Krevelen mechanism

The emission control catalysts used in the exhaust mufflers have precious metals supported over oxides. The support oxides serve several functions, including providing mechanical stability and necessary surface acidity. But one of the oxides, Ce02, is placed in the catalytic converters to serve as oxygen pump between the gas phase oxygen and surface reaction taking place over the precious metal. 

Oran and Uner (2004) have carried out extensive reaction tests and determined the reaction orders of CO oxidation as -2 and 1 with respect to CO and oxygen over Pt/y-Al203, while the corresponding values were reported as -1 and 0 over Pt/Ce02 and Pt/Ce02/y-Al203. Proposeand justify a mechanism for this system. 

As a result of the first assumption, the vacant site coverage could be written as 

Given that the first step proceeds under pseudoequilibrium conditions 

The third assumption renders the surface coverage of oxygen over ceria almost constant such that 

---

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 ... 

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. 

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... 

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). 

In this context, it is noteworthy that Over and co-workers (46,164-167) found the same types of Mars-van Krevelen mechanism for CO oxidation for ruthenium. Although the active surface was not characterized directly under high-pressure conditions in these investigations, it was found for the ruthenium(0 0 01) surface, which forms a RuO2(l 1 0) thin film in an oxygen-rich environment, that the activity of ruthenium as an oxidation catalyst is in fact primarily related to the RUO2 phase. 

Krevelen mechanism (1) is that the catalyst undergoes an oxidation-reduction cycle during reaction ... 

Indeed, this pathway is followed in many gas phase oxidations over metal oxide catalysts, such as vanadium pentoxide and bismuth molybdate. It is generally referred to as the Mars-van Krevelen mechanism after its original... 

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... 

From the extensive literature it can be concluded that a Mars-van Krevelen mechanism applies the oxidation of the hydrocarbons is achieved using the surface oxygens and the reoxidation of the catalysts is accomplished by reduction of oxygen at separate sites. 

In many of these reactions a lattice constituent — oxygen or hydrogen — is brought into reacting molecules and the lattice defect is subsequently removed by a reaction with another reaction component, or by a reaction with another reactive centre of the same molecule. This is the so-called Mars and van Krevelen mechanism. With some oxides peroxide groups are formed on the surface (from adsorbed oxygen molecules). Radicals may form at high temperatures (1000 K). 

The type (2) reactions are frequently of a very high selectivity, which is widely exploited by industry (about 20% of all chemicals are produced by oxidation reactions). The type (2) reactions proceed by the so called Mars and Van Krevelen mechanism [117], which is schematically shown for an oxidation reaction in Fig. 4.55. 

---