Electrochemical NOx Research Articles

Adsorption of NO2 on YSZ(111) and Oxygen-Enriched YSZ(111) Surfaces

The adsorption of NO2 onto yttrium-stabilized zirconia (YSZ) surfaces is rarely considered a necessary concern when designing devices or structures which rely on YSZ either as a high-temperature mechanical support or in its role as an oxide ion conducting electrolyte. However, in this work we show that the presence or absence of a subsurface oxygen ion vacancy can play a significant role in not only the type of adsorption which can occur but also the reaction mechanism of NO2 on the surface at different temperatures. In this work the binding energy, vibrational frequencies, density of states, magnetic moments, electron localization, and charge transfer, as determined using density functional theory calculations, are presented for all stable NO2 adsorption configurations on YSZ(111) and oxygen-enriched YSZ+O(111) surfaces. Additionally, ab initio molecular dynamics simulations performed at 298 and 773 K show how high temperatures influence the adsorption and desorption of NO2 onto YSZ(111) and YSZ+O(111) surfaces, with respect to their application as an oxide ion conducting electrolyte in high-temperature electrochemical gas sensors. It was found that, on an oxygen-enriched YSZ+O(111) surface, NO2 bonded with a surface oxygen atom, which could yield an adsorbed NO2, nitrate, or cis-peroxynitrite (OONO) molecule. Adsorbed NO2 was found to desorb from the YSZ(111) and YSZ+O(111) surfaces at 298 K. The adsorbed nitrate or cis-peroxynitrite species, however, are more stable, remaining on the surface at 298 K up to a simulation time of 3 ps. At 773 K, either NO3 desorbs from the surface or the OONO dissociates into O2 and NO, which can also desorb from the surface. Hence, the presence of subsurface O results in the oxidation of NO2, returning the surface to YSZ(111). A further oxide ion could then migrate to the vacancy, allowing reaction with further NO2 molecules. This work is of particular interest to those developing solid-state electrochemical NOx sensors and for understanding the adsorption and reaction of NO2 with YSZ surfaces.

Characterization of LSM/CGO Symmetric Cells Modified by NOx Adsorbents for Electrochemical NOx Removal with Impedance Spectroscopy

This study uses electrochemical impedance spectroscopy (EIS) to characterize an LSM/CGO symmetric cell modified by NOx adsorbents for the application of electrochemical NOx reduction. Three cells were prepared and tested: a blank cell, a cell impregnated with BaO, and a cell coated with a BaO-Pt-Al2O3 layer. The impedance analysis revealed that modification with the NOx adsorbents, either by impregnating the BaO into the electrode or by adding a BaO-Pt-Al2O3 layer on top of the electrode significantly enhanced the electrode activity. This activity enhancement was mainly due to the decrease in the resistance of the low-frequency processes, which were ascribed to adsorption, diffusion, and transfer of O2 species and NOx species at or near the triple phase boundary (TPB) region and the formation of the reaction intermediate NO2. The BaO impregnation improved the adsorption of NOx on the LSM/CGO electrode by selectively trapping NO2 in the form of nitrate over the BaO sites and provided availability for a direct reduction of the stored nitrate. The BaO-Pt-Al2O3 layer enhanced the NOx adsorption and promoted the formation of NO2 due to the NO oxidation ability of the Pt catalyst, but hindered the gas diffusion to the reaction sites.

Electrochemical NOx reduction on an LSM/CGO symmetric cell modified by NOx adsorbents

