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

Catalytic reaction pathways of NH3 on CuO/γ-Al2O3 catalysts during NH3 selective catalytic reduction reactions were investigated under oxygen-rich conditions. On 10 wt % CuO/γ-Al2O3, NH3 reacted with oxygen to produce NOx. In contrast, on the 0.5 wt % CuO/γ-Al2O3 catalyst, NH3 reacted primarily with NO to form N2 with a conversion efficiency of ∼80% at 450 °C. H2-temperature-programmed reduction (H2-TPR) results show that Cu species present in 10 wt % CuO/γ-Al2O3 can be easily reduced at ∼160 °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 % CuO/γ-Al2O3 catalyst does not show any measurable H2 consumption up to 700 °C, which suggests the presence of nonreducible isolated Cu species in this catalyst. Scanning transmission electron microscopy images collected from 10 wt % CuO/γ-Al2O3 show nanosized CuO clusters, but no evidence of cluster formation is seen in the images recorded from the 0.5 wt % CuO/γ-Al2O3 sample due to the intrinsic limitation of low Z contrast between highly dispersed Cu (atomic weight = 63.5) species 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/γ-Al2O3 catalyst, and an estimated coordination number of ∼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 O2 to produce atomic oxygen species. These reactive atomic oxygen species then react with NH3 to produce NOx. However, the nonreducible, isolated Cu species, modeled by γ-Al2O3-supported monomeric CuO, shows relatively weak interactions with both NO and O2. Most importantly, our calculations suggest that the dissociations of either NO or O2 are energetically unfavored on this latter catalyst. Therefore, molecularly adsorbed NO can react only with NH3 to produce N2 on the low (0.5 wt %) CuO-loaded catalyst.