Spectroscopic characterisation of Cu-Al2O3 catalysts for selective catalytic reduction of NO with propene

Copper-aluminium mixed oxide catalysts (Cu-Al2O3) for selective reduction of NO with propene (C3H6-SCR) were characterised as a function of Cu content. The results from different techniques, Cu K-edge and Cu L-3-edge XANES, diffuse reflectance UV-Vis, XPS, and XRD were combined to develop coherent description of the phases present and cation site distribution in Cu-Al2O3. Both the bulk and the surface of Cu-Al2O3 consist of Cu2+ cations at tetrahedral or octahedral sites of the Al2O3 matrix, forming an aluminate phase in a wide range of Cu content (1-30 wt.%). Cu L-3-edge XANES analysis succeeded in quantifying the fraction of tetrahedral and octahedral Cu2+ ions; the fraction of octahedral Cu2+ decreased as Cu concentration increased. The catalytic activity for C3H6-SCR per surface area of the catalysts increased as surface Cu2+ concentration increased, indicating that Cu2+ ions in the aluminate phase are responsible for the activity. The activity per surface Cu2+ decreased as surface Cu2+ concentration increased, which coincides with the decrease in the fraction of octahedra. The Cu2+ cations in octahedral sites should be responsible for the higher intrinsic activity.