Selective catalytic reduction of NOx by H2 using proton conductors as catalyst supports

Article abstract of DOI:10.1016/j.jcat.2007.02.001

The selective catalytic reduction (SCR) of NO_x by H₂ was studied using proton-conducting Sn_0.9In_0.1P₂O₇ as a catalyst support at 50-350 °C. When a mixture of 800 ppm NO, 1400-8400 ppm H₂, and 5% O₂ in Ar was introduced to the Pt/C working electrode, NO_x was reduced to N₂ with N₂ selectivity values > 80 %. The reaction mechanism was shown to be based on a mixed potential at the Pt/C working electrode. This mechanism was also applicable to a Sn_0.9In_0.1P₂O₇-supported Pt catalyst. H₂ SCR over the Pt/Sn_0.9In_0.1P₂O₇ catalyst was characterized by a wide operating temperature window (50-350 °C) and remarkable N₂ selectivity values (> 80 %). Catalyst performance could be further improved by the addition of Rh to Pt, in which NO_x conversion was a maximum 1.4 times higher than that over the Pt/Sn_0.9In_0.1P₂O₇ catalyst and N₂ selectivity was increased to > 89 % at all temperatures tested.

Full text of DOI:10.1016/j.jcat.2007.02.001

Journal of Catalysis 247 (2007) 137–144
www.elsevier.com/locate/jcat
Selective catalytic reduction of NOx by H2 using proton conductors
as catalyst supports
Atsuko Tomita a, Takeshi Yoshii b, Shinya Teranishi b, Masahiro Nagao b, Takashi Hibino b,∗
a
Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology,
Moriyama-ku, Nagoya 463-8560, Japan
b
Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
Received 13 November 2006; revised 25 January 2007; accepted 1 February 2007
Available online 2 March 2007
Abstract
The selective catalytic reduction (SCR) of NO by H was studied using proton-conducting Sn In P O as a catalyst support at 50–350 C.
◦
x
7
0.9 0.1 2
2
When a mixture of 800 ppm NO, 1400–8400 ppm H , and 5% O in Ar was introduced to the Pt/C working electrode, NO was reduced to
x
2
2
N with N selectivity values >80%. The reaction mechanism was shown to be based on a mixed potential at the Pt/C working electrode. This
2
2
mechanism was also applicable to a Sn In P O -supported Pt catalyst. H SCR over the Pt/Sn In P O catalyst was characterized by a
7
7
0.9 0.1 2
0.9 0.1
2
2
◦
wide operating temperature window (50–350 C) and remarkable N selectivity values (>80%). Catalyst performance could be further improved
2
by the addition of Rh to Pt, in which NO conversion was a maximum 1.4 times higher than that over the Pt/Sn In P O catalyst and N
x
7
0.9 0.1
2
2
selectivity was increased to >89% at all temperatures tested.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Selective catalytic reduction; NO ; Hydrogen; Proton conductor
x
1. Introduction
In recent years, the use of H2 as an alternate reducing agent
for NOx has received significant attention because of its pres-
ence in automobile exhaust and its high reactivity at tempera-
tures below 200 ◦C [9–15]. The following reactions occur in the
NO/H2/O2 system:
NOx (NO and NO2), produced by the combustion of fuels,
are major air pollutants responsible for photochemical smog,
acid rain, ozone depletion, and the greenhouse effect. Vehicular
and stationary combustion processes contribute up to 90% or
more of the global NOx production [1]. Consequently, a num-
ber of de-NOx techniques have been investigated, especially for
diesel and lean-burning gasoline engines that emit exhaust with
excess O2, for which the commonly used three-way catalyst
is not effective for the reduction of NOx to N2. Selective cat-
alytic reduction (SCR) of NOx using NH3 [2,3], urea [4,5], and
hydrocarbons [6–8] as reducing agents is widely viewed as a
promising de-NOx technology. However, NH3 and urea require
storage tanks and frequent replenishment. Moreover, hydrocar-
bons need to be operated at temperatures above 200 ◦C because
of their low reactivity.
