Development of Bimetallic Non-Platinum Group Metal Catalysts Based on Metal Organic Framework Precursors

Abstract

It has been postulated that platinum group metal free (PGM-free) catalysts for the oxygen reduction reaction (ORR) must meet four criteria: metal, carbon, nitrogen, and pyrolysis. Results from multiple studies indicate that metals surrounded by four pyridinic nitrogen atoms form at least part of the active site. Other groups have demonstrated enhanced activity in bimetallic catalysts using first and second-row non-PGM transition metals in metal-nitrogen-carbon (M-N4-C) coordination environments. The precise nature of the active site remains under debate. Many permutations of metal, nitrogen, and carbon are formed during pyrolysis, only a few of which are active. In nature, however, there is no known example of four electron reduction of oxygen at a single metal center. It may be that only multiple-metal centers, metals situated close enough to work cooperatively, are able to catalyze ORR. It stands to reason that the greater the number of M-N4-C sites formed, the greater the probability of producing centers close to one another. However, nanoparticle formation tends to compete as the metal content increases. The present research focuses on the use of zeolitic imidazolate frameworks (ZIFs), a subset of metal organic frameworks, as catalyst precursors. ZIFs are of interest because they are highly porous, contain a high density of M-N4-C centers, and are simple to prepare. Cobalt doping in ZIF precursors affects the porosity and graphitization of the pyrolyzed material. This research investigates the effect of cobalt doping level in ZIF precursors and non-PGM metal pairs (e.g. cobalt adjacent to iron) on catalytic activity. It is hypothesized that graphitization formed during pyrolysis will facilitate the cooperation of proximal metal sites in electron transfer to oxygen. This will be investigated by evaluating bimetallic Co-containing catalysts in ORR by varying the degree of graphitization while holding the ratio of Co and another metal (e.g. iron) constant. Comparison with similar catalysts prepared from amorphous ZIF precursors will elucidate the importance of local carbon structure in electron transfer between metal centers.