Unraveling Charge State of Supported Au Single-atoms during CO

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Unraveling Charge State of Supported Au Single-atoms during CO Oxidation Xiong Zhou, Qian Shen, Kaidi Yuan, Wenshao Yang, Qiwei Chen, Zhenhua Geng, Jialin Zhang, Xiang Shao, Wei Chen, Guoqin Xu, Xueming Yang, and Kai Wu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10394 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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Journal of the American Chemical Society

Unraveling Charge State of Supported Au Single-atoms during CO Oxidation Xiong Zhou†,‡, Qian Shen†,‡, Kaidi Yuan‡, Wenshao Yang , Qiwei Chen†, Zhenhua Geng , Jialin Zhang‡, Xiang Shao*, , Wei Chen*,‡, Guoqin Xu‡, Xueming Yang*, , Kai Wu*,† ∥

⊥

∥

∥

†

BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.

‡

Department of Chemistry, National University of Singapore, Singapore 117543, Singapore.

∥

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Dalian 116023, Liaoning, China.

⊥

Department of Chemical Physics, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China.

ABSTRACT: Thermally stable Au single-atoms supported by monolayered CuO grown at Cu(110) have been successfully prepared. The charge transfer from the CuO support to single Au atoms is confirmed to play a key role in tuning the activity for CO oxidation. Initially, the negatively charged Au singleatom is active for CO oxidation with its adjacent lattice O atom depleted to generate an O vacancy in the CuO monolayer. Afterwards, the Au single-atom is neutralized, preventing further CO reaction. The produced O vacancy can be healed by exposure to O2 at 400 K and accordingly the reaction activity is restored.

Ever since highly dispersed gold (Au) supported by metal oxide was used as a catalyst, extensive research has been devoted to unveiling the chemical nature for this remarkable activity of gold.1-10 Charge state of the Au species has been proposed to play a central role.11-20 Several reports11-16 claim that cationic gold is responsible for the enhancement of its catalytic activity due to its vacant d-orbital which activates carbon monoxide. Other studies17-20 propose that negatively charged gold is the active species due to its enhanced backdonation from Au d-electrons into the anti-bonding π* orbital of adsorbed carbon monoxide. A missed key point is that the charge state may dynamically change during reactions as previous computational results pointed out.21,22 Therefore, raveling the charge state and corresponding activity of the Au species is vital for understanding gold catalysis. For practical catalysts, many factors have been shown to affect the catalytic performance of gold.4-25 For instance, it is still in intense debate on whether single Au atoms are active for CO oxidation. Some studies such as individual Au atoms on FeOx,24 Au1,2 on TiO226 and Au8 on MgO17 state the inactivity of the single Au atoms for CO oxidation, while others like Au1/FeOx,14 Au doped aluminum oxide clusters,27 dispersed gold in a zeolite,28 Au1/CeO229 support the high activity of single Au atoms in CO oxidation. Thus fabrication of well-characterized Au singleatoms holds the key for exploration of gold catalysis. To prepare stabilized Au single-atoms, surface defects in oxide support and proper surface structures providing potential wells have been widely adopted.26-32 Monolayerd CuO film on

