Carbon black-supported FM–N–C (FM = Fe, Co, and Ni) single-atom catalysts synthesized by the self-catalysis of oxygen-coordinated ferrous metal atoms

Abstract

Carbon nanomaterials have abundant sources but are difficult to use directly as a support for single atom catalysts (SACs) due to the lack of strong anchoring forces to restrict the movement and aggregation of metal atoms during the high-temperature heat treatment. Herein, we report a “self-catalysis” method for the synthesis of ferrous metal single-atom catalysts (SACs) FM–N–C (FM = Fe, Co, and Ni) with ∼2 wt% metal loadings on a carbon black support. The combination of experimental and theoretical evidence reveals a self-catalytic process from FM atoms to FM–N₄ species, which involves (1) the adsorption of FM ions on the surface-oxidized carbon nanoparticles via oxygen coordination bonds (FM–O); (2) the catalytic decomposition of ammonia to nitrogen radicals on the FM atoms; (3) the replacement of coordinating oxygen with nitrogen. The precoordination of FM with oxygen is the key to the synthesis and reduces the energy barrier of ammonia decomposition and nitrogen bonding to the FM atoms. Thus, the formation of the FM–N₄ species at a mild temperature of 600 °C is enabled. The as-synthesized FM–N–C SACs exhibit high catalytic activities towards O₂ and/or CO₂ reduction reactions. In contrast to the metal–organic framework-based SACs in which the MOF support significantly changes in size and weight, the self-catalysis technique is “minimally invasive” to the carbon support, thus benefiting the structural design of SACs by taking advantage of the abundant morphologies of the carbon nanomaterials.