Lattice oxygen species

Based on the experimental data and some speculations on detailed elementary steps taking place over the catalyst, one can propose the dynamic model. The model discriminates between adsorption of carbon monoxide on catalyst inert sites as well as on oxidized and reduced catalyst active sites. Apart from that, the diffusion of the subsurface species in the catalyst and the reoxidation of reduced catalyst sites by subsurface lattice oxygen species is considered in the model. The model allows us to calculate activation energies of all elementary steps considered, as well as the bulk... 

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. 

Use of Halide Ions to Improve Selectivity. Earlier work has claimed that enhanced selectivities for alkene oxidation can be achieved by the inclusion of electronegative elements such as S, Se, or halogens. This has been reviewed elsewhere. " More recent work has demonstrated substantial improvements in selectivity for propene (25—70%) and isobutene (35—80%) oxidation when either chloride or bromide is present. Both elements are added to the catalyst in the form of trace levels of organo-halide in the process gas stream. The selectivity increase is the result of a decrease in the rate of complete oxidation rather than an increase in the partial oxidation rate. Since the reaction is first order in oxygen pressure and zero order with respect to alkene in the presence and absence of halide, the reaction mechanism is probably similar in both cases. In the light of Anshits recent work, the effect of the halide is presumably to reduce the relative number and/or reactivity of surface lattice oxygen species and thus reduce the amount of irreversibly adsorbed alkene. 

The exceptional activity exhibited by ion-exchanged copper ZSM-5 zeolite catalysts for nitric oxide (NO) decomposition, and for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) in the presence of excess oxygen is well documented [1-10]. The nature of the active copper species in the SCR reaction however still remains uncertain. We and others have recognised that there are two different types of copper species within the ZSM-5 zeolite channels [11]. Isolated copper ions exist in low symmetry environments, and small clusters, where the copper atoms are linked by extra-lattice oxygen species such as [Cu(II)-0-Cu(n)] dimers, are also present. Recent studies have also suggested that the isolated copper ions in ZSM-5 occupy two types of sites [11], which may have different SCR reactivity. It is likely... 

Lao.sSra2 Nio,sMa2 (M = Bi, Co, Cr, Cu, and Fe) MDR Highest stability with Lao Sro j, Nio,8Feo,203 due to high metal-support interaction and abundant lattice oxygen species minimizing carbon formation 54... 

Redox catalysts such as ceria modified with Group 1 or 2 metal oxides are also effective for the CO2-OCM reaction [30, 31]. The redox interchange between Ce" and Ce " oxidation states is crucial for CO2 dissociation to CO and lattice oxygen species. The modification of ceria with metal oxides such as... 

According to Ferreira-Aparicio et al. [190], the supply of surface oxygen species from the hydroxyls of the acidic supports can aid the formation of methoxo (CH ) species. Based on FTIR spectroscopy analysis of methane adsorption on alumina, Li et al. [191] observed the presence of two hydroxyl signals at 3750 and 3 665 cm which shifted to 3707 and 3640 cm upon adsorption of methane. Their results indicate the possibility of weak interaction between methane and surface hydroxyls, a phenomenon also observed with Ir catalysts during methane decomposition [192]. Similarly, on perovskite- and pyrochlore-type catalysts, the lattice oxygen species on the surface were found to assist the methane activatiOTi [167, 174, 193]. 

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