Noble-Metal Nanoparticle Materials

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Review

Hybrid Mesoporous Silica/Noble Metal Nanoparticle Materials - Synthesis and Catalytic Applications Malcolm Davidson, Yazhou Ji, G. Jeremy Leong, Nolan C Kovach, Brian G. Trewyn, and Ryan M. Richards ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00967 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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ACS Applied Nano Materials

Title: Hybrid Mesoporous Silica/Noble Metal Nanoparticle Materials - Synthesis and Catalytic Applications Authors: Malcolm Davidson,b,† Yazhou Ji,b,† G. Jeremy Leong,a Nolan C. Kovach,a Brian G. Trewyn,a,b and Ryan M. Richards*a,b a. Department of Chemistry, Colorado School of Mines, 1500 Illinois St, Golden CO 80401. b. Materials Science Program, Colorado School of Mines, 1500 Illinois St, Golden CO 80401. †

These authors contributed equally to this work.

Abstract: Due to the uniform and stable pore structure, mesoporous silica has attracted increasing research attention as a catalyst support material. As a large family of mesoporous silica supported materials, noble metal nanoparticles supported on mesoporous silica catalysts have demonstrated desirable properties across a broad platform of reactions. In this review article, we first introduce systems of metal nanoparticles dispersed on mesoporous silica then we focus on next generation of systems in which the noble metal is not supported on the mesoporous silica but rather entrapped/intercalated within the silica matrix, thus enhancing particle stability and in some cases, enhanced activity. Herein, research and future directions on both synthesizing hybrid noble metal nanoparticles/mesoporous silica composite catalysts and their resultant properties will be discussed. Keywords: Mesoporous silica, noble metals, nanoscale particles, hybrid support systems, heterogeneous catalysis,

intercalated

nanoparticles

1. Introduction As an important category of catalysts, supported nanoscale noble metal particles have been a cornerstone of the chemical industry beginning in the petrochemical industry and now broadly employed in areas ranging from pharmaceuticals and agrochemicals to environmental remediation.1-4 Research on nanoscale catalysts has encompassed nearly all noble metals and industrially-relevant reactions, including examples such as C-C coupling reactions catalyzed by Pd and water splitting catalyzed by Pt catalysts.5-13 In 1987, the catalytic activity of nanoscale Au in low temperature oxidation of CO was first reported.14 Since then, extensive research has been done on various reactions catalyzed by nanoscale Au. Typical

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reactions include oxidation of CO, partial oxidation of hydrocarbons, hydrogenation of carbon oxides, and reduction of NO.14-20 Despite their promising catalytic potential, nanoscale noble metal catalysts under demanding reaction conditions (temperature, pressure, etc.) often suffer from poisoning, coking, thermal degradation and sintering. Sintering is commonly observed with nanoscale particles because their melting points are generally significantly decreased as compared with their bulk counterparts and result in loss of activity. Furthermore, the activities and selectivities of nanoscale particles are size and shape (facet) dependent on the nanoscale.21-26 To prevent sintering of nanoscale particles, an effective approach is to support them on (or in) high surface area materials with high thermal stability such as silica and alumina.27-37 Beyond the chemistry of the active metal site, different support materials impact the catalytic properties of nanoscale catalysts and thus the choice of support material is of vital importance to the overall performance of the catalyst. For example, in the investigation of Pt-Ru bimetallic nanoparticles supported on carbon black, fullerene soot, and desulfurized carbon black, the formation of a Ru carbide phase at the support-nanoparticle boundary was observed.38 In this work, X-ray absorption spectroscopy (XAS) was employed to discern that while Pt preferentially migrates to the surface, a unique Ru-carbide phase forms. In addition, the amount of Pt affected the overall degree of support interaction by the metallic species. Likewise, the activity of Au nanoparticles for the water-gas shift reaction supported on TiO2, CeO2 and Al2O3 exhibited a support dependent trend in activity as TiO2 > CeO2 > Al2O3.39 It has also been reported that several noble metals (Pt, Rh, Ru, and Pd) on irreducible supports such as SiO2 are 1–2 orders of magnitude less active in the water-gas shift reaction as compared to catalysts supported on reducible materials such as TiO2 and CeO2.40 Most recently it has been demonstrated that ZnO supported Co3O4 nanoparticles are highly active catalysts in the transformation of renewable materials through carbonylation of glycerol with urea. The interface interaction between Co3O4 and ZnO was found to play an important role in the activity of the catalysts.41 Amongst the support materials used, mesoporous silica is an attractive material for noble metal


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nanoparticles because it is only mildly acidic, relatively inert, thermally and chemically stable, and can be prepared with very well-defined meso/micro porosity.42 By IUPAC definition, mesoporous silica refers to materials with pore diameters ranging from 2-50 nm. The well-defined pore structure of porous silica can function as a molecular sieve at small sizes and may ultimately be utilized to control substrate access to the catalyst which is very important in improving/tuning the selectivity. Thus, mesoporous silica supported metal nanoparticles have been the focus of research for a broad range of applications. Two materials highlight the range of synthetic conditions accessible for forming templated mesoporous silicas, base-catalyzed cationic-surfactant based MCM-41 and acidcatalyzed block-copolymer templated SBA-15. In an early report, MCM-41 was developed by Mobil Corporation as an important family of mesoporous silica materials.

43

Following the

Figure 1. Schematic illustration outlining the types of hybrid reduced noble metal supported on mesoporous silica materials covered in this review.

first report, other types of mesoporous silica have been developed including SBA-15.44 Research results indicate that the pore size, wall thickness and pore connectivity of mesoporous silica can be tuned by varying the parameter space of templated sol-gel chemistry. Numerous approaches to impart robustness and stability to noble metal nanoparticles by supporting them on mesoporous silica materials have been reported. In this review, after first introducing the state-of-the-art regarding stabilizing particles on silica, we will focus on methods to entrap noble metal nanoparticles within the mesoporous silica matrix: forming core-shell structures, metal growth inside the channel of mesoporous silica and intercalation in the wall of mesoporous silica. 1.1 Scope of Review The overall focus of this review is to analyze seminal and recent research in the area of supported



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metal nanoparticles on mesoporous silica and, in the end, with a thorough understanding of the limitations in this field; offer some paths forward and future directions that we, the authors, feel are exciting areas to pursue. To maintain focus and direction of this review, we will limit the papers we review to metal nanoparticles supported on mesoporous silica used in catalytic applications. We understand this means that a significant amount of important research on metal oxides, etc. supported on other scaffolding are omitted but our desire is to emphasize an area in which our collective experience can be leveraged. The breadth of supported noble metal – mesoporous silica hybrid materials that are covered in this review are illustrated in Figure 1.

