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CO2 reforming of CH4 over La–Ni based perovskite precursors
Applied Catalysis A: General 311 (2006) 164–171 www.elsevier.com/locate/apcata
CO2 reforming of CH4 over La–Ni based perovskite precursors Germa´n Sierra Gallego a,b, Fanor Mondrago´n b, Joe¨l Barrault a, Jean-Michel Tatiboue¨t a, Catherine Batiot-Dupeyrat a,* a
Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, Universite´ de Poitiers, Ecole Supe´rieure d’Inge´nieurs de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France b Universidad de Antioquia, Instituto de Quı´mica, A.A. 1226, Medellı´n, Colombia Received 23 May 2006; received in revised form 12 June 2006; accepted 12 June 2006 Available online 21 July 2006
Abstract LaNiO3 and La2NiO4 type perovskites were prepared by the ‘‘self-combustion’’ method and were used as catalyst precursors for the CO2 reforming of CH4 reaction at 700 8C. The catalysts were tested in reduced and non-reduced form. High CH4 and CO2 conversion were obtained without carbon deposition. This result was explained by the occurrence of the RWGS (reverse water gas shift) reaction. The La2NiO4 perovskite used as precursor presents the smallest nickel particles after the reduction treatment. Consequently the catalytic activity is higher than that obtained with Ni/La2O3 or LaNiO3. When La2NiO4 is used without treatment prior to the reaction high methane and carbon dioxide conversions are reached but a carbon deposition is observed. The perovskite structure is not completely transformed and the presence of metallic nickel particles at the surface of La2NiO4 would be responsible for the carbon deposition. It is assumed that the role of the support is to allow the activation of carbon dioxide, which is favoured over La2O3 whereas it is limited over La2NiO4. Consequently the reaction between the complex C–Ni species (resulting from methane activation at the surface of the nickel particle) and gaseous CO2 is inhibited over Ni/La2NiO4 leading to a carbon accumulation at the surface of the catalyst. As soon as the perovskite structure is completely transformed, after reductive treatment or during the reaction, a high activity is reached and no carbon deposition was further observed, the catalytic performances being optimal when the average nickel particles size is the smallest. # 2006 Elsevier B.V. All rights reserved. Keywords: Perovskite precursors; La2NiO4; LaNiO3; CO2/CH4 reforming; Coke formation
1. Introduction The process of carbon dioxide reforming of methane to produce synthesis gas (CO + H2) has received considerable attention in recent years as it constitutes a very attractive route for the conversion of two low-cost products which can be used for the production of liquid hydrocarbons in the Fischer– Tropsch reaction [1] or in the production of methanol. The reaction has been investigated over both, noble metal [2–6] and Ni based [7–11] supported catalyst. Industrially the metal of choice is nickel due to its inherent availability, low cost and high activity in comparison to noble metals. However, the major problem encountered with the reaction of CO2 reforming
* Corresponding author. E-mail address: [email protected] (C. Batiot-Dupeyrat). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.06.024
of methane is the coke formation leading to the catalyst deactivation especially when Ni is used. Nevertheless it has been shown that a high dispersion of the metal species over the support can limit the coke formation [12]. Shiozaki et al. [13] showed that a metal oxide with a welldefined structure can be a source of small metal particles. Hayakawa et al. used CaTi1xNixO3 as catalyst precursor for the partial oxidation of methane to syngas [14], highly dispersed Ni metals were formed in situ during the reaction resulting in a high activity and stability. In previous papers we have shown that LaNiO3 perovskite used as catalyst precursor, leads to a very active catalyst for the CO2 reforming of CH4 [15,16]. A reducing treatment under hydrogen is not necessary before reaction since the perovskite structure is completely destroyed under reactants and products during the reaction at 700 8C, the only species detected by XRD after 15 h of reaction being Ni0 and La2O2CO3. Moreover we showed that the average nickel
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particle size (measured after reaction) was smaller when the catalyst was used without pre-treatment (20 nm) than when it was reduced under hydrogen prior to the reaction (27 nm). It was expected that the better the dispersion of nickel, the higher is the catalytic activity [17]. In our case, the performance was as high as the thermodynamic equilibrium indicates. The aim of the present work was to investigate the activity and stability of the perovskite La2NiO4 and to compare its performance with that of LaNiO3 and Ni/La2O3. Ni/La2O3 was used as a reference catalyst as suggested by Zhang and Verykios [18]. Low nickel content in La2NiO4: 15 wt.% against 24 wt.% in LaNiO3 would allow to obtain smaller nickel particle size. For that purpose the reaction was conducted within the kinetic-controlling regime. The catalyst was characterized at the different steps of the reaction: before reaction, after reducing treatment and after reaction. 2. Experimental
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2.2.3. Temperature programmed reduction (TPR) and H2 pulses chemisorption TPR and H2 chemisorption experiments were carried out in a Micromeritics Autochem 2910 using about 160 mg of catalyst. TPR experiments were performed using 5% hydrogen in argon, 50 mL/min, with a heating rate of 5 8C/min, from room temperature to 900 8C. H2 consumption was obtained from the integrated peak area of the reduction profiles relative to the calibration curve. Before hydrogen chemisorption, the samples were reduced in a 5% H2/Ar flow while the temperature was risen at 5 8C/min from ambient to 700 8C and maintained at this temperature for 6 h. Then the system was purged in Ar at 720 8C for 3 h and cooled to 50 8C. Hydrogen pulse chemisorption was started at 50 8C using 5% H2/Ar and repeated at 5 min intervals until the hydrogen area picks became identical. The amount of hydrogen consumption was measured with a thermal conductivity detector. Metal dispersion on the surface was calculated assuming the adsorption stoichiometry of one hydrogen atom per nickel surface atom (H/Nis = 1).
2.1. Catalyst preparation The perovskite type oxides LaNiO3 and La2NiO4 were prepared by the self-combustion method [19]. Glycine (H2NCH2CO2H) was used as ignition promoter was added to an aqueous solution of metal nitrates with appropriated stoichiometry, in order to get a NO3/NH2 = 1 ratio. The resulting solution was slowly evaporated until a vitreous green gel was obtained. The gel was heated up to 250 8C, temperature at which the ignition reaction occurs yielding to the formation of a powdered precursor which still contains carbon residues. Calcination at 700 8C for 6 h eliminates all of the remaining carbon and leads to the formation of the perovskite structure. Catalysts containing 1, 5 and 17% Ni/La2O3 were prepared by the wet impregnation method using nitrate salt as metal precursor. Nickel nitrate was placed in a beaker with 60 mL of distilled water. After complete dissolution, the appropriated amount of La2O3 was added under continuous stirring. The slurry was then heated up to 90 8C until the water was evaporated. The residue was then dried at 120 8C for 12 h and subsequently calcinated at 700 8C for 6 h under N2. 2.2. Characterization 2.2.1. X-ray diffraction The catalysts were characterized by powder X-ray diffraction using a Siemens D-5000 diffractometer with Cu ˚ , operated at 40 kV Ka1 = 1.5406 and Cu Ka2 = 1.5439 A and 30 mA. The diffraction patterns were recorded in the 2u values range 10–908 with a step size of 0.018 and 1 s per step. 2.2.2. Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) was carried out on a Philips CM120 instrument, with LaB6 filament and equipped with an energy dispersive X-ray analyzer (EDX).
