Defective Mesocrystal ZnO-Supported Gold Catalysts: Facilitating CO

Jun 11, 2018 - Guo, Mei, Yuan, Wang, Liu, Yan, and Zhang. 0 (0), pp 6203–6215 ... Nie, Jiang, Wang, Luo, Janik, Chen, Guo, and Song. 2018 8 (6), pp ...
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Defective Mesocrystal ZnO-Supported Gold Catalysts: Facilitating CO Oxidation via Vacancy Defects in ZnO Ming-Han Liu, Yun-Wen Chen, Tien-Sung Lin, and Chung-Yuan Mou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01282 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Catalysis

Defective Mesocrystal ZnO-Supported Gold Catalysts: Facilitating CO Oxidation via Vacancy Defects in ZnO Ming-Han Liu,ξ, ‡ Yun-Wen Chen,⊥, ‡ Tien-Sung Linξ and Chung-Yuan Mouξ, ∗ ξ

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

⊥ ‡

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan

Equal Contribution

ABSTRACT: We present a strategy to prepare a highly active Au/ZnO catalyst for CO oxidation by introducing abundant Zn- and O-vacancy defects into a ZnO support of mesocrystal form. Two different ZnO supports were chosen for comparison; nearly defect-free ZnO nanorods (NR-ZnO) and twin-brush-like ZnO mesocrystals (TB-ZnO ) with rich Zn/O-vacancy defects gave Au/NR-ZnO and Au/TB-ZnO upon deposition of gold nanoparticles. The catalytic test of CO oxidation over Au/TB-ZnO catalyst showed an enhanced catalytic activity of 153 times higher than the activity of Au/NR-ZnO. The dramatic enhancement in CO oxidation is attributed to a room temperature Mars–van Krevelen (MvK) mechanism on the surface of the Au/TB-ZnO catalyst which was promoted by extensive vacancy defects in TB-ZnO. To elucidate the increase in activity, the vacancy ratio (i.e., [Vo•]/[VZn•]) of TB-ZnO was systematically modulated by adjusting calcination conditions. The defective ZnO support altered the tendency in the variation of size, valence state and activity of gold correlated to an increased vacancy ratio. Combining experimental results and theoretical modeling, it is concluded that the higher vacancy ratio [Vo•]/[VZn•] in support endows defective ZnO with accommodation of plenty of “Au-O-AuZn” linkages (AuZn denotes Au-substitution at a zinc site.) around gold nanoparticles. The O-atom extraction from “Au-O-AuZn” linkages formed by gold doping in ZnO lattice is energetically more favorable than typical “Au-O-Zn” linkages at the perimeter of gold, facilitating CO oxidation via MvK mechanism. Systematic manipulation of defects density in the support provides a method in improving catalytic properties of supported gold catalysts. KEYWORDS: CO Oxidation, ZnO, Mesocrystal, Defect, Gold, Catalysis

Oxide-supported gold nanoparticles show outstanding catalytic properties in many oxidation and energy-related conversion reactions at low temperatures.1-4 Active gold catalysts owe their excellent activity to the small size and specific metal-support interaction.5-8 The catalytic properties of supported gold can be tuned by controlling particle size9, 10 and valence state of surface gold11-14 through influence of the support. Theoretical studies and experimental evaluations all indicated that the lattice oxygen atoms at the boundary between a metal and an oxide are highly active in oxidation reaction.15 Understanding the interaction of gold and support therefore is a key topic in the development of highly effective supported gold catalysts. Supports have been found to have a prominent role in pinning particles, affecting their sizes.16 Zinc oxide is known to possess extensive vacancy defects17 which may affect its interactions with supported nanoparticles. Previously, we demonstrated a strong-metal-support -interaction (SMSI) in ZnO nanorod supported gold catalyst, termed Au/NR-ZnO, giving insights into the tuning of the

catalytic activity for CO oxidation under different atmosphere pretreatments.1 The activity of Au/NR-ZnO was shown to increase when the electrons transferred from gold clusters to the oxygen vacancies (Vo) of NR-ZnO upon oxygen atmosphere pretreatment, leading to a positively charged gold surface for promoting CO activation. Accordingly, it is envisaged that the activity of gold catalyst might be affected by tuning the amount of defects in supports.16, 18-20 Up to now, very few reports quantitatively discussed the influence of support defects for gold catalyzed oxidation reactions. The significant roles of defects in metal-support interaction in its catalysis still need to be understood. However, standard crystalline ZnO, including NR-ZnO, does not contain substantial amount of defects inside the bulk ZnO. We turn to a special form of mesocrystalline ZnO, containing rich amount of defects, to study their effects in supported Au catalyst. Mesocrystal superstructures are assemblies of defect-rich nanocrystals. They are formed from the lattice oriented

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ACS Catalysis a)

b)

a)

1

At 77 K:

1

V•O

0.1

0.1

V•Zn Spin Count. (x1014)

5x10

6

Spin Count. (x1014)

At 77 K: TB-ZnO NR-ZnO

b)

c)

d)

c) Intensity (a. u.)

