Influence of Solid-State Reactions at the Electrode− Electrolyte

May 19, 2007 - ... a non-Nernstian sensing element in combination with an upstream catalytic filter. David L. West , Fred C. Montgomery , Timothy R. A...
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J. Phys. Chem. C 2007, 111, 8307-8313

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Influence of Solid-State Reactions at the Electrode-Electrolyte Interface on High-Temperature Potentiometric NOx-Gas Sensors Jiun-Chan Yang and Prabir K. Dutta* Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210-1185 ReceiVed: January 30, 2007; In Final Form: April 4, 2007

The electrical signal from yttria-stabilized zirconia (YSZ) electrolyte-based potentiometric gas sensors is dependent on the electrochemical reactivity of the gases around the triple-phase boundary points. However, chemical reactions taking place at these triple-points can influence the performance of gas sensors, typically resulting in reduction of the electrochemical signal. In this paper, the focus is on a NOx sensor using Ptzeolite Y as the reference electrode and WO3 as the sensing electrode. Significant improvement of NOx sensitivity measured at 600 °C was noted if the sensor assembly was heated to 950 °C as compared to 700 °C. From temperature-programmed desorption (TPD), diffuse reflectance FT-IR (DRIFTS), and chemical reactivity measurements, NOx interaction with YSZ was found to be strong, forming surface nitrates on YSZ and exhibiting high activities toward NOx equilibration. On the contrary, NOx adsorption on heat-treated WO3YSZ mixtures was found to be weak and the surface was barely active toward NOx equilibration (2NO + O2 T 2NO2) even at 600 °C. From powder diffraction and Raman experiments, it was discovered that WO3 reacts with YSZ upon thermal treatment, resulting in the formation of polytungstates and monoclinic ZrO2. These interfacial species suppress the chemical reactions around the triple-phase boundary because of their lower NOx catalytic activities, thereby enhancing the electrochemical sensor signal.

1. Introduction Solid-state devices based on stabilized zirconia electrolyte are of significant importance in the energy industry. Zirconia oxygen sensors have long been used to monitor the performance of internal combustion engines in automobiles in order to increase fuel efficiency and minimize emissions.1 Solid-oxide fuel cells (SOFC) based on zirconia generate electricity via electrochemical oxidation of fuels, such as hydrogen and hydrocarbons and could play an important role in mitigating energy problems.2 Both applications exploit zirconia’s high ionic conductivity as well as mechanical and chemical stability. High-temperature potentiometric sensors are also emerging as an application of stabilized zirconia,3 especially NOx sensors.4-6 The demand for high-temperature NOx sensors comes from the entire combustion industry. With increasing fossil fuel demand and global warming alerts in the last few decades, new automotive engines with high air/fuel ratio are being developed in order to increase fuel efficiency. In an environment of excess oxygen, however, three-way catalysts traditionally used to reduce NOx, hydrocarbon, and CO emissions are not functional. Possible proposed solutions include using reductants for continuous NOx reduction or a chemical trap with periodic regeneration.7 Reliable NOx sensors are needed for controlling these processes.8 Applications of NOx sensors are also expected in the power, chemical, glass, and other high-temperature industries. The electrochemical reactions on zirconia devices occur at the triple-phase boundary (TPB), which is the junction between the electrode, electrolyte, and gas. Interfacial reactions between the electrode and electrolyte can influence the electrochemical reactions at the TPB and the overall device performance. For * Corresponding author. E-mail: [email protected].

example, with SOFC cathodes, interdiffusion driven by chemical potential gradients usually induces the formation of lowconductive layers and results in ohmic and polarization loss.9 For YSZ-based NOx sensors, the role of the electrolyte and interfacial reactions has not been extensively studied. Primary research effort is directed to discovering electrode materials with high sensitivity toward particular target molecules.5 The role of the chemical reactivity of NOx on a WO3 electrode was shown to be important in sensor response.10 Recent studies have concluded that the ideal materials for NOx-sensing electrodes should have high NOx electrocatalytic activities, slow electrochemical oxygen kinetics, and low heterogeneous catalytic activities toward NOx conversion.5,6 At elevated temperatures, the heterogeneous catalytic ability of electrolytes can also influence gas-sensing. Mukundan et al. have indicated that the response of mixed-potential hydrocarbon sensors can be greatly improved by decreasing the heterogeneous catalysis on the YSZ electrolyte.11 We have reported that sensors composed of WO3 electrode, YSZ electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NOx, are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments.12 Several reports have also described the superior NOx sensitivity when using WO3 electrodes with YSZ, especially at temperatures higher than 600 °C.10,13 Studies have indicated that the low catalytic activity toward NOx equilibration is responsible for the good performance of the sensor.10,12 In this study, the focus is on the electrode-electrolyte interface. The chemical reactions between WO3 and YSZ were investigated by powder diffraction and Raman spectroscopy. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), TPD, and NOx conversion measurements were carried out to examine NOx adsorption and chemical reactivity.

