二维材料限域单原子催化剂研究进展

近年来,单一原子或单一位点催化剂因其独特的结构和电子特性受到催化研究人员的广泛关注.目前,多种无机固体材料被用作限域该类单原子催化剂,包括传统的金属氧化物、沸石分子筛以及金属有机框架配合物等.载体的性质会显著地影响单原子的催化性能,因此具有独特物理化学性质的二维材料无疑是限域单原子的一类理想介质,并逐渐引起了人们在该领域的研究兴趣.二维材料兴起于石墨烯的成功剥离,随后其他类似物如氮化硼、氮化碳以及二硫化钼等蓬勃发展起来.结构简单明确且性质独特的二维材料自身就是一类新颖的催化剂,其与单原子的结合将会为催化带来更多新的可能.二维材料限域单原子催化剂的潜在优势如下:(1)二维材料独特的电子结构对单原子中心的电子特性有显著的调变作用,使其催化性能更为独特;(2)二维材料通常具有巨大的比表面积,这允许其锚定更多的单原子从而显著提高其活性位密度;(3)单原子层二维材料有利于反应物分子从双向接触其表面限域的单原子位点,增加碰撞几率并降低传质阻力;(4)二维材料限域单原子催化剂可被视为理想的模型催化剂,其结构均一的活性中心有利于催化剂构效关系的研究;(5)二维材料限域的单原子能够反过来促进或激活二维材料的本征催化活性.在这里,我们总结了二维材料限域单原子催化剂的最新进展,其中二维材料主要涉及石墨烯、氮化碳和硫化钼.我们围绕在二维材料限域单原子催化剂中什么是真正的活性位点及其如何协同催化等问题进行了讨论,进而展望了二维材料限域单原子催化剂的应用前景和挑战.     

Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: from photochemistry to multipurpose catalysis to sustainable chemistry

Polymeric graphitic carbon nitride materials (for simplicity: g-C(3)N(4)) have attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor H content only. In contrast to graphenes, g-C(3)N(4) is a medium-bandgap semiconductor and in that role an effective photocatalyst and chemical catalyst for a broad variety of reactions. In this Review, we describe the "polymer chemistry" of this structure, how band positions and bandgap can be varied by doping and copolymerization, and how the organic solid can be textured to make it an effective heterogenous catalyst. g-C(3)N(4) and its modifications have a high thermal and chemical stability and can catalyze a number of "dream reactions", such as photochemical splitting of water, mild and selective oxidation reactions, and--as a coactive catalytic support--superactive hydrogenation reactions. As carbon nitride is metal-free as such, it also tolerates functional groups and is therefore suited for multipurpose applications in biomass conversion and sustainable chemistry.     

Plasmonic heterogeneous catalysis for organic transformations

Plasmonic catalysis has been recognised as a promising alternative to many conventional thermal catalytic processes in organic synthesis. In addition to their high activity in fine chemical synthesis, plasmonic photocatalysts are also able to maintain control of selectivity under mild conditions by utilising visible-light as an energy source. This review provides an overview of the recent advances in organic transformations with plasmonic metal nanostructures, including selective reduction, selective oxidation, cross-coupling and addition reactions. We also summarize the photocatalysts and catalytic mechanisms involving surface plasmon resonance. Finally, control of reaction pathway and strategies for tailoring product selectivity in fine chemical synthesis are discussed.     

Microfabricated reactors for on-chip heterogeneous catalysis

Microfabricated devices constructed from glass and polydimethylsiloxane with integral heaters are described, which can be used for heterogeneous catalysis reactions. Sulfated zirconia is used as the catalyst in an open channel reactor, with either a syringe pump or electroosmotic flow being used to deliver the reactants. The results clearly demonstrate that very high conversion efficiencies are possible, however, the thermodynamics of the reactions are the same as in bulk systems. Ethanol and hexanol are dehydrated to ethene and hexene, respectively, with conversion efficiencies approaching 100%, and the esterification of ethanol is investigated. Yields of approximately 30% ethyl acetate are obtained by gas chromatographic analysis. This is the first time such a method for fabricating a catalyst micro reactor has been reported, yet it demonstrates sufficient robustness and resistance to leakage. The use of electroosmotic flow in a heated catalyst reactor is a significant advancement in reactor design.     

