Crystal Structural Effect of AuCu Alloy Nanoparticles on Catalytic CO

Supporting Information Placeholder. ABSTRACT: Controlling the physical and chemical properties of alloy nanoparticles (NPs) is an important approach t...
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Crystal Structural Effect of AuCu Alloy Nanoparticles on Catalytic CO Oxidation Wangcheng Zhan,† Jinglin Wang,† Haifeng Wang,† Jinshui Zhang,‡ Xiaofei Liu,†,‡ Pengfei Zhang,‡ Miaofang Chi,§ Yanglong Guo,† Yun Guo,† Guanzhong Lu,† Shouheng Sun,∥ Sheng Dai,*,‡,⊥ and Huiyuan Zhu*,‡ †

Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ Chemical Sciences Division and §Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States ⊥ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: Controlling the physical and chemical properties of alloy nanoparticles (NPs) is an important approach to optimize NP catalysis. Unlike other tuning knobs, such as size, shape, and composition, crystal structure has received limited attention and not been well understood for its role in catalysis. This deficiency is mainly due to the difficulty in synthesis and fine-tuning of the NPs’ crystal structure. Here, Exemplifying by AuCu alloy NPs with face centered cubic (fcc) and face centered tetragonal (fct) structure, we demonstrate a remarkable difference in phase segregation and catalytic performance depending on the crystal structure. During the thermal treatment in air, the Cu component in fcc-AuCu alloy NPs segregates more easily onto the alloy surface as compared to that in fct-AuCu alloy NPs. As a result, after annealing at 250 °C in air for 1 h, the fcc- and fct-AuCu alloy NPs are phase transferred into Au/CuO and AuCu/CuO core/shell structures, respectively. More importantly, this variation in heterostructures introduces a significant difference in CO adsorption on two catalysts, leading to a largely enhanced catalytic activity of AuCu/CuO NP catalyst for CO oxidation. The same concept can be extended to other alloy NPs, making it possible to fine-tune NP catalysis for many different chemical reactions.

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INTRODUCTION Alloy nanoparticles (NPs) have been studied extensively because of their unique synergistic effects derived from the parent metals on catalytic, optical, and magnetic properties.1−6 Significant progress has been made in synthesizing well-defined alloy NPs, and the properties of alloy NPs can be enhanced through controlling their composition, shape, and size.7−17 The NP parameter−property relationship can now be better correlated.18−22 On the contrary, the crystal structural effect of these alloy NPs on their physicochemical properties, especially the stability and catalytic behavior, has not been well understood. The crystallographic ordering of atoms is a critical structural factor to determine the properties of alloy NPs as the band structure of the NPs should be dependent critically on the directional overlap of atomic orbitals. This has motivated the search for the crystal structure−property relationship of alloy NPs.23−26 However, the nanoscale structural effect on alloy stability (phase segregation) and the consequent catalytic performance have not been fully revealed. It is well-known that the chemical composition at the surface of alloy NPs usually differs from that in the bulk because of the © 2017 American Chemical Society

surface segregation. Because the catalysis is dependent on the surface structure, both experimental and theoretical efforts have been made to explore the structural evolution of alloy upon their interactions with reactants in the reaction conditions.27−31 Despite these efforts, there has been comparatively less success to reveal the effect of the crystal structure on surface segregation of alloy NPs. Because of the complexity of alloy NP systems, it is still challenging to predict the surface segregation during the catalysis. Often, conflicting conclusions have been drawn, as shown in the studies of the influence of oxidative−reductive treatment of silica or SBA-15 supported AuCu catalysts on their surface and catalytic activity for CO oxidation.32−34 Previously, we have demonstrated that AuCu bimetallic NPs supported on silica show little activity for CO oxidation. Once treated in oxidative atmosphere at 400−500 °C, the AuCu phase segregated and the formation of CuO boosted the catalytic activity.33 A similar trend was observed for the AuCu/SBA-15 catalyst during the preferential oxidation of Received: February 20, 2017 Published: June 7, 2017 8846

DOI: 10.1021/jacs.7b01784 J. Am. Chem. Soc. 2017, 139, 8846−8854

Article

Journal of the American Chemical Society

configurations within the 2 × 2 × 2 supercell are taken to mimic fcc-AuCu (see type-I and type-II in Figure S1a). Correspondingly, on the derived fct-AuCu(111) with the O* (resulting from atmosphere O2 dissociation) occupying the optimal Cu2Au1 hollow site, the exchange of the subsurface or bulk-phase Cu atom with the surface Au atom was revealed to be evidently endothermic by 0.71 and 0.86 eV, respectively, implying the relative difficulty of Cu enrichment on surface for fct-AuCu alloy NPs (see Figure S1b). By contrast, on either type-I or type-II fcc-AuCu(111), the Cu segregation from the subsurface or bulk phase was much easier (see Table S1). For example, exchanging the subsurface (or bulk) Cu with the surface Au on type-I fcc-AuCu(111) corresponds to an energy cost as low as 0.13 eV (or 0.20 eV). Our calculations clearly show that Cu segregates more easily from fcc-AuCu than from fct-AuCu. To verify the theoretical prediction, we synthesized AuCu alloy NPs with fcc and fct structures by the seed-mediated diffusion route (Scheme 1). In a typical synthesis, 8 nm Au NPs

