DeNO<i>_x</i> Reactions on MgO(100), Zn<i>_x</i>Mg₁_-<i>_x</i>O(100), Cr<i>_x</i>Mg₁_-<i>_x</i>O(100), and Cr₂O₃(0001): Correlation between Electronic and Chemical Properties of Mixed-Metal Oxides

Abstract

The rational design of catalysts with a high efficiency for the destruction of NOx compounds (DeNOx process) is a major problem in environmental chemistry. The adsorption of NO, NO2, and N2O on MgO(100), Zn0.06Mg0.94O(100), Cr0.06Mg0.94O(100), and Cr2O3(0001) was studied using high-resolution photoemission and first-principles density-functional calculations. Important differences were found in the chemistry of the adsorbed NOx 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, Cr0.06Mg0.94O(100) and Cr2O3(0001), bond NO strongly and are able to induce the dissociation of NO2 and 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 Nx species are produced upon decomposition of N2O. The trends in the behavior of ZnxMg1-xO(100) and CrxMg1-xO(100) illustrate a basic principle for the design of mixed-metal oxide catalysts in DeNOx 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 NOx species and facilitate N−O bond cleavage. The validity of this basic principle is confirmed after examining the bonding of NO and N2O to a series of TM0.06Mg0.94O(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.