Temperature-programmed reaction and desorption of the sensor elements of a WO3/YSZ/Pt potentiometric sensor

Article abstract of DOI:10.1149/1.2188248

Temperature-programmed reaction (TPR) and desorption (TPD) were performed to determine the reaction and adsorption characteristics of N Ox, CO, O2, C O2, and their mixtures on the sensor elements of a W O3 /yttria-stabilized-zirconia (YSZ)/Pt potentiometric sensor. The results are discussed, focusing on how sensor performance is influenced by catalytic and adsorptive behavior of the electrode materials. The N O2 sensor response is most likely given by a dissymmetry of the N O2 decomposition activity between the W O3 and Pt electrode. No significant adsorption of N O2 and its related molecules is observed during the TPD of N O2 and N O2 -containing gas mixtures. Enhanced NO response by coexisting CO agrees with the increased NO adsorption by the addition of CO, indicating that the NO response is related to gas adsorption behavior.

Full text of DOI:10.1149/1.2188248

Temperature-Programmed Reaction and Desorption of the
Sensor Elements of a WO 3?⁄?YSZ?⁄?Pt Potentiometric
Sensor
Jiho Yoo, F. Martin Van Assche and Eric D. Wachsman
J. Electrochem. Soc. 2006, Volume 153, Issue 6, Pages H115-H121.
doi: 10.1149/1.2188248
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© 2006 ECS - The Electrochemical Society

Journal of The Electrochemical Society, 153 ͑6͒ H115-H121 ͑2006͒
H115
0013-4651/2006/153͑6͒/H115/7/$20.00 © The Electrochemical Society
Temperature-Programmed Reaction and Desorption of the
Sensor Elements of a WO3/YSZ/Pt Potentiometric Sensor
*
*
**,z
Jiho Yoo, F. Martin Van Assche, and Eric D. Wachsman
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida
32611-6400, USA
Temperature-programmed reaction ͑TPR͒ and desorption ͑TPD͒ were performed to determine the reaction and adsorption char-
acteristics of NOx, CO, O2, CO2, and their mixtures on the sensor elements of a WO3/yttria-stabilized-zirconia ͑YSZ͒/Pt poten-
tiometric sensor. The results are discussed, focusing on how sensor performance is influenced by catalytic and adsorptive behavior
of the electrode materials. The NO2 sensor response is most likely given by a dissymmetry of the NO2 decomposition activity
between the WO3 and Pt electrode. No significant adsorption of NO2 and its related molecules is observed during the TPD of NO2
and NO2-containing gas mixtures. Enhanced NO response by coexisting CO agrees with the increased NO adsorption by the
addition of CO, indicating that the NO response is related to gas adsorption behavior.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2188248͔ All rights reserved.
Manuscript submitted September 14, 2005; revised manuscript received January 31, 2006. Available electronically April 10, 2006.
An appropriate method for detecting harmful NOx gases is in
great demand both for the monitoring of air quality and for the
emission control of combustion processes.1,2 For the detection of
environmental NOx, the sensor should be sensitive to low concen-
trations of NOx ͑0–100 ppm͒.1 Enhanced selectivity and consis-
tency are critical requirements for the applications in combustion
exhausts containing hydrocarbons, CO2, H2O, O2, CO, and NOx.
One of the most practical ways is by using solid-state sensors be-
cause of their compact size, easy use, and low cost.3 Many solid-
state NOx sensors, based on the electrical response of the semicon-
ducting metal oxides ͑WO3, SnO2, ZnO, In2O3, LaFeO3, etc.͒, have
been made and tested.4-10
Among the above oxides, tungsten ͑VI͒ oxide is particularly in-
teresting because it has been known to be highly sensitive to NOx
without significant sensor signal drift.7,8,11 According to previous
studies, the response to NOx of the potentiometric sensor, built by
using the WO3 electrode, was fast and stable,7,12 and a linear rela-
tionship was obtained in a plot of the sensor electromotive force
͑emf͒ vs the logarithm of the NOx concentration.8,12 In addition, the
WO3-based sensor showed high sensitivity to as low as ϳ10 ppm
NO2 and selective NO2 detection in 3% O2 vs CO, CO2, and H2O;
however, its NO2 sensitivity was affected by the variation of O2
concentration.12 Moreover, the sensor became less selective and
consistent in combustion exhausts.12
A selective and reproducible NOx sensor has not been achieved
yet due in part to the lack of understanding of the sensing mecha-
nism. Non-Nernstian behavior of a potentiometric sensor working in
gas mixtures is conventionally explained based on the mixed-
potential theory.13,14 In the theory, when more than two heteroge-
neous electrochemical reactions take place concurrently, the poten-
tial at a steady state achieved by relative kinetics of competing
reactions determines a sensor voltage response.
