Proposed Active Sites and Mechanisms of n-Butane Oxidation

6 Proposed Active Sites and Mechanisms of n-Butane Oxidation 

The best VPO catalysts preferentially expose the (200) planes of (VO)2P207 [4, 10-13]. Therefore, the models of n-butane oxidation on the VPO catalysts proposed to date are based on the hypothetical active sites present on the surface (200) plane. The following types of hypothetical active sites have been proposed to exist on the surface (200) plane [13] (i) Bronsted acid sites, probably -POH groups (ii) Lewis acid sites, probably and (iii) one electron redox couple, yv/yiv, V /V (iv) two electron redox couple, (v) bridging oxygen, 

or VO(P)V (vi) terminal oxygen, = 0, V = 0 and (vii) activated molecular oxygen, OLperoxo and 0 -superoxo species. The roles of these species according to the proposed mechanisms of n-butane oxidation are discussed below. 

Pepera et al. [80] further argued that an active surface V site could be viewed as a transition-metal radical. While highly active oxygen species [85] would 

The hydrogen atom abstraction by the Lewis acid site in the model of Centi et al [10] may be broken up into the following steps  

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