Heat Capacity and Thermodynamic Properties of Photocatalitic Bismuth Tungstate, Bi2WO6

The photocatalytic activity of Bi2WO6 Aurivillius phase has been widely exploited for the degradation of a wide range of gaseous and aqueous molecules, as well as microorganisms, under the influence of visible irradiation. Strategies for the development of doped and co-doped bismuth tungstate materials require the thermodynamic data on this phase. The heat capacity of bismuth tungstate, Bi2WO6, was investigated using a DSC microcalorimeter on polycrystalline powder samples in the temperature range from 313 to 1103 K (40–830 °C) in two separate runs. The samples were synthesized by solid-state reaction from pure binary oxides at 1033 K (760 °C) in a platinum crucible with cover. The high temperature Cp(T) data were fitted by the Maier–Kelley equation and, from this relation, the standard molar heat capacity of γ-Bi2WO6 polymorph was estimated to be at 298.15 K 176.8 ± 3.9 J·K−1·mol−1. A reversible second-order transition of Bi2WO6 phase was observed in the experimental temperature range, with a peak close to 940 K (667 °C). Additionally, the extrapolation of Cp(T) to 0 K was proposed using a method based on the multiple Einstein model. Thermodynamic properties (heat capacity Cp(T), entropy S°(T), enthalpy H°(T), Gibbs energy G°(T)) of crystalline γ-Bi2WO6 were calculated in the temperature range of 298.15–1123 K (25–850 °C).