(49g) Fundamental Insight Into the Unique Reactivity of NO Oxidation On the Mn-Ce Mixed Oxide

NO oxidation is the key step for NOx trapping, soot combustion under NOx/O₂, and NH₃-SCR. Recently, MnOx-CeO₂ catalyst has been found to be a promising catalyst to substitute the precious metals (e.g., Pt) for NO oxidation for its high activity. To understand the synergistic effect of MnO₂/CeO₂ catalyst, NO oxidation over CeO₂, β-MnO₂ and MnO₂/CeO₂ catalysts was systematically investigated using DFT+U method. For the perfect CeO₂ (111), we found that the NO adsorption energy is strong (-1.14 eV), which suggests that the extraction of lattice oxygen from CeO₂ (111) to form NO₂ is preferred. However, NO₂ desorption from the CeO₂ (111) is very difficult, which needs 2.51 eV. On the other hand, the adsorption of NO over the β-MnO₂ (110) surface is also strongly exothermic with an adsorption energy of -2.50 eV. Compared with CeO₂ (111), the NO₂ desorption on the β-MnO₂ (110) surface is much lower (0.48 eV). This is consistent with the experimental observations that MnOx sample exerts obviously higher NO oxidation activity than CeO₂. For MnO₂/CeO₂, the interaction between MnO₂ and CeO₂ (111) surface forms the interface Mn—O—Ce structure. The NO adsorption energy increases as the Ce site (-1.10 eV) ≈ the Mn site (-1.00 eV) < the interface site (-0.47 eV). While the NO₂ desorption energy increases as the interface site (0.48 eV) < the Mn site (1.02 eV). This implies that the NO₂ desorption on MnO₂/CeO₂ has been significantly facilitated by the interface sites, which only requires the lowest NO₂ desorption energy. To complete the catalytic cycle, the oxygen vacancies generated by the NO₂ desorption have to be replenished. Our calculations show that the adsorption energy of single oxygen atom over MnO_2-x is -0.33 eV while it is -1.88 eV at the interface site over MnO_2-x/CeO₂ catalyst. This indicates that the MnO_2-x/CeO₂ catalyst regenerates the active lattice oxygen much easier than the MnO_2-x. Our theoretical modeling results elucidate the experimental observation well.