CO.32,41 More interestingly, a recent study reported that reduced AuCu alloy supported on SiO2, which underwent calcination at 500 °C in 20% O2 for 1 h followed by reduction at 400 °C in 15% H2 for 1 h, was significantly more active than the oxidized AuCu/SiO2 catalyst.34 A similar phenomenon was reported on AuCu supported on carbon.34 For the Au51Cu49/C system, the reduction treatment led to the formation of surface oxygenated Cu species that can provide activated Oδ− species in CO oxidation. In contrast, the active surface was completely blocked in the AuCu system when treated in air at 500 °C. On the basis of these previous results, we immediately notice that (1) the formation of the Au−CuO interface is crucial for CO oxidation as the alloy AuCu itself is not active for catalysis; and (2) the degree of phase segregation (i.e., the thickness of CuO on surface) also plays a vital role because the large amount of CuO on the surface may block the CO/O2 activation. This leaves us questions as how to control the surface segregation of AuCu alloy NPs and whether the crystal structure could impact this segregation and catalysis. Herein, on the basis of our theoretical calculations and experimental results, we report the crystal structural effect of alloy NPs on their surface segregation and catalytic activity. We choose to focus on AuCu as a model system to see if a bettercontrolled experiment can help to elucidate the real mechanism of the catalytic oxidation of CO on AuCu. Such studies may also help to understand the AuCu catalysis for the controlled oxidation of benzyl alcohol, propene, and methanol.4,5,13,35−38 In our studies, we first employ the first-principles density functional theory (DFT) calculations to predict the crystal structural effect of face-centered cubic (fcc) and face-centered tetragonal (fct) AuCu NPs on the barrier of Cu out-diffusion onto the alloy surface under O2 atmosphere. We then synthesize fcc- and fct-AuCu NPs in similar NP parameters (size, composition, and shape). Once annealed in O2, the fctand fcc-AuCu NPs tend to form AuCu/CuO and Au/CuO core/shell structure. This thermally induced segregation strategy for the synthesis of heterostructures leads to a wellcontrolled degree of the formation of surface CuO, beneficial for CO/O2 activation. The thickness of the formed CuO on either Au or AuCu surface in our system is in the range of 0.5− 0.65 nm depending on the crystal structure, approximately 2 atomic layers, favoring the diffusion and activation of reactants. Consequently, two types of alloy NPs show a remarkable difference in CO adsorption and oxidation. To the best of our knowledge, this is the first study of the alloy structural effect on NP phase segregation and catalytic CO oxidation. The concept can be extended to other alloy NPs, making it possible to finetune NP catalysis for many different chemical reactions.

Scheme 1. Schematic Illustration of the Synthesis of AuCu Alloy NPs and Structure Transformation after Being Annealed at 250 °C

were prepared according to the previously reported procedure with some modifications.39 In the presence of the Au NPs, Cu(acac)2 then was reduced by oleylamine to Cu that diffused into the Au seeds to form AuCu NPs with fcc structure at 210 °C. If the mixture was kept at 290 °C, AuCu NPs with fct structure were obtained. The synthesized AuCu NPs were dispersed in a nonpolar solvent such as hexane. The alloy compositions were controlled by the mass ratio of Au seed and Cu(acac)2, and can vary in the range of 0.8−1.1 Au/Cu atomic ratio. Herein, we chose fcc- and fct-AuCu NPs with ∼0.93 Au/ Cu atomic ratio as a model, measured by inductively couple plasma atomic emission spectroscopy (ICP-AES), as this close to 1 ratio is required for fct-formation. The formation of the AuCu NPs with fcc and fct structure was characterized by highangle annular dark field (HAADF)-scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and ultraviolet−visible (UV/vis) spectroscopy. As shown in the HAADF-STEM image (Figure 1a and c), the as-synthesized AuCu NPs with fcc and fct structures have an average size of 8.4 ± 0.5 and 8.2 ± 0.4 nm, respectively (Figure S2). Meanwhile, the intermetallic structure, represented by the periodic contrast change, is visible in the HAADF-STEM image of a single as-synthesized NP (Figure 1d), proving the formation of fct structure. The NP is at the [100] zone axis of fct structure, and the corresponding intensity line profile shows the alternative atomic occupancies of Au and Cu (Figure 1d). On the contrary, this ordering is absent in fcc-AuCu (Figure 1b), further confirming the formation of two types of crystal structures. The lattice spacing of the arbitrarily selected single NPs is found to be 0.226 and 0.224 nm in high-