Advanced synthesis for advancing heterogeneous catalysis

There are tremendous needs and opportunities for the understanding and application of heterogeneous catalysis in the solution of vexing technological problems. Nanoscale, catalytically active phases, particularly metal nanoparticles and metal oxide clusters, supported on high surface area oxides (supported catalysts) are one of the most important classes of heterogeneous catalysts. The problem of inhomogeneity and the limits it places on the understanding of catalytic chemistry has led to substantial efforts to produce more uniform catalyst systems via more synthetic control. This article highlights an approach adopted at Argonne National Laboratory for the synthesis of uniform supported metal and oxide particles.     

Anchoring Ni single atoms on sulfur-vacancy-enriched ZnIn2S4 nanosheets for boosting photocatalytic hydrogen evolution

Structure manipulation of photocatalysts at an atomic scale is a promising way to improve its photocatalytic performance.Herein,we realize the anchoring of sing...     

一步法合成g-C3N4纳米片用作苯酚可见光降解高效催化剂

石墨相氮化碳(g-C3N4)是一种在室温条件下最稳定的氮化碳.同时g-C3N4的带隙为2.7 eV,可以利用可见光催化很多反应,例如光解水、CO2还原、有机污染物降解和有机物合成.但普通体相g-C3N4的光催化性能不尽如人意,主要是由于普通体相材料的载流子复合效率高,可见光(<450 nm)利用率低且比表面积小.众所周知,半导体的光催化性能与材料表面状态密切相关,因此可以控制合成条件来制备有利于光催化形貌的g-C3N4材料.普通体相g-C3N4材料的比表面积较小,约为10 m2/g,导致传质作用较差,光生电子-空穴复合严重,因此制备高比表面积的g-C3N4材料是目前研究的热点.我们发现在550℃下将三聚氰胺和三聚氰酸一起煅烧可以一步热合成g-C3N4纳米片,合成温度较低,对材料带隙影响小,同时可以提高材料比表面积,从而极大地提高了材料的光降解苯酚性能.XRD测试发现,随着前驱体中三聚氰酸比例增加,材料的主峰从27.38°显著偏移到27.72°.这表明三嗪环面内相连构成CN平面,同时CN层也会有堆叠最终形成g-C3N4材料.通过BET测试,g-C3N4纳米片的比表面积为103.24 m2/g.采用AFM分析得到g-C3N4纳米片的厚度为3.07 nm.研究了该g-C3N4纳米片的光降解性能,结果显示,在可见光照射30 min后,使用这种g-C3N4纳米片作为催化剂的条件下,苯酚降解率达到最优的81%.在5次循环利用后,g-C3N4(1:9)的降解率还能保持在80%以上,说明材料有良好的循环稳定性.这主要得益于材料的纳米片结构,在对苯酚吸附时不会有很复杂的吸附与脱附过程.同时纳米片结构可为有机污染物的吸附和原位降解提供传质通道.光反应体系中的产物由HPLC检测,分析苯酚的降解产物及产物的产量可以大致推测苯酚可能的降解历程.在三聚氰酸作用下,CN聚合层弯曲,减少了CN层之间的相互结合,同时不会对材料的带隙产生影响.同时整个合成过程无需引发剂,也不会导致CN层的基本单元和连接方式发生改变,同时由于二维片层结构,提高了材料的电荷分离效率.通过苯酚的降解实验得知三聚氰胺与三聚氰酸的比例为1:9,在550℃下煅烧得到的g-C3N4纳米片的光降解性能最优,同时具有很好的催化稳定性.     

Methanol dissociation and oxidation on single Fe atom supported on graphitic carbon nitride

Considering environmental protection and sustainable development, fuel cells have always been an ideal choice for clean energy. For the performance of fuel cells, the anode catalysts are a key consideration. In the work reported, we designed a new type of anode catalyst, in which graphitic carbon nitride was used as the substrate and transition metal iron as the supported atom. The calculation results were characterized by adsorption energy, reaction energy barrier, potential energy surface and charge analysis. Based on density functional theory, the gradual decomposition of methanol molecules on the catalyst is realized; the decomposition products are oxidized to reduce the formation of CO, and the efficiency of anode catalysis is improved, which provides a new idea for the design of anode materials for methanol fuel cells.     

Homochiral metal-organic frameworks for heterogeneous asymmetric catalysis

Homochiral crystallizations of two enantiomeric metal-organic frameworks (MOFs) Ce-MDIP1 and Ce-MDIP2 were achieved by using L- or D-BCIP as chiral inductions, respectively, where the chiralities were characterized by solid state CD spectra. Ce-MDIPs exhibit excellent catalytic activity and high enantioselectivity for the asymmetric cyanosilylation of aromatic aldehydes; the homochiral Cd-TBT MOF having L-PYI as a chiral adduct exhibits stereochemical catalysis toward the Aldol reactions.     

