Effects of adsorbate-adsorbate interaction in the description of adsorption isotherm of hydrocarbons in micro-mesoporous carbonaceous materials

In this paper, we present a model accounting for the adsorbate-adsorbate interaction in the adsorbed phase in the description of adsorption of pure vapors on carbonaceous materials. The details of the adsorbate-adsorbate interaction of a particular species are obtained from the analysis of its adsorption data on non-porous carbon black. The predictability of the model is tested against the adsorption isotherm data for benzene, toluene, n-pentane, n-hexane, carbon tetrachloride, methanol and ethanol on microporous activated carbon. It was found that the model prediction for non-polar adsorbates are satisfactory while it under-predicts for polar adsorbates, which is attributed to their additional interaction with functional groups. (C) 2002 Elsevier Science B.V. All rights reserved.

Characterization of micro-mesoporous carbonaceous materials. Calculations of adsorption isotherm of hydrocarbons

In this paper we apply a method recently developed by Do and co-workers(1) for the prediction of adsorption isotherms of pure vapors on carbonaceous materials. The information required for the prediction is the pore size distribution and the BET constant, C, of a corresponding nonporous surface (graphite). The dispersive adsorption force is assumed to be the dominant force in adsorption mechanism. This applies to nonpolar and weakly polar hydrocarbons. We test this predictive model against the adsorption data of benzene, toluene, n-pentane, n-hexane, and ethanol on a commercial activated carbon. It is found that the predictions are excellent for all adsorbates tested with the exception of ethanol where the predicted values are about 10% less than the experimental data, and this is probably attributed to the electrostatic interaction between ethanol molecules and the functional groups on the carbon surfaces.

Drying of porous building materials: hydraulic diffusivity and front propagation

One-dimensional drying of a porous building material is modelled as a nonlinear diffusion process. The most difficult case of strong surface drying when an internal drying front is created is treated in particular. Simple analytical formulae for the drying front and moisture profiles during second stage drying are obtained when the hydraulic diffusivity is known. The analysis demonstrates the origin of the constant drying front speed observed elsewhere experimentally. Application of the formulae is illustrated for an exponential diffusivity and applied to the drying of a fired clay brick.

Comparison of nitrogen adsorption at 77 K on non-porous silica and pore wall of MCM-41 materials by means of density functional theory

Nitrogen adsorption on a surface of a non-porous reference material is widely used in the characterization. Traditionally, the enhancement of solid-fluid potential in a porous solid is accounted for by incorporating the surface curvature into the solid-fluid Potential of the flat reference surface. However, this calculation procedure has not been justified experimentally. In this paper, we derive the solid-fluid potential of mesoporous MCM-41 solid by using solely the adsorption isotherm of that solid. This solid-fluid potential is then compared with that of the non-porous reference surface. In derivation of the solid-fluid potential for both reference surface and mesoporous MCM-41 silica (diameter ranging front 3 to 6.5 nm) we employ the nonlocal density functional theory developed for amorphous solids. It is found that, to out, surprise, the solid-fluid potential of a porous solid is practically the same as that for the reference surface, indicating that there is no enhancement due to Surface curvature. This requires further investigations to explain this unusual departure from our conventional wisdom of curvature-induced enhancement. Accepting the curvature-independent solid-fluid potential derived from the non-porous reference surface, we analyze the hysteresis features of a series of MCM-41 samples. (c) 2005 Elsevier Inc. All rights reserved.

Mesoporous dye-doped titanium dioxide for micro-optoelectronic applications

Optically transparent, mesostructured titanium dioxide thin films were fabricated using an amphiphilic poly(alkylene oxide) block copolymer template in combination with retarded hydrolysis of a titanium isopropoxide precursor. Prior to calcination, the films displayed a stable hexagonal mesophase and high refractive indices (1.5 to 1.6) relative to mesostructured silica (1.43). After calcination, the hexagonal mesophase was retained with surface areas >300 m2 g-1. The dye Rhodamine 6G (commonly used as a laser dye) was incorporated into the copolymer micelle during the templating process. In this way, novel dye-doped mesostructured titanium dioxide films were synthesised. The copolymer not only directs the film structure, but also provides a solubilizing environment suitable for sustaining a high monomer-to-aggregate ratio at elevated dye concentrations. The dye-doped films displayed optical thresholdlike behaviour characteristic of amplified spontaneous emission. Soft lithography was successfully applied to micropattern the dye-doped films. These results pave the way for the fabrication and demonstration of novel microlaser structures and other active optical structures. This new, high-refractive index, mesostructured, dye-doped material could also find applications in areas such as optical coatings, displays and integrated photonic devices.

