The production of functionalized anilines by chemoselective hydrogenation of nitroarenes occupies an important position in the chemical industries. Recently, cobalt and nitrogen codoped carbon (Co–N–C) catalysts have been reported to show outstanding catalytic performance in this process, whereas the nature of their active sites is still at the center of intense debate. In this paper, based on the well-controlled experiments on four types of typical Co–N–C materials derived from different methodologies, several unusual generalities in the hydrogenation of nitroarenes are clearly revealed, including protic-solvent dependence, antipoisoning, acid resistance, and reversible deactivation. Further investigations indicate that the single-atom species Co–N x are the main active sites of the Co–N–C catalysts in this chemoselective hydrogenation process. The present work sheds light on the mechanism study on the metal–N–C catalysts and further contributes to the rational design of the high-performance single-atom catalysts.

The ubiquitous mineralization of calcium phosphate (CaP) facilitates biological organisms to produce hierarchically structured minerals. The coordination number and strength of Ca 2+ ions with phosphate species, oxygen-containing additives, and solvent molecules played a crucial role in tuning nucleation processes and surface stability of CaP under the simulated body fluid (SBF) or aqueous solutions upon the addition of oligomeric lactic acid (LACn, n=1, 8) and changing pH values. As revealed by ab initio molecular dynamics (AIMD), density functional theory (DFT), and molecular dynamics (MD) simulations as well as high-throughput experimentation (HTE), the binding of LAC molecules with Ca 2+ ions and phosphate species could stabilize both pre-nucleation clusters and brushite (DCPD, CaHPO4• 2H2O) surface through intermolecular electrostatic and hydrogen bonding interactions. When the concentration of Ca 2+ ions ([Ca 2+ ]) is very low, the amount of the formed precipitation decreased with the addition of LAC based on UV-Vis spectroscopic analysis due to the reduced chance for the LAC capped Ca 2+ ions to coordinate with phosphates and the increased solubility in acid solution. With the increasing [Ca 2+ ] concentration, the kinetically stable DCPD precipitation was obtained with high Ca 2+ coordination number and low surface energy. Morphologies of DCPD precipitation are in plate, needle, or rod, depending on the initial pH values that tuned by adding NH3•H 2O, HCl, or CH3COOH. The prepared samples at pH ≈ 7.4 with different Ca/P ratios exhibited negative zeta potential values, which were correlated with the surface electrostatic potential distributions and potential biological applications.3 INTRODUCTIONBones and teeth of vertebrates are known to come from mineralization of calcium phosphate (CaP) 1 through two possible nucleation processes, i.e., a classical aggregation or a two-step nucleation pathway. 2 The real nucleation process of CaP in biological conditions becomes much more complicated, attracting extensive interest both experimentally and theoretically.The first factor in controlling nucleation process of CaP is the pH value, which could affect the occurrence of different phosphate species, such as H3PO4 (called 3H for short), H2PO4 -(2H), HPO4 2-(1H), and PO4 3-(0H), shown in Figure 1a. Thus, properties of CaP precipitation, 3, 4 adsorption of organic acid 5 , phase composition 6 , morphology 7, 8 , zeta potentials [9][10][11][12] , and crystal size of CaP 13 could be varied with different pH values.Secondly, through the modulation of interfacial interaction and kinetic stability, the introduction of many kinds of additives, such as metal ions, acid or basic solutions, amino acids, dopamine, and citrate, could change the nucleation or phase transformation paths and morphologies 8, 14-19 . Starting from the ion pair CaHPO4 in a neutral or basic CaP solution, a Ca-deficient pre-nucleation cluster, Ca(HPO4)3 4-, is formed at the early stage of nucleation process. [20][21][22][23][24] Subsequently, those sm...

Single-atom catalysts (SACs) have received intensive interest due to the utmost atom utilization and unique catalytic behaviors. However, the catalytic mechanism of SACs is still unclear, especially in the reactions involving multiple substrates. Here, we report that the environment moieties, protic solvents, and external bases can boost the cobalt SAC-catalyzed chemoselective hydrogenation of nitroarenes. Systematical studies clearly reveal that the heterolysis of H2 between the metal center and the neighboring coordination sphere is significantly facilitated via the H-shuttling or deprotonation route with the participation of protic solvents or bases. Besides, the derived single-atom cobalt hydride species are confirmed to be specifically reactive for the reduction of the nitro group, affording a superior activity and general chemoselectivity in the hydrogenation of various nitroarenes. Finally, a catalytic cycle is proposed to elucidate the activity origin of SACs, which sheds light on the rational design of SACs by engineering the microenvironment of active sites.

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Mineralization of calcium phosphate (CaP) is ubiquitous in nature, which can facilitate biological organisms produce hierarchically structured minerals. The influences of the addition of oligomeric lactic acid (LACn, n=1, 8) and changing pH values on nucleation processes of CaP under the simulated body fluid (SBF) solution or aqueous solution were revealed by using ab initio molecular dynamics (AIMD) and molecular dynamics (MD) simulations. Through increasing pH value, the coordination between differently protonated phosphate species and Ca2+ ions could be tuned from the monodentate (η1) to the coexisting monodentate and bidentate (η2) modes. The carboxyl and hydroxyl groups of LAC molecules are capable of forming multiple interactions such as proton transfer, electrostatic interaction, and intermolecular hydrogen bonding with phosphate species and Ca2+ ions on both pre-nucleation clusters and hydroxyapatite (HA) surfaces. The high-throughput experimentations (HTE) with factors of the adding LAC, changing Ca/P ratios (1.25 ~ 2.50), using different solutions demonstrated that the UV-Vis absorbance values decreased with addition of LAC, indicating the inhibition of the nucleation process of CaP. At the physiological pH of 7.4, the CaP samples with different Ca/P ratios (1.50, 1.67 and 2.00) exhibited negative zeta potential values, which were correlated with the surface electrostatic potential distributions and potential biological applications. The precipitation was CaHPO4·2H2O (DCPD) in neutral condition at the early stage of nucleation process. Understanding the effects of different pH and Ca/P values on the nucleation process and interfacial interaction between LAC additive and the nanocluster is helpful to guide the rational design of biocompatible materials.