Influence of adsorption and catalytic reaction on sensing properties of a potentiometric La2CuO4/YSZ/Pt sensor

Article abstract of DOI:10.1149/1.2731305

This study describes the gas-sensing properties of a La2 CuO4 yttria-stabilized zirconia (YSZ)Pt potentiometric sensor and discusses its sensing mechanism in terms of the adsorptive and catalytic behavior of the electrode materials. The sensor is highly sensitive to NO, CO, and NO2, but virtually no response is observed for CO2 and O2 detection. Optimal NO sensing is obtained at ～450°C. Above 500°C, there is an abrupt decrease in sensitivity. Below 400°C, there is a saturation of the response. The sensor is NO selective against O2, but its sensitivity is affected significantly by CO2 and CO. This selectivity behavior is in good agreement with temperature-programmed desorption results, indicating that the NO response is very likely adsorption-related. For example, the NO chemisorption peak disappears in the presence of CO2, which agrees with the sensor results showing a significant decrease in the NO sensitivity by CO2 addition even though CO2 itself shows no sensor response.

Full text of DOI:10.1149/1.2731305

Influence of Adsorption and Catalytic Reaction on Sensing
Properties of a Potentiometric La 2CuO4?⁄?YSZ?⁄?Pt Sensor
Jiho Yoo, Suman Chatterjee, F. Martin Van Assche and Eric D. Wachsman
J. Electrochem. Soc. 2007, Volume 154, Issue 7, Pages J190-J195.
doi: 10.1149/1.2731305
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J190
Journal of The Electrochemical Society, 154 ͑7͒ J190-J195 ͑2007͒
0013-4651/2007/154͑7͒/J190/6/$20.00 © The Electrochemical Society
Influence of Adsorption and Catalytic Reaction on Sensing
Properties of a Potentiometric La2CuO4/YSZ/Pt Sensor
*
*
Jiho Yoo, Suman Chatterjee, F. Martin Van Assche, and
**,z
Eric D. Wachsman
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-6400,
USA
This study describes the gas-sensing properties of a La2CuO4/yttria-stabilized zirconia ͑YSZ͒/Pt potentiometric sensor and dis-
cusses its sensing mechanism in terms of the adsorptive and catalytic behavior of the electrode materials. The sensor is highly
sensitive to NO, CO, and NO2, but virtually no response is observed for CO2 and O2 detection. Optimal NO sensing is obtained
at ϳ450°C. Above 500°C, there is an abrupt decrease in sensitivity. Below 400°C, there is a saturation of the response. The sensor
is NO selective against O2, but its sensitivity is affected significantly by CO2 and CO. This selectivity behavior is in good
agreement with temperature-programmed desorption results, indicating that the NO response is very likely adsorption-related. For
example, the NO chemisorption peak disappears in the presence of CO2, which agrees with the sensor results showing a significant
decrease in the NO sensitivity by CO2 addition even though CO2 itself shows no sensor response.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2731305͔ All rights reserved.
Manuscript submitted October 11, 2006; revised manuscript received February 14, 2007. Available electronically May 9, 2007.
There is a tremendous environmental and industrial need for the
charge carriers, the accumulated ͑or depleted͒ electrons, via gas ad-
sorption, create an electrical field across the electrolyte.13,23
monitoring and control of NOx/CO emissions during combustion
processes.1,2 Various techniques have been attempted to detect these
harmful gases effectively.3,4 Among the many sensing devices,
solid-state sensors are of particular interest due to their small size,
ease of use, and low cost. Specifically, zirconia-based solid-state O2
sensors have been well proven in harsh combustion exhaust envi-
ronments. If they are tailored to measure NOx and CO selectively,
this type of sensor can be applied to combustion process optimiza-
tion, resulting in improved fuel utilization and reduced emissions.
