Materials Sciences and Applications, 2012, 3, 733-738
http://dx.doi.org/10.4236/msa.2012.310107 Published Online October 2012 (http://www.SciRP.org/journal/msa)
Direct Decomposition of NO into N2 and O2 over C-Type
Cubic Y2O3-Tb4O7-ZrO2
Toshiyuki Masui, Shunji Uejima, Soichiro Tsujimoto, Nobuhito Imanaka*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Japan.
Received July 14th, 2012; revised August 15th, 2012; accepted September 16th, 2012
ABSTRACT
Catalytic activities for direct NO decomposition were investigated over C-type cubic Y2O3-Tb4O7-ZrO2 prepared by a
co-precipitation method. The NO decomposition activity was enhanced by partial substitution of the yttrium sites with
terbium in a (Y0.97Zr0.03)2O3.03 catalyst, which shows high NO decomposition activity. Among the catalysts synthesized
in this study, the (Y0.67Tb0.30Zr0.03)2O3.33 catalyst exhibited the highest NO decomposition activity; NO conversion to N2
was as high as 67% at 900˚C in the absence of O2 (NO/He atmosphere), and a relatively high conversion ratio was ob-
served even in the presence of O2 or CO2, compared with those obtained over conventional direct NO decomposition
catalysts. These results indicate that the C-type cubic Y2O3-Tb4O7-ZrO2 catalyst is a new potential candidate for direct
NO decomposition.
Keywords: Nitrogen Monoxide; Direct Decomposition; Catalyst; Rare Earth Oxide; C-Type Cubic Structure
1. Introduction
Nitrogen oxides (NOx) are not only harmful to human
beings, but are also responsible for photochemical smog
and acid rain when present in relatively high levels in the
atmosphere. NOx is mainly produced by the high-tem-
perature combustion of fossil fuels such as petroleum in
the engines of vehicles and ships, or coke in large-size
boilers of factories. NOx species in exhaust gases emitted
at high temperatures are composed principally of ther-
modynamically stable NO and a negligible amount of
NO2. Therefore, studies on the catalytic decomposition of
NOx should focus on NO.
Several NO reduction processes have been proposed
for NOx removal. Among them, selective catalytic reduc-
tion (SCR) methods employing ammonia or urea have
been extensively studied and applied in diesel engines
and large-size boilers [1]. The SCR methods have suffi-
cient NO decomposition efficiency and a stable reaction
process at high temperatures. However, separate special-
ized equipment is necessary to supply the reducing
agents. Moreover, it is absolutely essential to ensure se-
cure control systems due to the high toxicity and flam-
mability of ammonia.
In contrast to the above processes employing ammonia
or urea, direct catalytic decomposition of NO into N2 and
O2 (2NO → N
2 + O2) is the most ideal route for NOx
removal, because no reducing agents and no special
equipment are required. A number of materials, such as
zeolites [2,3], perovskites [4-6], and other complex ox-
ides [7-18], have been proposed as active catalysts for
direct NO decomposition. However, the NO decomposi-
tion activity of these conventional catalysts is signifi-
cantly decreased in the presence of O2 and CO2, due to
strong adsorption of these molecules on the surface of the
catalysts. Oxygen molecules produced by NO decompo-
sition as well as those present in the gas phase adsorb on
the catalyst surface and interfere with the catalytic reac-
tion. In addition, a number of active direct NO decompo-
sition catalysts contain highly basic alkaline and alkaline
earth ions in the lattice. Among them, catalysts contain-
ing Ba typically exhibit high NO decomposition activities,
because NO is acidic and adsorption of NO on the sur-
face of the catalyst is significantly enhanced with increas-
ing basicity of the catalyst. However, the high basicity
also facilitates adsorption of CO2. As a result, catalyst
poisoning by CO2 adsorption becomes a serious problem.
To realize high activities for direct NO decomposition,
it is important to design a novel catalyst that is not influ-
enced by the presence of CO2 or O2. Accordingly, we
focused on C-type cubic rare earth oxides. Rare earth
oxides can form three types of crystal structures, A-
(hexagonal), B-(monoclinic), and C-types (cubic), de-
pending on the ionic size of the respective rare earth
element [19]. Among these, the C-type cubic structure
*Corresponding author.

Direct Decomposition of NO into N2 and O2 Over C-Type Cubic Y2O3-Tb4O7-ZrO2 734
has the largest interstitial open space. It has been gener-
ally accepted that large open spaces in a catalyst play an
important role in direct NO decomposition, and for this
reason the C-type cubic rare earth oxide is suitable as a
direct NO removal catalyst. Furthermore, in our previous
works, we found that several C-type cubic rare earth ox-
ide catalysts can exhibit high activities for direct NO
decomposition even in the presence of O2 [20-25] and
that exclusion of alkaline earth ions from the catalyst
lattice is significantly effective to induce CO2 tolerance
[25,26].