2NO + 4H2 + O2 → N2 + 4H2O
and
(1)
2NO + 3H2 + O2 → N2O + 3H2O.
(2)
Various studies have reported that Pt-based support catalysts
are active for the above reactions. NO is dissociated into N(ad)
and O(ad) on Pt clusters, promoted by previously adsorbed H(ad)
[9,10]. Burch and Coleman proposed that N2 and N2O are
formed through the combination of N(ad) with each other and
with another molecular NO, respectively [11]. NH3 formation
was also found to occur by the reaction of N(ad) with H(ad) at
high temperatures [12]. Shibata et al. found that the reaction of
NO with the NH3 formed is another important reaction pathway
for H2 SCR [13].
*
Corresponding author.
E-mail address: hibino@urban.env.nagoya-u.ac.jp (T. Hibino).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.02.001

138
A. Tomita et al. / Journal of Catalysis 247 (2007) 137–144
The above reaction mechanism is strongly dependent on the
and selectivity toward N2 in the temperature range of 50–
350 ◦C. Finally, we attempted to improve the performance of
the Pt/Sn0.9In0.1P2O7 catalyst by adding Rh to Pt.
catalyst support. Yokota et al. found that the activity order of the
tested supports was ZSM-5 ∼ mordenite > SiO2 > Al2O3 [14].
They also developed a Pt–Mo–Na/SiO2 catalyst with a wider
temperature window (especially for NOx conversion) compared
with the Pt/zeolite catalysts. Costa et al. investigated the use of
La0.5Ce0.5MnO3 as a potential support for Pt [15] and found
that it improved the selectivity toward N2 at 100–200 ◦C.
We recently investigated the feasibility and efficacy of us-
ing intermediate-temperature proton conductors as catalyst sup-
ports for H2 SCR. Sn0.9In0.1P2O7 is a promising anhydrous
proton conductor because of its high proton conductivity of
ꢀ0.1 Scm−1 under dehumidified conditions at 100–300 ◦C
[16,17]. This material has a cubic structure, with Sn(or In)O6
octahedra at the corners and P2O7 units at the edges. Protons
migrate via the dissociation of hydrogen bonds with oxide ions
in the P2O7 units by a hopping mechanism. We also applied
this material as a solid electrolyte in various electrochemical
devices. In a fuel cell, the H2 oxidation shown in reaction (3)
proceeded very easily over a Pt/C electrode [18]. On the other
hand, in an electrolyzer, the NO reduction shown in reaction (4)
was achieved with high current efficiencies over the same Pt/C
electrode [19],
2. Experimental
2.1. Electrochemical cell studies
Fig. 1 shows an electrochemical cell used for measur-
ing the activity and potential of a Pt/C working electrode.
Sn1−xInxP2O7 (x = 0–0.1) samples were prepared in the same
manner as reported previously [16–19]. The corresponding ox-
ides (SnO2 and In2O3) were mixed with 85% H3PO4 and
ion-exchanged water and held under stirring at 300 ◦C until
a high-viscosity paste was formed. This paste was calcined
in an alumina pot at 650 ◦C for 2.5 h and then ground in a
mortar with a pestle. The composition of Sn1−xInxP2O7 was
determined by X-ray fluorescence (XRF) analysis. X-ray dif-
fraction (XRD) analysis showed that Sn1−xInxP2O7 had a cubic
single phase in the tested range of x of 0–0.1. Finally, the com-
pound powders were pressed into pellets (12 mm i.d., about
1.5 mm thick) under 100 MPa pressure. Unless stated other-
wise, all electrochemical measurements were conducted using
Sn0.9In0.1P2O7 as the solid electrolyte. For comparison, silica
and yttria-stabilized zirconia (YSZ) pellets were also used as
solid electrolytes in the electrochemical cell. The working elec-
trode (area, 0.5 cm2) consisted of 10 wt% Pt/C or 10 wt% PtM,
(M = Rh, Ir, Ru, Pd, Co) catalyst (E-TEK) and carbon paper
(Toray, TGPH-090), wherein the atomic ratio of Pt to M was 4.