10.1021/jp070802h CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007

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Figure 1. Schematic representation of potentiometric sensors composed of Pt, YSZ, WO3, and Pt-zeoliteY (PtY) (a) with WO3 sensing electrode, (b) with Pt sensing electrode.

Correlations between the observed sensing responses with the chemical reactions occurring at the triple-phase boundary are developed. The results from this study suggest strategies for improvement in the sensitivity of high-temperature gas sensors. 2. Experimental Section 2.1. Preparation and Characterization of Materials. WO3 (Alfa Aesar, 99.8%), ZrO2 (Alfa Aesar, 99.7%), and 8 mol % YSZ (Tosoh, TZ-8Y) were obtained from commercial sources and used without any further treatment. The WO3-YSZ mixtures were prepared by ball-milling two powders with different ratios 2% and 26% WO3 (by weight), followed by heating at appropriate temperatures (700 or 950 °C) in air. Scanning electron microscopy (FEI XL30 FEG ESEM) was used to investigate the surface morphology of WO3 and Pt. A Rigaku Geigerflex X-ray diffractometer with Ni-filtered Cu KR radiation was used for powder diffraction experiments. Raman spectra were taken by a HORIBA-Jobin Yvon HR800 spectrometer with 514.5 nm laser excitation. The BET surface area was determined from the adsorption isotherms of N2 at -196 °C using a Micromeritics ASAP 2020 analyzer. 2.2. Sensor Fabrication and Electrical Measurements. A schematic design of the two-electrode electrochemical sensors based on YSZ electrolyte examined in this study is illustrated in Figure 1. The electrode and electrolyte materials are from the same sources as the samples discussed above for characterization. The YSZ pellets were prepared by pressing the YSZ powder at 8000 lbs, followed by sintering in air at 1450 °C for 2 h to form dense bodies. Two Pt lead wires (99.95%, 0.13 mm in diameter, Fischer Scientific) were attached to YSZ with a small amount of commercial Pt ink (Englehard, A4731). The end attached to YSZ was shaped into a disc of 2 mm diameter in order to increase the mechanical stability. The Pt ink was cured at 1200 °C for 2 h to secure bonding between the Pt wire and YSZ. For sensor (a) in Figure 1, WO3 powder was mixed with R-terpineol to form a paste, which was then painted on top of the Pt lead wire and YSZ. The WO3 layer was spread over as much YSZ as possible. After heating at 700 °C in air for 2 h, the WO3 layer was typically about 200 µm thick. The reference electrodes are fabricated by covering a layer of Pt-loaded zeolite Y (PtY). Zeolite Y (1.0 g) was added to 2.5 mM [Pt(NH3)4]Cl2 solution and stirred for 24 h at room temperature. The Pt-exchanged powder was centrifuged and washed with distilled water. After repeating the ion-exchange process three times, the powder was calcined at 300 °C for 3 h and exposed to 5% H2 at 400 °C for 5 h to reduce the platinum. The resulting powder was mixed with R-terpineol and painted on the top of the Pt lead wires to form the reference electrodes. The layer is around 100 µm thick after calcination at 700 °C in air for 2 h. Sensors were also prepared that were heat treated at 950 °C for 2 h. The gas-sensing experiments were performed within a quartz tube placed inside a tube furnace (Lindberg Blue, TF55035A). A computer-controlled gas delivery system with calibrated mass

