Ni–Ce–Al composite oxide catalysts synthesized by solution combustion method: Enhanced catalytic activity for CO methanation

Fuel 162 (2015) 16–22

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Ni–Ce–Al composite oxide catalysts synthesized by solution combustion method: Enhanced catalytic activity for CO methanation Yan Zeng, Hongfang Ma, Haitao Zhang, Weiyong Ying ⇑, Dingye Fang Engineering Research Center of Large Scale Reactor Engineering and Technology, Ministry of Education, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Solution combustion method is

effective for preparation of Ni–Ce–Al composite oxide catalysts.
The formation of Ni–Ce–Al composite oxides improved the Ni particle distribution. 4+ 3+
Electron transfer from Ce /Ce facilitated CO methanation at lowtemperature.
The addition of Ce enhanced the tolerance against carbon deposition at water free condition.

a r t i c l e

i n f o

Article history: Received 1 June 2015 Received in revised form 26 July 2015 Accepted 23 August 2015 Available online 2 September 2015 Keywords: Ni–Ce–Al composite oxides CO methanation Solution combustion method Ce doping

a b s t r a c t Ce-incorporated Ni/c-Al2O3 catalysts, with Ce loadings from 0 wt% to 8 wt%, were prepared by solution combustion method (SCM) and evaluated for CO methanation. Compared with Ce-free Ni/c-Al2O3 catalyst, Ce-doped Ni/c-Al2O3 catalysts retained higher low- and high-temperature catalytic activity. The Ce-doped Ni/c-Al2O3 catalyst with Ce content of 6 wt% (Ni–6Ce) maintained 100% CO conversion and nearly 100% CH4 selectivity throughout the low-temperature activity test. It is revealed that the promotional effects of Ce species were possibly attributed to the high dispersion and the resultant small sized Ni particles, owing to the formation of Ni–Ce–Al composite oxides. Furthermore, Ce4+/Ce3+ redox couples in Ce-doped catalysts enriched electrons on Ni atoms, facilitating CO dissociation, and thus the conversion. Moreover, Ni–Ce–Al composite oxide catalysts retained better coke-resistant performance at water free condition, leading to higher catalytic activity at high-temperature reaction. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Natural gas is an efficient and clean energy carrier for its high calorific value and complete combustion [1]. However, due to its poor reserves in some regions of the world, the production of synthetic natural gas (SNG) from coal or biomass is becoming of great interest [2–4]. CO methanation as a useful technology to synthesize natural gas from syngas, has attracted much attention ⇑ Corresponding author. Tel./fax: +86 21 6425 2192. E-mail address: [email protected] (W. Ying). http://dx.doi.org/10.1016/j.fuel.2015.08.046 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

from both academia and industry in recent few years [5–7]. In 1902, Sabatier and Senderens pioneered to discover that the reaction between CO and H2 over nickel catalyst could generate methane [8]. To date, a number of strategies have been suggested to meet the requirement of a high-efficiency catalyst, i.e., exhibiting high activity at low-temperature (ca. 300 °C) and excellent stability at high-temperature (ca. 600 °C). Despite the fact that ruthenium is more active to CO methanation, nickel is invariably chosen in industry for the wide availability and relatively lowcost [9–11]. In particular, Ni-based catalysts supported on Al2O3 have been widely used due to its high performance-cost ratio

