A comparison study on methane dry reforming with carbon dioxide over LaNiO3 perovskite catalysts supported on mesoporous SBA-15, MCM-41 and silica carrier

Catalysis Today 212 (2013) 98–107

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A comparison study on methane dry reforming with carbon dioxide over LaNiO3 perovskite catalysts supported on mesoporous SBA-15, MCM-41 and silica carrier Ning Wang a , Xiaopeng Yu a , Ying Wang a , Wei Chu a,∗ , Ming Liu b,∗ a b

Department of Chemical Engineering, Sichuan University, Chengdu 610065, China Analytical & Testing Center, Sichuan University, Chengdu 610064, China

a r t i c l e

i n f o

Article history: Received 13 March 2012 Received in revised form 27 June 2012 Accepted 22 July 2012 Available online 1 September 2012 Keywords: Mesoporous silica Supported LaNiO3 perovskite Methane dry reforming High performance Pore structure Hydrogen

a b s t r a c t The supported LaNiO3 perovskite catalysts on mesoporous carrier (LaNiO3 /SBA-15, LaNiO3 /MCM-41 and LaNiO3 /SiO2 ) with different pore structures have been synthesized via filling the pores of mesoporous silica with citrate complex precursors of nickel and lanthanum, with further treatments. The catalysts were characterized by means of N2 physisorption, XRD, HRTEM + EDX, TPR, temperature-programmed hydrogenation (TPH) and TGA techniques, and their catalytic performances were measured in methane dry reforming with carbon dioxide to hydrogen and synthesis gas (syngas). The results of low-angle XRD, N2 physisorption and TEM analysis showed that LaNiO3 perovskite was formed inside the channels of mesoporous supports, and the introduction of LaNiO3 perovskite did not destroy the mesoporous structure of support. The pore structure had a substantial influence on the catalytic performance. LaNiO3 /MCM-41 exhibited the higher initial catalytic activity, owing to the higher Ni dispersion, while LaNiO3 /SBA-15 was superior to LaNiO3 /MCM-41 in the long-term stability, which could be due to the stable silica matrix restricted the agglomeration of nickel species. The hexagonal mesopores of LaNiO3 /SBA-15 were still kept intact after reaction, while the mesoporous structure in LaNiO3 /MCM-41 was collapsed during the reaction, which resulted in metal particles aggregation to certain extent. For comparison, the carbon deposition was responsible for the remarkable decrease of catalytic activity over LaNiO3 /SiO2 sample, evidenced by TGA and TPH results. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction The dry reforming of methane with carbon dioxide to produce hydrogen and syngas (CO2 + CH4 → 2CO + 2H2 , H298 = 247 kJ/mol) has received considerable attention in recent years for its great benefit to both environment and economy [1–3]. The methane dry reforming not only converts carbon dioxide and methane which are recognized as undesirable greenhouse gases, but also produces syngas with low H2 /CO molar ratio which is suitable for obtaining valuable products through Fischer–Tropsch synthesis and oxo-synthesis [4]. In addition, it is also considered as a mean of converting solar and atomic energy into chemical energy, which is easier to store and transport [5,6]. Another advantage of this technology is the directly usage of natural gas with high CO2 levels, avoiding the complex gas-separation process [7]. The dry reforming reaction has been studied over both noble metals catalysts (Ru, Rh, Pt, Pd and Ir) [8,9] and nickel-based catalysts [10]. In comparison with noble metal based catalysts, although nickel-based

∗ Corresponding authors. Tel.: +86 28 85403836; fax: +86 28 85461108. E-mail addresses: [email protected] (W. Chu), [email protected] (M. Liu).

catalysts are less active and more sensitive to the coke formation, they are more practical in consideration of availability and economic cost. The major obstacle for industrialization of methane dry reforming over nickel-based catalyst is the rapid deactivation due to the coke deposition on the active sites and/or sintering of the metallic active phases [11,12]. Therefore, it is necessary to develop new nickel catalysts with improved activity and good stability with the aim of preventing the coke formation.A particularly attractive way is the utilization of perovskite type oxides (PTOs) as precursors, which have high thermal stability and relative low cost [13]. The PTOs materials have been synthesized for various reactions, such as hydrocarbons and CO oxidation [14,15], NOx reduction [16,17], water gas shift reaction [18], and so on. Among the various perovskite type materials, LaNiO3 and the related compounds have attracted special attention due to their excellent catalytic activity in dry reforming [13,19,20]. Unfortunately, the PTOs synthesized via conventional procedures exhibit rather low surface areas [13], which strongly restrict the applications of these materials as catalysts. The synthesis strategy plays an important role in the catalytic performance of methane reforming catalysts. The catalysts prepared using different methods have different structural and textural properties, which influence their catalytic behavior. Therefore, numerous methods, including spray pyrolysis [21],

