High Surface Area Silica Research Articles

The effect of heat-treatment on the physico-chemical properties of silica aerogel prepared by sub-critical drying technique

Synthesis of transparent and crack-free monoliths of silica aerogel by sub-critical drying technique is reported in the present article. Silane ageing with 50% tetraethylorthosilicate:ethanol followed by solvent exchange using ethanol was adopted. The effect of heat-treatment on the textural and physical characteristics of silica aerogel was evaluated. The chosen composition resulted in a high surface area silica aerogel of 1,000 m2 g−1 and a pore volume of 1.4 cm3 g−1 at room temperature. The aerogel heat-treated at 900 °C possessed a surface area of 450 m2 g−1 with a pore volume of 0.4 cm3 g−1. The decrease in surface area and pore volume was associated with the sintering process. The present technique seems advantageous in preserving the high surface area of the material at high temperatures. The XRD studies showed that the amorphous nature of aerogel matrix was retained till 1,400 °C, beyond which it crystallized to phase pure crystoballite.

A novel way for preparing high surface area silica monolith with bimodal pore structure

The crack-free silica monolith with macropores and mesopores has successfully been achieved in the presence of citric acid as nonsurfactant via sol-gel reactions of tetramethoxysilane (TMOS). Citric acid was removed by calcination to afford monolithic bodies with high specific surface area of 648 m2/g, pore volume of 0.9 cm3/g. Poly (ethyl glycol) has been used together with citric acid to control the particle aggregation and internal structure. Macropores in the micrometer range originate from the spinodal phase separation and gelation kinetics. Textural mesopores in the 2–8 nm range are controlled through adding citric acid and postsynthesis treatment in ammonia solution. By employing the glycerol as drying control chemical additives (DCCA), cracks of the materials can be successfully avoided.

Mechanistic Details of Atomic Layer Deposition (ALD) Processes

Several examples are provided where the reaction mechanisms of atomic layer deposition (ALD) processes have been characterized by a combination of surface-sensitive techniques. In the first, a proposal is advanced for the layer-by-layer growth of metal films using metal carbonyl precursors and hydrogen as a carbon monoxide displacement agent. For copper film growth, the use of amidinate complexes has being characterized. A third example comprises the deposition of TiN films from TiCl4 and ammonia. Finally, the use of Zr(EtMe)4 as a precursor for zirconium nitride ALD film growth on high surface-area silica is briefly explored.

Visible-light-induced hydrogen production over Pt-Eosin Y catalysts with high surface area silica gel as matrix

A new system for the production of hydrogen, constructed using silica gel as a matrix, Eosin Y as a photosensitizer, and Pt as a cocatalyst, has been reported. It was found that the rate of photosensitized hydrogen evolution in the presence of silica gel is enhanced about 10-fold relative to the homogeneous phase, i.e. in the absence of silica gel. The pH value of the solution and the concentration of Eosin Y have remarkable effects on the rate of hydrogen evolution. The optimal pH and concentration of Eosin Y are 7 and 3.60 × 10 −4 mol dm −3 (E/S = 1/3) to 7.24 × 10 −4 mol dm −3 (E/S = 1/1), respectively. Triethanolamine (TEOA) as an electron donor, the rate of hydrogen evolution and the apparent quantum efficiency in the silica gel system under visible-light irradiation ( λ ≥ 420 nm) can reach about 43 μmol h −1 and 10.4%, respectively. In addition, the roles of silica gel, Pt and TEOA, respectively; and the probable mechanism of photosensitized hydrogen evolution have been discussed.

