Lanthanides adsorption on controlled pore glass

Eu/III/ and Ce/III/ are adsorbed on controlled pore glass from neutral and basic solutions. The experiments have been carried out using 10^–5M solutions of chlorides. For 0.1 M NaCl as a supporting electrolyte very low adsorption is observed below pH 6 and above pH 8 the adsorption curve has a plateau corresponding to over 90% of europium /or cerium/ adsorbed. At lower NaCl concentrations the above pH values shift towards lower values. Europium adsorbs considerably better than cerium and the best selectivity is observed at pH=7.     

Characterization of pore size distribution in porous silicon by NMR cryoporosimetry and adsorption methods

The results of studying the pore size distribution of mesoporous silicon by NMR cryoporosimetry are described. These data are compared with the results obtained by adsorption methods.     

Calculation of pore size from adsorption data

Spherical pores (voids) with 1 to 6 round necks are used to estimate the probable disagreement between the average diameter and Kelvin equivalent size. This disagreement is due to the delay in irreversible capillary condensation in such pores [1, 2].     

Ethane adsorption on microporous carbon adsorbent with a wide pore size distribution

Russian Chemical Bulletin - A microporous carbon adsorbent MPU-007 with a wide pore size distribution (specific micropore volume W0 calculated within the framework of the theory of micropore volume...     

Laser densification of medium pore size gel-silica glass

Type VI sol-gel silica monoliths with 32 Å and 45 Å pore radii were made using tetramethylsiloxane (TMOS) with HF as a catalyst. The pore texture was characterized using nitrogen absorption/desorption isotherms and BET analysis. Monoliths thermally stabilized to specific bulk densities of 1.67 g/cc to 2.00 g/cc were irradiated with a CO₂ laser to create microlenses. The densified spots were characterized using microscopic Fourier transform infrared (FTIR) spectroscopy. Full density microlenses were achieved; the peak position of the Si-O-Si vibrational mode was at 1122 cm^–1, equivalent to commercial optical silicas. The refractive index in the center of the densified spots was 1.46 at a wavelength of 632.8 nm with a gradient in refractive index of 0.07–0.08. The combinations of laser power and substrate bulk density that yield full density microlenses is expanded when compared with conditions previously established for small pore, 12 Å, gel-silica substrates. Dimensions of the laser densified region, magnitude of the index gradient, and shape of the GRIN profile are easier to control with the medium pore size gel-silica substrates.     

Methane adsorption on microporous carbon adsorbent with wide pore size distribution

Methane adsorption on a microporous carbon adsorbent with a bimodal pore size distribution is studied at temperatures of 303–333 K at pressures up to 30 MPa. The total micropore volume of the adsorbent, as determined by the Dubinin method, is as large as 1.02 cm³/g. Maximum values of methane adsorption of ≈18 mmol/g are attained at a temperature of 303 K and a pressure of 30 MPa. Methane adsorption isosteres are plotted based on experimental data, and adsorption equilibria at low temperatures are calculated using the linearity of the plots. Experimental isotherms of methane adsorption are compared with the isotherms calculated by the Dubinin–Nikolaev equation with variations in parameters E and n. Temperature dependences of these parameters are determined. Specific characteristics of methane adsorption accumulation are calculated.     

Low-temperature adsorption of hydrogen on nanoporous aluminophosphates: effect of pore size

Several nanoporous aluminophosphates (AlPOs) have been used to analyze the effect of pore diameter on the hydrogen adsorption characteristics. The heat of adsorption and adsorption capacity per unit micropore volume increase with decreasing pore size. AlPOs with smaller micropores favorably adsorb hydrogen at relatively low pressures. This work demonstrates that small pore size and large micropore volume are beneficial for high hydrogen uptake.     

Evolution of pore size distribution and average pore size of porous ceramic membranes during modification by counter-diffusion chemical vapor deposition

The modification of porous ceramic membranes by counter-diffusion chemical vapor deposition (CVD) is studied theoretically and experimentally. Numerical simulations of the evolution of the membrane permeance, average pore size and pore size distribution as a function of extent of modification are presented and compared with experimetal data. It is found that the change of the average pore size of the membranes after modification strongly depends on the initial pore size distribution of the membrane, CVD reaction kinetics and characterization method. Experimental data suggest that CVD of zirconia (and yttria) inside porous ceramic membranes by reaction of zirconium (and yttrium) chlorides with steam/air at elevated temperatures proceeds by quasi-zero reaction kinetics with respect to the oxidant, typical of non-stoichiometric supply of the reactants from opposite sides of the membrane. Under such conditions, CVD modification may result in a modest increase of the average pore size of coarse-pore ceramic membranes as suggested by numerical calculations and experimental data.     

