γ-alumina nanocomposites by one-pot synthesis

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Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis Li Zhang a, Xueguang Wang a,b,*, Chenju Chen a, Xiujing Zou b, Weizhong Ding a,b, Xionggang Lu a,** a

State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072, China Shanghai Key Laboratory of Advanced Ferrometallurgy, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China

b

article info

abstract

Article history:

La-modified NiAl2O4/g-Al2O3
La composites with mesoporous structures were prepared by

Received 10 January 2017

one-pot template-free strategy and applied for dry reforming of methane (DRM) to syngas.

Received in revised form

The characterization results confirmed that these materials possessed high specific surface

24 February 2017

areas, large pore volumes and narrow pore size distributions. The reduced catalysts

Accepted 21 March 2017

exhibited excellent catalytic properties as well as long-term stability for DRM reaction.

Available online xxx

Addition of La showed little influence on the catalyst structure and the mean sizes of metal Ni particles, but could enhance the medium-strength basicity and the accumulation of Ni2þ

Keywords:

on the catalyst surface, resulting in the enhancement of intrinsic activity, the reduction of

Nickel catalyst

apparent activation energy, and the suppression of carbon deposition for DRM reaction.

Mesoporous g-alumina

The catalyst containing 3 wt% La possessed the best catalytic performance. The charac-

La-modifier

terization of spent catalysts also demonstrated that La could effectively prevent the phase

Nickel aluminate

transformation of g-alumina in the DRM process.

Methane dry reforming

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Recently, carbon dioxide reforming of methane (DRM), which can consume the two primary greenhouse gases into industrially important syngas (H2 and CO), has been an attractive subject [1‒4]. The produced syngas with a low H2/CO molar ratio of unity is more suitable for FischereTropsch synthesis

of higher hydrocarbons [5‒8], and increasing the selectivity of long chain hydrocarbons [9‒11]. Furthermore, its high endothermicity makes the DRM into a promising method for converting abundant renewable energies, especially solar energy, into easy-to-use chemical energy [2,12,13]. Therefore, it is highly desirable to develop an effective and economic catalyst for DRM [14‒16].

* Corresponding author. State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072, China. Fax: þ86 21 56338244. ** Corresponding author. Fax: þ86 21 56338244. E-mail addresses: [email protected] (X. Wang), [email protected] (X. Lu). http://dx.doi.org/10.1016/j.ijhydene.2017.03.140 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140

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Catalysts based on noble metals, such as Rh, Ru, Pt and Pd, possess high catalytic activity and excellent coke resistance for DRM reaction, but the high price and limited availability restrict their large-scale commercial application [17‒20]. Lowcost nickel-based catalysts, especially supported on alumina, have been extensively used in various reforming reactions [21‒24]. However, nickel catalysts are easy to suffer from rapid deactivation due to the sintering of Ni active sites and carbon deposition during the reforming reactions [7,25,26]. Numerous attempts have been conducted to develop robust Ni-based catalysts with stable and highly dispersed Ni particles. In this context, the mineral-type catalyst precursor of NiAl2O4 spinel with a confinement environment for nickel particles, excellent thermal stability and high chemical inertia has received considerable attention [27‒30]. The stable and highly dispersed Ni particles can be easily obtained based on the “solid phase crystallization” method, by which the Ni2þ ions are extracted from NiAl2O4 spinel after in suit reduction treatment [31‒33]. Nevertheless, the NiAl2O4 precursor via the conventional methods generally provides low surface areas and small pore volumes. Generally, the stability of alumina-supported Ni catalysts can be improved by the modification of alkaline (Na, K), alkaline earth (Mg, Ca, Sr) and rare earth (La, Ce, Sm) oxides [34‒38]. The incorporation of promoters can provide more Lewis basic sites and favor the activation of chemisorbed CO2 on the catalyst surfaces, leading to formation of CO and O species and accelerating the removal of deposited carbon. Additionally, the promoters can also effectively hinder the phase transformation of g-Al2O3 to low-surface-area a-Al2O3 during the reaction process [5,16,17,34,40]. Among them, the La oxides exhibit greater efficacy for Ni-based catalysts in DRM reaction [39]. In the present work, a series of mesoporous La-modified NiAl2O4/g-Al2O3 composites with large surface areas, pore volumes and narrow pore size distributions were facilely synthesized via the one-pot partial hydrolysis of nitrate salts with (NH4)2CO3 for DRM reaction. A comprehensive investigation into the influence of La content on their physicochemical properties and catalytic behaviors was conducted. The existence of moderate La could improve the catalytic activity and significantly suppress the coke deposition. The optimization of La content showed that the catalyst with 3 wt % La could achieve the highest catalytic activity and lowest apparent activation energy.

Experimental Catalyst synthesis Mesoporous g-Al2O3 and NiAl2O4/g-Al2O3
xLa samples were prepared based on our previously reported literature [41]. Accordingly, Al(NO3)3$9H2O (0.1 mol) and certain amounts of Ni(NO3)2$6H2O and La(NO3)3$6H2O were dissolved in 50 mL deionized water in a beaker at ambient temperature. After heating the mixed solution to 70  C, (NH4)2CO3 aqueous solution (1 mol/L) was dropped into the mixed solution under vigorous stirring until a gel was suddenly formed, where the gel pH was in the range of 5.0e5.8. Subsequently, the gel was

cooled to ambient temperature. After aged at ambient temperature for 48 h under a relative closed condition by covering plastic film over the gel beaker, the gel was dispersed in a glass dish at 100  C for 24 h. Then, the dried gel was heattreated at 150  C and 200  C for 10 h, respectively, and finally, was further calcined at 800  C for 10 h. The Ni content was fixed at 13 wt%, and the La loadings were 0 wt%, 1 wt%, 2 wt%, 3 wt%, 5 wt%, 7 wt%, respectively, denoted as x ¼ 0, 1, 2, 3, 5, and 7.

