Understanding the Role of Metal and Molecular Structure on the Electrocatalytic Hydrogenation of Oxygenated Organic Compounds

Abstract

Electrocatalytic hydrogenation is increasingly studied as an alternative to integrate the use of recycled carbon feedstocks with renewable energy sources. However, the abundant empiric observations available have not been correlated with fundamental properties of substrates and catalysts. In this study, we investigated electrocatalytic hydrogenation of a homologues series of carboxylic acids, ketones, phenolics, and aldehydes on a variety of metals (Pd, Rh, Ru, Cu, Ni, Zn, and Co). We found that the rates of carbonyl reduction in aldehydes correlate with the corresponding binding energies between the aldehydes and the metals according to the Sabatier principle. That is, the highest rates are obtained at intermediate binding energies. The rates of H2 evolution that occur in parallel to hydrogenation also correlate with the H-metal binding energies, following the same volcano-type behavior. Within the boundaries of this model (e.g., compounds reactive at room temperature and without important steric effects over the carbonyl group), the reported correlations help to explain the complex trends derived from the experimental observations, allowing for the correlation of rates with binding energies and the differentiation of mechanistic routes.

Upgrading of Bio‐Oil Model Compounds and Bio‐Crude into Biofuel by Electrocatalysis: A Review

“…Electrode commonly consists of catalysts and conductive material (catalyst support), which has a crucial effect on the bio‐oil conversion and products selectivity in bio‐oil ECH process. In detail, electrocatalysts like noble metal, [32b,33,39b,48] base metal, [32a,48a,c,49] alloy, [9a,50] oxide [51] and phosphide [40a] have been reported for upgrading bio‐crude and model compounds by ECH. Besides, graphite, activated carbon and other porous materials as catalyst supports can improve the electrocatalytic performance.…”

“…Large amounts of noble metals such as Pd, Rh, Ru, Au and Pt have been employed as cathode materials during ECH of aldehyde and phenolic model compounds [28,32b,39b,44b, 48a,c,52] . Among them, Pd, Rh, Au and Ru showed high ECH activity for aldehyde model compounds upgrading [48a,52,53] .…”

“…Among them, Pd, Rh, Au and Ru showed high ECH activity for aldehyde model compounds upgrading [48a,52,53] . Pd exhibited excellent ECH activity for aromatic aldehydes [48a,c] . For instance, the ECH rate [μmol products g −1 catalyst h −1 ] of benzaldehyde with different metal cathodes followed the order of Pd/CF>Ru/CF>Rh/CF under the same other conditions (electrolyte: 47.5 wt% isopropanol, 47.5 wt% deionized water and 5.0 wt% acetic acid; potential: −1.1 V vs Ag/AgCl; ambient temperature and pressure) [48a] .…”

Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase

“…The rates increased in the order Rh/C<Ru/C<Pd/C<Cu/C (Figure 5, Figure S12, S13 and Table S3). This trend is inversely proportional to the binding energies ( E b ) of benzaldehyde on charged metal surfaces as determined computationally: −0.7 eV (Cu)<−3.2 eV (Pd)<−4.8 eV (Ru)<−5.4 eV (Rh) [51] . Thus, the rate enhancement observed by phenol co‐adsorption is hypothesized to be related to the ability of phenol to adsorb at the metal surface displacing benzaldehyde.…”

“…This trend is inversely proportional to the binding energies (E b )o fb enzaldehyde on charged metal surfaces as determined computationally: À0.7 eV (Cu) < À3.2 eV (Pd) < À4.8 eV (Ru) < À5.4 eV (Rh). [51] Thus,t he rate enhancement observed by phenol co-adsorption is hypothesized to be related to the ability of phenol to adsorb at the metal surface displacing benzaldehyde.T his is also reflected in the changes of reaction order in benzaldehyde ranging from % 0i nt he absence of phenol to > 0i nt he presence of phenol ( Figure S14 and Figure S15). Surprisingly,HER rates also increased in the presence of phenol, despite the parallel hydrogenation of the organic substrate.Atpresent, we hypothesize that the higher rates are enabled by the increase of the concentration of hydronium ion induced by the sorption of molecules with polar groups or to lower kinetic barriers for their reduction through shuttling mechanisms similar to the mechanism leading to the enhancement of ECH rates.T he positive impact of phenol on HER rates is attributed to the increasing concentration of hydronium ions at the interface,which has been identified to cause the positive impact of proton carriers on HER rates on gold electrodes.…”