Gunter Hagen

Metal-Organic Frameworks for Sensing Applications in the Gas Phase

Several metal-organic framework (MOF) materials were under investigated to test their applicability as sensor materials for impedimetric gas sensors. The materials were tested in a temperature range of 120 °C - 240 °C with varying concentrations of O2, CO2, C3H8, NO, H2, ethanol and methanol in the gas atmosphere and under different test gas humidity conditions. Different sensor configurations were studied in a frequency range of 1 Hz -1 MHz and time-continuous measurements were performed at 1 Hz. The materials did not show any impedance response to O2, CO2, C3H8, NO, or H2 in the gas atmospheres, although for some materials a significant impedance decrease was induced by a change of the ethanol or methanol concentration in the gas phase. Moreover, pronounced promising and reversible changes in the electric properties of a special MOF material were monitored under varying humidity, with a linear response curve at 120 °C. Further investigations were carried out with differently doped MOF materials of this class, to evaluate the influence of special dopants on the sensor effect.

Conductometric soot sensor for automotive exhausts: initial studies.

In order to reduce the tailpipe particulate matter emissions of Diesel engines, Diesel particulate filters (DPFs) are commonly used. Initial studies using a conductometric soot sensor to monitor their filtering efficiency, i.e., to detect a malfunction of the DPF, are presented. The sensors consist of a planar substrate equipped with electrodes on one side and with a heater on the other. It is shown that at constant speed-load points, the time until soot percolation occurs or the resistance itself are reproducible means that can be well correlated with the filtering efficiency of a DPF. It is suggested to use such a sensor setup for the detection of a DPF malfunction.

Application of V2O5/WO3/TiO2 for Resistive-Type SO2 Sensors

A study on the application of V2O5/WO3/TiO2 (VWT) as the sensitive material for resistive-type SO2 sensor was conducted, based on the fact that VWT is a well-known catalyst material for good selective catalytic nitrogen oxide reduction with a proven excellent durability in exhaust gases. The sensors fabricated in this study are planar ones with interdigitated electrodes of Au or Pt. The vanadium content of the utilized VWT is 1.5 or 3.0 wt%. The resistance of VWT decreases with an increasing SO2 concentration in the range from 20 ppm to 5,000 ppm. The best sensor response to SO2 occurs at 400 °C using Au electrodes. The sensor response value is independent on the amount of added vanadium but dependent on the electrode materials at 400 °C. These results are discussed and a sensing mechanism is discussed.

In-Operation Monitoring of the Soot Load of Diesel Particulate Filters: Initial Tests

Diesel particulate filters (DPF) are indispensable parts of modern automotive exhaust gas aftertreatment systems due to the stringent emissions legislation. For a fuel-efficient control strategy, it would be beneficial to determine directly and in-operation their actual trapped soot mass. Two novel approaches—based on the electrical conductivity of trapped soot particles—emerged recently. By measuring the electrical resistance between different single walls inside the filter, the soot load is determined with local resolution. The microwave-based technique is a contactless approach that gives an integral value depending on the soot mass in the DPF. We present investigations on loading and regeneration of DPFs in a dynamometer test bench applying both methods. The results are compared with each other and correlated with the differential pressure and the soot mass. Especially the microwave-based technique has a potential for serial application.

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.

Combination of Wirebound and Microwave Measurements for In Situ Characterization of Automotive Three-Way Catalysts

Recently, a novel approach for the characterization of the state of automotive catalysts has been proposed, viz., the direct electrical characterization of the catalyst material itself. This direct approach has the potential to do away with the complex models needed by indirect measurement methods to infer the catalyst state, for instance from the oxygen partial pressure outside of the catalyst determined by lambda probes. In this work, we compare two ways of directly observing the catalyst state. Firstly, the electrical conductivity of the catalyst material is determined by wirebound sensors mounted inside of the catalyst and fabricated from catalyst-formulation coated planar interdigital electrodes. Secondly, we employ a contactless method based on the perturbation of the partially filled microwave cavity resonator formed by the catalyst and its metallic housing. For a better understanding of the observed effects, the data obtained at microwave frequencies from engine tests are compared with the local electrical conductivity measurements. It is shown that both methods give a coherent picture of the oxygen loading state of an automotive three-way catalyst and that both methods enable one to follow the progress of reaction fronts inside the catalyst. Furthermore, it is demonstrated that the resonance frequency is strongly correlated with the oxygen load.

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.

Microwave Cavity Perturbation as a Tool for Laboratory In Situ Measurement of the Oxidation State of Three Way Catalysts

Three-way catalyst-based automotive exhaust gas aftertreatment is of high importance to meet today’s emission standards. To determine in situ the oxygen loading state of three-way catalysts, a microwave cavity perturbation method is used. In this study, it is investigated whether this measurement setup that had originally been described for full-sized catalysts can be transferred to a lab test bench using cores of 1″ diameter. The initial tests were successful and a high correlation between the oxygen loading degree dependent resonance frequency and the conversion was found. As an application example of the new in situ characterization technique, the steady state degree of oxidation of a three way catalyst was measured as a function of the exhaust stoichiometry. The experimental results are compared with the prediction of a recently published improved kinetic model that takes into account the oxidation of reduced ceria by H2O and CO2. It is shown that the experimental observations agree very well with this improved model. This result provides evidence that under typical operating conditions, the degree of oxidation of the three way catalyst is controlled by equilibrium effects.

Four-Wire Impedance Spectroscopy on Planar Zeolite/Chromium Oxide Based Hydrocarbon Gas Sensors

Impedometric zeolite hydrocarbon sensors with a chromium oxide intermediate layer show a very promising behavior with respect to sensitivity and selectivity. The underlying physico-chemical mechanism is under investigation at the moment. In order to verify that the effect occurs at the electrode and that zeolite bulk properties remain almost unaffected by hydrocarbons, a special planar setup was designed, which is very close to real sensor devices. It allows for conducting four-wire impedance spectroscopy as well as two-wire impedance spectroscopy. Using this setup, it could be clearly demonstrated that the sensing effect can be ascribed to an electrode impedance. Furthermore, by combining two- and four-wire impedance measurements at only one single frequency, the interference of the volume impedance can be suppressed and an easy signal evaluation is possible, without taking impedance data at different frequencies.

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.