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Industrial catalysts are typically made of nanosized metal particles, carried by a solid support. The extremely small size of the particles maximizes the surface area exposed to the reactant, leading to higher reactivity. Moreover, the higher the number of metal atoms in contact with the support, the better the catalyst performance. In addition, peculiar properties have been observed for some metal/metal oxide particles of critical sizes. However, thermal stability of these nanostructures is limited by their size; smaller the particle size, the lower the thermal stability. The ability to fabricate and control the structure of nanoparticles allows to influence the resulting properties and, ultimately, to design stable catalysts with the desired characteristics. Tuning particle sizes provides the possibility to modulate the catalytic activity. Unique and unexpected properties have been observed by confining/embedding metal nanoparticles in inorganic channels or cavities, which indeed offers new opportunities for the design of advanced catalytic sytems. Innovation in catalyst design is a powerful tool in realizing the goals of more green, efficient and sustainable industrial processes. The present Review focuses on the catalytic performance of noble metal- and non precious metal-based embedded catalysts with respect to traditional impregnated systems. Emphasis is dedicated to the improved thermal stability of these nanostructures compared to conventional systems.
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Heterogeneous catalysis controls more than 90 % of the world’s chemical manufacturing processes. The production of energy sources, plastics, synthetic fibers, modern building materials, paper products, polymers, pharmaceuticals, and agricultural products are just some examples. Moreover, the use of heterogeneous catalysts is a widespread and well-established technology in the environment protection field (i.e., three-way catalysts, DeNOx catalysts, catalytic filters for diesel particulate abatement, photocatalysts for waste-water treatment).
Heterogeneous catalysts belong to the nanosized world, long before the recognition of nanotechnology. Indeed, the length scale ranging from approximately 1 to 100 nanometers is well known to be of relevance to heterogeneous catalysis since the active components are usually nanosized (metal or metal oxide) particles dispersed on high surface area solids.
A typical example of the importance of catalyst’s size is given by gold. It has been well recognized that this metal, which is not reactive in its bulk form, becomes catalytically active in very particular instances: when it presents to the chemical environment as discrete, nanoscale Au particles in a very peculiar size range, variously considered to be 2 to 3 nm1 or 3 to 5 nm.2 However, an optimum range of 7 to 8 nm for some reactions was also claimed3 and activity was even reported for particles in the 30 to 50 nm range.4 The type of support, in particular its reducible nature, plays one of the major roles in directing the final catalytic properties of gold nanoparticles. Whereas the upper cutoff of activity appears to be for particles of 50 nm diameter, the lowest cutoff appears to be close to 2 nm.5 The question of whether it is some special property of these nanophase particles that confers on them a catalytic ability, or it is the result of an interaction between particle and support, or even particle size and support are only indirect factors and it is some special site on the surface of the gold that does the work, is not yet agreed in the literature.
The reactions on the surface of heterogeneous catalysts typically involve atom–molecule interactions and the active sites (metal or metal oxide particles) are placed on highly porous and thermally stable supports where the access to the active sites are in the range of 10 to 100 nanometers. The issue of access path is familiar in heterogeneous catalysis. For example zeolites, which are widely applied catalysts, derive much of their unique catalytic functions by shape-selectivity.6 This is done by distinguishing molecules by their different diffusivities in and out of the zeolite particles due to their shapes and sizes relative to the channels, pores, and cages of the zeolite. The shape constraint occurs far away from the active site and influences the reaction by controlling access to or diffusion from the active sites.
In supported metal catalysts, the surface of the metal particles is populated by different types of metal atoms, such as corners, edges, or terraces. A surface atom, or a combination of atoms, has definite geometrical and electronic properties to act as an active site. In structure-sensitive reactions, different types of surface metal atoms possess totally different properties.7
Even though the key role of the catalyst’s size in catalytic processes does not represent a novelty, great impact on the development of new catalytic systems will be performed by more recent opportunities offered by nanotechnology and nanoscience. In particular, the innovative idea at the base of what is defined as nanocatalysis is the acquisition of the ability not only to synthesize, but also to design and stabilize the catalyst at nanometer scale. The capacity to customarily design active sites and site environments for perfect selectivity and desirable activity have been achieved in some systems whereas it continues to be a goal still strived in others. The ultimate control of a catalytic process remains a great challenge. New techniques made available by nanotechnology have resulted in some progress towards achieving this goal. Moreover, the development of analytical tools to accurately study chemical processes on a molecular scale opened the opportunity to better understand the mechanisms that govern catalytic reactions and surface science.
Traditionally, nanosized catalytic active phases were prepared by wet impregnations, ionic exchange, or thermal decomposition methods.8 In the last decades, a very large number of new techniques, based on the possibility of a much more sophisticated size control, have been developed. For example, nanoparticles with a narrow size distribution can be efficiently prepared by the dendrimer-assisted method.9 In this procedure, metal ions in a solution are complexed to a dendrimer, mostly with amine groups in the inner shell of an OH-terminated poly(amidoamine), for example.10 The complexed metal ions are subsequently reduced to metal atoms which agglomerate into a metal particle. Since dendrimers can be prepared with high purity and amine groups present high complexation ability of metal atoms, by opportunely tuning reaction conditions, it is possible to obtain metal particles within each dendrimer with near monodisperse dimensions. Furthermore, by tuning the binding sites of the dendrimer, it is possible to prepare uniform-sized metal particles over a wide range of sizes. This is an advantage compared with the preparation using well-defined organometallic complexes, which is limited by the availability of the metal complexes. The dendrimer-assisted method can be extended to bimetallic clusters. The second metal ion can be introduced by partial displacement of the first metal, or sequentially after the first, or together with the first by co-complexation.9 Although these methods succeeded in making bimetallic particles, it is not obvious whether they can generate uniform composition particles. Thus, new procedures or variations of the existing methods have to be developed.
Almost monodispersed particles can be prepared by using organic compounds as capping agents. These compounds (typically polymers, amines, thiols) act as protecting agents preventing the agglomeration of metal particles in solution. Moreover, they form an organic layer around the particles which might give novel properties to the final assemblies, such as solubility in organic solvents, different reactivity, and particular optical, thermal, and electrical properties.11
Nanosized metal oxide nanoparticles can be prepared using reverse micelles which are water-in-oil systems stabilized by a surfactant.12 In a nonpolar solvent, a surfactant aggregates into nanosized spherical structures made up by a core of polar headgroups and a shell of hydrophobic tails. When a small amount of polar solvent is added in the mixture, hydrophobic interactions favor its segregation in the interior of the micelles. If the polar solvent used is an alcohol, which contains an oxide precursor such as a metal alkoxide, controlled hydrolysis of the precursor achieved by mixing in a small amount of water would generate a small particle of oxyhydroxymetal gel inside the reverse micelle. After drying and calcination, small particles of metal oxide are therefore obtained. Modifications of this method also exist. Similarly, by chemical vapor deposition, it is possible to prepare supported metal nanoparticles in a controlled and reproducible manner.13 The procedure is conducted vaporizing a suitable metal precursor and adsorbing it on the supporting material.
Subsequently, as a result of a surface reaction with or without a co-reactant, the adsorbate is transformed in the catalytically active species. The key to control the metal dispersion is the understanding of the relationship between the precursor properties and surface reactivity. The formation of highly dispersed metal clusters can be achieved by controlling the surface concentration and reactivity of adsorption centres and deposition related parameters such as temperature, partial pressure, and precursor reactivity.
In an alternative approach, a nanosized catalytically active phase was prepared allowing its growth inside controlled cavities or channels. For this purpose, a carefully controlled and regular structural environment is necessary. Good examples of this approach are provided by the confinement of the nanoparticles inside zeolites or mesoporous materials, such as the MCM or SBA families. When the active phase is placed in the channels of about a few nanometers diameter, it was found that the access to the metal center is restricted by the concave wall of the channels.14 Since it is possible to tune the diameter of pores in zeolites or mesoporous materials, the selectivity can be nicely controlled using the environment provided by the shape of the channels. Thus, it is desirable to be able to construct cages in a way that could offer the best flexibility to modify the cage and window size. In this way, functional groups can be attached at specific positions. Such flexibility enables the positioning of different active sites and reactant binding sites at specific locations with respect to each other, such that specific points of a reactant molecule can be activated.
An alternative and fascinating way to control the active phase of a catalyst is its preparation inside carbon nanotubes. This topic has received significant attention in recent years due to the rapid advances in techniques to produce and purify them. Although separation and manipulation of carbon nanotubes require further improvements to reduce the variability in their properties, their potential for catalytic applications is evident. For example, it is possible to trap catalytic active sites inside a carbon nanotube.15 Methods have been discovered to control access to nanotube channels.16
Having established various methodologies for designing and controlling the size and the shape of the active phase, great attention has to be dedicated to the thermal stabilization of these nanostructures, if the application is a catalytic process. Catalytic activity often drops as a result of sintering of the active phase, of the support, or of both components. Metal sintering results in decreased metal surface area, with the obvious effects on the catalytic activity. Support sintering reduces the total surface area and the pore volume and it may trap the active catalyst inside the reorganized structure. Interactions between the metal and the support may further complicate matters.
