MgAl2O4 catalysts in dry reforming of methane

Journal of CO₂ Utilization 24 (2018) 40–49

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Study of LaxNiOy and LaxNiOy/MgAl2O4 catalysts in dry reforming of methane

T

⁎

Hassiba Messaoudia,b, , Sébastien Thomasb, Abdelhamid Djaidjaa,c, Samira Slyemia, Akila Baramaa a

Laboratoire des Matériaux Catalytiques et Catalyse en Chimie Organique, Faculté de Chimie, USTHB, BP 32 El Alia, 16111Bab Ezzouar, Alger, Algeria Institut de Chimie et Procédés pour l’Énergie, l’Environnement et la Santé, UMR 7515 CNRS-Université de Strasbourg, Groupe “Énergies et Carburants pour un Environnement durable”, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France c Laboratoire des Procédés pour Matériaux, Energie, Eau et Environnement, Faculté des Sciences et des Sciences Appliquées, Université de Bouira, rue Drissi Yahia, 10000 Bouira, Algeria b

A R T I C L E I N F O

A B S T R A C T

Keywords: Nickel Lanthanum Spinel Perovskite Dry reforming Kinetic modelling

Bulk LaxNiOy and supported LaxNiOy/MgAl2O4 (with x = 1 or 2 and y = 3 or 4) catalysts have been prepared respectively by sol-gel and impregnation methods The elaborated materials have been characterized by XRD, BET, H2-TPR, H2-chemisorption and TPO. The catalytic activity was evaluated in dry reforming of methane (DMR) with an equimolar ratio of CH4 and CO2. XRD analysis shows the presence of LaNiO3, La2NiO4 and MgAl2O4 phases. Higher specific surface areas and nickel dispersions were obtained for the supported catalysts. H2-TPR analysis revealed a low reducibility of the nickel in the supported solids. Supported catalysts were found more active and stable than bulk one in DMR in good agreement with higher Ni dispersion and the beneficial role of the basic support. The XRD analysis performed on the spent catalysts (after 65 h of catalytic test) revealed the presence of the initial phases with metallic nickel species. The TPO analysis showed a low carbon deposition for the supported catalysts. A kinetic study based on the reaction mechanism points out the participation of the reverse water gas shift reaction (RWGS) for the conversion of CO2 and of the produced H2 as well as contribution of steam methane reforming reaction for the conversion of CH4, with water produced from (RWGS), for temperatures over 1023 K.

1. Introduction The dry methane reforming reaction (DMR) is of great interest from an industrial and environmental point of view. This reaction converts two greenhouse gases (CO2 and CH4) into syngas (CO + H2) with a molar ratio CO/H2 = 1 (CH4 + CO2 ↔ 2CO + 2H2, ΔrH°298K = +247 kJ mol−1) suitable for various applications such as FischerTropsch reaction [1]. Moreover, the syngas produced from the DMR can be considered as solar and nuclear energy storage [2] and also DMR process can be fed by biogas (mixture of CO2, CO and CH4) to produce clean and environmentally friendly fuels [3]. However, it is generally observed that obtained H2/CO ratio is less than 1 which can be explained by a simultaneous production of CO from the reverse water gas shift (RWGS) which causes an increasing amount of CO with respect to H2 [4]. For all methane reforming reactions, noble metal based catalysts are the most efficient materials due to their high catalytic activity. However, because of their limited availability, and consequently their ⁎

high cost, their use is discouraged for an industrial application. For these reasons, more abundant metals must be considered. As a consequence, transition metals based catalysts represent the best alternative for the conversion of methane to syngas for industrial applications [5]. Among these catalysts, nickel-based systems have been found to be the most effective in the cleavage of the CeH and CeO bonds and demonstrate a high activity in the DMR. However, the disadvantage of nickel is its sensitivity to catalytic deactivation by the formation of Ni clusters and inactive carbon, which leads to blocking of the active sites [6]. In order to minimize the deactivation phenomena of the nickel based catalysts, a great attention is given to the choice and the use of the appropriate catalytic supports. Indeed, the nature of the support ameliorates the catalytic performances because it allows a better dispersion of the active species increases and their thermal stability. Moreover, redox properties of the support can also help to decrease carbon deposit on the catalyst surface. In addition, as the dry reforming process involves the adsorption and activation of CO2, the basic

Corresponding author at: Laboratoire des Matériaux Catalytiques et Catalyse en Chimie Organique, Faculté de Chimie, USTHB, BP 32 El Alia, 16111Bab Ezzouar, Alger, Algeria. E-mail address: [email protected] (H. Messaoudi).

https://doi.org/10.1016/j.jcou.2017.12.002 Received 26 September 2017; Received in revised form 14 November 2017; Accepted 4 December 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.

