Design and experimental validation of an extended Kalman filter-based NOx concentration estimator in selective catalytic reduction system applications

This paper presents an extended Kalman filter (EKF) based approach of integrating NO_x and NH₃ sensors to estimate the NO_x concentrations in Diesel engine selective catalytic reduction (SCR) aftertreatment systems. NO_x sensors have been commonly used by vehicles for aftertreatment system control and onboard diagnostics (OBD) purposes. However, most currently available NO_x sensors are cross-sensitive to ammonia. Based on the experimental observations and physical inferences, the cross-sensitivity characteristics may change with temperature and is hard to be predicted by a model. This feature limits the applications of NO_x sensors on urea-SCR systems where ammonia is the reductant for NO_x conversions. Grounded in the insight into SCR dynamics and NO_x sensor properties, a novel approach of using an extended Kalman filter to estimate the actual exhaust gas NO_x concentration was proposed. The estimator was examined by NO_x measurements from a Horiba gas analyzer under different engine operating conditions. The experimental results show that the EKF-based approach can significantly improve the accuracy of NO_x concentration measurements from the original NO_x sensor readings.     

Selective mixed potential ammonia exhaust gas sensor

Novel selective ammonia sensors with high potential for long-term stability in harsh exhaust environments are introduced. The sensor bases on the mixed potential effect. In contrast to common sensors, the electrode functionalities electrical conductivity, selective catalytic activity, and electrochemical activity combined with long-term stability are separated. For that reason, one of the two electrodes is covered by a well-known porous vanadia–tungstenia–titania-based SCR catalyst material, which has been developed for exhaust gas SCR applications. The resulting sensor signal depends semi-logarithmically on ammonia. The NO_x cross-sensitivity is marginal. If enough oxygen is in the exhaust, the sensor signal is independent of the oxygen concentration. Tests downstream of an SCR catalyst show that very small ammonia slips can be determined.     

Diesel engine dynamometer testing of impedancemetric NOx sensors

Prototype solid-state electrochemical sensors using a dense gold sensing electrode, porous yttria-stabilized zirconia (YSZ) electrolyte, and a platinum counter electrode (Au/YSZ/Pt) were evaluated for measuring NO_x (NO and NO₂) in diesel exhaust. Both electrodes were exposed to the test gas (i.e., there was no reference gas for the counter electrode). An impedancemetric method was used for NO_x measurements, where the phase angle was used as the response signal. A portion of the tailpipe exhaust from the dynamometer test stand was extracted and fed into a furnace containing the experimental sensor. The prototype sensor was tested along with a commercially available NO_x sensor. Simultaneous measurements for NO_x, O₂, CO₂, H₂O, CO, and CH₄ in a separate feed stream were made using Fourier transform infrared (FTIR) spectroscopy and an oxygen paramagnetic analyzer. The experimental sensor showed very good measurement capability for NO in the range of 25-250 ppm, with a response paralleling that of the FTIR and commercial sensor. The prototype sensor showed better sensitivity to NO_x at the lower concentration ranges. O₂ is an interferent for the experimental sensor, resulting in decreased sensitivity for measurement of NO_x. Methods to overcome this interference are discussed.     

Improvement of the NOx selectivity for a planar YSZ sensor

In recent years planar yttria-stabilized zirconia (YSZ) based electrochemical gas sensors for automotive exhaust applications have become a major source of interest. The present work aims to develop a sensor for industrialisation. For this reason planar YSZ-based electrochemical sensors using two metallic electrodes (platinum and gold) were fabricated using screen-printing technology and tested in a laboratory test bench for different concentrations of pollutant gas such as CO, NO, NO₂ and hydrocarbons in oxygen rich atmosphere. It was furthermore shown that the selectivity towards NO_x could be highly reinforced by deposing a catalytic filter consisting of 1.7-4.5 wt.% Pt dispersed on alumina directly on the sensing elements. This filter was characterized by the use of SEM, TPD and XRD.     

