Development of heterogeneous olympic medal metal nanoparticle catalysts for environmentally benign molecular transformations based on the surface properties of hydrotalcite

In this review, we describe the development by our research group of highly functionalized heterogeneous Olympic medal metal (gold, silver, and copper) nanoparticle catalysts using hydrotalcite as a support, aimed towards Green and Sustainable Chemistry. Olympic medal metal nanoparticles can cooperate with the basic sites on the hydrotalcite surface, providing unique and high performance catalysis in environmentally-benign organic transformations such as aerobic oxidation of alcohols, lactonization of diols and selective deoxygenation of epoxides and nitro aromatic compounds.     

Gold nanoparticle-catalyzed environmentally benign deoxygenation of epoxides to alkenes

We have developed a highly efficient and green catalytic deoxygenation of epoxides to alkenes using gold nanoparticles (NPs) supported on hydrotalcite [HT: Mg(6)Al(2)CO(3)(OH)(16)] (Au/HT) with alcohols, CO/H(2)O or H(2) as the reducing reagent. Various epoxides were selectively converted to the corresponding alkenes. Among the novel metal NPs on HT, Au/HT was found to exhibit outstanding catalytic activity for the deoxygenation reaction. Moreover, Au/HT can be separated from the reaction mixture and reused with retention of its catalytic activity and selectivity. The high catalytic performance of Au/HT was attributed to the selective formation of Au-hydride species by the cooperative effect between Au NPs and HT.     

Polymer particles with dendrimer@SiO2-Ag hierarchical shell and their application in catalytic column

Polymer particles with dendrimer@SiO₂–Ag hierarchical shell were prepared, and their application in the catalytic column for the reduction of 4-nitrophenol (4-NP) was also investigated. The PS microspheres with the carboxyl group were used as the supports for the immobilization of dendrimer@SiO₂–Ag shell. The polyamidoamine (PAMAM) dendrimer was grafted on the surface of PS microsphere through repetitive Michael addition reaction of methyl acrylate (MA) and amidation of the obtained esters with a large excess of ethylenediamine (EDA) successively. The silver nanoparticles formed inside the PAMAM shell. Then, the silver nanoparticles, which were used as center of nucleation, were coated with SiO₂ shell through improved Stöber method. Moreover, the more silver nanoparticles were dispersed on the surface of SiO₂ shell. The contents of silver element were measured using inductively coupled plasma (ICP-MS). The obtained PS@PAMAM@SiO₂–Ag nanoparticles were packed in stainless steel column, which has been used effectively for the catalytic reduction of 4-NP. Under column pressures, the rigid SiO₂ shell plays a better role in immobilization of silver nanoparticles than the soft PAMAM shell. This technique for packing catalytic nanoparticles in column improves the efficiency of application with metal catalysts as well as reduces the tedious separation processes in catalytic reaction.     

Synthesis of Triple‐Layered Ag@Co@Ni Core–Shell Nanoparticles for the Catalytic Dehydrogenation of Ammonia Borane

Triple‐layered Ag@Co@Ni core–shell nanoparticles (NPs) containing a silver core, a cobalt inner shell, and a nickel outer shell were formed by an in situ chemical reduction method. The thickness of the double shells varied with different cobalt and nickel contents. Ag_0.04@Co_0.48@Ni_0.48 showed the most distinct core–shell structure. Compared with its bimetallic core–shell counterparts, this catalyst showed higher catalytic activity for the hydrolysis of NH₃BH₃ (AB). The synergetic interaction between Co and Ni in Ag_0.04@Co_0.48@Ni_0.48 NPs may play a critical role in the enhanced catalytic activity. Furthermore, cobalt–nickel double shells surrounding the silver core in the special triple‐layered core–shell structure provided increasing amounts of active sites on the surface to facilitate the catalytic reaction. These promising catalysts may lead to applications for AB in the field of fuel cells.     

