OY

Langmuir 2001, 17, 1511-1517

1511

Relationship between Structure of CeNiXOY Mixed Oxides and Catalytic Properties in Oxidative Dehydrogenation of Propane L. Jalowiecki-Duhamel,*,† A. Ponchel,‡ C. Lamonier,† A. D’Huysser,† and Y. Barbaux‡ Laboratoire de Catalyse He´ te´ roge` ne et Homoge` ne, UPRESA C.N.R.S. Nο. 8010, Baˆ t. C3, Universite´ des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex, France Received July 31, 2000. In Final Form: December 6, 2000 The oxidative dehydrogenation (ODH) of propane has been studied on cerium-based mixed oxides. An optimum propene yield of 5.4% is obtained at a low temperature (648 K) on CeNi0.5OY, and this yield can be increased to 6.9% by treating the solid in situ under H2. In situ “under catalytic reaction” X-ray photoelectron spectroscopy evidences that the active Ni species present the characteristic of being able to be reduced and reoxidized easily and reversibly, allowed by their close interaction with Ce species. Moreover, the results obtained by X-ray diffraction analysis performed after a catalytic test are in agreement with the presence of hydrogen species in the anionic vacancies of the solid. So, a low coordination ODH active site is proposed to involve an anionic vacancy common to two cations of different natures, XM-YM′ (where X and Y are the numbers of unsaturation of each cation), allowing a heterolytic abstraction of a H- species from the alkane while the H+ species leads with the O2- species of the solid to a OH- group.

Introduction Selective oxidation is one of the promising routes of utilizing alkanes, which are relatively abundant in natural gas or in liquefied petroleum gas. A process based on the oxidative dehydrogenation (ODH) of low-cost hydrocarbons could contribute to satisfying the growing demand for high-purity light olefins.1 Moreover, ODH of alkanes offers advantages of minimal catalyst deactivation and energy savings compared to thermal dehydrogenation. Progress toward this end requires a thorough understanding of the reaction mechanism which would make possible the identification of factors limiting the selectivity of the reaction and ameliorate catalytic formulations. A large variety of catalysts have been claimed as being effective in the ODH of propane.2-11 Mainly, vanadiumbased catalysts such as VPO and VMgO solids have been developed. Recently, on silica-titania mixed oxide supported molybdenum catalysts a propane yield of 30% has been obtained at 823 K.10 However, utilization of rare earth catalysts in oxidation reactions also seems to be * To whom correspondence should be addressed. E-mail: [email protected]. † Universite ´ des Sciences et Technologies de Lille. ‡ Present address: Universite ´ d′Artois, SP 18, rue J. Souvraz, 62307 Lens Cedex, France. (1) Cavani, F.; Trifiro`, F. Catal. Today 1995, 24, 307. (2) Creaser, D.; Andersson, B.; Hudgins, R. R.; Siveston, P. L. Appl. Catal., A 1999, 187, 147. (3) Adamski, A.; Sojka, Z.; Dyrek, K.; Che, M.; Wendt, G.; Albrecht, S. Langmuir 1999, 15, 5733. (4) Viparelli, P.; Ciambelli, P.; Lisi, L.; Ruoppolo, G.; Russo, G.; Volta, J. C. Appl. Catal., A 1999, 184, 291. (5) Zhang, W. D.; Au, C. T.; Wan, H. L. Appl. Catal., A 1999, 181, 63. (6) Khodakov, A.; Olthof, B.; Bell, A.; Iglesia, E. J. Catal. 1999, 181, 205. (7) Bardin, B. B.; Davis, R. J. Appl. Catal., A 1999, 185, 283. (8) Alca´ntara-Rodrı´guez, M.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A. Langmuir 1999, 15, 1115. (9) Moro-oka, Y. Appl. Catal., A 1999, 181, 323. (10) Watson, R. B.; Ozkan, U. S. J. Catal. 2000, 191, 12. (11) Zhang, W.; Zhou, X.; Tang, D.; Wan, H.; Tsai, K. Catal. Lett. 1994, 23, 103.

