Krypton Adsorption Research Articles

Pore analysis of obliquely deposited nanostructures by krypton gas adsorption at 87 K

The porous nature of high surface area and large aspect ratio nanorods arrays make them strong candidates for a range of applications from catalyst supports to energy scavenging devices. The glancing angle deposition (GLAD) technique is a straightforward and flexible method for ensuring control of morphology and porosity in nanorod arrays. Quantifying the porous nature of nanostructured columnar arrays, fabricated using the GLAD technique, is critical for understanding and optimizing the performance of devices fabricated using this method. The mesoporosity and pore volume of these GLAD nanorod arrays have thus been characterized using gas adsorption (krypton adsorption at 87.3 K) and scanning electron microscopy (SEM) image analysis techniques. To characterize the obliquely deposited amorphous titanium dioxide (TiO 2) and silicon dioxide (SiO 2) samples, top-down SEM images of the films were also analyzed which allowed us to determine the mean distance between columns. SEM image analysis permitted for estimation of inter-column porosity of films deposited at angles from 65° to 85°, while krypton analysis allowed for the identification of mesoporosity in films from 45° to 85°. Our results provide a comprehensive characterization of GLAD film porosity which will be vital for the design of devices for a diverse set of applications from photonics to sensing to microfluidics.

Measurement of airborne nanoparticle surface area using a filter-based gas adsorption method for inhalation toxicology experiments

Measurement of the surface area of airborne nanoparticles as administered to an experimental subject is critical for characterizing exposures during inhalation experiments. A filter-based surface area measurement methodology is described herein that allows for such determinations. Krypton gas adsorption was used to determine total particle surface area. Track-etched polycarbonate 0.4 μm pore filters were chosen as the collection substrate for metal oxide particles due to their highly reproducible surface areas and low background weights. The subject nanomaterials included two different batches of ultrafine TiO2, TiO2 nanorods, and SiO2. The instrument detection limit for surface area was 200 cm2 (0.02 m2). Ninety percent confidence interval estimates of method accuracy were 17.7–23.5% with a point estimate of 20.8%. The filter-based surface area measurement strategy is demonstrated to be a viable sampling and analysis methodology that provides much needed physical characterization information of particles as administered in an animal inhalation chamber.

Krypton adsorption as a suitable tool for surface characterization of multi-walled CNTs

Multi-walled carbon nanotubes (CNTs), pristine and subjected to treatments, are comparatively characterized from N 2 and Kr (77 K) adsorption measurements. The CNTs are lab-synthesized by in situ chemical vapour deposition of an iron-based organometallic compound at 895 °C. The treatments applied to the CNTs include low temperature gas-phase oxidation, mild temperature annealing and ultrasonic dispersion in ethanol, in an attempt to examine possible changes in adsorption characteristics. N 2 and Kr adsorption measurements give rise to steadily increasing and stepped isotherms, respectively. The former are representative of a multilayer adsorption phenomenon, while the latter indicate successive monolayer condensation. The treatments affect differently gas adsorption capacities of the CNTs. Oxidation leads to CNTs with higher BET specific surface area and increased adsorption capacity, though the effect is more pronounced for Kr adsorption. Ultrasonic dispersion of the CNT brings about a significant reduction only in N 2 adsorption capacity. Modifications in the characteristic steps in Kr adsorption isotherms of the CNTs subjected to annealing can be appreciated, although no remarkable changes are observed in N 2 adsorption isotherms. Present results demonstrate that determination of Kr adsorption isotherms represents a more suitable tool to obtain a more reliable textural characterization of CNTs than does N 2 adsorption.

Krypton adsorption on microporous adsorbents at higher pressures

The krypton adsorption isotherms were measured on AUK microporous carbon adsorbent in the range of pressures from 1 Pa to 6 MPa and temperatures of 177.7—393 K. With adsorption isotherms in the same range of pressures and temperatures, the krypton adsorption isosteres were calculated. The derived curves are well approximated with linear functions. With the adsorption isosteres, differential molar isosteric adsorption heat curves were plotted vs. adsorption and temperature. The received results were compared with the reference data of the krypton adsorption on Carbolac active carbon.

