Catalytic property of AII2BIIBVIO6 double perovskites (BVI = Mo, W) for the reduction of nitric oxide with propane in the presence of oxygen

Article abstract of DOI:10.1016/S0025-5408(00)00238-5

Systematic series of double perovskites containing W and Mo, A^II₂B^IIB^VIO₆ (A^II = Ba, Sr, Ca; B^II = Mg, Ca, Co, Ni, Cu, Cd; B^VI = W, Mo, W_1-xMo_x)

Full text of DOI:10.1016/S0025-5408(00)00238-5

Pergamon
Materials Research Bulletin 35 (2000) 521–530
II II VI
Catalytic property of A B B O double perovskites
2
6
VI
(
B ϭ Mo, W) for the reduction of nitric oxide with
propane in the presence of oxygen
a
b,
b
M.-D. Wei , Y. Teraoka *, S. Kagawa
a
Department of Marine Resources Research and Development, Graduate School of Marine Science and
Engineering, Nagasaki University, Nagasaki 852-8521, Japan
b
Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852-8521, Japan
(Refereed)
Received 26 March 1999; 16 June 1999 accepted
Abstract
II II VI
II
Systematic series of double perovskites containing W and Mo, A B B O (A ϭ Ba, Sr, Ca;
2
6
II
VI
B
ϭ Mg, Ca, Co, Ni, Cu, Cd; B ϭ W, Mo, W1ϪxMo ) were synthesized, and their catalytic
x
activities for NO–C H –O reaction were investigated. All the oxides crystallized in the ordered double
3
8
2
II
VI
perovskite structure with the rock-salt arrangement of B and B cations, and the unit cell symmetry
was almost exclusively determined by A cations. The double perovskites catalyzed the nonselective
II
reduction of NO into N O and occasionally N specifically at 600°C in the gaseous mixture of NO
2
2
(
0.44%), C H (0.29%) and O (4.4%). The reduction activity of NO into N O and N depended on
3 8 2 2 2
II
II
II
VI
the sort of A , B , and B cations, but the effect of B cations was the most remarkable. The NO
reduction activity increased monotonically with an increase in the enthalpy of formation of B O
II
II
II
(Ϫ⌬H °), suggesting that the strength of B –O bond or the redox property of B ions is of primary
f
importance for the NO reduction activity in the NO–C H –O reaction. © 2000 Elsevier Science Ltd.
3
8
2
All rights reserved.
Keywords: A. Oxides; D. Catalytic properties; D. Crystal structure
*
Corresponding author. Tel.: ϩ81-95-848-9652; fax: ϩ81-95-848-9652.
E-mail address: yasu@net.nagasaki-u.ac.jp (Y. Teraoka).
0
025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0025-5408(00)00238-5

5
22
M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
II II VI
II
VI
Fig. 1. Structure of A 2B B O double perovskites with the rock-salt ordering of B and B cations.
6
1
. Introduction
Perovskite-type oxides, which have general formula of ABO , are an important class of
3
catalytic materials and have been studied extensively as catalysts for complete oxidation of
hydrocarbons, direct decomposition of NO, and nonselective reduction of NO by CO, H2,
and hydrocarbons [1–4]. In addition, many basic studies have been carried out to elucidate
relationships between the solid-state chemistry and the catalytic properties of perovskite-type
oxides [1–4]. So far, both unsubstituted (ABO ) and substituted (A AЈ BO ,
3
1Ϫx
x
3
A1ϪxAЈ B BЈ O , etc.) systems have been investigated, almost all of which contain rare
x
1Ϫy
y 3
earth cations, especially La, at the A sites and 3d transition metal ions, especially Co and Mn,
at the B sites.
A subclass of perovskites represented by AAЈBBЈO are referred to as a double
6
perovskite in which three B-cation sublattice types are possible: random, rock-salt, and
VI
layered [5]. It is known that hexavalent Mo and W (B ) are stabilized in the ordered
VI
II
double perovskites with the rock-salt arrangement of B and divalent B ions [5,6]. Fig.
II II VI
2 6
1
shows the crystal structure of A B B O ordered double perovskite. In this struc-
II
VI
ture, only the three-dimensional chain of the –B –O–B –O– linkage is possible. This
II
VI
specific arrangement of B and B cations is of great interest, because the catalytic
properties of perovskites are generally determined by the nature of B-site cations [3]. To
the best of our knowledge, however, the work by Voorhoeve et al. [7] is the only report
dealing with the catalytic properties of the ordered double perovskite; they reported the
catalytic activity of Ba CoWO for CO oxidation and NO reduction by CO and H . In
2
6
2
order to explore new perovskite catalysts as well as to collect more information about the
relationships between solid-state chemistry and the catalytic properties, the system-
atic synthesis and evaluation of catalytic activity of ordered perovskites are highly
desired.
II II
II
Systematic series of A B WO compounds have been already reported with A ϭ Ba
2
6
II
and Sr and B ϭ Mg, Ca, Co, Cu, Fe, Ni, and Zn [5,6]. In addition, we have recently
succeeded the synthesis of systematic series of Mo-containing oxides, A B MoO and
A B W Mo O [8]. In this paper, catalytic activities of Mo- and W-containing double
II II
2
6
II II
2
1Ϫx
x 6

