Partial oxidation of methane to synthesis gas over rhodium catalysts

The partial oxidation of methane to synthesis gas has been studied using the temporal-analysis-of-product (TAP) set-up on different Rh catalysts. Rh sponge diluted with either SiC or alpha-Al2O3, and 0.3 wt% Rh/alpha-Al2O3 were investigated using pulses and continuous flow of methane and oxygen. The effect of introducing a support in the catalyst system was given special concern. Blank experiments on the alpha-Al2O3 support showed no activity for the partial oxidation of methane in the 673-1123 K range. Oxygen multipulse experiments on the 0.3 wt% Rh/alpha-Al2O3 and on the pure support showed that alpha-Al2O3 is not inert concerning adsorption, desorption and storage of oxygen. Totally oxidized catalysts show low activity because of few sites for methane dissociation. Mainly, total oxidation products CO2 and H2O are formed. The formation of these products partially reduces the catalyst whereupon with time-on-stream a steep discontinuity is observed in the methane and oxygen conversion as reduced metal sites largely increase the sate of methane dissociation. The discontinuity is followed by a continuous change in the product distribution from CO2 and H2O towards CO and H-2. The time-on-stream, before an active catalyst was obtained, increased with increasing amount of oxygen in the feed. The time before obtaining an active catalyst also depended on the catalyst system and changed in the following way: Rh sponge mixed with SiC<Rh sponge mixed with alpha-Al2O3<Rh/alpha-Al2O3. The latter indicates a spillover effect of oxygen from the support to the metal which also affects the steady-state methane conversion and syngas selectivity. The CO, CO2 and H2O pulse responses shifted to shorter times with increasing amount of oxygen in the feed, while H-2 showed the opposite effect. This indicates a faster formation of the products containing oxygen but sorption effects may not be excluded. Removal of surface oxygen by methane pulsing led to oxygen migration from the Rh bulk resupplying the surface. When the catalyst is at steady state, CO and H-2 are formed as primary products on a Rh sponge mixed with SiC. Experiments on Rh sponge mixed with alpha-Al2O3 or a Rh/alpha-Al2O3 showed the same product sequence and unless sorption effects disguise the real primary product formation the sequence is not altered by the support. It is believed that the formation of primary products depends on the amount of oxygen available on the surface. (C) 1998 Elsevier Science B.V.