attractive. It has been reported that solids composed of oxides of Ce, Sm, Nd, or Y and CeF3 are able to preserve high selectivity at high conversion and that a propene yield of 33% can be obtained.11 The reaction mechanism is still not well-known. However, it is generally admitted that the breakage of the C-H bond from the alkane is the rate determining step and that the reaction mechanism is of the Mars-Van Krevelen type (redox).12 Moreover, it is often agreed that the oxidation of hydrocarbons over oxide catalysts involves surface oxygen/oxygen vacancy participation,5,8 and the oxygen mobility of metal oxide catalysts has something to do with catalytic activity. The fluorite oxides have been extensively studied as oxygen-ion-conducting materials because of their high oxygen vacancy concentration and mobility properties. It has been proposed that anionic vacancies associated to CeO2 and in interaction with metallic particles are the active sites in NO and CO conversion13 and for CO2 transformation into methane.14,15 In our laboratory, an active site has been proposed based on the formation of anionic vacancies in ceria, facilitated by the presence of transition cations.16 The CeMXOY mixed oxides (M ) Ni or Cu) have been described as a mixture of nickel oxide or copper oxide and ceria, modified by the insertion of a part of nickel or copper in its lattice. The size of the nickel oxide or copper oxide varies considerably from clusters to a crystallized material depending on the X value and on the experimental conditions (preparation conditions, calcination temperature, etc.).17 Particular sites where Ce and M are in close interaction have been (12) Mars, P.; Van Krevelen, D. W. Chem. Eng. Sci. Suppl. 1954, 3, 41. (13) Harrison, B.; Diwell, A. F.; Halett, C. Platinum Met. Rev. 1988, 32, 73. (14) Herrmann, J. M.; Ramaroson, E.; Tempere, J. F.; Guilleux, M. F. Appl. Catal. 1989, 53, 117. (15) Trovarelli, A.; de Leitenburg, C.; Dolcetti, G.; Llorca, J. J. Catal. 1995, 151, 111. (16) Lamonier, C.; Ponchel, A.; D’Huysser, A.; Jalowiecki-Duhamel, L. Catal. Today 1999, 50, 247. (17) Wrobel, G.; Lamonier, C.; Bennani, A.; D’Huysser, A.; Aboukaı¨s, A. J. Chem. Soc., Faraday Trans. 1996, 92, 2001.

10.1021/la001103y CCC: $20.00 © 2001 American Chemical Society Published on Web 02/02/2001

1512

Langmuir, Vol. 17, No. 5, 2001

proposed, in the solid solution and at the MO particlesupport interface. In the partially reduced state, anionic vacancies, common to two cations, able to receive hydrogen in a hydridic form are created in the bulk and at the surface of the solid.16 We have studied the transformation of propane to propene on cerium-based mixed oxides. This system was of interest to us at least in the perspective of a mechanistic study (although interesting yields are obtained at lowtemperatures, i.e., in conditions where gas-phase radical reactions are unlikely). Moreover, its performances can be improved by mixing with other metal oxides. To participate in the open debate on the ODH active site, various techniques were used to characterize the solids in the oxidized and H2 reduced states, “under catalytic reaction” and after catalytic reaction.

Jalowiecki-Duhamel et al.

Figure 1. Propane conversion ([) and propene yield (O) and selectivity (9) as a function of temperature on CeNi0.5OY.

Experimental Section The mixed oxides denoted CeMXOY, in which M represents Ni, Cu, Co, Cr, or Zn and X represents the M/Ce ratio, were prepared by coprecipitation of hydroxides from mixtures of cerium and metal nitrates using triethylamine (TEA) as a precipitating agent. The solids were dried at 363 K and calcined in air at 773 K. The loading was measured by microanalysis. Pure cerium and nickel oxides were also prepared and used as reference compounds. The catalytic oxidation of C3H8 was performed under atmospheric pressure in a fixed-bed stainless steel tubular reactor (length 300 mm, internal diameter 15 mm) by cofeeding the nitrogen-diluted reaction gases (C3H8/O2/N2 ) 5/15/80). Prior to the reaction, 0.10 g of each catalyst was mixed with SiC (1:1). The total flow rate was 100 mL min-1 (down flow), and the reaction temperature was in the range of 473-673 K. The temperature reported here is the applied temperature; one must note that a temperature rise can be observed (25 K) at high conversions. The experimental details have been published previously.18 When the oxidative dehydrogenation of C3H8 was performed over a solid previously reduced in-situ under H2 at various temperatures, the catalyst was purged under He for 2 h after the reduction step. The gases at the outlet of the reactor were taken out intermittently with the aid of a sampler directly connected to the system and analyzed by FID gas chromatography with porapak Q and molecular sieve 5A columns. Conversion and selectivity were collected after about 5 h of reaction, when the steady-state conversion and selectivity were obtained for each catalyst. Selectivity was calculated on the C3 (propane) basis. The XPS analyses were performed with a Leybold-Hereaus LHS 10 spectrometer using Al KR radiation (hν ) 1486.6 eV). The details of the spectrometer and the experimental procedure are given in ref 19. H2 reduction treatments and/or catalytic reactions (under 5% C3H8/15% O2/80% N2) were carried out in situ up to 573 K in a preparation chamber attached via ultrahigh vacuum chamber to the analysis chamber. Surface atomic ratios were obtained on the basis of the peak area intensities after correction for instrumental parameters, photoionization cross sections, and electron mean free paths. The structure of the oxides was analyzed by X-ray diffraction (XRD) in a Siemens D5000 diffractometer just after catalytic reaction. The catalyst (400 mg) was treated for 36 h at 548 K or 673 K under the reaction mixture (5% C3H8, 15% O2, 80% N2) in a catalytic reactor. The total flow rate was 6 L h-1. The decrease in temperature was performed under N2.

