Adsorption Branch Research Articles

Cu-phthalocyanine-mediated nanowindow production on single-wall carbon nanohorn

We report a route for opening nanowindows on graphene wall of single wall carbon nanohorn (SWCNH) aided by copper (II) 2,3,9,10,16,17,23,24-octakis (octyloxy)-29H, 31H-phthalocyanine (CuPc), which acts as a catalyst for nanowindow formation. CuPc is adsorbed on SWCNH from liquid phase and heat treated in oxygen atmosphere to produce nanowindows on the adsorption sites of CuPc. The adsorption isotherms of N2 and Ar on the CuPc-adsorbed SWCNH heat-treated have a low pressure adsorption hysteresis. The low pressure adsorption hysteresis gap of the adsorbed amount difference between the desorption and adsorption branches reaches a maximum at 573 K for both N2 and Ar adsorption. This marked low pressure adsorption hysteresis evidences formation of nanowindows whose size is comparable to the N2 and Ar molecules (0.33–0.34 nm).

Adsorption and diffusion of moisture and wet flue gas on silica gel

Moisture equilibrium and kinetics on silica gel, a potential adsorbent for drying wet flue gas, are measured using static volumetric and dynamic column breakthrough (DCB) methods. Isotherm models are identified that adequately describe the adsorption branch and hysteresis of the desorption branch. The transport mechanism and transport coefficient of moisture in silica gel pores are also presented. DCB experiments in the silica gel bed pre-equilibrated with wet helium are used to quantify N2 and CO2 adsorption reduction in the presence of moisture. Combining present results with those reported recently for equilibrium and transport models for N2 and CO2 (Goyal et al., 2019), a ternary simulation model is finally proposed in this study that sufficiently predicts complete adsorption and desorption breakthrough of wet flue gas (N2, CO2 and H2O mixture) in an initially clean silica gel bed. Two mixture equilibrium calculation methods are compared while retaining the independently established transport mechanism.

Removal of ammonium ion from aqueous solutions by using unmodified and H2O2-modified zeolitic waste

In the petroleum industry during a catalytic cracking process, the used zeolitic catalyst becomes waste. This article investigated the sorption capacities of ammonium ions from aqueous solutions onto the previously mentioned zeolitic waste by batch experiments. Three types of zeolitic waste were used: unmodified zeolitic waste with two different particle size distributions and H2O2-modified zeolitic waste. Several techniques, including X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) multilayer adsorption theory measurements, and X-ray fluorescence analysis (XRF) were used to demonstrate experimentally that the zeolitic waste could be used as a sorbent for the water decontamination of NH4+ ions. The morphology of zeolitic waste investigated by scanning electron microscopy (SEM) revealed particles with a spherical shape. The nitrogen adsorption–desorption isotherms showed an isotherm mixture of types I (pure microporous) and IV (mesoporous). This suggested that the investigated zeolitic materials were mesoporous (4.84 nm) and microporous (0.852 nm), as well as containing slit/cylindric pores, according to a quench solid density functional theory (QSDFT) adsorption branch model. Zeolitic waste from the oil industry showed good NH4+ sorption properties (removal efficiency of 72%), thus becoming a potential adsorbent to be used in the treatment of contaminated aqueous effluents polluted with ammonium ions. Simultaneous waste and water decontamination can be achieved, providing a new tool and enhanced capabilities for environmental remediation.

Synthesis of Nanoporous Titanosilicates and Their Structural Characterization by Physical Nitrogen Adsorption

Mesoporous titanosilicate structures synthesized by the sol–gel method using the cetylpyridinium template at different Ti/Si ratios are characterized by high specific surface areas ABET ≈ 800 m2/g and Aext ≈ 900–1000 m2/g, as well as the shape of the isotherms of physical nitrogen adsorption characteristic of the mesoporous molecular sieve MCM-41 with an adsorption branch rising and a section of reversible capillary condensation at a relative pressure of 0.21 < p/p0 < 0.42. An increase in the Ti/Si ratio and pH values leads to a decrease in the dispersion and surface area of the samples. FTIR spectra confirm the presence of titanium(IV) in the silicate framework. The incorporation of titanium(IV) affects the morphology of titanosilicate samples and reduces the ordering of cylindrical mesopores characteristic of MCM-41.

