Showing papers on "Equilibrium constant published in 2005"

Coke Formation over a Nickel Catalyst under Methane Dry Reforming Conditions: Thermodynamic and Kinetic Models

Abstract: The CO2 reforming of methane is studied over a 20 wt % Ni/USY-zeolite, and more specifically, a thermodynamic analysis of the formation of coke is used as a basis for the kinetic modeling of coke phenomena that exist under dry reforming conditions. Two thermodynamic parameters, α and β, are compared to the equilibrium constants for the CH4 decomposition and the CO disproportionation reactions and defined to determine whether coke formation is favored. This thermodynamic analysis elucidates the significance of the CO disproportionation reaction on the amount of coke deposited over the catalyst under consideration. A kinetic model with negative overall order of one, with respect to the partial pressure of carbon monoxide, is found as the most accurate prediction of the rate of coke formation. This type of kinetics strongly suggests the requirement of three adjacent free catalyst sites for the coking reaction to proceed under allowable thermodynamic conditions.

Methanation of CO over Nickel: Mechanism and Kinetics at High H2/CO Ratios†

Abstract: The CO methanation reaction over nickel was studied at low CO concentrations and at hydrogen pressures slightly above ambient pressure. The kinetics of this reaction is well described by a first-order expression with CO dissociation at the nickel surface as the rate-determining step. At very low CO concentrations, adsorption of CO molecules and H atoms compete for the sites at the surface, whereas the coverage of CO is close to unity at higher CO pressures. The ratio of the equilibrium constants for CO and H atom adsorption, K(CO)/K(H), was obtained from the rate of CO methanation at various CO concentrations. K(H) was determined independently from temperature programmed adsorption/desorption of hydrogen to be K(H) = 7.7 x 10(-4) (bar(-0.5)) exp[43 (kJ/mol)/RT] and hence the equilibrium constants for adsorption of CO molecules may be calculated to be K(CO) = 3 x 10(-7) (bar(-1)) exp[122 (kJ/mol)/RT]. Furthermore, the rate of dissociation of CO at the catalyst surface was determined to be 5 x 10(9) (s(-1)) exp[-96.7 (kJ/mol)/RT] assuming that 5% of the surface nickel atoms are active for CO dissociation. The results are compared to equilibrium and rate constants reported in the literature.

Theoretical Expression of the Average Activation−Deactivation Equilibrium Constant in Controlled/Living Free-Radical Copolymerization Operating via Reversible Termination. Application to a Strongly Improved Control in Nitroxide-Mediated Polymerization of Methyl Methacrylate

Abstract: The average activation−deactivation equilibrium constant, 〈K〉, was determined on a theoretical basis for controlled free-radical copolymerizations operating via a reversible termination mechanism (i.e., nitroxide-mediated polymerization or atom transfer radical polymerization), using the terminal model for the activation−deactivation equilibrium and the terminal model or the implicit penultimate unit effect model for the propagation reaction. From the equation, it was shown that the addition of a small fraction of an appropriate comonomer to a monomer with a very large activation−deactivation equilibrium constant, K, might lead to strong reduction of 〈K〉, providing the added comonomer exhibits a low K. In nitroxide-mediated polymerization, the monomers with a very high K, such as the methacrylic esters, do not lead to controlled polymerization in the presence of nitroxides like SG1, despite the absence of disproportionation reaction between the nitroxide and the growing radical, because of the too fast ir...

Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg 2+ – Cl – , OH – , CO 3 2– , SO 4 2– , and PO 4 3– aqueous systems (IUPAC Technical Report)

Abstract: This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic l ...

Selection of smart aptamers by equilibrium capillary electrophoresis of equilibrium mixtures (ECEEM).

Abstract: We coin a term of “smart aptamers”, which describes aptamers with predefined binding parameters of their interaction with the target. Here, we introduce a method for selection of smart aptamers with predefined values of Kd: equilibrium capillary electrophoresis of equilibrium mixtures (ECEEM). Conceptually, a mixture of a target with a DNA (RNA) library is prepared and equilibrated. A plug of the equilibrium mixture is injected into a capillary prefilled with a run buffer containing the target at the concentration identical to the target concentration in the equilibrium mixture. The components of the equilibrium mixture are separated by capillary electrophoresis while equilibrium is maintained between the target and aptamers. The unique feature of ECEEM is that aptamers with different Kd values migrate with different and predictable mobilities. Thus, collecting fractions with different mobilities results in smart aptamers with different and predefined Kd values. In this proof-of-principle work, we used E...

