9005
2007, 111, 9005-9007 Published on Web 06/08/2007
Interaction of Chlorine with Au-O Surface Complexes Weiwei Gao,† Ling Zhou,‡ Dilini S. Pinnaduwage,‡ and Cynthia M. Friend*,†,‡ Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street, Cambridge, Massachusetts 02138, and School of Engineering and Applied Sciences, HarVard UniVersity, 27 Oxford Street, Cambridge, Massachusetts 02138 ReceiVed: April 2, 2007; In Final Form: May 7, 2007
Chlorine effectively improves the selectivity of catalytic partial oxidation reactions on Au(111). Here, scanning tunneling microscopy was used to visualize the dispersal and redistribution of Au islands stabilized by oxygen on the surface. The dispersal of Au islands also prevents the ordering of oxygen on the surface at room temperature. This dynamic interaction stems from the through-metal nature, for example, incorporation of gold atoms in the adsorbates. The change of oxygen spatial organization is used to explain one of the major mechanisms for chlorine promotion of olefin oxidation by Au.
Recently, considerable attention has been focused on gold as an active catalyst for oxidation reactions, including low temperature CO oxidation,1,2 olefin epoxidation,3 and, most recently, oxidative amination.4 For the further development of gold catalysts for practical applications, improvement in selectivity is a challenging issue. In this regard, we recently demonstrated that chlorine dramatically increases the selectivity for styrene epoxidation on Au(111) by completely inhibiting secondary oxidation products, including acids, CO2, H2O, and residual carbon.5 In order to understand this effect, we have studied the interaction of chlorine and oxygen on Au(111). Gold(111) reconstructs into the so-called “herringbone” structure, which contains ∼4% more atoms than a (111) plane in the bulk.6 When an electronegative adsorbate such as O is deposited on Au(111), these excess gold atoms on the surface release and combine with oxygen to form Au-O complexes on the surface.7 A relationship between selectivity and surface morphology, for example, particle size and distribution, has been established for CO,2 propene, and acrolein oxidation on Au(111).8 Specifically, smaller and less ordered metastable Au-O particles formed by dosing ozone at lower temperature (200 K) show a much higher selectivity for epoxidation than those with larger size and a higher degree of ordering formed at higher temperature (400 K). Herein, we report the first scanning tunneling microscope (STM) evidence of the dynamic interaction between chlorine and Au-O complexes on Au(111) and discuss its role in the promotion of the selectivity by chlorine. The Au(111) surface initially exhibited the typical herringbone reconstruction. After the deposition of oxygen at 300 K, Au-O complexes agglomerate to form particles approximately 3 nm in diameter on both terraces and step edges (Figure 1a). In this specific case, the oxygen coverage was about 0.4 ML, as estimated by temperature programmed desorption (TPD). The * To whom correspondence should be addressed. E-mail: friend@ chemistry.harvard.edu. † Department of Chemistry and Chemical Biology. ‡ School of Engineering and Applied Sciences.
10.1021/jp072569t CCC: $37.00
Figure 1. STM images of (a) the initial oxygen covered Au(111) surface and after in situ dosing of Cl2: (b) 1.55 L; (c) 3.10 L; (d) 6.20 L. Cl2 is dosed at 300 K with a partial pressure of 4 × 10-9 Torr. Small squares cover the same area in different images, which is amplified and shown in the large square insets. The STM images are collected with a bias voltage of +0.1 V (sample biased, filled states) and tunneling current of 1 nA.
fact that both Au and O participate in forming particles is evidenced by previous studies showing that Au particles persist after removal of all oxygen on the surface by CO oxidation at room temperature (see the Supporting Information) and also by the fact that the presence of atomic oxygen stabilizes the roughened Au(111) surface.9 Real-time STM measurements of the surface morphology, while exposing Cl2 to the oxidized Au surface, show that Cl disperses the Au-O particles, yielding a highly mobile surface © 2007 American Chemical Society
9006 J. Phys. Chem. C, Vol. 111, No. 26, 2007
Figure 2. TPD spectra of 0.4 ML of oxygen (m/e ) 32) on Au(111) after exposure of the oxidized surface to Cl2: (a) 0 L; (b) 0.18 L, (c) 0.6 L; (d) 30 L. Oxygen is formed from O3 adsorption at 300 K. Cl2 was dosed from the background at 300 K.