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[12]. Since CO methanation is a strongly exothermic reaction, Ni/cAl2O3 catalysts suffer serious nickel sintering and carbon deposition [13]. Our previous work found that Ni/c-Al2O3 catalysts prepared by solution combustion method (SCM) retained better ability against sintering, due to that Ni particles were scattered and spatially isolated by Al2O3 [14]. To reduce coke formation and prolong the lifespan of the catalysts, water is commonly added into the reactor. When this method is applied to industrial-scale methane production, feeding a large sum of water would cause serious energy loss. Meanwhile, hot liquid water could lead to the structural changes of c-Al2O3-supported catalysts [15], and accelerate the loss of surface nickel atoms [16]. Thus, it is of significance to improve the resistance to carbon deposition at water free condition. It was reported that carbon formation could be prevented by (1) adding promoters like CeO2 etc. [17], and (2) modification of catalysts by forming solid solution or composite oxides, which can provide pathway to higher activity and better resistance to carbon deposition due to the synergistic effect [18]. CeO2 is often used as structural and electronic promoter or support in heterogeneous catalysis because of its several beneficial effects, such as enhancing the coke-resistance dynamically by the oxygen storage/release capacity, improving the thermal stability of alumina, promoting the dispersion of metal on the support, and changing the properties of metals through strong metal to support interaction (SMSI) [19–22]. Liu et al. [17] reported that CeO2-decorated Ni/c-Al2O3 catalysts for CO methanation (prepared by impregnation method followed by a modified deposition–preci pitation of CeO2) showed a high resistance to both coking and sintering, but relatively low catalytic activity. Even great efforts have been made to improve the Ni/Al2O3 catalysts [23–27], it is still hardly to inhibit coking and sintering of Ni particles simultaneously while maintain the high catalytic activity of the catalysts in CO methanation. Interestingly, it is reported that SCM is a promising method to achieve a solid solution or composite oxides due to its high-temperature during the combustion process [28–32], and Ni-based catalysts prepared by SCM retained higher catalytic activity than conventional methods, such as coprecipitation, impregnation, sol–gel and mechanical mixing [33], Thus, adding Ce into Ni/c-Al2O3 catalysts prepared by SCM would be an effective method to enhance the catalytic activity and the tolerance against sintering and coking. To the best of our knowledge, there is few reports concerning Ce-doped Ni/c-Al2O3 catalysts prepared by SCM for CO methanation. As a consecutive effort, we modified the method to prepare catalysts for improved catalytic performance. In this work, several Ni/c-Al2O3 catalysts with Ce contents in the range of 0–8 wt% were prepared by SCM, and catalytic performance for CO methanation was investigated and discussed. Both fresh and/or used catalysts were characterized by N2 adsorption, Inductive coupled plasma emission spectrometer (ICP), X-ray diffraction (XRD), Highresolution transmission electron microscopy (HRTEM), Carbon monoxide temperature programmed desorption (CO-TPD), X-ray photo-electron spectra (XPS), and Thermogravimetric (TG). The possible mechanism for promotional effect of Ce species was proposed.

the homogeneous clear solution placed in drying oven with constant temperature of 70 °C for 6 h, the concentrated viscous solution was transferred to an open china crucible was heated to 700 °C under static air in a muffle furnace with the heating rate of 2 °C min1 (combustion occurred spontaneously during the heating process), and then kept at 700 °C for 7 h. The as-obtained catalysts were denoted as Ni–xCe, in which x indicated the Ce weight content (wt%, x = 0, 2, 4, 6, and 8). 2.2. Characterization of catalysts Nitrogen physisorption was conducted on ASAP 2020 (Micromeritics, USA). Specific surface areas were measured by Table 1 Surface area, pore volume, average pore size, Ni particle size, and weight content for samples. Catalyst

SBET (m2 g1)

Vp a (cm3 g1)

Dpb (nm)

DNic (nm)

Ni–0Ce Ni–2Ce Ni–4Ce Ni–6Ce Ni–8Ce

135.5 103.1 89.6 83.3 75.0

0.28 0.62 0.61 0.53 0.38

9.0 24.0 26.8 23.8 18.7

32.3 13.6 11.1 10.9 9.8

Weight contentd (%) Ni

Ce

29.3 29.6 29.4 29.7 29.6

– 2.0 3.9 5.8 7.9

a

BJH desorption pore volume. BJH desorption average pore diameter. Measured by averaging the diameter of 200 randomly selected particles from P 3 P 2 TEM images (Ni average diameter = ni di = ni di ). d Calculated by ICP. b

c

2. Experimental 2.1. Catalyst preparation Ce-doped Ni/c-Al2O3 catalysts, with constant Ni loading of 30 wt% and Ce content from 0 wt% to 8 wt%, were prepared by SCM in glycol. In a typical experiment, certain amounts of Ni (NO3)26H2O, Ce(NO3)6H2O and Al(NO3)39H2O were dissolved in 200 mL glycol which was also used as combustible solution. After

Fig. 1. XRD patterns of calcined samples (a), and enlargement of peaks in the 2h range of 44–46° (b).

