Engineering Stable Surface Oxygen Vacancies on ZrO2 by Hydrogen

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Energy, Environmental, and Catalysis Applications

Engineering Stable Surface Oxygen Vacancies on ZrO by Hydrogen-Etching Technology: An Efficient Support of Gold Catalysts for Water-Gas Shift Reaction 2

Li Song, Xuebo Cao, and Lei Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07007 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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ACS Applied Materials & Interfaces

Engineering Stable Surface Oxygen Vacancies on ZrO2 by Hydrogen-Etching Technology: An Efficient Support of Gold Catalysts for Water-Gas Shift Reaction Li Song, Xuebo Cao and Lei Li* College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China KEYWORDS: gold catalysts; ZrO2; oxygen vacancies; water-gas shift reaction; hot-electron flow

ABSTRACT: The surface structure of supports is crucial to fabricate efficient supported catalysts for water-gas shift (WGS). Here, hardly-reducible ZrO2 was etched with hydrogen (H), aiming to modify surface structures with sufficient stable oxygen vacancies. After deposition of gold species, the obtained khaki ZrO2-H notably improved WGS catalytic activities and stabilities in comparison to the traditional white ZrO2. The characterization results and quantitative analysis indicate that sufficient surface oxygen vacancies of ZrO2-H support give rise to more metallic Au0 species and higher microstrain, which all boost WGS catalytic activities. Furthermore, optoelectronic properties were successfully used to correlate with their

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WGS thermocatalytic activities, and then a modified electron flow process was proposed to understand WGS pathway. For one thing, the introduction of surface oxygen vacancies narrowed band gap of ZrO2 and decreased Ohmic barrier, which facilitated the flow of “hot-electron”. For another thing, the conduction band electrons can be easily trapped by oxygen vacancies of ZrO2 supports, and then these trapped electrons immediately take part in reduction of H2O to H2. Thus, the electron recombination was suppressed and the WGS catalytic activity was improved. It is worth to extend H2-etching technology to improve other thermocatalytic reactions.

INTRODUCTION Hydrogen are considering as promising clean, efficient and sustainable fuel to avoid environment pollution and greenhouse gas emissions. Nowadays, the major hydrogen is generated from water-gas shift reaction (WGS, CO + H2O ↔ CO2 + H2). Whereas, the commercial WGS catalysts (Fe-Cr, Cu-Zn-Al and Co-Mo) commonly demand long and redundant pretreatments. For this reason, there is an urgent requirement for high-performance WGS catalysts. Currently, the supported catalysts had been paid considerable attentions due to stronger synergistic effect among multiple components than single-component catalysts. For instance, both precious metals (Pt, Rh, Pd, Au, Ir, Ru, etc.)1-2 and non-precious metals (Cu and Ni)3 were supported on miscellaneous supports, such as TiO2,4-10 CeO2,11-13 ZrO2,14-19 Mo2C,20 FeOx,21,22 Al2O3,23 CeO2-TiO224 and CeO2-ZrO2.25-29 Among them, the supported gold catalysts usually present high activity and selectivity.1 Meanwhile, ZrO2 is regarded as a promising support because of its excellent thermal stability, low cost and tunable physicochemical properties (e.g., surface acidity/basicity and redox properties), which can be modified by using


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different synthesis methods and adding different dopants.17,18 Thus, Au-ZrO2 should be potential catalysts to deliver high WGS activities. To develop an efficient Au-ZrO2 catalysts, we have to firstly discuss what factors affect the WGS catalytic performance. No one is going to argue against the important effect of metalsupport interface on catalytic performance,30 manifesting that WGS activities depend on appropriate supports besides the supported metal species. For one thing, miscellaneous supports can influence the properties of active metal, such as morphology, size, charge state and strain, thus affecting their activities and stabilities.31-33 For another thing, supports are no longer spectators, but they also directly take part in WGS reaction by dissociating H2O molecules into H2 on the surface oxygen vacancies of CeO2 and TiO2.7,34-36 As far as ZrO2 based catalysts are concerned, some similar conclusions were also reported in the literatures. For examples, the difference in ZrO2 crystal phases (tetragonal and monoclinic) induced the distinct performance of catalytic reactions,19 e.g., WGS reaction.16 Moreover, gold nanoparticles supported on the reduced CeOx and CeO2-ZrO2 supports show higher CO adsorption coefficient in comparison to their oxidized counterparts, and it is also suggested that electron transfer from the reduced supports to gold nanoparticles with the change in the chemical state of gold nanoparticles.37,38 As stated above, efficient ZrO2 based WGS catalysts require ZrO2 with abundant surface oxygen vacancies. On the one hand, oxygen vacancies of ZrO2 can be created by doping metals (e.g., Y, La, Mg, Mn, Fe).39-41 However, owing to insufficient dopants (e.g., maximum dopant concentration of nearly 8 mol % Y2O3),39 oxygen vacancies is usually not enough. Not only that, the dopants also complicate catalysts system and puzzle us concerning on the real catalytic mechanism. On the other hand, oxygen vacancies can be formed during the reduction of ZrO2 supports. However, it will be more challenging to reduce ZrO2, because ZrO2 is commonly