This study investigated the effect of modifying a (La0.85Sr0.15)0.99MnO3 (LSM)/Ce0.9Gd0.1O1.95 (CGO) symmetric cell by NOx adsorbents on the electrochemical reduction of NOx under O2-rich conditions. The modification was based on a full ceramic cell structure without any noble metals. Three cells were prepared and tested: a blank cell, a cell impregnated with BaO, and a cell coated with a BaO/Pt/Al2O3 layer. The electrochemical reduction of NOx on the three cells was studied by conversion measurement, degradation testing, and microstructure characterization. The modification, either by impregnating the BaO into the electrode or by coating the Ba/Pt/Al2O3 layer on the surface of the electrode, significantly increased the activity and selectivity of the NOx reduction on the LSM/CGO symmetric cell by enhancing the adsorption and storage of the NOx species or providing reaction sites for direct nitrate reduction. The cell with the BaO/Pt/Al2O3 layer exhibited a preferable performance at low temperatures (350 and 400 °C) and low voltages (1.5 to 2 V) due to the NO oxidation ability of the Pt catalyst, although its performance was relatively poor at elevated temperatures and voltages due to the impedance of the diffusion of NOx to the reaction sites by the adsorption layer. For lowering the operation temperature and minimizing the power consumption, adding an adsorption layer was shown to be the optimum approach for modifying the electrochemical cell by NOx adsorbents. The square wave (SV) polarization can balance the trapping and reduction rates of NOx species on the electrochemical cells to further improve the NOx reduction relative to the direct current (DC) polarization.

In-situ scanning photoelectron microscopy study of operating (La,Sr)FeO3-based NOx-sensing surfaces

Scanning photoelectron microscopy and impedance spectroscopy were used to study simultaneously the impact of NO gas on surface chemistry and the electrochemical response of electrolyte-supported (La1−xSrx)FeO3−δ (LSF) sensing layers in electrochemical NOx sensors, during high-temperature operation. Electrochemical model cells were fabricated by screen printing macro- and micro-patterned thin films on zirconia electrolyte. Cell bias, temperature and NO/O2 gas ratio were varied during the experiments.LSF/3YSZ sensing layers remained stable over a wide temperature-bias range in an oxygen-containing environment. LSF compositions with low strontium content (x=0.2) did not undergo any changes in their surface chemistry, while compositions with higher strontium level (x=0.4) showed partial reduction of surface iron at low oxygen chemical potential. Upon exposure to NO gas at low oxygen partial pressure, NO was observed to dissociate on the LSF surface and form chemisorbed O- and N-adatoms with strong interactions to surface iron or zirconium. Concentration and stability of these ad-complexes increased with the NO/O2 ratio, cathodic bias and decreasing temperature. At low NO level in the gas, NO was observed to interact preferentially with iron atoms at the LSF surface and the surface reaction was reversible. At high NO/O2 ratios and low temperatures, irreversible formation of surface ad-complexes was observed, with reduced iron and zirconium, and caused even new phase formation.Simultaneous assessment of surface chemistry and cell impedance demonstrated that modifications in surface chemistry directly affected the oxygen exchange rate in the cell. In the presence of NO, the sensing oxide resistance decreased for zero or small bias. For large bias, no additional decrease in cell resistance was observed beyond the corresponding intrinsic cell performance in oxygen. Under these conditions, again, the particular cell response depended on the strontium substitution level in LSF, favoring lower redox-stability and thus a more complicated cell response to NO gas for high strontium level.

Diffuse Reflectance Infrared Fourier Transform Study of NOx Adsorption on CGO10 Impregnated with K2O or BaO

In the present work diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy is applied to study the adsorption of NO(x) at 300-500 °C in different atmospheres on gadolinium-doped ceria (CGO), an important material in electrodes investigated for electrochemical NO(x) removal. Furthermore, the effect on the NO(x) adsorption when adding K(2)O or BaO to the CGO is investigated. The DRIFT study shows mainly the presence of nitrate species at 500 °C, whereas at lower temperature a diversity of adsorbed NO(x) species exists on the CGO. The presence of O(2) is shown to have a strong effect on the adsorption of NO, but no effect on the adsorption of NO(2). Addition of K(2)O and BaO dramatically affects the NO(x) adsorption and the results also show that the adsorbed NO(x) species are mobile and capable of changing adsorption state in the investigated temperature range.

