Effect of support on partial oxidation of methane to synthesis gas over supported rhodium catalysts

Two kinds of oxides, reducible and irreducible, were used as supports. It was found that the reducible-oxides-supported rhodium catalysts provide, in general, much lower activities and selectivities than those supported on irreducible oxides. The exposed metal surface areas of the reduced catalysts were determined, and the reaction behaviors of CH4/O-2 (2/1) over the pure supports and the precalcined 1% Rh(O)/MxOy catalysts were investigated in a pulse microreactor. It is suggested that the partial coverage of rhodium sites by the reducible oxides and the combustion of methane that occurs over these oxide sites are responsible for the lower activities and selectivities observed over this kind of supported catalysts. Among the irreducible metal oxides, gamma-Al2O3, La2O3, and MgO provided stable catalytic activities and selectivities during 100 h of reaction, and the activity increased in the sequence La2O3 < gamma-Al2O3 less than or equal to MgO. Possible explanations for stability are proposed on the basis of TPR and XRD experiments. In addition, the nature of the active sites is examined based on pulse reaction experiments. (C) 1999 Academic Press.