In Situ Confined Growth Based on a Self‐Templating Reduction Strategy of Highly Dispersed Ni Nanoparticles in Hierarchical Yolk–Shell Fe@SiO2 Structures as Efficient Catalysts

Ni‐based magnetic catalysts exhibit moderate activity, low cost, and magnetic reusability in hydrogenation reactions. However, Ni nanoparticles anchored on magnetic supports commonly suffer from undesirable agglomeration during catalytic reactions due to the relatively weak affinity of the magnetic support for the Ni nanoparticles. A hierarchical yolk–shell Fe@SiO₂/Ni catalyst, with an inner movable Fe core and an ultrathin SiO₂/Ni shell composed of nanosheets, was synthesized in a self‐templating reduction strategy with a hierarchical yolk–shell Fe₃O₄@nickel silicate nanocomposite as the precursor. The spatial confinement of highly dispersed Ni nanoparticles with a mean size of 4 nm within ultrathin SiO₂ nanosheets with a thickness of 2.6 nm not only prevented their agglomeration during catalytic transformations but also exposed the abundant active Ni sites to reactants. Moreover, the large inner cavities and interlayer spaces between the assembled ultrathin SiO₂/Ni nanosheets provided suitable mesoporous channels for diffusion of the reactants towards the active sites. As expected, the Fe@SiO₂/Ni catalyst displayed high activity, high stability, and magnetic recoverability for the reduction of nitroaromatic compounds. In particular, the Ni‐based catalyst in the conversion of 4‐nitroamine maintained a rate of over 98 % and preserved the initial yolk–shell structure without any obvious aggregation of Ni nanoparticles after ten catalytic cycles, which confirmed the high structural stability of the Ni‐based catalyst.     

Identification of Catalytic Sites for Oxygen Reduction in Metal/Nitrogen‐Doped Carbons with Encapsulated Metal Nanoparticles

The development of metal‐N‐C materials as efficient non‐precious metal (NPM) catalysts for catalysing the oxygen reduction reaction (ORR) as alternatives to platinum is important for the practical use of proton exchange membrane fuel cells (PEMFCs). However, metal‐N‐C materials have high structural heterogeneity. As a result of their high‐temperature synthesis they often consist of metal‐N_x sites and graphene‐encapsulated metal nanoparticles. Thus it is hard to identify the active structure of metal‐N‐C catalysts. Herein, we report a low‐temperature NH₄Cl‐treatment to etch out graphene‐encapsulated nanoparticles from metal‐N‐C catalysts without destruction of co‐existing atomically dispersed metal‐N_x sites. Catalytic activity is much enhanced by this selective removal of metallic nanoparticles. Accordingly, we can confirm the spectator role of graphene‐encapsulated nanoparticles and the pivotal role of metal‐N_x sites in the metal‐N‐C materials for ORR in the acidic medium.     

Synthesis of Supported Ultrafine Non‐noble Subnanometer‐Scale Metal Particles Derived from Metal–Organic Frameworks as Highly Efficient Heterogeneous Catalysts

The properties of supported non‐noble metal particles with a size of less than 1 nm are unknown because their synthesis is a challenge. A strategy has now been created to immobilize ultrafine non‐noble metal particles on supports using metal–organic frameworks (MOFs) as metal precursors. Ni/SiO₂ and Co/SiO₂ catalysts were synthesized with an average metal particle size of 0.9 nm. The metal nanoparticles were immobilized uniformly on the support with a metal loading of about 20 wt %. Interestingly, the ultrafine non‐noble metal particles exhibited very high activity for liquid‐phase hydrogenation of benzene to cyclohexane even at 80 °C, while Ni/SiO₂ with larger Ni particles fabricated by a conventional method was not active under the same conditions.     

Metal–Support Cooperative Catalysts for Environmentally Benign Molecular Transformations

Metal–support cooperative catalysts have been developed for sustainable and environmentally benign molecular transformations. The active metal centers and supports in these catalysts could cooperatively activate substrates, resulting in high catalytic performance for liquid‐phase reactions under mild conditions. These catalysts involved hydrotalcite‐supported gold and silver nanoparticles with high catalytic activity for organic reactions such as aerobic oxidation, oxidative carbonylation, and chemoselective reduction of epoxides to alkenes and nitrostyrenes to aminostyrenes using alcohols and CO/H₂O as reducing reagents. This high catalytic performance was due to cooperative catalysis between the metal nanoparticles and basic sites of the hydrotalcite support. To increase the metal–support cooperative effect, core–shell nanostructured catalysts consisting of gold or silver nanoparticles in the core and ceria supports in the shell were designed. These core–shell nanocomposite catalysts were effective for the chemoselective hydrogenation of nitrostyrenes to aminostyrenes, unsaturated aldehydes to allyl alcohols, and alkynes to alkenes using H₂ as a clean reductant. In addition, these solid catalysts could be recovered easily from the reaction mixture by simple filtration, and were reusable with high catalytic activity.     

Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphitic Layers as Oxygen Reduction Catalysts

Nonprecious metal catalysts for the oxygen reduction reaction are the ultimate materials and the foremost subject for low‐temperature fuel cells. A novel type of catalysts prepared by high‐pressure pyrolysis is reported. The catalyst is featured by hollow spherical morphologies consisting of uniform iron carbide (Fe₃C) nanoparticles encased by graphitic layers, with little surface nitrogen or metallic functionalities. In acidic media the outer graphitic layers stabilize the carbide nanoparticles without depriving them of their catalytic activity towards the oxygen reduction reaction (ORR). As a result the catalyst is highly active and stable in both acid and alkaline electrolytes. The synthetic approach, the carbide‐based catalyst, the structure of the catalysts, and the proposed mechanism open new avenues for the development of ORR catalysts.     

Structure‐activity relationship of supported Au catalysts with high catalytic activity by modifying the inactive supports

Three supported Au catalysts have been prepared by the deposition‐precipitation method by using the active carbon (AC), SiO₂‐AC, and SiO₂‐AC‐hollowed. The 3 supports were characterized by Brunauer‐Emmett‐Teller and scanning electron microscopy. Meanwhile, the supported Au nanoparticles were also characterized in detail by X‐ray powder diffraction, transmission electron microscopy, H₂‐TRP, and X‐ray photoelectron spectroscopy, and their catalytic activity and stability in CO oxidation was evaluated. The results demonstrated that Au supported on SiO₂‐AC‐hollowed exhibited much higher catalytic activity with acceptable stability for 72 hours than the other 2. We attributed to finer supported Au nanoparticles with abundant low‐coordinated Au atoms on the surfaces of hollowed supports with large special surface area and abundant pore structure. In summary, we successfully found an efficient and cheap method to prepare catalysts with high catalytic activity and acceptable stability by modifying the inactive supports.     

An improved preparation of mesoporous silica‐supported Pd as sustainable catalysts for phosphine‐free Suzuki–Miyaura and Heck coupling reactions

An improved and eco‐friendly procedure has been developed to generate mesoporous silica‐supported palladium nanoparticles (SiO₂@PdNP) that could be used as a sustainable heterogeneous Pd catalyst for phosphine‐free Suzuki–Miyaura and Heck coupling reactions with excellent turnover number and turnover frequency. The presence of Pd on the silica surface was detected by X‐ray diffraction and the structural morphology of SiO₂@PdNP was obtained by transmission electron microscopy. The heterogeneous catalytic system is recyclable and leaching of the metal after the reaction is not apparently observed. Copyright © 2013 John Wiley & Sons, Ltd.     

Ionic Liquids as Precursors for Efficient Mesoporous Iron‐Nitrogen‐Doped Oxygen Reduction Electrocatalysts

A ferrocene‐based ionic liquid (Fe‐IL) is used as a metal‐containing feedstock with a nitrogen‐enriched ionic liquid (N‐IL) as a compatible nitrogen content modulator to prepare a novel type of non‐precious‐metal–nitrogen–carbon (M‐N‐C) catalysts, which feature ordered mesoporous structure consisting of uniform iron oxide nanoparticles embedded into N‐enriched carbons. The catalyst Fe¹⁰@NOMC exhibits comparable catalytic activity but superior long‐term stability to 20 wt % Pt/C for ORR with four‐electron transfer pathway under alkaline conditions. Such outstanding catalytic performance is ascribed to the populated Fe (Fe₃O₄) and N (N2) active sites with synergetic chemical coupling as well as the ordered mesoporous structure and high surface area endowed by both the versatile precursors and the synthetic strategy, which also open new avenues for the development of M‐N‐C catalytic materials.     

Multiwalled carbon nanotubes drive the activity of metal@oxide core-shell catalysts in modular nanocomposites

Rational nanostructure manipulation has been used to prepare nanocomposites in which multiwalled carbon nanotubes (MWCNTs) were embedded inside mesoporous layers of oxides (TiO(2), ZrO(2), or CeO(2)), which in turn contained dispersed metal nanoparticles (Pd or Pt). We show that the MWCNTs induce the crystallization of the oxide layer at room temperature and that the mesoporous oxide shell allows the particles to be accessible for catalytic reactions. In contrast to samples prepared in the absence of MWCNTs, both the activity and the stability of core-shell catalysts is largely enhanced, resulting in nanocomposites with remarkable performance for the water-gas-shift reaction, photocatalytic reforming of methanol, and Suzuki coupling. The modular approach shown here demonstrates that high-performance catalytic materials can be obtained through the precise organization of nanoscale building blocks.     

