Competitive Adsorption Effects Research Articles

In situ determination of the adsorption characteristics of a zeolite membrane

A methodology based on adsorption-branch porosimetry is described for in situ measurement of the adsorption of condensable gases within the pore structure of inorganic membranes. The method is applied to the study of n-hexane and p-xylene adsorption in a high-silica, MFI zeolite membrane. The results, interpreted in terms of a simple model for competitive adsorption effects on the permeance of a non-adsorbing gas, yield Langmuir adsorption constants and Henry’s law constants for n-hexane and p-xylene that are in excellent agreement with measurements on bulk materials. The method is proposed for the fundamental study of fouling characteristics of inorganic membranes, especially in cases where a true bulk surrogate is not available.

Preparation and gas separation performance of zeolite T membrane

Zeolite T membranes were prepared by hydrothermal synthesis on porous mullite tubes seeded with zeolite T crystals, using milk-like aluminosilicate gel with a molar composition of SiO2∶Al2O3∶Na2O∶K2O∶H2O = 1∶0.05∶0.26∶0.09∶14. A zeolite T crystal layer of about 20 µm in thickness was formed on the outer surface of the support after the synthesis at 373 K for 30 h. Single-gas and mixed-gas permeation experiments through zeolite T membranes were carried out by a vacuum method at 303∼473 K using He, H2, CO2, O2, N2, CH4, C2H6 and C3H8 single-component gases and CO2/N2, CO2/CH4 and other CO2/hydrocarbon mixtures, respectively. In single-gas permeation experiments, with increasing kinetic diameter from 0.33 nm for CO2 to 0.43 nm for C3H8, the gas permeance decreased by four orders in magnitude, indicating a kind of molecular sieving behavior for the zeolite T membranes prepared in this study, that is, they had little defects and their permeation behavior was controlled by zeolitic pores of erionite. Permeance of CO2 was much higher than those of N2 and CH4 and the ideal selectivities for CO2/N2 and CO2/CH4 were 31 and 266 at 343 K, respectively. In mixed-gas permeation experiments, zeolite T membranes showed the high selectivities for CO2/N2 and CO2/CH4 pairs of 107 and 400, respectively, at 308 K. The selectivity α decreased with an increase in temperature, but was still in a high level of 20 and 52 for CO2/N2 and CO2/CH4, respectively, even at 473 K. This is due to the synergetic effects of competitive adsorption of CO2 and molecular sieving of zeolitic pores. Because of the increasing effect of single file diffusion, the selectivities for CO2/C2H6 (α = 61) and CO2/C3H8 (α = 17) were rather low.

Effects of competitive carbon monoxide adsorption on the hydrogen response of metal–insulator–semiconductor sensors: the role of metal film morphology

We have investigated the effects of carbon monoxide coadsorption on the performance of hydrogen-sensitive palladium metal–insulator–semiconductor (MIS) sensors, and have found that device performance is highly dependent on the morphology of the metal film. On smooth, continuous films, experiments utilizing H2/CO mixtures show that CO induces a time-delayed increase in response to H2. Based on a kinetic model of CO and hydrogen adsorption on the Pd film, this increase is interpreted as being due to trapping of H atoms at the Pd–SiO2 interface by CO adsorbed on the metal surface. However, MIS sensors fabricated so that a large concentration of voids exist in the metal film respond in a markedly different fashion: on these devices, the addition of CO causes a time-delayed decrease in the sensor response. Possible explanations for this attenuating CO effect are discussed. The experimental and modeling results presented here suggest that sensor behavior in the presence of competitively adsorbing mixtures can be quite complex, with film imperfections playing an important role in determining the response.

Preparation and gas separation properties of zeolite T membrane.

Zeolite T membranes were synthesized on tubular porous mullite tubes by hydrothermal synthesis. The membranes selectively permeated carbon dioxide from CO2/CH4 and CO2/N2 mixtures with high separation performances, which were due to combined effects of molecular sieving and competitive adsorption.

