Long-term stability test of Ni-based catalyst in carbon dioxide reforming of methane

Applied Catalysis A: General 474 (2014) 107–113

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Long-term stability test of Ni-based catalyst in carbon dioxide reforming of methane A. Serrano-Lotina ∗ , L. Daza Instituto de Catálisis y Petroleoquímica (CSIC), C/Marie Curie 2 L10, Campus Cantoblanco, 28049 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 30 January 2013 Received in revised form 12 August 2013 Accepted 14 August 2013 Available online 23 August 2013 Keywords: Biogas Reforming Hydrogen Hydrotalcite Carbon deposition

a b s t r a c t A La-NiMgAlO catalyst, obtained after calcination of a hydrotalcite precursor, was evaluated in dry reforming of methane at 650 and 700 ◦ C and compared with a previous test performed at 750 ◦ C. At 700 ◦ C the catalyst showed no sign of deactivation during 200 h, while it deactivated slowly afterwards. However, at 650 ◦ C conversion decay was detected from the beginning of the test. In both tests, CH4 and CO2 conversion were higher than thermodynamic equilibrium estimation which suggests the participation of other reactions such as methane decomposition or steam reforming of methane. The occurrence of reverse water-gas-shift reaction (RWGS) was responsible for the H2 /CO ratios below unity and for the higher CO2 conversion compared with CH4 . Used catalysts were characterized by several techniques (TEM, 27 Al MAS NMR, XRD, TPO and Raman spectroscopy) in order to study catalyst structure and to establish whether carbon was deposited and its nature. Ni particle diameter increased when reaction temperature decreased but no differences in 27 Al MAS NMR or XRD results were observed. A higher coke deposition rate was detected when the temperature was increased. At 700 and 650 ◦ C carbon species were mainly graphite ribbons, coating carbon and graphite nanoencapsulates, while at 750 ◦ C there were multi-walled carbon nanotubes (MWCNT), fibres and graphite ribbons. The deactivation of the catalysts tested at 650 and 700 ◦ C can be related to: (i) the presence of coating carbon, graphite nanoencapsulates and Ni particles embedded inside the carbon nanotubes (CNT); (ii) partial sintering and (iii) a lower hydrogen production what makes that carbon transportation away the surface was less favoured. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

CH4 + CO2  2H2 + 2CO H ◦ 298 = 247 kJ mol−1

The CO2 -reforming of CH4 with CO2 has been intensively studied due to its important application in the industry for producing synthesis gas [1–5]. Synthesis gas (H2 + CO) is an important feed for the petrochemical industry (e.g. in methanol synthesis and the Fischer–Tropsch process). Although the commercial method most commonly employed by industry to produce syngas is the steam reforming of methane (1), this process produces a rather high H2 /CO ratio for the methanol and Fischer–Tropsch syntheses. On the contrary, CO2 -reforming of CH4 (2) produces a lower H2 /CO ratio and the separation of CO2 , which is an energy intensive and a rather costly process, is not necessary. In addition, it introduces some other environmental benefits such as: upgrading of biogas (a renewable resource mainly composed by CH4 and CO2 ) and the removal of two greenhouse gases

The major drawback of this reaction is the rapid deactivation of nickel catalysts as a result of carbon deposition via the Boudouard reaction (3), CH4 decomposition (4) and/or the reverse carbon gasification reaction (5) [6]

CH4 + H2 O  3H2 + CO H ◦ 298 = 206 kJ mol−1

∗ Corresponding author. Tel.: +34 91 5854793; fax: +34 91 5854760. E-mail address: [email protected] (A. Serrano-Lotina). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.08.027

(1)

2CO  C + CO2 CH4  2H2 + C

H ◦ 298 = −171 kJ mol−1 H

◦

298

= 75 kJ mol

H2 + CO  C + H2 O H

◦

298

(3)

−1

= −131 kJ mol

(2)

(4) −1

(5)

