Heat Of Adsorption Values Research Articles

Heats of Adsorption of Linear and Multibound Adsorbed CO Species on a Pt/Al2O3 Catalyst Using in Situ Infrared Spectroscopy under Adsorption Equilibrium

The adsorption of CO (1 and 10% CO/He mixtures, 1 atm total pressure) on a reduced 2.9% Pt/Al2O3 catalyst in the temperature range 298–740 K is studied using Fourier transform infrared spectroscopy (FTIR). Several adsorbed CO species on Pt sites are detected at 300 K: linear (IR band at 2075 cm−1), bridged (three species; IR bands at 1878, 1835, and 1824 cm−1), and threefold coordinated CO species (IR band at 1800 cm−1) which are denoted by L, B, and 3FC, respectively. The evolution of the intensities of the IR bands with the adsorption temperature Ta allows the determination of the change of the coverage θ of each adsorbed CO species with Ta. The curves θ=f(Ta) permit the determination of the heats of adsorption of the three adsorbed species using the same adsorption model which considers: (a) a linear decrease in the heats of adsorption Eθ as a function of θ; and (b) immobile adsorbed species. The following heat of adsorption values were found (the subscript indicates the value of the coverage): for the L species, EL0=206 kJ/mol and EL1=115 kJ/mol; for the B species, EB0=94 kJ/mol and EB1=45 kJ/mol; for the 3FC species, E3FC0=135 kJ/mol and E3FC1=104 kJ/mol. In order to validate the assumptions of the adsorption model, the heats of adsorption obtained with the present procedure are compared to the isosteric heat of adsorption for the L and B species.

Adsorption behavior of copper(II) ion from aqueous solution on methacrylic acid-grafted poly(ethylene terephthalate) fibers

The adsorption behavior of methacrylic acid-grafted poly(ethylene terephthalate) fibers was studied toward the copper(II) ion in aqueous solutions by a batch equilibriation technique. The influence of treatment time, temperature, pH of the solution, metal ion concentration, and graft yield were considered. One hour of adsorption time was found sufficient to reach adsorption equilibrium for the copper(II) ion. It was found that the adsorption isotherm of Cu(II) fits Langmuir-type isotherms. The adsorption process is not affected by the temperature when treated with low ion concentration, but is remarkably decreased at a high ion concentration. The heat of adsorption value was calculated as 0.71 kcal/mol. It was found that the reactive fibers are stable and regenerable by acid without losing their activity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 766–772, 2000

Adsorption behavior of copper(II) ion from aqueous solution on methacrylic acid-grafted poly(ethylene terephthalate) fibers

The adsorption behavior of methacrylic acid-grafted poly(ethylene terephthalate) fibers was studied toward the copper(II) ion in aqueous solutions by a batch equilibriation technique. The influence of treatment time, temperature, pH of the solution, metal ion concentration, and graft yield were considered. One hour of adsorption time was found sufficient to reach adsorption equilibrium for the copper(II) ion. It was found that the adsorption isotherm of Cu(II) fits Langmuir-type isotherms. The adsorption process is not affected by the temperature when treated with low ion concentration, but is remarkably decreased at a high ion concentration. The heat of adsorption value was calculated as 0.71 kcal/mol. It was found that the reactive fibers are stable and regenerable by acid without losing their activity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 766–772, 2000

A calorimetric, IR, XANES and EXAFS study of the adsorption of NH 3 on Ti-silicalite as a function of the sample pre-treatment

The interaction of NH 3 with Ti-silicalite-1 (TS-1) was studied by means of adsorption microcalorimetry IR, XANES and EXAFS techniques. A specific interaction with Ti(IV) sites was revealed and quantitatively and energetically described, in comparison with silicalite taken as a reference material. The adsorption was found to be largely reversible but a minor amount of ammonia was irreversibly held at the surface of TS-1. The energetic spectrum obtained by the heat of adsorption versus coverage plot is quite wide in both TS-1 (40< q ads<130 kJ mol −1) and silicalite (40< q ads<90 kJ mol −1), indicating not only that different processes occur at the surface but that a few highly energetic sites are present on TS-1. The standard pre-treatment of the samples in ammonium acetate aqueous solution lowers the whole adsorption capacity of TS-1 surface (as well of silicalite at about the same degree), but not the number of NH 3 molecules adsorbed per Ti atom, which is close to two for both untreated and treated TS-1 samples. The most energetic sites are preferentially suppressed by the ammonium acetate treatment on both TS-1 and silicalite, as shown by the heat of adsorption values much lower for the treated samples than for the untreated ones. Parallel IR and XANES–EXAFS results were found to support these findings.

