Formic Acid Electroreduction Pathways on (111) Metal Surfaces

We explore HCOOH electroreduction pathways on seven (111) surfaces using density functional theory and experimental literature insight. Cu(111) mainly produces H₂, CH₃OH, and C₂ products in accordance with experiments. The most likely outcome is CH₃OH on Au(111), CH₃OH with low rate on Ag(111), CH₄ on Zn(111), and CO that poisons the catalyst on Pt(111), Pd(111), and Ru(111).

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.

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