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RESULTS AND DISCUSSION To explore the effect of AuCu alloy crystal structures on Cu surface segregation under O2 atmosphere, we calculated the reaction energetics accompanying the segregation process in fcc- and fct-AuCu alloy NPs by the first-principles DFT calculation (see computational details in the Supporting Information). Our calculations focused on the (111) surface that is mostly exposed on the polyhedral fcc- and fct-AuCu NPs, and described Cu diffusion from subsurface or bulk region to the surface by exchanging with the surface Au in the presence of adsorbed oxygen (O*). As shown in Figure S1, the bulk fct-AuCu possesses an alternating Au- and Cu-layer arrangement along the ⟨001⟩ direction, while for fcc-AuCu, Au and Cu atoms are randomly arranged, and two possible atomic 8847

DOI: 10.1021/jacs.7b01784 J. Am. Chem. Soc. 2017, 139, 8846−8854

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Journal of the American Chemical Society

Figure 1. HAADF-STEM images of as-synthesized AuCu alloy NPs with fcc (a,b) and fct structure (c,d); XRD patterns (e) and UV−vis spectra (f) of as-synthesized AuCu alloy NPs.

diffusion was evident in EELS element mapping of a single annealed fcc-AuCu NP (Figure S4e), in which the overlapped image suggests a higher concentration of Cu along the outer layer of the NP. The HAADF-STEM image of a single NP (Figure 2a and e) further demonstrates that the amorphous

resolution TEM (HRTEM) of the fcc- and fct-AuCu NPs, respectively (Figure S3), lying between the lattice spacing of the (111) plane of monometallic Au (0.235 nm) and Cu (0.208 nm), suggesting that these NPs are composed of Au and Cu. The solid solution structure of the as-synthesized AuCu alloy was further characterized by XRD and UV/vis spectroscopy. As shown in Figure 1e, the XRD peaks of AuCu NPs formed at 210 °C can be indexed as fcc structure, while a superlattice reflection of 201 and the splitting reflections of 200/002 and 220/202 are clearly visible for the AuCu NPs synthesized at 290 °C, suggesting the formation of fct structure, or, to be more exact, a partially ordered one mainly due to the small size of the AuCu NPs, making the phase transformation hindered as compared to bulk phase behavior.16,17 Furthermore, upon alloying Cu with Au, the (111) peaks shift slightly to a higher angle irrespective of nanocrystal structure, lying between that of pure fcc Au (JCPDS no. 04-0784) and that of pure fcc Cu (JCPDS no. 85-1326), indicating the reduction of structural lattice parameter due to the alloy formation.40 The d spacing of fcc- and fct-AuCu (111) peak is 0.223 and 0.222 nm, respectively, consistent with their HRTEM. The alloy formation was further proved by the comparison of UV−vis spectroscopy of Au and AuCu NPs, shown in Figure 1f. Both fcc- and fct-AuCu NPs exhibit the surface plasmon peak at 545 nm, between that of Au (520 nm) and Cu (570 nm), confirming the alloy structure and absence of Au and Cu NPs.4,16,17 To investigate the influence of alloy crystal structure on Cu oxidation and diffusion during the annealing process, the assynthesized AuCu NPs were annealed in air and characterized by HAADF-STEM and XRD. HAADF-STEM images of AuCu alloy NPs annealed at 200 °C (to prevent the oxidation of the carbon-coated cupper grid) are shown in Figure S4. The annealed AuCu NPs show no obvious NP morphology change, but their average sizes were slightly decreased to 7.9 ± 0.4 nm (from the fcc-AuCu) and 7.7 ± 0.3 nm (from the fct-AuCu) (Figure S2). Furthermore, a lightly contrasted shell is now visible around the annealed AuCu alloy NPs. According to previous studies on the AuCu alloy system, Cu with stronger bonding to O2 tends to diffuse out to form CuO on the alloy surface under O2 atmosphere.32,33,41−43 In this case, this lighter shell can be assigned to CuO species. This O2-induced Cu out-

Figure 2. HAADF-STEM and XRD patterns of AuCu alloy NPs with fcc (a,b) and fct structure (e,f) annealed at 200 °C in air for 1 h, and corresponding AuCu alloy NPs with fcc (c,d) and fct structure (g,h) supported on titanium dioxide after annealing at 250 °C in air for 1 h.

shell of CuO species on the surface of annealed fcc-AuCu NPs is about 0.65 nm, thicker than that on the surface of annealed fct-AuCu NPs (about 0.5 nm). The XRD patterns (Figure 2b and f) show that the (111) diffraction peaks for annealed fccand fct-AuCu NPs shifted from 40.4° to 38.6° and 40.6° to 39.2°, respectively. These results revealed that Cu in both alloy NPs can be segregated from the alloy structure during the annealing process at 200 °C, while the cores of both fcc- and fct-AuCu alloy NPs partially retain the corresponding alloy structure after annealing at 200 °C in air for 1 h, because the (111) diffraction peak for both annealed alloy NPs is at a higher angle than that (38.2°) of pure Au NPs. To see whether we can further segregate Cu from AuCu alloy NPs, we treated as-synthesized alloy NPs at a higher temperature. Correspondingly, the XRD patterns of assynthesized alloy NPs after annealing at 250 °C in air for different time were collected. As shown in Figures 3a,b and S5, when the as-synthesized AuCu NPs were annealed at 250 °C, the XRD patterns of both fcc- and fct-AuCu alloy NPs shifted gradually toward low angles with increasing annealing time, 8848