0920-5861/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.07.022

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surfactant-assisted method [22], freeze-drying, and sol–gel processes [23] have been developed to increase the specific surface area of PTOs. However, the surface area of PTOs prepared by most of these methods is still quite low, generally below 10 m2 /g, except for the so-called “amorphous citrate process” [24] and reactive grinding [25]. In addition, a considerable part of the perovskites still have not been successfully applied in industrial catalytic applications, due to their weak mechanical strength and easy poisoning by SO2 . An attractive strategy to overcome these difficulties is to develop a well-dispersed perovskite on a support with high specific surface area, as well as high thermal stability to prevent the sintering of active metal. Citric acid was reported to be beneficial for the formation of pure perovskite phase at relatively low temperatures [26,27]. This synthetic route of supported PTOs has been reported in previous reports [6,28–34]. It was first applied by Kaliaguine and co-workers [28] who prepared MCM-41 supported LaCoO3 perovskites with different La–Co oxide content. It was found that highly dispersed LaCoO3 perovskite was formed inside the channels of MCM-41. These nanostructured composites showed high catalytic activities and resistance to SO2 poisoning in complete methane oxidation. By a novel microwave-assisted method, Cao and co-workers [29] successfully loaded LaCoO3 perovskites nanocrystals on SBA-15 mesoporous silica. The catalytic activity of microwave-derived samples was superior to the LaCoO3 /SBA-15 prepared by impregnation method in complete methane oxidation. Alifanti et al. [27,30] reported that when the LaCoO3 was dispersed on Ce1−x Zrx O2 solid solutions, the reaction rates increased with an order of magnitude in comparison with bulk LaCoO3 perovskite for total oxidation of VOC. Makshina et al. [31,32] supported LaCoOx perovskite inside the MCM-41 channels by the citrate method and showed that the catalytic activity of supported LaCoOx was much higher than that of bulk LaCoO3 in methanol oxidation. In general, supported PTOs catalysts have advantages over the unsupported ones, e.g., better heat transfer character, better mechanical strength, controllable catalyst textures, and larger contact surface between the perovskite and the reactant. The type and nature of support also display significant influence on catalytic behaviors [35]. However, there was few published work about the structural aspects of supported PTOs for methane dry reforming. In this study, we focus our attention on the effects of pore topology on the LaNiO3 particles dispersion, reducibility and the catalytic behaviors. MCM-41 and SBA-15 are silica-based mesoporous materials with uniform hexagonal porosities, narrow pore size distribution and high surface area [36,37]. MCM-41 has the pore diameter of 2–6 nm and relatively smooth pore wall surfaces, while SBA-15 possesses larger pore size, typically in the range of 5–10 nm and exhibits considerable surface roughness due to the presence of microporous interconnections [38]. Three types of siliceous materials with different structural features were adopted as support materials of LaNiO3 catalyst for methane dry reforming reaction. By means of various characterization techniques, the physico-chemical properties of supported LaNiO3 catalysts were systematically investigated. The catalytic performances for methane dry reforming were evaluated and compared, and the relationships between catalytic behavior and property of catalysts have been established. In particular, the catalyst deactivation aspect has been discussed in detail.

according to the direct hydrothermal methods reported in the literatures [37,39]. The La–Ni perovskites loaded into mesoporous materials were prepared by the method described by Yi et al. [29]. 0.59 g of lanthanum nitrate, La(NO3 )3 ·6H2 O, and 0.39 g of nickel nitrate, Ni(NO3 )2 ·6H2 O, (atomic La/Ni = 1) were dissolved in deionized water, and then 0.85 g of citric acid was added to the solution. The molar ratio of citric acid to total metal ions was fixed at 1.5. A homogeneous solution was obtained after stirring for one night. Then 2.78 g of mesoporous materials treated in vacuum at 60 ◦ C for several hours was added to the solution. The mixture was then stirred for another several hours at 40 ◦ C until it became viscous, followed by vacuum drying at 80 ◦ C. To achieve the final Ni content of 10 wt%, this process was repeated following the same procedures for six times. The resulting La–Ni citrate complex precursors were calcinated at 700 ◦ C for 6 h in air. For comparison, a reference bulk LaNiO3 sample was also prepared by calcination of the La–Ni precursors, which was obtained by a conventional citrate method at the same temperature.

