Low-temperature Adsorption Isotherms Research Articles

Effect of Nanoparticles and Microparticles of Hexagonal Boron Nitride on Structure, Thermal and Mechanical Stability of Lithium and Calcium Greases

Hexagonal boron nitride is being considered as an additive for greases due to its structure and physical and chemical properties. In the context of the application of such lubricants in real tribological systems, it is important to recognise the effect of hexagonal boron nitride not only on tribological properties, but also on other functional properties of this group of lubricants. In the present study, tests including dropping point, penetration and mechanical stability were carried out. Additionally, particular focus was placed on the properties of the additive itself, including particle size distribution and adsorption properties, as determined by scanning electron microscopy and low-temperature adsorption isotherms. The introduction of hexagonal boron nitride particles into lithium and calcium greases resulted in enhanced resistance to high temperature and prolonged mechanical stress. This phenomenon was attributed to the type of base grease and the modifications in the configuration of the grease's spatial network that ensued as a result of the incorporation of solid particles. It was found that an additive with a smaller particle size and a significant proportion of nanoparticle fractions, and a more developed porous structure, was more effective. Microscopic observations of the structure of the greases confirmed that the solid particles were deposited in the spatial network of the greases. The distribution of hexagonal boron nitride in the grease structure was found to be contingent upon the physical and chemical properties of the additive. Furthermore, the type of base grease, including the arrangement of the soap fibre network, was identified as a contributing factor.Graphical

ANALYSIS OF THE INFLUENCE OF HEXAGONAL BORON NITRIDE ON TRIBOLOGICAL PROPERTIES OF GREASE

The article discusses the problem of using hexagonal boron nitride (h-BN) as a grease additive. The literature on the subject was analysed in terms of greases into which hexagonal boron nitride was added. Particular attention was paid to the nano h-BN, due to the topicality of this topic and the potential of nano-additives to lubricants noted in published scientific studies. It was found that in order to indicate the regularities describing the tribological interaction of hexagonal boron nitride, detailed studies and an analysis of its properties are required. The important factors determining the application of this additive include particle size distribution, morphology, specific surface area, and porosity. The mentioned properties were determined for four samples of hexagonal boron nitride, which were also objects of tribological experiments. For this purpose, a scanning electron microscope (SEM) and XRD method were used, and low-temperature adsorption isotherms were determined. The research on the influence of h-BN on the lubricity properties of lithium grease was carried out on a four-ball apparatus. Possible mechanisms of interaction of different types of h-BN in the friction zone were identified using the information collected on their important properties. Based on the results of the research, it was found that the use of nano h-BN in the discussed context seems promising. Article published in connection with the Autumn Tribological School.

Mechanism of solvent extraction-induced changes to nanoscale pores of coal before and after acidification

As the most widely used reservoir stimulation method, hydraulic fracturing requires the injection of large volumes of fluid with certain viscosity, mainly formed by water and various additives in a certain proportion, into coal seam for the purpose of improving the permeability of coal reservoir, and thus the selection of appropriate fracturing fluid is very important to improve the fracturing effect. In the study, using a medium-volatile bituminous coal as the research object, solvent extraction experiments of coal samples before and after acidification under different solvent concentrations were carried out. After the soluble low molecular weight compounds in coal were extracted and dissolved, and the differences in pore structure parameters and their mechanisms in coal were investigated by using low-temperature liquid nitrogen adsorption apparatus and capillary coalescence theory. The research results show that, after tetrahydrofuran (THF) extraction with different concentrations, the pore volume (PV) of each pore size section for raw coal decreases, and with the increase of the pore size, the decreasing PV tends to be stable. The PV of acidified coal increases significantly in all pore size ranges, especially in the range of 5 ∼ 10 nm. Compared with unacidified coals, the low-temperature nitrogen adsorption isotherm of acidified coals after extracted with different concentrations of THF were more prominent in the relative pressure of 0.8 ∼ 1.0. As THF concentration increases, the fractal dimension D of raw coal and nitrogen adsorption capacity decrease, while the fractal dimension D of acidified coal and nitrogen adsorption capacity increase and are significantly bigger than those of raw coal. The nitrogen adsorption capacity of raw coal and acidified coal shows an inverted “N” and “N” trend, respectively, and the nitrogen adsorption capacity of the residue with 50%THF extraction after acidification reaches the maximum value of 1.47 cm3/g. This study suggests that adding a certain proportion of organic solvent (THF) and acid into fracturing fluids can be beneficial to improve reservoir porosity and thus increasing gas production.

