Concentration Of Adsorption Sites Research Articles

A chemo-mechanical model for describing sorption hysteresis in a glassy polyurethane

Hysteretic sorption and desorption of water is observed from 0 to 95% relative humidity and 298–333 K on a glassy polyurethane foam. It is postulated that sorption-induced swelling of the glassy polyurethane increases the concentration of accessible hydrogen-bonding adsorption sites for water. The accessibility of sites is kinetically controlled due to the restricted thermal motions of chains in the glassy polymer, causing a difference in accessible site concentrations during sorption and desorption. This discrepancy leads to hysteresis in the sorbed concentrations of water. A coupled chemo-mechanical model relating volumetric strain, adsorption site concentration, and sorbed water concentration is employed to describe water sorption hysteresis in the glassy polyurethane. This model not only describes the final mass uptake for each relative humidity step, but also captures the dynamics of water uptake, which exhibit diffusion and relaxation rate-controlled regimes.

Protein corona of SiO2 nanoparticles with grafted thermoresponsive copolymers: Calorimetric insights on factors affecting entropy vs. enthalpy-driven associations

The mechanism of protein adsorption on various surfaces of tailored functionality is often accessed by isothermal titration calorimetry (ITC), the powerful tool yielding the full set of thermodynamic parameters in one label-free experiment. In this work, the non-ionic polymer brushes with thermo-switchable hydrophilic-hydrophobic balance, based on diethylene glycol methacrylate (DEGMA) and 4-vinyl pyridine (4VP) copolymers, grafted to SiO2 nanoparticles, were titrated with individual blood proteins, their mixture, and diluted human plasma. The concentration of adsorption sites was calculated from the sizes of protein molecules, assuming the formation of a monolayer. For the titrations with the protein mixture and diluted plasma, the concentration of titrant was an estimate, limiting the accuracy of thermodynamic parameters. The particles with grafted polymer brushes showed negative ξ-potentials similar to the blood proteins and their interactions occurred against the Coulomb forces. Two cases of exothermic hydrophobic and endothermic polar interactions were distinguished. The strength of adsorption, expressed with the affinity constant Ka, was dependent on brush composition and temperature. The adsorption of proteins from blood plasma was always exothermic and, therefore, the dominating hydrophobic interactions were assumed.

Role of silica fume on hydration and strength development of ultra-high performance concrete

Silica fume (SF) is the most commonly used ingredient in ultra-high performance concrete (UHPC) due to its inherited properties, which can effectively improve the rheology and mechanical performance. However, investigations with regard to its filler effect and pozzolanic reaction on UHPC are limited. To assess the contribution from SF, a chemically inert filler of titanium dioxide (TD), with a comparable particle size distribution and specific surface area (per unit volume), was selected as a reference ingredient to eliminate the influence of packing effect. Results showed the addition of SF will accerate the early hydation of UHPC and it is controlled by the combined effect of seeding effect and preferential adsorption. Moreover, the SF’s surface characteristic controlling the concentration of adsorption sites provided for hydration product is the key factor influencing the early hydration rather than surface specific area. Furthermore, the enhancement of compressive strength of UHPC after 3 days is attributed to the pozzolanic reaction of SF and its promoting effect on cement hydration degree. The total contribution is less than 20%, and it will enhances with the decrease of W/C ratio.

Investigation of carbon dioxide photoreduction process in a laboratory-scale photoreactor by computational fluid dynamic and reaction kinetic modeling

The production of solar fuels via the photoreduction of carbon dioxide to methane by titanium oxide is a promising process to control greenhouse gas emissions and provide alternative renewable fuels. Although several reaction mechanisms have been proposed, the detailed steps are still ambiguous, and the limiting factors are not well defined. To improve our understanding of the mechanisms of carbon dioxide photoreduction, a multi-physics model was developed using COMSOL. The novelty of this work is the computational fluid dynamic model combined with the novel carbon dioxide photoreduction intrinsic reaction kinetic model, which was built based on three-steps, namely gas adsorption, surface reactions and desorption, while the ultraviolet light intensity distribution was simulated by the Gaussian distribution model and Beer-Lambert model. The carbon dioxide photoreduction process conducted in a laboratory-scale reactor under different carbon dioxide and water moisture partial pressures was then modeled based on the intrinsic kinetic model. It was found that the simulation results for methane, carbon monoxide and hydrogen yield match the experiments in the concentration range of 10−4 mol·m−3 at the low carbon dioxide and water moisture partial pressure. Finally, the factors of adsorption site concentration, adsorption equilibrium constant, ultraviolet light intensity and temperature were evaluated.

