Cu/Al2O3 catalysts for soot oxidation: Copper loading effect

Cu/Al2O3 catalysts with metal loading from 0.64 to 8.8 wt.% have been prepared and characterized by different techniques: N2 adsorption at −196 °C (BET surface area), ICP (Cu loading), XRD, selective copper surface oxidation with N2O (Cu dispersion), TPR-H2 (redox properties), and XPS (copper surface species). The catalytic activity for soot oxidation has been tested both in air and NOx/O2. The activity in air depends on the amount of easily-reduced Cu(II) species, which are reduced around 275 °C under TPR-H2 conditions. The amount of the most active Cu(II) species increases with the copper loading from Cu_1% to Cu_5% and remains almost constant for higher copper loading. In the presence of NOx, the first step of the mechanism is NO oxidation to NO2, and the catalytic activity for this reaction depends on the copper loading. For catalysts with copper loading between Cu_1% and Cu_5%, the catalytic activity for soot oxidation in the presence of NOx depends on NO2 formation. For catalysts with higher copper loading this trend is not followed because of the low reactivity of model soot at the temperature of maximum NO2 production. Regardless the copper loading, all the catalysts improve the selectivity towards CO2 formation as soot oxidation product both under air and NOx/O2.