High Surface Area SiO2 Research Articles

Studies on the performances of silica aerogel electrodes for the application of supercapacitor

Silica aerogel (SiO2 aerogel) was prepared by sol–gel method from tetraethyl orthosilicate hydrolyzation and has been characterized by scanning electron microscopy and N2 adsorption for its surface structure, surface area, and pore-size distribution. Constant current charge–discharge technique, cyclic voltammetry, and electrochemical impedance spectrum were employed for its specific capacitance and equivalent series resistance. The results showed that the maximum specific capacitance of SiO2 aerogel electrode in 1 M Et4NBF4/PC electrolyte was 62.5 F g−1. In addition, the SiO2 aerogel capacitor exhibits excellent long-term stability with no significant degradation after 500 charging and discharging cycles. Therefore, the application of high surface area SiO2 aerogel as electrodes in supercapacitor devices is promising.

Molecular Layer Deposition of Nylon 66 Films Examined Using in Situ FTIR Spectroscopy

Molecular layer deposition (MLD) techniques can utilize sequential, self-limiting surface reactions with bifunctional monomers to grow polymer films. In this study, transmission in situ Fourier transform infrared (FTIR) investigations were utilized to monitor the growth of nylon 66 MLD films using adipoyl chloride (AC) and 1,6-hexanediamine (HD) as the bifunctional monomers. In situ FTIR measurements on high surface area SiO2 powders monitored the linear growth of the integrated absorbance of the N−H, C−H, C−N, and CO stretching vibrations and the CO−N−H bending vibrations versus the number AC and HD sequential exposures. The growth of the integrated absorbance for the nylon 66 features was also self-limiting as a function of the individual AC and HD exposures. In addition to the vibrational modes expected for nylon 66, vibrational features were also observed that were consistent with some ammonium chloride salt incorporated in the film during nylon 66 MLD. The temperature dependence of the nylon 66 growth rate for constant exposures revealed that the highest growth rates were observed at the lowest sample temperature of 62 °C. This temperature dependence was consistent with a precursor-mediated adsorption model. Nylon 66 MLD was studied with and without surface functionalization of the SiO2 substrate using (3-aminopropyl)triethoxysilane (APS). The APS functionalization facilitated the reaction of AC with the amine-terminated surface. The growth rate of the nylon 66 MLD films was estimated using in situ transmission FTIR measurements on flat KBr substrates with an amine-terminated Al2O3 ALD overlayer. The integrated absorbance of the C−H vibrational stretches was utilized to estimate the nylon 66 MLD growth rate per AC/HD reaction cycle. The observed growth rates were both above and below the expected growth rate for one nylon 66 bilayer. This range of growth rates illustrates the difficulties involved in using bifunctional monomers for MLD.

Bifunctional nature of a SiO 2-supported Ni 2P catalyst for hydrotreating: EXAFS and FTIR studies

A Ni 2P catalyst supported on a high-surface area SiO 2 (350 m 2 g −1) was prepared by temperature-programmed reduction, and its structural and surface properties were studied. X-ray diffraction and extended X-ray absorption fine structure measurements were used to obtain structural parameters for the supported Ni 2P phase, and Fourier transform infrared (FTIR) analysis with the probe molecules CO and pyridine was carried out to characterize the surface properties. The catalytic activity was measured at 573 K and 3.1 MPa in a three-phase fixed-bed reactor for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) using a model liquid feed. At standard conditions using 500 ppm S as 4,6-dimethyldibenzothiophene (4,6-DMDBT), 3000 ppm S as dimethyldisulfide, 200 ppm N as quinoline, and 1% tetralin in a tridecane solvent, the Ni 2P/SiO 2 gave an HDS conversion of 85%, an HDN conversion of 100%, and a tetralin conversion of 37%, which were much higher than those of a commercial Ni–Mo–S/Al 2O 3 catalyst, which gave an HDS conversion of 41%, an HDN conversion of 98%, and a tetralin conversion of 20% based on equal numbers of sites (240 μmol) loaded in the reactor. The sites were counted by CO chemisorption for the phosphide and by low-temperature O 2 chemisorption for the sulfide. The Ni 2P/SiO 2 catalyst favored the hydrogenation (HYD) pathway for 4,6-DMDBT HDS to generate methylcyclohexyltoluene and dimethylbicyclohexane with a relative HYD selectivity of 95%. It also favored hydrogenation for tetralin to give decalin with a relative HYD selectivity of 89%. The Ni 2P/SiO 2 catalyst also showed better resistance to N-compounds than the Ni–Mo–S/Al 2O 3 catalyst. The FTIR spectra of adsorbed CO showed that the Ni site in the Ni 2P phase gave rise to considerable π-back bonding, which was related to the high activity of the Ni 2P/SiO 2 catalyst in the hydrogenation of aromatics. The FTIR spectra of adsorbed pyridine showed that the Ni 2P phase had a P OH group associated with Brønsted acidity that was active for the protonation of N compounds. These results thus suggest that the supported Ni 2P catalyst has bifunctional properties that are beneficial for catalytic activity in hydroprocessing.

