Synthesis of Single-Atom and Dual-Atom Catalyst Using N-Defective C3N4

Abstract

Single-atom catalysts, which have been studied tremendously in the past decade, are emerging as a new class of heterogeneous catalysts. It is now generally known that defect sites play an important role in forming single-atomic structures. In the case of SACs, defective sites provide not only space for fixation of metal precursor, but also electrons for stabilizing positive metal ions. Graphitic carbon nitride (g-C3N4) is one of the ideal 2D materials for the synthesis of SACs. C3N4 has enough anchoring sites which originate from the ordered structure of tri-s-triazines connected together. Furthermore, abundant nitrogen atoms in C3N4 can provide electrons to single metal atoms. however, C3N4 is unsuitable as support for electrochemical catalysts due to its low electrical conductivity. Therefore, we made a C3N4 shell on the outer surface of carbon black (Ketjen Black EC-600JD) to obtain conductive support (C@C3N4).[1] Pt was supported on C@C3N4 at 1, 2, 4, and 8 wt% by wetness impregnation method. Through XRD, HAADF-STEM, and XAFS analysis, it was confirmed that a single-atomic structure was formed only when the Pt content was 1 or 2 wt%. In addition, I confirmed that the H2O2 selectivity (%) in the electrochemical oxygen reduction reaction was different according to the Pt content of the catalysts.In terms of controlling reaction sites at the atomic level, the challenge above creating single-atomic structures is creating structures in which two or three atoms are formed as dimer or trimer. Unlike single-atom catalysts, in a dimer or trimer structure, two or more atoms are adjacent to each other, so they can exhibit completely different catalytic properties. Finely controlling the defect site of the support can be a good strategy. Therefore, I adopted the strategy of making a defect in C3N4 to synthesize a dimer structure. It has been reported that N-vacancy can be selectively formed on C3N4 by additional heat treatment with NaBH4.[2, 3] In addition, DFT calculation results have been reported that Pd-Cu dimer structure can be stably formed on nitrogen-defective C3N4.[4]In this work, I create N-vacancy on a Pt1/C@C3N4 by additional heat treatment with NaBH4, and Co atoms were deposited as a secondary transition metal. The research strategy was to anchor the Co atom to the N-vacancy formed around the Pt single-atom, resulting in the formation of a dimer structure. First of all, the formation of N-vacancy on C@C3N4 support was clearly confirmed by FT-IR and XPS analysis. To confirm the dimer structure of Pt and Co, XAFS analysis was conducted. As a result, although not all Pt and Co atoms formed a dimer structure due to the limitation of the synthesis method, the intended structure of Co atom connected to a Pt atom was confirmed. In the Pt L3 edge EXAFS, a peak arising from Pt-Co scattering was observed at 2.4 Å. Furthermore, to confirm that PtCo alloy particles were not formed, high-result HAADF-STEM images were taken. All metal atoms were atomically dispersed, and nanoparticles were not found. This research demonstrated that defect engineering on support material can successfully modify active site on the atomic scale.