2. Experimental

2.3. Catalytic measurements in methane dry reforming with carbon dioxide

2.2. Characterizations of catalysts The specific surface areas, total pore volume and average pore diameter were determined by N2 adsorption/desorption isotherms at −196 ◦ C, using an automated gas sorption system (Quantachrome NOVA 1000e apparatus). Before each measurement, the sample was degassed in vacuum at 300 ◦ C for 3 h. The specific surface area of all the materials was calculated by BET equation and pore size distribution from desorption branch of isotherms using BJH method. The total pore volume was determined from the amount of nitrogen adsorbed at a relative pressure of 0.99. The X-ray diffraction patterns were carried on an X-ray diffraction apparatus (Philips X’ pert PRO) with Cu K˛ (45 kV, 50 mA) radiation. The unit cell√parameter (a0 ) was calculated using the formula a0 = 2 × d1 0 0 / 3, where d1 0 0 represented the d-spacing value of the (1 0 0) diffraction peak in XRD patterns of the samples. TEM measurements were operated on Tecnai G2 F20 transmission electron microscopy. The samples were dispersed in ethanol assisted by ultrasonic technique. TPR experiments were carried out in a fixed-bed reactor. 50 mg sample was loaded, and the reduction gas of 4.2% H2 /N2 with a flow rate of 30 ml/min was introduced. The temperature of the reactor was raised linearly from 100 ◦ C to 800 ◦ C at a rate of 10 ◦ C/min by a temperature controller. The hydrogen consumption was analyzed on-line by a SC-200 gas chromatograph with a thermal conductivity detector (TCD). The amount of carbon deposited on the used samples was determined with a thermo gravimetric analyzer (TGA Q500). The sample was heated in flowing air from room temperature to 800 ◦ C at a heating rate of 10 ◦ C/min. Carbon species formed during the stability test were also characterized by TPH. The TPH experiments were carried out in a fixed-bed reactor. 100 mg of sample after reaction was purged in Ar at 100 ◦ C for 1 h. Subsequently, pure H2 with a flow rate of 30 ml/min was introduced. The temperature of the reactor was raised linearly from 100 ◦ C to 900 ◦ C with a ramping rate of 10 ◦ C/min by a temperature controller. The products were analyzed on-line by a Hiden QIC-20 mass spectrometer.

2.1. Catalyst preparation Conventional silica (SiO2 ) was commercially available from Qingdao Haiyang Chemical Corporation and was treated at 500 ◦ C for 3 h for activation. SBA-15 and MCM-41 were synthesized

The catalytic activity measurements were carried out under atmospheric pressure using a continuous fixed-bed flow reactor. Typically, 100 mg of catalyst was loaded in the reactor using quartz wool. The molar ratio of CH4 to CO2 was 1:1 and GHSV was

100

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Fig. 1. Nitrogen physisorption isotherms and pore size distributions of supports and corresponding supported LaNiO3 catalysts.

36,000 mL/(h gcat ). The catalyst was reduced in the reactor under H2 at 700 ◦ C for 1 h before the catalytic experiment. Effluent gases from the reactor were analyzed on-line by a GC-1690 model gas chromatograph with a TDX01 column and a thermal conductivity detector (TCD). 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Physical properties The N2 adsorption/desorption isotherms and the corresponding pore size distributions of supports materials and supported LaNiO3 catalysts were shown in Fig. 1. For three supports materials, sharp increases in the adsorbed and desorbed volumes were registered in the relative pressure range of 0.6–0.7 for SBA-15, 0.3–0.4 for MCM-41 and 0.75–0.95 for SiO2 , suggesting uniformity of the pore size distribution of each mesoporous silica. All supported LaNiO3 catalysts exhibited type IV isotherms according to the IUPAC classification, which were characteristic of mesoporous materials. The isotherm of LaNiO3 /SBA-15 displayed a sharp increase of nitrogen uptake in the relative pressure (P/P0 ) range of 0.45–0.65 with a characteristic H1-type hysteresis loop, corresponding to the presence of a typical mesoporous structure with 1D cylindrical channels [40,41]. In comparison with pure silica SBA-15, the sorption value shifted slightly to lower P/P0 . As the P/P0 position of the inflection point correlated with the diameter of the mesopore, this observation clearly indicated confinement of LaNiO3 largely inside the pores, resulting in the decrease in average pore size. The broad hysteresis loop suggested that long mesopores limited the emptying and filling of accessible volume [42]. The isotherm of LaNiO3 /MCM-41 was not steep and vertical as in the case of MCM41 material, indicating that the uniformity of the pores was partly suppressed by the LaNiO3 molecules inside the channels. In the case of LaNiO3 /SiO2 , the capillary condensation showed a step increase in nitrogen uptake in the P/P0 range of 0.65–0.9 with a characteristic H1-type broad hysteresis loop. The height of pore condensation i.e., the amount of N2 adsorbed over all of the supported LaNiO3