Effect of Competitive Adsorption of Zn2+ on the Flocculation of Lauryl Sulfate Micelles by Al3+

In micellar flocculation of lauryl sulfate micelles with solutions containing Zn2+ and Al3+ cations, the charge ratios polyvalent cation/micellar surfactant may be greater than 1 above [Zn2+] = 0.031 M. This is unlike the case of flocculation of lauryl sulfate micelles with Al3+ or Al3+/Na+ solutions, where the charge ratio Al/sodium dodecyl sulfate in the floc is lower than or nearly equal to 1. Only below [Zn2+] = 0.031 M may the localized adsorption model be used to describe the binding of cations to micelles. The apparent charge inversion may be explained by assuming that a flocculating micelle drags the ions contained within its hydrodynamic radius. A reduction of the Stern and diffuse layer radii under increasing electrolyte concentrations may collapse these within the hydrodynamic radius of the micelle, allowing the hydrodynamic radius to reach even into the bulk solution. If all the solution included within the hydrodynamic radius is dragged into the floc, this would explain the apparent charge in...

Elucidation of the effects of competitive adsorption of Cl −and Br − ions on the initial stages of Pt surface oxidation by means of electrochemical nanogravimetry

At anodically polarized Pt electrodes in aqueous H 2SO 4 solution, Cl − and Br − ions are adsorbed competitively with the oxygen species that is deposited between 0.8 and 1.1 V, RHE, forming the initial stage of anodic oxide-film generation at Pt anodes. The Cl − adsorption leads to selective and progressive blocking of the first 50% of coverage by O species at the Pt surface with increasing Cl − concentration (commencing at 10 −7 M) while relatively little effect is seen on the place-exchanged Pt/O→O/Pt lattice that is formed beyond 1.10 V, except at Cl − concentrations greater than 10 −1 M, where evolution of Cl 2 commences and hence interferes. The states of co-adsorbed Cl and O-containing species, and their relative surface coverages, determine the nature of the electrocatalytic surface on which anodic Cl 2 evolution takes place. How this selective blocking behavior arises has not previously been well understood. In the present paper, complementary applications of the electrochemical quartz crystal nanobalance (EQCN) technique with cyclic voltammetry are brought to bear on this matter and provide new results at high levels of sensitivity and reproducibility which help to elucidate the competitive and selective chemisorption effects of Cl − at Pt anodes. The results obtained with Cl − ions exhibit some peculiarities which comparative experiments on Br − adsorption help to rationalize.

Effect of Ionic Strength and Competitive Adsorption of Na+ on the Flocculation of Lauryl Sulfate Micelles with Al3+

The binding of Al3+ to SDS micelles is best described by means of Guoy−Chapman−Stern isotherms. A method is described to estimate parameters for a Guoy−Chapman−Stern competitive adsorption isotherm for the system Na+/Al3+/SDS from data of the composition of micellar flocculates obtained in conditions of optimum flocculation. The dissociation coefficient KAl3+ is found to be equal to 17.67 if the bound fraction is defined as the ratio of bound Al3+ to SDS ions in the micelle. Experiments performed in the presence of a range of NaCl concentrations between 0 and 0.5 M show that the absence of flocculates in micellar solutions of lauryl sulfate in the presence of high [Al3+] cannot be explained by a drop in the chemical activity of Al3+ at high ionic strengths, although the chemical activity of Al3+ strongly influences the binding of Al3+ to SDS micelles. One effect of increasing [Na+] is to increase the critical micelle concentration of the surfactant in solution, from a value close to that of Al(DS)3 to another close to that of SDS. This reduces the proportion of surfactant in micellar form and susceptible therefore of being flocculated.

