4 years ago

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

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 (ΘOH = 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

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