Therefore, modification of the conventional O2 sensor has been at-
tempted repeatedly over the last two decades.5
Potentiometric gas sensors assembled by the combination of a
metal and semiconducting metal oxide electrode have shown high
sensitivity.6-12 However, improvement in selectivity, consistency,
and stability are still necessary. One of the critical factors determin-
ing the gas-sensing properties of the potentiometric sensor is the
catalytic activity of the electrodes.12,13 La2CuO4 is catalytically ac-
tive for NO reduction by CO as a three-way catalyst but has limited
catalytic selectivity in the presence of O2.14-17 The catalytic activity
of La2CuO4 has been attributed to its small bandgap and oxygen
defects in the lattice.18 As we have previously described in our pro-
posed potentiometric sensor mechanism “differential electrode equi-
libria”, the semiconducting behavior of the electrodes also contrib-
utes to the potentiometric response.19 La2CuO4 is a semiconducting
oxide and has been well established as a semiconducting NOx sensor
material.20,21 In this work, the resistivity increased with increasing
NO concentration but decreased with increasing NO2 concentration
due to its p-type semiconducting behavior.
The concept of differential electrode equilibria is a comprehen-
sive mechanism wherein, in addition to the generation of an electric
potential by the so-called “mixed-potential mechanism”, the sensing
potential can also be created as a result of a difference in homoge-
neous gas-phase reaction kinetics and the adsorption of an oxidizing
or reducing gas between two asymmetrical electrodes, even when
the two electrodes are exposed to the same gas environment.19,22
Particularly, the detection of gases can be accomplished by the ad-
sorption of a gaseous species and the accompanying electron trans-
fer into ͑or from͒ the semiconducting electrode material via the re-
sultant Fermi level change in the material. Because the electrode is
in contact with a solid electrolyte blocking the flow of electronic
A key issue that must be addressed for the sensor devices to be
useful in combustion exhaust gas is selectivity for discrimination of
the target gas. Specifically, the sensors should exhibit a highly se-
lective response to the parts-per-million level of NOx in the presence
of high and varying O2, H2O, CO2, CO, and hydrocarbon concen-
trations. In this work, the sensing properties of an asymmetric po-
tentiometric sensor ͓La2CuO4/yttria-stabilized zirconia ͑YSZ͒/Pt͔
are reported with an emphasis on the NO selectivity. In addition, gas
adsorption on and catalytic reactions by sensor elements of a
La2CuO4-based potentiometric sensor were investigated using
temperature-programmed desorption ͑TPD͒ and reaction ͑TPR͒. The
discussion focuses on how the gas-sensing response is affected by
the adsorptive and catalytic behavior of the electrodes.
Experimental
Synthesis and characterization of La CuO
4.— La2CuO4 powder
2
was synthesized by the autoignition technique.24 A stoichiometric
mixture of La͑NO3͒3·6H2O ͑Kanto Chemical Co., 99.99%͒ and
Cu͑NO3͒2·xH2O ͑Alfa Aesar, 99.999%͒ was dissolved in deionized
water and subsequently 0.25 mol % citric acid ͑Alfa Aesar, 99.5%͒
was added to the mixture. The solution was heated at ϳ70°C with
constant stirring until a precipitate was formed, and then the precipi-
tate was autoignited with further heating at ϳ90°C. The resulting
powder was calcined at 600°C for 10 h.
X-ray powder diffraction ͓XRD, Philips APD 3720 automated
diffractometer ͑Cu K␣ radiation, ␭ = 1.54178 Å͔͒ was utilized to
characterize the phase homogeneity of the La2CuO4. Step scans
were performed with 2␪ interval = 0.02° and a retention time of 5 s
at every step in the 20° Յ 2␪ Յ 60° range.
Sensor experiments.— Details of the procedure for the prepara-
tion of the sensor electrodes are described elsewhere.16,25 La2CuO4
slurry for screen-printing was prepared by mixing the powder with
polyethylene glycol 400 ͑PEG-400, Avocado Research Chemicals
Ltd.͒ as a dispersant. Asymmetric planar sensors were fabricated by
screen-printing the La2CuO4 slurry on one face of an 8 mol %
Y2O3-doped ZrO2 substrate ͑Marketech International Inc., YSZ-8Y,
20 ϫ 12 ϫ 0.1 mm͒ and platinum paste ͑Heraeus, Conductor paste
CL11-5349͒ on the other face. The screen-printed sensor was then
sintered at 800°C for 10 h. Micrographs of the electrodes were ob-
tained using a scanning electron microscope ͑SEM, JEOL JSM 6400
SEM͒.
Both electrodes of the sensor ͓͑+͒ La2CuO4/YSZ-8Y/Pt ͑−͔͒
were exposed to the same gas atmosphere. Sensing response was
monitored with increasing and then decreasing concentration steps
*
Electrochemical Society Student Member.