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Brunauer Emmet Teller (BET) method. Total pore volume and average pore diameter were evaluated using the standard Barrett–Joyner–Halenda (BJH) treatment. The metal compositions of the samples were analyzed by ICP (Agilent 725ES, USA). XRD was carried out on a Rigaku D/Max2550VB/PC X-ray diffractometer (Rigaku, Japan) with Cu Ka radiation (c = 1.54056 Å). CO-TPD was carried out on AutoChem II 2920 (Micromeritics, USA). HRTEM was performed on transmission electron microscope apparatus (JEOL JEM-2100, Japan) with an accelerating voltage of 200 kV. The binding energy of Ni and Ce was analyzed by XPS on an ESCALAB 250Xi system (Thermo Fisher Scientific, UK) with Al Ka radiation (hm = 1486.6 eV), and calibrated by referencing to the C 1s peak form the contaminated carbon that was assumed to have a binding energy of 284.6 eV. TG analysis was conducted on a Q600 analyzer (TA Instruments, USA) in a temperature range of room temperature to 800 °C at a heating rate of 10 °C min1 in air flow of 100 mL min1.

was added in both ends of the uniform temperature zone to keep the catalyst in the thermostatic area. 500 mg catalyst was used at a total gas flow rate of 100 mL min1, i.e., a weight hourly space velocity (WHSV) of 12,000 mL g1 h1. The catalyst was heated from ambient temperature to 700 °C in nitrogen flow within 300 min and reduced by pure hydrogen (100 mL min1) for 3 h. After the catalyst was cooled to reaction temperature under nitrogen flow, syngas (17.5 vol% CO, 8.9 vol% CO2, 67.5 vol% H2, and 6.2 vol% N2) was introduced to investigate the lowtemperature catalytic activity of Ni-based catalysts at 260– 340 °C, 0.1 MPa and 12,000 mL g1 h1. High-temperature stability tests were also conducted over the five catalysts at 600 °C, 2 MPa, 12,000 mL g1 h1, without adding water vapor. Here, we define:

CO conversion : X CO ð%Þ ¼

NCO;in  NCO;out  100 NCO;in

CH4 selectivity : SCH4 ð%Þ ¼  2.3. Catalytic evaluation

ð1Þ

NCH4 ;out    NCO;in  NCO;out þ NCO2 ;in  NCO2 ;out

 100 Syngas (carbon oxides and hydrogen) methanation reaction was carried out in a fixed-bed reactor (U14  2  500 mm). Inert Al2O3

Fig. 2. TEM images and Ni (111) fringes of Ni–0Ce (a), Ni–2Ce (b), Ni–4Ce (c), Ni–6Ce (d) and Ni–8Ce (e); Ni particle sizes distribution of the five catalysts (f).

ð2Þ

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CH4 formation rate : r CH4 ¼

NCH4 ;out mNi

19

ð3Þ

where Ni,in and Ni,out are the inlet and outlet mole flow rate (mol h1) of species i (i = CO, CO2, and CH4), and mNi is the weight of nickel (g). 3. Results and discussion 3.1. Catalyst characterization 3.1.1. N2-adsorption and ICP The texture properties of the Ce-doped Ni/c-Al2O3 catalysts with different Ce loadings were characterized using the N2 adsorption. The surface area (SBET), pore volume (VP) and average pore size (DP) are summarized in Table 1. Ni–0Ce had a surface area of 135.5 m2 g1, pore volume of 0.28 cm3 g1, and average pore size of 9.0 nm. As the loading of Ce increased, surface area of the Ce-doped catalysts decreased, while both pore volume and average pore size increased, possibly due to (1) the existence of Ce species on the surface of the catalyst leading to smaller surface areas, and (2) the presence of Ce species blocking some micropores. The weight contents of Ni and Ce of the reduced samples were determined by ICP. As listed in Table 1, each metal weight content from the test result was close to the theory design value, indicating that SCM is an efficient method with little metal loss. 3.1.2. XRD A careful XRD study was undertaken. Fig. 1a shows the XRD patterns of the five calcined samples. The diffraction peaks at 37.3°, 43.3°, 62.9°, 75.5° and 79.4° derive from NiO (JCPDS 47-1049),

Fig. 4. CO-TPD profiles for reduced samples (a); XPS spectra of Ni 2p3/2 for calcined samples (b).

Fig. 3. CO conversion (a), CH4 selectivity (b) and CH4 formation rate (c) of the five samples at 0.1 MPa and 12,000 mL g1 h1 in the temperature range of 260–340 °C.