The heterogeneity of pore structure in lacustrine shales: Insights from multifractal analysis using N2 adsorption and mercury intrusion

Abstract A series of parameters from a variety of techniques, such as pore size distribution, surface area, pore volume, etc., have been used to characterize the pore structure of unconventional shale reservoirs. However, the investigation on the quantification of pore structure heterogeneity is still scarce. In this aspect, a multifractal theory combined with N2 adsorption and mercury intrusion was used to obtain pore structure characteristics and quantify the heterogeneity of pore size distribution of nine lacustrine shale samples from the second member of Kongdian Formation (Ek2) in Cangdong Sag, Bohai Bay Basin, China. The results show that the Ek2 shale contains complex pore network, and pores with small pore size predominate in pore system. Both N2 adsorption branches and pore throat size distributions from mercury intrusion demonstrate multifractal nature. The width of singularity spectra from multifractal analysis is used to quantitatively characterize the heterogeneity degree of pore size distribution and shows good consistency with pore size distributions derived from N2 adsorption and mercury intrusion. The pore structure from mercury intrusion is much heterogeneous than that from N2 adsorption, indicating that an increased number of large pores will increase the heterogeneity of pore size distributions. Compared with other rock compositions, TOC has an obvious impact on the heterogeneity of pore structure in small pore size range.

Pore-Size Distribution of Silica Colloidal Crystals from Nitrogen Adsorption Isotherms.

Silica colloidal crystals are face-centered cubic structures comprised of silica spheres with the diameters ranging between tens and hundreds of nanometers. The voids between the spheres form pores, which can be probed by nitrogen adsorption porosimetry. Here, we prepared two mesoporous samples and a macroporous reference sample and then measured nitrogen adsorption and desorption isotherms for further characterization. We proposed a straightforward procedure for calculation of the pore-size distribution of silica colloidal crystals from nitrogen adsorption isotherms. The procedure is based on the adsorption integral equation solution with a kernel of theoretical isotherms, consistent with the procedure used for many other porous materials. The solution is carried out using the non-negative least squares (NNLS) regression with Tikhonov regularization. The kernel of mesoporous isotherms is built on the basis of the macroscopic Derjaguin-Broekhoff-de Boer (DBdB) theory of capillary condensation considering the voids as a network of spheres. Application of our procedure for the analysis of the adsorption branches of experimental isotherms resulted in bimodal distributions, where the modes matched well with the sizes of the voids in the colloidal crystals face centered cubic structure: the main mode corresponds to the octahedral voids and the second mode to the tetrahedral voids. Furthermore, we modified the surface of the samples with organics and repeated the characterization procedure for the modified samples. The resulting pore-size distribution for the samples with the modified surface matched the original one quite closely. It demonstrates the procedure as a simple and efficient technique to estimate the pore-size distribution and justifies the spherical shape approximation for the voids in the silica colloidal crystals.

A model for sorption hysteresis in hardened cement paste

In hardened cement paste (C-S-H gel), water intercalated between the C-S-H layers can be adsorbed and desorbed by varying relative humidity. Sorption in the interlayer space between the C-S-H sheets is supposed to be responsible for low pressure hysteresis of water sorption isotherms. Additionally, large hysteresis loops also appear in diagrams that depict the humidity-dependent volume change of the paste. In contrast to most other porous solids, the volume of the C-S-H gel is larger along the desorption branch (drying) than along the adsorption branch (rewetting). Based on a macroscopic approach, we use a simple model to describe hysteresis in sorption and volume change diagrams. Computed hysteresis loops, which depend on the lateral extension of the interlayer spaces, are similar to loops observed experimentally.

