Dissecting the steps of CO2 reduction: 1. The interaction of CO and CO2 with γ-Al2O3: an in situ FTIR study

Abstract

The adsorption of CO₂ and CO was investigated on a pure γ-Al₂O₃ support material that has been used in Pd and Ru catalysts for the reduction of CO₂. The adsorption of CO₂ resulted in the formation of carbonates, bicarbonates and linearly adsorbed CO₂ species. The amount and the nature of the adsorbed species were dependent on the annealing temperature of the alumina support. On γ-Al₂O₃ annealed at 473 K mostly bicarbonates formed, while no adsorbed CO₂ was seen on this highly hydroxylated surface. With increasing calcination temperature the amount of both surface carbonates and linearly adsorbed CO₂ increased, but still the most abundant surface species were bicarbonates. Surface carbonates and adsorbed CO₂ can readily be removed from the alumina surface, while bicarbonates are stable to elevated temperatures. The interaction of CO with γ-Al₂O₃ is much weaker than that of CO₂. At room temperature CO adsorbs only on Lewis acid sites, and can be readily removed by evacuation. At 100 K CO can probe different defect sites on the alumina surface. Under no conditions we have observed the formation of any carbonates or bicarbonates upon the interaction of CO with the pure alumina support. In co-adsorption experiments CO competes for adsorption sites with the linearly adsorbed CO₂ on the 773 K-annealed γ-Al₂O₃ surface, but it does not result in the desorption of CO₂, rather in the increased production of weakly held carbonates. After the removal of adsorbed CO, CO₂ moves back to its original adsorption sites, i.e., Lewis acidic Al³⁺ centers. The exposure of a CO₂-saturated γ-Al₂O₃ to H₂O did not affect any of the adsorbed surface species. The findings of this study will be used to rationalize the results of our ongoing in situ and in operando studies on the reduction of CO₂ on supported Pd and Ru catalysts.