state of the catalyst, it can be classified as three types: homogeneous catalyst, heterogeneous catalyst, and biological (enzyme) catalyst. [4,5] Among them, the heterogeneous catalyst is the most important one because of its strong stability under severe conditions (e.g., high temperature, high pressure) and easy recovery from the reactants and products. [6,7] Heterogeneous catalyst is usually in the form of welldispersed metal particles and clusters, each of which may have multiple active sites with different properties. [8] It is well known that the coordination environment and electronic structure of metal-centered sites are the decisive factors in affecting the catalytic performance. However, traditional heterogeneous catalysts usually suffer from the insufficient utilization of atom, because only a small fraction of surface metal atoms can participate in the reaction. To reduce the cost of heterogeneous catalysis and improve the utilization efficiency of active metals in the heterogeneous catalyst, to rationally design and develop catalytic materials with good activity, durability and selectivity is now of greatest concern.Last decades, the gradual development of various precision instruments and detection methods has greatly stimulated the interests in the observation and exploration of microcosms. [9][10][11][12][13][14][15][16][17][18] Therefore, nanocatalysis, a new era of catalytic research, has been introduced and promoted by the rapid development of nanoscience. Studies have shown that the performance of catalyst can be determined by the size of catalytic metal particles. By controlling the nucleation and growth processes at nanoscale, nanoparticles (NPs) with high surface-to-volume ratios can be obtained. [19][20][21][22][23] Moreover, the abundant coordination unsaturated surface atoms located at edges, corners, and steps along with specific morphology of nanoparticles both have an important effect on the adsorption, desorption and activation process of small molecule reactants, which can effectively modulate the catalytic efficiency. [24] To obtain high atomic utilization efficiency, the particle of active metal is continuously reduced from nanoscale to sub-nanometer-scale or even atomic scale. [25,26] To this end, single-atom catalysts (SACs) bring new opportunities to the study in molecularly and atomically catalytic mechanisms, which require the accurate structure design at atomic scale.Compared with NPs catalysts, the metal species of SACs are uniformly dispersed on the support with maximum atom dispersion. [5,7,[27][28][29] Furthermore, the catalytic activities As a new and popular material, single-atom catalysts (SACs) exhibit excellent activity, selectivity, and stability for numerous important reactions, and show great potential in heterogeneous catalysis due to their high atom utilization efficiency and the controllable characteristics of the active sites. The composition and coordination would determine the geometric and electronic structures of SACs, and thus greatly influence the catal...
The controllable synthesis of stable single-metal site catalysts with an expected coordination environment for high catalytic activity and selectivity is still challenging. Here, we propose a cation-exchange strategy for precise production of an edge-rich sulfur (S) and nitrogen (N) dual-decorated single-metal (M) site catalysts (M = Cu, Pt, Pd, etc.) library. Our strategy relies on the anionic frameworks of sulfides and N-rich polymer shell to generate abundant S and N defects during high-temperature annealing, further facilitating the stabilization of exchanged metal species with atomic dispersion and excellent accessibility. This process was traced by in situ transmission electron microscopy, during which no metal aggregates were observed. Both experiments and theoretical results reveal the precisely obtained S, N dual-decorated Cu sites exhibit a high activity and low reaction energy barrier in catalytic hydroxylation of benzene at room temperature. These findings provide a route to controllably produce stable single-metal site catalysts and an engineering approach for regulating the central metal to improve catalytic performance.
Herein, we report a negative pressure pyrolysis to access dense single metal sites (Co, Fe, Ni etc.) with high accessibility dispersed on three-dimensional (3D) graphene frameworks (GFs), during which the differential pressure between inside and outside of metal-organic frameworks (MOFs) promotes the cleavage of the derived carbon layers and gradual expansion of mesopores. In situ transmission electron microscopy and Brunauer-Emmett-Teller tests reveal that the formed 3D GFs possess an enhanced mesoporosity and external surface area, which greatly favor the mass transport and utilization of metal sites. This contributes to an excellent oxygen reduction reaction (ORR) activity (half-wave potential of 0.901 V vs. RHE). Theoretical calculations verify that selective carbon cleavage near Co centers can efficiently lower the overall ORR theoretical overpotential in comparison with intact atomic configuration.
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