Sol–gel synthesis and characterization of transition metal based mixed oxides and their application as catalysts in selective oxidation of propane

Applied Catalysis A: General 325 (2007) 244–250 www.elsevier.com/locate/apcata

Sol–gel synthesis and characterization of transition metal based mixed oxides and their application as catalysts in selective oxidation of propane Carlo Lucarelli a, Pietro Moggi a,*, Fabrizio Cavani b, Michel Devillers c b

a Department of Organic and Industrial Chemistry, University of Parma, Viale G.P. Usberti 17/A, 43100 Parma, Italy Department of Industrial Chemistry and Materials, University of Bologna, Viale del Risorgimento 4, 40136 Bologna, Italy c Unite´ de Chimie des Mate´riaux Inorganiques et Organiques, Universite´ Catholique de Louvain, Place Louis Pasteur 1/3, B-1348 Louvain-la-Neuve, Belgium

Received 20 July 2006; accepted 7 February 2007 Available online 3 March 2007

Abstract In this work binary and ternary mixed oxides containing Nb, V, Mo and Sb were prepared by a modified hydrolytic sol–gel technique and tested as catalysts for the selective oxidation of propane. All systems were active in the ODH of propane, with the appearance in some cases of oxygenated products. The best results as concerns the selectivity to acrylic acid (about 20%) have been obtained on ternary Mo–Nb–V oxide systems in the presence of Nb–Mo–O and V–Mo–O crystalline phases. # 2007 Elsevier B.V. All rights reserved. Keywords: Nb–V–Mo–Sb–O catalysts; Mixed oxides; Sol–gel synthesis; Propane selective oxidation; Acrylic acid

1. Introduction The direct conversion of alkanes, such as propane, to partial oxidation products is an economically viable alternative route to the conventional one from alkenes [1]. There is an increasing interest in the development of a process for direct oxidation of propane to acrylic acid as an alternative to the two-step conventional industrial process based on propene as feedstock, which represents about 90% of the total capacity of acrylic acid production plants [2]. Mixed oxides of four to six group transition metals are important inorganic materials used in many different fields; e.g. complex oxide formulations including Nb and Mo are extensively used in partial oxidation catalysis [3]. Vanadium oxide based catalysts have been widely employed in selective oxidation of alkanes due to the mobility of lattice oxygen bound to vanadium, which can be both involved in the extraction of hydrogen from hydrocarbons and in the oxidation of the reacting molecule [4]. It is well known that the catalytic behaviour of vanadium oxide supported on or mixed with other metal oxides depends on the interaction

* Corresponding author. Tel.: +39 0521905464; fax: +39 0521905472. E-mail address: [email protected] (P. Moggi). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.02.034

occurring between the two metals [5]. Vanadium is a very active metal in oxidation reactions but it is not selective to partial oxidation products; instead, niobium is very selective for the production of propene from propane but poorly active, as reported by Smits et al. [6]. In the work described in this paper we have prepared binary and ternary mixed metal oxide systems containing niobium, vanadium, molybdenum and antimony by sol–gel techniques to obtain a very fine interdispersion of the metal oxides and possibly the formation of mixed oxide phases. The aim is to study the synergetic effects between different pure or mixed oxide phases, which can contribute to the activity for the conversion of propane and the optimum selectivity to oxygenated products, particularly to acrylic acid. 2. Experimental The preparation of all catalytic systems has been performed by the sol–gel technique, following a standard methodology to warrant a good reproducibility. The samples were prepared under the same conditions using Nb(OPri)5, MoCl5, VO(OPri)3 and Sb(OPri)3 as starting materials, diluted with 2-propanol as solvent, to allow an intimate interdispersion of the metal oxide precursors. The synthesis was