Synthesis of hierarchical Polystyrene/Polyaniline@Au nanostructures of different surface states and studies of their catalytic properties

We synthesized hierarchical Polystyrene/Polyaniline@Au(PS/PANI@Au) catalysts through a seeded swelling polymerization and in-situ reduction procedure. PS/PANI@Au catalysts possess a core of PS as seed and template, a PANI shell with fibers and uniform gold nanoparticles on the surface. The configuration changes of the PANI chains resulting from the doping/ dedoping procedure led to various loading amounts of Au nanoparticles. Reduction of 4-nitrophenol was chosen as the probe reaction to evaluate the catalytic activity of supported Au nanocatalysts. The catalytic results indicated that dedoping treatment of the PS/PANI supports provides stronger coordinative ability to metal nanoparticles as well as more –N= groups, which results in a better catalytic performance towards the reduction of 4-nitrophenol.     

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.     

Oxygen dissociation on palladium and gold core/shell nanoparticles

Oxygen dissociation reaction on gold, palladium, and gold‐palladium core/shell nanoparticles was investigated with plane wave basis set, density functional theory. Bader population analysis of charge and electron distribution was employed to understand the change of catalytic activity as a function of the nanopaticle composition. The nanoparticles’ electronic properties were investigated and the degree of core/shell charge polarization was estimated for each composition. It was found that surface polarization plays an important role in the catalysis of the initial step of electrophile reactions such as oxygen dissociation. We have investigated the O₂ adsorption energy on each nanoparticle and the activation barrier for the oxygen dissociation reaction as a function of the nanoparticle structure. Furthermore, we have investigated the influence of surface geometry, that is., surface bond lengths on the catalytic activity. We have compared the electronic and the geometry effects on the oxygen activation and dissociation. Our design rules for core/shell nanoparticles offer an effective method for control of the surface catalytic activity. Palladium and gold are often used as catalysts in synthetic chemistry. First‐principles calculations elucidate the mechanisms that control the surface reactivity of gold, palladium, and gold‐palladium core shell nanoparticles in oxygen dissociation reactions. Oxygen dissociation is promoted on the gold surface of gold/palladium core‐shell nanoparticles by favorable electron transfer from the core to the shell. Such core‐shell electronic effects can be used for fine‐tuning the nanoparticles catalytic activity.     

Preparation and properties of silica–gold core–shell particles

Silica-metal core–shell particles, as for instance those having siliceous core and nanostructured gold shell, attracted a lot of attention because of their unique properties resulting from combination of mechanical and thermal stability of silica and magnetic, electric, optical and catalytic properties of metal nanocrystals such as gold, silver, platinum and palladium. Often, the shell of the core–shell particles consists of a large number of metal nanoparticles deposited on the surface of relatively large silica particles, which is the case considered in this work. Namely, silica particles having size of about 600 nm were subjected to surface modification with 3-aminopropyltrimethoxysilane. This modification altered the surface properties of silica particles, which was demonstrated by low pressure nitrogen adsorption at ?196 °C. Next, gold nanoparticles were deposited on the surface of aminopropyl-modified silica particles using two strategies: (i) direct deposition of gold nanoparticles having size of about 10 nm, and (ii) formation of gold nanoparticles by adsorption of tetrachloroauric acid on aminopropyl groups followed by its reduction with formaldehyde.The overall morphology of silica–gold particles and the distribution of gold nanoparticles on the surface of modified silica colloids were characterized by scanning electron microscopy. It was shown that direct deposition of colloidal gold on the surface of large silica particles gives more regular distribution of gold nanopartciles than that obtained by reduction of tetrachloroauric acid. In the latter case the gold layer consists of larger nanoparticles (size of about 50 nm) and is less regular. Note that both deposition strategies afforded silica–gold particles having siliceous cores covered with shells consisting of gold nanoparticles of tunable concentration.     