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Figure 3. Powder XRD patterns showing the transformation of as-synthesized (a) fcc-AuCu alloy NPs and (b) fct-AuCu alloy NPs annealed at 250 °C in air for different times; the corresponding 2 theta for the (111) reflection peak of as-synthesized AuCu alloy NPs annealed at 250 °C in air for different times is shown in (c); (d) Cu 2p XPS spectra of fcc- and fct-AuCu/TiO2 samples before and after annealing at 250 °C in air for 1 h.

which was attributed to Cu diffusion from alloy structure onto the NP surface, leaving the Au-rich core with larger lattice spacing. The diffraction peaks of CuO are absent in all XRD patterns, suggesting that copper in both fcc- and fct-AuCu alloy NPs is inclined to diffuse onto alloy surface as amorphous CuO. Furthermore, this shift is more discernible for fcc-AuCu alloy NPs (Figure 3a) as compared to fct-AuCu alloy NPs (Figure 3c), indicating that in the fct-AuCu structure, Cu diffusion onto the alloy surface during the annealing process is more difficult than in fcc-AuCu, consistent with our DFT prediction. Consequently, after annealing at 250 °C for 1 h, the (111) diffraction peak of fcc-AuCu NPs shifted from the alloy position (2θ = 40.4°) to the pure Au position (38.2°), signifying that all of the copper has left the gold lattice to form Au/CuO core/ shell structure. On the contrary, the (111) diffraction peak of fct-AuCu alloy NPs just shifted from 2θ = 40.6° to 38.8°, suggesting that the core of fct-AuCu alloy NPs retains the alloy structure to some extent and forms an AuCu/CuO core/shell structure after the annealing. We also annealed as-synthesized fct-AuCu alloy NPs at temperatures higher than 250 °C (300, 325, and 350 °C) and tracked their XRD patterns (Figure S6). Accordingly, we found that the critical annealing temperature for the complete segregation of Au and Cu in fct-AuCu alloy NPs is between 325 and 350 °C, much higher than that of fccAuCu (250 °C), further confirming fcc-AuCu is more vulnerable to phase separation during oxidative annealing. The annealing-induced diffusion of Cu from the TiO2 supported AuCu NPs was also studied. The as-synthesized AuCu NPs were deposited on TiO2 (P25, 70% anatase and 30% rutile) to obtain supported AuCu samples, and then the samples were annealed at 250 °C for 1 h in air, which are denoted as AuCu/TiO2. HAADF-STEM and X-ray photoelectron spectroscopy (XPS) were conducted to track the structural change of the AuCu/TiO2. HAADF-STEM images (Figure S7) show that the NPs in both fcc- and fct-AuCu/TiO2

catalysts retain a uniform size distribution after annealing. The HAADF-STEM image of a single NP (Figure 2c and g) revealed that there is a flocky shell on the surface of the annealed fcc- and fct-AuCu NPs. Furthermore, the lattice distance of the annealed fcc- and fct-AuCu NPs is 0.235 and 0.230 nm (Figure 2d and h), respectively. These results indicate that the structural evolution of AuCu alloy NPs during annealing is independent of the support TiO2. Figures 3d and S8 present the XPS spectra of AuCu/TiO2 catalysts before and after annealing at 250 °C in air. It is seen that gold exists in the metallic state in AuCu/TiO2 samples before annealing regardless of AuCu alloy structure, as proved by Au 4f peaks at a binding energy of ∼84.0 eV corresponding to metallic gold.19,44−47 After annealing at 250 °C in air, gold in both fccand fct-AuCu/TiO2 was kept in the metallic state. The Cu 2p spectra (Figure 3d) of both fcc- and fct-AuCu/TiO2 samples before annealing exhibit Cu 2p3/2 and 2p1/2 doublet peaks centered at ∼931.2 and ∼950.9 eV, indicating that Cu exists in the metallic state in AuCu/TiO2 samples before annealing.48−50 However, for both fcc- and fct-AuCu/TiO2 after annealing at 250 °C in air, Cu 2p3/2 and 2p1/2 doublet peaks shifted to the high binding energy of ∼932.5 and ∼952.5, respectively. Furthermore, Cu 2p3/2 and 2p1/2 satellite peaks, which are the characteristic peaks of Cu2+, appeared at 938−945 and 960− 965 eV. These results confirm the oxidation of Cu during the annealing in air. According to refs 50 and 51, the relative concentrations of Cu2+ species present on a surface can be roughly estimated on the basis of the ratio of the main peak/ shakeup peak area, which is 0.97 and 0.79 on the fcc- and fctAuCu/TiO2 samples surface, respectively. Furthermore, to qualitative investigate the metallic Cu in samples, Auger Cu LMM spectra of fcc- and fct-AuCu/TiO2 after annealing at 250 °C in air for 1 h were obtained. As shown in Figure S9, there are two peaks at 568.6 eV (Cu metal) and 570.5 eV (Cu2O) in the Cu LMM spectrum of fct-AuCu/TiO2 catalyst, while the 8849