Functional porous organic polymers for heterogeneous catalysis

Porous organic polymers (POPs), a class of highly crosslinked amorphous polymers possessing nano-pores, have recently emerged as a versatile platform for the deployment of catalysts. The bottom-up approach for porous organic polymer synthesis provides the opportunity for the design of polymer frameworks with various functionalities, for their use as catalysts or ligands. This tutorial review focuses on the framework structures and functionalities of catalytic POPs. Their structural design, functional framework synthesis and catalytic reactions are discussed along with some of the challenges.     

石墨相氮化碳负载钒作为多相催化剂用于苯直接氧化制苯酚

苯酚是一种重要的化工原料,目前苯酚的工业生产路线普遍存在工艺流程复杂、苯酚收率低和环境污染严重等问题.为实现苯酚的绿色生产,苯直接氧化制苯酚的合成路线受到各国研究者的广泛关注.在苯直接羟基化反应常用的 N2O, O2和 H2O2三类氧化剂中, N2O由于来源有限,其工业应用受到极大限制;而 O2不易活化,且反应过程中常需还原剂存在,苯酚收率低;相比之下, H2O2作为氧化剂,其唯一副产物是 H2O,而且反应条件温和,因而以 H2O2为氧化剂的苯羟基化反应是最具工业应用前景的苯酚合成路线.然而,由于苯分子中的 C?H键非常稳定,活化能较高,同时产物苯酚的反应活性要高于反应物苯,因此,为实现苯的高效转化,积极探索研究高活性和稳定性的催化剂变得尤为重要.在我们之前的研究中发现,包含大π体系的氧化石墨烯载体有利于具有同样π共轭体系的反应物苯的吸附,进而促进苯的转化,提高反应活性.而石墨相氮化碳(g-C3N4)具有与氧化石墨烯类似的π共轭体系,且表面具有大量的活性位点和缺陷位,对苯环类物质具有较好的活化作用,这使其可能成为更优异的载体材料.基于此,以 g-C3N4为载体,采用浸渍法制备了一系列不同钒含量的催化剂xV/g-C3N4,并通过 XRD, FT-IR, TEM, TG等表征技术对催化剂进行了系统研究,以期揭示催化剂结构与反应活性之间的构效关系. XRD的表征结果表明,xV/g-C3N4仍具有载体 g-C3N4的层状堆积结构,且该结构不受钒负载量变化的影响.同时, xV/g-C3N4中钒物种的分散度较高,未发生团聚晶化.更直观地,通过 TEM观察发现,xV/g-C3N4中的钒物种均匀分散. FT-IR的表征结果说明钒物种与 g-C3N4之间存在较强的相互作用.此外,通过 TG表征发现, g-C3N4高温稳定性较好,即使焙烧温度高达550°C,其结构仍不受影响.综上所述,在xV/g-C3N4催化剂中,载体 g-C3N4的结构非常稳定,经负载钒物种以及焙烧处理后仍能保持不变;而钒物种与 g-C3N4之间存在较强的相互作用,且均匀分散,使催化剂具有较高的稳定性和较好的催化性能.在以 H2O2为氧化剂,80 wt%醋酸溶液为溶剂的苯直接氧化制苯酚反应中,xV/g-C3N4催化剂显示了良好的催化活性,其中反应活性最高的是8V/g-C3N4催化剂,在最佳反应条件下,苯酚的收率和选择性分别达到24.4%和99.2%.同时,通过计算 TOF值发现,8V/g-C3N4的 TOF值高达13.1 h-1,远高于文献中报道的以 C3N4为载体的催化剂的 TOF值(0.52–0.59 h-1),这表明xV/g-C3N4催化剂具有优异的催化活性.此外,以8V/g-C3N4为代表又进一步考察了催化剂的稳定性,在回收重复实验中催化剂的活性保持稳定.     

Active-site coating for molecular discrimination in heterogeneous catalysis

A coherent layer of zeolite A has been applied to a SiO2-supported Pt-Fe oxidation catalyst, using total surface charge-reversal. The zeolite has tracked the metals into the pore structure of the support, covering the active sites. The zeolite channel size is large enough to allow access of CO and O2 to the active sites, and to allow CO2 to emerge, but it excludes larger molecules. The presence of the zeolite membrane transforms the supported Pt-Fe into a highly specific catalyst, which can discriminate between CO and butane, even after the macroscopic catalyst particles are crushed.     

