Low-temperature intermediates to oxygen reduction reaction catalysts based on amine-modified metal-loaded carbons. An XPS and ss-NMR investigation

Abstract

Carbon functionalization is a major subject of interest in a number of project applications. Herein we report results on the characterization of nitrogen- and metal-loaded (Me = Fe, Co) carbon derivatives from low-T reaction steps before they are converted to catalysts for electrochemical oxygen reduction by later high-T treatments. The aim is to shed light on the state of carbon and carbon-bonded moieties before thermal modifications take place during any chosen high-T treatment. Though necessary for end catalyst activation, such thermal treatments make difficult to establish a relation between the starting reactants and finally obtained catalysts. Of interest to the paper are 13C, 15N solid-state NMR (ss-NMR) and high-resolution X-ray Photoelectron Spectroscopy (XPS) results on a commercial carbon that was reacted first with aliphatic di- and tri-amines and then with Fe, Co ions in room-T water. Data from natural abundance ss-15N NMR in combination with XPS analysis were found especially relevant to assess that, in the adopted conditions, amines preferentially bind to carbon by creating alkylimino functional groups, which spontaneously form hydrous surface metal complexes with soluble Fe and Co ions. A chemical model is thus proposed for metal coordination in such C–N species.