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CeO2–ZrO2 composite catalysts for methane combustion: Correlation between morphology reduction properties and catalytic activity
Catalysis Communications 6 (2005) 329–336 www.elsevier.com/locate/catcom
Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite catalysts for methane combustion: Correlation between morphology reduction properties and catalytic activity L.F. Liotta b
a,* ,
G. Di Carlo b, G. Pantaleo b, G. Deganello
a,b
a ISMN-CNR via Ugo La Malfa, Sezione Di Palermo, 153, 90146 Palermo, Italy Dipartimento di Chimica Inorganica e Analitica ‘‘Stanislao Cannizzaro’’, Universita` di Palermo, Viale delle Scienze, Parco dÕOrleans II - 90128 Palermo, Italy
Received 29 July 2004; accepted 5 February 2005 Available online 17 March 2005
Abstract Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite catalysts have been prepared by two different techniques, co-precipitation by citrate method and impregnation with cobalt nitrate of pre-formed ceria and ceria–zirconia oxides. The materials, as prepared and after ageing at 750 °C 7 h, were tested for methane combustion and the catalytic performances were compared with those of a commercial Co3O4, used as reference. A significant improvement of the activity was observed in the composite oxide Co3O4(30 wt%)/ CeO2(70 wt%), prepared by citrate method, which exhibits the lowest light-off temperature of methane (T50 = 400 °C) and does not suffer deactivation after calcination at 750 °C 7 h. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Co3O4–ceria/ceria–zirconia composites; Methane combustion; Morphological and reduction properties
1. Introduction The virtues of catalytic combustion are well-known. Indeed, due to thermodynamic reasons, the catalytic flameless combustion of hydrocarbons can reduce thermal NOx emissions. In this respect, many studies on the development of new gas turbine combustors in which lean combustion is coupled with catalytic combustion have been performed in the recent years [1]. Catalytic combustion is also effective in the abatement of unburned hydrocarbons from natural gas fueled vehicles (NGVs). Methane, the main component of natural gas, is a potent greenhouse gas, therefore emissions
*
Corresponding author. Tel.: +39 091 6809371; fax: +39 091 6809399. E-mail address: [email protected] (L.F. Liotta). 1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.02.006
from NGVs must be reduced by catalytic after-treatment of exhaust gases [2,3]. Nobel metal based catalysts are well-known to be active for methane oxidation at low temperature [4], but they are expensive and easily sinterise. Among the possible substitutes for noble metals, in the last years attention has been paid to the activity of Cu-doped CeO2–ZrO2 solid solutions, Mn-doped ZrO2 [5,6]. It is also known that perovskite-type transition metal oxides and Co3O4 exhibit good catalytic activity for CO and methane combustion [7,8]. Based on our most recent results on the reduction properties of two classes of defective materials, cobalt oxide [9] and ceria–zirconia solid solutions [10,11], we report the methane combustion over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides. A correlation between morphological and reduction properties with the catalytic activity is suggested.
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2. Experimental All the used reagents from Aldrich were ACS chemicals. Two composite oxides Co3O4(2, 30 wt%)/CeO2, labeled as Co2Cecopr and Co30Cecopr, were prepared by co-precipitation method in presence of citrate at pH of 9 [12]. The resulting materials were calcined at 650 °C for 5 h. Three additional samples were synthesized by impregnating with cobalt nitrate the supports, a commercial ceria (CeO2Aldrich) with relatively high surface area (79 m2/g) and a ceria–zirconia oxide with nominal composition Ce0.6Zr0.4O2 obtained by sol–gel method (CeZrsol–gel) [10] having lower surface area (21 m2/g). The loading values are Co3O4(2 wt%) over ceria–zirconia (Co2CeZrimpr) and Co3O4(2, 30 wt%) over ceria Aldrich (Co2Ceimpr, Co30Ceimpr). These samples were calcined at 500 °C for 5 h. For comparison reason, a ceria oxide with low surface area was prepared by precipitation method [8] (CeO2prec) and calcined at 650 °C for 5 h. Our reference material was a commercial Co3O4 (from Aldrich) with surface area of 5 m2/g. The samples were characterized by X-ray diffraction measurements, carried out with a Philips (PW 1820) vertical goniometer using Ni-filtered Cu Ka radiation ˚ ). The spectra were collected using a step (k = 1.5418 A size of 0.05° and a counting time of 5 s per angular abscissa. The assignment of the crystalline phases was based on the ICSD data base (Co3O4, n° 28,158; CeO2, n° 28,785) [13]. Mean crystallite size (d) was estimated from the line broadening of the most intense reflections using the Scherrer equation [14]. In particular, for Co3O4 spinel the broadening of the (3 1 1) line in the diffraction pattern was considered, whereas for CeO2 fluorite the mean particle size was estimated from the (1 1 1) line. Specific surface area values have been measured by BET method. Temperature programmed reduction (TPR) experiments were carried out with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector (TCD), by using experimental conditions described elsewhere [9]. The above-mentioned oxides, as prepared (fresh) and after ageing at 750 °C 7 h (aged) were tested for methane combustion in a flow system equipped with a temperature programmer controller. A thermocouple in contact with the catalytic bed allowed the control of the temperature inside the catalyst. The reagent gas mixture consisted of 0.3% of CH4 + 4.8% of O2 in He and the weight hourly space velocity (WHSV) was typically 12,000 mL g1 h1. Over the most promising catalyst, as fresh and aged sample, catalytic tests at a space velocity of 60,000 mL g1 h1were also carried out. The inlet and outlet gas compositions TM were monitored by a mass quadrupole (Thermostar ,
Balzers) and an IR analyser (ABB Uras 14). The only reaction products were CO2 and H2O.
