Showing papers on "BET theory published in 1994"
TL;DR: In this article, a direct relationship between the degree of reduction and the BET surface area was established using the area of low-temperature TPR composite peak, the maximum of which was found to be constant at 810 K.
Abstract: Successive reduction steps of CeO2 particles by hydrogen between 300 and 1070 K have been followed by temperature-programmed reduction (TPR) and in situ magnetic measurements on several samples with different BET surface areas. The nature of the phases present in cerias reduced between 670 and 1270 K was determined by X-ray analysis. Finally, reoxidation by oxygen or air was studied at room temperature for all the reduced samples.Magnetic and TPR results show a direct relationship between the degree of reduction and the BET surface area. Indeed, for most of the samples, the degree of reduction at 620–670 K determined by magnetism corresponded to the creation of one layer of Ce3+ ions at the surface of the ceria. A similar relationship between the BET surface area and the extent of reduction was established using the area of the low-temperature TPR composite peak, the maximum of which was found to be constant at 810 K.When the reduction progresses further into the bulk, two main phases were evidenced: first, and expanded cubic CeO2 –x phase derived from the initial ceria by a dilatation of the whole structure and, for deeply reduced samples, the hexagonal Ce2O3 phase. A new intermediate phase, cubic Ce2O3, was also observed on samples reduced at 1070–1170 K.Complete reoxidation by oxygen occurs at room temperature, for all reduction percentages below ca. 60 %, i.e. as long as the reduced phase remained in the cubic form. When the hexagonal Ce2O3 phase has been formed, the reoxidation cannot be completed at 294 K.
275 citations
TL;DR: In this article, a series of activated carbons has been prepared by steam pyrolysis of apricot and cherry stones, almond shells, and grape seeds at relatively low temperatures of 600, 650, and 700°C for 1-3 h at each temperature.
220 citations
TL;DR: In this article, a sol-gel preparation method for Co3O4 spinel-type oxides was proposed, where the solution of Co(II)-carbonate in propionic acid is heated to form a resinic cobalt propionate.
170 citations
TL;DR: In this paper, the structural properties of molybdenum trioxide catalysts were examined and the authors concluded that the key to producing the highest surface area Mo nitrides was channeling the reaction through HxMoO3 (x ≤ 0.34) and γ-Mo2OyN1y intermediates.
129 citations
Patent•
23 May 1994Abstract: Oxidation catalyst compositions include a catalytic material having a BET surface area of at least about 10 m 2 /g and consisting essentially of a combination of bulk ceria and a bulk second metal oxide which may be one or more of titania, zirconia, ceria-zirconia, silica, alumina-silica and α-alumina The combination may optionally also include activated alumina having a BET surface area of at least about 10 m 2 /g The ceria, second metal oxide and optional activated alumina may be mixed together or provided as discrete layers Optionally, one of platinum or palladium metal may be dispersed on the catalytic material provided that the platinum, when used, is used in limited amounts to preclude excessive oxidation of SO 2 to SO 3 The catalyst compositions may be used for oxidation of oxidizeable components in a gas-borne stream, eg, in a method to treat diesel engine exhaust by contacting the hot exhaust with the catalyst composition to promote the oxidation of the volatile organic fraction The optional inclusion of platinum or palladium promotes the oxidation of gas phase components, eg, hydrocarbons and carbon monoxide
80 citations
TL;DR: In this article, the point of zero charge (pzc) has been measured by potentiometric titration using KNO3 as a supporting electrolyte, regardless of the source, the same pzc at 8.1 ± 0.1.
75 citations
TL;DR: In this paper, the authors used Activated Carbon Fibers (ACF) to adsorb ppmv concentrations of volatile organic compounds (VOCs) from laboratory generated gas streams.