The concept of differential electrode equilibria was recently sug-
gested to explain the potentiometric gas-sensing mechanism.10,15
This is a comprehensive idea claiming that in addition to the gen-
eration of an electrical potential by the mixed potential, the sensing
potential can be created as a result of a difference in homogeneous
gas-phase reaction kinetics and adsorption of an oxidizing or reduc-
ing gas between two asymmetrical electrodes even when the two
electrodes are exposed to the same gas environments. According to
the concept, a sensor signal can be produced by dissimilarity in the
kinetics of nonelectrochemical catalytic reactions between the two
electrodes.16 For CO gas detection using a Pt electrode catalytically
active for CO oxidation by O2 and an inert metal oxide electrode, as
an example, the higher oxygen partial pressure ͑pO2͒ will be created
at the inert electrode in consequence of its lower catalytic activities,
resulting in the higher potential at the metal oxide side. Based on the
Nernst equation, an order of magnitude difference in pO2 can pro-
duce 30–60 mV sensor response at an elevated temperature. The
detection of gases can be also accomplished as a result of adsorption
of a gaseous species accompanying electron transfers into ͑or from͒
semiconducting electrode materials, and therefore, the Fermi level
changes in semiconducting materials. Since the electrode is in con-
tact with a solid electrolyte blocking a flow of the electronic charge
carriers, the accumulated ͑or depleted͒ electrons, via gas adsorption,
can create an electrical field across the electrolyte.4,16
In this work, catalytic reactions on and gas adsorption by sensor
elements of a WO3-based potentiometric sensor were investigated
using temperature-programmed reaction ͑TPR͒ and desorption
͑TPD͒. The discussion focuses on how the gas-sensing response is
affected by the catalytic and adsorptive behavior of the electrode
materials.
Experimental
Sample preparation
.— Tungsten oxide ͑WO3, 99.8% purity, Alfa
Aesar͒ powder was purchased and calcined at 900°C for 10 h to
prevent microstructural changes during the TPR/TPD experiments.
The WO3 powder was then ballmilled for 24 h in ethanol. After
drying, the Brunauer–Emmett–Teller ͑BET͒ surface area was mea-
sured at ϳ4 m2/g.
For the TPR experiments, a WO3 thick film on an 8 mol %
Y2O3-doped ZrO2 substrate ͓Marketech International, Inc., yttria-
stabilized zirconia ͑YSZ͒-8Y, 20 ϫ 10 ϫ 0.1 mm͔ and a Pt thick
film on a YSZ-8Y were prepared by the screen printing method. The
details are described elsewhere.12 General procedures used for the
preparation of the sensing electrodes were repeated, except that only
one face of the YSZ-8Y substrate was screen printed with WO3
slurry or Pt paste ͑Heraeus, Conductor paste CL11-5349͒. The
screen printed films were sintered at 800°C for 10 h with heating
and cooling rate of 1°C/min. Micrographs of the sintered films
showed that the Pt layer was porous and deposited uniformly with a
thickness of ϳ9 ␮m, and the WO3 thick film exhibited a very po-
rous microstructure with 0.5–3 ␮m grains and 10–30 ␮m
thickness.12
TPR/TPD system.— A catalytic reactor was fabricated using
quartz tubes and a frit. As shown in Fig. 1a, the reactor was con-
structed by welding the two quartz tubes around a semiporous frit to
isolate the sample. To minimize the dead volume between the reac-
tor and a quadrupole mass spectrometer ͑QMS, Extrel 150-QC
QMS͒, a tube with small inner diameter ͑i.d.͒ = 1 mm was used at
the gas effluent end. A thermocouple ͑type-K͒ contained in a con-
centric quartz tube to prevent direct contact to the sample was
*
Electrochemical Society Student Member.
Electrochemical Society Active Member.
**
z E-mail: ewach@mse.ufl.edu

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Journal of The Electrochemical Society, 153 ͑6͒ H115-H121 ͑2006͒
Figure 2. TPR of NO2 + O2 over ͑a͒ Pt on YSZ ͑Pt + Y͒ and ͑b͒ WO3 on
YSZ ͑W + Y͒. Calcd: Calculated Equilibrium gas compositions based on
Eq. 1.
