Further on the influence of the presence of small amount of N2O in the reactant feed in the catalytic oxidation of methane over supported Rh catalysts

Further information on the effects caused by the addition of small quantities (300, 500 or 1000 ppm) of N2O in the feed during the catalytic partial oxidation of methane to syngas (CPO) and its relationship with the catalytic performances of Rh supported on γ-Al2O3, MgO and TiO2 are presented. Co-feed of N2O modifies significantly the catalytic performances, particularly the H2/CO ratio tends to increase respect to the H2/CO obtained for the CPO in the absence of N2O. The yields to H2 and CO are affected in a different manner when N2O is co-fed. At N2O low concentration (300 and 500 ppm), N2O acts as a reducing promoter but on increasing their concentration (1000 ppm) it acts as an oxidant. The highest performance of syngas is obtained when rhodium works in a reduced state, this happening at 300 ppm of N2O for Rh/γ-Al2O3 and at 500 ppm of N2O for Rh/MgO and Rh/TiO2. Reversible modifications exist on the surface of the catalysts. After removing the N2O in the gas feed, catalysts recover the H2 and CO performances observed in the absence of N2O. The modification of the catalytic performances (conversion and yields) is due to the beneficial effect of N2O as a gas promoter, which allows Rh to remain in a reduced state and more disperse. N2O is adsorbed and decomposed in the surface oxygen vacancies where usually molecular oxygen would adsorb forming electrophilic species.