Adsorption characteristics and surface free energy of AlPO 4 -5

The micropores and surface characteristics of aluminophosphate-type zeolite, AlPO4-5, were analyzed by examining the adsorption behavior of water and other adsorbates. Water adsorption on AlPO4-5 occurred on both structural defects and nonpolar surfaces. Adsorption on structural defects, accompanied by high heats of adsorption, is attributed to adsorption to surface hydroxyls. Water adsorption increased steeply at a certain relative pressure depending on the adsorption temperature, and this was considered attributable to capillary condensation. The contact angle of water on AlPO4-5 micropore surfaces can be determined quantitatively by applying the Kelvin equation. The surface free energy of AlPO4-5 calculated on the basis of the contact angle was revealed to be about 120 mJ/m2, in agreement with accepted values of the dispersion component of the surface free energy of metal oxides. Adsorption heat values of adsorbates with different polarities indicate that the AlPO4-5 surface is essentially nonpolar and interacts only with dispersion interaction. In the case of n-hexane the contact angle was assumed to be zero, showing high affinity with the result of enhanced adsorption due to pore filling.

Calorimetric and gravimetric measurements of hydrocarbon adsorption on a granulated active carbon

Adsorption isotherms of n-butane on a granulated activated carbon were measured by two different but complementary experimental methods: calorimetry and gravimetry. Adsorption heats were determined in different ways. For the system studied, the experimental results prove that the adsorbent offers a homogeneous site distribution. Besides, there can be differences between the adsorption heat values which might come from the way they are obtained (by calculation or direct measurements).

Adsorption of copper(II), cobalt(II), and iron(III) ions from aqueous solutions on poly(ethylene terephthalate) fibers

The adsorption behavior of poly(ethylene terephthalate) (PET) fibers towards copper(II), cobalt(II), and iron(III) ions in aqueous solutions was studied by a batch equilibriation technique. Influence of treatment time, temperature, pH of the solution, and metal ion concentration on the adsorption were investigated. Adsorption values for metal ion intake followed the following order: Co(II) > Cu(II) > Fe(III). One hour of adsorption time was found sufficient to reach adsorption equilibrium for all the ions. The rate of adsorption was found to decrease with the increase in the temperature. Langmuir adsorption isoterm curves were found to be significant for all the ions studied. The heat of adsorption values were calculated as −5, −2.8, and −3.6 kcal/mol for Cu(II), Co(II), and Fe(III) ions, respectively. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1935–1939, 1998

Microcalorimetric and IR spectroscopic studies of pyrindine sorption in NaH-mordenites

The adsorption of pyridine in NaH-mordenites of various Na/H exchange degrees was followed by adsorption microcalorimetry and IR spectroscopy. IR data evidenced that practically only Brönsted acid sites (OH groups) were present in our mordenites. The concentration of Lewis sites was very low. All the results obtained in this study concern therefore the properties of acid hydroxyls in large channels, which are active sites in reactions catalyzed by mordenites. The amounts of such hydroxyls determined volumetrically were found to be close to the amounts determined in a previous IR study. The average acid strength determined microcalorimetrically increased with exchange degree up to 47% and then was constant. It indicates that as long as new hydroxyls are formed in large channels, their average acid strength increases (probably more strongly acid hydroxyls are formed at higher exchange degrees). Above 47% of exchange, the acid strength of hydroxyls in large channels was constant, indicating that the formation of hydroxyls in side pockets (which takes place above 50% of cation exchange) did not influence the properties of hydroxyls in large channels. The differential heat of pyridine adsorption with most of acid sites (OH groups) did not depend on pyridine loading. Our earlier IR and TPD results of ammonia desorption suggested heterogeneity of OH groups in large channels of mordenites. However, the constant value of adsorption heat suggests that pyridine molecules reacted with all the accessible hydroxyls without selecting the most acid ones. This hypothesis was confirmed by IR studies of pyridine adsorption: the stretching frequency of hydroxyls consumed by pyridine was independent on the amount of pyridine adsorbed.

Interaction of nicotine and its pharmaceutical derivatives with acrylamide/itaconic acid hydrogels

Acrylamide/itaconic acid hydrogels (AAm/IA) containing different amounts of itaconic acid prepared by irradiating with g radiation were used in experiments on swelling, diffusion, and interactions of the pharmaceuticals nicotine, nicotinic acid, nicotinamide, and nikethamide. AAm/IA hydrogel containing 60 mg itaconic acid and irradiated at 4.65 kGy has been used for swelling and diffusion studies in water and aqueous solutions of the pharmaceuticals. For this hydrogel, swelling studies indicated that swelling increased with the following order: water u nicotine u nikethamide u nicotinamide u nicotinic acid. Diffusions of water and the pharmaceuticals within hydrogels were found to be non-Fickian in character. The uptake of the pharmaceuticals to AAm/IA hydrogels was studied by batch adsorption technique at 257C. In the experi- ments of the adsorption, C-type adsorption in Giles's classification system was found. Some binding and thermodynamic parameters for AAm/IA hydrogel-pharmaceutical systems were calculated by using the Klotz method. The values of adsorption heat, free energy, and entropy of this system were found as negative values. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 733-739, 1997

