Chromium-modified zinc oxides

Abstract

Zinc oxides modified with chromium were prepared by the co-precipitation method. Samples were calcined at 400 °C. Structural and surface properties of materials were studied by means of transmission electron microscopy, X-ray diffraction and low-temperature nitrogen adsorption methods. Temperature-programmed reduction methods were used for investigation of the influence of chromium on the redox properties of zinc oxides. The nature of surface sites and the course of surface reactions occurred during methanol decomposition were determined by the temperature-programmed desorption of methanol with detection of evolved gasses by mass spectrometry and in situ diffuse reflectance infrared Fourier transform spectroscopy. We have found that modification of ZnO with chromium led to the increase of the specific surface area of oxides and decrease of mean crystallite size of ZnO. Partial incorporation of chromium ions to the hexagonal ZnO structure, formation of strongly dispersed spinel (ZnCr2O4) and chromium zinc oxide species (ZnO·CrO3) was observed. We have stated that methanol was adsorbed on the surface of ZnO at 100 °C mainly via formation of methoxy species, and the main products of methanol desorption were carbon monoxide and hydrogen. The presence of chromium facilitated direct oxidation of methoxy groups to formate species at low temperatures (~100 °C) and their transformation to hydrogen and carbon dioxide during temperature-programmed desorption. The enhanced transformation of methoxy to formate species was attributed to the labile oxygen in chromium-modified zinc oxides.