In2O3 + xBaO (x = 0.5-5 at.%) - A novel material for trace level detection of NOx in the ambient

Indium oxide (In₂O₃) doped with 0.5-5 at.% of Ba was examined for their response towards trace levels of NO_x in the ambient. Crystallographic phase studies, electrical conductivity and sensor studies for NO_x with cross interference for hydrogen, petroleum gas (PG) and ammonia were carried out. Bulk compositions with x ≤ 1 at.% of Ba exhibited high response towards NO_x with extremely low cross interference for hydrogen, PG and ammonia, offering high selectivity. Thin films of 0.5 at.% Ba doped In₂O₃ were deposited using pulsed laser deposition technique using an excimer laser (KrF) operating at a wavelength of (λ) 248 nm with a fluence of ∼3 J/cm² and pulsed at 10 Hz. Thin film sensors exhibited better response towards 3 ppm NO_x quite reliably and reproducibly and offer the potential to develop NO_x sensors (Threshold limit value of NO₂ and NO is 3 and 25 ppm, respectively).     

Au-doped WO3-based sensor for NO2 detection at low operating temperature

Pure and Au-doped WO₃ powders for NO₂ gas detection were prepared by a colloidal chemical method, and characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The NO₂ sensing properties of the sensors based on pure and Au-doped WO₃ powders were investigated by HW-30A gas sensing measurement. The results showed that the gas sensing properties of the doped WO₃ sensors were superior to those of the undoped one. Especially, the 1.0 wt% Au-doped WO₃ sensor possessed larger response, better selectivity, faster response/recovery and better longer term stability to NO₂ than the others at relatively low operating temperature (150 °C).     

Development and experimental studies of a control-oriented SCR model for a two-catalyst urea-SCR system

This paper presents the development and experimental studies of a complete selective catalytic reduction (SCR) system control-oriented model of a two-catalyst SCR system with onboard NO_x and ammonia sensors. SCR catalysts have been popularly regarded as effective means for NO_x emission control in medium- and heavy-duty vehicles in recent years. However, control of urea dosing upstream of the SCR systems still remains a challenge in the field mainly due to the complicated SCR dynamics and limited/inaccurate feedback information. A control-oriented SCR model is thus indispensable for SCR control systems. A variety of experimental tests were examined using a Diesel engine-aftertreatment system consisting of a diesel oxidation catalyst (DOC)/diesel particulate filter (DPF), two-SCR catalysts (Fe-Zeolite type) in series, three NO_x sensors, and two NH₃ sensors. By utilizing multiple emission sensors and the two-catalyst SCR setup, the sensor properties and SCR system dynamics were studied. Grounded in the experimental investigations and the physical insights, a control-oriented model for a complete SCR system was developed and validated with experimental data.     

Necked ZnO nanoparticle-based NO2 sensors with high and fast response

The NO₂ gas sensing characteristics of semiconductor type gas sensors with channels composed of necked ZnO nanoparticles (NPs) were investigated in this study. The heat treatment of the NPs at 400 °C led to their necking and coarsening. The response of the necked-NP-based sensors was as high as 100 when exposed to 0.2 ppm of NO₂ at 200 °C. As the concentration of NO₂ increased to 5 ppm, their response was enhanced to approximately 400. During the repeated injection of NO₂ gas with a concentration of 0.4 ppm, the sensors exhibited stable response characteristics. Furthermore, the 90% response and recovery times of the gas sensor were as fast as 13 and 10 s, respectively. These observations indicate that the non-agglomerated necking of the NPs induced by the heat treatment significantly enhances the gas sensing characteristics of the NP-based gas sensors.     

Fabrication at wafer level of miniaturized gas sensors based on SnO2 nanorods deposited by PECVD and gas sensing characteristics

SnO₂ nanorods were successfully deposited on 3″ Si/SiO₂ wafers by inductively coupled plasma-enhanced chemical vapour deposition (PECVD) and a wafer-level patterning of nanorods layer for miniaturized solid state gas sensor fabrication were performed. Uniform needle-shaped SnO₂ nanorods in situ grown were obtained under catalyst- and high temperature treatment-free growth condition. These nanorods have an average diameter between 5 and 15 nm and a length of 160-300 nm. The SnO₂-nanorods based gas sensors were tested towards NH₃ and CH₃OH and gas sensing tests show remarkable response, showing promising and repeatable results compared with the SnO₂ thin films gas sensors.     

