Other Structure-Sensitive Oxidation Reactions

While the effect of crystallite size has been investigated for reactions on iron oxide, the dependence of the activity and selectivity of other oxidation reactions on the nature of the exposed surface planes has been investigated for reactions on Mo03 and V2Os catalysts. A list of these reactions, the catalysts used, and the major conclusions are listed in Table IX. It appears that all the reactions listed are structure sensitive, that is, different crystal faces catalyze different reactions, or special active sites are required. 

Structure Sensitive Oxidation Reactions on Transition Metal Oxides 

Oxidation of benzene to maleic anhydride 613-723 V205/Ti02, v2o5/ai2o3 Selectivity for maleic anhydride increases with the thickness of V2Os layers on the support Reaction rate is proportional to the density of surface V=0 65 

CO oxidation 613-745 v2o5, V205/Ti02 V—O on the (010) face has low activity surface defects have high activity 68 

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Azide polymers contain -N3 bonds within their molecular structures and burn by themselves to produce heat and nitrogen gas. Energetic azide polymers burn very rapidly without any oxidation reaction by oxygen atoms. GAP, BAMO, and AM-MOare typical energetic azide polymers. The appropriate monomers are cross-Hnked and co-polymerized with other polymeric materials in order to obtain optimized properties, such as viscosity, mechanical strength and elongation, and temperature sensitivities. The physicochemical properties GAP and GAP copolymers are described in Section 4.2.4. 

Several studies suggested that CH4 partial oxidation over Rh and other metals (that is, the combined activation of CH4 by O2. H2O and CO2) involves a network of structure-sensitive reaction steps (including C-H and C-0 bond breaking) [151—... 

This bimolecular coupling is a structure-sensitive reaction, and it illustrates a key characteristic of metal oxides multiple coordinative unsaturation of surface metal cations may facilitate coupling of ligands in a manner similar to that for unsaturated metal complexes in solution. Examples of other coupling reactions... 

We have seen how studies on the various faces of Fe and Ni have contributed to the understanding of the structure sensitivity of the ammonia synthesis and of the hydrogenolysis of alkanes. Clearly, the effect of crystal faces, steps, and kinks on other reactions should be pursued. In some cases, kinetics can be measured at vacuum conditions, permitting the use of transient methods. The example of CO oxidation is striking (315). 

The reason for this is the relative instability of Pt oxide and its tendency to decompose at temperatures <550 C even under oxidizing conditions. Platinum oxide does not penetrate into the subsurface of any support material. To achieve a high dispersion additives are used in which the oxygen ions are more reactive than in 7-AI2O3 as for instance CeC. Thus the addition of 2.6% ceria to 7-AI2O3 increases the surface concentration of PtO from 2.2 /imole Pt/m to 4.2 /imole Pt/m (BET) [3]. As will be shown below other more reactive metal oxide additives have a similar effect on Pt dispersion. This, in turn affects the behavior of the catalysts with respect to several structure sensitive reactions. 

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