The study of controlling pore size on electrospun carbon nanofibers for hydrogen adsorption

Polyacrylonitrile (PAN)-based carbon nanofibers (CNFs) were prepared by using electrospinning method and heat treatment to get the media for hydrogen adsorption storage. Potassium hydroxide and zinc chloride activations were conducted to increase specific surface area and pore volume of CNFs. To investigate the relation between pore structure and the capacity of hydrogen adsorption, textural properties of activated CNFs were studied with micropore size distribution, specific surface area, and total pore volume by using BET (Brunauer-Emmett-Teller) surface analyzer apparatus and the capacity of hydrogen adsorption was evaluated by PCT (pressure-composition-temperature) hydrogen adsorption analyzer apparatus with volumetric method. The surface morphology of activated CNFs was observed by SEM (scanning electron microscope) images to investigate the surface change through activation. Even though specific surface area and total pore volume were important factors for increasing the capacity of hydrogen adsorption, the pore volume which has pore width (0.6-0.7 nm) was a much more effective factor than specific surface area and pore volume in PAN-based electrospun activated CNFs.     

Kinetics of protein adsorption by nanoporous carbons with different pore size

Kinetics of bovine serum albumin and ovalbumin adsorption by nanoporous carbons with different main pore sizes (1.6, 5, 7.8 and 28 nm) was studied. Experimental kinetics curves were well described by multi-exponential equation with different number of exponents (from 1 to 4). Protein adsorption kinetics showed significant dependence on pore size of carbonaceous adsorbent. Correlation between pore size distribution and amount of protein adsorbed revealed threshold pore size 7.3 nm for BSA and 6.8 nm for OVA, which are close to hydrodynamic diameter of protein molecules. The fastest and the highest adsorption of proteins were observed in carbons having developed porosity with pore sizes larger than 15 nm.     

Comparison between different presentations of pore size distribution in porous materials

The different presentations of the pore size distribution derived from the gas adsorption method and the mercury porosimetry are connected with some problems. This concerns especially the use of the logarithmically differential pore volume distribution. The incorrect application of this distribution to bimodal pore systems involves the danger of an apparent overemphasizing of larger pores. This effect may also occur using the incremental pore size distribution in case the experimental point spacing considerably increases towards the larger pore radii. The pore volume density distribution defined as the linear derivative of the cumulative pore volume curve with respect to the pore radius has been found the most convenient form among the various kinds of pore volume distribution presentations. It has been shown that the direct comparison between this distribution and the logarithmically differential pore volume distribution is not allowed. Nevertheless, there is a clear connection between these definitions for the pore size distribution so that they are completely equivalent. Received: 15 May 1998 / Revised: 8 October 1998 / Accepted: 10 October 1998     

Comparison between different presentations of pore size distribution in porous materials

The different presentations of the pore size distribution derived from the gas adsorption method and the mercury porosimetry are connected with some problems. This concerns especially the use of the logarithmically differential pore volume distribution. The incorrect application of this distribution to bimodal pore systems involves the danger of an apparent overemphasizing of larger pores. This effect may also occur using the incremental pore size distribution in case the experimental point spacing considerably increases towards the larger pore radii. The pore volume density distribution defined as the linear derivative of the cumulative pore volume curve with respect to the pore radius has been found the most convenient form among the various kinds of pore volume distribution presentations. It has been shown that the direct comparison between this distribution and the logarithmically differential pore volume distribution is not allowed. Nevertheless, there is a clear connection between these definitions for the pore size distribution so that they are completely equivalent.     

Amine-functionalised SBA-15 of tailored pore size for heavy metal adsorption

This study examines the synthesis of SBA-15 with tailored pore sizes through controlled thermal treatment for the adsorption of Pb and Cd ions. The aim is to produce a material that can adsorb heavy metals at both high and low concentrations. The materials were characterised by means of N(2) physisorption, powder X-ray diffraction (PXRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), microanalysis and transmission electron microscopy (TEM). The surface areas ranged from 410 to 871 m(2)g(-1), and pore diameter was increased from 5.9 to 10.8 nm. This method allows for maximum adsorption of metal ions at very low concentrations. Metal ion adsorption was determined using an Atomic Absorption Spectrophotometer. The effects of pH were found to play a major role in the precipitation and, therefore, adsorption of metal ions. This method proved to be efficient at adsorbing large quantities of both metals (39 and 41 mg g(-1) for Pb and Cd, respectively).     