M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
523
Table 1
a
Ordered double perovskites used in this study
b
Cubic (2a type)
p
II
II
Ba B WO
B
B
B
B
B
Ba B MoO
2
6
2
6
II
II
II
II
II
II
ϭ Mg (1250°C, 30 h)
B
B
B
B
B
ϭ Mg (1250°C, 5 h)
ϭ Ca (1200°C, 10 h)
ϭ Co (1000°C, 20 h)
ϭ Ni (1000°C, 5 h)
ϭ Cd (900°C, 40 h)
II
II
II
II
ϭ Ca (1100°C, 10 h)
ϭ Co (1000°C, 5 h)
ϭ Ni (1100°C, 30 h)
ϭ Cd (900°C, 20 h)
Ba CdW0.5Mo0.5O (900°C, 30 h)
2
6
b
Tetragonal (2a type)
p
Ba CuWO (900°C, 40 h)
Sr CuWO (950°C, 30 h)
2 6
2
6
b
Tetragonal (͌2a ϫ ͌2a ϫ 2a type)
p
p
p
II
Sr B WO
Sr MgW Mo O
2 1Ϫx x 6
x ϭ 0.25 (1100°C, 20 h)
x ϭ 0.5 (1100°C, 60 h)
2
6
I
B ϭ Mg (1100°C, 30 g)
II
B
B
B
ϭ Co (1200°C, 20 g)
ϭ Ni (1100°C, 20 h)
II
II
x ϭ 0.75 (1100°C, 60h)
IIϭ Cd (950°C, 40 h)
Sr CoW0.5Mo0.5O (1200°C, 20 h)
2
6
Sr B MoO
2
6
II
B
B
B
ϭ Mg (1100°C, 60 h)
ϭ Co (1150°C, 25 h)
ϭ Ni (1100°C, 20 h)
II
II
b
Orthorhombic (͌2a ϫ ͌2a ϫ 2a type)
p
p
p
Sr CaWO (1000°C, 30 h)
Ca CoWO (1050°, 30 h)
2 6
2
6
Sr CaMoO (1050°C, 45 h)
Ca NiWO (1050°C, 20 h)
2 6
2
a
b
6
Synthesis conditions (temperature, time) are shown in parentheses.
a is the cell parameter of cubic perovskite of the primitive ABO type (a Ϸ 0.4 nm).
p
3
p
perovskites have been investigated for the reduction of NO by C H in the presence of excess
3
8
oxygen, and the relationship between the catalytic property and oxide composition is
discussed.
2
. Experimental
2
.1. Synthesis and characterization of double perovskites
Polycrystalline powders of double perovskites listed in Table 1 were synthesized from
appropriate mixtures of MoO , WO , and nitrates of other elements. Molybdenum and/or
3
3
tungsten trioxide was added into a mixed aqueous solution of metal nitrates, and the
suspended solution was evaporated to dryness under vigorous stirring. The obtained solid
mixture was ground and heat-treated at 350°C for 1 h in order to decompose remaining
metal nitrates. After regrinding, the mixture was calcined in air; the calcination tem-
perature and time necessary to obtain single-phase double perovskites are listed in Table
1
. Powder X-ray diffraction (XRD) patterns were recorded at room temperature using a

