Showing papers on "BET theory published in 1995"

Adsorption isotherms of microporous-mesoporous solids revisited

Abstract: By application of the modified BET isotherm on adsorption data, for adsorption isotherms which are neither purely of the I or IV IUPAC type, it is possible to determine simultaneously the net micropore volume, the monolayer capacity and the BET parameterC. Specific surface obtained from the monolayer capacity represents the mesopore surface area and is not overestimated as in the case when the presence of micropores is neglected. The obtained BET parameterC can serve for choosing the appropriate master isotherm which is required for calculation of the mesopore-size distribution. The net micropore volume characterises the adsorption capacity when the adsorption mechanism is volume filling of pores.

Physisorption of nitrogen by porous materials

Abstract: Nitrogen adsorption at 77 K is widely used for the determination of the surface area and pore size distribution of various porous materials. The first stage in the interpretation of a nitrogen isotherm is the identification of the physisorption mechanism(s): that is monolayer-multilayer adsorption, capillary condensation or micropore filling. The Brunauer-Emmett-Teller (BET) method cannot be used to provide a reliable evaluation of the surface area if the solid contains pores of molecular dimensions (i.e. narrow micropores). Furthermore, with such materials, it is necessary to use different probe molecules to determine the pore size distribution. Capillary condensation is involved as a secondary process in the filling of mesopores (pore width in the range 2–50 nm). Recent work on MCM-41, a model mesoporous adsorbent, has confirmed that capillary condensation-evaporation can occur reversibly in open-ended pores of about 4 nm width.

Tetramethylphosphonium- and Tetramethylammonium-Smectites as Adsorbents of Aromatic and Chlorinated Hydrocarbons: Effect of Water on Adsorption Efficiency

Abstract: Tetramethylphosphonium-smectite (TMP-clay) and tetramethylammonium-smectite (TMA-clay), were prepared and characterized as adsorbents for a series of aromatic and chlorinated hydrocarbons. The sorption of benzene, alkylbenzenes, and carbon tetrachloride as vapors and as solutes from water was studied to evaluate the effect of water on adsorption efficiency. Adsorption of organic vapors depended on the N2 BET surface area. TMA-clay was a slightly better adsorbent than TMP-clay, due to its somewhat higher surface area. The Langumir isotherms obtained indicated that adsorption occurred predominantly in the interlayer micropores, apparently on mineral surfaces between onium ions. Adsorption efficiency of both organo-clays decreased, compared to vapor sorptions, in presence of water. Lower sorption was apparently due to shrinkage of the interlayer pore or cavity sizes by hydration of interlayer TMA and TMP cations. Although sorption efficiencies of both organo-clays was reduced in presence of bulk water, the extent of reduction was much less for TMP-clay. Thus, TMP-clay was a better adsorbent than TMA-clay in presence of water, despite its lower surface area, in direct contrast to vapor sorption. The Langumir isotherms indicated interlayer sorption of benzene, alkylbenzenes and carbon tetrachloride from water by TMP-clay. The absence of Langumir isotherms for toluene, ethylbenzene and p-xylene uptake from water by TMA-clay indicated that these bulkier solutes were not adsorbed in the interlayers. These results indicate that hydration of TMA cations causes shrinkage of the interlayer pores to dimensions that exclude these solutes. The lower degree of hydration of TMP cations enables TMP-clay to maintain interlayer pores large enough to accommodate the bulkier alkylbenzenes.

Combustion synthesis of yttria

Abstract: Fine-particle, sinter-active yttria has been prepared by combustion of a redox compound, Y(N2H3COO)3·3H2O and mixtures of Y(N2H3COO)3·3H2O–NH4NO3 or NH4ClO4 as well as yttrium nitrate and hydrazine-based fuels. The fineparticle nature of the combustion-derived yttria has been investigated using powder density, particle size and BET surface area measurements. The uniaxially, cold-pressed fine-particle yttria when sintered at 1450–1500 °C achieved 98% theoretical density and showed a fine-grain (1–2 µm) microstructure.

