High Stability of Rh Oxide-Based Thermoresistive Catalytic Combustion Sensors Proven by Operando X-ray Absorption Spectroscopy and X-ray Diffraction.

Thermoresistive catalytic combustion sensors based on noble metals are very stable stable and highly sensitive devices to monitor potentially explosive atmospheres. We studied and proved the high stability of rhodium oxide-based sensors under working conditions in different CH4/air mixtures (up to 3.5 vol % methane) with the help of operando X-ray-based characterization techniques, DC resistance measurements, and IR thermography using a specially designed in situ cell. Operando X-ray diffraction and X-ray absorption spectroscopy showed that the active Rh species are in the oxidized state and their chemical state is preserved during operation under realistic conditions. The resistance correlated with the surface temperature of the pellistor and is related to the combustion of CH4, confirming the catalytic nature of the observed sensing process. Only under harsh operation conditions such as an oxygen-free atmosphere or enhanced working current, a reduction in the active Rh2O3 phase was observed. Finally, the effect of poisoning causing the lowered activity on the catalytic combustion of methane was investigated. While stable rhodium sulfate might form in a sulfur-poisoned pellistor, silicon dioxide seems to additionally physically block the pores in the alumina ceramics of the pellistor poisoned by hexamethyldisiloxane.