Figure 1. Schematic diagram of ͑a͒ catalytic reactor and ͑b͒ complete TPR/
TPD experimental system.
and 620 ppm NO2 + 1220 ppm O2 to W + Y. NO2 decomposition
is facilitated by increasing the temperature for both samples and
produces NO and O2. Equilibrium gas compositions were calculated
for comparison under the assumption that only gas-phase NO2 de-
composition ͑Eq. 1͒ is involved in the present system ͑Fig. 2͒
placed just above the sample to monitor the temperature. The reactor
was sealed gastight using Ultra-Torr fittings. Gas entered from the
top, reacted with the sample, and exited through the bottom to the
QMS.
An MKS eight-channel mass flow programmer/display ͑model
647C8RON͒ and six mass flow controllers ͑MFCs͒ were manifolded
and calibrated to control gas flow rate. The complete system is
shown in Fig. 1b.
1
NO2 ↔ NO + O2
͓1͔
2
This assumption is appropriate because formation of N2O and N2
are not observed during the TPR experiments and a corresponding
increase of O2 concentration, approximately half of the amount of
the NO2 concentration decrease, indicates that NO2 gases are de-
composed mainly via Eq. 1 rather than via heterogeneous reactions
between NO2 and oxygen defects in either YSZ or WO3. For the
calculations, 1200 ppm NO2 and 1900 ppm O2 in Fig. 2a were used
as initial concentrations and 650 ppm NO2 and 1150 ppm O2 in Fig.
2b. Pt + Y shows much higher catalytic activity for the NO2 decom-
position than W + Y. NO2 starts to decompose at ϳ410°C over
Pt + Y and is almost completely decomposed at Ͼ550°C, whereas
NO2 is not decomposed until ϳ530°C over W + Y and not fully
decomposed even at 800°C.
The effect of CO, CO2, and O2 on the NO2 TPR profile was
investigated ͑Fig. 3͒. Gas combinations used on Pt + Y were NO2
+ O2 ͑ϳ2000 ppm͒, NO2 + CO2 ͑ϳ900 ppm͒ + O2 ͑ϳ2000
ppm͒, and NO2 + CO ͑ϳ1100 ppm͒ + O2 ͑ϳ2000 ppm͒. A similar
NO2 profile obtained in the NO2 + O2 TPR with that in the NO2
TPR ͑Fig. 3a͒ indicates that NO2 decomposition is not affected by
the presence of ϳ2000 ppm O2. In the NO2 + CO2 + O2 TPR, a
small shift of the NO2 curve to 10–20°C higher temperature region
occurs, but this may be within experimental error. This indicates that
the NO2 decomposition takes place without significant influence of
ϳ900 ppm CO2 and ϳ2000 ppm O2. The earlier reduction of NO2
in NO2 + CO + O2 TPR is observed starting from ϳ150°C due to a
reaction of NO2 with CO ͑Eq. 2͒, which is most likely kinetically
favored over the NO2 decomposition ͑Eq. 1͒ at T Ͻ 450°C
TPR/TPD experiments
.— Gas reactions catalyzed by 45 mg
WO3 powder ͑W͒, a WO3 thick film ͑ϳ3 mg͒ on a YSZ-8Y sub-
strate ͑W + Y͒, a Pt film on a YSZ-8Y ͑Pt + Y͒, and a YSZ-8Y
substrate alone ͑Y͒ were analyzed by using the TPR technique. Ad-
ditionally, the TPR experiments were performed on a blank reactor
͑B͒ to determine the uncatalyzed reaction. Initially, 500–2000 ppm
of a reactant gas ͑NO2, NO, CO, O2, or CO2͒ or a gas mixture
flowed with a flow rate of 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. We scanned
m/e = 12 ͑C͒, m/e = 14 ͑N͒, m/e = 28 ͑CO or N2͒, m/e = 30 ͑NO͒,
m/e = 32 ͑O2͒, m/e = 44 ͑CO2 or N2O͒, and m/e = 46 ͑NO2͒ dur-
ing the TPR measurements.
The TPD experiments on WO3 powder ͑total BET surface area
ϳ0.18 m2͒ started from the adsorption of a gas ͑or a gas mixture͒ at
300°C for 30 min. The reactor was then cooled to 50°C at 5°C/min
and subsequently purged with 30 cm3/min helium. Temperature was
increased from 50 to 800°C under flowing 30 cm3/min helium and
at the same time, effluent gas concentration was recorded using the
QMS.
Measurements of NOx sensing properties.— Details of the mea-
surements can be found elsewhere.12 Screen printing slurry was pre-
pared by dispersing the metal oxide powder in polyethylene glycol
400 ͑PEG-400, Avocado Research Chemicals, Ltd.͒. An asymmetri-
cal planar sensor was made using
a tape-casted 8 mol %
Y2O3-doped ZrO2 substrate ͑YSZ, 20 ϫ 10 ϫ 0.1 mm͒ as an elec-
trolyte and the WO3 and Pt as an electrode. Both electrodes of the
sensor ͓͑Ϫ͒ Pt/YSZ/WO3 ͑ϩ͔͒ were exposed to the same gas atmo-
sphere and connected to a Keithley 2000 multimeter for emf mea-
surements. The total gas flow rate was fixed at 300 cm3/min. The
sensor response was measured with increasing and then decreasing
concentration steps of NOx. The retention time at every step was
200–300 s.
