Syngas Production via Methane Dry Reforming over La-Ni-Co and La-Ni-Cu Catalysts with Spinel and Perovskite Structures

In this paper, the catalytic properties of La-Ni-M (M = Co, Cu) based materials in dry reforming of methane (DRM) for syngas (CO + H2) production, were studied in the temperature range 773−1073 K. The LaNi0.9M0.1O3 and La2Ni0.9M0.1O4 (M = Co, Cu and Ni/M = 0.9/0.1) catalysts were prepared by partial substitution of Ni by Co or Cu using sol-gel method then characterized by XRD, H2-TPR and N2 physisorption. The XRD analysis of fresh catalysts showed, in the case of Co-substitution, the formation of La-Ni and La-Co perovskite and spinel structures, while only LaNiO3 and La2NiO4 phases were observed for the Cu-substituted samples. The substitution of these two structures by copper decreases the reduction temperature compared to cobalt. The reactivity results showed that the partial substitution of nickel by copper decreases the methane activation temperature, whereas a better stability of catalytic activity and syngas production was obtained via the cobalt-substituted catalysts, which is due to a synergistic effect between Ni and Co. The TPO analysis carried out on the spent catalysts indicated that the lowest carbon deposition was obtained for the cobalt substituted samples. Copyright © 2020 BCREC Group. All rights reserved


Introduction
Syngas (H2+CO) is an essential raw material for several reactions in the petrochemical indus-ing reaction since it allows at the same time the conversion of methane into synthesis gas (with an equimolar ratio H2/CO) by using CO2 (greenhouse gas) which also contributes to the reduction of this gas emissions in the environment [5].
The DRM process requires high energy because it is a highly endothermic reversible reaction [6]. Indeed, a very high temperature is necessary for the course of the reaction and the formation of synthesis gas as the main product of this reaction [7]. In this case, the purpose of using catalysts is to reduce the energy required to obtain a high yield of syngas.
However, like all the other catalytic processes which are carried out at high temperature, the DRM reaction is also confronted with problems of catalysts deactivation due to sintering and carbon deposition phenomena [8]. In fact, these problems cannot be completely avoided because they are linked to thermodynamics at high temperatures at which the reaction is performed. Whereas, these phenomena can be limited by bringing about changes in the operating conditions of the reaction or even on the structure and properties of the used catalysts [9].
Diverse catalytic systems are used in the DRM reaction in particular nickel supported catalysts [10] because of their high efficiency and low cost compared to noble metal catalysts [11]. Otherwise, the use of well-defined structures in methane reforming reactions is getting a lot of attention and is currently considered as one of the interesting alternative materials to minimize the problems of sintering and rapid deactivation of catalysts encountered with conventional supported nickel-based systems [12−14]. Indeed, the insertion of the nickel into a defined structure like spinel and perovskite has several advantages such as a well dispersion of nickel particles, a strong interaction that makes the nickel species less mobile than on the surface of a conventional support and furthermore a favored regeneration of nickel metallic sites by an in situ reduction. These properties should therefore improve the catalytic activity and stability of nickel species by limiting the formation of large aggregates and consequently reducing the coke deposition. Guo et al. [15] found that the Ni/MgO-Al2O3 and Ni/MgAl2O4 catalysts tested in the dry reforming of methane exhibit better stability and higher activity compared to Ni/Al2O3 system. In the one hand, these good catalytic performances were attributed to the formation of MgAl2O4 spinel in Ni/MgO-Al2O3, which stabilizes small Ni crystallites. On the other hand, the high activity of the Ni/MgAl2O4 catalyst compared to Ni/Al2O3 were mainly related to the characteristics of the MgAl2O4 spinel support, that reduces the sintering phenomena of nickel active phase. In addition, according to these authors, the formation of NiAl2O4 is completely avoided over MgAl2O4 spinel phase and that strong interactions between nickel and MgAl2O4 produce highly dispersed active Ni species leading to high catalytic reactivity [15].
Other studies reported that La and Ni spinel and perovskite structures were found to be very interesting catalysts in the DRM reaction because of the uniform dispersion of Ni thus reducing Ni 0 segregation and the generated La2O3 that interact with CO2 to form La2O2CO3 intermediates, which help to decrease coke formation [16−18]. Furthermore, it has been confirmed in our previous work, that the use of MgAl2O4 spinel as catalytic support for the LaNiO3 and La2NiO4 catalysts increases the CH4 and CO2 conversions and enhanced the catalytic stability [3].
In order to further improve the catalytic activity and the metal dispersion, the effect of nickel doping with another transition metal has also been the subject of many studies. Indeed, these bimetallic systems have exhibited very good catalytic performances [19−21] due to the synergistic effect between the nickel and the inserted metal, which contributes to the improvement of the activity and stability of the final catalyst. In a study on bimetallic catalysts, Fan et al. [22] compared the conversion rates of methane on Ni/MgO-ZrO2, Co/MgO-ZrO2, and Ni-Co/MgO-ZrO2 catalysts. Bimetallic Ni-Co reached 80% methane conversion at 1023 K, higher than those observed with monometallic catalysts Ni and Co (70% and 71%, respectively). The increase of the catalytic activity of the bimetallic compound relative to the monometallic ones has been synonymous with the synergistic effect due to the coexistence of small particles of both Ni and the added metal. Another work developed by Nataj et al. [23] studied the effects of nickel and copper loadings and reaction temperature on the reactivity of Ni-Cu/Al2O3 catalysts in methane dry reforming. The results indicated that the catalytic activity was strongly influenced by the amounts of Cu. Furthermore, sintering of active phases during the reaction was restricted by the formation of Ni-Cu alloy that leads to the improvement of both catalyst activity and stability. However, the promoting effects of copper were significantly faded by increasing Cu content. Samples with high amounts of copper presented lower activity and rapid deactivation due to the agglomeration of active phase and Ni covering.
In this context, the present work is focused on the study of the catalytic performances of Ni-M bimetallic materials (M = Co, Cu) of welldefined perovskite and spinel structures in the methane dry methane reaction for the production of synthesis gas. The bimetallic catalysts were prepared by the substitution of 10 mol% of Ni by cobalt or copper in the monometallic structure LaNiO3 and La2NiO4.