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2. Experimental

character of the support could improve the activity of the Ni-based catalysts [7–9]. For example, the calcined Mg-Al hydrotalcite are very attractive supports for DMR due to their basic properties, their large surface area and thermal stability [10]. It has also been observed that the addition of MgO to Ni/Al2O3 catalyst improves the basicity of support and leads to the formation of the MgAl2O4 spinel that contributes to the improvement of the stability toward carbon deposit at high temperature [11]. This higher alkanity of the catalyst allows a high CO2 adsorption on its surface, which allows a high resistance to carbon deposit [11,12]. The use of ordered mesoporous silica (OMS) such as SBA-15, MSUH, KIT-6, MSU-F [13,14] and ordered mesoporous alumina (OMA) materials as catalytic supports for nickel based catalysts [15] has been widely investigated by several authors and very promoting catalytic results were obtained. Among these studies, Wang et al. [14] found that the incorporation of cerium into SBA-15 promotes the dispersion of Ni nanoparticles on the silica matrix and the mobility of surface oxygen species in dry reforming of methane. Kim et al. [15] showed also the highly distributed Ni nanocrystals partially or fully utilizing doping to have stronger resistance to the carbon deposition under the oxygendeficient conditions. Addition of promoters can also lead to a strong reduction of surface carbon formation by modifying the acid-base and redox properties of the catalyst [7]. Rare earth oxides are suggested as good promoters of DMR [7,16] as the addition of lanthanum improves nickel dispersion and catalytic stability [7,17]. Dahdah et al. [18] have compared the catalytic performance of the catalysts Ni2Mg4Al2 and Ni2Mg4Al1.8La0.2 in DMR. The catalyst promoted by La was more efficient with a H2/CO molar ratio close to 1 at all the reaction temperatures. On the other hand, the presence of La2O3 strengthens the adsorption of CO2 on the support, which prevents the deposition of carbon by Boudouard reaction (C + CO2 ↔ 2CO) [19–21]. For every chemical industrial process, design of the reactors requires reliable kinetic data. This kind of data must be based on reaction mechanism in order to be correctly extrapolated to a large range of industrial conditions. Even if DMR reactions on nickel based catalysts have been widely studied as previously mentioned, only a few works report kinetic models based on reaction mechanism on nickel without a general agreement [22–26]. LaNiO3 and La2NiO4 structures are widely studied in dry methane reforming [27–31] because of their relatively small Ni particle size, which contributes to their high catalytic properties associate with a good stability toward sintering and carbon deposition phenomena commonly observed with other nickel based catalysts. In fact, it has been proved that the formation of the presence of La2O2CO3 intermediates in these two structures increases the activity of the lattice oxygen species which is beneficial to remove deposited carbon on the surface of Ni species [28]. Recently, a few studies have been carried out on the combination of these structures with a redox or basic support in the partial oxidation of methane [32] to favor the activation of CH4 or CO2. The use of a support can also enhanced metal dispersion by increasing the specific surface area. The aim of the present paper is to use a basic MgAl2O4 support and to evaluate the reactivity of LaxNiOy and LaxNiOy/MgAl2O4 catalysts (with x = 1 or 2 and y = 3 or 4) in DRM. The synthesized materials have been characterized by several techniques (before and after the catalytic tests) to get correlations between the physico-chemical and the catalytic properties. Based on our previous study on partial oxidation of methane (POM) which included a kinetic model based on reaction mechanism [33], a model for DMR was developed in order to provide a kinetic law of the DMR reaction and evaluate the contribution of secondary reactions.

2.1. Catalyst synthesis 2.1.1. Preparation of LaNiO3 and La2NiO4 samples 2.0 mmol of Ni(NO3)2, 6H2O and 2.0 or 4.0 mmol of La(NO3)3, 6H2O are dissolved in 10 ml of deionized water. Citric acid is then added to the nitrates solution with a molar ratio of citrate to metallic ions equal to 3.0. After stirring and heating at 353 K, a green gel is formed and dried at 393 K for 12 h [34]. The obtained powders are calcined in air at 1023 K during 6 h for perovskite (P 750) and 4 h for spinel (S 750) with a temperature ramp of 5 K min−1. 2.1.2. Preparation of supported catalysts MgAl2O4 support (M 700) has been prepared by sol-gel method as reported in [33]. 50%massLaNiO3/50%massMgAl2O4 (5P/5 M 650) and 50%massLa2NiO4/50%massMgAl2O4 (5S/5 M 750) catalysts have been prepared according to the incipient wetness impregnation method. MgAl2O4 was impregnated with a solution of nickel and lanthanum nitrates. Solution of 1.0 mmol of nickel and 1.0 or 2.0 mmol of lanthanum nitrates in a total volume of 10 ml of deionized water, where in citric acid was added with a molar ratio of 3.0 (nickel + lanthanum). After heating the solution to 353 K and drying at 393 K for 12 h, the residue is calcined at 923 K for 6 h to give the 5P/5 M 650 sample and at 1023 K for 4 h to give the 5S/5 M 750 solid. The calcination temperatures of the supported catalysts have been selected among several values based on X ray diffraction patterns (not shown here) in order to obtain spinel or perovskite phase with a good crystallinity and small crystallite sizes. 2.2. Catalyst characterizations The crystalline structure of the fresh, reduced and used samples has been determined by X-ray diffraction (XRD) using a Bruker AXS-D8 diffractometer with a Cu-Kα irradiation source (λ = 1.5406 Å). The scan range was 10° to 80° with a 0.020° step and an acquisition time of 0.80 s at each step. Surfaces area measurements have been performed by nitrogen adsorption-desorption 77 K using a Micromeritics ASAP 2420 instrument. Prior to analysis, the sample was outgassed under vacuum at 523 K for 12 h. The specific surfaces areas have been determined using the BET method in the range P/P0 of 0.05–0.30. A Micromeritics Auto ChemII 2920 equipment was used to evaluate the reducibility of the catalysts with a mass of sample around 50 mg. Temperature programmed reduction (H2-TPR) experiments were carried out from room temperature to 1173 K with a heating rate of 10 K min−1 under a flow of 10% H2 in Argon (50 mlSATP min−1). The metal dispersion on the catalyst surface was analyzed using the H2 chemisorption. This technique was performed on the same apparatus used for TPR analysis. Around 100 mg of catalyst was reduced at 873 K for 60 min (10 K min−1) in 50 mlSATP min−1 of 10% H2/Ar gas mixture. Then, the sample was cooled to 323 K under pure Ar flow. After 30 min, pulses of 5.0 μl of 10% H2/Ar were introduced every 5 min until areas of successive hydrogen peaks were found to be identical. Dispersion was then calculated assuming an atomic H/Ni0surface ratio of 1. Metallic surface was calculated assuming a mean exposed surface per surface nickel atom of 6.15 Å2. Temperature programmed oxidation (TPO) analysis were performed in a U-shaped quartz reactor under oxygen flow 15 mlSATP min−1 (1% in He) in a temperature range of 298–1123 K with a heating rate of 8 Kmin−1. Oxygen consumption and CO2 formation were monitored following the m/z = 32 and 44 signal (normalized to the m/z = 4 signal) respectively by mass spectrometry with a Pfeiffer vacuum instrument QMS 200 Prisma. The microstructures of samples were obtained using scanning electron microscopy (SEM). This analysis was carried out using a Zeiss 41

