A study of methane partial oxidation in annular reactor: activity of Rh/α-Al2O3 and Rh/ZrO2 catalysts

The activity of Rh/α-Al2O3 and Rh/ZrO2 in CH4 partial oxidation was investigated in an annular reactor, which operates at high space velocity, under kinetically controlled conditions and minimum temperature gradients along the catalyst bed. The effects of temperature, dilution, space velocity and CH4/O2 ratio were explored. Rh/α-Al2O3 provided higher H2 yields than Rh/ZrO2 at all conditions. Concerning the process kinetics, over Rh/α-Al2O3 an indirect-consecutive kinetic scheme for synthesis gas formation prevailed; the observed trends were in fact in line with the combined presence of methane total oxidation, methane reforming reactions and consecutive oxidations of H2 and CO. Over Rh/ZrO2 the additional contribution of a direct route to synthesis gas could not be excluded. The complexity of the reaction scheme seems associated with the existence of different active sites, whose concentration could be affected by metal-support interactions and Rh reconstruction during conditioning. Indeed, once exposed to the reaction atmosphere, the fresh catalysts showed an initial conditioning during which a specific activation of the high temperature routes (responsible for H2 production) was observed.