Results and Discussion Propane Oxidative Dehydrogenation. Propane conversion to propene has been studied on CeMXOY mixed oxides (M ) Ni, Cu, Co, Cr, Zn with X ) 0.5) as a function of temperature. Appropriate blank runs showed that under our experimental conditions the contribution of the gas(18) Boisdron, N.; Monnier, A.; Jalowiecki-Duhamel, L.; Barbaux, Y. J. Chem. Soc., Faraday Trans. 1995, 91, 2899. (19) Ponchel, A.; D’Huysser, A.; Lamonier, C.; Jalowiecki-Duhamel, L. Phys. Chem. Chem. Phys. 2000, 2, 303 and references therein.

Figure 2. Propene yield as a function of temperature on CeM0.5OY solids: M ) Ni (9), Cr (b), Co (×), Cu (O), Zn ([).

phase reaction is negligible. Propene and CO2 were the only products detected. No CO was observed whatever the temperature. Figure 1 shows, as an example, the evolution of the conversion and selectivity obtained on CeNi0.5OY versus temperature. For reaction temperatures equal to or higher than 523 K, propane conversion increases whereas propene selectivity decreases. The selectivity decreases from 50 to 25% in the 525-575 K temperature range, and a propene selectivity of 31% is obtained at 10% of propane conversion. The propene yield obtained on the CeM0.5OY compounds is presented in Figure 2. Among the solids studied, the best catalytic results are obtained on CeNi0.5OY, with an optimum yield of 5.4% obtained at 648 K. Moreover, as shown in Figure 3, when increasing the Ni content in CeNiXOY up to X ) 1 no better results are observed. For CeNiXOY with X g 0.7 and temperatures higher than 675 K, CH4 is also observed among the products obtained. For the sake of comparison, the catalytic activity of CeO2 was also evaluated. On CeO2, at 573 K a propane conversion of 3% is observed with a propene selectivity of 1.6%. As a function of temperature, the conversion and selectivity increase, and at 673 K a propane conversion of 10% is obtained with a propene selectivity of 6%. On NiO at 623 K, a propane conversion of 10% is obtained with a propene selectivity of 17%, and an optimum propene yield of 5.7% is obtained at 723 K, a yield quite similar to that observed on the mixed oxides but obtained at a higher temperature. Moreover, the oxidative dehydrogenation of C3H8 has been performed previously at different temperatures with in situ H2 reduced catalysts. In Figure 4, propene yield and selectivity are reported as a function of propane

Structure and Catalytic Properties of Mixed Oxides

Figure 3. Propene yield as a function of temperature on CeNiXOY, with X ) 0.2 (]), X ) 0.5 (O), X ) 0.7 ([), and X ) 1 (0), on CeO2 (b) and on NiO (/).

Figure 4. Propene yield (9, b, and [) and selectivity (0, O, and ]) as a function of propane conversion on CeNi0.5OY not treated (9 and 0) and previously treated under H2 at 433 K ([ and ]) and at 473 K (b and O).