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while the diffraction peaks around 19.1°, 31.4°, 37.2°, 45.2°, 59.7°and 65.8° are attributed to NiAl2O4 (JCPDS 10-0339). However, no peaks assigned to Ce species can be observed, suggesting high dispersion of Ce species. Interestingly, no peaks of NiO shifted, while diffraction peaks attributed to NiAl2O4 shifted toward lower 2h area. To further clarify this, the strongest diffraction peak of NiAl2O4 between 44° and 46° of each sample was enlarged and plotted separately in Fig. 1b. The 2h value for pristine Ni–0Ce is 45.14°, and shifted to lower region gradually with the increasing of the Ce contents. The negative shift can be ascribed to the formation of Ni–Ce–Al composite oxides during the combustion process, and the composite oxides may be the Ni–Ce–Al solid solution [34]. 3.1.3. HRTEM HRTEM images of reduced samples and corresponding particle sizes distribution were given in Fig. 2a–f. Ce-doped catalysts retained uniform dispersion and much smaller particle sizes than Ni–0Ce. Ni mean particle sizes (measured by TEM image) are listed in Table 1. Ni mean particle size of Ni–0Ce was 32.3 nm, while that of Ni–2Ce, Ni–4Ce, Ni–6Ce, and Ni–8Ce were 13.6 nm, 11.1 nm, 10.9 nm, and 9.8 nm, respectively. The smaller mean particle sizes for Ce-promoted catalysts may be due to the synergetic effect of Ni–Ce–Al composite oxides. It was reported that reduction of Ni2+ containing composite oxides or solid solution is realized as an effective way to maintain the Ni particle size small [34–37]. In addition, Fratello et al. reported that Ni and Ce can form CeNiO3 alloy, which was inactive to methanation [38]. Since ionic radii of Ce3+ (1.03 Å) and Ce4+ (0.92 Å) are larger than that of Ni2+ (0.72 Å), there would be a positive shift in the lattice spacing of Ni if some Ni cations were substituted with Ce cations [39,40]. Ni (1 1 1) fringes of the five samples are inserted in Fig. 2a–e. The planar lattice of Ni–0Ce was 0.203 nm, but became smaller for Ce-promoted samples (Ni–2Ce: 0.194 nm, Ni–4Ce: 0.191 nm, Ni–6Ce: 0.186, and Ni–8Ce: 0.183 nm). The lattice shrinkage provided additional evidences to the XRD results that no Ni–Ce solid solution had formed. Moreover, the lattice shrinkage may be due to a better spatial distribution and smaller particle size of Ni species over the Ce-doped catalysts. It was reported that small crystallite size could resulted in lattice shrinkage [32].

contents. It is supposed that small Ni particle size of catalysts would enhance CO adsorption capacity, due to more active surfaces. The four Ce-doped catalysts possessed similar particle sizes, but the amount of CO desorption decreased obviously with the increasing Ce content. It could be related to the partial coating of catalyst surface by excessive Ce species, causing the decrease in active sites. When the dopants reached an upper limit to the maximum possible dissolution into the host oxides, it tended to be enriched on the surface [40]. It was reported that peaks observed at low-temperature (<300 °C) contribute little toward methanation, while the relatively high-temperature (300–600 °C) desorption peaks significantly contribute toward methanation [41]. However, the four Ce-doped catalysts showed similar catalytic activity even retaining various CO adsorption capacity. The result indicated that CO adsorption capacity may not be the key factor for CO methanation when CO adsorption capacity reached a certain level. XPS curves of the five samples are showed in Fig. 4b, the binding energy of Ni 2p3/2 was 856.9 eV for Ni–0Ce, and shifted toward lower side with the addition of Ce species. Moreover, the shifts

