Effects of palladium particle size in hydrogenation of carbon dioxide to methanol over Pd/ZnO catalysts

Abstract

Pd/ZnO catalysts with various Pd particle sizes were prepared by water-in-oil microemulsion method and investigated as catalysts for CO 2 to methanol. The Pd particle size of the catalyst could be controlled by varying pH of microemulsions, regardless of Pd content. As pH varied from 1 to 7, the average Pd particle size increased from 2.1 to 10.1 nm. The distribution of metal particle sizes was considerably narrower compared with that of the catalysts obtained by the conventional precipitation method. The effects of Pd particle size were complicated by the formation of Pd-Zn alloy and coverage of Pd particles by ZnO, both of which were pronounced for small particles. Thus methanol selectivity increased with decreasing Pd particle size, showing a maximum at 3.4 nm, and then decreased for smaller particles. This selectivity pattern showed an excellent correspondence with the pattern of surface Pd fractions measured by EPMA.