Abstract: Activated carbon fibers (ACF) were used to adsorb ppmv concentrations of volatile organic compounds (VOCs) from laboratory generated gas streams. VOCs considered were benzene and acetone because the VOCs are commonly found in indoor air and have potential to increase health risks to humans. ACF were used as the adsorbent because they typically exhibit higher adsorption capacities and faster adsorption kinetics than commercially available granular activated carbons (GAC) and show potential as an adsorbent to effectively remove VOCs from indoor air. Adsorption models by Dubinin and coworkers (Dubinin, 1975), based on the theory of volume filling of micropores, and an empirical model by Freudlich were used to fit the measured adsorption isotherms. Agreement between the modeled and experimental results for acetone and benzene using the Dubinin-Radushkevich equation generally improved with increasing BET surface area and produced reasonable fits of the adsorption isotherms for both acetone and benzene. The Freundlich equation produced values for correlation coefficients (R) between modeled and experimental data from -.980 to 0.997, indicating the validity of using the Freundlich equation to model the adsorption isotherms over the concentration range of interest. These results indicate that ACF show potential as an adsorbent for removing low concentrations of VOCsmore » from indoor air. 14 refs., 5 figs., 4 tabs.« less
68 citations
TL;DR: In this article, a TiO2 supported copper catalysts with a K/Cu atomic ratio varying between 0 and 2 were calcined at 1073 K. The catalytic activity was measured in a microbalance by means of temperature-programmed oxidation in air or argon.
Abstract: We have prepared a TiO2 supported copper catalyst and studied the effect of potassium on its activity in the oxidation of soot particles. The catalysts, with a K/Cu atomic ratio varying between 0 and 2, were calcined at 1073 K. They were characterized by BET surface area measurements, X-ray diffraction and temperature-programmed reduction under hydrogen. The catalytic activity was measured in a microbalance by means of temperature-programmed oxidation in air or argon. The catalytic activity of copper was enhanced by the presence of potassium. This effect was attributed to the formation of mixed KTi oxides which inhibit the sintering of the TiO2 support and thus increases the surface area of the catalyst. Although a redox mechanism can explain the catalytic combustion, no correlation could be established between the reducibility of the different solids and their activity in soot combustion.
68 citations
01 Jan 1994
TL;DR: In this article, the authors used Activated Carbon Fibers (ACF) to adsorb ppmv concentrations of volatile organic compounds (VOCs) from laboratory generated gas streams.
Abstract: -~~~~~~ ~ Activated carbon fibers (ACF) were used to adsorb ppmv concentrations of volatile organic compounds ( VOCs) from laboratory generated gas streams. VOCs considered were benzene and acetone because the VOC are commonly found in indoor air and have potential to increase health risks to humans. ACF were used as the adsorbent because they typically exhibit higher adsorption capacities and faster adsorption kinetics than commercially available granular activated carbons (GAC) and show potential as an adsorbent to effectively remove VOCs from indoor air. Adsorption models by Dubinin and coworkers (Dubinin, 1975), based on the theory of volume filling of micropores, and an empirical model by Freundlich were used to fit the measured adsorption isotherms. Agreement between the modeled and experimental results for acetone and benzene using the DubininRadushkevich equation generally improved with increasing BET surface area and produced reasonable fits of the adsorption isotherms for both acetone and benzene. The Freundlich equation produced values for correlation coefficients (R) between modeled and experimental data from 0.980 to 0.997, indicating the validity of using the Freundlich equation to model the adsorption isotherms over the concentration range of interest. These results indicate that ACF show potential as an adsorbent for removing low concentrations of VOCs from indoor air,
63 citations
TL;DR: The TiO2 support is composed of particles with radii of about 1 μm, these forming aggregates of loosely coherent particles (radii 100 μm) as mentioned in this paper.
59 citations
TL;DR: In this article, vanadia-titania aerogels with high surface area have been synthesized by a two-stage sol-gel process with ensuing high-temperature supercritical drying.
TL;DR: In this paper, various Cu-Al catalysts with copper concentration ranging from 0 to 57.3 wt% were prepared by coprecipitation techniques for the production of H 2 from methanol.
Abstract: Various Cu-Al catalysts with copper concentration ranging from 0 to 57.3 wt% were prepared by coprecipitation techniques for the production of H 2 from methanol. These catalysts were calcined at various temperatures and then reduced in H 2 at 300°C. The catalysts were characterized thoroughly at appropriate stages of preparation. Results show that the dried catalysts contained crystalline species such as Cu 2 (OH) 2 CO 3 and Cu(OH) 2 and noncrystalline species such as Al(OH)CO 3 , Al(OH) 3 , and Cu 2 (OH) 2 CO 3 . Their proportions and thermal stability depended on copper concentration. Calcined catalysts contained Cu 2 (OH) 2 CO 3 , CuO, Cu 2 O, and CuAl 2 O 4 reducible species. Their proportions, reducibilities, and extents of reduction with hydrogen depended strongly on copper concentration and calcination temperature. The acid site distribution on calcined catalysts varied drastically with copper concentration and calcination temperature. On the other hand, BET surface area and pore structure depended not only on copper concentration and calcination temperature but also whether the catalyst was reduced or unreduced
TL;DR: In this article, the fine particle nature of combustion derived powders has been studied using density, particle size, and surface area measurements and scanning electron microscopy using powder X-ray diffraction, infrared spectroscopy, and Si-29 MAS-NMR.