Novel aspects of the physical chemistry of Co/SiO2 Fischer–Tropsch catalyst preparations

Abstract Co/SiO 2 catalysts were prepared by aqueous cobalt nitrate impregnations of silicas with different surface areas to study the effect of the support surface area on the reactions occurring during impregnation and calcination and to define the stage and mode of metal–support interactions. TPR analyses of samples calcined in dry air showed the presence of various quantities of cobalt silicate species, while cobalt silicate formation was not discernible by other analytical techniques. Our conclusion, confirmed in our later studies, is that cobalt silicate does not form during impregnation or calcination, but is created during the reduction in the TPR instrument. Because of these and other ambiguities of the TPR analyses, in our continuing studies we preferred alternative analytical approaches. These studies on the calcination stage resulted in the following unusual findings: (1) X-ray photoelectron spectroscopy revealed drastic decreases in the surface cobalt concentration after calcination of high surface silicas impregnated with cobalt nitrate solutions. (2) Infrared spectroscopy indicated much less than expected Co 3 O 4 formation upon calcination if high surface area silica was the support. (3) A method was devised to calculate the surface areas of individual components in mixtures. The calculations indicated about 20% surface area losses for the silica in calcined catalysts. (4) Scanning electron micrographs of a calcined catalyst on high surface area silica support showed smaller-sized decorations around the larger silica particles. Energy-dispersive X-ray analysis of the decorations showed Si as major, and Co as a minor component. Pure Co 3 O 4 phases were not found by EDX analyses of these decorations. These four seemingly unrelated findings are attributed to a common cause: silica migration and weak bond formation between CoO and SiO 2 . The extent of surface area losses (i.e. the extent of silica migration) is about an order of magnitude greater in CoO x –SiO 2 catalysts than in analogously treated SiO 2 . The migration of silica must have occurred in a relatively short time period during the thermal decomposition of cobalt nitrate, while simultaneous migration and oxidation of CoO to Co 3 O 4 aggregates also occurred. The CoO species intercepted by SiO 2 were unable to oxidize, resulting in reduced quantity of Co 3 O 4 formation. The extensive migration of silica is attributed to strong attraction between SiO 2 and CoO species, inducing the removal of silicic acid or silica molecules from the silica surface.

The oxygen-assisted transformation of propane to CO x/H 2 through combined oxidation and WGS reactions catalyzed by vanadium oxide-based catalysts

This paper reports about the gas-phase oxidation of propane catalyzed by bulk vanadium oxide and by alumina- and silica-supported vanadium oxide. The reaction was studied with the aim of finding conditions at which the formation of H 2 and CO 2 is preferred over that of CO, H 2O and of products of alkane partial oxidation. It was found that with bulk V 2O 5 considerable amounts of H 2 are produced above 400 °C, the temperature at which the limiting reactant, oxygen, is totally consumed. The formation of H 2 derived from the combination of: (i) oxidation reactions, with generation of CO, CO 2, oxygenates (mainly acetic acid), propylene and H 2O, all occurring in the fraction of catalytic bed that operated in the presence of gas-phase oxygen, and (ii) WGS reaction, propane dehydrogenation and coke formation, that instead occurred in the fraction of bed operating under anaerobic conditions. This combination of different reactions in a single catalytic bed was possible because of the reduction of V 2O 5 to V 2O 3 at high temperature, in the absence of gas-phase oxygen. In fact, vanadium sesquioxide was found to be an effective catalyst for the WGS, while V 2O 5 was inactive in this reaction. The same combination of reactions was not possible when vanadium oxide was supported over high-surface area silica or alumina; this was attributed to the fact that in these catalysts vanadium was not reduced below the oxidation state V 4+, even under reaction conditions leading to total oxygen conversion. In consequence, these catalysts produced less H 2 than bulk vanadium oxide.

Intumescent mineral fire retardant systems in ethylene–vinyl acetate copolymer: Effect of silica particles on char cohesion

Silica particles of different physical properties (particle size, morphology and specific surface area) were introduced into an ethylene–vinyl acetate copolymer filled with an intumescent mineral fire retardant system containing magnesium hydroxide and organo-modified montmorillonites. The effect of silica particles on the cohesion and strength of the foamed combustion residue was studied by micro-indentation, concurrently with the characterization of material's fire behavior. Silica particles seem to act as defects that generate cracks, decreasing the strength of the residue. This generation of cracks is connected to a lower cohesion of the residue, especially for silica particles of high specific surface area. In parallel, the presence of silica of high specific surface area, despite tending to decrease Time To Ignition, also improves self-extinguishing ability, thanks to a better thermal stability at higher temperatures, and reduces the maximum Heat Release Rate in cone calorimeter tests.

