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

摘要

A systematic first-principle study of CH_(3)OH oxidation along indirect and direct pathways on Pt(111) has been carried out, and some new insights into CH_(3)OH oxidation pathways in direct CH_(3)OH 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 CH_(3)OH oxidation step proceeds by the CH_(3)O intermediate under a nonelectrochemical environment, whereas it prefers to occur by the CH_(2)OH 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, CH_(3) can be easily formed through C–O bond cleavage in CH_(3)OH, which is a nonelectrochemical step. Thus, the CH_(x ) (x = 0–3) species is possibly formed on Pt(111) during CH_(3)OH oxidation regardless of being under an electrochemical or nonelectrochemical environment. The adsorbed CH_(x ) species will result in the blocking of the active sites and the prevention of further CH_(3)OH oxidation. Our present findings on the formation of carbonaceous deposits on Pt(111) are consistent with the experimentally observed C–O bond scission of CH_(3)OH into CH_(x ) species. Thus, we propose that the adsorbed residues that poisoned the Pt surface and impeded the performance of DMFCs may be CH_(x ) 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 CH_(x ) 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 CH_(3)OH oxidation mechanisms and designing strategies of Pt-based alloy catalysts for CH_(3)OH oxidation can be provided.