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N.D.

3400

3500

Magnetic field (Gauss)

NR-ZnO

TB-ZnO

Type of ZnO

Figure 1.SEM characterization and EPR analysis of ZnO supports. a) and b) show ZnO supports in the forms of nanorods (i.e., NR-ZnO) and twin-brush mesocrystals (i.e., TB-ZnO), respectively. c) Weight-normalized EPR spectra measured at 77K show the integrated peak area of TB-ZnO sample is about 18.5 times larger than that of NR-ZnO. The signal observed at g =1.96 (red bar) is assigned to singly charged oxygen vacancies Vo•, and g=2.002 (blue bar) is assigned to zinc vacancies VZn•. d) Spin counts per mg sample measured from Fig. 1c in comparison with DPPH the peak area of spin-count standard (diphenylpicrylhydrazyl). The spin counts were drawn in logarithm scale.

attachment of nanoparticles. The properties that make mesocrystals viable for catalytic applications are their rich endowment of defects, mesoporosity and single-crystal nature. Recently, we reported a new type of metal-support interaction mediated through rich vacancy defects in mesocrystals of ZnO for a gold catalyst (Au/TB-ZnO).21 We found gold atoms could interact strongly with the mesocrystal of TB-ZnO to the extent where gold atoms replaced some Zn sites in the ZnO wurtzite lattice, forming a mixed crystal. Surprisingly, gold atoms can easily diffuse in and out of the ZnO because of the large amount of zinc vacancies in the mesocrystal ZnO. ZnO mesocrystal thus behaved like a sponge to accommodate the large number of gold atoms. These gold atoms eventually moved out of the ZnO lattice to form highly dispersed Au nanoparticles of around 2 nm, after moderate thermal treatment. The Au NPs were deposited from below the surface (not from above!). DFT calculation showed the Au-doping and its migration in TB-ZnO are energetically favorable. The resulting Au/ZnO nanocatalyst showed excellent catalytic activity in CO oxidation compared to literature values of Au/TiO2.21 In order to get a better understanding of the catalytic oxidation reaction with this new class of defect-mediated Au/ZnO catalyst, we carry out the following defect dependent studies. We compare the catalytic oxidation of CO over ZnO supported gold catalysts with and without rich Zn vacancy defects. The

Figure 2. TEM images of supported gold catalysts prior to CO oxidation evaluation. a) and b) are the catalyst images of pretreated Au/NR-ZnO and Au/TB-ZnO catalysts, respectively. The respective size distributions are 4.1±1.1 nm (Fig. a) and 4.1±1.0 nm (Fig. b). c) CO conversion of Au/NR-ZnO (blue line) and Au/TB-ZnO (wine line) as a function of temperature from -40 o C to 180 oC.

detailed studies would allow us to uncover the effects of defects in the support on the catalytic oxidation of CO. Herein, we used two types of ZnO as supports to prepare gold catalysts possessing drastically different amount of defects. The two supports are nanorod ZnO (denoted as NR-ZnO, Figure 1a) and twin-brush-like ZnO mesocrystals (denoted as TB-ZnO, Figure 1b). The TB-ZnO particle (1.2 µm in size) shows assembled bundles of small nanorods at both halves of the twin-brush form of particles. Both materials possess rod-like morphology with (1010) nonpolar surfaces of ZnO being the main exposed facets. We choose this plane because it is known the nonpolar surface gives stronger metal-support interaction than of polar plane of ZnO.1 NR-ZnO nanorods were prepared via room temperature solution synthesis with the assistance of ethylenediamine as the capping agent. The nanorods have an average diameter of 89.2±19.6 nm with several hundreds of nanometers to few microns in length (Figure 1a). On the other hand, the twin-brush TB-ZnO mesocrystals were synthesized hydrothermally using gum Arabic as a structure-directing agent according to our previous report (Figure 1b).22 The mesocrystal TB-ZnO was formed via orientated attachment of nanocrystallites. A lot of inter-particle mesopores contributing in TB-ZnO result in a surface area of ~20.2 m2/g, which is five times higher than that of NR-ZnO (Figure S1). Electron paramagnetic resonance (EPR), a sensitive technique in detecting species with unpaired electrons, was used to investigate structural defects, i.e., atomic vacancies, in our specimens. Three types of oxygen vacancies, including Vo0, Vo• and Vo••, could co-exist in the ZnO structure. However, only singly charged oxygen vacancies (Vo•, g=1.96, red bar in