In addition to operation parameters [7–9], catalyst structure and properties affect carbon deposition. It is supposed to occur more easily on larger nickel particles than on smaller ones [10,11]. Furthermore, it is favoured by acidic supports. It has been suggested that carbon deposition can be attenuated or even suppressed when the metal is supported on a metal oxide with a strong Lewis basicity [12] or when a promoter is added [13,14]. This occurs because the ability of the catalyst to chemisorb CO2 is enhanced, and the adsorbed CO2 reacts with C to form CO, resulting in the reduction of coke formation. Ni/MgO possess good coke resistance ability due to formation of solid solution Ni–Mg–O [15,16], but the surface area

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and activity of Ni/MgO is low compared with other catalysts under the same reaction conditions [17]. The synthesis method is another important parameter to take into account. The use of precursors in which the metal is homogeneously distributed, may result, after calcination and reduction, in the formation of highly dispersed and stable metal particles on the surface [18]. Catalysts obtained after calcination of hydrotalcitelike precursors have shown high surface area and basic properties as well as high dispersion of the active metal [19]. However, they do not provide a sufficient degree of stability which is required by an industrial process. Previous works [20–22] reported the addition of lanthanum as a way to enhance catalytic stability in dry reforming of methane when calcined at 750 ◦ C. Lanthanides favour metal dispersion [23,24] and strengthen CO2 adsorption on the support [25]. The presence of oxycarbonates over La2 O3 also facilitates coke removal since it seems to act as a dynamic oxygen pool [14,26]. The aim of this work is to study the influence of reaction temperature over catalyst deactivation, as well as to evaluate and characterize the deposited carbon.

Fig. 1. CH4 and CO2 conversion vs. time in dry reforming of methane at 650, 700 and 750 ◦ C. Testing conditions: CO2 :CH4 = 1 and space velocity equal to 4800 cm3 gcat −1 h−1 .

2. Experimental 2.1. Catalyst preparation The catalyst was obtained after calcination at 750 ◦ C of a hydrotalcite precursor, which was prepared by co-precipitation, according to a previously reported method [20,21]. The final measured Mg/Al molar ratio was 2.3, whereas Ni and La contents were 2.8% and 1.9%, respectively. 2.2. Catalytic tests Catalytic tests were carried out in a Microactivity Reference PID Eng&Tech equipment, in a tubular fixed-bed stainless steel (Ni free) reactor with a CH4 :CO2 molar ratio of 1:1. Excessive pressure drop was avoided by choosing a catalyst particle size between 0.50 and 0.42 mm. A more detailed description of the testing protocol can be found elsewhere [20]. Reaction products were analyzed with an Agilent chromatograph 6890N connected in line, equipped with a TCD detector and Chromosob 102 and Porapak P5 Q columns. CH4 , N2 , H2 , and CO2 gases were fed from Praxair gas bottles with a purity of 99.5 for CH4 and 99.999% for the rest. The catalyst was pre-treated in situ at 650 ◦ C with pure hydrogen (100 mL min−1 ) during 1 h, in order to only reduce the segregated NiO [22]. The influence of reaction temperature was evaluated testing the catalyst at 650 and 700 ◦ C and comparing the results with an experiment performed at 750 ◦ C which has been already reported [27]. These tests were performed at the highest space velocity necessary to achieve the maximum conversion, i.e. 4800 cm3 gcat −1 h−1 . Therefore, deactivation would not be hidden as a consequence of an excess of active sites. 2.3. Characterization X-ray diffraction of tested catalysts was performed by an X-ray diffractometer (XPERT-PRO, PANanalytical) using Cu K␣ radiation ( = 0.154 nm). 27 Al MAS NMR analysis was carried out in a BRUKER AV 400 WB spectrometer, equipped with a CPMAS 4mm probe, at room temperature and 15 kHz. TEM studies were carried out in a JEOL Model JEM-1200 EXII transmission electron microscope equipped with an emission source of electron at 200 kV. Samples were ultrasonically dispersed in ethanol and a few drops were placed on a holey-carbon-coated copper grid, allowing the solvent to evaporate in air before TEM observation. Temperature programmed oxidation (TPO) were conducted in a Mettler-Toledo TGA/SDTA 851 thermo-balance with STAR 8.10

