Effect of dopants on the activity of Cu/M0.3Zr0.7O2 (M = Ce, Mn, and Pr) for CO hydrogenation to methanol

Previous investigations have shown that Cu/ZrO2 is an active catalyst for the hydrogenation of CO to methanol and that both components of the active play an active role in the reaction mechanism. It has also been shown that the substitution of Ce for Zr into the ZrO2 lattice results in significantly enhanced methanol synthesis activity. The present investigation was undertaken with the aim of understanding whether other substituents, such as Mn and Pr, could also enhance the activity of Cu/ZrO2. Zirconia and Ce-, Mn-, and Pr-substituted zirconia were prepared by forced hydrolysis at low pH, starting from nitrates of each metal, and Cu was then dispersed onto the surface of the calcined oxide by deposition–precipitation. All catalysts were characterized by XRD, XANES, and temperature-programmed reduction in H2. H2 and CO chemisorption capacities were also measured. The area-based activity of 3 wt% Cu/M0.3Zr0.7O2 decreased in the order 3 wt% Cu/Ce0.3Zr0.7O2 > 3 wt% Cu/Pr0.3Zr0.7O2 > 3 wt% Cu/Mn0.3Zr0.7O2 > 3 wt% Cu/ZrO2. Catalyst activity was found to correlate with H2 adsorption capacity and the proportion of bridge-bonded hydroxyl groups. The importance of the latter species is ascribed to their higher Brønsted acidity, which contributes to the rapid release of methoxide groups formed on the oxide surface and the formation of methanol. Dopant cations that can participate in redox cycles (e.g., Ce and Mn) are desirable, because they can enhance the methanol synthesis activity of Cu/M0.3Zr0.7O2 catalysts to a greater degree than cations that do not participate in redox cycles (e.g., Pr).