(678e) Effect of Subsurface Hydrogen and ?-PdHx Phase on the Selective Electrochemical Hydrogenation of Cis,Cis–Muconic Acid

Developing a detailed surface-level understanding of chemical reactions at the electrode-solvent interface is a key for electrocatalytic biomass upgrading.^1,2 Biomass-derived cis,cis-muconic acid (ccMA) has potential to produce adipic acid (AA) and performance-advantaged chemicals such as trans-3-hexenedioic acid (t3HDA) and trans-2-hexenedioic acid (t2HDA) through electrochemical hydrogenation (ECH).³ This ability to access novel species, as well as the ability to leverage “green” electricity from existing wind resources in the State of Iowa, makes electrochemical manufacturing an attractive platform.

Our current experiments at pH 4 on commercial 5 wt% Pd/C for ccMA ECH showed 100% selectivity to AA with >95% conversion, vs. <1% selectivity to AA with >95% conversion on Pd foil. Cyclic voltammograms suggest the presence of subsurface hydrogen on Pd/C, while no subsurface hydrogen peak was observed for Pd foil. In addition to metal-support interaction, the presence of different crystal facets of pallidum makes it complicated to answer: (i) What is the structure of the active phase for these Pd-based catalysts? (ii) How do subsurface hydrogen coverage and surface geometry impact the overall reaction energetics?

In this presentation, we will share DFT calculations of detailed reaction energetics for possible elementary steps involved in the ccMA ECH to t2HDA, t3HDA, and AA on β-PdH_0.16(111), β-PdH_0.20­(533), and Pd(111) and Pd(533) with varying amounts of subsurface H*. The presence of subsurface hydrogen generally reduces ΔG for the first hydrogenation step by ~0.10 eV. The effect of the β-PdH_x phase on ccMA ECH is sensitive to the surface geometry and subsurface H* coverage. Overall, reaction free energies for surface-mediated ECH are correlated to the binding strength of reactants, which enables us to comment on designing electrodes for tuning selectivity.

References:

1. 1. Chem. Rev. 120, 11370-11419 (2020).
2. 2. ACS Sustain. Chem. Eng. 4, 3575–3585 (2016).
3. 3. Green Chem. 22, 1517–1541 (2020).