Mars and Van Krevelen

The kinetics of selective CO oxidation over the Cu Cej r02, nanostructured catalysts can be well described by employing Mars and van Krevelen type of kinetic equation derived on the basis of a redox mechanism ... 

Figure 7.13 represents the calculated vs. experimental values of reaction rates for the Mars and van Krevelen model of the selective CO oxidation in excess of hydrogen over the catalyst used. From the figure one can see that most scatter of data represents the use of eight different catalyst samples the data obtained over one catalyst sample lie almost on a straight line, within 95% confidence limits. 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

A redox mechanism involving lattice oxygen originally proposed in 1954 by Mars and Van Krevelen (22) for hydrocarbon oxidation over V2O5 can be applied to a variety of catalytic oxidation reactions (23). The following illustrates a lattice redox mechanism for CO oxidation ... 

For a large group of metal oxide catalysts, it has been proved that a redox mechanism occurs, as originally proposed by Mars and van Krevelen [204], The oxidation of the hydrocarbons, methanol, etc. is effected by oxygen supplied by the catalyst, very often by oxygen contained in the crystal lattice a vacancy results which is reoxidized by oxygen from the gas phase, viz. 

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. 

To describe the oxidation kinetics, a reduction—oxidation model is generally accepted. Mars and van Krevelen [204] were the first to apply this model to the benzene oxidation. The overall oxidation rate is expressed by... 

A redox mechanism for oxidation catalysis was proposed by Mars and van Krevelen (34) for the oxidation of aromatics over V205. This mechanism introduced the concept that lattice oxygen of a reducible metal oxide could serve as a useful oxidizing agent for hydrocarbons. Moreover, it formed the basis for the early work at SOHIO which led to the development of the bismuth molybdate catalyst. Since that time there have been many reports which support the redox concept. 

A point common to all the models is that they are based upon a redox-type mechanism, in which the reoxidation of the catalyst is not a limiting factor. Corresponding, none of them employ the model expression of Mars and van Krevelen (37). On contrast newer works by Keulks (38,39) assume, at lower reaction temperatures, a limiting effect from the reoxidation which leads to a dependence on oxygen partial pressure for the acrolein formation and to a two to three-fold higher activation energy compared with the reaction at higher temperatures. 

On ceria, unreduced gold seems to play a major role in the reaction (Section 6.3.3.6). Ceria may provide sites for the formation of superoxide and peroxide-type species,100 or act as an oxygen supplier for a reaction of Mars and van Krevelen-type.97 It is occasionally suggested that the reaction might go entirely on the support itself modified by gold ions, and forming a solid solution of type Cei a Au3 02 5-98... 

In many of these reactions a lattice constituent — oxygen or hydrogen — is brought into reacting molecules and the lattice defect is subsequently removed by a reaction with another reaction component, or by a reaction with another reactive centre of the same molecule. This is the so-called Mars and van Krevelen mechanism. With some oxides peroxide groups are formed on the surface (from adsorbed oxygen molecules). Radicals may form at high temperatures (1000 K). 

Fig. 4.55. Mars and van Krevelen redox mechanism of the selective oxidation (left side) or selective reduction (right side). Both reactions can also be coupled into a system of two selective reactions...

The type (2) reactions are frequently of a very high selectivity, which is widely exploited by industry (about 20% of all chemicals are produced by oxidation reactions). The type (2) reactions proceed by the so called Mars and Van Krevelen mechanism [117], which is schematically shown for an oxidation reaction in Fig. 4.55. 

Since the decomposition is regulated by the redox cycle Fe"/Fem represented by Eqs. 1,2, the kinetics can be treated by the very classical Mars and van Krevelen model (23, 40). Moreover, it was experimentally demonstrated that inhibition by excess 02 is of low extend for Co- and Fe-zeolite, the rate law can thus be expressed as ... 

Some of the oxides of vanadium and molybdenum catalyze the selective oxidation of hydrocarbons to produce valuable chemical intermediates. In a reaction path proposed by Mars and van Krevelen (see Section 10.5), the hydrocarbon first reduces the surface of the metal oxide catalyst by reaction with lattice oxygen atoms. The resulting surface vacancies are subsequently re-oxidized by gaseous O2. The elementary steps of this process are shown below. Electrons are added to the sequence to illustrate the redox nature of this reaction. 

There is now a considerable body of evidence to indicate that propene oxidation to acrolein occurs via a Mars and van Krevelen [123] mechanism whereby the reacting hydrocarbon, or a species derived from it, extracts lattice oxygen from the encapsulating surface layer of bismuth molybdate. In a separate step this lattice oxygen is replenished at least in part by lattice oxygen transfer from the encapsulated phases. These phases in turn are reoxidized by gas phase oxygen. This process is shown in Scheme 5.2. 

Scheme 5.2. Illustration of Mars and van Krevelen Mechanism with incorporation of gas phase oxygen into one phase and transfer to a separate surface phase for reaction with propene [114].

In regimes (i) or (ii), if the concentration of catalyst is constant, the formal rate equations are the same as that used by Mars and Van Krevelen to model partial oxidation reactions. Here we have ... 

In most studies it has been stated that the hydrocarbon is oxidized by lattice oxygen of the oxidized form of the catalyst, KO. Subsequently, the reaction of the reduced form, K, with 0 regenerates the initial state according to the well-known Mars and van Krevelen mechanism (32) ... 

V. Oxidation catalysts have to be considered with a dynamical view under reaction conditions. This is related to the Mars and van Krevelen mechanism which involves a redox mechanism and also to the mobility of the oxide lattice. This dynamical phenomenon results in the wetting effect observed under catalytic reaction conditions for multicomponent and supported oxide catalysts [46, 47]. It follows that for many catalysts a certain time on stream is necessary before the catalyst reaches its steady state. It is frequent that in an industrial plant a... 

The kinetics of the ODH of n-butane has been investigated for unpromoted and cesium promoted a-NiMoOa catalysts. The reaction rates of dehydrogenation products as functions of the butane and oxygen partial pressures are described by a kinetic model based on the Mars and van Krevelen mechanism. The effects of Cs on the kinetic parameters can be interpreted on the basis of recently published results concerning the properties of those catalysts. 

The kinetic studies found in literature (cf Introduction) suggest that this reaction proceeds via a Mars and van Krevelen mechanism [18] and then the following steps are involved ... 

Taking into account all the above mentioned results, a generalized Mars and van Krevelen model was considered for butane ODH over both catalysts. Consequently, the two steps mentioned above can be generally described by the following redox scheme ... 

The vapor phase catalytic oxidation of toluene to benzaldehyde has been studied over V20s-K2S04-Si02 catalysts in an isothermal differential reactor. The experiments were carried out at atmospheric pressure, temperatures from 410 C to 470 C and the modified spatial time (W/Fto) ranging from 0 to 180 g cat/mol toluene/h. The experimental tests showed the best performance for the catalyst obtained by co-precipitation. These results may be due to a crystalline phase identified in the process of catalyst characterization. Reaction kinetics was determined using the Mars and van Krevelen model. 

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

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