Sebastian Reiß

Resistive Oxygen Gas Sensors for Harsh Environments

Resistive oxygen sensors are an inexpensive alternative to the classical potentiometric zirconia oxygen sensor, especially for use in harsh environments and at temperatures of several hundred °C or even higher. This device-oriented paper gives a historical overview on the development of these sensor materials. It focuses especially on approaches to obtain a temperature independent behavior. It is shown that although in the past 40 years there have always been several research groups working concurrently with resistive oxygen sensors, novel ideas continue to emerge today with respect to improvements of the sensor response time, the temperature dependence, the long-term stability or the manufacture of the devices themselves using novel techniques for the sensitive films. Materials that are the focus of this review are metal oxides; especially titania, titanates, and ceria-based formulations.

Determination of the NOx Loading of an Automotive Lean NOx Trap by Directly Monitoring the Electrical Properties of the Catalyst Material Itself

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.

Zeolite-based Impedimetric Gas Sensor Device in Low-cost Technology for Hydrocarbon Gas Detection

Due to increasing environmental concerns the need for inexpensive selective gas sensors is increasing. This work deals with transferring a novel zeolite-based impedimetric hydrocarbon gas sensor principle, which has been originally manufactured in a costly combination of photolithography, thin-film processes, and thick-film processes to a low-cost technology comprising only thick-film processes and one electroplating step. The sensing effect is based on a thin chromium oxide layer between the interdigital electrodes and a Pt-loaded ZSM-5 zeolite film. When hydrocarbons are present in the sensor ambient, the electrical sensor impedance increases strongly and selectively. In the present work, the chromium oxide film is electroplated on Au screen-printed interdigital electrodes and then oxidized to Cr2O3. The electrode area is covered with the screen-printed zeolite. The sensor device is self-heated utilizing a planar platinum heater on the backside. The best sensor performance is obtained at a frequency of 3 Hz at around 350 °C. The good selectivity of the original sensor setup could be confirmed, but a strong cross-sensitivity to ammonia occurs, which might prohibit its original intention for use in automotive exhausts.

Overview: Status of the Microwave-Based Automotive Catalyst State Diagnosis

The oxygen loading degree in TWCs, the amount of stored ammonia in SCR catalysts, the NOx loading degree in LNTs, or the soot loading of DPFs play a key role in automotive exhaust gas aftertreatment. Today’s methods determine the catalyst state indirectly. They utilize gas sensors installed up- or downstream of the catalysts and the catalyst state is inferred from the sensor signals. This overview reports on the status of an alternative approach based on the interaction of electromagnetic microwaves with the catalyst material. Since the catalyst state is strongly correlated with the electrical properties of the catalyst material itself, this concept shows a great potential.

Mikrowellengestützte Aufklärung elektrochemischer Vorgänge in Katalysatoren und verwandten Systemen

Zusammenfassung Technisch bedeutsame elektrochemische Reaktionen laufen oftmals bei hohen Temperaturen und im Volumen von Festkörpern ab. Die mit ihrer direkten Überwachung zusammenhängenden Probleme werden meist durch indirekte Messverfahren umgangen. Der vorliegende Beitrag berichtet über ein mikrowellenbasiertes direktes Messverfahren und die damit erzielten Ergebnisse bei Anwendung auf Messobjekte wie Dreiwegekatalysatoren (three-way catalysts, TWC), NOx-Speicherkatalysatoren (NSK, lean NOx traps, LNT) und Dieselpartikelfilter (DPF). Abstract Technically important electrochemical reactions often occur at high temperatures and inside bulky structures. The difficulties associated with their direct observation are usually circumvented by indirect measurement strategies. This contribution reports on a microwave-based direct measurement method and the results obtained when it was applied to systems such as three-way catalysts (TWC), lean NOx traps (LNT), and diesel particulate filters (DPF).

Planar Zeolite Film-Based Potentiometric Gas Sensors Manufactured by a Combined Thick-Film and Electroplating Technique

Zeolites are promising materials in the field of gas sensors. In this technology-oriented paper, a planar setup for potentiometric hydrocarbon and hydrogen gas sensors using zeolites as ionic sodium conductors is presented, in which the Pt-loaded Na-ZSM-5 zeolite is applied using a thick-film technique between two interdigitated gold electrodes and one of them is selectively covered for the first time by an electroplated chromium oxide film. The influence of the sensor temperature, the type of hydrocarbons, the zeolite film thickness, and the chromium oxide film thickness is investigated. The influence of the zeolite on the sensor response is briefly discussed in the light of studies dealing with zeolites as selectivity-enhancing cover layers.

Monitoring of Electrochemical Processes in Catalysts by Microwave Methods

Many electrochemical reactions occur at high temperatures and inside bulk materials. Because of the difficulties associated with a direct observation of the reaction, one often resorts to indirect measurement strategies. An example is the three-way catalyst (TWC) in the exhaust pipe of a gasoline engine which stores oxygen when it is abundant in the exhaust gas and releases it later to oxidize noxious gas components such as CO in oxygen-deficient (“lean”) exhaust gases. Currently, the oxygen loading of the TWC is derived indirectly from the output signals of two lambda probes, one upstream and the other downstream of the catalyst, which monitor the air-to-fuel ratio λ in the exhaust gas. We have investigated a microwave cavity perturbation approach towards the direct measurement of the catalyst state. It will be shown that the uptake or release of oxygen in the catalyst is observable in situ via the S-parameters of a cavity resonator.

Kontaktlose hochfrequenzbasierte Qualitätsanalyse von Harnstoff-Wasser-Lösungen für SCR-Anwendungen

SCR-Systeme (Selective Catalytic Reduction) tragen bei modernen Abgasnachbehandlungssystemen für Dieselfahrzeuge entscheidend Stickoxidemissionsminderung bei. Aktuell erreichen Nutzfahrzeuge nur durch den Einsatz von SCR-Systemen die geltenden Grenzwerte der Schadstoffemissionen. Dabei werden durch zusätzlich in den Abgasstrang eindosierten Ammoniak in einem Katalysator Stickoxide zu Stickstoff und Wasser reduziert. Das Reduktionsmittel Ammoniak wird in Form einer Harnstoff-Wasser-Lösung (HWL) bereitgestellt, welche nach der Eindosierung zu Ammoniak umgesetzt wird. Entscheidend für die korrekte Funktion des Systems ist die Harnstoffkonzentration der getankten HWL. Aktuell wird allerdings lediglich der Füllstand des Tanks und durch einen Stickoxid-Sensor nach SCR-Katalysator die Funktion des Systems überwacht. Vorteilhaft wäre es aber, wenn direkt im Tank der Harnstoffgehalt der HWL analysiert und auf die Eignung für das SCR-System überprüft werden könnte. Im Rahmen dieser Arbeit werden erste Versuche zu einem Messsystem zur Qualitätsanalyse einer HWL vorgestellt, das auf der Auswertung der elektrischen Eigenschaften beruht. Da die HWL korrosiv wirkt, ist ein kontaktloser mikrowellenbasierter Ansatz vielversprechend.