多相催化剂常以过渡金属为活性中心,当催化活性位点呈原子级分散时,金属利用率达到最高,故具有优异的催化性能.然而,铂族金属和惰性氧化物的结合较弱,所以在Al2O3 和SiO2 之类的载体上,铂很难被分散成单原子的形式.使用简单的溶胶-凝胶法,在惰性氧化物中掺杂铁元素可显著增强铂与载体之间的共价相互作用,从而制得单原子铂催化剂.在苯乙炔选择性加氢生成苯乙烯的反应中,0.2 wt%PtFe50/Al2O3-SiO2 单原子催化剂具有良好的选择性,其反应速率是 0.2 wt%Pt/Al2O3-SiO2 纳米颗粒催化剂的 1.4 倍.该单原子催化剂还在循环测试中表现出高稳定性,证明了载体的掺杂改性是一种将活性金属分散在惰性氧化物上,得到高活性和高稳定性的单原子催化剂的有效策略.

The functionalization of alkenes is one of the most important conversions in synthetic chemistry to prepare numerous fine chemicals. Typical procedures, such as hydrosilylation and hydroformylation, are traditionally catalyzed using homogeneous noble metal complexes, while the highly reactive and stable heterogeneous single-atom catalysts (SACs) now provide alternative approaches to fulfill these conversions by combining the advantages of both homogeneous catalysts and heterogeneous nanoparticle catalysts. In this review, the recent achievement in single-atom catalyzed hydrosilylation and hydroformylation reactions are introduced, and we highlight the latest applications of SACs for additive reactions, constructing new C-Y (Y = B, P, S, N) bonds on the terminal carbon atoms of alkenes, and then mention the applications in single-metal-atom catalyzed hydrogenation and epoxidation reactions. We also note that some tandem reactions are conveniently realized in one pot by the concisely fabricated SACs, facilitating the preparation of some pharmaceutical compounds. Lastly, the challenges facing single-atom catalysis for alkene conversions are briefly mentioned.

The catalytic approaches fulfilled by the singly dispersed metal atoms anchoring on the solid materials were categorized as single‐atom catalysis. In this perspective, after briefly introducing the history in this research field, we compare the differences between single‐atom catalyst and several terms indicating some heterogeneous highly dispersed metal catalysts. Single‐atom catalysts could provide the simplified models in exploring the catalytically active structures, and a new era of designing catalysts at the atomic scale is thus opened. In this context, single‐atom catalysis may be regarded as a bridge to combine the homogeneous and heterogeneous catalysis. Moreover, the materials containing isolated metal centers also provide great opportunities in many other applications, such as medicinal treatment, selective separation of gas molecule, and quantum devices. At last, challenges to the single‐atom catalysis are discussed.

Alcohol-based disinfectants have protected people in the coronavirus disease 2019 (COVID-19) pandemic, but olfactory stimuli of ethanol may evoke unpleasant memories associated with stressful situations in the devastating infectious disease. The smell of ethanol in household cleaning and disinfectant products can be covered up by the fragrance additives, and 3-hexenol is especially appreciated for the characteristic, strong odor of green plants. Industrial production of 3-hexenol relies on the selective hydrogenation of 3-hexyn-1-ol, where Lindlar catalyst is normally used for the superior selectivity. Although achieving such catalytic transformation in ethanol solution seems as a direct way to produce a disinfectant with green aroma, a popular consumer product in the post-COVID era, severe leaching of toxic Pb hinders Lindlar catalyst as a promising candidate. We find that the Fe 2 O 3 supported Pd single-atom catalyst is highly selective to fulfill semi-hydrogenation of 3-hexyn-1-ol in 75% ethanol, and the aforementioned household product is readily generated after filtrating the stable, solid catalyst out of reaction solution. Single-atom catalysts have been frequently utilized for fine-chemical synthesis, while in this work they make stunning debut in practical manufacture of daily used products.

单原子催化剂中所有金属原子都能作为活性位点,故表现出极高的原子利用率和优异的催化性能.以铜-乙二胺四乙酸配合物作为前驱体,高比表面的活性炭作为载体,通过简单的吸附浸渍-高温焙烧法制备了一系列Cu-N/C催化剂.结果发现,当铜负载量为0.2 wt％时,铜以单原子的状态分散在载体上,得到0.2 wt％Cu-N/C(1:1)单原子催化剂.该催化剂在芳基硼酸羟基化制苯酚的反应中具有优异的活性,可以直接用氧气作为氧化剂,反应效率达到3250 mmol产物/gCu,高于铜纳米颗粒催化剂6倍.此外,该单原子催化剂稳定性较好,并且该反应对各种带有吸电子或供电子基团的芳基硼酸都具有良好的底物适用性.

Single-atom catalysis has become a new branch in heterogeneous catalysis. Although the naturally produced SiO2 -based materials are abundant and stable, fabrication of single-atom catalysts on such supports with high loading remains as a formidable challenge due to the lack of bonding sites to anchor the isolated metal species. Herein, modifying the diatomite, a kind of pure SiO2 mineral, with CeO2 nanoparticles is demonstrated to increase the defect sites on the support. The enhanced metal-support interaction maintains the atomic dispersion of Pt species with above 1 wt.% loading, exhibiting good performance in the selective hydrogenation of phenylacetylene to styrene.

