Differential Heats Of Adsorption Research Articles

Development of a new analysis method evaluating adsorption energies for the respective ion-exchanged sites on alkali-metal ion-exchanged ZSM-5 utilizing CO as a probe molecule: IR-spectroscopic and calorimetric studies combined with a DFT method

For alkali-metal ion-exchanged ZSM-5 zeolites (MZSM-5; M: Li, Na, K, Rb, Cs) the analysis of ion-exchangeable sites was performed by means of a combined method based on IR spectroscopic and calorimetric measurements using CO as the probe molecule. The heat of adsorption of CO was found to be correlated with an IR frequency of stretching vibration of C-O in the adsorbed species. It was revealed that there exists at least two types of sites capable of ion-exchanging; for the lithium ion-exchanged ZSM-5 (LiZSM-5) CO adsorption on each type of site is evaluated to give a set of IR bands and heats of adsorption, 2195 cm(-1) and 49 kJ mol(-1), 2185 cm(-1) and 39 kJ mol(-1) with the aid of the newly developed method utilizing the data obtained from a combined microcalorimetric and IR-spectroscopic study. Such types of data were also obtained for Na- and K-ion-exchanged ZSM-5 samples. Furthermore, a linear relationship between the differential heat of adsorption (q(diff)) evaluated and the shift of wavenumber of the C-O stretching vibration from that of a gaseous CO molecule (Deltanu) was established for the systems of MZSM-5-CO, and the bonding nature of the CO molecule with each site can be explained in terms of the electrostatic force. The model of each adsorption site was also examined by the quantum calculation method (density functional theory: DFT). The trends obtained from the experimental data may be substantially supported by the calculation method even adopting a model as simple as the ZSM-5-type zeolite: the composition of MAlSi(4)O(4)H(12).

Statistical Thermodynamics and Surface Phase Transitions of Interacting Particles Adsorbed on One-Dimensional Channels Arranged in a Triangular Cross-Sectional Structure

Monte Carlo simulations and finite-size scaling analysis have been carried out to study the critical behavior in a submonolayer lattice-gas, which mimics a nanoporous environment. In this model, one-dimensional chains of atoms were arranged in a triangular cross-sectional structure. Two kinds of lateral interaction energies have been considered: (1) wL, interaction energy between nearest-neighbor particles adsorbed along a single channel and (2) wT, interaction energy between particles adsorbed across nearest-neighbor channels. We focus on the case of repulsive transverse interactions (wT > 0), where a rich variety of structural orderings are observed in the adlayer, depending on the value of the parameters kBT/wT (kB being the Boltzmann constant) and wL /wT. For wL /wT = 0, successive planes are uncorrelated, the system is equivalent to the triangular lattice, and the well-known [ ] ordered phase is found at low temperatures and a coverage, , of 1/3 [2/3]. In the more general case (wL /wT  0), the competition between interactions along a single channel and the transverse coupling between sites in neighboring channels leads to a three-dimensional adsorbed layer. Consequently, the and structures “propagate” along the channels and new ordered phases appear in the adlayer. The influence of each ordered phase on adsorption isotherms, differential heat of adsorption and configurational entropy of the adlayer has been analyzed and discussed in the context of the lattice-gas theory. Finally, the Monte Carlo technique was combined with the recently reported free energy minimization criterion approach (FEMCA) [F. Romá et al.: Phys. Rev. B Vol. 68 (2003), art. no. 205407] to predict the critical temperatures of the surface-phase transformations occurring in the adsorbate. The excellent qualitative agreement between simulated data and FEMCA results allows us to interpret the physical meaning of the mechanisms underlying the observed transitions.

The adsorption properties of titanium dioxide

The adsorption properties of titanium dioxide were studied by gas chromatography. We used organic compounds from different classes, namely, n-alkanes, n-alkenes (C6-C8), and polar compounds (electron donors and acceptors) as test adsorbates. The differential heats of adsorption and the contributions of dispersion and specific intermolecular interaction energies were determined for the systems from the experimental retention data. The electron-donor and electron-acceptor characteristics of the ultimately hydroxylated surface of TiO2 were evaluated.