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Figure 4. CO oxidation light-off curves (a) and corresponding activation energies (b) on the fcc- and fct-AuCu/TiO2 catalysts annealed at 250 °C in air for 1 h; (c) in situ DRIFT of CO+O2 coadsorption on the annealed fcc- and fct-AuCu/TiO2 catalysts at 75 °C for different times under 1 vol % CO−5 vol % O2−94 vol % Ar conditions; and (d) H2-TPR profiles of the annealed fcc- and fct-AuCu/TiO2 catalysts.

on Au/TiO2 with 0.6 wt % Au. Although the supported Au shows a slightly higher activity at the temperature lower than 80 °C, further increasing the reaction temperature from 80 to 250 °C brought about the appearance of a valley in the curve of CO oxidation, indicating a low stability for Au catalyst (Figure S11a). In contrast, the fct-AuCu/TiO2 catalyst presents a high stability for CO oxidation, and CO conversion can remain unchanged at both 85 and 125 °C within 50 h, as well as in the second run (Figure S11b). When TiO2 was replaced by SiO2, the AuCu NPs show a similar catalysis trend (Figure S12). When the as-synthesized fct-AuCu/TiO2 catalysts were pretreated by acetic acid at 77 °C for 24 h or annealed at 300 °C in H2 for 1 h only to remove the capping agents on AuCu, they exhibited a very low activity for CO oxidation (Figure S13), indicating that the formation of surface CuO plays a vital role during CO oxidation. This observation is consistent with previous studies that supported AuCu alloy NPs are inactive for the oxidation of CO unless an Au−CuO heterostructure is formed in the reaction condition.32,33,41,42 Additionally, different annealing temperature was probed on fcc-AuCu/TiO2 for catalysis study (Figure S14). We found that the catalytic activity of fcc-AuCu/TiO2 catalyst annealed at 200 °C for 1 h is much higher than that of fcc-AuCu/TiO2 annealed at 250 °C for 1 h, confirming the promoting effect of the AuCu alloy core. However, the catalytic activity of the fcc-AuCu/TiO2 catalyst annealed at 200 °C for 1 h is still much lower than that of the fct-AuCu/TiO2 catalyst annealed at 250 °C for 1 h; that is, the activity of the partially segregated fcc-AuCu is lower than that of partially segregated fct-AuCu. Our study shows that the AuCu catalysis can be further improved when the AuCu NPs are made in the fct-structure, in which the stabilization of Cu makes it possible to limit the degree of Cu oxidation, giving an fct-AuCu/CuO core/shell that is more active and stable in catalyzing CO oxidation.

peak assigned to Cu metal is absent in the Cu LMM spectrum of fcc-AuCu/TiO2 catalyst, confirming that all Cu was oxidized in fcc-AuCu, and, in contrast, some Cu was kept metallic in fctAuCu after the same thermal treatment.52 Taken cumulatively, XPS and Auger LMM spectra proved that almost all Cu in assynthesized fcc-AuCu alloy left the gold lattice after annealing at 250 °C in air for 1 h, but part of the Cu was kept as AuCu alloy in annealed fct-AuCu alloy, consistent with our XRD and HRTEM results. In addition, as compared to untreated AuCu/ TiO2 samples, the Au/Cu atomic ratio on the surface of annealed AuCu/TiO2 samples decreased rapidly (Table S2), implying the enrichment of Cu in the outer layer of the annealed alloy during the annealing process. More interestingly, we also found that annealing at 250 °C in air led to the partial retention of periodic alternating ordering in the fct-AuCu core (Figure S10). To investigate how the surface structure of the AuCu/TiO2 catalysts affects their catalytic properties, we chose to investigate CO oxidation as a probe reaction. The catalysts were prepared by the same procedure used to make XPS samples, while the loading of Au and Cu was decreased to 0.61 wt % Au−0.21 wt % Cu and 0.64 wt % Au−0.22 wt % Cu for the fcc- and fct-AuCu/TiO2 catalysts (measured by ICP-AES), respectively. The catalytic activities of the fcc- and fct-AuCu/ TiO2 catalysts for CO oxidation are shown in Figure 4a. Very interestingly, a significant difference exists in the catalytic activity for CO oxidation between the fcc- and fct-AuCu/TiO2 catalysts. A complete CO conversion is observed at 120 °C on the fct-AuCu/TiO2, but at 220 °C over the fcc-AuCu/TiO2 catalyst. The apparent activation energies of fcc- and fct-AuCu/ TiO2 catalysts were estimated to be 52 and 25 kJ/mol (Figure 4b), respectively, confirming the higher activity of the fctAuCu/TiO2 over the fcc-AuCu/TiO2 in catalyzing the CO oxidation. As a control, we also studied the oxidation reaction 8850