Electrochemical Society Active Member.
**
z E-mail: ewach@mse.ufl.edu
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Journal of The Electrochemical Society, 154 ͑7͒ J190-J195 ͑2007͒
of sensed gases. The total gas-flow rate was 300 cm3/min and the
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retention time at every step was 200–300 s. First, the electromotive
force ͑emf͒ changes were recorded while varying the concentration
of NO, CO, NO2, O2, and CO2, at 450°C. Then, the temperature
dependence of NO, CO, and NO2 sensitivity was measured at 350–
600°C. Finally, the NO selectivity against O2 in N2 balance and
against CO2, CO, H2O, and NO2 was investigated in 3% O2 with N2
balance.
TPD and TPR experiments.— A brief procedure of the TPD ex-
periments is described below.16,26 The TPD was performed to inves-
tigate the adsorption characteristics of exhaust gases such as NO,
CO, NO2, O2, CO2, and their mixtures on 0.043 g La2CuO4 powder
͓total Brunauer–Emmett–Teller ͑BET͒ surface area ϳ0.15 m2͔. The
powder was presintered at 800°C for 10 h before loading in the
quartz reactor to prevent microstructural changes during the TPD
measurements. The TPD experiments started with adsorption of a
gas ͑or gas mixture͒ at 300°C for 30 min. The reactor was cooled
down to 50°C at 5°C/min and then purged with helium at 50°C.
The temperature was increased from 50 to 800°C at 30°C/min un-
der helium while measuring the concentration of effluent gases with
a quadrupole mass spectrometer ͑QMS, Extrel 150-QC͒. We
scanned m/e = 12͑C͒, 14 ͑N͒, 28 ͑CO or N2͒, 30 ͑NO͒, 32 ͑O2͒, 44
͑CO2 or N2O͒, and 46 ͑NO2͒.
In order to determine the catalytic activity of the sensor elec-
trode, gas reactions catalyzed by the La2CuO4 thick film on a
YSZ-8Y substrate were analyzed using TPR.16,26 The same proce-
dure used in the sensor experiments was repeated for the preparation
of the TPR sample, except that only one face of the YSZ-8Y sub-
strate was screen-printed. A mixture of a reactant gas ͑NO2, NO,
CO, O2, or CO2͒ flowed with a flow rate = 30 cm3/min through the
reactor at 50°C. When the gas flow was stabilized, temperature was
increased from 50 to 800°C at 30°C/min with reactant gas flow
while measuring the concentration of effluent gases with the QMS.
The same masses were scanned in the TPR as in the TPD measure-
ments.
Figure 2. SEMs of ͑a͒ La2CuO4 electrode surface and ͑b͒ cross section.
K2NiF4-type structure corresponding to that of La2CuO4 is shown in
the powder diffractogram with very weak impurity peaks ͑asterisks
in the figure͒, due to a minor copper oxide phase.
Figure 2a and b shows scanning electron images of the La2CuO4
electrode surface and cross section, respectively, deposited on the
YSZ-8Y substrate. The film exhibits a very porous microstructure
with 0.1–0.5 ␮m grains and is 18–25 ␮m thick. The screen-printed
Pt is ϳ10 ␮m thick and also has a porous microstructure.25
Gas sensitivity
.— Response to NO, CO, NO2, CO2, and O2.—
The sensitivity to NO, CO, NO2, and CO2 was measured in 3% O2
with N2 balance, and to O2 in N2 balance, at 450°C ͑Fig. 3͒. The
sensor is highly sensitive to NO, CO, and NO2, whereas there is
virtually no response to O2 and CO2. Values of the emf measured for
650 ppm NO, CO, and NO2 are approximately 19, 57, and −47 mV,
respectively. The slopes are generally linear and the highest slope is
obtained in the CO plot.
The direction of the sensor response based on the p-type semi-
conducting La2CuO4 is positive for reducing gases ͑NO and CO͒
and negative for oxidizing gas ͑NO2͒. This is the exact opposite of
previous results obtained at 650°C with an n-type semiconducting
WO3-based sensor of the same configuration, showing a positive
response for NO2 detection and a negative for NO and CO,25 and is
an indication that the sensing mechanism is related to the semicon-
ducting behavior of the oxide electrode.
