NO reduction by H2 over perovskite-like mixed oxides
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Transcript of NO reduction by H2 over perovskite-like mixed oxides
NO reduction by H2 over perovskite-like mixed oxides
Davide Ferria, Lucio Fornia,*, Mark A.P. Dekkersb, Ben E. Nieuwenhuysb
aDipartimento di Chimica Fisica ed Elettrochimica, UniversitaÁ di Milano, via C. Golgi 19, 20133 Milano, ItalybLeiden Institute of Chemistry, Gorlaeus Laboratoria, Leiden University, PO Box 9502, 2300 RA Leiden, Netherlands
Received 12 July 1997; received in revised form 15 October 1997; accepted 19 October 1997
Abstract
Perovskite-like mixed oxides La0:9A0:1A0BO3�d (A0�Ce or Eu; B�Mn or Co) and La0.8Sr0.2BO3�� (B�Mn, Fe, Co or Ni)
prepared by the amorphous citrate method were used as catalysts for NO reduction by hydrogen. XRD patterns showed a fully
crystalline perovskitic structure only in the case of Ce- and Eu-substituted samples. The results suggest that the presence of
structural defects is important for the activity of these catalysts, as shown by pretreatment under different atmospheres (He and
O2). La0.9Ce0.1CoO3�� was the most active of the mixed oxide catalysts investigated and its activity was in¯uenced by the
presence of both anion vacancies and Co2� species. La0.8Sr0.2NiO3�� showed a particular catalytic behaviour, attributed to
surface Ni reduction. # 1998 Elsevier Science B.V.
Keywords: Perovskites; NO reduction; Defect structure
1. Introduction
The methods for eliminating NOx from combustion
exhaust gases may be grouped into two classes [1]: (i)
lowering or preventing NOx formation during com-
bustion, e.g., by decreasing the ¯ame temperature by
using a proper catalyst; (ii) selectively reducing NOx
by reaction with NH3 or another reducing gas. CO and
light hydrocarbons [2,3] have been proposed for this
purpose. The reduction by hydrogen can also be
considered, when this gas is already present in the
exhaust gas [4].
Perovskite-like mixed oxides have been extensively
studied for NOx reduction by CO [5±10], but only
scarce data are available for NOx reduction by hydro-
gen [8,11]. In the present work some perovskites with
general formula AA0BO3�� (hereafter referred to as
AA0BO3 for brevity) have been prepared and tested as
catalysts for this process. La0:9A00:1BO3 (A0�Ce or Eu;
B�Mn or Co) have been used for a feed consisting of a
gas mixture with a NO/H2 ratio of unity, diluted in He.
The same catalysts, together with La0.8Sr0.2BO3
(B�Mn, Fe, Co or Ni), have been tested for both 1/
1 and 1/3 NO/H2 feed ratios, in order to investigate the
in¯uence of excess hydrogen on the activity. The aim
of the work was to study the behaviour of different
perovskite-like mixed oxides, looking for correlations
between catalyst structure and activity. The in¯uence
of substituting Ce or Eu cations for La and of changing
the B ion has been investigated. Particularly, the
attention was focused on the La±Ce±Co system and
on La0.8Sr0.2NiO3, since Ni-containing perovskite-like
Applied Catalysis B: Environmental 16 (1998) 339±345
*Corresponding author. Tel.: +39 2 26603289; fax: +39 2
70638129; e-mail: [email protected]
0926-3373/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.
P I I S 0 9 2 6 - 3 3 7 3 ( 9 7 ) 0 0 0 9 0 - 8
catalysts showed activity for some environmentally
interesting reactions [12±15] including, e.g., NO
decomposition [16].
2. Experimental
2.1. Materials
Metal nitrates and citric acid were purchased
from Acros and Merck. The following gases were
used for catalyst testing: He (purity (�99.999 vol%),
4.12 vol% NO in He, 4.18 vol% H2 in He (Air
Products), 10 vol% N2 in He (Hoek Loos) and
3.97 vol% O2 in He (Aga). For H2-TPR experiments
a gas mixture of 8 vol% H2 in N2 (SIAD) was
used.