Yang and Dutta flow controllers (MFC) was used to introduce the test gases. Certified 2000 ppm NO and 2000 ppm NO2 cylinders balanced with nitrogen (Praxair) were used as NOx sources. Sensor tests were carried out with mixtures of dry air, NO2, and nitrogen with total gas flow rates of 200 cm3/min at 600 °C. A pair of Pt wires were used to connect the sensor to external leads. The open circuit potential of sensors was recorded by HewlettPackard data acquisition system (HP 34970A) with 10 GΩ internal impedance. PtY electrodes were connected to the negative terminal of the HP multimeter. 2.3. Catalytic NOx Conversion Measurements. The catalytic activity for NOx equilibration (2NO + O2 T 2NO2) was measured by a chemiluminescence NOx analyzer (Eco-Physics CLD 70S). A 100 mg sample was placed on a quartz wool support inside a U-shape quartz tube with a 4 mm diameter. The quartz tube was heated by a vertical tube furnace. Then, 600 ppm NO2 and 3% O2 (balance N2) were delivered through the quartz tube at a total flow rate of 200 cm3/min, and the product was examined by the chemiluminescence analyzer. The GHSV (gas hourly space velocity) is around 90 000 h-1. Mass transfer limitation was checked by changing the gas flow rate and sample mass. The NOx analyzer was calibrated daily with a 600 ppm NO primary standard. A blank experiment was carried out with only quartz wool to examine NOx conversion from the quartz wool and the quartz tube. 2.4. Temperature-Programmed Desorption Measurements. Temperature-programmed desorption (TPD) was performed to study the co-adsorption of NOx and oxygen. A 300 mg sample was placed on a quartz wool support inside a U-shape quartz tube (4 mm diameter). As the NOx source, 5000 ppm helium-balanced NO and NO2 cylinders from Praxair were used. Before gas adsorption, the sample was heated to 650 °C in 10% oxygen for 30 min and cooled to room temperature in He. Then, 2500 ppm NO or NO2 and 5% oxygen were delivered through the sample tube for 20 min at a flow rate of 60 cm3/ min for gas adsorption. Samples were purged with 30 cm3/min He for 10 min to evacuate NOx and O2. The sample temperature was then increased from room temperature to 600 °C at the rate of 10 °C/min for desorption. The desorbed species were analyzed by a gas chromatography-mass spectrometer (Shimadzu QP-5050). The mass signal of NOx from different samples was normalized by a 5000 ppm NO standard. 2.5. Diffuse Reflectance Infrared Fourier Transform Spectroscopy. NOx and O2 co-adsorption was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). A Bruker IFS 66/S spectrometer equipped with a high-temperature/high-pressure reflection chamber (ThermoElectron) and a liquid-nitrogen-cooled MCT detector were used in this study. The sample could be heated to temperatures up to 750 °C during infrared studies under controlled concentration of gases. The temperature was measured from the thermocouple located in the center of the sample cup. The gas flow rate was set to 30-60 cm3/min to decrease the temperature difference between the surface and the bulk temperature measured by the thermocouple. The temperature reported in this paper is the thermocouple reading. Before performing NOx adsorption, the sample was heated at 700 °C in 20% oxygen for 30 min and cooled to the appropriate temperature also in 20% oxygen. Background spectra at different temperatures were acquired during the cooling procedure. Then, 1000 ppm NO2 or 1000 ppm NO and 10% oxygen were delivered to the sample chamber for 20 min at a flow rate of 60 cm3/min for gas adsorption. Samples were purged with 60 cm3/min 20% O2 for 10 min to evacuate NOx.

High-Temperature Potentiometric NOx-Gas Sensors

Figure 2. Sensor response traces for 40-800 ppm NO2 in 3% O2 at 600 °C. (a) WO3 sensing electrode, (b) Pt sensing electrode. Insert is a recovery trace after NOx is turned off for the sensor with the WO3 sensing electrode.