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solids (5P/5M 650 and 5S/5M 750), patterns showed the presence of two phases MgAl2O4 (ICDD-PDF 21–1152) and NiAl2O4 (ICDD-PDF 100339) phases. It is difficult to differentiate between these two phases because their XRD peaks overlap. In fact, the heat treatment in air results in partial substitutions of Mg2+ by Ni2+ which leads to the formation of NiAl2O4 [33]. Besides MgAl2O4 and NiAl2O4 common phases, LaNiO3 and mixture of perovskite and spinel (LaNiO3 and La2NiO4) have been observed for 5P/5M 650 and 5S/5M 750 respectively. The XRD diffractograms, performed on H2-reduced samples (Fig. 1.b.), showed for both P 750 and S 750 catalysts a total decomposition of perovskite and spinel structures into lanthanum oxide (ICDD-PDF 005-0602) and metallic nickel (ICDD-PDF 004-0850). Whereas, in the case of the supported solids, XRD patterns, recorded after reduction, revealed in addition of large diffraction lines of Ni metallic phase, the presence of other diffraction lines assigned to the support and to LaNiO3 and La2NiO4 structures which are partially reduced due probably to their strong interaction with the support. Crystalline structures of the spent catalysts (Fig. 1c), exhibit some differences compared to fresh ones. Indeed, a total decomposition of the perovskite LaNiO3 and spinel La2NiO4 structures for the P 750 and S 750 solids respectively after 65 h of reaction was evidenced in parallel to the appearance of metallic nickel and also La2O2CO3 formed by in situ adsorption of CO2 (CO2 + La2O3 ↔ La2O2CO3) [28,36]. For the supported solids, the presences of the initial phases in addition to the characteristic peaks of the metallic nickel have been observed after catalytic tests. Moreover, we can notice that no carbon phase was detected on any sample after catalytic tests. In order to evaluate the crystallites size of the different samples, the Debye-Scherer formula has been employed using the full widths at half maximum (FWHM) of the peaks at 2θ = 47.0° for the oxide crystallites in the fresh samples and 2θ = 51.8° for the nickel crystallites in the reduced and the spent ones. However, diffraction peak of Ni0 at 51.8° was not enough intense on supported samples to perform DebyeScherer equation on it. The obtained results (Table 1) revealed that for the fresh samples, the bulk solids (P 750 and S 750) possess large crystallites domains (22 nm) compared to the supported ones (9–11 nm). After H2-TPR at 1173 K or after catalytic tests, relatively large particle of nickel are obtained due to sintering. The BET surfaces areas of the prepared samples are given in Table 1. The obtained values show that the specific surfaces areas of the solids are relatively low and around 8 m2.g−1 for both P 750 and S 750 samples. For the supported catalysts, as expected, a significant increase of the specific surface area is noted with 31 and 26 m2 g−1 for 5P/5 M 650 and 5S/5 M 750 respectively. The H2-TPR profiles normalized of the different samples are displayed in Fig. 2. P 750 sample exhibits two reduction peaks at around 613 and 773 K. The one at low temperature is broad (from 400 to 700 K) and corresponds to the reduction of both NiO particles present in low amount and with probably small crystallites and/or not well crystallized [27] as previously mentionned in the XRD section and of nickel in the pervoskite structure into La2Ni2O5 [29,35]. The second reduction peak corresponds to the reduction of nickel in La2Ni2O5 into Ni0 [29,37,38]. The ratio between hydrogen consumption at high temperature and low temperature is around 1.6 indicating that around 10% of the nickel is initially into NiO particles. The TPR profile of S 750 sample includes also a broad peak at low temperature corresponding to the reduction of small NiO particles into Ni0. The second peak at 856 K is attributed to the reduction of nickel in La2NiO4 to Ni0 [39]. Its higher temperature compared to the one of P 750 sample, indicates that the presence of a high proportion of lanthanum in S 750 sample leads to nickel (Ni2+) species reduction more difficult due to a more stable crystalline structure. Ratio of these peaks revealed that around 20% of the nickel may be present into NiO particles after calcination. The supported solids TPR profiles are more complex than the bulk