Temperature dependence of NO sensitivity.— The temperature de-
pendence of the NO response was investigated in 3% O2 at 350
Ͻ T Ͻ 600°C ͑Fig. 4͒. The sensor response is generally rapid at
T Ͼ 450°C, with a new steady-state attained within Ͻ70 s and re-
sponse time becoming more rapid as temperature increases ͑Fig. 4a͒.
The magnitude of the response increases with decreasing tempera-
ture. A linear relationship is obtained in the plot of the emf vs the
The present TPR conditions ͑gas concentration and flow rate͒
were chosen for experimental convenience and mass spectrometer
sensitivity. They are, however, somewhat different from the sensor
measurement conditions. In order to compare TPR and sensor re-
sults, the TPR experiments were also done under reduced flow rate
͑matching space velocity͒ and with 3% O2 concentration to more
closely match the sensor experimental conditions. The difference
made by the TPR experimental condition change was found to be
within general reproducibility limits for all the TPRs.26
Results and Discussion
Characterization of La2CuO4 powder and electrode.— The
XRD pattern of the La2CuO4 powder is shown in Fig. 1. The
Figure 1. ͑Color online͒ XRD pattern of La2CuO4 powder calcined at 600°C
for 10 h. * indicate peaks due to minor impurity phases.
Figure 3. ͑Color online͒ La2CuO4-based sensor response to NO, CO, NO2,
and CO2 in 3% O2 with N2 balance and to O2 in N2 balance at 450°C.
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Journal of The Electrochemical Society, 154 ͑7͒ J190-J195 ͑2007͒
Figure 4. ͑Color online͒ NO sensitivity of La2CuO4-based sensor in 3% O2
with N2 balance at 350–550°C: ͑a͒ plots of sensor emf ͑mV͒ vs time ͑s͒ and
͑b͒ vs NO concentration ͑ppm͒. The concentration steps in ͑a͒ are 50, 100,
200, 400, and 650 ppm NO at all the temperatures. The lines drawn in ͑b͒ are
logarithmic fits.
logarithm of NO concentration at T Ն 400°C and the slope in-
creases with decreasing temperature ͑Fig. 4b͒. Above 500°C, values
of the emf are very small; for example, less than 2 mV is produced
for 650 ppm NO at 550°C. Nearly no response was observed at
600°C ͑not shown͒. At 400°C and below, the response is less stable
and saturates in the higher NO concentration region. At 350°C, the
sensor response in fact switches to a small and negative slope, indi-
cating a change in mechanism.
Considering response time and sensitivity, optimal sensor re-
sponse is obtained at ϳ450°C. Therefore, further measurements ͑the
NO selectivity experiments͒ were performed at 450°C, as described
in the NO selectivity section. The measurements of the NO sensi-
tivity at 450°C show good reproducibility between different mea-
surements repeated over 300 h under the same condition, as shown
in Fig. 5. The slopes in the plots are similar to each other, but values
of the emf are slightly gas-history dependent, most likely due to
incomplete desorption of the gaseous species from previous mea-
surement.
Figure 6. ͑Color online͒ TPD of NO over La2CuO4 powder.
The NO desorption profile obtained from the NO TPD is shown
in Fig. 6. Initially, the powder was exposed to 1000 ppm NO in
helium. Nitric oxide is physically ͑desorption peak max. ϳ150°C͒
and chemically ͑desorption peak max. ϳ280°C͒ adsorbed onto the
oxide surface at T Ͻ 400°C, thus saturating the surface and sensor
response at low temperatures ͑Fig. 4͒.
Temperature dependence of CO sensitivity
.— The CO sensitivity at
300 Ͻ T Ͻ 550°C is presented in Fig. 7. The sensor response is
generally rapid, such that even at T = 350°C, the response to the CO
concentration change is completed within 150 s ͑Fig. 7a͒. The CO
response time becomes shorter with increasing temperature. The
plots of the sensor emf vs the logarithm of CO concentration are
generally linear at all the temperatures ͑Fig. 7b͒. The CO sensitivity
is strongly temperature-dependent and an abrupt decrease in the sen-
sitivity is shown at T Ն 500°C; values of the emf corresponding to
100 ppm CO are ϳ130 mV at 300°C, ϳ78 mV at 350°C, ϳ23 mV
at 450°C, and ϳ3 mV at 500°C, and negligible CO response is
observed at 550°C.