Copper and lead adsorption as influenced by organic matter in soils from a tropical toposequence with different chemical and mineralogical attributes

ABSTRACTThe aim of this work was to investigate the influence of the organic matter on copper and lead adsorption in soils with different physiochemical and mineralogical attributes. Suspensions (pH 6.0) of a Latosol, a Neosol and a Vertisol containing increasing amounts of copper or lead were used to obtain sorption isotherms while identical experiments were carried out with the soils previously treated with H2O2 to remove organic matter (OM). For the undisturbed soils, L-type and H-type isotherms were predominant for copper and lead respectively, showing that lead interacts more strongly with adsorption sites. For both metals, the non-linear Freundlich adsorption model revealed higher concentration of adsorption sites for Vertisol due to 2:1 clays. For the OM-removed soils, C-type isotherms were observed for copper with the permanence of less stable and more homogeneous sites. For this metal, a high correlation (R2 = 0.997) was observed between the decrease of adsorbent sites and the loss of organic carbon, evidencing the central role of organic matter on copper complexation, while lead may be able to interact efficiently with both organic matter and soil minerals.

Understanding Ni Promotion of MoS2/γ‐Al2O3 and its Implications for the Hydrogenation of Phenanthrene

AbstractThe chemical composition and structure of NiMo sulfides supported on γ‐Al2O3 and its properties for hydrogenation of polyaromatic compounds is explored. The presence of Ni favors the formation of disperse octahedrally coordinated Mo in the oxide precursors and facilitates its reduction during sulfidation. This decreases the particle size of MoS2 (measured by transmission electron microscopy) and increases the concentration of active sites up to a Ni/(Mo+Ni) atomic ratio of 0.33. At higher Ni loadings, the size of the MoS2 did not decrease further, although the concentration of adsorption sites and accessible Ni atoms decreased. This is attributed to the formation of NiSx clusters at the edges of MoS2. Nickel also interacts with the support, forming separated NiSx clusters, and is partially incorporated into the γ‐Al2O3, forming a Ni‐spinel. The hydrogenation of phenanthrene follows two pathways; by adding one or two H2 molecules, 9,10‐dihydrophenanthrene or 1,2,3,4‐tetrahydrophenanthrene are formed as primary products. Only symmetric hydrogenation, leading to 9,10‐dihydrophenanthrene, was observed on unpromoted MoS2/γ‐Al2O3. In contrast, symmetric and deep hydrogenation (leading to 9,10‐dihydrophenanthrene and 1,2,3,4‐tetrahydrophenanthrene, respectively) occur with similar selectivity on Ni‐promoted MoS2/γ‐Al2O3. The rates of both pathways increase linearly with the concentration of Ni atoms in the catalyst. The higher rates for symmetric hydrogenation are attributed to increasing concentrations of reactive species at the surface, and deep hydrogenation is concluded to be catalyzed by Ni at the edge of MoS2 slabs.

The effect of mixed HCl–KCl competitive adsorbate on Pt adsorption and catalytic properties of Pt–Sn/Al2O3 catalysts in propane dehydrogenation

The effect of competitive adsorbate concentration and combination on the adsorption of H2PtCl6 onto γ-Al2O3 in the preparation and performance of PtSnK/γ-Al2O3 catalyst for propane dehydrogenation was investigated. The catalysts were prepared by sequential impregnation of Sn and Pt precursors. The effect of competitor concentration on Pt adsorption was studied by using hydrochloric acid (0.1–0.3M) and the effect of pH was studied by using KCl/HCl mixtures at constant (0.1M) total chloride ion concentration. The catalysts were characterized by nitrogen adsorption/desorption, XRD, XRF, SEM and CO chemisorption. The catalytic performance tests were carried out in a fixed-bed quartz reactor under kinetic controlled condition for proper catalyst screening. It was found that the corrosive competitor HCl could be partially substituted with KCl without appreciable impact on catalyst performance with the advantage of lower acid attack on the support and reduced leaching of the deposited tin. A model based on initial concentration and uptake of the adsorbates was developed to obtain the adsorption parameters. Values of 890μmol/g and 600lit/mol were obtained for adsorption site concentration of the tin-impregnated support and equilibrium constant for Pt adsorption, respectively, for HCl concentration range of 0.1–0.3M.

