Interconversion of Formate/Bicarbonate for Hydrogen Storage/Release: Improved Activity Following Sacrificial Surface Modification of a Ag@Pd/TiO₂ Catalyst with a TiO_x Shell

Catalytic formate/bicarbonate interconversion is a promising means of achieving a reversible hydrogen storage/release energy cycle because this system is easily controlled by varying only the H2 pressure and the reaction temperature. The present study demonstrates that binary Pd-Ag alloy nanoparticles (NPs) supported on rutile TiO2 exhibit higher catalytic activity during both dehydrogenation of HCOONa/H2O and hydrogenation of NaHCO3 compared to various other combinations of supports and binary alloys. The electron-rich state of Pd resulting from the formation of an alloy was found to be an important factor providing this high activity, as indicated by X-ray photoelectron spectroscopy. The electronic state of Pd showed weakened correlation with surface modification via the application of a TiOx shell on TiO2-supported Pd-Ag alloy catalysts, indicating that the formation of Pd–TiO2 interface sites also affects activity. A TiOx shell-modified core–shell Ag@Pd catalyst supported on TiO2 was synthesized to optimize catalytic activity. This material exhibited significant activity during the dehydrogenation of HCOONa/H2O (with a turnover frequency of 6499 h–1 at 75 °C) and the hydrogenation of NaHCO3 (with a turnover number of 820 after 2 h at 80 °C, 3.0 MPa H2) based on the total amount of Pd loading. This activity resulted from both the formation of a Pd-Ag alloy and modification of the Pd-Ag alloy NPs with a TiO2 shell. Formation of the desired structure was confirmed by high-angle annular dark-field scanning transmission electron microscopy observations together with energy-dispersive X-ray spectroscopy mapping and line analysis. Kinetics studies during both reactions revealed that the presence of Ag sites and the TiOx shell in the vicinity of the Pd promoted the rate-limiting C–H bond dissociation step during dehydrogenation of HCOONa/H2O and facilitated bicarbonate adsorption and activation during the hydrogenation of NaHCO3.