The counterelectrode was the same Pt/C electrode as above.
H2 → 2H+ + 2e−
and
(3)
1
NO + 2H+ + 2e− → N2 + H2O.
(4)
2
These reactions suggest that if Sn0.9In0.1P2O7 is used as a cat-
alyst support, a new route to reduce NOx to N2 could be estab-
lished for H2 SCR. Namely, in the NO/H2/O2 system, a local
galvanic cell is formed at the catalyst–support interface, as dis-
played in Scheme 1. As a result, the two reactions (3) and (4)
proceed at the interface simultaneously due to self-discharge.
This is similar to the corrosion behavior of metals in acid so-
lutions, which in principle can be distinguished from conven-
tional H2 SCR.
The purpose of this study was to demonstrate the above
concept by various electrochemical techniques and to develop
a highly active and selective Sn0.9In0.1P2O7-supported cata-
lyst for H2 SCR. First, we fabricated an electrochemical cell
consisting of an Sn0.9In0.1P2O7 electrolyte and a Pt/C work-
ing electrode, to measure the activity and potential of the
working electrode under open-circuit conditions. We then pre-
pared a Pt/Sn0.9In0.1P2O7 catalyst and evaluated its activity
Fig. 1. Schematic illustration of the electrochemical cell.
Scheme 1.

A. Tomita et al. / Journal of Catalysis 247 (2007) 137–144
139
A Pt reference electrode was attached to the side face of the
electrolyte pellet.
in good agreement with the value (+20 mV) calculated from
Nernst’s equation (6) for a steam concentration cell [20]:
The working electrode was supplied with a mixture of
800 ppm NO, 1400–8400 ppm H2, 2–9% O2, 0–4% H2O va-
por, and 0–10% CO2 in Ar at a flow rate of 50 mL min−1. The
counter and reference electrodes were exposed to atmospheric
air. The concentrations of NOx and N2 in the outlet gas from
the working electrode were monitored using an online NOx gas
analyzer (Horiba, PG-225) and gas chromatograph (Shimazu,
GC-8A), respectively. The gas concentrations were obtained
after the reaction steady state was attained. The difference in
potential (i.e., open-circuit voltage [OCV]) between the work-
ing and counter electrodes was measured with a digital elec-
trometer (Hokuto Denko HE-104). Polarization and impedance
measurements were made by controlling the potential of the
working electrode versus that of the reference electrode with a
potentiostat (Hokuto Denko HA-501) and impedance analyzer
(Solartron SI-1260), respectively.
E = −(RT /4F)ln[PO (counter)/PO (working)],
(5)
2
2
E = −(RT /2F)ln[PH O(working)/PH O(counter)
]
2
2
1/2
× [PO (counter)/PO (working)
]
.
(6)
2
2
This means that Sn0.9In0.1P2O7 exhibits proton conduction
rather than oxide ion conduction under the present conditions.
In contrast, when a small amount of H2 coexisted with excess
O2, the OCV was shifted significantly toward the negative di-
rection, with the value dependent on the H2 concentration; for
example, the OCV reached −185 mV at 8400 ppm. It is im-
portant to note here that, assuming that all of the 8400-ppm
H2 is converted to H2O over the Pt/C working electrode, the
O2 concentration will change from 5% to 4.58%, and the H2O
vapor concentration will increase from 0.6% to 1.44%. Accord-
ing to Nernst’s equation (6), the OCV will change from +20
to −0.6 mV. The large discrepancy between the measured and
theoretical OCV values implies that the behavior observed for
the Pt/C working electrode is non-Nernstian [21], suggesting
that the reaction rate of reaction (3) is much higher than that of
the O2 reduction and H2O vapor oxidation. In contrast, when a
small amount of NO was present in an excess of O2, the elec-
trochemical cell showed moderately positive OCVs, indicating
that reaction (4) proceeded at a faster rate. Preliminary experi-
ments showed that about 9% of the supplied NO was converted
into NO2 through catalytic oxidation over the Pt/C working
electrode. It appears that the following reaction also contributes
to the observed OCV to some extent:
2.2. Catalyst studies
Pt/Sn0.9In0.1P2O7 (Pt content = 0.2–0.8 wt%) and PtRh/
Sn0.9In0.1P2O7 (PtRh content = 0.8 wt%; Pt/Rh atomic
ratio = 4) catalysts were prepared by impregnating Sn0.9In0.1
-
P2O7 powders with aqueous solutions of the corresponding
nitrates. After impregnation, the catalysts were dried in air at
110 ◦C for 12 h and then heated in Ar at 200 ◦C for 1 h. N2 ph-
ysisorption measurements (Bel Japan BELSORP-28SP) found
a BET surface area of the catalyst support of ꢁ0.5 m2 g−1
.
