Improving selectivity by the addition of N2O in the feed during partial oxidation of methane over supported rhodium catalysts

New results studying the effects brought about by the introduction of small amounts of N2O in the gas feed (promoter) during the partial oxidation of methane (POM) over Rh/γ-Al2O3 and Rh/MgO catalysts are presented. Standard tests were performed with 5 vol.% CH4, 2.5 vol.% O2, and balanced with N2. When small amounts of N2O (300 ppm, 1000 ppm) were added to the feed, yield and selectivity to H2 and CO increased. Under these conditions rhodium works in a reduced state, resulting in the enhancement of syngas selectivity and the inhibition of the total oxidation of methane. This effect tends to disappear after a certain period of time when N2O supply is cut. The effect is reversible. It is suggested that N2O is adsorbed on the vacancies where O2 does, inhibiting the formation of oxidant (electrophilic) oxygen species produced by dissociation of O2 on the same vacancies. In the presence of N2O, rhodium remains in a reduced oxidation state. Reduced rhodium modifies the kinetics of all reactions involved under POM reaction conditions and promotes synthesis gas.