2. Metal dispersion on mesoporous silica surface – strengths and limitations Wet impregnation methods to disperse oxidized metal species (usually in the form of a metal salt) have long been the mainstay to incorporate metal on a silica surface. The approach has been employed in the early reports of supporting platinum on the surface of silica as hydrogenation catalysts all the way to recent work on supporting platinum and palladium on silica for combustion catalysts for the removal of volatile organic compounds.45 Numerous recent studies have further developed this paradigm for numerous biomass upgrading reactions as well as imparting resistance to coking.46-47 While an effective method at distributing noble metals, wet impregnation on silica supports frequently leads to heterogeneous dispersion and polydisperse particle sizes. This becomes particularly troublesome as we move into porous supports where pores can easily be completely blocked, effectively limiting the active surface areas. As with all areas of material science and chemistry, methods have advanced to facilitate more uniform dispersion of noble metals on silica supports.42,48-75 Instead of allowing simple, and often times weak, metal salt – silanol/silicate interactions drive surface precipitation, new methods to focus the interactions have arisen. Prior to the dispersion of metal nanoparticles, functional groups are often decorated onto the mesoporous silica surface by a surface modification process. Initially amine or thiol groups were the main modifying groups to nucleate metal nanoparticles; however, ionic liquids have been employed in more recent years.57,63 The surface modification process can either occur after silica support


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synthesis (post-synthesis grafting) or during the hydrolysis and condensation process when the silica walls form (in situ co-condensation). The grafted functional groups act as anchors to enhance the interaction between metal precursor and silica wall. After surface modification, metal nanoparticles are produced through a chemical or physical reduction. Ultimately, the resulting nanoparticles are immobilized on the surface of the silica framework by a physical-chemical interaction with the surface groups to reduce sintering and nucleates nanoparticle formation. It is reported that when surface modification occurs during the condensation process, functional groups are more homogeneously distributed on the surface of silica framework.50 Post synthesis grafting of a silica framework surface is generally conducted by refluxing modifying organic groups and the pre-synthesized silica matrix in a solvent. Certain solvent interactions need to be considered in this case, for example, aqueous solutions are not typically used during the introduction of amine moieties, because of the possible hydrogen bond interactions between water molecules and amine groups. The reflux lasts several hours and the material is subsequently filtered to yield the functionalized silica matrix. After surface modification, the mesoporous silica is immersed in a metal precursor solution. A surface layer of metal precursor is formed on the mesoporous framework through complexation with the grafted functional group. This fluctuation in the coordination of the chosen metal ion can be observed by a color change in the precursor solution. Finally, the metal precursor is then reduced into metal nanoparticles through either chemical reduction (i.e. sodium borohydride) or physical reduction (i.e. photo-assisted). General procedures to make metal nanoparticles dispersed on silica materials have been proposed and a scheme of the process is shown in Figure 2.76 Factors including nature of the functional group, solution, and treatment conditions affect the size of formed metal nanoparticles, which ranges between 3 and 12 nm for these methods.49,51-52,57-59,61,67 Some results show a variation in the metal nanoparticle size that was associated with the density and identity of the functional group.77 First, surface modification with monoamine or diamine groups was employed using (3-aminopropyl)trimethoxysilane (APTMS) or N-[3(trimethoxysilyl)propyl] ethylene diamine,



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respectively, and refluxing in toluene for 24 h. The material was then recovered by filtration, washed with ethanol, and vacuum dried at 353 K for 6 h.

Figure 2. Schematic of a process to disperse nanoparticles on surface of mesoporous support nanomaterial (MSN). Modification from ref. 72.

Gold was introduced by suspending the functionalized mesoporous silica in a 10-4 M gold(III) chloride trihydrate solution formed in water or ethanol (Figure 2). Sonication was used to promote uptake and the materials were recovered by filtration, then dried in vacuo at 298 K. Finally, the samples were calcined at 473 K or 873 K using a 1 K min-1 ramp and holding for 1 h at the highest temperature. Compared with materials made in ethanol solution, aqueous environments tend to produce smaller metal nanoparticles. At a high calcination temperature, metal nanoparticles grew larger in size, and an increase of calcination temperature by 330 K increased the particle size upper limit by more than a factor of three. Using diamine groups instead of monoamine groups also stimulated nanoparticle growth. A higher degree of porosity is known to result in lower thermal stability in mesoporous silica, thus monoamine modified silica particles are expected to exhibit superior stability at high temperatures compared to diamine modified silica.78 Simple electrostatic interactions between protonated functional groups and metal salts with formal negative charges (as shown in Figure 2) have been advanced with improved organic synthesis skills recently.79 This advancement was demonstrated by producing an organic modifier (3-aminopropyl-


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methylsilane) that contains two different groups (a hydrido and a primary amine) placed on SBA-15 support in a geometrically controlled fashion. These groups are typically incompatible and are traditionally difficult to co-functionalize on a single support. Including this bi-functionality on the SBA15 led to onsite metal ion reduction, forming silica-supported single metal atoms as well as small metal nanoparticles (Ag, Pd, Au). This cooperative catalyst (primary amine and small metallic species) efficiently catalysed the dehydrogenation of formic acid at room temperature without additional base. In a recent study it was shown that ultrasmall (2.7-4.7 nm) Ni nanoparticles could be supported in mesoporous silica SBA-16 functionalized with -COOH via wet impregnation under basic conditions.80 It was demonstrated that at the appropriate conditions (pH 9) the carboxylic acid groups would deprotonate and allow effective incorporation of Ni2+ species resulting in well dispersed Ni nanoparticles in the mesoporous network. Here, the cage type mesopores of the SBA-16 confined the Ni nanoparticles and facilitated tuning of their sizes. These materials demonstrated remarkable catalytic activity for the hydrogenation of carbon dioxide.

