DeNO(x) reactions on MgO(100), ZnxMg1-x(100), Cr(x)Mg1-(x)(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 NOx compounds (DeNO(x) process) is a major problem in environmental che mistry. The adsorption of NO, NO2, and N2O on MgO(100), Zn-0.06- Mg0.94O(10 0), Cr0.06Mg0.94O(100), and Cr2O3(0001) was studied using high-resolution p hotoemission and first-principles density-functional calculations. Importan t differences were found in the chemistry of the adsorbed NO, species, but in general our results show a clear correlation between the electronic prop erties and reactivity of the oxide surfaces. Systems that have occupied ele ctronic states with a relatively low stability, Cr0.06Mg0.94O(100) and Cr2O 3(0001), bond NO strongly apd are able to induce the dissociation of NO2 an d N2O at temperatures as low as 80 K. On these oxide surfaces, adsorbed NO is a main product in the dissociation of NO2, whereas adsorbed N-x species are produced upon decomposition of N2O. The trends in the behavior of ZnxMg 1-xO(100) and CrxMg1-xO(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 o ccupied electronic states located well above the valence band of the oxide. These hybrid dopant states lead to large adsorption energies for NO, speci es and facilitate N-O bond cleavage. The validity of this basic principle i s confirmed after examining the bonding of NO and N2O to a series of TM0.06 Mg0.94O-(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.