2.2. Catalysts preparation
Catalysts were prepared by the citrate method [17].
Aqueous solutions of the nitrates of each metal com-
ponent were mixed with an aqueous solution of citric
acid, in such a way that the solution contained a 1:1
molar ratio of the total amount of precursor nitrates
and citric acid. The resulting solution was evaporated
in a rotary evaporator and then dried in a vacuum oven
overnight at 708C. The spongy material thus obtained
was crushed to ®ne powder and calcined for 2 h in
¯owing air at the temperature determined by thermo-
gravimetric analysis (TGA).
2.3. Catalysts characterisation
Catalysts precursors (10 mg) were analysed by a
Mettler Toledo TA 8000 TGA instrument with heating
rate of 108C/min from room temperature up to
10008C, in order to determine the minimum tempera-
ture for the phase transition to the perovskite-like
structure. After calcination at this temperature the
catalysts were analysed by XRD to examine the
formation of the perovskite-like crystalline structure
and the possible presence of other phases. XRD
patterns were obtained by means of a Siemens D-
500 diffractometer using Cu Ka (��1.5148 AÊ ) radia-
tion, Ni-®ltered, for all samples, except for
La0.9Ce0.1MnO3, for which Co Ka (��1.79026 AÊ )
was used. The patterns were then compared with the
JCPDS database [18] reference patterns. The BET
surface areas were determined by a Carlo Erba Sorp-
tomatic-1800 instrument.
2.4. H2-TPR experiments
The catalysts (ca. 50 mg samples) of the
La0.8Sr0.2BO3 series were analysed by temperature-
programmed reduction (TPR) with an 8 vol% H2 in N2
¯owing gas mixture, by means of the apparatus
described in detail elsewhere [19]. A temperature
ramp of 58C/min was used, from 508C up to 6008C.
H2 consumption was monitored by means of a
hot-wire-detector (HWD). Samples such as La2O3,
Ni2O3 and commercial SrCO3 (from Ciba) were
also tested as references, in order to investigate the
in¯uence of secondary phases on the reduction process
and on the TPR peak shape. H2-TPR runs were also
performed on the same catalysts to identify the various
reduction steps by monitoring the proper m/z signals
by means of a UTI 100C quadrupole mass spectro-
meter (MS).
2.5. Catalytic activity
NO reduction by H2 was carried out in a Pyrex ®xed
bed reactor (i.d.�10 mm) using 200 mg catalyst. Gas
¯ow rates were controlled by mass ¯ow meters/con-
trollers (Bronkhorst Hi-Tech BV). Temperature inside
the catalytic bed was monitored by a thermocouple
inserted in a thin Pyrex coaxial thermowell and con-
nected to a temperature controller (Shimaden SR52).
Analysis of the ef¯uent gas was performed by a gas
chromatograph (Chrompack), equipped with a column
packed with 5 AÊ molsieve. Prior to every run, the
catalyst was preheated in situ in a He stream (ca.
20 N cm3/min) for 1 h to remove adsorbed oxygen.
The total ¯ow of reactants was always kept at ca.
40 N cm3/min and the NO/H2 molar ratio was set at
either 1/1 or 1/3. The temperature range was set from
1508C to 3508C, ramping at 38C/min. Hysteresis
phenomena were investigated on the La0.8Sr0.2BO3
catalysts by monitoring the catalytic activity while the
sample was heated up to 3508C and then cooled down
to 1508C at the same rate.
Some runs were also carried out on the
La0.9Ce0.1BO3 (B�Mn and Co) samples after activa-
tion in an oxygen-rich atmosphere.
