Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition

Applied Catalysis A: General 180 (1999) 163±173

Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition H. Provendiera, C. Petita,*, C. EstourneÁsb, S. Libsa, A. Kiennemanna a

LERCSI-ECPM UMR 7515 ± 1, rue Blaise Pascal, 67008 Strasbourg, France b IPCMS-GMI UMR 7504, 23, rue du Loess, 67037 Strasbourg, France

Received 3 July 1998; received in revised form 23 September 1998; accepted 30 September 1998

Abstract Mixed LaNixFe(1ÿx)O3 perovskite oxides (0x1) have been prepared by a sol±gel related method, characterised by X-ray diffraction (XRD), speci®c surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly ef®cient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi0.3Fe0.7O3. # 1999 Elsevier Science B.V. All rights reserved. Keywords: LaNiO3; LaFeO3; Ni; Stability; Partial oxidation; Methane; Syngas

1. Introduction Catalytic partial oxidation of methane (CPO) to synthesis gas was ®rst investigated in the thirties and forgotten during almost 50 years, certainly because of the increasing interest for the steam reforming of methane [1±3]. Reviews have recently summarised the advances in the various ways for the conversion of methane into synthesis gas [4,5]. Like in the steam reforming of methane, metals such as nickel, cobalt or iron, deposited on various supports have been studied in detail [6±12]. Recently, Vermeiren et al. [7] indicated that the activity *Corresponding author. Fax: +33-3884-16863; e-mail: [email protected]

observed in the oxyreforming of methane was about 13 times as high as that of the steam reforming of methane, using the same catalytic system. Studies carried out by Aschcroft et al. [13,14] and Boucouvalas et al. [15] reported that catalysts containing ruthenium were active in the oxyreforming of methane. Catalysts containing other precious metals like rhodium [16±20], platinum [21,22] or palladium also perform well in this reaction. If the mechanism is considered, numerous authors [6,7,10,13,19] con®rm the initial proposal of Prettre et al. [2], supposing a total oxidation of methane into CO2 and H2O as a ®rst step, followed by a sequence of steam reforming and reverse gas shift reaction to give product yields corresponding to the thermodynamic equilibrium. However, other authors: Hickman and Schmidt [16,23],

0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00343-3

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Choudary et al. [24,25], and Holmen et al. [22] reported, in the CPO of methane, conversions and CO and H2 selectivities close to 100% for short residence times, indicating that these compounds are the primary products under kinetic control of the reaction. In most of these catalytic systems, the active part is reported to be the reduced metal. However, in number of recent works, the conventional catalysts combining a metal deposited on various supports are replaced by new and more sophisticated preparations, in which the metal is initially inserted in well-de®ned structures. So, in several works, ruthenium or iridium-containing pyrochlores, such as Pr2Ru2O7, Gd2Ru2O7 [26], or Eu2Ir2O7 [27] have shown to be interesting compared to Ru/Al2O3. The same observation is made for RhVO4 supported on SiO2 compared to RH/SiO2 [28]. Other combined structures like perovskite: LaCrO3, LaNiO3, LaRhO3, LaCoO3 [29,30] or spinel [31] show good potentials. The interactions created by the formation of bonds between Ni oxide and rare earth permit to increase the reduction temperature of nickel oxide. However, particularly for LaNiO3, the thermal stability of the perovskite is low under reductive atmosphere and the coke formation can still be important. In these conditions, Slagtern and Olsbye [30] conclude to the interest of the addition of a third metal into the perovskite structure to stabilise the catalytic system and to limit the metal particle growth. The aim of the use of a de®ned structure is the limitation of coke formation by the control of nickel particle size and the reaction of a stronger interaction with the support. So, plurimetallic perovskite structures like Ca0.8Sr0.2Ti0.8Ni0.2O3 [32], CaTi0.5Ni0.5O3 [33], La0.66Sr0.34Ni0.3Co0.7O3 [34] have been shown to be active in the oxyreforming of methane. Our work deals with the performances of LaNixFe1ÿxO3 perovskite catalysts in the CPO of methane. These three elements La, Ni, Fe were chosen in agreement with the literature data: lanthanum is a basic rare earth preventing coke formation, nickel is very active in the CPO of methane and the second metal (iron) is especially interesting because the ionic radius is close to that of Ni. The two metals (Ni, Fe) can be involved into perovskite structures with lanthanum: LaNiO3,