Recently, great attention has been dedicated to the development of novel synthetic methods for the preparation of nanostructured catalysts with higher activity and thermal stability than those available. The solid-phase crystallization (SPC) technique is one of the proposed approaches. The SPC strategy is based on the preparation of a crystalline oxide precursor (generally perovskite or hydrotalcite compounds) by sol-gel or co-precipitation methods in the presence of ions of the active metal. After calcination, the material contains species of the active metal, homogeneously dispersed inside the bulk. Subsequent reduction at high temperature leads to the migration of most of the metal atoms to the surface, forming small homogeneously dispersed metal particles. It has been indicated that the metal–support interaction is stronger than that obtainable by the usual impregnation or deposition methods. Using the SPC technique, active and thermally stable catalysts were produced for reforming reactions involving methane17–22 and methanol.23
The microemulsion synthesis route shows interesting advantages related to the possibility of controlling properties such as particle size distribution and morphology. Particles with a narrow nanosize distribution can often be achieved in this way with consequent benefits for catalytic performances. Although this synthetic strategy is quite successful in producing active and stable catalysts, it usually requires large quantities of expensive reagents which have to be then removed.
An innovative and promising approach, used by Budroni et al.,24 is based on the incorporation of the metal nanoparticles into an open shell of support (porous oxide) in order to limit the sintering of the particles at high temperatures. The porous nature of the support prevents the total occlusion of the nanoparticles, thus favoring the access of the reactants to the catalytic sites. The innovation of this approach relies on the covalent link between preformed metal nanoparticles and the growing support, which accounts for the superior catalytic activity and stability of such materials (Figure 1).
The thermal stabilization of metal nanoparticles involved in reactions operating at very high temperatures (ca. 900 °C) was also achieved through an alternative embedding approach, in which the metal nanoparticles were directly coated with a nanoporous oxide shell.25–28 These core-shell nanocatalysts are expected to have unique implications in catalysis that are not present in either core or shell materials.29 The outer shells isolate the catalytically active nanoparticle cores and prevent the possibility of sintering the core particles during reactions at high temperatures. Moreover, the particular connection in terms of physical and/or chemical interaction can create synergic effects. This interaction allows maximizing the metal–support interface where such interfaces are important in catalytic performances. Even though the fundamental feasibility of such an approach has been demonstrated by a number of reports, significant challenges remain in establishing control over particle size, shape, and composition and maximizing accessibility to the nanoparticle via porosity of the shell without compromising the particle stability.
Irrespective of the particular synthetic method used, much attention must be paid to the sizes of the channels or cavities in which metal nanoparticles are confined in order to ensure the access to the catalytically active phase and to minimize at the same time, diffusional resistance (mass transfer) of reactant molecules which can strongly affect the catalytic activity of the supported catalysts. When the channel/cavity, for example, is much bigger with respect to the average metal particle size, their mobility will increase and sinterization will turn into an important limiting factor. Even with these limitations, the embedded catalyst seems to be very promising especially for reactions which involve medium/small molecules.
Finally, it is important to recall that designing catalysts that are more efficient, more selective and more specific to a certain type of reaction can lead to significant savings in manufacturing expenses (i.e., reduction of raw material, energy consumption, and waste production). A higher activity will be reflected either in high productivity from relatively small reactors and catalyst volumes or in mild operating conditions, particularly temperature. Higher selectivity produces high yields of a desired product while suppressing undesirable side reactions. A reaction of high selectivity can not only reduce waste products, but also the energy and process requirements for separation and purification. Moreover, the ability to exercise greater control over the interaction of reactant with the catalyst surface can create new possibilities for heterogeneous catalysis in pollution control, for example, or in the provision of novel power sources.
This Review focuses on recent findings on noble metal and non-precious metal embedded catalysts. Notably, embedded noble metal-based catalysts have been more deeply investigated with respect to the other transition metals mainly due to their lower tendency to oxidize/redissolve during preparation conditions. Some short remarks are also dedicated to the use of core-shell oxides of potential interest in catalysis.
Gold is certainly one of the most used metals in the preparation of complex architectures and advanced materials because of several reasons: 1) its chemistry is well known; 2) there exists extensive research in the preparation of gold nanoparticles; 3) it has high stability in the colloidal state when adequately protected; 4) it demonstrates interesting catalytic properties in many reactions. For the purpose of this Review, although inorganic shells are the most interesting, we selected some examples in which organic shells were used to prepare active or promising catalysts. Most of the literature focuses on SiO2-based materials because many Si precursors are available and the characterization of the obtained materials, especially by high-resolution transmission electron microscopy (HRTEM), is generally more simple compared to other oxides.
Scott et al.30 prepared dendrimer-encapsulated Au and Pd particles, very small in size (1–2 nm), by complexation and reduction of chloride metal salts with ethylenediamine functionalities placed in the core of the dendrimers. Then, they used a sol-gel procedure to build a TiO2 layer around the nanoparticle-dendrimer system. After calcination, although a 2-fold increase in the average dimension of the particles was observed, the system was more stable against sintering than a conventional one prepared by impregnation of the nanoparticles-dendrimer assemblies onto a commercial TiO2 support.
The use of dendrimers was also reported by Wu et al.31 After grafting dendrimers onto the SiO2 microspheres, Au ions were bound to the dendrimers and carefully reduced to obtain Au nanoparticles. These particles were then embedded into alternately charged polyelectrolytes by layer-by-layer assembly. Finally, SiO2 cores were etched in order to obtain hollow spheres with Au particles (ca. 3 nm) embedded into thin layers of the polyelectrolytes. The system was found to be catalytically active in the reduction of 4-nitrophenol using NaBH4.
Auten et al. used dendrimers containing metal particles (Au, Pt, and bimetallic Au–Pt) to impregnate silica, titania, and alumina.32 After removing the dendrimer by calcination and subsequent reduction at 300 °C, the catalysts were tested for CO oxidation with the bimetallic catalysts being the more active and resistant to deactivation during an extended high temperature oxidation treatment. The support effects were found to be small and differences in support played only a minor role in modulating the nanoparticles’ activity.
Kònya et al. reported the synthesis of MCM-type materials incorporating gold particles of different sizes.33 They started from monodisperse gold colloids and prepared MCM-41 and MCM-48 supports around the particles by controlled hydrolysis of the silica precursors. Large particles (20 nm) or high metal concentration were not compatible with ordered materials, leading to significant deposits on the external surface of the mesoporous systems. However, 2 and 5 nm Au particles were effectively incorporated inside the mesoporous channels. It is important to underline that only 2 nm Au particles were readily accessible to the reactants whereas larger particles caused blockage of the pores. Unfortunately, no catalytic data are available.
The growth of MCM-type materials using particles as templates was reported also by Liu et al.34 In this case, cationic surfactants, which are typical templates for SiO2-based materials preparation, were used as protecting agents in the preparation of gold particles, which were then successfully incorporated inside the channels of MCM-41. However, the large dimension of the Au particles and the micron length of the MCM-41 channels resulted in a very poor CO oxidation ability of the catalyst mainly due to the blockage of the channels operated by the Au particles. Consistently, the preparation of shorter channels resulted in a remarkable increase in the catalytic activity. A better control in the initial metal particle size must still be achieved. In fact, the activity of these materials remained poor especially considering the very high metal loadings (up to 30 wt %). Therefore, to better control Au particle size,35 the same research group attempted the modification of preformed SiO2-based materials (MCM-41, MCM-48, and SBA-15) with appropriate silane coupling agents bearing amine groups at one end. These groups were used to complex a gold precursor (HAuCl4), which was then reduced directly into the channels to form particles located inside the pores. The channels acted as barriers to the growth of Au particles but, after calcination, there was an increase in particle size. The small wall thickness (about 1.0 nm) may account for the observed sintering.
An ion-exchange strategy was instead used by Yang et al.36 to adsorb the gold precursor onto a preformed SBA-15 support. The approach involved the functionalization of SBA-15 with a triethoxysilane bearing a quaternary ammonium fragment, which was used to exchange chloride anions with tetrachloroaurate(III) anions. This method was aimed to overcome the repulsion between a negatively charged silica surface and the AuCl4− anion, which is the most frequently used Au precursor. The subsequent reduction of AuIII species with NaBH4 should allow the formation of well-dispersed gold particles onto the support walls. Unfortunately, these materials showed CO oxidation activity of an order of magnitude lower than a similar sample prepared by chemical vapor deposition. Furthermore, dramatic deactivation was observed even under mild reaction conditions (160 °C). 10–50 nm Au particles located outside of SBA-15 channels were observed, indicating that the method was not effective for particles encapsulation.
