(326e) Nanostructured Transition Metal Carbide Based Electrocatalysts for Triglyceride Hydrogenation

More than 8,300,000 metric tons of partially hydrogenated vegetable oil are produced in the U.S. annually [1], using a high temperature, high pressure thermochemical process with H₂ and a nickel catalyst. The resulting partially hydrogenated oils typically contain significant amounts of trans-fatty acids (TFAs), which have been linked to coronary artery disease.

Recently an electrochemical approach has been reported [5]. The partial hydrogenation of soybean oil using noble metal electrocatalysts resulted in a significant decrease in TFA contents. While the mechanism for the thermochemical reaction has been extensively studied [2-4], much less is known about the electrochemical process. Research described in this paper explored the use of nanostructured transition metal carbides such as WC, NbC, VC, and Mo₂C, carbide-supported metals, and non-noble metals such as Cu, Fe, Ni, and Co, as electrocatalysts. Transition metal carbides have been shown to be active for a variety of reactions including CO hydrogenation [6] and alkane isomerization [7]. Carbides have also attracted attention for use as electrocatalysts [8] and electrocatalyst supports [9], due to their high surface areas and high electrical conductivities.

The electrochemical hydrogenation measurements were performed using a reactor very similar to a proton exchange membrane (PEM) fuel cell. Reaction rates and selectivities for triglyceride hydrogenation were evaluated at 70 °C and 1 atm H₂ under constant current (0.5 A) and constant potential (0.25 ? 1.0 V) conditions. The products were characterized using gas chromatography using industry standard procedures (AOCS Methods Ce 1f-96 and Ce 2-66). The early transition metal carbides demonstrated good coulombic efficiencies and selectivities for the electrochemical hydrogenation of the types of triglycerides in soybean oil. These carbide catalysts exhibited efficiencies in the range of 5-15% under constant current conditions. For comparison, the Ru and Ir black catalysts yielded similar efficiencies while Pt, Pt, and Rh exhibited efficiencies in the range of 60-90%.

Research described in this paper also explored the possibility that differences in coulombic efficiencies between the various electrocatalysts were due to differences in their mechanisms. Horiuti and Polanyi proposed a two-step mechanism for the thermochemical hydrogenation of unsaturated hydrocarbons [10], which involved adsorbed hydrogen reacting with the adsorbed unsaturated hydrocarbon. Previous descriptions of mechanisms for the electrochemical hydrogenation reaction also implicated surface hydrogen as reacting with adsorbed unsaturated species [11]. The hydrogen involved in this process, similar to the Volmer-Tafel hydrogen evolution reaction (HER) mechanism, appeared to be electrochemically generated, but the hydrogenation step appeared to be thermochemical.

Our results appear to be consistent with two different hydrogenation pathways. Carbides, Ru, and Ir, which likely evolve hydrogen via the Volmer-Heyrovsky mechanism [12], may also hydrogenate triglycerides via a Heyrovsky-like mechanism (Figure 1). But Pd, Pt, and Rh, which likely evolve hydrogen via the Volmer-Tafel mechanism [13], may use a second pathway, a Tafel-like mechanism. This difference in reaction pathways may be correlated to the difference in efficiencies between the two groups.