Catalyst characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ MAX-2200 diffractometer using Cu Ka radiation (l ¼ 0.15418 nm). The Scherrer equation was used to calculate the mean crystallite sizes of the metallic Ni. N2 sorption isotherms at
196  C were measured on a Micromeritics ASAP 2020 instrument after outgassing the samples at 250  C for 8 h. Specific surface area (SBET) was calculated using the BET method; pore size distribution (PSD) was estimated by the BJH method from the desorption branch of the isotherms; and pore volume (Vp) was calculated from the N2 total adsorbed amount at the P/P0 ¼ 0.990. SEM images were acquired by employing a Hitachi S-4800 electron microscope. TEM images were taken on a JEOL JEM2010F electron microscope, and the Ni particles size distributions were calculated by Nano Measurer. H2 Temperature-programmed reduction (H2-TPR) experiments were conducted on a fixed bed reactor equipped with a thermal conductivity detector (TCD). Typically, 100 mg of catalyst was pretreated at 200  C for 1 h in an Ar stream (30 mL/min). After cooling the reactor to 50  C, the 5 vol% H2/ Ar (30 mL/min) was introduced, and the temperature was increased to 1000  C at a heating rate of 10  C/min. X-ray photoelectron spectra (XPS) was collected on an ESCALAB 250Xi spectrometer equipped with an Al Ka radiation (hn ¼ 1486.6 eV). Charging effects were calibrated using the binding energy (BE) of C 1s peak at 284.8 eV. CO2 temperature-programmed desorption (CO2-TPD) measurements were carried out on a Micromeritics ASAP 2920. Firstly, the sample (100 mg) was pretreated at 200  C for 1 h in He flow (30 mL/min). After cooling to 40  C, a pure CO2 gas (30 mL/min) was introduced for 1 h. Then, the sample was purged in flowing He until the baseline was smooth. Finally, CO2-TPD was performed with a heating rate of 10  C/min to 800  C under He stream. NH3 temperature-programmed desorption (NH3-TPD) was performed with the same procedure of CO2-TPD. The difference was that a 10 vol% NH3/He mixed gas instead of pure CO2 gas was introduced. Temperature-programmed oxidation (TPO) characterizations were operated on the same setup as described for CO2TPD. The spent samples were purified with helium at 200  C for 0.5 h and then cooled down to 50  C. After this, the spent samples were subjected to heat treatment (10  C/min, up to 800  C) in a 10 vol% O2/He mixture gas (30 mL/min). Coke formation was quantitatively analyzed by thermogravimetric (TG) analyzer (Netzsch STA 4449 F3). The used catalysts were heated from 50  C to 900  C at a heating speed of 10  C/min in flowing air (30 mL/min).

Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140

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Catalyst test for DRM Dry reforming of CH4 was conducted in a continuous fixed-bed quartz reactor (inner diameter of 8 mm) at atmospheric pressure. Typically, 100 mg of catalyst (40e60 mesh) diluted with 900 mg of quartz particles (40e60 mesh) was used in each test. The catalyst was first reduced in situ at 800  C for 2 h in a 50 mL/min flow of 20 vol% H2/N2, and then cooled to the required reaction temperature. The mixture gas (CH4:CO2 ¼ 1:1, GHSV ¼ 1.8  105 mL/gcat h) without dilution were fed into the reaction system. After cooling in an ice-water bath and removing water, the effluent gases were analyzed by online GC-TCD gas chromatograph with a TDX-01 packed column. The estimations of the activity and H2/CO molar ratio of the catalyst for DRM and the turnover frequency (TOF) were all given in Supporting Information.

Results and discussion Catalyst characterization Fig. 1 illustrated the XRD patterns of the g-Al2O3 and NiAl2O4/ g-Al2O3
xLa materials. All the samples displayed three typical diffraction peaks centered at ca. 37 , 45 , 66 , corresponding to g-Al2O3 (JCPDS 10-0425) and/or NiAl2O4 spinel (JCPDS 10-0339). Owing to the overlap of XRD patterns for g-Al2O3 and NiAl2O4, it is difficult to identify the two phases clearly by XRD. Additionally, all the samples exhibited no characteristic diffraction peaks of NiO, proving that NiO species reacted with alumina to form NiAl2O4 after 800  C calcination or were highly dispersed with very small crystallites in the g-Al2O3 matrices below the detection limit of XRD. However, one most important evidence of the formation of NiAl2O4 spinel could come from the decrease in the intensity ratio of (400) and (311) peaks and slight shifts of (400) and (440) peaks to smaller angles for NiAl2O4/g-Al2O3
xLa samples compared with g-Al2O3 [40,42]. Moreover, no peaks associated with La species were found for

Fig. 1 e XRD patterns of g-Al2O3 and NiAl2O4/g-Al2O3¡xLa materials calcined at 800  C.

