Single-atom-catalyst (SAC), which was defined as the active metal site in a catalyst exist as isolated single atoms stabilized by a support or in an alloy by the second coordinate round atoms, is emerging as a new class of heterogeneous catalysts ^{[1, 2]}. Compared with nano- and subnano-catalysts, SACs could achieve the atom utilization efficiency up to 100%, offer the opportunities for achieving high activity/selectivity in reactions, and bridge the gap between homogeneous and heterogeneous catalysts. Therefore, SACs may provide a good platform to understand the structure-reactivity relationship at atomic scale.

The highly dispersed metal catalysts (extremely up to 100%) have been a long standing interest in heterogeneous catalysis field. In 2011, Zhang and co-workers reported the first Pt₁/FeO_x catalyst with single Pt atom dispersion ^[2]. The catalysts were prepared by a co-precipitation method and demonstrated the catalytic activity in CO oxidation and selective preferential oxidation (PROX) reaction with a turnover frequency (TOF) of 3-fold higher than the nano-counterpart. In the following years, they developed different noble metal SACs, such as Ir/FeO_x ^[3], Pd/ZnO ^[4], Au/CeO₂ ^[5] and Ag-Pd/SiO₂ ^[6]. These catalysts showed high activity in water-gas-shift reaction and high selectivity in hydrogenation reactions of acetylene and functionalized nitroarenes ^[7]. In particular for the chemoselective hydrogenation of functionalized nitroarenes, the Pt₁/FeO_x SAC afforded a TOF of ~1500 h^-1, 20-fold higher than the best result reported in Ref. ^[8], and a selectivity to 3-aminostyrene close to 99%, the best ever achieved over platinum group metals. Motivated by these work, some other researchers also made progress ^[9-13]. However, owing to the high surface energy of single atoms, the ever prepared SACs only have low metal loading (< 0.5 wt%), and moreover, the transition metal SACs, have rarely been developed.

Recently, this group has made further progress in preparing transition metal single-atom catalyst, Co-N-C, with relatively high metal loading ^[14]. The Co(Fe)-N-C catalysts have been widely investigated and demonstrated excellent catalytic performance in electro-chemical reactions, such as oxygen reduction reaction (ORR), hydrogen evolution reaction (HER) as well as CO₂ reduction reaction, and have been regarded as one of the most promising substitutes of Pt/C ^[15-18]. Despite intensive research, great debate still exists on the exact structure of the active sites because of the heterogeneity in composition. Usually, this type of catalysts are prepared by pyrolysis of a specific precursor containing Co(Fe), N, and C elements supported on activated carbon at high temperatures (600-900 ℃), which often leads to complex species, such as the mixture of Co(0), CoO_xand CoN_x with size of tens of nanometers, as well as the Co (Fe) single atoms only visible under atomic-resolution scanning electron microscope (STEM). Zhang and his colleagues employed Mg(OH)₂ instead of carbon as the sacrificial support and prepared the single-atom Co-N-C catalyst where cobalt exists exclusively as single atoms (Fig. 1(a)). By using a combination of subngstrm-resolution high angle annular dark field (HAADF)-STEM, X-ray absorption fine structure, and density functional therory calculations, the exact structure of the Co-N-C catalyst was identified as CoN₄C₈-1-2O₂, where Co center atom is coordinated with four pyridinic N atoms in the graphitic layer while two oxygen molecules are weakly adsorbed on Co atoms in perpendicular to the Co-N₄ plane (Fig. 1(b)). Different from the noble metal SACs reported earlier, the Co loadings in the Co-N-C SAC could be as high as 3.6 wt%.

Fig. 1

Fig. 1. (a) HAADF-STEMimages of Co-N-C catalyst. The white dots are Co single atoms. (b) Comparison between the K-edge XANES experimental spectrum of Co-N-C (solid red line) and the theoretical spectrum (black dotted line) calculated with the inset structure.

Notably, the Co-N-C SAC exhibited catalytic activity and selectivity comparable to noble metal catalysts in the chemoselective hydrogenation of functionalized nitroarenes to produce azo compounds. By using Co-N-C SACs as the catalysts, direct synthesis of azo compounds from nitroarenes through the green hydrogenation strategy could be achieved under mild conditions. The catalyst was also effective for a broad scope of substrates with different functional groups, such as -C=C, -I, -Br, etc.

It is of particular merit that the unrivalled uniformity of Co species in the atomically dispersed Co-N-C catalyst provides a good entry to the identification of active sites without much ambiguity. In this respect, revisiting other earlier reported Co-N-C catalysts containing mixed Co species will be helpful to elucidating the nature of genuine active sites ^[19-21].

The Co complex as a molecular catalyst has been widely used in homogeneous catalysis. However, in this work, Co site is fixed in a C, N containing matrix which is analogous to the molecular catalyst in nature, while a truly heterogeneous catalyst with highly dispersed single atom site. Here it could be anticipated that, in more general way, many other transition metal complex could be also possibly transformed into solid catalysts with single active sites following the strategy reported in this work.