Synthesis of carbon nanofibers and measurements of hydrogen storage

The synthesis of carbon nanofibers was carried out by catalytic decomposition of ethylene in presence of hydrogen. Bimetallic catalysts, e.g. Fe–Cu or Ni–Cu, were synthesized by coprecipitation, reduction–precipitation and reverse microemulsion techniques and were proven to have a strong influence on the morphology of the nanofibers. The best results in terms of synthesis homogeneity were obtained by supporting the bimetallic catalyst on a high surface area silica support by the “incipient wetness” method. The hydrogen storage capacity of carbon nanofibers was tested in a custom made Sievert apparatus operating up to 160 bar and 450 °C. Several “in situ” activation procedures were experimented, however according to our data carbon nanofibers do not seem a suitable candidate for hydrogen storage. With the purpose of promoting a “spillover” function, 2 wt.% Pd-doped nanofibers were prepared. After loading at 77 bar, a hydrogen storage of 1.38 ± 0.30 wt.% was measured at room temperature.

Spatial near‐infrared imaging of hydroxyl band coverage on ceria‐based catalysts

AbstractHigh‐throughput near‐infrared imaging was used to distinguish catalyst activity for low‐temperature methane steam‐reforming. Geminal hydroxyls of reduced ceria were depicted during methane reforming at 673 K. The changes in absorbance maps under various water partial pressures showed evidence of formate intermediate formations without redox exchanges. Higher resolution was observed in absorbance change images than that of thermal images obtained from catalyst surface self‐emissions. The experimental results illustrated higher activity of pure rhodium catalyst than that of bimetallic ones, likely because of the high dispersion of rhodium on the catalyst support. Moreover, the reaction was accelerated when high surface area silica was added because more reduced sites were exposed. Our filter bandwidths limited our interest in band‐shift distribution of geminal hydroxyl band during the reduction process. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Chemical origins of permanent set in a peroxide cured filled silicone elastomer – tensile and 1H NMR analysis

The aging of a commercial filled siloxane polymeric composite in states of high stress and Co-60 γ-radiation exposure has been studied. DC-745 is a commercially available silicone elastomer consisting of dimethyl, methyl-phenyl, and vinyl-methyl siloxane monomers crosslinked with a peroxide vinyl specific curing agent. It is filled with ∼ 30 wt.% mixture of high and low surface area silicas. This filled material is shown to be subject to permanent set if exposed to radiation while under tensile stress. Tensile modulus measurements show that the material becomes marginally softer with combined radiation exposure and tensile strain as compared to material exposed to radiation without tensile strain. In addition, the segmental dynamics as measured by both uniaxial NMR relaxometry and Multiple Quantum NMR methods indicate that the material undergoes radiatively-induced crosslinking in the absence of tensile strain. In the presence of tensile strain, relaxometry and MQ NMR studies show a strain dependent change in the dynamic order parameter and in the number of polymer chains associated with the filler surface. Solvent swelling measurements indicated no dependence on network crosslink density on strain ratio. Variable tau CPMG echo experiments indicate that a fraction of the polymer chains diffuses through areas of strong magnetic field gradients both at the filler–polymer interface and adjacent to micro-voids within the network. The population of the polymer chains influenced by the field gradients was observed to be dependent on the cumulative dose and degree of tensile strain applied during exposure. The relative change in crosslink density from the NMR and solvent swelling data deviates from that predicted from the Tobolsky model, particularly at higher doses. The likely reasons for this deviation are changes in the filler–polymer interface, increasing deviation from Gaussian chain statistics, and/or the formation of increased numbers of elastically ineffective network chains.

