Krypton Adsorption Research Articles

Bulk and surface chemical functionalities of type III PAN-based carbon fibres

PAN-based carbon fibres carbonised at relatively low temperature, i.e. type III carbon fibres, were submitted to heat treatment at 2300 °C (GR) or oxidation in nitric acid. The samples were characterised by XPS, FTIR, wetting measurements, gas adsorption, elemental analysis and acid/base titration. While oxidation only slightly affects the nitrogen concentration, it produces an appreciable change in the nature of the chemical functions, namely the conversion of pyridine-type nitrogen and quaternary nitrogen into aliphatic functions. Oxidation treatment modifies all the material constituting the fibre, the oxygen concentration being about 1.5 times higher at the fibre external surface compared with the whole material. Three components (531.2, 532.6 and 533.8 eV) are clearly identified in the oxygen XPS peak, allowing a comparison to be made between the whole material and the external surface regarding chemical species. The acidic groups are mainly carboxyl. Fibres submitted to extensive oxidation also show a high basicity, attributed mainly to calcium carboxylate. Although the acidic and basic groups present in the whole material can be titrated with aqueous solutions, the fibres develop only a very small surface area and no microporosity as determined by krypton adsorption. The material may be viewed as a sponge, collapsed when dry but able to swell in water and developing a high cation-exchange capacity.

Microcalorimetric measurement of the interactions between water vapor and amorphous pharmaceutical solids.

Use a microcalorimetric technique to measure the interactions between water vapor and amorphous pharmaceutical solids and describe the relationship between long-term physical stability and the storage relative humidity (RH) at constant temperature. A thermal activity monitor was used to characterize interactions of water vapor with spray-dried amorphous sucrose, lactose, raffinose, and sodium indomethacin. Differential scanning calorimetry was used to measure glass transition temperature, Tg. X-ray powder diffraction was used to confirm that the spray-dried samples were amorphous. Scanning electron microscopy was used to examine particle morphology. Specific surface area was determined by BET analysis of nitrogen and krypton adsorption isotherms. The moisture-induced thermal activity traces (MITATs) of the materials in this study exhibit general behavior that helps explain the effect of moisture content on the physical stability of the glassy phase at a given storage temperature. At some RH threshold, RHm, the MITAT exhibits a dramatic increase in the energy of interaction between water vapor and the glass that cannot be explained by a phase or morphology change. Calorimetric data indicate that water vapor-solid interactions are reversible below RHm; above RHm, energetic hysteresis is observed and water-water interactions predominate. In addition, the MITAT was deconvoluted into sorptive and nonsorptive components, making it possible to assign the observed heat flow to unique thermal events. Samples stored at a RH just below RHm for more than 2 months show no evidence of morphology or phase change. In addition, the MITAT can be deconvoluted into sorptive and nonsorptive components by using a twin-calorimeter arrangement. This analysis provides specificity to the microcalorimetric analysis and helps explain the nature of the physical changes that occur during the hydration glassy phase. The MITAT is a useful tool to determine the onset of moisture-induced physical instability of glassy pharmaceuticals and may find a broad application to determine appropriate storage conditions to ensure long-term physical stability.

Adsorption studies of a krypton film adsorbed on catalytically synthesized multiwalled carbon nanotubes: Dependence on the nanotube morphology

Krypton adsorption isotherms on multiwalled carbon nanotubes have been measured at 77 K and compared to those obtained under the same conditions on exfoliated graphite. The results are discussed in the light of the nanotube morphology, as determined by transmission electron microscopy and electron energy loss spectroscopy. The isotherms exhibit two steps representative of successive monomolecular layer condensations on the external walls of nanotubes, instead of five steps on graphite. The monolayer condensation pressures are higher than those observed with graphite, which probably limits their number at krypton bulk condensation pressure. Near the completion of the first condensed layer, a second transition towards a phase of higher density occurs, the nature of which is discussed.

Influence of lanthanide oxides on the catalytic activity of nickel

The catalytic activity of supported Ni/Ln 2O 3 (Ln=La, Ce, Gd, Ho, Yb) catalysts for the hydrogen/water isotopic exchange reaction has been studied. The metallic and total surface areas were measured by hydrogen and krypton adsorption, respectively. The Ni surface area evaluated by hydrogen chemisorption is higher than the total BET surface area for Ni/La 2O 3. The Ni surface area does not change significantly while the total surface area increases with the decreasing Ln 3+ ionic radius. The thermodesorption spectra for the adsorbed hydrogen with and without preadsorbed H 2O are reported. The mobility of the adsorbed hydrogen decreases from La 2O 3 to Yb 2O 3 as support and in the same direction decreases the water uptake normalised to the total surface area. The catalytic activity normalised to the Ni surface area decreases exponentially with the decreasing Ln 3+ ionic radii from La 2O 3 to Yb 2O 3 as support, in close relationship with the basicity and hydration degree of the support oxide.

