Catalytic combustion of methane on Co/MgO: characterisation of active cobalt sites

M.A. ULLA; R. SPRETZ; E. LOMBARDO; W. DANIELL; H. KNÖZINGER

APPLIED CATALYSIS B-ENVIRONMENTAL

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2001 vol. 29 p. 217 - 229

0926-3373

A series of Co/MgO catalysts with 3?12 wt.% Co were prepared by impregnation and calcined at 1073K for 10 h. The catalytic behaviour of these samples toward CH4 combustion was found to increase with cobalt loading, though a plateau was reached at ca. 9 wt.% Co content. Bulk characterisation was carried out using XRD, TPR and Raman spectroscopy, and showed that the solids were made up of a CoO?MgO solid solution and a MgO phase. A detailed examination of their surfaces was achieved through FTIR spectroscopy of adsorbed CO probe molecules, which indicated that at low cobalt loadings only a small proportion of the Co going into the solid solution was present on exposed faces as either Co2C oxo-species or pentacoordinated Co2C. However, as the cobalt content of the samples increased, a larger amount was exposed on the surface. This effect levelled off at 9 wt.% Co, after which the increase in exposed Co2C sites was countered by the masking effect of islands of MgO. In addition, at high cobalt loadings (9 and 12 wt.%) Co formed small clusters which showed bulk CoO-like behaviour. Consequently, the benefit of having surface Co2C species was balanced by the clustering effect of these species and the presence of MgO islands, negating their contribution to the overall catalytic activity of the samples.