2.2.4. BET All samples were degassed with He for 30 min at 623 K before measurement, the adsorption–desorption isotherm of N2 was measured using 30% N2/Ar as the adsorbate on a Micromeritics Flowsorb II 2300 apparatus at 196 8C. 2.2.5. Catalytic reaction The reaction was carried out by passing a continuous flow of CH4/CO2/He = 10/10/80 over the catalyst bed. The amount of catalyst was fixed at 20 mg with a total flow rate of 100 mL/min which corresponds to a space velocity of 3 105 mL h1 g1, in order not to reach the thermodynamic equilibrium (at 700 8C, the equilibrium state is reached for: CH4 conversion = 90% and CO2 conversion = 93%). All measurements were conducted using catalyst grain size of 180 mm in order to perform the experiments within the region of intrinsic kinetics. The temperature was increased from room temperature to 700 8C at a rate of 5 8C/min, and maintained at this temperature for the desired reaction time. The temperature was measured with a thermocouple located inside the reactor but without direct contact with the catalyst. In some experiments the perovskite catalyst was used without pre-treatment or after reduction under hydrogen at 700 8C for 1 h. The reaction products were analyzed by an online mass spectrometer. The detection limit was estimated to be around 0.04 mmol. According to the intensity of the signal measured by the MS, this value corresponds to about 1% of the total amount detected. 3. Results 3.1. X-ray diffraction characterization The X-ray diffraction patterns obtained for LaNiO3 and La2NiO4 are shown in Figs. 1 and 2, respectively. After
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Fig. 1. XRD patterns of the LaNiO3 precursor material and after various treatments: (a) PDF 88-0633; (b) prepared catalyst before reaction: LaNiO3; (c) after reduction under hydrogen at 700 8C; (d) after 1 h of reaction at 700 8C over the non-reduced material; (e) after 15 h of reaction using the non-reduced or reduced material. (*) Ni0, (5) La2O3 and (^) La2O2CO3 hexagonal.
Fig. 2. XRD patterns of the La2NiO4 precursor material—(a) PDF 89-3460; (b) prepared catalyst before reaction: La2NiO4; (c) after reduction under hydrogen at 700 8C; (d) after 1 h of reaction at 700 8C over the non-reduced material; (e) after 15 h of reaction using the non-reduced or reduced material. (*) Ni0, (5) La2O3, (^) La2O2CO3 hexagonal and (&) La2O2CO3 monoclinic.
calcination at 700 8C the only presence of the LaNiO3 perovskite structure with a rhombohedral symmetry (Fig. 1b) and La2NiO4 perovskite-like K2NiF4 tetragonal structure (Fig. 2b) were observed. After the reduction treatment under hydrogen at 700 8C, whatever the starting material (LaNiO3 or La2NiO4) the perovskite structure was completely destroyed the only phases detected being Ni0 and La2O3 (Figs. 1c and 2c). After 15 h of CO2/CH4 reforming the presence of lanthanum oxo-carbonate La2O2CO3 was observed with both starting catalysts LaNiO3 and La2NiO4 (Figs. 1e and 2e).
according to Ruckenstein and Hu [20], while the high temperature peak between 520 and 680 8C be due to the reduction of the perovskite phase. Thus, the complete reduction of La2NiO4 requires a higher temperature (680 8C) than for the complete reduction of LaNiO3 (610 8C).
3.2. Temperature programmed reduction The TPR profiles of LaNiO3 and La2NiO4 are shown in Fig. 3. The reduction of LaNiO3 proceeds in three steps, the successive changes of the perovskite structure were determined by an in situ XRD measurement. The results were reported in a previous paper [15]:
3.3. CO2 reforming of methane The reaction was performed using the perovskites LaNiO3 and La2NiO4 following two different procedures: (1) the catalyst was reduced for 1 h under hydrogen at 700 8C prior to the reaction; (2) the catalyst was used without treatment prior to the reaction.
4LaNiO3 þ 2H2 ! La4 Ni3 O10 þ Ni0 þ 2H2 O La4 Ni3 O10 þ 3H2 ! La2 NiO4 þ 2Ni0 þ La2 O3 þ 3H2 O La2 NiO4 þ H2 ! Ni0 þ La2 O3 þ H2 O Thus, the perovskite La2NiO4 is formed as an intermediary during the reduction of LaNiO3, at 610 8C the perovskite is completely reduced into metallic nickel and lanthanum oxide. The TPR profile of La2NiO4 shows two main peaks, the first peak could be assigned to the reduction of amorphous NiO
Fig. 3. TPR curves of the perovskites LaNiO3 and La2NiO4, catalyst weight: 160 mg; gas flow rate: 5% H2/Ar, 50 mL/min.
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Table 1 CH4, CO2 conversions, H2/CO ratio, carbon deposition determined by elemental analysis (in wt.%) after 15 h of reaction Catalyst precursor
LaNiO3 La2NiO4 1% Ni/La2O3 5% Ni/La2O3 17% Ni/La2O3 Fig. 4. Conversion of CH4 and CO2, molar ratio H2/CO; catalyst: 20 mg LaNiO3 reduced under hydrogen at 700 8C for 1 h; reaction temperature: 700 8C.
Conversion after 15 h of reaction (%) CH4
CO2
65 80 32 59 62
81 90 48 73 75
Molar ratio H2/CO
Carbon deposition (wt.%)
0.82 0.84 0.47 0.66 0.84
2.8 2.7 2.6 3.2 12
CH4/CO2/He: 10/10/80; catalyst weight: 20 mg; flow: 100 mL min1; T = 700 8C, catalyst reduced under hydrogen at 700 8C prior to the reaction.
loading on the Ni/La2O3 catalyst an increase of the CH4 and CO2 conversions is observed. The low molar ratio H2/CO obtained with 1% Ni/La2O3 can be due to the occurrence of reverse water gas shift reaction, this reaction being favoured at low methane conversion. The elemental analysis performed after reaction reveals that the amount of carbon is around 3 wt.%, which correspond to the percentage of carbon in La2O2CO3. The results confirm that no carbon deposition is observed after 15 h of reaction except for the 17% Ni/La2O3 catalyst (see Table 1).
