The origins of formic acid electrooxidation on selected surfaces of Pt, Pd, and their alloys with Sn

Abstract

Bimetallic and trimetallic catalysts enhance the catalytic process of formic acid oxidation (FAO) and its adsorbed CO* main intermediate. A comprehensive study, using DFT calculations and electrochemical experiments, was conducted on a series of five mono-, bi-, and tri-metallic catalysts for FAO: Pt(111), Pd(111), Pt₃Sn(111), Pd₃Sn(111), and Pt₃Pd₃Sn₂(111), considering both pathways: direct oxidation to CO₂ and the indirect pathway involving the oxidation of adsorbed CO*. The results show that H–COOH cleavage is thermodynamically predominant on most of the surfaces except Pd₃Sn(111). However, kinetically, O–H bond scissoring is preferred on all of the surfaces. The barriers for O–H and C–H cleavage for Pt(111) and Pt₃Pd₃Sn₂(111) are comparable thus the process is governed by thermodynamics. On Pt(111), C–H cleavage is favored, while on Pt₃Pd₃Sn₂(111), both O–H and C–H cleavage are equally viable due to similar thermodynamic profiles. Additionally, Pt-based alloys promote the indirect mechanism, whereas Pd enhances the direct mechanism. Alloying Pt with Sn and the combined effect of Pt, Pd, and Sn in a trimetallic alloy results in the weakening of the HCOOH adsorption and reduces all the activation barriers. The electrochemical findings support the computational results, demonstrating that Pd(111) exhibits oxidation peaks at a low potential, 0.37 V, (indicating a direct mechanism), while Pt-based catalysts display oxidation peaks at around 0.95 V, indicative of the indirect mechanism. Notably, Pt₃Pd₃Sn₂/C shows the highest overall performance towards FAO, with peak current densities of 225 mA mg_PGM⁻¹. Thus, Pt₃Pd₃Sn₂ is the most efficient catalyst, providing the lowest energetic pathways for FAO reaction.