Microkinetic modeling of CO TPD spectra using coverage dependent microcalorimetric heats of adsorption

CO adsorption on the ternary methanol synthesis Cu/ZnO/Al2O3 catalyst was studied in detail by means of adsorption microcalorimetry and flow temperature-programmed desorption (TPD). Based on these experimental data, we established a microkinetic analysis method, which provides information about the adsorption kinetics of CO on the catalyst surface. Experimentally derived microcalorimetric heats of adsorption were applied in a microkinetic model to simulate TPD curves with varying initial coverage. Two approaches were used: an integral approach based on evaluation of the integral heats of adsorption which predicts the experimental TPD curves roughly and provides first approximations for the preexponential factors. The second, more detailed approach was based on the simulation of the adsorption isotherm taking the experimentally determined coverage-dependence of the heat of adsorption into account. This approach led to a significantly improved agreement between experimental and simulated TPD curves. Moreover, it was possible to derive the standard entropy of adsorption. The general applicability of our approaches is demonstrated by analyzing the CO TPD and microcalorimetry data obtained with a binary ZnO-free Cu/Al2O3 catalyst.     

Study of Water Adsorption on Activated Carbons with Different Degrees of Surface Oxidation

A carbon of wood origin was oxidized with different oxidizing agents (nitric acid, hydrogen peroxide, and ammonium persulfate). The microstructural properties of the starting material and the oxidized samples were characterized using sorption of nitrogen. The surface acidity was determined using Boehm titration and potentiometric titration. The changes in the surface chemistry were also studied by diffuse reflectance FTIR. Water adsorption isotherms were measured at three different temperatures close to ambient (relative pressure from 0.001 to 0.3). From the isotherms the heats of adsorption were calculated using a virial equation. The results indicated that the isosteric heats of water adsorption are affected by the surface heterogeneity only at low surface coverage. In all cases the limiting heat of adsorption was equal to the heat of water condensation (45 kJ/mol).     

Adsorption of gases in carbon nanotubes: are defect interstitial sites important?

Molecular simulations are used to shed light on an ongoing controversy over where gases adsorb on single walled carbon nanotube bundles. We have performed simulations using models of carbon nanotube bundles composed of tubes of all the same diameter (homogeneous) and tubes of different diameters (heterogeneous). Simulation data are compared with experimental data in an effort to identify the best model for describing experimental data. Adsorption isotherms, isosteric heats of adsorption, and specific surface areas have been computed for Ar, CH 4, and Xe on closed, open, and partially opened homogeneous and heterogeneous nanotube bundles. Experimental data from nanotubes prepared from two different methods, electric arc and HiPco, were examined. Experimental adsorption isotherms and isosteric heats for nanotubes prepared by the electric arc method are in best agreement with simulations for heterogeneous bundles of closed nanotubes. Models including adsorption in defect interstitial channels are required to achieve good agreement with experiments. Experimental isosteric heats and specific surface areas on HiPco nanotubes are best described by a model consisting of heterogeneous bundles with approximately 11% of the nanotubes opened.     

Adsorption of normal pentane on the surface of rutile. Experimental results and simulations

Adsorption isotherms and differential heats of normal pentane adsorption on microcrystalline rutile were measured at 303 K. The heat of adsorption of n-pentane on rutile at zero occupancy is 64 kJ/mol. The differential heats have three descending segments, corresponding to the adsorption of n-pentane on three types of surfaces. At low coverage (first segment), the adsorption is restricted to the rows A of the (110) faces along the 5-fold coordinatively unsaturated (cus) Ti(4+) ions with differential heat showing a linear decrease with increasing occupancy. The second segment is attributed to bonding with atoms of the rows along the remaining faces exposed, (101) and (100). The third segment is related to a multilayer adsorption. The mean molar adsorption entropy of n-pentane is ca. -25 J/mol K less than the entropy of the bulk liquid, thus revealing a hindered state of motion of the n-pentane molecules on the surface of rutile. Simulations of the adsorption of n-pentane on the three most abundant crystallographic faces of rutile were also performed. The adsorption isotherm obtained from the combination of each face's isotherm weighted by the respective abundance was found to be in a good agreement with the experimental data. A structural characterization of n-pentane near the surface was also conducted, and it was found that the substrate, especially for the (110) face, strongly perturbs the distribution of n-pentane conformations, compared to those found for the gas phase. Adsorbed molecules are predominantly oriented with their long axes and their backbone zigzag planes parallel to the surface and are also characterized by fewer gauche conformations than observed in the bulk phase.     