3.3.1. CO2 reforming of methane using perovskites reduced prior to the reaction As mentioned above, the reduction step under hydrogen leads to the formation of Ni0/La2O3 whatever the nature of the precursor, as shown by XRD (Figs. 1c and 2c). Figs. 4 and 5 present the catalytic behaviour obtained with the reduced LaNiO3 and La2NiO4, respectively. High conversions of CH4 and CO2 are reached after 2 h of reaction at 700 8C, the catalyst precursor La2NiO4 being the most active. A possible explanation for the larger conversion of CO2 compared to CH4 can be attributed to the occurrence of the reverse water gas shift reaction (RWGS: CO2 + H2 ! CO + H2O). The value of the molar ratio: H2/CO lower than one supports this assumption. After reaction, the only phases detected by XRD are metallic nickel and the hexagonal phase lanthanum oxycarbonate: La2O2CO3, resulting from the CO2 adsorption on La2O3 during the reaction (Figs. 1e and 2e). The performances of perovskites were compared with a lanthanum oxide supported Ni catalyst (Ni0/La2O3) containing various amount of nickel, prepared by the wet impregnation method. The results obtained after 15 h of reaction are gathered in Table 1. The perovskite La2NiO4 used as catalyst precursor shows the best catalytic activity. By increasing the nickel
3.3.2. CO2 reforming of methane using perovskites without treatment prior to the reaction The catalytic behaviour of LaNiO3 and La2NiO4 are reported in Figs. 6 and 7, respectively. The methane and carbon dioxide conversions increase slowly after the temperature has reached 700 8C, particularly over the LaNiO3 perovskite. After 15 h of reaction high conversions were obtained with La2NiO4, but the H2/CO molar ratio remains slightly higher than one. The elemental analysis performed after reaction reveals the presence of a carbon deposition over the catalyst as shown in Table 2. After 15 h of reaction over La2NiO4 the composition of the exhaust gases was approximately the following: CH4, 8%; CO2, 4%; CO, 44%; H2, 44%, the amount of water could not be determined by mass
Fig. 5. Conversion of CH4 and CO2, molar ratio H2/CO; catalyst: 20 mg La2NiO4 reduced under hydrogen at 700 8C for 1 h; reaction temperature: 700 8C.
Fig. 6. Conversion of CH4 and CO2, molar ratio H2/CO; catalyst: 20 mg of nonreduced LaNiO3; reaction temperature: 700 8C.
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Fig. 7. Conversion of CH4 and CO2, molar ratio H2/CO; catalyst: 20 mg of nonreduced La2NiO4; reaction temperature: 700 8C.
spectrometry, nevertheless the oxygen and hydrogen balance strongly suggest that water is formed under this experimental condition according to the reverse water gas shift reaction (RWGS: CO2 + H2 ! CO + H2O). The contribution of the RWGS reaction was calculated from the amount of water formed while the carbon deposition was determined from the carbon balance. It was checked that the amount of carbon missing was in accordance with the amount of hydrogen analyzed (CH4 ! Cs + 2H2). The XRD analysis revealed that after 1 h at 700 8C, under reactant mixture, the perovskite phase La2NiO4 is not completely reduced but the material is composed of a mixture of Ni0, La2O2CO3 and non-reduced La2NiO4 (Fig. 2d). This result is in accordance with the TPR profiles showing that La2NiO4 is not as easy reducible as LaNiO3.
Fig. 8. TEM micrograph obtained after reduction of LaNiO3 under hydrogen at 700 8C.
20 nm. The distribution of the particle size is approximately the same after 15 h of reaction using the catalyst with or without reduction treatment prior to the reaction. Starting from La2NiO4 a narrower size distribution is observed: 2–30 nm, the majority of
3.4. Characterization by electron microscopy TEM analysis was performed after reduction of the catalyst, after 15 h of reaction using the reduced and non-reduced perovskites LaNiO3 and La2NiO4. The TEM micrograph obtained after reduction of LaNiO3 under hydrogen at 700 8C clearly shows the presence of spherical particles of nickel (Fig. 8). Nickel particle size distributions derived from TEM are reported in Fig. 9a and b for LaNiO3 and La2NiO4, respectively. The data are normalized to 100 particles while about 500 particles were measured. Starting from LaNiO3, after reduction under hydrogen, the diameter of the nickel particles ranges between 2 and 50 nm, most of the particles being smaller than Table 2 CH4, CO2 conversions, H2/CO ratio, carbon deposition determined by elemental analysis (in wt.%) after 15 h of reaction Catalyst precursor
LaNiO3 La2NiO4
Conversion after 15 h of reaction (%) CH4
CO2
60 80
75 90
Molar ratio H2/CO
Carbon deposition (wt.%)
0.68 1.03
2.7 70
CH4/CO2/He: 10/10/80; catalyst weight: 20 mg; flow: 100 mL min1; T = 700 8C, catalyst used without treatment prior to the reaction.
Fig. 9. Particle size distribution determined from TEM measurements: (a) LaNiO3 and (b) La2NiO4; hatched: catalyst reduced under hydrogen, black: after reaction using reduced catalyst and white: after reaction using non-reduced catalyst.
G.S. Gallego et al. / Applied Catalysis A: General 311 (2006) 164–171 Table 3 CH4, CO2 conversions, H2/CO ratio, average nickel particles size derived from TEM for LaNiO3 and La2NiO4 Catalyst
Average nickel particles size (nm)
LaNiO3 After reduction After reaction over reduced catalyst After reaction over unreduced catalyst
15 17 18
La2NiO4 After reduction After reaction over reduced catalyst After reaction over unreduced catalyst
7 10 20
1% Ni/La2O3 reduced 17% Ni/La2O3 reduced
12 Agglomerates + Ni average: 19
nickel particles having a diameter lower than 10 nm after the reduction step and after 15 h of reaction using the reduced material. When La2NiO4 is used without treatment prior to the reaction, the presence of large nickel particles with a diameter up to 140 nm is detected. The average nickel particle size is shown in Table 3. The formation of small Ni particles is obtained by reduction under hydrogen of the perovskite precursor, particularly when starting from La2NiO4. The nickel particles size slightly increases after reaction showing a small sintering of metallic nickel during the reaction. No significant differences were observed for LaNiO3 used with or without reductive treatment prior to the reaction, whereas for La2NiO4 the use of the non-reduced material results in an increase of the nickel particles size.
