Effect of spinel (ZnCr2O4) formation on the texture, electrical conduction and catalytic behaviour of the ZnOCr2O3 system

Structural and phase changes accompanying the calcination in air, up to 1000 °C, of the ZnOCr2O3 system were monitored using differential thermal analysis, differential scanning calorimetry, infrared spectrophotometry, X-ray diffractometry and chromium ion estimation. The texture was assessed by analyzing nitrogen sorption isotherms measured at −196 °C. The results indicate that ZnOCr2O3 solid solution becomes a major product at 500 °C, and the ZnCr2O4 spinel phase starts to form at 400 °C. The electrical conductance properties of the samples were investigated before and after admission of 2-propanol in the temperature range 100–400 °C. The electrical conduction is attributed to the existence of a surface mobile-electron Zener phase that maximizes the conductivity. The obtained interstitial Zn2+ cations and free electrons (charge carriers) are considered to be localized at the ions or vacant sites, and the conduction occurs via a hopping-type process, which implies a thermally activated electronic mobility. The catalytic activity data of the vapor-phase decomposition of 2-propanol were obtained in the temperature range 200–400 °C, using a flow system technique. It was found that propylene is the main reaction product, with a minor yield of acetone. In the solid solution of ZnO in Cr2O3, the Cr ion sites will be electronically more isolated than either disordered or ordered spinel phases. The neutralization of these sites will not occur by bulk electron transfer, but they tend to trap electrons from the oxygen end of the 2-propanol molecule, leading to a high dehydration activity. Correlations were attempted between the structure of the catalysts and their catalytic activity.