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

Abstract

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 CH 4 /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 CH 4 , 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 Rh 2 O 3 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.

Catalytic reactor for operando spatially resolved structure–activity profiling using high-energy X-ray diffraction

“…Although XAS allows the extraction of bulk metal atom oxidation states and coordination numbers, highly complementary methods such as XRD enable the deduction of crystalline phases in both a qualitative and a quantitative manner. The highly beneficial combination of XRD and XAS was shown to be an ideal tool to promote a holistical understanding of catalysts (Ressler et al, 2000;Frenkel et al, 2011;Gaur et al, 2019Gaur et al, , 2020Mu ¨ller et al, 2020). Nowadays, XAS/XRD is increasingly gaining relevance through advanced beamlines, allowing simultaneous measurement of absorption and diffraction data for highly efficient characterization.…”

Towards Low Temperature Operation of Catalytic Gas Sensors: Mesoporous Co3O4-Supported Au–Pd Nanoparticles as Functional Material

“…However, the measuring principle has some intrinsic disadvantages, such as poor stability, a small measuring range, and high susceptibility to catalyst poisoning [14,15]. The investigation of catalytic poisoning caused by silicon-containing gases, such as siloxanes, is essential due to their presence as trace contaminants in biogas, which can lead to irreversible inhibition of catalytic oxidation and sensor failures [1,16]. As an example, hexamethyldisiloxane (HMDSO) is stable up to a temperature of 300 • C, yet at temperatures above 300 • C, silicon dioxide is increasingly formed on the surface of the pellistor by the decomposing HMDSO, which passivates the functional material [17].…”

Methane Combustion Using Pd Deposited on CeOx-MnOx/La-Al2O3 Pellistors

“…Accordingly, a large diversity of CH 4 sensors, involving different technologies, have been developed [6]. Among them, pellistors have advantages related to the low fabrication costs and simplicity in operation, being typically used in a Wheatstone bridge arrangement where the out-of-balance voltage from the bridge is used as a signal measurement [7,8]. From the phenomenological point of view, CH 4 reduces the oxygen species from the catalytic material surface, leading to an exothermic reaction translated into the pellistor signal.…”