Differential Heats Of Adsorption Research Articles

Investigation of energetic heterogeneity of carbon black surfaces during adsorption of chromatographically low concentrations of n-pentane

Energetic heterogeneity of different types of industrial carbons was studied by gas-adsorption chromatography in regions of low surface coverages. The main thermodynamic characteristics of n-pentane adsorption on the carbon black surfaces were experimentally determined under the conditions close to the zero coverage of the surface (“Henry region”). These characteristics include the Henry constants, differential molar heats of adsorption, changes in the entropy of adsorption, and differences in molar heat capacities of the adsorbate in the adsorbed state and its vapors at a constant pressure. The specific features of the geometric structure and ratio of surface regions different in the adsorption energy make it possible to establish the presence of the “Henry region” in the adsorption isotherm. The gas-chromatographic criteria, which can be used to estimate the degree of energetic heterogeneity of the adsorbent surface, were considered.

Chain-Length Anomaly in the Two-Dimensional Ordering of the Cationic Surfactants CnTAB at the Graphite/Water Interface, Revealed by Advanced Calorimetric Methods

Surfactants adsorbed at the graphite/aqueous solution interface form half-cylindrical surface micelles templated by a flat, ordered monolayer. Here, the differential heats of adsorption of even-numbered alkyltrimethylammonium bromides CnTAB (n = 6−16) were measured in the monolayer region by frontal-flow calorimetry and pulsed-flow calorimetry. The monolayer self-assembles exothermically and with a negative change of heat capacity. Pure water and pure surfactant domains coexist in the submonolayer region. The differential heat of adsorption is independent of the surface coverage and increases linearly with n, but a step occurs between n = 10 and n = 12. The adsorption is reversible below this critical chain length, but is nonreversible above it. This anomaly has been interpreted in terms of a reorientation of the headgroup from nonbonding to bonding with respect to the surface, and the formation of two different two-dimensional crystal structures on the surface. Pulsed-flow calorimetry, ideally suited for the study of nonreversible adsorption processes, has a high potential for the investigation of a variety of solid/liquid interfacial phenomena that cannot be studied by other current calorimetric methods.

Adsorption thermodynamics of interacting particles on diffusion-limited aggregates

Grand canonical Monte Carlo simulations have been performed in order to study adsorption thermodynamics of pairwise interacting particles on fractal surfaces. Diffusion-limited aggregates (DLA) have been used as a substrate where interacting particles are adsorbed. In order to obtain aggregates with different morphologies, DLA clusters are generated on different strongly correlated surfaces. Adsorption isotherm, adsorption energy and differential heat of adsorption were calculated for attractive and repulsive nearest-neighbor (NN) lateral interactions. For the case of repulsive couplings and low temperatures, four novel ordered phases has been found in the adsorbate, each one corresponding to the formation of a chessboard-like structure on sites with one, two, three and four NN sites, respectively. The values of coverage at which these ordered phases emerge are not symmetrical around θ=0.5. This is a consequence of the non-equivalence between vacancy and particle in the case of adsorption on fractal structures. The influence of ordered structures on thermodynamic quantities associated to the adsorbed monolayer has been analyzed and discussed in the context of the Lattice-Gas model.

The influence of ZnO on the differential heat of adsorption of CO on Cu catalysts: a microcalorimetric study

The differential heat of adsorption of carbon monoxide on carefully reduced copper catalysts employed in methanol synthesis was measured by means of a Tian–Calvet calorimeter to probe the influence of ZnO. A ternary catalyst (Cu/ZnO/Al 2O 3) and two binary catalysts (Cu/ZnO and Cu/Al 2O 3) were prepared by coprecipitation and characterized by N 2 physisorption (BET surface area), temperature-programmed reduction (TPR), N 2O-reactive frontal chromatography (N 2O RFC), and methanol synthesis activity measurements. The shape of the adsorption isotherms, the initial heat of adsorption, and the coverage dependence of the heat of adsorption were found to be different for the catalysts with and without ZnO. The initial heat of adsorption turned out to be inversely correlated with the activity for methanol synthesis: Cu/ZnO/Al 2O 3 had the lowest initial heat of adsorption of 68 kJ mol −1 and was the most active catalyst for methanol synthesis. Cu/ZnO showed a somewhat higher heat of adsorption of 71 kJ mol −1 and a lower activity, and Cu/Al 2O 3 had the highest initial heat of adsorption of 81 kJ mol −1 and the lowest activity. The decrease in the heat of adsorption of CO induced by the presence of ZnO is rationalized by strong metal–support interactions (SMSI); i.e., ZnO x species are assumed to cover the Cu metal surfaces presumably as Zn+O coadsorbate under reducing conditions.