software. The thermobalance was coupled to a mass spectrometer (MS) detector Pfeiffer Thermostar GSD 301 T3. The test was performed between 25 and 900 ◦ C (5 ◦ C min−1 ) using a mixture of O2 /N2 10/40 mL N min−1 . Carbon gasification was monitored by CO2 + signal (m/z = 44). Raman spectra were collected in a Renishaw System 1000 spectrometer equipped with Ar ion laser (Spectra Physics,  = 514 nm, power 19 mW, 1 mW on sample), a cooled CCD detector (−73 ◦ C) and a holographic super-Notch filter to remove the elastic scattering. The spectral resolution was ca. 3 cm−1 and spectrum acquisition consisted of 10 accumulations of 30 s. The spectra were recorded at ambient temperature. 3. Results and discussion 3.1. Catalyst testing Fig. 1 shows the evolution of CH4 and CO2 conversion (XCH4 and XCO2 , respectively) at different reaction temperatures versus time. The experiments were kept for almost 300 h. All the tests started at 700 ◦ C in order to check that catalyst conversion achieved the expected value, and to complete the activation process of the catalyst which may be ascribed to the formation of new active sites when the catalyst was exposed to the reaction mixture [4,28]. Fig. 1 shows the catalytic performance afterwards. In all cases CO2 conversion was higher than CH4 conversion what may be due to reverse water shift reaction (RWGS) (6) [20] CO2 + H2  H2 O + CO H ◦ 298 = 41 kJ mol−1

(6)

Moreover, both conversions were higher than thermodynamic equilibrium estimation (650 ◦ C: 60%, 700 ◦ C: 76%, 750 ◦ C: 86%), what suggests that other reactions were taking place. These reactions could be methane decomposition (3) or methane steam reforming (1) which will consume water formed by means of RWGS reaction (6) [9]. When the catalyst was operated at 750 ◦ C no sign of deactivation occurred [27]. However, when reaction temperature decreases deactivation phenomena become evident. At 700 ◦ C the catalyst was stable during the first 200 h and afterwards methane and carbon dioxide conversions slightly decreased from 82% and 86% to 80% and 84%, respectively. The deactivation rate (kd ) was 2.3 × 10−4 h−1 . This rate was calculated according to a previously reported procedure [21]. At 650 ◦ C a higher deactivation was observed from the beginning of the test, being the deactivation rate kd = 4.3 × 10−4 h−1 .

A. Serrano-Lotina, L. Daza / Applied Catalysis A: General 474 (2014) 107–113

Fig. 2. TPO-MS characterization of used catalysts.