LaFeO3, but also into mixed perovskites like LaNixFe1ÿxO3 [35]. Considering that the LaFeO3 structure is thermodynamically more stable than LaNiO3 [36], the presence of iron in the structure could stabilise the nickel inside the bulk, particularly under reductive atmosphere [37]. The reactivity of the trimetallic La± Ni±Fe structures in the CPO has been studied focusing on the effect of the presence of iron on the stabilisation of the LaNiO3 structure in reductive conditions. An important point is the evolution of the catalytic structures under test and their possible regeneration. 2. Experimental 2.1. Catalyst preparation The mixed LaNixFe1ÿxO3 systems have been prepared via a sol±gel related method with x values varying from 0 to 1 with a 0.1 step. Lanthanum nitrate La(NO3)3
6H2O, nickel nitrate Ni(NO3)2
6H2O and iron nitrate Fe(NO3)3
9H2O were separately dissolved in hot propionic acid and stirred under re¯ux. The respective amounts of nitrate salts used were adapted to each x value (0x1). The nickel and iron propionic solutions were mixed and rapidly added to the lanthanum one. After a 30 min stirring, the resulting solution was evaporated until the formation of a resin. In some cases, the concentration of the solution leads to nitrate decomposition, producing nitrous vapours and foam formation. This step must be controlled due to the exothermic phenomenon. The resulting resin or foam was calcined at an increasing temperature with an increasing slope of 38C minÿ1 and maintained at 7508C for 4 h. 2.2. Characterisation of the formed oxides The catalysts were characterised by powder X-ray diffraction (XRD) on a Siemens D-5000 diffractometer using Cu K radiation for the determination of the crystalline phases and for the calculation of the lattice parameters. Transmission electron microscopy (TEM) analyses were performed on a TOPCON EM002 B apparatus coupled to an energy dispersive X-ray spectrometer (EDS) in order to determine the nanoscopic state of the catalyst, its homogeneity and the lattice parameters.

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The total amounts of La, Ni, Fe and the amount of C deposited on the catalysts during test were measured by elemental analysis in the CNRS Centre in Vernaison. Temperature programmed reduction (TPR) measurements were performed on a 50 mg sample placed in a U-shaped quartz reactor (6.6 mm ID) using a heating slope of 158C minÿ1 from 258C up to 9008C. The reductive gas used was a mixture of He and 3% H2 (total ¯ow 50 ml minÿ1). A thermal conductivity detector analysed the ef¯uent gas after water trapping and permitted to quantify the hydrogen consumption.

analysis were performed every 608C, by a stage of 1 h at 8008C (three GC analysis) and by a stepwise decrease every 508C, down to 4008C (one GC analysis at each stage). The second cycle was comparable to the ®rst one except that the initial temperature was equal to 4008C, at which the sample was staying all night long under argon atmosphere. The third cycle was limited to a temperature increase up to 8008C. The catalyst was taken after a rapid cooling. Methane conversion, CO selectivity and CO yield were calculated as follows: Conv

…CH4 † ˆ …CH4in ÿ CH4out †  100=CH4in ;

2.3. Reaction test

Select

…CO† ˆ CO  100=…CH4in ÿ CH4out †;

Yield

…CO† ˆ CO  100=CH4in :

The operating conditions were the following: ®xed bed quartz reactor (6.6 mm ID); inlet temperature: 400±8008C; feed ¯ow rate: 0.4 l hÿ1 CH4, 0.2 l hÿ1 O2 and 2.4 l hÿ1 Ar; weight of the catalyst: 200 mg. The outlet gas was analysed by two gas chromatographs used simultaneously (the one giving the amount of remaining CH4 and of produced CO, CO2; the second one quantifying CO and H2 production). The initial CH4 data (CH4 in) were measured by by-pass analysis. The temperature program was composed of an initial treatment and three cycles given in Fig. 1. The initial treatment consisted of a temperature increase from 258C to 5008C with a gradient of 108C minÿ1. During this step no synthesis gas formation was observed for each catalyst. The ®rst cycle was formed by a continuous increase of temperature from 5008C to 8008C (slope of 38C minÿ1), during which chromatographic

3. Results and discussion 3.1. Characterisation of the La±NixFe1xO3 systems before catalytic test The elemental composition in the series of LaNixFe1ÿxO3 perovskite systems (0x1), determined by elemental analysis, presents a good balance between the theoretical and the experimental values (error <1%). The BET surface areas of the series are close to 5 m2 gÿ1 catalyst (3.3±6.2 m2 gÿ1). The XRD diagrams of the LaNixFe1ÿxO3 series were compared to LaNiO3 [38] and LaFeO3 [39] reference diagrams. In each case, only one perovskite phase is obtained (Fig. 2).