Oxygen adsorption on Pt/TiO 2 catalysts

Oxygen adsorption over Pt/TiO 2 surfaces were investigated as a function of metal loading. Oxygen adsorption over pure TiO 2 was molecular at all pressure ranges investigated. On the other hand, differential heats of adsorption measured by adsorption calorimetry indicated that oxygen adsorption was dissociative over Pt/TiO 2 surfaces until the Pt surface was saturated with oxygen. The saturation coverage of atomic oxygen on Pt was determined as approximately one from adsorption calorimetry. The initial heat of oxygen adsorption was determined as ∼375 kJ/mol over 0.5 wt.% Pt/TiO 2. The differential heats of oxygen adsorption at coverages less than one monolayer of Pt surface was ∼200 kJ/mol consistent with the oxygen adsorption heats over Pt single crystals reported in the literature. As the pressure increased after the saturation of Pt surface, oxygen adsorption continued in a manner similar to oxygen adsorption over pure TiO 2. Isosteric heats of oxygen adsorption over pure and Pt/TiO 2 samples at coverages >5 μmol/g catalyst were determined to be around 10 kJ/mol. The amounts of oxygen adsorption over Pt/TiO 2 surfaces depended strongly on the pretreatment conditions. Most favorable oxygen adsorption was observed over calcined/reduced samples.

Microcalorimetric and IR spectroscopic studies of CO adsorption on molybdenum nitride catalysts

The adsorption of CO on both nitrided and reduced passivated Mo2N catalysts in either alumina supported or unsupported forms was studied by adsorption microcalorimetry and infrared (IR) spectroscopy. The CO is adsorbed on nitrided Mo2N catalysts on three different surface sites: 4-fold vacancies, Moδ+ (0 < δ < 2) and N sites, with differential heats of CO adsorption decreasing in the same order. The presence of the alumina-support affects the energetic distribution of the adsorption sites on the nitrided Mo2N, i.e. weakens the CO adsorption strength on the different sites and changes the fraction of sites adsorbing CO in a specific form, revealing that the alumina supported Mo2N phase shows lower electron density than pure Mo2N. On reduced passivated Mo2N catalysts the CO was found to adsorb mainly on Mo4+ sites, although some slightly different surface Moδ+ (0 < δ < 2) sites are also detected. The nature, density and distribution of surface sites of reduced passivated Mo2N/γ-Al2O3 were similar to those on reduced MoO3/γ-Al2O3.

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: II. Growth kinetics

AbstractThe experimental results of the effect of concentration of Mn(II) ions on the growth kinetics of different faces of ammonium oxalate monohydrate single crystals from aqueous solutions at a constant temperature and different predefined supersaturations are described and discussed. It was observed that: (1) at a given supersaturation σ, Mn(II) ions lead to a decrease in the growth rates of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σd but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, (3) the values of σd and σ* increase with increasing concentration of the impurity, and (4) the values of σd depend on the growth kinetics of a face but those of σ* are independent of face growth kinetics. The experimental R(σ) data for different Mn(II) concentrations ci were analysed according to the model involving complex source of cooperating screw dislocations and concepts of instantaneous and time‐dependent impurity adsorption. It was found that: (1) for a given face the differential heat of adsorption Qdiff is higher during instantaneous impurity adsorption than that during time‐dependent adsorption, and (2) the values of Qdiff involved during instantaneous adsorption are related with face growth kinetics but those during time‐dependent adsorption are independent of face growth kinetics.

