J. Phys. Chem. C 2007, 111, 11767-11775
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FEATURE ARTICLE Understanding Au-Catalyzed Low-Temperature CO Oxidation Mayfair C. Kung,*,† Robert J. Davis,‡ and Harold H. Kung*,† Department of Chemical and Biological Engineering, Northwestern UniVersity, EVanston, Illinois 60208-3120, and Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22904-4741 ReceiVed: March 15, 2007; In Final Form: May 17, 2007
The discovery of exceptionally high catalytic activities of small Au particles has initiated intense research activity to understand their origin. In spite of a large volume of work, the system is far from being fully understood. There are four major issues in Au-catalyzed CO oxidation that have not been resolved: (1) the importance of the nature of the support on catalyst activity; (2) the Au oxidation state necessary for high activity; (3) the sensitivity of the activity to the moisture level in the reaction feed; and (4) reasons for the high activity in small Au particle size and for the strong dependence on particle size and specific morphology. The current understanding of these issues based on available experimental evidence and computational investigations is discussed, as well as aspects that remain unresolved.
Introduction In 1989, Haruta et al.1 first reported that nanosize gold particles deposited on metal oxides catalyze CO oxidation at temperatures as low as 203 K. This report of exceptional activity sparked off two decades of intensive research in Au catalysis. In addition to CO oxidation, Au also catalyzes a wide array of reactions such as water-gas shift,2,3 vapor-phase epoxidation of propylene using H2 and O2,4,5 selective hydrogenation,6-9 selective CO oxidation,10 hydrochlorination,11 and hydrocarbon selective oxidation.12-15 There are two features of Au catalysis that make it particularly unique. One is that in some reactions its performance can be tuned simply by changing the catalyst loading. For example, in the vapor-phase oxidation of propylene in the presence of H2 and O2,16 the predominant product changes from propane to propylene oxide to CO2 depending on the catalyst Au loading. Another feature is that Au can catalyze reactions with a much higher selectivity than other catalysts, especially oxidation reactions using molecular oxygen. Whereas epoxidation of higher alkenes are generally accomplished using alkyl hydroperoxide or hydrogen peroxide as the oxidant,17 Au/C catalyzes cyclohexene epoxidation using molecular O2.15 Au also catalyzes the oxidation of glucose to gluconic acid with molecular O2 with 100% selectivity18 and the monooxygenation of ethane-1,2-diol with 93% selectivity at 80% conversion.19 Beside oxidation reactions, Au catalysts also catalyze demanding selective hydrogenation reactions, such as selective hydrogenation of CdC versus CdO in R,β-unsaturated aldehydes/ketones to the corresponding unsaturated alcohol. An example is the hydrogenation of benzalacetone to the corresponding unsaturated alcohol over Au/Fe2O3 with over 60% selectivity up to 100% conversion.9 * Corresponding authors. E-mail:
[email protected] (M.C.K.);
[email protected] (H.H.K.). † Northwestern University. ‡ University of Virginia.
Thus, supported Au catalysts stand apart from other metal catalysts in both its unusual activity and selectivity, especially in reactions involving molecular oxygen. In this article, we will focus on aspects of Au catalysis related to low-temperature CO oxidation. It is hoped that a thorough understanding of this apparently simple reaction could serve as a platform to understanding other selective oxidation reactions. Since the first publication of Haruta, the number of publications dealing with this research topic in recent years has been overwhelming, and many excellent review articles have been written.20-23 Thus, the purpose of this article is to critically analyze some of the unresolved issues, using both literature results and examples drawn from our own research. The sheer volume of recent reports necessitates selection of published results for discussion, and we make the selection based on two criteria: the reported catalytic activity must reflect those of highly active catalysts, which we define arbitrarily to be higher than supported Pt, and there must be either in situ, or at least postreaction, characterization of the catalyst. The activity criterion is necessary because the literature values for atomic rate (mol CO reacted (mol Au-s)-1) for low-temperature CO oxidation on apparently similar catalysts vary by orders of magnitude, even among the selected examples in Table 1 that meet our activity criteria. Some of the reported activities are comparable to that of Pt/TiO2,24 which suggest that the corresponding catalysts, although good, are no longer unusually active. Consequently, the corresponding spectroscopic characterization would not capture information relevant to the unusual active centers. In discussing catalytic activity, in addition to the term atomic rate used above, turnover frequency (TOF, mol CO reacted (s-mol surface Au or specific site that will be specified in the text)-1) will also be used. In situ characterization or characterization of used samples is necessary because it is known that Au samples change easily, especially under reaction conditions. For example, the Au species in Au/TiO2 is present as an ionic complex after deposition-precipitation (DP). This
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Mayfair C. Kung is Research Associate Professor in the Chemical and Biological Engineering Department at Northwestern University. She received her B.S. degree in biochemistry from the University of Wisconsin, Madison and both her M.S. and Ph.D. degrees in chemistry from Northwestern University. Her research interests include siloxane chemistry and synthesis and catalysis of nanostructured materials.
