Optical fiber evanescent wave absorption spectrometry of nanocrystalline tin oxide thin films for selective hydrogen sensing in high temperature gas samples

SnO₂ nanocrystalline material was prepared with a sol-gel process and thin films of the nanocrystalline SnO₂ were coated on the surface of bent optical fiber cores for gas sensing. The UV/vis absorption spectrometry of the porous SnO₂ coating on the surface of the bent optical fiber core exposed to reducing gases was investigated with a fiber optical spectrometric method. The SnO₂ film causes optical absorption signal in UV region with peak absorption wavelength at around 320 nm when contacting H₂-N₂ samples at high temperatures. This SnO₂ thin film does not respond to other reducing gases, such as CO, CH₄ and other hydrocarbons, at high temperatures within the tested temperature range from 300 °C to 800 °C. The response of the sensing probe is fast (within seconds). Replenishing of the oxygen in tin oxide was demonstrated by switching the gas flow from H₂-N₂ mixture to pure nitrogen and compressed air. It takes about 20 min for the absorption signal to decrease to the baseline after the gas sample was switched to pure nitrogen, while the absorption signal decreased quickly (in 5 min) to the baseline after switching to compressed air. The adhesion of tin oxide thin films is found to be improved by pre-coating a thin layer of silica gel on the optical fiber. Adhesion increases due to increase interaction of optical fiber surface and the coated silica gel and tin oxide film. Optical absorption spectra of SnO₂ coating doped with 5 wt% MoO₃ were observed to change and red-shifted from 320 nm to 600 nm. SnO₂ thin film promoted with 1 wt% Pt was found to be sensitive to CH₄ containing gas.     

Synthesis, characterization, and electrochemical properties of ordered mesoporous carbons containing nickel oxide nanoparticles using sucrose and nickel acetate in a silica template

New ordered mesoporous carbons containing nickel oxide nanoparticles have been successfully synthesized by carbonization of sucrose in the presence of nickel acetate inside SBA-15 mesoporous silica template. The obtained samples were characterized by X-ray diffraction (XRD), nitrogen adsorption-desorption, and transmission electron microscopy (TEM). The NiO nanoparticles were embedded inside the mesoporous carbon framework due to the simultaneous pyrolysis of nickel acetate during carbonization. The electrochemical testing of the as-made nanocomposites showed a large specific capacitance of 230 F g⁻¹ using 2 M KOH as the electrolyte at room temperature. This is attributed to the nanometer-sized NiO formed inside mesoporous carbons and the high surface area of the mesopores in which the NiO nanoparticles are formed. Furthermore, the synthetic process is proposed as a simple and general method for the preparation of new functionalized mesoporous carbon materials, for various applications in catalysis, sensor or advanced electrode material.     

Mesoporous silica SBA-15, a new adsorbent for bioactive polyphenols from red wine

The aim of this paper is to evaluate the ability of the mesoporous silica SBA-15 to adsorb polyphenols from red wine. The mesoporous molecular sieve silica SBA-15 was hydrothermally synthesized in acidic media and characterized by SAXRD, BET, EDX and SEM. The adsorption behavior of mesoporous silica SBA-15 was investigated at 5 °C for 24 h using an adsorbent dose of 8 g SBA-15 L⁻¹ red wine. The total polyphenols content expressed as mg of gallic acid equivalents (GAE L⁻¹) was estimated from the standard curve of gallic acid (absorbance at 280 nm). HPLC chromatograms of methanolic extract from mesoporous SBA-15 at 256, 280, 324, and 365 nm exhibits the strong retention of quercetin and cis-resveratrol and a reasonable retention of trans-resveratrol, catechin, epicatechin, rutin, and phenolic acids (meta- and para-hydroxybenzoic, vanillic, caffeic, syringic, salicylic and para-coumaric acids).     

Planting of bis(1,5‐cyclooctadiene) nickel upon silica to harvest NiO (<5 nm) nanoparticles in a silica matrix

Bis(1,5‐cyclooctadiene) nickel [Ni(COD)₂] was employed as a nickel precursor to prepare nickel oxide nanoparticles upon high‐surface‐area mesoporous silica. Under protection of argon, Ni(COD)₂ was dissolved in tetrahydrofuran (THF) to react with surface silanols of mesoporous silica SBA‐15, which formed a black powder after completion of the surface reaction. Calcination of the powder produced ultrafine NiO inside the mesoporous silica matrix, which was evidenced by X‐ray diffraction, N₂ adsorption–desorption, transmission electron microscopy and thermogravimetric analysis. The thermogravimetric analysis suggests that NiO formation is a result of surface nickel species calcination, whereas structural characterization clearly show that NiO nanoparticles of <5 nm are evenly distributed inside the silica SBA‐15 matrix and mesoporosity is well preserved upon calcinations and NiO formation. The surface reaction between Ni(COD)₂ and surface silanols was found for the first time, and the method used here may be extended conveniently to prepare other metal oxide nanoparticles upon high‐surface‐area supports as well. Copyright © 2005 John Wiley & Sons, Ltd.     

