Spectroscopic characterization of CuO_x/TiO₂-ZrO₂ catalysts prepared by a-step sol-gel method

Copper-titania-zirconia mixed oxides with TiO₂/ZrO₂ 1:1 atomic ratio and copper content of 1, 3 and 5 wt% were prepared by one-step sol-gel method. X-ray diffraction and gas-volumetric analysis showed poor crystallinity and high specific surface area (around 250 m²/g) of the catalysts calcined at 400 ° C. The nature of the copper species in the mixed oxides was characterized by temperature-programmed reduction in hydrogen (TPR), low-temperature electron paramagnetic resonance (EPR) and diffuse reflectance (DR) UV-Vis spectroscopies. These techniques suggested the presence of different copper species including distorted octahedral Cu²⁺ ions (isolated or quasi-isolated) in the solid solution bulk, characterized by a TPR peak above 400 ° C (labeled as T1), electronic d-d transitions around 800 nm and an anisotropic EPR signal with two hyperfine interactions in the g_|| region. The symmetry of this site changed from rhombic (S1) to axial (S3) while increasing copper loading. The inclusion of copper ions in the TiO₂-ZrO₂ matrix was also testified by changes in the mixed-oxide edge position. Two families of supported copper oxide were found. CuO "bulk-like" showed relatively high reduction temperature (T2, around 250 °C), while highly dispersed CuO_x nanoparticles/clusters resulted in TPR peaks around 170 and 200°C (T4 and T3). These species were responsible for the UV-Vis adsorption above 400 nm and for the broad and asymmetric axial EPR signal with g_|| in the range from 2.14 to 2.29 (S2and S2'). Finally, 2-propanol decomposition was employed as a test reaction to probe the catalyst's surface properties. The results suggested a marked effect of copper loading on the reactivity pattern, confirming the presence of CuO/CuOx particles on the surface of the TiO₂-ZrO₂ support.