Results
TPR of NO2 and NO -containing gas mixtures
.— NO2 + O2
2
TPR results obtained over Pt + Y and W + Y are shown in Fig. 2.
Initially 1300 ppm NO2 + 2000 ppm O2 were supplied to Pt + Y
Figure 3. Effect of O2, CO, and CO2 on the NO2 TPR profile: ͑a͒ Pt + Y
and ͑b͒ W + Y.

Journal of The Electrochemical Society, 153 ͑6͒ H115-H121 ͑2006͒
H117
Figure 5. Effect of O2 concentration ͑0.2–10%͒ on NO2 profile of NO2
+ O2 TPR over W + Y.
Figure 4. Comparison of NO2 TPR between Pt + Y, W + Y, W, Y, B, and
Calc.
sponding to 30 cm2/m volumetric flow of gas is ϳ7ϫ larger than
space velocity associated with 300 cm2/m gas flow in the sensor
reactor. Decreasing the flow rate in our TPR system within experi-
mental limitations ͑2 ϫ ͒ was found to make a negligible difference,
especially when compared to the difference in catalytic activity be-
tween the two materials.
NO2 + CO → NO + CO2
͓2͔
The NO2 TPR results, in the presence of O2, CO2, and CO over
W + Y, are shown in Fig. 3b. Gas mixtures tested were NO2 + O2
͑ϳ2000 ppm͒, NO2 + CO2 ͑ϳ400 ppm͒, and NO2 + CO ͑ϳ900
ppm͒ + O2 ͑ϳ1750 ppm͒. In general, NO2 starts to decompose at
ϳ550°C for the NO2, NO2 + O2, and NO2 + CO2 TPR, showing
that the NO2 decomposition is not affected by the addition of O2 and
CO2 and is mainly temperature-dependent. The onset of the NO2
reduction in the NO2 + CO + O2 TPR appears at the lower tempera-
ture, ϳ400°C, and slightly more NO2 reduction is shown at T
Ͼ 400°C; however, the NO2 profile in the NO2 + CO + O2 TPR is
not that different from the NO2 TPR.
NO2 + CO + O2 TPR of Pt + Y and W + Y are shown in Fig. 6.
Formations of NO and CO2, with a corresponding decrease in NO2
and CO concentration ͑Eq. 2͒, are shown at the lower temperature
region for both catalysts. Over Pt + Y, the onset of the reaction is
ϳ150°C and at 150 Ͻ T Ͻ 450°C, NO2 and CO concentration de-
crease with a moderate slope ͑Fig. 6a͒. A sudden change of the slope
in NO and NO2 curve at 450–500°C with a small increase of O2
concentrations coincides with the NO2 profile obtained without CO
͑Fig. 2-4͒ and seems to be due to an occurrence of the NO2 decom-
position ͑Eq. 1͒. Most of the NO2 molecules are reduced at T
Ͼ 550°C, as they are in the NO2, NO2 + O2, and NO2 + CO2
+ O2 TPR ͑Fig. 3a͒. CO oxidation is almost completed at the same
region as well. A reaction between NO2 with CO ͑Eq. 2͒ is most
likely dominant at T Ͻ 400°C, but the present results ͑Fig. 6a͒ can-
not distinguish which is dominant at T Ͼ 400°C among Eq. 1, 2,
and 4. All of the reactions are thermodynamically and kinetically
feasible at the higher temperature and therefore very likely involved
Results of the NO2 TPR experiments obtained on W, W + Y,
Pt + Y, Y, and B are compared in Fig. 4. The TPR pattern of Y is
very similar to that of W + Y; slow NO2 decomposition at 500
Ͻ T Ͻ 600°C, a steep drop above 600°C, and an incomplete de-
composition at 800°C. This indicates that the NO2 decomposition
activity of W + Y is not given by the screen printed WO3 but
mainly by the YSZ-8Y substrate. Higher catalytic activity of a sin-
tered YSZ-8Y substrate ͑Y͒ than an uncatalyzed case ͑B͒ is shown.