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Table 1. Catalytic activities (20 oC) of ZnO-supported gold catalysts

a)

b) -3

124 oC

2

Entry Catalyst 1 2

Rate × 10 Weight GHSV [mg] [mL/(gcat⋅h)] [molco/(gAu⋅h)]

4.5 61.1 wt%_Au/NR-ZnO 4.1 11.1 wt%_Au/TB-ZnO

2.9×104

1.91

TCD signal

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135 oC 77 oC

4.9×105

293

The feed gaseous mixture of CO (1 vol%) and O2 (6 vol%) was balanced with He. The total flow rates were 30 mL/min and 90 mL/min for Au/NR-ZnO and Au/TB-ZnO, respectively.

Figure 1c) are paramagnetic and EPR active; its intensity is therefore employed as a quantitative indicator of the amount of oxygen vacancies.21 Zn vacancies can also be detected at g=2.002 (assigned to singly charged Zn vacancy, denoted as VZn•, blue bar in Figure 1c) with a weaker signal.17 Measured at 77 K, the EPR result shows that TB-ZnO mesocrystals possess much more singly charged oxygen vacancies, which is shown in Figure 1c. To quantitatively elucidate the amount of oxygen vacancies for each sample, a diluted spin counter of diphenylpicrylhydrazyl (DPPH) was used as a standard to obtain the accurate quantity of Vo• and VZn•. The content of singly charged oxygen vacancies for NR-ZnO is 6.37 x 1012 mg-1, which is only 5.4% of the total Vo• defects of TB-ZnO (1.18 x 1014 mg-1) (see Figure 1d). For structural stability, zinc vacancies (VZn) should co-exist with oxygen vacancies. In the EPR characterization, Zn vacancies in TB-ZnO are detectable with a total amount of 2.44 x 1012 mg-1 while we cannot detect any Zn vacancy in NR-ZnO. In addition to EPR measurements, a cathodoluminescence (CL) characterization was employed to examine zinc vacancies of the supports qualitatively (Figure S2). The CL spectra show that other than near-band edge emission around 380 nm, there is an intense and broad emission at 510 nm which is ascribed to the presence of abundant Zn deficiencies VZn near the nonpolar surface.23 Furthermore, from XRD patterns (Figure S3), it reveals that the diffraction peak (101) of TB-ZnO is o down-shifted by about 0.12 in comparison with NR-ZnO samples. The shift of (101) Bragg peak of ZnO can be attributed to the stress of the unit cell in the crystal structure when ions with larger radii were used as the dopant. The unit cell of wurtzite ZnO crystal thus slightly expanded owing to the 0.3% elongation in a=b (from 3.2495 to 3.2594 Å) and c (from 5.2069 to 5.2230 Å) axes as compared to ideal ZnO(ICSD_776641). Accordingly, it can be expected that abundant Zn and O defects thus existed at the interfaces of orientated attachments. Through EPR analyses and CL characterization, TB-ZnO support shows much more Zn and O deficiencies than NR-ZnO support. Subsequently, both supports were used for preparing the gold catalyst via deposition-precipitation. Prior to gold deposition, the supports were calcined under oxygen atmosphere to remove the residual capping agents. The sizes of gold nanoparticles in both samples were controlled at ~4 nm (Figures 2a & 2b). Next, the catalytic performance of Au/NR-ZnO and Au/TB-ZnO for CO oxidation was evaluated (Figure 2c). The bare nanorod and twin-brush ZnO were first examined, showing no catalytic activity from room temperature to 300 °C. After Au deposition, Au/TB-ZnO showed much higher

2.5X10-2

10

100 200 Temperature (oC)

Absorbance (a. u.)