Investigation of CO and formaldehyde oxidation over mesoporous Ag/Co3O4 catalysts

CO and formaldehyde (HCHO) oxidation reactions were investigated over mesoporous Ag/Co₃O₄ catalysts prepared by one-pot (OP) and impregnation (IM) methods. It was found that the one-pot method was superior to the impregnation method for synthesizing Ag/Co₃O₄ catalysts with high activity for both reactions. It was also found that the catalytic behavior of mesoporous Co₃O₄ and Ag/Co₃O₄ catalysts for the both reactions was different. And the addition of silver on mesoporous Co₃O₄ did not always enhance the catalytic activity of final catalyst for CO oxidation at room temperature (20 °C), but could significantly improve the catalytic activity of final catalyst for HCHO oxidation at low temperature (90 °C). The high surface area, uniform pore structure and the pretty good dispersion degree of the silver particle should be responsible for the excellent low-temperature CO oxidation activity. However, for HCHO oxidation, the addition of silver played an important role in the activity enhancement. And the silver particle size and the reducibility of Co₃O₄ should be indispensable for the high activity of HCHO oxidation at low temperature.     

CeO2‐Based Pd(Pt) Nanoparticles Grafted onto Fe3O4/Graphene: A General Self‐Assembly Approach To Fabricate Highly Efficient Catalysts with Magnetic Recyclable Capability

New Pd(Pt) catalysts have been fabricated by assembling multicomponents of Fe₃O₄ and CeO₂/Pd(Pt) on the surface of reduced graphene oxide (RGO) nanosheets in layers. The as‐obtained Pd(Pt) catalysts exhibit extremely high catalytic activity in the selective hydrogenation reaction of nitrobenzene. Owing to the presence of Fe₃O₄, the catalysts can be easily recycled from the catalytic system through magnetic separation. Their high activity, stability, and magnetic recyclability make the as‐obtained hybrids very promising as catalysts in catalytic applications. Compared to other traditional multishell magnetic catalysts that were prepared by means of layer‐by‐layer technology, our process is much more facile and more easily controlled.     

Influence of Pore Structure on Catalytic Properties of Mesoporous Silica-supported Co-B Amorphous Alloys in Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol

Co‐B amorphous alloy catalysts supported on three kinds of mesoporous silica (common SiO₂, MCM‐41 and SBA‐15) have been systematically studied focusing on the effect of pore structure on the catalytic properties in liquid‐phase hydrogenation of cinnamaldehyde to cinnamyl alcohol (CMO). Structural characterization of a series of different catalysts was performed by means of N₂ adsorption, X‐ray diffraction, transmission electron microscopy, hydrogen chemisorption, and X‐ray photoelectron spectroscopy. Various characterizations revealed that the pore structure of supports profoundly influenced the particle size, location and dispersion degree of Co‐B amorphous alloys. Co‐B/SBA‐15 was found more active and selective to CMO than either Co‐B/SiO₂ or Co‐B/MCM‐41. The superior catalytic activity could be attributed to the higher active surface area, because most of Co‐B nanoparticles in Co‐B/SBA‐15 were located in the ordered pore channels of SBA‐15 rather than on the external surface as found in Co‐B/SiO₂ and Co‐B/MCM‐41. Meanwhile, the geometrical confinement effect of the ordered mesoporous structure of SBA‐15 was considered to be responsible for the enhanced selectivity to CMO on Co‐B/SBA‐15, inhibiting the further hydrogenation of CMO to hydrocinnamyl alcohol.     

Synthesis of Superacid‐Functionalized Mesoporous Nanocages with Tunable Pore Diameters and Their Application in the Synthesis of Coumarins