Gemini SEM 500 instrument equipped with an energy dispersive X-ray analyzer (EDS). 2.3. Catalytic tests Catalytic tests of DMR were carried out at atmospheric pressure (with a pressure drop due to the catalytic bed < 0.1 bar) in the experimental setup described in [33] and in the 773–1073 K temperature range. Sample (15.0 mg) was pre-treated under nitrogen (3.0 mlSATP min−1) from ambient to 773 K with a heating ramp of 5 K min−1. Then the reactive mixture was introduced to the reactor with a total flow rate was 28 mlSATP min−1 (GHSV around 5.6 × 104 h−1) with a feed ratio CH4/CO2 of 1.0 and nitrogen as internal standard (3.0 mlSATP min−1). Temperature was maintained constant during 80 min and then increased to 823 K with a heating ramp of 5 K min−1 and maintained constant during 80 min. The same procedure of temperature increase was repeated every 50 K up to 1073 K. The stability of the catalysts towards deactivation was also studied by performing catalytic tests with 5.0 mg for 65 h. Prior to test, pretreatment under N2 (5.0 mlSATP min−1) from ambient to reaction temperature (1073 K) was carried out, then the N2 gas was replaced by the reaction mixture (GHSV around 1.7 × 105 h−1). In order to perform a kinetic study of our catalyst in the dry methane reforming reaction, the catalyst mass was varied between 1.8 and 6.0 mg (diluted in around 13.2 and 9 mg of inert SiC, adjusted to keep the same height of catalytic bed, in order to limit temperature heterogeneity along the catalytic bed) and the total flow rate from 35 to 116 mlSATP min−1 (GHSV from 1.6× 105 to 1.2 × 106 h−1). In order to have similar pressures for all the experiments whatever the pressure drop due to the catalytic bed, the medium pressure was set to 1.3 bar (with pressure drops due to the catalytic bed < 0.2 bar). Before test, the catalyst was heated progressively from ambient to 1073 K under nitrogen flow (3–5 mlSATP min−1) and the catalytic tests were then carried out successively at 1073, 1023 and 973 K. Analysis of the reactants and reaction products was recorded by on line gas chromatography TCD gas chromatography line (Agilent 6890N with a TCD detector) equipped with a carbosphere column. The argon was used as the carrier gas while inert nitrogen as an internal standard to calculate the partial outflows. Conversions and yields were calculated according to the equations given below:

Conversion of CH 4 = −X CH4 =

Conversion of CO2 = −X CO2 =

Yield of H2 = YH2 =

F in (CH 4) − F out (CH 4) F in (CH 4) F in (CO2) − F out (CO2) F in (CO2)

F out (H2) 2 × F in (CH 4)

Yield of CO = YCO =

F out (CO) in 4) + F (CO2 )

F in (CH

3. Results and discussion The XRD patterns of the catalysts are shown in Fig. 1. The diffractograms of the fresh solids (Fig. 1.a.) show the presence of perovskite phase LaNiO3 (ICDD-PDF 033-0711) with a rhombohedric structure in the case of the P 750 solid, whereas for the S 750 sample, we note the formation of La2NiO4 spinel as the major phase (ICDD-PDF 034-0314). Besides La2NiO4, La2O3 free oxide has been observed. The presence of this latter oxide after calcination at 1023 K is due to the decomposition of La2O2CO3 formed during the preparation step [35]. The presence of La2O3 implies the presence of NiO not observed maybe due to small amount or low crystallite sizes. Concerning the supported 42

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Fig. 1. XRD patterns of the catalysts (a) fresh, (b) after reduction and (c) after reaction (65 h). (•) MgAl2O4, (○) NiAl2O4, (◊) La2NiO4, LaNiO3, (*) La2O3, NiO, (↓) La2O2CO3.

solids. In fact, we still observe peaks at relatively low temperature (580 K) corresponding to reduction of NiO particles in Ni0 and at medium temperature (between 630 and 900 K) attributed to reduction of LaxNiOy phase to Ni0. In addition to these peaks, another reduction signal at relatively higher temperatures (between 1153 and 1173 K) corresponding to the reduction of the Ni2+ in the NiAl2O4 structure [40] has been recorded. The reduction degree of nickel in the supported solids is between (58–61%) whereas it was total for the bulk samples. This fact is in good agreement with the XRD results obtained after H2TPR analysis which have shown a total decomposition of the bulk structures (LaNiO3 and La2NiO4) while a partial reduction of nickel (Ni2+) has been observed for the supported ones. Moreover, the intensity of Ni0 peaks after TPR and catalytic test for supported sample were less than the bulk ones which confirms this lower nickel reducibility. The percentages of nickel dispersion determined by hydrogen chemisorption after reduction of samples at 873 K are presented in Table 1.