Catalytic measurements were carried out in a fixed-bed flow re-
actor. A mixture of 800 ppm NO, 8400 ppm H2, and 5% O2 in
Ar was passed through the catalyst (weight, 0.1 g) at a flow rate
of 50 mL min−1. The analysis of the outlet gas was carried out
as described above.
1
NO2 + 4H+ + 4e− → N2 + 2H2O.
(7)
2
Based on the above results, if the Pt/C working electrode were
supplied with a mixture of NO, H2, and O2, then a galvanic cell
would be formed at the electrode–electrolyte interface. Because
the electrode also functions as a lead wire, the galvanic cell
would then be short-circuited. As a result, both reactions (3) and
(4) proceed simultaneously at the Pt/C working electrode, as
displayed in Scheme 1. We next provide experimental evidence
for this scheme.
We measured the transient change in the NOx concentra-
tion in the outlet gas from the Pt/C working electrode under
open-circuit conditions at 250 ◦C, with the NO and O2 concen-
trations maintained at 800 ppm and 5%, respectively. As shown
in Fig. 2, the reduction of NOx did not occur in the absence
of H2. This is due to inhibition of this reaction by oxygen ad-
sorbed over the Pt surface from the gas phase [22]. As the H2
concentration increased in the gas mixture, the NOx concentra-
tion decreased and N2 concentration increased. In this case, the
N2 selectivity was as high as ∼84% for all H2 concentrations
evaluated. We discuss the high N2 selectivity later. Another im-
portant result is that NOx was not reduced over a Pt-free carbon
electrode, indicating that the Pt catalyst plays an important role
in H2 SCR. In other words, Sn0.9In0.1P2O7 itself has no cat-
alytic activity for H2 SCR.
3. Results and discussion
3.1. Electrochemical cell studies
Table 1 shows OCVs generated from the electrochemical
cell by feeding various gas mixtures (0–1600 ppm pf NO, 0–
8400 ppm H2, 5% O2, and 0.6% H2O vapor in Ar) into the Pt/C
working electrode at 250 ◦C. In the absence of H2 and NO, the
electrochemical cell yielded an OCV of +20 mV. This OCV
value is more positive than the value (−16 mV) calculated from
Nernst’s equation (5) for an oxygen concentration cell and is
Table 1
OCV responses of the electrochemical cell with the mixtures of 0–1600 ppm
◦
NO, 0–8400 ppm H , and 5% O in Ar at 250 C
2
2
NO (ppm)
H
(ppm)
OCV (mV)
2
200
800
1600
0
0
0
0
50.6
53.5
55.2
2800
5600
8400
−141
−169
−185
0
0
We studied the catalytic activity of the Pt/C working elec-
trode for H2 SCR using various solid electrolytes at 250 ◦C.

140
A. Tomita et al. / Journal of Catalysis 247 (2007) 137–144
◦
Fig. 2. Transient changes in NO and N concentrations in the outlet gas from the Pt working electrode at 250 C. The reactant gas was 800 ppm NO, 0–8400 ppm
2
−1
H , and 5% O in Ar at a flow rate of 50 mL min
.