Air sampling with porous solid-phase microextraction fibers

A new, rapid air sampling/sample preparation methodology was investigated using adsorptive solid-phase microextraction (SPME) fiber coatings and nonequilibrium conditions for volatile organic compounds (VOCs). This method is the fastest extraction technique for air sampling at typical airborne VOC concentrations. A theoretical model for the extraction was formulated based on the diffusion through the interface between the sampled (bulk) air and the SPME coating. Parameters that affect the extraction process including sampling time, air velocity, air temperature, and relative humidity were investigated with the porous (solid) PDMS/DVB and Carboxen/PDMS coatings. Very short sampling times from 5 s to 1 min were used to minimize the effects of competitive adsorption and to calibrate the extraction process in the initial linear extraction region. The predicted amounts of extracted mass compared well with the measured amounts of target VOCs. Findings presented in this study extend the existing fundamental knowledge related to sampling/sample preparation with SPME, thereby enabling the development of new sampling devices for the rapid sampling of air, headspace, water, and soil.

Zeolite Catalysts as Solid Solvents in Fine Chemicals Synthesis: 2. Competitive Adsorption of the Reactants and Products in the Friedel–Crafts Acetylations of Anisole and Toluene

The liquid-phase acetylations of anisole by acetic anhydride and isopropenyl acetate and that of toluene by acetic anhydride, using zeolites HBEA, HMFI, and HFAU as catalysts, were carried out under batch conditions using various molar ratios of the reactants without added solvent. Evidence has been obtained for competitive adsorption effects of the reactants resulting from differences in their adsorption equilibrium constants (AEC). Estimated values of the AEC show that preferential adsorption is in the order toluene<acetic anhydride (AA)<anisole<isopropenyl acetate (IPA). In all cases, the acetylation products are adsorbed more strongly than the reactants, but the competitive adsorption of acetic acid and acetone appear to be negligible when AA and IPA are used as acetylating agents, respectively. The effect of dealumination was investigated for the acetylation of toluene by acetic anhydride, using HBEA as catalyst. Both the initial and the quasi-stationary-state reaction rates are proportional to Al content, i.e., the number of Brønsted sites at low Al content. The rates decrease at high aluminium content, indicating that competitive adsorption effects are enhanced due to the increased zeolite polarity and polarisability. Initial rate constants for the acetylation of anisole, derived by assuming a Langmuir–Hinshelwood model, show that the rate-determining step is most likely to be the reaction of anisole with an acyl cation equivalent formed by reaction of either acetic anhydride or isopropenyl acetate with the zeolite (HBEA). A possible mechanism is proposed in which the formation of an acyl cation–zeolite complex is the initial step. This proposal is based in part on the colour change of the zeolite when it is contacted with the reactant mixture and during reaction, the colour being attributed to the formation of the zeolite–acyl cation complex. The present work confirms that competitive adsorption effects, involving both reactants and products, need to be controlled in order to achieve maximum catalytic performance when zeolites are used as catalysts in the liquid phase.

Transient behavior of the enantioselective hydrogenation of a hydroxymethylpyrone

Various 2‐pyrone derivatives are important intermediates in the synthesis of biologically active compounds. Pd chirally modified by cinchona alkaloids has a potential in the enantioselective hydrogenation of 4‐hydroxy‐6‐methyl‐2‐pyrone to the corresponding 5,6‐dihydropyrone. A study of various parameters (solvent, temperature, pressure, concentration) and catalyst systems (Pd/alumina and Pd/titania, modified by cinchonidine or cinchonine) revealed striking variations of the reaction rate and enantioselectivity with conversion. This transient behavior is interpreted by the effect of competitive adsorption and hydrogenation of the substrate and modifier.