Electrochemical incorporation of hemin in a ZnO–PPy nanocomposite on a Pt electrode as NOx sensor

An electrochemical NO(x) sensor was fabricated based on the incorporation of hemin on a ZnO-PPy nanocomposite modified Pt electrode. Scanning electron microscopy, energy dispersive X-ray analysis and cyclic voltammetry were used to confirm the successful stepwise assembly procedure for the sensor. The electrocatalytical behavior of the sensor was investigated by cyclic voltammetry. The hemin-ZnO-PPy-Pt electrode exhibited characteristic hemin reversible redox peaks at 0.035 V and -0.11 V vs. Ag/AgCl respectively. The hemin-ZnO-PPy-Pt electrode exhibited 3-fold enhanced electrocatalytic activity towards NO(x) compared to the hemin-PPy-Pt electrode. The electrocatalytic response of the sensor was proportional to the NO(x) concentration in the range of 0.8 to 2000 μM (r(2) = 0.9974) with a sensitivity of 0.04 μA μM(-1) cm(-2) and detection limit of 0.8 μM for the hemin-ZnO-PPy-Pt electrode. The low detection limit, wide linear range and enhanced sensitivity of the present sensor make it valuable for potential applications. In addition, this sensor exhibited good reproducibility and stability.

Electrochemical NOx Reduction with Power Generation in Solid Oxide Fuel Cells with Cu-Added ( LaSr ) ( CoFe ) O3 – ( Ce , Gd ) O2 − x Cathode

A process for nitric oxides (, NO plus ) removal by solid oxide fuel cells (SOFCs) can easily reduce in the flue gases to zero and also generate an electrical current. An yttria-stabilized zirconia electrolyte-supported SOFC unit cell was constructed with Ni–gadolinia-doped ceria (GDC) as the anode and Cu-added –GDC composite as the cathode. tests were carried out at a fixed voltage of 0.55 V and . As the inlet concentration decreases, the conversion increases. The generated current density increases with increasing NO concentration from 150 to 5000 ppm. Increasing concentration from 4 to 8% increases the NO conversion due to increased current density, which increases the oxygen transfer rate. Adding can increase the NO conversion, and also due to increased current density. As the concentration increases from 2 to 8% and the concentration increases from 5 to 15%, the generated current density increases. Adding can decrease both the current density and the NO conversion.

Sensors for Environmental Compliance of Diesel Engines

Nitrogen oxides generated from the combustion of fossil fuels contribute to ground level ozone formation and acid rain deposition. To minimize emissions the robust sensor technologies required. Solid-state electrochemical NOx and NH3 sensors evaluated in this study showed significant cross-interference issues. In contrast, the semiconductor nanowires of Cu and Pb Pthtalocyanine and CdS thin films functionalized with Fe and Mn Porphyrines (Ph) exhibited negligible cross-interferences. The computational molecular modeling was utilized to analyze equilibrium structures and binding energies of O2, CO, NO, NH3, and these were shown to control the selectivity. The FePh exhibited the highest selectivity for NO (-190 kJ/mol), while MnPh was most selective to NH3 (-46.7 kJ/mol). The binding energies of gas molecules to the hybrid Ph-CdS system were approximately 30 kJ/mol greater than binding to the Ph alone. Therefore, the semiconductor surface buried under the Ph layer enhanced sensitivity, but it did not alter the selectivity.

Electrochemical NOx sensors based on stabilized zirconia: comparison of sensing performances of mixed-potential-type and impedancemetric NOx sensors

A tubular sensor was fabricated by using yttira stabilized zirconia (YSZ) and ZnCr2O4 sensing-electrode (SE) aiming for detection of NOx at high temperature. The sensing characteristics of this YSZ-based sensor were evaluated at 700∘C by means of potentiometric (mixed-potential) and impedancemetric methods with the variation of thickness of SE. A correlation between the thickness and the sensing performances was obtained for both types of NOx sensors. The mixed-potential-type sensor using ZnCr2O4-SE exhibited high NOx sensitivity when the SE thickness was small (4 μm). On the other hand, the impedancemetric sensor, employing the same oxide-SE, provided almost equal sensitivity to NO and NO2 when the SE thickness was large (39 μm). In this case, the total concentration of NOx can be measured. The comparison of sensing mechanisms for the both sensors was briefly discussed.