Catalysts Preparation
Non substituted La-Ni and substituted La-Ni-M (M = Co, Cu) with spinel and perovskite structures were prepared by the sol-gel method using nitrates salts and citric acid as raw materials [24]. The non substituted samples are perovskite LaNiO3 (noted LN3) and spinel La2NiO4 (noted LN4). While the nickel substituted solids by copper or cobalt are perovskite LaNi0.9M0.1O3 (noted LNM3) and spinel La2Ni0.9M0.1O4 (noted LNM4) with M = Co, Cu. The corresponding loadings for Co and Cu, respectively are 2.4 wt% and 2.5 wt% in the LNM3 samples and 1.5 wt% and 1.6 wt% in the LNM4 solids.
The preparation consists of dissolving the nitrate salts La(NO3)3.6H2O, and Z(NO3)2.6H2O (Z = Ni, Co, Cu) in 10 mL of deionized water, according to the stoichiometric ratios of perovskite and spinel structures. The citric acid is then added with a molar ratio of citrate to metallic ions equal to 3.0. The resulting solution is left under stirring and heating at 353 K until a green gel is obtained which is dried at 393 K for 12 hours. The final precursors were calcined at 1023 K (5 K.min −1 ) for 6 hours in the case of the perovskite and 4 hours for the spinel. The calcination conditions were fixed after several tests of heat treatments during different times and at various temperatures in order to obtain high crystallinity under moderately severe conditions.

Powder X-Ray diffraction (XRD)
The structure of the fresh, reduced and used catalysts was studied by the X-Ray diffraction analysis using a Bruker AXS-D8 diffractometer with a Cu-=1.5406 Å). The XRD patterns were recorded in a 2θ range of 10-80° with a step size of 0.020° and a time of 0.80 s per step. The phase composition was determined using powder files (PDF-ICDD).

Surface area measurements (BET)
The specific surface areas of the different samples were obtained by measuring nitrogen adsorption-desorption isotherms at 77 K using a Micromeritics ASAP 2420 instrument. Prior to measurements, around 100 mg of sample were outgassed under vacuum at 523 K for 12 h. The BET surface areas were calculated with the Brunauer, Emmett and Teller model in the range of P/P0 of 0.05-0.30. The pore size distribution was determined using the Barrett-Joyner-Halenda (BJH) method.

Temperature programmed reduction (H2-TPR)
The reducibility of the catalysts was studied by the temperature programmed reduction (H2-TPR) on a Micromeritics Auto ChemII 2920 apparatus. The catalyst (around 50 mg) was heated from room temperature to 1173 K (10 K.min −1 ) under a flow of 10% H2 in argon (50 mLSATP.min −1 ). The reduction temperatures, the H2-consumption as well as the reduction degree (called '% red') of the different samples were measured by this technique. The % red designates the ratio between the number of moles of experimental and theoretical H2 consumed by the sample.