conversion on CeNi0.5OY pretreated under H2 at 433 and 473 K. Propane ODH obtained on the H2-pretreated CeNi0.5OY at 433 K is equivalent to that obtained on the untreated solid. Besides, at 648 K an optimum yield of about 6.9% can be obtained on the CeNi0.5OY reduced at 473 K, whereas at the same temperature a yield of 5.4% is obtained on the untreated solid. At 10% of propane conversion, a propene selectivity of 41% is obtained. Clearly, the treatment at 473 K under H2 leads to a beneficial effect on the propene selectivity and yield. A similar effect has also been verified on CeNi0.2OY. So, the results obtained here confirm that for the ODH of propane the catalyst works in a partially reduced state and that a redox mechanism is involved. Thus, it is essential to characterize the solid under catalytic conditions. For such a low temperature (648 K), the result obtained is already slightly better than that obtained on some classical V-based ODH catalysts.20,21 Moreover, one must note that a particularly low mass of catalyst is used in the present study (0.1 g). At 673 K, some higher propene yields (up to 10%) have been reported on various catalysts,8,22-25 but, clearly, much better performances have been obtained (20) Grabowski, R.; Grzybowska, B.; Samson, K.; Stoch, J.; Wcislo, K. Appl. Catal., A 1995, 125, 129. (21) Savary, L.; Saussey, J.; Costentin, G.; Bettahar, M. M.; Gubelmann-Bonneau, M.; Lavalley, J. C. Catal. Today 1996, 32, 57.

Langmuir, Vol. 17, No. 5, 2001 1513

at 773 K on cerium-based catalysts (33%).11 Besides, it is always difficult to compare catalytic results obtained under different experimental conditions, as, for example, using a quartz reactor, which allows enhancement of the catalytic results. Characterization of the Catalysts. Among the different CeNiXOY systems studied, the CeNi0.5OY compound has been chosen not only because of its good performance in propane ODH but also because it presents the highest number of interactions between Ce4+ cations and Ni2+ cations located either in the solid solution of ceriumnickel or at the interface of NiO/CeO2.16 Ion sputtering followed by X-ray photoelectron spectroscopy (XPS) analysis has been very useful in order to estimate the size of NiO clusters present in the CeNiXOY, too small to be detected by XRD and transmission electron microscopy (TEM).19 For all the calcined CeNiXOY mixed oxides, the XRD analysis of the ceria-like phase indicates that the substitution of Ce4+ by Ni2+ (which has a smaller ionic radius) increases up to Ni/Ce ) 0.5, and for higher values of X the NiO phase crystallizes.19 It has been seen by XPS that all the oxides have slightly less nickel on the surface than in the bulk and the observed discontinuity near X ) 0.5 has evidenced different structural properties on both sides of this nickel concentration.16 To get some information about the state of the catalytic surface, a solid partially reduced in situ under H2 has been analyzed by XPS as well as under a catalytic test. The in situ reduction under H2 at different temperatures allows obtaining various reduction degrees and avoids a reoxidation of the solid under air. Such precautions are necessary because these solids are able to be easily and immediately reoxidized under air. CeNi0.5OY was first calcined at 773 K for 2 h and then treated for 10 h under H2 (2 L h-1). Two treatment temperatures under H2 were applied (473 and 573 K); the decrease in temperature was performed under H2 and in one case under N2 as reported in Figure 5. The spectra obtained for the oxidized state are reported on the same figure as a reference. A slight modification of the Ce 3d peak is evidenced when treating the solid at a temperature higher than 473 K, and new u′ and v′ components appear in the Ce 3d spectra. These u′ and v′ species characterize the existence of surface Ce3+ cations.26 Nevertheless, the Ce 3d spectra clearly show that the Ce4+ species largely remain in the majority even after the H2 treatment at 573 K. The O 1s peak is not modified; therefore, the oxygen species on the surfaces of the solids treated with H2 at 473 and 573 K remain the O2- species. In fact, the H2 treatment affects the surface nickel species much more than the cerium species. For a 473 K treatment under H2, the decrease of the intensity of the satellite peak and the spreading of the Ni 2p3/2 principal peak characterize the partial reduction of Ni2+ cations into metallic nickel. This process increases with the temperature increase, and only the 573 K treatment leads to a significant shift of the Ni 2p3/2 peak position to lower binding energies. The electron energy is in agreement with the presence of Ni0,26 and the semiquantitative ratios confirm the partial reduction of Ni2+ cations (Table 1). As a matter of fact, the decrease of the Ni/Ce atomic ratio under H2 can be explained by the agglomeration of metallic nickel clusters on the surface (22) Eon, J. G.; Olier, R.; Volta, J. C. J. Catal. 1994, 145, 318. (23) Komatsu, T.; Uragami, Y.; Otsuka, K. Chem. Lett. 1988, 1903. (24) Mizuno, N.; Suh, D. J. Appl. Catal. 1996, 146, L249. (25) Viparelli, P.; Ciambelli, P.; Volta, J.-C.; Hermann, J. M. Appl. Catal. 1999, 182, 165. (26) Wrobel, G.; Sohier, M. P.; D’Huysser, A.; Bonnelle, J. P.; Marcq, J. P. Appl. Catal. 1993, 101, 73.

