(488b) The Role of Surface Oxides in NOx Storage Reduction Catalysts: A DFT and kMC Study

NOx storage reduction (NSR) catalysts have drawn great attention due to their ability to accommodate NOx reduction under the lean burn conditions demanded by more fuel efficient engines [1].  When using Pd as the active metal, a monolayer PdO surface is suggested to be the thermodynamically favored surface during the oxidation catalysis process [2,3]. In this work, the first-principles Monte Carlo methods are used to explore the role of surface oxide during catalysis cycle in NSR catalysts. A reconstructed (√5×√5) R27°PdO(101)-Pd(100) which was used as a model surface in previous examinations of CO oxidation [2] has been used as our catalyst model. Density function theory (DFT) calculations have been employed to examine thermodynamics and kinetics of the atomic-level elementary steps for NSR process. The use of H₂ as a reductant has also been explored. A phase diagram has been constructed to determine the thermodynamic stability of the surface under the fuel-rich mode conditions of NSR. Based on the results from the DFT calculations, first-principles kinetic Monte Carlo (kMC) simulations have been used to investigate the stability range of different surface structures under both the oxidation (O₂ rich) and reduction (fuel rich) portions of the NSR process. kMC results show that surface oxide is stable in fuel-lean mode, though phase-diagram shows bulk PdO is the most phase under that particular thermodynamic condition[3]. kMC results indicate there is pressure difference of NO partial pressure for decomposition of surface oxide and that for forming surface oxide from prime metal at the same O₂ partial pressure. H-O phase-diagram produced by DFT linked to thermodynamic methods indicates under standard NSR operating condition, monolayer PdO is stable while preliminary kMC results show that thin film oxide starts decomposing around partial pressure of H₂ at 10^-3 atm with O₂ partial pressure of 1atm.

[1] N. Takahashi, H. Shinjoh, T. Iijima,et al, Catal. Today 27, 63 (1996)

[2] J. Rogal, K. Reuter, and M. Scheffler, Phys. Rev. B 75, 205433 (2007).

[3] J. Jelic and R. J. Meyer, Phys. Rev. B 79, 125410 (2009)