Analytical equation for the mesopore size distribution function of open cylindrical capillaries

For the characterization of porous structures, besides the specific surface area, the pore size distribution is of special interest. In this paper, the pore size distribution of mesoporous structures is calculated from the adsorption branch of the hysteresis loop, whereby only loops of type H1 are considered. The modeling is done only for cylindrical capillaries and the calculation of the pore size distribution is done by a combination of the percolation theory and the theory of capillary condensation. By applying percolation theory to the adsorption process, the pressure dependence of pore filling during capillary condensation is described. Because of a lack of strictly theoretically based equations for the adsorption process by capillary condensation, a mathematical adaptation to measured data is done by the Bernoulli equation. This is proposed by the authors for the first time for a description of adsorption–desorption processes. Thus a new, strictly thermodynamic method to calculate the mesopore size ...

On the canonical isotherms for bulk fluid, surface adsorption and adsorption in pores: A common thread

Kinetic Monte Carlo simulated isotherms calculated in the canonical ensemble, at temperatures below the critical temperature, for bulk fluid, surface adsorption and adsorption in a confined space, show a van der Waals (vdW) loop with a vertical phase transition between the rarefied and dense spinodal points at the co-existence chemical potential, µco. Microscopic examination of the state points on this loop reveals features that are common to these systems. At state points with chemical potentials greater than μco the microscopic configurations show clusters, which coalesce to form two co-existing phases along the vertical section of the loop (the coexistence line). As more molecules are added, the dense region expands at the expense of the rarefied region, to the point where the rarefied region becomes spherical (cylindrical for 2D-systems) with a curvature greater than that of the coexisting phases. This results in a decrease of chemical potential from µco to the liquid spinodal point where the rarefied region disappears. With a further increase in loading, the chemical potential and the density increase. The existence of a vdW loop is the microscopic reason for the hysteresis observed in the grand canonical isotherm, where the adsorption and desorption boundaries of the hysteresis loop are first-order transitions, enclosing the vertical section of the vdW loop of the canonical isotherm. However, a first-order transition is rarely observed in experiments where transitions are usually steep, but not vertical. From our extensive simulations, we provide two possible reasons: (1) the finite extent of the system and (2) the existence of high energy sites that localize the clusters. In the first case, the desorption branch, and in the second case the adsorption branch, either comes close to, or collapses onto the coexistence line. When both occur, the hysteresis loop disappears and the isotherm is reversible, as often observed experimentally.

In Ukrainian

The investigation of the adsorption behavior of water vapour on porous carbon matrices is still relevant because the humidity of the environment can significantly affect the adsorption capacity of the carbon adsorbents to certain substances. Furthermore, the measurements of water isotherms could also be used to analyze the pore structure of activated carbons. The correlation between the adsorption characteristics of series of activated carbon samples (Norit, NL) with similar values of the specific surface area, but different pore size distribution (different volumes of micro- and mesopores), towards water vapour and their internal structure, defined on the basis of nitrogen adsorption isotherms, as well as the chemical composition and the nature of primary adsorption centers of the surface has been researched in the study. By the methods of Thermogravimetric Analysis (TGA), Temperature-Programmed Desorption Mass Spectrometry (TPD-MS) and X-ray Photoelectron Spectroscopy (XPS) it has been determinated of quantitative and qualitative composition of the surface of investigated carbons; it has been shown that the mesoporous samples of activated carbon contain a greater total amount of surface oxygen-containing complexes than microporous samples. The adsorption and desorption branches of water adsorption isotherms for the microporous carbons come close to each other and give a plateau at high relative pressures wherein the water adsorption capacities for the microporous samples correspond to their micropore volumes determined from nitrogen adsorption isotherms. As opposed to this, a gradual rise of water uptake and the existence of wide hysteresis loops at high relative pressures are observed for the mesoporous carbons. Moreover, the maximum values of water adsorption in this case considerably exceed the micropore volumes, and are attributable to the capillary condensation in mesopores. Nevertheless, the total volumes of water uptake both for micro- and mesoporous activated carbons are significantly less than those determined from nitrogen adsorption. This could be due to a mechanism of cluster pore filling in which the water density is less than its bulk density. In addition, probably, the water can not condense in the larger mesopores.

