Thermochemical Study of the Reduction and Oxidation of Molybdovanadophosphoric Heteropoly Acids in Aqueous Solutions
Статья (Full article),
Russian Journal of Inorganic Chemistry
, E-ISSN: 1531-8613
Институт катализа им. Г.К. Борескова СО РАН
The differential enthalpy (-Delta(r)H(4j)) of the reduction of 0.2 M aqueous solutions of heteropoly acids H(3+x)PV(x)Mo(12-x)O(40) (HPA-x, 1 <= x <= 3) by hydrazine hydrate (HH) at 343 K was investigated as a function of x and of the degree of reduction m = [V(IV)]/[HPA-x]. For all of the HPA-x solutions studied, the initial -Delta(r)H(4j) value (at m similar to 0) approximated 436 +/- 14 kJ per mole of HH. For an HPA-3 solution at m approximate to 0.65, a maximum in -Delta(r)H(4) was observed (approximate to 483 kJ per mole of HH) due to the interaction of the VO(2)(+) and VO(2)(+) cations in the inner coordination sphere of the HPA-3 ion. This maximum is similar to that observed in the reduction of an 0.1 M solution of VO(2)ClO(4) in 4 M HClO(4). In the case of reduction of HPA-1, HPA-2, or salts of HPA-3, no -Delta(r)H(4j) maxima were observed, because the content of VO(2)(+) in the ICS of the corresponding heteropoly anion is too low. The -Delta(r)H(4j) values for solutions of HPA-3 salts depend only on the pH but not on the nature of the cation. The enthalpies of oxidation of solutions of the reduced forms of the HPA-x by dioxygen (-Delta(r)H(2j)) were calculated. The sequence of oxidation of various V(IV) species in solutions of the reduced HPA-x is opposite to the sequence of reduction of various forms of V(V) in the initial solutions of HPA-x.