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: forn@rs6.csrsrc.mi.cnr.it

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

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