Kinetic redox model

Kinetic redox models, as formulated by Mars and van Krevelen [204], have not been considered in any recent work. Although the combined dependence on both propene and oxygen pressures does arise in certain investigations, the authors seem to ignore redox mechanisms completely and correlate their data with Langmuir—Hinshelwood type models. 

Hinrichsen, Muhler, and co workers—micro kinetic analysis parameterized by redox model. Hinrichsen et al.317 investigated the elementary steps by micro kinetic analysis by applying temperature and concentration-programmed experiments over Cu/Zn0/Al203, and modeling the data with the simplified redox mechanism in the spirit of Ovesen, Topsoe, and coworkers.303 This included 3 steps (1) dissociative adsorption of H2 on Cu metallic surface (2) dissociative adsorption of H20 leading to an adsorbed H2 molecule and an O adatom and a reduction step by CO to form gas phase C02 and a free active site (see Scheme 71). 

Jakdetchai and Nakajima/Wang and coworkers—theoretical models favor redox mechanism. Beginning in 2002, a number of theoretical models were published in Theochem studying the water-gas shift reaction over Cu(110), Cu(lll), and Cu(100) surfaces. Perhaps the first was by Jakdetchai and Nakajima,325 relying on the AMI method. The main goal of the study was (1) to determine whether or not theoretical calculations are consistent with a redox or associative (e.g., formate) mechanism and (2) whether the kinetics are described best by a Langmuir-Hinshel-wood expression or an Eley-Rideal expression. That is, in the case of a redox model, does the adsorbed O adatom react with adsorbed CO or directly with gas phase CO Their approximate A//a[Pg.205]

The replenishment of the vacancy can be directly from the gas phase or indirectly from the catalyst. In the latter case, the oxygen mobility within the catalyst is so large that bulk oxygen can diffuse to the vacancy. Then oxygen from the gas phase reoxidizes the lattice on sites which differ from hydrocarbon reaction sites. In a steady state, the rate of catalyst oxidation will be equal to the rate of reduction by the substrate. The steady state degree of reduction, equivalent to the surface coverage with oxygen, is determined by the ratio of these two rates. Kinetic models based on these principles are called redox models, for which the simplest mathematical expression is... 

A large number of authors describe the oxidation kinetics by Langmuir—Hinshelwood type models. Depending on the particular L—H model selected, the mathematical difference between L—H models and redox models can be very small, although the former always contains more... 

With respect to the kinetics of aromatic oxidations, (extended) redox models are suitable, and often provide an adequate fit of the data. Not all authors agree on this point, and Langmuir—Hinshelwood models are proposed as well, particularly to describe inhibition effects. It may be noted once more that extended redox models also account for certain inhibition effects, for mixtures of components that are oxidized with different velocities. The steady state degree of reduction (surface coverage with oxygen) is mainly determined by the component that reacts the fastest. This component therefore inhibits the reaction of a slower one, which, on its own, would be in contact with surface richer in oxygen (see also the introduction to Sect. 2). 

A V2Os—K2S04 catalyst was used in a kinetic study by Jaswal et al. [165] with a differential flow reactor at 350—400° C and varying benzene/ oxygen ratio. The overall benzene oxidation rate was adequately described by a simple redox model. Kinetic parameters are given in Table 29. The initial selectivity is not reported by the authors, but a value of 50—60% can be derived from the stoichiometric number n, assuming C02 and maleic anhydride as the main products. 

The kinetics are adequately described by an extended redox model, in which the rate of each of the three reactions is described by... 

Regarding the kinetics, the oxidation of o-xylene and o-tolualdehyde were compared for catalysts with different V/Ti ratios (Table 36). The ratio between partial and complete oxidation (X for o-xylene and Y for o-tolualdehyde) are influenced similarly, indicating that a change in the catalyst structure influences all the reaction steps. The oxidation of o-tolualdehyde in mixtures with o-xylene revealed that o-tolualdehyde reduces the o-xylene oxidation rate by a factor of about 2. The authors conclude that a redox model is inadequate and that hydrocarbon adsorption cannot be rate-determining. Adsorption of various products should be included, and equations of the Langmuir—Hinshelwood type are proposed. It should be noted that the observed inhibition is not necessarily caused by adsorption competition, but may also stem from different... 