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viewed as stable hardly-reducible oxide despite in pure H2 at high temperature and pressure (> 900 °C, 120 kPa or even > 1500 °C, 22 atm).42,43 It was reported that ZrO2 became much more reducible when its particle size is decreased to nanoscale.44 It was also found that heating in H2O promoted surface reduction of ZrO2 because H2O dissociated into OH groups and the unstable hydroxide was formed.42 Accordingly, to create abundant oxygen vacancies at rather low temperature and pressure, we are trying to reduce ZrO2 with small crystal size and sufficient surface OH groups. Coincidentally, we recently reported that hardly-reducible TiO2 can be reduced by hydrogen-etching technology at 550 °C, forming sufficient surface oxygen vacancies as defect sites.45 Analogously, hardly-reducible ZrO2 should be worth trying to be reduced by hydrogen-etching at rather low temperature and pressure to engineer sufficient surface oxygen vacancies, which has been rarely reported for the enhancement of WGS catalytic performance. In addition, many attentions had been paid to metal-support interaction to improve catalytic performance.31 Recently, the “hot-electron flow” located at metal-oxide interface was reported to determine the catalytic activity of exothermic chemical reactions (e.g., CO oxidation).46,47 WGS is a common exothermic reaction, thus it is valuable to lunch a deeper understand on the electron flow process in WGS based on interface effect of support’s oxygen vacancies. Here, based on non-annealed ZrO2 with small crystal size and sufficient surface OH groups, hydrogen-etching at rather low temperature and pressure (550 °C, 1 atm) was successfully carried out to reduce hardly-reducible ZrO2 support. The engineered sufficient stable surface oxygen vacancies obviously improved the WGS catalytic performances over gold catalysts. A modified electron flow process was used to understand WGS pathway. Hereinto, surface oxygen vacancies narrowed band gap of ZrO2 and decreased Ohmic barrier, which facilitated the flow of “hot-electron”. Furthermore, the conduction band electrons can be easily trapped by oxygen


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vacancies located on the surface of ZrO2 supports, and then these trapped electrons immediately take part in reduction of H2O to H2. Thus, the electron recombination was suppressed and the WGS catalytic activity was enhanced. EXPERIMENTAL SECTION Preparation of ZrO2 supports and Au-ZrO2 catalysts Hydrothermal method was used to synthesize ZrO2 precursors. The ZrOCl2·8H2O aqueous solution (60 mL, 0.4 mol/L) was stirred in a Teflon-lined autoclave (100 mL of capacity) for 10 min. The hydrothermal reaction was performed in an oven at 150 °C for 6 h. The resultant product was washed six times with deionized water and dried at 70 °C for 12 h. The obtained samples were named as non-annealed ZrO2, which were further annealed at 550 °C for 4 h in static air (-A), hydrogen (-H) and air followed by hydrogen (-A-H), respectively. The obtained samples are denoted as ZrO2-A, ZrO2-H and ZrO2-A-H, respectively. A typical deposition-precipitation method was performed to prepare Au-ZrO2 catalysts with Au loading of 4 wt. %. The experiment details follow our previous work.45 In brief, the assynthesized ZrO2 were used as supports, HAuCl4 and NH3⋅H2O solution were used as the gold source and precipitant, respectively. After washing and drying the precipitates, the obtained powders were directly used as catalyst without any pretreatment. Similar to the denoted ZrO2 supports, these catalysts were denoted as Au-ZrO2-A, Au-ZrO2-A-H, and Au-ZrO2-H, respectively. The ZrO2 supports and Au-ZrO2 catalysts were characterized by a series of technologies. The detailed descriptions have been shown in Supporting Information.


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Evaluation of Catalytic Performance The evaluation conditions and calculations of catalytic performance are presented in the Supporting Information. Briefly, 0.15 g of catalysts was used to measure catalytic activities and stabilities. The dried feed gas consists of 12.5% CO and 87.5% N2. After the activity measurement of Au-ZrO2-H, the sample was kept under pure N2 for cooling and denoted as AuZrO2-H-used. RESULTS AND DISCUSSION Catalytic activities


