The effects of zirconia morphology on methanol synthesis from CO and H2 over Cu/ZrO2 catalysts: Part II. Transient-response infrared studies

The interactions of CO, CO/H2, H2, D2, and CH3OH with t-ZrO2, m-ZrO2, Cu/t-ZrO2, and Cu/m-ZrO2 were investigated by in situ infrared spectroscopy with the aim of understanding the nature of species involved in methanol synthesis and the dynamics of the formation and consumption of these species. With both phases of ZrO2, the primary surface species observed during CO hydrogenation were bidentate formate groups, b-HCOOsingle bondZr, and methoxide groups, CH3Osingle bondZr. Transient-response experiments indicated that the rate-limiting step for each catalyst is the reductive elimination of methoxide species. Relative to 1.2 wt% Cu/t-ZrO2, however, spillover of H atoms and the formation and reduction of formate and methoxide species proceeded more rapidly on the more active 1.2 wt% Cu/m-ZrO2. Steady-state intensities of surface species were also larger on 1.2 wt% Cu/m-ZrO2. These differences are attributed to the higher reactivity of the hydroxyl groups on the surface of m-ZrO2. Increasing the Cu surface area on m-ZrO2 increases the rate of reductive elimination of methoxide species up to a maximum value, determined by the eventual saturation of the ZrO2 surface with H atoms via spillover from Cu. The product of the apparent rate coefficient for reductive elimination of methoxide species and the surface concentration of these species increases linearly with increasing Cu surface area, which is consistent with the proportionality seen in the rate of methanol synthesis at steady state.