Effects of oxide promoters on metal dispersion and metal-support interactions in Ni catalysts supported on activated carbon

Various oxide-promoted Ni catalysts supported on activated carbon were prepared, and the effect of promoters on the surface structure and properties of Ni catalysts was studied. Physical adsorption (Na adsorption), thermogravimetric analysis (TGA), temperature-programmed desorption (TPD), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the catalysts. It is found that nickel is fairly uniformly distributed in the pores of the carbon support. Addition of promoters produces a more homogeneous distribution of nickel ion in carbon. However, distributions of promoters in the pores are varying. Addition of promoters increases the dispersion of nickel in carbon. Promoters also change the interaction between the carbon and Ni, resulting in significantly different behaviors of catalysts under various environments. CaO and MgO promoters improve the reactivity of nickel catalysts with O-2 but retard the interaction between nickel oxide and carbon. La2O3 shows some inhibiting effect on the interactions between nickel oxide and oxygen as well as carbon.

Heat of desorption measured by means of electrical heat excitation

A new method of measuring heat of desorption is proposed in this Letter. The principle of the method is to measure the amount of mass released when a controlled amount of energy is supplied directly to a solid adsorbent. This is in contrast to conventional methods such as microcalorimetry, where heat released upon adsorption is measured. In this method, a quantified heat supply is generated by passing a de current through a carbon pellet, which is equilibrated with a gas phase confined in a closed vessel. As a consequence of the heating, the particle temperature is increased, resulting in partial desorption of adsorbed molecules. The variations of pellet temperature and the system pressure with respect to time are used to determine the heat of desorption as a function of loading.

Heterogeneous model for gas transport in carbon molecular sieves

A dual resistance model with distribution of either barrier or pore diffusional activation energy is proposed in this work for gas transport in carbon molecular sieve (CMS) micropores. This is a novel approach in which the equilibrium is homogeneous, but the kinetics is heterogeneous. The model seems to provide a possible explanation for the concentration dependence of the thermodynamically corrected barrier and pore diffusion coefficients observed in previous studies from this laboratory on gas diffusion in CMS.(1.2) The energy distribution is assumed to follow the gamma distribution function. It is shown that the energy distribution model can fully capture the behavior described by the empirical model established in earlier studies to account for the concentration dependence of thermodynamically corrected barrier and pore diffusion coefficients. A methodology is proposed for extracting energy distribution parameters, and it is further shown that the extracted energy distribution parameters can effectively predict integral uptake and column breakthrough profiles over a wide range of operating pressures.

Comparative Analysis of Structural and Morphological Properties of Large-Pore Periodic Mesoporous Organosilicas and Pure Silicas

A systematic study on the structural properties and external morphologies of large-pore mesoporous organosilicas synthesized using triblock copolymer EO20PO70EO20 as a template under low-acid conditions was carried out. By employing the characterization techniques of SAXS, FE-SEM, and physical adsorption of N-2 in combination with alpha(s)-plot method, the structural properties and external morphologies of large-pore mesoporous organosilicas were critically examined and compared with that of their pure-silica counterparts synthesized under similar conditions. It has been observed that unlike mesoporous pure silicas, the structural and morphological properties of mesoporous organosilicas are highly acid-sensitive. High-quality mesoporous organosilicas can only be obtained from synthesis gels with the molar ratios of HCl/H2O between 7.08 x 10(-4) and 6.33 x 10(-3), whereas mesoporous pure silicas with well-ordered structure can be obtained in a wider range of acid concentration. Simply by adjusting the HCl/H2O molar ratios, the micro-, meso-, and macroporosities of the organosilica materials can be finely tuned without obvious effect on their structural order. Such a behavior is closely related to their acid-controlled morphological evolution: from necklacelike fibers to cobweb-supported pearl-like particles and to nanosized particulates.

