Identification of Carbonyl Species on Palladium Supported on Ceria in Complex Microenvironments

Herein, we present a systematic comparison between Pd carbonyl (Pd-CO) species, specifically over Pd/CeO2 based catalysts, observed during isothermal adsorption and in several prototypical catalytic reactions to identify and understand CO adsorption on palladium–ceria based catalysts. Pd-CO is observed via DRIFTS to probe the gas–solid conditions, while ATR-IR is used to probe the affinity of Pd-CO under more complex solvated gas–solid–liquid conditions to discern the influence of the microenvironments for carbonyl adsorption. We explore the presence of Pd-CO under several reactive environments, including CO adsorption, CO2 + H2, CO + H2, CH4 + CO2 and CO under gas–solid–liquid media, highlighting reactions with notable Pd-CO formation. The differences between palladium carbonyls and carbonate species show that carbonyl species are much more affected via a shifting of the peak position than carbonates, which remain static irrespective of the immediate chemical environment. By following the rate of CO accumulation via K–M mode DRIFTS, we observe migration from linear, 2095 cm–1, to bridge site, 1978 cm–1, as a function of time under a static CO atmosphere. With the use of DFT, we discerned changes in Pd-carbonyl stretches due to both coverage effects of CO under simulated reaction conditions and temperature effects. Regardless of whether CO is formed as an intermediate or a reactant, the competitive adsorption of *H and *CO affects the binding strength of *CO at all temperatures, with low temperature favoring atop binding and high temperature favoring the more stable FCC Pd-CO site.