Selective oxidation adsorbed oxygen, role

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 role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

A third group contains those metal catalysts which do not form specific crystal phases in an oxidized state. The common types of oxygen on the surface are then 02 (adsorbed) and O (adsorbed) which generally do not lead to selective oxidation. One of the exceptions is silver, which very probably catalyses the selective oxidation of ethylene by providing 02 on the surface. However, an active role of surface oxides, which may be formed particularly by the action of promotors, is not excluded. 

The existence of the molecular radical ion 02 , of atomic O-, and of the regular ions in the lattice O2- has been firmly established. A review by Lunsford (33) presents a summary of the experimental evidence which led to the discovery of 02 and O-. The participation of these various forms of oxygen in hydrocarbon oxidation is discussed in a review by Sachtler (11). It seems clear that both adsorbed and lattice oxygen species play an important role in the selective oxidation of hydrocarbons. 

Centi and coworkers [84] have suggested that, in addition to Lewis acidity, Br0nsted acidity plays an important role in the selective oxidation of butane to maleic anhydride. The surface phosphorus enrichment means that a number of P—OH groups are present on the catalyst surface. Centi offers three hypotheses for the role of Bronsted acidity the stabilization of reaction intermediates, the stabilization of an adsorbed oxygen species, or the generation of an organic surface species that is involved in oxygen activation or transport. 

The oxidation of -butane to maleic anhydride is a 14-electron oxidation. It involves the abstraction of eight hydrogen atoms, the insertion of three oxygen atoms, and a multi-step polyfunctional reaction mechanism that occurs entirely on the adsorbed phase. No intermediates have been observed under standard continuous flow conditions, although mechanisms for this process have been proposed based on a variety of experimental and theoretical findings. The description of the active site is linked to the mechanism and is the subject of considerable debate in the literature. The mechanisms are linked to the researchers hypotheses of the active site, which will be discussed in a separate section in this chapter. It is widely accepted that the (100) plane of vanadyl pyrophosphate, (VO)2P207, (referred to as the (020) plane by certain authors) plays an important role in the selective oxidation of butane. 

One of them is that the oxidation of organics over perovsldte oxides proceed by two reaction pathways the first suprafacial and the second intrafacial. In the suprafacial process, the reaction rate is correlated with the electronic configurations of the surfeice transition metal ions and takes place between the adsorbed species on the surface at relatively low temperatures. Conversely, the intrafacial mechanism takes over at high temperatures and the reaction rate appears to be correlated primarily with the thermodynamic stability of oxygen vacancies adjacent to a transition metal ion. However, the role of these oxygen species in complete and selective oxidation and their participation in combustion over various ceramic perovskite phases is still an open question. 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

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