Fig. 1. Temperature program used for the catalytic tests. P corresponds to a GC analysis.

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Fig. 4. Lattice parameter of the LaNixFe(1ÿx)O3 structure versus x. Fig. 2. XRD diagram of LaNi0.3Fe0.7O3 with 2 between 20 and 90.

However, a progressive and regular shift of the structure peaks between those of LaFeO3 and LaNiO3 with increasing x was noticed. This indicates the formation of a solid solution of LaFeO3 and LaNiO3 in all proportions (0x1). A zoom on the 2 diagram area between 32.0 and 33.5 shows this progressive shift for the most intensive diffraction peak of some LaNixFe1ÿxO3 structures (Fig. 3). The lattice parameter (a) has been calculated for each x value from the six most intensive diffraction peaks of the assumed pseudo-cubic structures. From Fig. 4, it must be noted that this parameter decreases

linearly with increasing x between 3.92 (LaFeO3) and Ê (LaNiO3), which con®rms the formation of the 3.84 A solid solution [40]. The obtained curve permits to reach the nickel content x of the perovskite system and will be used later as a calibration system to evaluate the migration of nickel from the structure during the catalytic tests. In order to estimate the homogeneity of the solid solutions obtained, the LaNixFe1ÿxO3 systems were examined by TEM and are presented in Fig. 5(a) and (b) for xˆ0.3 and xˆ0.7, respectively. On these micrographs, a regular succession of the atomic planes was observed but the interreticular distances measured for the LaNixFe1ÿxO3 perovskites do not permit to obtain directly the x value, because of the multiplicity of the

Fig. 3. XRD highest peak of the LaNixFe(1ÿx)O3 structures for (a) xˆ0, (b) xˆ0.3, (c) xˆ0.4, (d) xˆ0.5, (e) xˆ0.7, (f) xˆ0.8, (g) xˆ1 in the area 2 between 32.0 and 33.5 with the reference phases (*) LaFeO3 and (8) LaNiO3.

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Fig. 5. Transmission electron micrographs of (a) LaNi0.3Fe0.7O3 and (b) LaNi0.7Fe0.3O3.

observed planes. Electron dispersive X-ray spectroscopy (EDS) coupled to TEM measurements were carried out on different areas of the samples, using a broad focused beam (200 nm) or a ®ne focused beam (14 nm). As shown by Houalla et al. [41], the use of a ®ne focused beam in the scanning transmission mode for X-ray microanalyses gives information about the sample homogeneity. The large focused beam leads to the mean elemental proportions in the sample. Results for LaNi0.3Fe0.7O3 and LaNi0.7Fe0.3O3 are presented in Fig. 6(a) and (b). The combined XRD and TEM

characterisations demonstrate the formation of the La±Ni±Fe solid solution in all proportions and the good homogeneity of the prepared systems was shown in by the constant local elemental distribution given by the EDS. 3.2. Study on the reducibility of the La±Ni±Fe perovskites Numerous authors reported that the active species in CPO of methane is the reduced metal present at the

Fig. 6. La, Ni and Fe elemental distribution observed by energy dispersive X-ray spectroscopy for (a) LaNi0.3Fe0.7O3 and (b) LaNi0.7Fe0.3O3. (1) Broad focused beam. (2) Narrow focused beam.

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surface of the catalyst [26,27,30], the study of the reduction of our structures is necessary and particularly the effect of iron addition on the stabilisation of the active nickel by interaction with the structure. On Ni/Al2O3, the reduction from free NiO to metallic nickel is studied by Swaan et al. [10]. NiO was reduced at a relatively low temperature (lower than 4508C). The stability of LaNiO3 is not very high under reducing atmosphere [42]. The reduction properties of substituted perovskite with a no reductive element were studied by Slagtern et al. [43,44] and a progressive reduction of the structure has been suggested from mixed oxides, where Ni is stabilised. The temperature programmed reduction (TPR) curves of some prepared catalysts are given in Fig. 7 for xˆ0, 0.3, 0.7 and 1. From Fig. 7, it must be noticed that LaFeO3 is almost irreducible in our TPR conditions. On the contrary, LaNiO3 presents two reduction peaks. The ®rst one with a maximum consumption of hydrogen at 4208C, corresponds to La2Ni2O5 formation, so to an oxygen loss by the structure [45]. The hydrogen amount used for this ®rst reduction ®ts with the corresponding reduction of NiIII to NiII. The second peak with a maximum consumption of H2 at 5508C leads to metallic nickel deposited on lanthanum oxide