Chemical speciation of environmentally significant heavy metals with inorganic ligands part 1: The Hg2+- Cl-, OH-, CO32-, so42-, and PO 43- aqueous systems

Abstract: This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic ligands Cl-, OH -, CO32-, SO42-, and PO43-. The analysis used data from the IUPAC Stability Constants database, SC-Database, focusing particularly on values for 25°C and perchlorate media. Specific ion interaction theory (SIT) was applied to reliable data available for the ionic strength range Ic ≤ 3.0 mol dm-3. Recommended values of log10 β p,q,r° and the associated reaction enthalpies, Δ rHm°, valid at Im = 0 mol kg-1 and 25°C, were obtained by weighted linear regression using the SIT equations. Also reported are the equations and specific ion interaction coefficients required to calculate log10 βp,q,r values at higher ionic strengths and other temperatures. A similar analysis is reported for the reactions of H+ with CO32- and PO43-. Diagrams are presented to show the calculated distribution of Hg(II) amongst these inorganic ligands in model natural waters. Under typical environmental conditions, Hg(II) speciation is dominated by the formation of HgCl2(aq), Hg(OH)Cl(aq), and Hg(OH)2(aq).

A model for estimating CO2 solubility in aqueous alkanolamines

Abstract: Partial pressures of carbon dioxide (CO2) over aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), and N-methyldiethanolamine (MDEA) have been correlated using a simple approach where only one chemical equilibrium reaction is taken into account and assuming ideal gas and ideal liquid properties. The approach combines the Henry's law constant and the chemical reaction equilibrium constant for the formation of carbamate for primary and secondary alkanolamines (MEA, DEA) or bicarbonate for tertiary alkanolamines(MDEA), resulting in an explicit expression for calculating the partial pressure of CO2 over an aqueous alkanolamine solution. Accurate values for the partial pressure of CO2 are obtained for a limited loading, temperature, and pressure range that is useful in modeling CO2 capture from coal-fired power plants. Heat of absorption values derived from the model agree with experimental data from the literature.

NO2 inhibits the catalytic reaction of NO and O2 over Pt

Abstract: The rate equation for the overall reaction of NO and O2 over Pt/Al2O3 was determined to be r=kf[NO] 1.05±0.08[O2]1.03±0.08[NO2]0.92±0.07(1-β), with kf as the forward rate constant, β=([NO2]/K[NO][O2]1/2), and K as the equilibrium constant for the overall reaction. An apparent activation energy of 82 kJ mol−1 ± 9 kJ mol−1 was observed. The inhibition by the product NO2 makes it imperative to include the influence of NO2 concentration in any analysis of the kinetics of this reaction. The reaction mechanism that fits our observed orders consists of the equilibrated dissociation of NO2 to produce a surface mostly covered by oxygen, thereby inhibiting the equilibrium adsorption of NO, and the non-dissociative adsorption of O2, which is the proposed rate determining step.

Inhibition of calcite precipitation by natural organic material: kinetics, mechanism, and thermodynamics.

Abstract: The inhibition of calcite precipitation by natural organic material (NOM) in solutions seeded with calcite was investigated using a pH-stat system. Experiments were carried out using three NOMs with different physical/chemical properties. For each of the materials, inhibition was found to be more effective at lower carbonate/calcium ratios and lower pH values. The reduction in the precipitation rate could be explained by a Langmuir adsorption model using a conditional equilibrium constant. By identification of the type of site on the NOM molecules that is involved in the adsorption reaction, the "conditional" equilibrium constants obtained at different solution compositions converged to a single "nonconditional" value. The thermodynamic data determined at 25 degrees C and 1 atm suggest that the interaction between NOM molecules and the calcite surface is chemisorptive in nature and that adsorption is an endothermic reaction driven by the entropy change. The greatest degree of inhibition was observed for the NOM with the highest molecular weight and aromatic carbon content. For a given type of NOM, the degree of inhibition of calcite precipitation was dictated bythe balance between the enthalpy change and the entropy change of the adsorption reaction.

Sorption characteristics of heavy metal ions by a natural zeolite

Abstract: Zeolites have been shown to be effective adsorbents for the removal of heavy metals from aqueous solutions. A natural material from Cuba, containing zeolite, has been used for the removal of several metal ions, namely Cu2+, Zn2+, and Ni2+, to evaluate its potential use as a low-cost adsorbent. Batch experiments have been conducted to evaluate the process kinetics and the removal equilibrium at different pH values, metal and zeolite concentrations. Pseudo-second order kinetics and Freundlich equilibrium parameters have been obtained. Results suggested that this natural zeolite has a high potential for heavy metal retention. The selectivity of the studied metals was determined as Cu2+ ≫ Zn2+ > Ni2+, related to the first hydrolysis equilibrium constant. The metal removal efficacy was strongly dependent on pH, and to a lesser extent on metal/zeolite ratio. Copyright © 2005 Society of Chemical Industry

In situ infrared study of SBA-15 functionalized with carboxylic groups incorporated by a co-condensation route.

