Bismuth molybdate catalyst diffusion

III. Stability of the Multicomponent Bismuth Molybdate Catalyst Depending on the Bulk Diffusion of Oxide Ion... 

Improved catalytic performance, selectivity and resistance to fusion, over bismuth molybdate catalysts was reported by McClellan (90) for catalysts obtained by chemically combining bismuth, molybdenum, phosphorus, and silica. After calcination at 450°C, the bismuth phosphomolybdate-on-silica catalyst showed an X-ray pattern of mainly crystalline Bi2(Mo04)3 which subsequently was converted to a new, substantially amorphous, phase after calcination at 800°C. Substantially morphous meant that the X-ray diffraction lines were broad diffuse bands of low intensity. The pattern of lines for this novel phase indicated a scheelite structure. A special interaction of silica with bismuth molybdate was also suggested by Callahan et al. (91). 

In the preceding section, we explained that the bulk diffusion of oxide ion plays an important role in the enhancement of the catalytic activity of the multicomponent bismuth molybdate systems. Here, another important role of the oxide ion migration in increasing the stability of the catalyst system is introduced. 

When the oxygen vacancies in the catalyst are located primarily near the surface, the reoxidation is very rapid. The activation energy for the reoxidation of these surface vacancies was found to be about 1-2 kcal/mol for the a and p phases of bismuth molybdate. Reoxidation of subsurface vacancies is much slower, however, and is limited by the ability of the catalyst to transport the oxygen from the surface O2 chemisorption sites to these subsurface vacancies. For catalysts having structures that facilitate this type of diffusion the reoxidation proceeds rapidly. 

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