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

5 years ago

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

Pieter van Helden, Eric van Steen, Melissa A. Petersen, Denzil J. Moodley, Michael Claeys, Werner Janse van Rensburg

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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b01907

DOI: 10.1021/acs.jpcc.7b01907