Finite element analysis of steady-state natural convection problems in fluid-saturated porous media heated from below

In this paper, a progressive asymptotic approach procedure is presented for solving the steady-state Horton-Rogers-Lapwood problem in a fluid-saturated porous medium. The Horton-Rogers-Lapwood problem possesses a bifurcation and, therefore, makes the direct use of conventional finite element methods difficult. Even if the Rayleigh number is high enough to drive the occurrence of natural convection in a fluid-saturated porous medium, the conventional methods will often produce a trivial non-convective solution. This difficulty can be overcome using the progressive asymptotic approach procedure associated with the finite element method. The method considers a series of modified Horton-Rogers-Lapwood problems in which gravity is assumed to tilt a small angle away from vertical. The main idea behind the progressive asymptotic approach procedure is that through solving a sequence of such modified problems with decreasing tilt, an accurate non-zero velocity solution to the Horton-Rogers-Lapwood problem can be obtained. This solution provides a very good initial prediction for the solution to the original Horton-Rogers-Lapwood problem so that the non-zero velocity solution can be successfully obtained when the tilted angle is set to zero. Comparison of numerical solutions with analytical ones to a benchmark problem of any rectangular geometry has demonstrated the usefulness of the present progressive asymptotic approach procedure. Finally, the procedure has been used to investigate the effect of basin shapes on natural convection of pore-fluid in a porous medium. (C) 1997 by John Wiley & Sons, Ltd.

Theoretical and numerical analyses of convective instability in porous media with upward throughflow

Exact analytical solutions have been obtained for a hydrothermal system consisting of a horizontal porous layer with upward throughflow. The boundary conditions considered are constant temperature, constant pressure at the top, and constant vertical temperature gradient, constant Darcy velocity at the bottom of the layer. After deriving the exact analytical solutions, we examine the stability of the solutions using linear stability theory and the Galerkin method. It has been found that the exact solutions for such a hydrothermal system become unstable when the Rayleigh number of the system is equal to or greater than the corresponding critical Rayleigh number. For small and moderate Peclet numbers (Pe less than or equal to 6), an increase in upward throughflow destabilizes the convective flow in the horizontal layer. To confirm these findings, the finite element method with the progressive asymptotic approach procedure is used to compute the convective cells in such a hydrothermal system. Copyright (C) 1999 John Wiley & Sons, Ltd.

Characterization of pore size distributions of mesoporous materials from adsorption isotherms

In this article, a new hybrid model for estimating the pore size distribution of micro- and mesoporous materials is developed, and tested with the adsorption data of nitrogen, oxygen, and argon on ordered mesoporous materials reported in the literature. For the micropore region, the model uses the Dubinin-Rudushkevich (DR) isotherm with the Chen-Yang modification. A recent isotherm model of the authors for nonporous materials, which uses a continuum-mechanical model for the multilayer region and the Unilan model for the submonolayer region, has been extended for adsorption in mesopores. The experimental data is inverted using regularization to obtain the pore size distribution. The present model was found to be successful in predicting the pore size distribution of pure as well as binary physical mixtures of MCM-41 synthesized with different templates, with results in agreement with those from the XRD method and nonlocal density functional theory. It was found that various other recent methods, as well as the classical Broekhoff and de Beer method, underpredict the pore diameter of MCM-41. The present model has been successfully applied to MCM-48, SBA's, CMK, KIT, HMS, FSM, MTS, mesoporous fly ash, and a large number of other regular mesoporous materials.

Synthesis of anatase TiO2 supported on porous solids by chemical vapor deposition

Coating anatase TiO2 onto three different particle supports, activated carbon (AC), gamma -alumina (Al2O3) and silica gel (SiO2), by chemical vapor deposition (CVD) was studied. The effect of the CVD synthesis conditions on the loading rate of anatase TiO2 was investigated. It was found that introducing water vapor during CVD or adsorbing water before CVD was crucial to obtain anatase TiO2 on the surface of the particle supports. The evaporation temperature of precursor, deposition temperature in the reactor, flow rate of carrier gas, and the length of coating time were also important parameters to obtain more uniform and repeatable TiO2 coating. High inflow precursor concentration, high CVD reactor temperature and long coating time tended to cause block problem. Coating TiO2 onto small particles by CVD involved both chemical vapor deposition and particle deposition. It was believed that the latter was the reason for the block problem. In addition, the mechanism of CVD process in this study included two parts, pyrolysis and hydrolysis, and one of them was dominant in the CVD process under different synthesis route. Among the three types of materials, silica gel, with higher surface hydroxyl groups and macropore surface area, was found to be the most efficient support in terms of both anatase TiO2 coating and photocatalytic reaction. (C) 2001 Elsevier Science B.V. All rights reserved.

Porous clays and pillared clays-based catalysts. Part 2: A review of the catalytic and molecular sieve applications

Metal oxide pillared clay (PILC) possesses several interesting properties, such as large surface area, high pore volume and tunable pore size (from micropore to mesopore), high thermal stability, strong surface acidity and catalytic active substrates/metal oxide pillars. These unique characteristics make PILC an attractive material in catalytic reactions. It can be made either as catalyst support or directly used as catalyst. This paper is a continuous work from Kloprogge's review (J.T. Kloprogge, J. Porous Mater. 5, 5 1998) on the synthesis and properties of smectites and related PILCs and will focus on the diverse applications of clay pillared with different types of metal oxides in the heterogeneous catalysis area and adsorption area. The relation between the performance of the PILC and its physico-chemical features will be addressed.