samples were less than the corresponding support materials, which again verified the anchoring of LaNiO3 within the pores. The pore size distribution of LaNiO3 /SBA-15 sample was quite narrow and sharply distributed in the range 3–5 nm with a maximum at 4.4 nm (Table 1). In the case of LaNiO3 /MCM-41 sample, it was even narrower (2–3 nm) and centered at a lower pore size value. SiO2 possessed a much wider pore size distribution than that of SBA-5 and MCM-41. Compared to those support materials, the specific surface area and pore volume decreased significantly for supported catalysts, as summarized in Table 1. The decrease in surface area can be ascribed to the rise in the density of catalysts due to the incorporation of LaNiO3 into the mesopores of support materials. This was reflected by the values of the specific surface areas normalized to 1 g support materials (Table 1). The decrease of pore volume was an important evidence to prove anchoring of LaNiO3 inside the pores of MCM-41 and SBA-15, as the LaNiO3 molecule could occupy the mesopores without any constrictions. The insertion of LaNiO3 perovskite caused a slight reduction in average pore diameter of SBA-15 and MCM-41. However, for LaNiO3 /SiO2 , there was a slight increase in average pore diameter plausibly due to the blocking of the small pores. Similar phenomenon has been observed in the literature [43]. 3.1.2. XRD patterns of supports and supported LaNiO3 catalysts Fig. 2 illustrated the low-angle XRD patterns of the support materials as well as corresponding supported LaNiO3 catalysts. The SBA-15 and MCM-41 showed a well-developed ordered hexagonal pore structure. The three well-resolved characteristic peaks at 2 = 0.94◦ , 1.57◦ , 1.84◦ and 2.24◦ , 3.81◦ , 4.42◦ could be attributable to the (1 0 0), (1 1 0) and (2 0 0) diffraction of SBA-15 and MCM-41, respectively [2,44]. No such diffraction peaks were observed in the case of SiO2 (not shown). The LaNiO3 loading had significant effects on the pore structure of the SBA-15 and MCM-41. The (1 0 0), (1 1 0), and (2 0 0) diffraction peaks of LaNiO3 /MCM-41 catalysts decreased notably in intensity, which was due to partial blocking of mesopores and decline in long-range order of hexagonal arrange [45]. As for LaNiO3 /SBA-15,

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Table 1 Physical characteristics of supports materials and corresponding LaNiO3 catalysts before and after reaction. Sample

SBA-15 MCM-41 SiO2 LaNiO3 /SBA-15 LaNiO3 /MCM-41 LaNiO3 /SiO2 a

SBET (m2 /g)

NSBET (m2 /g SiO2 )

Before reaction

After reaction

612 1045 287 246 409 158

– – – 76 63 32

612 1045 287 423 703 272

Vp (cm3 /g)

Dp (nm)

a0

Before reaction

After reaction

Before reaction

After reaction

0.93 0.96 0.65 0.38 0.23 0.43

– – – 0.14 0.08 0.07

6.0 2.8 9.1 4.4 2.2 9.8

– – – 4.9 3.5 7.6

Ni cluster size (nm)a

11.0 – 4.6 – – – 9.9 7.7 4.5 12.5 – 14.4

Ni particle size of the spent samples was determined by TEM results.

the (1 1 0) and (2 0 0) peaks of SBA-15 disappeared completely. In comparison with MCM-41, SBA-15 possessed larger pore size and volume and could accommodate more LaNiO3 inside the pores, hence a more remarkable reduction in the long-rang order of hexagonal arrange was observed for SBA-15 supported LaNiO3 sample. According to Yi et al. [29] and Wang et al. [46], the reduced peak intensities did not reflect degradation of the mesoporous structure but was a result of X-ray scattering on the anchored LaNiO3 nanocrystals and reduced support concentration in the composites. Nonetheless, the (1 0 0) diffraction peak could still be detected over the supported catalysts. In other words, the pore structure was retained in the LaNiO3 loading catalysts. In addition, the diffraction peaks shifted to higher angles and the unit-cell parameter (a0 ) decreased (Table 1). It was due to the confinement of the LaNiO3 within the mesoporous channels and to the dilution effect induced by perovskite addition [47]. 3.1.3. Morphology analysis of LaNiO3 /SBA-15 and LaNiO3 /MCM-41 From the TEM images of the LaNiO3 /SBA-15 (Fig. 3a), it could be observed that the long ordered arrangement of SBA-15 has been well retained after LaNiO3 loading. From the high-magnification images, it could be clearly seen that the parallel fringes had a peri˚ which corresponded to the planes with odicity of 3.87 A˚ and 2.79 A, a d-spacing of d1 0 1 = 3.840 A˚ and d1 1 0 = 2.732 A˚ (JCPDS 33-0711), respectively, in the LaNiO3 perovskite structures [13]. According to EDX analysis, the approximate La/Ni atom ratio was 1:1, and four elements (La, Ni, Si, and O) were uniformly distributed. From the above results, it could be confirmed that LaNiO3 perovskite structures were formed in the LaNiO3 /SBA-15 sample. The TEM images and corresponding EDX spectrum of LaNiO3 /MCM-41