Fig. 2. H2-TPR patterns of the fresh catalysts.

Table 1 Characterizations of the samples. Catalysts

P 750 S 750 5P/5 M 650 5S/5 M 750 a b c d

Crystallite size (nm) Oxidesa

Ni0

22 22 11 9

25 27 – –

b

Ni0 23 15 – –

Nickel Reduction degree (%)

Dispersiond (%)

Ni0 aread (m2 g−1)

Specific surface areaa (m2 g−1)

C depositc (mmol g−1)

99 99 58 61

5 2 11 8

7 2 8 4

7 8 31 26

13.3 5.6 10.0 5.3

c

after calcination. after TPR analysis. after catalytic tests. after reduction in H2 at 873 K.

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Table 2 Effect of the reaction temperature on the reactivity of LaNiO3 and La2NiO4 catalysts. m = 15.0 mg, GHSV = 5.6×104 h−1. TReaction (K)

923 973 1023 1073 923 973 1023 1073

XCH4

XCO2

YCO

YH2

P 750

S 750

P 750

S 750

P 750

S 750

P 750

S 750

0.52 0.66 0.75 0.87 5P/5 M 650 < 0.01 < 0.02 0.69 0.85

< 0.01 0.62 0.74 0.87 5S/5 M 750 < 0.01 < 0.01 < 0.01 0.84

0.59 0.74 0.83 0.91 5P/5 M 650 < 0.01 < 0.02 0.79 0.90

< 0.01 0.73 0.83 0.91 5S/5 M 750 < 0.01 < 0.01 < 0.01 0.90

0.54 0.69 0.81 0.91 5P/5 M 650 < 0.01 < 0.02 0.75 0.90

< 0.01 0.70 0.81 0.91 5S/5 M 750 < 0.01 < 0.01 < 0.01 0.88

0.49 0.63 0.73 0.87 5P/5 M 650 < 0.01 < 0.02 0.63 0.81

< 0.01 0.59 0.71 0.85 5S/5 M 750 < 0.01 < 0.01 < 0.01 0.80

Thermodynamic equilibrium 923 973 1023 1073

0.59 0.74 0.84 0.90

0.71 0.83 0.91 0.95

0.65 0.78 0.88 0.93

0.53 0.69 0.81 0.88

single peak of O2 consumption and of CO2 formation. The S 750 catalyst presents less amount of C burnt during the TPO analysis compared to P 750 which exhibit high amount of carbon, not entirely burnt during the analysis as the signal decrease only when temperature stops to increase. The temperature of CO2 formation for S 750 sample is around 923 K and corresponds to relatively stable nanotube of carbon [42,43]. In the case of the supported catalysts, a peak towards 823 K and 873 K was recorded for 5P/5M 650 and 5S/5M 750 respectively which may corresponds to unstructured carbon and/or to nickel carbide [42]. SEM-EDS analysis were performed in order to investigate the morphology and the surface and sub-surface composition of the studied catalysts. The EDS measurements reveal, for all samples, that the measured the atomic percentages are close to the theoritical ones (Table 3).The obtained images are presented in Fig. 5. The fresh sample 5P/5M 650 shows uniform distribution of LaNiO3 particles on the MgAl2O4 support. The comparison between the fresh and the spent 5P/ 5M 650 (after 65 h of catalytic test), reveals a change in the morphology of the catalyst after DMR reaction. In fact, for the spent catalysts, there is apparition of small particles of metallic nickel Ni0 as well as carbon deposit (previously quantified by TPO analysis). The 5P/5M 650 sample which exhibit higher initial activity compared to 5S/5M 750 sample was selected to perform the kinetic study of the DMR reaction. For this purpose, it is necessary to deal with experimental data characteristic of chemical regime. As a consequence, internal and external diffusion limitations of heat and mass have been investigated at the higher temperature of this kinetic study (1073 K) as chemical steps exhibit higher activation energies than physical ones. External diffusion limitations have been investigated by performing two catalytic tests in the same reactor filled with 6.0 or 1.8 mg of 5P/ 5 M 650 sample and a total reactive flow rate of 116 and 35 cm3SATP min−1 respectively with the same composition described in section 2.3. These conditions correspond to identical GHSV but to an greater turbulence and higher heat consumed by the reaction in the case of the higher flow rate. As a consequence, if external diffusion limitation would occur, conversions and yields (far from thermodynamic limitations) of the two catalytic tests would be rather different which was not the case as presented Table 4. Thus, external diffusion limitations can be excluded in these experimental conditions. Internal diffusion limitations were investigated by performing two catalytic tests in the same conditions but with two different granulometric fractions of the same catalyst. In fact, decreasing the grains size will shorten diffusion path and will decrease the impact of internal transfer diffusion. As almost identical catalytic activities were found for both fractions (Table 4), one can neglect internal diffusion limitations

