PNNL

The Science 

In electrochemistry, applying an electric potential to a catalytic system can speed up reactions through accelerating charge transfer or by influencing thermocatalytic pathways via non-Faradaic effects. Researchers found that simultaneously applying H₂ and a cathodic potential increases the rate of Pd-catalyzed benzaldehyde hydrogenation to benzyl alcohol in water up to six-fold compared to the condition without an external bias. Through a combination of experimental measurements and computational modeling, the team elucidated the reaction mechanism at the molecular level: first proton and electron transfer to the adsorbed benzaldehyde molecule followed by hydrogen addition. Within this mechanistic framework, they identified that cathodic potential promotes hydrogenation through two routes: (i) increasing the surface coverage of the partially converted intermediate preceding the rate-determining step and (ii) potentially reducing the free energy barrier of kinetically relevant hydrogen addition.

The Impact 

Developing methods to boost the activity of chemical reactions is central to catalysis. Understanding how electrochemical enhancement works at the molecular level can help researchers develop additional approaches to catalysis at the solid–liquid interface. Overall, these findings demonstrate that electrode polarization can enhance heterogeneous catalytic hydrogenation in more ways than forcing electron-transfer chemistry and open a new approach to improving catalytic processes beyond conventional methods.

Summary 

Electrochemistry offers a promising route to drive chemical reactions, replacing high temperatures and pressures with electrical energy. One intriguing electrochemical phenomenon is the non-Faradaic promotion of catalysis, where applying an electrical potential to a catalyst enhances reaction rates through mechanisms that go beyond simply transferring electrons to reactants. However, understanding how and why this promotion occurs at solid–liquid interfaces has remained elusive. A team of researchers demonstrated that applying a cathodic (negative) potential to a palladium catalyst significantly accelerates the aqueous-phase hydrogenation of benzaldehyde to benzyl alcohol, which cannot be explained by charge transfer alone. By carefully decoupling the contributions of direct electron transfer from other promotion effects, the team showed that the enhancement is fundamentally non-Faradaic in character.

Through a combination of experiments and computational modeling, the researchers found that the reaction proceeds through a quasi-equilibrated proton-coupled electron transfer step, followed by rate-determining surface hydrogen addition. The cathodic potential promotes activity by increasing the coverage of key surface intermediates and potentially lowering the energy barrier of this rate-determining step. These mechanistic insights demonstrate that electrical polarization can serve as a powerful and distinct tool for enhancing catalytic activity, beyond charge transfer contribution. This understanding lays the groundwork for designing next-generation catalytic systems that harness non-Faradaic effects to achieve more energy-efficient chemical transformations at solid–liquid interfaces.

Contact 

Sungmin Kim, Pacific Northwest National Laboratory, sungmin.kim@pnnl.gov 

Udishnu Sanyal, Pacific Northwest National Laboratory, udishnu.sanyal@pnnl.gov

Funding 

This work was performed at Pacific Northwest National Laboratory (PNNL), which is a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy (DOE) under contract number DE-AC06-76RL0183. This work is supported by the U.S. Department of Energy (DOE), Office of Science (SC), Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences and Biosciences (Advancing key catalytic reaction steps for achieving carbon neutrality, FWP 47319). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by DOE under Contract No. DE-AC02-05CH11231 using NERSC award BES-ERCAP0032416 and BES-ERCAP0032412. Scanning transmission electron microscopy (STEM) analysis was performed at the TEM facility in the Environmental Molecular Sciences Laboratory (EMSL) and at the Radiological Microscopy Suite in Radiochemical Processing Laboratory, both located at PNNL.