Some differences were also observed in H2 /CO ratio. In all cases it was below unity what may be due to RWGS reaction (6). This ratio increased at higher temperatures due to the participation of steam reforming of methane (1) and methane decomposition (4) which are favoured at high temperatures [9]. 3.2. Post-reaction characterization Characterization of the used catalysts was performed in order to study the possible causes of deactivation. When the catalyst was operated at 650 ◦ C Ni particle diameters were mainly between 7 and 30 nm and some other particles were detected up to 50–60 nm. When the catalyst was tested at 700 ◦ C most of the particles lied between 3 and 20 nm and some other particles at 40 nm were detected. Ni particles sizes measured by TEM microscopy showed a bimodal distribution when the catalyst was operated at 750 ◦ C [27], with maximums at 8 and 13 nm. The maximum Ni particle diameter found was 21 nm. Then, partial sintering may have occurred at low temperatures, despite this phenomena is usually favoured at high temperatures. The used catalyst were also characterized by 27 Al MAS NMR (not shown) but no differences were observed (neither among the used catalysts, nor with respect to fresh catalyst). The ratio between tetrahedral and octahedral aluminium did not change after reaction, what may indicate that clusters of Mg-Al inverse spinel type [29] were still present. No differences were observed in crystalline structure characterized by XRD (not shown) with the exception of the appearance of Ni and graphite phases. TPO characterization showed that carbon deposited during the tests. The average coking rate was calculated from the weight loss from thermogravimetric results (Fig. 2), caused by CO2 formation, and divided by the duration of the test. Despite a higher deactivation was observed at low temperatures, carbon deposition rate was higher at high temperatures. Carbon deposition rate at 750 ◦ C was 2.3 mg C gcat −1 h−1 , while at 700 ◦ C was 1.8 mg C gcat −1 h−1 and at 650 ◦ C 1.4 mg C gcat −1 h−1 . Similar trend was observed by Tsyganok et al. [30]. According to equilibrium calculations performed by Gadalla and Bower [31], coking was expected to be reduced with increasing reaction temperature from 650 to 750 ◦ C. However, here the trend is the reverse. On one hand, methane cracking is known to start at 550 ◦ C and it is favoured at higher temperatures. On the other hand, Boudouard reaction becomes negligible at T > 700 ◦ C. This fact may indicate that in this case methane decomposition was the main route of carbon deposition. Pechimuthu et al. [32] reported that the amount of deposited coke over the catalyst varied with temperature in the following order: 650 ◦ C > 750 ◦ C > 700 ◦ C.

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They suggested that both CH4 cracking and CO disproportionation contributed to carbon formation. However, the higher amount of accumulated coke did not affect the good performance of the catalyst when it was operated at 750 ◦ C [27]. This suggests that some carbon species formed on the surface of the catalysts must be involved in the reaction to produce CO (i.e. acting as active sites in the reaction but not as a deactivating factor). Thus, the deposited carbon did not deactivate the Ni active phase and the catalytic activity was kept with reaction time because the active metal was still exposed to reactants. In addition, the diffusion of carbon species must be well matched with the rate of methane decomposition and then the carbon species are able to be transferred away in time, avoiding the coverage of the catalyst surface [33]. Although, no increase in pressure was detected in any of the tests, in the long run carbon deposition may lead to reactor plugging. An important fact from an industrial point of view would be to know how long the catalyst could be operated before reactor plugging. TPO is also a useful technique for evaluating the extent of carbon structural order, i.e., amorphous carbon and/or graphitic nature. It is well established that a higher crystallization degree of carbon species is accompanied by an elevation of the temperature at which gasification is induced [34]. Crystalline carbon can be only burnt out at temperatures as high as 700 ◦ C [35]. In all the samples an initial weight loss (very trace amount) between 25 and 150 ◦ C was detected, which was due to desorption of physically adsorbed water. A small desorption of CO2 at 250 ◦ C was observed also in the three used samples, though its contribution is higher in the catalyst tested at 750 ◦ C. This weight loss can be ascribed to C␣ which corresponds to the active species responsible for the formation of synthesis gas [36]. The peak at 450 ◦ C may be ascribed to C␤ which is probably produced by polymerization and rearrangement of C␣ [37]. At temperatures above 500 ◦ C a broad and asymmetric peak appeared which suggested the presence of crystalline filaments [38]. This peak was shifted to the right when the temperature decreased what may imply a higher graphitization degree, a less exposed location or a higher diameter of the filaments [39]. The broadness of this peak may be indicative of a range of nanofiber diameters where fibre diameter/availability of edge sites can impact on gasification characteristics [38]. TEM micrographs of used catalysts (Figs. 3 and 4) showed different types of deposited carbon: coating or shell-like encapsulating carbon (Figs. 3a and 4a), graphite ribbons (Figs. 3a,b,d and 4b), graphite nanoencapsulate (Figs. 3c and 4a), fibres and MWCNT (Figs. 3a,d and 4c,d). The most frequent species detected after reaction at 650 and 700 ◦ C were coating carbon, ribbons and graphite nanoencapsulates. Fibres and nanotubes were detected in a lesser extent, overall after reaction at 650 ◦ C. When the catalyst was operated at 750 ◦ C, the most frequent structures were nanotubes and ribbons [27]. Then, filaments were detected over the catalyst that showed a great stability, while coating carbon and nanoencapsulates were formed when deactivation occurred. These results agree with the finding of Suelves et al. [40]. A possible explanation would be that coating carbon prevents methane diffusion towards Ni surface and acts as a cement of catalyst particles favouring the sintering, as was observed by Ni particle size evaluation that showed higher diameters when the reaction temperature decreased. The growth rate of the filamentous carbon has been suggested to increases as the H2 concentration in the carrier gas increases [41]. According to Nolan et al. [42], H2 helps that filaments growth continues for a long period of time, since carbon is transported away from the active surface, preventing carbide formation. At 750 ◦ C H2 production is higher because of the higher conversion and then filament growth may be more favoured. Graphitic ribbons (Figs. 3a,b,d and 4b) were formed as a result of transformation of hexagonal diamond structures which leads to

