Structure–Activity Relationships of Hierarchical Meso–Macroporous Alumina Supported Copper Catalysts for CO2 Hydrogenation: Effects of Calcination Temperature of Alumina Support

Abstract

Although alumina-supported copper materials have been widely used as the catalyst in methanol synthesis from CO2 hydrogenation, the effect of calcination temperature of alumina support is not yet fully understood. In this work, hierarchical meso–macroporous alumina material prepared by a sol–gel process was calcined at different temperatures (600, 700, 800 and 900 °C) and used as the supported copper catalysts. XRD, SEM-EDS mapping, XANES and hydrogen temperature-programmed reduction studies suggested that highly dispersed CuO nanoparticles and strong interaction between CuO and alumina support were formed when the alumina support was calcined at 600 °C (Cu/H-600). H2 temperature-programmed desorption and CO2 temperature-programmed desorption results revealed that the strong metal-support interaction of Cu/H-600 created a larger number of active sites for H2 and CO2 adsorption at moderate temperature (100–300 °C), resulting in the maximum yield of methanol. Increasing calcination temperature (700–900 °C) caused an increase of CuO crystallite size and a weakened interaction between CuO and alumina support, resulting in a lower yield of methanol but enhancing the formation of CO. The plot of CO2 conversion to CO against copper surface area indicated that the surface of metallic copper acted as the active site in reverse water–gas shift reaction. The conversion of methanol to dimethyl ether was found to relate with the number of weak acid sites.