Gas transport parameters, density and free volume of nanocrystalline poly-2,6-dimethylphenylene oxide

Poly(2,6-dimethylphenylene oxide-1,4) (PPO) belongs to a fairly investigated group of polymers that, nevertheless, provokes permanent research interest. The current study is focused on a combination of various physicochemical methods of polymer testing (DSC; molecular weight distribution; surface area by low-temperature adsorption isotherms of nitrogen and carbon dioxide; X-Ray diffraction analysis; hydrostatic measurement of density; gas permeability and diffusion measurement) with variation of molecular weight and different states of polymer samples (amorphous, semi-crystalline, biaxial-oriented). The alteration of the density, gas permeability and diffusion parameters is evaluated through modeling of PPO as a polymer-polymer blend material with amorphous, nano-sized crystalline (≤7 nm), and so-called rigid amorphous fraction (RAF) phases. The estimation of the phase properties of PPO samples allows to conclude that the permeability and diffusion characteristics decline in a row crystalline > amorphous > RAF.

Self-adjusting binding pockets enhance H2 and CH4 adsorption in a uranium-based metal-organic framework.

A new, air-stable, permanently porous uranium(iv) metal-organic framework U(bdc)2 (1, bdc2- = 1,4-benzenedicarboxylate) was synthesized and its H2 and CH4 adsorption properties were investigated. Low temperature adsorption isotherms confirm strong adsorption of both gases in the framework at low pressures. In situ gas-dosed neutron diffraction experiments with different D2 loadings revealed a rare example of cooperative framework contraction (ΔV = -7.8%), triggered by D2 adsorption at low pressures. This deformation creates two optimized binding pockets for hydrogen (Q st = -8.6 kJ mol-1) per pore, in agreement with H2 adsorption data. Analogous experiments with CD4 (Q st = -24.8 kJ mol-1) and N,N-dimethylformamide as guests revealed that the binding pockets in 1 adjust by selective framework contractions that are unique for each adsorbent, augmenting individual host-guest interactions. Our results suggest that the strategic combination of binding pockets and structural flexibility in metal-organic frameworks holds great potential for the development of new adsorbents with an enhanced substrate affinity.

Crossover between Fickian and non-Fickian diffusion in a system with hierarchy

Abstract A hierarchical porous material was prepared by embedding microporous ZSM-5 zeolite into the mesoporous alumina matrix. The presence of the micropores with the size relevant to zeolite ZSM-5 and the mesopores of 4 nm in diameter were justified from the low-temperature N2 adsorption and desorption isotherm data. Transport of various diffusants with significantly different molecule size, e.g. methanol, isopropanol, n-hexane, benzene, cyclohexane, and cyclohexanol, through the as-prepared material was studied. The diffusants molecule sizes (d) are chosen to be smaller or larger compared to the ZSM-5 zeolite micropore size (dm). For the diffusants with d > dm, the mass transfer was found to follow the second Fick's law of diffusion. In contrast, the diffusants with d

СОВМЕСТНОЕ КАТАЛИТИЧЕСКОЕ СЖИГАНИЕ САЖИ И ВОДОРОДА В КИПЯЩЕМ СЛОЕ

Актуальность исследования обусловлена необходимостью обезвреживания выбросов сажи и водорода из печей нанесения углеродных покрытий на сорбенты, которые в основном сжигаются в факельном режиме. Это приводит к образованию термических оксидов азота и не обеспечивает полного сгорания сажи. Цель: определение оптимальных условий проведения процесса совместного сжигания продуктов пиролиза бутадиена в кипящем слое катализатора глубокого окисления на реальных выбросах смеси сажи и водорода из печи нанесения углеродных покрытий на сорбенты. Объекты: реальные выбросы смеси сажи и водорода из печи нанесения углеродных покрытий на сорбенты. Методы: хроматографический анализ отходящих газов из реактора с кипящим слое катализатора при сжигании смеси сажи и водорода, фотоколориметрический анализ газов на содержание оксидов азота с использованием реактива Грисса–Илосвая, весовой метод определения выбросов сажи. Удельную поверхность сажи определяли на основе данных изотермы низкотемпературной адсорбции азота. Результаты. Определены оптимальные условия проведения процесса совместного сжигания продуктов пиролиза бутадиена в кипящем слое катализатора глубокого окисления на реальных выбросах смеси сажи и водорода из печи нанесения углеродных покрытий на сорбенты. Показано, что при совместном каталитическом сжигании сажи и водорода в кипящем слое промышленного катализатора ИК-12-70 происходит практически полное окисление водорода при температуре на уровне 550 °С, а степень выгорания углерода в виде сажи достигает значения не ниже 99,9 %. Выбросы вредных веществ незначительны: NOx – не более 4,5 мг/м3, CO – не более 0,037 об. %.

