PNNL

The Science                                 

Converting biologically derived carbon-based, or organic, molecules into energy carriers or more valuable chemicals could help reach a carbon-balanced future. Many of these transformations require selectively adding hydrogen to oxygen-containing parts of molecules. Because there is relatively less renewable hydrogen available, researchers are exploring driving these reactions with electricity in electrocatalysis. A new study looked at electrocatalysis of carbonyls, molecules with a carbon–oxygen double bond, on palladium. Through a combination of spectroscopy, electrochemical characterization, and reaction kinetics, researchers found that small amounts of excess electrical energy favor hydrogenation. Increasing the amount of excess energy favors hydrogen formation in the same system.

The Impact

Closing the carbon cycle requires finding ways to effectively use and valorize biological carbon. This will involve targeted additions of hydrogen to carbon–oxygen double bonds. Through electrocatalysis in water, researchers mapped the reactivity of palladium (Pd) for hydrogenation or hydrogen evolution in the absence of external hydrogen. This could lead to a more sustainable route for chemical fuels, converting excess renewables-based electricity into chemical bonds.

Summary

The mismatch between the volume of available renewable carbon sources and required input for current chemical plants to perform hydrogen addition requires new approaches. Electrocatalytic hydrogenation does not need external molecular hydrogen and is particularly attractive as an alternative hydrogenation approach. Additionally, electrocatalysis is currently performed near room temperature.

Researchers performed electrocatalytic hydrogenation of a series of carbonyl compounds using a Pd catalyst. They quantitatively determined the influence of molecular structure and cathodic potential on electrocatalytic hydrogenation rates and selectivity. They combined spectroscopy, electrochemical characterization, and reaction kinetics measurements to show that pushing the negative electric potential has two major effects on hydrogenation. It increases charge transfer rates for electrons, directly influencing the overall rate, and decreases the surface coverage of the organic compound by increasing the hydrogen coverage.

Researchers found that organic compounds with higher standard free energies of adsorption induce higher hydrogenation rates. Fast hydrogenation kinetics produce a hydrogen-depleted environment that kinetically hinders hydrogen evolution and the transition of the Pd surface to a Pd hydride. As a consequence of strong organic–metal interactions, hydrogenation dominates when a small amount of extra energy is added. Researchers determined the range of electric potentials that favor hydrogenation on Pd. They also quantitatively deconvoluted the effects of organic compound sorption and proton-coupled electron transfer rates on the kinetics of both hydrogenation and hydrogen evolution. The results indicate that the presence of an electric field offers different and potentially faster hydrogenation pathways for polar molecules.

PNNL Contact

Oliver Gutierrez, Pacific Northwest National Laboratory, oliver.gutierrez@pnnl.gov

Funding

This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences program, Division of Chemical Sciences, Geosciences, and Biosciences.