A new microcalorimeter of adsorption for the determination of differential enthalpies

An adsorption microcalorimeter for the determination of the differential heat of adsorption is designed and built, and some of the results are presented here. For this purpose, a Calvet heat-conducting microcalorimeter is developed and is connected to a gas volumetric unit built in stainless steel to record adsorption isotherms. The microcalorimeter is electrically calibrated to establish its sensitivity and reproducibility, obtaining K = 154.34 ± 0.23 W V −1 and, chemically, it is examined with a THAM–HCL system, obtaining Δ f = −30.49 ± 0.05 kJ mol −1.The adsorption microcalorimeter is used here to obtain adsorption isotherms and the corresponding differentials heats, using as reference solid, a zeolite ZSM-5 type.

Thermodynamics of methane adsorption on the microporous carbon adsorbent ACC

The behavior of the thermodynamic functions (differential molar isosteric heat of adsorption, entropy, enthalpy, and heat capacity) of the adsorption system methane—microporous carbon adsorbent ACC was analyzed at different adsorption equilibrium parameters in the temperature interval from 177.65 to 393 K and in the pressure range from 1 Pa to 6 MPa. The influence of the nonideal character of the gas phase and noninertness of the adsorbent were taken into account, which resulted in the appearance of the temperature dependence of the isosteric heat of adsorption, especially in the region of high pressures of the adsorptive. For the system studied, nonideality of the gas phase exerts the main effect on the thermodynamic functions of the adsorption system. In this interval of the parameters of the adsorption system, the correction to the noninertness of the adsorbent is not higher than 2.5%.

Preparation and Characterization of Sulpho-Functionalized Ordered Mesoporous Silica

Sulpho groups have been successfully embedded in mesoporous silica with an ordered two-dimensional hexagonal structure. The sulpho-functionalized mesoporous silica was prepared by the oxidation of a thiol-functionalized mesoporous silica which was obtained via the hydrolysis and co-condensation of tetramethoxysilane (TMOS) and mercaptopropyl trimethoxysilane (MPTMS). The samples exhibited an ordered two-dimensional hexagonal pore array up to 30 mol% MPTMS loading. The large surface area of MCM-41 (ca. 1000 m2/g) was maintained after incorporation of the mercaptopropyl group and oxidation of its thiol group. Solid-state 29Si NMR spectra showed the existence a mercaptopropyl group in a silica framework and 13C NMR studies further proved that treatment with hydrogen peroxide led to oxidation of the sulpho groups. High ammonia uptakes with high differential heats of adsorption revealed the strong acidity of sulpho-functionalized mesoporous silica. The amounts of acid sites may be controlled by regulating the amount of MPTMS employed in the synthesis. The sulpho-functionalized mesoporous silica behaved as an acid catalysis for the isomerization reaction of but-1-ene to cis- and trans-but-2-ene.

Design, Calibration, and Testing of a New Tian-Calvet Heat-Flow Microcalorimeter for Measurement of Differential Heats of Adsorption

A Tian-Calvet heat-flow microcalorimeter system is described for measurement of heats of adsorption and reaction on porous solids. Heat-flow signals are measured in transducer assemblies consisting of several hundred Seebeck-effect thermoelements connected in series and arranged in a square configuration. These transducers surround a stainless steel calorimeter cell connected to a high-vacuum volumetric adsorption system. The sensitivity of the calorimeter was calibrated by measurement of electric response and of heats of adsorption for known processes. Microcalorimetric measurements have been conducted at temperatures from 273 to 298 K. The calorimetric cell described here facilitates sample handling under controlled atmosphere conditions, allowing the study of freshly treated materials and samples that have been exposed to catalytic reaction conditions.

Interrelation between the spatial structure of unsaturated hydrocarbon molecules and heats of their adsorption

The total area (sm) of Voronoi-Dirichlet polyhedron faces corresponding to all intermolecular contacts of one molecule in the structure of crystals and the total volume of pyramids (Vm) built on such faces and containing the nuclei of atoms participating in intermolecular contacts in their vertices were determined for 19 unsaturated hydrocarbons. The differential molar heat of adsorption of the hydrocarbons on graphitized thermal carbon black was found to be linearly related to the sm or Vm integral parameters of their Voronoi-Dirichlet polyhedra. Aromatic hydrocarbons on the one hand and saturated hydrocarbons, olefins, and acetylene on the other are characterized by different dependences because of the special shapes of their molecular Voronoi-Dirichlet polyhedra.

