Electrochemical Upgrading of Wastes to Products- Fundamental Studies Using a Combined Experimental and Computational Approach

Abstract

In recent years there has been growing interest in using electrochemical methods for hydrogenation of biomass-derived molecules for the formation of biofuels. Conventional hydrogenation processes require moderate temperatures between 433 and 678 K, pressures of 14,000 kPa, and external sources of dihydrogen for the stabilization and formation of biofuels. However, the same reaction can be performed using electrochemical reactors and much lower temperatures and pressures (293 K and 101 kPa) and with no supplied molecular hydrogen. In this work, we use a combined experimental–theoretical approach to identify atomistic features and parameters that affect the electrochemical hydrogenation (ECH) rates of different biomass organic compounds (such as aldehydes, ketones, and carboxylic acids) under ambient reaction conditions. In particular, we evaluated platinum group metals (PGM) and base-group metals (BGM) for the benzaldehyde ECH and correlated the activity with computationally-derived parameters. We then used these parameters to explain the dependence of molecule functionality on the ECH rates. The experiments were conducted using a continuous flow fixed bed reactor at room temperature and atmospheric pressure in the aqueous phase. We tested 7 metal catalysts with 12 substrates. Classical molecular dynamics simulations were performed on the metal’s most stable surfaces. The GROMACS package(83) was used for all classical molecular dynamics (MD) simulations. The results, summarized in Figure 1, indicate that the computationally-derived binding energy strongly correlates with the experimentally-derived ECH rates and hydrogen evolution reaction (HER) rates. The results suggest that Pd is the most active material for benzaldehyde ECH because it has an optimal organic reduction potential and binding energy compared to the other metals tested. However, Rh, which has a similar organic reduction potential, it is not active for benzaldehyde ECH due to the strong binding energy and high HER rates. These metrics were also used to evaluate the ECH activity of other biomass-derived molecules and we observed that the experimentally-derived parameters can be used to explain the differences in ECH activity with respect to benzaldehyde. For example, ketones such as acetophenone are not as active for ECH as benzaldehyde because of their binding energy. Figure 1