WO3 powder ͑W͒ exhibits higher NO2 decomposition activity than
W + Y and Y, where a faster start of the NO2 decomposition
͑ϳ450°C͒, following a steeper slope, makes most of the NO2 re-
duced at 800°C. The activity of W is, however, much lower than
that of Pt + Y. The uncatalyzed NO2 decomposition reaction ͑B͒ is
so small that it begins at ϳ700°C and only less than 16% of NO2 is
reduced at 800°C. Ignoring the incidence of gas-solid reactions such
as Eq. 3, where Kröger–Vink notation is used, gas compositions in
thermodynamic equilibrium ͑calc͒ of the NO2 decomposition ͑Eq. 1͒
were calculated with initial NO2 concentration = 900 ppm
1
CO + O2 → CO2
͓4͔
2
The NO2 + CO + O2 TPR result on W + Y is shown in Fig. 6b. The
beginning of the NO2 reduction at ϳ400°C, lower than ϳ550°C
observed in the NO2, NO2 + O2, and NO2 + CO2 TPR ͑Fig. 3b͒,
and a related decrease of CO concentration are consistent with the
incidence of a reaction of NO2 with CO ͑Eq. 2͒, as already observed
on Pt + Y. Both NO2 and CO concentration decrease continuously
at 400–800°C with increasing temperature. A simultaneous decrease
of NO2 and CO concentration at 500–700°C, is most likely due to
Eq. 1 or 2 but not due to Eq. 4, since CO oxidation by O2 over
NO2͑gas͒ + VOؠؠ͑lattice͒ + 2e− ↔ NO
+ Ox
͓3͔
͑gas͒
O͑lattice͒
According to the calculation, more than 90% of NO2 should be
decomposed to NO at T Ͼ 400°C, which is much more than ob-
served from the NO2 TPR of Pt + Y, W + Y, and Y.
In order to compare TPR and sensor results under similar gas
condition, TPR experiments were performed over a wide range of
gas concentrations. For example, NO2 TPR profiles obtained during
NO2 + O2 TPR over WO3 on the YSZ ͑W + Y͒ with 0.2, 3.3, and
10% O2 are compared in Fig. 5. As shown in the figure, an increase
in pO2 from 0.2 to 3.3% ͑or 10%͒ induces only a small decrease of
the NO2 decomposition activity. This is also the case for the TPR
over Pt on the YSZ ͑Pt + Y͒. Therefore, the change made by the
pO2 increase is small enough to be ignored, when compared to the
difference in the catalytic activity between W + Y and Pt + Y ͑Fig.
4͒. In order to compare TPR and sensor results, one also needs to
take into consideration flow rate or space velocity effects. Due to the
difference in diameter, space velocity in the TPR reactor corre-
Figure 6. TPR of NO2 + CO + O2 over ͑a͒ Pt + Y and ͑b͒ W + Y.

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Journal of The Electrochemical Society, 153 ͑6͒ H115-H121 ͑2006͒
Figure 7. TPR of NO + O2 over ͑a͒ Pt + Y and ͑b͒ W.
Figure 9. TPR of CO + O2 over ͑a͒ Pt + Y and ͑b͒ W.
W + Y cannot take place below 750°C, which is discussed in a later
clear change of NO and NO2 concentration, as shown in Fig. 8a.
Accordingly, Pt + Y catalyzes the CO2 formation, not via Eq. 5, but
selectively via Eq. 4. The activity is given by a Pt thick film because
no activity is exhibited by an YSZ-8Y substrate ͑Y͒. The start of CO
oxidation is at ϳ200°C and it is almost completed at T Ͼ ϳ
750°C. Catalytic activity of W + Y, for the CO oxidation ͑Eq. 4͒ is
so small that the onset of the CO2 formation occurs at ϳ650°C
without NO and NO2 concentration change, and less than 10% of
CO is oxidized at 800°C ͑Fig. 8b͒.
section.
A small increase of O2 concentration at 700 Ͻ T
Ͻ 800°C probably comes from O2 produced by the NO2 decompo-
sition ͑Eq. 1͒.
TPR of NO and NO-containing gas mixtures
.— Results of NO
+ O2 TPR experiments on Pt + Y and W are shown in Fig. 7. Equi-
librium compositions of a gas reaction ͑Eq. 1͒ calculated with
1200 ppm NO and 2000 ppm O2 as initial concentrations in Fig. 7a
and 600 ppm NO and 1500 ppm O2 in Fig. 7b are plotted together.
Although formation of NO2 from NO/O2 mixtures is thermody-
namically favorable, especially at T Ͻ 400°C, oxidation of NO ͑re-
verse reaction of Eq. 1͒ is not detected during the TPR on both Pt
+ Y and W. Instead, constant NO and O2 concentrations are shown,
regardless of temperature. As expected from the above results, W
+ Y, Y, and B, showing lower catalytic activity than Pt + Y and W,
do not produce NO2 during the NO + O2 TPR at 50–800°C.