At 300°C, the sensor shows the highest values of the emf, but the
response becomes unstable and varies depending on the direction of
the CO concentration change ͑increasing or decreasing͒, resulting in
an unsymmetrical plot. The sensitivity achieved at temperatures as
low as 300°C is surprising in terms of the mixed potential theory,
because at this temperature the exchange current is expected to be
extremely small. In addition, there are no reported mixed-potential-
type potentiometric sensors based on a stabilized zirconia electrolyte
working below 400°C.6-11 These results indicate that CO sensing at
We reported16 that in NO and NO + O2 TPR experiments of
La2CuO4 powder, no observable NO or NO2 concentration changes,
associated with homogeneous gas-phase or heterogeneous gas-solid
reactions ͑direct decomposition of NO to N2 and O2 or oxidation of
NO to NO2͒, were observed up to 800°C. Thus, any catalytic
mechanism seems to be unlikely and the NO sensitivity due to NO
adsorption and the resulting change in Fermi level of the oxide is the
more likely mechanism.19
Figure 7. ͑Color online͒ CO sensitivity of La2CuO4-based sensor in 3% O2
with N2 balance at 300–550°C: ͑a͒ plots of sensor emf ͑mV͒ vs time ͑s͒ and
͑b͒ vs CO concentration ͑ppm͒. The concentration steps in ͑a͒ are 50, 100,
200, 400, and 650 ppm CO at all the temperatures. The lines drawn in ͑b͒ are
logarithmic fits.
Figure 5. ͑Color online͒ Reproducibility of NO sensitivity measurements
repeated over 300 h under the same condition ͑in 3% O2 at 450°C͒. The
numbers labeled in the figure indicate a sequence of the measurements with
elapsed time.
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Journal of The Electrochemical Society, 154 ͑7͒ J190-J195 ͑2007͒
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Figure 8. ͑Color online͒ ͑a͒ TPD of CO + O2 over La2CuO4 powder and ͑b͒
TPR of CO + O2 over La2CuO4 on YSZ-8Y.
Figure 10. ͑Color online͒ NO2 sensitivity of La2CuO4-based sensor in 3%
O2 with N2 balance at 450–600°C: ͑a͒ plots of sensor emf ͑mV͒ vs time ͑s͒
and ͑b͒ vs NO2 concentration ͑ppm͒. The concentration steps in ͑a͒ are 50,
100, 200, 400, and 650 ppm CO at all the temperatures. The lines drawn in
͑b͒ are logarithmic fits.
low temperatures is probably not related to electrocatalytic phenom-
ena ͑“mixed potential”͒ but rather to asymmetric adsorption/
desorption phenomena.
nism was due to the differential heterogeneous catalytic activity for
CO oxidation, resulting in a differential decrease in local pO2 at the
TPB of the two electrodes and a corresponding Nernstian voltage,
then the sensor would exhibit a positive emf at one temperature
region transitioning to a negative emf at the other, with a zero emf at
ϳ300°C. In fact, an apparent maximum in sensor voltage is ob-
tained at ϳ300°C, ruling out this mechanism.
The mechanism and temperature dependence of CO sensing was
studied further by TPD of La2CuO4 powder and TPR of La2CuO4
electrodes on YSZ substrates. The results of the CO + O2 TPD on
the La2CuO4 powder are presented in Fig. 8a. For the TPD, an
adsorption of 500 ppm CO and 1% O2 was used. Broad desorption
of CO2 and CO occurs at the same temperature region ͑150–500°C͒
with the peak maximum positioned at ϳ300°C ͑Fig. 8a͒. Signifi-
cantly more CO2 is desorbed than CO, indicating that either the
surface CO is oxidized to CO2 in the desorption step or exists in an
oxidized surface COx species. The fact that CO and CO2 continue to
desorb up to 500°C indicates that a COx species is strongly bound to
the La2CuO4 surface. The rapid O2 evolution above 600°C is due to
lattice oxygen and indicates that the La2CuO4 surface was not re-
duced by CO in the presence of O2 and gives credence to the con-
cept of an oxidized surface COx species.