3. Results and discussion Fig. 1(a) shows the light-off curves of methane oxidation over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides along with Co3O4 and the supports only. As indicated by the lowering of the temperature at which the reaction starts (see Fig. 1(a)), Co30Ceimpr and Co30Cecopr composite oxides behave better than bulk Co3O4, the latter exhibiting the same catalytic activity of Co2Ceimpr and Co2CeZrimpr systems. Co2Cecopr appears the worst cobalt catalyst. The activities of pure supports, CeO2Aldrich, CeO2prec and CeZrsol–gel are lower than those of the corresponding cobalt catalysts and decrease in the same order as the surface areas (Fig. 1(a)). One of the major sources of catalyst deactivation, especially in the automobile catalytic converters, occurs following high-temperature exposure. Generally, thermal effects cause crystal growth and loss of catalytic surface. Depending on the nature of active components, in particular oxides, their thermo-chemical stability must be also considered. At the operating window of 700– 750 °C Co3O4 is at the limit of thermal stability and although its known good activity in methane combustion [7], this drawback cannot be neglected for practical applications. An attempt to evaluate the thermal resistance of our systems was done ageing the catalysts in air at 750 °C for 7 h. Light-off curves for methane oxidation over aged catalysts are displayed in Fig. 1(b). A pronounced deactivation (T50 = 470 °C, see Table 1) has been observed for the Co30Ceimpr composite oxide. Over the aged samples Co3O4, Co2Ceimpr, Co2CeZrimpr and the corresponding supports the 50% of methane conversion was attained at T > 500 °C (Table 1). On the contrary, very promising appears the catalytic behavior Co30Cecopr which exhibits almost stable lightoff temperature (410 °C) after calcination at 750 °C. As expected, at increased space velocity (60,000 mL g1 h1) the temperatures for 50% conversion of methane increase from 400 to 465 °C for the fresh Co30Cecopr and from 410 to 490 °C in the case of the aged one (see Table 1). Taking into account the relatively low surface area of the Co30Cecopr sample (Table 2), these results can be considered promising. In Table 2 morphological parameters of the abovementioned oxides, fresh and aged samples are listed. In Fig. 2(a) the XRD patterns of Co3O4, Co30Cecopr and Co30Ceimpr fresh and aged samples are displayed. In Fig. 2(b) a magnification in the angular range 25–50 2h of the patterns of Co30Cecopr fresh and aged samples is reported. Well-defined features of Co3O4 and CeO2 crystalline phases (ICSD, Co3O4, n° 28158; CeO2, n° 28785)
L.F. Liotta et al. / Catalysis Communications 6 (2005) 329–336 100
331
Co3O4 Co30Ce copr Co30Ce impr Co2Ce copr Co2Ce impr Co2CeZr impr CeO2 CeO2 sol-gel CeZrsol-gel
80
60
40
CH4 conversion (%)
20
(a) fresh samples 0 200
300
400
500
600
700
100
80
60
40
20
(b) aged samples 0 200
300
400
500
600
700
T (˚C) Fig. 1. Light-off of CH4 oxidation over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides, as fresh (a) and aged samples (b). The reaction gas mixture consisted of 0.3% of CH4 + 4.8% of O2 in He at WHSV of 12,000 mL g1 h1. Table 1 Temperatures of 50% of CH4 conversion over fresh and aged (750 °C, 7 h) samples at WHSV = 12,000 mL g1 h1
Table 2 Specific surface area and mean particle size (d) of the different phases in fresh and aged (750 °C, 7 h) samples
Catalyst
T50 (°C) (fresh sample)
T50 (°C) (aged sample)
Sample (fresh, aged)
Surface areaa (m2/g)
d Co3 O4 b (nm)
d CeO2 b/d ðCex Zr1x O2 Þ c (nm)
Co3O4 Co30Cecopr
445 400 465a 390 511 445 445 495 636 546
505 410 490a 470 n.d. 512 515 531 n.d. 571
Co3O4 Co30Cecopr Co30Ceimpr Co2Cecopr Co2Ceimpr Co2CeZrimpr CeO2Aldrich CeO2prec CeZrsol–gel
5 (3.9) 28 (20) 30 (15) 23 (15) 67 (39) 19 (16) 79 (39) 20 (3) 21 (16)
116 (171) 15 (29) 32 (56) n.d. n.d. n.d. – – –
– 13 (28) 24 (45) 19 (36) 23 (29) n.d. 22 (29) 30 (45) n.d.
Co30Ceimpr Co2Cecopr Co2Ceimpr Co2CeZrimpr CeO2Aldrich CeO2prec CeZrsol–gel a
WHSV = 60,000 mL g1 h1.
are evident and no definite phase between cobalt and ceria has been noticed, in accord with the literature [8]. No variation of the CeO2 and Co3O4 reflection lines was ob-
a
Determined by BET method. Calculated by using Scherrer equation. c At least two solid solutions CexZr1 xO2, a cubic CeO2-rich and a tetragonal ZrO2 rich phases, were detected in the pattern XRD (see Fig. 3) [10]. b
served by comparing the XRD patterns of Co30Cecopr fresh and aged, neither decomposition of Co3O4 into the CoO phase was detected. Upon ageing at 750 °C only
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(a)
(b)
CeO2
CeO2 Co3O4
Intensity (a.u.)
f e
Co3O4
d
d
c b c
a 30
40
50
60
70
80
25
30
35
40
45
50
2θ
2θ
Fig. 2. (a) XRD patterns of fresh and aged Co3O4 (a,b), Co30Cecopr (c,d) and Co30Ceimpr (e,f) catalysts; (b) magnification of the patterns (c,d) in the angular range 25–50 2h.
sintering of the crystallites occurs as it is evident by the sharpness of the peaks (see Figs. 2(a), (b)). In Fig. 3 the XRD patterns of two composite oxides with low cobalt loading (Co2Ceimpr and Co2CeZrimprfresh and aged) are displayed. No features of Co3O4 are visible in the patterns (a–d), only reflections of CexZr1 xO2 and CeO2 phases can be observed. In particular, the patterns (a, b) related to the sample CoCeZrimpr are characterized by broad and slightly asymmetrical peaks. According to our previous results [10], the presence of at least two solid solutions must be considered. In the patterns (c, d) (Figs. 3) related to the catalyst CoCeimpr only reflections of CeO2 particles were detected, which sinter upon ageing, as indicated by the narrowing of the peaks.
Therefore, the major effect of ageing treatment at 750 °C for 7 h is a loss of surface area and a crystal growth, which extent depends on the nature of the samples (Table 2, Figs. 2 and 3). In particular, it should be noted that both samples Co30Cecopr and Co30Ceimpr suffer a drop of surface area of 30% and 50%, respectively, and the crystallite sizes double. However, after ageing at 750 °C Co30Cecopr shows mean particle size (d) comparable to that of fresh Co30Ceimpr (Table 2). On this basis it could be inferred that finely dispersed Co3O4/CeO2 species mainly contribute to the methane oxidation activity. Therefore, we suggest that an important requisite for achieving good oxidation activity is an intimate contact between the species cobalt–ceria obtained via an appropriate preparation method.