Denny, R.A., and Sangaranarayan, M.V. 1998. Dynamics of electron hopping in redox polymer electrodes using kinetic Ising model. Jourrtal of Solid State Electrochemistry 2, 67-72. 

The catalysts which have presented the most suitable characteristics for this oxidation are the metal oxides and metal oxides mixtures of transition elements of the V and VI groups, and the hterature reports information related to the formulation, preparation and evaluation of the catalysts (2 - 6), although very few data have been pubhshed related to the reaction kinetics. Gunduz and Akpolat (5) present experimental kinetic data of gas phase oxidation of toluene to benzaldehyde over V2O5 catalysts. Their results are based on the redox model and are restricted to the temperature of 430 C. Also, it is not found in the hterature enough data which allow to analyze the activity and behavior of V2O5 catalysts based only on their physical characteristics. 

The reaction kinetics was analyzed using the Mars and van Krevelen (8) redox model. 

Several kinetic models have been used to describe the partial oxidation of o-Xylene over vanadium pentoxide catalysts, as shown earlier. Two of these models are presented in this section, the first is a redox model and the second a CDS model. 

Most gas-solid catalytic reactions follow some form of CSD kinetic model, except for partial oxidation reactions (and similar reactions) where CSD model and Redox models are still competing. In this first case these types of CSD kinetic models are illustrated using an extremely simple reaction, the unimolecular irreversible reaction. 

Figure 11.5 compares simulated and sampled Fe and Mn distributions when the model is only slightly extended from the state, presented by Holzbecher et al. (2001), i.e. in which Fe is included as additional redox process of minor priority. Both measured Fe and Mn concentrations are well reproduced by the model. Thus observed Fe and Mn concentrations in the Oderbruch can be simulated when the redox model is supplemented only by a kinetic precipitation/dissolution approach. 

Tronconi and coworkers have proposed a fast SCR kinetic mechanistic model that is based on a Redox mechanism [27, 57, 58]. Like the LH SCR model, the Redox SCR model has adsorbed NH3 reacting with gas phase HONO or surface nitrites, forming NH4NO2, which decomposes to N2 (cf. S22). The nitrites are formed through the reduction of nitrates by NO (step S9). Additional steps would include the formation of NH4NO3 and its decomposition to N2O, among others. 

That is, the role of NO is to convert nitrates to nitrites, which are rapidly converted to N2 in the presence of NH3. This follows from earlier works advocating the Redox model, such as Grossale et al. [27]. The analog of step S9 indeed represents a redox step involving the change in the formal oxidation state of N from -h5 (nitrates) to -h3 (nitrites). Earlier we showed transient kinetic evidence that the NO reduction of NH4NO3 (step S25) may be rate determining at lower temperature. A similar reaction is the NO reduction of nitric acid (S9) or nitrates. 

In bioenergetic systems, proton translocation across membranes is generally driven by electron transfers generated by redox reaetions (respiratory systems) or light energy (photosynthetic systems). Owing to their eomplexity and to the lack of detailed kinetic data, modelling these proeesses is eurrently a difficult task. 

While these calculations provide information about the ultimate equilibrium conditions, redox reactions are often slow on human time scales, and sometimes even on geological time scales. Furthermore, the reactions in natural systems are complex and may be catalyzed or inhibited by the solids or trace constituents present. There is a dearth of information on the kinetics of redox reactions in such systems, but it is clear that many chemical species commonly found in environmental samples would not be present if equilibrium were attained. Furthermore, the conditions at equilibrium depend on the concentration of other species in the system, many of which are difficult or impossible to determine analytically. Morgan and Stone (1985) reviewed the kinetics of many environmentally important reactions and pointed out that determination of whether an equilibrium model is appropriate in a given situation depends on the relative time constants of the chemical reactions of interest and the physical processes governing the movement of material through the system. This point is discussed in some detail in Section 15.3.8. In the absence of detailed information with which to evaluate these time constants, chemical analysis for metals in each of their oxidation states, rather than equilibrium calculations, must be conducted to evaluate the current state of a system and the biological or geochemical importance of the metals it contains. 

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