Similarly, if the sensing mechanism was electrochemical in na-
ture ͑e.g., mixed potential͒ via the anodic reaction13,26
CO
+ Ox
ꢀ CO2͑gas͒ + VؠOؠ
+ 2e−
͑lattice͒
͓2͔
͑gas͒
O
͑lattice͒
then the differential heterogeneous catalytic activity for CO oxida-
tion would result in a differential decrease in local pCO between the
TPB of the two electrodes and a corresponding mixed-potential re-
sponse. Therefore, the sensor would exhibit a positive ͑mixed-
potential͒ emf at one temperature region transitioning to a negative
͑mixed-potential͒ emf at the other, with a zero emf at ϳ300°C. Due
to the apparent maxima in sensor voltage obtained at ϳ300°C, this
does not appear to be the appropriate mechanism.
In addition, a positive CO response at temperatures greater than
300°C cannot be explained based on the difference in the CO oxi-
dation activity ͑Fig. 9͒, because according to the catalytic mecha-
nism, the lower CO concentration at the La2CuO4 electrode than at
the Pt should end up with a negative response, opposite to the ex-
perimental results.
Finally, the magnitude of the sensor response increases with de-
creasing temperature from 550 to 300°C ͑Fig. 7͒, which is consis-
tent with the CO/CO2 TPD profiles ͑Fig. 8a͒ that show a desorption
maximum at 300°C and extended but decreasing COx coverage up
to ϳ550°C. This appears to indicate that CO sensing is adsorption-
related, similar to NO sensing.
As can be seen in the TPD ͑Fig. 8a͒ and both TPRs ͑Fig. 8b and
9͒ La2CuO4 is highly catalytic for the oxidation of CO
1
CO + O ꢀ CO2
͓1͔
2
In fact, above ϳ320°C the activity of La2CuO4 on YSZ-8Y for CO
oxidation is higher than that of Pt on YSZ-8Y ͑Fig. 9͒.
This would imply a catalytic mechanism; however, at T
Յ 300°C the relative activity for CO oxidation over La2CuO4 on
YSZ-8Y is less than Pt on YSZ-8Y ͑Fig. 9͒. If the sensing mecha-
Temperature dependence of NO2 sensitivity.— The temperature de-
pendence of the NO2 sensitivity was studied, as shown in Fig. 10.
The NO2 response at T Ն 500°C is fast enough to attain a new
steady state in less than 100 s and is symmetrical between increas-
ing and decreasing NO2 concentration steps. However, the response
at 450°C, while significantly greater, becomes slow and varies de-
pending on whether the measurements were obtained with increas-
ing or decreasing concentration steps ͑Fig. 10a͒. As stated previ-
ously, this is probably due to the difference between adsorption and
desorption kinetics.25
At T Յ 400°C, the sensor response is so unstable and saturated
that no meaningful reading is achieved ͑not shown͒. The NO2 re-
sponse, as with NO and CO, is strongly temperature dependent, with
both magnitude and slope decreasing with increasing measurement
temperature. The emf value obtained for 650 ppm NO2 is approxi-
mately −47 mV at 450°C and −3 mV at 600°C ͑Fig. 10b͒.
The desorption profiles obtained in the NO2 TPD over La2CuO4
powder show significant, strong surface adsorption ͑Fig. 11a͒. Not
only does NO2 exhibit strong chemisorption but it involves multiple
Figure 9. ͑Color online͒ Comparison of CO + O2 TPR results between
La2CuO4 on YSZ-8Y ͑L + Y͒ and Pt on YSZ-8Y ͑Pt + Y͒.26
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Journal of The Electrochemical Society, 154 ͑7͒ J190-J195 ͑2007͒
Figure 11. ͑Color online͒ ͑a͒ TPD of NO2 over La2CuO4 powder and ͑b͒
TPR of NO2 + O2 over La2CuO4 on YSZ-8Y.
adsorbed NOx surface species. NO2 begins desorption at ϳ230°C
with a peak max ϳ280°C. Further, the NO2 desorption peak is
concomitant with an even larger NO desorption peak. A second NO
desorption peak at ϳ400°C is concomitant with an O2 desorption
peak, indicating decomposition of a surface nitrate species. Thus, a
series of complex reaction steps occur on the surface of La2CuO4 at
the lower end of the temperature range, where it is most sensitive to
NO2. Further, at the upper end ͑T Ͼ 600°C͒ lattice oxygen is emit-
ted, which would dramatically alter the sensing mechanism.
