Formic Acid Electroreduction Pathways on (111) Metal Surfaces.

Formic acid (HCOOH) electroreduction is poorly studied, even though it holds promise for energy conversion and storage applications. We use density functional theory to investigate the most likely products after four HCOOH electroreduction steps on Cu(111), Au(111), Ag(111), Zn(111), Pt(111), Pd(111), and Ru(111). On Cu(111), the formation of H2, CO (and/or CO-derived C2 products), CH3OH, and CH4 is thermodynamically allowed. The Cu(111) surface has low selectivity because HCOOH reduction intermediates can adsorb via both O and C atoms. Experimentally, formic acid reduction on Cu produces H2, C2 products, and CH3OH, but very little CH4. Interestingly, CO/CO2 reduction on Cu produces CH4 rather than CH3OH. The CO/CO2 reduction pathway can proceed via the *COH intermediate, whereas HCOOH reduction can only proceed via the *CHO or CH2O*OH intermediates, possibly explaining the different product distribution. Au(111) is the most promising catalyst with high suggested selectivity to methanol and low hydrogen evolution rates. Ag(111) could be selective to methanol, but the first reduction step is very costly, so the reaction rates will be low. Zn(111), most likely, reduces HCOOH to CH4, whereas Pt(111), Pd(111), and Ru(111) most likely produce CO poisoning the catalyst surfaces.