Effect of model potential of adsorptive bond on the thermodynamic properties of adsorbed CO molecules on Ni(111) surface

The effect of anharmonicity on the adsorption of CO molecules on the Ni(111) surface has been investigated. The DFT calculations are used to obtain the effective adsorption potential of the CO molecule on the Ni(111) surface. First, using an appropriate slab model, the geometry of adsorption system corresponding to hcp, fcc, bridge, and on-top sites with p(2 x 2) arrangement and coverage of 0.25 ML is optimized by the DFT calculations using a plane wave basis set and ultrasoft pseudopotentials; this gives the hcp site as the most stable site with De = 185 kJ/mol, for which the equilibrium distance of CO from the surface and C-O bond length on the surface are found to be 1.31 and 1.192 A, respectively. Then, the potential function of adsorption versus adsorptive bond distance was plotted, which is significantly different from that of a harmonic oscillator, i.e., the anharmonicity for the adsorptive bond is significant. Also the harmonic and anharmonic shifts of vibrational frequencies of adsorptive and C-O bonds are calculated to be -22.6 and 7.8 cm(-1), respectively. Hence, two potential models are selected for which their Schr?dinger equations are solved analytically, namely the hard repulsion-soft attraction (HS) and Morse potential (MP) models. The adsorption isotherms, internal energy, isochoric heat capacity, and entropy of adsorbed CO molecules have been calculated for the mentioned model potentials and compared with those of the harmonic oscillator (H). As a result, the adsorption isotherms are not considerably sensitive to the model potential. The anharmonicity of CO-Ni bond, which is included in HS and MP models, gives an average deviation in pressure as much as 1.4% for HS and 5.8% for MP, compared to 6.1% for the H model. However, isochoric heat capacity and entropy depend on the model potential significantly, and the differences may be as high as 69% and 55% for isochoric heat capacity and entropy, respectively.     

Adsorption of styrene at a binary solution-gas interface

The surface tension isotherms of styrene solutions in 2-propanol-water solvents are obtained experimentally. Isotherms of excess and absolute adsorption at the liquid-gas interfaces are also obtained. A thermodynamic model for the adsorption of styrene from 2-propanol-water binary solvent at the solutiongas interfaces is proposed on the basis of the stoichiometric approach. The isosteric adsorption heats are calculated using the experimental isotherms of surface tension and excess adsorption of styrene from 2-propanol-water binary solvent, and it is shown that the formation of the surface layer occurs according to a mechanism combining the processes of displacement adsorption between molecules of the components of the solvent, styrene, and 2-propanol.     

Sensitivity of PSA process performance to input variables

Mathematical models for pressure swing adsorption (PSA) processes essentially require the simultaneous solutions of mass, heat and momentum balance equations for each step of the process using appropriate boundary conditions for the steps. The key model input variables needed for estimating the separation performance of the process are the multicomponent adsorption equilibria, kinetics and heats of adsorption for the system of interest. A very detailed model of an adiabatic Skarstrom PSA cycle for production of high purity methane from a ethylene-methane bulk mixture is developed to study the sensitivity of the process performance to the input variables. The adsorption equilibria are described by the heterogeneous Toth model which accounts for variations of isosteric heats of adsorption of the components with adsorbate loading. A linear driving force model is used to describe the kinetics. The study shows that small errors in the heats of adsorption of the components can severely alter the overall performance of the process (methane recovery and productivity). The adsorptive mass transfer coefficients of the components also must be known fairly accurately in order to obtain precise separation performance.     

Modelling adsorbent heterogeneity for adsorption from binary liquid mixtures

It is demonstrated that the detailed structure of the surface energy or selectivity distribution function is not critical to obtaining adequate analytical expressions for surface excess isotherms for adsorption from binary liquid mixtures on heterogeneous adsorbents. The gamma and the uniform selectivity distribution functions, which are very different in form, were successfully used in conjunction with the monolayer-pore filling model for adsorption on a homogeneous site to describe adsorption of various binary liquid mixtures on silica gel. Both models described the salient features of the surface characteristics of the silica gel.     