IR spectra were collected at a spectral resolution of 4 cm-1 and were converted to Kubelka-Munk (KM) units for quantitative comparison. The spectra taken before NOx adsorption were used as the reference spectra for KM calculations. In diffuse reflectance measurements, to account for the scattering effects, KM (F(R∞)) units provide a better measure of the correlation between signal and concentration of adsorbing species and is calculated as follows: F(R∞) ≡ (1 - R′∞)2/2R′∞, where R′∞ is the reflectance of the sample divided by the reflectance of the reference material (Rsample/Rreference) at a particular wavelength. The intensity of NOx adsorption peaks was normalized by using the 8 mol % YSZ sample (700 °Ctreated) as the reference every time after adjusting the optical alignment. 3. Results 3.1. NO2-Sensing Behavior. Figure 2a is the sensor response with a WO3 sensing electrode (Figure 1a) and Figure 2b with a Pt sensing electrode (Figure 1b). The sensor with the WO3 electrode has a significantly stronger signal toward NO2; for example, for 100 ppm NO2, the signal is an order of magnitude higher than that of the sensor with the Pt electrode. Even though the response times of the WO3- and the Pt-electrode systems are comparable, the recovery time of the WO3 device is significantly longer. The insert in Figure 2 shows that the entire recovery with the WO3 electrode can take an hour. Figure 3 (plots c and d) compares the response of a WO3based sensor conditioned at 700 °C for 2 h with the same sensor heated to 950 °C for 2 h, with sensor testing of 40-800 ppm NO2 in 3% O2 at 600 °C. The log[NO2]-EMF plots in Figure 3 (plots a and b) are the average of signal from three sensors. Both plots display a logarithmic relation of EMF to NO2 concentration. The signal for the sensor heated to the higher temperature of 950 °C is almost double that of the 700 °C heattreated sensor. SEM micrographs in Figure 4 show the morphology of WO3, with a growth in grain size from ∼300 nm to ∼2 µm after 950 °C heat treatment. 3.2. Catalytic NOx Conversion Measurements. Examination of the activity of samples toward NOx equilibration was performed in 3% O2 with a chemiluminescence NOx analyzer for product analysis. Figure 5 compares the activity of YSZ, WO3-YSZ (26% WO3, heated to 700 °C), ZrO2, and WO3,

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Figure 3. EMF-log([NOx]) plots and response traces of sensors measured at 600 °C after heating sensor to (a)(c) 700 °C, (b)(d) 950 °C. Plots (a) and (b) are the average signal of three sensors. Plots (c) and (d) are response traces from one of the sensors. The concentrations of NOx follow the same profile as Figure 2(a).

with BET surface areas of 5.71, 4.52, 3.79, and 2.76 m2/g, respectively. YSZ exhibits the highest activity. At 600 °C, the working temperature of the sensors, 80% conversion of NO2 is observed on YSZ, whereas the WO3-YSZ sample exhibited 4% conversion. For monoclinic ZrO2, the surface area was 66% of the YSZ sample, but the conversion rate at 600 °C was only 16% of YSZ. WO3 exhibited the lowest activity. For the WO3YSZ (26% WO3) sample heated at 950 °C, the NOx equilibration activity was lower as compared to the sample heated to 700 °C, with ∼2% conversion at 600 °C (data not shown). 3.3. Temperature-Programmed Desorption. The release of NOx species from YSZ, WO3-YSZ, and WO3 after coadsorption of NOx and 5% O2 was investigated by temperatureprogrammed desorption. The adsorption of NO2 and NO both lead to the same desorption profile, so only the data from NO2 adsorption is shown. The relevant fragments monitored were m/z ) 30(NO) and 46(NO2). Only data for NO (m/z ) 30) are shown in Figure 6, since the peak for NO2 (m/z ) 46) exhibited similar features. No features from O2 (m/z ) 32) evolution were observed up to temperatures of 600 °C. In Figure 6, no desorption features due to NOx were observed from WO3 (plot c). Desorption profiles of YSZ show at least two desorption peaks, labeled as R peak at 400 °C and β peak (shoulder) at 465 °C (plot a). The 26% WO3-YSZ sample had much lower NOx adsorption, the desorption peak area being only 2.5% of YSZ (plot b), and the peak maximum was at 300 °C. 3.4. Diffuse Reflectance Infrared Fourier Transform Spectroscopy. IR spectroscopy was carried out to determine the nature of the adsorbed species after exposure to NOx. Insitu studies were done over the temperature range of 300600 °C. At 600 °C, the working temperature of sensors, the IR signal from all samples except YSZ was below the detection limit. The data obtained at 300 °C are presented in Figure 7. The general trend of the observed bands at all temperatures is that the signal is strongest on YSZ, followed by ZrO2, and then the WO3-YSZ sample. We focus primarily on the 1200-1800 cm-1 region, where the bands due to nitrate species are observed. NO/O2 coadsorption produced similar IR spectra similar to those of NO2/ O2. For the YSZ sample, there is a relative change in the intensity of bands in the 1500-1650 cm-1 region with longer

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Figure 4. SEM micrographs for (a) WO3 electrodes heated for 2 h in air at 700 °C, (b) at 950 °C.