Application of a generalized thermodynamic approach to characterize mesoporous materials

Equilibrium adsorption and desorption in mesoporous adsorbents is considered on the basis of rigorous thermodynamic analysis, in which the curvature-dependent solid-fluid potential and the compressibility of the adsorbed phase are accounted for. The compressibility of the adsorbed phase is considered for the first time in the literature in the framework of a rigorous thermodynamic approach. Our model is a further development of continuum thermodynamic approaches proposed by Derjaguin and Broekhoff and de Boer, and it is based on a reference isotherm of a non-porous material having the same chemical structure as that of the pore wall. In this improved thermodynamic model, we incorporated a prescription for transforming the solid-fluid potential exerted by the flat reference surface to the potential inside cylindrical and spherical pores. We relax the assumption that the adsorbed film density is constant and equal to that of the saturated liquid. Instead, the density of the adsorbed fluid is allowed to vary over the adsorbed film thickness and is calculated by an equation of state. As a result, the model is capable to describe the adsorption-desorption reversibility in cylindrical pores having diameter less than 2 nm. The generalized thermodynamic model may be applied to the pore size characterization of mesoporous materials instead of much more time-consuming molecular approaches. (c) 2005 Elsevier B.V. All rights reserved.

The physical and surface chemical characteristics of activated carbons and the adsorption of methylene blue from wastewater

Adsorption of a basic dye, methylene blue, from aqueous solutions onto as-received activated carbons and acid-treated carbons was investigated. The physical and surface chemical properties of the activated carbons were characterized using BET-N-2 adsorption, X-ray photoelectron spectroscopy (XPS), and mass titration. It was found that acid treatment had little effect on carbon textural characteristics but significantly changed the surface chemical properties, resulting in an adverse effect on dye adsorption. The physical properties of activated carbon, such as surface area and pore volume, have little effect on dye adsorption, while the pore size distribution and the surface chemical characteristics play important roles in dye adsorption. The pH value of the solution also influences the adsorption capacity significantly. For methylene blue, a higher pH of solution favors the adsorption capacity. The kinetic adsorption of methylene blue on all carbons follows a pseudo-second-order equation. (c) 2004 Elsevier Inc. All rights reserved.

A wavelet-based adaptive technique for adsorption problems involving steep gradients

A general, fast wavelet-based adaptive collocation method is formulated for heat and mass transfer problems involving a steep moving profile of the dependent variable. The technique of grid adaptation is based on sparse point representation (SPR). The method is applied and tested for the case of a gas–solid non-catalytic reaction in a porous solid at high Thiele modulus. Accurate and convergent steep profiles are obtained for Thiele modulus as large as 100 for the case of slab and found to match the analytical solution.

Modification of MCM-41 by surface silylation with trimethylchlorosilane and adsorption study

Siliceous MCM-41 samples were modified by silylation using trimethylchlorosilane (TMCS). The surface coverage of functional groups was studied systematically in this work. The role of surface silanol groups during modification was evaluated using techniques of FTIR and Si-29 CP/MAS NMR. Adsorption of water and benzene on samples of various hydrophobicities was measured and compared. It was found that the maximum degree of surface attachments of trimethylsilyl (TMS) groups was about 85%, corresponding to the density of TMS groups of 1.9 per nm(2). The degree of silylation is found to linearly increase with increasing pre-outgassing temperature prior to silylation. A few types of silanol groups exist on MCM-41 surfaces, among which both free and geminal ones are responsible for active silylation. Results of water adsorption show that aluminosilicate MCM-41 materials are more or less hydrophilic, giving a type IV isotherm, similar to that of nitrogen adsorption, whereas siliceous MCM-41 are hydrophobic, exhibiting a type V adsorption isotherm. The fully silylated Si-MCM-41 samples are more hydrophobic giving a type III adsorption isotherm. Benzene adsorption on all MCM-41 samples shows type IV isotherms regardless of the surface chemistry. Capillary condensation occurs at a higher relative pressure for the silylated MCM-41 than that for the unsilylated sample, though the pore diameter was found reduced markedly by silylation. This is thought attributed to the diffusion constriction posed by the attached TMS groups. The results show that the surface chemistry plays an important role in water adsorption, whereas benzene adsorption is predominantly determined by the pore geometry of MCM-41.

Adsorption of binary hydrocarbon mixtures in carbon slit pores: A density functional theory study

Adsorption of binary hydrocarbon mixtures involving methane in carbon slit pores is theoretically studied here from the viewpoints of separation and of the effect of impurities on methane storage. It is seen that even small amounts of ethane, propane, or butane can significantly reduce the methane capacity of carbons. Optimal pore sizes and pressures, depending on impurity concentration, are noted in the present work, suggesting that careful adsorbent and process design can lead to enhanced separation. These results are consistent with earlier literature studies for the infinite dilution limit. For methane storage applications a carbon micropore width of 11.4 Angstrom (based on distance between centers of carbon atoms on opposing walls) is found to be the most suitable from the point of view of lower impurity uptake during high-pressure adsorption and greater impurity retention during low-pressure delivery. The results also theoretically confirm unusual recently reported observations of enhanced methane adsorption in the presence of a small amount of heavier hydrocarbon impurity.