Ethylene Hydrogenation Shape controlled nanoparticles have demonstrated unique conversion and selectivity (as compared to their polycrystalline counterparts) in many chemical reactions.81-86 Pre-synthesized cubic-Pt nanoparticles with sizes between 1.7 to 7.1 nm, as seen in Figure 3B, have been supported on the surface of porous silica.66 The Pt colloid was mixed with SBA-15 and sonicated before calcination at 723 K under oxygen flow. The Pt nanoparticles were shown by electron microscopy to have retained their original size and shape while remaining evenly distributed on the surface of the mesoporous silica framework. The catalysts were then tested in the ethylene hydrogenation reaction under 20 torr C2H4, 200 torr H2, and 658 K. The Pt colloid on SBA-15 materials all out performed the Pt powder reference, but not quite as well as the Pt on SBA-15 prepared via ion exchange (IE). In particular, 0.6 % Pt (1.7 nm)/SBA-15 showed a TOF at 1.2 (100 × s-1) while 1 nm Pt nanoparticles supported on silica formed by ion exchange reached the highest TOF of 6.5 (100 × s-1) compared to 0.04 (100 × s-1) for Pt powder. The drop-in performance was



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attributed to a preference for low coordination surface Pt atoms prevalent on smaller particles, which are favoured to form by IE.

CO Oxidation More advanced tandem metal systems have also been explored. A novel photo-assisted deposition method to disperse Pt particles on silica surface, which can be observed in Figure 3C, has been reported whereby a Ti containing mesoporous silica was synthesized with titanium tetraisopropoxide as precursor and dodecylamine as the template.65 The Pt precursor was added to the Ti containing silica and was exposed to UV light for 24 h yielding Pt nanoparticles with controlled size of ∼4 nm. The authors also synthesized silica supported Pt particles made from an impregnation method as a control group, which had larger particles and a broader size distribution (2-30 nm) as seen in Figure 3C. Catalytic performance of the materials was evaluated by the oxidation of CO with the photo-generated catalyst demonstrating a faster reaction rate with 100 % conversion obtained at 399 K.

N2O Decomposition Noble metals have a history of serving as catalysts for N2O decomposition.87-89 With Europe’s commitment to the Kyoto protocol, there is a renewed fervour in the development of materials capable of reducing this powerful greenhouse gas. RhxOy have shown the most promise in remediating N2O, and there surface area constraints have been addressed by mesoporous silica supports.90 Current demonstrations include mesocellular silica foam (MCF) functionalized with Rh by wet impregnation which improves N2O abatement over similar SBA-15, MCM-41, and KIT-6 derivatives. This advance was attributed to the three-dimensional interconnectivity of the 40 nm pores present in MCF leading to highly dispersed and accessible 1 nm RhxOy particles. With this marked improvement stemming from unique support architecture, further development of mesoporous silica itself parallels the importance of exploring new metals species.

Silica Support Effects


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While metal nanoparticles dispersed on the surface of silica are used as catalysts for a broad range of chemical processes, the nature of the silica support also has an important influence on performance. Due to varying pore sizes, surface areas, dispersions, and phases provided by the chosen mesoporous silica: diffusion, selectivity, and the thermodynamics of the active sites can be altered. For example, the catalytic performance of Pt supported on silica of different pore sizes has been studied.54 FSM-10, FSM-16, and FSM-22 were synthesized with pore sizes ranging from 1.8 nm to 7.0 nm. Pt nanoparticles were dispersed on the surface and reduced by H2 at 473 K. The catalysts were tested on the oxidation of CO, and at 353 K the conversion rate decreased from 100 % in the system with 4.0 nm pores to 90 % in the system with 7.0 nm pores (Figure 3D). Additionally, for a constant Pt loading, as pore size increased CO uptake by Pt decreased from 0.39 to 0.22. The difference in reaction activity was attributed to the amount and distribution of surface silanol groups existing in different pore sized silica supports. Narrower pore size silica had more hydrogen bonded Si-OH groups, which formed a stable ring structure to affect the absorption of CO onto the surface. In a similar study MCM-41, SBA-15, and glycerol-templated KIE-6 were prepared and tested in the catalytic dehydrogenation of formic acid at 298 K.91 The support materials were post-synthetically amine functionalized and then decorated with Pd-MnOx particles by reduction of a precursor solution. KIE-6 is advantageous to templated MSN due to its textural properties depending on concentration of glycerol, enabling facile control. A TOF of 593 h-1 was achieved due to KIE-6 supports possessing three-dimensional pore network of high-volume pores which increased mass transport as opposed to the two-dimensional structure of the templated MSNs. Amongst the merits of supported nanoparticle catalysts is the ability to regenerate them while retaining particle size and activity. One example produces ultra-fine Au nanoparticles dispersed on silica by repeated Michael-type addition and amidation of ester moieties leading to polyamidoamine dendrimers (G4-PAMAM) on the surface of the silica.92 Particle size varied according to the amount of Au precursor added, with the narrowest range confirmed to be 1.3-1.6 nm by TEM (Figure 3E). The Au supported on silica catalyst was employed in the oxidation of 1-phenylethanol. Compared with Au on SBA-15 made



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from traditional incipient wetness methodology, which demonstrated a decrease of conversion from 99.5 % to 72.5 % after five recycles, the ultrafine Au nanoparticles retained a conversion of 97.7 %. In addition to regenerating catalysts, capturing and reusing supported catalytic metals is important in industrial processes. By first incorporating magnetic iron oxide in the MCM-41 supports, preformed Pt nanoparticles (1.5 nm) were introduced to the magnetic MCM-41 and supported within the channels of the mesoporous support.93 Detailed TEM analysis and 3D tomography is offered in this original research publication that shows the interplay between the nanoparticle and the pore wall along with nanoparticlenanoparticle repulsive forces result in a distribution of Pt nanoparticles throughout the MCM-41 channels. The Pt NPs were shown to be catalytically active for the hydrogenation of p-nitrophenol and, utilizing the magnetic properties, were recycled several times showing negligible loss in activity. This was the first demonstration of achieving a uniform distribution of preformed nanoparticles into MCM-41 channels (pore size 2.9 nm) through diffusion.

Fuel Cells Fuel cells are an important technology with demanding catalyst requirements. Synthesis of Pt nanoparticles dispersed on mesoporous silica can be coated with a conductive polymer layer for fuel cell applications by functionalizing the mesoporous silica with vinyl groups.94 After the surface modification process, polythiophene is coated on mesoporous silica with azobisisobutyronitrile and ethylene glycol dimethylacrylate as a conductive layer. Pt nanoparticles ranging from 7-9 nm (Figure 3F) are then deposited onto the surface. The electrochemical properties of the formed Pt-thiol-MS catalyst were investigated and compared with a PtRu nanopowder catalyst. It was determined that the maximum power density of the Pt dispersed on silica catalyst almost tripled to 35.96 mW cm-2 when compared to 12.5 mW cm-2 of the PtRu nanopowder.