5
24
M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
Cu K␣ radiation (Rigaku RINT-2200VL, 30 kV, 16 mA). The specific surface areas of
catalysts, which were measured by N adsorption at liquid N temperature (BET
2
2
2
Ϫ1
method), were very small, ranging between 1.0 and 0.2 m ⅐g
.
2
.2. Catalytic activity test
The catalytic activity was measured in a fixed-bed flow reactor between 300 and
00°C. A mixture of NO (0.44%), C H (0.29%), O (4.4%) and He (balance) was fed
8
3
3
8
2
Ϫ1
Ϫ1
to 0.5 g catalyst at a rate of 30 cm ⅐min ; the space velocity was 7000 h , or the
Ϫ3
contact time (catalyst weight/gas flow rate) was 1.0 g⅐s⅐cm . The effluent gas was
analyzed by a gas chromatograph equipped with a thermal conductivity detector (Shi-
madzu GC 14B). A molecular sieve 5A column was used to separate NO, O , N , and
2
2
CO, and a Porapak N column for CO and C H . The catalytic activity at a steady state,
2
3 8
which was attained after 1 h or more on stream, was evaluated by the following
parameters:
Y Conversion of NO into N (X[N ]/%) ϭ (2[N ] /[NO] ) ϫ 100
2
2
2 out
in
Y Conversion of NO into N O (X[N O]/%) ϭ (2[N O] /[NO] ) ϫ 100
2
2
2
out
in
Y Conversion of C H into CO (X[CO ]/%) ϭ ([CO ] /3[C H ] ) ϫ 100
3
8
2
2
2 out
3 8 in
Y Conversion of C H into CO (X[CO]/%) ϭ ([CO] /3[C H ] ) ϫ 100
3
8
out
3 8 in
The total conversion of NO (X[NO]), which is a sum of X[N ] and X[N O], is also used to
2
2
discuss the NO reduction activity.
3
. Results and discussion
3
.1. Crystal structure
Double perovskites with the rock-salt sublattice have unit cells with lattice constants
of 2a or ͌2a ϫ ͌2a ϫ 2a , where a is the lattice constant of cubic perovskite of
p
p
p
p
p
the primitive ABO type (a Ϸ 0.4 nm). In the present study, four types of unit cells were
3
p
discerned, and XRD pattern of the representative sample in each type is shown in Fig.
. The appearance of superlattice lines, for example, 111, 311, and 511 lines of Fig. 2(a),
2
II
VI
evidences the rock-salt ordering of B and B cations [5]. As can be seen from Table
1
almost all oxides with A ϭ Ba, Sr, and Ca have the structure of cubic 2a type,
II II VI
2 6
II
, the unit-cell type of A B B O is almost exclusively determined by the A cation;
II
p
tetragonal ͌2a ϫ ͌2a ϫ 2a type, or orthorhombic ͌2a ϫ ͌2a ϫ 2a type,
p
p
p
p
p
p
II
IV
respectively. Exceptions to this rule are oxides with B ϭ Cu and Sr CaB O . The
2
6
former crystallizes in the tetragonal 2a -type structure, due to the Jahn–Teller distortion
p
2
ϩ
VI
of the Cu ion [7]. The structure of SrCaB O , the orthorhombic ͌2a ϫ ͌2a ϫ 2a
type, is more distorted than that of other tetragonal SrB B O oxides, because the
combination of smaller A (Sr) and larger B (Ca) gives a smaller tolerance factor or
fitness factor [8], which results in the lattice distortion. Details of the crystal structure
6
p
p
p
II VI
6
II
II