Effects of pore structure and surface chemical characteristics on the adsorption of organic vapors on titanate nanotubes

Effects of pore structure and surface chemical characteristics of titanate nanotubes (TNTs) on their adsorptive removal of organic vapors were investigated. TNTs were prepared via a hydrothermal treatment of TiO₂ powders in a 10 M NaOH solution at 150?°C for 24?h, and subsequently washed with HCl aqueous solution of different concentrations. Effects of acid washing process (or the sodium content) on the microstructures and surface chemical characteristics of TNTs were characterized with nitrogen adsorption-desorption isotherms, FTIR, and water vapor adsorption isotherms. For the adsorption experiments, gravimetric techniques were employed to determine the adsorption capacities of TNTs for four organic vapors with similar heats of vaporization (i.e., comparable heats of adsorption) but varying dipole moments and structures, including n-hexane, cyclohexane, toluene, and methyl ethyl ketone (MEK), at isothermal conditions of 20 and 25?°C. The experimental data were correlated by well-known vapor phase models including BET and GAB models. Isosteric heats of adsorption were calculated and heat curves were established. Equilibrium isotherms of organic vapors on TNTs were type II, characterizing vapor condensation to form multilayers. The specific surface area (and pore volume) and hydrophilicity of TNTs were the dominating factors for the determination of their organic vapors adsorption capacity. The GAB isotherm equation fitted the experimental data more closely than the BET equation. The heats of adsorption showed that the adsorption of organic vapors on TNTs was primarily due to physical forces and adsorbates with larger polarity might induce a stronger interaction with TNTs.     

The thermodynamic characteristics of adsorption of hydrogen,sodium, and potassium chlorides at the aqueous solution-gas interface

Thermodynamic models of the adsorption of ions at the interphase boundary between a solution of a 1,1-electrolyte and a gas are suggested. The experimental surface tension isotherms and the isotherms of excess adsorption of hydrogen, sodium, and potassium chlorides from aqueous solutions were used to show that the formation of the surface layer followed both the mechanism of coadsorption of the anion and cation and the mechanism of predominant adsorption of one of the ions. The calculated total adsorption isotherms were used to obtain the dependences of the heats and entropies of adsorption on the amount of the ion adsorbed. The results are discussed in terms of the solvation and desolvation of electrolyte ions in bulk solution and at liquid-vapor interfaces.     

Argon and nitrogen adsorption in disordered nanoporous carbons: simulation and experiment

We report experimental measurements of the isosteric heats of adsorption for argon and nitrogen in two microporous saccharose-based carbons, using a Tian-Calvet microcalorimeter. These data are used to test recently developed molecular models of these carbons, obtained by a constrained reverse Monte Carlo method. Grand canonical Monte Carlo simulation is used to calculate the adsorption isotherms and isosteric heats for these systems, and the results for the latter are compared to the experimental data. For argon, excellent quantitative agreement is obtained over the entire range of pore filling. In the case of nitrogen, very good agreement is obtained over the range of coverage 0.25 < or = gamma/gamma 0 < or = 0.85, but discrepancies are observed at lower and higher coverages. The discrepancy at low coverage may be due to the presence of oxygenated groups on the pore surfaces, which are not taken into account in the model. The differences at high coverage are believed to arise from the presence of a few mesopores, which again are not included in the model. Pair correlation functions (argon-carbon and argon-argon) are determined from the simulations and are discussed as a function of pore filling. Snapshots of the simulations are presented and provide a picture of the pore filling process.     

Sorption of Benzene in CoAlPO4-5 Molecular Sieves

The sorption isotherms of benzene in CoAlPO₄-5 at temperatures of 323–399 K were measured by the gravimetric technique. The sorption capacity and isosteric heat were estimated and interpreted on the basis of the pore topologies of the molecular sieves. The isotherms followed the typical Type-I shape, and could be well fitted with Langmuir and Dubinin-Radushkevich equations. The initial heats of sorption at zero coverage could be well explained by the surface curvature model. The isosteric heats of sorption varied with the coverage, which may suggest a shift of molecular packing manner in the sorbed phase.     