Kung et al.
Harold H. Kung received his B.S. in chemical engineering at the University of Wisconsin, Madison, and Ph.D. in chemistry form Northwestern University. After two years at the Central Research Department at Dupont de Nemours & Co., he joined Northwestern and is currently Professor of Chemical and Biological Engineering at Northwestern University. His interest in heterogeneous catalysis covers solid acid, selective oxidation, and environmental catalysis, as well as developing synthetic methods for novel, nanoscale structures for catalysis.
TABLE 1: Selected Examples of Reported CO Oxidation Activities of Highly Active Supported Au Catalysts catalyst
Robert J. Davis is Professor and Chair of Chemical Engineering at the University of Virginia. He received his B.S. degree in chemical engineering from Virginia Tech and both his M.S. and Ph.D. degrees in chemical engineering from Stanford University. Prior to joining the faculty at UVa in 1990, he worked as a postdoctoral research fellow in the Chemistry Department at the University of Namur in Belgium. His research interests include synthesis, spectroscopic characterization, and kinetic evaluation of solid catalysts.
sample exhibits high activity for room-temperature CO oxidation without further treatment. However, in situ XANES characterization shows clearly that the Au cations are rapidly reduced to metallic Au in a reaction feed.25 Thus, conclusions based on characterization of prereaction samples would be misleading. In addition to experimental results, we will also draw on conclusions derived from surface science and computational studies. These studies have been very helpful in advancing the understanding of the field. There are four major issues in Au-catalyzed CO oxidation that have not been resolved: (1) the importance of the nature of support on catalyst activity; (2) the Au oxidation state necessary for high activity; (3) the sensitivity of the activity to the moisture level in the reaction feed; and (4) reasons for the high activity in small Au particle size and for the strong dependence on particle size and specific morphology. In the following sections, we will explore these issues, relying on our own as well as selected literature results and will discuss them with respect to a model of the active site and a proposed reaction mechanism. Nature of the Support There have been conflicting reports on the dependence of CO oxidation activity on the nature of the support. The surface
Pt/TiO2 Au/TiO2 Au/TiO2 Au/TiO2 Au/TiO2 Au/TiO2 Au/TiO2 Au/TiO2 Au/TiO2 Au/Al2O3 Au/Al2O3 Au/Al2O3 Au/CeO2 Au/CeO2 Au/ZrO2 Au/ZrO2 Au/Fe2O3 Au/Fe2O3
ratea mol CO metal diameter,b (mol metal‚s)-1 T, K nm 0.003 0.13 0.02 0.025 0.039d 0.16 0.35 0.03 0.1 0.03 0.05 50 m2/g). For example, the Au particles in a 1 wt % sample, present as 2 nm cubic crystallites with FCC packing, would occupy ∼0.01 m2 area, equivalent to 8 is repulsive. Indeed, the authors found that the reaction rates calculated per corner Au atom were the same for Au/TiO2 and Au/MgAl2O4 and 4 times lower for Au/Al2O3. They proposed that these low coordination sites are used to bind CO, whereas oxygen can be activated on Au or at the metalloxide-boundary.46,87 In their model, however, they did not address the role of moisture or the deactivation/regeneration behavior of Au catalysts. Conclusion Why Au can be an extremely active catalyst continues to be an intriguing question. The discussions above illustrate that, although great strides have been made in understanding the system, there still remain many unresolved issues, particularly with respect to the nature of the active site and the reaction mechanism. The lack of consensus arose in part because of the inherent difficulties in studying supported Au catalyst due to its high susceptibility to poisoning and low density of active sites. Small metallic Au particles are important, presumably because they have a higher density of active sites than large particles. Support appears to play an indirect (secondary) role in influencing reactivity but is unlikely to participate in the reaction directly by supplying active oxygen. Although metallic Au is deemed necessary for activity, the question of whether cationic Au is needed or constitutes a component of the active site continues to linger. The mode of oxygen activation is by far the least understood, yet this step is probably the most relevant in understanding and exploiting Au catalysis for other oxidation reaction, such as selective epoxidation and oxidation of hydrocarbons. Whereas potential for applications has expanded research into investigations to improve the catalysts, such as by addition of modifiers and alloying, improving our understanding of the issues identified here would undoubtedly help to craft more rational approaches to find better catalysts. Acknowledgment. Work cited here was supported by the Northwestern University Institute of Catalysis in Energy Processes funded by the Department of Energy (Grant No. DEFG02-03ER15457), the National Science Foundation (Grant No. CTS-0121619), and Department of Energy, Basic Energy Sciences (Grant No. DE-FG02-01ER15184). The DND-CAT is supported by E. I. DuPont de Nemours & Co., Dow Chemical Co., the National Science Foundation (Grant No. DMR9304725), and the State of Illinois through the Department of Commerce and the Board of Higher Education (Grant No. IBHE HECA NWU 96). The APS was supported by the U.S. Department of Energy, Office of Energy under Contract No. W-31-102-Eng-38.
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