Langmuir-Hinshelwood type catalytic

Model for zero order and Langmuir-Hinshelwood type catalytic reactions... 

The desire to understand catalytic chemistry was one of the motivating forces underlying the development of surface science. In a catalytic reaction, the reactants first adsorb onto the surface and then react with each other to fonn volatile product(s). The substrate itself is not affected by the reaction, but the reaction would not occur without its presence. Types of catalytic reactions include exchange, recombination, unimolecular decomposition, and bimolecular reactions. A reaction would be considered to be of the Langmuir-Hinshelwood type if both reactants first adsorbed onto the surface, and then reacted to fonn the products. If one reactant first adsorbs, and the other then reacts with it directly from the gas phase, the reaction is of the Eley-Ridel type. Catalytic reactions are discussed in more detail in section A3.10 and section C2.8. 

The kinetics of a coupled catalytic reaction can be well described by equations of the Langmuir-Hinshelwood type, since these are able to express mutual influencing of single reactions. Power-law equations are not suitable for this purpose. 

Many kinetic equations have been suggested for the description of catalytic reactions (9, JO). The best approximations are usually seen (77) in relations of the Langmuir-Hinshelwood type (7), which assume the adsorption equilibrium of all the components present in the reaction mixture on the catalyst surface. 

Capsule membrane PTC systems are more amenable to a mechanistic analysis than typical triphase systems where the mechanism of interaction between the aqueous and organic phases with the catalytic sites is complex and not understood. A mechanism for capsule membrane PTC involving mass transfer and smface reaction for both PTC and IPTC reactions has been developed by Yadav and Mehta(1993), Yadav and Mistry (1995). A Langmuir-Hinshelwood type model with the anchored quaternary-nucleophile complex as the active site was assumed to govern the overall rate of reaction... 

Suppression of the mutual correlation between parameters will be illustrated by a case study. Rates of catalytic reactions are very frequently measured in gradientless and differential reactors, and the rate expressions of the Langmuir-Hinshelwood type are frequently used in the interpretation of experimental data. The rate expression has the general form... 

The kinetics of the CO oxidation reaction over all three noble metals exhibit a Langmuir-Hinshelwood type behavior, due to competitive adsorption of CO and oxygen, characterized by the appearance of rate maximum with increasing CO partial pressure. This is a typical behavior which has been described by many investigators for this reaction system [19,20], The catalytic activity, presented as turnover number of CO2 production, was measured by varying either the partial pressures of CO or O2, or temperature, keeping the other parameters constant. 

Quantitative rate data on the catalytic reduction of nitrates in drinkable water are relatively scarce. One of the first works concerning kinetics is that of Tacke and Vorlop who employed a Pd-Cu bimetallic catalyst containing 5wt.% of Pt and 1.25 wt.% of Cu in a slurry reactor. Measurements of the initial rates resulted in a power-law rate expression. They reported a power of 0.7 with respect to the nitrate concentration, and an independency on the hydrogen partial pressure providing this pressure exceeded 1 bar. Pintar efa/. reported a complete kinetic model of the Langmuir-Hinshelwood type written in the form... 

The diffusion of adsorbates on the surface is a process which is important for reactions of the Langmuir-Hinshelwood type, where the molecules in question travel some distance on the surface before they react. The surface diffusion process is influenced by the structure and corrugation of the surface. Therefore the information which can be obtained by studying the diffusion process is of importance for the understanding of the surface structure and its influence on a number of processes, as adsorption/desorption, catalytic properties, and epitaxial growth. 

A dimer-dimer (DD) surface reaction scheme of the type (1/2)A2 + B2 B2A has been proposed in order to mimic the catalytic oxidation of hydrogen A2 is O2, B2 is H2, AB is OH and B2A is H2O. The model reaction proceeds according to the Langmuir-Hinshelwood... 

When a simple, fast and robust model with global kinetics is the aim, the reaction kinetics able to predict correctly the rate of CO, H2 and hydrocarbons oxidation under most conditions met in the DOC consist of semi-empirical, pseudo-steady state kinetic expressions based on Langmuir-Hinshelwood surface reaction mechanism (cf., e.g., Froment and Bischoff, 1990). Such rate laws were proposed for CO and C3H6 oxidation in Pt/y-Al203 catalytic mufflers in the presence of NO already by Voltz et al. (1973) and since then this type of kinetics has been successfully employed in many models of oxidation and three-way catalytic monolith converters... 

The theoretical calculations described have recently been supported by an extraordinary kinetic analysis conducted by Vanrysellberghe and Froment of the HDS of dibenzothiophene (104). That work provides the enthalpies and entropies of adsorption and the equilibrium adsorption constants of H2, H2S, dibenzothiophene, biphenyl, and cyclohexylbenzene under typical HDS conditions for CoMo/A1203 catalysts. This work supports the assumption that there are two different types of catalytic sites, one for direct desulfurization (termed a ) and one for hydrogenation (termed t). Table XIV summarizes the values obtained experimentally for adsorption constants of the various reactants and products, using the Langmuir-Hinshelwood approach. As described in more detail in Section VI, this kinetic model assumes that the reactants compete for adsorption on the active site. This competitive adsorption influences the overall reaction rate in a negative way (inhibition). 

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 reaction progress is monitored ofF-Une by HPLC. Flow rates, residence times and initial concentrations of 4-chlorophenol are varied and kinetic parameters are calculated from the data obtained. It can be shown that the photocatalytic reaction is governed by Langmuir-Hinshelwood kinetics. The calculation of Damkohler numbers shows that no mass transfer limitation exists in the microreactor, hence the calculated kinetic data really represent the intrinsic kinetics of the reaction. Photonic efficiencies in the microreactor are still somewhat lower than in batch-type slurry reactors. This finding is indicative of the need to improve the catalytic activity of the deposited photocatalyst in comparison with commercially available catalysts such as Degussa P25 and Sachtleben Hombikat UV 100. The illuminated specific surface area in the microchannel reactor surpasses that of conventional photocatalytic reactors by a factor of 4-400 depending on the particular conventional reactor type. 

Langmuir-Hinshelwood-Hougen-Watson (LHHW) Rate Equations (1947) Hougen and Watson analyzed several types of catalytic reactions with different ratedetermining steps (adsorption, surface reaction), different types of adsorption (one or more species, dissociative or molecular adsorption), and different types of reactions (mono- or bimolecular, reversible or irreversible). They derived a general rate equation based on three terms ... 

The influence of the Brpnsted and Lewis acidity on OBP was investigated using a ZSM-5 zeolite. A maximum in catalytic activity and selectivity was reached for steamed samples under mild conditions (30% conversion, 94% selectivity). The Brpnsted acid sites play a role of primary importance in OBP as they work in combination with the Lewis acidity. These sites, which are present in the vicinity of the A1 framework, are formed during the dealumination of the zeolite. A Langmuir-Hinshelwood mechanism seems to operate. An acid-catalyzed mechanism is proposed, passing through a Wheland-type intermediate stabilized within... 

It needs to be mentioned that, within the Mars—van Krevelen-type catalytic cycle, the elementary steps of CO oxidation by lattice oxygen may stiU follow a Langmuir—Hinshelwood or Eley—Rideal mechanism. In the Langmuir—Hinshelwood mechanism, the CO molecule adsorbs first on the ceria surface before undergoing the oxidation, whereas in the Eley— Rideal mechanism, the CO molecule attacks directly the surface oxygen from the gas phase. As we have already mentioned when we discussed... 

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