Mars Van Krevelen redox mechanism

The aikoxy group then ioses a second hydrogen and desorbs as an aldehyde or ketone. This series of transformations forms the first part of the Mars-van Krevelen redox mechanism, which is then followed by adsorption of oxygen from the gas phase, transfer of electrons from the solid to adsorbed oxygen molecule and incorporation of oxygen ions into the lattice of the oxide, which completes the redox cycle. The cycle involves two adsorbed redox couples ... 

It is now well established that, in the most active and selective catalysts used in fixed- or fluidized-bed reactors, only a few layers of VPO participate in the reaction. One may think that the Mars-van Krevelen redox mechanism is ruled out, or at least that it is confined to the outermost layers. The question arises as to how this picture holds when working deliberately in the transient state using alternate pulses or periodic feed, with the idea of separately optimizing the two steps of the redox mechanism, or when alternative reactors in which the redox system is physically separated by decoupling. The next paragraphs address this question, as well as show some important differences in using the same DuPont catalyst in different reactor configurations. 

Scheme 15.2 Proposed Mars-van Krevelen redox mechanism over reducible metal oxide catalysts showing the formation of an OH group and a carbanion, where Z is a vacant site...

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. 

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...

This Equation does not differ from the usual Mars-Van Krevelen redox equation. The rate constants of the separate steps of oxidation and reduction from Equation (11) are listed in Table 3. They are-compared in the same Table with the rate constants determined separately from the experiments on reduction and reoxidation. The coincidence between the calculated and experimental rate constants confirms the proposed redox mechanism of allyl alcohol oxidation over the rhombic phase of V-MoOg catalyst. 

The Pd-O bond also varies with the extent of oxidation of Pd. During the methane combustion reaction, the catalyst surface is a non-equilibrium, kineti-cally controlled structure. The oxygen concentration profile in the particle results from a combination of particle reconstruction, oxygen adsorption, bulk diffusion, and oxygen removal. This concentration profile varies as a function of time, and as the oxygen content increases, the Pd-O bond strength decreases. This increase is accompanied by an increase in the specific activity. The most widely accepted reaction pathway is the Mars and van Krevelen redox mechanism, which involves lattice oxygen and uneoordinated Pd centers as active species. Inhibition by products (H2O and CO2) and impurities (SO2) is a major drawback for low temperature combustion. The effect of sulfur is particularly important for catalytic converters for NGV applications because it drastically reduces the methane combustion activity. 

A completely different approach to selective oxidation, with respect to the H2O2/TS-I systems already described, was based on depletive, or redox, systems. This approach is obtained in atmospheres lacking an oxidant in the gas phase the oxidation of the organic substrate takes place through lattice oxygen atoms of a, usually, multi-metal oxide pseudo-catalyst (or cataloreactant), via a Mars-van Krevelen type mechanism, and is followed by the re-oxidation of the reduced oxide in a separate step, spatially or temporary, thereby formally closing a catalytic cycle (Reactions 15.10 and 15.11). 

In this process, which proceeds via a Mars-van Krevelen (redox) type mechanism [85], the role of CO2 as an oxidant is critical in re-oxidizing the lower oxidation state metal oxide (7.13) and as shown in Scheme 7.4 [37,47]. 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

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... 

It is known [41] that partial oxidation reactions in heterogeneous catalysis involves redox properties of the solid catalysts, allowing the well known Mars-van Krevelen mechanism [42] to occur, or at least to be facilitated. Acid-base properties are also an important feature, as they play a determining role in the activation of the reactants and in the desorption of the intermediate compounds, for instance, an acid surface will favor desorption of acid products, thus avoiding further over-oxidation, while a basic surface will favor desorption of basic products as olefins. It follows that heteropolyoxometallate compounds, in particular TMSP, appear as potential... 

The quantitative agreement between the rates of catalytic oxidation observed experimentally and those predicted from the reduction and oxidation rates of the catalysts measured independently demonstrated that the catalytic oxidation proceeded by redox cycles of the catalysts, that is, the redox mechanism or Mars-van Krevelen mechanism [3]. 

A redox mechanism (Mars-van Krevelen) is generally accepted for the ammoxidation reaction of methyl aromatic compounds, thus most catalysts applied contain transition metal oxides (e. g. vanadium, molybdenum) readily enabling changes in valence states. 

Many industrially important selective oxidation reactions are catalyzed by transition metal oxides. The activity of such catalysts is related to the reducibility of the transition metal ion, which enables the bulk oxide lattice to participate actively in the redox processes present in the Mars van Krevelen mechanism. Unfortunately, NMR spectroscopic investigations are severely limited by the occurrence of paramagnetic oxidation states. As a general rule, NMR signals from atoms bearing unpaired electron spins cannot be detected by conventional methtxls, and the spectra of atoms nearby are often severely broadened. For this reason, most of the work published in this area has dealt with diamagnetic vanadium(V) oxide-based catalysts. 

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... 

As shown by Golodets (p. 289), the formation of the carbonate intermediate also may be interpreted in terms of the more conventional redox or Mars-van Krevelen cycle, which explicitly involves the oxidation and reduction of the metal oxide." The Mars-van Krevelen mechanism of catalytic oxidations over metal oxides is a redox mechanism involving both gas phase and lattice oxygen ... 

The redox mechanism implied in Equations (9.1) and (9.II) is essentially the same as the Mars-Van Krevelen mechanism, with the difference that the oxygen used to oxidize the catalyst comes from the water rather than the gas-phase oxygen. 

In most cases, the selective oxidatitMi of hydrocarbons by oxides occurs through the so-called Mars-van Krevelen mechanism [68], in which the organic molecule reacts with the lattice oxygen, and then the oxygen vacancy is replenished by gaseous oxygen. As a result, the redox properties of oxides influence their ability to complete such a catalytic cycle. We have demonstrated that the activation of an alkane C-H bond usually undergoes H-abstraction mechanism on a transition metal oxide [69] ... 

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) ... 

Although the exact reaction mechanism is unclear, general concepts regarding the fuel oxidatiOTi reaction on LSCM have been described in the literature. Yamazoe and Teraoka pointed out that high-temperature oxidation reactions on perovskites typically occur through a reduction-oxidation cycle of the catalysts, otherwise known as a Mars-van Krevelen-type (MvK) mechanism [75], with the B-site elements serving as the redox centers. Studies suggest that this is the case for LSCM. 

The comparison of P- and Pd-promoted catalysts shows that Pd-promoted catalysts are more selective towards the formation of acetic acid. Other advantages are that they do not produce CO as a reaction product and display better redox behavior. A study of catalyst redox property revealed that Pd facilitated both the reduction of the MoVNb oxide catalyst with ethane and its re-oxidation with oxygen. This is very important because the reaction occurs via the redox mechanism of a Mars-van Krevelen type. Such a conclusion comes from the data presented in Fig. 11.1. With increasing the amount of oxygen removed from the surface of the MoVNbPd... 

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