and co-workers [22,23] using La2NiO4. Our study demonstrated that smaller nickel particles are formed by reduction of La2NiO4 (7 nm) compared to LaNiO3 (15 nm), it could be attributed to the smaller nickel content in La2NiO4 than in LaNiO3, 15 and 24 wt.%, respectively. Consequently, the catalytic performance of the catalyst precursor La2NiO4 is better than the one of LaNiO3. Our results are different from those obtained by Guo et al. [24], which have found that LaNiO3 was more active than La2NiO4. However, their results could be due to the too low temperature used during the reduction reaction (500 8C under hydrogen), which would not permit a complete reduction of the perovskite material particularly for La2NiO4. We showed that the perovskites used as catalyst precursor exhibited higher activity than Ni/La2O3 catalyst prepared by the wet impregnation method. The increase in the nickel loading leads to an increase in the CH4 and CO2 conversion, nevertheless a larger nickel content (17%) leads to carbon deposition. The TEM micrograph (Fig. 10) reveals the presence of nickel particles with a size up to 100 nm. The presence of such large particles can explain the coke deposition. The TPR profiles showed that the perovskite LaNiO3 is completely reduced at a lower temperature than La2NiO4, 610 and 680 8C, respectively. The presence of metallic nickel particles formed during the first reduction step between 250 and 360 8C, could explain the easier reducibility of LaNiO3. The nickel particles thus formed would activate molecular hydrogen promoting the reduction of LaNiO3 as well as that of the different intermediates. When starting from La2NiO4, the small
4. Discussion We have shown in a previous paper [16] that the use of the perovskite LaNiO3 prepared by the ‘‘self-ignition method’’ allows to obtain an active and stable catalyst for the carbon dioxide reforming of methane. The present work showed that better catalytic performances can be achieved using the perovskite La2NiO4 as catalyst precursor. The reduction of perovskites at 700 8C under hydrogen leads to the formation of lanthanum oxide and metallic nickel, the perovskite structure whatever the starting material LaNiO3 or La2NiO4 being completely transformed. Using reduced catalysts, the CH4 and CO2 conversion are, respectively, equal to 65 and 81% with LaNiO3, while they are equal to 80 and 90% with La2NiO4. The reverse water gas shift is suggested to occur according to the product composition in the exhaust gases. It results in a H2/CO molar ratio lower than one, the RWGS reaction being then responsible of the transformation of about 19% of the carbon dioxide introduced. This observation is supported by the fact that the presence of coke on the catalyst was not detected. The high activity using perovskite precursors can be explained by the formation of relatively small nickel particles by the reductive treatment, such conclusion is in accordance with the work of Choudhary et al. using LaNiO3 [21] and that of Gao
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Fig. 10. TEM micrograph obtained with 17% Ni/La2O3.
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amount of metallic nickel present after the reduction of NiO is not sufficient to favour the reduction, the temperature has to reach 520 8C in order to initiate the reduction of the perovskite. The composition of the exhaust gases, the carbon deposition and the hydrogen and oxygen balance allow us to propose which can be the main reactions involved (see Section 3). Thus after 15 h of reaction over La2NiO4 used without treatment prior to the reaction, the molar composition of the exhaust gases shows a important ‘‘oxygen’’ and ‘‘hydrogen’’ deficiency, which suggests that the amount of water would be very important (H2O could not be quantify by mass spectrometry). Water is formed in the reverse water gas shift reaction (CO2 + H2 ! CO + H2O). The molar ratio H2/CO is near to one, so the consumption of hydrogen by the RWGS reaction would be compensated by the methane cracking leading to the carbon deposition and the production of hydrogen (CH4 ! C + 2H2). The conversions obtained allow to propose some of the main reactions involved at 700 8C: Methane transformation: 38%
CH4 þ CO2 ! 2CO þ 2H2
42% CH4 ! C þ 2H2 Carbon dioxide transformation: 38%
CH4 þ CO2 ! 2CO þ 2H2
52%
CO2 þ H2 ! CO þ H2 O
This proposal is in accordance with the conversions of CH4 (80%) and CO2 (90%). It shows that the contribution of the reverse water gas shift is very important using La2NiO4 without treatment prior to the reaction since 52% of the introduced CO2 reacts with hydrogen to produce CO and H2O. Nevertheless the carbon deposition according to the Boudouard reaction (2CO ! C + CO2) cannot be completely excluded, but its contribution must be rather very small since the amount of carbon dioxide in the exhaust gases is very low. Our results agree very well with those published by Tsipouriari and Verykios, who showed by using isotopic tracing techniques that most of the carbon accumulated over a Ni/La2O3 catalyst at 750 8C originated from methane cracking [25]. Starting from the perovskite La2NiO4 without treatment prior to the reaction, XRD analysis performed after a reaction time of 1 h at 700 8C revealed that a mixture of La2NiO4 and Ni/La2O3 is present. The non-reduced perovskite could explain the importance of carbon deposition. It is well admitted by different authors that the mechanism of the reaction includes a step of methane activation by metallic nickel and a step of carbon dioxide adsorption on La2O3 to form La2O2CO3, which react with the deposited carbon according to [26]: 0
be present at the surface of the support La2NiO4, the carbon deposit at the surface of those nickel particles could not react with carbon dioxide consequently an irreversible carbon deposition occurs. The reactions involved when the unreduced La2NiO4 is used, can be the following: CH4 þ Ni=La2 O2 CO3 ! CNi=La2 O2 CO3 þ 2H2 La2 O2 CO3 þ NiC ! 2CO þ La2 O3 þ Ni0 and nCH4 þ Ni=La2 NiO4 ! nCNi=La2 NiO4 þ 2nH2 Therefore, in spite of the important carbon deposition, no deactivation was observed within 20 h of reaction. This can be attributed to the fact that catalytic reaction is occurring at the Ni–La2O2CO3 interfacial area which is not significantly affected by the carbon deposition during the time of reaction. Moreover this proposal is in perfect accordance with the different reactions described before, the carbon deposition resulting mainly from methane decomposition. We have also shown that large particles of nickel are formed when using La2NiO4 without reduction prior to the reaction, whereas small nickel particles were obtained as soon as the perovskite is completely transformed into Ni0 and La2O3 (or La2O2CO3). Thus, we can assume that the sintering of the metallic particles would depend on the nature of the support. The sintering of Ni0 would be limited over La2O3 or La2O2CO3, whereas it would readily occur over La2NiO4. 5. Conclusions We have shown that the use of the perovskites LaNiO3 and La2NiO4 as catalyst precursors allow to obtain high activity for the carbon dioxide reforming of methane. The best catalytic performances are obtained using La2NiO4 after reduction under hydrogen at 700 8C when methane and carbon dioxide conversion reached 80 and 90%, respectively, without carbon deposition. The reduction of La2NiO4 leads to the formation of nickel particles (7 nm), smaller than the particles obtained by reduction of LaNiO3 (15 nm), the presence of such small particles would be responsible for the high activity. The perovskite La2NiO4 used without reductive treatment prior to the reaction leads to an important carbon deposition resulting mainly from methane decomposition. The species Ni/ La2O3 would allow the CO2 reforming of CH4 whereas Ni/ La2NiO4 would be responsible for carbon formation. Acknowledgements
0
CH4 þ Ni ! CNi þ 2H2 CO2 þ La2 O3 ! La2 O2 CO3 CNi0 þ La2 O2 CO3 ! 2CO þ Ni0 þ La2 O3 The last step would occur at the Ni–La2O2CO3 interfacial area. If the perovskite La2NiO4 is not completely reduced when performing the reaction, isolated metallic nickel particles can
The authors are grateful to COLCIENCIAS (Colciencias’s project: 1115-06-17639) and the PICS program: ‘‘Valorization of natural gas and Fischer–Tropsch synthesis’’ for the financial support given. F. Mondragon and G. Sierra acknowledge the financial support from the project Sostenibilidad-2005 by the University of Antioquia. G. Sierra thanks COLCIENCIAS and the University of Antioquia for his PhD scholarship.