Intensity (a.u.)
CeO2
d c
CexZr1-xO2
b a 25
30
35
40
45
50
55
2θ Fig. 3. XRD patterns of fresh and aged Co2CeZrimpr (a,b) and Co2Ceimpr (c,d).
L.F. Liotta et al. / Catalysis Communications 6 (2005) 329–336
Taking into account the literature [7] the areal (per square meter) specific rates of methane combustion were calculated, always using low conversion values (below 10%) in order to satisfy the conditions of a differential reactor. In Figs. 4(a), (b) the calculated reaction rates (lmol/s1 m2) are plotted against the temperature for fresh and aged samples, respectively. From the inspection of Fig. 4(a) the inherent high activities of Co3O4 and both composite oxides Co30Ceimpr and Co30Cecopr clearly emerge. A decay of the reaction rate was observed for all the aged samples, with exception of Co3O4 and Co30Cecopr (Fig. 4(b)). For instance Co30Ceimpr after ageing at 750 °C for 7 h suffers a deactivation higher than 50%. It is well-known that high mobility of surface and bulk oxygen give a major contribution to the hydrocarbons oxidation activity [15–17]. Experimental observations indicate that oxygens octahedrally coordinated around Co3+ ions are active sites in CH4 oxidation
0.20
0.15
0.10
333
[7,16]. Finely dispersed and higher valence state CoOx species over CeO2 has been reported to be the main active sites of CO oxidation [8]. Moreover the increase of oxygen bulk mobility of ceria-based catalysts, by introducing defective sites, seems to be effective in the promotion of hydrocarbons oxidation reactions [5]. On this basis, we considered useful to study the reduction properties of the analysed samples in order to get insight in the reaction mechanism and deactivation process upon thermal ageing. Fig. 5 shows the TPR curves of fresh and aged samples, for the cobalt richer oxides and pure Co3O4. The reduction of Co3O4 is described as a two steps process (Co3O4 ) CoO ) Co) [18,19], although there are two types of TPR shapes described in the literature, one broad curve [18], or two defined peaks [19]. In our case the TPR profile of fresh Co3O4 is a broad peak slightly asymmetric in accord with our previous results [9], the aged sample shows a more asymmetric
(a)
Co3O4 Co30Cecopr Co30Ceimpr Co2Cecopr Co2Ceimpr Co2CeZrimpr CeO2 CeO2 sol-gel CeZrsol-gel
Reaction rate
-1
-2
molCH4 s m )
0.05
0.00
fresh samples 200
250
300
(b) 0.20
0.15
0.10
0.05
0.00
aged samples 200
250
300
T (˚C)
Fig. 4. Areal (per square meter) specific rates of CH4 oxidation over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides, as fresh (a) and aged samples (b).
L.F. Liotta et al. / Catalysis Communications 6 (2005) 329–336
H2 consumption as TCD signal (a.u.)
334
4a 4b 3a 3b 2
1a 1b 0
200
400
600
800
1000
T (˚C) Fig. 5. TPR (5% H2 in Ar) profiles of pure Co3O4 and Co30Ce oxides: (1) Co3O4 fresh (a), aged (b); (2) CeO2Aldrich as received; (3) Co30Cecopr fresh (a), aged (b); (4) Co30Ceimpr fresh (a), aged (b).
0.8
0.6
Co3O4 Co30Cecopr Co30Ceimpr Co2Ceimpr Co2CeZrimpr
(a)
-1
-1
-2
Reaction rate (µmolCH4 s mmolCo m )
0.4
0.2
0.0
fresh samples 200
250
300
(b)
0.8
0.6
0.4
0.2
0.0
aged samples 200
250
300
T (˚C) Fig. 6. Specific rates (per square meter, per mmol of Co) of CH4 oxidation over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides, as fresh (a) and aged samples (b).
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gen in the aged samples is responsible for the lower methane oxidation activity. Moreover, both bulk as well as surface properties determining the combustion activity, appear directly related to the sample preparation method and, therefore, to the morphological characteristics. This is the case of Co30Ceimpr and Co30Cecopr oxides, with the same nominal content of cobalt, but showing different crystallite sizes (see Table 2). Remarkable properties of ceria–zirconia oxides are the enhanced thermal stability and oxygen storage capacity with respect to pure ceria [20]. Recently, we have reported that ceria–zirconia mixed oxide obtained by sol–gel method is characterized by high oxygen mobility [10,11]. In order to assess the contribution to the activity of the supports, ceria and ceria–zirconia, the areal reaction rates were also normalized per gram of cobalt. The specific reaction rates (per square meter, per gram of cobalt) are plotted versus the temperature in Figs. 6(a), (b) for fresh and aged oxides, respectively. It is clear that the inherent activity of Co2CeZrimpr composite oxide was much higher than that of the others. It is worth noticing, in particular, the superiority of Co2CeZrimpr over Co2Ceimpr (Figs. 4(a), (b) and 6(a), (b)). With the aim to investigate the reactivity changes of ceria and ceria–zirconia supports upon cobalt addition, the H2 temperature programmed reductions were performed over two selected catalysts Co2CeZrimpr, Co2Ceimpr and over the corresponding supports, as fresh and aged samples. For the sake of clarity, the TPR curve of ceria Aldrich (as received), already shown in Fig. 5, has been repeated in Fig. 7.