Biosensing Dispersion methodologies also allow for probing the effects of loading on catalytic performance. While typical metal loadings are found in the range of 1-5 wt. %, higher loadings have shown unique


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properties in chemical and biological reactions. A catalyst with 18 wt. % Au on silica was obtained by adding KCl (3:1 ratio salt:Si) to the synthesis of the silica to tune the pore size.95 The tuned silica framework was then modified by APTMS in an ethanol solution for 24 h. Finally, the surface modified silica was immersed in a gold precursor solution and reduced to Au nanoparticles by sodium borohydride. The system composed of 18 wt. % Au on silica was characterized by UV-Vis and TEM demonstrating a uniform size distribution of ~7 nm Au nanoparticles as displayed in Figure 3G. The Au loaded silica was used as a biosensor for glucose by casting it into a polyvinyl acetate matrix upon a gold electrode. The sensor showed improved response time, lower limit of detection, and better sensitivity towards glucose detection compared to a Au-amine complexed SBA-15. This improvement was attributed first to the high dispersion of Au afforded by the MSN, which translated into increased enzyme loading. Additionally, immobilization of the enzyme may promote a preferred geometry of the enzyme and enhanced electron transfer efficiency between the enzyme and electrode may occur due to the channels of the MSN.

Biodiesel Production Finally, disordered mesoporous systems with monodisperse morphology and narrow pore size distributions are an emerging family of support materials.96 As the demand for green fuels grows, so does biodiesel production. Hence, finding ways to transform glycerol, considered a major byproduct of biodiesel production, has become relevant challenge in heterogeneous catalysis. Direct plasmonic photocatalytic oxidation of glycerol is a state-of-the-art implementation of these tunable materials.97 This phenomenon is described by the resulting Shottky barrier present at the metal-silica interface and the excitation of free electrons in the Au particles (plasmon resonance). The former process creates an electric field which facilitates charge transfer necessary for photocatalysis. The latter induces an increase in bandgap absorption of UV and visible energy necessary to initiate the catalysis.98 Due to the homogeneous pore structure present in monodisperse mesoporous silica spheres, better incorporation of metal-precursor solution was achieved. Additionally, the small cage-like pores retain nanoparticle more effectively after reduction, leading to evenly distributed 2 nm Au particles at loadings as high as 15 wt.



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%. The homogeneous distribution of Au present on the disordered MSN out performed KIT-6, SBA-15, and MCM-41 analogues by at least 10 % in the conversion of glycerol.

Figure 3. Metal nanoparticles dispersed on the surface of mesoporous silica structure. A. Enlarged Au nanoparticles with 823 K calcination and diamine group modification. Reprint from ref. 73; B. Shape controlled Pt nanoparticles dispersed on mesoporous silica. Reprinted with permission from Hydrothermal Growth of Mesoporous SBA-15 Silica in the Presence of PVP-Stabilized Pt Nanoparticles: Synthesis, Characterization, and Catalytic Properties Hyunjoon Song, Robert M. Rioux, James D. Hoefelmeyer, Russell Komor, Krisztian Niesz, Michael Grass, Peidong Yang, and and Gabor A. Somorjai, J. Am. Chem. Soc. 2006 128 (9), 3027-3037. Copyright 2006 American Chemical Society."; C. Pt nanoparticles photo deposited on mesoporous silica. Reprint from ref. 61; D. Pt nanoparticles dispersed on 7.0 nm large pored silica. Reprint from ref. 50; E. Ultra-fine Au nanoparticles (1.6 nm) dispersed on surface of mesoporous silica. Reprint from ref. 83; F. Pt nanoparticles on polythiophene functionalized MS. Reprint from ref. 84. G. Au nanoparticles on the surface of mesoporous silica with 18 % loading. Reprint from ref. 85.

3. Core-shell structure Core-shell structures are an interesting topic in material sciences due to their ability to incorporate desirable properties of two systems in complimentary ways. Typically, a core-shell structure is defined as a material possessing both an inner core and an outer shell layer similar to an egg. Core-shell structured silica supported materials can be synthesized either with metal nanoparticles as the core or with silica support as the core.99-109 Generally, there are two approaches to form core-shell structures, top-down and bottom-up as depicted in Figure 4. In the synthesis of core-shell metal nanoparticles and mesoporous silica structures, the bottom-up approach is widely used.110-117 The bottom-up approach in forming coreshell structures utilizes the chemical properties of reagents which undergo self-assembly processes (under proper conditions) to form desired structures (Figure 4). In a typical top-down synthesis with metal nanoparticles as the core, metal nanoparticles are presynthesized by any of the numerous existing colloidal methodologies. With the nanoparticle cores evenly


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distributed in solution, a template agent is introduced. These can adopt micellar structure under specific temperature, pH and concentration conditions. In common aqueous syntheses, the hydrophobic cores of the micelles encapsulate the metal core forming the basic structural template of the shell. Next, an added silica source undergoes

hydrolysis

and

co-condensation

processes to form the walls of the mesoporous silica over the micelle template. The remaining surfactant is removed afterwards by calcination or chemical etching. Conversely, in a bottom-up method with the silica support as the core, the silica core is synthesized first. The surface is then modified

to

with

chemical

motifs

Figure 4. A survey of top-down and bottom-up methods for producing noble metal/silica hybrid materials. Both processes occur in a one-pot synthesis. The bottom-up yields a surface functionalized MSN core, while the top-down produces a metal NP protected by a mesoporous silica sheath.

which

preferentially bind to metals; amines and thiol groups are popular for chelating noble metals. The functional groups then interact with the added metal precursor or metal nanoparticles in solution, producing an outer layer on the silica core upon reduction. As an example, a method was demonstrated in which Pt/mesoporous silica core-shell structure was further protected by a poly(vinylpyrrolidone) (PVP) layer for enhanced stability.100 In the experiments, Pt nanoparticles were formed in the presence of PVP as directing and protecting agent. With cetyltrimethylammonium bromide (CTAB) as the template in a basic solution, the silicates polymerized to form the outer shell. The Pt nanoparticles retained their original size between 2-4 nm after the coreshell formation and the facet orientation remained unchanged. At 573 K, the PVP/silica double layer



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protected the Pt particles from sintering. Compared with commercial Pt/C catalyst, the protected catalyst achieved three times the TOF for the hydrogenation of cinnamic acid. One frequent cause of nanoscale metal catalyst deactivation is sintering. When nanoparticles agglomerate, particle size increase causes corresponding decrease in activity. A new method to make a core-shell structure with a single particle in the core completely protected by the silica shell, was shown to be highly resistant to sintering.115 In the synthesis, AgNO3 was used as precursor and reduced by formaldehyde. The silica shell was templated by CTAB and formed by hydrolysis and condensation tetraethylorthosilicate (TEOS). Na2S was added into the already formed core-shell structure to allow for a sulfuration process, after which a single core of nano Ag2S is achieved. The results of this technique can be seen in Figure 5A. Of note, the silica shell thickness can be tuned by adjusting the amount of silica source and particle size of the single metal core averaged at ∼10 nm. The core particle size was reported to increase with a decrease in thickness of the silica wall. As compared with Au nanoparticles, Au nanorods possess unique optical properties due to localized surface

plasmon

resonance

(transverse

and

longitudinal).