Stable oxygen-selective sorbents for air separation

Commercial sorption-based air separation is usually done using nitrogen selective zeolites in pressure swing adsorption (PSA) systems. Separation of air by adsorption of less abundant oxygen is more desirable. In this study we have developed some stable oxygen selective sorbents with silver and cerium salts. AgCl, AgBr, AgI and CeCl 3 all showed stable adsorption characteristics with pure component selectivity of O 2 / N 2 ∼ 2.0 –3.0 at 1 atm. For these salts heat of adsorption of oxygen was found to be slightly higher than that of nitrogen, which was also predicted by ab initio molecular orbital calculations by Chen and Yang (Ind. Eng. Chem. Res. 35 (1996) 4020). The adsorption capacity of these salts was increased by thermal dispersion on high surface area SiO 2 support. AgBr thermally spread on SiO 2 is the best sorbent obtained in this study. AgBr / SiO 2 (1.0 g/g) showed a pure component oxygen selectivity of ∼ 3 at 1 atm and ∼ 5 at 7 atm. PSA simulations were used to show the feasibility of nitrogen production using AgBr / SiO 2 .

Oxime Carbapalladacycle Covalently Anchored to High Surface Area Inorganic Supports or Polymers as Heterogeneous Green Catalysts for the Suzuki Reaction in Water.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Kinetics of ethylene combustion in the synthesis of vinyl acetate over a Pd/SiO 2 catalyst

The kinetic parameters of ethylene combustion in the presence of acetic acid, i.e., during the synthesis of vinyl acetate and in the absence of acetic acid, have been measured in the temperature range 413–453 K over Pd supported on a high surface area SiO 2 (600 m 2/g) at pressures ranging from 5.0 to 15.0 kPa of C 2H 4 and 1.0 to 10.0 kPa of O 2. CO 2 formation as a function of the partial pressures of C 2H 4 and O 2 exhibited a negative reaction order with respect to C 2H 4 and a positive order with respect to O 2. The kinetic parameters of ethylene combustion in the presence and absence of acetic acid were essentially the same, consistent with ethylene combustion being primarily responsible for CO 2 formation in the synthesis of vinyl acetate from ethylene and acetic acid over unpromoted Pd within the pressure regime investigated.

Oxime carbapalladacycle covalently anchored to high surface area inorganic supports or polymers as heterogeneous green catalysts for the Suzuki reaction in water.

An oxime carbapalladacycle known as an extremely active homogeneous catalyst for the Suzuki coupling in water has been conveniently modified and anchored on high surface area SiO(2), MCM-41, and polystyrene-divinylbenzene and ethylenglycol dimethylacrylate polymers. The resulting solids were characterized by analytical and spectroscopic (UV-vis and IR) techniques and tested as catalysts for the reaction of 4-chloroacetophenone with phenylboronic acid in water, dioxane, and a mixture of both. Differences in activity depending on the support were remarkable, the palladium complex being more active for the reactions in water when supported on SiO(2) or MCM-41. The catalysts were truly heterogeneous (no leached palladium) and when anchored on SiO(2) were reused seven times without loosing activity. Palladium complex anchored in SiO(2) was also tested as Suzuki catalyst for a wide range of bromo-, chloro-, and even fluoroaromatics.

Anchoring Fe ions to amorphous and crystalline oxides: a means to tune the degree of Fe coordination.

