Academic literature on the topic 'Langmuir-Hinshelwood equation'

Abstract. In this work we have quantitatively measured the degradation of 4-phenoxyphenol adsorbed on silica particles following oxidative processing by gas-phase ozone. This was performed under dark conditions and in presence of 4-carboxybenzophenone under simulated sunlight irradiation of the particles surface. At mixing ratio of 60 ppb which corresponds to strongly ozone polluted areas, the first order decay of 4-phenoxyphenol is k1=9.95×10−6 s−1. At very high ozone mixing ratio of 6 ppm the first order rate constants for 4-phenoxyphenol degradation were the following: k1=2.86×10−5 s−1 under dark conditions and k1=5.58×10−5 s−1 in presence of photosensitizer (4-carboxybenzophenone) under light illumination of the particles surface. In both cases the experimental data do follow the modified Langmuir-Hinshelwood equation for surface reactions. Langmuir-Hinshelwood and Langmuir-Rideal mechanisms are also discussed along with the experimental results. Most importantly, the quantities of the oligomers such as 2-(4-Phenoxyphenoxy)-4-phenoxyphenol and 4-[4-(4-Phenoxyphenoxy)phenoxy]phenol formed during the heterogeneous ozonolysis of adsorbed 4-phenoxyphenol were much higher under solar light irradiation of the surface in comparison to the dark conditions.

Abstract. In this work we have quantitatively measured the degradation of 4-phenoxyphenol adsorbed on silica particles following oxidative processing by gas-phase ozone. This was performed under dark conditions and in the presence of 4-carboxybenzophenone under simulated sunlight irradiation of the particles surface. At the mixing ratio of 60 ppb which corresponds to strongly polluted ozone areas, the first order of decay of 4-phenoxyphenol is k1=9.95×10−6 s−1. At a very high ozone mixing ratio of 6 ppm the first order rate constants for 4-phenoxyphenol degradation were the following: k1=2.86×10−5 s−1 under dark conditions and k1=5.58×10−5 s−1 in the presence of photosensitizer (4-carboxybenzophenone) under light illumination of the particles surface. In both cases, the experimental data follow the modified Langmuir-Hinshelwood equation for surface reactions. The Langmuir-Hinshelwood and Langmuir-Rideal mechanisms for bimolecular surface reactions are also discussed along with the experimental results. Most importantly, the quantities of the oligomers such as 2-(4-Phenoxyphenoxy)-4-phenoxyphenol and 4-[4-(4-Phenoxyphenoxy)phenoxy]phenol formed during the heterogeneous ozonolysis of adsorbed 4-phenoxyphenol were much higher under solar light irradiation of the surface in comparison to the dark conditions.

Kinetics of methyl tert.butyl ether synthesis from methanol and isobutene was measured in gaseous phase at 85 °C and atmospheric pressure on macroreticular ion exchanger catalyst containing strongly acidic functional groups SO3H and on the same catalyst partially neutralized by sodium and iron ions. The form of the best Langmuir-Hinshelwood type kinetic equation suggests absorption of the reactants in the polymer mass causing ìswellingî of it and influencing the accessibility of active sites by the reactants. Neutralization of the catalyst by metal ions suppresses this effect.

The kinetics of the selective hydrogenation of acetylene in the presence of an excess of ethylene has been studied over a 0.05 wt. % Pd/α-Al2O3 catalyst. The experimental reaction conditions were chosen to operate under intrinsic kinetic conditions, free from heat and mass transfer limitations. The data could be described adequately by a Langmuir–Hinshelwood rate-equation based on a series of sequential hydrogen additions according to the Horiuti–Polanyi mechanism. The mechanism involves a single active site on which both the conversion of acetylene and ethylene take place.

An active dye, Methyl Orange (MO) was employed as the target pollutant to evaluate the photocatalytic activity of TiO2/schorl composite and the kinetics and thermodynamics of this process was emphasized in this work. Langmuir–Hinshelwood kinetic model was employed for the kinetic studies and the results revealed that the process of MO photocatalytic discoloration by TiO2/schorl composite followed one order reaction kinetic equation under different conditions. The reaction rate constant (k) increased with initial MO concentration decreasing. When the catalyst dosage or solution pH increased,kvalues increased and then decreased. The possible reasons for these phenomena were discussed. Finally, the thermodynamic parameters ΔG, ΔH, ΔSwere obtained by the classical Van't Hoff equation.

The mechanisms and kinetics of the sonochemical degradation of organic molecules in water are relatively complex since several parameters such as physicochemical properties, substrate concentration, water matrix, reactor geometry, ultrasound properties (frequency, power, emission system) all typically affect the process. In this work, simple kinetic models were used to predict the degradation of 2-chlorophenol and sodium dodecylbenzene sulphonate in aqueous solutions and verified against experimental data taken from previous studies. A pseudo-first order kinetic expression can adequately describe the degradation of the phenolic substrate, while a heterogeneous model based on the Langmuir-Hinshelwood equation is suitable for the surfactant degradation.