340 D. Ferri et al. / Applied Catalysis B: Environmental 16 (1998) 339±345
3. Results and discussion
3.1. Characterisation
The main characteristics of the catalysts employed
are listed in Table 1. All the samples showed the
perovskite-like structure (Fig. 1). However, the Sr
substituted catalysts showed also the presence of
SrCO3 in small amounts (Fig. 1(a)). Besides SrCO3,
very tiny amounts of other Ni-containing phases such
as NiO and La2NiO4 or LaNiO3 were found in
La0.8Sr0.2NiO3 (Fig. 1(b)), as has also been observed
by Zhao et al. [15] using a similar preparation. Ce- and
Eu-manganites (see Fig. 1(c)) and Eu-cobaltate
(Fig. 1(d)) were well-crystallised perovskites. CeO2
was found in La0.9Ce0.1CoO3 though in almost unde-
tectable amount. In previous works [20,21] it was
reported that the mutual solubility of CeO2 and
LaCoO3 lies within the 0.03±0.05 range for x; x being
the index of substitution degree in La1ÿxCexCoO3.
Tabata et al. [21] and Nitadori and Misono [22]
prepared this catalyst at 8508C and obtained a higher
segregation of CeO2. Therefore, the lower calcination
temperature for our catalyst (5808C) seems to lead to a
Ce-richer perovskite phase. The same result was
obtained with the other Ce- and Eu-containing cata-
lysts, for which 7408C was the highest calcination
temperature (see Table 1).
It is well known that substitution of a bivalent or a
tetravalent metal cation for La brings about a mod-
i®cation in the oxidation state of the B-site metal
cation. In particular, it has been shown that the intro-
duction of Sr in the perovskitic cell is accompanied by
the oxidation of the B cation, due to charge compen-
sation [23]. Furthermore, the stability of this oxidation
state can be different, according to the nature of B
metal. In fact Mn4� is more stable than Co4� and of
Fe4� and Ni3�, which are supposed to be formed in the
La0.8Sr0.2BO3 series. Instability in this case means
easy reducibility and thus formation of structural
defects, such as oxygen vacancies. These anion
defects are created in the cobaltate, while in the
manganite Mn3� oxidation is preferential with respect
to oxygen release [23]. Ferrates [24] and Ni-based
perovskites [15] behave in an intermediate way. In the
former system, it has been observed that a low Sr
content (as for x�0.2) leads to high concentration of
oxygen vacancies and low concentration of Fe4�. For
the latter catalyst, the substitution of Sr2� for La3� is
not accompanied by oxidation of Ni to Ni4� and
formation of anion vacancies occurs when introducing
a bivalent metal cation. In the manganites cation
vacancies, i.e. oxygen excess, are easier to form,
though their amount decreases with increase in the
substitution degree of Sr for La, ending into oxygen
vacancies only for x>0.4 [23]. The role played by Eu is
expected to be the same as that of Sr, since Eu can
assume the Eu2� valency state rather easily.
On the other hand, the case of Ce is different. As
reported by Nitadori and Misono [22], only after
formation of CeO2 the effect of Sr or Ce substitution
for La appears the same, because of formation of
Table 1
Main characteristics of the catalysts employed
Catalyst Tc (8C)a Phase(s)b SA (m2/g)c
La0.9Eu0.1MnO3 740 P 26
La0.9Ce0.1MnO3 740 P 32
La0.9Eu0.1CoO3 600 P 16
La0.9Ce0.1CoO3 580 P (�CeO2) 22
La0.8Sr0.2MnO3 740 P (�SrCO3) 45
La0.8Sr0.2FeO3 680 P (�SrCO3) 21
La0.8Sr0.2CoO3 580 P (�SrCO3) 19
La0.8Sr0.2NiO3 800 P (�SrCO3�NiOxd) 5
aCalcination temperature.bP�perovskite. The phases present in minor amounts are given in
parentheses.cBET surface area.dNix�various Ni±O or La±Ni±O phases (see text).