Figure 5. Measure of NOx equilibration as a function of temperature with 600 ppm NO2 in 3% O2. Sample mass ) 100 mg. Contribution from the quartz support and quartz tube has been subtracted. Dashed line: equilibrium values calculated from the equilibrium constants, (b) YSZ, (O) ZrO2, (4) 26% WO3-YSZ, (() WO3. All samples were heat treated at 700 °C.

exposure, whereas for ZrO2 and 2% WO3-YSZ (note change in WO3 loading from the chemical reactivity studies), the relative band intensities did not change with time. The bands observed around 1600 and 1200 cm-1 arise from nitrate species.14 Typically, the free nitrate ion has one ν3 vibration in this region, which is split due to symmetry lowering via interaction with the surface.15 Three peaks at ∼1560, 1585, and 1620 cm-1 are observed, indicating the presence of three different types of nitrates, bridging monodentate, chelating, and bridging bidentate groups, respectively.14,15 Two bands in the 1200 cm-1 region around 1280 and 1232 cm-1 are due to monodentate and bidentate groups, respectively.14,15 Figure 8 shows the spectra after evacuating the NOx from the sample chamber for 10 min. It demonstrates the marked influence of small levels of WO3 on decreasing the amount of NOx adsorption on YSZ. Four samples YSZ, ZrO2, 2% WO3YSZ, and WO3 were examined. The intensity of all bands decreased with addition of WO3 to YSZ. On pure WO3 (plot d), no bands from NO adsorption were observed; with ZrO2, the nitrate bands were considerably weaker than with YSZ. 3.5. X-ray Diffraction and Raman Scattering on WO3YSZ Samples. The phases present in the 26% WO3-YSZ sample were investigated by X-ray diffraction at room temperature after heating the sample at 700 and 950 °C for 2 h. As

Figure 6. TPD profiles of 700 °C-treated samples following adsorption of 2500 ppm NO and 5% O2 at room temperature. (a) YSZ, (b) 26% WO3-YSZ, (c) WO3.

Figure 7. DRIFTS spectra acquired after exposure to1000 ppm NO2 and 10% O2 at 300 °C. (a) YSZ, (b) monoclinic ZrO2, (c) 2% WO3YSZ. The three spectra in each figure represent 5 min, 10 min, and 20 min adsorption (peaks growing with time). All samples were heat treated at 700 °C.

can be seen from plot (a) in Figure 9, monoclinic WO3 and cubic YSZ were identified after heating at 700 °C. However, after the heat treatment at 950 °C for 2 h, the peaks due to WO3 decreased, and peaks due to monoclinic-phase ZrO2

High-Temperature Potentiometric NOx-Gas Sensors

Figure 8. DRIFTS spectra acquired at 300 °C after exposing at 1000 ppm NO2/10% O2 for 20 min and evacuating the chamber for 10 min (a) YSZ, (b) ZrO2, (c) 2% WO3-YSZ, (d) WO3. All samples were heat treated at 700 °C prior to NO2 exposure.

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Figure 10. Room-temperature Raman spectra: (a) WO3, (b) ZrO2, (c) 26% WO3-YSZ heated at 950 °C, (d) cubic YSZ, (e) the surface of a sensor heat treated at 950 °C and WO3 mechanically removed after heat treatment.

clusters, bands at ∼830-840 and 990-1000 cm-1 have been assigned to the WdO stretching and W-O-W stretching modes, respectively.18 With the above spectroscopic assignments, it is clear that for the sensor sample shown in Figure 10e, besides peaks due to WO3 and YSZ, bands due to ZrO2 and polytungstates are observed at the WO3-YSZ interface. 4. Discussion

