Ordered Mesoporous NiâFeâAl Catalysts for CO Methanation with

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Ordered Mesoporous Ni-Fe-Al Catalysts for CO Methanation with Enhanced Activity and Resistance to Deactivation Qing LIU, Yingyun Qiao, Yuanyu Tian, Fangna Gu, Ziyi Zhong, and Fabing Su Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02174 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017

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Industrial & Engineering Chemistry Research

Ordered Mesoporous Ni-Fe-Al Catalysts for CO Methanation with Enhanced Activity and Resistance to Deactivation

Qing Liua,b,*, Yingyun Qiaoc, Yuanyu Tiana,c, Fangna Gub,*, Ziyi Zhongd, and Fabing Sub,e,*

a

Key Laboratory of Low Carbon Energy and Chemical Engineering, College of Chemical and

Environmental Engineering, Shandong University of Science and Technology, Qingdao Shandong 266590, China b

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese

Academy of Sciences, Beijing 100190, China c

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China),

Qingdao Shandong 266580, China d

Institute of Chemical and Engineering Sciences, A*star, 1 Pesek Road, Jurong Island 627833,

Singapore e

Institute of Industrial Chemistry and Energy Technology, Shenyang University of Chemical

Technology, Shenyang 110142, China

*Corresponding author. Tel.: +86-10-82544850; Fax: +86-10-82544851. E-mail addresses: [email protected] & [email protected] (Q. Liu); [email protected] (F. Gu); [email protected] (F. Su).

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ACS Paragon Plus Environment


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ABSTRACT: A series of ordered mesoporous Ni-Fe-Al ternary oxide composites were prepared via a one-pot evaporation-induced self-assembly (EISA) method and applied in CO methanation reaction to produce synthetic natural gas. The results showed that the ordered mesoporous Ni-Fe-Al catalyst with proper amount of Fe species (10N1FOMA) had better both CO conversion and CH4 selectivity than the impregnation-derived 10N1FA catalyst with unordered mesopores and identical component, owing to the higher Ni dispersion and larger H2 uptake. In a 120 h atmospheric-pressure lifetime test, the ordered mesoporous 10N1FOMA catalyst showed significant enhancement in both anti-sintering and anti-coking properties in comparison with the unordered mesoporous 10N1FA, mainly because of the confinement effect of the mesopore channels, the weak acidity of ordered mesoporous alumina support, and the smaller Ni particle size (< 5.0 nm).

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1. INTRODUCTION The production of synthetic natural gas (SNG) from coal via syngas methanation reaction (CO + 3H2 → CH4 + H2O, ∆H298 K = −206.1 kJ mol−1) is of great importance in some regions such as China, where coal resource is abundant while its domestic natural gas cannot meet the enormous demand.1-3 Considering the highly exothermic nature of CO methanation reaction, efficient recovery of the reaction heat is essential for any industrial methanation technologies,4,

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thus

high-temperature methanation processes are usually employed in industry due to the economic evaluation.1, 2 As a result, the practical methanation catalysts often suffer from Ni sintering and coking at high temperatures.5-9 Great efforts to solve these issues have generally focused on one or the other individually,8, 10-14 although they often occur simultaneously. Few methods can achieve simultaneous inhibition of Ni sintering and coking, while maintaining high catalytic activity for high-temperature methanation.15, 16 Previous literatures have shown that encapsulating metal nanoparticles in ordered mesoporous materials is an effective strategy to overcome the abovementioned technical barriers,15-18 because the ordered mesoporous frameworks can not only exert a spatial restriction on metal nanoparticles to hamper their sintering and coke formation, namely nanoconfinement effect, but also offer a high surface area for the high dispersion of metals species as well as large channels to facilitate the mass transmission of reactants, thus promising to improve both the stability and activity of the catalysts simultaneously.19-22 Catalysts with silica-based ordered mesoporous support such as SBA-15, MCM-41, or MSN are widely applied in high-temperature reactions,21-24 however, the metal particles supported in the silica-based channels could still migrate from the mesopores to the external surface at high temperatures due to the weak interaction between metallic nanoparticles and silica support, and then the metal particles sintering is inevitable in these silica-based catalysts. 3



