Mechanism of sensing CO in nitrogen by nanocrystalline SnO2 and SnO2(Pd) studied by Mossbauer spectroscopy and conductance measurements

The mechanism of CO/N-2 sensitivity of undoped and Pd-doped nanocrystalline tin dioxide was studied in situ by coupled electrical measurements and Sn-119 Mossbauer spectroscopy. Nanocrystalline SnO2 with a grain size of about 6-8 nm was synthesized by the sol-gel method. SnO2(Pd) ([Pd] = 1 at%) was prepared from SnO2 by the impregnation method. Electrical properties of SnO2 and SnO2 (Pd) in the presence of CO/N-2 were studied at a fixed temperature in the range 50 less than or equal to T less than or equal to 380 degreesC. Addition of Pd strongly increases the sensor response to CO over the whole temperature range. It was shown by Mossbauer spectroscopy that two different mechanisms of gas sensitivity can exist in the presence of CO/N-2. The first "low temperature'' mechanism (T < 100 °C) may be attributed to redox reactions between surface oxygen and CO catalyzed by PdOx clusters. The second "high temperature'' one (125-380 °C) involves oxygen vacancies diffusing into the bulk of the tin dioxide grains which leads to reversible reduction of Sn(IV) to Sn(II).