Random surface statistical associating fluid theory: Adsorption of n-alkanes on rough surface.

Adsorption properties of chain fluids are of interest from both fundamental and industrial points of view. Density Functional Theory (DFT) based models are among the most appropriate techniques allowing to describe surface phenomena. At the same time, Statistical Associating Fluid Theory (SAFT) successfully describes bulk pressure-volume-temperature properties of chain-fluids. In this study, we have developed a novel version of the SAFT-DFT approach entitled Random Surface (RS)-SAFT which is capable of describing adsorption of short hydrocarbons on geometrically rough surfaces. A major advantage of our theory is the application to adsorption on natural rough surfaces with normal and lateral heterogeneity. For this reason, we have proposed a workflow where the surface of a real solid sample is analyzed using a theoretical approach developed in our previous work [T. Aslyamov and A. Khlyupin, J. Chem. Phys. 147, 154703 (2017)] and experimentally by means of low temperature adsorption isotherm measurements for simple fluids. As a result, RS-SAFT can predict adsorption properties of chain fluids taking into account the geometry of the surface sample under consideration. In order to test our workflow, we have investigated hexane adsorption on carbon black with the initially unknown geometry. Theoretical predictions for hexane adsorption at 303 K and 293 K fit corresponding experimental data well.

Valorization of Toxic Weed Lantana camara L. Biomass for Adsorptive Removal of Lead

Valorization of Lantana camara L., which is a recognized invasive plant, as a potential source of activated carbon is proposed in this study. Its stem and leaf have been utilized for the preparation of activated carbon (ACL and ACS) by following acid-impregnation technique, followed by thermal treatment. The developed activated carbon samples were characterized for their structural and surface related properties by low-temperature nitrogen adsorption isotherm, SEM techniques, and pHPZC method. The samples show reasonable high surface area and pore volume; nonetheless, these properties are higher in case of ACL as compraed to ACS. Both of these samples developed negative charge on their surface due to acid treatment that resulted in an increase in adsorption at pH > 5. The batch adsorption studies on these samples shows the Pb(II) ion adsorption capacities of ACL and ACS were 36.01 and 32.24 mg·g−1, respectively, at 25°C. The kinetics of adsorption with both the sample systems follow the pseudo-second-order model, whereas the experimental equilibrium isotherm data of ACL and ACS were explained by Freundlich and Langmuir models, respectively. For these samples, the HCl shows maximum desorption with which the recycling test on these samples shows that ACS has better recycling potential over ACL samples.

Characteristics of activated carbon prepared from waste PET by carbon dioxide activation

Poly(ethylene)terephthalate (PET) waste was subjected to carbonization in a nitrogen stream at 1098K. The coke was then activated in a carbon dioxide stream under various conditions: temperatures of 1173, 1198 and 1213K as well as process times of 4, 5, 6 and 8h. The activated carbons were characterized using various methods: the structure from Raman and XRD measurements, the porosity from low temperature nitrogen adsorption, the surface properties from cyclic voltammetry and the hydrogen storage capacity from the low temperature adsorption isotherm of H2. The results demonstrated the importance of the temperature and the duration of the process. Higher temperatures result in the etching of graphitic domains of better crystallinity. A relatively small increase in activation time at the highest temperature used yielded a significant increase in the degree of burn-off and porous structure development. The microporosity of these carbons is similar to that of commercial activated carbons. They also have a similar capacity to adsorb water pollutants (e.g. 4-chlorophenol). The PET carbon sample with maximum burn-off exhibited higher values of the microporous structure parameters and the electric double layer capacity in electrolyte solution than the other three samples. The same sample exhibits a sufficient hydrogen storage capacity, which after optimization of the activation conditions should yield an effective storage material. This confirms the possibility of producing activated carbon from waste PET with satisfactory properties by the simple processes of carbonization and activation. The activated carbons obtained have potential use as water pollutant adsorbents, low-cost materials for hydrogen storage and electrode materials in supercapacitors or fuel cells.

