A “competitive occupancy” strategy toward Co–N4 single-atom catalysts embedded in 2D TiN/rGO sheets for highly efficient and stable aromatic nitroreduction

Abstract

Single-atom catalysts (SACs) have been promising in various catalytic fields, but the controllable synthesis of SACs with high stability remains challenging. Here, we show a robust strategy toward the fabrication of highly efficient and stable SACs based on the “competitive occupancy” of two metal (M) species on graphene oxide (GO). The abundant M₁ (Ti) species predominantly occupy more groups of GO, thereby leaving a tiny number of groups in the gaps of the M₁ species to combine with M₂ (Co²⁺, etc.) species. Consequently, this guarantees the formation of Co–N₄ SACs embedded in 2D TiN/rGO sheets during nitridation. Further, TiN can act as a “spacer” to prevent Co–N₄ from aggregation, thereby improving catalyst stability. Co–N₄/TiN–rGO exhibits outstanding catalytic performance for the fast conversion of highly concentrated aromatic nitro compounds (∼0.3–2 mM) into amino compounds and excellent recyclability, which is the best among reported catalysts. In contrast, TiN/rGO has no obvious activity, and Co₄N–rGO prepared without Ti competition shows poor activity and stability, which indicates the vital role of Co–N₄ and TiN for obtaining the remarkable catalytic ability of Co–N₄/TiN–rGO. The reaction mechanism is also proposed based on theoretical calculations. The strategy can be used to design Ni (Fe, Cr, Cu)-based SACs.