Investigation of surface properties of lunar regolith part III

Recently we published results of investigations of the surface properties of three lunar soil samples from the Apollo 11, 12 and 16 missions. In the present paper we summarise and supplement the results of measurements of density using a helium pycnometer, krypton and nitrogen adsorption using volumetric techniques and of water, heptane and octane adsorption using gravimetric techniques. We present additional diagrams and electron micrographs.

Growth mechanism and photocatalytic properties for dye degradation of hydrophobic mesoporous ZnO/SDS films prepared by electrodeposition

The growth by electrodeposition of ZnO/SDS hybrid thin films from aqueous mixed solution of zinc chloride and sodium dodecylsulfate (SDS), an anionic surfactant, using molecular oxygen as a precursor, has been studied. Significant morphological changes have been observed with the deposition time. At low electrodeposition time the layer is composed of flake-shaped aggregated grains whereas for a time longer than 10–15 min an inner layer composed of aggregated particles appears below the flake-type one. The flakes have a lamellar structure oriented parallel to the substrate. Krypton adsorption experiments show that the mesoporosity of the films is due to pores larger than 8 nm and probably arises from the nanoparticulate phase. The as-deposited films are hydrophobic with contact angles ranging between 110° and 130°. The hydrophobic character of these mesoporous ZnO films is found to substantially improve the efficiency of ZnO films for methylene blue photodegradation, their performances being similar to those of thicker hydrophilic mesoporous films.

Argon and krypton adsorption on templated mesoporous silicas: molecular simulation and experiment

In this work we report molecular simulation results for argon and krypton adsorption on atomistic models of templated mesoporous silica materials. These models add atomistic levels of detail to mesoscale representations of these porous materials, which were originally generated from lattice Monte Carlo simulations mimicking the synthesis process of templated mesoporous silicas. We generate our atomistic pore models by carving out of a silica block a ‘mathematically-smooth’ representation of the pores from lattice MC simulations. Following that procedure, we obtain a model material with mean mesopore and micropore diameters of 5.4 nm and 1.1 nm, respectively (model A). Two additional model materials were considered: one with no microporosity, and with mesopores similar to those of model A (model B), and a regular cylindrical pore (model C). Simulation results for Ar and Kr adsorption on these model materials at 77 K and 87 K shows that model A provides the best agreement with experimental data; however, our results suggest that fine-tuning the microporosity and/or the surface chemistry (i.e., by decreasing the density of OH groups at the pore surface) of model A can lead to better agreement with experiments. The filling of the mesopores in model materials A and B proceeded via a classical capillary condensation mechanism, where the pores fill at slightly different pressures. This observation contrasts with what was observed in our previous study (Coasne, et al. in Langmuir 22:194–202, 2006), where we considered atomistic silica mesopores with an important degree of surface roughness at length scales below 10 A, for which we observed a quasi-continuous mesopore filling involving intermediate phases with liquid-like “bridges” and gas-like regions. These results suggest that pore surface roughness, and other morphological features such as constrictions, play an important role in the mechanism of adsorption and filling of the mesopores.

Vacancy Solution Model Formulated by the NRTL Equation for Correlation of Adsorption Equilibria

A vacancy solution model based on the NRTL equation was derived and its validity was examined using experimental isotherms for adsorption of krypton and xenon on MS5A and activated charcoal adsorbents. The experimental adsorption isotherms of pure component of Kr and Xe on the adsorbents were successfully correlated with the vacancy solution model based on the NRTL equation. The experimental results for the adsorption equilibrium of Kr–Xe binary component systems on the MS5A and activated charcoal adsorbents were also examined using the model. The binary adsorption equilibria were well predicted by the vacancy solution model without parameter fitting. The optimization of parameters of the model in the binary systems resulted in better correlation of the binary adsorption equilibria.

Adsorption and pore condensation of krypton on mesoporous silicas at 77 K

The adsorption and pore condensation behavior of krypton was studied on the series of well-defined mesoporous silicas (mode pore diameter 2.6–6.0 nm) and one sample of disordered mesoporous silica (mode pore diameter 7.5 nm) at 77 K. The similar adsorption measurement was carried out on non-porous silica Davisil. The results show clearly that the pore condensation of krypton occurs only in mesopores whose diameter does not exceed 6 nm. The krypton pore condensation vanishes in larger mesopores, which limits information obtainable from the isotherm. This phenomenon can be explained on the basis of Broekhoff and de Boer thermodynamic theory of capillary condensation.

