Hybrid Biotic/Abiotic Catalysts for Oxidation of Complex Biofuels

The electrochemical oxidation of oxalic acid on platinum is one of the main methods explored so far in the design of amperometric sensors for quantitative determination of oxalate and a commonly used approach for wastewater purification and energy production. Apart from that, herein we report the application of various platinum alloys and platinum nanoclusters as catalyst for oxidation of oxalic acid as a final step in the multistep oxidation of various organic compounds, such as fructose, glycerol, glyceric acid, glyceraldehyde, etc. This multistep oxidation processes can involve purely biotic or abiotic catalysts or a hybrid organo/metalo/enzyme catalysts, where the latter along with traditional electrocatalysis takes advantage of bioelectrocatalysis as well.

Various PtSn and PtRu alloys were electrochemically evaluated towards the oxidation of oxalic acid in neutral and acidic media (Fig. 1). The onset potential of oxalate oxidation was significantly shifted towards lower potentials for all platinum alloys tested indicating decreased activation energy for the rate-limiting step especially at low pH. Along with the improved thermodynamics of the process, the kinetic of oxalate oxidation was also enhanced when PtSn (1:1) was employed.

Density Functional Theory (DFT) calculations were explored to gain a better understanding of the reactivity of the platinum alloys [1]. The mechanism in which two electrons are transferred directly from oxalic acid molecule to the catalyst surface was considered:

 C₂O₄H₂→*C₂O₄H + H⁺ + e^-→*CO₂H+CO₂+H⁺+e^-→2CO₂+2H⁺+2e^-

where * represents the active site on the catalyst’s surface. Other mechanisms were also considered, which involve participation of adsorbed species such as OH or O. The free energy diagrams for the oxidation of oxalic acid on Pt based on these mechanisms were calculated using Perdew-Burke-Ernzerhof functional [1]. The results indicate that the adsorption of oxalic acid, which is accompanied by the release of H⁺and transfer of one electron to the electrode, requires the largest change in the free energy and can be considered as thermodynamically limiting step. Since the adsorption of oxalic acid is the potential-limiting step, an increase in the adsorption energy of oxalic acid on the different platinum alloys can explain the differences in the overpotential observed during the electrochemical evaluation. Based on the DFT calculations it was established that the presence of Sn and/or Ru increases oxalate adsorption and leads to decrease in the overpotential of oxalate oxidation in accordance to the experimentally observed results.

DFT also predicted that oxalate oxidation is highly sensitive to the size of the Pt clusters used as a catalyst, imposing the conclusion that decreasing the size of Pt would provide higher activity. Thus, platinum nanoclusters were synthesized using DNA as a template that provides improved cluster stability and were evaluated towards oxidation of oxalic acid. These nanoclusters could be further assembled with aldehyde dehydrogenase to provide multistep catalysis of glyoxalic acid to CO₂. In addition, Pt alloys were explored in combination with TEMPO for the subsequent and complete oxidation of glycerol.

[1] I. Matanovic, S. Babanova, A. Perry III, A. Serov, K. Artyushkova and P. Atanassov, Bio-inspired design of electrcatalysis for oxalate oxidation: a combined experimental and computational study of Mn-N-C catalyst, Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP00676G