Additionally, no changes of gas concentrations were observed
during the NO ͑ϳ1200 ppm͒ + CO2 ͑ϳ950 ppm͒ + O2 ͑ϳ2000
ppm͒ TPR experiment over Pt + Y. The TPR of NO ͑ϳ550 ppm͒,
NO ͑ϳ500 ppm͒ + CO2 ͑ϳ1250 ppm͒, and NO ͑ϳ950 ppm͒
+ CO ͑ϳ600 ppm͒ on WO3 powder ͑W͒ did not give rise to
changes in the gas concentrations, even though several gas reactions
such as Eq. 5 and 6 are thermodynamically allowed in the present
condition
TPR of CO and CO-containing gas mixtures.— The results of
CO + O2 TPR over Pt + Y are presented in Fig. 9a. An early onset
of the CO oxidation at ϳ150°C is followed by an abrupt decrease of
CO concentration at 200–300°C. The CO oxidation slows down at
300–400°C and a steady formation of CO2 is then shown up to
ϳ600°C. Most of the CO gases are oxidized above 600°C. The
general CO profile in the CO + O2 TPR on Pt + Y ͑Fig. 9a͒ is
similar to that in the NO + CO + O2 TPR on Pt + Y ͑Fig. 8a͒, but at
250–600°C faster CO oxidation is found in the CO + O2 TPR.
The CO oxidation activity of WO3 powder ͑W͒ is much lower
than that of Pt + Y ͑Fig. 9b͒. The start of CO oxidation over W
appears at ϳ350°C and full oxidation is achieved above ϳ700°C
with a matching change of O2 and CO2 concentration.
A CO profile of CO + O2 TPR on Pt + Y, W, W + Y, and Y are
compared in Fig. 10. Compared to Pt + Y and W, a negligible ac-
tivity is observed for W + Y, where CO oxidation begins at
ϳ750°C and less than 10% CO is converted to CO2. Oxidation of
CO is not observed for Y below 800°C.
In addition, TPRs of O2 and CO2 over Pt + Y, W, W + Y, and Y
were performed. None showed gas concentration changes during the
TPRs at 50–800°C.
1
NO + CO → CO2 + N2
͓5͔
͓6͔
2
1
2NO → NO2 + N2
2
Furthermore, no gas reactions were detected over Y during the TPR
of NO ͑ϳ850 ppm͒, ͑ϳ800 ppm͒ + CO ͑ϳ600 ppm͒, NO ͑ϳ750
Discussion
NO sensing behavior of WO -based sensor
.— The NO2 sensi-
ppm͒ + CO ͑ϳ850 ppm͒ + O2 ͑ϳ1550 ppm͒, and NO
ϳ900 ppm͒ + CO2 ͑ϳ200 ppm͒ + O2 ͑ϳ1900 ppm͒.
͑
2
3
tivity of a WO3-based potentiometric sensor was investigated at
The results of NO + CO + O2 TPR on Pt + Y and W + Y are
shown in Fig. 8. Formation of CO2 is shown over Pt + Y with a
corresponding decrease of CO and O2 concentration, but without a
Figure 10. Comparison of CO + O2 TPR between Pt + Y, W + Y, W, and
Y.
Figure 8. TPR of NO + CO + O2 over ͑a͒ Pt + Y and ͑b͒ W + Y.

Journal of The Electrochemical Society, 153 ͑6͒ H115-H121 ͑2006͒
H119
Figure 11. Variation of the NO and NO2 sensitivity as a function of tem-
perature in 3% O2: ͑a͒ NO2 at 550–800°C and ͑b͒ NO at 600–750°C.
Figure 12. TPD of NO2 + O2 over WO3 powder ͑W͒. 500 ppm NO2 and
1% O2 were used as adsorbates.
550 Ͻ T Ͻ 800°C and 10 Ͻ NO2 Ͻ 200 ppm ͑Fig. 11a͒.12 Values
of the emf ͑mV͒ are positive and they increase with increasing NO2
concentration, giving a positive slope in a plot of the emf vs the
logarithm of NO2 concentration. The present sensor is expected to
give a positive response when cathodic reactions are dominant at the
WO3 electrode, such as forward reactions of Eq. 3 and 716
the CO and NO2 have already achieved ϳ100% conversion at that
temperature. However, a small decrease of NO2 concentration at
650°C is shown for W + Y by the CO addition. The decrease of
NO2 concentration by CO is expected to be smaller in the sensor
experimental condition, where lower CO concentration ͑less than
300 ppm͒ is applied, than in the TPR condition ͑ϳ1000 ppm CO͒.
