Interfacial Engineering of PdAg/TiO2 with a Metal–Organic Framework to Promote the Hydrogenation of CO2 to Formic Acid

The interfacial postmodification of PdAg alloy nanoparticles (NPs) supported on TiO2 was performed, using the widely studied metal–organic framework ZIF-8. The resulting PdAg/TiO2@ZIF-8 enhanced the selective hydrogenation of CO2 to give formic acid (FA) even under relatively mild reaction conditions (2.0 MPa, 100 °C). The highest activity, which was almost double the value obtained from the unmodified catalyst, was obtained with about 1.6 nm-thick ZIF-8 layer. Density functional theory calculations show that the presence of the ZIF-8 framework on the PdAg(111) surface not only enhances the adsorption of HCO3– ions but also promotes electronic interactions between the C atoms of HCO3– ions and also changes the atomic charge of C atoms of adsorbed HCO3– ions, based on the vertical orientation of the 2-methylimidazole rings, which promotes electronic interactions with dissociated H atoms. This interfacial postmodification also increases the durability of the catalyst by suppressing the agglomeration of NPs during the catalytic reaction.