It appears that nickel dispersion is rather low for both bulk samples but higher in the case of LaNiO3 compared to La2NiO4 solid. An amelioration of the nickel dispersion is observed with impregnation of LaNiO3 and La2NiO4 on the MgAl2O4 support, with still higher values for LaNiO3 compared to La2NiO4 supported sample. According to Öksüzömer et al. [41], this can be due to the formation of NiAl2O4 (confirmed by H2-TPR analysis) which improves the nickel dispersion. The corresponding nickel surface areas of reduced samples containing metallic Ni0 are between 2 and 8 m2 g−1. It can be noted that metallic surface of 5P/5M 650 sample is higher than those of bulk solids despite its twice lower nickel content. The effect of reaction temperature on the catalytic performances of our samples was examined in the temperature range 773–1073 K (Table 2) in order to differentiate the catalytic activity of spinel and perovskite structures. The results show that the perovskite P 750 is active at 923 K with CH4 and CO2 conversions of 52 and 59% respectively, while the spinel catalyst S 750 is active from 973 K. For the supported samples the conversion of methane occurs since 1023 and 1073 K for 5P/5 M 650 and 5S/5M 750 respectively. This difference in the temperature of methane activation by CO2 can be explained by the H2-TPR results. In fact, the LaNiO3 sample has a lower temperature reduction compared to the La2NiO4 and less nickel reducibility was observed for the supported catalysts. The catalytic behavior of the samples becomes similar and close to thermodynamic equilibrium at 1073 K. The catalysts stability in DMR reaction was evaluated at 1073 K for 65 h (Fig. 3) at higher GHSV in order to limit effect of thermodynamic. All catalysts leads to stable activity upon time on stream. Supported catalysts exhibit higher conversions as well as CO and H2 yields compared to LaxNiOy bulk samples. The high catalytic activity observed with the supported catalysts can be attributed to (i) the good dispersion of nickel on the surface of the support consistent with the small size of Ni0 particles obtained by XRD after 65 h of reaction (Table 1) and (ii) the higher specific area which provide more basic sites abble to activate CO2 as previously mentioned in the first section. It can noticed that activities of S 750 and 5S/5M 750 samples increase with time before reaching a plateau which can be related to a progressive reduction during the first hours of reaction. This confirms the TPR profiles which demonstrate higher reduction temperatures for spinel structure compared to perovskite. The TPO analysis carried out on spent samples after 65 h of catalytic test (Fig. 4) reveals relatively low amounts of deposited carbon for all catalysts (Table 1) with average selectity to solid carbon lower than 0.005% for supported catalysts. The normalized TPO profiles exhibit

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Fig. 3. Conversions of CH4, CO2 and yields to CO and H2 at 1073 K, m = 5 mg, total flow rate = 28 mlSATP min−1(CH4/CO2 = 1).

Table 3 Theoretical and experimental atomic fraction measured by EDS analysis of fresh and spent catalysts. Atomic fraction

b

P 750 S 750 b 5P/5 M 650a 5P/5 M 650 b 5S/5 M 750 b a b

Theorical

Experimental

Ni

La

Mg

Al

Ni

La

Mg

Al

0.50 0.33 0.25 0.25 0.17

0.50 0.67 0.25 0.25 0.33

– – 0.17 0.17 0.17

– – 0.33 0.33 0.33

0.51 0.32 0.26 0.26 0.17

0.49 0.67 0.26 0.25 0.33

– – 0.17 0.18 0.18

– – 0.31 0.30 0.32

after calcination. after catalytic tests.

whatever the experimental conditions. Consequently, elementary steps considered are listed below: Dissociative adsorption of hydrogen:

Fig. 4. TPO profiles of spent catalysts.

H2 + 2 Ni* ⇄ 2HeNi

in these experimental conditions. Based on these results, all the catalytic tests used for the kinetic study were performed with flow rates and an average particle size for which external and internal diffusion limitation can be neglected and so with data reflecting chemical kinetics. On our previous work on a nickel based catalyst for partial oxidation of methane (POM) [33], a survey of the literature allowed us to consider several elementary steps involved in the reaction mechanism. All these steps were also considered in POM mechanism except oxygen adsorption as in the present study, no oxygen was detected in gas phase

(1)

Dissociative adsorption and decomposition of methane: CH4 + 2 Ni* ⇄ CH3eNi + HeNi

(2)

CH3eNi + Ni* ⇄ CH2eNi + HeNi

(3)

CH2eNi + Ni* ⇄ CHeNi + HeNi

(4)

CHeNi + Ni* ⇄ CeNi + HeNi

(5)

Formation of water: 45

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Fig. 5. SEM-EDS images of fresh (a) 5P/5 M 650, and after reaction (65 h) (b) 5P/5 M 650, (c) 5S/5 M 750, (d) P 750, (e) S 750.