Fig. 2. Low-angle XRD patterns of supports and supported LaNiO3 catalysts.

sample (Fig. 3b) also revealed the formation of LaNiO3 perovskite species in the channels of MCM-41. The places with darker contrast in the images of Fig. 3 could be attributed to the LaNiO3 perovskite particles with different dispersion. The small dark spots could be ascribed to LaNiO3 perovskite particles locating in the channels of supports. The larger dark areas over the pores corresponded to perovskite oxide agglomerates on the external surface, which was due to a migration of the perovskite species out of the supports during the calcination procedure [48]. 3.1.4. Reducibility of catalysts Shown in Fig. 4 were the H2 -TPR profiles of the bulk LaNiO3 perovskite and supported LaNiO3 samples. The type and nature of the silica support materials had a significant impact on the reducibility of supported samples. Both the bulk perovskite and supported samples showed three reduction peaks which were attributable to different Ni intermediate species through three-step reduction. For LaNiO3 /SBA-15 catalyst, the first reduction peak was ascribed to the formation of La4 Ni3 O10 , and the second peak was assigned to the formation of the spinel phase La2 NiO4 as follows [49]: 4LaNiO3 + 2H2 → La4 Ni3 O10 + Ni0 + 2H2 O La4 Ni3 O10 + 3H2 → La2 NiO4 + 2Ni0 + 2La2 O3 + 3H2 O The third peak was the complete reduction of perovskite to La2 O3 and Ni0 : La2 NiO4 + H2 → Ni0 + La2 O3 + H2 O During the reduction of LaNiO3 /MCM-41 sample, the first peak at 405 ◦ C indicated the formation of LaNiO2.75 without total destruction of the perovskite-type structure [50]. The second peak centered at 450 ◦ C corresponded to La4 Ni3 O10 phase, and the third larger peak at 641 ◦ C was attributed to the formation of La2 O3 and Ni0 [50]. The reduction steps were similar to that for the bulk perovskite but occurred at remarkably higher temperatures. The LaNiO3 /SiO2 sample exhibited different reduction patterns from the other two samples with three peaks at around 374, 569 and 726 ◦ C. The first peak corresponded to the reduction of NiO particles and two others were to the reduction of La2 NiO4 . For the supported LaNiO3 catalysts, a shoulder peak between 200 and 300 ◦ C was attributed to nickel oxide (NiO) from the Ni precursor salt during synthesis which did not form part of the perovskite structure [47]. In comparison with bulk LaNiO3 , the reduction of the LaNiO3 /SBA-15 and LaNiO3 /MCM-41 was remarkably shifted toward higher temperatures and the signal broadened obviously, indicating a strong interaction between the Ni in the perovskite and the supports. This shift on TPR profiles could be plausibly explained by a decrease of the particle size from bulk material to ultrafine particles.

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Fig. 3. TEM micrographs (left) and corresponding EDX spectrum (right) of (a) LaNiO3 /SBA-15 sample and (b) LaNiO3 /MCM-41 sample.

3.1.5. XRD patterns of the reduced samples The XRD patterns of bulk and supported LaNiO3 catalysts after reduction were shown in Fig. 5. The diffraction peaks belonging to La2 O3 and metallic Ni0 phase were detected after reduction, which

indicated that LaNiO3 was reduced and nickel existed mainly as Ni0 . Higher dispersion was obtained for mesoporous silicas supported samples with smaller pore size and higher surface areas compared to the bulk LaNiO3 perovskite.

Fig. 4. TPR profiles of bulk LaNiO3 and supported LaNiO3 catalysts.

Fig. 5. XRD patterns of bulk LaNiO3 and supported LaNiO3 samples after reduction.

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Fig. 6. Effect of temperature on the initial catalytic performance of bulk LaNiO3 and supported LaNiO3 catalysts.

3.2. Catalytic performances of mesoporous supported perovskite catalysts Blank test using pure siliceous supports showed negligible catalytic activity under the typical reaction condition of our experiments. The catalytic performances of various supported LaNiO3 perovskite and bulk LaNiO3 catalysts were shown in Fig. 6. The conversion was dependent on both the support materials and reaction temperature. Both CH4 and CO2 conversion increased with increasing reaction temperature for all of the samples, reflecting the endothermic character of methane dry reforming with CO2 . It was observed that the supported LaNiO3 catalysts showed higher catalytic activities than the bulk LaNiO3 perovskite. The

conversion values of CO2 and CH4 were improved obviously over supported LaNiO3 catalysts. Among all of the supported catalysts, LaNiO3 /MCM-41 presented the highest initial CH4 and CO2 conversion, followed by LaNiO3 /SBA-15 and LaNiO3 /SiO2 in most of the temperature range investigated. From above TPR results, it can be deduced that strong interaction between Ni and support materials existed over LaNiO3 /MCM-41 and LaNiO3 /SBA-15 samples which promoted the high dispersion of Ni nanoparticles on the silica matrix compared with LaNiO3 /SiO2 . For the bulk LaNiO3 catalyst, a noticeable higher conversion was observed for CO2 compared to CH4 , due to the contribution of reverse water gas shift (RWGS) reaction (CO2 + H2 → H2 O + CO, H298 = +41 kJ/mol). However, the general tendency for the

