Size-Dependent Catalytic Performance of CuO on gamma-Al2O3: NO Reduction versus NH3 Oxidation

Abstract

Catalytic reaction pathways of NH3 on CuO/gamma-Al2O3 catalysts. during, NH3 selective catalytic reduction reactions were investigated Under oxygen rich conditions. On 10 wt % CuO/gamma-Al2O3, NH3 reacted with oxygen to produce NON. In contrast, on the 0.5 wt % CuO/gamma-Al2O3 catalyst, NH3 reacted primarily with NO to form N-2 with a conversion efficiency of similar to 80% at 450 degrees C. H-2-temperature-programmed reduction (H-2-TPR) results show that Cu species present in 10 wt % CUO/gamma-Al2O3 can be easily reduced at similar to 160 degrees C, which suggests the formation of large CuO Clusters on the alumina surface. On the other. hand, the TPR spectrum obtained from the 0.5 wt % CnO/gamma-Al2O3 catalyst does not show any measurable H-2 consumption up. to 700 degrees C, Which suggests the presence of nonreducible isolated Cu species in this catalyst. Scanning transmission electron microscopy images collected from 10 wt % CuO/gamma-Al2O3 show nanosized CuO clusters, but no evidence Of cluster formation is seen in the images recorded from the 0.5 wt % CuO/gamma-Al2O3 sample due to the intrinsic limitation of low Z contrast between highly dispersed Cu (atomic weight = 63.5) specie's and the alumina Support (atomic weight of Al = 27). EXAFs data indicates the presence of Cu-Cu (Al) second shell at 0.35 nm only in the 10 wt % Cuo/gamma-Al2O3 catalyst, and an estimated coordination number of similar to 1.7. The XANES and EXAFS results suggest the formation of relatively highly dispersed Cu oxide nanoclusters, even at 10 wt % Cu loading. Density functional theory results show that supported CuO clusters, represented by a two-dimensional CuO monolayer, can effectively dissociate adsorbed NO and O-2 to produce atomic oxygen species. These reactive atomic oxygen specie's then react with NH3 to,produce NOx. However, the nonreducible, isolated Cu species, modeled by gamma-Al2O3-supported monomeric CuO, shows relatively weak interactions with both NO and O-2 Most importantly, our calculations suggest that the dissociations of either NO or O-2 are energetically unfavored on this latter catalyst. Therefore, molecularly adsorbed NO can react only with NH3 to produce N-2 on the low (0.5 wt%) CuO-loaded catalyst