Gas sensing properties of platinum derivatives of single-walled carbon nanotubes: A DFT analysis

The limitations of intrinsic carbon nanotube (CNT) based devices to examine toxic gases motivate us to investigate novel sensors which can possibly overcome sensitivity problems. Pt–CNT assemblies (with Pt deposited externally as well as internally Pt-doped ones) interacting with NO₂ and NH₃ are studied and compared with unmodified CNTs. DFT calculations show that Pt can enhance adsorption and charge transfer processes to a very large degree. Incoming gas molecules cause changes in the electronic structure and charge distribution of the Pt-substituted CNTs that are both larger and more far-reaching than in their unmodified counterparts. Their relatively high stability is unaffected by the complexation with NO₂ and NH₃. CNTs with defective surface were also investigated. The sensing performance of Pt-doped CNT is found to be superior to defected CNTs.     

Highly sensitive NO2 sensor based on square-like tungsten oxide prepared with hydrothermal treatment

The square-like WO₃ nanosheets were synthesized by hydrothermal treatment of irregular WO₃ nanosheets prepared through acidification of Na₂WO₄·2H₂O. The obtained square-like and irregular WO₃ nanosheets were characterized with field emission scanning electron microscopy, X-ray powder diffraction and transmission electron microscopy. The gas sensing properties of sensors based on as-prepared samples were investigated. The results indicated that both samples exhibited high response to NO₂. The sensor based on square-like WO₃ nanosheets exhibited remarkably enhanced response and faster response/recovery time for NO₂ compared with that based on irregular nanosheets. Especially, the sensor based on square-like WO₃ nanosheets could detect NO₂ down to 40 ppb, which covered environmental standard. A possible reason for the influence of unique structure on the sensing properties of sensors based on square-like WO₃ was proposed.     

New method for selectivity enhancement of SiC field effect gas sensors for quantification of NO x

A silicon carbide based enhancement type metal insulator field effect transistor with porous gate metallization has been investigated as a total NO _x sensor operated in a temperature cycling mode. This operating mode is quite new for gas sensors based on the field effect but promising results have been reported earlier. Based on static investigations we have developed a suitable T-cycle optimized for NO _x detection and quantification in a mixture of typical exhaust gases (CO, C₂H₄, and NH₃). Significant features describing the shape of the sensor response have been extracted and evaluated with multivariate statistics (e.g. linear discriminant analysis) allowing quantification of NO _x . Additional cleaning-cycles every 30?min improve the stability of the sensor further. With this kind of advanced signal processing the influence of sensor drift and cross sensitivity to ambient gases can be reduced effectively. Measurements have proven that different concentrations of NO _x can be detected even in a changing mixture of other typical exhaust gases under dry and humid conditions. In addition to that, unknown concentrations of NO _x can be detected based on a small set of training data. It can be concluded that the performance of GasFETs for NO _x determination can be enhanced considerably with temperature cycling and appropriate signal processing.     

Gas sensing properties of SnO2 nanowires on micro-heater

The SnO₂ nanowires (NWs) network gas sensors were fabricated on a micro-electrode and heater suspended in a cavity. The sensors showed selective detection to C₂H₅OH at a heater power during sensor operation as low as 30-40 mW. The gas response and response speed of the SnO₂ NWs sensor to 100 ppm C₂H₅OH were 4.6- and 4.7-fold greater, respectively, than those of the SnO₂ nanoparticles (NPs) sensor with the same electrode geometry. The reasons for these enhanced gas sensing characteristics are discussed in relation to the sensing materials and sensor structures.     