Adsorption and Diffusion Behavior of Ethane and Ethylene in Sol-Gel Derived Microporous Silica

High surface area silica (500 m2/g) was synthesized by the sol-gel method from tetraethyl orthosilicate. The total porosity of the sample was 37% and most of the pores were well below 2 nm in size. The adsorption characteristics of ethylene and ethane in the silica were measured from 300–350 K by gravimetry, and Langmuir adsorption constants and enthalpies and entropies of adsorption were determined. Quasielastic neutron scattering was used to determine the translation and rotational diffusivities of both adsorbates from 200–270 K. Based on the adsorption and translational diffusion characteristics of ethylene and ethane, separation factors of 1.1–2 for olefin to paraffin are predicted.

Oxidation Resistant Germanium Nanowires: Bulk Synthesis, Long Chain Alkanethiol Functionalization, and Langmuir−Blodgett Assembly

A simple method is developed to synthesize gram quantities of uniform Ge nanowires (GeNWs) by chemical vapor deposition on preformed, monodispersed seed particles loaded onto a high surface area silica support. Various chemical functionalization schemes are investigated to passivate the GeNW surfaces using alkanethiols and alkyl Grignard reactions. The stability of functionalization against oxidation of germanium for various alkyl chain lengths is elucidated by X-ray photoelectron spectroscopy. Among all schemes tested, long chain alkanethiols (> or = C12) are found to impart the most stable GeNW passivation against oxidation upon extended exposure to ambient air. Further, the chemically functionalized oxidation-resistant nanowires are soluble in organic solvents and can be readily assembled into close-packed Langmuir-Blodgett films potentially useful for future high performance electronic devices.

Adsorption Enhancement Mechanisms of Silica−Titania Nanocomposites for Elemental Mercury Vapor Removal

A novel nanocomposite that combines high-surface area silica with the photocatalytic properties of titania has been developed that allows for effective capture of elemental mercury vapor. The adsorption capability of the developed material has been found to improve after periods of photocatalytic oxidation. In this study, the mechanisms for adsorption enhancement were identified. BET nitrogen adsorption and mercury porosimetry were used to evaluate pore structure, and the results suggest that a decrease in contact angle was likely to be responsible for improved mercury capture over time. Contact angle measurements showed a significant change of more than 10 degrees, indicating greater attraction to mercury for the used pellets due to deposited mercuric oxide. ICP and TGA analyses showed that mercury was captured as both elemental mercury (Hg0) and mercuric oxide (HgO). In addition, it was shown that pellets used for nearly 500 h still showed greater than 90% removal efficiency and had an average capacity of 10 mg of Hg/g based on mass balance calculations, while some pellets had a capacity over 30 mg of Hg/g according to ICP and TGA analyses. Mercuric oxide doped pellets removed 100% of elemental mercury without pretreatment. The superior mercury removal efficiency combined with various advantages of the novel composite demonstrates its use as an effective alternative to conventional activated carbon injection technology.

Enzymes supported on ordered mesoporous solids: a special case of an inorganic–organic hybrid

The discovery in the late 1990s of ordered, high surface area silicas with pore sizes of 5 nm and above opened the way to the study of well-defined biomolecule–mesoporous silica hybrids. In particular, it has been possible to immobilize a range of small to medium size enzymes, such as proteases, lipases and peroxidases, via physisorption, encapsulation and tethering on the internal surfaces of the solids. Use has also been made of silicas functionalized for this purpose. In many cases the immobilized enzymes are both active and re-usable. Here we review the studies on enzymes immobilized on ordered mesoporous solids and assess the need for careful studies in real applications. Furthermore, we note the emerging applications of related biomolecule–mesoporous solid hybrids in other applications, such as intracellular drug delivery and transfection technology.

Nanobuilding blocks based on the [OSiO1.5]x (x= 6, 8, 10) octasilsesquioxanes

The dissolution of high surface area silica with tetralkylammonium hydroxides at concentrations greater than 0.2 M, produces polyanionic cage structures, [R4N+][−OSiO1.5]x or Qx, where x = 6, 8 or 10. The value x depends on the type of alkyl or alkanol group. If R = Me or Me3NCH2CH2OH+ group (choline) x = 8 is favored. These silicate anions can be functionalized by reaction with chlorosilanes or siloxanes to produce [RSiMe2OSiO1.5]x. The octafunctional molecules are 1.2–1.4 nm in diameter, each functional group occupies a different octant in Cartesian space and as such all functional groups are either orthogonal to or opposite each other. This highly symmetrical structure offers the opportunity to construct multiple kinds of materials one nanometer at a time in one, two or three dimensions. In this review, we discuss methods of synthesizing the polyanionic silicates, their transformation into octafunctional building blocks, “cubes,” and their use in formulating polyfunctional compounds and nanocomposites therefrom. Examples of tailoring global properties by tailoring the chemistries used to link cubes are given.

