A series of CuO x 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 SiO2−Al2O3 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 CuO x centers (<1 atomCu nm-2) are characterized by low stability and high mobility. Increasing the copper content diminished the mobility of the CuO x species, as larger CuO x aggregates were formed. The selective catalytic reduction of NO x with ethene in the presence of excess oxygen was studied in a flow apparatus at fixed reactant concentration (1500 ppm of NO x 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.

The acidity of a prereduced Cu/SiO 2 catalyst was extensively investigated by means of FT-IR of adsorbed pyridine and by titration with 2phenylethylamine in cyclohexane. Comparison with the parent CuO/SiO 2 material, which was already shown to exhibit Lewis acid sites due to the high dispersion of the CuO phase, provided evidence that reduction of this phase to the metallic state increases the acidity of the material. This allowed us to set up a bifunctional catalyst showing acidic and hydrogenation activity, both ascribable to the presence of the metal particle, without the need of an acidic support. This catalyst was tested in the one-pot transformation of γ−valerolactone into pentyl valerate and showed comparable activity (91% vs 92% conversion) and improved selectivity (92% vs 72%) with respect to the previously reported copper catalyst supported on acidic material. The role of Cu in activating the substrate was also evidenced through FTIR of adsorbed γ-valerolactone.

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Copper catalysts prepared by chemisorption-hydrolysis technique over silica (Cu/Si) and silica-alumina (Cu/SiAl) supports were studied to understand the role of the support on the nature and surface properties of the copper species stabilized on their surfaces. The morphological and surface properties of the copper phases have been characterized by complementary techniques, such as HRTEM, EXAFS-XANES, EPR, XPS, and FTIR. For the FTIR investigation, molecular probes (CO and NO) were also adsorbed on the surfaces to test the reactivity of the copper species. Moreover, the catalytic performances of the two catalysts have been compared in the HC-SCR reaction (NO reduction by C(2)H(4)) performed in highly oxidant conditions. The superior activity and selectivity of the supported silica-alumina catalyst with respect to that supported on silica could be related with the different nature of the copper species stabilized on the two supports, as emerged from the results obtained from the spectroscopic investigations. Small and well-dispersed CuO particles were present on silica, whereas isolated copper ions predominated on silica-alumina, likely in regions rich in alumina that made some exchangeable sites available, as indicated by FTIR spectra of adsorbed CO. The less positive global charge of copper species on Cu/SiAl than in Cu/Si has been confirmed by EPR, XPS, and EXAFS-XANES analyses.

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