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On the contrary, the ordered mesoporous alumina (OMA) has a great tendency to interact with metal particles. In fact, the metal nanoparticles are anchored in the frameworks with the strong interaction between metal and the support, obtaining the prominent anti-sintering property over OMA-based catalyst.16 In our previous works,15-17 the ordered mesoporous Ni-V-Al, Ni-Cr-Al and Ni-Zr-Al catalysts have displayed high anti-sintering and anti-coking performances or hydrothermal stability for CO methanation, however V15 and Cr16 species are hazardous for environment and health, meanwhile the activity of Ni-Zr-Al catalyst17 should be further enhanced. Hence, the highly efficient and safe promoters of the ordered mesoporous Ni-Al catalyst for CO methanation should be explored. Recently, the addition of secondary metals such as Ru,8, 25 Co20, 23 and Fe26 to Ni-based catalyst has been proposed as an effective strategy to improve its catalytic activity as well as stability. The metals can either form the new alloy nanoparticles or be in the state of isolated atomic clusters with single component according to their characteristics as well as different reduction conditions. From an economic point of view, the use of Fe would be highly desirable due to its low price compared with other different noble or non-noble metals. Also, the Ni-Fe based catalysts have been employed in methanation reaction,26-29 reforming reaction30,

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and CO2 hydrogenation reaction,32 which

exhibited high activity and stability in the reactions compared with single component catalyst. Andersson et al. reported that the optimal dissociative adsorption energy of CO could be obtained on the multicomponent surface active sites over bimetallic Ni-Fe, and the methanation reaction was proceeded through a COH species mechanism undercoordinated sites.33, 34 Kustov et al. found that the addition of Fe promoted the dispersion of Ni species and decreased the reduction temperature of the Ni-Fe bimetallic catalyst,28 which was similar with the results of Tian et al.27 and Cheng et al..35 Also, the synergy between Ni and Fe could significantly reduce carbon formation, for example, the 4


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Fe oxide lattice oxygen as well as the dissociated surface oxygen over Ni accelerated carbon gasification from the spent Fe-Ni catalysts in methane dry reforming reaction.30 However, there are also some problems left to further study, for instance, increase of Fe content over Ni-based catalyst would lead to the enhanced the water-gas shift reaction and hydrocarbon formation.36 Furthermore, the sintering of metal nanoparticles at high temperatures could be severe especially over the bimetallic catalysts prepared by the conventional method such as impregnation or precipitation method, because the melting points or Tammann temperature of the alloys were lower than that of monometallic ones.37, 38 Therefore, the stability as well as the catalytic activity of the bimetallic catalyst should be further improved. Herein, to continue our research work on catalysis,13, 15-17,

39-41

especially for simultaneously

enhancing the anti-coking and anti-sintering properties as well as the catalytic activity of the Ni-based catalysts for CO methanation reaction, a series of ternary NiO-Fe2O3-Al2O3 materials with ordered mesoporous structure were synthetized via a one-pot evaporation-induced self-assembly (EISA) method. Also, in order to highlight the advantages of the ordered mesoporous catalysts, an unordered mesoporous Ni-Fe/Al2O3 catalyst prepared by the conventional impregnation method was used as reference. To our knowledge, there is no report in the literature of the design and synthesis of the ordered mesoporous NiO-Fe2O3-Al2O3 catalysts for CO methanation. In order to get insights into the structure-catalytic performance relationship, and to know how the Fe species impacts the activity and stability, e.g. to determine the optimum Fe loading, to reveal the true state of the Fe species, and to clear the location and distribution of both the Ni and Fe species, various techniques were performed to characterize the catalysts before and after the CO methanation reaction in this work.

