DeNOx Reactions on MgO(100), ZnxMg1-xO(100), CrxMg1-xO(100), and Cr2O3(0001): Correlation between Electronic and Chemical Properties of Mixed-Metal Oxides

The rational design of catalysts with a high efficiency for the destruction of NO x compounds (DeNO x process) is a major problem in environmental chemistry. The adsorption of NO, NO 2, and N 2 O on MgO(100), Zn 0.06 Mg 0.94 O(100), Cr 0.06 Mg 0.94 O(100), and Cr 2 O 3 (0001) was studied using high-resolution photoemission and first-principles density-functional calculations. Important differences were found in the chemistry of the adsorbed NO x species, but in general our results show a clear correlation between the electronic properties and reactivity of the oxide surfaces. Systems that have occupied electronic states with a relatively low stability, Cr 0.06 Mg 0.94 O(100) and Cr 2 O 3 (0001), bond NO strongly and are able to induce the dissociation of NO 2 and N 2 O at temperatures as low as 80 K. On these oxide surfaces, adsorbed NO is a main product in the dissociation of NO 2, whereas adsorbed N x species are produced upon decomposition of N 2 O. The trends in the behavior of Zn x Mg 1 - x O(100) and Cr x Mg 1 - x O(100) illustrate a basic principle for the design of mixed-metal oxide catalysts in DeNO x operations. The general idea is to find metal dopants that upon hybridization within an oxide matrix produce occupied electronic states located well above the valence band of the oxide. These hybrid dopant states lead to large adsorption energies for NO x species and facilitate N−O bond cleavage. The validity of this basic principle is confirmed after examining the bonding of NO and N 2 O to a series of TM 0.06 Mg 0.94 O(100) surfaces (TM = Zn, Sn, Ni, Co, Fe, Mn, or Cr). The effects of different metal dopants on the electronic properties of MgO are discussed.