Figure 9. Room-temperature XRD of the 26% WO3-YSZ sample, (a) 700 °C-treated, (b) 950 °C-treated. Symbols: (1) monoclinic ZrO2, (() monoclinic WO3, (3) cubic YSZ.

appeared. Several weak peaks between 2θ ) 20° and 30° are assigned to yttrium tungsten oxides (based on the Raman data shown below). Figure 10e shows the Raman spectrum of the surface of a sensor that was heated to 950 °C, with the WO3 layer mechanically removed after the heat treatment. In order to interpret the spectrum, Raman spectra of a number of model samples were also recorded. Figure 10a is the spectrum of WO3, characterized by bands at 272, 719, and 808 cm-1. Figure 10b is the spectrum of monoclinic ZrO2, with prominent bands at 179, 191, 335, 349, 477, 617, and 639 cm-1, in agreement with the literature.16 For YSZ, the major features are similar to ZrO2, and bands at 150, 263, 473, and 623 cm-1 shown in Figure 10d are characteristic of YSZ.17 The Raman spectrum of the WO3-YSZ (26%) sample (Figure 10c) exhibited features of WO3, YSZ, and ZrO2 and new bands at 840 and 998 cm-1. Consistent with XRD, there is a chemical reaction between WO3 and YSZ, which results in ZrO2 formation. Also, the reaction between WO3 and Y2O3 forms polytungstates, as evidenced by the bands at 840 and 998 cm-1. For oligomeric tungstate

4.1. NOx Adsorption and Conversion on WO3, ZrO2, and YSZ. Thermal desorption profiles of NO from YSZ (Figure 6) suggest significant adsorption on the surface as evidenced by the strong features over the temperature range of 300-500 °C. Intense nitrate peaks between 1200 and 1700 cm-1 were also found on the YSZ surface at 300 °C, indicating that nitrates are formed on the YSZ surface. The IR data show the presence of mono- and bidentate nitrates, and the TPD data show two peaks. We propose that these TPD peaks correspond to mono(R) and bidentate (β) nitrates, with the latter being held more tightly on the surface and hence being released at the higher temperature. Fewer adsorption sites are available on the ZrO2, since the IR peaks are considerably weaker. Doping ZrO2 with yttria has been reported to enhance NOx sorption properties.19 Zhu et al. studied catalytic activities of YSZ and ZrO2, and they concluded that YSZ is more reactive than ZrO2 for partial oxidation of methane to synthesis gas due to higher density of oxygen vacancies.20 The anionic vacancies in YSZ also led to higher catalytic activity for total oxidation of propane and toluene.21 Thus, we propose that the oxygen vacancies in YSZ provide NOx adsorption sites, which promotes the higher NOx equilibration activity at 600 °C. In contrast to YSZ, the WO3 surface provides fewer reactive adsorption sites, and hence it is relatively inert for NOx equilibration. 4.2. Interfacial Reactions between WO3 and YSZ and Their Influence on NOx Adsorption at the Triple-Phase Boundary. Raman and XRD data suggest that at 950 °C yttrium tungsten oxides and monoclinic ZrO2 are found at the interface of WO3 and YSZ. Surface segregation of Y2O3 on calcined YSZ at temperatures higher than 950 °C has been reported.22 Auger and XPS analysis has revealed that the Y2O3 can reach up to 23 mol % on the surface of calcined YSZ, regardless of the bulk Y2O3 content.23 From the NOx equilibration measurements

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(Figure 5), it is clear that the addition of WO3 suppresses the NOx reactivity on YSZ. The solid-state reaction between WO3 and YSZ on the sensor surface lead to formation of interfacial ZrO2 and yttrium tungsten oxides, altering the reactivity of the YSZ surface and leading to lower NOx equilibration activity. At 950 °C, these interfacial species are identified by Raman and diffraction studies, and their absence at 700 °C could be due the fact that the extent of reaction is smaller, thus making it difficult to detect the products by Raman or XRD studies. 4.3. Influence of Interfacial Species on NOx Sensing. For the sensor, a steady-state potential arises when the redox reactions (1) and (2) occur simultaneously on the same electrode: where O2- represents an oxygen ion in the YSZ lattice. The

O2 + 4e- T 2O2-

(1)

2NO + 2O2- T 2NO2 + 4e-

(2)

