@Au.
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Figure 4. The EPR spectra (A) and digital photos (B) of Cu2O/TiO2, Au@Cu2O/TiO2, Au@TiO2/Cu2O@Au, and Au@TiO2/Cu2O; UV-Vis absorption spectra (C) and Fluorescence emission spectra (D) of TiO2, Cu2O/TiO2, Au@Cu2O/TiO2, Au@TiO2/Cu2O@Au, and Au@TiO2/Cu2O. The excitation wavelength is 350 nm.
Figure 4. The EPR spectra (A) and digital photos (B) of Cu2O/TiO2, Au@Cu2O/TiO2, Au@TiO2/Cu2O@Au, and Au@TiO2/Cu2O; UV-Vis absorption spectra (C) and Fluorescence emission spectra (D) of TiO2, Cu2O/TiO2, Au@Cu2O/TiO2, Au@TiO2/Cu2O@Au, and Au@TiO2/Cu2O. The excitation wavelength is 350 nm.
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Figure 5. Photocurrent-time traces of photoelectrodes at a bias of 1.23 V vs. RHE under chopped AM 1.5G illumination (100 mW·cm−2), back illumination. Solution: 0.1 M phosphate buffer (pH 7).
Figure 5. Photocurrent-time traces of photoelectrodes at a bias of 1.23 V vs. RHE under chopped AM 1.5G illumination (100 mW·cm−2), back illumination. Solution: 0.1 M phosphate buffer (pH 7).
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Catalysts 10 00539 g006 550
Figure 6. (A) H2 and acetaldehyde production rate via the photoreforming of ethanol under simulated solar light (AM 1.5G). (B) The cycle tests of Au@TiO2/Cu2O for photocatalytic hydrogen evolution. Conditions: suspension of the photocatalyst (1 g/L) in ethanol under stirring were irradiated with simulated solar light at Ar atmosphere (1.4 bar) for 6 h.
Figure 6. (A) H2 and acetaldehyde production rate via the photoreforming of ethanol under simulated solar light (AM 1.5G). (B) The cycle tests of Au@TiO2/Cu2O for photocatalytic hydrogen evolution. Conditions: suspension of the photocatalyst (1 g/L) in ethanol under stirring were irradiated with simulated solar light at Ar atmosphere (1.4 bar) for 6 h.
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Figure 7. In situ FTIR spectra of ethanol adsorption on the catalysts during the photoreforming: (A) Au@Cu2O/TiO2, (B) Au@TiO2/Cu2O, (C) Au@TiO2/Cu2O@Au.
Figure 7. In situ FTIR spectra of ethanol adsorption on the catalysts during the photoreforming: (A) Au@Cu2O/TiO2, (B) Au@TiO2/Cu2O, (C) Au@TiO2/Cu2O@Au.
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Figure 8. Mott-Schoottky plots of (A) TiO2 nanorods and (B) Cu2O nanocubes measured at varied frequencies and room temperature. Solution: 0.1 M phosphate buffer (pH 7).
Figure 8. Mott-Schoottky plots of (A) TiO2 nanorods and (B) Cu2O nanocubes measured at varied frequencies and room temperature. Solution: 0.1 M phosphate buffer (pH 7).
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Scheme 2. Diagram of energy levels and charge transfer on Au@TiO2/Cu2O catalyst.
Scheme 2. Diagram of energy levels and charge transfer on Au@TiO2/Cu2O catalyst.
Catalysts 10 00539 sch002
Table
Table 1. Ti 2p XPS analyses of the samples.
Table 1. Ti 2p XPS analyses of the samples.
SampleB.E. in 2p3/2(eV)B.E. in 2p1/2(eV)Ti3+/Ti4+ Ratio 1
Ti3+Ti4+Ti3+Ti4+
TiO2/Cu2O457.9458.3463.4464.31.17/1
Au@TiO2/Cu2O457.9458.3463.3464.20.96/1
Au@TiO2/Cu2O@Au457.9458.3463.4464.31.03/1
Au@Cu2O/TiO2457.8458.2463.4464.21.11/1
1 The value refers to the ratio of the respective integral peak area.
Table
Table 2. O 1s XPS analyses of the samples.
Table 2. O 1s XPS analyses of the samples.
SampleB.E. (eV)OV/OL Ratio 1
OLOVOC
TiO2/Cu2O530.0532.0533.50.86
Au@TiO2/Cu2O530.0532.0533.50.71
Au@TiO2/Cu2O@Au530.0532.0533.50.74
Au@Cu2O/TiO2530.0532.0533.50.81
1 The value refers to the ratio of the respective integral peak area.
Table
Table 3. Au 4f XPS analyses of the samples.
Table 3. Au 4f XPS analyses of the samples.
SampleB.E.(eV)△B.E.(eV) 1
Au@TiO2/Cu2O83.4−0.6
Au@TiO2/Cu2O@Au83.7−0.3
Au@Cu2O/TiO284.0-
1 The value refers to the shift compared to that of Au@Cu2O/TiO2.
Table
Table 4. H2 production via photoreforming of ethanol on the catalysts under simulated solar light.
Table 4. H2 production via photoreforming of ethanol on the catalysts under simulated solar light.
CatalystsProduction Rate (μmol gcat−1 h−1) 1
Gas PhaseLiquid Phase
H2CH4COCO2CH3CHOCH3COOH
Au@TiO2/Cu2O85485-38806-
Au@Cu2O/TiO2693212397239-
Au@TiO2/Cu2O@Au734813-77561-
Au@TiO271437-77356-
Cu2O/TiO2830---1020-
TiO2136---273-
1 Suspension of the photocatalyst (1 g/L) in ethanol under stirring were irradiated with simulated solar light (100 mW cm−2) at Ar atmosphere (1.4 bar) at 25 °C for 6 h.

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MDPI and ACS Style

Luo, L.; Zhang, T.; Zhang, X.; Yun, R.; Lin, Y.; Zhang, B.; Xiang, X. Enhanced Hydrogen Production from Ethanol Photoreforming by Site-Specific Deposition of Au on Cu2O/TiO2 p-n Junction. Catalysts 2020, 10, 539. https://doi.org/10.3390/catal10050539

AMA Style

Luo L, Zhang T, Zhang X, Yun R, Lin Y, Zhang B, Xiang X. Enhanced Hydrogen Production from Ethanol Photoreforming by Site-Specific Deposition of Au on Cu2O/TiO2 p-n Junction. Catalysts. 2020; 10(5):539. https://doi.org/10.3390/catal10050539

Chicago/Turabian Style

Luo, Lan, Tingting Zhang, Xin Zhang, Rongping Yun, Yanjun Lin, Bing Zhang, and Xu Xiang. 2020. "Enhanced Hydrogen Production from Ethanol Photoreforming by Site-Specific Deposition of Au on Cu2O/TiO2 p-n Junction" Catalysts 10, no. 5: 539. https://doi.org/10.3390/catal10050539

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