Fig. 7. Catalytic stability of bulk LaNiO3 and supported LaNiO3 catalysts for the methane dry reforming with CO2 at 700 ◦ C for 60 h.

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Fig. 8. TG/DTG profiles of spent catalysts after 60 h reaction.

supported LaNiO3 catalysts was that CH4 conversion was lower than that of CO2 at or below 600 ◦ C; whereas it was the other way around above 600 ◦ C. Luo et al. [51] also reported that CH4 conversion was higher than CO2 conversion at 700 and 800 ◦ C over Ni–La2 O3 /5A catalyst, which was due to the presence of both CH4 decomposition and CO2 complete dissociation (i.e., CO2 → CO + O and then CO → C + O; the surface oxygen species adsorbed could enhance CH4 decomposition). Similarly, Daza et al. [52] found the similar catalytic behavior over Ce-promoted Ni/Mg–Al and ascribed it to conversion of CH4 into light hydrocarbons (ethane and ethylene) according to the (n + 2) CH4 → (CH2 )n CH3 + (2n + 2)H2 reaction and/or the formation of carbon residues via the cracking of methane. In addition, H2 selectivity and H2 /CO ratio were shown in Fig. 6c and d, respectively. For all of the samples, H2 selectivity increased with increasing reaction temperature. This was consistent with the previous thermodynamics studies [54], in which high temperature favored the formation of H2 through various reactions, such as water gas shift reaction, methane decomposition and carbon gasification. Both LaNiO3 /MCM-41 and LaNiO3 /SBA-15 exhibited analogous H2 selectivity while LaNiO3 /SiO2 showed obviously lower selectivity toward H2 even compared to the LaNiO3 catalyst. The H2 /CO ratio was less than unity over the whole temperature range tested, especially at low temperatures. This could be due to the occurrence of side reaction such as RWGS and methanation reaction at low temperatures which consumed H2 [53]. 3.3. Stability investigation of the mesoporous supported catalysts The long-term stability evaluation of the prepared catalysts was conducted at 700 ◦ C and the results were presented in Fig. 7. For both LaNiO3 /MCM-41 and LaNiO3 /SBA-15 catalysts, the conversion of CH4 and CO2 was higher than 75% and 70%, respectively, and it was relatively stable at 700 ◦ C for 60 h of time on stream (TOS) with LaNiO3 /SBA-15 being more stable than LaNiO3 /MCM-41. It revealed that the better anchoring effect of LaNiO3 /SBA-15 could have contributed to the higher stability. The observed smoothness of the experimental points indicated that the SBA-15 could minimize the heat diffusion problems of the highly endothermic reforming reaction, avoiding the formation of cold point in the catalyst bed and

the sintering of nickel particles during reaction [47]. The initial conversion of CH4 and CO2 was about 68% and 64% for the LaNiO3 /SiO2 catalysts, but it gradually declined with the increase of TOS due to the formation of a large amount of carbon deposited on the surface of LaNiO3 /SiO2 . It could be verified by TG and TPH profiles of spent catalysts, which would be discussed in Section 3.4. The bulk LaNiO3 sample exhibited the poorest stability. It manifested that the perovskite particles being dispersed on support with high specific surface area was favorable for the enhancement of catalytic performance. Rivas et al. [47] ascribed the improved catalytic performance of loaded perovskite materials to higher dispersion of the perovskite oxides and the dilution effect exerted by the mesoporous materials which was favorable for the heat transfer. Again the detection of H2 O in the outlet indicated the occurrence of the RWGS, giving rise to higher CO2 conversion than that of CH4 for the bulk LaNiO3 sample, while a higher CH4 conversion was observed than that of CO2 for the supported LaNiO3 catalysts, due to the rapid thermal decomposition of methane. This was in good agreement with the results reported by Tian et al. using LaNiOx /ZSM-5 catalyst [6]. The H2 selectivity and H2 /CO ratio during the stability analysis were shown in Fig. 7c and d, respectively. Both LaNiO3 /MCM-41 and LaNiO3 /SBA-15 catalysts were maintained high H2 selectivity and H2 /CO ratio during 60 h of TOS. A significant decrease in H2 selectivity and H2 /CO ratio was also observed on LaNiO3 /SiO2 catalyst.