3

NiAl2O4/g-Al2O3
xLa, demonstrating that the La species existed in the amorphous form, or were well dispersed. With raising La content, the peak intensities for NiAl2O4/gAl2O3
xLa became weaker, while the peak widths increased. This result meant lower crystallization degree and smaller crystallite size for the samples containing higher La content. The surface chemical properties of the samples were examined by XPS measurement, and the results were presented in Fig. 2 and Table 1. As shown in Fig. 2(a), the NiAl2O4/g-Al2O3
xLa showed main Ni 2p3/2 peaks located around 856.2 eV with satellite peaks around 862.6 eV, while the BE of Ni 2p3/2 over pure NiO obtained by roasting the nickel nitrate hexahydrate was 854.6 eV. This result suggested that Ni2þ ions were incorporated into g-Al2O3 lattice to form NiAl2O4 spinel [42]. Fig. 2(b) showed that all the La-containing samples presented a main La 3d5/2 peak positioned around 835.3 eV with a satellite peak. However, the BE of 835.3 eV was higher than the reference values for crystalline La2O3 (834.3 eV) and LaAlO3 (833.8 eV) [43]. According to previous report [44], the higher BE of ca. 835.3 eV could be assigned to highly-dispersed La2O3. It was also found that the calculated surface La/Al molar ratios were almost identical to the nominal values, as listed in Table 1. This phenomenon was indicative of a good diffusion at surface level of La ions into the gAl2O3 framework. However, each sample showed a lower surface Ni/Al molar ratio than the corresponding nominal one. This result further verified that Ni2þ ions were transformed into NiAl2O4 in the calcined materials. Additionally, Ni element seemed to have a preference to enrich on the catalyst surface. The Ni/Al was up to 0.098 at La content of 3wt%. Fig. 3 showed the H2-TPR profiles of NiAl2O4/g-Al2O3
xLa samples. As previously reported, the pronounced H2 consumption peaks of all the samples, which centered in the range of 805e825  C, were attributed to the reduction of Ni2þ ions in the crystal lattice of NiAl2O4 [19,28], and the typical reduction peaks for free NiO or complex NiOx species in the lower temperature region less than 750  C did not emerge, regardless of La content. This phenomenon verified that all the Ni2þ ions were uniformly diffused into g-Al2O3 lattice to form single-phase NiAl2O4/g-Al2O3 solid solutions. Fig. 4(a) and (b) showed the N2 adsorptionedesorption isotherms and corresponding pore size distributions of the materials, respectively. All the typical type IV isotherms with apparent hysteresis loops and narrow pore size distributions indicated that the mesoporous structures with uniform mesopores were successfully formed in the frameworks [17,45]. However, the addition of La showed no apparent influence on the mesoporous structures of the materials. As we can see from Table 2, the NiAl2O4/g-Al2O3
xLa (x  3) possessed similar SBET of ca.188 m2/g and Vp of ca.0.25 cm3/g. When the La content increased to 7 wt%, the SBET and Vp decreased to 171 m2/g and 0.21 cm3/g, respectively. This result was due to the variation in mass density of the catalyst and the distribution of La species into the pore system [42,45]. The large specific surface area and big pore volume could result in a great deal of nickelesupport interface and effectively enhance the mass and heat transfer properties [46]. CO2-TPD and NH3-TPD were used to characterize the surface acidebase properties of the fresh samples, and the profiles were displayed in Fig. 5(a) and (b), respectively. It was reported that CO2 adsorbed on weak basic sites was desorbed

Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140

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Ni2p3/2

a

b

La3d5/2

satellite peak

satellite peak La3d3/2

x=7

x=7

x=5

Intensity (a.u.)

Intensity (a.u.)

x=5 x=3 x=2

x=3

x=2

x=1 x=0

845

850

855 860 865 Binding energy (eV)

870

x=1

830

835 840 Binding energy (eV)

845

Fig. 2 e (a) Ni 2p and (b) La 3d XPS spectras of the NiAl2O4/g-Al2O3¡xLa materials.

Table 1 e XPS results for the various samples. Sample

NiOb NiAl2O4/g-Al2O3 NiAl2O4/g-Al2O3
1La NiAl2O4/g-Al2O3
2La NiAl2O4/g-Al2O3
3La NiAl2O4/g-Al2O3
5La NiAl2O4/g-Al2O3
7La a

b

BE of BE of Ni 2p3/2 La 3d5/2 (eV) (eV) 854.6 856.2 856.2 856.3 856.1 856.3 856.3

e e 835.5 835.4 835.3 835.3 835.5

La/Al atomic ratioa e e 0.003 (0.004) 0.008 (0.009) 0.014 (0.014) 0.021 (0.024) 0.034 (0.034)

Ni/Al atomic ratioa e 0.082 0.085 0.087 0.098 0.096 0.079

(0.135) (0.137) (0.139) (0.141) (0.145) (0.149)

The values in parentheses are the nominal surface Ni/Al and La/ Al molar ratios estimated from the nominal composition used for the gel precursor. The NiO obtained by roasting the nickel nitrate hexahydrate at 600  C.