A single-site model for divalent transition and heavy metal adsorption over a range of metal concentrations

Metal adsorption data over a range of surface coverages typically are characterized by curvilinear metal adsorption isotherms. These isotherms generally have a slope of 1 at low surface coverage and a shallower slope at higher surface coverages. The curvature of metal adsorption isotherms with increasing surface coverage is frequently interpreted in terms of sequential adsorption onto different types of surface sites, multinuclear surface complexation, or nonideality of metal adsorption. We demonstrate that the curvature of metal adsorption isotherms can also be attributed to changes in surface charge and potential that depend on the predominant type of metal surface complex. A single-site extended triple-layer model is used to reinterpret previously studied metal adsorption isotherms and pH edges for a wide variety of metals (Cd2+, Co2+, Cu2+, Pb2+, and Zn2+) and solids (goethite, hydrous ferric oxide, corundum, and magnetite) in different electrolyte solutions (NaNO3 and NaClO4). Only metal adsorption on ferrihydrite at very low surface coverages is not consistent with the single-site triple-layer model. This discrepancy might be explained if ferrihydrite is in fact not a single phase but a mixture of two or more phases. Metal surface coverages ranging from 10(-4) to 10.2 mmol/m2 on the other minerals can be accounted for with a single-site extended triple-layer model if appropriate metal adsorption reactions are chosen. In addition, several examples suggest that, within the context of the model, surface complexation schemes can be established that describe metal adsorption over both a wide range of surface coverage and a wide range of ionic strength.     

Adsorption of Carbon Dioxide on Activated Carbon

The adsorption of CO2 on a raw activated carbon A and three modified activated carbon samples B, C, and D at temperatures ranging from 303 to 333 K and the thermodynamics of adsorption have been investigated using a vacuum adsorption apparatus in order to obtain more information about the effect of CO2 on removal of organic sulfur-containing compounds in industrial gases. The active ingredients impregnated in the carbon samples show significant influence on the adsorption for CO2 and its volumes adsorbed on modified carbon samples B, C, and D are all larger than that on the raw carbon sample A. On the other hand, the physical parameters such as surface area, pore volume, and micropore volume of carbon samples show no influence on the adsorbed amount of CO2. The Dubinin-Radushkevich (D-R) equation was the best model for fitting the adsorption data on carbon samples A and B, while the Preundlich equation was the best fit for the adsorption on carbon samples C and D. The isosteric heats of adsorption on carbon samples A, B, C, and D derived from the adsorption isotherms using the Clapeyron equation decreased slightly increasing surface loading. The heat of adsorption lay between 10.5 and 28.4 kJ/mol, with the carbon sample D having the highest value at all surface coverages that were studied. The observed entropy change associated with the adsorption for the carbon samples A, B, and C (above the surface coverage of 7 ml/g) was lower than the theoretical value for mobile adsorption. However, it was higher than the theoretical value for mobile adsorption but lower than the theoretical value for localized adsorption for carbon sample D.     

Calorimetric determination of the acidic character of amorphous and crystalline aluminosilicates

Aluminosilicates can present different structures such as crystalline true zeolite molecular sieves or amorphous silica–aluminas. With a large surface area available, both can be involved as catalysts, adsorbents or catalyst supports, and the determination of their surface acidic properties is an important parameter in the study of such materials.

The number, strength and strength distribution of the acidic sites were determined using microcalorimetry linked to a volumetric line. Ammonia was used as a basic probe molecule. The adsorption temperatures ranged from 353 K up to 473 K. The samples consisted of two amorphous silica–aluminas (Si/Al ≈ 6.5) and three microporous zeolites H-β, H-ZSM-5 and H-MCM-22 with similar Si/Al ratios (Si/Al ≈ 13).

The differential heats of ammonia adsorption versus coverage and the corresponding isotherms are given. The H-ZSM-5, H-MCM-22, H-β samples display a plateau of constant adsorption heats near 150 kJ mol⁻¹, while the silica–alumina samples present continuously decreasing heats from 150 kJ mol⁻¹ at zero coverage to 40 kJ mol⁻¹ at high coverage, due to their surface heterogeneity. For amorphous silica–aluminas, the number of acid sites is dependent of the aluminum distribution at the surface.