Sampling procedures for intrinsically valid volatile organic compound measurements

The concept of an intrinsically valid volatile organic compound (VOC) measurement is described, whereby the final data contain all the information needed to calibrate and validate the determination. To realise such a system, all modes of failure within the measurement system need to be tested as the analysis proceeds, and the results of the tests incorporated into the final analytical output. The sources of error and uncertainty associated with the seven principal stages of a VOC analysis sequence are considered. In particular, the effect of competitive adsorption, arising from a transient concentration excursion, on the determination of other analytes is highlighted with an example showing a 97.5% reduction in sensitivity towards benzene in air that, in many recommended methods, would remain undetected. An experimental approach is presented which indicates how the problem of validating adsorbent-based VOC measurements may be addressed by adopting an intrinsically valid measurement approach. A sampling manifold is used that enables two internal standards, fluoropentane and fluorononane, to be added to sampled air in a reproducible and traceable manner before the sample is presented to six adsorbent traps. Thus, these internal standards are simultaneously trapped on the adsorbent alongside the airborne VOC. Upon analysis, the internal standards provide a reference within the chromatogram, allowing the validity of the sampling and subsequent steps in the measurement sequence to be assessed. Fluoropentane and fluorononane were chosen because, amongst other reasons, they bracket the adsorbent capacities of Tenax towards benzene, toluene and xylene. Thus, by comparing the behavour of the fluorinated standards within the analysis, it was possible to identify the range of faults that may arise during active sampling, thermal desorption, chromatographic separation and detection. The chromatogram obtained from this approach had associated with it all the features of an intrinsically valid VOC measurement. Furthermore, the system described resulted in 21–51% reductions in the relative standard deviations for the compounds studied, with the exception of 1-xylene.

Electrodeposition of Iron‐Group Metals and Binary Alloys from Sulfate Baths. II. Modeling

A mechanism of iron‐group elemental metal and binary alloy electrodeposition is proposed. The one‐dimensional diffusion model of Grande and Talbot is used to determine near‐surface concentrations of the ionic species deemed important for electrodeposition; the current model expands upon the surface kinetics by including the effects of competitive adsorption, site blockage by hydrogen atoms, and a variance in the number of adsorption sites. Fitting of the model kinetic parameters to the elemental electrodeposition data was found to simulate the partial current densities extremely well. The proposed model was found to be extensible to the iron‐group binary alloys. Use of the elemental electrodeposition parameters in alloy codeposition was found to effectively characterize the experimental results, e.g., partial current densities, weight fractions, and the effect of electrode rotational rate. © 2000 The Electrochemical Society. All rights reserved.

Zeolite Catalysts as Solid Solvents in Fine Chemicals Synthesis: 1. Catalyst Deactivation in the Friedel–Crafts Acetylation of Anisole

The liquid phase acetylation of anisole by acetic anhydride (anisole:acetic anhydride in a molar ratio of 2:1) using zeolite HBEA (Si/Al=11) as catalyst was carried out in a batch reactor at 90°C, without and with addition of the product, p-methoxyacetophenone (p-MOAP) (molar ratio anisole/p-MOAP=3 and 6). As expected, acetic acid and p-MOAP are produced in equal amounts during the initial stages of the reaction but acetic acid is consumed at long reaction time and high conversion. Partial zeolite dealumination of the used catalyst was evidenced by 27Al MAS NMR spectroscopy and the regenerated catalyst showed a lower activity agreeing with its reduced Al content, i.e., acidity. Without added p-MOAP, acetylation occurs rapidly at low conversion but deactivation becomes important as conversion increases. The reaction rate is largely decreased when p-MOAP is added to the reaction mixture, indicating inhibition of the reaction by p-MOAP. A detailed kinetic analysis using a Langmuir–Hinshelwood model was performed to quantify the nature and extent of the reaction inhibition by p-MOAP. It shows that the adsorption equilibrium constant for p-MOAP exceeds by a factor of at least 6 the adsorption equilibrium constant for any of the reactants and that the occupancy of the intracrystalline volume of the zeolite by p-MOAP increases rapidly with conversion, thereby reducing the access of the reactants to the catalytic sites. Comparison of our results with literature data enabled us to derive an approximate activation energy for this reaction, i.e., ca. 11 kcal mol−1. A good agreement is observed between the calculated and experimental reaction rates as a function of conversion. It is concluded that the deactivation of the catalyst as conversion increases is mainly due to product inhibition, i.e., the competitive adsorption of the reactants and products in the zeolite intracrystalline volume which can be rationalized in terms of the zeolite acting as a solid solvent. Our work suggests that the application of zeolites and other microporous solids as catalysts to fine chemicals synthesis would be better performed using catalytic reactor configurations minimizing the role played by competitive adsorption effects.