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2400

CO2

III

Au

Auδ+

2100 -1 Wavenumber (cm )

Figure 3. H2-TPR analyses and CO-adsorbed DRIFT spectra of ZnO supported gold catalysts. a) H2-TPR profiles of Au/TB-ZnO (▬), Au/NR-ZnO (▬), TB-ZnO (▬), cyanide-treated Au/TB-ZnO (▬) and O2-pretreated CN-treated Au/TB-ZnO (▬). (Except CN-Au/TB-ZnO sample, others were underwent O2-pretreatment at 200 oC for 1 h prior to TPR measurements.) b) DRIFT spectra of CO adsorption on Au/TB-ZnO (▬) and Au/NR-ZnO(▬) obtained after 5 min of CO/He gas purging.

activity in comparison with the performance of Au/NR-ZnO. Because of its extremely high activity, a much higher flow rate and lower catalyst loading were applied to examine Au/TB-ZnO samples. As shown in Figure 2c, the CO conversion increased when the temperature was raised for both catalysts. However, Au/TB-ZnO showed much greater catalytic activity than Au/NR-ZnO between 10 oC to 80 oC. These catalytic activities at 20 oC are tabulated in Table 1. When a very small amount of catalyst (about 11 mg) of Au/TB-ZnO and an extremely high space velocity (GHSV=4.9 x 105 mL⋅gcat-1⋅h-1) were used, the specific rate of CO conversion at 20 oC was observed to be ~153 times higher than that of the nanorod-supported catalyst. The high conversion rate in Au/TB-ZnO is comparable to or even better than conventional active gold catalysts such as Au/TiO224, 25 and Au/CeO226. We further examine the catalytic stability of Au/TB-ZnO in which the gold nanoparticles were chosen to be 2 nm for long-term catalysis under various pretreatment conditions (Figure S4). And the long-term stability test was carried out at 25 oC. The activity of Au/TB-ZnO rapidly evolved to the same steady state after the system underwent four cycles of changing pretreatments atmosphere of O2-H2 -O2-H2 at 200 oC. The size of active gold just increased slightly from 1.9±0.3 nm to 2.4±0.4 nm. Au/TB-ZnO catalyst performed again stably in catalysis due to its anti-sintering strong interaction between active gold and ZnO support. Similarly, a ZnO mesocrystal-supported gold catalyst pretreated at 120 oC and 300 oC (the typical pretreatment temperature is 200 oC.) in oxygen atmosphere also gave high dispersion of gold particles (Figure S5) and steadily superior activities. A temperature-programed reduction (TPR) study was carried out to investigate the surface gold species. Figure 3a indicates no consumption peak of H2 found in pristine TB-ZnO sample. Nor can we find reduction peak in the gold loaded nanorod sample of ZnO (Au/NR-ZnO). No reducible species were present in the TB-ZnO sample even at elevated temperatures. The TPR profile for nanorod-supported catalyst reveals that the gold nanoparticles in Au/NR-ZnO were already fully reduced upon oxygen pretreatment. However, in the

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Figure 4. The effect of degree of calcination on the amount of defects of TB-ZnO supports, reaction rate, size and active Au(III) content of gold particles of catalysts. (a) Vacancy amount of TB-ZnO supports obtained under different calcination conditions. (b) Vacancy ratio of [Vo•] to [VZn•] (calculated from Fig. 4a, grey bars) and corresponding specific rate of CO oxidation at 20 oC of Au/TB-ZnO catalysts (dark cyan line). (c) Percentage of Au(III) species derived from deconvoluted XPS spectra of Au/TB-ZnO catalysts (brown bars) and corresponding particle size of active gold (dark cyan line).

O2-pretreated Au/TB-ZnO catalyst, two separate TPR peaks can be clearly resolved at 77 oC and 135 oC. To identify the two reduced species, O2-pretreated Au/TB-ZnO sample was treated with a cyanide solution to remove gold nanoparticles. TEM examination of cyanide-washed sample confirmed Au nanoparticles were stripped from the surface (Figure S6). After the removal of metallic gold, only one H2-consumption peak at 124 oC (closer to 135 oC) remained in the TPR profile (Figure 3a, brown line). Next, the cyanide-washed sample was again treated under oxygen atmosphere to drive out the residual gold species from the lattice, forming gold nanoparticles again. The TPR profile shows a broad peak from 50 to 125 oC, confirming that the low temperature TPR peak (at 77 oC) can be associated with reducible gold species linked with the gold nanoparticles and the high temperature peak (135 oC) is probably associated with the Au(III) species further away from the gold nanoparticles in Au/TB-ZnO. As these Au(III) species are easily reducible, one can suspect the reducible gold species near surface may be associated with a Mars-van Krevelen mechanism15 in the extremely high activity in Au/TB-ZnO. According to a study by Zhang and co-workers,27 a low-temperature Au-assisted Mars-van Krevelen mechanism was proposed for a Au/FeOx catalyst, leading to higher catalytic activity in CO oxidation. Zhang et al. proposed that increased activity in CO oxidation is attributed to CO molecules reacting with activated surface lattice oxygens accompanied by the reduction of Fe3+ to Fe2+. Thus, a CO-adsorbed surface characterization was conducted by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) to probe the existence of the MvK mechanism. The CO adsorption experiment was carried out under an oxygen-free CO atmosphere. Figure 3b shows that two peaks at 2110 cm-1 and 2160 cm-1 were observed in both Au/TB-ZnO and Au/NR-ZnO samples, which are attributed to CO molecules adsorbed onto the partial positively charged and trivalent states of gold, respectively. Meanwhile, an obvious double peak (at ~2330 cm-1 and ~2370 cm-1) from vapor CO2 molecules was found only in the