Synthesis and physicochemical properties of cobalt aluminium hydrotalcites

Abstract: Cobalt aluminium hydrotalcites with different compositions were prepared by a coprecipitation method under low supersaturation conditions. The compounds were characterized by X-ray diffraction (XRD), infrared absorption (IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and BET surface area measurements. XRD and IR studies revealed that all the compounds are single-phase crystallized under a hydrotalcite-like network. Hydrothermal treatments given to the aged sample increased the crystallinity of the samples. TG studies showed two stages of weight loss, the first due to the removal of interlayer water and the second ascribed to the removal of water molecules from the brucite sheet and CO2 from the interlayer carbonate anion, whose transition temperature depends on the Co/Al atomic ratio. Thermal calcination of these materials results in the formation of high surface area non-stoichiometric spinel phase whose crystallinity increases with increase in the calcination temperature attributed to the sintering of the particles.

New silica-pillared material prepared from the layered silicic acid of ilerite

Abstract: A new siliceous porous material of high BET surface area is derived from an intercalation of octylamine and tetraethylorthosilicate into the layered silicic acid of ilerite; the resultant pillared material has a BET surface area of over 1000 m2 g–1 at 600 °C and 580 m2 g–1 even at 900 °C.

Mechanically Activated MoO3. 1. Particle Size, Crystallinity, and Morphology

Abstract: Physicochemical changes induced in MoO 3 by mechanical activation in a planetary mill were investigated. During the milling process, the BET surface area increases from about 1.3 to 32 m 2 /g. Energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS) of the powdered MoO 3 samples reveal that no agate is abrased during the milling process. An estimation of the mean particle size using the BET data or scanning electron microscopic (SEM) images indicates a decrease from about 1 μm to about 50 nm. The primary crystallite size calculated from X-ray diffraction (XRD) line broadening also shows a decreasing size from about 160 nm to about 80 nm. The difference between the particle size of unmilled MoO 3 , as determined from the BET surface area or by scanning electron microscopy (SEM), and the calculated primary crystallite size using X-ray line broadening is explained by MoO 3 particles consisting of smaller primary crystallites. The smaller average particle size of MoO 3 milled for 600 min calculated from BET data, on the other hand, as compared to the XRD primary crystallite size is ascribed to the presence of ultrafine amorphous material which is X-ray amorphous and, therefore, does not contribute to the X-ray line broadening. This formation of amorphous material is also indicated by an increasing amorphous scattering background in the X-ray diffraction patterns. In SEM pictures, these particles appear to have an amorphous overlayer. The strange behavior of both the X-ray diffraction pattern quality and the diffuse scattering background, which do not coincide, during mechanical activation probably indicates a complex process of particle size reduction. Variation of X-ray reflection profiles, intensity ratios, and additional X-ray reflections may point toward the formation of shear defects during this process. A Warren-Averbach analysis of the most intense X-ray reflections of milled MoO 3 reveals that internal strain is only marginally enhanced by mechanical activation.

Physico-chemical properties and catalytic activities of the spinel series MnxFe3 –xO4 towards peroxide decomposition

Abstract: Samples of the spinel series, MnxFe3 –xO4(O ⩽x⩽ 3), have been synthesized by the hydroxide coprecipitation technique and characterised by chemical analysis; XRD and SEM techniques, and measurements of their BET specific surface areas, saturation magnetisations and electrical resistivities. The specimens with values of x⩽ 1.5 are single-phase cubic spinels and those with x > 1.5 are two-phase cubic and tetragonal spinels. The relative catalytic activities of the specimens towards the decomposition of H2O2 in both alkaline and neutral media, measured on the basis of the energies of activation, a microstructure-independent parameter, and the rate constants normalised to unit BET surface area, a microstructure normalised parameter, follow the order, (given as values of x) 1.45 > 2.5 ≈ 2.0 > 1.0 > 1.2 > 0.8 > 0.5. All the MnxFe3 –xO4 prepared specimens are potential substitutes for CoFe2O4, the high-performance catalyst for the decomposition of H2O2; the specimen with x= 1.45 is the most powerful and is significantly more active than CoFe2O4. The activity appears to depend on the availability and efficacy of redox conversion of redox conversion of MnII centres in the sublattices of the MnxFe3 –xO4 catalysts.