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monolayer-CuO(111)/fct-AuCu(111) interface structures were constructed to mimic fcc- and fct-AuCu, respectively; see the optimized configurations in Figure S19. First, for fctAuCu alloy catalyst, it was revealed that CO preferentially adsorbs on the substrate AuCu(111) with an adsorption energy of −0.80 eV (Figure 5a), and the adsorbed CO* can readily

It is widely accepted that CO oxidation on supported Au− CuO heterostructure mainly follows the Mars−van Krevelen mechanism.32−34,41,42 In this mechanism, CO adsorbed on the catalyst surface and then reacted with the lattice oxygen to produce CO2 and the oxygen vacancies. Simultaneously, the gas-phase oxygen is activated on the catalyst to replenish the oxygen vacancies. Therefore, the key factors that influence the CO oxidation activity on AuCu/TiO2 can be attributed to CO adsorption ability and the activity of the lattice oxygen. To verify this hypothesis, we performed in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy studies of the CO adsorption and H2-TPR experiments to identify the surface effect of the fcc- and fct-AuCu/TiO2 after air annealing at 250 °C on CO adsorption and the reducibility of the lattice oxygen. Figure S15 presents the DRIFT spectra of the fcc- and fct-AuCu/TiO2 catalysts under a 5 vol % CO−95 vol % Ar flow for different times. Although the adsorption band at 2101 cm−1, assigned to CO adsorbed on metallic Au0, can be observed on both the fcc- and the fct-AuCu/TiO2 surfaces, the intensity of the adsorption band is much lower on the former than that on the latter, indicating more available Au sites for CO adsorption on the fct-AuCu/TiO2 catalyst.46,53,54 When CO and O2 were coadsorbed on the AuCu/TiO2 surfaces, besides the adsorption band at 2101 cm−1, one new adsorption band at 2175 cm−1 assigned to gas-phase CO can be observed (Figure 4c).53 Although the intensity of the adsorption band at 2101 cm−1 decreased due to the competitive adsorption between CO and O2, the intensity of the adsorption band at 2101 cm−1 is 10 times higher on fct-AuCu/TiO2 than on fcc-AuCu/TiO2. This difference in CO adsorption ability may be attributed to the different coverage of CuO layer and available Au sites for CO adsorption. On the contrary, the H2-TPR profiles over the fccand fct-AuCu/TiO2 catalysts are similar (Figure 4d). Both catalysts have two reduction peaks centered at about 200 and 350 °C, which can be assigned to highly dispersed CuO with a difference in strong NPs−support interaction.43,55,56 Furthermore, the intensity of the corresponding reduction peaks is slightly lower for the fct-AuCu/TiO2 catalyst than for the fccAuCu/TiO2 catalyst, due to a low content of CuO in the former. These results reveal that the fcc- and fct-AuCu/TiO2 catalysts after air annealing possess similar reducibility. In other words, the activity of oxygen species is quite close over the two catalysts, based on the H2-TPR results. To confirm the activity of oxygen species in CuO shell for CO oxidation, CO pulse experiments (Figure S16) on fcc-AuCu/TiO2 catalysts annealed in air and TiO2 support were carried out. We found that CO was consumed in the first 20 pulses on our TiO2 supported AuCuO catalysts without the supply of gas-phase oxygen, while the CO signal did not change on the pure TiO2 support, proving that lattice oxygen participated in CO oxidation in our system. In summary, there are more available Au sites exposed for CO adsorption on fct-AuCu/TiO2 than on fcc-AuCu/TiO2 after annealing, leading to the higher catalytic activity of the fctAuCu/TiO2 over the fcc-AuCu/TiO2 in the CO oxidation reaction. We also noticed that the difference in CO adsorption and the similar activity of oxygen species were retained after CO oxidation, as shown in Figures S17 and 18. To further understand the underlying rationale for the difference in catalytic activity of fcc and fct-typed AuCu alloy NPs, the elementary reactions involved in CO oxidation following the acknowledged Mars−van Krevelen mechanism were systematically calculated. On the basis of the experimental characterizations, the two-layer-CuO(111)/Au(111) and

Figure 5. Energy profiles of CO oxidation on fcc- and fct-AuCu catalyst annealing in O2 without (a) and with entropy effects of gasphase CO, O2, and CO2 included (b, T = 400 K).

couple with the lattice oxygen of CuO at the interface into CO2* with a very low barrier of 0.13 eV; moreover, the formed CO2* can easily desorb from the NP surface, leaving a lattice O vacancy. Second, at the generated O vacancy, O2 can be efficiently adsorbed via a bidentate configuration with the Cu sites at the CuO/fct-AuCu(111) interface, with an adsorption energy as high as −1.01 eV; the adsorbed O2 can easily react with CO and produce CO2 and accomplish the whole catalytic cycle. The whole energy profile is given in Figure 5a. We can see that there is no apparently high barrier in the whole process, implying the high catalytic activity of CO oxidation on the fctAuCu alloy catalyst. For comparison, the whole energy profile of CO oxidation at the CuO(111)/Au(111) interface was also calculated, as illustrated in Figure 5a. It shows that the lattice O (or O2* adsorbed at the lattice O vacancy) of the two-layer CuO oxide film can react with the CO* easily with a very low barrier, with the main differences focused on CO and O2 adsorption, which correspond to a much reduced binding strength (−0.58 and 8851

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Journal of the American Chemical Society −0.32 eV, respectively) on the Au(111) substrate. Specifically, with the gas-phase entropy effect of CO and O2 reactants under the realistic conditions (T = 400 K), we can recognize from the derived free energy profiles (see Figure 5b) that the fcc-AuCu alloy catalyst exhibits a higher effective activation energy than the fct-AuCu alloy catalyst, due to the low CO and O2 adsorption ability at the CuO/Au(111) interface and, therefore, the low activity of the fcc-AuCu catalyst.