layer at room temperature (Figure 1b-d). Previous studies showed that Cl2 dissociatively adsorbs on Au(111). The strong interaction between Cl and Au exists suggested by its high desorption temperature, for example, above 600 K, from Au(111).10,11 The insets in Figure 1a-c are enlargements of the same area in different images. After dosing 1.55 L of Cl2 (Figure 1b) onto the preoxidized Au(111) surface, STM shows a decrease of particle size from ∼3 to ∼1 nm on both step edges and terraces. When the Cl2 dosage reaches 3.10 L, no particles remain (Figure 1c). It is also observed that the step edges become serrated after dosing Cl2 as particles attached to steps before are removed by chlorine. Therefore, the STM results clearly demonstrate the interaction between chlorine and the Au-O complexes, which leads to a dispersion of the Au-O particles initially present. Comparison of parts a-c of Figure 1 shows no evidence of growth along the step edge. Therefore, the dispersed Au-O complex may transform into highly mobile species which move faster than the STM tip and cannot be imaged under the current conditions.12 When the chlorine dosage reaches 6.20 L, the STM image (Figure 1d) has many streaks along the scanning direction, particularly at step edges, suggesting the formation of mobile species. The image is similar to those obtained by dosing chlorine on a clean Au(111) surface under the same conditions. Therefore, at this stage, the interaction of chlorine with the Au(111) surface is dominant. The interaction between chlorine and oxygen on Au is also apparent during temperature programmed desorption of O2 (m/e ) 32, Figure 2). As the amount of coadsorbed chlorine is increased, the O2 peak splits, with an additional peak appearing at a lower temperature (510 K) which increases in intensity with increasing Cl2 dosage. That oxygen desorbs at lower temperature upon coadsorbing chlorine has also been observed on Pt(111) and attributed to repulsive interactions between oxygen and chlorine.13 Although the repulsive nature of adsorbed species
Letters is often invoked to explain such effects, the model may be oversimplified for this case. If metal atoms are incorporated into the metal-oxygen, the desorption kinetics may be affected by changes in the local coordination of the oxygen. The high and low temperature peaks in Figure 2 exhibit different peak shapes, indicating a difference in the rate-limiting step for desorption with and without coadsorbed chlorine. In the absence of chlorine, a second-order desorption mechanism would be expected if the recombination of oxygen atoms on the surface is the rate-limiting step. However, the asymmetric peak shape and the independence of desorption temperature on the oxygen coverage indicates a pseudo-first-order process.14 The first-order kinetics would result if oxygen desorption is rate-limited by the decomposition of an ordered Au-O overlayer, a process also suggested by other investigators.15 In the presence of coadsorbed chlorine, oxygen desorption exhibits a symmetric peak shape at a lower temperature, characteristic of secondorder recombination of adsorbed oxygen atoms. The recombination of atomic oxygen may become the rate-limiting step due to the chlorine-induced decomposition of the Au-O complexes, which disperses the adsorbed oxygen. This dispersion also appears to increase the rate of O2 desorption. It is also noted that coadsorption of chlorine and oxygen does not affect chlorine desorption (see the Supporting Information). The use of halogens, in particular Cl, has been reported in several important heterogeneous catalytic partial oxidations but not for Au catalysis.16 