On the Role of Hydroxyl Groups on Cu/Al₂O₃ in CO₂ Hydrogenation

Understanding the nature of adsorbates and intermediates on catalytic surfaces is critical for the development of highly efficient systems for CO2 conversion. However, the role of structural hydroxyl groups in CO2 hydrogenation is still under debate. This paper describes an experimental study on the nature of surface-bound hydroxyl groups over Al2O3-supported Cu catalysts for CO2 reduction. A model 5 wt % Cu/Al2O3 catalyst with stable hydroxyl groups was prepared. The structural hydroxyl groups were modulated by the hydration of unsaturated Al3+ sites, which were introduced by the evaporation-induced self-assembly method. Terminal hydroxyl groups (T-OH*) were found to significantly facilitate the conversion of CO2, while the AlVI-T-OH and AlIV-T-OH species promoted the formation of Al-b-HCOO and Al-m-HCOO species, respectively, accompanied with the increase in methanol and dimethyl ether yields. The reaction behavior of hydroxyl species was further investigated by in situ IR experiments with HD isotope exchange on Cu/Al2O3. T-OH* were identified as active sites participating in the formation of key intermediates, while more hydroxyl sites were occupied by formate species with the H coadsorbed on the Cu surfaces.