Observed Dramatically Improved Catalysis of Ag Shell on Au/Ag Core‐shell Nanorods is Due to Silver Impurities Released During Etching Process

Core/shell bimetallic nanoparticles are highly popular in electrocatalysis; it is argued that the core metal enhances the catalytic properties of the shell. We have investigated the electrocatalytic properties of Au/Ag core‐shell nanorods (Au/Ag NRs) where Ag shell was thinned by aging in the presence of cetyltrimethylammonium bromide. We observed excellent electrocatalysis toward hydrogen peroxide electroreduction upon decreasing the Ag shell thickness, which would, at first, appear to imply a strong synergistic effect of the Au core with the Ag shell for electrocatalysis. We show, however, that this electrocatalysis is not caused by particular Au/Ag core/shell structures but rather by the presence of residual silver impurities in the form of Ag nanoparticles (Ag NPs) formed during the preparation of the thin‐layer silver shell/gold core nanorods.     

High Carbon-Resistance Ni@CeO<Subscript>2</Subscript> Core–Shell Catalysts for Dry Reforming of Methane

Ni@CeO₂ core–shell catalysts were synthesized via a facile surfactant-assisted hydrothermal method and their catalytic performance in the dry reforming of methane (DRM) reaction was evaluated. A variety of techniques including XRD, N₂ adsorption–desorption, SEM, TEM, TPO, TGA were employed to characterize the prepared or spent catalysts. The encapsulation by the CeO₂ shell, on one side, can restrict the sintering and growth of Ni nanoparticles under harsh reaction conditions. On the other side, compared to the conventional shell material of SiO₂, CeO₂ can provide more lattice oxygens and vacancies, which is helpful to suppress coke deposition. Consequently, the Ni@CeO₂ core–shell catalysts exhibited better catalytic activity and stability in the DRM reaction with respect to the referenced Ni@SiO₂ core–shell catalysts and Ni/CeO₂ supported catalysts.     

Efficient Tandem Synthesis of Methyl Esters and Imines by Using Versatile Hydrotalcite‐Supported Gold Nanoparticles

Efficient basic hydrotalcite (HT)‐supported gold nanoparticle (AuNP) catalysts have been developed for the aerobic oxidative tandem synthesis of methyl esters and imines from primary alcohols catalyzed under mild and soluble‐base‐free conditions. The catalytic performance can be fine‐tuned for these cascade reactions by simple adjustment of the Mg/Al atomic ratio of the HT support. The one‐pot synthesis of methyl esters benefits from high basicity (Mg/Al=5), whereas moderate basicity greatly improves imine selectivity (Mg/Al=2). These catalysts outperform previously reported AuNP catalysts by far. Kinetic studies show a cooperative enhancement between AuNP and the surface basic sites, which not only benefits the oxidation of the starting alcohol but also the subsequent steps of the tandem reactions. To the best of our knowledge, this is the first time that straightforward control of the composition of the support has been shown to yield optimum AuNP catalysts for different tandem reactions.     

Efficient tandem synthesis of methyl esters and imines by using versatile hydrotalcite-supported gold nanoparticles

Efficient basic hydrotalcite (HT)-supported gold nanoparticle (AuNP) catalysts have been developed for the aerobic oxidative tandem synthesis of methyl esters and imines from primary alcohols catalyzed under mild and soluble-base-free conditions. The catalytic performance can be fine-tuned for these cascade reactions by simple adjustment of the Mg/Al atomic ratio of the HT support. The one-pot synthesis of methyl esters benefits from high basicity (Mg/Al=5), whereas moderate basicity greatly improves imine selectivity (Mg/Al=2). These catalysts outperform previously reported AuNP catalysts by far. Kinetic studies show a cooperative enhancement between AuNP and the surface basic sites, which not only benefits the oxidation of the starting alcohol but also the subsequent steps of the tandem reactions. To the best of our knowledge, this is the first time that straightforward control of the composition of the support has been shown to yield optimum AuNP catalysts for different tandem reactions.     

Silver and gold nanoparticles in silica matrices: synthesis,properties, and application

Methods of synthesis, optical characteristics, morphology, and catalytic and bactericidal characteristics of composite materials based on silica (films and powders) containing nanoparticles of silver, gold, and their binary compounds with alloy or core–shell structure are examined. The photochemical reduction of Au³⁺ and Ag⁺ with a photocatalyst (a film of SiO₂ with adsorbed benzophenone) makes it possible to generate stable nanoparticles of gold and silver in solutions for subsequent introduction into adsorbents and catalysts. Examples of the use of nanosized composites in catalysis and in microbiological experiments are presented.     

