Optimization of tailoring of CuOx species of silica alumina supported catalysts for the selective catalytic reduction of NOx

A series of CuOx catalysts dispersed on SiO2-Al2O3 support with a copper content from 0.2 to 12 wt % corresponding to 0.04-3.7 atomCu nm-2 was prepared by chemisorption-hydrolysis method from copper solutions. The catalysts were characterized in their bulk (XRD, redox cycles with H2 and O2) and surface (N2 adsorption, SEM, XPS, and DRS) properties. Copper species were found to be uniformly spread on the SiO2Al2O3 support as small aggregates both in the low and high copper loaded samples. Spectroscopic evidence agrees with Cu2+ presence in an axially distorted octahedral environment of O-containing ligands. At high copper loading, the existence of copper centers in closer interaction occurred forming structures of oxocations-like type. Temperature programmed reduction (TPR) experiments confirmed the presence of dispersed copper species which underwent complete reduction. Comparing the position and shape of two successive TPR profiles, carried out interposing an oxidation run (temperature programmed oxidation), it was found that the smallest CuOx centers (<1 atomcu nm-2) are characterized by low stability and high mobility. Increasing the copper content diminished the mobility of the CuOx species, as larger CuOx aggregates were formed. The selective catalytic reduction of NOx with ethene in the presence of excess oxygen was studied in a flow apparatus at fixed reactant concentration (1500 ppm of NOx and C2H4 and 15000 ppm of O2) and contact time (8 g s mmol-1), with an online FT-IR analytical device. Catalysts containing up to about 1 atomcu nm-2 displayed very little activity, while catalysts with higher copper content were active and selective. Maximum activity was associated with samples containing 1.5-2 atomCu nm-2, while samples with higher copper concentration were less active. Results point to the need to individuate relationships between structure and catalyst properties and activity to optimize the preparation of suitable tailored copper-containing catalysts.