The effect of the thermal pretreatment on the performance of ZnO/Cr2O3 catalysts applied in high-temperature methanol synthesis

Co-precipitated ZnO/Cr2O3 catalyst precursors were thermally pretreated in synthetic air (calcination), N-2 (pyrolysis) and 10% H-2 diluted in N-2 (reduction) at temperatures in the range from 320 degrees C to 600 degrees C and then applied in methanol synthesis from CO and H-2 at 260-300 degrees C and 60 bar. The X-ray diffraction patterns showed that increasing temperatures used in the thermal pretreatment lead to sintering of both the resulting non-stoichiometric Zn-Cr spinel and ZnO phases. However, reduced and pyrolyzed catalysts exhibit a much slower increase in particle size of the Zn-Cr spinel phase compared with the calcined catalyst. Correspondingly, at the same temperature of thermal pretreatment the specific surface areas of the catalyst follow the trend reduced catalyst> pyrolyzed catalyst> calcined catalyst. Chromate species were found to be present in the calcined catalyst, while their amount decreased with increasing calcination temperature. Infrared spectroscopy and temperature-programmed reduction confirmed the existence of carbonate species in the catalysts reduced or pyrolyzed below 500 degrees C. NH3 temperature-programmed desorption showed that the calcined catalysts exhibit more acid sites due to the existence of chromate species. The reduced and pyrolyzed catalysts demonstrate significantly improved performance in high temperature methanol synthesis compared with the calcined catalysts, and the maximum methanol productivity was achieved subsequent to reduction at 500 degrees C. It is suggested that the thermal pretreatment in reducing or inert atmosphere favors the formation of oxygen vacancies, which are considered to be the active sites for the hydrogenation of CO. (C) 2017 Elsevier B.V. All rights reserved.