Proton conductivity of superacidic sulfated zirconia

Three types of superacidic sulfated zirconia (S-ZrO2) were prepared by different methods using hydrated zirconia and sulfuric acid. Their proton conductivities were evaluated at 20–150 °C under saturated water vapor pressure. We found that the SOx concentration on S-ZrO2 varied largely depending on the method of preparation. The S/Zr atomic ratio changed from 0.046 for the sample prepared through a mixture of hydrated zirconia powder and sulfuric acid to 0.35 for the sample prepared through a mixture of hydrated zirconia sol and sulfuric acid. A powder compact of the former S-ZrO2 showed a proton conductivity of 4×10−2 S cm−1 at 70 °C and 8×10−3 S cm−1 at 150 °C, whereas that of the latter S-ZrO2 exhibited a high conductivity of 5×10−2 S cm−1 at 60–150 °C. Our experiment confirmed from IR spectra that there exists SOx bonded tightly with Zr on a heat-treated S-ZrO2 surface. The electronic polarization induced by such SOx was assumed to improve the Lewis and Brønsted acidities on Zr, leading to higher proton conductivities.