3.4. Deactivation analysis 3.4.1. TGA profiles of spent samples The coke formation on the active centers during the methane dry reforming was one major reason which led to the deactivation of catalysts, resulting in low durability and activity. The most probable reactions [54] leading to the carbon formation in methane dry reforming were listed below: 2CO → CO2 + C(Boudouardreaction)

(R-1)

CH4 → 2H2 + C(methanedecomposition)

(R-2)

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Fig. 9. (a) TPH profiles and (b) N2 physisorption isotherms of spent supported LaNiO3 catalysts after 60 h reaction.

The deposited carbon could be diminished by carbon gasification (H2 O + C → CO + H2 ), which was endothermic reaction and favored at high temperatures [53]. In this study, the spent catalysts were characterized by TG, TPH and TEM to probe the impact of support materials on the carbon deposition. The thermogravimetric analysis under oxidative atmosphere was performed to quantify the total amount of carbon formed on the spent catalysts during the reforming reaction. Fig. 8 shows the TG/DTG profiles of the spent catalysts after 60 h of TOS. It should be mentioned that the weight loss peak around 100 ◦ C was associated with the pronounced desorption of moisture, and the weight loss caused by the carbon deposition was calculated only the weight loss above 110 ◦ C to exclude the interference of moisture [2]. The slight weight gain caused by the oxidation of metallic Ni particles was also accounted for the calculation. Due to the similar Ni loading for all catalysts, the weight gain related to Ni oxidation was expected to be the same. The total amount of carbon deposition over the spent catalysts increased in the following sequence: LaNiO3 /SBA15 (4.47%) < LaNiO3 /MCM-41 (4.83%) < LaNiO3 /SiO2 (5.67%). For LaNiO3 /SBA-15 and LaNiO3 /MCM-41 samples, two weight loss peaks were observed, indicating that two different types of carbon were deposited. The weight loss peak at low temperatures (400–550 ◦ C) could be attributed to the oxidation of amorphous carbon, and the one at high temperatures (>550 ◦ C) could be attributed to the oxidation of the coke deposits in the filament form with different degree of graphitization [55,56]. From the DTG profiles, it could be found that there was a significant weight loss around 500 ◦ C for LaNiO3 /SBA-15 sample, while the significant weight loss for LaNiO3 /SiO2 was around 600 ◦ C. It was well known that amorphous carbon was more reactive and could be easily removed by carbon gasification, while graphitic carbon needed higher temperature to be oxidated [57]. The results revealed that the more carbonaceous species formed over LaNiO3 /SBA-15 were relatively active carbon species. The active carbon participated in the reaction as intermediate species for the formation of the desired product and was beneficial to prevent the deactivation of sample to some extent [55], whereas over the LaNiO3 /SiO2 sample, more graphitic carbon species were

formed. The deposition of a large amount of graphitic carbon on the surface of LaNiO3 /SiO2 decreased the catalytic activity during the stability test. A significant filament formation was observed over LaNiO3 /SiO2 catalyst by TEM, as further shown in Fig. 10. 3.4.2. TPH profiles and physical properties of spent samples The type and the quantity of the coke were also determined by TPH in which the coke was quantified from the CH4 formed during its combustion and classified according to the combustion temperature. The TPH profiles of supported LaNiO3 catalysts after stability tests were shown in Fig. 9a. Depending on the removal temperature, three hydrogenation peaks were identified for the spent catalysts at around 530 ◦ C, 630 ◦ C, and 750 ◦ C, designated as ␣-carbon, ␤-carbon, and ␥-carbon, respectively. The hydrogenation temperature indicated the activity of surface carbon, and the lower hydrogenation temperature showed the higher activity of surface carbon. For the spent LaNiO3 /SBA-15 sample, the peak around 520 ◦ C could be attributed to amorphous carbon species on Ni atoms, which participated in the sequence of steps to form CO. Another broad peak centered around 630 ◦ C was clearly observed, which corresponded to the C␤ , and a small amount of graphitic coke (C␥ ) was also found. Over the spent LaNiO3 /SiO2 sample, a strong relative intensity of graphite was observed at 785 ◦ C, suggesting more inert carbon was deposited on LaNiO3 /SiO2 compared to the other two samples. The TPH results reflected that the less stable C␤ species over LaNiO3 /SBA-15 played a role as the reactive intermediate in the catalytic stability enhancement, and the presence of difficultly removed C␥ was responsible for the deactivation of LaNiO3 /SiO2 . Although a large amount of carbonaceous species formed over LaNiO3 /MCM-41 catalyst, its catalytic stability was well maintained. This was most likely due to the absence of the pore-mouth plugging or the low probability of covering the active nickel species by carbon deposition over the LaNiO3 /MCM-41 [58]. The tendency of carbon deposition over the spent samples was in good agreement with the TG analysis. The easily oxidized carbons were preferentially formed during the early stage of reforming, mainly via CH4 decomposition [55]. The prolonged exposure under reaction conditions lead to