H2 consumption as TCD signal (a.u.)
peak shifted to higher temperature. The TPR curves of fresh Co30Cecopr and Co30Ceimpr are characterized by two main features, a small peak at lower temperature than pure Co3O4 and a second big peak at T °C P than the reference Co3O4. In order to investigate the reducibility changes of Co30Ce samples with respect to ceria support, in Fig. 5 the TPR pattern of ceria Aldrich is also displayed. As reported in the literature, ceria is characterized by a low-temperature reduction peak, at 437 °C attributed to the reduction of surface oxygen species and a high-temperature peak centered at 825 °C due to the reduction of bulk oxygen [5,20]. On both Co30Ce samples the intense peak at 825 °C disappears. A thorough analysis of the Fig. 5 evidences only a small peak at about 750 °C, the main hydrogen consumption occurring in the range of temperature 200–500 °C. On this basis, it can be deduced that in our composite oxides the proper cobalt loading (30 wt%) leads to an intimate contact between ceria and cobalt oxide enhancing the ceria bulk reducibility. Finally, by comparing TPR profiles of fresh and aged Co30Cecopr,impr and Co3O4 samples (Fig. 5) the most important difference upon ageing is the shift of the peak at low temperature to a higher temperature, except one catalyst. Indeed, in accord with catalytic results, Co30Cecopr oxide is the only sample that does not show increase of temperature of reduction after ageing. Even a better contact cobalt–cerium oxide could be inferred from the shape of the curve 3b that is more symmetrical compared to 3a (Fig. 5). From the data so far reported, we may surmise that a decreased reducibility of the surface/bulk oxy-
335
4a 4b 3a 3b
2a 2b 1a 1b 0
200
400
600
800
1000
T(˚C) Fig. 7. TPR (5% H2 in Ar) profiles of fresh and aged Co2CeZrimpr and Co2Ceimpr oxides along with the respective ceria–zirconia and ceria supports: (1) CeZrsol–gel fresh (a), aged (b); (2) Co2CeZrimpr fresh (a), aged (b); (3) CeO2Aldrich as received (a), aged (b); (4) Co2Ceimpr fresh (a), aged (b).
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As revealed by TPR profiles (see Fig. 7) oxygen surface and bulk mobility seems to play a key role in determining the catalytic activity also in the cobalt ceria–zirconia. Fresh and aged Co2CeZrimpr samples exhibit the main reduction peak below 400 °C, that accounts for the reduction of cobalt as Co3O4 along with the reduction of overall ceria–zirconia, surface and bulk. The presence of peaks at temperature P700 °C is due to the presence of ceria rich phase [10], as observed for ceria–zirconia alone (Fig. 7, curves 1a,b). Conversely, the fresh sample Co2Ceimpr exhibits a reduction peak around 250–300 °C, which hydrogen consumption agrees for the reduction of Co3O4 only [9]. Upon ageing of Co2Ceimpr at 750 °C the peak at low temperature shifts to 350 °C, probably due to sintering of cobalt species [9]. As previously observed (Fig. 5) no significant reduction of ceria surface occurs below 400 °C in CeO2Aldrich neither in Co2Ceimpr (Fig. 7, curves 3a,b and 4a,b). In conclusion, the main points coming from this study are: I The reducibility of the surface and bulk oxygen species reflects the methane combustion activity of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides. II The role of the synthetic procedure is important in order to disperse the active component and to maintain high oxygen mobility, as deduced by comparing the systems Co30Cecopr with Co30Ceimpr. III An improved oxygen reducibility and a higher inherent activity (areal specific rate) have been noticed for Co2CeZrimpr with respect to Co2Ceimpr. IV A higher oxygen reducibility of ceria has been observed on Co30Ceimpr with respect to Co2Ceimpr, indicating that a proper cobalt oxide loading enhances the reduction of ceria surface and bulk. On these basis, the study of catalytic performances of Co3O4/CeO2–ZrO2 systems with higher cobalt content is in progress. In order to evaluate a possible future application of these systems, life tests with long contact time have been performed [21].
Acknowledgements Thanks are due to the scientific staff of Centro Ricerche Fiat (Orbassano, Torino) in particular to Ing. Paolo Faraldi for valuable discussion and suggestions.
References [1] M. Zwinkels, S.G. Jaras, P.G. Menon, in: A. Cybulski, J. Moulijn (Eds.), Structured Catalysts and Reactors, Marcel Dekker, New York, 1998, p. 149. [2] J.K. Lampert, M. Shahjahan Kazi, R.J. Farrauto, Appl. Catal. B 14 (1997) 211. [3] R.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control, second ed., John & Sons Inc., New York, 2002. [4] P. Ge´lin, M. Primet, Appl. Catal. B 39 (2002) 1. [5] D. Terribile, A. Trovarelli, C. de Leitenburg, A. Primavera, G. Dolcetti, Catal. Today 47 (1999) 133. [6] V.R. Choudhary, B.S. Uphade, S.G. Pataskar, Appl. Catal. A 227 (2002) 29. [7] J. Kirchnerova, M. Alifanti, B. Delmon, Appl. Catal. A 231 (2002) 65. [8] M. Kang, M. Woo Song, C. Ha Lee, Appl. Catal. A 251 (2003) 143. [9] L.F. Liotta, G. Pantaleo, G. Di Carlo, G. Marcı`, G. Deganello, Appl. Catal. B 52 (2004) 1. [10] L.F. Liotta, A. Macaluso, A. Longo, G. Pantaleo, A. Martorana, G. Deganello, Appl. Catal. A 240 (2003) 295. [11] L.F. Liotta, G. Pantaleo, A. Macaluso, G. Marcı`, S. Gialanella, G. Deganello, J. Sol-Gel Sci. Technol. 28 (2003) 119. [12] Q. Xu, D. Huang, W. Chen, J. Lee, H. Wang, R. Yuan, Scripta Mater. 50 (2004) 165. [13] Inorganic Crystal Structure Data Base (ICSD), National Institute of Standards and Technology (NIST), release 2004/2. [14] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [15] G.I. GolodetsHeterogeneous Catalytic Reactions Involving Molecular Oxygen, Studies is Surface Science and Catalysis, vol. 15, Elsevier, Amsterdam, 1983, pp. 437–469. [16] A. Bielanski, J. Haber, Oxygen in Catalysis, Marcel Dekker, New York, 1991. [17] D. Wolf, Catal. Lett. 27 (1994) 207. [18] H.F.J. vanÕt Blik, R. Prins, J. Catal. 97 (1986) 188. [19] B. Sexton, A. Hughes, T. Turney, J. Catal. 97 (1986) 390. [20] J. Kaspar, P. Fornasiero, M. Graziani, Catal. Today 50 (1999) 285, and references therein. [21] L.F. Liotta, G. Di Carlo, G. Pantaleo, G. Deganello, Appl. Catal. B (submitted).
Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite catalysts for methane combustion: Correlation between morphology reduction properties and catalytic activity L.F. Liotta b
a,* ,
G. Di Carlo b, G. Pantaleo b, G. Deganello
a,b
a ISMN-CNR via Ugo La Malfa, Sezione Di Palermo, 153, 90146 Palermo, Italy Dipartimento di Chimica Inorganica e Analitica ‘‘Stanislao Cannizzaro’’, Universita` di Palermo, Viale delle Scienze, Parco dÕOrleans II - 90128 Palermo, Italy
Received 29 July 2004; accepted 5 February 2005 Available online 17 March 2005
Abstract Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite catalysts have been prepared by two different techniques, co-precipitation by citrate method and impregnation with cobalt nitrate of pre-formed ceria and ceria–zirconia oxides. The materials, as prepared and after ageing at 750 °C 7 h, were tested for methane combustion and the catalytic performances were compared with those of a commercial Co3O4, used as reference. A significant improvement of the activity was observed in the composite oxide Co3O4(30 wt%)/ CeO2(70 wt%), prepared by citrate method, which exhibits the lowest light-off temperature of methane (T50 = 400 °C) and does not suffer deactivation after calcination at 750 °C 7 h. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Co3O4–ceria/ceria–zirconia composites; Methane combustion; Morphological and reduction properties
1. Introduction The virtues of catalytic combustion are well-known. Indeed, due to thermodynamic reasons, the catalytic flameless combustion of hydrocarbons can reduce thermal NOx emissions. In this respect, many studies on the development of new gas turbine combustors in which lean combustion is coupled with catalytic combustion have been performed in the recent years [1]. Catalytic combustion is also effective in the abatement of unburned hydrocarbons from natural gas fueled vehicles (NGVs). Methane, the main component of natural gas, is a potent greenhouse gas, therefore emissions
*
Corresponding author. Tel.: +39 091 6809371; fax: +39 091 6809399. E-mail address: [email protected] (L.F. Liotta). 1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.02.006
from NGVs must be reduced by catalytic after-treatment of exhaust gases [2,3]. Nobel metal based catalysts are well-known to be active for methane oxidation at low temperature [4], but they are expensive and easily sinterise. Among the possible substitutes for noble metals, in the last years attention has been paid to the activity of Cu-doped CeO2–ZrO2 solid solutions, Mn-doped ZrO2 [5,6]. It is also known that perovskite-type transition metal oxides and Co3O4 exhibit good catalytic activity for CO and methane combustion [7,8]. Based on our most recent results on the reduction properties of two classes of defective materials, cobalt oxide [9] and ceria–zirconia solid solutions [10,11], we report the methane combustion over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides. A correlation between morphological and reduction properties with the catalytic activity is suggested.
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2. Experimental All the used reagents from Aldrich were ACS chemicals. Two composite oxides Co3O4(2, 30 wt%)/CeO2, labeled as Co2Cecopr and Co30Cecopr, were prepared by co-precipitation method in presence of citrate at pH of 9 [12]. The resulting materials were calcined at 650 °C for 5 h. Three additional samples were synthesized by impregnating with cobalt nitrate the supports, a commercial ceria (CeO2Aldrich) with relatively high surface area (79 m2/g) and a ceria–zirconia oxide with nominal composition Ce0.6Zr0.4O2 obtained by sol–gel method (CeZrsol–gel) [10] having lower surface area (21 m2/g). The loading values are Co3O4(2 wt%) over ceria–zirconia (Co2CeZrimpr) and Co3O4(2, 30 wt%) over ceria Aldrich (Co2Ceimpr, Co30Ceimpr). These samples were calcined at 500 °C for 5 h. For comparison reason, a ceria oxide with low surface area was prepared by precipitation method [8] (CeO2prec) and calcined at 650 °C for 5 h. Our reference material was a commercial Co3O4 (from Aldrich) with surface area of 5 m2/g. The samples were characterized by X-ray diffraction measurements, carried out with a Philips (PW 1820) vertical goniometer using Ni-filtered Cu Ka radiation ˚ ). The spectra were collected using a step (k = 1.5418 A size of 0.05° and a counting time of 5 s per angular abscissa. The assignment of the crystalline phases was based on the ICSD data base (Co3O4, n° 28,158; CeO2, n° 28,785) [13]. Mean crystallite size (d) was estimated from the line broadening of the most intense reflections using the Scherrer equation [14]. In particular, for Co3O4 spinel the broadening of the (3 1 1) line in the diffraction pattern was considered, whereas for CeO2 fluorite the mean particle size was estimated from the (1 1 1) line. Specific surface area values have been measured by BET method. Temperature programmed reduction (TPR) experiments were carried out with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector (TCD), by using experimental conditions described elsewhere [9]. The above-mentioned oxides, as prepared (fresh) and after ageing at 750 °C 7 h (aged) were tested for methane combustion in a flow system equipped with a temperature programmer controller. A thermocouple in contact with the catalytic bed allowed the control of the temperature inside the catalyst. The reagent gas mixture consisted of 0.3% of CH4 + 4.8% of O2 in He and the weight hourly space velocity (WHSV) was typically 12,000 mL g1 h1. Over the most promising catalyst, as fresh and aged sample, catalytic tests at a space velocity of 60,000 mL g1 h1were also carried out. The inlet and outlet gas compositions TM were monitored by a mass quadrupole (Thermostar ,
Balzers) and an IR analyser (ABB Uras 14). The only reaction products were CO2 and H2O.