Multicomponent supercritical adsorption in microporous activated carbon materials

In this paper, a new technique for predicting multicomponent adsorption equilibria of supercritical fluids in microporous carbons is presented. In difference from adsorption on a surface, which is a function of the fluid-solid interaction only, adsorption in porous media is influenced by the proximity of the pore walls, resulting in the enhancement in adsorption affinity. The degree of this enhancement is different for different adsorbates, and it increases with a decrease in pore size. The theory is applied to a number of carbonaceous systems with good success.

Characterization of micro-mesoporous carbon media

A simple method to characterize the micro and mesoporous carbon media is discussed. In this method, the overall adsorption quantity is the sum of capacities of all pores (slit shape is assumed), in each of which the process of adsorption occurs in two sequential steps: the multi-layering followed by pore filling steps. The critical factor in these two steps is the enhancement of the pressure of occluded 'free' molecules in the pore as well as the enhancement of the adsorption layer thickness. Both of these enhancements are due to the overlapping of the potential fields contributed by the two opposite walls. The classical BET and modified Kelvin equations are assumed to be applicable for the two steps mentioned above, with the allowance for the enhanced pore pressure, the enhanced adsorption energy and the enhanced BET constant,all of which vary with pore width. The method is then applied to data of many carbon samples of different sources to derive their respective pore size distributions, which are compared with those obtained from DFT analysis. Similar pore size distributions (PSDs) are observed although our method gives sharper distribution. Furthermore, we use our theory to analyze adsorption data of nitrogen at 77 K and that of benzene at 303 K (ambient temperature). The PSDs derived from these two different probe molecules are similar, with some small differences that could be attributed to the molecular properties, such as the collision diameter. Permeation characteristics of sub-critical fluids are also discussed in this paper. (C) 2001 Elsevier Science B.V. All rights reserved.

Study of wet porous filtration

A growing demand for efficient air quality management calls for the development of technologies capable of meeting the stringent requirements now being applied in areas of chemical, biological and medical activities. Currently, filtration is the most effective process available for removal of fine particles from carrier gases. Purification of gaseous pollutants is associated with adsorption, absorption and incineration. In this paper we discuss a new technique for highly efficient simultaneous purification of gaseous and particulate pollutants from carrier gases, and investigate the utilization of Nuclear Magnetic Resonance (NMR) imaging for the study of the dynamic processes associated with gas-liquid flow in porous media. Our technique involves the passage of contaminated carrier gases through a porous medium submerged into a liquid, leading to the formation of narrow and tortuous pathways through the medium. The wet walls of these pathways result in outstanding purification of gaseous, liquid and solid alien additives. NMR imaging was successfully used to map the gas pathways inside the porous medium submerged into the liquid layer. (C) 2002 Elsevier Science B.V. All rights reserved.

Adsorption of supercritical fluids in non-porous and porous carbons: analysis of adsorbed phase volume and density

In this paper, we revisit the surface mass excess in adsorption studies and investigate the role of the volume of the adsorbed phase and its density in the analysis of supercritical gas adsorption in non-porous as well as microporous solids. For many supercritical fluids tested (krypton, argon, nitrogen, methane) on many different carbonaceous solids, it is found that the volume of the adsorbed phase is confined mostly to a geometrical volume having a thickness of up to a few molecular diameters. At high pressure the adsorbed phase density is also found to be very close to but never equal or greater than the liquid phase density. (C) 2003 Elsevier Science Ltd. All rights reserved.

Stress crrosion cracking fracture mechanisms in rock bolts

Rock bolts have failed by Stress Corrosion Cracking (SCC). This paper presents a detailed examination of the fracture surfaces in an attempt to understand the SCC fracture mechanism. The SCC fracture surfaces, studied using Scanning Electron Microscopy (SEM), contained the following different surfaces: Tearing Topography Surface (TTS), Corrugated Irregular Surface (CIS) and Micro Void Coalescence (MVC). TTS was characterised by a ridge pattern independent of the pearlite microstructure, but having a spacing only slightly coarser than the pearlite spacing. CIS was characterised as porous irregular corrugated surfaces joined by rough slopes. MVC found in the studied rock bolts was different to that in samples failed in a pure ductile manner. The MVC observed in rock bolts was more flat and regular than the pure MVC, being attributed to hydrogen embrittling the ductile material near the crack tip. The interface between the different fracture surfaces revealed no evidence of a third mechanism involved in the transition between fracture mechanisms. The microstructure had no effect on the diffusion of hydrogen nor on the fracture mechanisms. The following SCC mechanism is consistent with the fracture surfaces. Hydrogen diffused into the material, reaching a critical concentration level. The thus embrittled material allowed a crack to propagate through the brittle region. The crack was arrested once it propagated outside the brittle region. Once the new crack was formed, corrosion reactions started producing hydrogen that diffused into the material once again. (C) 2003 Kluwer Academic Publishers.