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conducted in an inert atmosphere to prevent the early hydrolysis of the alkoxide or chloride precursors. A solution of mixed water and 2-propanol (hydrolysis ratio water-toalcohol R = 2) was added dropwise to a stirred mixture of the metal oxide precursors getting a gel formation in few hours. In the case of the sample with Nb/V = 1/1 the gel was not obtained probably because the hydrolysis of vanadyl isopropoxide is much faster than the hydrolysis of niobium isopropoxide; then, in order to avoid the early precipitation of vanadium hydroxide species, the synthesis was repeated by adding a stoichiometric amount of citric acid as complexing agent. The xerogels obtained after ageing in air and vacuum drying, on the basis of a preliminary differential thermal analysis (DTA) test to evaluate the solid phase transformations at increasing temperatures, were thermally treated in air as follows: 15 h at 623 K, 2 h at 723 K, and finally 4 h at 823 K or 5 h at 773 K or 4 h at 973 K or 4 h at 1173 K. The obtained mixed oxides were characterized by BET surface area determinations, Raman spectroscopy, X-ray powder diffraction (XRD) analyses, UV–vis spectroscopy and scanning electron microscopy (SEM). The powder X-ray diffraction patterns were recorded on a Philips PW 3710 diffractometer using the Cu Ka ˚ ) radiation. The crystalline phases were (l = 1.54178 A identified with reference to the powder diffraction data files (JCPDS-ICDD). The BET specific surface area measurements were carried out on a Micromeritics Pulse Chemisorb 2705 analyser using nitrogen at 77 K (‘‘single point’’ method). The samples were previously outgassed under helium flow at 473 K.

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Raman spectroscopy was performed on a Renishaw 1000 spectrometer equipped with green laser (l = 514 nm, power 25 mW, cofocal objectives 5, 20, 50). SEM micrographs were taken with a JEOL 3120 instrument equipped with a Philips XL 30 ESEM. The selective oxidation catalytic tests were performed in a quartz reactor filled with 0.3–0.5 g of 30–40 mesh pre-screened catalyst, in the temperature range 623–823 K and atmospheric pressure, at a contact time t = 2 s. The gas feed composition was (vol%): propane/oxygen/H2O/He = 20/20/20/40. 3. Results In Table 1 the synthesized systems are reported, with the XRD detected crystalline phases and the measured BET surface area values. All samples are characterized by low surface areas, only the samples containing Sb or with molar ratios Nb/V > 1/1 or Nb/Mo > 5/1 showing values higher than 10 m2 g 1. A homogeneous gel was obtained from all preparations with the only exception of the sample with Nb/V = 1/1. From these preliminary results it was concluded that a high Nb(OPri)5 or MoCl5 amount, or the presence of a V complexing agent such as citric acid, are necessary conditions for preparing gels containing Nb, V, Mo and Sb mixed oxide precursors. The sample Nb/V 1/1 (precipitate) shows the stoichiometric crystalline phase NbVO5; instead, the sample Nb/V 1/1cit, synthesized by adding citric acid as V complexing agent, shows a second crystalline phase (Nb18V4O55) in addition to NbVO5, as reported in Table 1. It is possible to assume that the gel formation had some influence on the structure of the material because it can favour a better atomic dispersion. The other

Table 1 Synthesized samples, crystalline phases, surface area and final calcination temperature: (a) 823 K, (b) 973 K, and (c) 1173 K Sample

Crystalline phases

Surface area (m2 g 1)

Nb/V (a) 1/1 Nb/V (a) 1/1cita Nb/V (a) 2/1 Nb/V (a) 3/1 Nb/V (a) 4.5/1 Nb/V (a) 9/1 Nb/Mo (a) 12/1 Nb/Mo (a) 8/1 Nb/Mo (c) 14/3 Nb/Mo (b) 14/3 Nb/Mo (b) 4/1 Nb/Mo (c) 2/3 Mo/V (a) 3/2 Mo/Nb/V (a) 3/1.5/0.5 Mo/Nb/V (a) 3/1/1 Mo/Nb/V (a) 3/0.5/1.5 Mo/Nb/Sb (a) 3/1.5/0.5 Mo/Nb/Sb (a) 3/1/1 Mo/Nb/Sb (a) 3/0.5/1.5 Mo/Sb/V (a) 3/1.5/0.5 Mo/Sb/V (a) 3/1/1 Mo/Sb/V (a) 3/0.5/1.5 Nb/Sb/V (a) 2/1/1 Nb/Sb/V (a) 1/1/1

