The effects of zirconia morphology on methanol synthesis from CO and [formula omitted] over [formula omitted] catalysts : Part I. Steady-state studies

Abstract

The effect of zirconia phase on the activity and selectivity of Cu/ZrO 2 for the hydrogenation of CO was investigated. Relatively pure t-ZrO 2 and m-ZrO 2 were prepared with high surface areas (∼145 m 2/g). Copper was then deposited on the surface of these materials by either incipient-wetness impregnation or deposition-precipitation. For a fixed Cu surface area, Cu/m-ZrO 2 was ten times more active for methanol synthesis than Cu/t-ZrO 2 from a feed of 3:1 H 2/CO at 3.0 MPa and temperatures between 473 and 523 K. Cu/m-ZrO 2 also exhibited a higher selectivity for methanol. Increasing the Cu surface area on m-ZrO 2 resulted in further improvement in activity with minimal change in selectivity. Methanol productivity increased linearly for both Cu/t-ZrO 2 and Cu/m-ZrO 2 with increasing Cu surface area. The difference in inherent activity of each phase paralleled the stronger and larger CO adsorption capacity of the Cu/m-ZrO 2 as quantified by CO-TPD. The higher CO adsorption capacity of Cu/m-ZrO 2 is attributed to the presence of a high concentration of anionic vacancies on the surface of m-ZrO 2. Such vacancies expose cus-Zr 4+ cations, which act as Lewis acid centers and enhance the Brønsted acidity of adjacent Zr OH groups. The presence of cus-Zr 4+ sites and adjacent Brønsted acidic Zr OH groups contributes to the adsorption of CO as HCOO Zr groups, which are the initial precursors to methanol.