Mars-van Krevelen model

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... 

In selective oxidations, the presence or absence of spillover oxygen has an effect on the facetting and reconstruction of MoC crystallites, as demonstrated using SEM and AFM. These results highlight the necessity to keep the surface smicture in an optimal configuration during the whole catalytic cycle. A modified Mars-van Krevelen model can account better than previous ones for the reduction-oxidation cycle and the change in number of the active sites as a function of the reaction conditions. 

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). 

Wang et al and Lou and Lee assumed the reaction occurred between oxygen on the active site and Cl-VOC from the gas phase as in the Rideal-Eley mechanism. This is coincident with the equation proposed by the Mars—van Krevelen model... 

Dumesic et al. proposed a model involving six steps based on the general Mars-van Krevelen mechanism for oxidations ... 

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. 

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 ... 

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. 

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. 

Eley—Rideal or VCI component predominating at high PKOH may also be accommodated within this set of possible surface reactions [e.g. eqns. (45f) and (45h) are predominantly Langmuii Hinshelwood in character, whereas eqns. (45c) and (45g) could introduce Eley—Rideal or VCI character]. It is worth noting that no comparably successful explanation of the dependence upon isopropanol pressure could be achieved using models of the Mars—van Krevelen type [256]. 

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... 

The heterogeneously catalyzed reaction of starting materials A and B can follow different mechanisms [Claus 1996, Ertl 1990]. The models of Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen are widely used in practice. 

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 experimental initial rate curves in the Berty reactor (Fig. 2.b) shows a trend that can represent either type b ox e among the theoretical graphs in the Figure 1. Since the model Ml 7 belongs to type c it can be removed in the final selection from likely mechanisms. The two retained models. Ml 8 and M20 both are of Mars-van Krevelen types where the steady-state assumption involves the equality of the rate of three elementary steps as it has been shown in the Table 1. 

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) ... 

In the case of the Standard SCR reaction (R.6 or R.8), a dual-site Mars-Van Krevelen rate expression assuming that NH3 blocks the red-ox sites for NO activation was adopted in line with previous work performed over both vanadium-based [30, 36] and Fe-zeolite catalysts [12], in order to explain the observed inhibiting effect of NH3 on such a reaction at low temperatures. The model has been then extended to include also the SCR reactivity in the presence of excess NO2 and particularly to describe also the reactivity of N2O (R.14 and R.15). 

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... 

It is worth noting at this point that a H-W model invoking product desorption as the RDS could also give equation 7.30 for values of n = 1 or 2, provided the fraction of vacant sites is very low and the fraction of sites covered by the two reactants is very high. It may also be possible to propose a more realistic sequence of steps comprised of reversible and irreversible elementary steps (perhaps even including some quasi-equilibrated steps) that could result in a rate expression of the Mars-van Krevelen form. Consequently, the Mars-van Krevelen rate expression should be considered to be only a mathematical fitting equation with no theoretical basis. 

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 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. 

---