Cobalt-doped silica membranes for gas separation

Cobalt-doped silica membranes were synthesized using tetraethyl orthosilicate-derived sol mixed with cobalt nitrate hexahydrate. The cobalt-doped silica structural characterization showed the formation of crystalline Co₃O₄ and silanol groups upon calcination. The metal oxide phase was sequentially reduced at high temperature in rich hydrogen atmosphere resulting in the production of high quality membranes. The cobalt concentration was almost constant throughout the film depth, though the silica to cobalt ratio changed from 33:1 at the surface to 7:1 at the interface with the alumina layer. It is possible that cobalt has more affinity to alumina, thus forming CoOAl₂O₃. The He/N₂ selectivities reached 350 and 570 at 160 °C for dry and 100 °C wet gas testing, respectively. Subsequent exposure to water vapour, the membranes was regenerated under dry gas condition and He/N₂ selectivities significantly improved to 1100. The permeation of gases generally followed a temperature dependency flux or activated transport, with best helium permeation and activation energy results of 9.5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and 15 kJ mol⁻¹. Exposure of the membranes to water vapour led to a reduction in the permeation of nitrogen, attributed to water adsorption and structural changes of the silica matrix. However, the overall integrity of the cobalt-doped silica membrane was retained, given an indication that cobalt was able to counteract to some extent the effect of water on the silica matrix. These results show the potential for metal doping to create membranes suited for industrial gas separation.     

Thermal stability of high surface area silicon carbide materials

The synthesis of mesoporous silicon carbide by chemical vapor infiltration of dimethyl dichlorosilane into mesoporous silica SBA-15 and subsequent dissolution of the silica matrix with HF was investigated. The influence of the synthesis parameters of the composite material (SiC/SBA-15) on the final product (mesoporous SiC) was determined. Depending on the preparation conditions, materials with specific surface areas from 410 to 830 m² g⁻¹ and pore sizes between 2 and 10 nm with high mesopore volume (0.31-0.96 cm³ g⁻¹) were prepared. Additionally, the thermal stability of mesoporous silicon carbide at 1573 K in an inert atmosphere (argon) was investigated, and compared to that of SBA-15 and ordered mesoporous carbon (CMK-1). Mesoporous SiC has a much higher thermal textural stability as compared to SBA-15, but a lower stability than ordered mesoporous carbon CMK-1.     

Co(Ⅲ)官能化SBA-15的制备、表征及其催化环己烯环氧化

通过环戊二烯基修饰的SBA-15(SBA-15-Cp)与马来酸酐的Diels-Alder反应及水解合成了邻二羧酸官能化的SBA-15,并将原位生成的Co(Ⅲ)络合物负载于其上制得Co(Ⅲ)官能化SBA-15样品SBA-15-Co(Ⅲ).傅里叶变换红外光谱、元素分析和X射线光电子能谱法结果证实羧酸官能团和Co(Ⅲ)成功地...     

Palladium-citric acid-ammonium fluoride as a matrix modifier for overcoming of interferences occurring during the direct determination of Sn in aqua regia extracts from environmental samples by D2-ETAAS

When tin is to be determined in such a complex matrix like aqua regia extracts of environmental samples by electrothermal atomic absorption spectrometry (ETAAS), spectral interferences occur when deuterium-lamp (D₂) background correction is used, even using high pyrolysis temperature of 1400 °C achieved with palladium with citric acid chemical modifier. We have found that the further addition of NH₄F to palladium with citric acid chemical modifier is essential for overcoming the above-mentioned problems for which aluminium oxide is most probably responsible. It is supposed, that NH₄F enables volatilization of the alumina matrix formed by hydrolysis from the chloride salt and interfering in a gas phase via the formation of AlF₃ which could be, in contrast to aluminium oxide, removed from the graphite furnace during the pyrolysis stage. Using the proposed chemical modifier, the direct and accurate determination of Sn in aqua regia extracts from rocks, soils and sediments is possible even when using matrix free standard solutions. This presumption was confirmed by the analysis of certified reference samples and by the comparison with inductively coupled plasma time of flight mass spectrometry (ICP-TOFMS) method. Characteristic mass and LOD value for the original sample (10-μL aliquots of sample) was 17 pg and 0.055 μg g⁻¹, respectively.     

