Mars-van Krevelen-type

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]. 

Lewandowski M, Groot IMN, Shaikhutdinov S, Freund HJ. Scanning tunneling microscopy evidence for the Mars-van Krevelen type mechanism of low temperature CO oxidation on an FeO(lll) film on Pt(lll). Catal. Today. 2012 181 52-55. 

An Eley-Rideal/Mars-van Krevelen type of mechanism was found for the partial oxidation of methane to formaldehyde. The differences in the rate equations were due to differences in the amounts of oxygen present in methane rich and lean conditions. Methanol was not an intermediate in the reaction. Methanol oxidation experiments indicated that methanol was oxidised sequentially to formaldehyde, carbon monoxide and hence to carbon... 

In the gas phase a Mars-van-Krevelen type mechanism can compete successfully with free radical autoxidation because if free radicals are formed they are not surrounded by substrate molecules (RH) as in the liquid phase, i.e. free radical chains are very short. Conversely, a Mars-van-Krevelen type mechanism is difficult to achieve with 02 in the liquid phase (but see later) due to competing facile autoxidation. The key to designing selective catalysts for liquid phase oxidations is to create a gas phase environment in the liquid phase. 

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... 

At first sight, Ri and R2 mechanisms appear to be better probes of Mars-van Krevelen type processesli i l in which substrates are oxidised via lattice oxygen, generating vacancies which are replenished from the gas-phase. However, despite the different experimental approaches... 

Scheme 3.18 Mars van Krevelen-type mechanism for benzyl alcohol oxidation by molecular oxygen promoted by GO.

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. 

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. 

Scheme 9.11 Mars-van Krevelen-type oxygenation of anthracene and xanthene.

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... 

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). 

Mars-van Krevelen type redox mechanism is widely suggested for the ODH of light alkanes over reducible metal oxides at low temperatures. During this type of reaction, the light alkanes and intermediate products react with lattice... 

Great care should therefore be exercised when data from the catalytic literature relating to the Mars-van Krevelen type of catalysts... 

It needs to be mentioned that, within the Mars—van Krevelen-type catalytic cycle, the elementary steps of CO oxidation by lattice oxygen may stiU follow a Langmuir—Hinshelwood or Eley—Rideal mechanism. In the Langmuir—Hinshelwood mechanism, the CO molecule adsorbs first on the ceria surface before undergoing the oxidation, whereas in the Eley— Rideal mechanism, the CO molecule attacks directly the surface oxygen from the gas phase. As we have already mentioned when we discussed... 

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