at low temperature and that adsorbed on strong basic sites was desorbed at high temperature [5,17,42]. Therefore, it could be speculated that the CO2 desorption peaks around 100  C might be derived from the CO2 adsorbed on weak basic sites

and physically adsorbed CO2. The desorption regions located in 160e420  C, which included two overlapped desorption peaks, corresponded to medium-strength basic sites. The former ones centered at 283  C could be assigned to CO2 desorbed from the g-Al2O3 support and Ni oxides [42,45]. The latter CO2 desorption peaks only appeared in the Lacontaining samples could be associated with highly dispersed La2O3. However, the CO2 desorption peaks corresponding to strong basic sites higher than 450  C shifted towards low temperatures with the La content. It could be acquired that the addition of La principally enhanced the strength and number of the medium-strength basic sites on the catalyst surfaces. NH3-TPD profiles provided in Fig. 5(b) also presented three clear bands corresponding to different types of acidic sites. Depending on the desorption temperature, these desorption peaks could be assigned to weak acidic sites and physically adsorbed NH3 (below 250  C), mediumstrength acidic sites (between 250 and 500  C) and strong acidic sites (between 500 and 700  C), respectively. It was obvious that the incorporation of La visibly shifted all the peak centers to lower desorption temperatures. Table 2 listed the surface acidic and basic parameters of NiAl2O4/g-Al2O3
xLa. One could see that the amount of the desorbed CO2 primarily increased with La content, and reached the maximum value at 3 wt% La. When further increasing La content, the desorbed CO2 amount showed a slightly decline, which was likely due to their lower surface areas and smaller pore volumes and the decrease in the exposed La atoms. As is expected, the amount of desorbed NH3 clearly presented a contrary trend compared with that of desorbed CO2, and likewise reached the minimum value at 3 wt% La. These results powerfully indicated that the addition of La could effectively improve the surface acidebase properties and mainly promoted the medium-strength basicity of NiAl2O4/g-Al2O3 materials.

Catalyst evaluation

Fig. 3 e H2-TPR profiles for the NiAl2O4/g-Al2O3¡xLa materials.

A set of preliminary experiments showed that the catalytic activities of the Ni/g-Al2O3
xLa catalysts would gradually decrease in the initial ca. 80e100 h reaction, followed by

Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140

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Fig. 4 e (a) Nitrogen adsorptionedesorption isotherms, and (b) pore size distributions of the NiAl2O4/g-Al2O3¡xLa materials.

Table 2 e Textural properties and the acid-base parameters of the NiAl2O4/g-Al2O3¡xLa materials. Sample

NiAl2O4/g-Al2O3 NiAl2O4/g-Al2O3
1La NiAl2O4/g-Al2O3
2La NiAl2O4/g-Al2O3
3La NiAl2O4/g-Al2O3
5La NiAl2O4/g-Al2O3
7La a

SBET Vp (m2/g) (cm3/g)

187 188 187 189 176 171

0.26 0.25 0.24 0.24 0.23 0.21

Relative Relative amount amount of CO2 of NH3 desorptiona desorptiona 100 105 108 114 112 111

100 92 91 74 83 89

Defined as the total amount of CO2 or NH3 desorbed per gram of each catalyst divided by the total amount of CO2 or NH3 desorbed (100 as the reference) per gram of NiAl2O4/g-Al2O3 by the integration of TPD curves.

almost constant conversions of CO2 and CH4. Considering this situation, the activity evaluation in the temperature range of 575e800  C was conducted over Ni/g-Al2O3
xLa (x ¼ 0, 1, 2, 3, 5, and 7) stabilized after having in situ experienced 100-h DRM under the reaction conditions: GHSV ¼ 1.8  105 mL/gcat h, CO2/CH4 ¼ 1, 750  C, 1 atm. As shown in Fig. 6(a)e(c), the CO2 and CH4 conversions significantly increased with elevating reaction temperature and reached the biggest values at 800  C due to the strong endothermicity of the DRM reaction. Over the temperature range investigated, the CO2 conversion was higher than the CH4 one, and the H2/CO molar ratio was less than the unity. This was attributed to the concomitance of reverse water-gas shift (RWGS) reaction (CO2 þ H2 ¼ CO þ H2O). Furthermore, as the reaction temperature increased, the CO2 and CH4 conversions became more and more close, and the H2/CO molar ratio also increased towards stoichiometric ratio (1/1). This was because that the RWGS reaction was gradually inhibited with the increase in reaction temperature based on the thermodynamic analysis. Additionally, high temperature favors the cracking of methane over reduced catalysts, which also could lead to the CH4 conversion and H2/CO ratio increase.

Fig. 5 e (a) CO2-TPD profiles and (b) NH3-TPD profiles for the NiAl2O4/g-Al2O3¡xLa materials.

Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140

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The experimental data at the reaction temperature of 750  C were selected to indicate the influence of La content on the catalytic properties over Ni/g-Al2O3
xLa catalysts. It was widely known that the addition of La could promote the dispersion of Ni active sites and enhance the chemisorption and activation of CO2 on the surface of the catalysts, improving the catalytic performance of the catalysts. As shown in Fig. 6(d), the CO2 and CH4 conversions gradually increased with increasing La content up to 3 wt%, and then declined with further increasing the La content to 7 wt%. The change trend of H2/CO molar ratio was the same to that of CO2 and CH4 conversions over Ni/g-Al2O3
xLa catalysts, and the higher H2/CO ratio over La-incorporated catalysts might be due to the presence of less unreacted CO2 in the product gas, suppressing the RWGS reaction. These conclusions exhibited that only a small amount of La around 3 wt% was necessary for achieving the best catalytic performance for the present nickel aluminate/g-alumina catalysts. Similar results were observed over Ni-based catalysts in several previous studies. Oemar et al. [47] had reported that only 1 wt% of La was sufficient for enhancing both CO2 and CH4 conversions by more than 15%. Ma et al. [16] also reported the highest initial activity and H2 selectivity were gained over the catalyst containing 3 wt% La. To investigate the effect of La-modification on the stability of the catalysts, 400-h stability tests were conducted over Ni/

