Copper Cluster Size Effect in Methanol Synthesis from CO₂

Abstract

Size-selected Cun catalysts (n = 3, 4, 20) were synthesized on Al2O3 thin films using mass-selected cluster deposition. A systematic study of size and support effects was carried out for CO2 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 Cu4/Al2O3 catalyst shows the highest turnover rate for CH3OH production. With only one atom less than Cu4, Cu3 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 Al2O3 support for Cu3 than for Cu4 leads to remarkably reduced binding strength between the adsorbed intermediates and supported Cu3, which subsequently results in a less favorable energetic pathway to transform carbon dioxide to methanol.