Non-linear effects of Cu loading on the structure and low-temperature CO oxidation activity of CuO/Al₂O₃ catalysts:...

This study provides new insights into the non-linear catalytic activity of CuO/Al₂O₃ upon Cu loading for low-temperature CO oxidation, a trend often attributed to CuO particle growth and reduced dispersion. CuO/Al₂O₃ catalysts with different Cu loadings were prepared via a wet-impregnation method, showing a substantial increase in activity from 3.2 wt% to 5.8 wt% despite the larger particle size (∼10 nm). However, the increase in activity became less pronounced with further Cu loading, reaching a maximum at 8.0 wt%, and a further increase in the Cu loading to 10.0 wt% resulted in reduced activity. To explain this non-linear behavior-which cannot be accounted for solely by particle growth-multiple characterization techniques, including ToF-SIMS, XAS, TEM, XRD, N₂ adsorption/desorption, and H₂-TPR, were employed. ToF-SIMS revealed that the surface Cu population is the governing factor for apparent catalytic activity, displaying a non-linear dependence on Cu loading. With increasing Cu loading, particle growth occurred; however, larger CuO particles extended into the Al₂O₃ matrix, decreasing the accessible surface Cu population. ToF-SIMS also enabled direct examination of the intrinsic activity of surface Cu atoms. The 1-Cu sample, despite its lowest Cu loading and the absence of measurable CuO domains, exhibited the highest intrinsic activity. This was attributed to strong Cu-Al-O interfacial interactions coupled with weaker Cu-N and Cu-C interactions. This work highlights the capability of ToF-SIMS to unravel complex surface phenomena and emphasizes that while the apparent activity is governed by the surface Cu population, the intrinsic activity is governed by interfacial Cu-element interactions.