g-Al2O3 and Ni/g-Al2O3
3La, respectively, under the reaction conditions: GHSV ¼ 1.8  105 mL/gcat h, CO2/CH4 ¼ 1, 750  C, 1 atm. As shown in Fig. 7(a) and (b), in the case of Ni/g-Al2O3, the conversions of CO2 and CH4 decreased from ca. 75.5% and ca. 66.8% to ca. 60.9% and ca. 48.0%, respectively, in the initial stage of 100 h, followed by almost constant conversions. For Ni/g-Al2O3
3La, the conversions of CO2 and CH4 gently decreased from ca. 89.5% and ca. 81.9% to ca. 81.0% and ca. 70.0%, respectively, in the initial stage, and also kept unchanged during the following time. The H2/CO molar ratio also firstly decreased in the initial stage, and then kept unchanged at ca. 0.86 and ca. 0.92 for Ni/g-Al2O3 and Ni/g-Al2O3
3La, respectively. It seemed that the La-modification had no influence on the catalytic stabilities of the catalysts, but the amount of the carbon deposition on the two catalysts was very different. The coke deposited on Ni/g-Al2O3 was more severe than that on Ni/g-Al2O3
3La, nearly blocked the quartz tube after long-term reaction. However, these evaluations of catalytic stability, carbon deposition and Ni sintering at low GHSVs are not sufficient. To better understand the application potential of the prepared catalysts, another 400-h stability test over Ni/g-Al2O3
3La with much lower conversions was carried out under a higher GHSV of 3.6  105 mL/gcat h at 750  C, and the result was shown in Fig. 7(c). The initial conversions of CO2 and CH4 became ca. 70.5% and ca. 61.4%, probably due to the reduction in the residence time of the reactants on the

Fig. 6 e (a) CO2 and (b) CH4 conversions, (c) H2/CO ratios as functions of reaction temperature for DRM and (d) the catalytic properties at 750  C over the Ni/g-MA¡xLa catalysts stabilized after the 100-h reaction. Reaction conditions: GHSV ¼ 1.8 £ 105 mL/gcat h, CO2/CH4 ¼ 1, 1 atm. The thermodynamic equilibrium values (dashed lines) were calculated by HSC software. Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140

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Fig. 7 e Catalytic properties as a function of time for DRM over the (a) Ni/g-Al2O3 and (b) Ni/g-Al2O3¡3La catalysts under GHSV ¼ 1.8 £ 105 mL/gcat h, and (c) Ni/g-Al2O3¡3La catalyst under the GHSV ¼ 3.6 £ 105 mL/gcat h. Reaction conditions: 750  C, CO2/CH4 ¼ 1, 1 atm.

surface of the catalyst. After the initial maturation period, the catalyst exhibited rather stable CO2 and CH4 conversions around 58.5% and 47.4%, and also showed no apparent coke deposited after the stability test. These results powerfully demonstrated that the addition of La enhanced not only the catalytic properties but also the coking-resistance of the catalysts in DRM reaction. The steady-state TOFs were measured at low conversions and high GHSVs over Ni/g-Al2O3
xLa catalysts stabilized in situ after the 100-h DRM under the reaction conditions: GHSV ¼ 1.8  105 mL/gcat h, CO2/CH4 ¼ 1, 1 atm, and 750  C. As presented in Table 3, the TOFCH4 values increased from 3.45 s
1 to 5.77 s
1 with the La content increased from 0 to 3 wt

% and then decreased to 4.16 s
1 when La content further increased to 7 wt%, which showed identical variations to the catalytic activities in Fig. 6(d). The TOFCH4 values calculated here were higher than those for DRM reaction reported in the previous literatures. Liu et al. [26] obtained the TOFCH4 values of 0.7e1.2 s
1 over MCM-41-supported Ni-based catalysts. Using rare earth oxides modified NiMgAl catalysts, the TOFCH4 values between 2024 and 2349 h
1 after 8 h reaction were

Table 4 e Ni particle sizes and deposited carbon over the Ni/g-Al2O3¡xLa¡St catalysts for the DRM reactions.a Sample

Ni particle size (nm) by XRD

Table 3 e TOFCH4 values of the Ni/g-Al2O3‒xLa catalysts for the dry reforming of methane.a Sample Ni/g-Al2O3 Ni/g-Al2O3
1La Ni/g-Al2O3
2La Ni/g-Al2O3
3La Ni/g-Al2O3
5La Ni/g-Al2O3
7La a

CH4 conversion (%)

TOFCH4 (s
1)

10.7 13.7 15.2 17.1 15.3 12.6

3.45 4.37 4.98 5.77 4.80 4.16

TOFCH4 is defined as the number of converted CH4 molecules per surface Ni atom and second. Reaction conditions: GHSV ¼ 1.8  106 mL/gcat h (STP), CO2/CH4 ¼ 1.0, 750  C, 1 atm.

Ni/g-Al2O3
S100 Ni/g-Al2O3
1La
S100 Ni/g-Al2O3
2La
S100 Ni/g-Al2O3
3La
S100 Ni/g-Al2O3
5La
S100 Ni/g-Al2O3
7La
S100 Ni/g-Al2O3
S400 Ni/g-Al2O3
3La
S400 b Ni/g-Al2O3
3La
S400 a

b

9.1 8.5 8.7 9.1 8.4 8.8 9.5 8.9 8.6

TEM 9.7 ± 8.9 ± 9.2 ± 9.1 ± 8.8 ± 9.3 ± 9.8 ± 9.0 ± 9.0 ±

2.6 2.0 2.0 1.8 2.0 1.9 2.1 2.0 2.1

Carbon amount by TG (mg/gcat) 709 560 402 307 317 351 1641 373 492

Reaction conditions: GHSV ¼ 1.8  105 mL/gcat h, CO2/CH4 ¼ 1, 750  C, 1 atm. Reaction conditions: GHSV ¼ 3.6  105 mL/gcat h, CO2/CH4 ¼ 1, 750  C, 1 atm.

Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3

given [48]. Additionally, Xie et al. [49] found the steady-state TOFCH4 values over various Ni/SBA-15 catalysts at 450  C after 8 h reaction decreased to 169.4e198.2 h
1. Similar TOF values have been also reported. Over Ni/MgO catalysts, TOF values were in the range of 3.8e5.4 s
1 under the steady-state conditions [50]. It has reported that the TOFs were closely related to the sizes of Ni particles, and the TOFs always increased with reducing Ni particle sizes [28,51]. However, all the Ni/g-Al2O3
xLa catalysts showed comparable Ni crystallite sizes after 100-h DRM reaction in Table 4 and similar mesoporous frameworks in Fig. 4. Therefore, it could be

2.0

1.8

1.8

y=-4.2121x+5.8082 2 R =0.9905

1.4 1.2

y=-7.0611x+8.1666 2 R =0.991

0.8

y=-3.8939x+5.6107 2 R =0.9952

1.4

CO2

0.2

CH4

0.0 0.96

1.00

1.04 1.08 -1 1000/T (K )

1.12

1.2

ECH4=53.43kJ/mol y=-6.4263x+7.7732 R2=0.9876

1.0 0.8

0.4

0.6

CO2

0.4

CH4

0.2

1.16

0.96

1.00

1.12

1.16

2.4

c

ECH4=49.09 kJ/mol y=-5.9056x+7.3777 2 R =0.9924

1.2 1.0

1.8

-1

LnTOF (s )

-1

1.4

ECO2=27.23 kJ/mol y=-3.2754x+5.2923 2 R =0.9908

2.0

y=-3.778x+5.563 2 R =0.9964

1.6

d

2.2

ECO2=31.41 kJ/mol

1.8

1.6 ECH4=44.56 kJ/mol y=-5.3597x+6.973 2 R =0.9966

1.4 1.2

0.8

CO2

1.0

CO2

0.6

CH4

0.8

CH4

0.96

1.00

1.04

1.08

1.12

0.6

1.16

0.96

1.00

-1

1.04

1.08

1.12

1.16

-1

1000/T (K )

1000/T (K ) 2.2

2.4

e

2.2

1.4 1.2 1.0

CO2

0.8

CH4 0.96

1.00

ECH4=46.95 kJ/mol y=-5.6469x+7.1224 2 R =0.9831

1.6 -1

LnTOF (s )

-1

1.6

ECO2a=40.84 kJ/mol y=-4.1925x+5.9987 2 R =0.9930

1.8

y=-3.3434x+5.204 2 R =0.9951

1.8

f

2.0

ECO2=27.80 kJ/mol

2.0

LnTOF (s )

1.08 -1

2.0

0.6

1.04

1000/T (K )

2.2

0.4

b

-1

ECH4=58.71 kJ/mol

1.0

0.6

LnTOF (s )

ECO2=32.37 kJ/mol

1.6

LnTOF (s )

-1

a

ECO2=35.02 kJ/mol

1.6

LnTOF (s )

deduced that it's the improvement of the surface basicity of the catalyst mainly influenced the intrinsic activities of Ni/gAl2O3
xLa catalysts. Fig. 8 showed the Arrhenius plots of Ni/g-Al2O3
xLa catalysts, and the corresponding apparent activation energies (Ea) calculated on the basis of the intrinsic reaction rateetemperature relations. The activation energies for CO2 and CH4 consumption were reported to typically varied in the range of 29.3e360 kJ/mol [34,51]. Our results are of the same order of magnitude. As shown in Fig. 8, regardless of the catalysts studied, the Ea for CH4 consumption was higher than

1.4 1.2

ECH4=59.49 kJ/mol y=-7.155x+8.4266 2 R =0.9942

1.0 0.8

CO2

0.6

CH4

0.4 0.2 1.04

1.08 -1

1000/T (K )

1.12

1.16

0.0

0.96

1.00

1.04

1.08

1.12

1.16

-1

1000/T (K )

Fig. 8 e Arrhenius plots and apparent activation energies over Ni/g-Al2O3¡xLa catalysts. (a) x ¼ 0, (b) x ¼ 1, (c) x ¼ 2, (d) x ¼ 3, (e) x ¼ 5, (f) x ¼ 7. Reaction conditions: GHSV ¼ 1.8 £ 106 mL/gcat h, CO2/CH4 ¼ 1 and 1 atm. Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3

Fig. 9 e XRD patterns of the spent catalysts for the DRM reaction under GHSV ¼ 1.8 £ 105 mL/gcat h: (a) Ni/gAl2O3¡S100, (b) Ni/g-Al2O3¡1La¡S100, (c) Ni/gAl2O3¡2La¡S100, (d) Ni/g-Al2O3¡3La¡S100, (e) Ni/gAl2O3¡5La¡S100, (f) Ni/g-Al2O3¡7La¡S100, (g) Ni/gAl2O3¡3La¡S400, (i) Ni/g-Al2O3¡S400, and GHSV ¼ 3.6 £ 105 mL/gcat h: (h) *Ni/g-Al2O3¡3La¡S400. Reaction conditions: CO2/CH4 ¼ 1, 750  C, 1 atm.