We report on an IR spectroscopic study on the room-temperature adsorption of NO on different iron(II)-containing siliceous matrices. Fe2+ hosted inside the channels of MFI-type zeolites (Fe-ZSM-5 and Al-free Fe-silicalite) exhibits pronounced coordinative unsaturation, as witnessed by the capability to form, at 300 K, [Fe2-(NO)], [Fe2+(NO)2] and [Fe2+(NO)3] complexes with increasing NO equilibrium pressure. Fe2+ hosted on amorphous supports (high surface area SiO2 and MCM-41) sinks more deeply into the surface of the siliceous support and thus exhibits less pronounced coordinative unsaturation: only [Fe2+(NO)2] complexes were observed, even at the highest investigated NO equilibrium pressures. Activation at higher temperature (1073 K) of the Al-free Fe-silicalite sample resulted in the appearance of Fe2+ species similar to those observed on SiO2 and MCM-41, and this suggests that local (since not detectable by X-ray diffraction) amorphisation of the environment around Fe2+ anchoring sites occurs. The fact that this behaviour is not observed on the Fe-ZSM-5 sample activated at the same temperature suggests that framework Al species (and their negatively charged oxygen environment) have an important role in anchoring extraframework Fe2+ species. Such an anchoring phenomenon will prevent a random migration of iron species, with subsequent aggregation and loss of coordinative unsaturation. These observations can thus explain the higher catalytic activity of the Fe-ZSM-5 system in one-step benzene to phenol conversion when compared with the parent, Al-free, Fe-silicalite system with similar Fe content. The nature of the support and the activation temperature can therefore be used as effective means to tune the degree of Fe coordination.

Promoting effect of water vapor on catalytic oxidation of methane over cobalt/manganese mixed oxides

Methane oxidation was conducted in a fixed bed quartz tubular reactor on a series of mixed oxides of cobalt/manganese prepared by a sol–gel method. A unique promoting effect of water vapor on methane conversion was observed for the first time on these cobalt/manganese mixed oxides calcined at 450 or 600 °C. However, these mixed catalysts lost their catalytic activities after being calcined at 850 °C. The catalytic activity of methane oxidation was significantly improved by supporting the cobalt/manganese mixed species onto the high surface area SiO 2 or Al 2O 3–SiO 2 materials. It was noteworthy that the water enhancement effect was retained on these supported catalysts.

Nanostructured Silica-Gel Doped with TiO 2 for Mercury Vapor Control

A novel high surface area SiO2–TiO2 composite has been developed for elemental mercury vapor removal from combustion sources. The composite exhibits synergistic adsorption and photocatalytic oxidation. Mercury vapor in the gas stream is adsorbed, oxidized and stays on the composite. The composite has demonstrated a high mercury capacity (1512ug/g) although in its current 3-mm pellet form only the outer layer is effectively utilized. The loading of 13% TiO2 shows the best removal, both with and without UV irradiation. Increasing TiO2 loading beyond this level does not enhance the removal further. It has also been observed that the composite after being 'activated' by photocatalytic oxidation has better performance, probably due to the change of surface functional groups. The examination of the effects of flow velocity reveals that mass transfer is the rate limiting step. Relative humidity has been found to impede adsorption therefore decreasing the overall removal efficiency. By rinsing with acid, both the deposited mercury and composite can be regenerated.

On a Simple Nonisothermal Adsorption Experiment with Organic Vapors and an Inertial Microbalance To Study the Surface Properties of Hybrid (Organic/Inorganic) Porous Materials

A nonisothermal adsorption experiment using a controlled flow of cyclopentane in the 333–313 K range is used to simultaneously estimate the specific surface area and micropore volume of a hybrid (organic/inorganic) alcogel. For reference, the method is also applied to an all-inorganic material with a more rigid structure, namely, a high surface area SiO2–Al2O3. The proposed data analysis provides guidelines to determine whether adsorption data on a certain adsorbate/adsorbent system can be modeled effectively as a convolution of BET (meso- and macropore) and Dubinin–Radushkevitch (DR, micropore) contributions.

Ethylbenzene Oxidative Dehydrogenation on MnOx/SiO2 Catalysts

Supported MnOx on high surface area SiO2 (300 m2/g) catalysts were successfully used in ethylbenzene oxidative dehydrogenation. X-ray diffraction and X-ray photoelectron spectroscopy were employed to characterize the structure of fresh and used MnOx/SiO2 catalysts. The MnOx catalysts were prepared using the pore volume impregnation method, with MnO2 loading varying from 0.7 wt % (Mn/Si = 0.005, atomic ratio) to 30 wt % (Mn/Si = 0.14). The changes in the crystalline structure and dispersion of supported MnOx were related to the precursor/support interaction and the conditions used during catalyst preparation. A possible mechanism for ethylbenzene conversion to styrene on MnOx/SiO2 catalysts is proposed, where lattice oxygen from crystalline MnO2 can be used in the oxidation or oxidative dehydrogenation processes. The high selectivity in styrene (at 723 K, 24% conversion with 76% selectivity in styrene) was related to the high concentration in the MnO2 phase from the MnOx/SiO2 catalysts. The formation of the Mn3O4 phase, observed on the used catalysts, supports the proposed mechanism. These findings are of potential use for industrial applications, particularly in optimization of various oxidative dehydrogenation processes.