Modeling of Adsorption of Gases on Graphite Surfaces Accounting for the Solid−Fluid Nonadditivity Correction

A series of graphitized carbon blacks have been studied using argon and nitrogen adsorption at their boiling points. Analysis of adsorption isotherms was performed with nonlocal density functional theory (NLDFT) accounting for the Axilrod-Teller equation to describe the effect of nonadditivity of the gas-solid interaction. In our previous study [Ustinov, E. A. J. Chem. Phys. 2010, 132, 194703] we have shown that the nonadditivity effect decreases the attractive component of Ar-Ar interaction in the first molecular layer adjacent to the graphite surface by about 23%. This is a source of a large error (up to 40%) when a standard NLDFT is applied to fitting the low-temperature Ar adsorption isotherm on a graphitized carbon black. A new approach that incorporates the Axilrod-Teller equation into the standard NLDFT diminishes the relative error from 40 to 4%, which suggests that the nonadditivity correction should not be ignored in most adsorption systems including crystalline and amorphous solids. The present study is an extension of our approach to N(2) adsorption isotherms at 77.3 K on graphitized carbon blacks. We show that the approach allows to reliably determine the gas-solid molecular parameters, the gas-solid nonadditivity coefficient, the Henry coefficient, and the specific surface area. The surface areas of different carbon blacks determined with the N(2) at 77.35 K and Ar at 87.29 K are very close to each other, though in the former case the values proved to be slightly smaller presumably due to nonspherical shape of the nitrogen molecule. A comparison with the Brunauer, Emmett, and Teller method is provided.

Effect of three-body interactions on Ar adsorption on graphitized carbon black

The Axilrod-Teller equation is incorporated into nonlocal density functional theory (NLDFT) to improve the description of low-temperature argon adsorption isotherm on graphitized carbon black. Using the NLDFT based on the Tarazona smoothed density approximation and the test-particle method, the pair and triplet distribution functions for liquid argon were analyzed. The latter allowed us to quantitatively account for the three-body Axilrod-Teller nonadditive term. An analogous scheme to that of Weeks, Chandler, and Andersen is proposed to incorporate the Axilrod-Teller equation into the NLDFT in the framework of mean field approximation. The three-body nonadditive term appeared to decrease the internal energy of liquid argon by about 5%, which is in agreement with molecular simulation results. The new approach has been applied to the vapor-liquid coexistence and to argon adsorption on graphitized carbon black at 87.3 K. The approach has been shown to fit the experimental adsorption isotherm within the relative error of about 4% as opposed to 37% in the case of the standard NLDFT. This is due to weakening of the intermolecular interaction potential by about 20% in the molecular layer nearest to the graphite surface resulted mainly from the positive Axilrod-Teller contribution of triplets comprising one carbon atom.

Studies of the Kinetic Characteristics of HSA Adsorption by Different Types of Activated Carbon Fibres

Adsorption of human serum albumin (HSA) from aqueous solution on to activated carbon fibres (ACF) with different degrees of activation in the range 0-70% has been studied at 293±1 K. The various parameters for the porous structure were calculated by different methods on the basis of the low-temperature adsorption isotherms of nitrogen measured at 77±1 K. It was shown that an increase in the degree of ACF activation led to the development of a porous structure which exhibited increasing values of the micropore and mesopore volumes. Relationships between the structure, the physicochemical properties of the surface and the adsorption characteristics of ACF were established. The kinetic curves for HSA adsorption from aqueous solution at an initial concentration of 0.5 mg/ml were characterized by negative adsorption ranges. This phenomenon may be explained, on the one hand, by the energetic heterogeneity of the protein-surface interaction and, on the other hand, by the possibility of alterations in the conformations of the protein molecules at the interface. The latter suggestion was confirmed by the hyperchromous shift observed in the UV spectrum of the HSA solution after protein-surface contact had been established.

Hydrogen adsorption on alkali metal surfaces: Wetting, prewetting and triple-point wetting

We report the results of a quartz microbalance study of the multilayer adsorption of H2 and D2 on sodium and on rubidium. On sodium H2 and D2 display the well known phenomenon of triple-point wetting. Interestingly low temperature adsorption isotherms of H2 on the evaporated Na surface show sharp layering transitions. On rubidium a weaker substrate H2 and D2 undergo a wetting transition in the liquid phase followed by sharp presetting transitions away from liquid-vapor coexistence. The prewetting phase diagram of H2 on Rb has been quantitatively mapped out.

Liquid-film instabilities in confined geometries.

