Chemisorbed Oxygen Enables a Bicarbonate-Mediated Pathway for Formic Acid Oxidation on IrO2(110)

Abstract

Abstract Understanding the mechanisms for formic acid oxidation on transition-metal oxides is essential for designing catalysts with improved activity and resistance to CO poisoning. Here, we investigated how chemisorbed oxygen (Ot) influences formic acid oxidation on IrO2(110) using temperature-programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS), 18O-labeling and density functional theory (DFT) calculations. On the stoichiometric surface, RAIRS and DFT show that formic acid undergoes facile deprotonation by lattice oxygen (Obr) to form a bridging formate intermediate that subsequently produces CO2 during TPRS at temperatures near 500 K. In contrast to Obr, chemisorbed oxygen (Ot) redirects HCOOH oxidation through a distinct bicarbonate-mediated pathway that becomes increasingly dominant with increasing Ot coverage as observed by RAIRS and TPRS. In this pathway, HCOOH reacts with adjacent Ot and Obr atoms to produce a bridging bicarbonate intermediate that subsequently decomposes to CO2 near room temperature. Both DFT and 18O-labeling experiments show that an Ot atom is incorporated into the HCO3 intermediate and that Ot atoms are also incorporated into a large fraction of the room-temperature CO2 products, providing direct evidence that chemisorbed oxygen participates in the elementary steps leading to low-temperature CO2 formation. These results demonstrate that chemisorbed oxygen can fundamentally alter oxidation chemistry on IrO2(110) by creating a new reaction pathway rather than simply promoting oxidation of pre-existing intermediates. These findings expand the mechanistic understanding of formic acid oxidation on transition-metal oxides and provide design principles for oxide catalysts or co-catalysts that exploit chemisorbed oxygen to suppress CO poisoning through alternative oxidation pathways.