3. Results and discussion Fig. 1(a) shows the light-off curves of methane oxidation over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides along with Co3O4 and the supports only. As indicated by the lowering of the temperature at which the reaction starts (see Fig. 1(a)), Co30Ceimpr and Co30Cecopr composite oxides behave better than bulk Co3O4, the latter exhibiting the same catalytic activity of Co2Ceimpr and Co2CeZrimpr systems. Co2Cecopr appears the worst cobalt catalyst. The activities of pure supports, CeO2Aldrich, CeO2prec and CeZrsol–gel are lower than those of the corresponding cobalt catalysts and decrease in the same order as the surface areas (Fig. 1(a)). One of the major sources of catalyst deactivation, especially in the automobile catalytic converters, occurs following high-temperature exposure. Generally, thermal effects cause crystal growth and loss of catalytic surface. Depending on the nature of active components, in particular oxides, their thermo-chemical stability must be also considered. At the operating window of 700– 750 °C Co3O4 is at the limit of thermal stability and although its known good activity in methane combustion [7], this drawback cannot be neglected for practical applications. An attempt to evaluate the thermal resistance of our systems was done ageing the catalysts in air at 750 °C for 7 h. Light-off curves for methane oxidation over aged catalysts are displayed in Fig. 1(b). A pronounced deactivation (T50 = 470 °C, see Table 1) has been observed for the Co30Ceimpr composite oxide. Over the aged samples Co3O4, Co2Ceimpr, Co2CeZrimpr and the corresponding supports the 50% of methane conversion was attained at T > 500 °C (Table 1). On the contrary, very promising appears the catalytic behavior Co30Cecopr which exhibits almost stable lightoff temperature (410 °C) after calcination at 750 °C. As expected, at increased space velocity (60,000 mL g1 h1) the temperatures for 50% conversion of methane increase from 400 to 465 °C for the fresh Co30Cecopr and from 410 to 490 °C in the case of the aged one (see Table 1). Taking into account the relatively low surface area of the Co30Cecopr sample (Table 2), these results can be considered promising. In Table 2 morphological parameters of the abovementioned oxides, fresh and aged samples are listed. In Fig. 2(a) the XRD patterns of Co3O4, Co30Cecopr and Co30Ceimpr fresh and aged samples are displayed. In Fig. 2(b) a magnification in the angular range 25–50 2h of the patterns of Co30Cecopr fresh and aged samples is reported. Well-defined features of Co3O4 and CeO2 crystalline phases (ICSD, Co3O4, n° 28158; CeO2, n° 28785)
L.F. Liotta et al. / Catalysis Communications 6 (2005) 329–336 100
331
Co3O4 Co30Ce copr Co30Ce impr Co2Ce copr Co2Ce impr Co2CeZr impr CeO2 CeO2 sol-gel CeZrsol-gel
80
60
40
CH4 conversion (%)
20
(a) fresh samples 0 200
300
400
500
600
700
100
80
60
40
20
(b) aged samples 0 200
300
400
500
600
700
T (˚C) Fig. 1. Light-off of CH4 oxidation over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides, as fresh (a) and aged samples (b). The reaction gas mixture consisted of 0.3% of CH4 + 4.8% of O2 in He at WHSV of 12,000 mL g1 h1. Table 1 Temperatures of 50% of CH4 conversion over fresh and aged (750 °C, 7 h) samples at WHSV = 12,000 mL g1 h1
Table 2 Specific surface area and mean particle size (d) of the different phases in fresh and aged (750 °C, 7 h) samples
Catalyst
T50 (°C) (fresh sample)
T50 (°C) (aged sample)
Sample (fresh, aged)
Surface areaa (m2/g)
d Co3 O4 b (nm)
d CeO2 b/d ðCex Zr1x O2 Þ c (nm)
Co3O4 Co30Cecopr
445 400 465a 390 511 445 445 495 636 546
505 410 490a 470 n.d. 512 515 531 n.d. 571
Co3O4 Co30Cecopr Co30Ceimpr Co2Cecopr Co2Ceimpr Co2CeZrimpr CeO2Aldrich CeO2prec CeZrsol–gel
5 (3.9) 28 (20) 30 (15) 23 (15) 67 (39) 19 (16) 79 (39) 20 (3) 21 (16)
116 (171) 15 (29) 32 (56) n.d. n.d. n.d. – – –
– 13 (28) 24 (45) 19 (36) 23 (29) n.d. 22 (29) 30 (45) n.d.
Co30Ceimpr Co2Cecopr Co2Ceimpr Co2CeZrimpr CeO2Aldrich CeO2prec CeZrsol–gel a
WHSV = 60,000 mL g1 h1.
are evident and no definite phase between cobalt and ceria has been noticed, in accord with the literature [8]. No variation of the CeO2 and Co3O4 reflection lines was ob-
a
Determined by BET method. Calculated by using Scherrer equation. c At least two solid solutions CexZr1 xO2, a cubic CeO2-rich and a tetragonal ZrO2 rich phases, were detected in the pattern XRD (see Fig. 3) [10]. b
served by comparing the XRD patterns of Co30Cecopr fresh and aged, neither decomposition of Co3O4 into the CoO phase was detected. Upon ageing at 750 °C only
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(a)
(b)
CeO2
CeO2 Co3O4
Intensity (a.u.)
f e
Co3O4
d
d
c b c
a 30
40
50
60
70
80
25
30
35
40
45
50
2θ
2θ
Fig. 2. (a) XRD patterns of fresh and aged Co3O4 (a,b), Co30Cecopr (c,d) and Co30Ceimpr (e,f) catalysts; (b) magnification of the patterns (c,d) in the angular range 25–50 2h.
sintering of the crystallites occurs as it is evident by the sharpness of the peaks (see Figs. 2(a), (b)). In Fig. 3 the XRD patterns of two composite oxides with low cobalt loading (Co2Ceimpr and Co2CeZrimprfresh and aged) are displayed. No features of Co3O4 are visible in the patterns (a–d), only reflections of CexZr1 xO2 and CeO2 phases can be observed. In particular, the patterns (a, b) related to the sample CoCeZrimpr are characterized by broad and slightly asymmetrical peaks. According to our previous results [10], the presence of at least two solid solutions must be considered. In the patterns (c, d) (Figs. 3) related to the catalyst CoCeimpr only reflections of CeO2 particles were detected, which sinter upon ageing, as indicated by the narrowing of the peaks.
Therefore, the major effect of ageing treatment at 750 °C for 7 h is a loss of surface area and a crystal growth, which extent depends on the nature of the samples (Table 2, Figs. 2 and 3). In particular, it should be noted that both samples Co30Cecopr and Co30Ceimpr suffer a drop of surface area of 30% and 50%, respectively, and the crystallite sizes double. However, after ageing at 750 °C Co30Cecopr shows mean particle size (d) comparable to that of fresh Co30Ceimpr (Table 2). On this basis it could be inferred that finely dispersed Co3O4/CeO2 species mainly contribute to the methane oxidation activity. Therefore, we suggest that an important requisite for achieving good oxidation activity is an intimate contact between the species cobalt–ceria obtained via an appropriate preparation method.