A common feature of the examples described above is the use of secondary interactions (templating effects, complexation of metal ions, hydrophobic interactions, electrostatic interactions) for the formation of the oxide layer around the metal particles. In the case of gold, small particles (<100 nm) do not show an appreciable affinity towards SiO2 and its precursors. This is mainly due to the fact that gold metal is hardly oxidized and it does not easily form passivating oxide films in solution. These oxide films are generally used, in the case of other metals, to drive the covalent binding of SiO2 precursors onto the surface of the particles. Furthermore, organic molecules used to prevent agglomeration in the solution of the gold particles (i.e., carboxylic acids, amines, surfactants, alkyl thiols) contribute to decrease the affinity of the particle surface for silica (vitreophobic effect). To overcome this problem, Liz-Marzán et al.37 used the bifunctional capping agent (3-aminopropyl)trimethoxysilane (APS). This organic compound contains an amine group, which serves to complex Au atoms, and a silanol group, which can act as a primer to grow a SiO2 shell around the particles. The synthesis started from citrate-stabilized preformed gold particles and exchanged citrate ligands with APS. Following a first step of thin SiO2 layer formation using APS, as anchor for the growing layer, they used the classical Stöber method to prepare a thicker SiO2 layer around the particles. By changing the reaction/hydrolysis conditions, it was possible to tune the thickness of the SiO2 layer around the particles.
A similar approach was elegantly adopted by Aprile et al.38 to prepare Au embedded in mesoporous silica (mp-Au@SiO2) using a quaternary ammonium salt bearing two long alkyl chains and a triethoxy fragment at the other end. The role of the alkyl chains is to favor the interaction with the templating agent (cetyltrimethylammonium bromide (CTAB)) used for MCM-41 preparation. The growth of MCM-41 was then assessed using typical procedures and employing tetraethoxysilane (TEOS) as a silica source. Transmission electron microscopy (TEM) analysis demonstrated the good dispersion of Au particles on the support, in particular inside the hexagonal channels, without evidences of a segregation of Au or SiO2 phases. The materials with 2.5 wt % of Au content were tested for the aerobic oxidation of primary and secondary alcohols and compared with a traditional Au/SiO2 catalyst prepared by impregnation. When the oxidation was carried out in water, a remarkable deactivation was observed for both materials. This phenomenon was probably due to a collapse in the pore structure caused by the aqueous basic medium. However, when the oxidation of 1-phenylethanol was performed under solventless conditions, very good results were obtained with the encapsulated mp-Au@SiO2 catalyst, whereas the impregnated sample showed remarkably lower conversion and selectivity. Moreover, the recovered embedded catalyst was reused without any loss in activity, whereas the impregnated catalyst was totally inactivated after the first run.
The preparation method was improved by replacing the amine functionality with thiols by Budroni et al.24 The major advantage was the synthesis of smaller particles with narrower size distributions. Mixed-monolayer protected Au particles were prepared by using a 13:1 mixture of 1-dodecanthiol (DT) and (3-mercaptopropyl)trimethoxysilane (MPMS). DT acted as a spacer to avoid total occlusion of the Au particles inside the SiO2, whereas the role of MPMS was to help the condensation of TEOS around the Au particles. After removing the organic moiety by calcination in air at 450 °C, the average dimension of gold particles showed a very small increase (from 2.9 to 3.5 nm). This demonstrated the efficacy of the inorganic barrier in preventing high temperature sintering. After mild in situ activation, the material exhibited remarkable activity for CO oxidation, at least one order of magnitude higher than similarly prepared Au/SiO2 catalysts.
Arnal et al.39 synthesized rather large citrate-stabilized gold particles (15–17 nm), which were subsequently encapsulated into hollow ZrO2 spheres in three steps. First, a silica shell of variable thickness was formed around citrate-stabilized particles by adequately tuning the hydrolysis process of TEOS. The process, which is based on the typical Stöber preparation of silica spheres, was optimized to obtain silica shells with homogeneous diameters. TEM analysis showed that approximately 95 % of single gold particles were encapsulated in the center of the SiO2 spheres. The Au@SiO2 system was then coated with a thin layer of zirconia by using zirconium butoxide as an oxide precursor and a surfactant as a structure-directing agent. A calcination step at 900 °C produced Au@SiO2@ZrO2 without a significant increase in the dimensions of the encapsulated gold particles. Finally, the SiO2 layer was removed by etching in NaOH solution and the gold particles were found to lie on the walls of the very thin (about 20 nm) ZrO2 layer. Interestingly, this system showed remarkable activity for CO oxidation, despite gold usually being active only with particles smaller than about 5 nm. This effect was associated with the peculiar faceting of gold particles generated during high-temperature calcination steps. Furthermore, the thermal stability of the as-prepared and calcined (800 °C) Au@ZrO2 system is significantly higher when compared with the same sample crushed under very high pressures (1 GPa), in which thin zirconia shells are broken and gold particles are able to move outside their shells to sinter. This last observation well highlights the advantage of using a protective oxide shell (even very thin) to limit the aggregation of particles after harsh thermal treatments.
Shevchenko et al.40 reported the preparation of Au particles embedded into a Fe2O3 shell (Figure 2). The strategy involved the preparation of dodecanthiol-protected Au particles and their subsequent use as seeds to grow the magnetite layer around them. Fe2O3 coating was achieved by decomposition of Fe(CO)5 in octadecene, catalyzed by the same Au particles, and the following oxidation of Fe by means of the Kirkendall effect. The growth of the oxide layer was controlled by using a combination of organic capping ligands such as oleylamine and oleic acid. Interesting magnetic properties of the assemblies were observed, while no catalytic data have been reported so far.
Zhong et al.41 also used porous Fe2O3 as support but prepared Au catalysts in which small Au particles were inserted inside the pores. Fe(NO3)3⋅9H2O and tetraethylammonium hydroxide (TEAOH) were adopted as metal precursor and precipitant/structure directing agent, respectively. The subsequent hydrothermal treatment allowed the preparation of FeO(OH) nanorods, which were then calcined at 300 °C to obtain porous α-Fe2O3 materials whose pore size was in the range of 1–5 nm.
The deposition of Au particles inside the pores was achieved by a one-pot reaction. The particles were first formed by reduction with NaBH4 in the presence of lysine as capping agent. Then they were adsorbed without any further treatment onto porous α-Fe2O3 materials. The deposition of particles inside the pores was facilitated using sonication during the preparation. TEM analysis confirmed the presence of Au particles located inside the pores; however Brunauer-Emmett-Teller (BET) surface area analysis also confirmed that some pores were blocked by the occurrence of half-buried particles in the channels. Despite this finding, the catalyst showed very good performance in CO oxidation, with a complete removal of CO in air at 30 °C for the Au(3 %)/α-Fe2O3-nanorod catalyst. More importantly, TEM measurements suggested that no sintering was observed in the aged catalysts after the reaction.
SnO2 was chosen as a protective shell by Yu et al.42 Au particles were prepared to be used as seeds for the subsequent reduction of SnCl2 to form intermetallic AuSn nanoparticles. By heat treatment, Sn was selectively oxidized to form Au@SnO2 materials. Au particles (ca. 10 nm) surrounded by a thin layer of SnO2 (ca. 6–7 nm) were obtained. Some cracks, observed in the SnO2 layer, which were probably formed during the selective oxidation step, were responsible for the accessibility of the Au surface to the reactant molecules. The final materials showed high thermal stability after calcination at 850 °C and promising catalytic activity in CO oxidation. The authors addressed the high catalytic activity to confinement effects. In fact, Au particles, with similar dimensions and just deposited onto SnO2, showed much lower catalytic activity.
A physical method was described by Sinha et al. to partially cover Au nanoparticles in a thin layer of MnO243 and to prepare a catalyst for volatile organic compounds (VOCs) removal. A high-surface area mesoporous γ-MnO2 support was prepared through a surfactant-assisted wet-chemistry route, which involved the precipitation of MnII, by using NaOH in the presence of CTAB. Using the vacuum ultraviolet radiation-assisted laser ablation method, they formed small Au nanoparticles partly buried under the surface of the support. The TEM investigation confirmed the presence of small Au particles (3-6 nm) embedded into the MnO2 lattice. X-ray absorption fine structure (XANES) and X-ray photoelectron spectroscopy (XPS) data confirmed the presence of metallic Au, although binding energies were slightly shifted as a result of the strong metal-support interaction obtained during the deposition process. During the experiments of VOCs elimination, γ-MnO2/Au showed a 95 % toluene removal and 30 % n-hexane removal, that is, enhancement of about 2-fold and 15-fold after Au deposition (2.8 wt %). Moreover, an even increase in conversion was obtained by raising the temperature up to 85 °C, with 99 % toluene removal and 77 % n-hexane removal.
