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.