Direct Decomposition of NO into N2 and O2 Over C-type Cubic Y2O3-Tb4O7-ZrO2

doi:10.4236/msa.2012.310107

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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.

Email: *[email protected]

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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