Cu(110) surface contains rectangular unit cell consisting of copper and oxygen ions can also create such potential wells to trap Au atoms.33-36 In this work, combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) techniques were employed to explore the reaction mechanism of CO oxidation on Au single-atoms at the atomic level. The evolution of charge state and activity of the Au single-atoms during CO oxidation were scrutinized in detail. Though monolayered CuO film is not a common support for Au catalysis and the results for negatively charged Au should not be simply used to explain those for positively charged Au,11-16 this well-characterized single Au atom model may help understand the CO oxidation and the importance of the dynamic Au charging state in the activity of highly dispersed Au on supports. The monolayered CuO film was prepared by exposure of Cu(110) to O2 at room temperature (RT) and subsequent annealing to 500 K in vacuum. The CuO coverage was controlled by the O2 exposure (Fig. S1). As shown in Fig. 1a, the CuO monolayer is atomically flat with a unit cell of 0.36 nm × 0.51 nm, being 1 × 2 times of the Cu(110) unit cell. Such a CuO structure has been studied for several decades35-40 and clearly identified that each protrusion in Fig. 1a corresponds to a surface Cu cation, as schematically shown in Fig. 1c. Monolayered CuO film on Cu(110) is a necessity to stabilize the Au single-atoms which routinely aggregate into clusters when bulk-like CuO film is established (Fig. S2). On the intrinsic periodic trapping grid, single Au atoms (orange dots in Fig. 1b) were prepared by deposition of 0.05 ML Au at RT. Here one monolayer is defined as one Au atom per CuO unit cell. The dots were extremely uniform in shape and size, showing the monoatomic nature of the deposited Au atoms (their line-profile being shown in Fig. S3a). Previously, Co was deposited to form atomic species at 15~20 K on similar CuO film.38 Large-scale STM image (Fig. S4) demonstrates that the Au single atoms randomly distribute and spread over the whole surface. STM images of the as-prepared Au single-atoms annealed at 400 and 500 K are given in Fig. S5. At 400 K, there were still a plenty of Au atoms on the CuO film, indicating that the Au single-atoms could even survive 400 K treatment. Such an approach turns out to be simple and highly reproducible, providing us a facile way to prepare supported Au single-atoms.

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point to that the Au single-atoms are negatively charged with respect to the Au layers.

Figure 1. STM images of the Au single-atoms. Scanning condition: V = -0.5 V, I = 30 pA. (a) Image of the CuO monolayer with the Cu cation lattice structure. (b) Au singleatoms on the CuO monolayer by deposition of 0.05 ML Au at RT. Inset: enlarged image of the Au single-atoms. White and blue cycles represent the Cu and O ions, respectively. (c) Model for the Au single-atoms on the CuO monolayer. The Au single-atoms were geometrically determined by highresolution STM image, as shown by the inset in Fig. 1b. The white and blue circles indicate the positions of surface Cu and O ions, respectively. Each Au atom precisely sits at the mid-point of two neighboring O anions along the 110 direction, and positions inside the rectangle formed by four topmost neighboring Cu cations, as illustrated by the tentative model in Fig. 1c. It’s proposed that the Au-O interaction (O-O distance being about 0.51 nm, leading to a bidentate binding configuration for the Au atom) imposes a strong adhesion of the single Au atom to the substrate with an integrated lattice. The same arguments also apply to Co38 and Ni39 atoms on the same surface. Corresponding XPS data shows features originating from Au 4f and O 1s, providing evidence for the Au-O interaction (Fig. 2). The main O 1s peak (red in Fig. 2b) from pure CuO monolayer locates at a binding energy (BE) of 529.4 eV, attributed to O2- anions in CuO.41 A small O 1s peak (blue) locates at 532.6 eV, attributed to dissociatively chemisorbed O. Two types of samples with different Au loadings were prepared: one was ~ 0.05 ML Au, forming Au single-atoms; the other, ~5 ML Au, forming Au layers (full XPS spectra of both samples being shown in Fig. S6). The chemical state of the Au layers resembles that for neutral and bulk Au. By reference to the Au 4f peaks for the Au layers (4f5/2 87.8 eV and 4f7/2 84.1 eV), the Au 4f peaks for Au single-atoms shifts downward in BE by 0.3 eV (4f5/2 87.5 eV and 4f7/2 83.8 eV). The final state effect may affect this XPS results,42 however, it is more inevitable for extended systems and causes upward shift of the XPS peaks towards relatively higher BE for smaller particles.43-45 Another effect that XPS may contain subsurface signals20 is avoided by that only single-atoms exist on the surface. Thus the downward shift in BE of the Au single-atoms should be reliable. Meanwhile, both samples exhibit a new O1s feature located at 531.0 eV (green). The upward shift in BE by 1.6 eV with respect to the main peak at 529.4 eV means that the O anions be less charged. Together with the STM results, a charge transfer, at least partially, from O anions to Au single-atoms can be safely confirmed. Both XPS data for Au 4f and O 1s exclusively

Figure 2. XPS spectra of (a) Au 4f and (b) O 1s peaks of different samples. CuO: pure CuO monolayer on Cu(110); Au layers: thermal deposition of ~ 5 ML Au on CuO monolayer; Au single-atoms: thermal deposition of ~0.05 ML Au on CuO monolayer; CO RT and O2 400 K: Au single-atoms exposed to 36 L CO at RT and subsequent exposure to 60 L O2 at 400 K, respectively.

