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

• Pd-doped SnO2 obtained by powder impregnation (PI) and flame spray pyrolysis (FSP).
• Homogenous dispersion of Pd2+ in the SnO2 lattice during FSP.
• PI leads to PdO particles on the SnO2 surface.
• Sensor performance strongly influenced by structure of Pd.
• In operando characterization by X-ray absorption techniques for deriving structure–performance relationships.

Impregnation and flame spray pyrolysis (FSP) as complementary preparation routes were applied for the 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.