Sol-gel methods combined with specific thermal treatments were used to fabricate porous glasses with extremely uniform pore diameters and high internal surface areas. Low-temperature adsorption isotherm measurements reveal the dynamic and thermodynamic stability limits of the liquid films as a function of evolving pore diameter. Contrary to classical predictions, the results are in remarkable agreement with a universal hydrodynamic theory of film behavior which includes substrate interactions.

Low-temperature adsorption of gases on heterogeneous solid surfaces: Effects of surface topography

For the adsorption of gases on a solid surface we derive equations for the low-temperature adsorption isotherms, isosteric heats of adsorption, and heat capacities of the adsorbed phase for both a patchwise and a random surface topography. A model of localized monolayer adsorption with nearest neighbor interactions between admolecules is used. The equations are illustrated by appropriate model calculations.

Low-temperature adsorption of gases on heterogeneous solid surfaces: Surfaces with random topography

Equations for low-temperature adsorption isotherms, isosteric heats of adsorption, and heat capacities of the adsorbed phase are developed for monolayer localized adsorption of monoatomic gases on heterogeneous solid surfaces. The model assumes that adsorption sites having various adsorption energies are distributed on the adsorbent surface completely at random. The interactions between adsorbed molecules are treated in the quasichemical approximation. The equations are illustrated by numerous model calculations.

Low-temperature adsorption isotherms of 3He and para-H2 on grafoil and their normalization

Measurements have been made of the adsorption isotherms of 3He and of para-hydrogen (p-H2), on bare Grafoil, the former in the temperature range 0.88–1.23 K, and the latter in the temperature range 10.0–20.0 K. For p-H2 the quantity of material adsorbed plotted against (p/p 0) T yields a single temperature-independent curve, as would follow from the potential theory for multilayer adsorption, and as emphasized by Chester et al. in 1974. The same behavior was found for 3He on Grafoil, except that deviations from a single temperature-independent curve occurred at the higher temperatures. For p-H2 on Grafoil it is to be noted that the isotherms taken below the triple point of free, bulk p-H2 fit the same single temperature-independent curve as do the isotherms measured at temperatures between the triple point and the boiling point. From the data, the isosteric heats of adsorption were calculated over a wide range of coverage and this provided further experimental evidence for the similarity between the uppermost layers of thick adsorbed films and the free liquid of the same substance.

The surface and porosity of adsorbents

1. Methods of determination of the specific surfaces of adsorbents, based on the method of citation of the adsorption isotherms, are discussed. 2. The bases of the theory of the de Boer t-method and the Singα-method are proposed for the initial region of the low-temperature adsorption isotherm of N2 (p/Ps < 0.4). In contrast to the BET method, these methods are applicable only to adsorbents, the adsorption isotherms on which are practically similar to the standard curve, i.e., essentially for an adsorbent with a known surface. 3. An improvement of the t- andα-methods was proposed on the basis of the BET equation; in the new variations they became equivalent to the BET method and applicable to the adsorption isotherms of various curvature. 4. An analysis and an evaluation of the possibilities of the Kadlec t/F method for the determination of the surface of mesopores (transitional pores) and macropores of adsorbents containing micropores were given. 5. A differentiation of the finest variety of pores of adsorbents into micropores with equivalent radii r less than 6–7 a and supermicropores (6–7 a < F < 15–16 a) was proposed. The peculiarities of supermicropores as a transitional region of porosity between micropores and mesopores were noted. 6. A detailed analysis was given of the initial premises of the MP method of analysis of microporous structures, proposed by Brunauer, and the possible causes of its universal applicability. The absence of quantitative criteria for evaluating the physical reality of the results obtained was demonstrated. In principle, the question can be raised only of the roughly approximate applicability of the MP method to supermicroporous adsorbents.

Chlorides of nickel, cobalt and barium as adsorbents in gas chromatography

The adsorptive and chromatographic properties of structurally different chlorides of bivalent metals: NiCl2, CoCl2 and BaCl2, have been studied. Low temperature adsorption isotherms of xenon were used to discover the number and area of different homogeneous crystal faces of the adsorbents. For NiCl2 and CoCl2 which have layer structures with chloride ions situated on one of the crystal face, the contribution of the specific intermolecular interaction energy to the total adsorption energy of aromatic hydrocarbons is far lower than that for BaCl2, all the crystal faces of which contain both chloride and barium ions. The salts studied, either in pure form or coated on the surface of Silochrom have been used to separate compounds which do not differ greatly in their physicochemical properties.