Reexamination of formic acid decomposition on the Pt(111) surface both in the absence and in the presence of water, from periodic DFT calculations

The calculated results in the literature for the decomposition of formic acid into CO on Pt(111) cannot rationalize the well-known facile CO poisoning of Pt-based catalysts. The present work reexamines formic acid decomposition on Pt(111) by considering both the monomer and dimer pathways both in the absence and in the presence of water molecules. Upon a thorough search, we locate some new adsorption configurations of formic acid for the subsequent C-H or O-H cleavage, which were ignored in previously published literature. From the calculated minimum energy pathways (MEPs), we propose that formic acid decomposition on Pt(111) is initiated by C-H bond cleavage rather than O-H bond cleavage. Monomeric formic acid preferentially decomposes to CO2 regardless of the absence or presence of water, while the dimeric form favors the formation of CO in the gas phase, but competitively reacts to form CO and CO2 in the presence of water molecules. The surrounding water molecules and the second formic acid molecule in the dimer play substantial roles in the process of formic acid decomposition, and can be regarded as promoters, assisting formic acid decomposition on Pt(111). In all situations, that is, regardless of the presence of a monomer or dimer, or the presence or absence of water, the calculated barrier differences in the rate-determining steps of CO2 and CO formation are much smaller than those reported in previously published studies. These results improve our understanding of the mechanism of formic acid oxidation catalyzed by Pt-based catalysts, and rationalize the performance durability problem of Pt-based catalysts used in direct formic acid fuel cells.