For

example,

a

core-shell

Au

nanorod/mesoporous silica structure, as seen in Figure 5B, was synthesized through an existing seedmediated method.102 To form the micelle template and completely encapsulate the rod-shaped nanoparticle, the researchers produced chiral anionic surfactants from several amino acids. The surfactant was used along with a directing agent, N-trimethoxylsilylpropyl-N, N, Ntrimethylammonium chloride, to form the desired micelle structure. After the sol-gel process with TEOS, which forms the silica wall, the chiral surfactant was removed by HCl/ethanol reflux. The Au nanorods were found to be ∼70 nm in length and ∼45 nm in diameter with the silica wall constituting a 15 nm shell. The core-shell composite was used in near field plasmonic analysis. As compared with the polarization independent absorption of nanorods dispersed in solution without silica shell, the shelled structure exhibited several circular dichroism (CD) signals of approximately 10 mdeg in the visible light


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region from 500-900 nm, attributed to the coupling of the chiral surfactant with the metal nanoparticles. Additionally, the CD activity was tuneable by manipulating the aspect ratio of the particles. Core-shell syntheses which comprise a noble metal core encapsulated by a mesoporous silica have garnered much attention in recent years in fields such as in vivo medical imaging, catalysis and photovoltaics.118 Recently, a method to synthesize a core-shell system comprised of a Pt core encapsulated by a doubly-functionalized mesoporous silica framework. First, Pt cores were synthesized through hydrothermal reduction. An aqueous suspension of the particles was mixed with CTAB templates, forming a micellar coating around the core. TEOS was then added and hydrolysed to form the silica

shell.

The

surface

was

then

APTES

functionalized

followed

by

addition

of

diethylenetriaminepentaacetic acid (DTPA). The core metal was shown to exhibit photothermal excitation, a key aspect in photothermal therapy, by 808 nm NIR light. Further, amine the modified silica was successfully functionalized with DTPA. This was evidenced by the system’s ability to selectively bind to Gd complexes, of which are used extensively in magnetic resonance imaging (MRI) as contrast agents.119 An example of these multifunctionalized nanoparticles are shown in Figure 5C. A core-shell embodiment of Ag@silica were shown to be synthesized in a wet chemical, one-pot reaction; typical metal-silica core-shell frameworks are produced in multiple step processes.120 In one vessel, an AgNO3 precursor was added to formaldehyde, a reducing agent, CTAB, TEOS and catalytic NaOH. The resulting particles were shown to be promising candidates for their optical limiting properties and were shown to be dispersible into silicone rubber, a prototypical hybrid material for supporting such materials. When tested on their ability to attenuate nanosecond 532 nm light, their performance was comparable to that of Ag@silica particles prepared through many-step reaction schemes. Owing to the simplicity of fabrication, core-shell systems realized by one-pot reactions can greatly improve their production feasibility. In addition to spherical core-shell structures, dumbbell and lollipop core-shell structures has been achieved by controlled addition of Ag and dimethylformamide (DMF).99 Au nanorods are first



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synthesized by a seed mediated process followed by centrifugation to remove CTAB that is not bound to the Au nanorod surface. Silver ions, introduced as AgNO3, preferentially interact with the exposed (110) surface of the Au nanorod body as opposed to the (111) endcap. The lack of Ag ions on the end caps leads to a weaker interaction between the CTAB and the Au nanorod. Over a 1-3 week aging time, this results in a slower disordering of the adsorbed CTAB layer along the nanorod body as opposed to the end cap. When the DMF/TEOS sol-gel solution is then introduced the intercalation of DMF into CTAB, an agent shown to accelerate TEOS hydrolysis, occurs more rapidly in the disordered endcap region. This leads to accelerated formation of silica at the endcaps, and a dumbbell structure overall. The silica shell can encapsulate just one end, or both ends of the Au nanorods. The surface plasmon resonance bands of the special shape material exhibited a blue shift of 20-40 nm, having potential application in optical fields. To synthesize the structure, Au nanorods were first made through a seed-mediated approach. Sodium borohydride was first used to form Au seeds then ascorbic acid was used to slowly form a rod structure. To further form dumbbell or lollipop structure, the nanorods were dispersed in a methanol solution with CTAB as template and TEOS as silica source. DMF was used as shape directing agent and without it, the silica wall completely encapsulated the rod. With the DMF amount increased to 50 µL, the dumbbell structure formed. By adding an extra 25 µL of DMF, the lollipop structure was achieved. The authors proposed a mechanism in which the DMF molecules penetrated the CTAB template layer at the end of the Au rod more easily due to the curvature. Since DMF was reported to accelerate the condensation of the silica source, the silica wall forms quicker on DMF penetrated rod end area leading to a dumbbell structure. As another example of complex structure, a special core-shell reactor with Au nanocages as the core was synthesized.104 The synthesis began with silver nanocubes encapsulated by a mesoporous silica shells formed through Na2CO3 etching. The Au precursor was then added and went through spatially confined galvanic replacement to replace silver, forming an Au cage in the core area. The unique morphology of these particles is displayed in Figure 5-D. Interestingly, after the caged core-shell particle was coated with