3.2. Catalytic performance of the catalysts at low-temperature The catalytic performance of the five catalysts were investigated at 260–340 °C, 0.1 MPa and 12,000 mL g1 h1, and the results are exhibited in Fig. 3a–c. Clearly, the addition of Ce species to a Ni/c-Al2O3 catalyst significantly improved the catalytic performance of CO methanation (Fig. 3a). Ni–6Ce reached CO conversion 100% at the initial reaction temperature of 260 °C, much higher than that of Ni–0Ce (38.5%). However, CO conversion over the four Ce-doped catalysts showed no big difference. Simultaneously, as presented in Fig. 3b, the four Ce-doped catalysts retained a similar CH4 selectivity in the measured temperature range, which was higher than that of Ni–0Ce, especially at the starting reaction temperature. For instance, at the beginning, CH4 selectivity is 98.3% for Ni–6Ce, but only 71.0% for Ni–0Ce. CH4 formation rates of the catalysts are provided in Fig. 3c. Compared with Ni–0Ce, Ce-promoted catalysts possessed higher CH4 formation rate in the whole temperature range. Moreover, Ni–6Ce had a relatively higher CH4 formation rate of 308 mmol g1 h1 at reaction temperature below 290 °C. To clarify the promotion effects of Ce species in the catalysts, CO-TPD and XPS were employed to obtain more insights. Fig. 4a presents the CO-TPD of the five catalysts. The amounts of desorbed CO at high-temperature (300–600 °C) of Ni–2Ce catalyst was more than that of Ni–0Ce, and decreased gradually with increase of Ce

Fig. 5. XPS spectra of Ni 2p and Ce 3d for calcined samples.

Y. Zeng et al. / Fuel 162 (2015) 16–22

21

Fig. 7. XRD patterns of the five catalysts before (a) and after (b) lifetime tests.

Fig. 6. Stability test over all the samples at 600 °C, 2 MPa, and 12,000 mL g1 h1 (a); TG curves of the five catalysts before and after 150 h stability tests (b).

changed little with further increase of Ce contents. The decrease of binding energy could be attributed to enrichment of electron cloud density of Ni atoms by electron transfer from Ce4+/Ce3+ redox couples (see Fig. 5) [18]. The negative charge of Ni atoms could promote CO dissociation due to the enhanced Ni–C bond and subdued C–O bond [42,43]. Since CO dissociation is the ratedetermining step in CO methanation [41], the addition of Ce species could facilitate CO methanation. Ce-doped catalysts having about the same binding energy retained similar catalytic activity, and Ni–0Ce catalyst with higher binding energy showed lower catalytic activity. Thus, the decreasing binding energy of Ni 2p3/2 over the four Ce-doped catalysts possibly explained their higher catalytic performance, and CO dissociation was the key factor for CO methanation over Ce-promoted Ni/c-Al2O3 catalysts prepared by SCM, other than CO adsorption capacity. 3.3. Stability test The stability tests over the five catalysts were conducted at water free condition, 600 °C, 2 MPa, and 12,000 mL g1 h1. CO conversions are plotted as the function of time-on-stream, as presented in Fig. 6a. No obvious deactivation in the 150 h lifetime tests can be found among all the five catalysts. Although both Ni–0Ce and Ce-doped catalysts retained good stability at high-

temperature, Ce-promoted catalysts showed a little higher CO conversion than Ni–0Ce during the stability test. CO conversion for an optimal Ni–6Ce catalyst maintained at 94.6%, while that for Ni– 0Ce declined to 87.9%. Herein, XRD and TG were employed to demonstrate this phenomenon. Diffraction peaks of Ni of the five catalysts before and after lifetime tests retained no obvious changes (Fig. 7a and b), indicating that the differences of CO conversion were not attributed to sintering. TG curves of the five catalysts are given in Fig. 6b, and the difference of weight losses between before and after lifetime tests was attributed to carbon deposition [44]. Obviously, Ce-free Ni–0Ce catalyst retained more serious carbon deposition than that of the four Ce-promoted catalysts. The carbon amount of Ni–0Ce (3.9 wt%) was as 13 times higher as Ni–6Ce (0.3 wt%). The result indicated that Cepromoted catalysts possessed a better ability to resist carbon deposition, and the more serious carbon deposition of Ni–0Ce was possibly responsible for its relatively lower catalytic activity at hightemperature reaction. In addition, the higher tolerance to coking of Ce-doped catalysts may be attributed to their small Ni particle sizes [45], and the enriched oxygen vacancies by Ce species [46].

4. Conclusions In summary, Ce-promoted Ni/c-Al2O3 catalysts with Ce contents ranging from 0 wt% to 8 wt% have been synthesized by solution combustion method, and directly used to catalyze CO methanation. The addition of Ce led to better Ni particle distribution and smaller Ni particle size, and Ce4+/Ce3+ redox couples in the catalysts enriched electron cloud density of Ni atoms, thus improving the low-temperature catalytic activity for Ce-doped catalysts. Moreover, adding Ce can enhance the tolerance against to

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