Recent Advances in Preferential Oxidation of CO in H2 Over Gold Catalysts

Recent studies have revealed that supported gold catalysts exhibit comparable or superior catalytic performance relative to platinum group metals, especially at low temperatures, in the preferential oxidation of CO under excess H₂ (CO-PROX). Complete conversion of CO with good selectivity of O₂ for CO₂ and highly stable catalyst performance in the presence of CO₂ and H₂O are considered to be essential for the successful development of CO-PROX catalysts for application in polymer electrolyte membrane fuel cells. The performance of supported gold catalysts in the CO-PROX reaction has been shown to be dependent on the characteristics of gold (size, oxidation state, and its interaction with other metal/oxides), nature of the support (size, composition, preparation method, presence of promoters, and doping with other metal ions), and reaction conditions (temperature and feed composition). Complete CO conversion has been achieved in the presence of certain gold catalysts below 100 °C. The unresolved issues in CO-PROX include the undesired oxidation of H₂, detrimental effects of CO₂ and/or H₂O, and long-term stability of the catalysts. To address these issues, the catalytic activity of gold supported on simple oxides such as TiO₂, CeO₂, Al₂O₃, and Fe₂O₃ has been improved dramatically by the addition of promoters, alteration of the gold-oxide support interface, and modification of the oxide supports. Recently, nanoporous gold has also been recognized to be promising for this reaction. This review highlights recent developments of unsupported and supported gold catalysts for the CO-PROX reaction.     

Intramolecular hydroamination of 2-(2-phenylethynyl)aniline catalyzed by gold nanoparticles

The possibility of using heterogeneous catalysts with a low content of gold in the intramolecular hydroamination of 2-(2-phenylethynyl)aniline was shown. The catalysts with size of gold particles lower than 3 nm exhibit catalytic activity. The study of efficiency of the heterogeneous Au-containing catalysts on different supports (MCM-41, γ-Al₂O₃(F), NH₄ ⁺-Beta, Diasorbamine-60, APTES-MCM-41, and SH-PMO) revealed that the maximum yields of 2-phenylindole were achieved with gold supported on mesoporous silicate materials. A high degree of dispersion of gold in these catalysts is explained by the presence of amino or thiol anchor groups in the support composition. It was found by diffuse reflectance infrared Fourier transform spectroscopy and XPS and XRD methods that gold in these catalysts exists on the support surfaces as small metal particles and due to their size dimensions they show a decreased electron density, i.e., they are electron-deficient Au^δ+ nanoparticles.     

Catalytic oxidation of glyoxal to glyoxalic acid over Au-Pd alloy nanoparticles on hydrotalcite

Synthesis of glyoxalic acid by selective oxidation of glyoxal at ambient temperatures with O₂ as an oxidant is an important problem. We found that gold nanoparticles supported on hydrotalcite (Au/HT) exhibit an appreciable catalytic activity for this reaction in the liquid phase. Moreover, Au-Pd/HT, prepared by the deposition-precipitation method is superior in the catalytic behavior to monometallic Au/HT and Pd/HT catalysts. Introduction of palladium enhances ability of the catalysts to oxidize carbonyl to carboxyl, weakens the power to rupture C-C bond and in this way improves the catalytic performance. Furthermore, the Au: Pd ratio also influences the properties of the alloy catalysts. The 1.5Au-1.5Pd/HT catalysts show the highest activity for the selective oxidation at ambient temperature producing glyoxalic acid in 13.4% yield at pH 7.7. Moreover, due to basic properties of hydrotalcite, glyoxalic acid could be synthesized over 1.5Au-1.5Pd/HT in 8.0% yield without adding a base. It is hoped that results of this study can fuel further research in designing new catalysts with alloy nanoparticles supported by hydrotalcite that can be used for the selective oxidation of other useful compounds.     