Synergistic effects between Cu metal–organic framework (Cu-MOF) and carbon nanomaterials for the catalyzation of the thermal decomposition of ammonium perchlorate (AP)

In this study, a novel Cu-MOF@Carbon nanomaterial composite was prepared to catalyze the thermal decomposition of ammonium perchlorate (AP). The structure was characterized by using scanning electron microscope (SEM), X-ray energy-dispersive spectrum (EDS), and X-ray diffraction (XRD); the specific surface area was estimated by Brunauer–Emmett–Teller (BET) method; and the pore volumes and pore size distributions were derived from the adsorption branches of isotherms using the Barrett–Joyner–Halenda (BJH) model. And the thermal decomposition behavior was investigated by using differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). The results indicated that all products showed excellent catalytic activity. Among the samples investigated here, Cu-MOF@CNT-rGO exhibited the best catalytic activity, since the high-temperature decomposition peak of AP decreased to 313.8 °C, which is reduced nearly 100 °C than the raw material (409.7 °C). And this was attributed to the high thermal and electrical conductivities of carbon nanomaterials, and the large surface area of both Cu-MOF and carbon nanomaterials. This study provides a new choice to be used as the promising catalysts in modifying the burning performance of AP-based composite propellant.

Gas adsorption on commercial magnesium stearate: The origin of atypical isotherms and BET transform data

The causes of atypical nitrogen adsorption isotherm features of commercial magnesium stearate grades have been investigated through a combination of alternative adsorbates and variation of analytical conditions. Atypical features have previously been identified as having poor linearity of adsorption isotherms at relative pressures applied to the calculation of BET surface area and low pressure hysteresis between adsorption and desorption branches. These features are most pronounced for di-hydrated MgSt and are absent for the anhydrous form: the presence of hydrated water causing atypical features. The possible interaction between hydrated water molecules and nitrogen via the nitrogen quadrupole is investigated through application of non-polar adsorbates; argon and krypton. Low pressure hysteresis is investigated through alteration of equilibration conditions and through truncating adsorption isotherms to successively lower maximum relative pressures before commencing desorption. It is found that atypical gas sorption isotherms are not limited to nitrogen. The effects of these conditions on adsorption and desorption volumes are discussed and a model for the origin of atypical isotherm features is presented. The difference in gas sorption behaviour of anhydrous, mono-hydrated and di-hydrated magnesium stearate is discussed and ultimately the consequences on the calculation of BET surface area is considered.

Characterisation and comparative analysis of zeotype water adsorbents for heat transformation applications

This paper presents the experimental characterisation of three zeotype materials, namely, AlPO-18, FAPO-34 and SAPO-34, suitable for adsorption heat transformation applications, employing water as refrigerant. Morphological and thermo-physical analyses were performed on each sample. In particular, the adsorption capacity of each material was evaluated by measuring a complete set of water vapour adsorption/desorption isotherms, in order to investigate the hysteresis effect. The obtained equilibrium data were fitted by means of Dubinin-Astakhov equation, both for adsorption and desorption branches. Finally, a thermodynamic analysis of the achievable performance of these three working pairs was performed under three main operating conditions, namely, thermal energy storage, air conditioning and heat pumping. The achieved results confirmed the potentiality of these adsorbents, showing promising cooling and thermal COP, up to 0.80 and 1.60 respectively, and heat storage capacities both for daily and seasonal operation.

Adsorption on alumina nanopores with conical shape.

