Electronic and chemical properties of mixed-metal oxides: basic principles for the design of DeNO x and DeSO x catalysts

Abstract

Correlations between the electronic and chemical properties of perovskites, molybdates, and metal-doped MgO or CeO 2 are examined. Simple models based on band-orbital mixing can explain trends found for the interaction of these catalytic materials with adsorbates: the less stable the occupied levels of a mixed-metal oxide, the higher its chemical reactivity. Metal↔oxygen↔metal interactions are common in mixed-metal oxides and can lead to substantial changes in the electronic and chemical properties of the cations. This is particularly true in the case of ABO 3 perovskites (A=Pb, Ca, Sr, Li, K, Na; B=Ti, Zr, Nb), and it is an important phenomenon that has to be considered when mixing AO and BO 2 oxides for catalytic applications. In systems like Ce 1− x Zr x O 2 and Ce 1− x Ca x O 2, the structural stress induced by the dopant (Zr or Ca) leads to perturbations in the electronic properties of the Ce cations. The trends in the behavior of metal-doped MgO illustrate a basic principle in the design of mixed-metal oxide catalysts for DeNO x and DeSO x operations. The general idea is to find metal dopants that upon hybridization within an oxide matrix remain in a relatively low oxidation state and at the same time induce occupied electronic states located well above the valence band of the host oxide. Electronic effects should not be neglected a priori when explaining the behavior or dealing with the design of mixed-metal oxide catalysts.