2NO + O2 T 2NO2

(3)

measured potential is most often referred to as a non-Nernstian or mixed-potential because of the deviation from the Nernstian relation. A mixed-potential arises when a nonequilibrium state exists involving two or more electrochemical reactions, and it is the steady-state potential where the partial currents for each reaction (icathodic + ianodic) are equal to zero. Reaction (3) on the surface of electrodes and electrolytes competes with the electrochemical reaction. Potentiometric devices measure the thermodynamic driving force (free energy) of chemical reactions. With increasing levels of NOx equilibration via reaction (3), the sensor signal arising from reaction (2) will progressively become weaker. When WO3 is used as the sensing electrode, NOx can pass through the porous structure to arrive at the triple-phase boundaries without equilibrating with oxygen. That is why sensors with WO3 electrodes have a stronger signal over Pt sensing electrodes, which is a good NOx equilibration catalyst.4,12 However, when NOx adsorbs on YSZ at the triple-phase boundary, reaction (3) can occur on the YSZ surface. The transformation from YSZ to monoclinic ZrO2 and the yttrium tungsten oxides induced by WO3 at 950 °C lead to lower chemical catalytic activity around the triple-phase boundary, and hence the stronger sensor signal as compared to the samples heated to 700 °C, where this solid-state reaction is less pronounced. The interfacial reactions between the SOFC cathode and the YSZ electrolyte are undesirable because they mostly result in ohmic and polarization losses. For potentiometric gas sensors, ohmic and polarization losses are not concerns because the sensing performance does not depend on the current density. However, the electrochemical reaction still has to occur at the interface, and O2- migration across the interface will have to occur. The lower oxygen ion conductivity of the interfacial layers (due to the presence of ZrO2 and polytungstates) necessitates that the interfacial film thickness has to be small for obtaining a sensor signal with adequate time resolution. The long recovery times (Figure 2) may be a result of the interfacial species, which is supported by the observation that sensor recovery was longer in the 950 °C-heated sensor than for the 700 °C system. Figure 11a is a schematic diagram of the electrodeelectrolyte interface, and Figure 11b shows the reactions occurring at these sites. By using WO3 as electrode, path (a) in Figure 11b is reduced. The interfacial compounds suppress the reaction of path (b). Thus, the chemical equilibration of NOx is

Figure 11. (a) A schematic representation of the interfacial reactions and their influence on NOx equilibration chemistry. (b) Role of the reactions that influence the measured EMF using WO3 as the sensing electrode with YSZ. Paths (a) and (b) represent chemical reactions, and paths (c) and (d) represent electrochemical reactions.

minimized, thereby promoting the electrochemical reaction of path (c) and resulting in increased signal. However, the recovery time is compromised as O2 equilibration with O2- in path (d) is slowed because of the interfacial compounds. 5. Conclusion The influence of interfacial reactions between the WO3 electrode and YSZ electrolyte on NOx sensing was investigated. XRD and Raman data show that WO3 reacts with YSZ at 950 °C, leading to the formation of yttrium tungsten oxides and monoclinic ZrO2. Sensors prepared with WO3 electrodes on YSZ lead to a substantially stronger sensor signal. From TPD, DRIFTS, and catalytic activity measurements, the amount of NOx adsorption on WO3-YSZ surfaces was found to be smaller, and these surfaces were almost inactive toward NOx equilibration, even for samples heated to 700 °C, where the presence of the new phases is not observed in the XRD or Raman studies. On the contrary, NOx adsorption on YSZ results in an increased amount of surface nitrates, and YSZ is highly active toward NOx equilibration. The formation of yttrium tungsten oxides and monoclinic ZrO2, promoted at higher temperatures, largely decreases the number of adsorption sites on YSZ that can lead to NOx equilibration and, in turn, increases the NOx signal. The interfacial reactions between the electrode and electrolyte could be an important factor affecting the performance of solid-state potentiometric NOx sensors. Acknowledgment. We acknowledge funding from NETL DOE. We also thank Professor Umit Ozkan for access to TPD and Raman instrumentation. References and Notes (1) Ramamoorthy, R.; Dutta, P. K.; Akbar, S. A. J. Mater. Sci. 2003, 38, 4271. (2) McIntosh, S.; Gorte, R. J. Chem. ReV. 2004, 104, 4845. (3) Kyriakou, G.; Davis, D. J.; Grant, R. B.; Tikhov, M. S.; Keen, A.; Pakianathan, P.; Lambert, R. M. J. Phys. Chem. B 2006, 110, 24571; Hibino, T.; Hashimoto, A.; Mori, K.; Sano, M. J. Phys. Chem. B 2001, 105, 10648.