Adsorption on porous solids depends on the morphology of the pores, the cylindrical one being the most studied in the literature. In this work, we present the first experimental investigation of adsorption and evaporation on conical nanopores produced by anodization of aluminium oxide. The pores are about 50 μm long, with the wide ends having a diameter of ∼79 nm and the narrow ones of ∼30 nm. Three different pores configurations are considered: open at both ends, open only at the narrow end and open only at the wide end. Despite the very small value of the conical angle α, estimated to be ∼0.06°, just barely above α = 0° corresponding to a cylindrical pore, the adsorption isotherms look strikingly different from those measured on cylindrical pores of similar size. First of all, the hysteresis loops of the conical pores with two open ends and with open wide ends practically coincide. Furthermore, they are narrower and the adsorption and evaporation branches are broader than those of the cylindrical pores with similar size. Finally, conical pores with open narrow ends exhibit a large hysteresis indicative of pore blocking. To unravel the mechanisms underlying adsorption and evaporation in such conical pores, we also report complementary results obtained using on-lattice grand canonical Monte Carlo simulations.

Adsorption on Nanopores of Different Cross Sections Made by Electron Beam Nanolithography.

Adsorption on nanoporous matrices is characterized by a pronounced hysteresis loop in the adsorption isotherm, when the substrate is loaded and unloaded with adsorbate, the origin of which is a matter of immense debate in the literature. In this work, we report a study of argon adsorption at 85 K on nonconnecting nanopores with one end closed to the surrounding where the effects of different pore cross sections fabricated by electron beam lithography (EBL) are investigated. A polymethylmethacrylate (PMMA) resist is deposited on the electrodes of a sensitive quartz crystal microbalance without degradation of the resonance quality factor or the long-term and short-term stabilities of the device even at cryogenic temperatures. Four different pores' cross sections: circular, square, rectangular, and triangular, are produced from EBL, and the isotherms for these pore shapes exhibit pronounced hysteresis loops whose adsorption and desorption branches are nearly vertical and have almost the same slopes. No difference is observed in the hysteresis loops of the isotherms for the pores with triangular and square cross sections, whereas the hysteresis loop for the pore with circular cross sections is much narrower, suggesting that they are more regular than the other pores. All of these observations suggest that the hysteresis behavior resulted mainly from microscopic geometric irregularities present in these porous matrices.

A new method for analysis of dual pore size distributions in shale using nitrogen adsorption measurements

A new method is proposed to distinguish between cylindrical pores and slit pores of porous materials and obtain their individual pore size distributions (PSDs). The proposed method is based on nitrogen adsorption isotherms at 77.3K. First, we develop the conventional methods by utilizing both adsorption and desorption branches of the isotherms and deriving the volume distribution of the two types of pores. The method formulation and implementing procedure is given in detail. Then the isotherms of SBA-15 sample are used to verify the proposed method in the case of single-pore-type. Finally, the proposed method is applied to analyze the pore structure characterization of shale sample. The results depict that both the adsorbed quantity and pore volume are separated into two distinct parts contributed by cylindrical pores and slit pores respectively. That's to say, the proposed method can distinguish clearly between the two types of pores. This study can provide us a better understanding of the complicated pore structure of shale. Furthermore, it may have implications for shale gas production.

Sorption Hysteresis Characterization of CH4 and CO2 on Anthracite, Bituminous Coal, and Lignite at Low Pressure

Sorption hysteresis characterization of CH4 and CO2 on lignite, bituminous coal, and anthracite were studied to improve the understanding of the interaction between gas molecules and different ranks of coal and further improve the precision of the adsorption methods in characterizing pore structure at low pressure. Pore structure of three ranks of coal was investigated with scanning electron microscopy (SEM) and nitrogen (N2) adsorption. Then, CH4 and CO2 sorption isotherms were measured using the gravimetric method under 288, 308, and 328 K. The N2 sorption isotherms show that a wide distribution of pore size existed in three coal samples, and with the process of coalification, the specific surface area (SSA) decreased and then increased, while the pore size of coal monotonically decreased. This is confirmed by SEM observation. The measured sorption isotherms were then decomposed into simultaneously running adsorption and absorption branches based on the assumption that the former is totally reversible and the latter completely irreversible. The reconstructed adsorption branches can be well described by both Langmuir model and Dubinin–Radushkevich (D–R) equation. The absorption, which represents the sorption hysteresis portion, increased with pressure, but decreased with temperature. The absorbed amount of gas increased with pressure, but the absorption of CO2 increased concavely with gas pressure while CH4 followed an upward exponential function. Also, the absorption varied with coal rank, following a U-shaped function. This study can provide new insights to CH4 and CO2 sorption hysteresis on coal and other organic geomaterials.