Catalytic Ceramic Filters for Flue Gas Cleaning. 1. Preparation and Characterization

Abstract: This paper deals with the preparation and the characterization of ceramic porous filters, whose pores were deposited with a γ-Al 2 O 3 layer via the so-called «urea method», in order to increase their specific surface area. Once activated with a suitable catalytic principle, these filters can find a potential application in flue gas cleaning according to a combined action: mechanical particulate removal+catalytic abatement of chemical pollutants (nitrogen oxides, volatile organic compounds, etc.). Both the obtained filters and the bulk γ-Al 2 O 3 powder synthesized through the above method, were characterized from either a structural (BET surface area measurement, Hg porosimetry, differential thermal analysis-thermal gravimetry analysis, X-ray diffraction, scanning electron miscroscopy (SEM) observation, gas permeation) or a catalytic viewpoint. In this last context, isopropyl alcohol dehydration was chosen as a model reaction since it is directly catalyzed by γ-Al 2 O 3 (thanks to its acidic properties) with no need of further catalytic activation. A reaction mechanism is proposed for the test reaction, based on the existence of two types of active sites (A and B). On A-sites isopropyl alcohol gives an intermediate adsorbate and decomposes provided vicinal B-sites are available for water adsorption. A kinetic rate expression is worked out on the basis of experimental runs performed on a batch-operated differential reactor. The knowledge of this rate expression and of the structural characterization parameters are of major importance for the interpretation and the modeling of the results of a pilot-plant study on the above filters, performed using the same model reaction and presented in part 2 of this series. The urea method is demonstrated to be a reliable tool to deposit a γ-Al 2 O 3 layer all over the pore walls of the filter, markedly increasing its specific surface area. Drawbacks of the procedure employed are though the occurrence of pore blocking after a few deposition cycles and the occasional presence of cracks in the deposited layer

The role of anion in the preparation of nickel catalyst detected by TPR and FTIR spectra

Abstract: The role of NO 3 - anion in the preparation of nickel catalysts detected by temperature-programmed reduction (TPR) and Fourier transform infrared spectroscopy was studied. Thermogravimetric analysis-mass spectrometry, X-ray photoelectron spectroscopy, and BET surface area were also used to evaluate the effect of calcination temperature. The role of NO 3 - anion is one of the key factors which determine the activities and properties of the prepared catalysts. Two reduction peaks, NO 3 - anion and NiO, of TPR were obtained for nickel catalyst drying at 110°C. The results indicated that the reduction of NO 3 - anion occurs at 278°C, and then nickel nitrate converts to Ni 2 O 3 under hydrogen atmosphere. Nickel nitrate decomposed into NiO only and exhausted both NO and NO 2 when it was calcined in air. On the basis of the results of BET, both surface area and pore volume of catalysts slightly increased and then decreased with calcination temperature. The average pore size was almost constant when calcination temperature was less than 1100°C, after which it significantly increased

Synthesis and characterization of MTiO3(M = Mg, Ca, Sr, Ba) sol-gel

Abstract: Multiple oxide titanates have been prepared via the sol-gel method by the reaction of an alcoholic solution of titanium n-butoxide, Ti(OC4H9)4, with nitrate solutions, M(NO3)2(M = Mg, Ca, Sr, Ba), at room temperature. The amorphous gels so obtained under acidic conditions were calcined at temperatures between 700 and 900 °C to obtain high-purity, crystalline MTiO3 powders. The samples were characterized by powder X-ray diffraction (XRD), scanning (SEM) and energy-dispersive electron microscopy, thermogravimetric (TG) and differential thermal analysis (DTA) and BET surface area measurements. The stability and large surface area obtained for the Sr and Ba compounds make them suitable for use as catalyst supports.

