Effects of Support Composition on the Activity of Cu/Mxzr1-Xo2 (M = Ce, Pr, Mn) for Methanol Synthesis Via Co Hydrogenation
Alexis T. Bell and Konstatin A. Pokrovski. University of California, Berkeley, Department of Chemical Engineering, 107 Gilman Hall, Berkeley, CA 94720
The effects of Ce, Pr, and Mn incorporation into ZrO2 on the catalytic performance of Cu/ZrO2 for the hydrogenation of CO were investigated. MxZr1-xO2 solid solutions were synthesized by forced hydrolysis at low pH and Cu was then deposited onto the surface of the calcined solid by deposition-precipitation. 3 wt% Cu/CexZr1-xO2 exhibited H2 consumption peaks at low temperature (< 500 K) during H2-TPR indicating a significant fraction of Ce4+ is reduced to Ce3+. The reducibility and the activity of 3 wt% Cu/CexZr1-xO2for CO hydrogenation go through the maximum with increased Ce content. 3 wt% Cu/Ce0.5Zr0.5O2 catalyst is 4 times more active for methanol synthesis than 3 wt% Cu/ZrO2 at 3.0 MPa at temperatures between 473 and 523 K and exhibits a higher selectivity to methanol. In-situ infrared spectroscopy shows that the primary surface species on Cu/CexZr1-xO2 during CO hydrogenation are formate and methoxide species. A shift in the band position of the bridged methoxide species with increased Ce content indicated that some of these groups were bonded to both Zr4+ and Ce3+ cations. The rate-limiting step for methanol synthesis is the reductive elimination of methoxide species regardless of the catalyst composition. The differences in methanol synthesis on Cu/CexZr1-xO2 was primarily due to a higher apparent rate constant, kapp, for methoxide hydrogenation, which is attributed to the higher surface concentration of H atoms on the surface of the catalysts with intermediate Ce content. The increased capacity of the Ce-containing catalysts is attributed to interactions of H atoms with Ce-O pairs present at the surface of the oxide phase. Very similar results were obtained with 3 wt% Cu/Pr0.3Zr0.7O2 and 3 wt% Cu/Mn0.3Zr0.7O2 catalysts. On the basis of methanol synthesis activity per unit of oxide surface area, the rate of methanol synthesis increased in the order 3 wt% Cu/Ce0.3Zr0.7O2 < 3 wt% Cu/Mn0.3Zr0.7O2 < 3 wt% Cu/Pr0.3Zr0.7O2.