Grain shape influence on semiconducting metal oxide based gas sensor performance: modeling versus experiment

A model for sensing with semiconducting metal oxide (SMOX)-based gas sensors was developed which takes the effect of the shape of the grains in the sensing layers into account. Its validity is limited to materials in which the grains of the SMOX sensing layer are large enough to have an undepleted bulk region (large grains). This means that in all experimental conditions, the SMOX properties ensure that the influence of surface phenomena is not extended to the whole grain. The model takes the surface chemistry and its impact on the electrical properties of the sensing material into consideration. In this way, it relates the sensor signal--defined as the relative change of the sensor's conductance--directly to the concentration of the target gas and also exhibits meaningful chemical parameters, such as the type of reactive oxygen species, the reaction constants, and the concentration of adsorption sites. The validity of the model is confirmed experimentally by applying it to data gathered by measuring homemade sensors in relevant conditions.

Momentary Equilibrium in Transient Kinetics and Its Application for Estimating the Concentration of Catalytic Sites

We describe the novel concept of momentary equilibrium (ME), a special event during a pulse-response transient experiment in which the non-steady-state rates of adsorption and desorption of a probe molecule are instantaneously balanced. In the absence of other reactions, any system with reversible adsorption will always pass through ME during a pulse-response experiment with effusion. We also suggest a new method for measuring the concentration of adsorption sites on heterogeneous catalysts and the corresponding equilibrium constant by observing momentary equilibrium in thin-zone temporal analysis of products (TZTAP) pulse-response experiments with modulated pulse intensity. The suggested method employs reversible adsorption of probe molecules, contrary to traditional methods of counting adsorption sites which utilize irreversible reactions.

Effect of Grain Size on Uranium(VI) Surface Complexation Kinetics and Adsorption Additivity

The contribution of variable grain sizes to uranium adsorption/desorption was studied using a sediment from the US DOE Hanford site. The sediment was wet sieved into four size fractions: coarse sand (1-2 mm), medium sand (0.2-1 mm), fine sand (0.053-0.2 mm), and clay/silt fraction (<0.053 mm). For each size fraction and their composite (sediment), batch and flow-cell experiments were performed to determine uranium adsorption isotherms and kinetic uranium adsorption and subsequent desorption. The results showed that uranium adsorption isotherms and adsorption/desorption kinetics were size specific, reflecting the effects of size-specific adsorption site concentration and kinetic rate constants. The larger-size fraction had a larger mass percentage in the sediment but with a smaller adsorption site concentration and generally a slower uranium adsorption/desorption rate. The same equilibrium surface complexation reaction and reaction constant could describe uranium adsorption isotherms for all size fractions and the composite after accounting for the effect of adsorption site concentration. Mass-weighted, linear additivity was observed for both uranium adsorption isotherms and adsorption/desorption kinetics in the composite. One important implication of this study is that grain-size distribution may be used to estimate uranium adsorption site and adsorption/desorption kinetic rates in heterogeneous sediments from a common location.

On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO2 Sorption

To increase the CO2 capture capacity of hydrotalcites, the influence of alkali (K, Na) metal carbonate loading of activated supported and unsupported hydrotalcites (HTact) on their CO2 capture properties was investigated. The alkali-loaded supported hydrotalcites adsorb at 523 K, depending on the alkali metal (Na or K) and the preparation method, 1.7−2.2 mmol CO2 g−1HT which exceeds the capacity of unloaded supported HT (1.3 mmol CO2 g−1HT) and K-loaded unsupported HT (∼0.3 mmol.g−1). The key for the increase in capacity in alkali-loaded HT is a close contact at least at a mesoscopic level between HT and alkali metal carbonate. The alkali metal carbonate could be successfully introduced either by impregnation of a K2CO3 solution on as-synthesized HT or by leaving residual K (or Na), from the synthesis, in the final material. The latter method is advocated since it omits a washing step after precipitation. The increase in capture capacity for alkali loaded HTs points to a higher concentration of defects (low-coordination oxygen sites) on the surface of the activated alkali loaded HTs compared to the unloaded HT. We propose that the higher concentration of adsorption sites is caused by the presence of Na+/K+ on the surface of Mg(Al)Ox.

