Role of the surface state of Ni/Al2O3 in partial oxidation of CH4

Abstract

The effect of gas phase O2 and reversibly adsorbed oxygen on the decomposition of CH4 and the surface state of a Ni/Al2O3 catalyst during partial oxidation of CH4 were studied using the transient response technique at atmospheric pressure and 700°C. The results show that, when the catalyst surface is completely oxidized under experimental conditions, only a small amount of CO and H2 can be produced from non‐selective oxidation of CH4 by reversibly adsorbed oxygen which is more active in oxidizing CH4 completely than NiO via the Rideal–Eley mechanism and both the conversions of CH4 and O2 and the selectivities to CO and H2 are very low. Therefore, keeping the catalyst surface in the reduced state is the precondition of high conversion of CH4 and high selectivities to CO and H2. The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH4. During partial oxidation of CH4, no oxygen species but a small amount of carbon exists on the catalyst surface, which is favorable for maintaining the catalyst in the reduced state and the selectivity of CO. The results also indicate that direct oxidation is the main route for partial oxidation of CH4, and the indirect oxidation mechanism is not able to gain dominance in the reaction under the experimental conditions.