Surface area of compressed expanded graphite

In a previous work [Carbon (2002) in press], the densification of expanded graphite was modelled. This very light material consists of worm- or accordion-like particles in a loose packing. Worm-like particles are made up of thin disc-shaped graphite sheets, and the average disorientation angle of the thin discs was estimated based on the excluded volume concept. Then, using a simple picture of particle densification, the mosaicity of compressed expanded graphite was calculated and compared with the results of XRD experiments. In the present paper, the same model and assumptions are taken into account for predicting the surface area variations of two kinds of expanded graphite undergoing compaction. The constitutive graphite sheets are assumed to bend and to make flat contacts with their neighbours, thus decreasing the surface of the material. It is worth noting that the proposed modelization is free of any adjustable parameter. The calculations are shown to be in good agreement with experimental results obtained from krypton adsorption at liquid nitrogen temperature.

Determination of the Specific Surface Areas of Non-Porous and Macroporous Carbons

The current work is focused on the examination of a new method for the determination of the specific surface areas of non-porous and macroporous carbons from gas adsorption isotherms. This method has rigorous foundations since it utilizes the adsorption potential distribution which is a unique thermodynamic characteristic for a given adsorption system. Nitrogen adsorption isotherms at 77 K for a series of low-surface area graphitized and non-graphitized carbons were used to examine this method and to compare it with the BET procedure. Additional testing was undertaken for argon, nitrogen and krypton adsorption isotherms at 87 K on a highly graphitized carbon. It was shown that surface area analysis based on the adsorption potential distribution is very promising for the characterization of various carbons.

Structural Properties and Catalytic Behaviour of Iridium Black in the Selective Reduction of NO by Hydrocarbons

Several mechanical mixtures of Ir black or IrO2 with various mixing materials like silica xerogel, alumina, and H-ZSM-5 were tested for their activity in the selective reduction of NO by propene. Independent of the mixing material used the unsupported Ir showed outstanding DeNOx activity compared to their supported counterparts. The conversion of NO to N2 under lean conditions as well as oxidizability and reducibility of the Ir–IrO2 system was found to depend strongly on the Ir crystallite size. With increasing crystallite size, selectivity to N2 is greatly enhanced mainly by the suppression of a reaction pathway leading to the production of NO2. Tailoring of Ir crystallite size by controlled sintering of Ir black afforded highly active Ir catalysts without the need for further conditioning on stream. Among unsupported platinum group metals iridium showed the best performance in NOx removal. The catalytic behaviour of Ir black was compared to the behaviour of Pt black, Pd black, and Rh black. Unsupported IrO2 with relatively large crystallite size (24 nm) was inactive for the selective reduction of NO with propene. Only IrO2 with small crystallite size (below ca. 11 nm) and high surface area could be activated by on-stream conditioning. Ir black and IrO2 were characterized by XRD, TEM, SEM, thermal analysis, and krypton adsorption measurements. It was shown that it is unlikely that NO2 is an intermediate in the reduction of NO.

Reference potentials for adsorption of helium, argon, methane, and krypton in high-silica zeolites

Quantitative agreement with experimental data for adsorption of argon, krypton, and methane in high-silica zeolites is achieved with a molecular model, which (1) assumes pairwise additivity of intermolecular forces; (2) ignores partially concealed silicon atoms and lumps their dispersion energy with the oxygen atoms; (3) neglects energy induced by the weak electric field inside silicalite; (4) uses the Lennard–Jones 12-6 potential for gas–gas and gas–solid interactions. The parameters of the Ar–O gas–solid potential are ε/ k=93.0 K and σ=3.335 Å. The gas–solid potential parameters for krypton and methane were calculated from the Ar–O reference potential using Lorentz–Berthelot mixing rules and the principle of corresponding states. The pore volume of silicalite determined from the He–O potential is 0.175 cm 3 g −1. The local density of gases adsorbed in the pores varies with the strength of the gas–solid potential; at 306 K and 10 bar, the maximum absolute density of methane is about one half the density of liquid methane at its normal boiling point.