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2. EXPERIMENTAL 2.1. Catalyst preparation Aluminum isopropoxide 98%, nickel (II) nitrate hexahydrate 99%, ferric nitrate nonahydrate 99%, nitric acid 67 wt% and anhydrous ethanol 99.5% were purchased from Sinopharm Chemical Reagent Co. Ltd., China, and triblock copolymer Pluronic P123 (EO)20(PO)70(EO)20 was purchased from Sigma-Aldrich. The commercial porous γ-Al2O3 (purity >97%, 172 m2 g–1) was purchased from Wenzhou Jingjing Alumina Co. Ltd., China. All the gases (purity > 99.999 %, Beijing Haipu Gas Company Ltd., China) were used as received. The ordered mesoporous NiO-Fe2O3-Al2O3 ternary oxides were prepared by the one-pot EISA method following our previous method.15, 16 Typically, P123 (2.0 g) was dissolved in anhydrous ethanol (40.0 mL), followed with addition of 67 wt% nitric acid (3.2 mL), Al(OPri)3 (4.08 g) and stoichiometric quantities of Ni(NO3)2·6H2O and/or Fe(NO3)3·9H2O in sequence under vigorous stirring. The final mixture was stirred for 4 h at room temperature in a beaker covered with PE film. After that, the mixture was placed into an oven to undergo the slow EISA process at 60 °C for 48 h. The final gel was calcined in a tubular furnace under flowing air at 550 °C for 4 h with a heating rate of 1 oC min–1. The obtained samples were denoted as 10NxFOMA (x = 0, 1, 2 and 4), in which ‘x’ represented the mass percentage of Fe2O3, and ‘OMA’ was the abbreviation of ordered mesoporous alumina. The NiO content was fixed at 10 wt% in all catalysts in this work. In addition, the ordered mesoporous 10FOMA sample was prepared with the same EISA method with the Fe2O3 content of 10 wt%. NiO-Fe2O3-Al2O3 samples with unordered mesoporous structure were prepared by excessive impregnation method to compare with those obtained by one-pot synthesis. Typically, Ni(NO3)2·6H2O (0.79 g) and Fe(NO3)3·9H2O (0.23 g) was dissolved in anhydrous ethanol (40.0 6


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mL), then the γ-Al2O3 support (2.04 g) was dispersed in the solution under stirring. The obtained mixture was vigorously stirred at room temperature overnight, and then heated to 70 oC to evaporate the solvent, dried at 100 oC for 24 h, followed with calcination at 550 oC for 4 h in air with a heating rate of 1 oC min–1. The collected sample was denoted as 10N1FA with the NiO and Fe2O3 loading of 10 wt% and 1 wt%, respectively, in which ‘A’ was the abbreviation of γ-alumina. Moreover, the complex of NiO and Fe2O3 with the weight ratio of 10 : 4 was prepared by the same impregnation method, and the obtained sample was denoted as 10N4F. The bulk Fe2O3 was obtained through the calcination of Fe(NO3)3·9H2O at 550 oC for 4 h with a heating rate of 1 oC min–1. In addition, a commercial Ni/Al2O3 catalyst HTJ-103H (CMC, 18 wt% Ni loading) was purchased from Liaoning Haitai Sci-Tech Development Co., Ltd., China, and was evaluated as the reference catalyst.

2.2. Catalysts characterization Nitrogen physisorption was performed in a Quantachrome NOVA 3200e instrument at −196 °C. Prior to the measurement, the sample was degassed at 300 ºC for 3 h under vacuum. The specific surface area was determined according to the using the Brunauer–Emmett–Teller (BET) method. The pore size distribution (PSD) and pore volume were calculated based on the Barrett–Joyner– Halenda (BJH) method using the adsorption isotherm branch. The crystalline phases of the samples were determined by X-ray diffraction (XRD) on a PANalytical X'Pert PRO MPD diffractometer with a step of 0.02o using Cu Kα radiation (λ=1.5418 Å) at 40 kV and 40 mA. The crystallite size of the sample was estimated using the Debye-Scherrer equation. H2 temperature-programmed reduction (H2-TPR), H2 temperature-programmed desorption 7



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(H2-TPD), NH3 temperature-programmed desorption (NH3-TPD), and temperature-programmed oxidation (TPO) were performed on a Quantachrome chemBET pulsar TPR/TPD chemisorption analyzer. For H2-TPR, the sample (0.10 g) was pretreated at 300 oC for 1 h under He flow to remove moisture and impurities. After cooling to room temperature, the sample was heated to 1000 oC with a heating rate of 10 oC min–1 in 10.0 vol% H2/Ar flow (30 mL min–1), and the TCD signal was recorded continuously.16 For H2-TPD, the sample (0.20 g) was pre-reduced in situ under 10.0 vol% H2/Ar flow at 700 oC for 1 h, and then cooled down and saturated with H2 for 1 h at room temperature. In order to remove the physically adsorbed H2, the Ar flow was introduced to purge the sample for 2 h. After that, the sample was heated to 600 oC at 10 oC min–1 in an Ar flow (30 mL min–1). The Ni dispersion (D%) was calculated using a formula described in our previous works.15, 42 For NH3-TPD, the sample (0.2 g) was pre-reduced in situ by 10.0 vol% H2/Ar flow at 700 oC for 1 h, and cooled to 100 oC in a He flow (30 mL min−1). Then, the sample was saturated with ammonia (3.0 vol% NH3/He) for 0.5 h. After removing the physically adsorbed ammonia in a He flow for 1 h, the sample was heated to 600 oC (10 oC·min−1) under a continuous He atmosphere.11 For O2-TPO, the sample after the lifetime test was pre-treated in He flow at 300 oC for 1 h and then cooled down to 200 oC. After that, the 5.0 vol% O2/He flow (30 mL min−1) was introduced and the sample was then heated up to 850 oC with a heating rate of 10 oC min−1.43 For the sequential CO2-TPO/O2-TPO, the procedures were similar to that of O2-TPO. The sample was pre-treated in He flow at 300 oC for 1 h and then cooled down to 200 oC. The 10.0 vol% CO2/He flow (30 mL min−1) was fed and the sample was then heated up to 900 oC with a heating rate of 10 oC min−1. Sequentially, the sample was cooled down to 200 oC in He flow, and then 8


