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

Abstract

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 H2. Notably, the effect of H2 is quite prominent since it could react with chemisorbed O at a pressure as low as 2 × 10–9 Torr, and the presence of H2 at ambient pressure prevented CO2 dissociation. DFT calculations showed that in the presence of background H2, 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 H2. DFT also identified the possible variation of surface species on simultaneous exposure of CO2 and H2 over Pd(111) depending on temperature and pressure, which opens alternative opportunities to tune the CO2 hydrogenation catalysis by controlling the reaction conditions