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CO2 reforming of CH4 over La–Ni based perovskite precursors Germa´n Sierra Gallego a,b, Fanor Mondrago´n b, Joe¨l Barrault a, Jean-Michel Tatiboue¨t a, Catherine Batiot-Dupeyrat a,* a
Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, Universite´ de Poitiers, Ecole Supe´rieure d’Inge´nieurs de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France b Universidad de Antioquia, Instituto de Quı´mica, A.A. 1226, Medellı´n, Colombia Received 23 May 2006; received in revised form 12 June 2006; accepted 12 June 2006 Available online 21 July 2006
Abstract LaNiO3 and La2NiO4 type perovskites were prepared by the ‘‘self-combustion’’ method and were used as catalyst precursors for the CO2 reforming of CH4 reaction at 700 8C. The catalysts were tested in reduced and non-reduced form. High CH4 and CO2 conversion were obtained without carbon deposition. This result was explained by the occurrence of the RWGS (reverse water gas shift) reaction. The La2NiO4 perovskite used as precursor presents the smallest nickel particles after the reduction treatment. Consequently the catalytic activity is higher than that obtained with Ni/La2O3 or LaNiO3. When La2NiO4 is used without treatment prior to the reaction high methane and carbon dioxide conversions are reached but a carbon deposition is observed. The perovskite structure is not completely transformed and the presence of metallic nickel particles at the surface of La2NiO4 would be responsible for the carbon deposition. It is assumed that the role of the support is to allow the activation of carbon dioxide, which is favoured over La2O3 whereas it is limited over La2NiO4. Consequently the reaction between the complex C–Ni species (resulting from methane activation at the surface of the nickel particle) and gaseous CO2 is inhibited over Ni/La2NiO4 leading to a carbon accumulation at the surface of the catalyst. As soon as the perovskite structure is completely transformed, after reductive treatment or during the reaction, a high activity is reached and no carbon deposition was further observed, the catalytic performances being optimal when the average nickel particles size is the smallest. # 2006 Elsevier B.V. All rights reserved. Keywords: Perovskite precursors; La2NiO4; LaNiO3; CO2/CH4 reforming; Coke formation
1. Introduction The process of carbon dioxide reforming of methane to produce synthesis gas (CO + H2) has received considerable attention in recent years as it constitutes a very attractive route for the conversion of two low-cost products which can be used for the production of liquid hydrocarbons in the Fischer– Tropsch reaction [1] or in the production of methanol. The reaction has been investigated over both, noble metal [2–6] and Ni based [7–11] supported catalyst. Industrially the metal of choice is nickel due to its inherent availability, low cost and high activity in comparison to noble metals. However, the major problem encountered with the reaction of CO2 reforming
* Corresponding author. E-mail address: [email protected] (C. Batiot-Dupeyrat). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.06.024
of methane is the coke formation leading to the catalyst deactivation especially when Ni is used. Nevertheless it has been shown that a high dispersion of the metal species over the support can limit the coke formation [12]. Shiozaki et al. [13] showed that a metal oxide with a welldefined structure can be a source of small metal particles. Hayakawa et al. used CaTi1xNixO3 as catalyst precursor for the partial oxidation of methane to syngas [14], highly dispersed Ni metals were formed in situ during the reaction resulting in a high activity and stability. In previous papers we have shown that LaNiO3 perovskite used as catalyst precursor, leads to a very active catalyst for the CO2 reforming of CH4 [15,16]. A reducing treatment under hydrogen is not necessary before reaction since the perovskite structure is completely destroyed under reactants and products during the reaction at 700 8C, the only species detected by XRD after 15 h of reaction being Ni0 and La2O2CO3. Moreover we showed that the average nickel
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particle size (measured after reaction) was smaller when the catalyst was used without pre-treatment (20 nm) than when it was reduced under hydrogen prior to the reaction (27 nm). It was expected that the better the dispersion of nickel, the higher is the catalytic activity [17]. In our case, the performance was as high as the thermodynamic equilibrium indicates. The aim of the present work was to investigate the activity and stability of the perovskite La2NiO4 and to compare its performance with that of LaNiO3 and Ni/La2O3. Ni/La2O3 was used as a reference catalyst as suggested by Zhang and Verykios [18]. Low nickel content in La2NiO4: 15 wt.% against 24 wt.% in LaNiO3 would allow to obtain smaller nickel particle size. For that purpose the reaction was conducted within the kinetic-controlling regime. The catalyst was characterized at the different steps of the reaction: before reaction, after reducing treatment and after reaction. 2. Experimental
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2.2.3. Temperature programmed reduction (TPR) and H2 pulses chemisorption TPR and H2 chemisorption experiments were carried out in a Micromeritics Autochem 2910 using about 160 mg of catalyst. TPR experiments were performed using 5% hydrogen in argon, 50 mL/min, with a heating rate of 5 8C/min, from room temperature to 900 8C. H2 consumption was obtained from the integrated peak area of the reduction profiles relative to the calibration curve. Before hydrogen chemisorption, the samples were reduced in a 5% H2/Ar flow while the temperature was risen at 5 8C/min from ambient to 700 8C and maintained at this temperature for 6 h. Then the system was purged in Ar at 720 8C for 3 h and cooled to 50 8C. Hydrogen pulse chemisorption was started at 50 8C using 5% H2/Ar and repeated at 5 min intervals until the hydrogen area picks became identical. The amount of hydrogen consumption was measured with a thermal conductivity detector. Metal dispersion on the surface was calculated assuming the adsorption stoichiometry of one hydrogen atom per nickel surface atom (H/Nis = 1).
2.1. Catalyst preparation The perovskite type oxides LaNiO3 and La2NiO4 were prepared by the self-combustion method [19]. Glycine (H2NCH2CO2H) was used as ignition promoter was added to an aqueous solution of metal nitrates with appropriated stoichiometry, in order to get a NO3/NH2 = 1 ratio. The resulting solution was slowly evaporated until a vitreous green gel was obtained. The gel was heated up to 250 8C, temperature at which the ignition reaction occurs yielding to the formation of a powdered precursor which still contains carbon residues. Calcination at 700 8C for 6 h eliminates all of the remaining carbon and leads to the formation of the perovskite structure. Catalysts containing 1, 5 and 17% Ni/La2O3 were prepared by the wet impregnation method using nitrate salt as metal precursor. Nickel nitrate was placed in a beaker with 60 mL of distilled water. After complete dissolution, the appropriated amount of La2O3 was added under continuous stirring. The slurry was then heated up to 90 8C until the water was evaporated. The residue was then dried at 120 8C for 12 h and subsequently calcinated at 700 8C for 6 h under N2. 2.2. Characterization 2.2.1. X-ray diffraction The catalysts were characterized by powder X-ray diffraction using a Siemens D-5000 diffractometer with Cu ˚ , operated at 40 kV Ka1 = 1.5406 and Cu Ka2 = 1.5439 A and 30 mA. The diffraction patterns were recorded in the 2u values range 10–908 with a step size of 0.018 and 1 s per step. 2.2.2. Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) was carried out on a Philips CM120 instrument, with LaB6 filament and equipped with an energy dispersive X-ray analyzer (EDX).