Abstract: Two samples of SBA-15 mesoporous silica with a different content of carboxylic groups (-COOH) were prepared by a co-condensation route, using 4-(triethoxysilil)butyronitrile as the organosilane agent, then treating the samples with sulfuric acid, which removes the template and simultaneously hydrolyses the -CN to -COOH groups, as shown by IR spectroscopy. Both incorporation of organosilane agents and subsequent acid treatment do not affect the ordered SBA-15 structure. The proton-donor ability of carboxylic groups, as well their accessibility to reactants, has been studied in the IR by dosing ammonia, which forms reversibly COO- groups and NH4+ ions. The related equilibrium constants have been determined by elaboration of IR data. Outgassing the samples at progressively increasing temperatures destabilizes to an increasing extent the ammonium/carboxylate ion pair, because of the decrease in surface polarity brought about by dehydration: this decreases the related equilibrium constants. The amount of carboxylic groups undergoing reaction appears instead to be constant with dehydration, and to coincide with the whole population of COOH groups. Titration with alkali solutions yielded surface concentrations for the two samples of ca. 1.0 and 0.45 COOH/nm2. Proportionality between the surface concentrations and the intensities of the C=O IR band is observed: this suggests that COOH groups are noninteracting with each other and allows the computation of the related extinction molar coefficient.

Estimation of the bond dissociation energies from the kinetic characteristics of liquid-phase radical reactions

Abstract: Three methods used in the kinetics of liquid-phase radical reactions for estimating the dissociation energies of individual bonds in polyatomic molecules are described. The first approach is based on the study of the equilibrium in radical abstraction reactions involving stable radicals and measurements of the equilibrium constants. The second method is based on the study of the kinetics of homolytic decomposition of molecules. Measuring the activation energy of these reactions makes it possible to estimate the dissociation energies of the weakest bonds, e.g., the O–O bonds in various peroxides. The essence of the third approach developed in the framework of the model of intersecting parabolas is calculations of the bond dissociation energy from the activation energy of a radical reaction involving the molecules under consideration. This method allowed the dissociation energies of the C–H, N–H, O–H and S–H bonds in a large number of organic compounds to be estimated. The scope and the specific features of application of each method are discussed and the bond dissociation energies determined by these methods are given.

Water dependence of the HO2 self reaction : Kinetics of the HO2-H2O complex

Abstract: Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O HO2-H2O was determined to be K2 = (5.2 +/- 3.2) x 10(-19) cm3 molecule(-1) at 297 K. Substituting K2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2-H2O was estimated to be (1.5 +/- 0.1) x 10(-11) cm3 molecule(-1) s(-1) at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2-H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2-H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2-H2O]/[HO2] was estimated from the value of K2 to be 0.19 +/- 0.11.

A Modified Poisson−Boltzmann Model Including Charge Regulation for the Adsorption of Ionizable Polyelectrolytes to Charged Interfaces, Applied to Lysozyme Adsorption on Silica

Abstract: The equilibrium adsorption of polyelectrolytes with multiple types of ionizable groups is described using a modified Poisson-Boltzmann equation including charge regulation of both the polymer and the interface. A one-dimensional mean-field model is used in which the electrostatic potential is assumed constant in the lateral direction parallel to the surface. The electrostatic potential and ionization degrees of the different ionizable groups are calculated as function of the distance from the surface after which the electric and chemical contributions to the free energy are obtained. The various interactions between small ions, surface and polyelectrolyte are self-consistently considered in the model, such as the increase in charge of polyelectrolyte and surface upon adsorption as well as the displacement of small ions and the decrease of permittivity. These interactions may lead to complex dependencies of the adsorbed amount of polyelectrolyte on pH, ionic strength, and properties of the polymer (volume, permittivity, number, and type of ionizable groups) and of the surface (number of ionizable groups, pK, Stern capacity). For the adsorption of lysozyme on silica, the model qualitatively describes the gradual increase of adsorbed amount with pH up to a maximum value at pHc, which is below the iso-electric point, as well as the sharp decrease of adsorbed amount beyond pHc. With increasing ionic strength the adsorbed amount decreases (for pH > pHc), and pHc shifts to lower values.