Temperature programmed oxidation (TPO)
The analysis of the carbon deposits on the surface of spent samples (after catalytic test in DRM) was carried out by TPO analysis coupled to a mass spectrometer with a Pfeiffer Vacuum instrument QMS 2000 Prisma. Around 10 mg of used sample were subjected to a flow of helium (15.0 mLSATP.min −1 ) from room temperature to 1073 K (15 K.min −1 ) to desorb the carbonates. After cooling to 323 K, the TPO was carried out under 1.0% mol O2 in He (15.0 mLSATP.min −1 ) up to 1073 K with a temperature ramp of 8 K.min −1 .

Catalytic reaction
Catalytic tests in methane dry reforming using a molar ratio CH4/CO2 = 1.0 ( Figure 1) were conducted at atmospheric pressure using a quartz reactor in form I (length 300 mm, inner diameter 7.0 mm) placed in an experimental setup previously described [4], in the temperature range 773−1073 K. The catalyst (15.0 mg) was pretreated under nitrogen from room temperature to 773 K (3.0 mLSATP.min −1 ) then the gas mixture is introduced at rate of 28.0 mLSATP.min −1 (GHSV around 5.610 4 h −1 ). The equations used to calculate conversions of CH4, CO2 and yields of H2, CO are given below [3]: where Fin is the inlet molar flow and Fout is the outlet molar flow.

Catalysts Characterizations
The BET surface areas and the average pore diameters of the calcined samples are reported in Table 1. It can be seen that for all samples, average pore diameters around 5 nm were obtained indicating a mesoporous texture of our solids. The elaborated samples also revealed very low specific surfaces between 5−7 m 2 .g −1 , which evidences that the partial substitution of nickel by cobalt or copper on both perovskite and spinel structures has no significant effect on the evolution of the BET surface. Indeed, these results are also similar to those reported by Valderrama et al. [25], who studied the texture of LaNiO3 and LaNi0.8Co0.2O3 cobalt-type perovskite catalysts, calcined at 1023 K. These authors have measured specific surfaces of the order of 7 m 2 .g −1 for both catalysts. The nitrogen adsorption-desorption isotherms of the calcined samples, shown in Figure 2-a, are similar to type IV (IUPAC classification [26]) characteristic of mesoporous materials (2−50 nm), which agrees with the obtained pore diameter (around 5 nm). The pore size distribution represented by the BJH curves (Figure 2-b) also confirms the mesoporous character of the elaborated samples with a porous distribution between 2 and 10 nm.    Table 1. Specific surface area (SBET), average pore diameter (dpore), experimental H2 consumption (after H2-TPR) and reduction degree (%red) of the calcined samples. Figure 3 shows the XRD patterns of the different fresh catalysts. The diffractogram of non-substituted perovskite LaNiO3 and spinel La2NiO4 structures obtained in our previous work [3] are also given as references. As can be seen from the XRD results, the substitution of nickel in perovskite and spinel structures by cobalt or copper affects the crystalline structure. In the case of fresh LNCo3 and LNCu3 samples calcined at 1023 K, the diffractograms reveal the formation of perovskite phases, on the contrary LNCo4 and LNCu4 which present the formation of mixture of spinel phase and lanthanum oxide La2O3 (PDF-ICDD 05-0602) with a predominant of La2O3 in the case of the cobalt-substituted solid. It is also important to point out that it is difficult to accurately determine the spinel and perovskite phases formed in the case of cobalt-substituted samples. Indeed, an overlap between the diffraction lines of LaNiO3 (PDF-ICDD 33-0711) and LaCoO3 (PDF-ICDD 48-0123) was observed for LNCo3 and between those of La2NiO4 (PDF-ICDD 