Copper Cluster Size Effect in Methanol Synthesis from CO 2

Size-selected Cu n catalysts ( n = 3, 4, 20) were synthesized on Al 2 O 3 thin films using mass-selected cluster deposition. A systematic study of size and support effects was carried out for CO 2 hydrogenation at atmospheric pressure using a combination of in situ grazing incidence X-ray absorption spectroscopy, catalytic activity measurement, and first-principles calculations. The catalytic activity for methanol synthesis is found to strongly vary as a function of the cluster size; the Cu 4 /Al 2 O 3 catalyst shows the highest turnover rate for CH 3 OH production. With only one atom less than Cu 4, Cu 3 showed less than 50% activity. Density functional theory calculations predict that the activities of the gas-phase Cu clusters increase as the cluster size decreases; however, the stronger charge transfer interaction with Al 2 O 3 support for Cu 3 than for Cu 4 leads to remarkably reduced binding strength between the adsorbed intermediates and supported Cu 3, which subsequently results in a less favorable energetic pathway to transform carbon dioxide to methanol.