Understanding Metal Structure Sensitivity during the Electrocatalytic Hydrogenation and Oxidation of Biomass-Derived Molecules at Normal Temperature and Pressure

Abstract

In recent years there has been growing interest in using electrochemical methods for the upgrading of biomass-derived molecules for the formation of biofuels. Conventional upgrading 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 dihydrogen. In this work, we evaluate the use of Pd, Pt, Rh, and Cu-based heterogeneous electrocatalysts for the electrocatalytic hydrogenation (ECH) and oxidation (ECO) of organic molecules present in biocrude. In particular, we studied the effect of electrolyte composition, half-cell (WvR) potential, and metal particle size on the Faradaic efficiency (FE) and catalytic activity (TOFECH and TOFECO) for different bio-oil derived-molecules such as aldehydes, ketones, phenolics, and carboxylic acids. Our results show that whereas the reduction of aldehydes and ketones into alcohols can be achieved electrocatalytically at normal temperature and pressure, carboxylic acids cannot be reduced. However, carboxylic acids can be oxidized to alcohols, olefins, and ketones via Kolbe and non-Kolbe mechanism. Further, we observed there the TOF and FE depend on metal, fractional exposure, substrate concentration and WvR potentials. For example, as shown in Figure 1, the FE and TOFECH are the highest when using Pd electrodes with low Pd weight loading (and small metal particle size). The FE and TOFECH decreases as the Pd weight loading (and particle size) increases. Eley-Rideal-type kinetics were used to understand the structure sensitivity of the Pd electrocatalysts and revealed that it is related to changes in rate constants and substrate surface coverage. That is, small Pd nanoparticles have higher ECH rate constant and higher benzaldehyde surface coverage compared to large Pd particles. Structure sensitivity was also observed on Pt and Rh-based electrocatalysts, but with opposite trend. That is, large Pt and Rh nanoparticles were more active than small nanoparticles. Cu-based catalysts were structure insensitive. Figure 1. TOFECH (A) and Faradaic Efficiency (B) as a function of WvR potentials under normal conditions for the benzaldehyde ECH to benzyl alcohol. Catalysts with different Pd loadings were used where represents 0.05wt% Pd/CF, represents 0.10wt% Pd/CF, represents 0.50wt% Pd/CF, represents 1.0wt% Pd/CF, and represents 4.0wt% Pd/CF. Figure 1