| The nature of metal surface of palladium catalysts in NOx storage reduction (NSR) system during cyclic operation has been investigated by ab initio atomistic thermodynamics and first principle kinetic Monte Carlo methods.;For the fuel rich cycle of NSR, the stability of the surface termination of Pd(100) over a wide range of H2 oxidation conditions has been examined with density functional theory based ab initio thermodynamics. Results indicate that like the case for CO oxidation, the product is expected to desorb and that the bulk oxide can be reduced at mild reductant pressures. in contrast to CO oxidation, H2 oxidation results in the presence of a stable non-volatile surface intermediate (OH) which can cover the surface oxide for large portions of the phase diagram. Furthermore, it was found that although H2 binds weakly to the PdO(101) surface, it may dissociate and react with surface oxygen to form hydroxyl groups. Disproportionation of hydroxyl to form water (which then desorbs) results in the creation of oxygen vacancies on the surface which accelerates the oxide decomposition process.;For the fuel lean cycle, first-principles kinetic Monte Carlo simulations have been applied to examine the stability of Pd(100) and the (v5xv5)R27°PdO(101)-Pd(100) surface oxide in an NO oxidation environment from ultra-high vacuum conditions to ambient pressures. Reaction orders showed good agreement with experiment for both O2 and NO. In addition results show a region of bistability exists over a wide pressure range and both Pd(100) and PdO(101) could be the active surface for NO oxidation in practical operation. Simulations employing different temperatures (450 K, 600 K) show that the surface oxide stability increases as the temperature increases as desorption of the reductant is favored over reaction with the surface. However if the temperature is further increased above 1000 K then thermal decomposition may result. |