1514

Langmuir, Vol. 17, No. 5, 2001

Jalowiecki-Duhamel et al. Table 2. Superficial Reduction Rate of CeNi0.5OY Treated under H2 at Different Temperatures species

treated at 473 K (%)

treated at 573 K (%)

Ce3+ Ni0 a Ni0 b Ni0 c

< 20 20 20 20-30

< 20 60 50 50-55

a Calculated, taking into account the reduction of Ce. b Calculated, neglecting the reduction of Ce. c Estimated by spectral decomposition of Ni 2p3/2 peak.

Figure 5. (A) Ce 3d and Ni 2p XPS spectra and (B) Ni 2p3/2 XPS spectra obtained on CeNi0.5OY (a) calcined, (b) H2 reduced at 473 K (*temperature decrease under N2), (c) H2 reduced at 473 K, and (d) H2 reduced at 573 K. Table 1. Binding Energies and Surface Atomic Ratios of CeNi0.5OY after Various Treatments treatment oxidized H2 (N2)c H2 H2 ODH H2 at 473 K and ODH H2 at 573 K and ODH

T (K)

E1 Ni 2p3/2 (eV)a

E1 O 1s (eV)a

473 473 573 673 673

854.5 (4.5) 854.3 (4.5) 854.4 (5.2) 852.7 (5.1) 854.4 854.5

529.1 (3.2) 529.2 (3.2) 529.4 (3.1) 529.2 (3.0) 529.1 529.2

0.37 0.27 0.23 0.20 0.32 0.30

2.00 2.02 1.92 1.78 1.89 2.01

529.3

0.28

2.00

673 854.8

[Ni]/[Ce]b [O]/[Ce]b

a In parentheses are the full width at half-maximum values of the lines. b Atomic ratios calculated from Ni 2p3/2, Ce 3d, and O 1s peaks. c The temperature decrease is performed under N2.

of the catalyst. Besides, the decrease of the O/Ce ratio with increasing temperature under H2 is correlated to the loss of oxygen species of the solid and the creation of anionic vacancies. The effect of the treatment under H2 on the size of the nickel clusters has been evaluated from the Ni/Ce atomic ratio, by the use of the Moulijn and Kerkhof model for supported catalysts.27 One can recall that for the calcined CeNi0.5OY solid (oxidized state) the size of the NiO clusters, (27) Kerkhof, F. P. M. J.; Moulijn, J. A. J. Phys. Chem. 1979, 83, 1612.

according to this model, has been estimated at about 15 Å.19 After treatment under H2 at 573 K, the size of the clusters becomes close to 40 Å (Ni/Ce ) 0.5), which shows that the agglomeration of the crystallites is not so important. On the basis of previous works,19,26 the cerium reduction rate in the CeNi0.5OY solid reduced under H2 at 473 and 573 K does not exceed 20%. The metallic nickel content can be estimated by comparing the O/Ce ratio obtained after the calcined state and the H2 reduced state taking into account or neglecting the reduction of cerium as presented in Table 2. Moreover, a decomposition of the Ni2p3/2 line also permits deduction of the Ni2+ and Ni0 contents. It appears that the CeNi0.5OY catalyst treated under H2 at 473 K can have at the surface between 20 and 30% of Ni0 species, and when treated under H2 at 573 K it can have between 50 and 60% of Ni0 species. The decrease in temperature under N2, after the treatment under H2 at 473 K, leads to a complete recovery of the Ni 2p3/2 spectral features of the initial calcined solid. Moreover, the surface atomic ratios of the calcined CeNi0.5OY solid are almost recovered. These results show that the Ni0 species are reversibly reduced after the treatment under H2 at 473 K, in agreement with a redox process involving the Ni2+/Ni0 couple.16,17 In this case, the Moulijn and Kerkhof model leads to a NiO cluster size close to 24 Å, showing that the NiO dispersion does not vary so much, after the H2 treatment followed by the N2 purge, compared to the calcined state. To analyze the behavior of the catalyst under catalytic conditions, CeNi0.5OY was first calcined at 773 K for 2 h and then in situ catalytically tested with and without a previous treatment under H2. The solid was tested for 3 h under C3H8/O2/N2 at 20% of propane conversion (verified by gas chromatography) without exposure to ambient air, and the decrease in temperature was performed under N2. Figure 6 shows the Ce 3d and Ni 2p lines obtained after the catalytic test, and the surface compositions are given in Table 1. Without H2 pretreatment, after the catalytic test (Figure 6a′) the oxidation state observed for Ce is +IV and the resolution of the u and v′′′ peaks is slightly better than those obtained after H2 treatment and the catalytic test (Figure 6c′,d′). When the solid is previously treated under H2, after the catalytic test the spectral characteristics of the Ni 2p3/2 and O 1s peaks involve the existence, at the surface, of Ni2+ cations (different from those in an NiO environment) and of lattice O2- species. Therefore, the Ni2+ species reduced under H2 at 573 K are reoxidized under catalytic conditions, which confirms their redox properties. The Ni/Ce ratio obtained is almost the same as that in the oxidized state (0.32), in agreement with the reoxidation of the metallic nickel clusters formed during the H2 treatment. However, the increase of the cluster size leads to a worse dispersion. Besides, an increase of the O/Ce ratio is observed after the catalytic test when

