DFT and experimental investigations into mechanistic aspects of CO-PROX reaction over Co3O4 nanocubes - activation of reactants and evaluation of the role of surface carbonate intermediates

Abstract

Mechanistic investigations based on a combination of periodic-DFT+U and first principle thermodynamic modeling combined with IR and catalytic isotopic studies were used to ascertain the pathways of CO-PROX reaction over cobalt spinel nanocube catalyst. Structural, energetic, electronic, and magnetic characteristics of all surface intermediates (O₂/O, H₂, H₂O, CO, CO₂, CO₃ adspecies) interacting single octahedral and tetrahedral (Co^tet and Co^oct) and double (Co^tet‿Co^oct and Co^oct‿Co^oct) active sites, was determined and their thermodynamic stabilities and mechanistic role were established. For the bare surface, CO and H₂ oxidation were modeled based on an intrafacial Mars-van Krevelen mechanism, with the activation barriers of ΔE_a = 1.21 for CO and 1.71 eV for H₂. For the surface covered with different monoatomic oxygen species (Co^tet_2 C-O and Co^oct_5 C-O), various variants of the Eley-Rideal scheme of direct CO and H₂ oxidation mechanisms were examined. These processes exhibit lower activation energies (ΔE_a = 0.65 for CO and 1.11 eV for H₂) than the intrafacial ones but are controlled by demanding prior dissociation of O₂. The most stable in the CO-PROX conditions diatomic oxygen species give rise to the formation of Co^tet-CO₃^2--Co^oct and Co^oct-CO₃^2--Co^oct carbonate adspecies (with ΔE_a = 0.42 and 0.35 eV, respectively), observed in IR. The Co^tet-CO₃^2--Co^oct adduct being more stable acts as a spectator (CO₂ release occurs with ΔE_a = 1.21 eV), whereas the less stable Co^oct-CO₃^2--Co^oct, as a key CO-PROX reaction intermediate (ΔE_a of CO₂ release equals to 0.81 eV). The prime role of the carbonate intermediates was substantiated by the isotopic experiments of the CO-PROX reaction over the Co₃O₄ nanocubes using ¹⁸O₂ as an oxidant.