Stabilization of Palladium Nanoparticles on Nanodiamond–Graphene Core–Shell Supports for CO Oxidation

Nanodiamond–graphene core–shell materials have several unique properties compared with purely sp²‐bonded nanocarbons and perform remarkably well as metal‐free catalysts. In this work, we report that palladium nanoparticles supported on nanodiamond–graphene core–shell materials (Pd/ND@G) exhibit superior catalytic activity in CO oxidation compared to Pd NPs supported on an sp²‐bonded onion‐like carbon (Pd/OLC) material. Characterization revealed that the Pd NPs in Pd/ND@G have a special morphology with reduced crystallinity and are more stable towards sintering at high temperature than the Pd NPs in Pd/OLC. The electronic structure of Pd is changed in Pd/ND@G, resulting in weak CO chemisorption on the Pd NPs. Our work indicates that strong metal–support interactions can be achieved on a non‐reducible support, as exemplified for nanocarbon, by carefully tuning the surface structure of the support, thus providing a good example for designing a high‐performance nanostructured catalyst.     

Preparation and characterization of Fe3O4@Boehmite core‐shell nanoparticles to support molybdenum or vanadium complexes for catalytic epoxidation of alkenes

Fe₃O₄ core nanoparticles were prepared via a solvothermal process, and then they were covered with a surface hydroxyl‐rich boehmite shell via the hydrothermal‐assisted sol–gel processing of aluminum 2‐propoxide. The outer surface of the boehmite shell was subsequently covalently functionalized with 3‐(tri‐methoxysilyl)‐propylamine or 3‐(tri‐methoxysilyl)‐propyl chloride, and the terminal chlorine groups were treated with imidazole. These compounds were used to support the hexa‐carbonyl molybdenum and oxo‐sulfato vanadium (IV) complexes. The supported catalysts were characterized by the FT‐IR, CHN, ICP, and TEM analysis techniques. They were then used in the epoxidation of cis‐cyclooctene. The catalytic procedures were optimized for different parameters such as the solvent, oxidant, and temperature. The reaction progress was investigated by the gas–liquid chromatography analysis. The catalysts used were simply recovered from the solution by applying a magnet, and recycling the experiments revealed that the heterogeneous nanocatalysts could be repeatedly used for the epoxidation of cis‐cyclooctene. The optimized conditions were also successfully used for the epoxidation of some other alkenes.     

Preparation of Magnetically Recyclable Yolk/Shell FexOy/PdPt@CeO2 Nanoreactors with Enhanced Catalytic Activity

Noble metal nanoparticles (NPs) have recently received considerable attention from researchers working in the field of catalysis. However, the development of new methods allowing these materials to reach their maximum catalytic properties remains challenging. Nanoreactors could lead to dramatic improvements in activity with the help of the intrinsic confinement effect. In this study, we designed a series of yolk/shell Fe_xO_y/PdPt@CeO₂ composites, where the Fe_xO_y NPs acted as a movable core, allowing for the uniform distribution of the PdPt alloys on the inner surface of the CeO₂ shell. The high porosity and existence of hollow voids in the CeO₂ shell allowed these Fe_xO_y/PdPt@CeO₂ composites to be used as nanoreactors in catalytic reactions. As well this confinement effect, we identified two structural features that led to enhanced catalytic activity, including (i) the replacement of monometallic NPs with a bimetallic PdPt alloy and (ii) the replacement of a chemically inert support with a reactive CeO₂ shell. The resulting nanoassembled catalysts displayed higher activities toward the catalytic reduction of dyes than the reference samples. Moreover, these catalysts were readily recovered and reused because of the magnetic Fe_xO_y core.     

Unique catalysis of gold nanoparticles in the chemoselective hydrogenolysis with H2: cooperative effect between small gold nanoparticles and a basic support

Gold nanoparticles on hydrotalcite act as a heterogeneous catalyst for the chemoselective hydrogenolysis of various allylic carbonates to the corresponding terminal alkenes using H(2) as a clean reductant. The combination of gold nanoparticles and basic supports elicited significantly unique and selective catalysis in the hydrogenolysis.