Structure and Catalytic Properties of Mixed Oxides

Figure 6. Ce 3d and Ni 2p XPS spectra under in situ propane ODH obtained on CeNi0.5OY previously calcined (a′) and on CeNi0.5OY previously H2 reduced at 473 K (c′) and 573 K (d′). Table 3. ∆(2θ) Values Obtained on CeNi0.5OY after Catalytic Test under C3H8/N2/O2a temp ∆(2θ) (deg) ∆(2θ) (deg) ∆(2θ) (deg) (K) (111) peak = 28.62° (200) peak = 33.18° (220) peak = 47.61° 548 673

0.09 0.09

0.08 0.11

0.06 0.12

a ∆(2θ) ) initial peak position - peak position after catalytic test.

the solid is previously treated under H2 compared to the results obtained after the catalytic test without pretreatment. The XPS results obtained after the catalytic test in situ suggest that the nickel species, belonging to the small clusters and/or to the solid solution, participate actively in the catalytic reaction. It has already been shown that this type of nickel is in close interaction with cerium,16 and it is clear that these nickel species possess the ability to be reduced and reoxidized easily and reversibly. The evolution of the XRD spectra when treating the solid in situ under H2 as a function of the temperature has been studied. A careful examination has shown that each diffraction line is shifted toward lower angles for treatment temperatures higher than 423 K and clearly an expanded phase has been formed during the reduction treatment.16,17 After propane ODH reaction at 548 and 673 K, XRD analysis of the CeNiXOY solid shows only two crystallographic phases, CeO2 (JCPDS 34-394) and NiO (JCPDS 4-835). Therefore, once again no drastic crystallographic modification is apparent, but a careful examination of the results obtained on CeNi0.5OY shows that each diffraction line follows the same tendency observed on the H2-treated compound (Table 3). As a matter of fact, after propane ODH reaction at 673 K values of ∆(2θ) (with ∆(2θ) ) initial peak position - peak position after catalytic test) between 0.09° and 0.12° are observed for the most intense diffraction lines. For comparison, similar values are obtained for a treatment temperature under H2 of about 500 K.16 By comparison to the in situ XRD performed under H2, the results obtained after propane ODH can be explained by the reduction of some Ni2+ cations and/or by the insertion of hydrogen of a hydridic nature in the CeO2 lattice. As a matter of fact, considering the relative sizes of H- and O2- (1.54 Å and 1.32 Å, respectively) the lattice expansion observed in the reduction step under H2 has been attributed to the substitution of an O2- species by an H- species even if the contribution of some reduced cations also has to be considered.17 It is important to recall that CeNi0.5OY presents the highest values of ∆(2θ) when

Langmuir, Vol. 17, No. 5, 2001 1515

it is treated under H2 as a function of temperature whereas the Ni2+ species are not more reduced in this compound compared to the other solids studied (X ) 0.2, 0.7). Therefore, it is possible that some hydrogen species of a hydridic nature are maintained in this solid even after exposure to air and despite the violent reactivity of Hwith O2. Propane ODH Active Site and Mechanism. The propane activation mechanism is still under controversy. It is well-known that the C-H bond activation can result from different mechanisms. If one does not take into account the radical process, even if the abstraction of a hydride species from the alkane has already been proposed in the literature on solid superacid catalysts28,29 the transfer of the H+ species has been much more often proposed. In the case of a heterolytic rupture, the two species H+ and H- can coexist but the detection of the hydride species is much more difficult mainly because of its high reactivity. The importance of O2- species and unsaturated sites has already been proposed in the literature for propane activation on V-Mg-O catalysts.30-32 In previous studies, it has been shown by work function measurements that propane reacts with O2species located at the surface of oxide catalysts.18 Therefore, the O2-Mn+0 site can be involved in the alkane activation as has been proposed for ODH of propane32,33 and isobutane.34 Clearly, the treatment under H2 at 473 K leads to a beneficial effect on the propene yield. It has been shown previously that this treatment allows the CeNiXOY mixed oxides to accept large quantities of hydrogen by the presence of anionic vacancies.16 Taking into account that dehydrogenation requires the abstraction of hydrogen species from the hydrocarbon, the ability of the solid to accept hydrogen can be important. Therefore, by analogy to the heterolytic dissociation of H2 the heterolytic dissociation of propane can be envisaged on a low coordination site involving an anionic vacancy. As with H2, a hydride species will be localized in the anionic vacancy and a H+ species will form with the O2- species a hydroxyl group:

