Effect of Cu Substitution on the Structure and Reactivity of CuxCo3-xO4 Spinel Catalysts for Direct NOx Decomposition

Abstract A Cu-substituted, Co-based spinel catalyst (CuxCo3-xO4) is introduced for direct NO decomposition to N2 and O2. A series of CuxCo3-xO4 catalysts with varying Cu content (0 ≤ x ≤ 1) were synthesized via a co-precipitation method. Reactivity for direct NO decomposition was measured at 450 °C, with the maximum activity of 2.8 × 10-2 [(µmol NO to N2) g-1 s-1] and selectivity to N2 of 61% occurring over the Cu0.4Co2.6O4 (x = 0.4) catalyst. Additionally, the CuxCo3-xO4 catalysts demonstrated the ability to mitigate N2O formation as all traces of this greenhouse gas were decomposed regardless of Cu content. Characterization by X-ray diffraction and X-ray absorption spectroscopy revealed the effects of Cu substitution on the occupancies and valencies of the Co and Cu ions in the spinel structure. Activity was shown to correlate with increasing incorporation of Cu2+ into the tetrahedral sites of the normal spinel structure; however, significant formation of a segregated CuO phase caused the activity to decrease when x ≥ 0.4. The bulk structure-activity relationships we elucidate are expected to provide a guide for the design of improved direct NO decomposition catalysts and other bulk oxide catalyst systems based on careful design of the cation arrangements in oxides.