TB-ZnO-supported gold catalyst. This production of vapor CO2 under oxygen-free condition is evidence for the presence of the MvK mechanism on the catalyst surface as schematized in Figure S7. In addition, the peak from CO2 was not detected in the nanorod-supported catalyst. Although ZnO is an irreducible support, Au/TB-ZnO presents two reduction peaks (blue line in Figure 3a). We thus propose that the two TPR peaks possibly originate from two types of reducible gold-species: an oxidized gold species (i.e., Au-O-Au) emerged from substitutional gold during thermal annealing (77 oC) which is linked to Au nanoparticle and the surface lattice Au(III) of –Au–O–Zn– associated (135 oC). The evidences of Au substitutional defects where Au replaces Zn in the lattice has been extensively studied in our previous paper by STEM, EXAFS and Raman.21 We refer to ref. 21 for detailed descriptions. To further show evidences, we here examine atomic-resolution STEM images of microtome-sliced Au/TB-ZnO in Figure S8. One can clearly see sub-surface Au clusters in ZnO lattice. The picture (Figure S8b) and its corresponding fast Fourier transform (FFT) pattern (inset in Figure S8b) show that the doped gold atoms in cluster line up along the a-axis of ZnO wurtzite structure. Gold atoms doping in ZnO crystal structure did not disturb the original lattice of ZnO wurtzite structure. Therefore, we believe the zinc sites of ZnO crystal are substitutionally replaced by gold. Upon gentle heating, the gold will emerge from sub-surface to the surface of TB-ZnO as shown previously.21 Combining the results of DRIFTs characterization and TPR analysis, it can be concluded that there are two kinds of gold species on the Au/TB-ZnO surface: metallic gold nanoparticles and substitutional gold within ZnO lattice. However, for the nanorod-supported catalyst, like conventional gold catalysts, only metallic gold species were observed on its surface. It has been proposed that surface vacancy defects play a crucial role in the catalysis of CO oxidation for a significant enhancement of its activity. Recent works propose that the presence of oxygen vacancies in the support strongly affects the catalytic activity in CO