Figure 3. In-situ STM images of Au single-atoms in CO atmosphere at RT. CO pressure: 1×10-9 Torr. V = -0.6 V, I = 50 pA. (a) Original Au single-atoms on CuO monolayer. Sample treated in CO for (b) 17 min and (c) 35 min, respectively. In situ STM imaging was acquired in 1×10-9 Torr CO at RT. Continuous STM images were collected, during which three typical images are shown in Fig. 3. Fig. 3a shows the original Au single-atoms. New dark features appear next to some Au single-atoms (Fig. 3b, c) and their quantity increases as the time elapses in CO. Small molecules such as CO could not be imaged at RT due to their high surface mobility. At low temperature (77.4 K, Fig. S7), the adsorbed CO molecules on CuO surface could be feasibly identified as small chains35 and protrusions on the single Au atoms. In order to figure the dark features out, the sample was exposed to CO at RT, and then imaged at 4.2 K in order to achieve high-resolution STM images. The CO exposure was increased to 1×10-8 Torr for 30 minutes (18 L) to generate

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Journal of the American Chemical Society a large number of dark features (Fig. 4a). As anticipated, a great number of dark features appeared nearest to the Au single-atoms. The line profile in Fig. S3b shows that each dark feature is about 0.3 Å in depth.

Figure 4. Mechanism of CO oxidation activated by Au singleatoms on CuO monolayer. V= 1.5 V, I= 50 pA. (a) STM image of Au single-atoms exposed to 18 L CO at RT. (b) Enlarged STM image of the square area marked in (a). (c) Sample (a) exposed to 60 L O2 at 400 K. (d) Schematic illustration of the CO oxidation process activated by Au single-atoms on CuO monolayer. I Negatively charged Au single-atom (δ-). II Generation of CO2 due to CO reaction with neighboring lattice O anion at RT. III Creation of Ov after CO oxidation, leading to neutralization of the Au single-atom. IV Healing of Ov by 400 K oxygen exposure, resuming the activity of the Au single-atom. Zoom-in STM image in Fig. 4b reveals that the dark features are precisely generated at the position of the O anions nearest to the single Au atoms and therefore ascribed to O vacancies created by the reaction of CO with adjacent lattice O anions. Actually, in our previous study of CO oxidation activated by Pt single atoms and nanoclusters,46 CO2 product could be clearly detected by temperature programmed desorption (TPD) technique and these dark features were identified to be O vacancies according to combined STM and TPD measurements. Such a TPD measurement was also performed for Au singleatoms (Fig. S8), but no CO2 could be detected because the adsorbed CO on single Au atoms desorbed below 180 K, well below the temperature (close to RT) of its reaction with lattice O anions. Moreover, the dark features could be completely healed by exposure of the sample to O2 at higher temperatures. When the sample in Fig. 4a was heated at 400 K in 1×10-7 Torr O2 for 10 min (60 L), most dark features disappeared (Fig. 4c). This strongly indicates that the formed O vacancies are healed at 400 K in O2. XPS experiments can also provide evidence for the assignment of the O vacancies. In Fig. 2b, the O 1s peak at 531.0 eV (green) nearly disappears upon exposure to 36 L CO at RT,