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poly(N-isopropylacrylamide), the material exhibited a heating effect under NIR irradiation at a wavelength of 808 nm making it potentially useful for cancer treatment. Apart from hydrothermal syntheses, assisted laser ablation (ALA) has been shown to direct the fabrication of size-tunable silica onto colloidal Ag nanoparticles.121 In a water-ethanol solution, a silicon nanoparticles were produced through ablating an Si target with 9 ns Nd:YAG laser pulses (100 mJ/pulse) for 2 min. The silicon particles immediately oxidized in the solution, producing silica. Then, an Ag salt was added to the mixture. The surfaces of the immersed silica acts as a reducing agent, which allows for the tandem Ag reduction and formation of silica shell around the resulting reduced Ag. To note, ALA is able to produce uniformly passivated Ag cores with nearly identical coverage as those produced via the Stöber method. However, traditional Stöber syntheses involve toxic chemicals, of which ALA does not require. As an entrance to our next section, a method for the production of core-shell-like composite noblemetal nanorods within a mesoporous sheath will be introduced.122 Initially, Au nanorods were prepared by a seed mediated system utilizing a binary surfactant system of CTAB and sodium oleate.123 These nanorods were then encapsulated with a mesoporous silica shell by a single step process under basic conditions over several days at a TEOS concentration of 8 mM.124 In similar work, elasticity was observed in these types of mesoporous silica shells.125 An oxidative etch was executed using a hydrochloric acid in methanol system, where progress was tracked by observation of the UV-Vis absorption spectra. After the initial etching, noble metals such as Au, Pt, or Pd could be grown in the voids between the silica shell and the Au rod. It was also found that the texture of the Pt coating could be varied between smooth and abrasive by adjusting the growth speed. The combination of noble metals and surface conditions affords tuneable plasmonic behaviour. Owing to the differences in free electrons in various noble metals, their alloying on specific surfaces enables one to tune the absorption properties of incident UV and visible energy of the functionalized silica. This has direct implication on the efficiency of the photocatalytic system at hand.



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4. Nanowires/nanoparticles directly formed in the channel of mesoporous silica When using porous materials to cast noble metals into nanoscale particles or rods, accessibility of the entire interior pore volume ensures adequate metal precursor impregnates the template and exit pathways for hydrolysis products, while adjustment of the pore geometry can influence the final morphology of the noble metal domain.126 Fortunately, in mesoporous silica materials the pore size and connectivity can be adjusted by varying the parameters of the sol-gel chemistry.42 For example, it is reported that pore diameter can be adjusted within the range of 4-50 nm thus providing a readily tuneable parameter for hybrid systems. The synthesis of nanowires or nanoparticles inside the channels of mesoporous silica is commonly accomplished by a seed-mediated growth process.27,127-132 Nobel metal precursors dispersed in solution adhere onto the channel walls of the mesoporous framework by intermolecular force interactions with amine or thiol functional groups, followed by a reduction process which develops metal seeds from the precursor. Particles or wires then continue to grow by propagation from the metal seeds. To form particles or wires only in the channel, growth conditions must be carefully controlled by mild reducing agents and soft templates to decelerate growth speed and acquire desired shape.133-139 Figure 6 illustrates a general process of producing nanowires inside the channel of mesoporous silica. In addition to nanorods, shaped Pt nanoparticles directly formed inside the channel of mesoporous silica support have been realized.128


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E

Figure 5. Selected core-shell structure of metal nanoparticles/mesoporous silica composite. A. Core-shell composite with single Au particle as the core. Reproduced from Ref. 108 with permission from The Royal Society of Chemistry; B. Au nanorods encapsulated by mesoporous silica shell. Reprint from ref. 98; C. Au@mesoporous silica hollow structures composite. Reprint from ref. 96; D. Au nano cage encapsulated by mesoporous silica shell during galvanic displacement. Reprinted with permission from Spatially Confined Fabrication of Core–Shell Gold Nanocages@Mesoporous Silica for NearInfrared Controlled Photothermal Drug Release Jianping Yang, Dengke Shen, Lei Zhou, Wei Li, Xiaomin Li, Chi Yao, Rui Wang, Ahmed Mohamed El Toni, Fan Zhang, and Dongyuan Zhao, Chem. Mater. 2013 25 (15), 3030-3037. Copyright 2013 American Chemical Society.

Pt acetylacetonate as precursor was mixed with (3-mercaptopropyl)trimethoxysilane (MPTMS) surface modified mesoporous silica. The MPTMS served as crucial anchoring sites for forming Pt seeds, enhancing nucleation. With the presence of 1-octadecene, the metal seeds formed by stirring at 393 K. At elevated temperature, the Pt seeds grew to shaped nanoparticles. The formed Pt particles were found to be cubic, with a size distribution of 4.0 – 5.6 nm which can be seen in Figure 7A, verified by physisorption, the cubic Pt nanoparticles formed inside the channel and caused a blockage in the pore structures. However, the use of SBA-16 with its three-dimensional pore structure permitted uninhibited access to the catalysts interior. Catalytic performance of the material was tested by hydrogen uptake and the in-situ growth material achieved an uptake ∼10 times higher than shape-controlled Pt post-dispersed on silica support. This was attributed to the capping agents necessary for synthesis of cubic particles outside of a porous support, blocking active sites on the Pt cube surface.



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In addition to metal-salt precursor wet-chemical processes used to grow nanowires and nanoparticles inside the channels of mesoporous silica peculiar physicochemical methods such as electrodeposition have been investigated as techniques to form silver nanowires in mesoporous silica.133 To achieve silver nanorods as displayed in Figure 7B, a columnar mesophase silica system was formed by a evaporation induced self-assembly (EISA) followed by electrodeposition. In this method, acid catalyzed prehydrolysis silica precursor solutions are combined with surfactant solutions in the presence of an anodic alumina membrane, producing tendrils of silica with an ordered mesostructured within the membrane.140 Deposition of silver into the pore was carried out by a two-electrode system. Under constant nitrogen flow through the electrolyte solution, silver was deposited into the pore and formed a wire structure inside the mesopores. Dissolving the silica matrix with a hot sodium hydroxide solution resulted in standalone silver nanowires with a diameter of ∼10 nm, similar to the mesopore size. This methodology enables synthesis of unique metallic morphologies at vanishingly small length-scales, providing new components for microelectromechanical systems or electrode-based biosensors and chemical detectors. While novel methods such as electrodeposition provide unique controls during synthesis, wetchemical methods provide necessary scalability. A one-pot method to synthesize Au nanoparticles in the channel of mesoporous silica has been contrived that reduces the complexity of production.131 In the formation process, the P123 copolymer template served as the reducing agent. The formed Au nanoparticles were found to be in the size range of 2-4 nm (Figure 7C). Since the particles were in the channel of the silica network, the pore volume and surface area of the silica network were reduced by ∼20 % after growing Au nanoparticles. The catalyst was examined for the oxidation of CO, and achieved 100 % conversion at 573 K. Since no surface modification was utilized in producing this high temperature active catalyst, this approach is a novel way to support Au nanoparticles imparting thermal stability. Although there are numerous literature techniques reported that form rods, synthetic parameters which exert control over the length or diameter are still under investigation.