NbVO5 [46-0046] Nb18V4O55 [46-0087]/NbVO5 [46-0046] Nb18V4O55 [46-0087] Nb18V4O55 [46-0087] Nb18V4O55 [46-0087]/Nb2O5 [05-0352] Nb2O5 [05-0352] Nb2O5 [05-0352] Nb2O5 [05-0352] Mo13O33 [82-1930], Nb2O5 [05-0352] Mo13O33 [82-1930] Mo13O33 [82-1930], Nb12O29 [16-0734] Mo13O33 [82-1930] MoV2O8 [74-1510], MoO3 [05-0508], V4O9 [23-0720] Nb18V4O55 [46-0087], MoO3 [05-0508] (Nb0,09Mo0,91)O2,80 [27-1310], (V0,07Mo0,93)5O14 [31-1437], MoO3 [05-0508] (Nb0,09Mo0,91)O2,80 [27-1310], V2MoO8 [74-1510], MoO3 [05-0508] MoO3 [05-0508], Nb2O5 [05-0352] MoO3 [05-0508], Nb2O5 [05-0352] MoO3 [05-0508], Nb2O5 [05-0352] MoO3 [05-0508], V2MoO8 [74-1510] MoO3 [05-0508], V2MoO8 [74-1510] MoO3 [05-0508], V2MoO8 [74-1510] Sb0,67Nb2O6 [19-0080], VNb9O24,9 [81-2343] (Sb0,958V0,958)O4 [81-1218], Nb18V4O55 [46-0087]

5.1  0.3 6.3  0.1 20.7  0.2 22.4  0.2 23.7  0.1 20.2  0.4 38.8  0.1 17.9  0.4 4.1  0.2 4.3  0.2 2.5  0.1 4.1  0.3 5.8  0.1 6.7  0.1 7.2  0.2 6.3  0.4 15.8  0.1 16.3  0.2 15.1  0.3 17.9  0.1 15.3  0.3 20.4  0.1 8.2  0.2 14.5  0.1

a

Sample synthesized adding citric acid as complexing agent.

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Fig. 1. UV–vis analysis of the sample Nb/V 9/1.

samples containing Nb and V also show the presence of the crystalline mixed phase Nb18V4O55, with the sample Nb/V 9/1 showing only Nb2O5. The presence of vanadium in all samples has been detected by UV–vis spectroscopy. The sample with Nb/V ratio = 9/1 even shows the presence of a microcrystalline V4+ phase (UV band at about 420 nm) as reported in Fig. 1; the presence of V4+ was determined by recording the spectrum of the sample and subtracting to this one the spectrum of pure Nb2O5 calcined at the same temperature. For the samples containing Nb and Mo, mixed oxide phases have not been detected, but only either Nb2O5 or a slightly reduced molybdenum oxide or a mixture of the two single oxides, depending on the Mo/Nb ratio and the calcination temperature. Instead, the sample with Mo/V = 3/2 actually showed a mixed oxide phase containing Mo and V. The ternary systems containing Mo, Nb and Sb have not shown mixed oxides at all, but only crystalline molybdenum oxide and niobium oxide phases. On the contrary, in the Mo– Nb–V ternary systems mixed oxide phases containing Nb–V, Mo–Nb and Mo–V have been detected in addition to molybdenum oxide, as shown in Fig. 2a–c for the samples with Mo/Nb/V = 3/1.5/0.5, 3/1/1 and 3/0.5/1.5, respectively. In the XRD patterns of Mo–Sb–V ternary systems V2MoO8 and MoO3 phases were present, while Sb oxide phases were not detected. Finally, mixed oxide phases Nb–Sb, Nb–V and Sb–V were detected in the Nb–Sb–V ternary systems. The Raman analyses have also shown the presence of different metal oxides. In the samples with molar ratios Nb/ V > 1/1 or Nb/Mo > 5/1, as reported in Table 2, only the bands due to the presence of Nb2O5 according to Zhao et al. [7] have been detected; in the other samples containing Mo and Nb the Raman bands due to Mo O bonds vibration (990–1000 cm 1) and MoO3 (660 cm 1), in addition to Nb oxides have been detected. The samples containing Mo, Nb and V showed the bands due to the terminal V O bonds (970, 998 cm 1), the bands due to MoO3 (853, 658 cm 1) and the bands due to the Nb oxides (950, 753, 246 cm 1). In the case of the sample with Mo/Nb/V = 3/1/1 some bands probably due to a VxNb2 xO5 solid solution (700 cm 1 [V–O–Nb; Nb–O–Nb], 290 cm 1 [VxNb2 xO5 distortions]) have also been detected [7], while in