Mesoporous silica powders and films—Pore size characterization by krypton adsorption

An experimental correlation was established between pore size derived from N 2 adsorption, D N 2 , and capillary condensation/evaporation pressure, ( P/ P 0) t,Kr , obtained from Kr adsorption/desorption isotherms on powdery siliceous MCM-41, MCM-48 and SBA-15 samples, and was analyzed by the DBdB theory. The correlation was then used to estimate the pore size distribution from the Kr adsorption isotherm of mesoporous silica layers on Si(1 0 0) wafer prepared using P123 and Brij ®56 surfactants. It was found by TEM analysis that the pore shape in the thin films was distorted from circular to ellipsoidal, leading to different pore sizes estimated from the adsorption and desorption branches of the Kr isotherm.

Study of mixed valence iron borate catalysts in ethyl acetate oxidation process

The catalytic behavior of two mixed-valence borates was studied in ethyl acetate oxidation in respect to their different electronic structure. The studied compounds ferroferriborate (Fe 3BO 5) and magnesium ferroferriborate (Fe 2MgBO 5) are members of a solid solution series, which allows to synthesize samples with different mixed valence ions content. A number of methods, such as XRD, ISP, krypton adsorption, XPS and Moessbauer spectroscopy are used for sample characterisation. The catalytic experiments, as well as in situ Moessbauer investigation of the studied oxides in the course of ethyl acetate oxidation are also performed. It is established that ferroferriborate having higher mixed valence cations content shows higher catalytic activity. It is observed, that electron exchange (EE) in mixed valence metal oxide catalyst provides optimal conditions for ethyl acetate total oxidation. However, in respect to the observed selectivity against by-products formation, the electron delocalization (ED) should be located in a limited range. A partial oxidation of iron ions in the studied compounds, mainly in their surface, is observed after catalytic tests.

Krypton adsorption technique for assessment of structural properties of mesoporous silica and titania thin films

Krypton adsorption technique for assessment of structural properties of mesoporous silica and titania thin films

Surface characterisation of template-synthesised multi-walled carbon nanotubes

Physisorption studies and transmission electron microscopy have been used to characterise multi-walled carbon nanotubes (MWNTs) made by a template-synthesis technique. Microscopic investigations revealed formation of ‘branched nanotubes’ with significant irregularities in diameters and with structural defects on the external surfaces of the tubes. Krypton adsorption isotherms at 77 K were recorded; comparison of these isotherms with those obtained under the same conditions on well defined MWNTs made by the catalytic chemical vapour deposition (CCVD) technique is discussed in the light of the sample morphologies. The effect of annealing on the crystallinity of the surface is reported.

Ozone decomposition kinetics on alumina: effects of ozone partial pressure, relative humidity and repeated oxidation cycles

Abstract. The room temperature kinetics of gas-phase ozone loss via heterogeneous interactions with thin alumina films has been studied in real-time using 254nm absorption spectroscopy to monitor ozone concentrations. The films were prepared from dispersions of fine alumina powder in methanol and their surface areas were determined by an in situ procedure using adsorption of krypton at 77K. The alumina was found to lose reactivity with increasing ozone exposure. However, some of the lost reactivity could be recovered over timescales of days in an environment free of water, ozone and carbon dioxide. From multiple exposures of ozone to the same film, it was found that the number of active sites is large, greater than 1.4x1014 active sites per cm2 of surface area or comparable to the total number of surface sites. The films maintain some reactivity at this point, which is consistent with there being some degree of active site regeneration during the experiment and with ozone loss being catalytic to some degree. The initial uptake coefficients on fresh films were found to be inversely dependent on the ozone concentration, varying from roughly 10-6 for ozone concentrations of 1014 molecules/cm3 to 10-5 at 1013 molecules/cm3. The initial uptake coefficients were not dependent on the relative humidity, up to 75%, within the precision of the experiment. The reaction mechanism is discussed, as well as the implications these results have for assessing the effect of mineral dust on atmospheric oxidant levels.

