Role of Transient Co-Subcarbonyls in Ostwald Ripening Sintering of Cobalt Supported on γ‑Alumina Surfaces

The stability and mobility of atomic cobalt and of cobalt subcarbonyl species on γ-Al₂O₃ surfaces have been investigated using density functional theory (DFT) with a view to elucidate possible mobile species on these surfaces, which can act as agents in the Ostwald ripening process. The two most stable alumina surfaces γ-Al₂O₃(100) and γ-Al₂O₃(110) were probed at different levels of hydration. The stability of cobalt subcarbonyl species on γ-Al₂O₃(100) at high partial pressure of CO (10 bar) increases with increasing number of CO ligands attached to the central cobalt atom up to Co­(CO)₃ but exhibits a more complex behavior on γ-Al₂O₃(110). The effect of the hydration level on the stability of cobalt subcarbonyls was investigated. The interpretation of the DFT results in a thermodynamic model shows that at equilibrium the main cobalt subcarbonyl species present on the alumina surface at ca. 500K in the presence of CO are Co­(CO)₃ and Co­(CO)₄, with Co­(CO)₃ being the dominant species on dry γ-Al₂O₃(100) and wet γ-Al₂O₃(110). The fractional coverage of these species on a wetted alumina surface is lower than that on a dry alumina surface. The mobility of surface species was probed by exploring the potential energy surface of the adsorbed species on γ-Al₂O₃(100) and γ-Al₂O₃(110) at different hydration levels (Θ_OH = 8.5 and 17.7 OH/nm², respectively). Cobalt subcarbonyl species have a high mobility with activation barriers as low as 0.5 eV. It is argued that these species may contribute to the sintering process