Fig. 2. X-ray diffraction patterns of: (a) Mo/Nb/V 3/1.5/0.5; (b) Mo/Nb/V 3/1/ 1; (c) Mo/Nb/V 3/0.5/1.5.

the sample with Mo/Nb/V = 3/1.5/0.5 the bands due to V4Nb18O55 have also been detected [8]. The samples containing Mo–Nb–Sb and Mo–Sb–V only showed the bands assignable to MoO3 and Nb2O5; only the sample with Mo/ Sb/V = 3/0.5/1.5 showed a band at 701 cm 1 assignable to antimony oxide. To evaluate the effective relative amount of different metals and their interdispersion SEM microanalyses have been performed. The samples calcined at temperatures higher than 973 K showed a molybdenum amount lower than the theoretical one, probably because at the highest temperature Mo oxide volatilizes from the lattice. An example of SEMEDS microanalysis is reported in Table 3 for the sample with Nb/Mo = 4/1. In the other samples the theoretical amount of different metals was respected. The effective presence of metal oxides, yet not detectable by XRD and Raman spectroscopy, was detected by SEM microanalysis in all samples. All the samples showed a very good metal interdispersion. Some results of the catalytic tests performed are visually reported in Figs. 3–7.

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Table 2 Raman bands attributions Sample

Raman shift (cm 1)

Attribution

Nb/V (a) 1/1 Nb/V (a) 1/1cit

990; 900; 742; 703; 634; 472; 424; 359; 314; 232; 147 148; 232; 315; 702; 950

Nb2O5 Nb2O5

Nb/V (a) 2/1

989; 780; 703; 470; 314; 236; 145 790; 731; 525

Nb2O5 Nb/V/O

Nb/V (a) 3/1

952; 703; 317; 233; 147 732; 239

Nb2O5 Nb18V4O55

Nb/V (a) 4.5/1

957; 900; 742; 634; 472; 423; 361; 314; 235; 150 730; 240

Nb2O5 Nb18V4O55

Nb/V (a) 9/1 Nb/Mo (a) 12/1 Nb/Mo (a) 8/1

952; 703; 310; 233; 147 952; 775; 704; 311; 233 955; 718; 310; 240

Nb2O5 Nb2O5 Nb2O5

Nb/Mo (c) 14/3

990; 865; 660 910; 745; 360; 262; 240

MoO3 Nb2O5

Nb/Mo (b) 14/3

990; 874; 632 916; 751; 471; 367; 264

MoO3 Nb2O5

Nb/Mo (b) 4/1

987; 674 934; 900; 730; 630; 470; 310; 258

MoO3 Nb2O5

Nb/Mo (c) 2/3

988; 879; 650 970; 914; 743; 629; 470; 419; 369; 263

MoO3 Nb2O5

Mo/V (a) 3/2

989; 817; 659; 378; 330; 281; 233; 149

MoO3

Mo/Nb/V (a) 3/1.5/0.5

950; 753; 246 853; 658

Nb2O5 MoO3

Mo/Nb/V (a) 3/1/1

956; 756; 246 866; 656

Nb2O5 MoO3

Mo/Nb/V (a) 3/0.5/1.5

950; 753; 246 853; 658

Nb2O5 MoO3

Mo/Nb/Sb (a) 3/1.5/0.5

991; 818; 705; 666; 334; 287;243; 161

MoO3

Mo/Nb/Sb (a) 3/1/1

995;703; 218; 194 995; 818; 666; 472; 377; 337; 289; 245; 157; 128

Nb2O5 MoO3

Mo/Nb/Sb (a) 3/0.5/1.5

997;703; 215; 197 997; 820; 667; 471; 378; 339; 287; 248; 160; 128

Nb2O5 MoO3

Mo/Sb/V (a) 3/1.5/0.5 Mo/Sb/V (a) 3/1/1

992; 817; 662; 449; 372; 333; 288; 239; 152 992; 924; 814; 659; 446; 372; 330; 288; 239; 159

MoO3 MoO3

Mo/Sb/V (a) 3/0.5/1.5

701 992; 817; 666; 375;333; 291; 243; 155

Sb2O5 MoO3

Nb/Sb/V (a) 2/1/1

732; 238 997

Nb18V4O55 Nb2O5

Nb/Sb/V (a) 1/1/1

992; 934; 704; 635; 520; 496; 475; 401; 281; 147

Nb2O5

Final calcination temperatures: (a) 823 K, (b) 973 K, and (c) 1173 K.