On the difficulty of assessing the specific surface area of magnesium stearate

The water content of as-received commercial magnesium stearate batches from animal and vegetable sources have been modified by ageing in humid air at room temperature or by vacuum treatment. The complete adsorption–desorption isotherms of nitrogen and krypton vapours by samples of these as received and modified materials have been measured at liquid nitrogen temperature after standardised vacuum degassing. They are greatly affected by the initial water content of the material. In particular: (a) the BET surface area values computed from the adsorption branch vary widely and is increasing with increasing water content; (b) anomalous hysteresis of varying amplitude is observed in all cases except adsorption of krypton on the material with the lowest water content; (c) the hysteresis loops extend down to very low desorption pressure values and cannot be accounted for by capillary condensation. Lastly, the surface area value of a given material computed from nitrogen and krypton adsorption may differ by a factor as high as six. Accordingly, the very significance of BET surface area values obtained from routine adsorption experiments should be regarded as questionable, at least until the mechanisms of adsorption are fully clarified.

Intrinsic and extrinsic precursors to adsorption: Coverage and temperature dependence of Kr adsorption on Pt(111)

The kinetics of krypton adsorption on Pt(111) were investigated using supersonic molecular beam techniques. Krypton adsorbs at defects via an intrinsic precursor below a surface temperature of 85 K. The difference in activation energies for desorption and migration of a Kr atom on the terrace seeking a defect site is 10.7 kJ/mol, indicating that at 80 K, a Kr atom makes about 107 site hops before desorbing or finding a binding site. Below 60 K stable adsorption occurs on terraces, where the initial adsorption probability is independent of surface temperature. The activation energy for zero-order desorption from Pt(111) terraces is 12.9 kJ/mol; the activation energy for Kr migration on the terraces is then calculated to be ⩽2.2 kJ/mol. Krypton adsorption proceeds at nonzero coverages via an extrinsic precursor. The adsorption probability of Kr increases with self-coverage, and is described by the modified Kisliuk model [H. C. Kang, C. B. Mullins, and W. H. Weinberg, J. Chem. Phys. 92, 1397 (1990); C. R. Arumainayagam, M. C. McMaster, and R. J. Madix, J. Phys. Chem. 95, 2461 (1991)]. The Kr overlayer on terraces compresses between 60 and 42 K, then forms a second state before forming stable multilayers at 34 K.

Water adsorption at high temperature on core samples from The Geysers geothermal field, California, USA

For the first time, water sorption on representative geothermal reservoir rocks from The Geysers steam field has been determined in the laboratory at actual reservoir temperature. The Oak Ridge National Laboratory (ORNL) isopiestic apparatus has been used to measure quantities of water retained at various temperatures and relative pressures by plug samples of three representative reservoir metagraywacke cores. The measurements were made at 150, 200 and 250°C as a function of relative pressure in the range 0.00 ⩽ p/ p 0 ⩽ 0.98, where p 0 is the saturated water vapor pressure. Both adsorption (increasing pressure) and desorption (decreasing pressure) runs were made in order to investigate the phenomenon of hysteresis. Low-temperature gas adsorption analyses were completed on the same rock samples. Nitrogen or krypton adsorption and desorption isotherms at 77 K were used to obtain BET (Brunauer, Emmet, Teller) specific surface areas and pore volumes and their distributions with respect to pore sizes. Mercury-intrusion porosimetry was also used to obtain similar information extending to very large pores (macropores). A qualitative correlation was found between the surface properties obtained from nitrogen adsorption and the mineralogical and petrological characteristics of the solids. In general, however, there is no direct proportionality between BET specific surface areas obtained from nitrogen adsorption and the capacity of the rocks for water adsorption at high temperatures. An analysis of the temperature dependence of adsorption/desorption indicates that multilayer adsorption rather than capillary condensation is the dominant water storage mechanism in The Geysers reservoir rocks at high temperatures.

