Recently, it has been shown that the degree of loading of several types of automotive exhaust aftertreatment devices can be directly monitored in situ and in a contactless way by a microwave-based method. The goal of this study was to clarify whether this method can also be applied to NOx storage and reduction catalysts (lean NOx traps) in order to obtain further knowledge about the reactions occurring in the catalyst and to compare the results with those obtained by wirebound NOx loading sensors. It is shown that both methods are able to detect the different catalyst loading states. However, the sensitivity of the microwave-based method turned out to be small compared to that previously observed for other exhaust aftertreatment devices. This may limit the practical applicability of the microwave-based NOx loading detection in lean NOx traps.
Abstract. This study reports on a sensor concept to measure in situ sulfur poisoning (sulfidation) of refinery catalysts, in this case, of commercial silica pellets loaded with highly dispersed nickel. Catalyst pellets were poisoned in diluted H 2 S between 100 and 400 • C and the sulfidation of the catalyst was observed. During this process, nickel sulfides are formed on the catalyst according to X-ray diffraction spectra and energy dispersive X-ray spectroscopy data. The sulfidation kinetics was quantitatively described by a shrinking core model. Representative catalyst pellets were electrically contacted, and their impedance was recorded in situ during sulfidation. At the beginning, the particles are highly insulating and behave capacitively. Their conductivity increases by decades during sulfidation. At high temperatures, an almost constant slope in the double-logarithmic representation vs. time can be found. At low temperatures, the conductivity remains constantly low for a long time but changes then rapidly by decades, which is also indicated by the phase that drops from capacitive to ohmic behavior. Since nickel sulfides exhibit a lower conductivity than nickel, the conductivity increase by decades during sulfidation can only be explained by electrically conducting percolation paths that form during sulfidation. They originate from the increased volume of sulfides compared to the pure nickel metal.
In order to study the sulfidation of a catalyst fixed bed, an in operando single pellet sensor was designed. A catalyst pellet from the fixed bed was electrically contacted and its electrical response was correlated with the catalyst behavior. For the sulfidation tests, a nickel catalyst was used and was sulfidized with H2S. This catalyst had a very low conductivity in the reduced state. During sulfidation, the conductivity of the catalyst increased by decades. A reaction from nickel to nickel sulfide occurred. This conductivity increase by decades during sulfidation had not been expected since both nickel and nickel sulfides behave metallic. Only by assuming a percolation phenomenon that originates from a volume increase of the nickel contacts when reacting to nickel sulfides, this effect can be explained. This assumption was supported by sulfidation tests with differently nickel loaded catalysts and it was quantitatively estimated by a general effective media theory. The single pellet sensor device for in operando investigation of sulfidation can be considered as a valuable tool to get further insights into catalysts under reaction conditions.
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