Table 3 SEM-EDS microanalysis for the sample Nb/Mo = 4/1 Element

wt% Theoretical

Nb Mo O Totals

Experimental

55.01 14.20 30.79

57.07 12.23 30.69

100.00

99.99

Fig. 3 shows the conversion of propane (a) and the selectivity to propene (b) at different temperatures for the binary systems containing Nb and V. The selectivity to propene generally increases at increasing temperatures, the best results being obtained for the sample with a Nb/V ratio = 9:1. CO and CO2 were the other main products. Only traces of oxygenated products have been observed. Fig. 4 shows the conversion of propane (a) and the selectivity to propene (b) at different temperatures for the ternary systems containing Mo–Nb–Sb and Mo–Sb–V. The most active as catalyst is the sample Mo/Sb/V 3:1.5/0.5. The catalysts based on Mo, Nb and Sb oxides show the best selectivity to propene at

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Fig. 5. Catalytic activity results at different temperatures for the sample with Mo/Nb/V = 3:1:1: (a) conversion of C3H8 and selectivity to C3H6, CO, CO2; (b) selectivity to oxygenated products. Fig. 3. Catalytic activity results at different temperatures for the Nb–V systems: (a) conversion of C3H8; (b) selectivity to propene.

lower temperatures and the selectivity increases at increasing Sb content, while the catalytic systems containing Mo, Sb and V show the best results at higher temperatures and V contents. For these ternary systems oxygenated products have also been observed, but the highest selectivity values did not exceed 6%; the products observed in higher amounts were acetic acid and acrylic acid at lower temperatures, while acrolein was the principal oxygenated product at higher temperatures, probably due to the higher consumption of oxygen in combustion reactions. The catalyst with Mo/Nb/V = 3:1:1 showed a high conversion of propane and a good selectivity to propene at the highest temperature, as reported in Fig. 5(a). In Fig. 5(b) the selectivity values to oxygenated products are reported; a higher value of selectivity to acrylic acid, which reaches about 20%, can be observed at the lowest temperature, while this ternary system does not show a significant value of selectivity to acetic acid and to acrolein, like all other systems containing Sb. The crystalline phase which probably favours the formation of acrylic acid is the mixed oxide (Nb0.09Mo0.91)O2.80. In the absence of Nb, as shown in Fig. 6(a) and (b) for the Mo/V = 3/2 system, the selectivities to propene, CO and CO2 were almost the same of the catalyst with Nb, while acrylic acid has been obtained in a very lower amount also at the lowest temperature, the maximum value being only 4.5%. 4. Discussion

Fig. 4. Catalytic activity results at different temperatures for the Mo–Nb–Sb and Mo–Sb–V systems: (a) conversion of C3H8; (b) selectivity to propene.

The hydrolytic sol–gel method was adopted in this work to prepare mixed oxide systems containing Nb, V, Mo and Sb as potential catalysts for the selective oxidation of propane.

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Fig. 7. Comparison of C3H8 conversion between all samples tested.

Fig. 6. Catalytic activity results at different temperatures for the sample with Mo/V = 3:2: (a) conversion of C3H8 and selectivity to C3H6, CO, CO2; (b) selectivity to oxygenated products.