indicating that the CO oxidation involves lattice O in the CuO support. Subsequent exposure to 60 L O2 at 400 K led to the reappearance of the O 1s peak, implying that previously generated O vacancies were healed by the introduced O2 at 400 K. Therefore the CO oxidation at Au single-atoms on CuO follows a typical Mars-van Krevelen (MvK) mechanism. Since all O vacancies appeared at the positions nearest to the Au singleatoms, and almost no O vacancies appeared on a bare CuO monolayer upon its exposure to 60 L CO at RT (Fig. S9), it is concluded that the CO oxidation is indeed activated by the Au single-atoms. The CO oxidation activity is enhanced by single Au atoms through two possible ways. One is to activate neighbouring lattice oxygen so that the single Au atoms obtain charge from neighbouring oxygen that subsequently weakens the Cu-O bonds. The other is to activate the adsorbed CO molecules because negatively charged Au atoms enhance back-donation from the Au d-electrons to the anti-bonding π* orbital of the adsorbed CO molecules.17-20 Such a scenario is, however, a static picture for the apparent reaction process. What is remarkable here is that the reaction processes can be stepwise explored by both STM and XPS measurements so that the elusiveness of the charge state of the Au single-atoms during the reaction process can be uncovered. It is noticed that each Au atom symmetrically binds to two neighboring lattice O anions in the CuO monolayer. A statistics of the O 1s peak area in XPS spectra yields that the percentage of the charge-transferred O anions (green, peak area: 300 counts⋅eV) in total O anions (peak area: 2460 counts⋅eV) is about 12%, which is about double the total number of the deposited Au atoms (~0.05 ML). That means both neighboring lattice O anions in the CuO monolayer undergo a charge transfer to the Au single-atom. Moreover, the O vacancy randomly appears at either side of a single Au atom. These experimental facts point out the equivalent chemical state of the neighboring lattice O anions prior to the CO oxidation. Another prominent experimental phenomenon is that only one of the two lattice O atoms nearest to the Au is depleted. Thus, once the O vacancy forms, the chemical states of the single Au atom and its surrounding lattice O anions change and are prevented from reacting with incoming CO molecules. Corresponding XPS spectra provide further clues on this. In Fig. 2a, the Au 4f peaks shift back to the BE positions for neutral Au (4f5/2 87.8 eV and 4f7/2 84.1 eV) upon exposure to CO. The new O 1s peak (green) also disappears, meaning that after generation of the O vacancies, the charge transfer between the single Au atoms and the CuO support no longer exists, i.e., the Au single-atom does not bind to the remaining surface oxygen anymore. A possible reason is that underlying metallic Cu in the subsurface is exposed to the Au single-atom which may slightly migrate toward the vacancy and bind to the exposed Cu atoms,47 leading to that the Au-O connection is weakened and the Au charge state becomes neutral. Therefore, the Au single-atoms are deactivated and accordingly the oxidation reaction is suppressed with the left neighboring O anions. After treated in O2 at 400 K, the Au 4f peaks shifted back to the BE positions for negatively charged Au, and the O 1s peak at 531.0 eV restored. The mechanism for the CO reaction at the Au single-atoms on the CuO monolayer is schematically illustrated in Fig. 4d. The charge state of the Au single-atoms plays a key role in their activity for CO oxidation, namely, the Au single-atom gains partial negative charge (δ-) from its neighboring lattice O anions

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to activate RT CO oxidation. Once the O vacancy is created, the charge state of the Au single-atom changes from negative to neutral. Consequently, the other left neighboring lattice O is prohibited from further participation in the RT reaction because the original underlying metallic Cu substrate is exposed so that the negatively charged Au can no longer hold the charge. Finally, the O vacancy can be healed by exposure to O2 at 400 K, and the Au single-atom resumes its original activity. This study actually elucidates the charge transfer between the single Au atoms and the support (CuO in this study), which is a very important issue in Au catalysis. It may shed new light on the understanding of the extraordinary activity of highly dispersed Au on supports.

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T.; Liu, J. Nano Research 2015, 8, 2913. (15) Hutchings, G. J.; Hall, M. S.; Carley, A. F.; Landon, P.;

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Supporting Information. Experimental methods, STM images of the Cu-CuO strip structures, thermal stability test, in situ LT CO adsorption and a control experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author *

Email: [email protected], [email protected], [email protected]

[email protected],

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT (24)

This work was jointly supported by MOST (2017YFA0204702) and NSFC (21333001, 91527303), China.

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