Selective catalytic hydrogenation has wide applications in both petrochemical and fine chemical industries, however, it remains challenging when two or multiple functional groups coexist in the ...substrate. To tackle this challenge, the “active site isolation” strategy has been proved effective, and various approaches to the site isolation have been developed. In this review, we have summarized these approaches, including adsorption/grafting of N/S-containing organic molecules on the metal surface, partial covering of active metal surface by metal oxides either via doping or through strong metal–support interaction, confinement of active metal nanoparticles in micro- or mesopores of the supports, formation of bimetallic alloys or intermetallics or core@shell structures with a relatively inert metal (IB and IIB) or nonmetal element (B, C, S, etc.), and construction of single-atom catalysts on reducible oxides or inert metals. Both advantages and disadvantages of each approach toward the site isolation have been discussed for three types of chemoselective hydrogenation reactions, including alkynes/dienes to monoenes, α,β-unsaturated aldehydes/ketones to the unsaturated alcohols, and substituted nitroarenes to the corresponding anilines. The key factors affecting the catalytic activity/selectivity, in particular, the geometric and electronic structure of the active sites, are discussed with the aim to extract fundamental principles for the development of efficient and selective catalysts in hydrogenation as well as other transformations.
The annual incidence of suicide among Chinese doctoral students remains high. We reviewed the statistics for doctoral student suicides over the past 12 years and administered a questionnaire survey ...and interviews to relatives and friends of doctoral students who committed suicide. The results indicated that suicide among doctoral students is closely related to academic pressure, the teacher–student relationship, and the academic environment, reflecting the current condition of China's doctoral tutorial system. This study provides a theoretical basis for society to understand Chinese doctoral student suicide and the Chinese doctoral tutorial system as well as the relationship between the two. Our results may serve as a warning to the academic community to prevent the worsening of the suicide contagion.
Nanostructured Fe-N-C materials represent a new type of "platinum-like" non-noble-metal catalyst for various electrochemical reactions and organic transformations. However, no consensus has been ...reached on the active sites of the Fe-N-C catalysts because of their heterogeneity in particle size and composition. In this contribution, we have successfully prepared atomically dispersed Fe-N-C catalyst, which exhibited high activity and excellent reusability for the selective oxidation of the C-H bond. A wide scope of substrates, including aromatic, heterocyclic, and aliphatic alkanes, were smoothly oxidized at room temperature, and the selectivity of corresponding products reached as high as 99%. By using sub-ångström-resolution HAADF-STEM in combination with XPS, XAS, ESR, and Mössbauer spectroscopy, we have provided solid evidence that Fe is exclusively dispersed as single atoms via forming FeNx (x = 4-6) and that the relative concentration of each FeNx species is critically dependent on the pyrolysis temperature. Among them, the medium-spin FeIIIN5 affords the highest turnover frequency (6455 h-1), which is at least 1 order of magnitude more active than the high-spin and low-spin FeIIIN6 structures and 3 times more active than the FeIIN4 structure, although its relative concentration in the catalysts is much lower than that of the FeIIIN6 structures.
Metal-organic frameworks (MOFs) with high surface areas and uniform microporous structures have shown potential application in many fields. Here we report a facial strategy to synthesize ...Fe2O3@NiCo2O4 porous nanocages by annealing core-shell Co3Fe(CN)62@Ni3Co(CN)62 nanocubes in air. The obtained samples have been systematically characterized by XRD, SEM, TEM and N2 adsorption-desorption analysis. The results show that the Fe2O3@NiCo2O4 porous nanocages have an average diameter of 213 nm and a shell thickness of about 30 nm. As anode materials for Li-ion batteries, the Fe2O3@NiCo2O4 porous nanocages exhibit a high initial discharge capacity of 1311.4 mA h g(-1) at a current density of 100 mA g(-1) (about 0.1 C). The capacity is retained at 1079.6 mA h g(-1) after 100 cycles. The synergistic effect of the different components and the porous hollow structure contributes to the outstanding performance of the composite electrode.
The catalytic hydrogenation of nitroarenes is an environmentally benign technology for the production of anilines, which are key intermediates for manufacturing agrochemicals, pharmaceuticals and ...dyes. Most of the precious metal catalysts, however, suffer from low chemoselectivity when one or more reducible groups are present in a nitroarene molecule. Herein we report FeOx-supported platinum single-atom and pseudo-single-atom structures as highly active, chemoselective and reusable catalysts for hydrogenation of a variety of substituted nitroarenes. For hydrogenation of 3-nitrostyrene, the catalyst yields a TOF of ~1,500 h(-1), 20-fold higher than the best result reported in literature, and a selectivity to 3-aminostyrene close to 99%, the best ever achieved over platinum group metals. The superior performance can be attributed to the presence of positively charged platinum centres and the absence of Pt-Pt metallic bonding, both of which favour the preferential adsorption of nitro groups.
Co-N-C catalysts are promising candidates for substituting platinum in electrocatalysis and organic transformations. The heterogeneity of the Co species resulting from high-temperature pyrolysis, ...however, encumbers the structural identification of active sites. Herein, we report a self-supporting Co-N-C catalyst wherein cobalt is dispersed exclusively as single atoms. By using sub-Ångström-resolution HAADF-STEM in combination with XAFS and DFT calculation, the exact structure of the Co-N-C is identified to be CoN
C
-1-2O
, where the 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-N
plane. This single-atom dispersed Co-N-C catalyst presents excellent performance for the chemoselective hydrogenation of nitroarenes to produce azo compounds under mild reaction conditions.