Synthesis of AuCu/TiO2 and AuCu/SiO2. To perform the CO oxidation reaction tests, AuCu NPs were deposited on P25 and silica support, respectively. A certain amount of support was dispersed in hexane solution by sonication, and AuCu NPs in hexane dispersion were then added into the mixture under magnetically stirred. After the hexane was completely evaporated, the supported AuCu samples were collected by centrifugation (9000 rpm, 6 min), washed with ethanol, and dried in air at 40 °C. Before the test for CO oxidation, the supported AuCu samples were calcinated at 250 °C for 1 h in air to remove the OAm surfactant. Characterization. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) measurements were carried on a PerkinElmer Optima 2100 DV spectrometer. X-ray diffraction (XRD) measurements were performed on a PANalytical X’Pert Pro MPD diffractometer using an X’Celerator RTMS detector. HRTEM and STEM analysis were performed on an aberration corrected FEITitan 80/300 microscope, and STEM-EDX was carried out on a JEOL 2200FS-AC STEM (200 kV) equipped with an bruker SDD-EDX detector. UV−vis spectra were recorded on a Varian Carry 500 UV− vis−NIR spectrophotometer in the range of 300−800 nm. The XPS spectra were measured on a Kratos AXIS Ultra DLD XPS system equipped with monochromatic Al Kα (1486.6 eV) as X-ray source operated at 15 keV and 120 W. A charge neutralizer (CN) was used to compensate for the surface charge, and the binding energy in the XPS spectra was calibrated with a carbon signal (C 1s at 284.8 eV). In situ DRIFT measurements were performed on a Nicolet 6700 FT-IR spectrometer with an MCT detector. In the DRIFT cell with ZnSe windows connected with a gas flow system, the sample was pretreated at 200 °C in Ar for 2 h and then cooled to 75 °C in Ar. After the background spectra were recorded at 75 °C, Ar gas was replaced by the mixed gas of 1 vol % CO−5 vol % O2−94 vol % Ar (50 mL/min), and in situ DRIFT spectra of the samples were taken at different times. Temperature-programmed reduction of H2 (H2-TPR) experiments were conducted on a PX200 apparatus with a TCD. 50 mg of sample was directly heated from room temperature to 500 °C at a rate of 10 °C/min in a flow of 5 vol % H2/N2 (40 mL/min). The hydrogen consumption was quantitatively evaluated by the TCD signal. CO pulse experiments were performed on a Micromeritics AutoChem II 2920 chemisorption analyzer with He as carrier gas. Prior to the measurement, 50 mg of sample was first treated in He flow (50 mL/ min) from room temperature to 200 °C at a rate of 5 °C/min and maintained at 200 °C for 1 h. Subsequently, the sample was cooled to 75 °C in He flow and purged with He for 30 min. Then quantitative mixed gas of 1% CO/He was injected into the sample cell until CO was no longer consumed, to obtain the pulse curves. The signal of CO (M/Z = 28) was monitored by an online Hiden HAL301 mass spectrometer (MS). Catalytic Activity Measurement. The catalytic activity of supported AuCu catalysts for CO oxidation was evaluated. CO oxidation was carried out in a temperature-controlled microreactor (Altamira AMI 200) equipped with an online gas chromatograph. A gaseous mixture of CO (1.0 vol %) balanced in dry air was passed through 30 mg of the catalyst at a total flow rate of 10 mL min−1 (corresponding to a space velocity of 20 000 mL gcat−1 h−1). The inlet and outlet gas compositions were analyzed online by a gas chromatograph equipped with a thermal conductivity detector (TCD). For the kinetic measurements, the amount of the catalyst was reduced to 4 mg, and the flow rate of the feed gas was changed to ensure the CO conversion is below 15%. The CO conversion was averaged at 10, 20, 30, and 40 min to calculate the reaction rate. The reaction rate (r) was calculated as follows: r = CO conversion rate × [CO]in/n(Au). Here, the CO conversion rate is the percentage of CO oxidized to CO2 after the reaction. [CO]in is the total molar flow of CO per second. n(Au) stands for total moles of Au atoms. Theoretical Calculation. All of the spin-polarized calculations were performed with the Perdew−Burke−Ernzerhof (PBE) functional using the VASP code.57,58 The project-augmented wave (PAW) method was used to represent the core−valence electron interaction. The valence electronic states were expanded in plane wave basis sets with cutoff energy of 400 eV. The ionic degrees of freedom were

