(258b) CO Oxidation on Bifunctional Catalysts: The Au/TiO2 Interface and the Critical Role of Water

Multiple site-specific functionalities can significantly improve the performance of heterogeneous catalysts under steady-state reaction conditions. The most commonly studied systems consist of two distinct sites that catalyze different reaction steps independently and are often referred to as bifunctional catalysts. In previous work we have used density functional theory (DFT) and microkinetic modeling to understand the mechanisms and limitations of bifunctional catalysis over bimetallic alloys. Our results indicate that the existence of universal scaling relations limits the achievable activity improvement and bifunctional catalysts do not necessarily outperform single-site catalysts. More specifically, for CO oxidation on bimetallic systems we found that the overall activity is not significantly altered when bifunctional metal alloy catalysts are considered, but equally active bifunctional catalysts may be tailored from less active and cheaper components.

In contrast, metal/oxide interfaces are not restricted to the same universal scaling relationships inherent to transition metal surfaces, but their complexity requires more detailed studies to gain a fundamental understanding of the roles of each component. The prototype example for such system is room-temperature CO oxidation over Au/TiO₂catalyst, which continues to be one the most intriguing topics ever since its discovery by Haruta and co-workers [1]. Despite the vast amount of research performed on this system, considerable debate remains regarding the nature of the active site and the role of water, which can dramatically increase catalytic activity [2,3].

In order to better understand the roles of water, surface hydroxyls, and the metal-support interface, the detailed reaction mechanism for CO oxidation must be known. Here, we use kinetic measurements, isotopic labeling, in-situ IR spectroscopy, and first-principles simulations to study CO oxidation on Au/TiO₂ and our results provide direct evidence for a novel water-mediated reaction mechanism. A large kinetic isotope effect (KIE, k_H/k_D = 1.8) implicates O-H(D) bond breaking in the rate determining step. Kinetics and in-situ IR spectroscopy experiments showed that the TiO₂ coverage of weakly adsorbed water largely determines catalyst activity by changing the number of active sites.

Our DFT simulations showed that adsorbed water at the metal/support interface greatly facilitates oxygen adsorption via a barrierless proton transfer. Oxygen activation in the resulting Au-OOH species on the Au cluster is facile; the reaction of Au-OOH with Au-CO to form Au-COOH requires an activation energy of only E_a=0.10 eV. Subsequently, the decomposition of Au-COOH to form CO₂ involves O-H(D) bond scission and a proton transfer step to water. This step was found to be rate determining with a calculated KIE of 2.55. DFT also indicates that direct involvement of surface hydroxyl group is unlikely as the calculated barrier for transferring a Ti-OH group from the support to Au (E_a = 1.63 eV) is too large to be feasible at room temperature. Furthermore, generating Au-COOH from Au-CO and Au-OOH (ΔE = -2.23 eV, E_a = 0.10 eV) is thermodynamically and kinetically far more favorable than the reaction between Au-CO and Ti-OH (ΔE = 0.60 eV, E_a  = 0.72 eV). Instead, the more likely role of Ti-OH groups is to anchor and activate water near the metal/support interface. These experimental and theoretical results provide a fresh framework for interpreting the literature, and clearly define the mechanistic roles of water, Ti-OH groups and the metal/support interface.

References

1. M. Haruta, T. Kobayashi, H. Sano, N. Yamada 1987 Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0°C. Chem. Lett. , 405–408 (1987).
2. B.D. Chandler, S. Kendell, H. Doan, R. Korkosz, L.C. Grabow, C.J. Pursell, NaBr Poisoning of Au/TiO₂ Catalysts: Effects on Kinetics, Poisoning Mechanism, and Estimation of the Number of Catalytic Active Sites.  ACS Catal. 2, 684-694 (2012).
3. M. C. Kung, R. J. Davis, H. H. Kung, Understanding Au-Catalyzed Low-Temperature CO Oxidation. J. Phys. Chem. C 111, 11767-75 (2007).