Kinetics of Oxidation of Reduced Phosphorus–Molybdenum–Vanadium Heteropoly Acid Species with Dioxygen in Aqueous Solutions
Kinetics and Catalysis
, E-ISSN: 1608-3210
Boreskov Institute of Catalysis SB RAS
The kinetics of dioxygen oxidation of the reduced heteropoly acid H7 + m PV m IV V 4 − m V Mo8O40 (HmHPA-4) in aqueous solutions at atmospheric pressure and 313–363 K was studied, and a rate equation was proposed for this reaction. The reaction was of first order with respect to dioxygen. However, the order of the reaction with respect to HmHPA-4 was close to 1 at [HmHPA-4] > 0.20 mol/l but increased to ≈2.8 at [HmHPA-4] < 0.20 mol/l. Variable orders of reaction with respect to H+ ions with high negative values were observed. The introduction of overstoichiometric VO2+ cations into HmHPA-4 accelerated the oxidation of the solution. The apparent activation energy of the reaction dramatically increased as the degree of reduction of HmHPA-4 decreased. This can be explained by the fact that the heats of endothermic preequilibrium steps of the disproportionation of weakly reduced heteropoly anions are added to the true activation energy of formation of an active intermediate complex of the heteropoly anion with O2.