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CONCLUSIONS In summary, we designed and synthesized two types of AuCu alloy NPs with similar particle size, shape, and composition but different crystal structure (fcc vs fct), to reveal the effect of crystal structure on alloy surface segregation and their catalytic activities. The fcc- and fct-AuCu alloy NPs tend to form Au/ CuO and AuCu/CuO core/shell structures, respectively, during the thermal pretreatment of alloy NPs at 250 °C in air for 1 h due to the different thermodynamic barriers for Cu diffusion onto the alloy surface. As a result, there is a remarkable difference in the catalytic activity of fcc- and fct-AuCu alloy NPs supported on TiO2 for CO oxidation, which is mainly dependent on their different ability for CO adsorption. This discovery may open a new avenue for fine-tuning catalytic activities of alloy NPs in many chemical reactions.

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EXPERIMENTAL SECTION

Materials. Oleylamine (OAm, >70%), oleic acid, 1-octadecene (ODE), 1,2,3,4-tetrahydronaphthalene (tetralin), tert-butylamine-borane complex (TBAB, 97%), chloroauric acid (HAuCl4·3H2O), Cu(acac)2 (acac = aceylacetonate), hexane, acetone, 2-propanol, ethanol, titania P25 (70% anatase and 30% rutile) powder, and silica powder were all purchased from Sigma-Aldrich and used without further purification. Synthesis of Au NPs. In a typical synthesis of Au NPs, an orange precursor solution of tetralin (10 mL), OAm (10 mL), and HAuCl4· 3H2O (0.2 g) was prepared in air at room temperature and magnetically stirred at 2−5 °C (ice−water bath) under N2 flow for 20 min. A reducing solution containing 90 mg of TBAB, tetralin (1 mL), and OAm (1 mL) was mixed by sonication and injected into the precursor solution. The reduction was instantaneously initiated, and the solution changed to a deep purple color within 5 s. The mixture was allowed to react at 2−5 °C for 2 h before acetone (50 mL) was added to precipitate the Au NPs. The Au NPs were collected by centrifugation (9000 rpm, 6 min), washed with ethanol, and redispersed in hexane. The Au NPs synthesized were very uniform with a size of ∼6 nm. Subsequently, the Au NPs were enlarged to ∼8 nm by a secondary growth. A precursor solution of ODE (6 mL), OAm (6 mL), and HAuCl4·3H2O (0.1 g) was prepared in air at room temperature and magnetically stirred at 80 °C under N2 flow for 20 min. Thirty milligrams of OAm-coated Au NPs in hexane dispersion was injected into the precursor solution. The mixture was allowed to react at 80 °C for 2 h before 2-propanol (50 mL) was added to precipitate the Au NPs. The Au NPs were collected by centrifugation (9000 rpm, 6 min), washed with ethanol, and redispersed in hexane. The Au NPs synthesized were very identical in size of ∼8 nm. Synthesis of AuCu NPs. 40 mg of Cu(acac)2 was mixed with 15 mL of OAm and magnetically stirred at 80 °C under N2 flow for 30 min. Twenty-eight milligrams of OAm-coated Au NPs in hexane dispersion was then added into the reaction system, and the reaction mixture was heated to 210 °C (for synthesizing AuCu NPs with fcc structure) or 290 °C (for synthesizing AuCu NPs with fct structure) at a rate of 3 °C/min. After 1 h, the reaction solution was cooled to room temperature under N2 flow. The AuCu NPs were precipitated out by the addition of 35 mL of ethanol and centrifugation at 9000 rpm for 6 min, followed by ethanol wash and centrifugation. The AuCu NPs were dispersed in hexane for further use. 8852

DOI: 10.1021/jacs.7b01784 J. Am. Chem. Soc. 2017, 139, 8846−8854

Article

Journal of the American Chemical Society relaxed using the BFGS minimization scheme until the Hellman− Feynman forces on each ion were less than 0.05 eV/Å.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01784. Figures S1−S19 and Tables S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Jinshui Zhang: 0000-0003-4649-6526 Miaofang Chi: 0000-0003-0764-1567 Guanzhong Lu: 0000-0003-2031-785X Shouheng Sun: 0000-0002-4051-0430 Sheng Dai: 0000-0002-8046-3931 Huiyuan Zhu: 0000-0002-9962-1661 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.Z., J.Z., P.Z., and S.D. were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Electron microscopy work was performed at the Center for Nanophase Materials Science at ORNL, which is sponsored by the Scientific User Facilities Division, DOEBES. W.Z., Y.G., and G.L. appreciate the financial support from the National Key Basic Research Program of China (2013CB933200), the National Key Research and Development Program of China (2016YFC0204300), 111 project (B08021), and the Fundamental Research Funds for the Central Universities (222201717003). Y.G. thanks the National Natural Science Foundation of China (21571061).

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DOI: 10.1021/jacs.7b01784 J. Am. Chem. Soc. 2017, 139, 8846−8854