Kinetics of palladium particles on LiNbO 3 : an origin of the polarization-dependent catalysis
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Kinetics of palladium particles on LiNbO3: an origin of the polarization-dependent catalysis Seungchul Kim, Michael Rutenberg Schoenberg and Andrew M. Rappe The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, U.S.A. ABSTRACT Using ab-initio calculations and kinetic Monte Carlo simulations, we demonstrate that the deposition geometries of palladium are strongly dependent on the polarization direction of the LiNbO3 substrate. Different stoichiometries and atomic structures of the positively and the negatively polarized substrates cause substantially different bonding configurations of palladium and energy barriers for the movement of Pd clusters. Our simulations predict that palladium atoms form bulky clusters on the positive surface, while they are deposited in a dispersed or planar manner on the negative surface at moderate temperature. We suggest that Inoue and coworkers’ observation [J. Phys. Chem. 88, 1148 (1984)] that the catalytic activity of palladium depends on polarization direction of LiNbO3 substrate is, at least in part, due to differences in the geometric structures of palladium and the LiNbO3 surface. INTRODUCTION A transition metal that is supported on a ferroelectric oxide is a special subcategory of metal-supported oxides, which are widely used as heterogeneous catalysts. However, it has been expected that a ferroelectric support has one degree of freedom that may alter catalytic activity: switchable polarization. Inoue and coworkers demonstrated this idea in 1984 [1]; they showed that the activation barriers of CO oxidation differ by 30 kJ/mol when the reactions are catalyzed by Pd supported on the positively and the negatively poled lithium niobate (LiNbO3) substrates. The traditional explanation of polarization-dependent catalysis is that the opposite signs of surface charges on oppositely polarized surfaces alter the electronic structure of the supported metal differently; in turn, catalytic activity will be changed. However, surface charges of any polarized materials must be passivated to prevent the divergence of the electrostatic potential energy [2]. Thus, there might not be enough surface charge to alter the electronic structure of the catalytic metal. Then, what could alter the activity of this kind of catalyst? Since the catalytic activity of a transition metal can be affected both electronically and geometrically [3], we speculate that this activity difference is more geometric in origin rather than electronic. In fact, it has been experimentally demonstrated that particle sizes of Pd are different when deposited on the positive and negative surfaces of LiNbO3 (0001) [4, 5]. Furthermore, polarization-dependent CO temperature programmed desorption (TPD) is found in the cases that show large differences in particle size [4]. These experiments seem to support our speculation. We investigate whether and how the polarization direction of the LiNbO3 (0001) substrate affects the geometric structure of deposited Pd. First, we te
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