Rational design of cathode materials for the electrocatalytic hydrogenation of organic substrates by modulating the binding strength of H surface coverages Anna Ciotti 1 , Motiar Rahaman 2 , Yeung Wing See 2 , Arjun Vijeta 2 , Erwin Reisner 2 *, Max García-Melchor 1 1 School of Chemistry, The University of Dublin, Ireland, 2 Yusuf Hamied Department of Chemistry, University of Cambridge, UK Industrial hydrogenations of platform chemicals often rely on the use of organometallic catalysts and organic solvents. 1 An attractive alternative to cut down the associated costs and chemical waste is the electrification of reduction processes. In fact, the tunability of the applied potential mimics the selectivity effect of specific catalyst functionalization. 1 Additionally, organometallic catalysts are substituted with earth-abundant metal electrodes, and organic solvents with H 2 O and alcohols. 1 However, the use of aqueous electrolytes for electroreductions has to face the competition with the hydrogen evolution reaction (HER). 2 As HER is hindered under alkaline conditions, 2 we propose that the ideal catalyst for electrochemical hydrogenations (ECH) should work efficiently at basic pH, hydrolysing H 2 O to source the H atoms for the reaction. To facilitate the hydrogenation steps, H atoms should be retained on the surface with close to thermoneutral binding strength (ΔG *H ) at the applied potential. The ideal catalyst should also be abundant and cheap. In this talk, I will present our computational and experimental investigations on the ECH of acetophenone (AC) under alkaline conditions. As H 2 O hydrolysis is unfavourable on the substrate, 3 a catalyst exhibiting a H surface coverage was sought to achieve this model ECH. To prove our hypothesis, we employed metal electrodes covering a wide range of ΔG *H , i.e. Ag, Au, Cu, In, Ni and Pt. We determined that Ag and Au presented a H coverage of 25% of the fcc sites on the (111) surface under relevant ECH conditions, with moderately endergonic ΔG *H , Cu presented a H coverage of 75% of the fcc sites with moderately exergonic ΔG *H , and Ni and Pt presented a coverage of all the fcc sites with highly exergonic ΔG *H ( Fig.a ). As the ΔG *H on the (101) facet of In was highly endergonic, the In surface was considered clean ( Fig.a ). Therefore, we expected the best catalytic activity on Ag, Au and Cu, and poor yields on In, which lacked the H surface coverage, Ni, which had to pay a higher energy penalty for H desorption and AC hydrogenation, and Pt, due to its outstanding HER activity. 4 These predictions were corroborated both experimentally ( Fig.b ) and computationally, confirming that earth-abundant, efficient electrocatalysts for alkaline ECHs possess a basin of H on the surface, bound with moderately endo- or exergonic ΔG *H . Furthermore, mechanistic investigations unveiled the role of explicit H 2 O molecules shuttling H from the surface coverage to AC ( Fig.c ). Figure. a) Resting state of the electrodes under reaction conditions. b) Yields (%) of AC electroreduction on the electrodes. c) Transition state of AC hydrogenation on Ag, Au and Cu.
References 1. Schäfer, H. J.; Comptes Rendus Chim. 2011 , 14 , 745; 2. Ledezma-Yanez, I. et al.; Nat. Energy 2017 , 2 , 17031; 3. Hayon, E. et al.; J. Phys. Chem. 1972 , 76 , 2072; 4. Seh, Z. W. et al.; Science 2017 , 355 , 6321.
E02
© The Author(s), 2021
Made with FlippingBook Learn more on our blog