2.2.4. BET All samples were degassed with He for 30 min at 623 K before measurement, the adsorption–desorption isotherm of N2 was measured using 30% N2/Ar as the adsorbate on a Micromeritics Flowsorb II 2300 apparatus at 196 8C. 2.2.5. Catalytic reaction The reaction was carried out by passing a continuous flow of CH4/CO2/He = 10/10/80 over the catalyst bed. The amount of catalyst was fixed at 20 mg with a total flow rate of 100 mL/min which corresponds to a space velocity of 3 105 mL h1 g1, in order not to reach the thermodynamic equilibrium (at 700 8C, the equilibrium state is reached for: CH4 conversion = 90% and CO2 conversion = 93%). All measurements were conducted using catalyst grain size of 180 mm in order to perform the experiments within the region of intrinsic kinetics. The temperature was increased from room temperature to 700 8C at a rate of 5 8C/min, and maintained at this temperature for the desired reaction time. The temperature was measured with a thermocouple located inside the reactor but without direct contact with the catalyst. In some experiments the perovskite catalyst was used without pre-treatment or after reduction under hydrogen at 700 8C for 1 h. The reaction products were analyzed by an online mass spectrometer. The detection limit was estimated to be around 0.04 mmol. According to the intensity of the signal measured by the MS, this value corresponds to about 1% of the total amount detected. 3. Results 3.1. X-ray diffraction characterization The X-ray diffraction patterns obtained for LaNiO3 and La2NiO4 are shown in Figs. 1 and 2, respectively. After
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Fig. 1. XRD patterns of the LaNiO3 precursor material and after various treatments: (a) PDF 88-0633; (b) prepared catalyst before reaction: LaNiO3; (c) after reduction under hydrogen at 700 8C; (d) after 1 h of reaction at 700 8C over the non-reduced material; (e) after 15 h of reaction using the non-reduced or reduced material. (*) Ni0, (5) La2O3 and (^) La2O2CO3 hexagonal.
Fig. 2. XRD patterns of the La2NiO4 precursor material—(a) PDF 89-3460; (b) prepared catalyst before reaction: La2NiO4; (c) after reduction under hydrogen at 700 8C; (d) after 1 h of reaction at 700 8C over the non-reduced material; (e) after 15 h of reaction using the non-reduced or reduced material. (*) Ni0, (5) La2O3, (^) La2O2CO3 hexagonal and (&) La2O2CO3 monoclinic.
calcination at 700 8C the only presence of the LaNiO3 perovskite structure with a rhombohedral symmetry (Fig. 1b) and La2NiO4 perovskite-like K2NiF4 tetragonal structure (Fig. 2b) were observed. After the reduction treatment under hydrogen at 700 8C, whatever the starting material (LaNiO3 or La2NiO4) the perovskite structure was completely destroyed the only phases detected being Ni0 and La2O3 (Figs. 1c and 2c). After 15 h of CO2/CH4 reforming the presence of lanthanum oxo-carbonate La2O2CO3 was observed with both starting catalysts LaNiO3 and La2NiO4 (Figs. 1e and 2e).
according to Ruckenstein and Hu [20], while the high temperature peak between 520 and 680 8C be due to the reduction of the perovskite phase. Thus, the complete reduction of La2NiO4 requires a higher temperature (680 8C) than for the complete reduction of LaNiO3 (610 8C).
3.2. Temperature programmed reduction The TPR profiles of LaNiO3 and La2NiO4 are shown in Fig. 3. The reduction of LaNiO3 proceeds in three steps, the successive changes of the perovskite structure were determined by an in situ XRD measurement. The results were reported in a previous paper [15]:
3.3. CO2 reforming of methane The reaction was performed using the perovskites LaNiO3 and La2NiO4 following two different procedures: (1) the catalyst was reduced for 1 h under hydrogen at 700 8C prior to the reaction; (2) the catalyst was used without treatment prior to the reaction.
4LaNiO3 þ 2H2 ! La4 Ni3 O10 þ Ni0 þ 2H2 O La4 Ni3 O10 þ 3H2 ! La2 NiO4 þ 2Ni0 þ La2 O3 þ 3H2 O La2 NiO4 þ H2 ! Ni0 þ La2 O3 þ H2 O Thus, the perovskite La2NiO4 is formed as an intermediary during the reduction of LaNiO3, at 610 8C the perovskite is completely reduced into metallic nickel and lanthanum oxide. The TPR profile of La2NiO4 shows two main peaks, the first peak could be assigned to the reduction of amorphous NiO
Fig. 3. TPR curves of the perovskites LaNiO3 and La2NiO4, catalyst weight: 160 mg; gas flow rate: 5% H2/Ar, 50 mL/min.
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Table 1 CH4, CO2 conversions, H2/CO ratio, carbon deposition determined by elemental analysis (in wt.%) after 15 h of reaction Catalyst precursor
LaNiO3 La2NiO4 1% Ni/La2O3 5% Ni/La2O3 17% Ni/La2O3 Fig. 4. Conversion of CH4 and CO2, molar ratio H2/CO; catalyst: 20 mg LaNiO3 reduced under hydrogen at 700 8C for 1 h; reaction temperature: 700 8C.
Conversion after 15 h of reaction (%) CH4
CO2
65 80 32 59 62
81 90 48 73 75
Molar ratio H2/CO
Carbon deposition (wt.%)
0.82 0.84 0.47 0.66 0.84
2.8 2.7 2.6 3.2 12
CH4/CO2/He: 10/10/80; catalyst weight: 20 mg; flow: 100 mL min1; T = 700 8C, catalyst reduced under hydrogen at 700 8C prior to the reaction.
loading on the Ni/La2O3 catalyst an increase of the CH4 and CO2 conversions is observed. The low molar ratio H2/CO obtained with 1% Ni/La2O3 can be due to the occurrence of reverse water gas shift reaction, this reaction being favoured at low methane conversion. The elemental analysis performed after reaction reveals that the amount of carbon is around 3 wt.%, which correspond to the percentage of carbon in La2O2CO3. The results confirm that no carbon deposition is observed after 15 h of reaction except for the 17% Ni/La2O3 catalyst (see Table 1).