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Figure 1. (a) Photographs of ZrO2 support annealed in different atmosphere; (b) CO conversions of Au-ZrO2 catalysts for activity evaluation; (c) Arrhenius-type plots of TOFs over Au-ZrO2 catalysts (reaction temperature: 160-210 °C); (d) CO conversions at 350 °C for stability evaluation. As presented in Figure 1a, when ZrO2 precursor was annealed in air, traditional white ZrO2-A support was obtained. Interestingly, after annealing at 550 °C under high-purity H2 atmosphere, the white ZrO2 precursor and ZrO2-A support transformed into khaki ZrO2-H and light khaki ZrO2-A-H support, respectively. To our best knowledge, these unusual khaki ZrO2 supports have been rarely reported. It means that H2 atmosphere resulted in colored ZrO2 supports compared with air atmosphere. More impressively, as presented in Figure 1b, their CO conversions can be ranked as: AuZrO2-H >> Au-ZrO2-A-H > Au-ZrO2-A, especially at 200 °C. The results indicate that the annealing under hydrogen atmosphere of ZrO2 support has significantly boost WGS catalytic activities of the corresponding Au-ZrO2 catalyst. Thus, hydrogen atmosphere resulted in not only colored ZrO2 supports but also enhanced catalytic activities. For instance, the CO conversion of Au-ZrO2-H noticeably increased by 128% (from 33.2% to 75.7% at 200 °C) compared with AuZrO2-A. Furthermore, methane as the most probable byproduct was not detected for all catalysts, suggesting that the selectivity of H2 is 100%. The realistic differences in catalytic activities were further investigated by measuring and comparing TOFs. As presented in Figure 1c, Au-ZrO2-H catalyst presents the highest TOF and the smallest activation energy (Ea), followed by Au-ZrO2-A-H, and Au-ZrO2-A shows the lowest TOF and the biggest Ea. In detail, the TOF of Au-ZrO2-H (0.46 s-1 at 210 °C, Table S1) is more


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than twofold that of Au-ZrO2-A (0.21 s-1 at 210 °C). There is also a certain decrease in Ea from 48.4±1.3 kJ/mol (Au-ZrO2-A) to 39.5±0.9kJ/mol (Au-ZrO2-H). Furthermore, our Au-ZrO2-H catalyst presented higher TOF and smaller Ea values compared with other Au-ZrO2 catalysts presented in literatures (Table S2). Even the activity of our Au-ZrO2-H catalyst still comes top in comparison to other candidate catalysts. Figure 1d presents the WGS catalytic stabilities of Au-ZrO2 catalysts at 350 °C. It is found that Au-ZrO2-H shows a certain decrease in CO conversion (for 50 h, decreased by 36.7% from 91.8% to 58.1%). Nevertheless, the stability of Au-ZrO2-H is notably higher than those of Au-ZrO2-A (for 32 h, decrease by 71% from 70.3% to 20.3%) and Au-ZrO2-A-H catalyst (for 50 h, decrease by 52.3% from 84% to 40.1%). Meanwhile, the stability of our Au-ZrO2-H catalyst is higher than those of the communicated Au/CeZrO4 catalysts.26,27 Structural properties


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Figure 2. (a, b) XRD patterns, (c) crystal size and microstrain of various ZrO2 supports and AuZrO2 catalysts. SEM images of ZrO2 supports: (d) ZrO2-H; (e) ZrO2-A-H and (f) ZrO2-A. As shown in Figure 2a, all samples only present diffraction peaks of monoclinic ZrO2 (JCPDS file no. 07-0343). After deposition of Au on the ZrO2 supports, there are no changes in all XRD patterns of ZrO2. There are no observable Au species (Figure 2b), suggesting the high dispersion of Au particles. This observation is consistent with the TEM images in Figure 3. As presented in Figure 2d-f and Figure 3a-c, it was found that the disklike ZrO2 supports with some sieve pores were obtained. As shown in Figure S12, various ZrO2 show double hysteresis loops and two kinds of pore sizes that have most probable distributions of 6 nm and 30 nm, respectively. Thus, a kind of double-mesoporous ZrO2 was synthesized. By comparison, morphology and pore size distribution were scarcely influenced by H2 atmosphere. Also, as listed in Table S5, catalytic activities don't have the one-to-one corresponding relations with their SBET. But hydrogen indeed influences their colorations and activities. To seek this reasons, some characterizations were further carried out to investigate the differences in their structural and optoelectronic properties.


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Figure 3. TEM images of ZrO2 supports: ZrO2-H (a, d); ZrO2-A-H (b, e) and ZrO2-A (c, f). TEM images of Au-ZrO2 catalysts: Au-ZrO2-H (g, j); Au-ZrO2-A-H (h, k) and Au-ZrO2-A(i, l). The dash line means the boundary between crystalline core and disorder layer. The insets are Fast Fourier transform (FFT) images of square region (dot line). The lattice spaces are calculated