Investigations into the utility of high-surface area silica pellets as potential solid-phase synthesis supports

Grace-Davidson silica pellets (SMR-057-015) are found to be optimal for surface grafting of (RO) 3Si(CH 2) 3FG reagents. While loadings of up to 2.1 mmol g −1 can be attained (ca. 0.05 mmol per pellet) access of further reagents to the graft sites is problematic above loadings of 0.8 mmol g −1. 13C CPMAS NMR studies may be carried out on individual pellets (using natural abundance substrates) and the resulting spectra are diagnostic in identifying successful subsequent coupling reactions.

Room Temperature Solid Surface Water with Tetrahedral Jumps of 2H Nuclei Detected in 2H2O-Hydrated Porous Silicates

The nature of water adjacent to solid silicate surfaces in kanemite, rehydrated zeolite A, rehydrated highly porous glass, rehydrated high surface area silica gel, and the hydration products of tricalcium silicate has been investigated with 2H NMR techniques and calorimetry. The room temperature 2H NMR quadrupole echo spectra of all samples show a sharp central resonance that corresponds to 2H2O. The kanemite, porous glass, and silica gel spectra show in addition powder patterns that were assigned, based on previous findings for kanemite, to silanol −OH groups experiencing rapid 3-fold jumps or rotational diffusion about the Si−O bond. The spectrum of hydrated tricalcium silicate shows in addition to the aqueous peak a rigid powder pattern that is assigned to Ca(O2H)2. Spectra obtained at −120 and −150 °C for the kanemite sample show that the qcc values for the aqueous deuterons range from 180 to 210 kHz, as would be expected for the different water environments observed in the X-ray structure of kanemite...

Synthesis and characterization of dendrimer templated supported bimetallic Pt-Au nanoparticles.

Bimetallic dendrimer-stabilized nanoparticles (DSNs) were used to prepare supported Pt-Au catalysts within the bulk miscibility gap for this binary system. Hydroxy-terminated generation 5 PAMAM dendrimers were used to prepare Cu(0) nanoparticles (NPs). The Cu(0) NPs were subsequently used to reduce K(2)PtCl(4) and HAuCl(4), preparing stabilized bimetallic Pt-Au NPs with a 1:1 stoichiometry. The stabilized NPs were adsorbed onto a high surface area silica support and thermally activated to remove the dendrimers. Transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and infrared spectroscopy of adsorbed CO showed that this preparation route resulted in NPs in which the two metals are intimately mixed and that the majority of the bimetallic NPs were smaller than 3 nm. Further, the bimetallic NPs were highly active for CO oxidation catalysis near room temperature and showed evidence of CO induced restructuring at ambient temperatures.

Preparation of Silane‐Grafted Pellets: Silica Bound Reagents in a Very Convenient Form.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation and characterization of nano-structured silica from rice husk

Abstract Uniformly sized ultrafine silica powder can be obtained by nonisothermal decomposition of rice husk in an oxidizing atmosphere. The properties of reactant and product including morphology, particle size, surface area, pore volume and pore distribution, have been investigated by TEM, SEM, XRD, FTIR, ICP-MS, and EA. At a heating rate of 5 K/min, the specific surface area of the silica powder was 235 m 2 /g, the average pore diameter was 5.4 nm, and the average particle size was 60 nm. The products obtained from various heating rates were all amorphous. By using a thermogravimetric analysis technique, a mechanism including two reaction stages was observed for the thermal decomposition of rice husk in air. The activation energy was found to be 166±10 kJ/mol. This method can conveniently provide preparation of silica of high surface area and nanometer grade.