that for CO2 consumption, which illustrated that the activation of CH4 was more temperature sensitive than that of CO2 in this catalyst system and the dissociative adsorption of CH4 was considered to be the rate-controlling step [28]. In this work, it was obvious that La had a significant effect on the Ea values of Ni/g-Al2O3, and the Ni/g-Al2O3
3La presented the lowest Ea values (27.23 kJ/mol and 44.56 kJ/mol for CO2 and CH4 consumption, respectively), demonstrating that La was an efficient promoter for Ni/g-Al2O3 catalyst applied in DRM reaction. The Ea values over Ni/g-Al2O3
xLa catalysts were also lower than many previously reported activation energies for various catalysts, such as, ECO2 of 98.8 kJ/mol and ECH4 of 106.8 kJ/mol for Ni/CaO
Al2O3 [52], ECO2 of 77.0 kJ/mol and ECH4 of 68.0 kJ/mol for Ni/La2O3 [53]. The considerable variations of the activation energies for Ni-based catalysts were reported to be depended on the nature of the support, the addition of promoters and the catalytic tests conditions [51]. This excellent result could be attributed to the unique nature of the three-dimensional mesoporous g-Al2O3 frameworks and the high dispersion of strong Lewis base of La2O3 due to the onepot hydrolysis method.

Characterization of spent catalysts The spent Ni/g-Al2O3
xLa catalysts after t h of DRM reaction (denoted as Ni/g-Al2O3
xLa
St) were characterized by various techniques to investigate the factors influenced the stability performance of Ni/g-Al2O3
xLa catalysts in DRM reaction. From the XRD patterns in Fig. 9, all the catalysts showed similar characteristic reflections corresponding to g-Al2O3 and

9

metal Ni phases, revealing that NiAl2O4 spinel was reduced into metal Ni by H2 and the metal Ni could not be re-oxidised during DRM procedure. In terms of Ni/g-Al2O3eS400, a group of new reflections matched with a-Al2O3 (JCPDS 11e0661) was observed, and the relative ones for g-Al2O3 were accordingly weakened. This result demonstrated that g-Al2O3 was gradually transformed to a-Al2O3 during the stability test. However, the addition of La had effectively prevented the phase transformation of g-Al2O3. Chen et al. [44] declared that the highlydispersed La2O3 species could enhance the g-Al2O3 lattice stability, retard both sintering and phase transformation of gAl2O3. But, Yamamoto et al. [54] proposed that the change of strong Lewis acid sites to new Lewis acid sites with mediumstrength was the key point to enhance the thermal stability of alumina. On the basis of the characterized results of XPS and NH3-TPD, we thought that it was the synergistic effect of the two mechanisms accounted for the stabilization of g-Al2O3 in our catalysts. Table 4 summarized the Ni crystallite sizes over the spent Ni/g-Al2O3
xLa
St catalysts. The mean sizes of the Ni crystallites over Ni/g-Al2O3
xLa
S100 were all around 9.0 nm. This result reflected that the Ni crystallite sizes had nothing to do with the La content. Moreover, the similar Ni crystallite sizes of 9.0 nm over Ni/g-Al2O3
xLa
S400 were also obtained, verifying the excellent anti-sintering ability of the Ni particles, which was ascribed to the strong metal-support interaction (SMSI) derived from the reduction of NiAl2O3 precursor. In addition, the diffraction peaks around ca. 26 were attributed to graphitic carbon (JCPDS 26-1080), one of the major reasons contributed to the catalyst deactivation. Fig. 10 displayed the representative TEM and SEM images of the spent catalysts. From Fig. 10(a)‒(d), the mesoporous g-Al2O3 frameworks and highly-dispersed Ni particles with relatively narrow particle size distributions were observed, and no severe agglomeration of Ni crystallites was found over the samples. As listed in Table 4, the average sizes of Ni particles obtained from TEM images were in good agreement with the XRD results. Additionally, some filamentous coke over Ni/gAl2O3
S400 catalyst was clearly observed at outside of the mesopores in Fig. 10(b), but no noticeable accumulation of carbon deposition was observed in the other TEM images. The morphology and amount of carbon deposition could be further corroborated by SEM observations. As presented in Fig. 10(e) and (f), the coke over both Ni/g-Al2O3
S400 and Ni/gAl2O3
3La
S400 was all existed in the dominant form of whiskers or filaments instead of disordered carbonaceous species. A large amount of long and thick filamentous carbon was accumulated on Ni/g-Al2O3
S400 surface, but the surface of Ni/g-Al2O3
3La
S400 was relatively clean, indicating a more excellent coking-resistance of Ni/g-Al2O3
3La. It had been reported that the filamentous carbon did not encapsulate the active metallic sites, but large amount of filamentous carbon could block the reactor and cause catalyst destruction [40]. The agglomeration of Ni particles over Ni/g-Al2O3
xLa
St was effectively inhibited (Table 4), therefore the investigation of carbon deposition on Ni/g-Al2O3
xLa
St for the DRM reaction was crucial. The TG profiles of the spent catalysts were illustrated in Fig. 11(a). All the samples exhibited a large weight loss from 500  C to 800  C, which was used to quantify the amount of deposited carbon. The types of carbon species could be identified by O2-TPO profiles in Fig. 11(b). According to