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from FFT. Distributions of Au cluster diameters (dTEM) in Au-ZrO2-H (m), Au-ZrO2-A-H (n) and Au-ZrO2-A (o). From Figure 3g-i, all the Au-ZrO2 catalysts present excellent dispersion of Au particles. The average sizes (about 2 nm) of uniform Au particles (narrow range of 1~3 nm) are very similar to each other (Figure 3m-o). It was suggested that the different catalytic activities should be due to the distinct ZrO2 supports instead of size and dispersion of Au particles. As presented in Figure 3d and 3e, the surfaces of ZrO2-H and ZrO2-A-H nanocrystals became disordered, forming the disorder layers about 2-6 nm and 2.5 nm in thickness, respectively. Nevertheless, there are no obvious disorder layers on the surface of ZrO2-A (Figure 3f). After deposition of gold species, Au-ZrO2-H and Au-ZrO2-A-H retained the microstructure composed of disorder layer and crystalline core (Figure 3j and 3k), despite the fact that their disorder layers have decreased to around 1-4 nm and 1-2 nm in thickness, respectively. No obvious disorder layer was observed in Au-ZrO2-A (Figure 3l). H2 atmosphere leads to the similar disorder layer as reduced black TiO2,45,48 so the annealing under H2 atmosphere can be termed as the hydrogen-etching. In addition, as shown in Figure 3d-f, the lattice spaces of ZrO2 (100) gradually decrease in the turn of ZrO2-A (0.507 nm), ZrO2-A-H (0.489 nm) and ZrO2-H (0.479 nm). Meanwhile, as presented in Figure 3j-l, the lattice spaces of ZrO2 (100) also gradually decrease according to the sequence: Au-ZrO2-A (0.509 nm) > Au-ZrO2-A-H (0.502 nm) > Au-ZrO2-H (0.482 nm). Thus, hydrogen-etching has given rise to the decrease in the lattice spaces, which presents the same trend as their catalytic activities and stabilities. In addition, from the H2-TPR results (Figure S2), their big reduction peaks occur at 450-600 °C, which coincides well with hydrogen-etching temperature (550 °C). And their peak areas can be ranked as: ZrO2-A > ZrO2-A-H > ZrO2-H, suggesting that the oxygen species gradually decreased according to the above mentioned


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sequence. Therefore, H2 reduction should be responsible for the disorder layer because of oxygen species evacuation (i.e., surface hydroxy).49 To further investigate the effects of hydrogen-etching technology on the structures of ZrO2 supports and Au-ZrO2 catalysts, Rietveld analysis of XRD results were performed to calculate the crystal size, microstrain value and cell parameters, as presented in Figure 2c, Figure S1 and Table S3. In the sequence of ZrO2-A (or Au-ZrO2-A), ZrO2-A-H (or Au-ZrO2-A-H) and ZrO2-H (or Au-ZrO2-H), the crystal sizes of ZrO2 in ZrO2 supports and Au-ZrO2 catalysts gradually decreased from around 15 nm to 11 nm, and there were also the outstanding increases (from about 0.41 to 0.66) in microstrain (∆d/d) of ZrO2. As stated in above paragraph, hydrogenetching has resulted in the decrease in lattice space of ZrO2 (100) according to the same sequence. Hence, the contraction of lattice spaces led to lattice distortion, embodying as the increase in microstrain. Our previous studies communicated that larger microstrain gave rise to stronger metal-support interactions and higher surface energy, as well as higher catalytic activities.45,50,51 Therefore, H2 reduction led to surface lattice distortion (e.g., higher microstrain, smaller lattice spaces and more surface disorder layer) of ZrO2-H support, stronger interaction between Au and ZrO2-H and higher surface energy, so as to improve catalytic activities of Au-ZrO2-H.


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Figure 4. EPR spectra of ZrO2 (a) and Au-ZrO2 (b). EPR spectra were measured to further confirm oxygen vacancies. As shown in Figure 4, ZrO2H and Au-ZrO2-H led to an obvious EPR peak at g-value of 2.003, which was ascribed to single electron trapped at surface oxygen vacancy with the formation of F-centers.49,52 However, there are no peaks in ZrO2-A and Au-ZrO2-A. Thus, hydrogen-etching created more surface oxygen vacancies on ZrO2-H and Au-ZrO2-H. Raman spectroscopy is usually sensitive to the local disorder and lattice defects. Impressively, hydrogen-etching leads to remarkable alteration of Raman spectra, as shown in Figure S3. The ZrO2-A shows one set of typical Raman peaks of monoclinic ZrO2.53 After hydrogen-etching, the peaks of ZrO2-A-H and ZrO2-H become more and more broad and weak. The results support that partial Zr-O lattices have been rearranged and become disordered,53 indicating huge oxygen vacancies have been formed during hydrogenetching process. There is similar evolution of Raman spectra in corresponding Au-ZrO2 catalysts. Hereinto, ZrO2-H and Au-ZrO2-H show extremely weak peaks. Since the supports were immersed in water during the deposition process of gold species, the surface oxygen vacancies


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show higher stability. Besides, the stability of ZrO2-H was also evidenced by the unchanged khaki coloration in air over several months.