3.3.1. CO2 reforming of methane using perovskites reduced prior to the reaction As mentioned above, the reduction step under hydrogen leads to the formation of Ni0/La2O3 whatever the nature of the precursor, as shown by XRD (Figs. 1c and 2c). Figs. 4 and 5 present the catalytic behaviour obtained with the reduced LaNiO3 and La2NiO4, respectively. High conversions of CH4 and CO2 are reached after 2 h of reaction at 700 8C, the catalyst precursor La2NiO4 being the most active. A possible explanation for the larger conversion of CO2 compared to CH4 can be attributed to the occurrence of the reverse water gas shift reaction (RWGS: CO2 + H2 ! CO + H2O). The value of the molar ratio: H2/CO lower than one supports this assumption. After reaction, the only phases detected by XRD are metallic nickel and the hexagonal phase lanthanum oxycarbonate: La2O2CO3, resulting from the CO2 adsorption on La2O3 during the reaction (Figs. 1e and 2e). The performances of perovskites were compared with a lanthanum oxide supported Ni catalyst (Ni0/La2O3) containing various amount of nickel, prepared by the wet impregnation method. The results obtained after 15 h of reaction are gathered in Table 1. The perovskite La2NiO4 used as catalyst precursor shows the best catalytic activity. By increasing the nickel
3.3.2. CO2 reforming of methane using perovskites without treatment prior to the reaction The catalytic behaviour of LaNiO3 and La2NiO4 are reported in Figs. 6 and 7, respectively. The methane and carbon dioxide conversions increase slowly after the temperature has reached 700 8C, particularly over the LaNiO3 perovskite. After 15 h of reaction high conversions were obtained with La2NiO4, but the H2/CO molar ratio remains slightly higher than one. The elemental analysis performed after reaction reveals the presence of a carbon deposition over the catalyst as shown in Table 2. After 15 h of reaction over La2NiO4 the composition of the exhaust gases was approximately the following: CH4, 8%; CO2, 4%; CO, 44%; H2, 44%, the amount of water could not be determined by mass
Fig. 5. Conversion of CH4 and CO2, molar ratio H2/CO; catalyst: 20 mg La2NiO4 reduced under hydrogen at 700 8C for 1 h; reaction temperature: 700 8C.
Fig. 6. Conversion of CH4 and CO2, molar ratio H2/CO; catalyst: 20 mg of nonreduced LaNiO3; reaction temperature: 700 8C.
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Fig. 7. Conversion of CH4 and CO2, molar ratio H2/CO; catalyst: 20 mg of nonreduced La2NiO4; reaction temperature: 700 8C.
spectrometry, nevertheless the oxygen and hydrogen balance strongly suggest that water is formed under this experimental condition according to the reverse water gas shift reaction (RWGS: CO2 + H2 ! CO + H2O). The contribution of the RWGS reaction was calculated from the amount of water formed while the carbon deposition was determined from the carbon balance. It was checked that the amount of carbon missing was in accordance with the amount of hydrogen analyzed (CH4 ! Cs + 2H2). The XRD analysis revealed that after 1 h at 700 8C, under reactant mixture, the perovskite phase La2NiO4 is not completely reduced but the material is composed of a mixture of Ni0, La2O2CO3 and non-reduced La2NiO4 (Fig. 2d). This result is in accordance with the TPR profiles showing that La2NiO4 is not as easy reducible as LaNiO3.
Fig. 8. TEM micrograph obtained after reduction of LaNiO3 under hydrogen at 700 8C.
20 nm. The distribution of the particle size is approximately the same after 15 h of reaction using the catalyst with or without reduction treatment prior to the reaction. Starting from La2NiO4 a narrower size distribution is observed: 2–30 nm, the majority of
3.4. Characterization by electron microscopy TEM analysis was performed after reduction of the catalyst, after 15 h of reaction using the reduced and non-reduced perovskites LaNiO3 and La2NiO4. The TEM micrograph obtained after reduction of LaNiO3 under hydrogen at 700 8C clearly shows the presence of spherical particles of nickel (Fig. 8). Nickel particle size distributions derived from TEM are reported in Fig. 9a and b for LaNiO3 and La2NiO4, respectively. The data are normalized to 100 particles while about 500 particles were measured. Starting from LaNiO3, after reduction under hydrogen, the diameter of the nickel particles ranges between 2 and 50 nm, most of the particles being smaller than Table 2 CH4, CO2 conversions, H2/CO ratio, carbon deposition determined by elemental analysis (in wt.%) after 15 h of reaction Catalyst precursor
LaNiO3 La2NiO4
Conversion after 15 h of reaction (%) CH4
CO2
60 80
75 90
Molar ratio H2/CO
Carbon deposition (wt.%)
0.68 1.03
2.7 70
CH4/CO2/He: 10/10/80; catalyst weight: 20 mg; flow: 100 mL min1; T = 700 8C, catalyst used without treatment prior to the reaction.
Fig. 9. Particle size distribution determined from TEM measurements: (a) LaNiO3 and (b) La2NiO4; hatched: catalyst reduced under hydrogen, black: after reaction using reduced catalyst and white: after reaction using non-reduced catalyst.
G.S. Gallego et al. / Applied Catalysis A: General 311 (2006) 164–171 Table 3 CH4, CO2 conversions, H2/CO ratio, average nickel particles size derived from TEM for LaNiO3 and La2NiO4 Catalyst
Average nickel particles size (nm)
LaNiO3 After reduction After reaction over reduced catalyst After reaction over unreduced catalyst
15 17 18
La2NiO4 After reduction After reaction over reduced catalyst After reaction over unreduced catalyst
7 10 20
1% Ni/La2O3 reduced 17% Ni/La2O3 reduced
12 Agglomerates + Ni average: 19
nickel particles having a diameter lower than 10 nm after the reduction step and after 15 h of reaction using the reduced material. When La2NiO4 is used without treatment prior to the reaction, the presence of large nickel particles with a diameter up to 140 nm is detected. The average nickel particle size is shown in Table 3. The formation of small Ni particles is obtained by reduction under hydrogen of the perovskite precursor, particularly when starting from La2NiO4. The nickel particles size slightly increases after reaction showing a small sintering of metallic nickel during the reaction. No significant differences were observed for LaNiO3 used with or without reductive treatment prior to the reaction, whereas for La2NiO4 the use of the non-reduced material results in an increase of the nickel particles size.
