Effect of Missing-Linker Defects on CO2 Hydrogenation to Methanol by Cu Nanoparticles in UiO-66

Catalytic reduction of CO2 can be an important component of a decarbonization strategy, and recent experiments have studied reduction of CO2 by hydrogen on Cu nanoparticles supported on the UiO-66 metal-organic framework (MOF) with missing-linker defects. In quantum mechanical calculations accompanying that report, we identified the importance of the Zr-O-Cu interface for CO2 hydrogenation. Here, we report a detailed quantum mechanical study in which periodic density functional calculations are carried out to systematically examine the quantitative effects of missing-linker defects at the Cu-Zr-based node interface of Cun@UiO-66, on the activation of H2 and CO2, on the reaction pathway for CO2 hydrogenation to produce methanol, and on two possible reaction pathways for CO production. We examine the full catalytic cycle in the limit of low hydrogen coverage on the Cu nanoparticle. A general finding is that the kinetics, catalytic efficiency, and side-product formation are very sensitive to the presence and number of linker defects. The presence of missing linkers reduces the steric hindrance and thereby allows better access to the catalytic interfacial sites and, in addition, results in the formation of vacancy sites on the Zr. However, if there are too many missing-linker defects, CO2 binds strongly on the Zr node rather at the interfacial sites, and this impedes CO2 hydrogenation. There is an optimum number of missing linkers that balance the steric effects and the strong binding on the node, and we find that the optimum number of missing linkers is about 5-7 per unit cell. We also find that increasing the hydrogen coverage results in a reconstruction of the Cu nanoparticle and further changes the energetics. This suggests that effective strategies for tuning defect structures and controlling the hydrogen coverage should be core elements of catalytic optimization in this kind of system.