2
2
Plots of the NOx conversion as a function of H2 concentration
are shown in Fig. 3. Comparing the results for Sn0.9In0.1P2O7
with those for silica and YSZ (Fig. 3a) shows that using
Sn0.9In0.1P2O7 significantly enhanced NOx conversion. Note
that under the present conditions, Sn0.9In0.1P2O7 shows a
proton conductivity of 0.2 S cm−1, whereas the other two
can be seen from Fig. 4b, NOx conversion also was strongly in-
fluenced by temperature. High-temperature operation was unfa-
vorable for the present H2 SCR. Also, in this case, a decrease in
H2 concentration at increasing temperature, causing decreased
NOx conversion, was confirmed for the outlet gas from the Pt/C
working electrode. However, the fact that reaction (8) was cat-
alyzed at increasing temperature, which also can decrease NOx
conversion, cannot be neglected. It is well known that the ca-
thodic reaction corresponding to reaction (8) is promoted by an
increase in temperature in polymer electrolyte fuel cells (PE-
FCs) [24]. On the other hand, NOx conversion was shown to be
independent of CO2 concentration (see Fig. 4c). This is strong
evidence that CO2 functions as an inert gas for both H2 com-
bustion and reactions (3), (4), and (8).
If the reaction mechanism for the present H2 SCR is based
on Scheme 1, then it can be assumed that reactions (3) and (4)
proceed simultaneously at the Pt/C working electrode. These
reaction rates are equal due to the formation of a local cell and
its subsequent self-discharge, resulting in the appearance of a
mixed potential [25–27]. To clarify this mechanism, we mea-
sured the anodic polarization curve of 8400 ppm H2 diluted
with Ar and the cathodic polarization curve of a mixture of
800 ppm NO and 5% O2 in Ar at 250 ◦C. The results summa-
rized in Fig. 5, show that the direction of the cathodic current
was reversed. The intersection point of the anodic and cathodic
polarization curves, which corresponds to the mixed potential
[25–27], was found to be at −203 mV. This mixed potential
was somewhat more negative than the OCV value of −177 mV
found when a mixture of 800 ppm NO, 8400 ppm H2, and 5%
O2 in Ar was supplied to the Pt/C working electrode at 250 ◦C.
One possible explanation for this discrepancy is that whereas
all of the 8400-ppm H2 could participate in the anodic reaction
in the polarization measurement, a part of the H2 was burned
in the OCV measurement. Therefore, to eliminate any effect
due to the lost H2, the above two measurements were made
in the absence of O2. As shown in Fig. 5, the mixed poten-
tial was at −680 mV, almost in agreement with the observed
OCV value of −689 mV. This near agreement demonstrates
electrolytes show extremely low ion conductivities (10−6
–
10−4 Scm−1). This result supports the validity of Scheme 1.
In contrast, our previous study demonstrated that the proton
conductivity of Sn1−xInxP2O7 (x = 0–0.1) increased with in-
creasing amounts of In3+ dopants and H2O vapor concentration
in the atmosphere [17]. In both cases (Figs. 3b and 3c), the in-
creased proton conductivity enhanced NOx conversion. This
result can be explained by assuming that one or more of the
three steps shown in Scheme 1 (H2 oxidation, proton migra-
tion, and NO reduction) is promoted by increasing the proton
conductivity of Sn1−xInxP2O7. It is also suggested that H2O
vapor coexisting in the exhaust imparts a positive effect on H2
SCR, which is very useful under actual conditions that include
% level of H2O vapor.
We also studied the catalytic activity of the Pt/C work-
ing electrode for H2 SCR under different operating conditions.
Fig. 4a shows that NOx conversion was strongly influenced by
O2 concentration. The Pt/C working electrode became less ac-
tive as the O2 concentration increased. GC measurement of the
outlet gas from the Pt/C working electrode showed that the H2
concentration decreased from 8400 to 3200 ppm with increas-
ing O2 concentration from 2% to 9%. We hypothesize that this
is due to the catalytic combustion of H2 decreasing the NOx
conversion. Another possible reason is that the following reac-
tion could be enhanced by increasing the O2 concentration:
O2 + 4H+ + 4e− → 2H2O.