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Fig. 3. TEM micrographs of used catalyst at 700 ◦ C: (a) general micrograph, (b) graphite ribbons, (c) graphite nanoencapsulates, (d) graphite ribbons, MWCNT, pear-shape and embebed Ni particles.

bent graphitic forms, which consists of carbon platelets oriented in an arrangement that is parallel to the fibre axis. This structure is of short-range order [43]. The ribbon structures are similar to PAN-based carbon fibres and they present a lattice spacing of d002 = 0.34 nm which means that this carbon is also turbostratic. There were no differences between graphitic ribbons formed at 650, 700 or 750 ◦ C. Figs. 3c and 4a showed an encapsulated Ni particle (also called nanoencapsulates). These graphite nanoencapsulates together with coating carbon may be responsible of catalyst deactivation by blocking the metal surface [44]. It can be seen that the graphite shell conforms to the shape of the encapsulated particle. There are different mechanisms which try to explain this structure. Audier et al. [45] reported that small radial cracks in the graphite shells occurred providing a path for reaction gases to the metal. However, it was not observe any crack in the nanoencapsulates. Nolan et al. [42] proposed that in an intermediate step of the formation of the layer of graphite, edges of graphite basal planes are oriented perpendicular to the catalyst surface. The top portion of the particle remains uncovered by carbon and then exposed to the gas phase. The bottom of the particle is still attached to the support. The graphite layer terminates by turning-in to the catalyst metal. The edge of the basal plane is bonded to the metal surface, instead of closing upon itself. A second graphite layer may then precipitate, nucleate and spread underneath the first layer. Additional carbon atoms are added from the catalyst metal to the edge of the first layer so that its radius and thus surface area increase. Carbon eventually forms on the bottom

surface of the particle, separating it from the support. The closing process is driven by the increased stability of a closed formation, with all C C bonds. When a higher amount of hydrogen is present nanoencapsultes are not produced because this gas may satisfy graphite edge valences, then such edges will be stable in the final product and carbon atoms dos not need to close upon themselves to eliminate graphite edges [47]. This fact may explain why this structure was not found when the catalyst was tested at 750 ◦ C [27]. Fibres and CNTs were arranged in a random fashion in the manner of fibres formed at 750 ◦ C [27]. Most of these filaments exhibited some degree of curvature, probably owing to the unequal diffusion of graphitic carbon through the nickel particles [48]. The walls of the filaments were inclined with respect to the fibre axis, i.e. they showed fish-bone structure. However, the carbon nanofibres and nanotubes (CNTs) deposited after reaction at 750 ◦ C had multiple walls (MWCNTs) parallel with the axis. Fibre diameters were similar in both samples, 15–58 nm for the filaments deposited at 700 ◦ C and 13–51 nm for the fibres formed at 650 ◦ C. At 750 ◦ C, fibre diameters varied between 14 and 33 nm. According to TPO characterization, a wider range should be present at 650 ◦ C but TEM images showed no significant differences between filaments formed at 650 and 700 ◦ C. Consequently, the carbon species may gasify at higher temperatures because they are less exposed to the oxidizing agent or they are more crystalline. At least, three types of CNTs formed at 700 ◦ C were observed by TEM: (i) CNTs with mouth filled with Ni particle having pear-shape (Fig. 3d); (ii) CNTs with closed or opened end but without Ni