O2-Mn+0 + C3H8 f OH-Mn+H- + C3H6

(1)

2H- + O2 f O2- + H2O + 0

(2)

2OH- f H2O + O2- + 0

(3)

This finally gives the equation generally proposed for propane ODH:

C3H8 + 1/2O2 f C3H6 + H2O

(4)

The high reactivity of the hydride species permits consumption of O2, forming water, and therefore the solid can present an enhancement at the surface of the hydroxyl groups’ concentration; it also permits transformation of O2 into selective oxygen species O2-, which regenerates (28) Hatori, H.; Takahashi, O.; Takagi, M.; Tanabe, K. J. Catal. 1981, 68, 132. (29) Takahashi, O.; Hatori, H. J. Catal. 1981, 68, 144. (30) Wang, R.; Xie, M.; Li, P.; Ng, C. F. Catal. Lett. 1994, 24, 67. (31) Soenen, V.; Herrmann, J. M.; Volta, J. C. J. Catal. 1996, 159, 410. (32) Pantazadis, A.; Auroux, A.; Hermann, J.-M.; Mirodatos, C. Catal. Today 1996, 32, 81. (33) Jalowiecki-Duhamel, L.; Ponchel, A.; Barbaux, Y. Stud. Surf. Sci. Catal. 1997, 110, 383. (34) Jalowiecki-Duhamel, L.; Monnier, A.; Barbaux, Y.; Hecquet, G. Catal. Today 1996, 32, 237.

1516

Langmuir, Vol. 17, No. 5, 2001

Jalowiecki-Duhamel et al. Chart 1. Modeling of Sites A (Solid Solution) and B (NiO Particle Support Interface)a

Figure 7. Propane ODH measured at 648 K reported in mol C3H6 g-1 h-1 and in mol mol(Ni)-1 h-1 as a function of the Ni content present in the CeNiXOY solids.

the active site, O2-Mn+0. Indeed, an increase of the O/Ce ratio is observed by XPS after the catalytic test when the solid is previously treated under H2 compared to the results obtained after the catalytic test without pretreatment. The dynamic comportment of the solid that is a hydrogen reservoir can also explain the phenomenon observed when the catalytic reaction is roughly stopped by replacing the reaction mixture by N2. As a matter of fact, a high degree of coordinative unsaturations under H2 can exist, but changing the gas phase leads to the migration of hydroxyl groups OH- to the surface of the solid.35 The metallic nickel species are reoxidized into Ni2+ species. The progressive blocking of the coordinative unsaturations leads to the saturation of the surface by oxygen species, and this phenomenon explains why the O/Ce ratios are found to be similar to those obtained after calcination or those obtained after reduction under H2 followed by a purge under N2. These experiments confirm that the CeNiXOY catalysts possess particular redox properties through the presence of the Ni2+/Ni0 couple in close vicinity to Ce cations but also through good diffusion properties of hydroxyl groups. Propane ODH measured at 648 K can be reported in mol C3H6 g-1 h-1 as a function of the Ni content present in the CeNiXOY solids, as presented in Figure 7. A propene optimal formation is obtained when the nickel content reaches about 20%, which corresponds to the CeNi0.5OY catalyst. The solid studied containing the lowest nickel content (0.03%) corresponds to the CeNi0.01OY catalyst. For nickel contents higher than 20%, the activity decreases. Now, when the ODH activity is reported in mol mol(Ni)-1 h-1 (which corresponds to the ODH activity in mol C3H6 g-1 h-1 divided by the Ni content of the solid) a continuous decrease of the activity is obtained as a function of the Ni content of the solid. In the laboratory, a model has been developed on hydrotreating MoS2 catalysts, allowing correlation of the activity to the position of the active sites for different geometrical structures.36 In agreement with this previous study, the curve obtained can be easily explained if only the Ni atoms located at the periphery of the NiO clusters are responsible for the ODH activity. The smaller the NiO crystallites, the higher the proportion of active sites. Therefore, ODH of propane certainly involves particular sites where Ce and Ni are in close interaction. Such a site has already been modeled, as presented in Chart 1, by an (35) Sene, A.; Jalowiecki-Duhamel, L.; Wrobel, G.; Bonnelle, J. P. J. Catal. 1993, 144, 544 and references therein. (36) Kasztelan, S.; Toulhoat, H.; Grimblot, J.; Bonnelle, J. P. Appl. Catal. 1984, 13, 127.