Correlation between the spatial structure of molecules of saturated hydrocarbons and their heats of adsorption

The total area sm of Voronoi-Dirichlet polyhedron (VDP) faces corresponding to all intermolecular contacts of a molecule in a crystal structure and the total volume Vm of pyramids whose bases are such faces and whose vertices are the nuclei of atoms involved in intermolecular contacts were determined for 16 saturated hydrocarbons, including alicyclic and cage structures. In the Henry region, the differential molar heat of hydrocarbon adsorption on graphitized carbon black depends linearly on the integral parameter sm or Vm of its molecular VDPs. These parameters were shown to be universal descriptors characterizing both the spatial structure of molecules and their ability to participate in intermolecular interactions during adsorption from the gas phase.

Effect of Pt and Pd promoter on Ni supported catalysts—A TPR/TPO/TPD and microcalorimetry study

The promoting effect of Pt and Pd in bimetallic Ni–Pt and Ni–Pd catalysts supported on alumina nano-fibre (Alnf) were tested for the liquid phase reforming of sorbitol to produce hydrogen. The mono- and bimetallic catalysts were studied by different characterisation techniques such as: temperature programmed reduction, oxidation, CO desorption, microcalorimetry, TEM and STEM/EDX. Although bimetallic catalysts have long been the subject of great interest because of their exceptional properties compared to the monometallic catalysts, the reason behind their improved activity is still a question of debate. Experimental evidence showed that the addition of both Pt and Pd—even in a very small fraction—to the Ni catalyst increases its reducibility significantly. The TEM and STEM/EDX analysis confirmed that Pt and Ni are present as alloys in nano-sized rod shaped particles. At the same time it was found that the CO differential heat of adsorption is appreciably lowered in the bimetallic catalysts. This is substantial because reducing the CO binding strength can avoid the poisoning of the active metal sites. As a result, we demonstrate that the rate of H 2 formation from sorbitol reforming was 3 to 5 times higher for bimetallic catalysts when compared to the monometallic catalysts.

Determination of Differential Enthalpy and Isotherm by Adsorption Calorimetry

An adsorption microcalorimeter for the simultaneous determination of the differential heat of adsorption and the adsorption isotherm for gas-solid systems are designed, built, and tested. For this purpose, a Calvet heat-conducting microcalorimeter is developed and is connected to a gas volumetric unit built in stainless steel to record adsorption isotherms. The microcalorimeter is electrically calibrated to establish its sensitivity and reproducibility, obtaining K=154.34±0.23 WV−1. The adsorption microcalorimeter is used to obtain adsorption isotherms and the corresponding differential heats for the adsorption of CO2 on a reference solid, such as a NaZSM-5 type zeolite. Results for the behavior of this system are compared with those obtained with commercial equipment and with other studies in the literature.

New Method for Atomistic Modeling of the Microstructure of Activated Carbons Using Hybrid Reverse Monte Carlo Simulation

We propose a new hybrid reverse Monte Carlo (HRMC) procedure for atomistic modeling of the microstructure of activated carbons whereby the guessed configuration for the HRMC construction simulation is generated using the characterization results (pore size and pore wall thickness distributions) obtained by the interpretation of argon adsorption at 87 K using our improved version of the slit-pore model, termed the finite wall thickness (FWT) model (Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532) . This procedure overcomes limitations arising from the use of short-range potentials in the conventional HRMC method, which make the latter unsuitable for carbons such as activated carbon fibers that are anisotropic with medium-range ordering induced by their complex pore structure. The newly proposed approach is applied specifically for the atomistic construction of an activated carbon fiber ACF15, provided by Kynol Corporation (Nguyen, T. X.; Bhatia, S. K. Carbon 2005, 43, 775) . It is found that the PSD of the ACF15's constructed microstructure is in good agreement with that determined using argon adsorption at 87 K. Furthermore, we have also found that the use of the Lennard-Jones (LJ) carbon-fluid interaction well depth obtained from scaling the flat graphite surface-fluid interaction well depth taken from Steele (Steele, W. A. Surf. Sci. 1973, 36, 317) provides an excellent prediction of experimental adsorption data including the differential heat of adsorption of simple gases (Ar, N(2), CH(4), CO(2)) over a wide range of temperatures and pressures. This finding is in agreement with the enhancement of the LJ carbon-fluid well depth due to the curvature of the carbon surface, found by the use of ab initio calculations (Klauda, J. B.; Jiang, J.; Sandler, S. I. J. Phys. Chem. B 2004, 108, 9842) .