Fig. 1. XRD patterns of the catalysts: (a) La0.8Sr0.2MnO3; (b)
La0.8Sr0.2NiO3; (c) La0.8Sr0.2CoO3; and (d) La0.9Eu0.1CoO3. (5)
Perovskitic phase.
D. Ferri et al. / Applied Catalysis B: Environmental 16 (1998) 339±345 341
defect structures. As mentioned, CeO2 was hardly
detected in our samples. We can suppose that if some
of the added Ce ®lls the perovskite, a
La1ÿxCexÿy�yCoO3 structure, where 0�y<0.1 and �represents a cation vacancy, is created. Furthermore,
anion vacancies could also be created. In fact, the
formation of cation vacancies, owing to the low
amount of Ce4� present, is accompanied by cobalt
oxidation. Unstable Co4� ions can easily reduce
releasing oxygen, as in a Sr-substituted catalyst, thus
creating anion vacancies. Moreover, even a low con-
centration of Ce4� substituting La in the structure
results in cobalt reduction to Co2�, whereas Co3�
being the Co oxidation state in the unsubstituted
LaCoO3. Introduction of Ce4� in the manganite
should lead to the same effect, but to our knowledge
the presence of Mn2� has never been reported,
because CeO2 segregation always occurred.
More complex is the case of La0.8Sr0.2NiO3, where
anion vacancies might be created when substituting Sr
for La. Segregation of SrCO3 and of other Ni-contain-
ing phase (particularly NiO) may cause the formation
of different defects. The segregation of the Sr-contain-
ing phase can be attributed to the de®ciency of Ni in
the ®nal perovskite structure, since cation vacancies
are usually not observed in such materials [25]. Also
the formation of La±Ni±O species is probably due to
this phenomenon, because of charge compensation
within the ®nal structure of the mixed oxide.
3.2. Catalytic activity
Results are shown in Figs. 2 and 3 for a NO/H2 ratio
of 1/1 or 1/3, respectively. The order of activity for the
various samples is almost the same for both ratios. The
excess of hydrogen in the 1/3 ratio leads for all
catalysts to higher conversion. For the NO/H2�1/3
ratio, La0.9Ce0.1CoO3 and La0.8Sr0.2CoO3 showed
complete conversion of nitric oxide already at ca.
3008C. La0.9Ce0.1CoO3 appeared to be the most active
catalyst, but with a low selectivity to nitrogen. In
general, cobaltates appeared to be more active than
the other perovskites, especially compared to Mn-
based ones. In particular, Ce-containing catalysts were
more active than Eu-containing ones. The order of
activity for the La0.8Sr0.2BO3 perovskites was found to
be Co>Ni>Mn>Fe.
Voorhoeve et al. [11] proposed that the mechanism
of NO reduction with H2 includes reduction of the
catalyst, followed by NO adsorption, which is
favoured by the lower oxidation state of the B-site
metal cation. This involves the catalyst in a redox
cycle exploiting its defect structure. The higher activ-
ity found for cobaltates could then be attributed to the
presence of oxygen vacancies suitable for NO adsorp-
tion. However, the oxidation state of the B ion alsoFig. 2. Catalytic behaviour for NO/H2�1/1. La0:9A00:1BO3 (A0�Ce,
Eu, B�Mn, Co).
Fig. 3. Catalytic behaviour for NO/H2�1/3. (a) La0:9A00:1BO3
(A0�Ce, Eu; B�Mn, Co); (b) La0.8Sr0.2BO3 (B�Mn, Fe, Co, Ni)
and La0:9A00:1BO3 (A0�Ce, Eu; B�Mn, Co).
