Redox kinetics of bismuth molybdate ammoxidation catalysts*1

The response of several bismuth molybdate-based catalysts to reduction under propylene ammoxidation conditions in the absence of gaseous oxygen, and to reoxidation by gaseous oxygen, was studied using a pulse microreactor method. The catalysts investigated were Bi2Mo3O12, Bi2Mo2O9, Bi2MoO6, Bi3FeMo2O12, and a multicomponent system (Ma2+Mb3+BixMoyOz). The unit area rates of lattice oxygen participation at 430 °C decrease in the order: multicomponent system > Bi2Mo2O9 > Bi2Mo3O12 > Bi3FeMo2O12 ≳ Bi2MoO6. Maximum selective utilization of reactive lattice oxygen occurs after partial reduction for the multicomponent system, Bi2Mo3O12 and Bi2Mo2O9. These results are consistent with a mechanism requiring coordinately unsaturated metal ions in complex shear domains for selective ammoxidation. Conversely, Bi3FeMo2O12 and Bi2MoO6 show maximum lattice oxygen activity at their highest oxidation states. The overall reoxidation rates of partially reduced catalysts at 430 °C decrease in the order: Bi2MoO6 > Bi2Mo2O9 > Bi2Mo3O12 > Bi3FeMo2O12 ≳ multicomponent system. These reoxidation rates of partially reduced catalysts are first order in oxygen vacancy concentration and half order in gaseous oxygen. A general mechanism for catalyst reoxidation is proposed based on these kinetics. This series of catalysts exhibits two reoxidation regimes. One regime is characterized by a low activation energy at low degrees of initial reduction and involves the reoxidation of surface vacancies. A second regime is observed for deeper degrees of reduction which is characterized by a higher activation energy and involves the reoxidation of anion vacancies in the bulk of the catalyst. The observed activation energies for the reoxidation of the catalyst bulk are strongly dependent upon the structure and composition of the catalyst.