Effect of the addition of chromium- and manganese oxides on structural and catalytic properties of copper/zirconia catalysts for the synthesis of methanol from carbon dioxide

Abstract

The effect of chromium- and manganese oxide on the structural and catalytic properties of copper/zirconia used for methanol synthesis from carbon dioxide and hydrogen has been investigated by several techniques ( TG DTA , XRD, TPR, XPS, N 2O-titration, nitrogen physisorption). The methanol selectivity of all catalysts is governed by the competition of the simultaneously catalyzed methanol synthesis and reverse water gas shift reaction. The chromium containing sample produces predominantly methanol, whereas the manganese containing catalyst is most active for CO formation. For reaction temperatures 443–513 K and 1.7 MPa total pressure, methanol formation decreases in the order Cu/ZrO 2 > Cu/CrO x /ZrO 2 > Cu/MnO x /ZrO 2 for catalysts dried at 403 K. After calcination at 623 K in air, methanol synthesis activity is similar for all catalysts. For temperatures exceeding 523 K Cu/CrO x /ZrO 2 shows highest activity for methanol production. The addition of chromium oxide and less pronounced manganese oxide, to Cu ZrO 2 retards sintering of the copper component and shifts the crystallization of amorphous zirconia to higher temperatures, thus, resulting in an increased thermal stability of the catalyst under reaction conditions. X-ray photoelectron spectroscopy (XPS) was applied to examine the relative surface concentrations and the oxidation state of the catalyst components. XPS shows that there is no apparent correlation between the oxidation states of the metals and the catalytic properties of the catalysts. In situ diffuse reflectance FTIR studies were performed to identify the species present on the catalyst surface under CO 2 hydrogenation conditions and to elucidate the reaction mechanism. With all catalysts, surface carbonate and formate species were formed rapidly. Evidence is given that over copper/zirconia based catalysts methanol is formed mainly from bidentate surface carbonate via adsorbed CO, π-bound formaldehyde and surface-bound methylate.