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First Principles Study of Pd-Au Alloy Catalyst for Low-temperature CO Oxidation

Author: CaiYingYing
Tutor: CaoYiLin
School: Henan Normal
Course: Physical and chemical
Keywords: Density functional theory CO oxidation Pd-Au alloy surface Dissociation of O2 Ligandeffect Ensemble effect
CLC: X701
Type: Master's thesis
Year: 2012
Downloads: 52
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Abstract


CO which called the first public nuisance of car emissions is a potential threat to human health andliving environment, so the elimination of CO is particular important. CO oxidation is the most simple,cheap and effective method to eliminate CO, while its use in various fields of environmental, industrial,military and human life. In the catalysts of CO oxidation, bimetallic catalyst is the preferred catalystbecause of its high catalytic activity for CO oxidation. In particular, Pd-Au alloy is a highly efficientcatalyst at low temperature and shows good stability towards sintering and oxidation. On Pd-Au bimetallicsurface and particles, the synergistic effects are presented due to the “ensemble effects” of active metalconstituents and “ligand effects” of the second metal. Therefore, microscopic understanding of the catalyticactivity of Pd-Au bimetallic system and oxidation process of CO on the alloy surface is of greatsignificance, which can supply additional theoretical information for the rational design of robust Pd-Aubimetallic catalyst.In this paper, the reactions of CO oxidation on different Pd-Au bimetallic alloy surfaces have beeninvestigated at the density functional theory （DFT） by employing a slab model. The main findings are:（1） On Pd-decorated Au （111） surface Pd atoms tend to disperse. CO adsorption on the top of the Pdatom is mainly due to donation from the CO-5σ orbital to the d states of Pd atom, at the same time, backdonation from Pd-d states to the anti-bonding orbital of the CO molecule. On Pd/Au （111） surface, wefound that the CO adsorption energy at the top site increases with increasing concentration of the morereactive metal Pd, which is caused by the ligand effect. On the other hand, with the increase of Pd atoms atthe adsorption sites, the CO adsorption energy increases, which is caused by the ensemble effect.（2） O2adsorbed on Au/Pd （100） to take the “Yeager model”. Different number of Pd atoms on thealloy surface led to the different adsorption sites for O2. The adsorption energy and O-O bond lengths of O2molecule on the Au/Pd （100） are related to the number of Pd atoms on the top layer. The O2and Pd interactchiefly through hybridization between the O2-2π orbital and Pd-d2zorbital.（3） The dissociation barrier is dependent on the geometrical distribution of Pd ensembles on the Au/Pd（100） surface. Only the existence of continuous Pd atoms, O2will be able to break down on the surface. The energy barrier of0.84eV through multi-step decomposition of O2on Au7Pd2/Pd （100） surface is lessthan1.70eV through direct decomposition of O2.（4） On Au/Pd （100） alloy surface of continuous Pd atoms, CO catalytic oxidation reaction can becarried by the following procedure.①O2→2O;②CO+O→CO2. On Au6Pd3/Pd（100） surface, theactivation energy of O2dessociation is small, the reaction. Oxidation of CO is easy with continuous Pdensembles.

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