The NO2 + O2 TPR results over the La2CuO4 on YSZ-8Y show
that above 350°C, NO2 is decomposed to NO ͑Eq. 3͒, with a steep
slope at 400 Ͻ T Ͻ 500°C, and by 600°C, NO2 is almost com-
pletely decomposed ͑Fig. 11b͒.
Figure 13. ͑Color online͒ Effect of O2, CO2, CO, H2O, and NO2 on the NO
response at 450°C: ͑a͒ O2 effect ͑3–30%͒, ͑b͒ CO2 effect ͑0–16%͒, ͑c͒ CO
effect ͑0–200 ppm͒, ͑d͒ H2O effect ͑0–3%͒, and ͑e͒ NO2 effect ͑0–400 ppm͒,
measured in N2 balance for ͑a͒ and in 3% O2 with N2 balance for ͑b–e͒.
1
NO2 ꢀ NO +
O
͓3͔
2
Interestingly, the NO2 decomposition activity of the La2CuO4 on
YSZ-8Y is identical ͑within experimental error͒ to that of the Pt on
YSZ-8Y ͑Fig. 12͒.26 This similarity rules out NO2 sensing due to a
catalytic difference for Eq. 3 between the electrodes creating any
differential pNO2 or pO2 between the electrodes.
The TPD desorption profiles ͑Fig. 11a͒ show strongly adsorbed
NOx species to just beyond ϳ450°C, with a trailing NO shoulder
extending to almost ϳ600°C. At 450°C the sensor exhibits a large
but asymmetric signal. This is probably due to the nonequilibrium
associated with desorption kinetics that are relatively slow as com-
pared to the rate of adsorption. This nonequilibrium becomes more
severe below 400°C, and therefore, meaningful sensor results were
not achieved, most likely due to surface saturation.
Above ϳ450°C faster desorption rates would allow attainment
of equilibria with adsorption. Because La2CuO4 exhibits a p-type
semiconducting sensor response to NO and NO2, a corresponding
change in Fermi level could explain the potentiometric sensor
mechanism in this temperature range. In addition, differential elec-
trocatalytic reactions such as Eq. 4 may also contribute to symmetric
NO2 sensing
NO2͑gas͒ + VؠOؠ
+ 2e− ꢀ NO
+ Ox
͓4͔
͑gas͒
O
͑lattice͒
͑lattice͒
NO selectivity against O , H O, CO , CO, and NO
2.— The NO
2
2
2
selectivity of the sensor was studied at 450°C against O2 in N2 and
against CO2, CO, H2O, and NO2 in 3% O2 ͑Fig. 13͒. The NO sen-
sitivity is not significantly affected by 3–30% O2 concentration
change ͑Fig. 13a͒. The difference in the emf values made by the O2
change is always small ͑at most 2 mV͒ at the same NO concentra-
tion steps. The NO desorption profile obtained from the NO + O2
TPD over the La2CuO4 powder after exposing to 1000 ppm NO and
1% O2 for adsorption is presented in Fig. 14a. Two well-defined NO
desorption peaks in the NO + O2 TPD profiles are attained at same
the temperatures ͑ϳ150 and ϳ280°C͒ where the desorption peaks
are obtained in the NO TPD ͑Fig. 6͒. Additionally, the magnitude of
the two desorption peaks shown in the NO + O2 TPD is very similar
to that in the NO TPD. These results indicate that the NO adsorption
on the La2CuO4 is not affected by the addition of O2. Therefore, the
idea that the NO sensitivity is adsorptive in nature ͑Gas sensitivity
section͒ seems reasonable, because the NO adsorption is not influ-
enced by the presence of O2, consistent with the selective NO de-
tection against pO2 change. In contrast, the gas sensitivity at higher
temperatures, where gas adsorption is negligible due to fast desorp-
tion kinetics and the catalytic behavior is believed to be dominant, is
strongly affected by a change in the pO2.25,27,28
Figure 12. ͑Color online͒ Comparison of NO2 + O2 TPR results between
La2CuO4 on YSZ-8Y and Pt on YSZ-8Y.26 L + Y and Pt + Y correspond to
La2CuO4 on YSZ-8Y and Pt on YSZ-8Y, respectively.
