An investigation of the factors influencing the activity of Cu/Ce xZr 1− xO 2 for methanol synthesis via CO hydrogenation

Abstract

An investigation was carried out to identify the effects of incorporating Ce into ZrO 2 on the catalytic activity and selectivity of Cu/Ce x Zr 1− x O 2 for the hydrogenation of CO to methanol. A series of Ce x Zr 1− x O 2 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 H 2 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 = 0.5 , and the activity of 3 wt% Cu/Ce 0.5Zr 0.5O 2 catalyst is four times higher than that of 3 wt% Cu/ZrO 2 when tested at total pressure of 3.0 MPa and temperatures between 473 and 523 K with a feed containing H 2 and CO (H 2/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 Ce 3+ O(H) Zr 4+ species by dissociative adsorption of H 2 on particles of supported Cu followed by spillover of atomic H onto the oxide surface and reaction with Ce 4+ O Zr 4+ centers. In situ infrared spectroscopy shows that formate and methoxide groups are the primary adspecies present on Cu/Ce x Zr 1− x O 2 during CO hydrogenation. The rate-limiting step for methanol synthesis is the elimination of methoxide species by reaction with Ce 3+ O(H) Zr 4+ species. The higher concentration of Ce 3+ O(H) Zr 4+ 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/Ce 0.5Zr 0.5O 2 relative to 3 wt% Cu/ZrO 2.