Modification of the Surface Defects

The N20 molecule is a donor of atomic oxygen (the heat of the reaction N20= N2+0(3P) is 40kcal/mol [32]) and it is used to obtain the paramagnetic species like O- from the F-sites on various oxide surfaces [33,34]  

However, only for the system =Si + N20, the intermediate product of this reaction was registered experimentally [30]  

When heated above 350 K, the complex is decomposed. Molecular nitrogen is evolved to the gas phase in an amount comparable with the number of decomposed paramagnetic centers (PCs) and the (=Si-0-)3Si-0 radicals are formed  

In addition to the reverse reaction (the abstraction of nitrous oxide), another possible channel for the transformation of the complex is its isomerization and decomposition resulting in the formation of the oxy radical =Si-0 and molecular nitrogen. These products correspond to the absolute minimum on the potential energy surface of the =Si + NzO system. 

one of the possible channels to the final products of the reaction of silyl-type radicals with nitrous oxide (oxy radical and molecular nitrogen) is the formation of a metastable complex and its further isomerization and decomposition. 

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NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

The results of the study of the decomposition of nitrous oxide at 250° on a divided nickel oxide confirm therefore that the surface structure of a catalyst may be modihed in the course of the catalytic reaction at a moderate temperature. They have shown, moreover, that different gases (nitrous oxide and oxygen) which are both oxidizing agents, may induce, at the same temperature, different modifications of the surface defect structure of the solid and therefore, change its catalytic activity in different ways. 

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