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ACS Catalysis oxidation.28-30 It is claimed that the promotion in CO oxidation is ascribed to the facile formation of peroxo or superoxo species at oxygen vacancy sites, which effectively accelerates the CO oxidation reaction by weakening the O-O bonding and through fast dissociation of the oxygen molecules. This defect-mediated activation mechanism indeed could explain why defective metal oxides supported gold catalysts perform well in CO oxidation. To further elucidate surface vacancy-mediated catalysis, a series of TB-ZnO supports with increasing vacancy defects were examined. The temperature and duration of calcination pretreatment were utilized to control the amounts of defect vacancies in the TB-ZnO supports. In different calcination processes, we found the content of oxygen vacancies was roughly constant (red bars in Figure 4a) while in contrast the density of zinc vacancy defects increased with an elevated temperature and prolonged time of pretreatment (black bars in Figure 4a). The increasing content of zinc vacancies during a rigorous calcination process originated from the crystal structure truncation, especially at the ZnO nonpolar dihedral edges.31-33 From these findings, the TB-ZnO samples (Figure 4a) with different amounts of zinc vacancy are used as comparative supports for gold catalyst preparation. With the same gold deposition and CO oxidation evaluation process, the catalytic activities of these four TB-ZnO supported catalysts are shown in Figure 4b. Quite intriguingly, an increasing specific rate of CO oxidation correlates very well to the increasing vacancy ratio of [Vo•] to [VZn•]. At the same time, the size of the supported gold particles was examined by using high-resolution scanning electron microscope (HRSEM). X-ray photoelectron spectroscopy (XPS) profiles were examined to determine oxidized gold species (Figure S9) in the catalyst. The content of surface Au(0), Au(I) and Au(III) (Table S1), was acquired by deconvolution of the XPS spectra (Figure S9). For the TB-ZnO that was treated most mildly, e.g. 3 h at 300 0C there were equal amount of Au(I)(13.7%) and Au(III)(13.3%). This is reasonable since the substituted Zn(II) is divalent. Upon longer treatment of high temperature, One observe more Au(I) than Au(III) which means the substitution of Zn(II) is not stoichiometric and Au(III) is more prone to losing neighboring oxygen and get reduced. The Au(III) is particularly interesting for we find it is a strong marker of high activity. The activity dominating factors, i.e., particle size and amount of Au(III) species are presented in Figure 4c. The catalyst with the highest activity in CO oxidation was prepared using the TB-ZnO support underwent 3 h of calcination at 300 o C (denoted as TB-ZnO-300_3h). Such a highly active catalyst apparently possesses two crucial factors in accelerating the oxidation reaction, i.e., smaller gold particles at around 3 nm and plentiful trivalent gold species, Au(III) (Figure 4c). Interestingly, according to a previous report,34 Morgan et al. studied gold deposited on a gold-doped mixed oxide, i.e., Au/Au-doped CuMnOx for CO oxidation evaluation. They found that gold on Au-doped CuMnOx showed significantly increased activity with activated surface lattice oxygen close to the implanted gold. It has been suggested such surface Au-doping sites at the interface of gold NPs possess much higher activity in CO oxidation.35, 36 Noteworthy, Figures 4b and 4c show that gold deposited over higher ratios of [Vo•] to [VZn•] of TB-ZnO support gives not only more Au(III) species but also smaller active gold particles. When supports with defect density ratios ([Vo•]/ [VZn•]) from 1.8 to 10.5 were

utilized for catalyst preparation, the particle size reduced from ca. 6 nm to ca. 3 nm, respectively, simultaneously with Au(III) content variation from 0.5% to 13.3%. We have systematically tuned the vacancies (i.e., Vanion/Vcation) in the metal oxide supports for controlling the active gold cluster size and the content of trivalent gold species for the first time. To understand the excellent catalytic activity of CO oxidation over defective ZnO-supported Au on a molecular level, we performed density functional theory (DFT) calculations of CO oxidation near an Au cluster on ZnO (1010) . All calculations were performed by using Vienna Ab Initio Simulation Package (VASP)37, 38 with electron wave functions expanded in plane-wave basis sets in conjunction with pseudopotential via the projector augmented-wave method.39 The exchange-correlation energies were calculated in Perdew−Burke−Ernzerhof generalized gradient approximation.40 The energy barriers were estimated with the aid of climbing nudged elastic band method.41 More simulation parameters are given in supporting information. Three different ZnO nonpolar surface (1010) models are constructed including pristine ZnO nonpolar surface, ZnO nonpolar surface with Au3 cluster-deposition (i.e., forming “Au-O-Zn” at the perimeter) and nonpolar ZnO surface

a)

b)

c)

Figure 5. DFT simulated energy barrier of catalyzed CO oxidation by (a) pristine ZnO, (b) deposition-formed Au/ZnO with clustering gold consisted of three Au atoms and (c) Au/ZnO combining deposited gold cluster and one surface zinc substitution by gold. The energy barriers for surfaces (a) to (c) are 3.3, 1.9 and 0.01 eV, respectively.

