Chemical Reactivity of Reducible Oxides

A key question in selective oxidation catalysis is whether the reactive surface-oxygen species is used to activate C-H bonds or is used, instead, to aid in the addition of oxygen to the reactive surface intermediates to form oxygenated products. We analyze the results for studies carried out over both M0O3 or V2O5 surfaces. 

The topmost layer in Fig. 5.18 is made up of the neutral coordinatively saturated cations in the (001) surface for the layered V2O5 bulk material. In contrast, the (110) surface is formed by the cleavage of covalent V-0 bonds at the surface, thus resulting in coordinatively unsaturated V and 0 atoms at its side faces as shown in Fig. 5.18. The less reactive (001) surface is thought to be responsible for the selective oxidation of hydrocarbons such as toluene, whereas the reactive (110) surface predominantly results in total oxidation. This surface is also easily hydroxylated and, hence, contains both Brpnsted acid and base sites. 

As a final example in selective oxidation, we summarize the current understanding of the mechanism involved in the removal of NO with NH3 over vanadium oxideI l The reaction is important for the selective catalytic reduction (SCR) processes which target reducing vehicular emissions of combustion systems rich in air. 

As observed from Fig. 5.21, the NH cleavage reaction is the rate-limiting step that occurs on the surface in the formation of NH2NO. This reaction proceeds after a succession of isomerization steps, which are assisted by the presence of acidic VOH groups. The overall calculated activation barrier for this reaction is close to that of the NH2NO formation reaction. The preference of the catalytic reaction over the gas-phase reaction 

The energetics predicted may depend on the cluster size used to model this system or on the actual particle size. For oxidative dehydrogenation of alkanes catalyzed by vanadia, it has been shown that the activity per vanadium atom increases with increasing size of the vanadium particle. This is due to reduced electron transfer of oxygen to vanadium on the smaller particle and, hence, to a the lower reducibility of the smaller nano-sized particles . The lower coordination number of vanadium implies that an increase in charge is less easily accommodated on the smaller particle. 

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