Electrospray ionization mass spectrometric analyses of rhodium tetraphenylporphyrin complexes as electrocatalysts for CO oxidation by tandem mass spectrometry and hyphenated method with capillary electrophoresis

Abstract

Carbon monoxide (CO) poisoning of platinum (Pt)-based electrocatalysts is a huge obstacle in the development of proton-exchange membrane fuel cells. Fuel cell performance can be improved by removing CO through electrolytic oxidation. Yamazaki et al. synthesized several rhodium (Rh) tetraphenylporphyrin complexes as electrocatalysts to reduce CO poisoning. Experimental results showed that these complexes display higher electrochemical CO oxidation activity than previously synthesized Rh octaethylporphyrin complex and that the differences in the structures of p-substituted groups influence their activity. To confirm the formation of these Rh tetraphenylporphyrin complexes, analyses were performed using electrospray ionization mass spectrometry (ESI-MS), electrospray ionization tandem mass spectrometry (ESI-MS/MS), and capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESI-MS). In ESI-MS, peaks other than that of the Rh tetraphenylporphyrin complex were occasionally detected at different m/z values. The ESI-MS/MS and CE-ESI-MS results suggest that these peaks correspond to the complexes coordinating CO or other ions, or to remaining uncoordinated tetraphenylporphyrins. Coordination of CO is supported by the ESI-MS/MS results for the Rh octaethylporphyrin complex. Separation of the uncoordinated tetraphenylporphyrin from Rh tetraphenylporphyrin complex was achieved by CE-ESI-MS.