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supported Au3 cluster connecting with one substitutional Au atom through O atom (thus, forming “Au-O-AuZn” linkage). Figures 5a to c show the energy barriers of CO oxidation via MvK mechanism on those surface models. CO oxidation on pristine ZnO nonpolar surface via MvK (i.e., CO molecule was oxidized by lattice oxygen of ZnO surface.) is almost prohibited due to the very high energy barrier of 3.3 eV, although the whole reaction is slightly exothermic with ∆E of 0.24 eV (Figure 5a). As expected, the CO-assisted oxygen extraction from pristine ZnO lattice is very difficult. Then we deposit an Au3 cluster on top of the ZnO surface. When the surface oxygen near the deposited Au3 cluster was involved in a MvK-like oxidation (a CO-assisted O-extraction as shown in Figure 5b), the energy barrier reduced to 1.9 eV. The overall reaction is more exothermic with ∆E of 0.39 eV. Then in the 3rd model, we put an extra substitutional Au near Au3 cluster. CO oxidation catalyzed by Au3 clusters connected to a substitutional Au atom with O extraction from Au-O-AuZn (Figure 5c) shows a very small energy barrier of about 0.01 eV. There is a shallow metastable state indicating the weak adsorption of the CO2 molecule on the surface model(c). It costs a small energy of 0.09 eV to de-adsorb the CO2 molecule from the surface; a negligible barrier for a reaction at room temperature. The simulation results agree with our hypothesis that the superior catalytic activity of Au/TB-ZnO could be attributed to the existence of “Au-O-AuZn” linkages. Both XPS analysis (Figure 4c) and H2-TPR (Figure 3a) results reveal that there are reducible gold species on the catalyst surface which perform catalytic activities in CO oxidation. The Au(III) species possibly originated from substitutional gold (denoted as AuZn) sitting at the perimeter of gold nanoparticles act a crucial role in the high CO oxidation rate as depicted in Figure 5. To show that gold substitution of Zn site is energetically favorable for ZnO containing vacancies, we performed DFT simulations. We created oxygen vacancies in the Au doped ZnO bulk models as used in our previous study.21 The formation energies of Au doping are calculated by the following formula: E E  Au k˗V   E  ZnO k˗V   nEAu  nE Zn!  where E  Au "˗#$  is the total energy of a defective ZnO model with n Au dopings and k oxygen vacancies. In viewing of the experimental observation of larger number of oxygen vacancy than Zn vacancy, we mostly let k>n. E  ZnO "˗#$  is the total energy of a defective ZnO models having k O vacancies. EAu  is one Au atom self-energy of respective face-centered cubic (fcc) crystal. E Zn!  is one Zn atom self-energy of respective hexagonal close packing (hcp) crystal. Because the O hopping energy (2.4 eV) is much larger than that of Zn (1.1 eV) and Au (0.4 eV),21 we can take E  ZnO "˗#$  as the bulk energy reference for calculating formation energies. Figure 6 shows the correlation plot for simulated formation energy versus (k/n) ratio. The positive formation energies indicate the Au atoms tend to diffuse out to the ZnO surface once the temperature is high enough for Zn atom to diffuse and fix the VZn. In Figure 6, we see there is a clear trend of favorable energy for the formation of Au-doping with increasing k/n ratio (i.e., VO/AuZn); i.e. having lower formation energies. At the same time, we expect models with higher k/n ratio have a slower rate of Au atom diffusion out of lattice, indicating a smaller

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Figure 6. Correlation plot of DFT simulated formation energy versus the ratio of no. of oxygen vacancies (k) to no. of substitutional Au atoms (n).

Au nanoparticles emerging on the corresponding ZnO surface. In our previous work, we had shown that it is easy for Au atoms to diffuse into mesocrystal ZnO and occupy the Zn vacancies (VZn).21 The number of AuZn should be proportional to the number of Zn vacancy when mesocrystal ZnO was first prepared. Since the O/Zn stoichiometric ratio is higher than one in the mesocrystalline ZnO (more O-vacancy than Znvacancy), one can expect more favorable Au(III) substitution to restore the optimum metallic valency of II. With supported Au nanoparticle, one can expect higher dispersion and more “Au-O-AuZn” linkages around gold particles in comparison with the case of typical Au/NR-ZnO system. This may then result in higher reaction rate in CO oxidation with an easier O-extraction from the peripheral of Au on meso-ZnO. Combining the results of Bader charges42 analysis on Au atoms (Figure S10) and CO oxidation barriers (Figure 5), we find that surface AuZn play a pivotal role in CO oxidation from two aspects: (1) the doped AuZn (Figure S10b) assures the total charge of deposited Au3 clusters is more positively charged (Figure S10c in comparison with Figure S10a); (2) the bonding of lattice oxygen connecting to the deposited Au and doped AuZn would become weaker in comparison with the bonding of “Au-O-Zn” linkages (Figure 5c in comparison with Figure 5b). This interpretation is further supported by the XPS spectrum analyzing of the O 1s orbital of Au/ZnO catalysts. According to Figure S11, the XPS peak from lattice oxygen of Au/TB-ZnO shows a down-shift to a lower binding energy relative to the lattice oxygen of Au/NR-ZnO, and therefore, giving weaker linkages of lattice oxygen in the Au/TB-ZnO catalyst. In summary, we have elucidated the role of support defects in the preparation of the highly active Au/meso-ZnO catalyst and the catalytic mechanism in facilitating CO oxidation. A comparative study was achieved by depositing gold on two different ZnO supports, NR-ZnO and TB-ZnO, and catalytic tests of CO oxidation show enhanced catalytic activity for Au/TB-ZnO at about ~153 times (in specific rate) higher than that of Au/NR-ZnO. Through experimental analysis (H2-TPR