Synthesis and catalytic properties of porous α-Fe2O3/SiO2 catalyst

The synthesis of α-Fe2O3 supported on high specific surface area SiO2 has been studied. The hydroxylation of phenol to hydroquinone/catechol mixture is described with the help of BET and XRD. The result indicates that the dispersity and specific surface area of support have an important influence on the catalytic activity.

Atomic layer controlled deposition of SiO 2 and Al 2O 3 using ABAB… binary reaction sequence chemistry

Sequential ABAB… surface chemical reactions can be employed for atomic layer controlled deposition. We have examined the binary reactions SiCl 4+2H 2O⋌SiO 2+4HCl for SiO 2 deposition and 2Al(CH 3) 3+3H 2O ⋌ Al 2O 3+ 6CH 4 for Al 2O 3 deposition. Each binary reaction (A + B ⋌ products) was performed sequentially by individual exposures to the A reactant and then the B reactant. If each surface reaction is self-limiting, repetitive ABAB… cycling may produce layer-by-layer controlled growth. For example, the individual “A” and “B” surface reactions for SiO 2 deposition can be described by (A) Si–OH + SiCl 4 ⋌ Si–O–SiCl 3 + HCl, (B) Si–Cl + H 2O ⋌ Si-OH + HCl. We have studied ABAB… binary reaction sequences for SiO 2 and Al 2O 3 deposition using laser-induced thermal desorption, temperature-programmed desorption and Auger electron spectroscopy techniques on single-crystal Si(100) surfaces. Fourier transform infrared spectroscopy techniques were also employed to examine these binary reaction schemes on high surface area SiO 2 and Al 2O 3 samples. Controlled deposition of SiO 2 and Al 2O 3 was demonstrated and optimized utilizing the above techniques. Under the appropriate conditions, each surface reaction was self-terminating and atomic layer controlled growth was a direct consequence of the binary reaction sequence chemistry.

Surface coverage of ALE precursors on oxides

The work with single molecular layers on porous materials adds to the atomic level understanding of the film growth in ALE. In this paper the primary factors affecting the surface coverage of three different ALE precursors (TiCl 4, Cr(acac) 3 and hexamethyldisilazane (HMDS) on porous, high surface area SiO 2 were studied by element determinations and FTIR spectroscopy. Also a preliminary low-energy ion scattering (LEIS) study on Ti/silica samples was carried out. First, the surface coverage of metal species achievable at a certain reaction temperature on an oxide is determined by the number of specific reactive sites for the precursor. TiCl 4 utilizes both isolated and H-bonded OH groups while Cr(acac) 3 and HMDS react preferentially with isolated OH groups. Second, the bonding mode affected the surface coverage of titanium species. The bifunctional reactivity of TiCl 4, on the one hand, consumed two OH groups per one TiCl 4 molecule lowering the surface coverage on silica preheated at elevated temperatures, and on the other hand, enabled the utilization of the H-bonded OH groups present on silica preheated at lower temperatures thereby increasing the surface coverage. Third, the steric hindrance between the precursor and the already chemisorbed metal complexes was shown to have an effect on the surface coverage of Cr(acac) 3 because of the size of two remaining acac ligands of bound chromium complex. Hence, at saturating reaction conditions of compound precursors, where undecomposed, reproducible and selectively chemisorbed surface species are obtained, the full monolayer coverage on oxides is seldom achieved.