A lot of systems with different compositions have been prepared, obtaining in all cases the formation of a gel. In only one case (Nb/V = 1:1) a gel was not obtained, probably because the hydrolysis of the vanadyl isopropoxide is much faster than the hydrolysis of the niobium isopropoxide used as precursors. Therefore, the large amount of partially hydrolyzed vanadyl isopropoxide, which quickly precipitates, has influence on the subsequent polycondensation reaction between the two metal hydroxide precursor species. In order to avoid the precipitation of vanadium hydroxide species, another sample with the same Nb/V ratio has been prepared by adding during the synthesis a stoichiometric amount of citric acid as complexing agent. Many systems showed, after calcination, crystalline mixed oxide phases. The sample with Nb/V = 1:1 showed a stoichiometric crystalline phase NbVO5, such as the Nb/ V = 4.5:1 system, which presented the stoichiometric crystalline phase Nb18V4O55. The XRD pattern of the sample with Nb/ V = 9:1 only showed the Nb2O5 phase, but the presence also of a microcrystalline V4+ phase was observed by UV–vis analysis. The patterns of the catalysts based on Mo–Nb and Mo–Nb–Sb have not evidenced mixed phases, probably because microcrystalline or amorphous phases were prevailingly formed. In the other ternary systems two-component mixed oxide phases were observed, in addition to single oxide phases. The Raman analyses have supplemented the XRD results. In some cases the Raman analyses have shown vibration bands

assignable to different mixed phases with respect to XRD; these results suggest as hypothesis that some crystalline phases are not detectable by XRD analysis because they are microcrystalline with a size lower than 4 nm. In all cases the effective metal amounts and a good metal interdispersion have been verified by SEM-EDS microanalyses. The catalytic activity in the propane oxidation reaction resulted very similar for all samples as shown in Fig. 7. It could be observed that the catalysts containing only Nb and V were the most active at lower temperatures, while the Mo/Nb/V 3:1:1 system was the most active at higher temperatures. The selectivity to propene increases at increasing temperatures and it reaches the maximum value at 490 8C, in correspondence to the achievement of the total conversion of oxygen. The increase of the selectivity to propene at increasing temperatures corresponds to the decrease of amount of COx products and it is probably due to the contribution of gas phase free-radical reactions that start on the catalyst surface at temperatures higher than 450 8C. At temperatures higher than 530 8C, the selectivity to propene further increases probably because the contribution also increases of heterogeneously initiated homogeneous gas phase reactions. The selectivity to oxygenated products resulted generally low. Only in one case, i.e. for the sample with Mo/Nb/V = 3:1:1, the selectivity to acrylic acid reaches a 20% level; in the other cases small amounts of acrylic and acetic acids were detected at the lower temperatures and of acrolein at the higher ones, also probably due to the contribution of free-radical reactions, which is favourable to the formation of aldehydes as products. 5. Conclusions The sol–gel method of preparation is a good synthesis route to obtain binary and ternary mixed oxide catalysts from Mo, Nb, Sb and V. A lot of crystalline two-component mixed phases have been obtained in this work, but the problem has been found of the stoichiometry control. The compositions of the

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mixed phases obtained were generally not corresponding to the atomic ratios between the precursors used. All the synthesized systems have shown a very similar catalytic activity in the selective oxidation of propane. The values of selectivity to propene were in all cases lower than 40%. Only the sample with Mo/Nb/V = 3:1:1 has shown a good selectivity to acrylic acid at relatively low temperatures. References [1] J.M. Oliver, J.M. Lo`pez Nieto, P. Botella, A. Mifsud, Appl. Catal. A: Gen. 257 (2004) 67.

[2] T. Balasco, P. Botella, P. Concepcio`n, J.M. Lo`pez Nieto, A. Martinez-Arias, C. Pietro, J. Catal. 228 (2004) 362. [3] P. Botella, E. Garcia-Gonza`les, A. Dejoz, J.M. Lo`pez Nieto, M.I. Va`zquez, J. Gonza`les-Calbet, J. Catal. 225 (2004) 428. [4] S. Albonetti, F. Cavani, F. Trifiro`, Catal. Rev. Sci. Eng. 38 (1996) 413. [5] M. Cutaro, C. Pagliuca, L. Lisi, G. Ruoppolo, Termochim. Acta 381 (2002) 65. [6] R.H.H. Smits, K. Seshan, J.R.H. Ross, L.C.A. van den Oetelaar, J.H.J.M. Helwegen, M.R. Anantharaman, H.H. Brongersma, J. Catal. 157 (1995) 548. [7] Z. Zhao, X. Gao, I.E. Wachs, J. Phys. Chem. B 107 (2003) 6333. [8] P. Moggi, S. Morselli, C. Lucarelli, M. Sarzi-Amade`, M. Devillers, Stud. Surf. Sci. Catal. 155 (2005) 427.