Palladium is a widely used metal in catalysis both in oxidation and hydrogenation reactions, in CC bond-forming reactions, in hydrogen-purification reactions and more. However, it is relatively easily reduced and oxidized and thus its stabilization in high temperature environments is rather difficult.
As already described for Au (§ Gold), silica has been extensively used as a support also to encapsulate Pd particles. Among some strategies, the use of microemulsions emerged as particularly attractive to obtain the encapsulation of metal particles into this oxide.
Xue et al.44 prepared a Pd-Cu-O/SiO2 catalyst in which Pd–Cu–O cores were protected by a silica layer. The preparation involved the dissolution of PdII and CuII salts in the aqueous phase of the microemulsion (also containing ammonia as a hydrolysis catalyst) and the subsequent hydrolysis of TEOS. The formation of core-shell structures after calcination in air at 700 °C was observed with TEM indicating the presence of Pd–Cu–O cores surrounded by silica shells of 50–150 nm. In the oxidative carbonylation of phenol, this material showed superior activity compared to traditional systems prepared by classical impregnation and sol-gel methods, and also remarkable increased stability in recycling.
More recently, Park et al. presented a similar approach to prepare highly sintering-resistant Pd@SiO2 catalysts45 by using two different silicon alkoxides as support precursors: TEOS and n-octadecyl trimethoxysilane as pore forming agents. Pd particles were first prepared in the aqueous droplets of the microemulsion by reduction of Pd(NO3)2 with hydrazine, then the mixture of Si alkoxides was hydrolyzed around the preformed metal particles. Despite the use of very high metal loading (6.12 wt %), Pd cores were quite small (4.2±2.0 nm) and surrounded by a uniform, porous SiO2 shell approximately 10 nm thick. A Pd/SiO2 with the same metal loading was prepared by impregnation. After high-temperature calcination at 700 °C, the impregnated sample showed noticeable agglomeration of Pd particles, whereas the embedded catalyst evidenced only a slight increase in Pd particle size (Figure 3).
The fresh samples (before calcination) exhibited very different activity, with the impregnated sample being much more active than the embedded one. However, after calcination, in agreement with the increase of particle size, the impregnated system evidenced a strong deactivation, whereas Pd@SiO2 showed negligible loss of activity. Moreover, in acetylene hydrogenation, the fresh embedded catalyst already showed higher activity compared with the impregnated system.
Wada et al. proposed a more complex method to perform the encapsulation of Pd nanoparticles into SiO2 or mixed SiO2–TiO2 materials.46 PdII ions were coordinated by a silsesquioxane moiety and subjected to direct calcination at 550 °C or to adsorption onto TiO2 and calcination at the same temperature as for the pure SiO2 support. The mixed oxides catalysts exhibited excellent activity for the aerobic oxidation of benzyl alcohol in water. Furthermore, the thin silica layer acted as a barrier to prevent the growth of the Pd particles, as evidenced by TEM analysis.
Li et al. reported the preparation of Pd nanoparticles inside the channels of SBA-15.47 The synthesis is based on the adsorption of a cationic Pd precursor at pH 8-11 onto the negatively charged surface of non-calcined SBA-15, followed by calcination at 550 °C and reduction of PdII in H2 at 300 °C. Clearly, the pH of the solution played a key role in the adsorption of the Pd precursor onto the support. An impregnated catalyst was prepared in a similar way but by using a calcined SBA-15 support, where the effect of the pH is less important. In fact, TEM analysis showed the presence of small Pd particles (3.3–3.9 nm) in the embedded catalyst located inside the pores, whereas bigger particles (ca. 10 nm) were found in the impregnated sample on the outer surface of SBA-15. Even by changing the metal loading from 0.7 to 2.2 wt %, good dispersions of the Pd particles were obtained, and in the most concentrated sample, all the particles seemed to be located inside the channels. CO chemisorption experiments confirmed the accessibility of Pd particles. Accordingly, the embedded sample displayed a superior activity in the aerobic oxidation of benzyl alcohol under solvent-free conditions when compared to the corresponding impregnated sample. Interestingly, the embedded sample with a Pd content of 1.3 wt % showed the best results, whereas an increase in the Pd content resulted in a decrease of the conversion, probably due to diffusional problems of reactants into the SBA-15 channels.
Similarly, Wang et al.48 prepared Pd nanoparticles inside SBA-15 channels. The nanoparticles were prepared by using a block copolymer as a capping agent and formalin as a reducing agent. The copolymer was then used as a templating agent for SBA-15 preparation. Pd particles showed dimensions in the range of 6–10 nm and were located inside the silica channels. As a result, the channels were slightly enlarged by the presence of embedded Pd particles but their ordered structure was still maintained and, moreover, there was a decrease in the surface area caused by the incorporation of Pd. Nevertheless, particles were accessible to the reactants, and the materials exhibited excellent activities and selectivities for the Heck CC coupling reactions and high stability due to the low degree of Pd leaching.
Qian et al. reported the formation of SiO2 nanotubes filled with high loadings of fine metal particles.49 The preparation method involved the initial formation of pure ultralong carbonaceous nanofibers by hydrothermal carbon coating of preformed Te nanowires and the subsequent dissolution of the Te core. The carbon fibers thus obtained were strongly reductive due to the presence of residual CO and OH groups on their surface. Therefore, the simple mixing of the fibers with noble metal (Pd, Pt, Au) precursors produced the formation of metal particles preferably entrapped inside the channels of carbon tubes. Interestingly, while Pd and Pt formed nanoparticles located inside the pores, Au formed also structures onto the outer surface of the fibers, creating what the authors called “golden fleece”. The dimension of the particles was in the range 7–15 nm and can be tuned by changing the reaction conditions (such as metal loading and temperature). The tubes may then be covered by a layer of amorphous silica through a sol-gel procedure, with carbon fibers acting as a templating agent, and the carbon portion removed by calcination in air at 550 °C. The reduction of the metal phase was obtained through alcoholthermal treatment at 120 °C, with the obtainment of metal particles embedded into the silica nanotubes. The authors proved that the precursor noble metal/carbon systems are able to oxidize CO, with the Pt/C system being the most effective, completely depleting CO at 160 °C. Moreover, the Pd/C system, subjected to recycling runs, showed good thermal stability.
An interesting approach to Pd@SiO2 materials was provided by Budroni et al.50 They first prepared Pd nanoparticles protected by a mixture of an alkyl thiol and another one bearing a triethoxysilane moiety at one end. This last functionality was used to condense SiO2 precursors around the particles to yield a sponge-like silica framework with embedded Pd particles inside. The materials showed significant catalytic activity in the Suzuki—Miyaura coupling of electron-rich aryl bromides. Furthermore, it was possible to reuse the catalyst with only a gradual decrease in the catalytic activity.
The so-called evaporation induced self-assembly (EISA) technique was used by Hampsey et al. to produce spherical SiO2 particles with entrapped metal nanoparticles.51 The method used a solution of TEOS, a surfactant, HCl, and a metal salt that was sprayed and quickly evaporated using an aerosol-assisted method. The rapid hydrolysis of TEOS and the co-assembly of silica into ordered mesoporous structures incorporated metal precursors, and after calcination and reduction, the materials were found to present metallic particles embedded into the silica spheres. By changing reaction parameters, it was possible to obtain different structures and to tune the metal loading in the materials. Catalytic activity of the samples were assessed in the hydrodechlorination reaction of 1,2-dichloroethane with 100 % conversion at 350 °C.
Similarly, Cortial et al.52 used the EISA technique to encapsulate Pd and Au inside SiO2 and TiO2 mesoporous structures but starting from the preformed metal nanoparticles. Their materials proved to be catalytically more active than the starting nanoparticles for oxidation (Au@TiO2) and allylic amination (Pd@SiO2) reactions. This particular reactivity was addressed to confinement effects.
Only few studies deal with Pd/PdOx encapsulated in oxides other than silica. The microemulsion approach to prepare Pd embedded into ZrO2, TiO2, or Al2O3 was described by Kim et al.53 Preformed Pd particles were formed in the aqueous phase of the microemulsion by reducing PdII precursor with hydrazine. The support precursor, an appropriate alkoxide (zirconium or titanium butoxide or aluminium isopropoxide), was then hydrolyzed in the micelles at the oil/water interface, with surfactant molecules acting as pore-forming agents. Mild calcination led to the obtainment of regular spheres of the corresponding oxides with encapsulated small Pd particles (ca. 3 nm). These samples showed higher catalytic activity than standard impregnated systems in the hydrogenation of carbon monoxide.
An electrochemical method was employed by Reetz et al. to prepare Pd particles, which were embedded into a mixed magnesium–silicon oxide.54 Alkoxide precursors of the support were hydrolyzed in the presence of alkylammonium-protected Pd particles to encapsulate them in the gel.