Here we demonstrate for the first time the preparation of a triflic acid (TFA)‐functionalized mesoporous nanocage with tunable pore diameters by the wet impregnation method. The obtained materials have been unambiguously characterized by XRD, N₂ adsorption, FTIR spectroscopy, and NH₃ temperature‐programmed desorption (TPD). From the characterization results, it has been found that the TFA molecules are firmly anchored on the surface of the mesoporous supports without affecting their acidity. We also demonstrate the effect of the pore and cage diameter of the KIT‐5 supports on the loading of TFA molecules inside the pore channels. It has been found that the total acidity of the materials increases with an increase in the TFA loading on the support, whereas the acidity of the materials decreases with an increase in the pore diameter of the support. The acidity of the TFA‐functionalized mesoporous nanocages is much higher than that of the zeolites and metal‐substituted mesoporous acidic catalysts. The TFA‐functionalized materials have also been employed as the catalysts for the synthesis of 7‐hydroxy‐4‐methylcoumarin by means of the Pechmann reaction under solvent‐free conditions. It has been found that the catalytic activity of the TFA‐functionalized KIT‐5 is much higher than that of zeolites and metal‐substituted mesoporous catalytic materials in the synthesis of coumarin derivatives. The stability of the catalyst is extremely good and can be reused several times without much loss of activity in the above reaction.     

Boosting the Catalytic Performance of Metal–Organic Frameworks for Steroid Transformations by Confinement within a Mesoporous Scaffold

Solid‐state crystallization achieves selective confinement of metal–organic framework (MOF) nanocrystals within mesoporous materials, thereby rendering active sites more accessible compared to the bulk‐MOF and enhancing the chemical and mechanical stability of MOF nanocrystals. (Zr)UiO‐66(NH₂)/SiO₂ hybrid materials were tested as efficient and reusable heterogeneous catalysts for the synthesis of steroid derivatives, outperforming the bulk (Zr)UiO‐66(NH₂) MOF. A clear correlation between the catalytic activity of the dispersed Zr sites present in the confined MOF, and the loading of the mesoporous SiO₂, is demonstrated for steroid transformations.     

Textural manipulation of mesoporous materials for hosting of metallic nanocatalysts

The preparation and stabilization of nanoparticles are becoming very crucial issues in the field of so-called "nanocatalysis". Recent developments in supramolecular self-assembled porous materials have opened a new way to get nanoparticles hosted in the channels of such materials. In this paper, a new approach towards monodisperse and thermally stable metal nanoparticles by confining them in ordered mesoporous materials is presented, and three aspects are illustrated. Firstly, the recent progress in the functional control of mesoporous materials will be briefly introduced, and the rational tuning of the textures, pore size, and pore length is demonstrated by controlling supramolecular self-assembly behavior. A novel synthesis of short-pore mesoporous materials is emphasized for their easy mass transfer in both biomolecule absorption and the facile assembly of metal nanocomposites within their pore channels. In the second part, the different routes for encapsulating monodisperse nanoparticles inside channels of porous materials are discussed, which mainly includes the ion-exchange/conventional incipient wetness impregnation, in situ encapsulation routes, organometallic methodologies, and surface functionalization schemes. A facile in situ autoreduction route is highlighted to get monodisperse metal nanoparticles with tunable sizes inside the channels of mesoporous silica. Finally, confinement of mesoporous materials is demonstrated to improve the thermal stability of monodisperse metal nanoparticles catalysts and a special emphasis will be focused on the stabilization of the metal nanoparticles with a low Tammann temperature. Several catalytic reactions concerning the catalysis of nanoparticles will be presented. These uniform nanochannels, which confine monodisperse and stable metal nanoparticles catalysts, are of great importance in the exploration of size-dependent catalytic chemistry and further understanding the nature of catalytic reactions.     

Synthesis of Supported Bimetal Catalysts using Galvanic Deposition Method

Supported bimetallic catalysts have been studied because of their enhanced catalytic properties due to metal‐metal interactions compared with monometallic catalysts. We focused on galvanic deposition (GD) as a bimetallization method, which achieves well‐defined metal‐metal interfaces by exchanging heterogeneous metals with different ionisation tendencies. We have developed Ni@Ag/SiO₂ catalysts for CO oxidation, Co@Ru/Al₂O₃ catalysts for automotive three‐way reactions and Pd−Co/Al₂O₃ catalysts for methane combustion by using the GD method. In all cases, the catalysts prepared by the GD method showed higher catalytic activity than the corresponding monometallic and bimetallic catalysts prepared by the conventional co‐impregnation method. The GD method provides contact between noble and base metals to improve the electronic state, surface structure and reducibility of noble metals.     

