Modeling Non-Idealities in Oxygen Vacancy Formation on Reducible Oxides
Andrew (Bean) Getsoian, Ford Motor Company
Modeling Non-Idealities in Oxygen Vacancy Formation on Reducible Oxides: Insights from N2O Decomposition over Co3O4-based Catalysts
Yongchun Hong1, Xueqiang Zhang1, Andrew (Bean) Getsoian2, and Enrique Iglesia1
1Department of Chemical and Biomolecular Engineering, University of California, Berkeley CA
2Research and Advanced Engineering, Ford Motor Company, Dearborn MI
The direct decomposition of N2O to N2 and O2 is an attractive approach to mitigation of greenhouse gas emissions from automotive aftertreatment systems. The most active known catalysts for this reaction are derived from spinel phase Co3O4, typically promoted by additional elements. Decomposition of N2O on Co3O4-based catalysts reveals kinetics strictly first order in N2O partial pressure and exhibiting approximately inverse half order inhibition by gas phase O2 for all unsupported, supported, and/or promoted catalysts investigated. These kinetics are consistent with N-O bond activation at oxygen vacancy sites on catalyst surfaces predominantly covered by dissociatively adsorbed oxygen. Careful examination of the enthalpy and entropy of adsorption of oxygen implied by conventional, Langmuir-isotherm-based kinetic modeling, however, produces results that are not physically sensible and are inconsistent with oxygen vacancy titration and TPD experiments. The cause of the inconsistency is revealed to be a strongly coverage-dependent heat of adsorption of oxygen, which renders a Langmuir treatment inappropriate. Rigorous consideration of the coverage dependence of the enthalpy of adsorption obtained via independent TPD measurements combined with a lattice model for configurational effects on the entropy of adsorption enables development of a model containing physically sensible parameters and which accurately describes N2O decomposition kinetics. The approach applied here is expected to extend naturally also to oxygen vacancy formation and reaction kinetics on cerium oxide, which is likewise known to exhibit thermodynamically “non-ideal” oxygen vacancy formation.