Figure 5. XPS spectra of ZrO2 and Au-ZrO2: (a) O 1s spectra of ZrO2, (b) O 1s spectra of AuZrO2, (c) Au 4f spectra of Au-ZrO2; TOF at 190 °C and CO conversion at 300 °C vs. concentration of surface OH species (%) (d) and Au0 species (%) (e). As shown in Figure S4a and S4b, there are identical elements in all XPS spectra, suggesting that other impurities were not introduced by H2 atmosphere. Thus, the khaki coloration of ZrO2H cannot be attributed to the existence of other impurities. For Zr 3d spectra (Figure S5a-f), according to the sequence of ZrO2-A (or Au-ZrO2-A), ZrO2-A-H (or Au-ZrO2-A-H) and ZrO2-H (or Au-ZrO2-H), their binding energies of Zr 3d gradually downshift from 181.9 eV (or 181.8 eV) to 181.5 eV (or 181.4 eV), suggesting slight increase in electron density of Zr atoms and existence of some Zr3+ ions in ZrO2-H and Au-ZrO2-H. In order to confirm the ratio of Zr3+ ions, the Zr 3d5/2 spectra can be resolved into Zr4+ and Zr3+ spectra at about 181.8 eV and 181.4 eV. As presented in Figure S5, in the turn of ZrO2-A (or Au-ZrO2-A), ZrO2-A-H (or Au-ZrO2-A-H)


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and ZrO2-H (or Au-ZrO2-H), the ratio of the fitted Zr3+ ions gradually increased from 12.7% (or 13.7%) to 30.2% (or 29.5%). Furthermore, owing to the charge balance (2 Zr4+ + O2- = 2 Zr3+ + VӦ (oxygen vacancies)), Zr3+ ions can be also confirmed by the existence of oxygen vacancies (insufficient surface OH species). The existence of surface oxygen vacancies and decrease in surface OH species had been proven by the obvious EPR signal (Figure 4), Raman spectra (Figure S3) and O 1s spectra (Figure 5a-b), respectively. Thus, it is reasonable that the ratio of Zr3+ ions has a good linear correlation with the concentration of surface OH species, as presented in Figure S5i. From the fitted O 1s spectra, oxygen species consist of O atoms bonded with Zr atoms (about 529.7 eV) and surface OH species (531.3 eV). As illustrated in Figure 5a and 5b, in the turn of ZrO2-A (or Au-ZrO2-A), ZrO2-A-H (or Au-ZrO2-A-H) and ZrO2-H (or Au-ZrO2-H), the areas of surface OH species gradually decreased from 43.6% (or 44.1%) to 23.1% (or 23.9%). As stated in H2-TPR results (Figure S2), H2 reduction resulted in the evacuation of surface oxygen species, suggesting the generation of surface oxygen vacancies. For Au 4f spectra (Figure 5c), there are both Au0 and Au3+ species in all Au-ZrO2 catalysts. Interestingly, the ratio of Au3+ species gradually decrease according to the sequence: Au-ZrO2-A (37.2%) > Au-ZrO2-A-H (24.1%) > Au-ZrO2-H (9.3%). Thus, Au-ZrO2-H mainly presents metallic Au0 species. After Au(OH)3 species were deposited on ZrO2-H with sufficient oxygen vacancies, some electrons located on oxygen vacancies transferred into Au(OH)3 species to form metallic Au0 particles.37,38,54 Because surface oxygen vacancies of ZrO2 supports remain the trend: ZrO2-A < ZrO2-A-H < ZrO2-H, the amount of metallic Au0 species of Au-ZrO2 catalysts gradually increases following the same above sequence. Toward a high WGS activity and CO oxidation, the metallic Au0 has been deemed to be a requisite,6,7,55 thus Au-ZrO2-H present


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higher catalytic activities. Those results demonstrate that the different catalytic activities are dominantly influenced by the amount of metallic Au0, which depends on the amount of support’s surface oxygen vacancies. As shown in Figure 5d-e and Figure S5g-h, the effect of some factors on the catalytic activities were quantitatively analysed. Impressively, TOF at 190 °C and CO conversion at 300 °C correlates almost in a linear fashion with the concentration of surface OH species (Figure 5d), the ratios of Zr3+ ions in ZrO2 (Figure S5g) and Au-ZrO2 (Figure S5h) and the concentration of Au0 species (Figure 5e), respectively. From above structural studies, hydrogen-etching indeed brought about sufficient surface oxygen vacancies, forming surface disorder layer (Figure S6). Similar to the reported black TiO2 nanocrystals,45,48 the surface disorder layer should be responsible for the khaki coloration of ZrO2-H. For the reduction of hardly-reducible ZrO2 at rather low temperature and pressure, the interpretation can be supported by XRD and FT-IR spectrum of non-annealed ZrO2 (Figure S7). As shown in Figure S7a, the non-annealed ZrO2 shows quite lower crystallinity (i.e., smaller crystal size) than the annealed ZrO2-A. Meanwhile, the non-annealed ZrO2 shows much more surface OH groups or physically absorbed H2O molecules (absorbed around at 3400 cm-1 in Figure S7b) than the annealed ZrO2-A. As reported in the literatures,42,44 the reduction of ZrO2 can be improved by small crystal size and abundant sufficient surface OH groups or physically absorbed H2O molecules. Thus, these properties make the reduction of hardly-reducible ZrO2 come true at rather low temperature and pressure (550 °C, 1 atm). Optoelectronic properties