as determined by XRD. The ®rst reduction peak is similar for all catalysts containing nickel, even though the temperature of the maximum hydrogen consumption decreases slightly with decreasing x (from 4208C to 3708C). The second peak shifts from 5508C for LaNiO3 up to above 9008C for LaFeO3 with decreasing x. During this second reduction step, the nickel and part of the iron are reduced to metals and form a Ni±Fe alloy. In order to have more information concerning the reduction of Ni and Fe and the formula of the alloys, as shown by Imerik and Vedrine [46] the amount of reduced metal at different temperatures can be measured by magnetic methods. The reduction of Ni and Fe during the TPR treatment has been completed by in situ magnetization and MoÈssbauer [47] and these methods con®rm the results of TPR and XRD. In Fig. 8, the temperatures of the maximum hydrogen consumption for the second reduction peak are reported. This value decreases with the addition of nickel into the structure (increasing x) and is above 8008C (maximum temperature of the catalytic test) for x0.5. So the nickel poor catalysts (x0.5) are more stable under reductive conditions than the nickel-rich ones. This shows the stabilising effect of iron addition on the structure under these conditions. The LaNi0.3-

Fig. 7. Temperature programmed reduction curves of (a) LaFeO3, (b) LaNi0.3Fe0.7O3, (c) LaNi0.7Fe0.3O3 and (d) LaNiO3.

H. Provendier et al. / Applied Catalysis A: General 180 (1999) 163±173

Fig. 8. Temperature of the maximum consumption of hydrogen in the second reduction peak for the LaNixFe(1ÿx)O3 structures versus x.

Fe0.7O3 and LaNi0.7Fe0.3O3 catalysts will be taken as representatives of each stability area in order to study their differences in behaviour. 3.3. Study on the catalytic partial oxidation of methane with the perovskite systems LaNixFe1ÿxO3 As indicated in Section 2, all the catalytic tests were performed following the same temperature program composed of two cycles and a third heating slope without previous reduction of the catalyst (Fig. 1). A change in the catalytic behaviour is observed between the ®rst heating and cooling ramps. Indeed, reporting the CO yields versus the temperature in the total ®rst cycle, a hysteresis curve was observed. During the ®rst heating ramp, the CO production starts around 7258C, whereas it is still observed at lower temperatures (6508C) during the cooling down ramp. This has been

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shown to be due to the in situ reduction of the catalyst. As soon as the temperature is suf®cient and the inlet oxygen is totally consumed, the CO formation begins (Fig. 9). Experimentally, the ®rst cycle permits to activate the catalytic system. In our opinion, the nickel present in the catalytic system is slowly reduced and migrates progressively from the bulk of the perovskite to the surface. This phenomenon is observed between 7258C and 7508C in our testing conditions. After activation of the catalysts, differences in the catalytic behaviours have been evidenced depending on x. Fig. 10 presents the CO yields versus x for 0x1 at different temperatures in the second cycle. These values have been obtained at steady state during the second cooling ramp after a 24 h reaction. At 8008C, LaFeO3 and LaNi0.1Fe0.9O3 systems give low CH4 conversions (31% and 34%, respectively) with high CO2 selectivities (80% and 73%), and thus, low CO yields (6.3% and 9.3%). For the other catalysts of the series (x>0.1), high activities to syngas are obtained at 8008C in the three cycles. The CO yields are around 82% for the mixed perovskites and reach 96.3% for LaNiO3. The catalytic behaviours of the perovskite systems with 0.2x0.9 are comparable, except LaNi0.3Fe0.7O3, which emerges with a CO yield of 87%. It must be pointed out that this catalyst gives almost as good results as LaNiO3 in the formation of syngas. The composition of the products at 8008C is close to that obtained in the thermodynamic equilibrium corresponding to the model of Vermeiren [7], which combines combustion, steam reforming and water gas shift reactions.