(8)
Because reaction (8) competes with reaction (4) in reacting with
protons at the electrolyte–electrode interface, NOx conversion
can be considered decreased due to an increase in O2 con-
centration. A similar competition has been reported for NOx
electrolyzers using proton and oxide-ion conductors [23]. As

A. Tomita et al. / Journal of Catalysis 247 (2007) 137–144
141
(a)
(a)
(b)
(b)
(c)
Fig. 3. Plots of NO conversion as a function of H concentration (a) us-
(c)
x
2
Fig. 4. Plots of NO conversion as a function of H concentration (a) at O
concentrations of 2–9%, a CO concentration of 0% and at 250 C, (b) at
temperatures of 100–350 C and at an O concentration of 5% and a CO
x
2
2
◦
ing Sn In P O , silica glass and YSZ as electrolyte substrates, (b) using
7
2
0.9 0.1
2
◦
Sn
In P O (x = 0–0.1) as electrolyte substrates, and (c) at H O-vapor
x
7
1−x
2
2
2
2
◦
concentrations of 0–4%. The operating temperature was maintained at 250 C.
The reactant gas was 800 ppm NO, 0–8400 ppm H , and 5% O in Ar at a flow
rate of 50 mL min
concentration of 0%, and (c) at a CO concentration of 0–10% and an O con-
2
2
◦
2
2
centration of 5% at 250 C. The reactant gas was 800 ppm NO, 0–8400 ppm
H , 2–9% O , 0–10% CO in Ar at a flow rate of 50 mL min
−1
−1
.
.
2
2
2
absence of O2, higher than the theoretical current of 3 mA cal-
culated from Faraday’s law when all of the 800-ppm NO was
electrochemically reduced to N2 according to reaction (4). This
that reactions (3) and (4) actually occur at the Pt/C working
electrode. More importantly, the short-circuit current shown in
Fig. 5 reached 50 mA in the presence of O2 and 12 mA in the

142
A. Tomita et al. / Journal of Catalysis 247 (2007) 137–144
Fig. 5. Anodic polarization curve for 8400 ppm H in Ar and cathodic polar-
2
ization curves for a mixture of 800 ppm NO and 5% O in Ar and 800 ppm
NO in Ar at the Pt/C working electrode at 250 C. The direction of the cathodic
current is reversed.
2
◦
Fig. 6. Plots of NO conversion as a function of temperature over the Pt/Sn
-
x
0.9
In P O catalyst (Pt content = 0.2–0.8 wt%) and the Pt/C working electrode.
7
2
0.1
The reactant gas was 800 ppm NO, 8400 ppm H , and 5% O in Ar at a flow
2
2
−1
rate of 50 mL min
.
result implies that the short-circuit current was sufficiently high
to explain the NOx conversions shown in Figs. 3 and 4, thus
making Scheme 1 more reliable.
Here it is interesting to compare the NOx reduction based
on the mixed potential in the present study with the NOx re-
duction in the electrolyzer reported previously [19]. For in-
stance, the NOx conversion reached 6% at a H2 concentration
of 4200 ppm, as shown in Fig. 3. On the other hand, equivalent
hydrogen species production would be expected at the cathode
on application of a current density of 30 mA to the electrochem-
ical cell. The previous study showed a NOx conversion of 9%
at 30 mA, higher than the above conversion. The difference can
be explained by the more significant catalytic combustion of H2
in the present method compared with the previous method.
3.2. Pt/Sn0.9In0.1P2O7 catalyst
To clarify whether Scheme 1 is applicable to the Pt/Sn0.9
In0.1P2O7 catalyst, catalyst performance was tested at 50–
350 ◦C. Fig. 6 shows the NOx conversion over the Pt/Sn0.9In0.1
-
Fig. 7. Plots of N selectivity as a function of temperature over the Pt/Sn
-
0.9
2
In P O catalyst (Pt content = 0.8 wt%) and the Pt/C working electrode. The
7
2
0.1
reactant gas was 800 ppm NO, 8400 ppm H , and 5% O in Ar at a flow rate of
-
2
2
−1
50 mL min
.
