Microwave Enabled Incorporation of Singlemetal Atoms in Holey Graphene

Abstract

Single-atom catalysts (SACs) can simultaneously maximize the utilization of metal atoms and minimize side reactions, achieving high catalytic activity and selectivity, which are not attainable for the conventional nanoparticle-based catalysts. SACs have been intensively investigated for a wide range of reactions and great progress has been achieved. However, most of the carbon substrates supporting the SACs are highly defective carbon materials, which are not only low conductivity but also low stability against oxidation. In the recent review articles, carbon matrix oxidation has been identified as the most important degradation mechanism of PGM-free catalysts in PEMFC cathodes. The stability issue is also especially critical when the SACs are used in Fenton-like catalytic reactions. In this work, we present a novel approach to quickly fabricate SACs in which the single metal atoms are incorporated in highly conductive holey graphene. This novel approach takes advantages of the rapid heating capability of microwave when highly conductive holey graphene is used as the microwave absorber. The initial assumption of the method was based on the rapid heating/cooling capability of microwave heating to quickly carbonize the organic linkers in a metal-organic framework (MOF) into a conductive carbon network, leaving no time for the diffusion of the metal atoms, therefore aggregation of the metal atoms to form clusters can be largely avoided. Interestingly, instead of stopping the diffusion of metal atoms in the MOF, microwave heating appears causing the metal atoms flying out from the MOF particles and incorporated into the basal planes of the holey graphene. Using PCN222-Fe and PCN222-Cu MOF as the precursors, Fe and Cu SACs were fabrications, in which the Fe and Cu catalytic centers are incorporated into the basal planes of the holey graphene. It is worth to mention that the non-hole regions of the holey graphene are nearly defect free, which is in high contrast to the highly defect carbon materials, ensuring high stability of the catalysts against oxidation. The catalytic performance of thus fabricated Cu and Fe SACs are studied in oxygen reduction reaction. Most importantly, the hydrogen peroxide reduction/oxidation reactions were also studied. These results provide fundamental basis for these catalysts in Fenton-like catalytic reactions. Figure 1