Ordered mesoporous Pt/Fe3O4–CeO2 heterostructure gel particles with enhanced catalytic performance for the reduction of 4-nitrophenol

The unique physicochemical properties of ordered mesoporous transition metal oxides have attracted more and more attention. The hydrolysis process of metal oxide precursors is difficult to control, and it is difficult to synthesize an ordered mesoporous transition metal oxide material using the conventional template method. Ordered mesoporous Pt/Fe₃O₄–CeO₂ heterostructure gel materials with excellent catalytic properties were successfully prepared using aerogel technology and the chemical deposition method. The Pt/Fe₃O₄–CeO₂ material was an n–n combined heterostructured semiconductor material which consisted of a magnetic Fe₃O₄ layer, a CeO₂ core and Pt noble metal doped nanoparticles. A layer of Fe₃O₄ thin film was formed on the surface of ordered mesoporous Pt/CeO₂ gel matrix material using the chemical deposition method. The intriguing heterostructural features could facilitate reactant diffusion and exposure of active sites which could enhance synergistic catalytic effects between the Pt nanoparticles and CeO₂ nanoparticles. Compared with Pt/CeO₂, the prepared Pt/Fe₃O₄–CeO₂ showed enhanced catalytic activity in the reduction of 4-nitrophenol at room temperature. The catalytic activity of the heterostructure catalysts was systematically investigated using 4-nitrophenol reduction as a model reaction. The results showed that the Pt (0.1%)/Fe₃O₄–CeO₂ sample exhibited the optimal catalytic performance toward catalytic reduction of 4-nitrophenol to 4-aminophenol. The study provided a method for the preparation of heterostructure nanocatalysts with high efficiency, which would be effective for application in various catalytic reactions.     

Probing the Electronic Effect of Carbon Nanotubes in Catalysis: NH3 Synthesis with Ru Nanoparticles

Carbon nanotubes (CNTs) have been shown to modify some properties of nanomaterials and to modify chemical reactions confined inside their channels, which are formed by curved graphene layers. Here we studied ammonia synthesis over Ru as a probe reaction to understand the effect of the electron structure of CNTs on the confined metal particles and their catalytic activity. The catalyst with Ru nanoparticles dispersed almost exclusively on the exterior nanotube surface exhibits a higher activity than the CNT‐confined Ru, although both have a similar metal particle size. Characterization with TEM, N₂ physisorption, H₂ chemisorption, temperature‐programmed reduction, CO adsorption microcalorimetry, and first‐principles calculations suggests that the outside Ru exhibits a higher electron density than the inside Ru. As a result, the dissociative adsorption of N₂, which is an electrophilic process and the rate‐determining step of ammonia synthesis, is more facile over the outside Ru than that over the inside one.     

Metal–Organic Framework‐Templated Porous Carbon for Highly Efficient Catalysis: The Critical Role of Pyrrolic Nitrogen Species

Metal‐free catalysts are of great importance and alternative candidates to conventional metal‐based catalysts for many reactions. Herein, several types of metal–organic frameworks have been exploited as templates/precursors to afford porous carbon materials with various nitrogen dopant forms and contents, degrees of graphitization, porosities, and surface areas. Amongst these materials, the PCN‐224‐templated porous carbon material optimized by pyrolysis at 700 °C (denoted as PCN‐224‐700) is composed of amorphous carbon coated with well‐defined graphene layers, offering a high surface area, hierarchical pores, and high nitrogen content (mainly, pyrrolic nitrogen species). Remarkably, as a metal‐free catalyst, PCN‐224‐700 exhibits a low activation energy and superior activity to most metallic catalysts in the catalytic reduction of 4‐nitrophenol to 4‐aminophenol. Theoretical investigations suggest that the content and type of the nitrogen dopant play crucial roles in determining the catalytic performance and that the pyrrolic nitrogen species makes the dominant contribution to this activity, which explains the excellent efficiency of the PCN‐224‐700 catalyst well.     

Encapsulated ultrafine and highly dispersed molybdenum dioxide nanoparticles in hollow mesoporous silica spheres as an efficient epoxidation catalyst for alkenes

A facile post-synthetic strategy was developed to functionalize the preformed hollow mesoporous silica spheres by encapsulating the molybdenum dioxide (MoO₂) nanoparticles inside the interior cavity. Hollow mesoporous silica spheres were prepared and employed as carriers, and the encapsulation of MoO₂ nanoparticles was achieved through a one-pot hydrothermal protocol. After characterization, the encapsulated MoO₂ nanoparticles were certified to be ultrafine and highly dispersed, which greatly promoted the catalytic activity. The as-prepared catalysts were utilized in epoxidation of alkenes and exhibited as a promising catalyst in this reaction. After reacting for 10 h, the optimal catalyst MoO₂@SiO₂-1 achieved a conversion above 95% and selectivity above 95%, respectively. Moreover, the catalysts also exhibited good reusability, conversion of 78% and selectivity of 89% (reaction time 4 h) were still obtained after recycling for 5 times. Meanwhile, the employed facial and efficient hydrothermal approach could be expanded to other molybdenum modified heterogeneous catalysts in various applications.