X-ray and Raman study of spray pyrolysed vanadium oxide thin films

Vanadium oxide thin films were prepared by spray pyrolysis using solutions of vanadium chloride (VCl₃) with different concentrations on glass substrates heated at 200 and 250 °C. The influence of substrate temperature (T_s) and solution concentration (molarity) on structural and vibrational properties is discussed by using X-ray diffraction and Raman spectroscopy. The results revealed that at 0.05 M and T_s = 200 °C, V₄O₉ thin films are obtained. At 250 °C, V₂O₅ phases with preferential orientation are observed and the films become polycrystalline when the molarity increases.     

PMo or PW heteropoly acids supported on MCM-41 silica nanoparticles: Characterisation and FT-IR study of the adsorption of 2-butanol

Mesoporous silica, prepared in basic conditions, has been loaded (20% weight) with 12-molybdophosphoric (PMo) or 12-tungstophosphoric (PW) acid and calcined at different temperatures ranging between 250 and 550 °C. The samples have been characterised by N₂ adsorption-desorption at −196 °C, transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), UV-visible diffuse reflectance, Raman spectroscopy and temperature programmed reduction (TPR). The acidity and catalytic activity have been, respectively, examined by monitoring the adsorption of pyridine and 2-butanol by FT-IR spectroscopy. The results indicate that PW and PMo acids are highly dispersed on mesoporous silica MCM-41 spherical nanoparticles. While PMo retains its Keggin structure up to 550 °C, PW decomposes at this temperature into crystalline WO₃ and phosphorous oxides. In both cases, the morphology, hexagonal symmetry and long-range order observed for the support are preserved with calcination up to 450 °C. The Brönsted-type acid sites found in all samples, whose surface concentration decreases as the calcination temperature increases, are responsible for the selective formation of cis-butene detected upon adsorption of 2-butanol. The sample containing PW calcined at 450 °C also shows selectivity to methyl ethyl ketone.     

The effect of Zn, Al layered double hydroxide on thermal decomposition of poly(vinyl chloride)

Poly(vinyl chloride)/layered double hydroxide (LDH) composite was prepared by mixing 4 wt% Zn₂Al-CO₃-LDH with PVC and fluxing at 180 °C. The thermal decomposition behaviour of the LDH + PVC composite in air and nitrogen environments was systematically investigated. We found that mixing Zn₂Al-CO₃-LDH into PVC facilitates dehydrochlorination from ca. 300 to 270 °C but reduces the reaction extent to leave more chlorine on the polyene backbones both in air and N₂. We have also found that at 400-550 °C, both in air and N₂, LDH assists the formation of char-like materials and decreases the release of volatile hydrocarbons. From 550 to 800 °C, the char-like materials are mostly retained in N₂ while they are almost completely thermo-oxidized (burned) in air. Thus, addition of Zn₂Al-CO₃-LDH to PVC does not increase the thermal stability, but does promote charring to retard the generation of flame. The influence of LDH on PVC thermal properties has been also addressed mechanically.     

Sol-gel synthesis of hydrous ruthenium oxide nanonetworks from 1,2-epoxides

Hydrous ruthenium oxide (RuO₂·xH₂O) xerogels were synthesized through the addition of a 1,2-epoxide, propylene oxide, to commercial hydrated ruthenium chloride, “RuCl₃·xH₂O,” in ethanol. After a blue-black monolithic gel formed in 4 h, the samples were allowed to age for 24 h and were dried in ambient conditions. The dried samples were then characterized by XPS, XRD, DTA and TGA. XPS showed the Ru(3d_5/2) peak at a binding energy of 281.7 eV, corresponding to that of hydrous ruthenium oxide. XRD data revealed the synthesized material as amorphous. Heating the sample in inert atmospheres caused the complete reduction of the oxide to the zero-valent state, whereas heating the sample in air resulted in both crystalline anhydrous RuO₂ and zero-valent ruthenium, depending on the method of heating. DTA traces showed an endotherm ending at 150 °C, corresponding to the loss of coordinated water, as well as two higher temperature crystallization exotherms when the sample was heated in both inert and oxygen-rich atmospheres. TGA runs also confirmed the complete reduction of the hydrous oxide when heated in nitrogen below 270 °C and the formation of anhydrous ruthenium oxide when heated in air, confirming the XRD results.     