Moreover, the NO2/NO profile in the NO2 + CO TPD ͑0.67% NO2
and 0.33% CO adsorbate͒ over W was found to be similar to that in
the NO2 + O2 TPD ͑Fig. 12͒, excluding the occurrence of CO influ-
ence on the adsorption behavior of NO2 or its related species. Con-
sequently, the sensor can detect NO2 selectively over CO.
The NO2 emf values decrease considerably at 650°C with in-
creasing O2 concentration ͑3–21%͒, as shown in Fig. 13c. However,
oxygen cross-sensitivity was found to be insignificant for the
WO3-based sensor, no response to O2 concentration change alone,
indicating symmetric catalytic and/or adsorptive behaviors of O2
between WO3 and Pt electrode.20 Therefore, provided that the mixed
potential, conventionally used to explain non-Nernstian sensing be-
havior, is a dominant mechanism in the present system,21 enhanced
contribution of the O2 electrode reactions ͑Eq. 7͒ by increased pO2,
1
O2,͑gas͒ + VOؠؠ ͑lattice͒ + 2e− ↔ OxO ͑lattice͒
͓7͔
2
The lower catalytic activity of WO3 for the NO2 decomposition ͑Eq.
1͒ at 500–700°C relative to Pt ͑Fig. 4͒ results in higher local NO2
concentration at the WO3 electrode than at the Pt.16 Thus, a greater
forward reaction of Eq. 3 at the WO3 than at the Pt is expected,
leading to the present positive NO2 sensor response. A difference in
the NO2 decomposition activity between the WO3 and Pt electrode
decreases with increasing temperature at 550–750°C and the differ-
ence becomes insignificant at ϳ800°C, which agrees with the NO2
sensitivity results ͑Fig. 11a͒ showing the decrease in the response
with increasing temperature at 550–750°C and the negligible re-
sponse at 800°C.17
The unsymmetrical catalytic activity for Eq. 1 also creates a pO2
16
gradient across the YSZ ͑a higher pO2 at the Pt side͒ and by this
uneven pO2, a negative response should be shown in the present
sensor provided that a reaction ͑Eq. 7͒ is governing. This is, how-
ever, opposite to the experimental observation that shows a positive
response. The effect of the pO2 gradient produced by the different
catalytic activity is not expected to be significant in the present
condition, even though they possibly bring about the sensor re-
sponse, due to the sensor measurements that were performed under
much higher pO2 = 3% O2, relative to the produced pO2 difference
͑at most 200 ppm͒ by the catalytic reactions ͑Eq. 1͒.18 Ionosorption
of NO2, or its modified species, probably via electron-accepting
adsorption, can affect the sensor response.4,19 However, only small
adsorptions of NO2 and NO are shown in the NO2 + O2 TPD on W
͑Fig. 12͒, where 500 ppm NO2 and 1% O2 were used as adsorbate
gases.
Selective detection of NO2 against CO2 ͑0–20%͒ is accom-
plished in 3% O2 ͑Fig. 13a͒. In the TPR experiments, the effect of
ϳ1000 ppm CO2 on the NO2 decomposition activity of Pt + Y and
W + Y is small enough to be ignored ͑Fig. 3͒. Additionally, a NO2
and a NO profile obtained in the NO2 + CO2 TPD ͑500 ppm NO2
and 1000 ppm CO2 adsorbate͒ over W were similar to those in the
NO2 + O2 TPD ͑Fig. 12͒, indicating no significant interaction of
CO2 with NO2. These TPR/TPD results can explain the
CO2-independent NO2 sensing behavior together with much smaller
cross-sensitivity of the sensor to an inert CO2 gas.8
Values of the NO2-sensing emf obtained with 100 and 300 ppm
CO, which was found to be much less sensitive to the sensor than
NO2, were ϳ0.5 and ϳ3 mV smaller than those without CO, re-
spectively ͑Fig. 13b͒, but the difference is still within the general
range of reproducibility.20 As shown in Fig. 3, NO2 concentration at
650°C is almost CO-independent for catalytically active Pt + Y, as
Figure 13. Effect of CO2, CO, and O2 on the NO2 response at 650°C and in
3% O2: ͑a͒ CO2 effect ͑0–20%͒, ͑b͒ CO effect ͑0–300 ppm͒, and ͑c͒ O2
effect ͑3–21%͒.

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Journal of The Electrochemical Society, 153 ͑6͒ H115-H121 ͑2006͒
Figure 14. Effect of CO and O2 on the NO response at 650°C and in 3% O2:
͑a͒ CO effect ͑0–400 ppm͒ and ͑b͒ O2 effect ͑3–30%͒.