342 D. Ferri et al. / Applied Catalysis B: Environmental 16 (1998) 339±345
seems to be important. Indeed, in the experiments after
activation under different atmospheres a different
activity was shown by La0.9Ce0.1CoO3 in the low
temperature range. The catalyst performed better after
pretreatment in He, while at high temperature the
catalyst was slightly more active after pretreatment
in O2. With La0.9Ce0.1MnO3 the behaviour was the
opposite. This can be attributed to the different nature
of structural defects in these two types of perovskites.
The activation in oxygen produces an oxygen-rich
catalyst, because of the presence of anion vacancies.
Hence, higher activity at higher temperature, with
respect to pretreatment in He, can be due to oxygen
desorption, favouring NO oxidation to surface NO2, so
explaining the lower yield to N2. Moreover, the for-
mation of a lower oxidation state (Co2�), accompa-
nied by an increase in concentration of anionic
defects, as mentioned above, are needed in the former
catalyst to achieve a better adsorption and thus a faster
N±O dissociation.
In the La0.8Sr0.2BO3 series the cobaltate showed the
highest activity. In this case the difference in activity is
probably due to the different stability of the cation
formed by the partial substitution of a bivalent cation
such as Sr2� for La3�. Mn4� is more stable in the
perovskitic structure due to Jahn±Teller distortion
[26]. On the other hand, it has been shown [23] that
cation vacancies are mainly produced in the manganite
after substitution. Co4� is less stable than Mn4�, thus
providing a structure with more oxygen vacancies. NO
adsorption is certainly less favoured in the presence of
cationic vacancies than in the presence of anionic
ones. Reducibility is also much easier for the cobaltate
than for the manganite [24]. Reduction of the B-site
metal cation is possible because of the presence of
both gaseous H2 and of a rather unstable B4� cation.
The latter phenomenon is strongly connected with
oxygen release [27] and thus with the formation of
oxygen vacancies. Oxygen release is less marked for
the manganite [28] than for the cobaltate and Mn4� is
more stable than Mn3�. Hence, the higher activity of
La0.8Sr0.2CoO3 with respect to La0.8Sr0.2MnO3 is
easily explained.
From the nitrogen mass balance it can be deduced
that in addition to N2, N2O or NH3 must be formed
when using La0.9Ce0.1CoO3, La0.9Eu0.1CoO3,
La0.8Sr0.2CoO3 and La0.8Sr0.2NiO3 as catalysts. This
means that Mn- and Fe-based catalysts are more
selective towards N2 than the corresponding Co-
and Ni-based perovskites.
Using La0.8Sr0.2NiO3 and La0.8Sr0.2MnO3, a
shoulder in the kinetic curve appeared at 2808C and
3208C, respectively (Fig. 3). This phenomenon is
probably related to a change in the reaction mechan-
ism. Ni-based perovskites showed a similar behaviour
also for NO decomposition [15]. From Fig. 4(b) some
additional information may be drawn about the
mechanism of NO reduction by H2. An hysteresis
was observed when carrying out the reaction during
cooling from 3508C. Probably, after heating the cat-
alyst in the NO/H2 mixture, the reduced surface thus
created is more active, due to the presence of active
sites such as Ni2� (Ni3� being the average oxidation
state in the perovskite). An alternative explanation
could be the presence of a higher concentration of
surface defects on the reduced catalyst, favouring an
easier NO adsorption and dissociation. This would
con®rm the mechanism suggested by Voorhoeve et al.
[11]. H2-TPR experiments on La0.8Sr0.2BO3 can be
Fig. 4. Behaviour of La0.8Sr0.2NiO3 for NO/H2�1/3. (a) NO
conversion and N2 yield; and (b) hysteresis cycle. In (b) the
experiment was performed by increasing (&) and decreasing (&)
temperature with the same ramp.