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3

Fig. 10 e TEM images of the spent catalysts: (a) Ni/g-Al2O3¡S100, (b) Ni/g-Al2O3¡S400, (c) Ni/g-Al2O3¡3La¡S100, (d) Ni/gAl2O3¡3La¡S400. SEM images of the spent catalysts: (e) Ni/g-Al2O3¡S400, (f) Ni/g-Al2O3¡3La¡S400. Reaction conditions: GHSV ¼ 1.8 £ 105 mL/gcat h, CO2/CH4 ¼ 1, 750  C, 1 atm.

pioneer literatures [17,28,51,55], the higher the activity of carbon species was, the lower the carbon combustion temperature became. Therefore, combustion peaks located less than 350  C were attributed to the oxidation of amorphous carbon (Ca), which was the active intermediate and responsible for the formation of CO. All the catalysts presented the nearly identical combustion peaks of amorphous carbon, implying that Ni/ g-Al2O3
xLa
S100 still remained relative strong ability in carbon transformation from feedstock gas to syngas. The peaks around 520  C could be assigned to less-active carbon species (Cb) derived from further dehydrogenation, polymerization and rearrangement of part of amorphous carbon, and the peaks above 595  C corresponded to the inert whisker-like filamentous carbon (Cg) with the lowest reactivity. Therefore, the carbon species formed on Ni/g-Al2O3
S100 was mainly Cg, that

formed on Ni/g-Al2O3
3La
S100 was Cb, and Ni/gAl2O3
7La
S100 was found to exhibit an overlapped peak of Cb and Cg. It was obvious that Ni/g-Al2O3
3La possessed the strongest carbon transformation capability. Table 4 listed the deposited amount of the carbon estimated from TG profiles. It was found that La content had a close relationship with the amount of carbon deposition on Ni/g-Al2O3
xLa
S100 catalysts. The amount of carbon deposition on Ni/g-Al2O3
S100 was 709 mg/gcat, and significantly decreased to 301 mg/gcat when the La content increased to 3 wt%. With further increasing the La content, the deposited carbon just became a little severer. In addition, as expected, the carbon deposition on Ni/g-Al2O3
S400 was accordingly much heavier than that on Ni/g-Al2O3
3La
S400. The Ni/gAl2O3
3La
S400 exhibited a similar deposited amount of

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3

11

addition, instantaneous La2O2CO3 had likely formed in the reaction procedure, which could oxidize the surface carbon, but was not detected by the XRD probably due to its low concentration [15].

Conclusions NiAl2O4/g-Al2O3
xLa (x ¼ 0, 1, 2, 3, 5, and 7) composite oxides possessed crystalline mesostructure with high BET surface areas and homogeneous pore size distributions were synthesized through one-pot template-free hydrolysis and condensation of metal nitrates with (NH4)2CO3 and investigated for DRM. Characterization results verified that the NiAl2O4 spinel was successfully formed after high-temperature of 800  C calcination. Reduction of NiAl2O4 generated uniformly distributed Ni particles with strong metal‒support interaction, which was crucial for the anti-sintering and -coking ability of the obtained catalysts. However, the addition of La had no apparent effect on the Ni particle sizes, but was advantageous to the enrichment of nickel ions on the catalyst surface and increased the strength and number of mediumstrength basic sites. The catalytic performance of Ni/gAl2O3
xLa for the DRM had a strong dependence on the La content, and the results showed that only a small amount of La around 3 wt% was enough to improve the intrinsic activity of Ni/g-Al2O3. Additionally, the incorporation of La significantly suppressed the formation of filamentous carbon and was beneficial to the obtaining of a relative balance between carbon formation and elimination. The maintenance of Ni particle sizes after 400-h reaction was another vital factor to keep the activity unchanged during the long-term stability test. Over all, the mesoporous Ni/g-Al2O3
3La catalyst could be considered as an ideal candidate for the industrialized application of dry reforming of methane. Fig. 11 e (a) TG profiles of the spent catalysts, (b) O2-TPO profiles of the representative spent catalysts.

carbon to Ni/g-Al2O3
3La
S100 catalyst, indicating that the carbon deposition and elimination over Ni/g-Al2O3
3La reached a relative equilibrium after 100-h DRM reaction. Interestingly, *Ni/g-Al2O3
3La
S400 (under GHSV ¼ 3.6  105 mL/gcat h) also deposited a similar amount of carbon to Ni/ g-Al2O3
3La
S400 (under GHSV ¼ 1.8  105 mL/gcat h). This observation implied that the addition of La was indeed beneficial to inhibit and eliminate the carbon deposition even under more harsh reaction conditions. Under high reaction temperature, carbon resource mainly came from CH4 cracking (CH4 / C þ 2H2). The released CHx species would optionally occupy the proper active sites to form carbidic carbon (*C), then they could convert to graphitic carbon through further polymerization and recrystallization [42,56]. It was reported that the induction of pyrolytic carbon on alumina-supported Ni catalysts was closely related to the g-Al2O3 surface acidity [40]. Because La could improved the Lewis acidity of the support, the addition of La would not only prevent the formation of pyrolytic carbon but also enhance the adsorption of CO2 and increase the elimination rate of carbon deposition. In

Acknowledgement This research was supported by Innovation Program of Shanghai Municipal Education Commission, the Major State Basic Research Development Program of China (No. 2014CB643403), National Science Fund for Distinguished Young Scholars (No. 51225401, 51574164) and Basic Major Research Program of Science and Technology Commission Foundation of Shanghai (No. 14JC1491400).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.140.

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Please cite this article in press as: Zhang L, et al., Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.140