Effect of La2CuO4 electrode area on potentiometric NOx sensor response and its implications on sensing mechanism

The intent of this work is to look at the effects of varying the La₂CuO₄ electrode area and the asymmetry between the sensing and counter electrode in a solid state potentiometric sensor with respect to NO_x sensitivity. NO₂ sensitivity was observed at 500-600 °C with a maximum sensitivity of ∼22 mV/decade [NO₂] observed at 500 °C for the sensor with a La₂CuO₄ electrode area of ∼30 mm². The relationship between NO₂ sensitivity and area is nearly parabolic at 500 °C, decreases linearly with increasing electrode area at 600 °C, and was a mixture of parabolic and linear behavior 550 °C. NO sensitivity varied non-linearly with electrode area with a minima (maximum sensitivity) of ∼−22 mV/decade [NO] at 450 °C for the sensor with a La₂CuO₄ electrode area of 16 mm². The behavior at 400 °C was similar to that of 450 °C, but with smaller sensitivities due to a saturation effect. At 500 °C, NO sensitivity decreases linearly with area.We also used electrochemical impedance spectroscopy (EIS) to investigate the electrochemical processes that are affected when the sensing electrode area is changed. Changes in impedance with exposure to NO_x were attributed to either changes in La₂CuO₄ conductivity due to gas adsorption (high frequency impedance) or electrocatalysis occurring at the electrode/electrolyte interface (total electrode impedance). NO₂ caused a decrease in high frequency impedance while NO caused an increase. In contrast, NO₂ and NO both caused a decrease in the total electrode impedance. The effect of area on both the potentiometric and impedance responses show relationships that can be explained through the mechanistic contributions included in differential electrode equilibria.     

Development of SAW-based multi-gas sensor for simultaneous detection of CO2 and NO2

A 440 MHz wireless and passive surface acoustic wave (SAW)-based multi-gas sensor integrated with a temperature sensor was developed on a 41° YX LiNbO₃ piezoelectric substrate for the simultaneous detection of CO₂, NO₂, and temperature. The developed sensor was composed of a SAW reflective delay lines structured by an interdigital transducer (IDT), ten reflectors, a CO₂ sensitive film (Teflon AF 2400), and a NO₂ sensitive film (indium tin oxide). Teflon AF 2400 was used for the CO₂ sensitive film because it provides a high CO₂ solubility, with good permeability and selectivity. For the NO₂ sensitive film, indium tin oxide (ITO) was used. Coupling of mode (COM) modeling was conducted to determine the optimal device parameters prior to fabrication. Using the parameters determined by the simulation results, the device was fabricated and then wirelessly measured using a network analyzer. The measured reflective coefficient S₁₁ in the time domain showed high signal/noise (S/N) ratio, small signal attenuation, and few spurious peaks. The time positions of the reflection peaks were well matched with the predicted values from the simulation. High sensitivity and selectivity were observed at each target gas testing. The obtained sensitivity was 2.12°/ppm for CO₂ and 51.5°/ppm for NO₂, respectively. With the integrated temperature sensor, temperature compensation was also performed during gas sensitivity evaluation process.     

Gas sensing properties at room temperature of a quartz crystal microbalance coated with ZnO nanorods

Gas sensors based on a quartz crystal microbalance (QCM) coated with ZnO nanorods were developed for detection of NH₃ at room temperature. Vertically well-aligned ZnO nanorods were synthesized by a novel wet chemical route at a low temperature of 90 °C, which was used to grow the ZnO nanorods directly on the QCM for the gas sensor application. The morphology of the ZnO nanorods was examined by field-emission scanning electron microscopy (FE-SEM). The diameter and length of the nanorods were 100 nm and 3 μm, respectively. The QCM coated with the ZnO nanorods gas sensor showed excellent performance to NH₃ gas. The frequency shift (Δf) to 50 ppm NH₃ at room temperature was about 9.1 Hz. It was found that the response and recovery times were varied with the ammonia concentration. The fabricated gas sensors showed good reproducibility and high stability. Moreover, the sensor showed a high selectivity to ammoniac gas over liquefied petroleum gas (LPG), nitrous oxide (N₂O), carbon monoxide (CO), nitrogen dioxide (NO₂), and carbon dioxide (CO₂).     