Metal atoms dispersed on the oxide supports constitute a large category of single-atom catalysts. In this review, oxide supported single-atom catalysts are discussed about their synthetic procedures, characterizations, and reaction mechanism in thermocatalysis, such as water-gas shift reaction, selective oxidation/hydrogenation, and coupling reactions. Some typical oxide materials, including ferric oxide, cerium oxide, titanium dioxide, aluminum oxide, and so on, are intentionally mentioned for the unique roles as supports in anchoring metal atoms and taking part in the catalytic reactions. The interactions between metal atoms and oxide supports are summarized to give a picture on how to stabilize the atomic metal centers, and rationally tune the geometric structures and electronic states of single atoms. Furthermore, several directions in fabricating single-atom catalysts with improved performance are proposed on the basis of state-of-the-art understanding in metal-oxide interactions.

The hydroformylation of olefins is one of the most important homogeneously catalyzed industrial reactions for aldehyde synthesis. Various ligands can be used to obtain the desired linear aldehydes in the hydroformylation of aliphatic olefins. However, in the hydroformylation of aromatic substrates, branched aldehydes are formed preferentially with common ligands. In this study, a novel approach to selectively obtain linear aldehydes in the hydroformylation of styrene and its derivatives was developed by coupling with a water-gas shift reaction on a Rh single-atom catalyst without the use of ligands. Detailed studies revealed that the hydrogen generated in situ from the water-gas shift is critical for the highly regioselective formation of linear products. The coupling of a traditional homogeneous catalytic process with a heterogeneous catalytic reaction to tune product selectivity may provide a new avenue for the heterogenization of homogenous catalytic processes.

The catalytic activities of titanium alkoxides and alkali metal alkoxides for hydrosilylation of unfunctionalized olefins have been studied. Titanium(IV) alkoxides showed excellent catalytic activity, while alkali metal alkoxides have low catalytic activity for the hydrosilylation of olefins. However, by using titanocene dichloride as an additive, alkali metal alkoxides showed also excellent catalytic property for hydrosilylation. In comparison with titanium alkoxides, no α-adduct was obtained by using alkali metal alkoxides/Cp2TiCl2 as catalysts.

Single-atom catalysis has rapidly progressed during the last few years. In 2017, single-atom catalysts (SACs) were fabricated with higher metal loadings and designed into more delicate structures. SACs also found wide applications in C1 chemical conversion, such as selective oxidation of methane and conversion of carbon dioxide. Both experimental characterizations and computational modeling revealed the presence of tunable interactions between single atom species and their surrounding chemical environment, and thus SACs may be more effective and more stable than their nanoparticle counterparts. In this mini-review, we summarize the major achievements of SACs into three main aspects: a) the advanced synthetic methodologies, b) catalytic performance in C1 chemistry, and c) strong metal-support interaction induced unexpected durability. These accomplishments will shed new light on the recognition of single-atom catalysis and encourage more efforts to explore potential applications of SACs.

Surface-supported isolated atoms in single-atom catalysts (SACs) are usually stabilized by diverse defects. The fabrication of high-metal-loading and thermally stable SACs remains a formidable challenge due to the difficulty of creating high densities of underpinning stable defects. Here we report that isolated Pt atoms can be stabilized through a strong covalent metal-support interaction (CMSI) that is not associated with support defects, yielding a high-loading and thermally stable SAC by trapping either the already deposited Pt atoms or the PtO 2 units vaporized from nanoparticles during high-temperature calcination. Experimental and computational modeling studies reveal that iron oxide reducibility is crucial to anchor isolated Pt atoms. The resulting high concentrations of single atoms enable specific activities far exceeding those of conventional nanoparticle catalysts. This non defect-stabilization strategy can be extended to non-reducible supports by simply doping with iron oxide, thus paving a new way for constructing high-loading SACs for diverse industrially important catalytic reactions.

Hydrosilylation of olefins catalyzed by Cp2TiCl2/Sm (Cp=cyclopentadienyl) under solvent free conditions have been investigated. By using Cp2TiCl2/Sm as catalyst system, -adducts and hydrogenation products were detected. Hydrosilylation of olefins catalyzed by Cp2TiCl2/LiAlH4 under room temperature has also been studied. The influence of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) on Cp2TiCl2/Sm and Cp2TiCl2/LiAlH4, respectively, indicated that hydrosilylation of olefins catalyzed with Cp2TiCl2/Sm went through a free radical reaction pathway while a coordination mechanism was applied for Cp2TiCl2/LiAlH4 catalyst system.