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Figure 6. The optical absorption edges of ZrO2 (a) and Au-ZrO2 (b); XPS valence band spectra of ZrO2 (c) and Au-ZrO2 (d). Based on UV-visible absorption spectra (Figure S8), the band gap energy (Eg) values were calculated. As shown in Figure 6a, Eg value of ZrO2 supports can be ranked as: ZrO2-A (4.63 eV) > ZrO2-A-H (4.29 eV) > ZrO2-H (3.92 eV). Meanwhile, Eg value of Au-ZrO2 catalysts gradually decrease according to the sequence: Au-ZrO2-A (3.17 eV) > Au-ZrO2-A-H (2.89 eV) > Au-ZrO2-H (2.37 eV). It had been communicated that the band gap narrowing can be attributed to the oxygen vacancies and structural disorder in the lattice.41,56 Therefore, in our case, the narrowed Eg should be associated with the surface oxygen vacancies and disorder layer on the hydrogen-etched ZrO2 supports.


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The valence band spectra measurements of ZrO2 and Au-ZrO2 were further-performed. As presented in Figure 6c-d, the valence band maximum (VBM) of ZrO2 supports are almost identical (2.68 eV below the Fermi energy), while Au-ZrO2 catalysts present smaller VBM, which are 1.59 eV, 1.32 eV and 1.78 eV for Au-ZrO2-A, Au-ZrO2-A-H and Au-ZrO2-H, respectively. Accordingly, the conduction band minimum (CBM) of ZrO2 supports and Au-ZrO2 catalysts were calculated and listed in Table S4. Their CBMs gradually locate lower according to the sequence: Au-ZrO2-A (-1.58 eV above the Fermi energy) > Au-ZrO2-A-H (-1.3 eV) > AuZrO2-H (-0.59 eV).

Figure 7. Photoluminescent spectra of ZrO2 (a) and Au-ZrO2 (b). Photoluminescence (PL) emissions were used to investigate the recombination of electronhole pair. As illustrated in Figure 7, the PL spectra of all ZrO2 and Au-ZrO2 have been excited by UV-light at 300nm. There are similar emission spectra in all samples. Impressively, in the turn of ZrO2-A (or Au-ZrO2-A), ZrO2-A-H (or Au-ZrO2-A-H) and ZrO2-H (or Au-ZrO2-H), their PL peak intensities gradually decreased. Hereinto, Au-ZrO2-H presents the lowest PL peak intensity, indicating that Au-ZrO2-H has the lowest recombination rate of electrons and holes.


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Figure 8. Mott-Schottky plots of ZrO2 supports (a) and Au-ZrO2 (b). TOF at 190 °C and CO conversion at 300 °C vs. electron density (Nd) in ZrO2 supports (c) and Au-ZrO2 catalysts (d). Electrochemical impedances were measured to investigate their electronic properties influenced by hydrogen-etching. The positive slope can be seen in all Mott-Schottky plots (Figure 8), suggesting all samples are n-type semiconductors. Basing on the slopes of MottSchottky, electron densities were estimated by equation as follows57 Nd = 2(εe0ε0)-1[dV/d(1/C2)] where Nd is the electron density, ε is the dielectric constant of ZrO2 (ε = 28),58 e0 is the electron charge, ε0 is the permittivity of vacuum (8.85E-12 F/m), and V is the applied bias at the electrode. According to the sequence of ZrO2-A (or Au-ZrO2-A), ZrO2-A-H (or Au-ZrO2-A-H) and ZrO2-H (or Au-ZrO2-H), the slopes of Mott-Schottky plot gradually decreased. Their calculated electron


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densities gradually increased according to the sequence: 2.5 × 1022 cm-3 (or 1.7 × 1023 cm-3) < 9.6 × 1022 cm-3 (or 7.0 × 1023 cm-3) < 3.4 × 1023 cm-3 (4.3 × 1024 cm-3). The improved electron densities should be derived from the increase in famous electron donors (i.e., oxygen vacancies).57 Hence, the charge separation and transportation at the semiconductor/metal interface of Au-ZrO2-H were accelerated,57 consistent with the lowest PL intensity of Au-ZrO2-H (Figure 7). In this sense, the enhanced WGS catalytic activities are attributed to the efficient charge separation and transportation. The inference is supported by the phenomena that TOF at 190 °C and CO conversion at 300 °C shows a positive linear correlation with electron density (Nd) in ZrO2 (Figure 8c) and Au-ZrO2 (Figure 8d). Proposed electron flow process Lastly, a modified electron flow process contributes to understand WGS reaction pathway based on their optoelectronic properties. For the traditional WGS reaction pathway, supported gold catalysts are bifunctional as follows:54,57 CO adsorbs on the Au particles, H2O adsorbs and dissociates on the oxide and/or metal-oxide interfaces. Sastre and co-workers recently communicated a kind of Au-TiO2 photocatalysts for WGS reaction and propounded a photocatalytic WGS reaction pathway:59 the electrons (-) of Au particles will be excited by visible light irradiation and jump to TiO2, improving H2 generation from H2O reduction, while the holes (+) remained on Au particles oxidized adsorbed CO. Moreover, the “hot-electron flow” formed on metal-oxide interfaces was reported to determine the catalytic activity of exothermic chemical reactions (e.g., CO oxidation).46,47 Therefore, according to optoelectronic properties, we proposed a modified electron flow process based on traditional and photocatalytic WGS reaction mechanism to understand WGS pathway, as presented in Figure 9.


