New Insights into the Pt-Catalyzed CH3OH Oxidation Mechanism: First-Principle Considerations on Thermodynamics, Kinetics, and Reversible Potentials.

Abstract

A systematic first-principle study of CH3OH oxidation along indirect and direct pathways on Pt(111) has been carried out, and some new insights into CH3OH oxidation pathways in direct CH3OH fuel cells (DMFCs) are presented. The thermodynamics, kinetics, and reversible potentials for all possible elementary steps, initializing with C–H, O–H, and C–O bond cleavages and proceeding via sequential decomposition and oxidation from the reaction intermediates, are analyzed. Some key reactive intermediates are identified. By comparing the activation energies and reversible potentials of various possible elementary reaction steps, we can speculate that the initial CH3OH oxidation step proceeds by the CH3O intermediate under a nonelectrochemical environment, whereas it prefers to occur by the CH2OH intermediate under electrochemical environment. Furthermore, CHO hydroxylation into HCOOH along a direct pathway is more facile to occur than CHO dehydrogenation into CO along an indirect pathway at the nonelectrochemical interface, whereas the indirect and direct pathways may be parallel pathways on Pt(111) under the present simulated electrochemical environment. Simultaneously, CH3 can be easily formed through C–O bond cleavage in CH3OH, which is a nonelectrochemical step. Thus, the CHx (x = 0–3) species is possibly formed on Pt(111) during CH3OH oxidation regardless of being under an electrochemical or nonelectrochemical environment. The adsorbed CHx species will result in the blocking of the active sites and the prevention of further CH3OH oxidation. Our present findings on the formation of carbonaceous deposits on Pt(111) are consistent with the experimentally observed C–O bond scission of CH3OH into CHx species. Thus, we propose that the adsorbed residues that poisoned the Pt surface and impeded the performance of DMFCs may be CHx species, rather than CO species, since the direct pathway is more favorable on Pt(111) at the nonelectrochemical interface. However, the poisonous species that occupied the active sites of the Pt surface may be CHx and CO species due to the simultaneous occurrence of oxidation pathways on Pt(111) under the present simulated electrochemical environment. Based on the present study, some new insights into CH3OH oxidation mechanisms and designing strategies of Pt-based alloy catalysts for CH3OH oxidation can be provided.