Physico-chemical study of selective catalytic reduction vanadia-titania catalysts prepared by the equilibrium adsorption method

Abstract: Sub-monolayer V 2 O 5 /TiO 2 catalysts were prepared by the equilibrium adsorption method using a high surface area TiO 2 anatase powder. They were characterized by the following techniques: X-ray diffraction, BET surface area measurements, electrical conductivity, electron paramagnetic resonance, UV-VIS, laser-Raman spectroscopy and 51 V solid state nuclear magnetic resonance (NMR). The physico-chemical characterization shows that vanadium is present in the 5+ oxidation state and that the vanadium oxide phase is well dispersed on the surface of the support. The conditions of preparation markedly influence the nature of the surface species: alkaline pH promotes the formation of isolated tetrahedral species, whereas with acidic pH more condensed vanadium oxide species, containing octahedrally coordinated vanadium, are formed. The calcination gives rise to a further aggregation of the surface vanadium oxide phase. The activity of the catalysts in the selective reduction of NO by NH 3 in the presence of O 2 was determined. The analysis of kinetic data combined with the results of physico-chemical characterization leads to a good relationship between the reaction rate and the amount of octahedrally coordinated vanadium, as evaluated by NMR spectra under ambient conditions.

Characterization of Industrial Coke Structures

Abstract: The morphological characteristcs of macro and microstructures of commercial cokes were investigated prior to reaction and after reaction with air and CO2 using BET surface area measurements, electron microscopy, and X-ray powder diffraction . The results show that the initial coke morphology is dominated by the type of coking process and by the chemical composition. Upon oxidation, different types of areas accessible for oxidation are developed.

Characterization of α-(Fe,Al)2O3 solid-solution powders

Abstract: Monophasic powders constituting α-(Fe,Al)2O3 solid solutions have been prepared by a coprecipitation method and have been characterized, from the point of view of their solid-state and surface structures, by X-ray diffraction (XRD), Fourier-transform (far) infrared (FTIR/FT-FIR) and diffuse reflectance ultraviolet–visible (DR-UV–VIS) spectroscopy. BET surface area and porosity and adsorption of probe molecules. Their bulk and surface properties have been compared with those observed for pure α-Al2O3 and pure α-Fe2O3 as well as with biphasic materials constituting mixtures of two α-(Fe,Al)2O3 saturated solid solutions, with overall compositions external to the solubility limits of these phases. The maximum solubility of Fe2O3 in α-Al2O3 is near 11%, while the maximum solubility of Al2O3 in α-Fe2O3 does not exceed 7%, for samples calcined at 1173 K. Metastable solid-solution phases with higher solute concentrations can be obtained if the calcination temperature is limited to 673 K. Al3+ added to α-Fe2O3 tends to hinder sintering and stabilize the pore structure, while Fe3+ added to α-Al2O3 accelerates the γ→α phase transition and causes a reduction in the surface area. In spite of their structures, which forecast only octahedral cation coordination, tetrahedral Al3+ and Fe3+ rations are found, mainly at the surfaces of the solid solutions.

Effect of pre-drying on the pore structure development of sewage sludge during pyrolysis

Abstract: Pyrolysis of digested sewage sludge was conducted in a nitrogen atmosphere in this study. The effects of temperature and air pre-drying on the surface area and pore structure development were investigated. It is found that the BET surface area of the chars derived increases with increasing temperature. Pre-dried sewage sludge results in higher surface area in the char than the raw sludge. It is demonstrated that pre-drying has an effect of promoting micropore structure development, whereas there are more mesopores and macropores developed in the raw sludge char due to possible pore enlargement resulting from volatile gas bubbling out of the softened intermediate product during pyrolysis in the temperature range of 550°C – 650°C.

Oxidation Rates of Industrial Cokes with CO2 and Air

Abstract: The reaction rates of different sized of commercial cokes with CO2 and air were measured using a combined thermogravimetric and differential thermal analysis technique. The temperature range for air oxidation was varied from 873 to 1073 K, and for the reactions with CO2 it varied from 1173 to 1573 K. The oxidation rates were evaluated on the basis of the percentage weight loss and per unit surface area. Changes in reaction rate were found to depend on changes in the BET surface area.

