Sabatier Phenomenon in Hydrogenation Reactions Induced by Single-Atom Density.

The Sabatier principle is a fundamental concept in heterogeneous catalysis that provides guidance for designing optimal catalysts with the highest activities. For the first time, we here report a new Sabatier phenomenon in hydrogenation reactions induced by single-atom density at the atomic scale. We produce a series of Ir single-atom catalysts (SACs) with a predominantly Ir1-P4 coordination structure with densities ranging from 0.1 to 1.7 atoms/nm2 through a P-coordination strategy. When used as the catalysts for hydrogenation, a volcano-type relationship between Ir single-atom density and hydrogenation activity emerges, with a summit at a moderate density of 0.7 atoms/nm2. Mechanistic studies show that the balance between adsorption and desorption strength of the activated H* on Ir single atoms is found to be a key factor for the Sabatier phenomenon. The transferred Bader charge on these Ir SACs is proposed as a descriptor to interpret the structure-activity relationship. In addition, the maximum activity and selectivity can be simultaneously achieved in chemoselective hydrogenation reactions with the optimized catalyst due to the uniform geometric and electronic structures of single sites in SACs. The present study reveals the Sabatier principle as an insightful guidance for the rational design of more efficient and practicable SACs for hydrogenation reactions.

Regulating the electronic structure through charge redistribution in dense single-atom catalysts for enhanced alkene epoxidation.

Inter-site interaction in densely populated single-atom catalysts has been demonstrated to have a crucial role in regulating the electronic structure of metal atoms, and consequently their catalytic performances. We herein report a general and facile strategy for the synthesis of several densely populated single-atom catalysts. Taking cobalt as an example, we further produce a series of Co single-atom catalysts with varying loadings to investigate the influence of density on regulating the electronic structure and catalytic performance in alkene epoxidation with O2. Interestingly, the turnover frequency and mass-specific activity are significantly enhanced by 10 times and 30 times with increasing Co loading from 5.4 wt% to 21.2 wt% in trans-stilbene epoxidation, respectively. Further theoretical studies reveal that the electronic structure of densely populated Co atoms is altered through charge redistribution, resulting in less Bader charger and higher d-band center, which are demonstrated to be more beneficial for the activation of O2 and trans-stilbene. The present study demonstrates a new finding about the site interaction in densely populated single-atom catalysts, shedding insight on how density affects the electronic structure and catalytic performance for alkene epoxidation.

Uniform single atomic Cu1-C4 sites anchored in graphdiyne for hydroxylation of benzene to phenol.

For single-atom catalysts (SACs), the catalyst supports are not only anchors for single atoms, but also modulators for geometric and electronic structures, which determine their catalytic performance. Selecting an appropriate support to prepare SACs with uniform coordination environments is critical for achieving optimal performance and clarifying the relationship between the structure and the property of SACs. Approaching such a goal is still a significant challenge. Taking advantage of the strong d-π interaction between Cu atoms and diacetylenic in a graphdiyne (GDY) support, we present an efficient and simple strategy for fabricating Cu single atoms anchored on GDY (Cu1/GDY) with uniform Cu1-C4 single sites under mild conditions. The Cu atomic structure was confirmed by combining synchrotron radiation X-ray absorption spectroscopy, X-ray photoelectron spectroscopy and density functional theory (DFT) calculations. The as-prepared Cu1/GDY exhibits much higher activity than state-of-the-art SACs in direct benzene oxidation to phenol with H2O2 reaction, with turnover frequency values of 251 h-1 at room temperature and 1889 h-1 at 60°C, respectively. Furthermore, even with a high benzene conversion of 86%, high phenol selectivity (96%) is maintained, which can be ascribed to the hydrophobic and oleophyllic surface nature of Cu1/GDY for benzene adsorption and phenol desorption. Both experiments and DFT calculations indicate that Cu1-C4 single sites are more effective at activating H2O2 to form Cu=O bonds, which are important active intermediates for benzene oxidation to phenol.

Unprecedentedly high activity and selectivity for hydrogenation of nitroarenes with single atomic Co1-N3P1 sites.

Transition metal single atom catalysts (SACs) with M1-Nx coordination configuration have shown outstanding activity and selectivity for hydrogenation of nitroarenes. Modulating the atomic coordination structure has emerged as a promising strategy to further improve the catalytic performance. Herein, we report an atomic Co1/NPC catalyst with unsymmetrical single Co1-N3P1 sites that displays unprecedentedly high activity and chemoselectivity for hydrogenation of functionalized nitroarenes. Compared to the most popular Co1-N4 coordination, the electron density of Co atom in Co1-N3P1 is increased, which is more favorable for H2 dissociation as verified by kinetic isotope effect and density functional theory calculation results. In nitrobenzene hydrogenation reaction, the as-synthesized Co1-N3P1 SAC exhibits a turnover frequency of 6560 h-1, which is 60-fold higher than that of Co1-N4 SAC and one order of magnitude higher than the state-of-the-art M1-Nx-C SACs in literatures. Furthermore, Co1-N3P1 SAC shows superior selectivity (>99%) toward many substituted nitroarenes with co-existence of other sensitive reducible groups. This work is an excellent example of relationship between catalytic performance and the coordination environment of SACs, and offers a potential practical catalyst for aromatic amine synthesis by hydrogenation of nitroarenes.

Two-step modification towards enhancing the adsorption capacity of fly ash for both inorganic Cu(II) and organic methylene blue from aqueous solution.

A new adsorption material from fly ash (FA) was prepared by a two-step surface modification process, which showed higher ability for the removal of both inorganic and organic cationic pollutants from aqueous solution, i.e., Cu2+ and methylene blue (MB). Firstly, FA was modified by hydrothermal method in alkaline solution at 80 °C (FA80) to have a larger BET surface area. Afterwards, FA80 was further modified by sodium dodecyl benzene sulfonate (SDBS), of which a layer of anionic functional groups were grafted on the surface. The adsorption performance of SDBS@FA80 for removal of Cu2+ and MB were detailedly investigated. The results showed that SDBS@FA80 presented the optimal adsorption capacity at pH 7.0. Additionally, the maximum adsorption capacities of SDBS@FA80 for the removal Cu2+ and MB were up to 227.3 and 50.76 mg g-1 at 70 °C, respectively, as well as being about three times higher than that of FA. When the initial concentrations of Cu2+ and MB were lower than those of 20 and 10 ppm, their removal efficiencies were as high as 99.75 and 96.4%, respectively. The pseudo-second-order model was well applied to describe the adsorption kinetics, indicating that chemisorption was taking place. Furthermore, a plausible mechanism is proposed by XPS studies, where the high adsorption capacity is mainly contributed to the electrostatic attraction and π-π stacking interaction between the cationic Cu2+/MB and anionic functional groups of SDBS. Due to the low-cost and high adsorption capacity, SDBS@FA80 is regarded as a promising adsorbent for the removal of cationic pollutants.