Activation of O 2 and CH 4 on yttrium-stabilized zirconia for the partial oxidation of methane to synthesis gas

Abstract

The isotopic exchange reaction on ZrO 2 and yttrium-stabilized ZrO 2 (YSZ) during catalytic partial oxidation of methane to synthesis gas (CPOM) was studied with transient pulse experiments. The results reveal, surprisingly, that CPOM over both oxides proceeds via a Mars–van Krevelen mechanism. Despite the presence of adsorbed oxygen species, as confirmed by isotopic exchange experiments under reaction conditions, methane is selectively oxidized by lattice oxygen ions on the surfaces of YSZ and ZrO 2. At 900 °C, about 8 and 14% of lattice oxygen in the outermost surface layer of ZrO 2 and YSZ, respectively, can be extracted by methane. Extraction of lattice oxygen results in the formation of surface oxygen vacancies. However, the routes for replenishing oxygen differ for the two oxides. For ZrO 2, the extracted lattice oxygen ions are replenished by direct activation of molecular oxygen at the site of the surface vacancy. The presence of a high concentration of surface oxygen vacancies on YSZ, generated by doping of ZrO 2 with Y 2O 3, permits fast activation of oxygen molecules and fast lattice diffusion of oxygen. The two effects together lead to a rapid replenishment of the surface lattice oxygen extracted by methane. The proposed mechanism explains both the comparatively high activity of YSZ in CPOM and the observation that, in contrast to ZrO 2, lattice oxygen is found exclusively in oxidation products of methane over YSZ during pulse experiments.