In the present study, (Y0.97–xTbxZr0.03)2O3.03+δ solid so-
lutions (x = 0, 0.10, 0.20, 0.30, and 0.40), which adopt a
C-type cubic structure, were designed as novel NO de-
composition catalysts. In these catalysts, a fraction of the
yttrium sites in (Y0.97Zr0.03)2O3.03, which showed the
highest NO decomposition activity in (Y1–yZr y)2O3+y (0 ≤
y ≤ 0.10) [24], was substituted with terbium to inhibit
catalyst poisoning by O2 and CO2, utilizing the redox
property of Tb4+/3+ [23,25] for effective direct NO de-
composition.
2. Experimental
2.1. Catalyst Preparation
The C-type cubic (Y0.97–xTbxZr0.03)2O3.03+δ (x = 0, 0.10,
0.20, 0.30, and 0.40) catalysts were synthesized by a
co-precipitation method, where the zirconium content
was fixed at 3 mol% for the reason mentioned above. A
stoichiometric mixture of 1 mol·dm–3 Y(NO3)3, 0.1
mol· dm–3 Tb(NO3)3, and 0.1 mol·dm–3 ZrO(NO3)2 aque-
ous solutions was added to a 1.0 mol·dm–3 ammonium
carbonate solution with stirring. The pH of the mixture
was adjusted to 10 by dropwise addition of 9% ammonia
solution. After stirring at room temperature for 6 h, the
resulting precipitate was collected by filtration, washed
several times with deionized water, and then dried at
80˚C for 6 h. The powder was then ground in an agate
mortar and finally calcined at 900˚C in air for 6 h.
2.2. Characterization
The catalysts were characterized by X-ray powder dif-
fraction (XRD; Rigaku SmartLab) with CuKα radiation.
XRD patterns were recorded in the 2α range from 10˚ to
70˚. The sample compositions were analyzed by X-ray
fluorescence spectrometry (XRF; Rigaku, ZSX100e) and
the specific surface area was measured by the Brunauer-
Emmett-Teller (BET) method using nitrogen adsorption
at −196˚C with a Micromeritics TriStar 3000 adsorption
analyzer.
2.3. Catalyst Test
The NO decomposition reaction was carried out in a
conventional fixed-bed flow reactor with a 10-mm-di-
ameter quartz glass tube. A gas mixture of 1 vol% NO
and He (balance) was fed at a rate of 10 cm3·min−1 over
0.5 g of catalyst. The W/F ratio, where W and F are the
catalyst weight and gas flow rate, respectively, was ad-
justed to 3.0 g·s·cm−3. The gas composition was analyzed
using a gas chromatograph (Shimadzu GC-8A) with a
thermal conductivity detector (TCD), a molecular sieve 5
A column for NO, N2, and O2, and a Polapak-Q column
for N2O separation. The activity of each catalyst was
evaluated in terms of NO conversion to N2.
The effect of the presence of O2 or CO2 was measured
by mixing each gas species with the reactant gas. The
concentrations of the additional gases and NO were con-
trolled by changing the feed rate of He as the balance gas
to maintain a total reactant flow rate of 10 cm3·min–1.
2.4. Temperature Programmed Desorption
Temperature-programmed desorption (TPD) measure-
ments of O2 was carried out after adsorption of O2 at
600˚C for 1 h. After heating the catalyst in a flow of He
(30 cm3·min−1) at 600˚C for 30 min, the catalyst was
exposed to O2 (1 atm) at the same temperature for 1 h,
and then cooled to 50˚C. After evacuation at 50˚C for 30
min, the catalyst was heated under a flow of He at a
heating rate of 10˚C·min−1 and the desorbed gas was mo-
nitored using a gas chromatograph with a catalysis ana-
lyzer (BELCAT-B BEL JAPAN). In the case of
CO2-TPD, the catalyst was heated in a flow of He (30
cm3·min−1) at 600˚C for 1 h, and subsequently in a flow
of H2 (30 cm3·min−1) at 600˚C for 30 min. The catalyst
was cooled in a flow of He to 50˚C, and was then ex-
posed to CO2 (1 atm) at this temperature for 1 h. After
evacuation at 50˚C for 30 min, the catalyst was heated
under a flow of He at a heating rate of 10˚C·min−1.