Characterization of Water Adsorption onto Carbonaceous Materials Produced from Food Wastes

The recycling of organic wastes has become very important and the development of technology for recycling organic wastes needs to sustain industrial development. In this study, techniques for producing carbonaceous materials from organic wastes are described and water adsorption is characterized. The organic wastes used are coffee grounds and oolong tea leaves carbonized at 673 to 1073 K. The iodine adsorption capacity of the carbonaceous materials increased with increased carbonization temperature. The amount of water adsorbed onto the carbonization materials produced from oolong tea leaves at 873 K for 2 h was the highest. The Freundlich constant 1/ n and the differential heat of adsorption of the carbonaceous materials produced from oolong tea leaves were greater than that of the carbonaceous materials produced from coffee grounds. The ability to humidity control can be estimated by the difference between the amount of water adsorbed relative pressure 0.90 and that at relative pressure 0.55. The ability to humidity control was the greatest for the carbonaceous materials produced from the oolong tea leaves at 873 K for 2 h and did not depend upon the adsorption temperature. These results indicated that the carbonaceous materials produced from oolong tea leaves at 873 K for 2 h could have more humidity control.

The determination of interaction parameters in the characterization of polyetherimide gas separation membranes using the Horvath-Kawazoe model

The Horvath-Kawazoe (HK) model is widely used to characterize microporous carbons and zeolites. The use of the HK model in this application has been extensively studied and interaction parameters exist to relate gas adsorption isotherms to the microporosity of these materials. However, interaction parameters for the HK model do not exist for polymeric materials, which limits its use in characterizing such systems. In this work, the HK parameters for the nitrogen-polyetherimide system were determined from molecular properties and computational chemistry. The interaction energy coefficients required to calculate interaction parameters in the HK model were determined from the polarizability and diamagnetic susceptibility of PEI using the Kirkwood-Muller equations. The parameters were used in the HK method to model a nitrogen adsorption isotherm at 77.2°K and determine the micropore size distribution of a PEI gas separation membrane. The relation between the differential heat of adsorption and pore size distribution, which is the basis of the HK model, was studied for nitrogen adsorption on PEI using different interaction parameters and estimates of the diameter of the PEI chain. The influences of these parameters on the calculated pore size distribution were discussed. The validity of the pore size distribution was examined by comparing the dominant pore size with the d-spacing obtained from X-ray diffraction on PEI. Good agreement was obtained between these values on using a suitable structural model that represents the wall thickness of the adsorbent and includes the polar property of PEI in determining HK parameters.

An Examination of Brønsted Acid Sites in H-[Fe]ZSM-5 for Olefin Oligomerization and Adsorption

The adsorption and reaction properties of an Al-free H-[Fe]ZSM-5 were examined and compared to an H-[Al]ZSM-5 sample with the same site density. H-[Fe]ZSM-5 was shown to have Brønsted acid sites in a concentration equal to the framework Fe concentration. Differential heats of adsorption for ammonia and pyridine were shown to be identical to that obtained in H-[Al]ZSM-5, with differential heats of ∼150 kJ/mol for ammonia and 200 kJ/mol for pyridine. For H-[Al]ZSM-5, adsorption of either propylene or 1-butene at room temperature resulted in rapid oligomerization. TPD–TGA measurements of the oligomers in H-[Al]ZSM-5 showed evidence for hydride-transfer reactions, in addition to simple oligomer cracking. By contrast, it was necessary to heat H-[Fe]ZSM-5 to 370 K for rapid oligomerization of propylene and oligomerization of 1-butene occurred only slowly at 295 K. TPD–TGA measurements of the oligomers in H-[Fe]ZSM-5 showed no evidence for hydride-transfer reactions and H-[Fe]ZSM-5 formed much less coke than H-[Al]ZSM-5 during steady-state reaction in 1-butene at 573 K. Adsorption measurements of 1-butene on D-[Fe]ZSM-5 suggested that the protonated complexes of 1-butene were formed but that these were relatively stable toward reaction, implying that the carbocation transition states were relatively unstable.