Abstract: Oxide materials like perovskite, zirconolite, hollandite, pyrochlore, NASICON and sphene which are used for nuclear waste immobilization have been prepared by a solution combustion process. The process involves the combustion of stoichiometric amount of corresponding metal nitrates and carbohydrazide/tetraformyl trisazine/diformyl hydrazide at 450 degrees C. The combustion products have been characterized using powder X-ray diffraction, infrared spectroscopy, and Si-29 MAS-NMR. The fine particle nature of the combustion derived powders has been studied using density, particle size, BET surface area measurements and scanning electron microscopy. Sintering of combustion derived powder yields 85-95% dense ceramics in the temperature range 1000 degrees-1300 degrees C.
TL;DR: In this article, an acid-catalyzed titania gel was prepared from tetrabutoxy-titanium(IV) with methanol as solvent, and the titania matrix consisted of well-developed anatase crystallites of about 8-9 nm mean size.
TL;DR: In this article, the electron paramagnetic resonance (EPR) linewidth ΔBpp of the active carbons in the presence of various pressures of oxygen was analyzed. And the results showed a reversible dependence of the EPR characterization on the pressure of oxygen, Po2, and a dependence of response d(ΔBpp)/dPo2 on the BET surface area.
TL;DR: In this article, Monte Carlo (MC) computer simulations for a realistic model of carbon dioxide adsorbed on the exposed graphite basal plane were reported. But the authors did not consider the effect of the coverage dependence of the average tilt angle on the completed monolayer.
TL;DR: In this paper, the surface area and pore structure of alkyl-substituted silica aerogels and xerogels were determined by multipoint surface area measurement.
Abstract: High surface area alkyl-substituted silica aerogels and xerogels were successfully prepared by sol-gel processing and supercritical drying. The gels were further heat treated in inert atmosphere to temperatures as high as 1000°C. Surface areas and pore structure of the gels and gels pyrolyzed at high temperatures were determined by multipoint BET surface area measurement. The aerogels and xerogels exhibited surface areas of about 1100 m2/g. No significant effect of pH was found on the surface areas of gels in the two step sol-gel process, but gels of low pH showed smaller pore diameter and higher density. Xerogels showed smaller surface area, pore size, and pore volume compared to aerogels. Upon pyrolyzing in inert atmosphere, the surface areas of all the gels decreased with temperature as a result of collapse of micropores and shrinkage of mesopores. Unlike pure silica gel, which loses almost all surface area and densifies at 1000°C, the advantage of the alkyl-substituted gels is that they maintained a high surface area of 400 m2/g at 1000°C.
TL;DR: In this paper, four kinds of silica-containing mixed oxide gels (silica-zirconia, silica alumina, silicas-titania and silicamagnesia) were prepared by a coprecipitation method and studied as column packing materials for liquid chromatography, and the physico-chemical characteristics of the gels such as the BET surface area and acidity of the solid surface were determined.
TL;DR: In this paper, a disordered defective spinel-type phase, γ-FeAlO3, has been found, containing Fe3+ in octahedral sites and Al3+ at both octagonal and tetrahedral sites, both in the bulk and on the surface.
Abstract: Powders belonging to the Fe2O3–Al2O3 system prepared by a coprecipitation method have been characterized from the point of view of their solid-state and surface structure by X-ray diffraction (XRD), Fourier-transform (far) infrared (FTIR/FT-FIR), diffuse reflectance ultraviolet–visible (DR-UV–VIS) Spectroscopy, BET surface area and porosity and adsorption of probe molecules. In samples calcined at 673 K a disordered defective spinel-type phase, γ-FeAlO3, has been found, containing Fe3+ in octahedral sites and Al3+ in both octahedral and tetrahedral sites, both in the bulk and on the surface. Samples calcined at 1173 K are composed of mixtures of two saturated solid solutions with corundum–haematite-type structures. The surface of these biphasic materials is dominated by the higher reactivity of α-Fe2O3 with respect to α-Al2O3, although tetrahedral Al3+ is also found at the surface of the saturated solid solution of Fe2O3 in α-Al2O3. Al3+ added to Fe2O3 tends to stabilize the spinel-type phase and to hinder sintering, while Fe3+ added to Al2O3 accelerates the γ→α phase transition and sintering.