Figure 15. TPD of NO + CO over WO3 powder ͑W͒. 0.67% NO and 0.33%
CO were used as adsorbates.
compared to the NO2 electrode reactions ͑Eq. 3͒, may reduce the
sensor response, which is believed to be given by the unsymmetrical
NO2 electrochemical reactions. Similar NO2 TPD ͑1% NO2 adsor-
bate͒ results with the NO2 + O2 TPD ͑Fig. 12͒ rule out the chance
of the NO2 adsorption modified by O2.
This appears to be due to the O2-independent NO adsorption behav-
ior, in addition to an absence of the NO oxidation reaction with O2
͑Eq. 1͒.
Conclusion
NO-sensing behavior of WO3-based sensor.— Values of the NO
response are negative and the sensitivity decreases with increasing
temperature at 600 Ͻ T Ͻ 750°C ͑Fig. 11b͒. This negative response
is expected when preferred anodic reactions ͑a reverse reaction of
Eq. 3 or Eq. 7͒ take place at the WO3 side, which may not be
obtained in the present system because any clear changes in NO/O2
or NO/NO2 concentration associated with either direct decomposi-
tion of NO to N2 and O2 or oxidation of NO to NO2 are not ob-
served up to 800°C during the TPR of NO-containing gas mixtures
͑Fig. 7 and 8͒. Thus, the NO response is not due to any catalytic
mechanism ͑e.g., mixed potential͒. WO3 is a semiconductor, and
adsorption of a gas changes the carrier concentration, thus changing
the Fermi level.3,15 In differential electrode equilibria, when a reduc-
ing gas such as NO or CO is adsorbed on WO3, a negative NO
response is expected due to an associated Fermi-level change. We
believe this to be the mechanism for our observed NO response.
However, in NO TPD experiments, no significant NO desorption
peaks were observed. This does not mean that NO does not adsorb,
rather it means it only weakly adsorbs and that the rate of desorption
over the temperature investigated is greater than or equal to the rate
of adsorption.
The downward shift of the NO response by the addition of
200–400 ppm CO ͑Fig. 14a͒ can be explained by coadsorption of
NO and CO. Less than 10 ppm desorption of NO or its related
species ͑NO2, N2, and N2O͒ were observed at 50 Ͻ T Ͻ 800°C
during the NO TPD ͑1% NO adsorbate͒ and the NO + O2 TPD
͑500 ppm NO and 1% O2 adsorbate͒ over W. The intensity of the
NO desorption peak is, however, increased dramatically by CO ad-
dition, as shown in the NO + CO TPD ͑0.67% NO and 0.33% CO
adsorbate͒ over W ͑Fig. 15͒, where a series of strong NO peaks are
observed. In the NO + CO TPD ͑Fig. 15͒, a broad NO desorption
maximum is shown at ϳ550°C and a sharp and narrow NO peak at
ϳ250°C. At the same temperatures, smaller peaks of CO2 and CO
are observed, indicating further interactions between NO and CO.
This enhanced NO adsorption by CO appears to lead to the in-
creased NO response ͑Fig. 14a͒.
Sensing behavior of
a WO3-based potentiometric sensor
͑temperature-dependent NO2/NO sensitivity and NO2/NO selectiv-
ity at 650°C͒ and its related mechanisms are discussed based on
results obtained on the sensor elements using TPR/TPD techniques.
An NO2 concentration gradient across the electrolyte occurs as a
result of the unsymmetrical NO2 decomposition activity between the
WO3 and Pt electrode. This may bring about dissymmetry in the
heterogeneous electrode reactions, resulting in the sensor response.
Furthermore, the difference in the NO2 concentration between the
WO3 and Pt electrode is temperature-dependent, increasing with de-
creasing temperature, which is consistent with the NO2 sensitivity
results showing the increase in the NO2 response with decreasing
temperature.
The NO response due to any catalytic mechanism ͑e.g., mixed
potential͒ is less likely because any clear changes in relevant gas
concentration associated with catalytic reactions are not observed in
the TPR experiments and therefore, the NO response is most likely
given by an adsorption of the gas on the WO3 electrode. The in-
creased NO adsorption due to the interaction with CO agrees with
enhanced NO response by coexisting CO, supporting that the NO
sensing mechanism is adsorption-related.
Acknowledgments
This work was supported by the Department of Energy under
contract no. DE-FG26-02NT41533 and DE-FC26-03NT41614.
The University of Florida assisted in meeting the publication costs of this
article.
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When NO and CO coexist, the sensor response may also be due
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