Fig. 9. Methane conversion, CO and CO2 selectivities obtained in the second cycle versus the temperature for LaNi0.3Fe0.7O3.

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Fig. 10. CO yields at various temperatures in the second cycle versus x.

3.4. Characterisation of LaNixFe1ÿxO3 after catalytic test In order to understand the evolution of the system during the test, the catalysts were characterised after test by XRD and TEM. Table 1 gathers the crystalline phases detected by XRD after test for each system. The evolution of the catalyst under test depends deeply on the initial Ni/Fe ratio in the perovskite. Six different species were determined by XRD after test: a mixed perovskite LaNiyFe1ÿyO3, for which the y value depends on x and is lower than the initial value of x, and the other phases LaFeO3, La2O3, La2O2CO3, Ni and NiO. According to the results presented in Table 1, the two less active catalysts LaFeO3 and LaNi0.1Fe0.9O3 remained unchanged after the reaction. Table 1 Crystalline phases detected by XRD after test. x

Crystalline phases detected by XRD after catalytic test

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

LaFeO3

*

LaFeO3 LaFeO3 LaFeO3 LaFeO3*

LaNi0.1Fe0.9O3 LaNiyFe1±yO3 LaNizFe1±zO3 LaNiwFe1±wO3

La2O2CO3

Corresponds to phases present as traces.

La2O3* La2O3 La2O3 La2O3 La2O3 La2O3 La2O3

NiO* NiO NiO NiO NiO NiO*

Ni Ni

The XRD diagrams of the used catalysts with 0.2x0.4 show only a mixed perovskite phase LaNiyFe1ÿyO3, whose XRD peak positions have shifted to the LaFeO3 reference in comparison to the initial structure. This con®rms that a corresponding amount of nickel has left the structure during the catalytic test. However, this free nickel was not detected by XRD, either because the quantity was too low, or because the nickel was well dispersed or amorphous. For 0.5x0.8, the catalytic systems went to La2O3, LaFeO3 and NiO with an increasing La2O3/LaFeO3 ratio versus increasing x. For LaNi0.9Fe0.1O3 and LaNiO3, the perovskite structure has totally been transformed into nickel metal, La2O3 and La2O2CO3, and a low amount of LaFeO3 probably remained for xˆ0.9, but could not be detected by XRD. In these testing conditions, the catalytic systems containing iron leads to the LaFeO3 structure and no iron in the metallic state was observed. This shows that the catalyst was less reduced in the CPO testing conditions than in the TPR conditions (H2) and that the activation of the catalytic systems corresponds to a controlled nickel migration from the bulk of the mixed perovskite to the surface. This free nickel lost by the structure then appeared either as NiO (for x8) or as metallic Ni (for 0.8x1). Two hypotheses can be proposed to explain this difference. The ®rst one is the formation of NiO from the perovskite structure directly under test, as the ®rst reduction step prior to the possible reduction of NiO to metallic Ni. A similar observation has been described by Hayakawa et al. [48] for Ca0.8Sr0.2Ti1.0NiO3 and these authors have shown particles of tight agglomerates, which might be formed by sintering of small oxide particles, not decomposed during the reaction. However, in our case, according to the stability of the perovskite, the formation of nickel oxides does not constitute a driving force for the destruction of the perovskite compared to the formation of metallic Ni under CH4 or syngas. Therefore, the alternative possibility is the secondary formation of NiO under decreasing temperature after test, when oxygen is no more totally consumed, or at the storage conditions. Indeed, the lower the reduction temperature is, the more the formed nickel metal particles react with oxygen until pyrophoric behaviour [49]. This indicates that the Ni particles

H. Provendier et al. / Applied Catalysis A: General 180 (1999) 163±173

Fig. 11. La, Ni and Fe elemental distribution observed by energy dispersive X-ray spectroscopy for the used catalysts: (a) LaNi0.3Fe0.7O3 and (b) LaNi0.7Fe0.3O3.

obtained after test for the series x0.8 were small enough to be sensitive to O2 and appeared as NiO, while only the surface of larger Ni particles was oxidised by O2. Complementary characterisations of LaNi0.3Fe0.7O3 and LaNi0.7Fe0.3O3 after test have been obtained by EDS, as shown in Figs. 11 and 12. After test, both catalysts show the same mean La, Ni and Fe composition as before test, considering a large area (200 nm diameter) (Fig. 11(a) and

Fig. 12. La/Fe distribution observed by energy dispersive X-ray spectroscopy for the used catalysts: (a) LaNi0.3Fe0.7O3 and (b) LaNi0.7Fe0.3O3.