Interaction of Water and other Polar Substances with Hydrophilic Centres on the Surface of Hydrophobic Adsorbents

The adsorption of water and other polar substances on to the surface of graphitized carbon black and macroporous silica modified by polymethylhydride silicone has been studied using gas chromatography. The adsorption heat values of the substances studied were determined and the concentration of surface hydrophilic centres estimated. Conclusions have been drawn concerning the number of water molecules adsorbed on to hydrophilic centres at low surface occupation values. In the case of water adsorption on the graphitized carbon black surface, the chromatographic data for the adsorption heat have been compared with the energy of water molecule interaction with the surface clusters modelling the micropores existing on the basal surface of carbon black particles as calculated using quantum chemical methods.

Surface Heterogeneity on Hydrophilic and Hydrophobic Silicas: Water and Alcohols as Probes for H-Bonding and Dispersion Forces

The hydrophilic/hydrophobic behavior of two silica polymorphs (a finely ground cristobalite and a pyrogenic amorphous Aerosil) was studied by means of adsorption microcalorimetry, as a function of nature and extension of the surface hydroxylated layer. Water and two alcohols (methanol and tert-butyl alcohol) were used as probes for polar and apolar sites, available for H-bonding and for dispersion forces interactions. It was found that in most cases small portions of the surface were irreversibly modified upon the earliest contact with the probe, owing to dissociative reactions occurring at 303 K. However, the main process involved during the interaction was a reversible molecular adsorption: on both hydrophilic and hydrophobic SiO2 the process was by far more energetic for the two alcohols than for water, as indicated by the heat of adsorption values. The contribution of the dispersion forces to the overall energy of interaction on hydrophilic silica was ≈16 kJ/mol for methanol and ≈34 kJ/mol for tert-butyl alcohol. For hydrophobic silica the dispersive forces interaction was found to be a major component of the overall process, and the energetic described for hydrophilic silica was confirmed.

Potential energies and IR spectra for homopolar diatomic adsorbates over ion-exchanged ZSM-5 zeolites

Potential energies for N2, D2, and O2 entrapped in pores of MeZSM-5 (Me = Li, Na, K, Rb, and Cs) zeolites have been investigated theoretically as a basis for understanding the characteristic IR spectra of the adsorption systems. The adsorption potential proposed gives a reasonable value for the initial heat of adsorption q st/0 and enables us to predict the mode of orientation of an admolecule at the cation site. Specific predictions include an end-on orientation for N2, a lying-down orientation for D2, and a random orientation for O2. Analysis of the IR spectral intensity of the adsorbed molecule proves also that the predicted molecular orientation is realistic. Furthermore, the IR peak shifts predicted theoretically by using the adsorption potential are in good agreement with the experimental results. The influence of the orientation of the admolecule upon the IR peak shift is thus clarified.

Liquid Wetting on Metal Surfaces and the Heat of Immersion/Adsorption in Lubrication

Theories explaining the way in which liquid-surface interaction affect boundary lubrication are reviewed. In all views, adsorption is a critical step governing the end result—albeit through differing mechanism. This review collects persuasive qualitative experimental support to this effect. A quantitative treatment deriving the friction or wear coefficients as functions of the heats of adsorption of boundary lubricant molecules is discussed in more detail, and recommendations for testing existing models are made. Methods for measuring heats of adsorption are described and the interrelation between the different measurements is explained. Heat of adsorption values predicted from sliding friction and wear measurements are higher than the integral heats of adsorption measured for the lubricant. This discrepancy could mean that a differential heat (i.e., specific for a certain surface coverage) determines the friction and wear characteristics of the lubricant-surface combination or that the surface temperatur...

Effect of 1-Hydroxyethylidene-1, 1-Diphosphonic Acid on the Corrosion of SS 41 Steel in 0.3% Sodium Chloride Solution

Abstract The effectiveness of 1-hydroxyethylidene-1, 1-diphosphonic acid (HEDP) as a corrosion inhibitor was investigated by measuring corrosion weight losses and polarization curves of SS 41 steel in 0.3% NaCl. In concentrations lower than 50 ppm of HEDP, the corrosion of SS 41 steel was inhibited in accordance with the Langmuir adsorption isotherm of HEDP on the steel. The adsorption was believed to be a chemical adsorption resulting from the value of adsorption heat. In concentrations greater than 50 ppm of HEDP, the iron complex of HEDP developed, and the inhibition effect was decreased. Instead, it caused rust to be removed from the steel surface. The mutual inhibition effect by adding both HEDP and the Zn2+ ion in 0.3% NaCl solution was progressively more increased. It was shown that HEDP does not easily decompose.