The differences observed in the adsorption behavior of ammonia over the three zeolites arise from differences in their morphology, i.e. the total free volumes, pore geometries and electric field gradients at the adsorption sites. The adsorption isosteres have also been calculated from the adsorption isotherms, and the isosteric heats of adsorption have been compared with the heats measured by calorimetry.     

Heterogeneity of multiwalled carbon nanotubes based on adsorption of simple aromatic compounds from aqueous solutions

The surface heterogeneity of multiwalled carbon nanotubes (MWCNTs) is studied on the basis of adsorption isotherms from dilute aqueous phenol and dopamine solutions at various pH values. The generalized Langmuir–Freundlich isotherm equation was applied to investigate the cooperative effect of the surface heterogeneity and the lateral interactions between the adsorbates. The theoretical isosteric heats of adsorption were obtained assuming that the heat of adsorption profile reveals both the energetic heterogeneity of the adsorption system and the strength of the interactions between the neighboring molecules. The adsorption energy distribution functions were calculated by using algorithm based on a regularization method. The great advantage of this method is that the regularization makes no assumption about the shape of the obtained energy distribution functions. Analysis of the isosteric heats of adsorption for MWCNTs showed that the influence of the surface heterogeneity is much stronger than the role of the lateral interactions. The most typical adsorption heat is 20–22 kJ/mol for both phenol and dopamine. After purification of nanotubes, heat value for phenol dropped to 16–17 kJ/mol. The range of the energy distribution is only slightly influenced by the surface chemistry of the nanotubes in the aqueous conditions.     

Adsorption of chlorobenzene and benzene on γ-Al2O3/atl>

Adsorption of chlorobenzene and benzene on -Al₂O₃ was investigated in the 413--572 K temperature region at an adsorbate partial pressure ranging from 2 to 1000 Pa. The adsorption isotherms were measured and the isosteric heats and the entropy characteristics of adsorption were determined. The experimentally found and theoretically calculated entropy changes upon adsorption were compared. The mobility of the molecules of both adsorbates in the adsorption layer was limited with respect to that predicted by the ideal two-dimensional gas model. The mechanism of adsorption of benzene and chlorobenzene is discussed.     

Determination of the average heat and characteristic energy from the adsorption isotherm

The isotherms of the total content, isosteric and average heats of adsorption, as well as characteristic energies of adsorption were determined from the isotherms of excess adsorption of carbon dioxide on six different carbon adsorbents at temperature ranging from 293 to 423 К at pressures up to 6 MPa. The average isosteric heats are in agreement with the average heats of adsorption, which were determined from the equation relating the heat of adsorption with the characteristic energy of adsorption.     

Calorimetric study of water adsorption on saponite

The isotherms and differential heats of water vapor adsorption on Na- and Ca-saponite samples are measured. It is shown that the successive stages of hydration of the internal surface of this mineral appear as waves or inflections in the adsorption isotherms and maxima in the dependences of the differential heat on the adsorption value. The compositions of Na⁺ and Ca²⁺ aquacations in single- and double-layer saponite hydrates are determined. The heats of interaction between water molecules and interlayer Na⁺ cations in single-layer saponite hydrates and between Na⁺ cations and the ion exchanger matrix are estimated.     

Heat of Adsorption of Pure Sulfur Hexafluoride on Micro-Mesoporous Adsorbents

The isotherms and the isosteric heats of adsorption of pure SF₆ were measured on two microporous zeolites (NaX and Silicalite), one mesoporous alumina, and two activated carbons (BPL and PCB) at 305 K. The adsorption isotherms were Type I by Brunauer classification. The PCB carbon adsorbed SF₆ most strongly and the alumina adsorbed SF₆ most weakly. The adsorption of SF₆ on the other three materials were comparable in the low pressure region despite their drastic differences in the physicochemical properties. The heat of adsorption of SF₆ on the silicalite and the alumina remained practically constant over a large range of coverage. The heat of adsorption of SF₆ increased with increasing adsorbate loading on the NaX zeolite in the high coverage region. The heat of adsorption of SF₆ on the activated carbons decreased with increasing adsorbate loading before leveling off in the high coverage region.