Pseudopotentials for hybridized carbon atoms

Pseudopotentials for hybridized atoms are extracted and combined. The study focuses on sp² carbon atoms and their combination to give rise to a π electronic system. As electrons of interest are treated explicitly, any ab initio method can be used, thus, configuration interaction methods and time‐dependent density functional theory are used and compared. All electron and pseudopotential calculations are in good agreement for electronic transition spectroscopy (0.2 eV difference), geometrical parameters (error of 0.8 pm), and reaction energies. © 2012 Wiley Periodicals, Inc.     

Hydroxyl improving the activity,selectivity and stability of supported Ni single atoms for selective semi-hydrogenation

Atomically dispersed metal catalysts with high atomic utilization and selectivity have been widely studied for acetylene semi-hydrogenation in excess ethylene among others. Further improvements of activity and selectivity, in addition to stability and loading, remain elusive due to competitive adsorption and desorption between reactants and products, hydrogen activation, partial hydrogenation etc. on limited site available. Herein, comprehensive density functional theory calculations have been used to explore the new strategy by introducing an appropriate ligand to stabilize the active single atom, improving the activity and selectivity on oxide supports. We find that the hydroxyl group can stabilize Ni single atoms significantly by forming Ni₁(OH)₂ complexes on anatase TiO₂(101), whose unique electronic and geometric properties enable high performance in acetylene semi-hydrogenation. Specifically, Ni₁(OH)₂/TiO₂(101) shows favorable acetylene adsorption and promotes the heterolytic dissociation of H₂ achieving high catalytic activity, and it simultaneously weakens the ethylene bonding to facilitate subsequent desorption showing high ethylene selectivity. Hydroxyl stabilization of single metal atoms on oxide supports and promotion of the catalytic activity are sensitive to transition metal and the oxide supports. Compared to Co, Rh, Ir, Pd, Pt, Cu, Ag and Au, and anatase ZrO₂, IrO₂ and NbO₂ surfaces, the optimum interactions between Ni, O and Ti and resulted high activity, selectivity and stability make Ni₁(OH)₂/TiO₂(101) a promising catalyst in acetylene hydrogenation. Our work provides valuable guidelines for utilization of ligands in the rational design of stable and efficient atomically dispersed catalysts.

Hydroxyl group can stabilize significantly Ni single atom by forming Ni₁(OH)₂ complexes on anatase TiO₂(101), which displays high catalytic performance in acetylene semi-hydrogenation.     

Gas phase metal cluster model systems for heterogeneous catalysis

Since the advent of intense cluster sources, physical and chemical properties of isolated metal clusters are an active field of research. In particular, gas phase metal clusters represent ideal model systems to gain molecular level insight into the energetics and kinetics of metal-mediated catalytic reactions. Here we summarize experimental reactivity studies as well as investigations of thermal catalytic reaction cycles on small gas phase metal clusters, mostly in relation to the surprising catalytic activity of nanoscale gold particles. A particular emphasis is put on the importance of conceptual insights gained through the study of gas phase model systems. Based on these concepts future perspectives are formulated in terms of variation and optimization of catalytic materials e.g. by utilization of bimetals and metal oxides. Furthermore, the future potential of bio-inspired catalytic material systems are highlighted and technical developments are discussed.     

Indications for a surface temperature excess in heterogeneous catalysis

We explored the surface temperature excess DeltaT = T(r) - T(m) (real reaction temperature T(r), measured catalyst temperature T(m)) on the basis of experimental data, a gradually curved Arrhenius plot for CO oxidation reactions over Pd/gamma-Al(2)O(3) catalysts. Such a plot could be an indication of the surface temperature excess in the 2-dimensional reaction surfaces of catalysts. The positive or negative surface temperature excess could be developed to be a general explanation for a gradually curved Arrhenius plot of a gas-solid catalytic system. This is a new insight into solving the puzzle on such common phenomena in heterogeneous catalysis. By using the reciprocal of the real reaction temperature T(r) in the hypothetical 2-D reaction surface, instead of the experimentally determined catalyst temperature T(m) or the gas temperature T(g), the gradually curved Arrhenius plot becomes linear. We investigated the implications of such a difference among T(r), T(m), and T(g). The surface temperature excess could be the effect of coupling between the fluxes of a chemical reaction and heat transport in the 2-D reaction surface. Its order of magnitude is 10 K for the present model system.(1) The surface temperature excess increases exponentially with the reaction temperature.