Oxidation behaviour of a multi-layered ceramic-matrix composite (SiC) f/C/(SiBC) m

The oxidation behaviour of a 2.5D multi-layered ceramic-matrix composite (SiC) f/C/(SiBC) m was investigated in a dry atmosphere O 2 (20 vol.%) CO 2 (5 vol.%) He and in the presence of water vapour H 2O (2.3 vol.%). The matrix denoted (SiBC) m is constituted of three phases: silicon carbide, boron carbide and a SiBC phase. The aim of boron incorporation is to improve the oxidation resistance of the composite by boron oxide or borosilicate formation. The transformations were followed by thermogravimetry during isothermal experiments of 20 h exposure in the range 600–1200°C and the changes of the specific area of the samples were measured by krypton adsorption at 77 K. The results of this study show that effective protection occurs in the range 650–900°C and is mainly related to the oxidation of boron carbide. At higher temperatures, boron oxide is no longer protective because of its volatilisation and consumption by reaction with water vapour. However, the two other constituents of the matrix, SiC and SiBC, lead to self-healing behaviour by both borosilicate and silica formation.

The surface chemistry of amorphous silica. Zhuravlev model

A review article is presented of the research results obtained by the author on the properties of amorphous silica surface. It has been shown that in any description of the surface silica the hydroxylation of the surface is of critical importance. An analysis was made of the processes of dehydration (the removal of physically adsorbed water), dehydroxylation (the removal of silanol groups from the silica surface), and rehydroxylation (the restoration of the hydroxyl covering). For each of these processes a probable mechanism is suggested. The results of experimental and theoretical studies permitted to construct the original model (Zhuravlev model-1 and model-2) for describing the surface chemistry of amorphous silica. The main advantage of this physico-chemical model lies in the possibility to determine the concentration and the distribution of different types of silanol and siloxane groups and to characterize the energetic heterogeneity of the silica surface as a function of the pretreatment temperature of SiO 2 samples. The model makes it possible to determine the kind of the chemisorption of water (rapid, weakly activated or slow, strongly activated) under the restoration of the hydroxyl covering and also to assess of OH groups inside the SiO 2 skeleton. The magnitude of the silanol number, that is, the number of OH groups per unit surface area, α OH, when the surface is hydroxylated to the maximum degree, is considered to be a physico-chemical constant. This constant has a numerical value: α OH,AVER=4.6 (least-squares method) and α OH,AVER=4.9 OH nm −2 (arithmetical mean) and is known in literature as the Kiselev–Zhuravlev constant. It has been established that adsorption and other surface properties per unit surface area of silica are identical (except for very fine pores). On the basis of data published in the literature, this model has been found to be useful in solving various applied and theoretical problems in the field of adsorption, catalysis, chromatography, chemical modification, etc. It has been shown that the Brunauer–Emmett–Teller (BET) method is the correct method and gives the opportunity to measure the real physical magnitude of the specific surface area, S Kr (by using low temperature adsorption of krypton), for silicas and other oxide dispersed solids.

Methane and Krypton Adsorption on Single-Walled Carbon Nanotubes

Methane and krypton adsorption measured on single-walled nanotubes at 78.7 and 77.3 K, respectively, give rise to stepwise isotherms, representative of the adsorption on two types of comparatively uniform patches during the first stages of the adsorbate condensation. The values of the methane isosteric heat of adsorption on these two fractions of quasi-uniform surface, determined from the dependence of the equilibrium pressure on temperature between 78 and 110 K, are in very good agreement with those measured at 77.3 K by means of isothermal microcalorimetry in quasi-equilibrium. Their respective mean values (18.3 ± 1 and 11.2 ± 0.5 kJ mol-1) enclose that of methane adsorption on graphite in the monolayer range (14.9 kJ mol-1). A phase transition occurring in the adsorbed film on the less attractive quasi-uniform part of the surface can be predicted from volumetric measurements.

Adsorption of Krypton and Cyclopentane on C60: An Experimental Study

The adsorption of krypton and cyclopentane on C60 has been compared with that on reference carbon adsorbents. From krypton adsorption isotherms it follows that the fresh surface of C60 has similar adsorption properties as electrochemical carbon. Cyclopentane molecules penetrate into the bulk of C60 crystals and are localized in the octahedral interstices between fullerene molecules.

Low-energy electron diffraction study of krypton on Cu(110)

Low-energy electron diffraction (LEED) was used to study the adsorption of krypton on Cu(110). Adsorption isobars indicate that the isosteric heat of adsorption for one-half monolayer is 114±10 meV. A dynamical LEED study of the c(2×8) higher-order commensurate structure at 25 K indicates that rows of krypton atoms are localized on top of the copper substrate rows. The krypton rows are situated such that they have the lowest-coordinated position along the rows. The overlayer–substrate spacing is 3.36±0.07 Å and the top two interlayer spacings of the substrate are 1.18±0.04 Å and 1.31±0.02 Å, respectively.

