Understanding of Novel Surface Events on a Supported Co2+-Ensemble Catalyst Promoted by Gas-Phase Molecules: Increases in the Amount and Reactivity of Adsorbed NO by CO Undetectable at the Catalyst Surface

摘要

The paper reports an understanding of the novel surface phenomenon that the amount and reactivity of adsorbed NO on a Co~2+-ensemble/-Al_2O_3 catalyst are increased by gas-phase CO molecules undetectable at the catalyst surface. The nature of adsorbed NO species and the structures of divalent Co species were characterized by FT-IR, DR-UV/vis, XAFS, and DFT calculations. The Co~2+-ensemble structure attached on the -Al2O3 surface was crucial for the increase in the NO adsorption equilibrium and the promotion of NO reactivity at the surface by gas-phase CO, while monomeric Co~2+ sites did not bring out the promotional effects of CO. DFT calculations and FT-IR study revealed that the coordination structure of Co~2+ sites was restructured to form a more stable geminal dinitrosyl species, showing Co-OAl (surface) bond breaking, which is an origin of the increase in NO adsorption. Direct interaction of the adsorbed dinitrosyl with gas-phase CO (Eley-Rideal mechanism) was conceived to be involved in the transition state of the NO reduction to form N_2O + CO_2. The reactive NO species were assigned to cis-(NO)_2 dimeric species (mononitrosyl pair) with the 2π*-2π* bonding between the two NO adsorbates on the adjacent two Co~2+ sites with unoccupied d orbitals directed to each other. The energy profiles for the NO adsorption and reduction on the Co~2+-ensemble catalyst were presented. The present study documents how the ambient gas affects the surface species for adsorption and catalysis, which is regarded to be an essence of catalysis at surfaces.