Operando X-ray absorption spectroscopy studies on Pd-SnO2 based sensors

SnO2 gas sensors with palladium as additive in the range of 0.2 wt% and 3 wt% were studied by in situ X-ray absorption spectroscopy under idealized and real operating conditions. Simultaneously to the structural studies, measurements of the sensing properties were undertaken allowing for the determination of structure-function relationships. For this purpose a new in situ spectroscopic cell was designed which permitted on the one hand sensing on conventional screen printed 50 mm thick sensing layers and on the other hand structural analysis with X-rays provided by an insertion device at a 3rd generation synchrotron facility in fluorescence mode. Pd K-edge XANES and EXAFS results on gas sensors showed that palladium, present in an oxidized state, is finely dispersed if it is added in small quantities ( 0.2 wt%) while it forms clusters at higher concentrations ( 3 wt%). This is also rejected by the much easier reduction of palladium in the latter, higher concentrated ones. Under realistic sensing conditions (30-200 ppm H-2; 10-50 ppm CO in dry and humid air at 200 and 300 degrees C) for the low additive concentration samples, no change in oxidation state was observed, i.e. palladium remained in its oxidized state. This has important consequences on the understanding and modeling of the gas sensing mechanism.