M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
525
Fig. 2. Powder XRD patterns of double perovskites: (a) cubic 2a -type Ba CoW0.5Mo0.5O , (b) tetragonal
p
2
6
2
a -type Ba CuWO , (c) tetragonal (͌2a ϫ ͌2a ϫ 2a )-type Sr CoW0.5Mo0.5O , and (d) orthorhombic
p 2 6 p p p 2 6
(
͌2a ϫ ͌2a ϫ 2a )-type Sr CaWO .
p p p 2 6
and cell parameters of the present Mo- and W-containing double perovskites are reported
elsewhere [8].
3
.2. Catalytic activity
Fig. 3 shows temperature dependence of NO–C H –O reaction over Sr MgW Mo O
0.5 6
3
8
2
2
0.5
and Ba CoW Mo O . The former is representative of the catalysts showing the higher NO
2
0.5
0.5 6
reduction activity with the low conversion of C H at 600°C, while the latter is that showing
3
8
the lower NO reduction activity with the high conversion of C H . For both catalysts, the
3
8
reaction proceeded above 500°C, and the reduction of NO into N O and N was observed at
2
2
6
00°C. This temperature dependency was common to all the catalysts tested, and, therefore,
the catalytic performances at 600°C were used in this study to discuss the catalytic properties
of the double perovskites.
Fig. 4 shows the effect of oxygen concentration on the catalytic performance of Sr NiMoO6
2
at 600°C in the NO(0.44%)–C H (0.29%)–O (variable between 0 and 6.6%) atmosphere. In-
3
8
2
creasing the O concentration caused monotonic increase and decrease of X[CO ] and X[N ],
2
2
2
respectively, and the formation of N O was observed with the maximum conversion at 2.2% of
2
O . In the absence of O , only CO and N were detected with the CO /N molar ratio being close
2
2
2
2
2
2
to 3/5. This indicates the occurrence of the following reaction:

5
26
M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
Fig. 3. NO–C H –O
reaction over Sr MgW0.5Mo0.5O6 (A) and Ba CoW0.5Mo0.5O6 (B).
2 2
3
8
2
NO(0.44%)–C H (0.29%)–O (4.4%)–He(balance).
3
8
2
C H ϩ 10NO 3 5N ϩ 3CO ϩ 4H O
(1)
3
8
2
2
2
In the presence of oxygen in the feed gas, the conversion of NO into N decreased
2
progressively with increasing O concentration. This indicates that C H reacts with O in
2
3
8
2
preference to NO. At the O concentration of 2.2 and 3.5%, the formation of CO was
2
observed due to the incomplete oxidation of C H , reaction (2); the conversion of C H into
3
8
3 8
CO was 3.6% (2.2% O ) and 0.6% (3.5% O ). Assuming that the reduction of NO into N O
2
2
2
by CO, reaction (3), proceeds rapidly, the enhancement of the N O formation at the
2
intermediate O concentration would be reasonably explained.
2
C H ϩ ͑7/2͒O 3 3CO ϩ 4H O
(2)
(3)
(4)
3
8
2
2
CO ϩ 2NO 3 N O ϩ CO
2
2
C H ϩ 5O 3 3CO ϩ 4H O
3
8
2
2
2
Fig. 4. Effect of oxygen concentration on the catalytic performance of Sr NiMoO for the NO–C H –O reaction
2
6
3
8
2
at 600°C. NO(0.44%)–C H (0.29%)–O (0–6.6%)–He(balance).
3
8
2