Mixed-potential-type zirconia-based NO2 sensor with high-performance three-phase boundary

This paper focuses on the gas sensing properties of the mixed-potential-type NO₂ sensor based on yttria stabilized zirconia (YSZ) and NiO electrode. The sensing performance of the sensor was improved by modifying the three-phase boundary (TPB). Hydrofluoric acid with different concentrations (10%, 20% and 40%) was used to corrode YSZ substrate to obtain large superficial area of TPB. The scanning electron microscope and atomic force microscopic images showed that the 40% HF could form the largest superficial area at the same corroding time (3 h). The sensitivity of the sensor using the YSZ plate corroded with 40% hydrofluoric acid to 20-500 ppm NO₂ was 76 mV/decade at 850 °C, which was the largest among the examined HF concentrations. It was also seen that the sensor showed a good selectivity and speedy response kinetics to NO₂. On the basis of the measurements of anodic and cathodic polarization curves, as well as the complex impedance of the device, the sensing mechanism was confirmed to involve a mixed potential at the oxide sensing electrode.     

A route to high sensitivity and rapid response Nb2O5-based gas sensors: TiO2 doping, surface embossing, and voltage optimization

We report a novel route for the fabrication of highly sensitive and rapidly responding Nb₂O₅-based thin film gas sensors. TiO₂ doping of Nb₂O₅ films is carried out by co-sputtering without the formation of secondary phases and the surface area of TiO₂-doped Nb₂O₅ films is increased via the use of colloidal templates composed of sacrificial polystyrene beads. The gas sensitivity of Nb₂O₅ films is enhanced through both the TiO₂ doping and the surface embossing. An additional enhancement on the gas sensitivity is obtained by the optimization of the bias voltage applied between interdigitated electrodes beneath Nb₂O₅-based film. More excitingly, such a voltage optimization leads to a substantial decrease in response time. Upon exposure to 50 ppm CO at 350 °C, a gas sensor based on TiO₂-doped Nb₂O₅ film with embossed surface morphology exhibits a very high sensitivity of 475% change in resistance and a rapid response time of 8 s under 3 V, whereas a sensor based on plain Nb₂O₅ film shows a 70% resistance change and a response time of 65 s under 1 V. Thermal stability tests of our Nb₂O₅-based sensor reveal excellent reliability which is of particular importance for application as resistive sensors for a variety gases.     

High-sensitivity NO2 gas sensors based on flower-like and tube-like ZnO nanomaterials

Hierarchical flower-like and 1D tube-like ZnO architectures were synthesized by a microemulsion-based solvothermal method. Technologies of XRD, SEM and TEM were used to characterize the morphological and structural properties of the products. The influence of the flower-like and tube-like morphologies on their NO₂ sensing properties was investigated. The experimental results showed that high-sensitivity NO₂ gas sensors were fabricated. The sensitivity of the tube-like ZnO gas sensor was much higher than that of the flower-like ZnO gas sensor and the tube-like ZnO gas sensor exhibited shorter response time. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique was employed to investigate the NO₂ sensing mechanisms. Free nitrate ions, nitrate and nitrite were the main adsorbed species during the adsorption, and NO also existed in the initial period of surface reoxidation. Furthermore, N₂O was formed via NO⁻ and N₂O₂⁻ stemmed from NO and increased upon rising temperature. Moreover, the PL spectra and the XPS spectra further proved that the intensity of donors (oxygen vacancy (V_O) and zinc interstitial (Zn_i)) and surface oxygen species (O₂⁻ and O₂) involved in the gas sensing mechanism leaded to the different sensitivities.     

Preparation and gas sensing properties of Fe-doped yttrium manganate nanoparticles

Fe-doped yttrium manganate (YMn_(1−x)Fe_xO₃) nanoparticles were synthesized by the precursor method. X-ray diffractions showed that the structures of the as-prepared powders were crystallized in the normal yttrium manganate phase (space group: P6₃cm) when doping concentration was low, while they were crystallized into the high-temperature yttrium manganate phase (space group: P6₃/mmc) at a high doping concentration. Then, the gas sensing properties of YMnO₃ and YMn_(1−x)Fe_xO₃ nanoparticles were studied for the first time. Both YMnO₃ and YMn_(1−x)Fe_xO₃ exhibited sensor response to alcohols, organic amines, dichloromethane, acetone, acetonitrile, methylbenzene, THF and so on. Interestingly, high-concentration Fe-doped yttrium manganate showed much better sensor response than that of normal yttrium manganate phase. We conclude that the multiferroic material of YMn_(1−x)Fe_xO₃ is a promising potential new ABO₃ type gas sensing material.