Surface Engineering of a Supported PdAg Catalyst for Hydrogenation of CO2 to Formic Acid: Elucidating the Active Pd Atoms in Alloy Nanoparticles.

The hydrogenation of carbon dioxide (CO2) to formic acid (FA; HCOOH), a renewable hydrogen storage material, is a promising means of realizing an economical CO2-mediated hydrogen energy cycle. The development of reliable heterogeneous catalysts is an urgent yet challenging task associated with such systems, although precise catalytic site design protocols are still lacking. In the present study, we demonstrate that PdAg alloy nanoparticles (NPs) supported on TiO2 promote the efficient selective hydrogenation of CO2 to give FA even under mild reaction conditions (2.0 MPa, 100 °C). Specimens made using surface engineering with atomic precision reveal a strong correlation between increased catalytic activity and decreased electron density of active Pd atoms resulting from a synergistic effect of alloying with Ag atoms. The isolated and electronically promoted surface-exposed Pd atoms in Pd@Ag alloy NPs exhibit a maximum turnover number of 14 839 based on the quantity of surface Pd atoms, which represents a more than 10-fold increase compared to the activity of monometallic Pd/TiO2. Kinetic and density functional theory (DFT) calculations show that the attack on the C atom in HCO3- by a dissociated H atom over an active Pd site is the rate-determining step during this reaction, and this step is boosted by PdAg alloy NPs having a low Pd/Ag ratio.