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ACS Catalysis and XPS analyses) and DFT simulations, we believe the activity improvement of Au/TB-ZnO in comparison with NR-ZnO supported catalyst is mainly attributed to the occurrence of the MvK mechanism at the perimeter of gold particles via O-extraction from “Au-O-AuZn” linkages, which are formed by gold implanting in zinc vacancies of the ZnO crystal lattice. More importantly, these “Au-O-AuZn” linkages (occurring in Au/TB-ZnO) are weaker than the “Au-O-Zn” bonding (occurring in Au/NR-ZnO) at the perimeter of gold nanoparticles. With varying calcination conditions, the catalytic performance of gold catalyst in CO oxidation is systematically manipulated for the first time by tuning the vacancy ratio of [Vo•] to [VZn•], resulting in the decreasing of gold size and increasing of the content of active Au(III) species. In this report, we have successfully manipulated catalytic activity via controlling support vacancy contents. The presence of vacancies endows reducibility of the non-stoichiometric TB-ZnO and facilitates easy participation of O-atom from lattice in a MvK mechanism of oxidation. Mesocrystals of metal oxide may offer one an excellent support for metal catalyst through its higher density of vacancy and subsequent activation of lattice oxygen around the metal nanoparticle in other metal/oxide system.

ASSOCIATED CONTENT Supporting Information The following file is available free of charge on the ACS Publications website at DOI: xxxx. Experimental details for preparation, characterization, including BET, CL, XRD, TEM and XPS analysis (PDF)

AUTHOR INFORMATION

3. Comotti, M.; Pina, C. D.; Matarrese, R.; Rossi, M.; Siani, A. Oxidation of alcohols and sugars using Au/C catalysts: Part 2. Sugars. Appl. Catal. A 2005, 291, 204-209. 4. Fu, Q.; Kudriavtseva, S.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Gold–ceria catalysts for low-temperature water-gas shift reaction. Chem. Eng. J. 2003, 93, 41-53. 5. Comotti, M.; Li, W.-C.; Spliethoff, B.; Schüth, F. Support Effect in High Activity Gold Catalysts for CO Oxidation. J. Am. Chem. Soc. 2005, 128, 917-924. 6. Tang, H.; Wei, J.; Liu, F.; Qiao, B.; Pan, X.; Li, L.; Liu, J.; Wang, J.; Zhang, T. Strong Metal–Support Interactions between Gold Nanoparticles and Nonoxides. J. Am. Chem. Soc. 2016, 138, 56-59. 7. Portale, G.; Sciortino, L.; Albonetti, C.; Giannici, F.; Martorana, A.; Bras, W.; Biscarini, F.; Longo, A. Influence of metal-support interaction on the surface structure of gold nanoclusters deposited on native SiOx/Si substrates. Phys. Chem. Chem. Phys. 2014, 16, 6649-6656. 8. Schumann, J.; Eichelbaum, M.; Lunkenbein, T.; Thomas, N.; Álvarez Galván, M. C.; Schlögl, R.; Behrens, M. Promoting Strong Metal Support Interaction: Doping ZnO for Enhanced Activity of Cu/ZnO:M (M = Al, Ga, Mg) Catalysts. ACS Catal. 2015, 5, 3260-3270. 9. Haruta, M. Size- and support-dependency in the catalysis of gold. Catal. Today 1997, 36, 153-166. 10. Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis. Acc. Chem. Res. 2013, 46, 1740-1748.

Corresponding Author ∗E-mail: [email protected]

Author Contributions M.-H. Liu conceived and performed the experiment; Y.-W. Chen conducted ab initio calculations to support the experimental observations; T.-S. Lin joined the discussion of the results; C.-Y. Mou directed the research. The paper was written by M.-H. Liu, Y.-W. Chen and C.-Y. Mou.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research is financially supported by MOST (Ministry of Science and Technology), Taiwan. We thank C.-Y. Tang, S.-J. Ji, and C.-Y. Chien of Instrument Center of MOST for assistance in TEM, SEM/EDX observations. We thank Dr. Jer-Lai Kuo of IAMS for offering extensive computational resources for the DFT calculations. We also thank Dr. M.-W. Chu for STEM micrographs, Dr. Xiaoyan Liu for valuable discussions and Dr. Sabiha Runa for helps in manuscript editing.

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