M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
527
II II
Fig. 5. NO reduction activity of A B WO double perovskites for the NO–C H –O reaction at 600°C.
2
6
3
8
2
NO(0.44%)–C H (0.29%)–O (4.4%)–He(balance).
3
8
2
With increasing O concentration, the fraction of C H to be completely oxidized, reaction
2
3 8
(4), increases, leading to the decrease of the CO formation and thus the conversion of NO
into N O. The consecutive reaction of C H 3 CO 3 CO might also be the case; however,
2
3
8
2
this can be regarded as the complete oxidation of C H . At the highest O concentration
3
8
2
(
6.6%), the complete oxidation of C H by O proceeds exclusively.
3 8 2
The reduction of NO into N by hydrocarbons in the presence of excess oxygen is referred
2
to as the selective reduction of NO with hydrocarbons (HC-SCR). After the pioneering works
by Iwamoto et al. [9] and Held et al. [10], the HC-SCR reaction has attracted great attention
as a novel NO removal process for exhaust treatment. The most characteristic phenomenon
x
of the HC-SCR reaction is that the reduction of NO into N by hydrocarbons is enhanced by
2
coexisting O . From this point of view, it can be said that the Mo- and W-containing double
2
perovskites catalyze the nonselective reduction of NO by C H , but not the HC-SCR
3
8
reaction.
3
.3. Relation between the NO reduction activity and oxide composition
II
The effect of the A cation on NO reduction activity is depicted in Fig. 5 for the
II II
A B WO series oxides. Nitrous oxide was a main reduction product, and the formation of
2
6
II
N was observed only for catalysts with B ϭ Ca, Mg, and Ni. As can be seen for
2
II
2
II
2
II
A CaWO and A MgWO , the oxides with A ϭ Sr show a higher conversion of NO into
6
6
II
N than those with A ϭ Ba. With regard to the total reduction of NO, which is a sum of
2
conversions into N and N O, the Ba compounds show higher activity than the Sr compounds
2
2
II II
2
in all the systems. This relation is also true in the A B MoO series oxides. When the Ca
compounds are taken into account (B ϭ Co and Ni), however, the NO reduction activity
follows the order of Ca Ͼ Ba Ͼ Sr. This activity order has no direct relation with ionic
6
II
radius, Ba Ͼ Sr Ͼ Ca, or enthalpy of the oxide formation (Ϫ⌬H °), Ca Ͼ Sr Ͼ Ba (vide
f
II
infra). The reason for the effect of the A cation is not clear at present.
II
As seen from Fig. 5, the total NO reduction activity depended more largely on B cations

5
28
M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
II
Fig. 6. Relation between the total conversion of NO (X[NO]) and the standard enthalpy of formation of B O,
Ϫ1
Ϫ⌬H °. Reaction condition: NO(0.44%)–C H (0.29%)–O (4.4%)–He(balance), 600°C. Ϫ⌬H °/kJ ⅐ mol : 634.9
f
3
8
2
f
(CaO), 601.6 (MgO), 258.4 (CdO), 239.7 (NiO), 237.9 (CoO), 157.3 (CuO).
II
II
than A cations, and oxides with B ϭ Ca and Mg showed higher NO reduction activity than
II
those with 3d transition metal ions, implying that the redox property of the B ion influences
the NO reduction activity. Fig. 6 shows the relationship between the total conversion of NO
II
(
X[NO]), and the standard enthalpy of formation of B O, Ϫ⌬H ° [11,12]. In all of the
f
systems, a good correlation between the two quantities is obtained, and X[NO] tends to
increase with increasing Ϫ⌬H °. In catalytic chemistry, the standard enthalpy of formation
f
of metal oxide per oxygen atom, Ϫ⌬H °/O-atom, is often used as a parameter of metal–
f
oxygen bond strength, the ability of oxygen activation, or the redox property of metal ions
(
oxides). For example, it was reported on simple metal oxide catalysts that the catalytic
activity for the complete oxidation of propylene [13] and the amount of desorbed oxygen
14] increased with a decrease of Ϫ⌬H °/O-atom. These results indicate that the ability of
[
f
oxygen activation increases or the metal–oxygen bond strength weakens with decreasing
Ϫ⌬H °/O-atom. As stated above, the complete oxidation of C H by O , Eq. (4), competes
f
3
8
2
with the other reactions, Eqs. (1)–(3), to reduce NO into N and N O in the present
2
2
NO–C H –O reaction system. Accordingly, the tendency observed in Fig. 6 can be ex-
3
8
2
pressed in another way that the selectivity to the complete oxidation of propane or the
oxidation ability of the catalysts increases with decreasing Ϫ⌬H °. This is consistent with the
f
above discussion about the relation between Ϫ⌬H ° and catalytic activity. It can thus be
f
II
IV
II II IV
concluded that, when A and B ions are fixed, the catalytic property of A B B O in the
NO–C H –O reaction is almost exclusively determined by the redox nature of B ions, and
that the oxide containing B ion with larger Ϫ⌬H ° shows a higher activity of, or selectivity
to, NO reduction. The comparison between Ba Co WO and Ba Co WO might be
2
6
II
3
8
2
II
f
II
III
2
2
6
3
9
significant because both oxides are composed of the same metal cations but contain Co ions