Catalytic oxidation method

Abstract: Oxidation catalyst compositions include a catalytic material having a BET surface area of at least about 10 m 2 /g and consists 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, e.g., 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, e.g., hydrocarbons and carbon monoxide.

Effect of Precipitated Silica Physical Properties on Silicone Rubber Performance

Abstract: Nitrogen BET surface area is a highly accurate predictor of the physical performance of silica-filled and silica and carbon black-filled, cured organic rubber compounds. The effectiveness of predicting performance of a high-temperature-vulcanized, high-consistency silicone rubber, based on precipitated silica physical properties, is presented. Correlation and regression analyses were performed for twenty-five silica physical properties and fourteen silicone rubber performance characteristics. Silica properties studied included measurements of surface area, structure, impurities, particle size, pH and adsorbed moisture. Silicone rubber performance included measures of processing, curing, reinforcement, resiliency, stress whitening, color and clarity. The present study confirms that no single silica physical property can predict silicone rubber performance. A combination of surface area, oil absorption, particle size, pH, adsorbed moisture, residual sodium and/or residual salt can qualitatively pre...

Surface segregation and morphology of Mg-doped α-alumina powders

Abstract: This work considers the effect of a thermal treatment, at a temperature equal to or higher than 1200 °C, on the physical and chemical characteristics of submicron α-alumina powders, doped with 550 and 1650 ppm in weight of MgO. The powders have been characterized by TEM, SEM, BET, XPS and chemical analysis. TEM observations show that the grains have elongated and rounded forms and exhibit considerable agglomeration. The BET surface area of the powder decreases with increasing annealing time and temperature and increases with the amount of MgO. XPS analysis shows a strong segregation of Mg and this effect is more pronounced when the size of the grains increase. In some cases XPS also detected the presence of Na, Ca and Si in the surface region of the grains.

Preparation of alumina supported ceria. I: Selective measurement of the surface area of ceria and free alumina

Abstract: In a general study of the role played by each component of three-way catalysts, a selective measurement of alumina and ceria surfaces area has been developed on model ceria-alumina supports. Three CeO2/Al2O3 solids (6.5, 13.4, 21 wt.% CeO2) were prepared by grafting Ce acetylacetonate on the surface of an alumina and calcination at 673 K. They were characterized by BET and X-ray diffraction measurements. The hydrogen temperature-programmed reduction (TPR) resulted in profiles similar to those of unsupported cerias, thus allowing an estimation of the equivalent surface area of ceria alone from the hydrogen uptake at low temperature. By adsorbing CO2 on the OH groups of the alumina surface, hydrogenocarbonates species were formed and the IR band at 1235 cm−1 was selected to determine quantitatively the unpertubed alumina surface. In that case, by comparison with the BET surface area, it was possible to deduce an other value of the ceria surface area. The two sets of values obtained by these independent methods were found in good agreement, thus giving a good support for the reliability of each method. They show that the unpertubed alumina surface decreases when increasing the cerium content. Only 20% of the original alumina surface is preserved in the support containing 21 wt.% CeO2.

Contrasting adsorption exhibited by lignite-based activated carbons

Abstract: The adsorption of sodium benzoate, 2,4-dichlorophenol and 2,4,6-trichlorophenol onto five activated carbons made from Northern Ireland lignite is reported. A two-stage process consisting of chemical activation during the charring of the lignite, followed by steam activation was used to produce the activated carbons. The adsorption equilibrium isotherms obtained for the carbons have been plotted to obtain the Freundlich, Langmuir and Redlich-Peterson constants. Chemical and steam activation methods both produced improvements to the carbon performance on their own, with the combination of the two treatments providing the best results. In the main the chemical treatment providing the best improvement on its own also gave the biggest improvement when combined with steam treatment. BET surface area measurements made using nitrogen adsorption correlate strongly with the capacity for benzoate adsorption.