and co-workers [22,23] using La2NiO4. Our study demonstrated that smaller nickel particles are formed by reduction of La2NiO4 (7 nm) compared to LaNiO3 (15 nm), it could be attributed to the smaller nickel content in La2NiO4 than in LaNiO3, 15 and 24 wt.%, respectively. Consequently, the catalytic performance of the catalyst precursor La2NiO4 is better than the one of LaNiO3. Our results are different from those obtained by Guo et al. [24], which have found that LaNiO3 was more active than La2NiO4. However, their results could be due to the too low temperature used during the reduction reaction (500 8C under hydrogen), which would not permit a complete reduction of the perovskite material particularly for La2NiO4. We showed that the perovskites used as catalyst precursor exhibited higher activity than Ni/La2O3 catalyst prepared by the wet impregnation method. The increase in the nickel loading leads to an increase in the CH4 and CO2 conversion, nevertheless a larger nickel content (17%) leads to carbon deposition. The TEM micrograph (Fig. 10) reveals the presence of nickel particles with a size up to 100 nm. The presence of such large particles can explain the coke deposition. The TPR profiles showed that the perovskite LaNiO3 is completely reduced at a lower temperature than La2NiO4, 610 and 680 8C, respectively. The presence of metallic nickel particles formed during the first reduction step between 250 and 360 8C, could explain the easier reducibility of LaNiO3. The nickel particles thus formed would activate molecular hydrogen promoting the reduction of LaNiO3 as well as that of the different intermediates. When starting from La2NiO4, the small
4. Discussion We have shown in a previous paper [16] that the use of the perovskite LaNiO3 prepared by the ‘‘self-ignition method’’ allows to obtain an active and stable catalyst for the carbon dioxide reforming of methane. The present work showed that better catalytic performances can be achieved using the perovskite La2NiO4 as catalyst precursor. The reduction of perovskites at 700 8C under hydrogen leads to the formation of lanthanum oxide and metallic nickel, the perovskite structure whatever the starting material LaNiO3 or La2NiO4 being completely transformed. Using reduced catalysts, the CH4 and CO2 conversion are, respectively, equal to 65 and 81% with LaNiO3, while they are equal to 80 and 90% with La2NiO4. The reverse water gas shift is suggested to occur according to the product composition in the exhaust gases. It results in a H2/CO molar ratio lower than one, the RWGS reaction being then responsible of the transformation of about 19% of the carbon dioxide introduced. This observation is supported by the fact that the presence of coke on the catalyst was not detected. The high activity using perovskite precursors can be explained by the formation of relatively small nickel particles by the reductive treatment, such conclusion is in accordance with the work of Choudhary et al. using LaNiO3 [21] and that of Gao
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Fig. 10. TEM micrograph obtained with 17% Ni/La2O3.
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amount of metallic nickel present after the reduction of NiO is not sufficient to favour the reduction, the temperature has to reach 520 8C in order to initiate the reduction of the perovskite. The composition of the exhaust gases, the carbon deposition and the hydrogen and oxygen balance allow us to propose which can be the main reactions involved (see Section 3). Thus after 15 h of reaction over La2NiO4 used without treatment prior to the reaction, the molar composition of the exhaust gases shows a important ‘‘oxygen’’ and ‘‘hydrogen’’ deficiency, which suggests that the amount of water would be very important (H2O could not be quantify by mass spectrometry). Water is formed in the reverse water gas shift reaction (CO2 + H2 ! CO + H2O). The molar ratio H2/CO is near to one, so the consumption of hydrogen by the RWGS reaction would be compensated by the methane cracking leading to the carbon deposition and the production of hydrogen (CH4 ! C + 2H2). The conversions obtained allow to propose some of the main reactions involved at 700 8C: Methane transformation: 38%
CH4 þ CO2 ! 2CO þ 2H2
42% CH4 ! C þ 2H2 Carbon dioxide transformation: 38%
CH4 þ CO2 ! 2CO þ 2H2
52%
CO2 þ H2 ! CO þ H2 O
This proposal is in accordance with the conversions of CH4 (80%) and CO2 (90%). It shows that the contribution of the reverse water gas shift is very important using La2NiO4 without treatment prior to the reaction since 52% of the introduced CO2 reacts with hydrogen to produce CO and H2O. Nevertheless the carbon deposition according to the Boudouard reaction (2CO ! C + CO2) cannot be completely excluded, but its contribution must be rather very small since the amount of carbon dioxide in the exhaust gases is very low. Our results agree very well with those published by Tsipouriari and Verykios, who showed by using isotopic tracing techniques that most of the carbon accumulated over a Ni/La2O3 catalyst at 750 8C originated from methane cracking [25]. Starting from the perovskite La2NiO4 without treatment prior to the reaction, XRD analysis performed after a reaction time of 1 h at 700 8C revealed that a mixture of La2NiO4 and Ni/La2O3 is present. The non-reduced perovskite could explain the importance of carbon deposition. It is well admitted by different authors that the mechanism of the reaction includes a step of methane activation by metallic nickel and a step of carbon dioxide adsorption on La2O3 to form La2O2CO3, which react with the deposited carbon according to [26]: 0
be present at the surface of the support La2NiO4, the carbon deposit at the surface of those nickel particles could not react with carbon dioxide consequently an irreversible carbon deposition occurs. The reactions involved when the unreduced La2NiO4 is used, can be the following: CH4 þ Ni=La2 O2 CO3 ! CNi=La2 O2 CO3 þ 2H2 La2 O2 CO3 þ NiC ! 2CO þ La2 O3 þ Ni0 and nCH4 þ Ni=La2 NiO4 ! nCNi=La2 NiO4 þ 2nH2 Therefore, in spite of the important carbon deposition, no deactivation was observed within 20 h of reaction. This can be attributed to the fact that catalytic reaction is occurring at the Ni–La2O2CO3 interfacial area which is not significantly affected by the carbon deposition during the time of reaction. Moreover this proposal is in perfect accordance with the different reactions described before, the carbon deposition resulting mainly from methane decomposition. We have also shown that large particles of nickel are formed when using La2NiO4 without reduction prior to the reaction, whereas small nickel particles were obtained as soon as the perovskite is completely transformed into Ni0 and La2O3 (or La2O2CO3). Thus, we can assume that the sintering of the metallic particles would depend on the nature of the support. The sintering of Ni0 would be limited over La2O3 or La2O2CO3, whereas it would readily occur over La2NiO4. 5. Conclusions We have shown that the use of the perovskites LaNiO3 and La2NiO4 as catalyst precursors allow to obtain high activity for the carbon dioxide reforming of methane. The best catalytic performances are obtained using La2NiO4 after reduction under hydrogen at 700 8C when methane and carbon dioxide conversion reached 80 and 90%, respectively, without carbon deposition. The reduction of La2NiO4 leads to the formation of nickel particles (7 nm), smaller than the particles obtained by reduction of LaNiO3 (15 nm), the presence of such small particles would be responsible for the high activity. The perovskite La2NiO4 used without reductive treatment prior to the reaction leads to an important carbon deposition resulting mainly from methane decomposition. The species Ni/ La2O3 would allow the CO2 reforming of CH4 whereas Ni/ La2NiO4 would be responsible for carbon formation. Acknowledgements
0
CH4 þ Ni ! CNi þ 2H2 CO2 þ La2 O3 ! La2 O2 CO3 CNi0 þ La2 O2 CO3 ! 2CO þ Ni0 þ La2 O3 The last step would occur at the Ni–La2O2CO3 interfacial area. If the perovskite La2NiO4 is not completely reduced when performing the reaction, isolated metallic nickel particles can
The authors are grateful to COLCIENCIAS (Colciencias’s project: 1115-06-17639) and the PICS program: ‘‘Valorization of natural gas and Fischer–Tropsch synthesis’’ for the financial support given. F. Mondragon and G. Sierra acknowledge the financial support from the project Sostenibilidad-2005 by the University of Antioquia. G. Sierra thanks COLCIENCIAS and the University of Antioquia for his PhD scholarship.
G.S. Gallego et al. / Applied Catalysis A: General 311 (2006) 164–171
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