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Fig. 10. TEM results for spent samples. (1) TEM images for (a) spent LaNiO3 /SBA-15, (b) spent LaNiO3 /MCM-41 and (c) spent LaNiO3 /SiO2 , respectively. (2) Nickel particle size distributions for (d) spent LaNiO3 /SBA-15, (e) spent LaNiO3 /MCM-41 and (f) spent LaNiO3 /SiO2 , respectively.

the increase of C␥ species due to its relative inertness. This inert carbon species came from both CO2 and CH4 , since both reactants might lead to the CO formation and, then, to carbon deposition via Boudaouard reaction. Therefore, when only inert carbonaceous species was considered, the amount of carbon deposited over the used samples increased as the following sequence: LaNiO3 /SBA-15 < LaNiO3 /MCM-41 < LaNiO3 /SiO2 . These results showed that even though no considerable differences in the catalytic activity over LaNiO3 /SBA-15 and LaNiO3 /MCM-41 were observed, it was evident that the effect of LaNiO3 /SBA-15 was superior in the inhibition of coke formation in comparison with the LaNiO3 /MCM-41. The nitrogen sorption isotherms of spent samples were shown in Fig. 9b, and the textural properties were also summarized in Table 1. There was a decrease in the specific surface area and the pore volume to some extent after the reaction due to carbon deposition, metal sintering and local collapse. Nevertheless, the N2 physisorption isotherms of used supported LaNiO3 samples were close to those of the fresh ones, and the capillary condensation was still clearly observed at the mesoporous range with the corresponding average pore diameters of 4.9, 3.5 and 7.6 nm for LaNiO3 /SBA-15, LaNiO3 /MCM-41 and LaNiO3 /SiO2 , respectively.

3.4.3. TEM study of catalyst morphology and coke deposits The TEM images of spent supported catalysts and the corresponding particle size distributions obtained from TEM were shown in Fig. 10. The Ni average particle size was also summarized in Table 1. The nickel particle size was a crucial factor affecting activity of the catalysts. Smaller and relative uniform nickel particle size can be observed on the LaNiO3 /MCM-41 with LaNiO3 /SBA15 sample which possessed the smallest average particle size of 7.8 nm among all the spent catalysts. The LaNiO3 /SiO2 sample presented the largest nickel particle diameter of 14.5 nm and a wider size distribution due to the large pore diameter and heterogeneity of pore diameters. Filamentous carbon structures could be easily found in TEM images of LaNiO3 /SiO2 catalyst, which was in line with the large amount of coke deposition over LaNiO3 /SiO2 catalyst evidenced by TGA and TPH results.

Combining the catalytic results and the nickel particle size of various samples, it was manifested that the stable silica matrix restricted the agglomeration of nickel species. The smaller Ni particle inhibited the formation of coke and was beneficial to the catalytic performance. Similar trend which correlated particle size with catalytic performance has been reported in the literature [59,60]. Liu et al. [61] investigated the catalytic properties of SBA-15 and MCM-41 grafted Ni catalysts for CH4 dry reforming. It was found that the anchoring of the Ni particles on the silica matrix were the main reasons for stabilizing the catalysts against agglomeration. In our studies, it was postulated that similar anchoring effect was responsible for the formation of highly dispersed nickel particles. The high structural stability, much uniform mesopore distribution and unique pore structure endowed SBA-15 material with distinct advantages over the MCM-41 as the supporting material for CH4 dry reforming. 4. Conclusions The LaNiO3 perovskite catalysts supported on different mesoporous silica (SBA-15, MCM-41 and SiO2 ) were prepared by filling pores of silica host with La–Ni citrate complex precursors, and the catalytic properties of these catalysts were tested for methane dry reforming. The results showed LaNiO3 perovskite was dispersed inside the pore channels of mesoporous support, and the mesoporous structure of silica support was not destroyed by introducing LaNiO3 perovskite. The supported samples showed much better catalytic performance (both activity and stability) in the methane dry reforming than the conventional bulk LaNiO3 perovskite. Among the supported samples, LaNiO3 /MCM-41 exhibited the highest initial catalytic activity, due to the small Ni particle size. However, LaNiO3 /SBA-15 surpassed the other catalysts in the long-term stability, which was due to that the strong anchoring effect restricted the migration of nickel clusters. The N2 physisorption and TEM results for spent samples implied that the hexagonal mesopores of SBA-15 were still kept intact after reaction and the pore walls of SBA-15 prevented the formation of large nickel particles, while the collapsed mesoporous structure for LaNiO3 /MCM-41

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