Comparison of different methods for characterizing multi-walled carbon nanotubes

Four complementary and independent methods are used to characterize nanotube samples: (i) TEM observation coupled with image analysis, (ii) nitrogen and krypton adsorption isotherm analysis, (iii) thermogravimetry and (iv) wide angle X-ray scattering. The methodology is discussed on the basis of two multi-walled carbon nanotube samples produced by the CCVD technique with very different reaction rates. It is shown that the total amount of deposited carbon is larger for the sample produced at the higher rate, that the fraction of nanotubes in the deposited carbon does not seem to be significantly different, but that the crystallinity of the nanotubes wall is larger for the sample produced at the lowest reaction rate.

Modeling of pore wall amorphous structures: influence of wall heterogeneity on the mechanism of adsorption: Krypton and Argon adsorption in MCM-41 pore model

Influence of the pore wall heterogeneity on the adsorption mechanism has been studied. The heterogeneity has been modeled by adsorption site distributions. Two aspects of these distributions have been analyzed: energetic and structural. It has been shown that the energy site distribution determines the low-pressure adsorption whereas the structural distribution affects the multi-layer adsorption. In the case of the adsorption of Ar atoms, one can find a distribution of adsorption sites that allows one to modify the shape of adsorption isotherms and reproduce the experimental data. In the case of Kr adsorption, we observe that the intermediate structures, between one and two layers, are not stable.

An analysis of supercritical adsorption in the context of continuum mechanics

To illustrate a new extension of continuum mechanics to the nanoscale (not the molecular scale), we analyze the supercritical adsorption of argon, krypton, and methane on Graphon. We compare our results both with existing experimental data and with prior molecular-based theories.

Monte Carlo simulations of krypton adsorption in nanopores: Influence of pore-wall heterogeneity on the adsorption mechanism

We present molecular simulation results of the adsorption of krypton in a model of the mesoporous material MCM-41. The adsorption isotherm and adsorption enthalpies are studied at 77 K. Comparison of the experimental and simulation data allows us to analyze how the available interaction models (Kr–Kr and Kr–walls) are able to reproduce the experimental situation. The role of the heterogeneous interactions versus the homogenous model is studied and compared with the previous simulation results of nitrogen adsorption in MCM-41. The results show that a model of ideal cylindrical pores gives qualitatively and quantitatively different results. A distribution of the adsorption sites must exist to explain the loading at low pressure (below capillary condensation). Such a distribution in MCM-41 is a consequence of inhomogenous walls that contain a wide variety of attractive sites ranging from weakly attractive silica-type to highly attractive regions. In our simulations the MCM-41 structure is modeled as an amorphous array of oxygen and silicon atoms, each one interacting with an adsorptive atom via the atom–atom potential. The distribution of the adsorption sites is merely a consequence of the local atomic structure. Such a model of the wall reproduces the smooth increase in loading seen experimentally.

Opening of single-walled carbon nanotubes: evidence given by krypton and xenon adsorption

Adsorption isotherms of krypton between 77 and 93 K and xenon between 110 and 120 K on mechanically-opened single-walled carbon nanotubes (SWNTs) have been measured. The comparison of the results with those obtained under the same conditions on closed nanotubes is discussed. Evidence of adsorption inside the tubes is given through the appearance of an additional adsorbed amount at very low relative pressure on the isotherms. Different orderings are predicted for xenon and krypton, based on their respective adsorbed amounts inside the nanotubes, and on geometrical considerations. Finally, the fraction of opened nanotubes accessible to xenon and krypton was estimated, as well as the impurity content originating from the cutting process.

Adsorption of Supercritical Fluids on Graphitised Thermal Carbon Black: Molecular Layer Structure Theory versus Grand Canonical Monte Carlo Simulation

This paper presents a detailed analysis of adsorption of supercritical fluids on nonporous graphitized thermal carbon black. Two methods are employed in the analysis. One is the molecular layer structure theory (MLST), proposed recently by our group, and the other is the grand canonical Monte Carlo (GCMC) simulation. They were applied to describe the adsorption of argon, krypton, methane, ethylene, and sulfur hexafluoride on graphitized thermal carbon black. It was found that the MLST describes all the experimental data at various temperatures well. Results from GCMC simulations describe well the data at low pressure but show some deviations at higher pressures for all the adsorbates tested. The question of negative surface excess is also discussed in this paper.