Adsorption properties of lamellar silica

Isotherms and heats of adsorption of water, n-heptane, trimethylamine, and methanol (at 303 K) vapors and isotherms of adsorption of nitrogen (at 77 K) on the lamellar silica prepared by removing metals from natural mineral vermiculate were measured. The surface concentration of adsorption sites and their energetic characteristics were found to be similar to those of ordinary silicas. The distinction of the lamellar silica from ordinary silicas manifests itself through extended desorption branches, a feature that makes it possible to classify the lamellar silica under study as a limitedly swellable sorbent.

Application of the Statistical Rate Theory of Interfacial Transport to Interpret the Relaxation Time of Proton Adsorption from Solution onto Oxides

A new way to analyze the rate of hydrogen ion adsorption from solution onto oxides was described. The statistical rate theory of interfacial transport (SRT) was applied to interpret relaxation time of ion adsorption. The new procedure for determination of rate constants of surface reaction was compared with the classical theory of activated adsorption and desorption (TAAD). It was found that for adsorption of uncharged species, both models give the same result, but for ion adsorption, their predictions differ considerably. Influence of surface potential and total concentration of adsorption sites on calculated rate constants was also discussed.

The Oxygen Reduction Kinetics of Mixed Conducting Electrodes: Model Considerations and Experiments on La0.6Sr0.4Co0.8Fe0.2O3 Microelectrodes

A model is presented that yields current-voltage relations for electron transfer reactions at the electrode/gas interface of mixed conducting electrodes. The approach introduced appears necessary since the relevant electrostatic potential step at the gas/solid interface is not equivalent to the applied overpotential. As a consequence, surprising features such as an additional factor of two in the exponents of the current-voltage (I-V) relation or limiting currents even if charge transfer is the rate determining step are deduced depending on adsorption site concentration, type and equilibrium concentration of adsorbed surface species, etc. Experiments with well-defined La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 microelectrodes on yttria-stabilized zirconia electrolytes were performed to investigate the mechanism of the oxygen reduction reaction. A comparison of the measured I-V characteristics with the model calculations suggests that the electron charge transfer to an adsorbed O ad atom is the rate limiting step of the corresponding reduction reaction. Further measurements, however, are required to verify this conclusion.

Does the Dubinin–Serpinsky theory adequately describe water adsorption on adsorbents with high-energy centers?

The paper is an attempt to explain the mechanism of water adsorption on carbonaceous adsorbents with high-energy centers (surface or cation-exchange groups). The equations formulated previously by Dubinin et al. and Barton et al. are analyzed. Thus, four types of empirical factors defining the decrease in adsorption site concentration are taken into account. The isotherms of water adsorption measured on two microporous activated carbons containing various densities of hydrophilic sites are described by the above-mentioned theoretical isotherm equations. It is shown that none of these models provides an adequate description of the experimental data, especially at low relative pressures. This leads to the conclusion that with regard to the Dubinin and Serpinsky theory, the chemisorption of water at very high-energy adsorption sites needs to be taken into account. In view of the above, a new theoretical relationship is proposed which includes a correct description of the decreasing number of active sites. As a consequence, a good correlation between the new theoretical model and experimental data is observed over the whole range of relative pressures.

Effect of ionic strength and hydrogen peroxide on the photocatalytic degradation of 4-chlorobenzoic acid in water

Assisted-photocatalytic degradation of the pesticide precursor 4-chlorobenzoic acid (4-CBA) in water was investigated using different TiO 2 powders and near-UV radiation. Experiments were performed at pH=3.0 and at different TiO 2 loadings, solution ionic strengths, and concentrations of hydrogen peroxide as an oxidant additive. Dark adsorption equilibrium studies revealed that surface area, ionic strength, and catalyst heterogeneity influenced the adsorption capacity and adsorption mechanism. This was attributed to the type and concentration of adsorption sites and the electrostatic interactions in the solution-semiconductor interface. However, the rates of the photocatalytic reactions were not significantly affected by an increase in the ionic strength by a factor of 50. On the other hand, the reaction rate was a strong function of catalyst crystallinity and loading. This suggested that the reactions were mostly controlled by the rate of formation of the oxidizing species rather than the extent of electric double layer (EDL) compression. Addition of hydrogen peroxide up to 248 mg/l resulted in an increase of the reaction rates with a corresponding increase in photonic efficiency by ≈20%. Above this concentration, hydrogen peroxide either did not enhance or caused a significant inhibition of the mineralization rates.