Effects of disulfide bridges and backbone connectivity on water sorption by protein matrices

Understanding the water sorption behavior of protein powders is important in applications such as the preservation of protein-based pharmaceuticals. Most globular proteins exhibit a characteristic sigmoidal water adsorption isotherm at ambient conditions. However, it is not well understood how water sorption behavior is influenced by intrinsic factors that are related to structural properties of proteins. We investigate computationally how structural constraints on proteins influence the water sorption isotherms of amorphous protein powders. Specifically, we study the effects of non-local disulfide linkages and backbone connectivity using pheromone ER-23 and lysozyme as model proteins. We find that non-local disulfide linkages can significantly restrict structural changes during hydration and dehydration, and this in turn greatly reduces the extent of hysteresis between the adsorption and desorption branches. Upon removing the backbone connectivity by breaking all peptide bonds in lysozyme, we find that the hysteresis shifts towards the lower humidity regime, and the water uptake capacity is significantly enhanced. We attribute these changes to the higher aggregation propensity of the constraint-free amino acids in dehydrated condition, and the formation of a spanning water network at high hydration levels.

Methodology for determining the thermodynamic properties of smectite hydration

This study proposes an improved methodology for extracting the thermodynamic hydration properties of clay minerals from adsorption isotherms performed at two temperatures, by removing the capillary water contribution. The work focuses on Na-smectite. Its thermodynamic hydration properties were extracted as follows: (i) experimental measurement of water vapor adsorption isotherms at two temperatures (25 °C and 45 °C); (ii) removal of the capillary water contribution; (iii) calculation of the thermodynamic properties. The values obtained from the adsorption branch compare favorably with calorimetric measurements. We parameterized a strict regular solid solution model similar to that developed by Ransom and Helgeson (1994). From this we see how the thermodynamic properties of hydration evolve, so we can explain and sort the values obtained by previous authors. The calculation process, based on structural constraints from the literature, considers only half of the total adsorbed water as structurally related to the mineral, when relative humidity conditions are close to 100%. The approach proposed in this work enables generalization to other types of clay minerals and implementation of thermodynamic databases with thermodynamic properties that are intrinsic to each clay mineral and independent from contextual constraints.

Molecular simulation and experiments of water adsorption in a high surface area activated carbon: Hysteresis, scanning curves and spatial organization of water clusters

Despite decades of research, quantitatively accurate molecular models of water sorption on activated carbons remain elusive, while many phenomena are not properly understood even qualitatively. Here we focus specifically on scanning phenomena. Scanning isotherms, obtained by reversing either adsorption or desorption process before the closure of the hysteresis loop, have been recently explored as a sensitive experimental probe for structural characterization of activated carbons and at the same time have been subject of several competing theories of adsorption. We employ adsorption experiments and molecular simulations to understand the nature of the states along the adsorption and scanning curves for water at 308 K in a high surface area activated carbon, Maxsorb. The molecular model considered here is based on a random packing of fullerene-like fragments, functionalized with carboxylate groups. The model is able to reproduce reasonably well the shape of the adsorption and scanning desorption isotherms. We investigate spatial organization, structure, size and interaction between the clusters on the adsorption branch. Furthermore, we show that the scanning desorption isotherm consists of a series of states of the system where a single cluster of water shrinks in size as water evaporates from its surface, before it finally disintegrates into separate smaller clusters.