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It seems that these differences in assessment of the catalytic properties of the bismuth molybdate phases are due to the very high mobility of Mo03 on the one hand and to the considerable reactivity in the solid state of these phases on the other hand. As the result, dynamic changes of the surface structure may occur in the course of pretreatment and during the catalytic reaction. Indeed, the results of the photoelectron spectroscopic studies of the surface composition of the three bismuth molybdate phases after different vacuum and thermal treatments as well as after having worked in the catalytic reactor indicate [90] that on annealing in vacuum the surface layer of grains becomes enriched in molybdenum, whereas in the oxidizing atmosphere of the reacting mixture the composition of the surface remains the same as that of the fresh sample. Therefore the surface composition of the three molybdates will depend strongly on the redox properties of the gas phase, i.e., they may strongly depend on the conditions under which the catalytic reaction is carried out. This may be an important factor determining the catalytic properties of molybdate catalysts as indicated by the observation that strictly stoichiometric molybdates are inactive in the oxidation of propene and the presence of excess Mo03 is necessary to render them active [79].