Interdependence of Point Defects and Reaction Kinetics: CO and CH₄ Oxidation on Ceria and Zirconia

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.

Design Aspects of Doped CeO2 for Low-Temperature Catalytic CO Oxidation: Transient Kinetics and DFT Approach

“…On the other hand, zeroth-order dependence (or independence) was found with respect to the oxygen composition ( P O 2 ), as illustrated in Figure S14. These findings agree very well with recent kinetic work on various doped-ceria solids (Pr-, Gd-, or Nb-doped ceria), where the dependence of the CO oxidation rate on P CO varied in the 0.6–0.9 range (350 °C; 1% CO/1% O 2 /He), while a zeroth-order rate dependence was found on P O 2 . These findings strongly suggest that adsorbed CO-s on the surface participates in the rate-determining step (RDS) in the reaction path, which is not the case for the adsorption of oxygen, and the required reoxidation of the catalyst is faster than the RDS .…”

“… 61 These findings strongly suggest that adsorbed CO-s on the surface participates in the rate-determining step (RDS) in the reaction path, which is not the case for the adsorption of oxygen, and the required reoxidation of the catalyst is faster than the RDS. 61 These results are corroborating for an MvK type of mechanism for the CO oxidation reaction, where formation of an oxygen vacancy is included in the RDS.…”

“…These findings agree very well with recent kinetic work on various doped-ceria solids (Pr-, Gd-, or Nb-doped ceria), where the dependence of the CO oxidation rate on P CO varied in the 0.6–0.9 range (350 °C; 1% CO/1% O 2 /He), while a zeroth-order rate dependence was found on P O 2 . These findings strongly suggest that adsorbed CO-s on the surface participates in the rate-determining step (RDS) in the reaction path, which is not the case for the adsorption of oxygen, and the required reoxidation of the catalyst is faster than the RDS . These results are corroborating for an MvK type of mechanism for the CO oxidation reaction, where formation of an oxygen vacancy is included in the RDS.…”

Recent Advances in Complete Methane Oxidation using Zeolite‐Supported Metal Nanoparticle Catalysts

“…[11,12,[20][21][22] Figure 2 illustrates a typical Mars-van Krevelen mechanism on Pd/Al 2 O 3 accompanied by the three most common types of deactivation. [23][24][25] Although several excellent reviews have covered most of the recent literature on methane oxidation, [20,22] the recent advances in zeolite-based systems have not been covered as extensively. P. Gélin and M. Primet thoroughly reviewed the work done with zeolites for complete methane oxidation until 2002.…”

“…Specifically, Pd/Al 2 O 3 has attracted much attention over the years [11,12,20–22] . Figure 2 illustrates a typical Mars‐van Krevelen mechanism on Pd/Al 2 O 3 accompanied by the three most common types of deactivation [23–25] . Although several excellent reviews have covered most of the recent literature on methane oxidation, [20,22] the recent advances in zeolite‐based systems have not been covered as extensively.…”