Hydrogen Langmuir-Hinshelwood kinetics

GP 11] [R 5] Langmuir-Hinshelwood kinetics adequately describe the observed results as a parity diagram (Figure 3.52), comparing experimental with theoretical values (2.0-7.0 mmol 1 hydrogen 3.6 mmol oxygen 48-70 °C) [121]. 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

Figure 2.30 Parity plot based on the Langmuir-Hinshelwood kinetics of the 02 reaction rate r02 for hydrogen oxidation as determined over microstructured Pt-impregnated stainless-steel foils [61] (by courtesy of O. Gorke).

The hydrogenation of CO and the formation of each individual C hydrocarbon (e.g., methane) obey Langmuir-Hinshelwood kinetic expressions (32,64,66,67.75)-. 

If the system is well represented by Langmuir-Hinshelwood kinetics, then the relative values of the adsorption equilibrium constants of various unsaturated compounds can be obtained from the relative rates determined individually and in competition. The expression for the competitive reaction between two compounds A and B results from the division of the corresponding rate expressions for each compound (equation 3) to give equation (4). The ratio of the individual rates of hydrogenation, k/Jk, permits the abstraction of the ratio K/JK from the competitively determined ratio kkK/Jk K (equation 5). The assumption that the adsorption of the unsaturated compound is rapid and reversible relative to the rates of the surface-catalyzed reactions may not be correct. In that case, KfJK is an apparent relative adsorption equilibrium constant (see Section 3.1.3.2.2). 

This study presents the kinetic parameters and reactivity profiles for steam gasification of birch and beech char. The inhibition effect of hydrogen is also studied using Langmuir-Hinshelwood kinetics. In addition, the influence of the treatment of the experimental results is analysed by comparing the kinetic parameters differently obtained from the same experiments. 

Hydrogen inhibits the steam gasification reaction. The char gasification reaction with steam and hydrogen can be modelled based on Langmuir-Hinshelwood kinetics. The model fits well the results. 

Selective oxidation of CO in hydrogen over different catalysts has been extensively examined. Most research to date has occurred with formulations that include a precious metal component supported on an alumina carrier. The catalyst-mediated oxidation of CO is a multistage process, commonly obeying Langmuir-Hinshelwood kinetics for a single-site competitive mechanism between CO and 02. Initially, CO is chemisorbed on a PGM surface site, while an 02 molecule undergoes dissociative chemisorption either on an adjacent site or on the support in order for surface reaction between chemisorbed CO and O atoms to produce C02. 

Along with dissolved hydrogen and some admixtures, a liquid reaction mixture contains one or several reactants, reaction products, and sometimes also a solvent consisting of one or several compounds. All these compounds may affect the kinetics of the hydrogenation reaction. For the purposes of this study, the aim of which consists in a quantitative description of changes in composition of the reaction mixture, it is sufficient to employ the Langmuir-Hinshelwood kinetics (7) in its simplest form, where the process of hydrogenation is characterized by the rate constant and adsorption coefficient obtained from the simplest kinetic equations. 

A higher form of interpretation of the effect of solvents on the rate of heterogeneously catalyzed reactions was represented by the Langmuir-Hinshelwood kinetics (7), in the form published by Hougen and Watson (2), where the effect of the solvent on the reaction course was characterized by the adsorption term in the kinetic equation. In catalytic hydrogenations in the liquid state kinetic equations of the Hougen-Watson type very frequently degrade to equations of pseudo-zero order with respect to the concentration of the substrate (the catalyst surface is saturated with the substrate), so that such an interpretation is not possible. At the same time, of course, also in these cases the solvent may considerably affect the reaction. As is shown below, this influence is very adequately described by relations of the LFER type. 

The same explanation as for selective hydrogenation of nitroaromatics can be given assuming that the reaction follows a Langmuir Hinshelwood kinetics... 

Langmuir-Hinshelwood kinetics involving competetive adsorption of acetone molecules and hydrogen atoms were posmlated, and it was assumed that adsorbed acetone dominates. The SMSI effect is explained by the fact that the oxygen atom of the carbonyl group is more effectively activated than in conventional platinum catalysis. It is assmned that the oxygen atom is adsorbed on particularly active Ti " cen-... 

A major advantage of the models of Rieckmann and Keil [44] and Wood et al. [16] was that, unlike earlier studies, any general reaction kinetics could be incorporated into the computer simulation. This means that reactions of industrial interest, which are typically represented by Langmuir-Hinshelwood kinetics, could be represented. Wood et al. [16] chose the hydrodesulfurization of thiophene as an example reaction, using the kinetic measurements of Van Parijs et al. [45] in their simulations. The kinetic model assumes that first thiophene is hydrogenated to butene, then butene is hydrogenated to butane ... 

Although the tested catalyst shows a good intrinsic selectivity for butadiene hydrogenation, the results evidenced the presence of severe diffusion limitations in spite of the thin active shell (230 im). The experimental data were modeled by Langmuir-Hinshelwood kinetic expressions derived from an elementary mechanism. Nine kinetic parameters were reliably estimated by means of a regression analysis and it is concluded that the proposed kinetic model provides a good fitting of the experimental observations. 

Based on the Langmuir-Hinshelwood expression derived for a unimolecular reaction system (6) Rate =k Ks (substrate) /[I + Ks (substrate)], Table 3 shows boththe apparent kinetic rate and the substrate concentration were used to fit against the model. Results show that the initial rate is zero-order in substrate and first order in hydrogen concentration. In the case of the Schiff s base hydrogenation, limited aldehyde adsorption on the surface was assumed in this analysis. Table 3 shows a comparison of the adsorption equilibrium and the rate constant used for evaluating the catalytic surface. 

Deviations from this simple expression have been attributed to mechanistic complexity For example, detailed kinetic studies have evaluated the relative importance of the Langmuir-Hinshelwood mechanism in which the reaction is proposed to occur entirely on the surface with adsorbed species and the Eley-Rideal route in which the reaction proceeds via collision of a dissolved reactant with surface-bound intermediates 5 . Such kinetic descriptions allow for the delineation of the nature of the adsorption sites. For example, trichloroethylene is thought to adsorb at Ti sites by a pi interaction, whereas dichloroacetaldehyde, an intermediate proposed in the photo-catalyzed decomposition of trichloroethylene, has been suggested to be dissociatively chemisorbed by attachment of the alpha-hydrogen to a surface site... 

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