Nitridation under ammonia of high surface area vanadium aerogels

Vanadium pentoxide gels have been obtained from decavanadic acid prepared by ion exchange on a resin from ammonium metavanadate solution. The progressive removal of water by solvent exchange in supercritical conditions led to the formation of high surface area V₂O₅, 1.6H₂O aerogels. Heat treatment under ammonia has been performed on these aerogels in the 450-900 °C temperature range. The oxide precursors and oxynitrides have been characterized by XRD, SEM, TGA, BET. Nitridation leads to divided oxynitride powders in which the fibrous structure of the aerogel is maintained. The use of both very low heating rates and high surface area aerogel precursors allows a higher rate and a lower threshold of nitridation than those reported in previous works. By adjusting the nitridation temperature, it has been possible to prepare oxynitrides with various nitrogen enrichment and vanadium valency states. Whatever the V(O,N) composition, the oxidation of the oxynitrides in air starts between 250 and 300 °C. This determines their potential use as chemical gas sensors at a maximum working temperature of 250 °C.     

Can mono- or di-butyltin chlorides produce tributyltin chloride at elevated temperatures? Implications for applications in chemical vapour deposition

The thermolysis of the butyltin chlorides at 200-300 °C in the liquid phase has been investigated by ¹H, ¹³C, and ¹¹⁹Sn NMR spectroscopy. The stabilities follow the order: Bu₂SnCl₂ > Bu₃SnCl > BuSnCl₃. Only tributyltin chloride showed any evidence of redistribution, giving dibutyltin dichloride, together with metallic tin, butane, and but-1-ene, which would be formed by decomposition of tetrabutyltin. Dibutyltin dichloride decomposed to give mainly butane with no other apparent liquid organotin compound. Butyltin trichloride gave butane, some butene, and metallic tin, and showed no evidence of forming tributyltin chloride by the redistribution reaction, which would have environmental implications for its use in the CVD coating of glass.     

Preparation and characterization of (SBA-15)-La2O3 host-guest composite materials

Lanthanum oxide was successfully incorporated into an SBA-15 mesoporous molecular sieve via the microwave-assisted synthesis method (MASM) for the first time, and was compared with liquid-phase grafting and thermal diffusion methods. A series of characterizations were used to characterize the prepared materials. The results showed that the preparation of (SBA-15)-La₂O₃ host-guest composite materials by MASM has the advantages of simpler operation, higher efficiency and more plentiful lanthanum oxide could be incorporated into SBA-15 compared with other methods. In the prepared host-guest (SBA-15)-La₂O₃ materials, the frameworks of the host molecular sieve were kept intact, their structures were still kept high ordered and the guest lanthanum oxide locates inside the pores of the SBA-15. The sizes of the prepared (SBA-15)-La₂O₃ samples were 340-357 nm. The prepared host-guest composite materials show the properties of luminescence, and the luminescent intensities are about 2 times of bulk La₂O₃.     

An NMR study of the species produced by the reactions of dialkyl magnesium and HCl with calcined silica and the polymerisation performance of Ziegler-Natta catalysts produced from this support material

The effect of calcinations on the silica surface groups and thereby on the activity of Ziegler-Natta catalysts in ethylene homopolymerisation has been studied. Silica was calcined at different temperatures and treated with MgR₂ and HCl. Silica surface groups were identified by using ¹H MAS NMR and ¹³C and ²⁹Si CP MAS NMR techniques. Magnesium, titanium and chlorine were measured by elemental analysis. Ziegler-Natta catalysts were prepared from these supports and subsequently used in ethylene homopolymerisation. Maximum activity was obtained with the catalyst based on 590 °C calcined silica. The results indicate that MgR₂ reacts with siloxane-groups (Si-O-Si) in the 300 °C calcined silica, leaving the hydrogen-bonded hydroxyl-groups unreacted. Low activity Si-O-Ti(Cl)₂-O-Si species are formed after reacting with TiCl₄. The higher activity in the catalyst based on 590 °C calcined silica can be explained by the formation of -Si(R)-O-Si-O-TiCl₃ groups, originating from the siloxane bridges which cannot form in 300 °C calcined silica. Other explanations for the higher activity are a higher Mg/Ti ratio or small amounts of crystal water formed in the 590 °C calcined silica.     

