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

Abstract

Copper–titania–zirconia mixed oxides with TiO2/ZrO2 1:1 atomic ratio and copper content of 1, 3 and 5wt% were prepared by one-step sol–gel method. X-ray diffraction and gas-volumetric analysis showed poor crystallinity and high specific surface area (around 250m2/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 800nm 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 CuOx nanoparticles/clusters resulted in TPR peaks around 170 and 200°C (T4 and T3). These species were responsible for the UV–Vis adsorption above 400nm and for the broad and asymmetric axial EPR signal with g|| in the range from 2.14 to 2.29 (S2 and 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.