Structure-function relationships of conventionally and flame made Pd-doped sensors studied by X-ray absorption spectroscopy and DC-resistance

Impregnation and flame spray pyrolysis (FSP) as complementary preparation routes were applied forthe synthesis of Pd-doped SnO2 based sensors. The structure of the sensors, in particular the palladium constituent, was found to be significantly different as analyzed by X-ray diffraction, BET surface area, electron microscopy, X-ray absorption spectroscopy, temperature-programmed reduction (TPR), and their sensing behavior by DC-resistance measurements. XAS was additionally applied under in situ and sensing conditions to derive structure-function relationships. The impregnation of sol-gel derived and calcined SnO2 powder led mainly to Pd/SnO2 sensors where PdO particles were located on the SnO2 surface as evidenced by X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and TPR-XANES. Flame-spray pyrolysis led to a mostly homogeneously mixed Pd-SnO2 powder with Pd being mainly dispersed in the SnO2. In this case both a directly deposited and a screen printed Pd-SnO2 layer were prepared. Sensing of hydrogen in air showed that its sensing could be significantly improved by direct deposition of the FSP-derived Pd/SnO2. Comparison of the screen printed FSP and the samples synthesized by impregnation supports earlier reports that small PdO clusters decorating the surface of the SnO2 grains is beneficial for sensing. (C) 2015 Elsevier B.V. All rights reserved.