34-0314) and La2CoO4 (PDF-ICDD 72-0937) for LNCo4. These overlap phenomena were also observed by G. Valderrama et al. when substituting Ni by Co with a substitution ratio ranging from 0 to 1 [25]. On the other hand, the substitution of both perovskite and spinel structures by copper gives rise to the formation of LaNiO3 and La2NiO4 phases. No copper based phase was identified. According to the literature, the absence of copper phase or Ni-Cu alloy in the case of Cu-substituted samples can be explained by a good dispersion of copper species in the calcined sample [27−28].
To examine the effect of H2-reduction on the structure of our materials, samples after H2-TPR analysis were also characterized by XRD (Figure 4). The diffractograms show the presence of La2O3 phase accompanied by the characteristic peaks of metallic nickel Ni 0 (PDF-ICDD 04-0850) for both LNM3 and LNM4 structures. No characteristic peak of metallic cobalt or copper has been detected which may be due to their very low content and /or their good dispersion. We also note the absence of alloy between nickel and cobalt or copper which is confirmed by the H2-TPR analysis ( Figure 5) that highlighted two reduction peaks attribut-  ed to the reduction of Ni 2+ . Indeed, according to the literature works [26,29], the alloy formation is confirmed by the presence of a single reduction peak indicating that the reduction of bimetallic catalysts (Ni-Co) occurs in one step.
The H2-TPR profiles of the elaborated LNM3 and LNM4 catalysts are represented in Figure  5. In order to highlight the effect of nickel substitution by cobalt or copper on the reducibility, the profiles of non-substituted spinel La2NiO4 and perovskite LaNiO3 samples are also given. For all samples, two reduction peaks are observed. For LNM3 sample, the first one corresponds to the reduction of Ni 3+ to Ni 2+ in La2Ni2O5 (2 LaNiO3 + H2 → La2Ni2O5 + H2O) and also to that of NiO species, while for LNM4 solid, it represents the reduction of NiO to Ni 0 . Concerning the second peak, it is attributed to the reduction of Ni 2+ species present in form of La2NiO5 (in LNM3) and La2NiO4 (in LNM4) to  lanthanium oxide and metallic nickel as follows: La2Ni2O5 + 2 H2 → 2 Ni + La2O3 + 2 H2O and La2NiO4 + H2 → La2O3 + Ni + H2O. The LNCo3 profile is similar to that of LaNiO3 perovskite sample. In the case of LNCo4, a 30 K decrease of temperature of both reduction peaks was noted (663 and 823 K for the substituted sample instead of 693 K and 853 K for the La2NiO4 oxide). On the contrary, for copper substituted samples, a significant decrease of temperature for the low temperature peak was observed for both structures compare to the non-substituted samples. Indeed, the first reduction peak is observed at 603 K for LNCu3 and at 668 K for LNCu4, which correspond to around 20 K temperature decrease compared to LaNiO3 and 25 K in comparison with La2NiO4 respectively. The second peak of reduction was recorded at 768 K for LNCu3 and 853 K for LNCu4 which is close to the temperature of the corresponding non substituted structure. According to Moradi et al. [28] and Nataj et al. [23], Cu 2+ is reduced in Cu 0 at temperatures lower than those corresponding to the reduction of Ni 3+ and Ni 2+ , and that the presence of Cu 0 crystallites facilitates the reducilibilty of nickel cations. The experimental H2 consumption and reduction degree (%) given in Table 1 indicate that the substituted LNM3 samples are more reducible than LNM4 ones. Indeed, regardless the incorporated metal (Co, Cu), the reduction is almost total in the case of LNM3 while for substituted LNM4 structures, it is estimated at 90% for LNCo4 and 84% for LNCu4.