Substantiating the influence of pore surface functionalities on the stability of Grubbs catalyst in mesoporous SBA-15 silica

The influence of pore surface functionalities in mesoporous SBA-15 silica on the stability of a model olefin metathesis catalyst, namely Grubbs I, is substantiated. In particular, it is demonstrated that the nature of the interaction between the ruthenium complex and the surface is strongly depending on the presence of surface silanols. For this study, differently functionalized mesoporous SBA-15 silica materials were synthesized according to standard procedures and, subsequently, the Grubbs I catalyst was incorporated into these different host materials. All of the materials were thoroughly characterized by elemental analyses, nitrogen physisorption at -196 °C, thermogravimetric analyses, solid-state NMR spectroscopy, and infrared spectroscopy (ATR-IR). By such in-depth characterization of the materials, it became possible to achieve models for the surface/catalyst interactions as a function of surface functionalities in SBA-15; for example, in the case of purely siliceous silanol-rich SBA-15, octenyl-silane modified SBA-15, and silylated equivalents. It was evidenced that large portions of the chemisorbed species that are detected spectroscopically arise from interactions between the tricyclohexylphosphine and the surface silanols. A catalytic study using diethyldiallylmalonate in presence of the various functionalized silicas shows that the presence of surface silanols significantly decreases the longevity of the ring-closing metathesis catalyst, whereas the passivation of the surface by trimethylsilyl groups slows down the catalysis rate, but does not affect significantly the lifetime of the catalyst. This contribution thus provides new insights into the functionalization of SBA-15 materials and the role of surface interactions for the grafting of organometallic complexes.     

Adsorptive desulfurization by copper species within confined space

Copper species were incorporated into SBA-15 by solid-state grinding precursor with as-prepared mesoporous silica (SPA). The obtained materials (CuAS) were well-characterized by XRD, TEM, N(2) adsorption, H(2)-TPR, IR, and TG and compared with the material derived from calcined SBA-15 (CuCS). Surprisingly, CuO up to 6.7 mmol·g(-1) can be highly dispersed on SBA-15 by use of SPA strategy. Such CuO forms a smooth layer coated on the internal walls of SBA-15, which contributes to the spatial order and results in less-blocked mesopores. However, the aggregation of CuO takes place in CuCS material containing 6.7 mmol·g(-1) copper, which generates large CuO particles of 21.4 nm outside the mesopores. We reveal that the high dispersion extent of CuO is ascribed to the abundant silanols, as well as the confined space between template and silica walls provided by as-prepared SBA-15. The SPA strategy allows template removal and precursor conversion in one step, avoids the repeated calcination in conventional modification process, and saves time and energy. We also demonstrate that the CuAS material after autoreduction exhibits much better adsorptive desulfurization capacity than CuCS. Moreover, the adsorption capacity of regenerated adsorbent can be recovered completely.     

Carbon nanotube networks as gas sensors for NO2 detection

Networks of different carbon nanotube (CNT) materials were investigated as resistive gas sensors for NO₂ detection. Sensor films were fabricated by airbrushing dispersions of double-walled and multi-walled CNTs (DWNTs and MWNTs, respectively) on alumina substrates. Sensors were characterized by resistance measurements from 25 to 250 °C in air atmosphere in order to find the optimum detection temperature. Our results indicate that CNT networks were sensitive to NO₂ concentrations as low as 0.1 ppm. All tested sensors provided significantly lower response to interfering gases such as H₂, NH₃, toluene and octane. We demonstrate that the measured sensitivity upon exposure to NO₂ strongly depends on the employed CNT material. The highest sensitivity values were obtained at temperatures ranging between 100 and 200 °C. The best sensor performance, in terms of recovery time, was however achieved at 250 °C. Issues related to the gas detection mechanisms, as well as to CNT network thermal stability in detection experiments performed in air at high operation temperatures are also discussed.     

High-temperature synthesis of highly hydrothermal stable mesoporous silica and Fe-SiO2 using ionic liquid as a template

Mesoporous silicas and Fe-SiO₂ with worm-like structures have been synthesized using a room temperature ionic liquid, 1-hexadecane-3-methylimidazolium bromide, as a template at a high aging temperature (150-190 °C) with the assistance of NaF. The hydrothermal stability of mesoporous silica was effectively improved by increasing the aging temperature and adding NaF to the synthesis gel. High hydrothermally stable mesoporous silica was obtained after being aged at 190 °C in the presence of NaF, which endured the hydrothermal treatment in boiling water at least for 10 d or steam treatment at 600 °C for 6 h. The ultra hydrothermal stability could be attributed to its high degree of polymerization of silicate. Furthermore, highly hydrothermal stable mesoporous Fe-SiO₂ has been synthesized, which still remained its mesostructure after being hydrothermally treated at 100 °C for 12 d or steam-treated at 600 °C for 6 h.