M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
529
II II
Fig. 7. NO reduction activity of A B W Mo O double perovskites as a function of the Mo composition x.
2
1Ϫx
x 6
Reaction condition; NO(0.44%)–C H (0.29%)–O (4.4%)–He(balance).
3
8
2
III
in different oxidation states. Ba Co WO was prepared by the same procedure as for
3
2
9
II
Ba Co WO and had an ordered structure with a 1:2 arrangement of W and Co layers.
Ba Co WO was more active than Ba Co WO for the complete oxidation of C H by O
C H –O reaction), because of the easy redox change between Co and Co [15]. Conse-
quently, Ba Co WO showed lower total NO conversion than Ba Co WO (14.7% for the
2
6
III
2
II
3
9
2
6
3
8
2
III
II
(
3 8 2
III
2
II
3
9
2
6
former and 35.3% for the latter at 600°C).
Finally, the dependence of catalytic property on W and Mo is discussed. As shown in Fig.
, the effect of composition of W or Mo is not so large, but the NO reduction activity tends
7
to decrease with an increase in the Mo composition x. This might be explained by the fact
Ϫ1
that the Ϫ⌬H °/O-atom of WO (281.0 kJ ⅐ mol O ) is larger than that of MoO (248.4 kJ ⅐
f
3
3
Ϫ1
mol O ). Provided that the reaction proceeds via a redox cycle of the catalyst and that oxide
ions bridging B and B ions (–B –O–B –) take part in the catalysis, the catalytic property
should be dependent on the redox properties of both B and B ions. Thus, when A and
B ions are fixed, the redox property of B ions controls the catalytic activity. Because the
Ϫ⌬H ° value of B O is more wide-ranging than that of B O , the catalytic activity of the
present catalyst systems presumably depends largely on B ions.
II
VI
II
VI
II
VI
II
II
VI
II
VI
f
3
II
References
[1] R.J.H. Voorhoeve, in: J.J. Burton, R.L. Garten (Eds.), Advanced Materials in Catalysis, Academic Press,
New York, 1977, pp. 129–180.
[
[
[
2] L.G. Tejuca, J.L.G. Fierro, J.M.D. Tasc o´ n, Adv Catal 36 (1989) 237.
3] N. Yamazoe, Y. Teraoka, Catal Today 8 (1990) 175.
4] L.G. Tejuca, J.L.G. Fierro, Properties and Applications of Perovskite-Type Oxides, Marcel Dekker, New
York, 1993.
[
[
[
5] M.T. Anderson, K.B. Greenwood, G.A. Taylor, K.P. Poeppelmeier, Prog Solid State Chem 22 (1993) 197.
6] G. Blasse, J Inorg Nucl Chem 27 (1965) 993.
7] R.J.H. Voorhoeve, L.E. Trimble, C.P. Khattak, Mater Res Bull 9 (1974) 655.

5
30
M.-D. Wei et al. / Materials Research Bulletin 35 (2000) 521–530
[8] Y. Teraoka, M.-D. Wei, S. Kagawa, J Mater Chem 8 (1998) 2323.
[9] M. Iwamoto, H. Yahiro, Y. Yu-U, S. Shudo, N. Mizuno, Shokubai (Catalyst) 32 (1990) 430.
[
[
[
10] W. Held, A. K o¨ nig, T. Richter, L. Puppe, SAE Paper 900496, 1990.
11] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 75th ed., CRC Press, Boca Raton, FL, 1994.
12] D.D. Wagman, W.H. Evans, V.B. Parker, I. Halow, S.M. Bailey, R.H. Schumm, NBS Technical Note
2
70–4, 1969.
[13] T. Seiyama, N. Yamazoe, M. Egashira, Proc Int Congr Catal 2 (1973) 997.
[14] M. Iwamoto, Y. Yoda, N. Yamazoe, T. Seiyama, J Phys Chem 82 (1978) 2564.
[15] M.-D. Wei, Y. Teraoka, S. Kagawa, unpublished data.