Abstract Zn-doped NaA molecular sieves were prepared by hydrothermal method using zinc nitrate, sodium metaaluminate, silica sol, etc. as raw materials. The samples were characterized by XRD, SEM, FT-IR, etc., using 300W xenon lamp as the light source. Rhodamine B (RhB) was used as the target degradant for photocatalytic experiments for its photocatalytic activity. The results showed that zinc ion doping increased the number of active sites on the NaA molecular sieve, effectively reduced the photoelectron-hole recombination probability in the molecular sieve, and improved the photocatalytic activity. The degradation rate of rhodamine B was 50.8% after 2 hours of light exposure, and its photocatalytic process complied with the Langmuir-Hinshelwood first-order kinetic equation.

Pharmaceuticals are considered among the group of emerging contaminants. Paracetamol is a moderate painkiller, which has been detected in ground and surface water. Photodegradation of paracetamol at a wavelength of radiation of 254 nm with TiO2 nanotubes was studied by UV-spectroscopy, HPLC and measurement of the potential zeta in dependence of the solution pH. The efficiency of the photodegradation of paracetamol (20 mg L−1) was 99% after 100 min exposure. Application of the Langmuir-Hinshelwood equation allowed the evaluation of the rate constant. Non-organic by-products were detected under the conditions of the chromatographic analysis. The photoreaction was faster at pH 6.5, a value at which adsorption was favored, leading to higher efficiency.

Les objectifs de ce travail de thèse étaient le développement de nouvelles Pd NPs (nanocatalyseurs) par des méthodes vertes et leur application en flux continu à la valorisation de dérivés de la biomasse. Nous avons d'abord développé la synthèse de nanocatalyseurs constitués de NPs de Pd, qui sont stabilisés avec des ligands oxygénés, la famille de l'acide hydroxyl méthylène bisphosphonique (HMBP). Ces molécules nous ont permis de maintenir les solutions de NPs de Pd stables dans l'eau et sous conditions aérobies à 4 °C pendant une période de temps supérieure à 6 mois après leur préparation sans aucune perte d'activité catalytique. Ces solutions de Pd NPs ont pu catalyser jusqu'à 6 réactions organiques différentes en milieu aqueux dans des conditions bénignes, comme la réduction du 4-nitrophénol. Les NPs de Pd présentent une efficacité catalytique élevée pour la réduction catalytique du 4-nitrophénol en utilisant du borohydrure de sodium comme réducteur, contrairement à de nombreux catalyseurs mNP différents. La cinétique complète de la procédure de réduction a été examinée en changeant un facteur particulier à chaque fois, comme la quantité de Pd NPs, la concentration de NaBH4 et le 4-nitrophénol initial dans plusieurs circonstances expérimentales. L'évaluation du contrôle de la diffusion a été montrée par le comptage des secondes Nombre de Damköhler. Les valeurs théoriques obtenues à partir de l'équation de Langmuir-Hinshelwood ont été ajustées avec succès aux données expérimentales. Le présent travail de thèse a évalué une approche alternative pour maximiser l'accessibilité des sites catalytiques et empêcher la lixiviation de Pd par l'utilisation de NPs de Pd non supportées dans un microréacteur. Cette étude a été réalisée en utilisant différentes configurations de microréacteurs tels que le microréacteur capillaire en spirale PTFE (SCM) et leurs applications sur le modèle de réaction de référence de la réduction du 4-nitrophénol. Brièvement, en termes de synthèse de furfuryl éthyl éther, nous avons testé différents catalyseurs hétérogènes en réacteur à flux continu. L'éthérification réductrice du furfural a été effectuée sur divers métaux supportés sur du charbon actif. En raison de la meilleure sélectivité de l'éther furfurylique éthylique, le catalyseur Pd/C commercial qui contient des NP de Pd a été choisi. Nous avons utilisé un réacteur à lit fixe commercial (H-cub) sur un catalyseur Pd/C commercial contenant des NP de Pd. Le catalyseur Pd/C a été conservé dans une cartouche qui est placée dans le module de réacteur, à travers laquelle le solvant avec les réactifs est passé. Dans toutes les études, les processus d'optimisation se sont concentrés sur plusieurs points clés tels que la température, le pourcentage d'acide TFA et les catalyseurs Pd/C. Indépendamment des résultats prometteurs et des avantages de l'étude, de nombreuses possibilités peuvent être recommandées pour poursuivre le travail commencé dans cette thèse. Ainsi, des recherches supplémentaires peuvent être effectuées pour découvrir des nanocatalyseurs alternatifs, idéalement hétérogènes lors de l'utilisation d'un flux continu
The objectives of this PhD work were the development of new Pd NPs (nanocatalysts) by green methods and their application on continuous flow to valorisation of biomass derivatives. We first developed the synthesis of nanocatalysts made up of Pd NPs, which are stabilized with oxygen-based ligands, the family of hydroxyl methylene bisphosphonic acid (HMBP). These molecules have enabled us to keep the solutions of Pd NPs stable in water and under aerobic conditions at 4 oc for a period of time more than 6 months after their preparation without any loss in catalytic activity. These solutions of Pd NPs were able to catalyse up to 6 different organic reactions in an aqueous medium under benign conditions, such as the reduction of 4-nitrophenol. The Pd NPs, show high catalytic efficiency for catalytic reduction of 4-nitrophenol by utilizing sodium borohydride as the reducer in contrast to numerous different mNPs catalysts. The complete kinetics of the reduction procedure has been examined by changing a particular factor each time, as the quantity of Pd NPs, NaBH4 concentration, and initial 4-nitrophenol at several experimental circumstances. The assessment of diffusion control was shown by the counting of second Damköhler number. The theoretical values obtained from the Langmuir-Hinshelwood equation were successfully fitted to the experimental data. The present thesis work evaluated an alternative approach to maximize the accessibility of catalytic sites and prevent Pd leaching by the use of unsupported Pd NPs in a microreactor. This study was done by using different microreactors configurations such as PTFE spiral capillary microreactor (SCM) and their applications on the benchmark reaction model of 4-nitrophenol reduction. Briefly, in terms of furfuryl ethyl ether synthesis, we tested different heterogeneous catalysts in continuous flow reactor. The reductive etherification of furfural was done over various metals supported on activated carbon. Due to the best furfuryl ethyl ether selectively, the commercial Pd/C catalyst that contains NPs of Pd was the chosen one. we used a commercial packed-bed reactor (H-cub) over a commercial Pd/C catalyst that contains NPs of Pd. The PUC catalyst was kept in a cartridge that is placed in the reactor module, through which the solvent with reagents is passed. In all studies, the optimization processes was focused on several key points such as temperature, percentage of TFA acid and Pd/C catalysts. Regardless of the study's promising results and advantages, numerous possibilities can be recommended to proceed with the work commenced in this PhD. Thus, additional research can be done to discover alternative nanocatal Sts, ideall hetero eneous ones when utilizin continuous flow