Selective hydrogenation of acetylene to ethylene is an industrially important reaction. Pd-based catalysts have been proved to be efficient for the acetylene conversion, while enhancing the ...selectivity to ethylene is challenging. Here, we chose Cu as the partner of Pd, fabricated an alloyed Pd single-atom catalyst (SAC), and investigated its catalytic performance for the selective hydrogenation of acetylene to ethylene under a simulated front-end hydrogenation process in industry: that is, with a high concentration of hydrogen and ethylene. The Cu-alloyed Pd SAC showed ∼85% selectivity to ethylene and 100% acetylene elimination. In comparison with the Au- or Ag-alloyed Pd SAC, the Cu-alloyed analogue exceeded both of them in conversion, while the selectivity rivaled that of the Ag-alloyed Pd SAC and surpassed that of the Au-alloyed Pd SAC. As Cu is a low-cost metal, Cu-alloyed Pd SAC would minimize the noble-metal usage and possess high utilization potential for industry. The Cu-alloyed Pd SAC was verified by EXAFS, with the Pd/Cu atomic ratio lowered to 0.006, corresponding to the loading of Pd at 494 ppm. The microcalorimetric measurement results demonstrated that the adsorption of C2H4 over the Cu-alloyed Pd SAC was weaker than that over the catalyst with large Pd ensembles; thus, the selectivity to ethylene was greatly enhanced. At the same time, the adsorption of H2 was stronger than that over the corresponding monometallic Cu catalyst; thus, the activation of H2 was obviously promoted. On the basis of the above results, a possible reaction path over the Cu-alloyed Pd SAC was proposed. Furthermore, by systematic comparison of the IB-metal-alloyed Pd SACs, we found that the apparent activation energies of the IB-metal-alloyed Pd SACs were close to each other, indicating similar active sites and/or catalytic mechanisms over the three catalysts. The isolation of the Pd atoms by the IB metal distinctly contributed to both the conversion and the selectivity. Further DFT calculation results suggested that electron transfer between the IB metal and Pd might be responsible for their different selectivities to ethylene.
Abstract
Heterogeneous single-atom catalyst (SAC) opens a unique entry to establishing structure–performance relationship at the molecular level similar to that in homogeneous catalysis. The ...challenge lies in manipulating the coordination chemistry of single atoms without changing single-atom dispersion. Here, we develop an efficient synthetic method for SACs by using ethanediamine to chelate Pt cations and then removing the ethanediamine by a rapid thermal treatment (RTT) in inert atmosphere. The coordination chemistry of Pt single atoms on a Fe
2
O
3
support is finely tuned by merely adjusting the RTT temperature. With the decrease in Pt-O coordination number, the oxidation state of Pt decreases, and consequently the hydrogenation activity increases to a record level without loss of chemoselectivity. The tunability of the local coordination chemistry, oxidation states of the metal, and the catalytic performance of single atoms reveals the unique role of SACs as a bridge between heterogeneous and homogeneous catalysis.
Abstract
Single-atom catalysts (SACs) have emerged as a frontier in heterogeneous catalysis due to the well-defined active site structure and the maximized metal atom utilization. Nevertheless, the ...robustness of SACs remains a critical concern for practical applications. Herein, we report a highly active, selective and robust Ru SAC which was synthesized by pyrolysis of ruthenium acetylacetonate and N/C precursors at 900 °C in N
2
followed by treatment at 800 °C in NH
3
. The resultant Ru
1
-N
3
structure exhibits moderate capability for hydrogen activation even in excess NH
3
, which enables the effective modulation between transimination and hydrogenation activity in the reductive amination of aldehydes/ketones towards primary amines. As a consequence, it shows superior amine productivity, unrivalled resistance against CO and sulfur, and unexpectedly high stability under harsh hydrotreating conditions compared to most SACs and nanocatalysts. This SAC strategy will open an avenue towards the rational design of highly selective and robust catalysts for other demanding transformations.
Hydrothermally stable, acid‐resistant nickel catalysts are highly desired in hydrogenation reactions, but such a catalyst remains absent owing to the inherent vulnerability of nickel under acidic ...conditions. An ultra‐durable Ni‐N‐C single‐atom catalyst (SAC) has now been developed that possesses a remarkable Ni content (7.5 wt %) required for practical usage. This SAC shows not only high activities for hydrogenation of various unsaturated substrates but also unprecedented durability for the one‐pot conversion of cellulose under very harsh conditions (245 °C, 60 bar H2, presence of tungstic acid in hot water). Using integrated spectroscopy characterization and computational modeling, the active site structure is identified as (Ni‐N4)⋅⋅⋅N, where significantly distorted octahedral coordination and pyridinic N constitute a frustrated Lewis pair for the heterolytic dissociation of dihydrogen, and the robust covalent chemical bonding between Ni and N atoms accounts for its ultrastability.
Ni‐N‐C SAC: A high‐loading (7.5 wt %) Ni single‐atom catalyst is developed for hydrogenation reactions and one‐pot conversion of cellulose to ethylene glycol under harsh hydrothermal conditions. The catalyst has a well‐defined active site with a significantly distorted structure in which the isolated Ni cation and the adjacent non‐coordinated pyridinic N atom constitute a frustrated Lewis pair (FLP) for heterolytic dissociation of dihydrogen.
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