a See ref 16. The number of anionic vacancies is arbitrary. Mn+ ) Ni2+, Niδ+. M′n+ ) Ce4+, Ce3+.

ensemble of two cations in close interaction whether in the solid solution (site A) or at the NiO/CeO2 interface (site B) (the number of anionic vacancies presented is arbitrary).16 The different behavior of the catalysts depends, of course, on the number of active sites but also on the different natures of the metallic cations, which can have various oxidation degrees and so possess different unsaturation degrees. In particular, it has been shown in the laboratory that a 3 coordinatively unsaturated site (CUS) (3M) is a prerequisite condition for alkadiene hydrogenation activity, whereas 2 and 4 CUS (2M and 4 M) are related to alkadiene isomerization. Thus, different ensembles XM-YM′ (where X and Y are the number of unsaturations on each cation) have been proposed to be the active sites, each elementary ensemble being associated with a particular reaction.35 Therefore, H2 reduced CeO2 which has no hydrogenation activity cannot possess Ce cations with more than two coordinative unsaturations, whereas nickel species in NiO which have hydrogenating functions can be surrounded by three coordinative unsaturations. So, by analogy one can suppose that under an oxido-reduction mixture the XCe-YNi sites would be less unsaturated than some XNi-YNi sites and lead to a better selectivity in propene. Moreover, the reducibility of Ni2+ cations is influenced by the presence of Ce in close proximity which forbids a total and irreversible reduction of the Ni species in interaction, even in some strong reducing conditions. The presence of metallic nickel species that are not reoxidizable is a poison for the ODH of propane and is certainly related to the cracking phenomenon observed at high conversions (CH4, C2H4). In an oxido-reduction environment, the number of anionic vacancies at the surface of the solid is certainly limited. A possible representation of the reaction mechanism is presented in Scheme 1 that involves a heterolytic dissociation of propane. This propane activation mechanism is relatively close to the concerted mechanism proposed by G. Busca et al. on V-P-O systems for the activation of n-butane,37 with the difference that the abstraction of a hydride species is proposed here. Moreover,

Structure and Catalytic Properties of Mixed Oxides

Langmuir, Vol. 17, No. 5, 2001 1517

Scheme 1. Modeling of Propane ODH Mechanism

the propane activation mechanism on a site involving an anionic vacancy common to two cations of different natures is in agreement with the mechanisms proposed in the literature which require an oxido-reduction cycle on several sites and also permits explanation of the synergic effect observed in propane ODH. Conclusion In the series of CeMXOY solids studied, the CeNi0.5OY catalyst gives, at low temperature, results similar to those obtained on some classical propane ODH catalysts. Moreover, pretreating CeNi0.5OY solids under H2 ameliorates propene selectivity. The characterization of the solids has already evidenced that CeNi0.5OY presents the highest number of interactions between Ce and Ni cations located either in the solid solution of cerium-nickel or at the interface of NiO/CeO2. The XPS results obtained after the catalytic test in situ suggest that the nickel species, (37) Busca, G.; Centi, G.; Trifiro, F. Appl. Catal. 1986, 25, 265.

belonging to the small clusters and/or to the solid solution, participate actively in the catalytic reaction. The active Ni species present the characteristic of being able to be reduced and reoxidized easily and reversibly, allowed by their close interaction with Ce species. Although the dehydrogenation step requires the abstraction of hydrogen from the hydrocarbon, XRD analysis performed on CeNi0.5OY after the catalytic test is in agreement with the presence of hydrogen species of a hydridic nature in anionic vacancies of the solid. Therefore, an active site has been proposed based on the formation of anionic vacancies common to two Ce and Ni cations. Moreover, by analogy to the heterolytic dissociation of H2 a mechanism of alkane dehydrogenation is proposed, involving a heterolytic abstraction of a hydride species. Acknowledgment. One of the authors is grateful for an MRT grant from Ministry. LA001103Y