Solid catalysts by their very nature involve diffusion of reactant fluids within their matrix. These fluids react even as they diffuse. Thus the problem of internal diffusion accompanied by reaction becomes important. Another problem of equal importance is the transport of the reactant from the fluid bulk to the catalyst surface—often referred to as external diffusion. Together these constitute the microenvironment of the catalyst pellet and form the subject matter of this chapter. Consider the reaction between aniline and methanol on pellets of alumina to give dimethylaniline: Let the pellets be placed in a flowing stream of reactants inside a tubular reactor. Restricting our attention now to a single pellet and its immediate environment, the various steps involved in the overall process are shown in Figure 7.1. This physical-chemical circuit is built in analogy with the electrical circuit shown at the bottom of the figure. Clearly, the overall process is a complex combination of chemical and physical steps. Note, however, that the mathematical analysis of the parallel pathways (diffusion and reaction) is not always based on the addition of reciprocal resistances as in parallel electrical circuits but on the fact that the two occur simultaneously on a single pathway, that is, the molecule reacts even as it diffuses. The construction of a model based on one of the adsorption-reactiondesorption steps being the limiting step constitutes the core of the semiempirical approach considered in this chapter. In this approach, the microscopic origins of the observed macroscopic effects of catalysts (as described by many authors, e.g., Plath, 1989) are ignored. The models thus developed are commonly known as Langmuir-Hinshelwood models among chemists and as Hougen-Watson models among chemical engineers. We choose to call them Langmuir-Hinshelwood-Hougen-Watson (LHHW) models. For a more basic approach to kinetic modeling of catalytic reactions in general, reference may be made, among others, to Compton (1991) and Van Santen and Niemantsverdriet (1995). In the interest of generality, we consider hypothetical reactions and derive rate equations for a few typical LHHW models (Hougen and Watson, 1947; Yang and Hougen, 1950; Satterfield, 1980, Butt, 1980; Doraiswamy and Sharma, 1984; Boudart and Djéga-Mariadassou, 1984)..