D. Ferri et al. / Applied Catalysis B: Environmental 16 (1998) 339±345 343
used to check the status of the surface in order to
understand the reaction mechanism occurring on the
catalyst. Fig. 5 shows two reduction peaks for B�Co
or Ni, while for B�Mn or Fe the reduction process
seems more complex. Surface reduction is very slow
below 1508C. It starts to increase around 2508C in the
case of Ni. For the other catalysts it starts at higher
temperatures and in the case of Fe and Mn it appears
less pronounced. The H2 consumption peak below
4008C is splitted into two components and it can be
attributed to the Ni3�!Ni2� step. Recently, Fierro et
al. [29] attributed this splitting to a reduction process
controlled by H2 and/or H2O mass transport. In
La0.8Sr0.2CoO3 the ®rst peak has been assigned [30]
to a reduction step from Co3� to Co2� and the second
one from Co2� to metallic cobalt. According to Fig. 5,
Ni3� seems less stable than Co3�. This indicates that
the Ni-containing catalyst is more active. However,
XRD analysis showed that the concentration of Ni in
the structure is lower than theoretically expected, due
to segregation of other Ni-containing phases. Less
Ni2� sites during surface reduction by hydrogen will
then be created.
The shoulder in Fig. 4(a) could be attributed to
the formation of N2O as an intermediate. At low
temperature the concentration of active sites may
be low, thus leading preferentially to N2O and
N2 after NO adsorption. Among the following reac-
tions
NO�&! NOA (1)
NOA � NO! N2O� OA (2)
2NOA ! 2NA � O2 (3)
NOA � NA ! N2O�& (4)
2NOA ! N2 � 2OA (5)
2OA ! O2 � 2& (6)
where & represents an oxygen vacancy and the sub-
script A indicates adsorbed species; reactions (2) and
(4) are favoured at low temperature, while reaction (5)
is favoured at higher temperature, where active sites
are abundant and the concentration of NOA becomes
lower. This hypothesis can also include the presence of
different catalytic sites at the surface.
Zhao et al. [15] reported that the increase in NO
conversion could also be due to the reaction between
NO and the O2 released by the catalyst. In our catalyst
O2 uptake takes place below 3008C, as previously
shown [31] by O2-TPD experiments. However, Co-
and Fe-based perovskites released oxygen at almost
the same temperature, without showing this shoulder
in NO conversion. This fact con®rms a different
mechanism occurring at the surface of the Ni-contain-
ing catalyst.
4. Conclusions
The treatment in oxygen-rich or in inert atmosphere
showed that different defect structures are present in
manganites, with respect to cobaltates. In particular, it
has been possible to correlate this structure with the
NO adsorption behaviour and hence with the catalytic
activity for NO reduction by H2. NO adsorption is less
favoured on cationic than on anionic vacancies, thus
accounting for the lower activity of the manganites.
The in¯uence on NO reduction activity of substitution
at A-site with a bivalent (Eu) or tetravalent (Ce) metal
cation has been shown. This leads to an increase in
activity for cobaltates, when Ce is substituted for La,
and for manganites when Eu is introduced. This
behaviour may be attributed to the formation of
different kinds of defects.
In the La0.8Sr0.2BO3 series, the stability of B4�
ionic species seems the key factor for the order of
activity of the catalysts. A reaction mechanism of NO
reduction by H2 over La0.8Sr0.2NiO3 has also been
proposed, based on surface reduction by H2, followed
by NO adsorption, taking into account the hysteresis
phenomenon observed with such a catalyst.
Fig. 5. H2-TPR experiments on La0.8Sr0.2BO3 (B�Mn, Fe, Co,
Ni). H2±He gas flow rate: 15 N cm3/min.
344 D. Ferri et al. / Applied Catalysis B: Environmental 16 (1998) 339±345
References
[1] Z.R. Ismagilov, M.A. Kerzhentsev, Catal. Rev. -Sci. Eng. 32
(1990) 51.
[2] J.N. Armor, Catal. Today 22 (1995) 97.
[3] T. Tabata, M. Kokitsu, H. Ohtsuka, O. Okada, L.M.F.
Sabatino, G. Bellussi, Catal. Today 27 (1996) 91.