Patent•
05 Dec 1994TL;DR: A bioremediation support for the support of microorganisms used in the biotreatment of an aqueous waste stream or contaminated vapor is made of a low-density siliceous glassy material.
Abstract: OF THE DISCLOSURE A bioremediation support for the support of microorganisms used in the biotreatment of an aqueous waste stream or contaminated vapor is made of a low-density siliceous glassy material. This material has a cellular or frothy texture, large pores of greater than 1,000 Anstrom units in diameter dispersed throughout the material, a high macropore volume in pores of greater than 1,000.ANG. of more than 0.3 cc/cc and a BET surface area of greater than 10 m2/g. A preferred material is pumice.
TL;DR: In this paper, the durability of Cu-ZSM5 as a catalyst of selective reduction of NOx by hydrocarbons from the exhaust of stationary natural gas engines was examined, and the results of the durability test of SOx concentration dependence, SOx was not considered to be the cause of the deterioration.
Abstract: The durability of Cu-ZSM5 as a catalyst of selective reduction of NOx by hydrocarbons from the exhaust of stationary natural gas engines was examined. Even 100% ion-exchanged Cu-ZSM5 also showed a deterioration tendency at 773 K under wet conditions simulating lean-burning gas engine exhaust. From the results of the durability test of SOx concentration dependence, SOx was not considered to be the cause of the deterioration. The deteriorated Cu-ZSM5 was characterized by the measurements of BET surface area, CO adsorption, XRD, NMR, EXAFS, XANES as well as carbon analysis. Neither dealumination, carbon deposition nor loss of micropore was observed in the deteriorated sample, and CuO cluster formation and resultant loss of surface area of copper were clearly shown, therefore, the CuO cluster formation is considered to lead to deterioration of the activity of Cu-ZSM5.
TL;DR: Niobia-doped vanadia-titania aerogels with 20 wt% nominal V2O5 and 2-6 wt % nominal Nb2O 5 have been prepared by a two-stage solgel process with subsequent supercritical drying as discussed by the authors.
TL;DR: In this article, a titania gel was prepared by the addition of an acidic hydrolysant to tetrabutoxytitanium(IV) in methanol.
TL;DR: In this article, a Bulgarian lignite from the Chukurovo deposit was used to produce an activated carbon by one-step steam pyrolysis at 650 °C and the effects of the presence of water vapour on the yield and composition of the solid, liquid and gas products were investigated.
TL;DR: In this article, high resolution N2 adsorption isotherms of pitch-based activated carbon fibers, activated mesocarbon microbeads (a-MCMB), and a high surface area carbon powder(Super Sorb), which have apparent BET surface area greater than a classical upper limit of the surface area for carbonous materials, were measured.
Abstract: The high resolution N2 adsorption isotherms of pitch-based activated carbon fibers, activated mesocarbon microbeads (a-MCMB), and a high surface area carbon powder(Super Sorb), which have apparent BET surface area greater than a classical upper limit of the surface area for carbonous materials, were measured. The true surface area of these high surface area carbons was determined by the subtracting pore effect (SPE) method with the aid of the high resolution αs-plots. The BET surface area depended sensitively on the relative pressure region and the routine BET analysis using the data of the relative pressure region of 0.1-0.3 gave the seriously overestimated surface area.
Abstract: The generation of porous metal oxides by removal of template molecules from inorganic polymers formed by sol-gel type hydrolysis and condensation of metal alkoxides is described. The template molecules include organic polymers, copper (II) ions in hybrid copper oxide/silica sols and copper (II) bis(hexafluorocetylacetonate) (hfac). Neutron scattering experiments on the system in which polyacrylic acid (Mw=2,000 Daltons) is used as an organic template to generate microporous tin oxide show that removal of the template generates skeletal voids. In a second series of experiments, mixed copper/silicon oxide xerogels were prepared by hydrolysis of mixtures of Si(OEt)4 and Cu(OCH2CH(CH3)N(CH3)H)2 in the ratios of Si:Cu=2:1, 4:1, 9:1. Selective removal (etching) of the copper component generates porous silica. Neutron scattering data and BET surface area measurements are consistent with the creation of pores with molecular dimensions (micropores, 10 A or less). In the third strategy, Si(OEt)4 is hydrolyzed in the presence of Cu(hfac)2, a volatile, inert inorganic template, in a 4 to 1 molar ratio. Removal of the template from the xerogel at 100°C in vacuo affords microporous silica.