This paper discusses numerical analysis of heat and mass transfer characteristics in autothermal fuel reformer. Assuming local thermal equilibrium between bulk gas and surface of catalyst, one medium approach for energy equation is incorporated. Also, mass transfer between concentrations of bulk gas and near the surface of catalyst is neglected due to relatively low gas mixture velocity. For surface chemical reaction Langmuir-Hinshelwood reaction is incorporated when methane (CH4) is reformed to hydrogen-rich gases by autothermal reforming (ATR) reaction. Complete combustion, steam reforming, water gas shift and direct methane steam reforming reactions are included in the chemical reaction model. Under two operating conditions (O/C and S/C), ATR reactions are estimated from the numerical calculations. Mass, momentum, and energy equations are simultaneously calculated with chemical reactions. From the predicted results, we can estimate optimum operating conditions for high hydrogen yield.

This paper presents the work done to date on a modeling study of the Non-Selective Catalytic Reduction (NSCR) system. Several recent experimental studies indicate that the voltage signal from the heated exhaust gas oxygen sensor commonly used to control these emission reduction systems may not be interpreted correctly because of the physical nature in the way the sensor senses the exhaust gas concentration. While the current signal interpretation may be satisfactory for modest NOX and CO reduction, an improved understanding of the signal is necessary to achieve consistently low NOX and CO emission levels. The increasingly strict emission regulations may require implementing NSCR as a promising emission control technology for stationary spark ignition engines. Many recent experimental investigations that used NSCR systems for stationary natural gas fueled engines showed that NSCR systems were unable to consistently control the emissions level below the compliance limits. Modeling of NSCR components to better understand, and then exploit, the underlying physical processes that occur in the lambda sensor and the catalyst media is now considered an essential step toward improving NSCR system performance. This paper focuses only on the lambda sensor that provides feedback to the air-to-fuel ratio controller. The goals of this modeling study are: • Improve the understanding of the transport phenomena and electrochemical processes that occur within the sensor. • Investigate the cross-sensitivity of exhaust gases from natural gas fueled engines on the sensor performance. • Serve as a tool for improving NSCR control strategies. This model simulates the output from a planar switch type lambda sensor. The model consists of three modules. The first module models the multi-component mass transport through the sensor protective layer. A one dimensional mass conservation equation is used for each exhaust gas species. Diffusion fluxes are calculated using the Maxwell-Stefan equation. The second module includes all the surface catalytic reactions that take place on the sensor platinum electrodes. All kinetic reactions are modeled based on the Langmuir-Hinshelwood kinetic mechanism. The third module is responsible for simulating the reactions that occur on the electrolyte material and determining the sensor output voltage. The details of these three modules as well as a parametric study that investigates the sensitivity of the output voltage signal to various exhaust gas parameters is provided in the paper.

The increasingly strict emission regulations may require implementing Non-Selective Catalytic Reduction (NSCR) system as a promising emission control technology for stationary rich burn spark ignition engines. Many recent investigations used NSCR systems for stationary natural gas fueled engines showed that NSCR systems were unable to consistently control the emissions level below the compliance limits. Modeling of NSCR components to better understand, and then exploit, the underlying physical processes that occur in the lambda sensor and the catalyst media is now considered an essential step toward the required NSCR system performance. This paper presents the work done to date on a modeling of lambda sensor that provides feedback to the air-to-fuel controller. Several recent experimental studies indicate that the voltage signal from the lambda sensor may not be interpreted correctly because of the physical nature in the way the sensor senses the exhaust gas concentration. Correct interpretation of the sensor output signal is necessary to achieve consistently low emissions level. The goal of this modeling study is to improve the understanding of the physical processes that occur within the sensor, investigate the cross-sensitivity of various exhaust gas species on the sensor performance, and finally this model serves as a tool to improve NSCR control strategies. This model simulates the output from a planar switch type lambda sensor. The model consists of three modules. The first module models the multi-component mass transport through the sensor protective layer. Diffusion fluxes are calculated using the Maxwell-Stefan equation. The second module includes all the surface catalytic reactions that take place on the sensor platinum electrodes. All kinetic reactions are modeled based on the Langmuir-Hinshelwood kinetic mechanism. The model incorporates for the first time methane catalytic reactions on the sensor platinum electrode. The third module is responsible for simulating the reactions that occur on the electrolyte material and determine the sensor output voltage. The model results are validated using field test data obtained from a mapping study of a natural gas-fueled engine equipped with NSCR system. The data showed that the lambda sensor output voltage is influenced by the reducing species concentration, such as carbon monoxide (CO) and hydrogen (H2). The results from the developed model and the experimental data showed strong correlations between CO and H2 with the sensor output voltage within the lambda operating range between 0.994 to 1.007 (catalytic converter operating window). This model also showed that methane does not significantly influence the lambda sensor performance compared to the effect of CO and H2.