Characteristics of Silver Ions Exchanged in ZSM-5-Type Zeolite, Aluminosilicate, and SiO2 Samples: In Comparison with the Properties of Copper Ions Exchanged in These Materials

The IR technique in combination with adsorption microcalorimetry was used to picture the bonding nature of silver ion exchanged or supported on solid materials, such as ZSM-5-type zeolite, aluminosilicate, and SiO2, by utilizing CO as a probe molecule. It has become apparent that there exists an adsorption site on which the CO molecule is adsorbed to give an IR absorption band: IR band at around 2193 cm-1 for silver-ion exchanged ZSM-5 (AgZSM-5) and aluminosilicate (Ag/SiO2·Al2O3), and at 2177 cm-1 for silver-ion supported SiO2 (Ag/SiO2). The CO adsorption took place accompanying a large heat evolution of 100−80 kJ mol-1 on the former two samples and relatively small heat of 70−40 kJ mol-1 on the latter sample; silver ions exchanged with protons acting as Bronsted acid sites are responsible for the strong adsorption sites for CO adsorption. Taking account of the relationship between the differential heat of adsorption (qdiff) and the stretching vibrational frequency of adsorbed CO (νCO), it was concluded...

A calorimetric and statistical mechanics study of water adsorption in zeolite NaY

Adsorption isotherms and differential heats of adsorption of water in the zeolites NaY and NaY(Br) were measured at 303 K. The latter zeolite was obtained from NaY by introducing NaBr into the β-cages thus rendering them impenetrable to water molecules.The results obtained lead to the conclusion that initially water is accomodated in the small β-cages up to 4 H2O/cage before adsorption in the large supercages sets in and proceeds to a maximum of 26 H2O/supercage. The experimentally obtained adsorption and heat data for water in both types of cages can be quantitatively reproduced on the basis of the statistical-mechanical concept of independent subsystems which are identified with the β-cages and, on the other hand, with the supercage-Na+ ions. For each of the one-to-four H2O/β-cage units and each of the monomeric to hexameric H2O/Na+ ion clusters residing in the supercages, the partition functions and energetic quantities such as the energies of formation and of successive addition as well as entropies are evaluated. Potential energies of the oligomeric clusters are estimated. The data for the zeolite water/cation clusters are compared with experimental and theoretical (ab initio) results of free (gaseous phase) water/cation clusters available in the literature.

Dynamic adsorption of ammonia on activated carbons measured by flow microcalorimetry

Several activated carbons of different textural and chemical surface characteristics have been used to study the ammonia adsorption. The textural characteristics were determined by N 2 and CO 2 adsorption at 77 and 273 K. The chemical surface groups were estimated by thermal programmed desorption followed by mass spectrometry (TPD-MS) and by selective titrations in aqueous solutions. The ammonia adsorption was studied under dynamic conditions from N 2 diluted flow using a flow adsorption microcalorimeter. The ammonia adsorption consists of reversible and irreversible components. The former is assigned to physisorption process while the latter is adsorption on chemical groups. The plots of the differential heats of adsorption versus the cumulative adsorption point to the existence of a wide distribution of acid sites some of which are very strong. However, they are not always easily accessible to NH 3 because constrictions in the pores-network hinder the access which forces to re-arrangement.

Calorimetric study of adsorption of non-ionic surfactants on silica gels: Estimating the role of lateral interactions between surface aggregates

A new approach to surfactant adsorption, based on applying the scaled particle theory, is used here for modeling non-ionic surfactant adsorption. In the case of adsorption of ionic surfactants investigated in our previous publications, only the excluded area interactions were taken into account. This is because coulombic interactions between identically charged polar heads hinder close contacts between surface aggregates, thus decreasing the role of possible short-range interactions. This cannot be assumed a priori in the case of non-ionic surfactants. Thus, the present generalized approach also takes into account the possible short-range interactions between the adsorbed surfactant aggregates and accounts for the effects of the surface energetic heterogeneity of a polar substrate. The developed equations are used for a simultaneous quantitative analysis of both adsorption isotherms and the accompanying differential heats of adsorption of three non-ionic surfactants adsorbed on a porous glass surface. The analysis shows that introducing additional terms describing the short-range interactions does not improve the fit of adsorption isotherms and heats of adsorption. This would suggest that also in the case of non-ionic surfactants, the overwhelming contribution to the free energy of adsorption comes from the excluded area interactions, and that the short-range interactions between the surface aggregates play only a negligible role.