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Fig. 4. TEM micrographs of used catalyst at 650 ◦ C: (a) general micrograph, (b) graphite ribbons, (c) MWCNT, (d) MWCNT.

particle on the tip (not shown); (iii) CNTs with embedded small size of Ni particle inside their tubes (Fig. 3d). Pear-shaped Ni particles are produced as the consequence of the movement of the C atoms along the Ni particle through both the surface and the bulk towards the interface [49]. The occurrence of Ni inclusions in the growing fibre is a common feature to carbon growth. The movement of carbon atoms through the lattice necessitates a displacement of Ni atoms where the pressure exerted on the Nisupport interface due to graphite formation must be of sufficient magnitude to extract Ni particles from the substrate. The presence of filamentous carbon with Ni on the top could explain why severe deactivation did not occur since metallic sites remain uncovered and accessible to the reactants. They present a lattice spacing of d002 = 0.34 nm, while well-crystallized graphite d-spacing is 0.334 nm, that is this carbon is turbostratic. Apparently, within the growth process, ordering of atoms within a graphene layer was high, but between layers, it was limited [50]. Only type ii was detected in the used catalyst operated at 650 ◦ C. MWCNT formed at 650 and 700 ◦ C exhibited fishbone structure while at 750 ◦ C walls were parallel to the filament axis [27]. The Raman spectra (Fig. 5) show the following bands: the D band (∼1350 cm−1 ), the G band (∼1580 cm−1 ) and the G band (∼2700 cm−1 ) [51,52]. Two less intense bands (2D) were also observed (2940 and 3240 cm−1 ), which are supposed to be combinations of the D and G modes [53,54]. The so-called G-mode is assigned to the in-plane displacement of carbon atoms (C sp2 ) in the hexagonal sheets. G band is indicative of long-range order in a sample and arises from the two-phonon, second order scattering

process that results in the creation of an inelastic phonon [55]. D,D and 2D-modes are assigned to the non-zone-centred phonons associated to the disorder-induced vibration of the C C bond [56–58]. D-modes indicate the presence of disordered graphite, that can be referred to defects as pentagons and heptagons in graphite, edges of the graphite crystal, carbonaceous impurities with sp3 bonding, broken sp2 bonds in the sidewalls and amorphous carbonaceous products [52,59,60]. The high intensity of the D-band illustrates the large amount of carbonaceous disordered species as byproducts of the reaction. The appearance of D peak points out that these carbon nanotubes are multiwalled, which corroborates the results obtained by TEM. The ratio of intensities of the D and G maximums (ID and IG ) gives an indication of the crystallinity of the studied material. The lower value of ID /IG points out on higher crystallinity, while the higher ratio suggests on the higher disorder within the crystalline structure [61,62]. Highly oriented pyrolytic graphite (HOPG) has an ID /IG ratio approaching 0, while amorphous carbon has a value near 3.3. Used catalyst after reaction at 700 ◦ C showed an average ID /IG value (taken from five different Raman spectrum spots) of 1.3, that is, higher than the ratio of deposited carbon when the catalyst was tested at 750 ◦ C [27]. Different Raman spectra were obtained when different areas of used catalyst after operation at 650 ◦ C were analyzed. Two examples of these Raman spectra (650C-1 and 650C-2) are shown in Fig. 5. ID /IG ratios of the five spectra varied between 0.7 and 1. It can be observed that in 650C-1 spectrum, D band showed almost the same intensity that G band. However, in 650C-2 D band is less intense that G band. It seemed that carbon species were not uniformily distributed over

