Mars-Krevelen

The mechanism of the oxidation reaction involving oxygen of the lattice is well known as Mars-Krevelen model. Thus, for example, the oxidation of carbon monoxide on oxides can be explained schematically as follows [6-9] ... 

Catalytic oxidations on the surface of oxidic materials usually proceed according to the Mars-Van Krevelen mechanism [P. Mars and D.W. van Krevelen, Chem. Eng. Sci. 3 (1954) 41], as illustrated in Fig. 9.17 for the case of CO oxidation. Instead of a surface reaction between CO and an adsorbed O atom, CO2 is formed by reaction between adsorbed CO and an O atom from the metal oxide lattice. The vacancy formed is filled in a separate reaction step, involving O2 activation, often on defect sites. 

Figure 9.17. Mars-Van Krevelen mechanism for the oxidation of CO on a metal oxide surface. A characteristic feature is that lattice oxygen is used to oxidize the CO, leaving a defect that is replenished in a separate step by oxygen from the gas phase.

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

Explain the Mars-van Krevelen mechanism. In what sense does it differ from a metal-catalyzed reaction ... 

Can the hydrodesulfurization reaction also be considered to be a Mars-van Krevelen reaction ... 

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. 

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. 

Irreversible adsorption The LH mechanisms assume that the adsorption of all gas-phase species is in equilibrium. Some mechanisms, however, occur by irreversible steps. In these cases, the intermediates are again treated in the same manner as reactive intermediates in homogeneous mechanisms. An example is the Mars-van Krevelen (1954) mechanism for oxidation, illustrated by the following two steps ... 

A mechanism for the catalytic oxidation of hydrocarbons is due to Mars Van Krevelen (Spec Suppl Chem Eng Sci 3 41, 19541. These assumptions are made ... 

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. 

Figure 7.15 Scheme of the Mars-van Krevelen mechanism for selective CO oxidation in excess hydrogen over Cu01Ce0 9O2 ynanostructured catalyst. (Reprinted from [62], With permission from Elsevier.)... 

The major drawbacks to standard sol-gel synthesis include slow growth rate and the typically amorphous product, rather than defined crystals, which requires crystallization and post annealing steps. Growth rate and crystallization of the fabricated hybrid can be improved via solvothermal, reflux [224], sonication, and microwave [225] treatment. However, the air oxidation of CNTs (600 °C) and graphene (450 °C) may still be lower than MO crystallization temperature. Moreover, it has been shown that the MO coatings on CNTs can drastically affect their thermal oxidation, particularly with easily reducible metal oxides (e.g. TiOz = 520 °C, Bi203 = 330 °C) [180]. It appears that metal oxides can catalyze the oxidation of CNTs via a Mars van Krevelen mechanism, limiting the maximum temperature of their synthesis as well as applications (i.e. catalysis, fuel cells). 

Many-body effects, 34 214-215 on deep core-level spectra of metals, 34 215 Many-body Hartree-Fock approach, 34 244 Mars-van Krevelen mechanism, 41 211 reaction, 32 120-121 Mass spectrometry, 30 302-304 of C-labeled hydrocarbons, 23 22-25 in detection of surface-generated gas-phase radicals, 35 142-148 apparatus, 35 145... 

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... 

P. Mars and D. W. van Krevelen, Oxidations carried out by means of vanadium oxide catalysts,... 

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 ... 

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. 

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