P2O7 catalyst as a function of temperature, with the data ob-
served for the electrochemical cell with the Pt/C working elec-
trode included for comparison. For all Pt values, the catalyst
showed relatively high NOx conversion between 50 and 350 ◦C,
with NOx conversion reaching a maximum at about 100 ◦C.
These temperature profiles are qualitatively similar to those
observed for the electrochemical cell. Another similarity be-
tween the catalyst and electrochemical cell, in N2 selectivity,
was confirmed. As shown in Fig. 7, both the catalyst and elec-
trochemical cell showed N2 selectivity values of >80% over
the whole temperature range. These similarities suggest that H2
SCR proceeded in a similar manner over the catalysts and the
electrochemical cell. On the other hand, Fig. 6 also showed that
higher NOx conversions over the catalysts than over the elec-
trochemical cell at all temperatures tested, with this tendency
growing with increasing Pt content. This finding can be ex-
plained by the fact that the Pt/Sn0.9In0.1P2O7 interface assigned
to an electrochemical reaction site is more highly dispersed for
the catalyst than for the electrochemical cell, because the work-
ing electrode was merely placed on the electrolyte surface.
As described above, the Pt/Sn0.9In0.1P2O7 catalyst was char-
acterized by a wide operating temperature window and remark-
able N2 selectivity. It can be speculated that these characteris-
tics can be ascribed to the electrochemical activation of both
H2 and NO resulting from reactions (3) and (4), respectively.
This suggests that the catalyst performance would be further
improved by promoting reactions (3) and (4). Fig. 5 shows
that the polarization resistance of 30 ꢀcm2 for reaction (4) is
much higher than the resistance of 12 ꢀcm2 for reaction (3),
indicating that reaction (4) rather than reaction (3) should be
promoted. Toward this end, we first evaluated various work-
ing electrodes using the electrochemical cell. Fig. 8 shows the
plots of NOx conversion as a function of the H2 concentration
at 250 ◦C. Only the PtRh/C working electrode showed higher

A. Tomita et al. / Journal of Catalysis 247 (2007) 137–144
143
Fig. 8. Plots of NO conversion as a function of H concentration over the
x
Fig. 10. Plots of NO conversion as a function of temperature over PtRh/Sn
-
-
2
x
0.9
◦
PtM/C working electrodes (M = Rh, Ir, Ru, Pd, Co) at 250 C. The reactant
In P O (PtRh content = 0.8 wt%, Pt/Rh atomic ratio = 4) and Pt/Sn
7
0.1
2
0.9
gas was 800 ppm NO, 0–8400 ppm H , and 5% O in Ar at a flow rate of
In P O (Pt content = 0.8 wt%) catalysts. The reactant gas was 800 ppm
2
2
7
2
0.1
−1
−1
50 mL min
.
NO, 8400 ppm H , and 5% O in Ar at a flow rate of 50 mL min
.
2
2
two working electrodes, indicating that Rh does not contribute
to H2 oxidation at the electrode–electrolyte interface. To the
best of our knowledge, no previous study has shown that Rh is
a useful catalyst for the anodic reaction of H2 in PEFCs. On
the other hand, the difference between the two working elec-
trodes was more pronounced for the cathodic polarization curve
corresponding to reaction (4). Clearly, the addition of Rh to
Pt enhanced the catalytic activity for NOx reduction. This re-
sult is consistent with results from our previous study in which
the PtRh/C working electrode showed higher sensitivities in
the mixed-potential-type NOx sensor compared with the Pt/C
working electrode [28].
