Solid state reactions in catalysts: An approach to real active systems and their deactivation

Solid state reactions are involved in many stages of catalyst life, from preparation to deactivation. Reactions taking place at the catalyst surface during catalysis may relate to those occurring during preparation. They are the origin of several deactivation processes. The present contribution is aimed at enlarging the concept of deactivation by solid state reactions by considering all those involved in the bulk and surface over the entire life of the catalyst. The main example will be that of the behaviour of molybdenum oxides in selective oxidations. It will be shown that the formation of shear planes explains (i) the activation of MoO3 to the very flexible structure of the active suboxide (Mo18O52 or Mo8O23), (ii) the exceptional catalytic activity of these two intermediary oxides and (iii) the deactivation due to the formation of Mo4O11. This conclusion concerning the similitude of solid state reactions at various stages of catalyst life can be extended to molybdates of various metals and other catalysts active in selective oxidation. This suggests that a comprehensive view of the whole catalyst life may enable a better control of useful catalytic properties. The beneficial role of spillover oxygen, adequate supports, or the advantage of alternate reduction and oxidation stages in the industrial process (e.g. the so-called "riser" reactor) are, among others, examples of useful, practical applications which emerge when this comprehensive view is adopted.