Downloaded on 2012-12-05 to IP 138.87.11.21 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

Journal of The Electrochemical Society, 154 ͑7͒ J190-J195 ͑2007͒
J195
increases noticeably by 50 ppm NO2 addition, but among the plots
obtained with 50–400 ppm NO2 the slopes are similar.
Conclusions
The sensing behavior of a potentiometric sensor built using a
combination of La2CuO4 and Pt electrodes is demonstrated. The
sensor shows relatively high sensitivity to NO, CO, and NO2 at
450°C but no sensitivity to O2 or CO2. The NO sensitivity increases
with decreasing temperature from 600 to 400°C. However, sensor
operation is limited to T Ͼ 400°C, because NO does not fully des-
orb from the surface of the metal oxide below 400°C, resulting in a
saturation of the sensor response. The NO selectivity results support
the idea that the NO sensitivity is adsorptive in nature. The NO
adsorption is not influenced by the presence of O2, which is consis-
tent with the selective NO sensing against pO2 change. Removal of
the NO chemisorption peak by CO2 addition agrees with the sensor
results, showing a significant decrease in the NO sensitivity by CO2.
Moreover, by CO addition, the sensor emf values for NO detection
increase, but with a decrease in the slope ͑NO sensitivity͒, which
seems to be due to surface saturation caused by the enhanced NO
adsorption in the presence of CO.
Figure 14. ͑Color online͒ TPD over La2CuO4 powder: ͑a͒ TPD of NO
+ O2 and ͑b͒ TPD of NO + CO2.
Even though the sensor shows no response to CO2 ͑Fig. 3͒, the
effect of CO2 on the NO sensitivity is significant, reducing the sen-
sor signal and reversing the direction of response by adding 16%
CO2 ͑Fig. 13b͒. This coincides with the results of the NO + CO2
TPD ͑Fig. 14b͒. When NO and CO2 coexist, a broad and intense
CO2 peak replaced the NO chemisorption peak observed in the
NO-related TPD ͑Fig. 6 and 14a͒, and only the weaker NO phys-
isorption peak remains at ϳ150°C.16 Therefore, the decrement in
the NO sensitivity, by nonsensed CO2 ͑Fig. 3͒, occurs due to elimi-
nation of NO chemisorption by CO2. In contrast, potentiometric
sensors based on different oxide electrodes ͑WO3͒ are capable of
detecting NO selectively against CO2 in the regime of catalysis con-
trol ͑T Ͼ 600°C͒.25
Acknowledgment
This work was supported by the DOE under contract no. DE-
FG26-02NT41533 and DE-FC26-03NT41614.
University of Florida assisted in meeting the publication costs of this
article.
Addition of CO ͑30–200 ppm͒ decreases the slope ͑i.e., the NO
sensitivity͒ gradually but increases the emf values ͑Fig. 13c͒. This is
surprising, because the response of the sensor to NO and CO are
both positive. Thus, one would expect an additive response. We find
by TPD ͑Fig. 15a͒ that the presence of CO enhances NO adsorption,
so that NO continues to desorb up to 800°C ͑compare Fig. 15a and
14a͒.16 Thus, the decrease in sensitivity with CO addition can be
described in terms of the surface saturation that is exhibited for the
NO response alone below 400°C ͑Fig. 4͒. The intense CO2 desorp-
tion peak at 200–500°C in the NO + CO + O2 TPD probably forms
by the surface reactions of CO with adsorbed oxygen or lattice oxy-
gen. During the NO + CO + O2 TPR, formation of CO2 ͑Eq. 1͒ is
shown over the La2CuO4 on the YSZ-8Y, with a corresponding de-
crease of CO and O2 concentration at T Ͼ 200°C but without a
clear change of NO concentration ͑Fig. 15b͒.26 A different CO effect
was observed at high temperature ͑650°C͒ with the WO3-based po-
tentiometric sensor, where the sensor showed a negative NO re-
sponse and, by CO addition, the emf response increased, but without
a significant change of slope.25
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Figure 15. ͑Color online͒ ͑a͒ TPD of NO + CO + O2 over La2CuO4 powder
and ͑b͒ TPR of NO + CO + O2 over La2CuO4 on YSZ-8Y.
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