Non-ideal Behavior in Solid State Electrochemical NOx Sensors

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電気化学的NOx除去システム用積層型セラミック複合体のin-situプロセッシング

Electrochemical NOx reduction system composed of three-layered laminate, 8 mol%Y 2 O 3 -ZrO 2 (YSZ) // 70 vol %YSZ/BaMnAl 1 1 O 1 9 (BMA) // 30vol%YSZ/BMA, could be successfully synthesized at 1550°C by taking into account the shrinkage mismatch between YSZ and YSZ/BMA layers. Two types of alumina, α- and γ-phase, were used as an alumina source for the in-situ processing of the laminate. The total shrinkage of the precursor powder for the composite using γ-alumina was closer to that of YSZ than that using α-alumina. The shrinkage rate of the precursor powder was adjusted to that of YSZ at the heating rate of 2°C/min. The shrinkage curve of YSZ/BMA composite consisted of the first shrinkage region, a plateau region and the second shrinkage region. The plateau region, in which the formation of BMA crystallites inhibited the growth and sintering of YSZ, decreased with increasing YSZ/BMA ratio. The control of heating rate and the use of the intermediate layer with compositional gradient lead to the development of the three-layered laminate tightly bonded with each other without any separation, which can be applied for the electrochemical NOx reduction system.

Electrochemical NO2 Sensors with WO3 Electrodes for High Temperature Applications.

Electrochemical NOx sensors for high temperature applications were prepared coupling pellets of yttriastabilized zirconia (YSZ), an oxygen ion conductor, with a thick film of a semiconducting oxide (WO3) as an auxiliary phase. The obtained sensors were exposed to the same atmosphere without the use of reference air. Electromotive force (EMF), polarization curves and aperometric measurements were performed in air and at different concentrations of NO2 in air, at selected temperatures. Stable EMF values and fast response times were obtained. The sensors showed non-linear polarization curves. The role played by the electrolyte/electrode interface was determined by electrochemical impedance spectroscopy (EIS) tests: exposure to NO2 did not affect the bulk but only the electrolyte/electrode interface.

Electrochemical NOx Sensors for Automotive Diesel Exhaust

ABSTRACTNew emissions regulations will increase the need for compact, inexpensive sensors for monitoring and control of automotive exhaust gas pollutants. Species of interest include hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx). The current work is directed towards the development of fast, high sensitivity electrochemical NOx sensors for automotive diesel applications. We have investigated potentiometric NO sensors with good sensitivity and fast response when operated in 10% O2. The sensors consist of yttria-stabilized zirconia substrates attached with NiCr2O4 sensing electrodes and Pt reference electrodes. A composite NiCr2O4:Rh sensing electrode is shown to give significantly faster response than NiCr2O4 alone. The exact role of the Rh in enhancing the response speed is not clear at present. However, the Rh appears to accumulate at the contacts between the NiCr2O4 particles and may enhance the inter-particle electronic conduction. Ongoing testing of these sensors is being performed to elucidate the sensing mechanisms and to quantify cross sensitivity to, for example, NO2.

NO<sub>x</sub> Sensors Based on Interfacing Yttria Stabilized Zirconia with p and n-Type Semiconducting Oxides

Electrochemical NOx sensors based on coupling yttria stabilized zirconia (YSZ), an oxygen-ion conductor, with different semiconducting oxides as auxiliary phase were studied. Nanosized LaFeO3 perovskite-type oxide, a p-type semiconductor, and n-type WO3 were used as auxiliary phases because of their NO2 sensing properties in resistive-type devices. LaFeO3 or WO3 thick films were deposited using a screen printing oil on one side of the solid-electrolyte pellets. The sensors were fully exposed to the same atmosphere. Electromotive force (EMF), polarization curves, amperometric and electrochemical impedance spectroscopy (EIS) measurements, were performed in air and at different concentrations of NO2 in air, at selected temperatures.

Approach to High-performance Electrochemical NOx Sensors Based on Solid Electrolytes

Approach to High-performance Electrochemical NOx Sensors Based on Solid Electrolytes