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Figure 6. Schematic illustration of Au nanorods growing inside the channel of mesoporous silica by Li et al.120 Reprinted with permission from Size Tunable Gold Nanorods Evenly Distributed in the Channels of Mesoporous Silica Zhi Li, Christian Kübel, Vasile I. Pârvulescu, and Ryan Richards, ACS Nano 2008 2 (6), 1205-1212. Copyright 2008 American Chemical Society.

Efforts to produce long nanowires with high aspect ratios have been challenging because the diameter of the wire will also grow. The formation of several hundred nanometer, thermally stable nanowires has been demonstrated.135 The diameter of the nanowire did not increase due to the physical constraint of the silica walls serving as a hard template. The Pt seeds were formed through an ion-exchange process by mixing MCM-41 in [Pt(NH3)4](NO3)2 solution. The seeds were reduced with hydrogen at high temperature and immersed into a [Pt(NH3)4](NO3)2 solution with higher concentration repeatedly to allow Pt wire growth. The Pt nanowires were calcined at 773 K to test thermal stability. After calcination, the Pt nanowires did not agglomerate or alter shape. In similar work, length-tunable Au nanorods were also evenly distributed in the pores of mesoporous silica.134 By tailoring the amount of Au precursor, the length of Au nanorods could range from 3 nm to 200 nm while maintaining a fixed diameter of 6 nm, which is exemplified in Figure 7D. In the synthesis, APTS modified mesoporous silica support was immersed in Au precursor solution. Au seeds were immobilized in the channel by calcination at 623 K. The seeded sample was again put into precursor solution, with slow reduction by ascorbic acid and softtemplated by CTAB, Au nanorods formed evenly in the channel of mesoporous framework. Since the 21 ACS Paragon Plus Environment


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diameter of nanorods was restricted by the silica pores, the aspect ratio of the resulting rods could be tuned by the pore diameter of the silica. This proved useful since the longitudinal plasmon wavelength is strongly coupled with the aspect ratio, and diffuse-reflectance ultra-violet (UV) spectroscopy indicated the surface plasmon resonance absorbance peak red-shifted with increasing rod length. Since the chemistry of a catalyst surface depends on the exposed facet, controlled crystal orientation has long been an interesting topic in catalysis.141-142 As a new handle on resultant catalyst, the channel phase and morphology of the mesoporous silica has been leveraged to gain control of the orientation. Pt has a face-centred-cubic structure at temperatures relevant to catalysis, with planar atomic densities of 0.125 Å-2 for (100) and 0.088 Å-2 for (110). With the ability to form a Pt catalyst with the lower density facet exposed, engineering of substrate adsorption could be possible. Further, research on the synthesis of single crystalline Pt wires with preferential exposure of the (110) plane has been reported.137 In a typical experiment, silica support (FSM-16) was impregnated upon immersion into H2PtCl6 solution. After the impregnation, the seeded sample was exposed to CO/H2O at high temperature and illuminated by Hg lamp for 36 hrs. The length of the synthesized Pt wires ranged from 30 – 100 nm (Figure 7E). Diffraction analysis showed the nanowire to be a single crystal with cubic symmetry and fringes attributed to the (110) plane were observed by TEM. The exposure of a preferred orientation of the nanowire was completed by growth of cubic Pt single crystals directly inside the mesoporous channel, which shielded the non-growth axis facets. This then exposed the (110) facet, which tended to bond with CO at lower temperatures, resulting in a four times activity increase in water-gas shift reaction compared with control groups.

5. Intercalation of metal nanoparticles into the wall of mesoporous silica Often, nanoparticles of metals such as Au and Pd are found to be extremely mobile on silica surfaces and readily form large (50+ nm) unreactive particles when treated at high temperatures.143-144 Numerous strategies have been developed to impart stability to nanoparticles through varying their interface between


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mesoporous silica including functionalizing the surface of the silica and intercalating the nanoparticles in the silica framework.145-147 Immobilization of monodisperse Au nanoparticles on mesoporous silica MCM-41 was achieved by use of APTMS and MPTS.148 The supported Au nanoparticles were found to be highly active in hydrogenation reactions and no leaching of Au was found in the reaction.149 Following the report of surface modification of mesoporous silica, methodology for intercalation of Au and later Pd intercalated into the wall of mesoporous silica (GMS) was reported.37,150 The synthesis approach was a one-pot sol-gel process with P123 as the template and TEOS as the silica source, a surface modification agent (bis[3-(triethoxysilyl)propyl]-tetrasulfide (TESPTS) or APTMS) along with metal precursor was added into the solution.

Figure 7. Selected nanowire/nanoparticles directed synthesized in the channel of mesoporous silica. A. Shape controlled Pt nanoparticles directed formed in the channel of mesoporous silica. Reproduced from Ref. 131 with permission from The Royal Society of Chemistry; B. Silver wires synthesized in the channel of mesoporous silica. Reprinted with permission from Electrodeposition of Copper and Silver Nanowires in Hierarchical Mesoporous Silica/Anodic Alumina Nanostructures Andreas Keilbach, James Moses, Ralf Köhn, Markus Döblinger, and Thomas Bein Chemistry of Materials 2010 22 (19), 54305436. Copyright 2010 American Chemical Society; C. Au nanoparticles confined in the channel of mesoporous silica. Reprint from ref. 134; D. Single crystalline Pt nanowires formed in the channel of mesoporous silica. Reprint from ref. 140; E. Au nanorods evenly distributed in the channel of mesoporous silica. Reprinted with permission from Size Tunable Gold Nanorods Evenly Distributed in the Channels of Mesoporous Silica Zhi Li, Christian Kübel, Vasile I. Pârvulescu, and Ryan Richards ACS Nano 2008 2 (6), 1205-1212. Copyright 2008 American Chemical Society.