Langmuir‐hinshelwood model of soil phosphorus kinetics1

A mathematical model is needed to relate the dynamics of soil phosphorus (P) chemistry in a batch reactor to the soil/solution ratio and to initial P in the reactor. The Langmuir‐Hinshelwood model appears to describe this system quite well. According to this model reversible adsorption of P from solution to the colloidal surface (Langmuir component) is followed by an irreversible reaction of the surface species (Hinshelwood component). The system is an example of heterogeneous catalysis. Adsorption follows 2nd order kinetics related to solution P concentration and concentration of available surface sites for adsorption. Desorption and reaction are assumed to follow 1st order kinetics. The system is described by three simultaneous equations, two of which are 1st order nonlinear ordinary differential equations. Numerical integration is by the Euler finite difference method. Data from the literature are used to calibrate the model and to demonstrate its characteristics for a sandy soil. Rapid drop in solution P concentration is followed by a gradual decline. The first step in calibration is to assume quasi‐equilibrium in the early phase (analogous to that in enzyme kinetics), which leads to values for the total concentration of adsorption sites, So, and the Langmuir coefficient, potassium (K). The second step then estimates the adsorption coefficient, ka, and the reaction coefficient, kr. Full simulation of the transient process over time requires great care in size of time step, t, to maintain stability and accuracy in the procedure. Analysis shows a linear correlation between total surface sites, So, and soil/solution ratio, M, as expected.

The Influence of Thermal Treatment of Silica Gel on Surface–Molecule Interactions: I. Finite Coverage Region

Adsorption isotherms ofn-octane, benzene, toluene, and chloroform were determined by a gas chromatographic method on the surface of silica gel heated at 200, 400, and 800°C. From the isotherms, the film pressure of the adsorbates was determined, and then the components of silica surface free energy were calculated. It was determined that the thermal treatment provided almost no changes in nonspecific interactions and a considerable decrease in acid–base ones; however, electron–acceptor interactions were always stronger than electron–donor ones. Based on these data, the surface concentration of adsorption sites was estimated. The adsorption energy distribution function (χ) was also determined for silica gels according to the asymptotically corrected condensation approximation method. The influence of the thermal treatment on distribution of adsorption sites was demonstrated for adsorbates possessing various types of interactions.

Adsorption of Tungsten on Alumina from Sodium Tungstate Solutions. Estimation of Equilibrium and Kinetic Parameters

The tungsten adsorption isotherm from aqueous solutions of sodium tungstate on to alumina at 20°C has been studied. The isotherm shape was in agreement with that predicted by the classic Langmuir equation. This model was therefore fitted to the experimental data in order to determine the adsorption equilibrium constant and the total concentration of adsorption sites. Kinetic parameters of adsorption were also determined. To do so, a theoretical model that predicts the tungsten concentration profiles in impregnated thin layers was fitted to the experimental profiles. The diffuse reflectance spectra of dried samples used for isotherm determination and of thin alumina layers impregnated with tungsten solutions exhibit a band at 215 nm which is accompanied by a shoulder at 250 nm. For samples having a greater tungsten concentration, the shoulder develops and then transforms into a second band. The maximum at the lowest wavelength (first maximum) is assumed to correspond to tungsten in a tetrahedral coordination (monomeric species) while the second maximum is due to distortion in the tetrahedral symmetry caused by strong interactions between the support and the monomeric species.

Sensing mechanism of SnO2 gas sensors

Studies have been made of the gas sensing properties of both steady and unsteady state SnO2 thin film gas sensors in contact with CH4 and H2 in air from 400 to 500° C. The results suggest a new sensing mechanism model for SnO2 semiconductor flammable gas sensors. This model is based on the following points: (i) Sensor conductivity is determined by the concentration of carrier electrons. (ii) Carrier concentration is controlled by surface unsaturated oxygen adsorption site concentration which is decided by the balance between oxygen adsorption and the surface reaction between oxygen adsorbate and flammable gases. (iii) The activation energy of the reaction is changed by the Fermi energy change for any change in sensor conductivity. This model explains all experimental results.