Pt and Rh were supported on Sn0.9In0.1P2O7, and the cata-
lyst performance for H2 SCR was measured. As in the case of
the electrochemical cell, the PtRh/Sn0.9In0.1P2O7 catalyst ex-
hibited a beneficial effect of Rh on catalytic activity, as shown
in Fig. 10. One obvious finding shown in this figure is that the
effect was greater at higher temperatures; for example, NOx
conversion reached about 40% at 350 ◦C. Another obvious fea-
ture is that two conversion maxima were observed at 100 and
200 ◦C. This is a well-known behavior for various SCR cata-
lysts reported thus far; the low-temperature peak is assigned to
the direct reduction of NO, whereas the high-temperature peak
is assigned to the reduction of NO2 generated in situ [29,30].
Considering these results, the addition of Rh to the catalyst
can be proposed to catalyze reaction (7) as well as reaction
(4). In contrast, Fig. 11 shows higher N2 selectivity values for
PtRh/Sn0.9In0.1P2O7 than for Pt/Sn0.9In0.1P2O7 at 50–350 ◦C.
In particular, it is noteworthy that a N2 selectivity of 89% was
achieved at 50 ◦C. As described earlier, N2 formation is due to
the reaction of two N(ad) atoms with one another [11]. There-
fore, we suggest that N(ad) formation is increased by the pro-
motion of reactions (4) and (7), improving N2 selectivity.
Based on the results described above, it can be concluded
that proton conduction in Sn0.9In0.1P2O7 is a contributing fac-
tor to the high NOx conversion and N2 selectivity over a wide
temperature range. This can be well understood by consider-
Fig. 9. Anodic polarization curves for 8400 ppm H in Ar and cathodic polar-
2
ization curves for 800 ppm NO in Ar at the PtRh/C and Pt/C working electrodes
◦
at 250 C. The direction of the cathodic current is reversed.
catalytic activity for H2 SCR than the Pt/C working electrode;
NOx conversion was about 1.5 times greater at the PtRh/C
working electrode than at the Pt/C working electrode at all
tested H2 concentrations. The impedance spectra of the electro-
chemical cells obtained with these working electrodes further
clarify this point; the order of electrode-reaction resistance was
PtRh (3.0 ꢀ) < Pt (3.7 ꢀ) < PtRu (4.1 ꢀ) < PtPd (6.6 ꢀ) <
PtIr (6.8 ꢀ) < PtCo (7.4 ꢀ), which is roughly in agreement
with the order of the NOx conversion shown in Fig. 8. This
result suggests that adding Rh to Pt activates one or both of
reactions (3) and (4), whereas adding other metals to Pt deacti-
vates these reactions.
To better understand the beneficial effect of Rh on the cat-
alytic activity for H2 SCR, we measured the anodic polarization
curve of 8400-ppm H2 diluted with Ar and the cathodic polar-
ization curve of 800-ppm NO diluted with Ar at 250 ◦C. Fig. 9
shows the comparative results observed for the PtRh/C and Pt/C
working electrodes. There was little difference in the anodic
polarization curve corresponding to reaction (3) between the

144
A. Tomita et al. / Journal of Catalysis 247 (2007) 137–144
cell was formed at the Pt/C working electrode, followed by
self-discharge. This scheme also could be realized in the H2
SCR for Pt/Sn0.9In0.1P2O7. Two major catalytic properties were
observed: catalytic activity over a wide temperature range (50–
350 ◦C) and remarkably high N2 selectivity (>80%). A further
improvement in catalytic performance was achieved by adding
Rh to Pt. NOx conversion was a maximum 1.4 times greater
than those observed for Pt/Sn0.9In0.1P2O7; N2 selectivity was
>89% at all temperatures tested. Our findings demonstrate that
the beneficial effect of Rh is based on promotion of the cathodic
reaction of NO.
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The present study has proposed a new scheme for H2 SCR
based on proton conduction in Sn0.9In0.1P2O7, which was used
as a catalyst support. In a mixture of NO, H2, and O2 in Ar,
H2 dissociated to protons and electrons at an anodic site of the
Pt/C working electrode, causing a negative potential. In con-
trast, NOx reacted with protons and electrons to form N2 and
H2O at a cathodic site of the Pt/C working electrode, result-
ing in a positive potential. As a result, an electrochemical local