Surface Functionalization and Stabilization of Mesoporous Silica Spheres by Silanization and Their Adsorption Characteristics

Siliceous MCM-48 spheres are tailored with silane coupling agents, n-octyldimethylchlorosilane (C8) and (3-aminopropyl)triethoxysilane (APTS). The changes in adsorption features of water and acetaldehyde with the modifications are elucidated by aid of adsorption calorimetry. The MCM-48 surface exhibits a hydrophobic feature after the C8 treatment, and the hydrophobicity is retained by repetition of water sorption, although the surface of crude MCM-48 easily changes to hydrophilic by water adsorption. The APTS treatment drastically increases an adsorptivity of acetaldehyde vapor. The adsorbed amount of acetaldehyde is saturated at ca. 5 mmol/g for crude MCM-48; however, that for the APTS-treated sample is three times higher at equilibrium pressure (Pe) of ca. 6 kPa and monotonically increases at Pe > 6 kPa. Differential heats of adsorption during the mesopore filling of acetaldehyde are 53−70 kJ/mol, which is much higher than the heat of liquefaction (27.2 kJ/mol) of acetaldehyde, suggesting that chemical reactions of adsorbed acetaldehyde take place on the surface.

Adsorption of n-Alkanes on Plasma-Oxidized High-Strength Carbon Fibers

The objective of this work was to characterize the degree of heterogeneity brought about by oxygen plasma treatment of carbon fibers by studying its effects on the adsorption of n-alkanes. Untreated and unsized high-strength carbon fibers were subjected to oxygen plasma treatments with different degrees of severity. A sample of the same material oxidized following a standard industrial method was also studied for comparison. Adsorption of C5–C10n-alkanes at 303–353 K was measured by inverse gas chromatography (IGC). Elution peaks were symmetrical for the fresh and industrially oxidized samples; however, a large extent of asymmetry was observed for the plasma-treated fibers. Differences in surface heterogeneity were quantified in terms of several adsorption thermodynamic magnitudes. Differential heats of adsorption exhibited values similar to those corresponding to the probe–basal plane interaction. The dispersive component of the surface tension of the solids increased clearly upon plasma oxidation, the increase being systematic according to the severity of plasma treatment. It can be concluded that plasma oxidation generates high-surface-energy sites responsible for trapping of n-alkane molecules, this effect being more marked as the chain length increases. The possibility of this effect being associated to creation of micropores was ruled out on the basis of volumetric CO2 adsorption experiments and IGC measurements at finite dilution. Scanning tunneling microscopy observations allowed us to establish a possible connection between fiber surface nanostructure and IGC results. The sites accessible to n-alkane molecules in the industrially oxidized sample seem to be highly disordered, thus leading to a weaker interaction with the adsorbate.

Preparation of a Water-Resistant Siliceous MCM-41 Sample, through Improvement of Crystallinity, and Its Prominent Adsorption Features