Modification of Fagus sylvatica (L.) with 1,3-dimethylol-4,5-dihydroxyethylene urea (DMDHEU): Part 1. Estimation of heat adsorption by the isosteric method (Hailwood-Horrobin model) and by solution calorimetry

Abstract The differential heat of adsorption (ΔH s ) of Fagus sylvatica wood modified with 0.8 M, 1.3 M, and 2.3 M solution of 1,3-dimethylol-4,5-dihydroxyethylene urea (DMDHEU) was determined at different equilibrium moisture contents (EMCs) by both the isosteric method, using the Hailwood-Horrobin model at 20, 30, and 40°C and by solution calorimetry. Both methods revealed that at 1% EMC, the 2.3-M modified wood releases approximately double the amount of energy as does unmodified wood. At EMC below approximately 5%, DMDHEU-modified wood appears to be more hygroscopic than unmodified wood, indicating that more OH groups are available for adsorption. The bulking effect of the cell wall and the increase of OH groups caused by the modification with DMDHEU are probably the reason for this observation. As expected, higher EMC led to a reduction of ΔH s .

Role of adsorption activation energy in heat change of irreversible dissociative adsorption with coverage on energetically heterogeneous solid surface

The differential heat of gas adsorption on solid surface is often used to determine surface energy heterogeneity. In this paper, the theoretical analysis of irreversible dissociative adsorption on double-site surfaces shows that the change of differential adsorption heat with coverage is strongly dependent on the difference of adsorption activation energies between active sites. Furthermore, it was found that a discontinuous double-site surface can exhibit continuous changes of differential adsorption heat with coverage. This indicates that the distribution functions obtained from the relationship between differential adsorption heat and coverage may provide an incorrect characteristic of surface energetic heterogeneity.

Dissociative adsorption ofO2molecules on O-precovered Fe(110) and Fe(100): Density-functional calculations

The dissociative adsorption of ${\mathrm{O}}_{2}$ molecules on clean and oxygen-precovered Fe(110) and Fe(100) surfaces has been studied from first principles. For the relatively open Fe(100) surface, we find that along the most favorable reaction channel, ${\mathrm{O}}_{2}$ dissociation remains a nonactivated process up to almost full monolayer coverage. The differential heat of adsorption decreases only slowly with increasing oxygen precoverage. The potential energy profile for dissociation shows a dip, which is indicative of the formation of a very short-lived nonmagnetic peroxo precursor. On the close packed Fe(110) surface, the differential heat of adsorption begins to decrease already at a modest precoverage. For dissociation on a surface precovered with about 0.44 monolayer of oxygen, a low barrier begins to appear in the entrance channel. In the transition state, the incoming molecule is in a superoxo state with a magnetic moment of $1{\ensuremath{\mu}}_{B}$. Our results are discussed in relation to the electronic and magnetic properties of the partially precovered surfaces and to the available experimental results.

Monte Carlo study of multicomponent adsorption on triangular lattices

The adsorption process of interacting binary gas mixtures containing particles A and B on triangular substrates is studied through grand canonical Monte Carlo simulation in the framework of the lattice-gas model. The energies involved in the adsorption process are four: (1) ϵ0, interaction energy between a monomer (type A or B) and a lattice site; (2) wAA, nearest-neighbor interaction energy between two A particles; (3) wAB (=wBA), nearest-neighbor interaction energy between an A particle and a B particle and (4) wBB, nearest-neighbor interaction energy between two B particles. The process is monitored through partial and total isotherms, differential heats of adsorption and energy of the system, which appear as very sensitive to all lateral interactions. We focus on the case of repulsive lateral interactions, where a rich variety of structural orderings are observed in the adlayer, depending on the value of the parameters wAA, wAB and wBB. Results are rationalized through the determination of the phase diagrams characterizing second order phase transitions in the system. A nontrivial interdependence between the partial surface coverage of both species is observed.