Enzymatic degradation of plant cell wall polysaccharides: the kinetic effect of competitive adsorption.

Insoluble potato dietary fibre, isolated from potato pulp, can be enzymatically hydrolysed with the pectolytic enzyme preparation Pectinex Ultra SP from Novo Nordisk A/S, in order to produce soluble fibre. The soluble fibre has valuable functional properties for the food industry. Cloned monocomponent enzymes from Pectinex Ultra SP (arabinofuranosidase, endoglucanase II, pectin lyase, polygalacturonase I, rhamnogalacturonan acetyl esterase, rhamnogalacturonase a, rhamnogalacturonase b and xylanase I) were added in order to increase the yield. Surprisingly, however, the yield is not increased when any of the monocomponent enzymes are added. To describe the results a new model designated 'the competitive activity adsorption model' is proposed. The model is based on the fact that the enzymes are adsorbed to the substrate before action. A combination of the Langmuir adsorption isotherm and basic enzyme kinetics shows that different enzymes that adsorb competitively will have an inhibitory effect on each other and consequently decrease the hydrolysis rate and thereby the yield. The model has been confirmed by an experiment in which the fibre has been pre-treated with rhamnogalacturonan acetyl esterase.

Adsorption−Desorption of Chlordimeform on Montmorillonite: Effect of Clay Aggregation and Competitive Adsorption with Cadmium

Effect of the aggregation state of montmorillonite clays of types SAz-1 and SWy-1 on the adsorption of the monovalent organic cation chlordimeform was studied. The shapes of the adsorption isotherms were related to the degree of dispersion of the clay, changing from S- to L-type by decreasing clay concentration. Unlike monovalent organic cations denoted dyes, chlordimeform adsorption did not exceed the cationic exchange capacity of the clay (CEC). At larger Ca2+/Na+ charge ratio (≈0.5), chlordimeform exhibited low apparent affinity for adsorbing to the SAz-1 clay, due to steric inhibition of its penetration between closely opposed clay platelets. The apparent affinity increased dramatically at smaller Ca2+/Na+ charge ratios (<0.06) for Ca2+-montmorillonite, or by switching to Na+-montmorillonite. The desorption process of chlordimeform shows an apparent hysteresis in Ca2+-montmorillonite. An adsorption model which combines electrostatic equations with specific binding in a closed system is able to account...

Computer simulation of active air sampling for volatile organic compound analysis

A study of the literature reveals that published breakthrough volumes do not take into account the findings of other studies into the effects of competitive adsorption in active sampling for volatile organic compound analysis. A model-based approach for estimating breakthrough volumes under different sampling conditions is proposed and described. Simulations of sampling a binary mixture of volatile compounds illustrate how breakthrough volumes can vary significantly. Further, tests under laboratory conditions indicate that changes in breakthrough volumes may be reliably estimated by this approach, although the predictions were not as accurate when used to estimate 5% breakthrough volumes based on previously published work. Nevertheless, the shortcomings of using published safe sampling volumes in the development of sampling protocols are clearly demonstrated, and the approach described is a practical response to overcoming this problem.

New Insight into the Anomalous Codeposition of Alloys

SUMMARYA new modelfor the anomalous codeposition of alloys is based on the kinetic characteristics of metal electrodeposition and the effect of hydrogen evolution accompanying the deposition. The less noble metal has intrinsically faster kinetics and so its deposition rate can be controlled mainly by diffusion mass transfer in a certain range of overpotentials; the convection induced by hydrogen evolution can speed up the relative deposition rate of this less noble metal when it is mainly diffusion-influenced. In contrast with previous models, such as the hydroxide suppression hypothesis, the monohydroxide-discharge-controlled mechanism, and the competitive adsorption effect, our model can explain naturally and completely anomalous codeposition, i.e. the preferential deposition of the less-noble metal, a problem in alloy electrodeposition that has persisted for a century.