As the hydrolysis and condensation of silica proceeds, the metal precursor is trapped within the forming silica walls. After a calcination process, metal nanoparticles were found to be intercalated into the welldefined wall of SBA-15. Figure 8 shows a schematic of the process of the formation. The synthesized catalysts possess an unaltered pore structures, indicating mesoporous channels are still open and unblocked. Even after high temperature calcination, the pore network remains intact as confirmed by nitrogen physisorption. Metal nanoparticles observed in the intercalation structure had diameters from 4-7 nm. The role of tuning the TESPTS to surfactant ratio and its impact on the final catalyst structure was also extensively investigated.151 Following the typical acid-catalyzed surfactant-templated condensation scheme for producing MSN, the amount of TESPTS to P123 was held at 0.125, 0.25, 0.5, 1, 2, and 30 mol ratio. Through the application of TEM, nitrogen sorption, and SAXS it was observed that for a TESPTS/P123 ratio below 0.5 mesoporosity was maintained, while larger ratios favoured a cellular foam. The foam appeared to have some mesoporosity, yet due to the lack of pore ordering, these high surface area regions were inaccessible to substrates during catalytic benchmarking. Results assisted in elucidating how high Au content samples demonstrated reduced catalytic activity. Intercalated materials prepared per the co-condensed thioether approach outlined in Figure 8 possess two noteworthy traits, recyclability and thermal robustness. It was demonstrated that after calcination at 850 ˚C for 3 h, Pd nanoparticles retained surface area, dispersion and average diameter that were almost unchanged according to TEM, BET, and XRD measurements (Figure

8).37,151-152

To

demonstrate

recyclability, the catalyst was applied to

Figure 8. TEM micrographs of SBA-15 type mesoporous silica with 24 nanostructured Pd embedded in the pore walls. Shown above are images capturedPlus at various temperatures during an in situ heating experiment, ACS Paragon Environment yielding non-detectable amounts of particle migration/sintering on the MSN@Pd system at all temperatures investigated.

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a benchmark reaction based on the oxidation of benzyl alcohol. The reaction condition was set at 170 ˚C with catalyst recycled for three times. The intercalation system showed consistent conversion and selectivity after recycling three times, exhibiting advantage over other Au catalysts in the same reaction. The catalyst was also used in the reduction of 4-nitrophenol with CS2 poisoning. Results showed that the molar ratio of poisoning agent over Au active site was ~20:1 for complete deactivation. The enhanced chemical stability, manifested as poisoning resistance, was because metal nanoparticles were inside the wall of mesoporous framework. Encapsulation in silica network enshrouds the nanoparticles, limiting CS2 exposure and inhibiting poisoning. Progress in the partial intercalation of metallic nanoparticles within the walls constituting the pore channels of MSN has also been made possible.153 The approach focused on forming templated organosilane networks which possessed dithioether functionality. It was reported that by forming a bissilylated disulphide precursor with a hydrophilic core, the long-range ordering of the 2D mesoporous material was preserved as observed by SAXS. Figure 9 shows a graphic example of a synthesis on incorporating Au metal into the pore walls of mesoporous silica.

Figure 9. Schematic illustration of synthesis of Au intercalated in the wall of mesoporous silica based on work by Hu et al.137

The precursor was believed to have successfully co-condensed with the template surfactant due to variation in the T and Q sites observed by

29

Si CP-MAS solid-state NMR. Once a suitable method for

incorporating disulphide groups into the MSN was discerned, quantitative reduction to thiols was



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performed by the addition of triphenylphosphine. Finally, an organo-gold precursor was introduced at ratio of at most 0.5 Au/S to prevent outer-pore growth and reduced via NaBH4 to produce ~2 nm nanoparticles within the framework of the support. Extension of this methodology may one day enable controlled placement of catalytically active particles partially within the wall of the support, offering enhanced thermal stability. Most recently, a soft-enveloping approach has enabled controlled growth of metal nanoparticles inside mesoporous silica templates.154 The method utilizes a soft-enveloping reaction at the solid, liquid interface to grow nanostructures of wires, alloys and super lattices of nanoparticles. Thus, this approach seemingly opens a new pathway to metal nanoparticle/mesoporous silica hybrid materials with increasing complexity. 6. Conclusion and Future Outlook Modern research has produced a variety of methods which successfully support noble metal nanoparticles on mesoporous silica producing catalytic materials exhibiting high activity in harsh conditions and substantial recyclability characteristic. The research of noble metal nanoparticles in porous structures is transformational for the field of catalysis as it could provide a paradigm whereby these materials could be employed in rigorous reaction conditions in which they would typically exhibit decreased performance as active metal domains grow. Additionally, support by mesoporous silica offers a scalable approach to potentially independently alter the catalyst and porous environment. Since catalytic processes are of vital economic importance and ubiquitously employ noble metals, the benefits of increased performance and recyclability could be far reaching. Finally, added selectivity or unique productivity afforded by this class of catalyst may lead to the “green” chemistry principal of increased atom economy. Despite these benefits, the synthetic conditions to produce the discussed hybrid mesoporous silica materials have room to improve with respect to expense and time. Discovery of less expensive, greener surfactants for mesoporous silica nanoparticle templating is a major hurdle. Relative to current templating surfactants, less expensive candidate surfactants will ideally continue to produce


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particles with a narrow particle and pore size distribution, are non-toxic and easily handled, and undergo negligible side-reactions during templating. The high cost associated with noble metals is significant compared to non-precious/earth abundant metal alternatives which are currently an important active area of research to replace current noble metal catalysts. Examples of catalytically active metal species that have the potential to replace some noble metals are molybdenum and tungsten carbides/nitrides. Combining their valence sp electrons with metal spd bands results in structures with chemical reactivity that resembles that of noble metals. This field of study has garnered varied success, but some applications are more challenging in this pursuit. Investigating the distribution of metal carbides and metal nitrides on inorganic supports, either mesoporous or nonporous, is an important area of scientific exploration.155-157 Potential areas of exploration include, but obviously not limited to, homogenous distribution of metal carbides/nitrides within the silica matrix or incorporation of individual metal carbide/nitride entities (nanoparticles) distributed on the silica surface or intercalated in the silica matrix, analogous to the noble metals in this review. An area of great interest to the authors is the 2- and 3- dimensional organization of multiple types of porosity (with controlled connectivities) and functionalities (including ie. homo- and heterogeneous catalysts, magnetic, size and shape selectivity, plasmonics, etc.). A further area that has potential to transform the field but is just beginning to be utilized in catalysis is additive manufacturing.107,158-160 Selection and utilization of supported noble metals for industry-scale catalysis have impacts beyond their monetary cost. The global distribution of these metals is not uniform and security of demand is a substantial issue for companies and countries as a whole. Hopefully, we will soon have methods to use noble metals more efficiently and alternative active materials that can act as substitutes to our current metal technology. Hybrid mesoporous silica/ metal materials will have a profound positive impact on the environment and improve the economic feasibility and efficiency of important industrial processes.

Acknowledgements



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The authors acknowledge NSF for financial support under grants CHE-1214068 and CHE1508728.

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