Table 4 Conversion and yields for diffusion limitations catalytic tests at 1073 K. m (mg)

Qtot (cm3STP min−1)

6.0 116 1.8 35 6.0 116 Thermodynamic equilibrium

r+DMR=

GHSV (h−1)

dmindmax (μm)

XCO2

XCH4

YH2

YCO

5.8 × 105 5.8 × 105 5.8 × 105

50–80 50–80 > 125

0.43 0.46 0.42 0.95

0.29 0.28 0.29 0.90

0.20 0.21 0.19 0.88

0.36 0.39 0.34 0.93

HeNi + OeNi ⇄ OHeNi + Ni*

(6)

OHeNi + HeNi ⇄ H2O + 2 Ni*

(7)

in which DEN represent the surface occupation term. Based on our previous study on POM [33], on the FTIR work of Bradford et al. [25] and on the DFT calculations of Wang et al. [46], the most abundant adsorbed species taken into account were C-Ni, H-Ni and CO-Ni species which enables to write:

DEN=1+

(8)

OHeNi + CeNi ⇄ Ni-CO + Ni-H

(9)

COeNi ⇄ CO + Ni*

r

+DMR=

The influence of thermodynamic was taken into account with the rate of the reverse reaction noted r-DMR, which can be expressed as:

(11)

r−DMR = r+DMR .

Water gas shift reaction COeNi + OHeNi ⇄ CO2 + Ni-H + Ni*

kDMR KCH4PCH4 PCO2 DEN2.KCOPCOK 2H P2H 2 2

(10)

Dissociative adsorption of CO2 CO2 + 2 Ni* ⇄ COeNi + OeNi

K CH4 PCH4 +K COPCO+ K H2PH2 K 2H2 P 2H2

As steps (2) to (5) represent methane dissociative adsorption and decomposition, the product K2 K3 K4 K5 was simplified as KCH4. Moreover, as adsorption constant of CO2 (K11) is not present in DEN, the product k8 K11 was considered as kDMR. Based on these considerations, the forward reaction of DMR reaction can be written as follow:

Formation and desorption of CO CeNi + OeNi ⇄ COeNi + Ni*

k 8K2K3K 4 K5PCH4 K11PCO2 L² DEN 2.K10PCO.K12.P 2H2

ADMR = r+DMR . βDMR KDMR

where ADMR, KDMR and βDMR are respectively the reaction quotient, the thermodynamic constant of the DMR reaction at the considered temperature and the approach to equilibrium. This leads to the following expression of the global DMR rate noted rDMR:

(12)

DMR reaction is limited by thermodynamic in the considered range of temperature and pressure. The CO formation by DMR is reported to be limited by reaction (8) on nickel catalysts [44,45] which enables to write the expression of the forward rate r+DMR as follow:

r

DMR=

kDMR KCH4PCH4 PCO2 DEN2.KCOPCOK 2H P2H 2

r+DMR = k8.(OeNi).(CeNi)

.(1- βDMR )

2

This expression is consistent with the study of Das et al. [47] in which the authors developed a power law for the kinetic for DMR reaction. The order found for CO2 was around 0.8 consistent with the order 1 proposed this present study and around 0.4 for CH4 also consistent with the presence of PCH4 in the term DEN. Water Gas Shift (WGS) and Reverse Water Gas Shift (RWGS) reactions are included in this reaction mechanism with the steps (1), (7),

Assuming reactions (1)–(5) and (7), (10) and (11) to be close to thermodynamic equilibrium in steady conditions, one can express r+DMR as a function of k8, of thermodynamic equilibrium constants of elementary reactions (Ki) and of surface concentration of nickel active sites L (assumed to be constant over the range of temperature studied).

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Fig. 6. Experimental conversions of CO2 (•) and CH4 (▲) at 1073 K (a), 1023 K (c) and 973 K (e); experimental yields of H2(•) and CO (▲) at 1073 K (b), 1023 K (d) and 973 K (f). Full lines: simulated data for CO2 conversion and H2 yield (▬) for CH4 conversion and CO yield (▬). Dashed lines: contribution of DMR (–), SMR (- - -). Dotted lines: RWGS: CO2 conversion and H2 yield (•••), CO yield (•••).

(10) and (12) and its rate is believed to be controlled by elementary reaction (12) [22,48] with other three steps at the thermodynamic equilibrium. The forward reaction rate r+WGS can then be expressed as:

included in the apparent kinetic constant kSMR):

r+SMR =

r+WGS = k12.(OHeNi).(COeNi)

The influence of thermodynamic was also taken into account with the rate of the CO methanation reaction with rate r-SMR, which can be expressed as:

Using the same methodology than before, global reaction rate for WGS reaction, noted rWGS can be expressed as follow (with K7 and k12 included in the apparent kinetic constant kWGS):

r+WGS =

r−SMR = r+SMR .

kWGSPH2OK COPCO K H2.PH2 .DEN 2

rSMR=

AWGS = β WGS KWGS

kWGSPH2OK COPCO K H2.PH2 .DEN 2

kSMR PH2OK CH4 PCH4 .(1- βSMR ) 2.5 2 K 2.5 H2 .P H2 .DEN

At each temperature, expression of the rates of the 3 considered reactions implies 3 kinetic constants and 3 adsorption constants. Assuming an Arrhenius form for each kinetic constant and no variation of heats of adsorption for H2, CH4 and CO in the considered range of temperature (973–1073 K), 12 adjustable parameters are implied in the model. In our previous work [33], parameters of adsorption of methane on nickel were determined in the same experimental conditions. However, in this previous wok, adsorption constant of hydrogen was included in the one of methane. To be consistent with this work, we used the numerical parameters obtained in [33] to express adsorption constant of methane as a function of the one for hydrogen. Moreover, we used the numerical parameters of CO adsorption on nickel (supported on alumina) from the well known work of Xu and Froment [49]. It was not possible to use corresponding values for hydrogen adsorption as the