Gas phase catalytic hydrodechlorination of chlorophenols using a supported nickel catalyst

The gas phase hydrodechlorination of single component and mixtures of o-, m- and p-chlorophenol was studied over the temperature range 423 K≤ T≤573 K using a 1.5% (w/w) Ni/SiO 2 catalyst. The variation of catalyst activity with time-on-stream for each isomer is illustrated and the role of reaction temperature and thermodynamic limitations are addressed. The catalyst exhibits both short term and irreversible long term deactivation which is accounted for in terms of competitive adsorption and electronic effects. The hydrogen treatment of both phenol and chlorobenzene under the same reaction conditions are also considered for comparative purposes. The presence of the hydroxyl function enhances the rate of hydrodechlorination via an inductive effect. The relationship between rate and isomer structure is discussed on the basis of reactant adsorption where steric hindrance, in the case of the ortho-form, appreciably restricts dechlorination to such an extent that o-chlorophenol remains unreacted in equimolar o-/ p- and o-/ m-chlorophenol mixtures. Catalytic hydrodechlorination is viewed as a non-destructive low energy methodology for handling concentrated chlorine gas streams and relationships that describe the dependence of dechlorination rate on chlorine concentration are provided and can be used to evaluate the productive lifetime of the catalyst.

Effect of Ca2+ Exchange on Adsorption of N2−O2 Mixtures by NaCaX Zeolite

Steady-state isotopic transient kinetic analysis (SSITKA) was used in this study to compare the adsorption behavior of pure N2 and O2 with the adsorption of binary N2−O2 mixtures at different PN2/PO2 ratios on a series of NaCaX zeolites with different degrees of Ca2+ exchange. A sharp increase in relative N2 selectivity was observed when more than 49% of Na+ was exchanged with Ca2+. For the single adsorbate case, there was also a sharp increase in N2 uptake at this point. This was probably due to Ca2+ cations starting to be in accessible locations in the zeolite for adsorption and the stronger field-quadrupole interactions between N2 and Ca2+. A similar increase was observed for O2 uptake for zeolite samples with more than 71% of the Na+ exchanged. It is suggested that, during the exchange process, the first Na+ cations to be replaced are the ones located at sites I and I‘ which are inaccessible to either of the adsorbate molecules. At exchange degrees higher than 49%, the drastic increase in adsorption uptake, particularly for N2, suggests that the Ca2+ also replaces Na+ cations located at sites II and III in the supercages of the X zeolite. The differences in molecular interactions between adsorbate molecules and cationic sites are reflected by differences in the isosteric heats of adsorption of N2 which were found to be around 3.8 kcal/mol on the Na zeolite with no Ca2+ and 6.3 kcal/mol on the one having 94% of the Na+ replaced by Ca2+. For the zeolite with 94% of Na+ exchanged, the N2 uptake in the presence of O2 at a PN2/PO2 ratio of 1/4 was significantly smaller than the one measured for pure N2. Similarly, for the same zeolite, the O2 uptake in the presence of N2 at a PN2/PO2 ratio of 4/1 was significantly smaller than the one measured for pure O2. These phenomena are attributed to competition between N2 and O2 adsorption. They were not observed at lower levels of Na+ replacement (higher Na+ content), probably due to lower adsorbate coverages under the conditions studied which minimized the competitive adsorption effects.

Competitive adsorption effects in the metalorganic vapor phase epitaxy of GaN

The growth mechanisms of GaN grown on a GaN buffer deposited onto sapphire substrates are studied here. The growth rate was measured at different temperatures versus ammonia flow and was found to decrease with increasing NH3 flow. This surprising behavior is modeled in terms of competitive adsorption of species on the growing surface. Very good agreement is obtained between the model and the experimental data.