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Fig. 12(a), column 1). Other EDS analyses performed on smaller areas of these samples (14 nm diameter) present various elemental distributions, indicating the evolution of the catalyst under test (Fig. 11(a) and Fig. 12(a), columns 2-). After test, LaNi0.3Fe0.7O3 presents a higher homogeneity than LaNi0.7Fe0.3O3. However, some areas containing a high amount of nickel (Fig. 11(a), column 5) were detected, which indicates the presence of free nickel areas in the sample. For LaNi0.3Fe0.7O3, other analyses performed on different areas without apparent pure nickel zones, show mixed perovskite phases LaNiyFe1ÿyO3 containing various amounts of Ni. The similar La/Fe ratios observed in all the analysed areas of the used LaNi0.3Fe0.7O3 (Fig. 12(a)) con®rm the presence of a stable phase containing La and Fe. According to the XRD, the globally formed perovskite is an LaNiyFe1ÿyO3 phase with y close to 0.1. However, for xˆ0.7, the La/ Fe ratios observed in various areas (Fig. 12(b)) are very different, ®tting fairly well with the XRD observation of the two formed phases, La2O3 and LaFeO3. The amount of carbon deposited on the catalyst after test was measured by elemental analysis and the values found are lower than 0.3 wt% of the total catalyst (analysis precision) for all x tested values between 0 and 1. No carbon production was observed in our testing conditions, even after a run of 264 h. As reported by Rostrup-Nielsen [50], carbon formation requires 16 neighbouring nickel metal sites, whereas syngas production needs only 12 sites. Thus, the signi®cant absence of coke in our conditions could be an indicator of the formation of small metal particles. Another explanation is the burning of coke by oxygen under test. This absence of coke permits to predict good ageing properties. After a 250 h test at 8008C, no deactivation was noticed for LaNi0.3Fe0.7O3 (CH4 conversion of 94% and CO selectivity of 96%). LaNiO3, which was transformed into Ni/La2O3 with traces of La2O2CO3 after catalytic test can be regenerated to LaNiO3 after a 12 h calcination at 7508C, as evidenced by XRD, but the regenerated system is much less homogeneous than before test, which can lead to the sintering of the system. LaNi0.4Fe0.6O3, which went after test to LaNiwFe1ÿwO3 with wˆ0.15 and traces of NiO and La2O3, has been calcined for 12 h at 7508C and analysed by XRD. Only one mixed perovskite phase was observed by XRD. Moreover, a zoom in the area of the most intensive peak

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lower amount of LaFeO3. By recalcination, the easy combination of NiO with La2O3 leads to LaNiO3, while LaFeO3 remains. Only the systems with a low nickel content (x<0.5) can be regenerated by a mere calcination. 4. Conclusion

Fig. 13. XRD highest peak of the LaNi0.4Fe0.6O3 structure in three different states: (a) before test, (b) after test and recalcination, (c) directly after test in the area 2 between 31.5 and 33.5 with the reference phases (*) LaFeO3 and (8) LaNiO3.

32.0233.0 (Fig. 13) shows that the regenerated catalyst has almost recovered its initial structure by reinsertion of most part of the free nickel into the perovskite. Indeed, the obtained mixed structure is close to the initial one: the XRD determined structure corresponds to LaNi0.3Fe0.7O3. Probably some free nickel remains at the surface of the perovskite, but as it has not been detected by XRD, it could be well dispersed. TEM±EDS shows that the elemental distribution in the regenerated system LaNi0.4Fe0.6O3 was almost as homogeneous as before the test (Fig. 14). For the initially nickel-rich mixed catalysts like LaNi0.7Fe0.3O3 resulting in NiO, LaFeO3 and La2O3 after test, it was not possible to get the mixed perovskite structure back, but the formation of two separate perovskite phases LaNiO3 and LaFeO3 was observed by XRD and TEM coupled to EDS. Indeed, in the case of x0.5, the catalyst was mainly transformed, under test, into La2O3 and NiO with a

Fig. 14. La, Ni and Fe elemental distribution observed by energy dispersive X-ray spectroscopy for the LaNi0.4Fe0.6O3 catalyst after test and recalcination.

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