Intensity (a.u.)
CeO2
d c
CexZr1-xO2
b a 25
30
35
40
45
50
55
2θ Fig. 3. XRD patterns of fresh and aged Co2CeZrimpr (a,b) and Co2Ceimpr (c,d).
L.F. Liotta et al. / Catalysis Communications 6 (2005) 329–336
Taking into account the literature [7] the areal (per square meter) specific rates of methane combustion were calculated, always using low conversion values (below 10%) in order to satisfy the conditions of a differential reactor. In Figs. 4(a), (b) the calculated reaction rates (lmol/s1 m2) are plotted against the temperature for fresh and aged samples, respectively. From the inspection of Fig. 4(a) the inherent high activities of Co3O4 and both composite oxides Co30Ceimpr and Co30Cecopr clearly emerge. A decay of the reaction rate was observed for all the aged samples, with exception of Co3O4 and Co30Cecopr (Fig. 4(b)). For instance Co30Ceimpr after ageing at 750 °C for 7 h suffers a deactivation higher than 50%. It is well-known that high mobility of surface and bulk oxygen give a major contribution to the hydrocarbons oxidation activity [15–17]. Experimental observations indicate that oxygens octahedrally coordinated around Co3+ ions are active sites in CH4 oxidation
0.20
0.15
0.10
333
[7,16]. Finely dispersed and higher valence state CoOx species over CeO2 has been reported to be the main active sites of CO oxidation [8]. Moreover the increase of oxygen bulk mobility of ceria-based catalysts, by introducing defective sites, seems to be effective in the promotion of hydrocarbons oxidation reactions [5]. On this basis, we considered useful to study the reduction properties of the analysed samples in order to get insight in the reaction mechanism and deactivation process upon thermal ageing. Fig. 5 shows the TPR curves of fresh and aged samples, for the cobalt richer oxides and pure Co3O4. The reduction of Co3O4 is described as a two steps process (Co3O4 ) CoO ) Co) [18,19], although there are two types of TPR shapes described in the literature, one broad curve [18], or two defined peaks [19]. In our case the TPR profile of fresh Co3O4 is a broad peak slightly asymmetric in accord with our previous results [9], the aged sample shows a more asymmetric
(a)
Co3O4 Co30Cecopr Co30Ceimpr Co2Cecopr Co2Ceimpr Co2CeZrimpr CeO2 CeO2 sol-gel CeZrsol-gel
Reaction rate
-1
-2
molCH4 s m )
0.05
0.00
fresh samples 200
250
300
(b) 0.20
0.15
0.10
0.05
0.00
aged samples 200
250
300
T (˚C)
Fig. 4. Areal (per square meter) specific rates of CH4 oxidation over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides, as fresh (a) and aged samples (b).
L.F. Liotta et al. / Catalysis Communications 6 (2005) 329–336
H2 consumption as TCD signal (a.u.)
334
4a 4b 3a 3b 2
1a 1b 0
200
400
600
800
1000
T (˚C) Fig. 5. TPR (5% H2 in Ar) profiles of pure Co3O4 and Co30Ce oxides: (1) Co3O4 fresh (a), aged (b); (2) CeO2Aldrich as received; (3) Co30Cecopr fresh (a), aged (b); (4) Co30Ceimpr fresh (a), aged (b).
0.8
0.6
Co3O4 Co30Cecopr Co30Ceimpr Co2Ceimpr Co2CeZrimpr
(a)
-1
-1
-2
Reaction rate (µmolCH4 s mmolCo m )
0.4
0.2
0.0
fresh samples 200
250
300
(b)
0.8
0.6
0.4
0.2
0.0
aged samples 200
250
300
T (˚C) Fig. 6. Specific rates (per square meter, per mmol of Co) of CH4 oxidation over Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides, as fresh (a) and aged samples (b).
L.F. Liotta et al. / Catalysis Communications 6 (2005) 329–336
gen in the aged samples is responsible for the lower methane oxidation activity. Moreover, both bulk as well as surface properties determining the combustion activity, appear directly related to the sample preparation method and, therefore, to the morphological characteristics. This is the case of Co30Ceimpr and Co30Cecopr oxides, with the same nominal content of cobalt, but showing different crystallite sizes (see Table 2). Remarkable properties of ceria–zirconia oxides are the enhanced thermal stability and oxygen storage capacity with respect to pure ceria [20]. Recently, we have reported that ceria–zirconia mixed oxide obtained by sol–gel method is characterized by high oxygen mobility [10,11]. In order to assess the contribution to the activity of the supports, ceria and ceria–zirconia, the areal reaction rates were also normalized per gram of cobalt. The specific reaction rates (per square meter, per gram of cobalt) are plotted versus the temperature in Figs. 6(a), (b) for fresh and aged oxides, respectively. It is clear that the inherent activity of Co2CeZrimpr composite oxide was much higher than that of the others. It is worth noticing, in particular, the superiority of Co2CeZrimpr over Co2Ceimpr (Figs. 4(a), (b) and 6(a), (b)). With the aim to investigate the reactivity changes of ceria and ceria–zirconia supports upon cobalt addition, the H2 temperature programmed reductions were performed over two selected catalysts Co2CeZrimpr, Co2Ceimpr and over the corresponding supports, as fresh and aged samples. For the sake of clarity, the TPR curve of ceria Aldrich (as received), already shown in Fig. 5, has been repeated in Fig. 7.
