Interdependence of Point Defects and Reaction Kinetics: CO and CH<sub>4</sub> Oxidation on Ceria and Zirconia
Maximilian Schaube, Rotraut Merkle, Joachim Maier
Abstract
The rate of CO and CH4 oxidation is measured on systematically Pr-, Gd-, or Nb-doped ceria, and Y- or Pr-doped zirconia to investigate the impact of point defects on heterogeneously catalyzed oxidation kinetics. The oxidation reactions proceed via the Mars–Van Krevelen mechanism. The CO oxidation rate is found to be independent of dopant content for Pr- and Gd-doped ceria, while it increases with [Pr]tot for Pr-doped zirconia. Under certain conditions (low dopant concentrations and/or low temperatures) the competition between the lattice oxygen consumption by CO and the oxygen replenishment by gaseous O2 decreases the effective oxygen activity (p(O2)eff) inside the catalyst particles by up to 8 orders of magnitude. The increased point defect concentrations in the catalyst accelerate the oxygen incorporation until steady state is reached. Owing to the lower reactivity of CH4, no decreased p(O2)eff was observed for CH4 oxidation. We demonstrate that the contributions of point defect concentrations, which themselves depend on the (effective) oxygen partial pressure, must properly be included in the reaction rate expression to obtain correct apparent reaction orders for CO and O2.