TL;DR: In this paper, surface areas calculated from nitrogen sorption on aluminum oxide supports with varying MoO 3 and WO 3 loadings were consistently higher than surface area calculated from mercury penetration data.
Abstract: Surface areas (BET) calculated from nitrogen sorption on aluminum oxide supports with varying MoO 3 and WO 3 loadings were consistently higher than surface areas calculated from mercury penetration data. Increasing the MoO 3 or WO 3 content from 0% to about 20% resulted in a gradual decrease in the surface areas calculated from mercury penetration even though the BET surface area remained constant. The mercury contact angles which provided agreement between BET and mercury penetration surface areas decreased with increasing molybdenum or tungsten oxide loading up to the concentration corresponding to about one monolayer coverage, and approached that of unsupported MoO 3 or WO 3 . The best agreement of data from the two techniques was obtained for samples with pores in the 100-200 A region.
Patent•
21 May 1994
TL;DR: In this article, a method of making heat-insulating material from silicate-containing (siliceous) material, calcium hydroxide (hydrated lime, slaked lime), water, foam and a rapid-setting cement containing reactive aluminates, from which there is produced a castable raw mixture which is cast into moulds, with the green bodies formed in the moulds being removed from the molds after sufficient setting and autoclaved.
Abstract: The invention relates to a method of making heat-insulating material from silicate-containing (siliceous) material, calcium hydroxide (hydrated lime, slaked lime), water, foam and a rapid-setting cement containing reactive aluminates, from which there is produced a castable raw mixture which is cast into moulds, with the green bodies formed in the moulds being removed from the moulds after sufficient setting and autoclaved. To achieve very low bulk densities, it is provided for the silicate-containing material used to be a quartz flour having a specific surface area by BET of at least about 3 m /g, in particular from about 4 to 5 m /g, the raw mixture to be prepared with a weight ratio of water/solid (without foam) of at least about 1.5 and an essentially stoichiometric amount, based on the virtually complete reaction of the quartz flour and the reactive aluminates, of calcium hydroxide having a BET surface area of at least about 15 m /g, and the green bodies cast into moulds to be removed from the mould, apart from a mould bottom, after sufficient setting and to be autoclaved on the mould bottom.
TL;DR: In this paper, a series of organo-montmorillonites and organohectorites were prepared by complete ion exchange from the pure sodium form of the parent smectites.
Abstract: A series of organo-montmorillonites and organo-hectorites was prepared by complete ion- exchange from the pure sodium form of the parent smectites. The organic cations were tetramethylam- monium, trimethylated quaternary ammonium derivatives of the lysine and of the ornithine methyl esters, quaternarized polyammonium cations, or tetraphenylphosphonium (TPP). These organo-smectites were used as packing material for gas chromatography columns. Mixtures of light hydrocarbons (methane to n-butane; "CI" to "C4") could be separated. The degree of separation depends on the presence of micropores or of organophilic mesopores. The BET surface area, the micropore and mesopore volumes, as well as the size distribution of micropores and mesopores were measured for several systems. As a general trend, the retention times of the light hydrocarbons decrease with an increase of the micropore volume. In the case of TPP-montmorillonite, characterized by a large mesopore volume but for which no microporosity could be detected, separation of longer linear hydrocarbons (n-pentane to n-octane) could also be achieved.
TL;DR: In this paper, high-dispersed iron-based catalysts are examined for the initial stage of direct coal liquefaction, and a variety of methods are used to estimate the particle size of the catalyst precursor and, in some instances, the phases found consequent to the reaction process.
Abstract: Highly dispersed iron-based catalysts are being examined for the initial stage of direct coal liquefaction. Ultrafine (< 10-nm) particles are produced by a variety of methods. Mossbauer spectroscopy, magnetometry, XAFS, electron microscopy, XRD line broadening, and BET surface area measurement are used to estimate the particle size of the catalyst precursor and, in some instances, of the phases found consequent to the reaction process. With Mossbauer and magnetization techniques, data have to be obtained over a range of temperatures, usually from 4 to 300 K. In addition, obtaining the particle size by Mossbauer spectroscopy requires a knonnledge of the magnetic anisotropy energy. For some XRD lines, the broadening is a consequence of the presence of fault planes rather than crystallite size