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

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


Wang, X. - Presenter, Tianjin University
Mei, D., Pacific Northwest National Laboratory

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 MnO2-x is -0.33 eV while it is -1.88 eV at the interface site over MnO2-x/CeO2 catalyst. This indicates that the MnO2-x/CeO2 catalyst regenerates the active lattice oxygen much easier than the MnO2-x. Our theoretical modeling results elucidate the experimental observation well.