Catalysts Reactivity
The reactivity results of LN3, LN4, LNM3, LNM4 (M = Cu or Co) in the dry reforming of methane, obtained in the temperature range (773−1073 K), are presented in Figure 6. Firstly, it can be noted that the nickel substitution by cobalt or copper has a significant effect on the initial conversion temperature of both methane and carbon dioxide. Indeed, the substitution by cobalt delays the activation of CH4 contrary to copper for which, a decrease in the initial activation temperature (Tinit) is observed. Comparing the initial activation temperature of both reactants CH4 and CO2, the cobalt substituted samples LaxNi0.9Co0.1Oy (x = 1, 2 and y = 3, 4) show an increase of Tinit (1023 K) com-  pared to the copper substituted catalyst LaxNi0.9Cu0.1Oy (Tinit = 873 K). At the reaction temperature of 1073 K, the highest CH4 and CO2 conversions, estimated respectively at 88 and 91%, are obtained over the copper substituted sample with spinel structure (LNCu4).
The decrease of the activation temperature of both reactants, observed in the case of copper substituted catalysts, can be explained by the formation of Cu 0 crystallites that favors the reducibility of nickel cations to active metallic sites Ni 0 . Similar results have been observed by Valderrama et al. [25] for LaCoO3 catalyst and Moradi et al. [28] in the case of copper substituted catalysts.
The yields of reaction products (CO + H2) are shown in Figure 7. The measured yields at 1073 K revealed that the copper catalysts LNCu3 and LNCu4 have similar CO and H2 yields compared to those obtained for corresponding non-substituted LN3 and LN4 samples. No differences were observed between the two structures in the case of copper containing samples (90 and 86% for perovskite LNCu3) and (86 and 79% for spinel LNCu4). However, for the catalysts substituted by cobalt, a significant difference in the product yields is noted for the two structures compared to the nonsubstituted samples with a higher CO/H2 ratio at the reactor outlet. This high ratio, indicating an important CO production, suggests the important participation of the reverse water gas (RWGS) reaction for the cobalt based catalysts [31].
The study of the effect of the reaction time on the stability of catalytic performances was carried out at 1073 K during 65 h of catalytic test (Figure 8). Results obtained on nonsubstituted LN3 and LN4 catalysts show that both catalysts exhibit relatively stable conversions of reactants (76% of CH4 and 86% of CO2) and product yields (around 70% of H2 and 80% of CO) for both samples. However, it is also important to note that the spinel catalyst LN4 is slightly more stable over time than perovskite LN3. This difference in stability may be due to the presence of a larger amount of La2O3 in the spinel catalyst, on which CO2 can adsorb and then migrate in the bulk to form La2O2CO3 intermediate; this latter prevents carbon deposition and reduces the deactivation of active nickel species according to the following equation: La2O2CO3 + Ni-C → La2O3 + 2 CO + Ni [32].
For the substituted catalysts, the obtained stability results show that the cobaltsubstituted samples with spinel structure (LNCo4) exhibit the best and most stable conversions (CH4 and CO2) and syngas production (H2 + CO), despite the fact that copper substituted samples activate the methane at lower temperature compared to the Ni-Co catalysts. This better stability of cobalt-substituted catalysts could be explained by a synergistic effect between Ni and Co in the case of nickel-cobalt bimetallic catalysts [25,33−35]. In fact, the coexistence of these two metals contributes to the improvement of the catalytic performances and the stability of the catalysts.
In order to examine the structure evolution and coke deposition, XRD and TPO analyses were carried out on the spent catalysts after 65 h of catalytic test in DRM at 1073 K. The XRD patterns ( Figure 9) reveal a total decomposition of the initial phases and the appearance of characteristic peaks of La2O2CO3 (PDF-ICDD 048-1113) and Ni 0 in addition to La2O3 phase. In fact, as previously mentioned, the formation of the La2O2CO3 phase is due to the in situ adsorption and bulk migration of CO2 [36]. Furthermore, no diffraction lines of copper, cobalt or carbon phases were detected for the spent samples, indicating no segregation of copper nor cobalt and no significant formation of structured carbon.
The TPO profiles ( Figure 10) giving the oxygen consumption as a function of temperature, show for all the spent catalysts that the oxygen consumption occurred in the temperature range 833−973 K indicating the formation of C  -type carbon due to the high reaction temperature and the long reaction time corresponding to nickel carbide NiC and/or carbon nanotubes [37−38].
The Measurements of carbon amounts deposited after 65 h of catalytic test in DRM were performed by TPO analysis. The obtained results reveal that the spinel La2NiO4 catalyst leads to low carbon formation (5.6 mmol.g −1 ) compared to LaNiO3 perovskite (13.3 mmol.g −1 ), which is in good agreement with the good stability of the La2NiO4 sample observed during the DRM reaction. The comparison between the cobalt and copper-substituted solids for the two structures shows that the substitution with cobalt leads to a carbon deposition of 8.2 mmol.g −1 in the case of the perovskite LNCo3 and 9.9 mmol.g −1 for the spinel LNCo4. On the other hand, the presence of copper in the structure gives rise to amounts of 11.2 and 20.0 mmol.g −1 for LNCu3 and LNCu4 respectively. The lower carbon depositions obtained over the Co-substituted catalysts are in good agreement with the reactivity results that have revealed more stable catalytic performances in the case of cobalt samples.

Conclusion
The reactivity of LaNi0.9M0.1O3 (LNM3) and La2Ni0.9M0.1O4 (LNM4) catalysts with M= Co, Cu, prepared by partial substitution of Ni by Co or Cu using sol-gel method, was studied in the dry reforming of methane (DRM) for syngas (CO + H2) production and their performances were compared to those of LaNiO3 perovskite and La2NiO4 spinel. The obtained results highlighted the effect of: (i) the structure type, and (ii) the substitution of nickel by cobalt or copper on the catalytic activity and stability. The reducibility and activity results revealed better catalytic performances in the case of perovskite structure (LN3) due to its high reducibility compared to the spinel one (LN4). The nickel substitution in the LaNiO3 perovskite and La2NiO4 spinel structures by cobalt or copper strongly influences the activation temperature of methane and the stability of the catalytic performances over reaction time. The presence of cobalt improves the catalytic stability leading to higher and more stable conversions of CH4 and CO2 and syngas (H2 + CO) production with a low carbon deposition compared to copper samples. These results are strongly related to a synergistic effect between Ni and Co that enhances the catalytic performances in the DRM reaction.