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

Abstract

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 H 2 pressure and the reaction temperature. The present study demonstrates that binary Pd-Ag alloy nanoparticles (NPs) supported on rutile TiO 2 exhibit higher catalytic activity during both dehydrogenation of HCOONa/H 2 O and hydrogenation of NaHCO 3 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 TiO x shell on TiO 2 -supported Pd-Ag alloy catalysts, indicating that the formation of Pd−TiO 2 interface sites also affects activity. A TiO x shell-modified core−shell Ag@Pd catalyst supported on TiO 2 was synthesized to optimize catalytic activity. This material exhibited significant activity during the dehydrogenation of HCOONa/H 2 O (with a turnover frequency of 6499 h −1 at 75 °C) and the hydrogenation of NaHCO 3 (with a turnover number of 820 after 2 h at 80 °C, 3.0 MPa H 2 ) 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 TiO 2 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 TiO x shell in the vicinity of the Pd promoted the rate-limiting C−H bond dissociation step during dehydrogenation of HCOONa/H 2 O and facilitated bicarbonate adsorption and activation during the hydrogenation of NaHCO 3 .

Formate‐Bicarbonate Cycle as a Vehicle for Hydrogen and Energy Storage

“…In an another recent study focused on formate-bicarbonate, interconversion for hydrogen storage using Ag@Pd/TiO 2 catalyst and surface modification of the catalyst with TiO x shell led to the improved activity of the catalyst. [41] Modification of the surface significantly enhanced the activity of material during the dehydrogenation of formate and achieved TOF of 6499 h À 1 (i. e. 2 h at 80 °C, 3.0 MPa H 2 ). Similarly, TOF of 820 h À 1 was achieved during the hydrogenation of bicarbonate.…”

“…No bimetallic catalytic systems were demonstrated to promote both the reactions efficiently. To tackle this issue, Yamashita and co‐workers studied a binary Pd−Ag alloy nanoparticle system embedded on TiO x support [79b] . This core‐shell Ag@Pd catalyst exhibited excellent catalytic activity for both the dehydrogenation of HCOONa/H 2 O and hydrogenation of NaHCO 3 (Figure 11).…”

“…In the recent years most of the work on advancement in formate‐bicarbonate‐based energy systems were either carried out on bimetallic/alloy‐based materials or utilizing different carbon support for metals [79b,c,82] . However, these works were mostly focused on enhancing the catalytic activity, and recyclability at mild or ambient conditions.…”

“…While investigating mechanism of the Ag@Pd@TiO x (TTIP)/TiO 2 catalyst it was found that combination of Pd with both the Ag and TiO 2 are important for the good activity leading to dehydrogenation of HCOONa/H 2 O and the hydrogenation of NaHCO 3 at ambient and low pressure, respectively (Figure 21). [79b] Electron deficient TiO 2 or Ag sites accept electrons from oxygen atom of HCO 2 − when it adsorbs on catalyst surface followed by nucleophilic attack by water molecule on the carbon atom of HCO 2 − to eliminate an H atom. Subsequently, C−H bond of formate get dissociated due to Pd sites and release HCO 3 − (Figure 21A).…”

Combined Effect of Functionality and Pore Size on Dehydrogenation of Ammonia Borane via Its Nanoconfinement in Polyacrylamide-Grafted Organically Modified Mesoporous Silica

“…Lastly, it was concluded that LiAlH 4 can be utilized to enhance the dehydrogenation of Mg(BH 4 ) 2 . Masuda et al 31 reported dehydrogenation of HCOONa/H 2 O and hydrogenation of NaHCO 3 using Pd−Ag alloy nanoparticles supported on rutile TiO 2 . Pd, due to its electron-rich state in the formed alloy, was considered an important factor for high activity.…”