Theoretical Investigations On Reaction Mechanisms Of Methanol Partial Oxidation Catalyzed By Platinum-based Binary Alloys

Direct methanol fuel cell (DMFC) is one of the proton exchange membrane fuelcells(PEMFCs) with methanol as a liquid fuel. It is an efficient technology to convertchemical energy into electrical and thermal energy. DMFCs have received tremendousattention due to its advantages at many aspects, such as simple structure, lowoperating temperature, zero pollution, high efficiency, safety, reliability and so on. The development and application of direct methanol fuel cells can effectively alleviate theenergy crisis and reduce environmental pollutions. It is well known that the platinumis the most widely and leading catalyst in DMFCs. However, the usefulness ofDMFCs is limited by other factors such as electro-catalyst. On the one hand, he costof platinum is expensive, and platinum reserve on earth is limited. On the other hand,pure platinum catalysts are easily poisoned by carbon monoxide, which is formed bypartial oxidation of methanol either as a direct intermediate or a side reaction product.The adsorption of carbon monoxide on platinum is very stable, and thus reduces thecatalytic activity and utilization of platinum catalyst. Therefore, it is very important todevelop the catalysts of high catalytic activity and strong resistance to carbonmonoxide poisoning for the development of direct methanol fuel cell. In order todesign better catalyst, it is very significant to understand the mechanism of methanolelectro-oxidation reaction on anode in DMFCs. In this work, we have investigated thereaction mechanism of methanol partial oxidation on Pt6M (M=Pt, Ru, Sn) binaryalloy catalysts by using the density functional theory (B3PW91).We compared thecatalytic activity of Pt6M (M=Pt, Ru, Sn) binary alloy catalysts for methanol partialoxidation reaction. This research is expected to provide valuable information for themethanol electro-oxidation mechanism on anode catalyst of DMFCs. The main resultsof this work can be summarized as follows: Using the density functional theory (B3PW91), we have investigated the reactionmechanism of the initial adsorption and dehydrogenation steps of methanol on Pt6M(M=Pt, Ru, Sn) clusters. Pt6M (M=Pt, Ru, Sn) clusters are used to simulate thecatalysts. Two adsorption and dehydrogenation mechanisms of methanol were studied:one used the hydroxyl-hydrogen atom and methyl-hydrogen atom approach the Pt andM atoms of Pt6M, and the other used hydroxyl-oxygen atom and hydroxyl-hydrogenatom attack the M and Pt atoms of Pt6M, respectively. The reaction potential energysurfaces (PES) show that on pure Pt7cluster the favorable dehydrogenation channel isthe decomposition of CH3adsorption complex. On the Pt6Sn cluster thedecomposition of hydroxyl adsorption complex is more favorable. On Pt6Ru cluster,both pathways are dominant. The adsorption energy, energy barrier, dissociationenergy, NBO analysis and frontier molecular orbital analysis are discussed atB3PW91/LANL2DZ level of theory. The addition of Ru and Sn enhance the catalyticactivity of pure Pt for methanol partial oxidation reaction. We may conclude thatPt6Ru bimetallic catalyst is most effective among Pt6M (M=Pt, Ru, Sn) catalysts forinitial adsorption and dehydrogenation reaction of methanol in DMFCs.