We have prepared MCM-41 samples having good crystallinity and possessing a small amount of surface hydroxyl groups (0.48 OH nm-2) through extensive aging of a mixed solution of silica and surfactant in the temperature range from 308 to 413 K. The MCM-41 sample thus prepared exhibits excellent resistance to water, and its structure survives even after treating it in boiling water for more than 1 week. The surface and acidic properties of this sample were examined by X-ray diffraction, 29Si magic-angle spinning NMR, adsorption isotherms of dinitrogen and water molecules, and Fourier transform infrared measurements to determine the factors governing an adsorption feature and affecting the acidic property of the MCM-41 samples. The synthesized sample gives a sharp band centered at 3745 cm-1 tailing toward the lower wavenumber side. This band was deconvoluted into three components (3749, 3736, and 3715 cm-1), which were assigned to the stretching vibrations of the respective OH groups: a free OH (silanol), a geminal OH, and a terminal OH group of the hydrogen-bonded species. The data obtained by utilizing CO as a probe molecule provide useful information on the acidic nature of the surface OH groups. A linear relationship between ΔνOH (shift in wavenumber of OH stretching vibration) and νCO (stretching vibration of adsorbed CO) has been found to hold in the present system. It is concluded that the newly formed OH groups, which give strong IR bands at 3736 and 3715 cm-1, after water vapor and boiling water treatments, are responsible for the sites that are generally recognized as the strongly acidic sites existing in MCM-41, compared with the acidity of free silanol groups. The freshly prepared sample shows characteristic behavior in the differential heat of adsorption of water, qdiff, due to lateral interaction of the adsorbed water molecules. This is a different behavior from water adsorbed on the hydrated and boiled samples, indicating the homogeneous nature of the surface of the as-prepared sample for adsorbing water molecules. All properties of such materials should be discussed by taking account of the types of surface OH groups; the surface of MCM-41 having a smaller number of acid sites exhibits a prominent feature for water adsorption. From our results, it was concluded that the surface crystallinity of the sample plays a pivotal role in protecting the sample against attack by water and also in the adsorption behavior of adsorbates, such as water and CO molecules.

MOLECULAR SIEVE EFFECT OF SLIT-SHAPED GRAPHITIC NANOSPACE STUDIED BY GRAND CANONICAL MONTE CARLO SIMULATION

The differential heat of adsorption of Ar, O 2 , and N 2 on slit-shaped graphitic nanospace was calculated by grand canonical Monte Carlo (GCMC) simulation. The average kinetic energy of the adsorbed molecules was obtained by the difference between the potential minimum and the differential heat of adsorption. The slit width which provided minimum kinetic energy for adsorbed molecule was 1.6-1.7 times larger than each molecular diameter. The minimums of average kinetic energy were 0.440, 0.443, and 0.435 kJ mol −1 for Ar, O 2 , and N 2 , respectively. They correspond to lower temperature than ambient temperature. This phenomenon can be called "cooling effect" for the adsorbed molecule, and this effect corresponds to molecular sieve effect.

Molecular Sieve Effect of Slit-Shaped Graphitic Nanospace Studied by Grand Canonical Monte Carlo Simulation

The differential heat of adsorption of Ar, O 2 , and N 2 on slit-shaped graphitic nanospace was calculated by grand canonical Monte Carlo (GCMC) simulation. The average kinetic energy of the adsorbed molecules was obtained by the difference between the potential minimum and the differential heat of adsorption. The slit width which provided minimum kinetic energy for adsorbed molecule was 1.6-1.7 times larger than each molecular diameter. The minimums of average kinetic energy were 0.440, 0.443, and 0.435 kJ mol −1 for Ar, O 2 , and N 2 , respectively. They correspond to lower temperature than ambient temperature. This phenomenon can be called "cooling effect" for the adsorbed molecule, and this effect corresponds to molecular sieve effect.

Acid-base properties of alumina-supported M2O3 (M=B, Ga, In) catalysts

The acid-base properties of γ-Al2O3 and alumina-supported B2O3, Ga2O3 and In2O3 have been determined by microcalorimetry of ammonia and sulfur dioxide adsorption. From the adsorption of NH3, it was found that the addition of B2O3 on alumina leads to an increase of the number of acid sites, while Ga2O3 and In2O3 additives caused a decrease in the acidity of alumina. Using SO2 as a probe molecule to study the basicity, the number of surface basic sites on alumina was found to be strongly decreased by the addition of boron oxide, while it was only slightly affected by the addition of gallium oxide and decreased by the addition of indium oxide. The differential heats of adsorption are discussed as a function of the coverage by the probe molecules. The electronic properties of the oxides are examined in order to explain the acid-base properties of the supported oxides.