It is generally observed that, in the presence of halogens, partial oxidation by the addition of a single oxygen atom is enhanced, whereas further oxidation to acids and CO2 is suppressed.17,18 For example, on Pd catalysts, methane can be selectively oxidized to CO and formaldehyde in the presence of chlorine, whereas total combustion occurs when there is no chlorine.19 Monte Carlo simulations on this system led to the postulate that chlorine disperses and redistributes oxygen on the surface, creating a locally lower active oxygen concentration. This lower concentration in turn hinders attack by a second oxygen and therefore inhibits the formation of secondary oxidation products.18 Our STM observations provide direct evidence for this dispersion on Au. In summary, our results demonstrate that chlorine effectively disperses clustered Au-O complexes on Au(111), redistributing the oxygen on the surface. The dramatic promotion effect of chlorine observed in partial oxidation on Au(111) may result from this dispersion of oxygen on the surface by chlorine. The dispersal and redistribution of the oxygen on Au would render secondary oxidation steps that lead to combustion less possible. On the other hand, the presence of Cl clearly alters the Au-O bonding, which may also contribute to the enhanced selectivity for epoxidation over combustion and acid formation, recently observed for styrene oxidation over Au(111).5 X-ray photoelectron experiments and theoretical studies are planned to probe the changes in Cl-Au bonding in more detail. Acknowledgment. D.S.P. gratefully acknowledges support from NSF via the Harvard MRSEC, Grant No. DMR-02-13805. We thank the U.S. Department of Energy, Basic Energy Sciences, for support of this work under Grant No. DE-FG02ER-13289. Valuable discussions with X. Y. Liu are also acknowledged. Supporting Information Available: STM experiments of CO oxidation on Au(111), experimental details, and chlorine TPD. This material is available free of charge via the Internet at http://pubs.acs.org.
Letters References and Notes (1) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (2) Min, B. K.; Alemozafar, A. R.; Pinnaduwage, D.; Deng, X.; Friend, C. M. J. Phys. Chem. B 2006, 110, 19833. (3) Deng, X.; Friend, C. M. J. Am. Chem. Soc. 2005, 127, 17178. (4) Zhu, B.; Angelici, R. J. J. Am. Chem. Soc. 2006, 128, 14460. (5) Pinnaduwage, D. S.; Zhou, L.; Gao, W.; Friend, C. M. J. Am. Chem. Soc. 2007, 129, 1872. (6) Woll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. ReV. B 1989, 39, 7988. (7) Min, B. K.; Alemozafar, A. R.; Biener, M. M.; Biener, J.; Friend, C. M. Top. Catal. 2005, 36, 77. (8) Min, B. K.; Deng, X.; Liu, X.; Alemozafar, A. R.; Pinnaduwage, D. S.; Friend, C. M. Manuscript to be submitted for publication. (9) Biener, J.; Biener, M. M.; Nowitzki, T.; Hamza, A. V.; Friend, C. M.; Zielasek, V.; Baumer, M. ChemPhysChem 2006, 7, 1906.
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9007 (10) Spencer, N. D.; Lambert, R. M. Surf. Sci. 1981, 107, 237. (11) Kastanas, G. N.; Koel, B. E. Appl. Surf. Sci. 1993, 64, 235. (12) Mitsui, T.; Rose, M. K.; Fomin, E.; Ogletree, D. F.; Salmeron, M. Science 2002, 297, 1850. (13) Hohenegger, M. Surf. Sci. 1998, 412/413, 184. (14) Deng, X.; Min, B. K.; Guloy, A.; Friend, C. M. J. Am. Chem. Soc. 2005, 127, 9267. (15) Saliba, N.; Parker, D. H.; Koel, B. E. Surf. Sci. 1998, 410, 270. (16) Ueda, W.; Lin, S. W. Catalysis 2002, 16, 198. (17) Qian, M.; Emig, G.; Liauw, M. A. Catal. Today 2005, 99, 209. (18) Wang, Y.-N.; Marcos, J. A.; Simmons, G. W.; Klier, K. J. Phys. Chem. 1990, 94, 7597. (19) Cullis, C. F.; Keene, D. E.; Trimm, D. L. J. Catal. 1970, 19, 378.