Probing the Intrapore Surface of Phenyl-Substituted Nanoscale Mesoporous SilicaPiezoelectric Sorption Measurements in Thin Films

The incorporation of organic moieties into siliceous frameworks leads to a wide variety of adsorbate-adsorbent interactions including weak van-der-Waal attractions as well as strong interactions such as Coulomb forces. Depending on the desired properties of such substituted highly porous matrix materials, optimized synthesis routes can be established to enhance the desired internal pore surface-affinity toward certain volatile compounds. On the basis of a fundamental knowledge of the host-guest system, sorption-related applications may benefit from individually fine-tuned and modified sample materials. The sorption isotherms of vaporized toluene on nonmodified and phenyl-functionalized mesoporous silica samples were determined on an acoustic wave device at different temperatures. The mesoporous silica was modified by in situ co-condensation and postsynthesis grafting approaches, respectively. All samples were thoroughly characterized by nitrogen sorption, thermogravimetric analysis (TGA), scanning and transmission electron microscopy (SEM, TEM), solid-state nuclear magnetic resonance (29Si NMR), dynamic light scattering (DLS), Raman spectroscopy, and toluene adsorption on a quartz crystal microbalance (QCM). The different heats of adsorption of toluene on the various modified silica surfaces obtained by the sorption data make it possible to gain additional information about the degree and type of surface functionalization. It is thus demonstrated that QCM studies can be a powerful and convenient tool for efficient investigations of functionalized mesoporous silica particles that yield valuable quantitative information on molecule-surface interactions.

Evaluation of adsorbent materials for heat pump and thermal energy storage applications in open systems

The evaluation of solid adsorbents in open sorption systems for heating, cooling and thermal energy storage (TES) applications is crucial for the ecological and economical performance of these systems. An appropriate adsorbent has to reach the temperature limit given by the heating/cooling system of the consumer. It has to provide high energy efficiency and a high energy density for storage applications. A method for an easy evaluation of different adsorbents for a specific application has been developed. The method is based on the adsorption equilibrium of the adsorbent and water vapor. The crucial property for the discussed field of applications is the differential heat of adsorption. Criteria for the evaluation of the adsorbent are the breakthrough curves (responsible for the dynamics of the process), the possible temperature lift (or the dehumidification) of the air, the thermal COP and the storage capacity.

Influence of Si/Al ratio on hexane isomers adsorption equilibria

In this work we aim to have a better knowledge of the influence of silica/alumina ratio (SAR) on the adsorption equilibrium of hexane isomers on MFI zeolites. With a manometric set-up coupled with a micro-calorimeter we did address adsorption properties and heats of adsorption of n-hexane, 2-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane on three samples with different Si/Al ratio. The branched isomers all show a type I isotherm on the three samples, while n-hexane presents a dual site for SAR 1600 and 100, and type I for SAR 50. The Si/Al ratio has a large influence on the adsorption equilibrium properties, in addition the Henry’s constant and initial heats of adsorption increase with aluminium content. The saturation loading decreases with the increasing aluminium content. The differential heat of adsorption of n-hexane increases slightly with loading, whereas the heat of adsorption of branched hexanes presents a decrease with loading. The three MFI zeolite samples present equilibrium selectivity towards n-hexane.

Thermodynamics of Carbon Monoxide Adsorption on Polycrystalline Titania Studied by Static Adsorption Microcalorimetry

The adsorption of CO on polycrystalline TiO2 was investigated by static adsorption microcalorimetry. The initial differential heat of adsorption (qdiff,0) of CO on polycrystalline titania is 40 kJ/mol, and the standard adsorption entropy (Deltas0) is -104 J mol(-1) K(-1). These results are consistent with those derived from temperature-programmed desorption and FTIR results in the literature. The good reproducibility of the isotherms and the stable qdiff indicate that the lattice oxygen and hydroxyl groups on titania surface are basically not reactive to adsorbed CO.