Mechanistic Analysis of Electrocatalytic CO Oxidation By a Rh Porphyrin on a Carbon Support

Abstract

Electrochemical CO oxidation (CO + H2O → CO2 + 2H+ + 2e–) is important to counteract CO poisoning of Pt-based anode catalysts, since the reaction can reduce CO concentration on an anode of PEFC. However, the CO oxidation at anode potentials hardly occurs on conventional Pt-Ru electrocatalysts. From these backgrounds, we have developed Rh-porphyrin-based electrocatalysts for CO oxidation [1, 2]. In the catalysts, molecules of Rh porphyrin are dispersed on a carbon black support. These catalysts can oxidize CO at much lower overpotentials than the Pt-Ru electrocatalysts [1-3]. A membrane-electrode assembly (MEA) that uses a Rh porphyrin as an anode catalyst gave significant power (>40 mW/cm2) when CO was supplied as a fuel [4]. A combined catalyst of a Rh porphyrin and Pt-Ru catalyst functions as a CO-tolerant anode catalyst [5]. However, the reason why Rh porphyrins can catalyze CO electro-oxidation at low overpotentials remains unclear. Mechanistic analyses on carbon-supported Rh porphyrin are needed to clarify the reason. The analysis would also contribute to further improvement of this catalyst. In this work, we report a possible reaction mechanism of CO oxidation by the catalyst. First, we examined the reactivity of Rh porphyrins dissolved in solution. This analysis enables us to discuss the CO activation mechanism by Rh porphyrins without the effect of carbon black support. A Rh porphyrin was treated with CO gas in dichloromethane to generate CO adduct of the Rh porphyrin. We characterize this CO-adduct, a possible intermediate of CO oxidation, by IR, NMR, and X-ray crystallography. The results indicate that CO coordinated on Rh atom is feasible to nucleophilic attack by water. Actually, the CO-adduct reduces an soluble electron acceptor in the presence of water. A possible reaction mechanism is shown in Fig. 1. An electrode works as an electron acceptor in the electrocatalytic CO oxidation. In actual electroctalysts using Rh porphyrins, molecules are adsorbed on a carbon black support. Configuration of molecules on a carbon support would be also important for the catalytic activity. Then, we examined possible effects of the configuration by observing molecules on a highly oriented pyrolytic graphite with an atomic force microscope. The results show that certain ligand structure facilitates the configuration desirable for the high catalytic activity. These analyses would be an answer for the question; why does Rh porphyrin can oxidize CO at lower potentials than conventional Pt-based catalysts. This work was supported by Grant-in-Aids for Scientific Research (B) (No. 15H03853).