The organic products were extracted using ethanol, and the material thus obtained was characterized by TEM, diffuse reflectance infrared fourier transmission spectroscopy (DRIFTS), and CO chemisorption, which suggested the effective encapsulation of the particles. The hydrogenation of 1,5-cyclooctadiene to cyclooctene was chosen as a test reaction. The embedded catalyst exhibited higher activity and selectivity compared to a commercial Pd/Al2O3 sample.
Kwon et al.55 used silicon, titanium, or aluminium alkoxides to synthesize, through a sol-gel-type process, embedded Pd materials to be used as heterogeneous catalysts for organic reactions. A palladium precursor was reduced to Pd0 in ethylene glycol in the presence of an appropriate alkoxide precursor. TEM analysis indicated that small particles (2–5 nm) were dispersed in the network formed by the hydrolyzed support. The material did not need any further treatment and exhibited high activity in a broad spectrum of organic reactions, such as hydrogenations of alkenes and alkynes, aerobic oxidation of alcohols, and selective α-alkylation of ketones with alcohols. Furthermore, it could be simply removed by filtration and it was reusable without loss of activity.
Platinum particles embedded in silica-type materials have been widely reported in the literature. Small Pt nanoparticles, protected by polymers and with narrow size dispersion and specific shape, were embedded in the channels of SBA-15 by a controlled hydrolyzing silica precursor by Somorjai et al.56 The hydrogenolysis of ethane was found to be strongly dependent on the sizes of the particles: higher activity was observed for smaller particles (ca. 1 nm). On the other hand, ethylene hydrogenation was insensitive to particle size.
Employing a similar strategy,57 Pt nanoparticles were synthesized through the reduction of a PtII salt in the presence of a triblock copolymer. The polymer was also used as a templating agent for the formation of ordered channels of SBA-15 around preformed metal particles via hydrolysis of TEOS. Although particles located inside the channels were accessible to H2 in chemisorptions experiments, it was found that the activity of the catalyst in toluene hydrogenation was lower than that obtained by a similar sample prepared by classical impregnation. This was related to the encapsulation process, which led to a coverage of part of the Pt by silica and thus to its partial inaccessibility.
Mastalir et al.58 prepared Pt-doped MCM-41 using the cationic surfactant CTAB, which can act both as structure directing agent in MCM-41 formation and as a coordinating agent for the complexation of PtCl42− species in solution. Organic residues were removed by extraction and TEM images of the dried material showed that only 2.5 % of the Pt particles were over 3 nm and mostly located inside the channels. The catalyst exhibited good activity in liquid phase hydrogenation of various alkenes, although diffusion problems were observed. In addition, the material showed good selectivity for hydrogenations of alkynes to alkenes.
A very recent approach was provided by Joo et al.59 to produce Pt particles encapsulated in silica spheres. The proposed method consisted in the preparation of Pt particles protected by a surfactant as a structure-directing agent and subsequent silica formation around Pt particles. The role of the surfactant was to define the porous structure of the surrounding silica layer so that the trapped Pt nanoparticles would be completely accessible to the reactant molecules. The system presented high thermal stability, with Pt particles showing no agglomeration even after calcination in air at 750 °C. The fact that encapsulated Pt particles exhibited an activity for CO oxidation and ethylene hydrogenation similar to bulk Pt demonstrated that the accessibility of the metal to the reactants was assured.
In a different approach, Ikeda et al.60 covered PVP-protected Pt nanoparticles with a double shell of silica and mesoporous silica. After calcination at 550 °C, silica-encapsulated Pt nanoparticles were obtained. This material was impregnated with a carbonaceous source, heated at 900 °C under vacuum, and finally the silica was etched in HF to obtain Pt nanoparticles embedded in hollow carbon layers. The size of the Pt particles increased from 1.8 to 2.2 nm after thermal treatments and the final material showed extremely high surface area (1800 m2 g−1), in accordance with its carbonaceous nature. Significantly, it resulted very active for hydrogenation reactions and its activity was completely maintained even after reuse.
An interesting modification of this route, based on the use of titania as a template instead of silica, was proposed by Ng et al.61 Pt and phenol were deposited onto titania powder by means of photoirradiation to produce Pt nanoparticles surrounded by a phenolic polymer. Titania was then removed by treatment in HF to leave Pt nanoparticles encapsulated in the hollow carbon spheres. Also in this case, this material was active for the hydrogenation of alkenes.
Ikeda et al.62 employed a microemulsion technique to obtain the encapsulation of Pt into Al2O3. The addition of quaternary ammonium salts was used both for the complexation of Pt precursors and as templating agents. After calcination and reduction, Pt particles embedded into an Al2O3 support showed higher catalytic activity in the NO+CO reaction compared to a sample prepared by a standard incipient wetness impregnation method.
Pt nanoparticles embedded in a barium hexaaluminate matrix were prepared through a microemulsion technique63 and tested in methane partial oxidation reaction showing excellent stability. Unfortunately, the major drawback of this system is the rather high metal loading required (ca. 8 wt %), which precludes its utilization in a real industrial process.
Wang et al.64 reported the preparation of Pt nanoparticles entrapped into the pores of TiO2 films. The synthesis involved a combined photo- and sonochemical method to photodeposit Pt in the presence of methane on the surface and in the inner pores of the film. Sonication significantly increased the penetration of PtII precursors inside the pores. The obtained film exhibited good activity and stability in CO oxidation using an excess of O2. The film was also found to be active in the depletion of bacteria cells.
Yeung et al.65 used microemulsion to deeply encapsulate Pt into CeO2. The particles were not preformed, but a precipitation inside the micelles of Pt(OH)x and Ce(OH)x species, the subsequent aging of the mixture, and final calcination allowed to obtain small metal particles completely buried inside ceria. CO chemisorption data indicated the total inaccessibility of the active metallic phase even if metal loadings used were quite high (typically 5 wt %). Despite this fact, the catalysts were active under water–gas shift reaction conditions. However, while conventional Pt/CeO2 catalysts showed the tendency to form CH4 as a sideproduct of this reaction, the catalyst prepared by microemulsion did not. This unexpected behavior was explained through an electronic promotion of the CeO2 shell by the encapsulated Pt particles, whereas the high barrier for methane formation reaction was attributed to the absence of exposed active sites on ceria. These aspects need more investigation, in connection with structural characterization.
Ng et al.66 recently described the preparation of Pt nanoparticles embedded in microporous hollow carbon shells fabricated by using a photocatalytic approach. A titania powder was used as carrier and on its surface, PtIV precursor and phenol were adsorbed. By UV-light irradiation of the suspension, photogenerated electrons and holes in TiO2 caused reduction of PtIV and oxidation of phenol, with the formation of a phenolic polymer adsorbed onto titania powder with Pt clusters entrapped in the organic matrix. Carbonization in vacuum at 700 °C and subsequent dissolution of TiO2 formed hollow carbon spheres (thickness 3–5 nm) in which small Pt nanoparticles (ca. 3 nm) were entrapped (Figure 4). High temperature treatments resulted in only small changes in the average dimension of Pt particles, while a Pt/C sample prepared by impregnation showed, under the same conditions, large sintering of Pt. The catalytic activity of the samples was assessed in hydrogenation of 1-hexene. Higher turnover frequencies (TOFs) were obtained for the embedded sample with respect to the impregnated catalyst. Furthermore, after thermal treatments up to 700 °C, the activity of the embedded catalyst was maintained, whereas that of the impregnated sample dropped significantly.
Wen et al.67 presented a procedure that used Pt@SiO2 as a template for the preparation of Pt@C structures. Pt nanoparticles were formed by reduction with glucose inside the channels of SBA-15. Glucose was then polymerized around the particles and onto the walls and surface of SBA-15. The subsequent carbonization and dissolution of the silica template yielded the desired encapsulation of Pt particles into a carbonaceous layer. Even though the typical ordering of SBA-15 channels was not maintained after thermal treatments, Pt particles did not show any sign of agglomeration and the carbon layer showed a good degree of mesoporosity. The final Pt@C system showed high activity and stability for methanol-tolerant oxygen electroreduction, which is an important process in low-temperature fuel-cell systems.
Kishida et al. encapsulated Rh nanoparticles into a SiO2 matrix by a microemulsion approach68 and investigated the effects of hydrolysis conditions of TEOS, used as precursor for silica, on the properties of the catalysts. According to the synthesis conditions, rhodium particles can be partly or completely embedded in SiO2, with consequent significant differences in catalytic activity and stability. For example, CO hydrogenation for the production of oxygenated compounds (i.e., ethanol, acetaldehyde, acetic acid), a well-known structure-sensitive reaction, was affected by physical features of the catalyst due to the specificity of the Rh particles surfaces in contact with the gas phase.
Similarly, Rh@SiO2 systems with Rh particle size tuned in the range of 2–14 nm and a narrow size distribution were reported by Tago et al.69 Notably, the change of the metal loading did not influence appreciably the metal particle size.