Effect of fluid-substrate attraction and pore geometry on fluid adsorption

We employ grand canonical ensemble Monte Carlo simulations to investigate the impact of substrate curvature on the phase behavior of an adjacent fluid. The substrates consist of a periodic sequence of grooves in the x direction; the grooves are infinitely long in the y direction. The shape of the grooves is controlled by a parameter eta. For eta = 0 the substrates are planar. If eta = 1, the grooves are wedge shaped. If eta > 1 the grooves become concave and in the limit eta = infinity rectangular. The fluid-substrate potential representing a groove consists of two contributions, namely, that of the homogeneous substrate base corresponding to a semi-infinite solid and that of a finite piece of solid with nonplanar surfaces. Whereas the former contribution can be calculated analytically, the latter needs to be evaluated numerically. For very large values of eta, that is in (almost) rectangular grooves, we observe capillary condensation of that portion of fluid located inside the grooves. As eta decreases capillary condensation gives way to continuous filling. In all cases, a nearly planar film-gas interface eventually forms in the direction normal to the surface of the substrate base and outside the grooves if one increases the chemical potential sufficiently.     

Adsorption phenomena on glass surfaces. III. a microcalorimetric study of adsorption of n-butanol on heat treated surfaces of controlled pore glass

Flow microcalorimetry was used to study the adsorption of butanol on controlled pore glass (CPG) surfaces. Heats of adsorption and some thermodynamic data are reported for the adsorption process at the original, hydroxylated and hydrated surfaces and for those heat treated at temperatures of 450. 650 and 900°C. It was found that the molar free energy of adsorption is the same, 17 kJ mole^?1 for all the surfaces studied. The molar enthalpy and entropy of adsorption are indicative of steric effects caused by water molecules. For surfaces heat treated to 650°C monolayer coverage of butanol is close to 5 molecules per nm², the same figure as reported for the total number of vicinal and isolated silanol groups on the surface of silica.     

Cation control of pore and channel size in cage-based metal-organic porous materials

Porous materials resembling zeolites that are composed of organic and inorganic building units were synthesized and characterized. Control of pore and channel size was achieved by using different-sized cations. The metal-assembled, anionic cage molecule, Co(4)1(2)(8-), with a hydrophobic cavity and four carboxylate rich arms, was used as a structural unit for the formation of materials with pores and channels. When assembled into a solid material with dications (Mg(2+), Ca(2+), Sr(2+), and Ba(2+)), Co(4)1(2)(8-) arranges into sheets of cages linked together by cations. The series of materials based on Co(4)1(2)(8-) and containing alkaline earth cations was characterized using X-ray crystallography. The magnesium material packs with cages close together, has small channels, and has cation-carboxylate linkages in three dimensions. The calcium material has cages packed with voids between them and has 5 x 10 A channels and 10 x 21 A pores. The strontium and barium materials also pack with voids between the cages and similarly to each other. They have 11 x 13 A and 11 x 11 A channels and 10 x 27 A and 9 x 27 A pores, respectively. Each of these materials has many (20-50) solvent water molecules associated with each cage. The associated water can be removed from and adsorbed by the materials. The heat of water binding has been measured to be -52 kJ/mol (Mg(4)Co(4)1(2)); -47 kJ/mol (Ca(4)Co(4)1(2)); -48 kJ/mol (Sr(4)Co(4)1(2)); -49 kJ/mol (Ba(4)Co(4)1(2)).     

Calcium mediated polyelectrolyte adsorption on like-charged surfaces

Monte Carlo simulations within the primitive model of calcium-mediated adsorption of linear and comb polyelectrolytes onto like-charged surfaces are described, focusing on the effect of calcium and polyion concentrations as well as on the ion pairing between polymers and calcium ions. We use a combination of Monte Carlo simulations and experimental data from titration and calcium binding to quantify the ion pairing. The polymer adsorption is shown to occur as a result of surface overcharging by Ca(2+) and ion pairing between charged monomers and Ca(2+). In agreement with experimental observations, the simulations predict that the polymer adsorption isotherm goes through a maximum as the calcium or the polymer concentration is increased. The non-Langmuir isotherms are rationalized in terms of charge-charge correlations.