Phase stability of noble metal loaded WO3 for SO2 sensor applications

A thermodynamic phase stability investigation of semiconductor oxide materials for SO2 gas sensing applications has been performed to provide insight into understanding the working principles for SO2 sensing. Complex phase stabilities of noble metals (Au, Ag, Pd, and Pt) loaded WO3 semiconductor oxides for SO2 sensors have been investigated through the use of the CALPHAD (CALculation of PHAse Diagram) computational thermodynamics approach. Thermodynamic descriptions for the individual phases, i.e. SO2 gas, oxides, sulfides, and metal sulfates, are obtained from the Scientific Group Thermodata Europe (SGTE) Substance Database. Calculated isothermal sections of W-S-O show a significant phase transformation for WS2 at 400 °C. This phase transformation agrees with the previously reported experimental findings that the SO2 response of WO3 increases notably at that temperature. Ag-loaded WO3 in equilibrium with the SO2 gas exhibits very complex phase stabilities for the binary sulfide and ternary sulfate, while the other metals (Au, Pd and Pt) did not exhibit any notable phase transformations. The thermodynamic model predicts an allotropic phase transformation of the Ag2S phase near 500 °C in Ag/WO3-SO2 regardless of the Ag content, which differs from the reported experimental observation that 1.0 wt% Ag/WO3 exhibited SO2 sensor response at 450 °C. This discrepancy may be attributed to the fact that the solubility of Ag in the WO3 phase has not been considered in the thermodynamic model.