Rh nanoparticles embedded into AlO(OH) nanofibers were prepared by Park et al.70 through the reduction of rhodium chloride in butanol, followed by hydrolysis of Al(sec-ButO)3. After aging and drying, well-dispersed metal nanoparticles entrapped into the oxo-hydroxo matrix were obtained. These catalysts showed good activity in hydrogenation of arenes and ketones at low temperatures (25–75 °C). An important advantage of these systems is represented by the easy separation of the metal phase from the organic products and its reuse without an appreciable loss of activity. Since the catalysts were not subjected to high temperature treatments, which can result in metal sintering, a possible catalytic loss could be due to the leaching of the metal nanoparticles.
A simple and low-cost strategy was designed by our group for the synthesis of efficient and stable embedded Rh-based catalysts to be used in methane partial oxidation and ethanol steam reforming.25–27 In particular, Rh@Al2O3 systems with low metal loadings (ca. 1 wt %) and high surface areas (100 m2 g−1) were prepared by a precipitation procedure composed of two steps. First, a stable suspension of protected metal nanoparticles was prepared using HEAC16Br (N-hexadecyl-N-(2-hydroxyethyl)-N,N-dimethyl ammonium bromide) as surfactant and NaBH4 as reducing agent according to the method reported by Schulz et al.71 The role of the cationic surfactant is to modulate the particle size, to prevent their aggregation, and to control the encapsulation of the preformed metal particles, which represents the second part of the synthesis. During this phase, the growth of the porous oxide layers around the metal particles also takes place (Figure 5).
The embedded Rh catalysts proved to be thermally more stable under methane partial oxidation conditions than a reference catalyst prepared by conventional incipient wetness impregnation.25, 26 The significant improvement was correlated to the protection offered to the active metal phase by the surrounding layer of aluminum oxide, which prevented extensive metal sintering. Moreover, the partial deactivation, observed in the embedded system after prolonged aging at high temperature, was reversible as it was essentially due to coke deposition. This fact offered the possibility of catalyst regeneration with simple oxidative treatment. On the contrary, the effectiveness of a similar protocol was significantly lowered in the case of a conventional impregnated catalyst, in which (besides coke deposition), the sinterization of the metal phase and/or the incorporation of Rh into the Al2O3 lattice were the main causes of deactivation. Remarkably high activity and stability were also observed on the embedded systems by decreasing the metal loading to 0.5–0.25 wt %.28 Finally, the proposed method has strong flexibility offering the possibility to modulate the nature of the support and its texture and the inclusion of extra components (e.g., ceria-based mixed oxides as promoters) in the catalyst formulation. Indeed, Rh@CexZr1−xO2-Al2O3 nanocomposites were successfully prepared and showed high activity and stability in ethanol steam reforming.26, 27 In addition to the benefits of the embedding strategy, there was a further improvement related to the introduction of the CeXZr1−XO2 component, which provided reactive oxygen contributing to prevent deactivation.
Recently, it was proposed by Harada et al.72 the fabrication of Rh nanoparticles encapsulated in a hollow porous carbon shell having well-developed porosity. Rh nanoparticles protected with polyvinylpyrrolidone (PVP) and with a narrow size distribution centered at about 2.8 nm, were mixed in ethanol with TEOS to form Rh@SiO2. Subsequently, the silica-covered Rh nanoparticles were treated with phenol-formaldehyde resin as a carbon source. The catalyst prepared in this way (Figure 6) showed high activity for the hydrogenation of various aromatic and unsaturated heterocyclic rings in water. The porous structure of the carbon shell guaranteed channels for the efficient mass transfer of species into a hydrophobic void space where Rh nanoparticles were located. Since it is possible to tune the porosity inside the shell, the core-shell strategy is promising for designing catalysts with molecular shape-selective properties towards conversion of organics in general.
Interestingly, Pittelkow et al.73 reported the use of a chiral poly(amidoamine) dendrimer as a molecular scaffold for the encapsulation of Rh metal particles. The resulting particles formed by the reduction of the dendrimer-metal precursor complexes had a mean diameter of 1.7 nm. Depending on the size of the dendrimer, it was possible to obtain well-defined particles of nanometer dimensions. Even if no applications in catalysis are present in literature yet, this procedure provides an attractive route for designing catalysts.
Core-shell materials with Ag as the core are widely reported in literature, especially when the metal is encapsulated in silicon and titanium oxides. Different composite core-shell materials were obtained, such as Ag/C/TiO2 composites,74 Ag@TiO2 nanoparticles75 and nanowires,76 Ag@ZrO2,77 reverse SiO2/Ag systems,78 or reverse FeOOH/Ag composites.79 However, their applications in catalysis are still rather scarce.
Pastoriza-Santos et al.80 described the coating of Ag nanoparticles with a thin TiO2 layer in a one-step method. The preparation was a combination of two sequential processes that occurred consecutively in a single reaction mixture. AgI ions were reduced by N,N-dimethylformamide, while a modified titanium butoxide (by means of acetylacetone) was slowly hydrolyzed around the forming Ag particles. This method produced Ag particles with bimodal size distribution (4 and 30 nm), homogeneously coated with a very thin (1-2 nm) layer of amorphous TiO2 (Figure 7). Furthermore, stratified films of these core-shell nanoparticles could be realized through layer-by-layer deposition with a polyelectrolyte. The deposition of Ag@TiO2 is pH dependent and the best ordering was obtained at pH 2.0 with the particles forming fairly closely packed films. The position of the surface plasmon band in the deposited layers remained unchanged with respect to that observed in solution, which confirmed the insulation between Ag cores provided by the presence of the TiO2 layer.
Hirakawa et al.81 presented the preparation of Ag@TiO2 and Ag@SiO2 by simultaneous reduction of AgI to Ag particles and hydrolysis of a modified titanium alkoxide. The titanium complex contained a triethanolaminato ligand, which was able to coordinate Ag(0) formed during the reduction process, thus furnishing the driving force for the obtainment of the core-shell structures. The reduction was carried out using dimethylformamide as solvent and reducing agent. The conditions were optimized to have small particles without aggregation of the metal phase. In the same way, Ag@SiO2 was obtained by replacing titanium precursor with a silica precursor. Ag particles mostly showed small dimensions (3–4 nm), though some larger agglomerates (30–65 nm) were also formed. However, such large agglomerates showed the presence of a thin layer of TiO2 around the metallic Ag core. The authors found that when the concentration of the starting Ti precursor was increased, independent TiO2 particles without Ag cores were obtained. This suggests the synergic effect in the reduction–hydrolyzation process between the growing Ag cores and TiO2 shells. Both Ag@TiO2 and Ag@SiO2 systems exhibited a strong absorption in the visible region due to the surface plasmon band of the Ag core. Photocatalytic tests carried out in the reduction of C60 to C60⋅−, following laser pulse excitation at 308 nm, demonstrated a lower activity for the Ag@TiO2 system compared to bare TiO2. Although metal core-semiconductor shell structures are quite efficient for storing photogenerated electrons, their ability to catalyze a reduction process is limited.
This approach was then recently modified by Awazu et al.82 The presence of a direct contact between metallic Ag and TiO2 might lead to the oxidation of the metal with the contextual creation of a thin layer of AgO around Ag core, responsible for the low photocatalytic activity of the Ag@TiO2 samples. Therefore, a very thin layer (3–4 nm) of SiO2 around the Ag particles was first created to prevent their oxidation, and then a thicker TiO2 layer was formed in order to take advantage of the enhanced near-field amplitudes of localized surface plasmon. The authors called this innovative approach “plasmonic photocatalysis”. In this case, the presence of SiO2 greatly enhanced photocatalytic activity of the Ag/SiO2/TiO2 system with respect to bare TiO2 in the decomposition of methylene blue. The authors attributed the enhanced activity to the localization of plasmon resonance and also to the delay in the recombination between electron and hole pairs formed after irradiation. Furthermore, the effect of SiO2 coating of different thicknesses on the photocatalytic activity was examined. There was a decrease in the activity with an increase in the SiO2 thickness, thus demonstrating that the effect obtained with thin silica coatings was due to the localized surface plasmon resonance from the Ag nanoparticles.
Recently, Wang et al.83 prepared a Ag/anatase TiO2 core-shell system for photocatalytic applications. The synthesis involved the preparation of Ag nanoparticles by using 1-dodecylamine as a capping agent and their coating with a TiO2 layer by a solvothermal procedure. By varying the temperature during the solvothermal procedure, it was possible to obtain first the particles and then to crystallize the TiO2 species around them. Ag@TiO2 showed a slight increase in the photocatalytic activity for the degradation of alizarin red when compared to bare TiO2.