3. Results and Discussion
3.1. Characterization of the Catalysts
Figure 1 shows XRD patterns of the
(Y0.97–xTbxZr0.03)2O3.03+δ catalysts (x = 0, 0.10, 0.20, 0.30,
and 0.40). C-type cubic rare earth oxide with a single
phase structure (PDF-ICDD 41-1105 for Y2O3) was suc-
cessfully obtained for all samples and no crystalline im-
purities were observed. The catalyst compositions, as
determined using XRF, the lattice constants, and the BET
surface area of the (Y0.97–xTbxZr0.03)2O3.03+δ catalysts are
summarized in Table 1. The cubic lattice parameter of
(Y0.97–xTbxZr0.03)2O3.03+δ gradually decreased with the x
value, because ionic sizes of Y3+ and Tb4+ with a six-fold
coordination are 0.1040 nm [27] and 0.0900 nm [27],
respectively. When the smaller Tb4+ occupies the lattice
position of Y3+ in (Y0.97Zr0.03)2O3.03, the lattice parameter

Direct Decomposition of NO into N2 and O2 Over C-Type Cubic Y2O3-Tb4O7-ZrO2 735
Figure 1. XRD patterns of the (Y0.97−xTbxZr0.03)2O3.03+δ cata-
lysts (x = 0, 0.10, 0.20, 0.30, and 0.40).
Table 1. Composition, lattice constant, and BET surface
area of the (Y0.97–xTbxZr0.03)2O3.03+δ catalysts.
Composition Lattice constant BET surface area
/nm /m2·g−1
(Y0.97Zr0.03)2O3.03 1.0609 20.6
(Y0.87Tb0.10Zr0.03)2O3.13 1.0600 15.6
(Y0.77Tb0.20Zr0.03)2O3.23 1.0588 16.0
(Y0.67Tb0.30Zr0.03)2O3.33 1.0573 22.0
(Y0.57Tb0.40Zr0.03)2O3.43 1.0561 21.5
decreases monotonically with increasing Tb4+ content.
The results indicate that C-type cubic solid solutions
were successfully formed for all samples. The BET spe-
cific surface area was slightly affected by the introduc-
tion of Tb4+ into the (Y0.97Zr0.03)2O3.03 lattice.
3.2. NO Decomposition Activity
Figure 2 depicts the temperature dependencies of the N2
yield for the (Y0.97–xTbxZr0.03)2O3.03+δ catalysts (x = 0,
0.10, 0.20, 0.30, and 0.40). Initial NO decomposition
activity appeared at 550˚C and the N2 yield increased
monotonically with reaction temperature. The formation
of N2O was not detected between 400 and 900˚C. Figure
3 shows the dependence of NO conversion into N2 at
900˚C on the composition of the C-type cubic
(Y0.97–xTbxZr0.03)2O3.03+δ catalysts. The catalytic activity
increased with the x value, and the highest catalytic ac-
tivity was obtained for the (Y0.67Tb0.30Zr0.03)2O3.33 com-
position, where the N2 yield obtained over this catalyst
was 67%.
3.3. Effect of the Presence of O2 or CO2
The effect of the presence of O2 or CO2 on the N2 yield
Figure 2. Temperature dependence of the N2 yield obtained
over the (Y0.97–xTbxZr0.03)2O3.03+δ catalysts (x = 0, 0.10, 0.20,
0.30, and 0.40) (NO: 1 vol%, He; balance, W/F = 3.0
g·s·cm−3).
Figure 3. Dependence of NO conversion into N2 at 900˚C on
the terbium concentration in the (Y0.97–xTbxZr0.03)2O3.03+δ
catalysts (NO: 1 vol%, He; balance, W/F = 3.0 g·s·cm−3).
obtained over the (Y0.67Tb0.30Zr0.03)2O3.33 catalyst at
900˚C was also examined, and the results are presented
in Figure 4. In the presence of O2, the N2 yield for
(Y0.67Tb0.30Zr0.03)2O3.33 decreased from 67 to 53% until
the content reached 1 vol%, but became almost constant
in the range from 1 to 5 vol%. As a result, NO decompo-
sition activity as high as 47% was maintained, even in
the presence of 5 vol% O2, and the activity did not dete-
riorate during the 10-h catalytic test. This conversion
ratio in the presence of 5 vol% O2 (47%) was higher than
that for C-type cubic (Yb0.50Tb0.50)2O3±δ (35%) [25].
In contrast, the effect of CO2 on the NO decomposi-
tion activity was relatively larger than that for O2, but the
decreasing tendency of the N2 yield with the increase in
the partial pressure of CO2 was similar to that observed
for O2. Since CO2 is acidic, adsorption of CO2 on the
basic sites of the catalyst surface will inhibit NO adsorp-
tion on the open space sites of the catalyst. However, for
the (Y0.67Tb0.30Zr0.03)2O3.33 catalyst, a high N2 yield of
36% was maintained even in the presence of 5 vol% CO2,
which is higher than those for the (Yb0.50Tb0.50)2O3±δ
(34%) [25], Ba0.8La0.2Mn0.8Mg0.2O3 (20% at 850˚C) [12],
and La0.8Sr0.2CoO 3 (10% at 800˚C) catalysts [15].

