Theoretical Investigations on the Effect of the Functional Group of Pd@UiO-66 for Formic Acid Dehydrogenation

It is well-known that functional groups should greatly affect the electronic properties of the metal cluster-encapsulated metal-organic frameworks and thus change their catalytic properties; however, theoretical understanding is rare. Herein, four different functionalized Pd-28@UiO-66-X (X = NO2, Cl, H, and NH2) catalysts were investigated for their properties on the formic acid (FA) dehydrogenation using a density functional theory-based method, aiming to unravel the reaction mechanisms of the two competing pathways giving rise to CO2 and CO. With the functional groups from NO2 to Cl, H, and NH2, the charge transfer from the confined Pd-28 cluster to the framework decreases, i.e., 2.52, 2.14, 2.00, and 1.33 vertical bar e vertical bar, respectively, leading to the different reaction pathways for the FA decomposition. Interestingly, the results suggest that the -NO2 group acts as an acceptor for the H desorbed from the cis-COOH intermediate, giving rise to the formation of CO2, which greatly decreases the energy barrier from 0.62 to 0.15 eV, much lower than the value of 0.54 eV for the CO pathway by the decomposition of cis-COOH into CO + OH. In contrast, without the aid of the functional groups as an H acceptor, the undesired CO reaction pathway is dynamically more favorable or competitive to the CO2 reaction pathways for the four Pd-28@UiO-66-X catalysts. Our theoretical investigations imply that a rational design of functionalized groups for the catalysts would facilitate the H-2 production from FA dehydrogenation.