A computational investigation of the UiO-66 confinement effect on copper clusters: the initial hydrogenation of CO₂

Formate (HCOO) is a key intermediate in CO₂-to-CH₃OH conversion, especially on Cu-based catalysts. We recently demonstrated how UiO-66 confinement modulates the H₂ dissociation capacity of Cu clusters at the electronic level. Nonetheless, an understanding of the direct involvement of UiO-66 moieties (i.e., Zr nodes and organic linkers) in CO₂ hydrogenation pathways remains unclear. Here, we investigate the role of UiO-66 moieties in CO₂-to-HCOO conversion by comparing non-confined and UiO-66-confined Cu_n clusters (n = 4, 8, 16, or 32). Three main insights are obtained: (i) the poor activation of CO₂ on Cu clusters originates from limited overlap between the Cu HOMO and CO₂ LUMO+1; (ii) UiO-66 moieties enhance CO₂ adsorption but contribute little to its activation and hydrogenation; and (iii) the confinement effect mainly influences the hydrogenation step by facilitating H-desorption and strengthening C-H interactions.