Development of composite adsorbents of carbon and intercalated clay for N2 and O2 adsorption: A preliminary study

Composite adsorbents of carbon and alumina intercalated montmorillonite were prepared and characterized by adsorption of N-2 and O-2 at various temperatures. The effects of pyrolysis, temperature, heating rate, subsequent degassing, and doping of cations and anions were investigated. The adsorption capacities of the composite adsorbents developed at higher temperatures (0 and -79 degrees C) are found to be larger than those of normal alumina pillared clays. The experimental results showed that the framework of these adsorbents is made of alumina particles and clay sheets while the pyrolyzed carbon distributes in the space of interlayers and interpillars. The pores between the carbon particles, clay sheets, and alumina pillars are very narrow with very strong adsorption forces, leading to enhanced adsorption capacities at 0 and -79 degrees C. The composite adsorbents exhibit features similar to those of carbonaceous adsorbents. Their pore structures, adsorption capacities, and selectivities to oxygen can be tailored by a controlled degassing procedure. Meanwhile, ions can be doped into the adsorbents to modify their adsorption properties, as usually observed for oxide adsorbents like zeolite and pillared clays. Such flexibility in pore structure tailoring is a potential advantage of the composite adsorbents developed for their adsorption and separation applications. (C) 1999 Academic Press.

A pore-size-dependent equation of state for multilayer adsorption in cylindrical mesopores

The chemical potential of adsorbed film inside cylindrical mesopores is dependent on the attractive interactions between the adsorbed molecules and adsorbent, the curvature of gas/adsorbed phase interface, and surface tension. A state equation of the adsorbed film is proposed to take into account the above factors. Nitrogen adsorption on model adsorbents, MCM-41, which exhibit uniform cylindrical channels, are used to verify the theoretical analysis. The proposed theory is capable of describing the important features of adsorption processes in cylindrical mesopores. According to this theory, at a given relative pressure, the smaller the pore radius is, the thicker the adsorbed film will be. The thickening of adsorbed films in the pores as the vapor pressure increases inevitably causes an increase in the interface curvature, which consequently leads to capillary condensation. Besides, this study confirmed that the interface tension depends substantially on the interface curvature in small mesopores. A quantitative relationship between the condensation pressure and the pore radius can be derived from the state equation and used to predict the pore radius from a condensation pressure, or vice versa.

Carbon structure and porosity of carbonaceous adsorbents in relation to their adsorption properties

A number of carbonaceous adsorbents were prepared by carbonisation at 600 degrees C following acidic oxidation under various conditions. Effects of the chemical nature of the precursor, such as the ratio of aromatic to aliphatic carbons and oxygen content, on the chemical and structural characteristics of the resultant chars were investigated using C-13 NMR and Raman spectroscopy, respectively. The C-13 NMR spectral parameters of the coal samples show that as the severity of oxidation conditions increased, the ratio of aromatic to aliphatic carbons increased. Furthermore, it was also found that the amount of disorganised carbon affects both the pore structure and the adsorption properties of carbonaceous adsorbents. It is demonstrated that higher amount of the disorganised carbon indicates smaller micropore size. (C) 1999 Elsevier Science Ltd. All rights reserved.

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.

Adsorption hysteresis and criticality in regular mesoporous materials

In the present work, various theories predicting the critical diameter for the absence of capillary condensation and hysteresis are applied to experimental adsorption isotherms of vapors on regular mesoporous materials. Among the various theories studied, the tensile strength approximation proposed by the authors was found to be the most successful. Reversibility of nitrogen adsorption at 77.4 K was studied on pure MCM-41 of various pore sizes, as well as mixtures of pure MCM-41 samples in a 1:1 ratio. The results of PSD and hysteresis on MCM-41 mixtures are close to that expected from studies of the pure materials. The estimates of hysteresis critical temperature and diameter of MCM-41, HMS, FSM and KIT materials are also provided.