H2 consumption as TCD signal (a.u.)
peak shifted to higher temperature. The TPR curves of fresh Co30Cecopr and Co30Ceimpr are characterized by two main features, a small peak at lower temperature than pure Co3O4 and a second big peak at T °C P than the reference Co3O4. In order to investigate the reducibility changes of Co30Ce samples with respect to ceria support, in Fig. 5 the TPR pattern of ceria Aldrich is also displayed. As reported in the literature, ceria is characterized by a low-temperature reduction peak, at 437 °C attributed to the reduction of surface oxygen species and a high-temperature peak centered at 825 °C due to the reduction of bulk oxygen [5,20]. On both Co30Ce samples the intense peak at 825 °C disappears. A thorough analysis of the Fig. 5 evidences only a small peak at about 750 °C, the main hydrogen consumption occurring in the range of temperature 200–500 °C. On this basis, it can be deduced that in our composite oxides the proper cobalt loading (30 wt%) leads to an intimate contact between ceria and cobalt oxide enhancing the ceria bulk reducibility. Finally, by comparing TPR profiles of fresh and aged Co30Cecopr,impr and Co3O4 samples (Fig. 5) the most important difference upon ageing is the shift of the peak at low temperature to a higher temperature, except one catalyst. Indeed, in accord with catalytic results, Co30Cecopr oxide is the only sample that does not show increase of temperature of reduction after ageing. Even a better contact cobalt–cerium oxide could be inferred from the shape of the curve 3b that is more symmetrical compared to 3a (Fig. 5). From the data so far reported, we may surmise that a decreased reducibility of the surface/bulk oxy-
335
4a 4b 3a 3b
2a 2b 1a 1b 0
200
400
600
800
1000
T(˚C) Fig. 7. TPR (5% H2 in Ar) profiles of fresh and aged Co2CeZrimpr and Co2Ceimpr oxides along with the respective ceria–zirconia and ceria supports: (1) CeZrsol–gel fresh (a), aged (b); (2) Co2CeZrimpr fresh (a), aged (b); (3) CeO2Aldrich as received (a), aged (b); (4) Co2Ceimpr fresh (a), aged (b).
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As revealed by TPR profiles (see Fig. 7) oxygen surface and bulk mobility seems to play a key role in determining the catalytic activity also in the cobalt ceria–zirconia. Fresh and aged Co2CeZrimpr samples exhibit the main reduction peak below 400 °C, that accounts for the reduction of cobalt as Co3O4 along with the reduction of overall ceria–zirconia, surface and bulk. The presence of peaks at temperature P700 °C is due to the presence of ceria rich phase [10], as observed for ceria–zirconia alone (Fig. 7, curves 1a,b). Conversely, the fresh sample Co2Ceimpr exhibits a reduction peak around 250–300 °C, which hydrogen consumption agrees for the reduction of Co3O4 only [9]. Upon ageing of Co2Ceimpr at 750 °C the peak at low temperature shifts to 350 °C, probably due to sintering of cobalt species [9]. As previously observed (Fig. 5) no significant reduction of ceria surface occurs below 400 °C in CeO2Aldrich neither in Co2Ceimpr (Fig. 7, curves 3a,b and 4a,b). In conclusion, the main points coming from this study are: I The reducibility of the surface and bulk oxygen species reflects the methane combustion activity of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides. II The role of the synthetic procedure is important in order to disperse the active component and to maintain high oxygen mobility, as deduced by comparing the systems Co30Cecopr with Co30Ceimpr. III An improved oxygen reducibility and a higher inherent activity (areal specific rate) have been noticed for Co2CeZrimpr with respect to Co2Ceimpr. IV A higher oxygen reducibility of ceria has been observed on Co30Ceimpr with respect to Co2Ceimpr, indicating that a proper cobalt oxide loading enhances the reduction of ceria surface and bulk. On these basis, the study of catalytic performances of Co3O4/CeO2–ZrO2 systems with higher cobalt content is in progress. In order to evaluate a possible future application of these systems, life tests with long contact time have been performed [21].
Acknowledgements Thanks are due to the scientific staff of Centro Ricerche Fiat (Orbassano, Torino) in particular to Ing. Paolo Faraldi for valuable discussion and suggestions.
References [1] M. Zwinkels, S.G. Jaras, P.G. Menon, in: A. Cybulski, J. Moulijn (Eds.), Structured Catalysts and Reactors, Marcel Dekker, New York, 1998, p. 149. [2] J.K. Lampert, M. Shahjahan Kazi, R.J. Farrauto, Appl. Catal. B 14 (1997) 211. [3] R.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control, second ed., John & Sons Inc., New York, 2002. [4] P. Ge´lin, M. Primet, Appl. Catal. B 39 (2002) 1. [5] D. Terribile, A. Trovarelli, C. de Leitenburg, A. Primavera, G. Dolcetti, Catal. Today 47 (1999) 133. [6] V.R. Choudhary, B.S. Uphade, S.G. Pataskar, Appl. Catal. A 227 (2002) 29. [7] J. Kirchnerova, M. Alifanti, B. Delmon, Appl. Catal. A 231 (2002) 65. [8] M. Kang, M. Woo Song, C. Ha Lee, Appl. Catal. A 251 (2003) 143. [9] L.F. Liotta, G. Pantaleo, G. Di Carlo, G. Marcı`, G. Deganello, Appl. Catal. B 52 (2004) 1. [10] L.F. Liotta, A. Macaluso, A. Longo, G. Pantaleo, A. Martorana, G. Deganello, Appl. Catal. A 240 (2003) 295. [11] L.F. Liotta, G. Pantaleo, A. Macaluso, G. Marcı`, S. Gialanella, G. Deganello, J. Sol-Gel Sci. Technol. 28 (2003) 119. [12] Q. Xu, D. Huang, W. Chen, J. Lee, H. Wang, R. Yuan, Scripta Mater. 50 (2004) 165. [13] Inorganic Crystal Structure Data Base (ICSD), National Institute of Standards and Technology (NIST), release 2004/2. [14] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [15] G.I. GolodetsHeterogeneous Catalytic Reactions Involving Molecular Oxygen, Studies is Surface Science and Catalysis, vol. 15, Elsevier, Amsterdam, 1983, pp. 437–469. [16] A. Bielanski, J. Haber, Oxygen in Catalysis, Marcel Dekker, New York, 1991. [17] D. Wolf, Catal. Lett. 27 (1994) 207. [18] H.F.J. vanÕt Blik, R. Prins, J. Catal. 97 (1986) 188. [19] B. Sexton, A. Hughes, T. Turney, J. Catal. 97 (1986) 390. [20] J. Kaspar, P. Fornasiero, M. Graziani, Catal. Today 50 (1999) 285, and references therein. [21] L.F. Liotta, G. Di Carlo, G. Pantaleo, G. Deganello, Appl. Catal. B (submitted).