Investigating the Elusive Nature of Atomic O from CO₂ Dissociation on Pd(111): The Role of Surface Hydrogen

CO2 dissociation is a key step in CO2 conversion reactions to produce value-added chemicals typically through hydrogenation. In many cases, the atomic O produced from CO2 dissociation can potentially block adsorption sites or change the oxidation state of the catalyst. Here, we used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations to investigate the presence of surface species from the dissociation of CO2 on Pd(111). AP-XPS results show that CO2 was dissociated to produce adsorbed CO, but dissociated atomic O was not observed at room temperature. We were only able to observe atomic O when CO2 was introduced at 500 K. Further investigations of O-covered Pd(111) revealed that chemisorbed O could be easily removed by low pressures of CO and H-2. Notably, the effect of H-2 is quite prominent since it could react with chemisorbed O at a pressure as low as 2 x 10(-9) Ton, and the presence of H-2 at ambient pressure prevented CO2 dissociation. DFT calculations showed that in the presence of background H-2, facile CO2 dissociation took place via the reverse water-gas shift (rWGS) reaction, which resulted in the formation of adsorbed CO and removal of O by H-2. DFT also identified the possible variation of surface species on simultaneous exposure of CO2 and H-2 over Pd(111) depending on temperature and pressure, which opens alternative opportunities to tune the CO2 hydrogenation catalysis by controlling the reaction conditions.