This leads to the following expression of the global WGS rate noted rWGS:

rWGS=

ASMR = βSMR KSMR

This leads to the following expression of the global SMR rate noted rSMR:

Similarly to DMR, the influence of thermodynamic is taken into account with the rate of the Reverse Water Gas Shift (RWGS) reaction noted rRWGS, which can be expressed as:

rRWGS = r+WGS.

kSMR PH2OK CH4 PCH4 2.5 2 K 2.5 H2 P H2 .DEN

.(1- β WGS)

Last reaction included in reaction mechanism is steam reforming of methane (SMR) into CO and H2 with water formed by RWGS. For SMR the rate determining step is assumed to be step (9) which leads to: r+SMR = k9.(OHeNi).(CeNi) Using the same methodology than before, global reaction rate for SMR reaction, noted rSMR can be expressed as follow (with k9 and K7 47

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Table 5 Kinetic parameters obtained for modelling curves presented Fig. 4. kDMR

kWGS

kSMR

Ea kJmol−1

37.1

107

294

A unit

7.4 × 10−2 molbar−1 s−1 g−1

6.3 × 104 mol bar−1 s−1 g−1

7.0 × 1014 mol bar−1 s−1 g−1

Qads kJ mol−1 A unit

KCO [49]

KH2 [50]

KCH4

70.7

43

111

8.2 × 10−5 bar−1

1.5 × 10−5 bar−1

1.9 × 10−9 bar−1

hydrogen formed in the inlet part of the catalytic bed. This fact is one of the cause of difference between CO2 and CH4 conversions and between CO and H2 yields. In fact, when plotting the approach to thermodynamic equilibrium βWGS as a function of the GHSV (Fig. 7), it can clearly be seen that for GHSV lower than 3 × 105 h−1 RWGS almost , reaches its equilibrium which is consistent with the observation of Rostrup et al. [51] whereas the two others reaction implied (DMR and SMR) does not approach to thermodynamic equilibrium values higher than 0.05 for GHSV higher than 105 h−1 (not shown). For this value of GHSV, at 1073 and 1023 K, only half the conversion of CO2 is due to DMR, the other half produces, via RWGS, H2O which allows SMR to represent a third of the CH4 converted. This last statement is less pronounced at 923 K because of the high activation energy of SMR which leads to low kinetic at this temperature. Fig. 7. Approach to thermodynamic equilibrium of WGS at 1073 K (full line), 1023 K (dotted line) and 973 K (dashed line).

4. Conclusion

authors did not consider the same dissociative adsorption kinetic. Hydrogen adsorption parameters on nickel were taken from [50]. All these considerations allowed to reduce the number of adjustable parameters to only 6 which is more realistic as data from 18 different experimental conditions were used to perform the data fitting. Modeling was performed with the same method than reported in [33] assuming a plug flow behavior of the catalytic reactor which was decomposed into 200 slices. For a given set of parameters mass balance equations allows to obtained conversions and yields a function of the GHSV for the 3 considered temperatures. Adjustment of the parameters were first performed manually at each temperature with the following procedure: (i) all the 3 kinetic constants were set at zero, (ii) as DMR is the only primary reaction, kDMR was adjusted to fit CH4 conversions at high GHSV, (iii) as RWGS is the secondary reaction, kWGS was adjusted to fit the CO2 conversions at high GHSV and (iv) the kinetic constant of the tertiary reaction, kDMR, was adjusted to fit all the data. Pre-exponential factors and activation energies were then calculated with Arrhenius type plot. Then the Levenberg–Marquardt algorithm was then used to minimized differences between experimental and simulated conversions and yields for all the temperatures simultaneously by varying pre-exponential factors and activation energies. Results of the fitting are presented Fig. 6 and the corresponding parameters are summarized in Table 5. Apparent activation energies include heats of adsorption and are consequently different from the values of activation energies presented in Table 5. In fact, apparent activation energy found for DMR is equal to 83 kJ mol−1 which is close to typical values from literature between 74 and 118 kJ mol−1 as reported by Wei et al. for several Ni based catalysts [24] and to the value reported by Bradford et al. of 109 kJ mol−1 [25]. For WGS and SMR, the apparent activation energies found are equal to 58 and 290 kJ mol−1 respectively which are very close from the values obtained in our previous work on POM on a nickel based catalysts and which were reported to be in agreement with values of the literature. Once the optimized parameters obtained, integration of the rates of each reactions allows to calculate the contributions of DMR, RWGS and SMR in the conversions and yields as presented Fig. 6. On Fig. 6, it appears that for T higher than 1023 K and even at high GHSV (over 106 h−1), RWGS reaction occurs significantly with

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