Mars-van Krevelen reaction

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

The initial work which promoted interest in conventional heterogeneous catalysis for methane coupling chemistry was first reported in 1982 [56]. In this work, the subject reaction was evaluated using 23 metal oxides supported on a-alumina over the temperature range 500-1000 °C. Methane and oxygen were, however, sequentially introduced into the reaction vessel, suggesting that the overall chemical reaction involved the general Mars-van Krevelen reaction sequence ... 

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. 

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

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. 

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... 

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. 

At lower temperatures the Mars-van Krevelen mechanism no longer applies. Sancier et al. (440) studied propylene oxidation in the presence of 1802 over bismuth molybdate and found that the acrolein product contained 180 and not exclusively leO from the oxide lattice in contrast with results obtained by Keulks and co-workers (441, 442) at higher temperatures. This lower-temperature oxidation must involve adsorbed oxygen in some form but the nature is not clear. It is now accepted that not all these oxidation reactions do involve lattice oxygen (442,443). 

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. 

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. 

Oxide ions of the support were shown to participate in the reaction through a Mars-van Krevelen mechanism (Section 1.4), and the supports also acted as structural promoters to stabilise the small gold particles.55... 

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 selective oxidation of propylene to acrolein was suggested to occur via a Mars-van Krevelen mechanism (i.e. reaction of the hydrocarbon with lattice oxygen), where in the first step bismuth dehydrogenates propene to form an allylic species. This has mainly been concluded from isotopic scrambling studies. The oxidation step itself is believed to occur via the mechanism shown in Figure 7.11. 

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... 

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