Sorbent and Catalyst Development for Direct Air Capture and Formate-Based Hydrogen Carrier

Zhun Dong

doi:10.7273/000005325

Hydrogen becomes one of the most promising energy carriers with high gravimetric energy density (~120 MJ/kg). Its low volumetric density (~10 kJ/L) increases the transport cost through conventional compressed gaseous or liquefied hydrogen, limiting practical application on a large scale. Ammonium formate aqueous solution is a liquid organic hydrogen carrier that can uptake and release hydrogen through the redox reaction cycles of formate/bicarbonate under near ambient conditions and is easy to transport. Two challenges exist in formate-based hydrogen carrier system: 1) develop the formate generation from a highly concentrated CO2 source to a reversible carbon source, such as direct air capture; 2) sluggish kinetics of dehydrogenation and stability issues, especially for Pd/C catalysts. Metal-organic-frameworks (MOFs) serve as promising adsorbents because of their remarkably high surface area, controllable pore structures, and predictable functional groups. However, the effect that the identical MOFs materials display changeable CO2 adsorption capacities and rare works are reported to investigate ionic liquid (ILs) loaded on MOFs composites applied on direct air capture of CO2 attracts us to study the mechanism behind them deeply. Thus, we prepared zirconium-based MOFs (UiO-66) and treated them with methanol solvent and thermal activation approaches, which showed ~3 times enhanced CO2 capacity from 15.1 mg/g to 45 mg/g at 1 bar of CO2 pressure and excellent recyclability of 10 cycles. When we load ionic liquids ([P2228][2CNPyr], [P2228][6BrBnIm], [P4444][Im], and [Na][Im]) on the methanol-treated UiO-66, the CO2 capacity increases from 0.003 mmol/g to the maximum of 0.212 mmol/g at 0.04 mbar of CO2 partial pressure. The [Na][Im]@UiO-66 displayed a good recyclability of 5 cycles. This work deeply explains the MOFs activation mechanism and its applications in direct air capture CO2. On the dehydrogenation side, we investigate the kinetics of the surface-functionalized Pd on carbon catalysts for formate dehydrogenation and the impact of O-functional groups and Ag/Pd surface structure. The fraction of the distinguished O-functional groups was modulated by the different concentrated HNO3 solutions treatment or by H2 reduction. And a carbon-supported Pd-Ag bimetallic nanoparticles (NPs) catalyst through galvanic replacement and co-reduction methods. The as-prepared Pd5/re-ACA (reduced activated carbon washed by acid) and Pd3Ag10/ACA-G (a mass ratio 3:10 of Pd: Ag bimetallic NPs prepared by galvanic replacement method) exhibited significant activity with turnover frequencies of 4053 h-1 and 5202 h-1, respectively, higher than commercial Pd/C (1966 h-1). This study provides valuable guidelines for the efficient catalyst design and troubleshooting of the deactivation mechanism in formate-based hydrogen carriers.

Sorbent and Catalyst Development for Direct Air Capture and Formate-Based Hydrogen Carrier

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