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Surface

Bulk

(a)

Φs=5.05 eV

Φm=4.8 eV

ECB=-0.59 eV

EOB=0.59 eV

E0

H 2O EVö=-0.42 eV

Ef

CO Eg=2.37 eV

H2

2.2 eV

CO2 EVB=1.78 eV

Au

ZrO2-H Bulk

(b)

Surface

E0

Φs=5.05 eV

Φm=4.8 eV

ECB=-1.58 eV H O 2 EOB=1.58 eV Ef

CO Eg=3.17 eV CO2

H2

Energy loss EVB=1.59 eV

Au

ZrO2-A

Figure 9. Proposed electron flow process of Au-ZrO2-H (a) and Au-ZrO2-A (b) for the WGS reaction. Φs: work function of semiconductor; Φm: work function of metal; VÖ: oxygen vacancy; E0: vacuum level; Ef: Fermi level; EOB: Ohmic barrier, EVB: valence band; ECB: conduction band.

On the one hand, sufficient oxygen vacancies of ZrO2-H resulted in a high proportion of metallic Au0 species in Au-ZrO2-H (XPS results). As presented in Figure 9a, because metal Au (Φm = 4.8 eV) has a smaller work function compared with semiconductor ZrO2 (Φs = 5.05 eV),60 the electrons flow from the metallic Au to ZrO2 support.61 This kind of “hot-electron flow”46,47,62


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or “chemicurrent”63,64 commonly generated in catalytic reactions,46,47 or nonthermal processes.62 For instance, electron transfer also occurs in atomic or molecular gas chemisorption processes.63,64 In our case, the heat of CO adsorption on coordinately unsaturated Au sites will reach to 52.3-76.6 kJ/mol.,65,66 Besides, the electrons can also be excited by absorption heat (QAu = 4.4 kJ/mol at 200 °C) and/or WGS reaction heat (molar reaction enthalpy at 200 °C: -41.7 kJ/mol). Accordingly, energetic electrons will receive a strong energy (≥ 98.4 kJ/mol ≈ 1.02 eV) that is competent for electron flow from gold to ZrO2-H. As a consequence, the ZrO2-H carries negative charge (-) and gold remains positive charge (+). It was reported that the midlly oxidized Au-(OH)x species (Auδ+) were viewed as active gold species.5,67,68 What’s more, it was communicated that the electrons of metallic Au0 transfer to the near carriers and the corresponding Au+ were formed under the WGS feed gas, while the positive Au+ can return to metallic Au0 under an inert gas.68 Accordingly, in our case, the as-synthesized Au-ZrO2-H mainly presents metallic Au0 species, but positive Au+ should be formed during the reaction process due to the hot-electron flow from the metals into the supports. Then, the positive Au+ species will oxidize adsorbed CO reactant (as electronic donors) to CO2 product. On the other hand, after forming the same Fermi level,46,61 energy distance between metallic Au0 and ECB of ZrO2 support was named as Ohmic barrier (EOB). As above stated, the obtained energy (98.4 kJ/mol ≈ 1.02 eV) is bigger than Ohmic barrier (EOB = 0.59 eV, Figure 9a), suggesting that the electrons can overcome the Ohmic barrier and move to the conduction band (CB) of ZrO2-H with formation of hot-electron flow. Based on the calculated data of Figure 6 and Table S4, Au-ZrO2-H has smaller Ohmic barrier (EOB = 0.59 eV) compared with Au-ZrO2-A (EOB = 1.58 eV, Figure 9b). Accordingly, Au-ZrO2-H generate a stronger ‘‘hot-electron flow’’ than Au-ZrO2-A. It was reported that the energy level of defect (i.e., oxygen vacancies) states


