An investigation of the factors influencing the activity of Cu/CexZr1−xO2 for methanol synthesis via CO hydrogenation

An investigation was carried out to identify the effects of incorporating Ce into ZrO2 on the catalytic activity and selectivity of Cu/CexZr1−xO2 for the hydrogenation of CO to methanol. A series of CexZr1−xO2 solid solutions was synthesized by forced hydrolysis at low pH. The resulting catalysts were characterized to determine the structure of the mixed oxide phase, the H2 and CO adsorption capacities of the catalyst, and the reducibility of both oxidation states of both Cu and Ce. The methanol synthesis activity goes through a maximum at x=0.5x=0.5, and the activity of 3 wt% Cu/Ce0.5Zr0.5O2 catalyst is four times higher than that of 3 wt% Cu/ZrO2 when tested at total pressure of 3.0 MPa and temperatures between 473 and 523 K with a feed containing H2 and CO (H2/CO=3). The maximum in methanol synthesis activity is paralleled by a maximum in the hydrogen adsorption capacity of the catalyst, an effect attributed to the formation of Ce3+O(H)Zr4+ species by dissociative adsorption of H2 on particles of supported Cu followed by spillover of atomic H onto the oxide surface and reaction with Ce4+OZr4+ centers. In situ infrared spectroscopy shows that formate and methoxide groups are the primary adspecies present on Cu/CexZr1−xO2 during CO hydrogenation. The rate-limiting step for methanol synthesis is the elimination of methoxide species by reaction with Ce3+O(H)Zr4+ species. The higher concentration of Ce3+O(H)Zr4+ species on the oxide surface, together with the higher Brønsted acidity of these species, appears to be the primary cause of the four-fold higher activity of 3 wt% Cu/Ce0.5Zr0.5O2 relative to 3 wt% Cu/ZrO2.