Structural Evolution and Catalytic Properties of Nanostructured Cu/ZrO2 Catalysts Prepared by Oxalate Gel-Coprecipitation Technique

Abstract

A series of zirconia-supported copper oxide catalysts synthesized by decomposition of the oxalate precursors formed by oxalate gel-coprecipitation in alcoholic solution were extensively investigated in relation to their performance in methanol steam reforming. The combination of different techniques (N2 adsorption, X-ray diffraction (XRD), N2O titration, H2-TPR, diffuse reflectance Fourier transform infrared, Raman, and X-ray photoelectron spectroscopy) in the characterization of Cu/ZrO2 catalysts shows that the surface and structural characteristics of the zirconia phase as well as the dispersion and nature of the copper species depend strongly on the calcination temperature. Temperature-programmed reduction patterns reveal the presence of three types of copper species on the ZrO2 support. XRD results indicate that, depending on the calcination temperature, a substantial incorporation of Cu species into the zirconia lattice leading to a strong Cu−ZrO2 metal−support interaction may occur. The N2O titration reveals that the 550 °C calcined material exhibits the highest metallic copper surface area as compared to other samples, as opposed to in situ XRD analysis showing that the lower the calcination temperature the higher the copper dispersion. Spectroscopic measurements reveal that the phase transformation of zirconia from tetragonal to monoclinic takes place initially at the surface regions of the Cu−ZrO2 sample, as evidenced from the fact that the monoclinic phase can be detected first by Raman spectroscopies for the samples calcined at a lower temperature than by XRD. The highest activity was achieved for the 550 °C calcined material, illustrating that the creation of monoclinic phase enriched on the surface of tetragonal zirconia in Cu/ZrO2 are beneficial for the generation of copper catalyst with enhanced activities.