[4] K. Taylor, Catal. Rev. -Sci. Eng. 35(4) (1993) 457.
[5] N. Mizuno, M. Yamato, M. Tanaka, M. Misono, Chem. Mater.
1 (1989) 232.
[6] M.W. Chien, I.M. Pearson, K. Nobe, Ind. Eng. Chem. Prod.
Res. Dev. 14 (1975) 131.
[7] N. Mizuno, M. Tanaka, M. Misono, J. Chem. Soc. Faraday
Trans. 88 (1992) 91.
[8] A. Lindsteld, D. StroÈmberg, M. Abul Milh, Appl. Catal. 116
(1994) 109.
[9] Y. Teraoka, H. Nii, S. Kagawa, K. Jansson, M. Nygren, Chem.
Lett. (1996) 323.
[10] L. Simonot, F. Garin, G. Maire, Appl. Catal. B 11 (1997) 181.
[11] R.J.H. Voorhoeve, J.P. Remeika, L.E. Trimble, A.S. Cooper,
F.J. Disalvo, P.K. Gallagher, J. Sol. State Chem. 14 (1975)
395.
[12] A.K. Ladavos, P.J. Pomonis, J. Chem. Soc. Faraday Trans.
87(19) (1991) 3291.
[13] Z. Yu, L. Gao, S. Yuan, Y. Wu, J. Chem. Soc. Faraday Trans.
88(21) (1992) 3245.
[14] J.L.G. Fierro, J.M.D. TascoÂn, L.G. Tejuca, J. Catal. 93 (1985) 83.
[15] Z. Zhao, X. Yang, Y. Wu, Appl. Catal. B 8(3) (1996) 281.
[16] Y. Teraoka, H. Fukuda, S. Kagawa, Chem. Lett. (1990) 1.
[17] M.S.G. Baythoun, F.R. Sale, J. Mater. Sci. 17 (1982) 2757.
[18] Selected Powder Diffraction Data, Miner. DBM (1±40),
JCPDS, Swarthmore, PA, 1974±1992.
[19] L. Forni, E. Magni, E. Ortoleva, R. Monaci, V. Solinas, J.
Catal. 112 (1988) 444.
[20] L. Forni, C. Oliva, F.P. Vatti, M.A. Kandala, A.M. Ezerets,
A.V. Vishniakov, Appl. Catal. B 7 (1996) 269.
[21] K. Tabata, I. Matsumoto, S. Kohiki, M. Misono, J. Mater. Sci.
22 (1987) 4031.
[22] T. Nitadori, M. Misono, J. Catal. 93 (1985) 459.
[23] L.G. Tejuca, J.L.G. Fierro, J.M.D. TascoÂn, Adv. Catal. 36
(1989) 237.
[24] Y. Wu, T. Yu, B.S. Dou, C.X. Wang, X.F. Xie, Z.L. Yu, S.R.
Fan, Z.R. Fan, L.C. Wang, J. Catal. 120 (1989) 88.
[25] J.M. Thomas, W.J. Thomas, Principles and Practice of
Heterogeneous Catalysis, VCH, Weinheim, 1997.
[26] N. Yamazoe, Y. Teraoka, Catal. Today 8 (1990) 175.
[27] N. Yamazoe, Y. Teraoka, T. Seiyama, Chem. Lett. (1981)
1767.
[28] L. Marchetti, L. Forni, Appl. Catal. B 15 (1998) 179.
[29] R. Lago, G. Bini, M.A. PenÄa, J.L.G. Fierro, J. Catal. 167
(1997) 198.
[30] M. Futai, C. Yonghua, X. Louhui, React. Kinet. Catal. Lett.
31(1) (1986) 47.
[31] D. Ferri, L. Forni, Appl. Catal. B 16 (1998) 119.
D. Ferri et al. / Applied Catalysis B: Environmental 16 (1998) 339±345 345