FeIII-doped Ag@TiO2 core-shell systems were synthesized by Wang et al.84 The preparation of Ag nanoparticles was first accomplished by reduction of AgI in the presence of CTAB as a capping agent. Then, the FeIII precursor was added, followed by titanium tetraisopropoxide as the TiO2 source which was hydrolyzed around the Ag cores. The final system comprised of 15 nm Ag particles coated by a 10 nm thick TiO2 layer doped with FeIII cations. The Ag@Fe-TiO2 showed a red-shift in its absorption spectrum and, moreover, FeIII doping resulted in an enhanced colloidal stability of the systems as prepared. An optimal ratio between the components (1 wt % Ag @ 0.5 % Fe-TiO2) was found to result in the best activities in the photodegradation of methyl orange.
Nickel based catalysts are widely used in many important industrial processes, such as steam reforming or hydrogenation reactions. Unfortunately, the main problem with Ni is its ease of deactivation induced by carbon formation and metal sintering.85 Therefore, several attempts have been made to improve the catalytic performance of Ni catalysts by reducing coke deposition tendency and increasing the thermal stability of the metal phase. In this respect, encapsulation of Ni particles in a suitable and porous oxide can offer a valuable alternative to the usual strategy, which uses high metal loadings to compensate the deactivation phenomena.
Various methods of synthesis of stable suspension of Ni nanoparticles were published in literature.86 Here, we mostly focus our attention to the synthesis of embedded Ni nanoparticles with potential applications in catalysis.
An interesting approach for the preparation of Ni nanoparticles embedded in montmorillonite clays was proposed by Ayyappan et al.87 Ni nanoparticles, synthesized via in situ reduction of NiII acetate precursor in ethylene glycol within the montmorillonite, according to the procedure previously described by Malla et al.,88 had a diameter between 8 and 45 nm. Chemical analysis revealed that the maximum amount of intercalated Ni(metal) is approximately 10 wt %. However, no applications in catalysis were reported.
Carreño et al.89 compared the catalytic properties of Ni nanoparticles embedded in silica matrix with conventional impregnated samples in the methane reforming reaction. The nanocomposite samples were prepared using TEOS, as a source of silica, and nickel nitrate hexahydrate in the presence of citric acid and ethylene glycol. The nanocomposite catalysts exhibited higher catalytic activity and stability than the conventional impregnated samples because the embedding approach significantly reduced both carbon deposition and metal sintering. The same research group described a novel chemical route for obtaining highly dispersed nanometric Ni particles embedded in different matrices based on Al2O3, MgO, and TiO2 and in the heterogeneous matrices CeO2-doped Al2O3 and MgO-doped Al2O3.90 The Ni nanoparticles (in the range of 1–40 nm) were obtained in a single process, without the use of an external reducing agent.
Nanosized Ni metallic particles encapsulated in silica and carbon aerogels were presented by Martínez et al.91 The attractive advantage of the proposed method is that no particular reduction step is required to obtain metallic particles of Ni. However, the samples tested towards CC coupling reactions did not show appreciable catalytic activity. Unfortunately, the origin of their catalytic inert behavior was not discussed.
An effective approach for dispersing NiO onto ordered mesoporous silica (SBA-15) was proposed by Park et al.92 In this study, polyethylene oxide (PEO) was used with the purpose of NiII pre-encapsulation, in order to keep the method as simple as possible and to overcome limitations in the NiO loading (Figure 8). It was suggested that the attraction between the templating agent and NiII ions implicated mainly a crown-ether type conformation of the free PEO. Furthermore, it was also demonstrated that the presence of both NiII ions and the encapsulating agent (PEO) did not interfere with the self-assembly route of SBA-15, but led to the rapid and easy deposition of NiO on the mesostructured silica. This material showed a higher catalytic activity in the hydrodechlorination (HDC) of chlorobenzene to benzene with respect to the corresponding sample prepared by the wetness impregnation method.
Lee et al.93 embedded Ni particles within a three-dimensional mesoporous SBA-15. The material thus obtained exhibited higher activity, lower coke formation, and higher thermal stability in the temperature range of 600–800 °C during CO2 reforming of methane, compared with those of conventional Ni supported on SiO2 and γ-Al2O3.
Silica-coated Ni catalysts were synthesized using the water-in-oil microemulsion technique by Takenaka et al.94 The material presented significantly higher and more stable activity in methane partial oxidation with respect to a conventional supported Ni catalyst. The superior performances were ascribed to a change in the chemical property of Ni metal due to its strong interaction with the surrounding silica, which prevented metal sintering. Furthermore, in embedded Ni catalyst, there was limited room for coke formation.
Highly active Ni nanoparticles embedded into TiO2/SiO2 mesoporous mixed oxides were prepared by Zhang et al.95 in a one-pot process. The sample with NiO loading of 10 wt % and TiO2/SiO2 (w/w) of 50:50 showed the best performance in CO2 reforming of methane attributed to the strong metal-support interactions which prevented the nickel from sintering. The participation of a carbon species as an intermediate in CO2 reforming was proposed since high activity and stability were observed, although large amounts of coke were detected on the catalyst surface during the reaction. This peculiar behavior has also recently been reported for nickel–carbon nanocomposites developed by Carreño et al. under ethanol steam reforming conditions.96
Other inorganic embedded systems
An example of Ir embedded in CeO2 catalyst was recently reported by Huang et al.97 Coprecipitation in NaOH was used to obtain a mixed Ir(OH)x and Ce(OH)x phase which, during calcination at 400 °C, led to the formation of Ir species mostly embedded inside the support. Reduction in H2 caused the formation of metallic Ir. The catalyst showed high activity in preferential oxidation (PROX) of CO in the presence of H2, with a CO conversion of about 80 % and CO2 selectivity of about 70 %. When a mixture containing also CO2 and H2O was used, the activity of the catalyst dropped, but its activity and selectivity were still higher if compared to an impregnated Ir/CeO2 catalyst.
Yang et al.98 described a convenient one-step method for the synthesis of mesoporous silica, incorporating in the channels different metal oxide nanoparticles. The method was based on the hydrolysis of TEOS in the presence of a metal precursor (generally nitrates) and the block copolymer Pluronic 123 as a templating agent. In this way, Cr2O3, MnO, Fe2O3, Co3O4, NiO, CuO, ZnO, CdO, SnO2, and In2O3 particles inside silica channels were prepared. After calcination of the materials at 550 °C, the nanoparticles were found to be highly crystalline and uniform in diameter with rod-like shapes. This method was used by Reddy et al. to obtain cobalt species encapsulated inside SBA-15.99 The channels of the support accommodated Co3O4 particles without substantial loss of order, and temperature programmed reduction (TPR) experiments indicated a strong contact between the encapsulated nanoparticles and the support. A 2 wt % Co-based catalyst was used in the oxidation of cyclohexane to cyclohexanone, showing good activity (ca. 10 %) and selectivity (ca. 80 %), while a comparison sample prepared by an impregnation technique showed almost no activity.
The microemulsion technique was used by Hayashi et al. to synthesize iron/iron oxide particles embedded in SiO2.100 FeIII was precipitated inside the micelles as Fe(OH)3 by means of triethylamine and then, TEOS was added to encapsulate the formed Fe(OH)3 precipitate. The catalysts prepared with different iron loadings exhibited high activity and stability for CO hydrogenation. The improved performance with respect to the corresponding impregnated systems was ascribed to the presence of stable FeOx species during the reaction. Consistently, TPR experiments indicated the existence of FeOx species in the catalysts even after reduction in H2. Notably, the product distribution is strongly affected by the synthesis procedure. Indeed, whereas conventional catalysts produced mainly hydrocarbons, the embedded ones resulted in much higher yields of the C2+ oxygenates under all temperatures.
Martinez et al.101 reported the encapsulation of iron inside the channels of SBA-15. The preparation involved the co-condensation of iron (as FeCl3) and silica precursors (as TEOS) under acidic conditions by the templating effect of Pluronic 123. The final material, constituted by Fe2O3 crystallites (30–300 nm) dispersed inside the channels of SBA-15, was tested towards the photo-assisted degradation of phenol in the presence of hydrogen peroxide, and high total organic carbon (TOC) conversions were observed. Moreover, the catalyst behavior against iron leaching was also studied. The results indicated good stability of the iron phase in the encapsulated catalyst, with modest leaching under the highest catalyst and hydrogen peroxide concentrations.
Another interesting approach involving the encapsulation of iron oxide particles was described by Mori et al.102 The iron oxide nanoparticles were incorporated into titanium-modified hexagonal mesoporous silica (HMS) by means of two steps: first, a thin layer of silica was grown around the particles in order to facilitate the second step, the sol-gel polymerization of silicon and titanium alkoxides around the particles in the presence of 1-dodecylamine, to yield the FexOy@Ti-HMS system (Figure 9).