High-Temperature Potentiometric NOx-Gas Sensors (4) Szabo, N. F.; Dutta, P. K. Sens. Actuators, B 2003, 88, 168. (5) Miura, N.; Zhuiykov, S.; Ono, T.; Hasei, M.; Yamazoe, N. Sens. Actuators, B 2002, 83, 222. (6) Martin, L. P.; Pham, A. Q.; Glass, R. S. Sens. Actuators, B 2003, 96, 53; Szabo, N. F.; Dutta, P. K. Solid State Ionics 2004, 171, 183; Zhuiykov, S.; Ono, T.; Yamazoe, N.; Miura, N. Solid State Ionics 2002, 152, 801. (7) Shelef, M. Chem. ReV. 1995, 95, 209; Olsson, L.; Persson, H.; Fridell, E.; Skoglundh, M.; Andersson, B. J. Phys. Chem. B 2001, 105, 6895. (8) Docquier, N.; Candel, S. Prog. Energy Combust. Sci. 2002, 28, 107. (9) Badwal, S. P. S. Solid State Ionics 2001, 143, 39. (10) Yoo, J.; Van Assche, F. M.; Wachsman, E. D. J. Electrochem. Soc. 2006, 153, H115. (11) Mukundan, R.; Brosha, E. L.; Garzon, F. H. J. Electrochem. Soc. 2003, 150, H279. (12) Yang, J.-C.; Dutta, P. K. Sens. Actuators, B 2007, in press. (13) Dutta, A.; Kaabbuathong, N.; Grilli, M. L.; Di Bartolomeo, E.; Traversa, E. J. Electrochem. Soc. 2003, 150, H33; Park, C.; Miura, N. Sens. Actuators, B 2006, 113, 316. (14) Hadjiivanov, K. I. Catal. ReV.sSci. Eng. 2000, 42, 71.

J. Phys. Chem. C, Vol. 111, No. 23, 2007 8313 (15) Hadjiivanov, K.; Avreyska, V.; Klissurski, D.; Marinova, T. Langmuir 2002, 18, 1619; Weingand, T.; Kuba, S.; Hadjiivanov, K.; Knozinger, H. J. Catal. 2002, 209, 539. (16) Kuba, S.; Heydorn, P. C.; Grasselli, R. K.; Gates, B. C.; Che, M.; Knozinger, H. Phys. Chem. Chem. Phys. 2001, 3, 146. (17) Hirata, T.; Asari, E.; Kitajima, M. J. Solid State Chem. 1994, 110, 201. (18) Barton, D. G.; Shtein, M.; Wilson, R. D.; Soled, S. L.; Iglesia, E. J. Phys. Chem. B 1999, 103, 630. (19) Matsukuma, I.; Kikuyama, S.; Kikuchi, R.; Sasaki, K.; Eguchi, K. Appl. Catal. B 2002, 37, 107. (20) Zhu, J.; Albertsma, S.; van Ommen, J. G.; Lefferts, L. J. Phys. Chem. B 2005, 109, 9550. (21) Labaki, M.; Lamonier, J. F.; Siffert, S.; Zhilinskaya, E. A.; Aboukais, A. Kinet. Catal. 2004, 45, 227. (22) Stanek, C. R.; Grimes, R. W.; Rushton, M. D.; McClellan, K. J.; Rawlings, R. D. Philos. Mag. Lett. 2005, 85, 445; Nowotny, J.; Bak, T.; Nowotny, M. K.; Sorrell, C. C. AdV. Appl. Ceram. 2005, 104, 165. (23) Winnubst, A. J. A.; Kroot, P. J. M.; Burggraaf, A. J. J. Phys. Chem. Solids 1983, 44, 955; Theunissen, G. S. A. M.; Winnubst, A. J. A.; Burggraaf, A. J. J. Mater. Chem. Lett. 1989, 8, 55.