Formic acid (FA), a nontoxic and stable liquid under ambient conditions with a gravimetric hydrogen content of 4.4 wt %, has been identified as a secure and convenient carrier of hydrogen and could be well compatible with the existing infrastructures used for gasoline and diesel. Particularly noteworthy is that FA has a high volumetric hydrogen density of 53 g L^–1, which exceeds the 2017 target of 40 g L^–1 for on-board hydrogen storage updated recently by the US Department of Energy (DOE). Moreover, modern industrial biomass processes and catalytic reduction of CO₂ with H₂ can produce FA on a large scale and at low cost. Therefore, the efficient and selective conversion of liquid FA into H₂ and CO₂ under mild conditions, without CO generation, will enable the use of FA as an ideal hydrogen carrier.

Currently, Pd and Pd-based nanomaterials are the most widely used catalysts for H₂ generation from FA systems. However, the activities and selectivities of these catalysts are still far from those required for practical applications. Moreover, the procedures presently used for the synthesis of catalysts generally involve multiple steps and strictly controlled reaction conditions, including inert gas protection, heating, and post-processing, greatly hindering their use in practical applications. Thus, a simple and facile strategy that allows easy control of the nucleation and growth of metal nanocatalysts with distinguished performance for FA dehydrogenation under ambient conditions is vital for practical applications.

Moreover, the procedures presently used for the synthesis of catalysts generally involve multiple steps and strictly controlled reaction conditions, including inert gas protection, heating, and post-processing, greatly hindering their use in practical applications. Thus, a simple and facile strategy that allows easy control of the nucleation and growth of metal nanocatalysts with distinguished performance for FA dehydrogenation under ambient conditions is vital for practical hydrogen storage.

—Zhu et al.

The researchers developed an approach that uses anhydrous methanol not only as the solvent for the growth of palladium nanoparticles (Pd NPs) but also as a mild reductant and a weakly capping agent to control the nucleation and prevent the aggregation of the dispersed ultrafine Pd NPs.

Schematic representation for the preparation of Pd/C_m catalyst. Credit: ACS, Zhu et al. Click to enlarge.

When weakly capped by methanol molecules, the Pd NPs can be immobilized on a carbon support to create a stable surfactant-free catalyst,\. The resulting catalyst exhibits high activity for complete and efficient H₂ production without CO impurity from aqueous FA under mild conditions—an important consideration for hydrogen used in a PEM fuel cell.

The catalyst affords the highest turnover frequency (TOF; 7256 h^–1 at 60 °C) for the selective decomposition of FA in heterogeneous systems. The small size and clean surface of the naked Pd NPs account for their excellent catalytic properties, the researchers suggested in their paper.

The average rate of H₂ evolution was 43 L H₂ min^–1 gPd^–1, which corresponds to a high energy density of 58 W min^–1 gPd^–1. Owing to the low operating temperature, the hydrogen release of this system can be driven by the waste heat of a standard PEM fuel cell without additional heat treatment.

The catalyst is believed to give a tremendous boost to the practical application of FA for chemical hydrogen storage. Meanwhile, the utilization of this facile and economical reduction approach to obtain and subsequently deposit ultrafine metal NPs on various solid supports is expected to open new routes for designing highly efficient nanocatalysts.

—Zhu et al.

Resources

• Qi-Long Zhu, Nobuko Tsumori and Qiang Xu (2015) “Immobilizing Extremely Catalytically Active Palladium Nanoparticles to Carbon Nanospheres: A Weakly-Capping Growth Approach” Journal of the American Chemical Society doi: 10.1021/jacs.5b06707