ABSTRACT
An entirely different picture is obtained in the case of nickel molybdate. At lower temperatures the main products are acrolein and acetaldehyde. However, when the temperature is raised, the amount of acetaldehyde decreases and the amount of acrolein increases, appearing with higher and higher selectivity so that at about 5000 e acrolein becomes practically the only product. As shown in Section III. B of this chapter, active and selective oxidation catalysts must perform two functions: activate propene and insert nucleophilic oxygen into the activated species, this latter function being very efficiently performed by group V, VI, and VII transition metal oxides and the corresponding anionic sublattices in their oxysalts. A question may be raised as to which properties of these oxides are responsible for the very high activity and selectivity involved in the insertion of oxygen. The chemistry of such elements as vanadium or molybdenum is dominated by the consequences of the
considerable extension of their d orbitals and positions of the delectron redox potentials relative to the anion valence band edge [66] . In the octahedral coordination of oxide ions, in which d 2sp3 hybridized orbitals are used by the metal to form a bonds, the remaining Qxy' Qyz' and Qxz orbitals extend far enough to considerably overlap with the TI-p orbitals of oxygen. As the result TI bonds with oxygen atoms are formed and the cation becomes displaced from the center of the octahedron toward terminal oxygen atoms. The softness of the metal-oxygen potential is the cause of large cation displacement polarizabilities, which give rise to the high relaxation energy. As a result it becomes energetically more favorable for such an oxide to change on reduction the stoichiometry by the change of the mode of linkage between the coordination polyhedra instead of generating the point defects, the usual mechanism responsible for the nonstoichiometry of transition metal oxides. Namely, removal of
FIGURE 7.13 Selectivity of the oxidation of propene to acrolein, acetaldehyde, and C02 on NiMo04 and Ni3 (P04) 2 as a function of temperature.
