Redox-active phenanthrenequinone as a molecular electron bridge for ultrastable Pd-catalyzed formic acid electrooxidation

To further enhance the electrocatalytic performances of Pd-based electrocatalysts in small organic molecule oxidation reactions, Pd/C was modified by a redox-active organic molecule (9,10-phenanthrenequinone, PQ), resulting in a novel Pd/C-PQ catalyst. The catalyst was prepared by first physically adsorbing PQ onto the surface of Vulcan XC-72 carbon black (C-PQ), followed by the deposition of Pd nanoparticles on this modified C-PQ support. Results demonstrated that the Pd nanoparticles within the Pd/C-PQ catalyst were well-dispersed with a particle size distribution ranging from 2.4 to 4.6 nm. This size was significantly smaller than that of Pd nanoparticles in the unmodified Pd/C catalyst. Electrochemical testing revealed a substantial enhancement in performance. The mass and specific activity for formic acid oxidation achieved by the Pd/C-PQ catalyst were 2.67 and 2.07 times higher than that of the unmodified Pd/C catalyst, respectively. In addition, the CO stripping peak of the Pd/C-PQ catalyst exhibited a negative shift of 26 mV compared to Pd/C, indicating weaker CO adsorption. The remarkably improved stability was also observed for the Pd/C-PQ catalyst. Furthermore, the charge transfer kinetics for the formic acid oxidation reaction were significantly accelerated. The enhanced electrochemical performance of the Pd/C-PQ catalyst can be attributed to several synergistic factors of uniformly dispersed Pd with relatively smaller particle sizes, electronic interaction/modification between Pd and PQ, and the cooperative role of the PQ molecules acting as efficient redox mediators.