Charge transfer during redox of bismuth molybdate catalysts

Abstract

Charge transfer to and from silica-supported bismuth molybdate catalysts MoO 3, Bi 2O 3 · 3 MoO 3 + 0.25 Bi 2O 3, 3 Bi 2O 3, · MoO 3, and Bi 2O 3 (designated as M, B/M-0.7, B/M-6, and B, respectively) has been investigated by measuring the changes of the electrical conductance and of the ESR line due to Mo 5+ during exposure of the catalysts to propylene or oxygen and during steady-state catalysis. For reduction by propylene at 600 and 660 K, the order of the initial charge transfer rates ( t < 5 sec) of the catalysts are: B/M-0.7 > M > B/M-6 > B. Upon extended reduction ( t > 5 sec) the order is unchanged at 660 K and at 600 K is reversed for catalysts B/M-0.7 and M. The initial rapid rate exhibited by catalyst B/M-0.7 is most likely due to reactive surface oxygen, while subsequent rates appear to be limited by bulk diffusion. Upon extended reduction of catalyst B/M-07 at 600 K, the electrical conductance increases continuously but according to ESR data the formation of Mo 5+ ceases. At 660 K, the initial charge transfer rates upon reduction of catalysts M and B/M-0.7 are nearly the same. This behavior is ascribed to rapid exchange of bulk and surface oxygen. The kinetics of charge transfer upon oxidation is analyzed in terms of a model in which the rate of incorporation of oxygen in the lattice depends on the oxygen vacancy concentration. Such an analysis demonstrates dissociative chemisorption of oxygen as a rate-limiting process. The temperature dependence of the charge transfer rates upon initial oxidation and reduction gives apparent activation energies of 18 kcal/mole for oxidation and 23 kcal/mole for reduction. The charge-transfer kinetics exhibit half-order dependence on oxygen and first-order dependence on propylene pressures. During catalytic oxidation of propylene, the process of charge transfer is postulated to be associated with hydrogen removal from propylene by surface oxygen of the catalyst. Subsequent steps in the oxidation mechanism determine catalyst selectivity.