The authors found that the final materials possessed remarkable superparamagnetic properties. They also explored the potential catalytic ability of the FexOy@Ti-HMS sample in the oxidation of various organic compounds, such as styrene, cyclooctene, 2,6-di-tert-butyl phenol, and cyclohexane, with excellent results. Other advantages of FexOy@Ti-HMS are the easy recovery from the reaction mixture and the high reusability. Upon completion of the oxidation reaction, the magnetic properties of the system afforded a straightforward way of isolating the catalyst. In fact, by external application of a permanent magnet, the catalyst was easily recovered. It was recycled without significant loss of activity.
Neatu et al.103 proposed an alternative method to encapsulate Fe3O4 particles into silica. Iron oxide particles (12 nm average dimension) were first prepared by wet chemical methods, modified with cinchonidine and finally embedded in silica through a sol-gel method. The material was tested for the selective hydrogenolysis of bicyclo[2.2.2]oct-7-enes with good selectivity towards one of the possible products but without any enantioselectivity provided by the cinchonidine ligand.
Fe2O3 nanoparticles were embedded in montmorillonite by Kakuta et al.104 using different methods through which nanoparticles of distinct polymorphism (i.e., amorphous and α) were successfully fabricated. The activity in photocatalytic oxidation of water was then investigated. The use of clay as a support resulted in a more active and efficient material in comparison with the corresponding neat species. The preparation procedures and iron oxide polymorphs did not kinetically affect the reaction, while the best Fe loading was found to be 3 wt %.
Cu/CuOx nanoparticles embedded in TiO2 were recently synthesized by our group by using a simple microemulsion approach in which the preparation of a stable suspension of Cu metal nanoparticles was followed by hydrolysis and polycondensation of tetraisopropyl orthotitanate (Ti(iPrO)4).105 The activity of the catalyst thus obtained, was studied in the photocatalytic hydrogen production from methanol/water solution and compared with that of the conventional impregnated systems. The superior performances observed in the case of the embedded sample can be discussed in terms of a combination of several chemical/physical parameters. The experimental data suggested a better dispersion of the active Cu/CuOx species and a better metal–support interaction, which consequently influenced the electron/hole transfer mechanism. Moreover, it is reasonable to expect better protection against the adsorption of poisoning species during the reaction offered by TiO2 layers surrounding Cu metal particles.
Shanmugam et al.106 presented an easy single-step route to prepare MnxOy particles encapsulated in amorphous carbon. The material obtained by direct solid-state thermolysis of the cetyltrimethylammonium permanganate consisted of Mn3O4/MnOOH nanoparticles (with an average size of 9 nm) coated with amorphous carbon, which formed sheet-like structures in a two-dimensional fashion, when the reaction was carried out at 400 °C. Although in this work only the magnetic properties of the sample thus synthesized were evaluated, potential application in catalysis, as suggested by the same authors, can be explored.
The preparation of RuO2 nanoparticles embedded in a TiO2 matrix through the hydrolysis of ruthenium and titanium alkoxide mixture was described by Osman et al.107 Basic or neutral conditions led to powders consisting of 2–10 nm diameter crystalline RuO2 nanoparticles embedded in a matrix of crystalline (anatase) and amorphous TiO2. Acid hydrolysis conditions gave gels containing smaller, amorphous RuO2 nanoparticles (1-3 nm). Unfortunately, potential interesting catalytic applications were suggested but not investigated so far.
A set of Ce1−xZrxO2@ Al2O3 core-shell nanopowders with particle sizes <20 nm, were prepared by the research group of Laine in a single-step procedure via liquid-feed flame spray pyrolysis (LF-FSP).108 In NOx reduction and propane/propene oxidation processes, these materials showed activities which interestingly, approached those of traditional Pt containing systems but without the need for Pt as a cocatalyst. The formation of (Ce/Zr)3+ was suggested as responsible for their high catalytic activity.
Shiraishi et al. described the synthesis of TiO2 particles embedded in mesoporous silica by a surfactant-templating method starting from a TiO2 colloidal suspension.109 The final core-shell structures contained mesopores with widths of 2–5 nm and TiO2 cores of about 160–170 nm. The photocatalytic activity of theses materials towards several kinds of aromatic molecules in water showed a dependence on the polarity of the reactants, with higher conversions obtained for less polar compounds (i.e., benzene). This behavior was explained on the basis of the particular catalyst morphology which influenced the diffusion capacity of reactants into the pore of the catalysts and their subsequent reaction with short-lived hydroxyl radicals (⋅OH) formed at the surface of the inner TiO2 particles.
The entrapping of photosensitizers (i.e., metallophthalocyanines and triphenylpyrylium ion) in different inorganic supports, such as Y zeolite, mesoporous MCM-41, TiO2-SiO2, and SiO2 was recently reported by Cojocaru et al.110 A relevant decrease in the band-gap of these supramolecular ensembles was detected, which suggested the existence of a significant electronic interaction of the photosensitizer with the support. Furthermore, the formation of the supramolecular structure generated a stable environment, in which the oxidation state of the photosensitizer’s metal can be influenced by reaction of the metal center with gas molecules. The photocatalytic activity exhibited in the decomposition of dipropyl sulphide was correlated with the band gap of the porous host.
Confining metal or metal oxide nanoparticles in an inorganic shell (i.e., channels or cavities), has proven to be an alternative and very promising approach for the design of a novel class of heterogeneous catalysts with superior activity and thermal stability than those currently available.
A large variety of flexible methods to synthesize metal and metal oxide nanoparticles with tunable size and shape have been reported in literature due to their potential applications in catalysis, energy and magnetic data storage, and so on. However, the development of methodologies, to embed nanoparticles in an inorganic matrix, has attracted great interest especially in recent years.
Before a large scale application of this approach would be possible, the synthesis must be optimized to 1) use cheaper starting materials (often expensive metal precursors have been proposed), 2) reduce the large volume as typically used in many preparation methods (i.e., microemulsions are typically carried out in dilute solutions), 3) solve filtration problems, and 4) improve the texture of the catalysts by optimizing post-synthetic treatments to enhance the accessibility of the embedded active phase.
In our opinion, the design of catalysts based on metal nanoparticles embedded in stable matrices can significantly contribute to the realization of more sustainable industrial processes.
Professor Mauro Graziani (University of Trieste) is kindly acknowledged for his constant and stimulating discussions. We gratefully acknowledge the University of Trieste, ICCOM-CNR, INSTM, MIUR (Rome), PRIN2007 “Sustainable processes of 2nd generation for the production of H2 from renewable resources” Project and Fondo Trieste for financial support.
Paolo Fornasiero obtained his PhD in heterogeneous catalysis in 1996. After his postdoctoral fellow at the University of Reading (UK) in 1997, he became an assistant professor in 1998 and an associate professor of Inorganic Chemistry in 2006 at the University of Trieste (Italy). His scientific interests are in the technological applications of material science and environmental heterogeneous catalysis. He is a co-author of 115 publications. He was awarded in 1994 the Stampacchia Prize and the Nasini Gold Medal in 2005, awarded by the Italian Chemical Society, for his contribution to the research in the field of inorganic chemistry.
Matteo Cargnello obtained the Laurea Specialistica in Chemistry in 2008 at the University of Trieste (Italy). In 2008, he was a visiting graduate student at the University of Pennsylvania with Prof. R. J. Gorte. He is currently a PhD candidate at the Graduate School of Nanotechnology (University of Trieste). His activity relates to the design of embedded Au and Pd catalysts and also to the field of photocatalysis.
Tiziano Montini obtained a PhD in heterogeneous catalysis in 2005. Since 2005, he holds a research position at the University of Trieste (Italy). His scientific interests relates to material science and environmental heterogeneous catalysis. He is a co-author of 35 publications. He was awarded in 2003 the M. Forchiassin award for his undergraduate thesis.
Loredana De Rogatis graduated from Chemistry in 2004 at the University of Trieste (Italy), where she then obtained her PhD in nanotechnology in 2008. Her research interests are in the design of nanostructured catalysts. At the present, she holds a post-doctoral position at the University of Udine (Italy). She is a co-author of 15 publications and 3 book chapters. In 2005, she received the GIC Prize awarded by the Interdivisional Catalysis Group of the Italian Chemical Society for her undergraduate thesis, and in 2009, the ENI Award 2009-Debut in Research for PhD activity.
Valentina Gombac obtained the Laurea degree in Chemistry at the University of Trieste (Italy) in 1992. From 1993 to 2001, she worked as a researcher in the Area Science Park of Trieste. From 2002, she holds a research position at the University of Trieste. Her recent scientific interests are in the technological application of materials science and photocatalysis to the solution of environmental problems. She is co-author of 15 publications and 3 patents.
Barbara Lorenzut is a PhD candidate in nanotechnology at the University of Trieste (Italy). Her research activity deals with the design of embedded Rh and Ru catalysts for hydrogen production through methane partial oxidation, reforming reaction, and ammonia decomposition. She is the coauthor of three publications and one patent.