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

Abstract

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 further increase in 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 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 dictated by interfacial Cu–element interactions.