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

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 M (Ti) species predominantly occupy more groups of GO, thereby leaving a tiny number of groups in the gaps of the M species to combine with M (Co, etc.) species. Consequently, this guarantees the formation of Co-N SACs embedded in 2D TiN/rGO sheets during nitridation. Further, TiN can act as a "spacer" to prevent Co-N from aggregation, thereby improving catalyst stability. Co-N/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 CoN-rGO prepared without Ti competition shows poor activity and stability, which indicates the vital role of Co-N and TiN for obtaining the remarkable catalytic ability of Co-N/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.