Direct Decomposition of NO into N2 and O2 Over C-Type Cubic Y2O3-Tb4O7-ZrO2 736
Furthermore, the catalytic activity was recovered when
the catalytic test was carried out again in the absence of
CO2 (1 vol% NO/He), which indicates that the decrease
in NO decomposition activity is caused by the adsorption
of CO2 on the catalyst. In addition, the XRD patterns of
the (Y0.67Tb0.30Zr0.03)2O3.33 catalyst were the same before
and after the reactions.
3.4. O2 and CO2 Desorption Profiles
As mentioned above, the present (Y0.67Tb0.30Zr0.03)2O3.33
catalyst showed relatively high catalytic activity even in
the presence of O2 or CO2. To facilitate direct NO de-
composition, it is important that these coexisting gases
desorb from the surface of the catalyst easily. Therefore,
desorption behavior of O2 and CO2 adsorbed on
(Y0.67Tb0.30Zr0.03)2O3.33 was characterized by TPD meas-
urements.
Figure 5 shows O2 and CO2 desorption profiles
(O2-TPD and CO2-TPD) for (Y0.67Tb0.30Zr0.03)2O3.33. In
references [5,6,12], catalysts that demonstrated low-
temperature O2 desorption exhibited high activities for
NO decomposition. In the present case, a single O2 de-
Figure 4. Effect of the presence of O2 or CO2 on the N2 yield
obtained over the (Y0.67Tb0.30Zr0.03)2O3.33 catalyst at 900˚C
(NO: 1 vol%, O2: 0 - 5 vol%, CO2: 0 - 5 vol%, He; balance,
W/F = 3.0 g·s·cm−3).
Figure 5. O2 and CO2 desorption profiles (O2-TPD and
CO2-TPD) for (Y0.67Tb0.30Zr0.03)2O3.33.
sorption peak was observed at 470˚C for the
(Y0.67Tb0.30Zr0.03)2O3.33 catalyst, whereas that for
(Yb0.50Tb0.50)2O3±δ was 510˚C. In general, catalysts with
weak oxygen adsorption exhibit higher NO decomposi-
tion activities. Therefore, the (Y0.67Tb0.30Zr0.03)2O3.33 cata-
lyst showed high NO decomposition activity even in the
presence of O2.
In the case of CO2, the CO2 desorption peak was ob-
served at 115˚C, which is much lower than those seen for
Ba0.8La0.2Mn0.8Mg0.2O3 (700˚C) [14] and La0.8Sr0.2CoO3
(750˚C) [15]. The adsorption strength of CO2 correlates
with the desorption temperature observed in TPD, and
the lower the CO2 desorption temperature is, the weaker
the adsorption strength becomes. Although the weakly
adsorbed CO2 may block the active site for NO decom-
position in the (Y0.67Tb0.30Zr0.03)2O3.33 catalyst, the ad-
sorption strength of CO2 on this catalyst is quite weak.
As a result, the NO decomposition activity was not sig-
nificantly suppressed by the CO2 coexistence. This con-
sideration is supported by the result mentioned above
that the N2 yield obtained at 900˚C in the presence of 5
vol% CO2 over the present (Y0.67Tb0.30Zr0.03)2O3.33 cata-
lyst (36%) was almost equivalent to that over
(Yb0.50Tb0.50)2O3±δ (34%), because the CO2 desorption
peak was observed at 115˚C and 125˚C for the former ant
the latter, respectively.
4. Conclusion
C-type cubic (Y0.97–xTbxZr0.03)2O3.03+δ (x = 0, 0.10, 0.20,
0.30, and 0.40) catalysts, in which the yttrium site was
partially substituted with terbium, were found to exhibit
high catalytic activity for direct NO decomposition. The
highest catalytic activity was obtained for
(Y0.67Tb0.30Zr0.03)2O3.33. It is noteworthy that the catalytic
activity was maintained at a high conversion ratio even in
the presence of O2 or CO2. Therefore, the
(Y0.67Tb0.30Zr0.03)2O3.33 catalyst is expected to be a new
potential candidate as a direct NO decomposition catalyst.
5. Acknowledgements
This research was supported by the Industrial Technol-
ogy Research Grant Program ’08 (Project ID: 08B42001a)
from the New Energy and Industrial Technology Devel-
opment Organization (NEDO) of Japan, and by the Steel
Industry Foundation for the Advancement of Environ-
mental Protection Technology (SEPT).
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