(561a) A Molybdenum Carbide Catalyst for Formic Acid Decomposition: Improving Selectivity by Surface Modification

Formic acid is a promising energy source for fuel cells. However, fuel cells rely on platinum group catalysts which are expensive, of low-abundance, and susceptible to poisoning by impurities and reaction byproducts, namely CO. A promising catalytic alternative to platinum group metals are transition metal carbides. Here, we show how carburization of the Mo(110) single crystal surface changes the activity and improves the selectivity for the decomposition of formic acid. We have investigated the kinetics and mechanisms of the surface reactions of formic acid utilizing temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and reactive molecular beam scattering (RMBS). Metallic Mo(110) generally induces decomposition via bimolecular dehydration (yielding CO, H₂O, H₂, and CO₂) as well as total decomposition to atomic C, H, and O. The undesirable reactions occur via C-O bond cleavage which produces a formyl (HCO) intermediate which decomposes to form CO. However, the molybdenum carbide model catalyst selectively promotes dehydrogenation, forming the desired products H₂ and CO₂, by unimolecular deprotonation [O-H bond cleavage followed by C-H bond cleavage]. Molybdenum carbide achieves higher selectivity by stabilizing the formate (HCOO) surface intermediate and by significantly hindering the rate of C-O bond cleavage preventing the formation of CO. Consequently, formate preferentially decomposes through C-H bond dissociation leading to CO₂ and H₂ (by the recombinative desorption of atomic hydrogen). Our research suggests that molybdenum carbide catalysts may be effective electrochemical catalysts for fuel cells.