Carbon Dioxide Conversion to Methanol over Size-Selected Cu₄ Clusters at Low Pressures

The activation of CO₂ and its hydrogenation to methanol are of much interest as a way to utilize captured CO₂. Here, we investigate the use of size-selected Cu₄ clusters supported on Al₂O₃ thin films for CO₂ reduction in the presence of hydrogen. The catalytic activity was measured under near-atmospheric reaction conditions with a low CO₂ partial pressure, and the oxidation state of the clusters was investigated by <i>in situ</i> grazing incidence X-ray absorption spectroscopy. The results indicate that size-selected Cu₄ clusters are the most active low-pressure catalyst for catalytic CO₂ conversion to CH₃OH. Density functional theory calculations reveal that Cu₄ clusters have a low activation barrier for conversion of CO₂ to CH₃OH. This study suggests that small Cu clusters may be excellent and efficient catalysts for the recycling of released CO₂

Size-Dependent Ligand Quenching of Ferromagnetism in Co₃(benzene)_<i>n</i> ⁺ Clusters Studied with X‑ray Magnetic Circular Dichroism Spectroscopy

Cobalt–benzene cluster ions of the form Co₃(bz)_<i>n</i> ⁺ (<i>n</i> = 0–3) were produced in the gas phase, mass-selected, and cooled in a cryogenic ion trap held at 3–4 K. To explore ligand effects on cluster magnetic moments, these species were investigated with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectra yield both the spin and orbital angular momenta of these clusters. Co₃ ⁺ has a spin magnetic moment of μ_S = 6 μ_B and an orbital magnetic moment of μ_L = 3 μ_B. Co₃(bz)⁺ and Co₃(bz)₂ ⁺ complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co₃ ⁺. However, coordination of the third benzene to form Co₃(bz)₃ ⁺ completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co₃(bz)_<i>n</i> ⁺ species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for <i>n</i> = 0 → 3