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~2.2 eV above valence band (VB) in monoclinic ZrO2.40 In this case, the energy level of oxygen vacancies locate at about -0.42 eV (2.2 - 1.78 = 0.42 eV, Figure 9a), which is lower than CB of ZrO2-H. As a result, the ‘‘hot-electrons’’ of CB can easily move to the oxygen vacancies because oxygen vacancies are known as electron traps.57 Accordingly, the surface oxygen vacancies of ZrO2-H and Au-ZrO2-H suppress the electron recombination from CB to VB. As a result, it is reasonable that PL spectra of ZrO2-H and Au-ZrO2-H are weaker than those of ZrO2-A and AuZrO2-A (Figure 7). The sufficient ‘‘hot-electrons’’ reserved in oxygen vacancies immediately take part in reduction of H2O reactant (as electronic acceptors) to H2 product. In short, owing to more oxygen vacancies and lower Ohmic barrier, Au-ZrO2-H shows more efficient flow of ‘‘hotelectron’’, which is reflected in higher electron density in Mott-Schottky plots (Figure 8). Thus, Au-ZrO2-H presents higher WGS catalytic activity. However, Au-ZrO2-A has bigger Ohmic barrier (Figure 9b) because of larger forbidden band gap (Figure 6), forming a weaker “hotelectron flow”. Moreover, owing to scare surface oxygen vacancies, these “hot-electrons” cannot immediately move to oxygen vacancies for dissociation of H2O. It is terrible that some electrons prefer to transfer from CB to VB and lose energy (Figure 9b). Thus, Au-ZrO2-A catalyst has a poor activity. To sum up, a lower Ohmic barrier (i.e., narrowed Eg) and sufficient surface oxygen vacancies (i.e., more efficient flow of ‘‘hot-electron’’) lead to a stronger “hot-electron flow” of Au-ZrO2-H. In a sense, stronger “hot-electron flow” is reflected in higher electron density in Mott-Schottky plots (Figure 8). Impressively, TOF at 190 °C and CO conversion at 300 °C shows a positive linear correlation with electron density (Nd) in ZrO2 (Figure 8c) and Au-ZrO2 (Figure 8d). Therefore, we inferred that higher TOF and smaller Ea of Au-ZrO2-H (Figure 1c) mainly stem from stronger “hot-electron flow”.47,62


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In terms of catalyst’s deactivation, for one thing, the deactivation can be ascribed to the decrease of oxygen vacancies due to smaller EPR intensity of Au-ZrO2-H-used than Au-ZrO2-H (Figure S9). For another thing, the aggregation of Au particles should be responsible for the deactivation, which is reflected in stronger diffraction peak of Au particles in Figure S10 and bigger Au particles (4~6 nm) in Figure S11. Furthermore, Au-ZrO2-H shows the highest stability (Figure 1d). The interpretation is that the aggregation of Au particles was hampered because Au particles were anchored on the surface with sufficient oxygen vacancies.69 Thus, besides catalytic activity, the stability can be also improved by sufficient surface oxygen vacancies. Clearly, although a modified electron flow process contributes to understand WGS reaction pathway, more detailed studies of intermediate productions and elementary steps are necessary to further investigate the real WGS reaction mechanism. Anyhow, it is also valuable to lunch a deeper understand on the electron flow process in WGS based on interface effect of support’s oxygen vacancies. CONCLUSIONS The unusual khaki ZrO2 with surface disorder layer and sufficient stable oxygen vacancies were fabricated by H2-etching method. The H2-etched ZrO2-H has been viewed as an excellent support of Au catalysts, because of the improvement of both WGS catalytic activities and stabilities. At first, for structural properties, sufficient stable oxygen vacancies were indeed obtained by hydrogen reduction of surface oxygen species and led to more metallic Au0 species and higher microstrain, which are responsible for higher WGS activities. Secondly, from optoelectronic properties, sufficient oxygen vacancies narrow the band gap with the enhancement of electron density and decrease of electron recombination. The last discussion


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contributes to understand a modified electron flow process in WGS reaction. The narrowed band gap decreases the Ohmic barrier and improves the flow of “hot-electron”. The conduction band electrons can be easily trapped by surface oxygen vacancies of ZrO2 supports, and then these trapped electrons immediately take part in reduction of H2O to H2, thus suppressing the electron recombination from conduction band to valence band. Thus, the WGS catalytic activity was improved. Therefore, in consideration of fascinated interface effect of support’s oxygen vacancies and deeper understand on the electron flow process in WGS, the presented conclusions are fundamental and valuable. ASSOCIATED CONTENT Supporting Information Characterization methods; physical properties, TOFs and comparison of WGS rates of AuZrO2 catalysts; microstructure parameters and XRD Rietveld analysis of ZrO2 and Au-ZrO2; H2TPR profiles of various ZrO2 supports; Raman spectra, XPS survey spectra, C 1s spectra, Zr 3d spectra and Energy bands of various ZrO2 supports and Au-ZrO2 catalysts; activities vs. ratio of Zr3+ in ZrO2 and Au-ZrO2; concentration of surface OH species vs. ratio of Zr3+ in ZrO2 and AuZrO2; Schematic structure of hydrogen-etched ZrO2; XRD and FT-IR spectrum of non-annealed ZrO2; UV-visible absorption spectra of ZrO2 and Au-ZrO2; EPR spectra and XRD of Au-ZrO2H-used catalysts; Adsorption-desorption isotherms, pore size distribution and SBET of various ZrO2 supports and Au-ZrO2 catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Authors *Email: [email protected] ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (21503092 and 21673102) and Zhejiang Provincial Natural Science Foundation of China (LY19B030008 and LQ18B030006). REFERENCES 1.

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SYNOPSIS (TOC)


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Engineering Stable Surface Oxygen Vacancies on ZrO2 by Hydrogen

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