First-principles Study of PdAu Segregation with CO Coverage
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1177-Z02-06
First-principles Study of PdAu Segregation with CO Coverage
Bin Shan1*, Jangsuk Hyun1, Neeti Kapur1, and Kyeongjae Cho2 Nanostellar, Inc., 3696 Haven Avenue, Redwood City, CA 94063 Department of Materials Science and Engineering and Department of Physics, University of Texas at Dallas, Richardson, TX, 75080 [email protected] ABSTRACT Alloying has been one of the strategies to develop alternatives to Pt based CO oxidation catalyst. PdAu bimetallic alloy has recently been shown to have better reactivity and thermal stability toward CO oxidation for diesel engine applications as compared to pure metal catalysts. The key factor for low temperature light off in diesel engine catalysis is reactivity of alloy catalysts under CO environment, which in turn depends on the alloy surface composition and morphology. We explored the segregation processes in bimetallic 1:1 ratio Pd-Au alloy using first-principles calculations, assisted by a Monte-Carlo (MC) scheme that combines an improved Embedded Atom Method (EAM) and an atomistic treatment for adsorbed CO molecules for searching low energy states. Our simulation results show that PdAu surface changes from Au-rich to Pd-rich with increase in CO coverage up to 0.75 ML, beyond which additional CO adsorption is no longer favorable. A quantitative relationship between CO coverage and Pd concentrations on the surface is also revealed. INTRODUCTION Bimetallic alloys have been widely used in electrochemistry and surface catalysis. A higher reactivity can be achieved by optimizing the additional engineering flexibility in bimetallic systems which are not readily accessible in pure metal catalysts. PdAu bimetallic alloy has recently attracted much interest due to their potential applications in aromatics hydrogenation and CO oxidation [1,2]. These alloys also show good stability towards sintering and oxidation. To understand their physical and chemical properties, many density function theory (DFT) calculations have been carried out to study catalytic performance of PdAu alloys. However, most of the studies focus on well ordered bulk phase PdAu alloy and under low CO coverage conditions [3,4]. A systematic investigation of CO adsorption patterns on PdAu and the adsorbate-induced segregation of PdAu is still lacking. In order to gain insight into the CO adsorption processes and effects of CO molecules on PdAu bimetallic alloy segregation, we have performed DFT calculations and Monte-Carlo (MC) simulations to search for the ground states of 1:1 ratio PdAu alloy under different CO coverage conditions. Based on the simulation results, Pd concentration in the top layer gradually increases with CO coverage and reaches a monolayer at CO saturation coverage of 0.69 ML. The surface segregated Pd atoms come from either sub-surface layer or from deeper layers depending on the CO coverage. A quantitative relation between CO concentration and Pd surface concentration is observed. COMPUTATIONAL METHODS
The DFT calculations were done using the Vienna Ab-initio Simulation Package (VA
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