Chromatographic determination and molecular-statistical calculation of retention volumes of oxygen-containing organic substances on graphitized thermal carbon black part 1: Cyclic ethers and ketones

The paper is concerned with molecular-statistical determination of retention volumes of oxygen-containing organic substances such as cyclic ethers and ketones by presenting the potential energy of intermolecular interaction of the molecules with a monoatomic adsorbent having homogeneous flat surface (graphitized thermal carbon black) as a sum of atom-atom potentials. In order to eliminate the specific interactions of ethers and ketones with the traces of surface oxygen compounds on graphitized thermal carbon black, the latter was treated additionally with hydrogen. Comparison is made between the experimentally measured retention volumes and the Henry constants calculated for reference molecules of ethers (dioxane) and ketones (cyclohexanone) to correct the atom-atom potentials. It is shown that the atom-atom potentials obtained for the reference molecules can be used for other ethers and ketones of known structure with the subsequent calculation of the corresponding values of the retention volumes in terms of molecular statistics. When the structure is not known its parameters are optimized using the chromatostructural method (chromatoscopy). The calculated heat of adsorption values agree well with those determined from the chromatographic measurements. Chromatograms showing the full separation of mixtures of the ethers and ketones studied were obtained.

The Adsorption of Chloromethanes on KCl. The Role of the Surface Ions as Adsorption Sites

Abstract The adsorbed amounts of chloro-substituted methanes, such as CH2Cl2, CHCl3, and CCl4, for KCl particles were measured by a volumetric method at various temperatures, and the isosteric heats of adsorption and differential entropies of the adsorbed molecules were estimated from the adsorption isotherms. The order of magnitude of the heats of adsorption at a coverage of θ=0.5 was found to be CH2Cl2&amp;gt;CHCl3&amp;gt;CCl4. Moreover, a plot of the dipole moments of the three adsorbates vs. the adsorption heat values gave a straight line. Therefore, the differences among the adsorption heats of the three adsorbates is mainly caused by an orientation of their dissimilar dipoles to the surface. It may be concluded that the chloro-substituted methanes are adsorbed on surface anions of KCl powder.

Approximation of the adsorption heats of hydrogen on transition metals by means of an empirical model

The adsorption heats of hydrogen on transition metals were approximated using a first order polynomial containing variables of basic physical importance, which were determined recently by means of the empirical method of pattern recognition: the heat of fusion, heat of vaporization, Pauling electronegativity, molar thermal conductivity, first and second ionization energies, electrical conductivity, Debye temperature and atomic volume of the metal. The agreement with experimental values is closer than that obtained with the Pauling relation. Values of adsorption heats are predicted for seven transition metals.

Dependence of heat capacity of adsorbate on surface coverage on the basis of BET theory assumptions

From a consideration of elementary quasi-chemical equilibria between adsorption complexes, an equation has been obtained within the framework of the BET theory for the dependence of the average molar and differential heat capacities of an adsorbed substance on the relative vapor pressure. Approximate dependences of the average molar heat capacities of benzene adsorbed by a basal graphite face on the adsorption value have been calculated using this equation and the independently determined values for heat of adsorption and heat capacity at low surface coverage. The results are compared with direct calorimetric measurements of the heat capacity of adsorbed benzene.

Gas chromatographic determination of the absolute values of retention volumes and of adsorption heats for hydrocarbons on silica gels of various structures

1. 1. From chromatograms obtained at various temperatures, the adsorption heat values of C 1-C 10 hydrocarbons were determined on silica gels of varying porosity. For fine pore silica gels increased adsorption heats were observed with a reduction of mean pore diameter and with an increase in the number of carbon atoms in the n-paraffin molecule. With enlargement of pores, a maximum linear dependence is established of differential adsorption heats on the number of carbon atoms in the molecule. 2. 2. The difference in the adsorption heats of unsaturated and saturated hydrocarbons with the same numbers of carbon atoms, which characterizes the effect of specific interactions of π-electron bonds with the hydroxyl groups of the silica gel surface, is practically independent of geometrical structure. 3. 3. Absolute values (related to unit surface) of the retention volumes V Rs were obtained for normal hydrocarbons. On large pore silica gels V Rs is practically independent of s and on fine pore silica gels it increases with s, due to the increase of adsorption energy with constriction of pores, particularly for larger molecules.