Interaction of water, hydrogen and their mixtures with SnO2 based materials: The role of surface hydroxyl groups in detection mechanisms

DRIFTS, TGA and resistance measurements have been used to study the mechanism of water and hydrogen interaction accompanied by a resistance change (sensor signal) of blank and Pd doped SnO₂. It was found that a highly hydroxylated surface of blank SnO₂ reacts with gases through bridging hydroxyl groups, whereas the Pd doped materials interact with hydrogen and water through bridging oxygen. In the case of blank SnO₂ the sensor signal maximum towards H₂ in dry air (R₀/R _g) is observed at ∼345 °C, and towards water, at ∼180 °C, which results in high selectivity to hydrogen in the presence of water vapors (minor humidity effect). In contrast, on doping with Pd the response to hydrogen in dry air and to water occurred in the same temperature region (ca. 140 °C) leading to low selectivity with a high effect of humidity. An increase in water concentration in the gas phase changes the hydrogen interaction mechanism of Pd doped materials, while that of blank SnO₂ is unchanged. The interaction of hydrogen with the catalyst doped SnO ₂ occurs predominantly through hydroxyl groups when the volumetric concentration of water in the gas phase is higher than that of H₂ by a factor of 1000.