CO oxidation is of great importance in both fundamental study and industrial application. Supported noble metal catalysts are highly active for CO oxidation but suffer from the scarcity and high cost. Single-atom catalysts (SACs) can maximize the metal atom efficiency. Herein, ZnO nanowire (ZnO-nw) supported Rh, Au, and Pt SACs were successfully developed to investigate their CO oxidation performance. Interestingly, it was found that Rh-1/ZnO-nw showed much higher activity than the other noble metals which are usually regarded as good candidates for CO oxidation. In addition, the Rh SAC possessed high stability in high-temperature CO oxidation under simulated conditions in the presence of water and hydrocarbons. The high activity and stability make Rh-1/ZnO-nw promising for practical applications, especially in the automotive exhaust emission control. Theoretical calculations indicate that the CO oxidation proceeds via the Mars-van Krevelen mechanism and the lowest barrier for the rate-limiting O-2 dissociation at a surface oxygen vacancy site is a key factor in determining the observed highest activity of Rh-1/ZnO-nw amongst the studied SACs. (C) 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

The synthesis of bio-based linear diols is the subject of many research studies. However, one of the main obstacles in industrial development is the difficulty in controlling product selectivity. Here, we report the catalytic conversion of furfural to 1,4-pentanediol (PD) in the presence of Ru supported on an ordered mesoporous carbon (CMK-3) under pressure of H-2 and CO2 in water. In contrast to previous catalytic pathways, this work is distinct in that it yields 1,4-PD as an exclusive product, instead of a mixture of 1,2-and 1,5-PD as usual. Under optimized conditions, 1,4-PD was obtained in 90% yield, and in a one-pot reaction, directly from furfural. We disclose that the conversion of furfural to 1,4-PD followed an unusual catalytic route. It implies a bifunctional catalytic pathway based on sequential catalytic hydrogenation reactions and an acid-catalyzed Piancatelli's rearrangement.

Single-atom catalysis, the catalysis by single-atom catalysts (SACs), has attracted considerable attention in recent years as a new frontier in the heterogeneous catalysis field. SACs have the advantages of both homogeneous catalysts (isolated active sites) and heterogeneous catalysts (stable and easy to separate), and are thus predicted to be able to bridge the homo- and heterogeneous catalysis. This prediction was first experimentally demonstrated in 2016. In this mini-review, we summarize the few homogeneous catalysis progresses reported recently where SACs have exhibited promising application: a) Rh/ZnO and Rh/CoO SAC have been used successfully in hydroformylation of olefin of which the activity are comparable to the homogeneous Wilkinson's catalyst; b) a Pt/Al2O3 SAC has shown excellent performance in hydrosilylation reaction; and c) M-N-C SACs (M = Fe, Co etc.) have been applied in the activation of C–H bonds. All of these examples suggest that fabrication of suitable SACs could provide a new avenue for the heterogenization of homogeneous catalysts. These pioneering works shed new light on the recognition of single-atom catalysis in bridging the homo- and heterogeneous catalysis.

The idea that single metal atoms dispersed on a solid support can act as an efficient heterogeneous catalyst was raised in 2011 when single Pt atoms on an FeOx surface were reported to be active for CO oxidation and preferential oxidation of CO in H2. The last six years have witnessed tremendous progress in the field of single-atom catalysis. Here we introduce the major achievements on this topic in 2015 and 2016. Some particular aspects of single-atom catalysis are discussed in depth, including new approaches in single-atom catalyst (SAC) synthesis, stable gold SACs for various reac-tions, the high selectivity of Pt and Pd SACs in hydrogenation, and the superior performance of non-noble metal SACs in electrochemistry. These accomplishments will encourage more efforts by researchers to achieve the controllable fabrication of SACs and explore their potential applications.

Irgendwo, hinter dem Regenbogen …… schlagen Katalysatoren, in denen einzelne Atome auf einer Trägeroberfläche vorliegen, eine Brücke zwischen homogener und heterogener Katalyse. In ihrer Zuschrift auf S. 16288 ff. nutzen B. Qiao, T. Zhang und Mitarbeiter einzelne Rhodiumatome auf ZnO-Nanodrähten für die Olefin-Hydroformylierung. Bezüglich ihrer Effizienz sind diese Katalysatoren vergleichbar mit dem Wilkinson-Homogenkatalysator.

Homogeneous catalysts generally possess superior catalytic performance compared to heterogeneous catalysts. However, the issue of catalyst separation and recycling severely limits their use in practical applications. Single-atom catalysts have the advantages of both homogeneous catalysts, such as "isolated sites", and heterogeneous catalysts, such as stability and reusability, and thus would be a promising alternative to traditional homogeneous catalysts. In the hydroformylation of olefins, single-atom Rh catalysts supported on ZnO nanowires demonstrate similar efficiency (TON≈40000) compared to that of homogeneous Wilkinson's catalyst (TON≈19000). HAADF-STEM and infrared CO chemisorption experiments identified isolated Rh atoms on the support. XPS and XANES spectra indicate that the electronic state of Rh is almost metallic. The catalysts are about one or two orders of magnitude more active than most reported heterogeneous catalysts and can be reused four times without an obvious decline in activity.

Somewhere over the rainbow single-atom catalysts dispersed on a support surface offer a tantalizing bridge between homogeneous and heterogeneous catalysts. In their Communication on page 16054 ff., B. Qiao, T. Zhang, and co-workers describe rhodium single-atom catalysts supported on ZnO nanowires that show an olefin hydroformylation efficiency comparable to that of homogeneous Wilkinson's catalyst.