Preparation and characterisation of nickel-modified ceramic filters

A nickel-activated alumina candle filter shows potential for the combined removal of particles, tars and ammonia from biomass gasification gas at high temperature. To deposit the nickel catalyst in the filter, only methods applicable to a preformed support can be considered. In this paper, we investigated the incorporation of a nickel precursor into small-scale α-Al 2O 3 filter substrates with a mean pore radius of 26 μm. Two preparation routes were used and compared: deposition–precipitation with urea and nickel nitrate and conventional impregnation with nickel nitrate. The substrates were characterised by X-ray mapping (SEM/EDX), mercury porosimetry and krypton adsorption. Preliminary experiments with the urea method have shown that an appropriate selection of the reaction time and the urea/nickel molar ratio is necessary to obtain a high controlled fixation of precursor during the slow decomposition of urea. The urea method gave a fairly uniform distribution of the nickel precursor throughout the support bodies. On the contrary, with the conventional impregnation method, most of the precursor was found in the outer pores of the substrate. Both methods can be repeated to increase the catalyst loading. Performing twofold deposition cycles, it was found that the urea method was more reliable and still gave a good distribution of the precursor.

Adsorption of krypton at 77 K on fullerene C 60, graphitized carbon black and diamond

Adsorption of krypton at 77 K on fullerene C 60, graphitized carbon black and nanocrystalline synthetic diamond has been investigated. The shape of the isotherm on nanocrystalline synthetic diamond indicates that the adsorption mechanism follows predominantly monolayer–multilayer adsorption. The isotherm on graphitized carbon black is characterized by a step-like form, at the lowest surface coverage the amount adsorbed obeying Henry's equation. The isotherm on fullerene is similar to that on graphitized carbon black in the low-pressure region. At relative pressures lower than 0.005, both isotherms have a sigmoidal shape, the only difference being that the cooperative build-up of the first layer ends at a lower coverage on fullerene. The comparison of isotherms on fullerene and graphitized carbon black has shown that krypton is adsorbed on both solids by the same mechanism at a surface coverage below 0.3.

Differences in crystallization behavior between quenched and ground amorphous ursodeoxycholic acid.

To study the crystallization of ground and quenched ursodeoxycholic acid (UDCA) and to characterize their amorphous states. Amorphous UDCA was prepared by grinding and also by rapid cooling of the melt. These samples were characterized by powder X-ray diffraction (XRD), near IR spectra and dynamic water sorption. The heat associated with crystallization was measured in an isothermal microcalorimeter at 25 degrees C at various relative humidities (RH) (50%-100%) and, in the presence of the vapour from a mixed solvent of ethanol and water (ethanol conc. 10%-100%). The specific surface area was calculated from krypton adsorption. Contact angles were measured by using a Wilhelmy plate to calculate the surface energy of the samples. Ground and quenched samples yielded amorphous XRD patterns. Differential scanning calorimetry thermographs of the milled sample revealed that crystallization occurred at around 80 degrees C, whereas the quenched sample did not crystallize. Exposure to humid air did not result in crystallization of either amorphous sample during the microcalorimetric experiments. In the presence of ethanol vapour, the ground sample did, but the quenched sample did not, crystallize. The amount of water sorption into the quenched sample was larger than that of the ground sample at low RH. The surface energy of the quenched material was different to that of the ground. Peak shifts were observed in the NIR spectra at around 1450, 2100 nm, allowing differentiation between the ground and quenched samples. It can be concluded that different molecular states of amorphous UDCA were obtained depending on the preparation method. The crystallisation of amorphous UDCA was related to the molecular state of disorder.

Influence of time and temperature of hydrothermal treatment on glass fibers surface

Leaching appears to be a suitable method of obtaining a high hydroxyl population at the surface of borosilicate fibers. Several procedures were investigated and the treated fibers have been characterized by Fourier transform infrared spectroscopy (silanol band), and elemental analysis (carbon content) after trimethylsilylation. The texture of leached fibers was examined by scanning electron microscopy and the result was compared to that of original fibers. The modified fibers which exhibited the highest hydroxyl population with minimum alteration of the texture, as could be seen by scanning electron microscopy, were characterized by using nitrogen, krypton and water adsorption techniques in order to get information on surface area and porosity. The procedure revealed textural modifications: the surface area was increased and a micropore volume was formed.