Incorporation of titania into preshaped silica monolith structures

A series of titania-silica honeycombs with various titanium profiles across the walls of the cells have been prepared by impregnation and homogeneous precipitation-deposition methods. Although the latter method seems to be very useful to deposit an atomic layer of TiO 2 on high surface area SiO 2 microspheres, it does not work if the silica is preshaped in monolith structures. However, quite varied titanium profiles can already be prepared by impregnating the silica monoliths with acid solutions of TiCl 3. As the transport of Ti 3+ from the solution to the pores is controlled by diffusion, convex profiles are usually obtained at short impregnation times, but these tend to flatten off at very long times. Moreover, the amount of titanium incorporated can be reasonably increased by consecutive impregnation steps. Drying of impregnates is also very critical. If drying takes place at 398 K, very sharp titanium profiles are obtained owing to the fast evaporation of water, and the simultaneous Ti(OH) 4 precipitation, on the outer walls surface. Scanning electron microscopy-energy dispersive X-ray and X-ray photoelectron spectroscopy techniques appear to be very useful to monitor the macrodistribution of TiO 2 across the monolith walls. Further information on TiO 2 distribution, including crystal size and location, is derived from X-ray diffraction and mercury porosimetry.

Elementary surface reactions in the preparation of vanadium oxide overlayers on silica by chemical vapor deposition

Elementary processes of the formation of vanadium oxide overlayers by the adsorption-decomposition of VO(OC 2 H 5 ) 3 vapor on high-surface area SiO 2 were studied by IR (infrared) spectroscopy, TPD (temperature-programmed decomposition), and the stoichiometry of the surface reactions. In this study the adsorption-decomposition process will be called a CVD (chemical vapor deposition) cycle. The structure of the vanadium oxide prepared by repeated CVD cycles was characterized by XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy)

Transient Raman detection of excited state electron, transfer to methyl viologen from photoexcited molecular reagents in fluid solution and anchored to SiO 2

The lowest electronic excited state of the complexes [Ru(2,2′-bipyridine) 3] 2+, fac-[ClRe (CO) 3(2,2′-bipyridine)], and fac-[(pyridine) Re (CO) 3(2,2′-bipyridine)] + can be quenched by methyl viologen, MV 2+, N, N′-dimethyl-4,4′-bipyridinium, in fluid solutions. The quenching obeys Stern—Volmer kinetics as deduced from plots of relative luminescence quantum yield vs [MV 2+], and the data are consistent with a quenching process that is essentially diffusion controlled. Pulsed laser excitation (18 ns, 354.7 nm frequency tripled Nd: YAG) of the metal complexes in the presence of MV 2+ shows that a detectable fraction of the quenching results in net electron transfer to form MV +. The MV + is detectable by resonance Raman scattering from the trailing portion of the excitation pulse. Excited state electron transfer to MV 2+ from a photo-excited complex anchored to SiO 2 has also been detected by transient Raman spectroscopy. High surface area SiO 2 was functionalized by reaction with 4-[2-(trimethoxysilyl)ethyl]pyridine to give [SiO 2]-SiEtpyr. Reaction of [SiO 2]-SiEtpyr with [(CH 3CN)Re(CO) 3(2,2′-bipyridine)] + then yields [SiO 2]-[(SiEtpyr) Re (CO) 3 (2,2′-bipyridine)] +. Electron transfer quenching of the photo-excited immobilized Re complex occurs when suspended in CH 3CN solutions of MV 2+ to yield MV + as detected by resonance Raman scattering and by lifetime attenuation in the presence of MV 2+.

Vibrational relaxation of adsorbed molecules: Comparison with relaxation rates of model compounds

Time-resolved measurements of vibrational relaxation rates are reviewed. Depopulation of the vibrationally excited OH oscillator (on high surface area SiO2, ZSM-5, and ZnO) was found to occur on a time scale of 200 ps or less, depending on the substrate. The temperature dependence of both the OH and OD vibrational relaxation rates on SiO2 was found to follow simple multiphonon relaxation models. However, the relative rates for the two isotopes did not scale as predicted by the same theoretical models. Measurements of isotopically substituted silanols and alcohols [i.e., R3SiOH(D) and R3COH(D) dilute in CCl4] also find OD decay times comparable to those of OH. Based on results from these model compounds, the observed relaxation rates for OH/SiO2 appear to be dominated by processes which proceed through a SiOH bending mode.

Infrared spectra of NO, N2O, NO2 and O2 adsorbed on SiO films

Infrared spectra of nitrogen and oxygen adsorbed on high surface area SiO films were investigated at different temperatures and surface coverages. On adsorption of NO at 77 K and at low coverage, decomposition of the molecule takes place with simultaneous oxidation of the surface. At higher coverages N2O is formed. On partially or completely oxidized surface absorptions due to NO dimers are obtained. On adsorption of NO2 at 136, 250, 273 and 300 K on freshly prepared SiO films decomposition of the molecule takes place with formation of NO and here again complexes with the surface are formed which are similar to those obtained when NO is adsorbed.