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In addition to the bands ascribed to deposited carbon, other two bands were detected (1054 and 3100 cm−1 ). First band can be assigned to OH bending vibrations and/or symmetric stretching vibrations of carbonates [68] while second band can be ascribed to stretching water molecules [19]. 4. Conclusions

Fig. 5. Raman spectroscopy characterization of used catalysts.

the catalyst surface. The higher crystalinity of the coke deposited at 750 ◦ C may be due to the structure of MWCNT. At this temperature, MWCNT exhibited parallel walls while at lower temperatures the structure was fishbone-like. These results agree with Zheng et al. [63] finding. They explained this fact by a higher density of edge atoms exposed when graphene layers are slightly inclined. Several works [53,64] reported that the straight CNTs with concentric parallel carbon sheets have the carbon layer connected with each other along the axial direction. Thus the in-plane carbon crystalline size can be increased significantly and defects inside the carbon layers can be diminished significantly. However, CNTs of the fish-bone type has a length of the carbon sheets that is naturally limited and the in-plane carbon crystalline size cannot increase significantly along the axial direction as compared with the concentric parallel sheet CNTs. Furthermore, the helical tubes may also have many more graphitic sheets with a distorted angle than the straight tubes, which would lead to defects inside the as-grown CNTs. Attending to G1 band, intensity increase in the following order: 750C > 650C-2 > 650C-1 > 700 C, i.e. the more intense D band is, the lower intense G1 becomes. This is because when a sample become less ordered, G1 band intensity decreases since the coupling effect, which is necessary for the two-phonon, decreases. Baird et al. [65] claimed that the high carbon deposition rate is related to the wellformed carbon morphologies, which also may explained why at 750 ◦ C higher carbon deposition rate was detected despite no deactivation was observed. However, comparing these spectra with others reported by other authors [52,66,67], G1 band in this case is much more intense despite a higher disorder degree. These may be explained by the different carbon species present, some better ordered that others.

A La-NiMgAlO catalyst, obtained after calcination of a hydrotalcite precursor, was evaluated in dry reforming of methane at 650 and 700 ◦ C during 300 h. The catalyst operated at 700 ◦ C showed no sign of deactivation during 200 h, while it deactivated slowly afterwards. At 650 ◦ C deactivation was evident from the beginning of the test. In both cases, CH4 and CO2 conversions were higher than thermodynamic equilibrium estimation due to participation of secondary reactions such as CH4 decomposition or steam reforming. RWGS was responsible of the higher CO2 conversion compared with CH4 conversion. Characterization of the used catalyst showed that Ni particles diameters increased when temperature decreased, probably due to the coverage of shell-like carbon which acts as a cement leading to Ni particle agglomeration. No differences were detected in catalyst structure excepting the appearance of Ni and graphite phases. Carbon deposition rate increases with the increase in temperature. It may be due to the favourable growth of filaments when H2 concentration increased. In addition, when deactivation occurred there were less active sites to catalyze methane decomposition, so less carbon was formed. Different types of carbon were detected by TEM microscopy: shell-like encapsulating carbon, graphite ribbons, graphite nanoencapsulates, fibres and MWCNTs. The presence of nanoencapsultes and shell-like carbon may be responsible for the deactivation of the catalyst operated at 650 and 700 ◦ C. The higher H2 concentration was, the more prone the filaments were to grow. The deactivation of the catalyst tested at 650 and 700 ◦ C may be attributed to: (i) the presence of coating carbon, graphite nanoencapsulates and Ni particles embedded inside the CNT, which would be inaccessible to the reactants (ii) partial sintering and (iii) a lower hydrogen production what makes that carbon transportation away the surface was less favoured. Acknowledgements Financial support from Comunidad de Madrid (DIVERCEL-CM Programme, S-2009/ENE-1475) is gratefully acknowledged. The ˜ authors thank Professor M.A. Banares, Dr. E. Rojas and Dr. Ricardo López-Medina from Instituto de Catálisis y Petroloeoquímica (CSIC) for their help with the Raman analysis and for their helpful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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