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A series of well-defined M–N–C based SACs (M = Fe, Ni, Co, Mn and Cu) was synthesized via a facile pyrolysis of metal-doped ZIF-8 and applied in electrocatalytic CO2 reduction to CO. Fe–N–C based SACs possess highest activity for CO2RR.

Electrochemical CO2 reduction reaction (ECO2RR) is an important route for global carbon abatement. However, a comprehensive picture of the structure evolution of metal active sites is currently lacked in ECO2RR. Here, we present the first full view of Ni single-atom catalyst for ECO2RR over a broad potential range. Comprehensive X-ray absorption spectroscopy (XAS) analyses confirmed the Ni coordinated with pyrrole nitrogen in the form of Ni-N4 attached with an axial O2 ligand. Operando XAS revealed the precise structure of the Ni single-atom catalyst that dynamically changes with the shift of applied potentials. Such changes ultimately contributed to the CO selectivity variation ranging from 20%-99%. Interestingly, the Ni center was found to move toward the N4 plane during the ECO2RR, which played a crucial role of reaching near-unity CO selectivity. Together with theoretical calculations, a clear quantitative correlation between the dynamic configuration and the catalytic properties was established.

Isolated cobalt single atoms with a high loading of 5.9 wt% are generated through an in situ preparation strategy., Single-atom catalysts (SACs) have shown great potential in a wide variety of chemical reactions and become the most active new frontier in catalysis due to the maximum efficiency of metal atom use. The key obstacle in preparing SAs lies in the development of appropriate supports that can avoid aggregation or sintering during synthetic procedures. As such, achieving high loadings of isolated SAs is nontrivial and challenging. Conventional methods usually afford the formation of SAs with extremely low loadings (less than 1.5 wt%). In this work, a new in situ preparation strategy that enables the synthesis of isolated cobalt (Co) SAs with an exceptionally high metal loading, up to 5.9 wt%, is developed. The approach is based on a simple one-step pyrolysis of a nitrogen-enriched molecular carbon precursor (1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile) and CoCl2. Furthermore, due to the successful electron transfer from carbon nitride to the isolated Co SAs, we demonstrate a high-performance photocatalytic H2 production using Co SAs as a co-catalyst, and the evolution rate is measured to be 1180 μmol g–1 h–1. We anticipate that this new study will inspire the discovery of more isolated SACs with high metal loadings, evidently advancing the development of this emerging type of advanced catalysts.

Small gold nanoparticles (∼3 nm) loaded onto various supports have been prepared by a deposition–precipitation method and studied for direct methylation of N-methylaniline with CO2/H2. Among the catalysts examined, an acid–base bifunctional support γ-alumina supported gold catalyst (Au/γ-Al2O3) exhibits the best catalytic performance. Au/γ-Al2O3 catalysts with controlled mean Au particle sizes (1.8–8.3 nm) have also been successfully prepared by regulating the concentration of HAuCl4 in solution, aging time, aging temperature and mole ratio of urea to gold in the process of deposition–precipitation with urea. The turnover frequency (TOF) values for direct methylation of N-methylaniline with CO2/H2 increase on decreasing the mean size of Au nanoparticles (from 8.3 to 1.8 nm), showing that methylation of N-methylaniline with CO2/H2 is a structure-sensitive reaction. A fast increase in TOF occurs when the mean Au particle size becomes smaller than 3 nm. Through TEM (transmission electron microscope), gold L3-edge XAFS (X-ray absorption fine structure) and CO2- and NH3-TPD (temperature programmed desorption) analysis, we can conclude the Au particle sizes, oxidation state of the gold species and acid–base properties of the supports are responsible for the high catalytic activity of direct methylation of N-methylaniline with CO2/H2.