Mars-van-Krevelen kinetics

At 500°C, the reaction is confined to the surface of the catalyst and can be described with Mars-van Krevelen kinetics. The rate is insensitive to the state of the catalyst bulk and hence there was no relationship between e.m.f. and rate. 

Combined with their kinetic measurements, the authors proposed CO from the gas phase could directly react with oxygen atoms in the surface oxides, accounting for relatively high reactivity of this phase for CO oxidation. This mechanism, termed as Mars-Van Krevelen mechanism, challenges the general concept that CO oxidation on Pt group metals is dominated by the Langmuir-Hinshelwood mechanism, which proceeds via (1) the adsorption of CO and the dissociative adsorption of 02 and (2) surface diffusion of COa(j and Oa(j atoms to ultimately form C02. 

Kinetics There have been few comprehensive studies of the kinetics of selective oxidation reactions (31,32). Kinetic expressions are usually of the power-rate law type and are applicable within limited experimental ranges. Often at high temperature the rate expression is nearly first order in the hydrocarbon reactant, close to zero order in oxygen, and of low positive order in water vapor. Many times a Mars-van Krevelen redox type of mechanism is assumed to operate. 

Eley-Rideal) mechanism, one of the reactants comes directly from the fluid phase to react with the other, which is already chemisorbed. This procedure was devised to explain the kinetics of the hydrogen-deuterium reaction on certain metals (see Section 9.2), but has also been suggested for other reactions. The Mars-van Krevelen mechanism applies to oxidations catalysed by oxides that are easily reducible, and are therefore able to release their lattice oxide ions for the purpose of oxidising the other reactant they are then replaced by the dissociation of molecular oxygen. With gold catalysts supported on such oxides, it is sometimes proposed that this mechanism plays a part in the total process. 

The Langmuir-Hinshelwood kinetic model describes a reaction in which the rate-limiting step is reaction between two adsorbed species such as chemisorbed CO and 0 reacting to form C02 over a Pt catalyst. The Mars-van Krevelen model describes a mechanism in which the catalytic metal oxide is reduced by one of the reactants and rapidly reoxidizd by another reactant. The dehydrogenation of ethyl benzene to styrene over Fe203 is another example of this model. Ethyl benzene reduces the Fe+3 to Fe+2 whereas the steam present reoxidizes it, completing the oxidation-reduction (redox) cycle. This mechanism is prevalent for many reducible base metal oxide catalysts. There are also mechanisms where the chemisorbed species reacts... 

The kinetics of the heterogeneously catalysed vapor-phase oxidation of a,p-unsaturated aldehydes to the a,p-unsaturated acid has been investigated for a Mo-V-Cu-oxid catalyst. The reaction rates of aldehyde and oxygen consumption as function of the aldehyd, oxygen, acid and water partial pressure are described by a kinetic model based on a modified Mars-van Krevelen mechanism Also the rate of the a, p-unsaturated acid oxidation has been measured and described for varied partial pressures of acid, oxygen and water. 

A kinetic study has been carried out on the partial oxidation of methane to formaldehyde over a silica-supported vanadia catalyst. The results indicate that oxygen was adsorbed on the catalyst and took part in the reaction in an Eley-Rideal or Mars-van Krevelen maimer. The nature of the interaction with the catalyst was dependent on whether the reaction took place in methane rich (pcH4 = 80 kPa) or lean (po,4 = 4 kPa) conditions. A reaction mechanism for the partial oxidation of methane to formaldehyde is proposed, which is consistent with the data reported here. Methanol oxidation experiments over this catalyst suggested that it was not an intermediate under the conditions employed during this study. 

The experimental results were well described by a kinetic model based on a redox mechanism of the Mars-van Krevelen type, where the quinone surface groups are reduced to hydroquinone by adsorbed ethylbenzene, and reoxidized back to quinone by oxygen [46,63,64], as shown in Figure 6.3. A recent... 

It is possible to incorporate in this model the kinetics of the mechanism described in eq. (8) and (9), namely the so-cdled Mars-van Krevelen mechanism. The reduction and oxidation rates are represented by... 

Equations of this form (with n=0.5) represent satisfactorily results of careful kinetic measurements in the oxidation of isobutene to methacrolein on mixtures of separately prepared a-Sb204 and M0O3 of different compositions (34,35). Until this, published lanetic models in selective oxidation were either purely empirical or, if based on the Mars-van Krevelen model, contained fractional exponents which had no meaning in terms of mechanism (34,35). 

Many kinetics studies have been performed for methanol to formaldehyde oxidation over iron molybdates. The majority of authors accept a Mars van Krevelen mechanism. However a great number of kinetic laws have been presented for this reaction. 

The kinetics of methane combustion over a perovskite catalyst (Lao.9Ceo.iCo03) has been studied in Micro-Berty and fixed bed reactors. Discrimination among twenty-three rival kinetic models from Eley-Rideal, LHHW and Mars-van Krevelen (MVK) types has been achieved by means of (a) the initial rate method as well as by (b) integral kinetic data analysis. Two MVK type models could be retained as a result of the two studies, with a steady-state assumption implying the equality of the rate of three elementary steps. 

The transient kinetic model of the standard SCR reaction over a commercial V-based catalyst for vehicles reported in Reference (101) is the only treatment available so far accounting both for the redox nature of the SCR catalytic mechanism and for the ammonia inhibition effect. It relies on a dual-site redox scheme, whereby ammonia is first adsorbed onto acidic sites, but reacts with NO on different redox sites associated with the vanadium component. The redox sites can, however, be blocked by excess ammonia. Adopting a Mars-Van Krevelen formal approach, the following modified redox (MR) rate expression was derived (27) ... 

Miranda et a/. studied the kinetics of TCE oxidation using four different models Langmuir-Hinshelwood (reaction between the two adsorbed species was the rate-controlling step), Rideal-Eley (assuming that oxygen reacts from the gas phase), Mars-van Krevelen (Eq. 4.10), and a CI2 inhibition model (Eq. 4.11). The last of these resulted in the best fit with the experimental laboratory data... 

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