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heated up to 1000 oC with a heating rate of 10 oC min−1 in a 5.0 vol% O2/He flow (30 mL min−1).43 The catalyst surface chemical composition analysis was performed on an X-ray photoelectron spectroscopy (XPS) spectrometer (VG ESCALAB 250, Thermo Electron, U.K.) equipped with a non-monochromatized Al Kα X-ray source (1486 eV).13 The field emission scanning electron microscope (SEM) (JSM-6700F, JEOL, Japan) with a liquid nitrogen cooled energy-dispersive X-ray spectroscopy (EDS) detector was used to collect the morphology and elemental analysis of the samples. In addition, the transmission electron microscope (TEM) images were obtained on by JEM-2010F under a working voltage of 200 kV. The total amount of deposited coke on the spent catalysts was measured using Thermogravimetric (TG) analysis on a Seiko Instruments EXSTAR TG/DTA 6300, in which the sample (10 mg) was heated in a flow of air (200 mL min–1) from room temperature up to 1000 oC (10 oC min–1). A Thermo Scientific iCAP 6300 inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to determine the exact composition of all the samples, and the results are listed in Table S1.

2.3. Catalytic activity measurements The catalytic measurements were carried out in a fixed-bed tubular quartz reactor (8 mm i.d.) at 0.1 MPa.10, 17, 44 For each catalytic experiment, the catalyst (0.10 g, 20−40 mesh) diluted with quartz sands (5.0 g, 20−40 mesh) was packed and in-situ pre-reduced at 700 oC in pure H2 (100 mL min–1) for 1 h. After cooling to the starting reaction temperature (300 oC) in H2, the mixed H2 and CO as well as N2 (as an internal standard) at a molar ratio of H2/CO/N2 = 3/1/1 were introduced to perform the CO methanantion. The outlet gas stream from the reactor was cooled using a cold trap, followed 9



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by a dryer, and then analyzed by online on-line by a Micro GC 3000A (Agilent Technologies) equipped with a TCD detector. In addition, a 120h-lifetime test of CO methanation was performed at 550 oC, 0.1 MPa. The weight hourly space velocity (WHSV, mL·min–1·h–1), CO conversion, CH4 selectivity and CH4 yield were calculated as follows:10, 17, 44 Fmixedgas ,in

WHSV ( mL ⋅ g −1 ⋅ h −1 ) =

mcatalyst

CO conversion: X CO (%) =

CH4 selectivity: SCH4 (%) =

CH4 yield: Y CH4 (%) =

× 60

FCO ,in − FCO ,out × 100 FCO ,in

FCH4 ,out FCO ,in − FCO ,out

X CO × SCH4 100

=

× 100

FCH4 ,out FCO ,in

× 100

Where, X is the conversion of CO; S is the selectivity of CH4; Y is the yield of CH4; Fi, in and Fi, out

are the volume flow rates of species i (i=CO or CH4) at the inlet and outlet, respectively; mcatalyst

is the weight of the catalyst, g. Moreover, the turnover frequency (TOF) value at 220 oC was calculated based on a formula described in our previous work.45 In order to confirm the low CO conversion (below 10%), the assessments were carried out at 0.1 MPa, 200 mL min‒1, 220 oC with catalyst (0.5 g, 20−40 mesh) diluted with quartz sands (3.0 g, 20−40 mesh). TOFCO =

VCO ,in − VCO ,out mCat −TPD A M CuO × × CuO −TPR × RH 2 / Ni × Vm mCat − R e action ACat −TPD mCuO −TPR

where, VCO,in and VCO,out are the volume flow rates of CO at the inlet and outlet of the reactor at standard temperature and pressure (STP), mL s–1; Vm = 22414 mL mol–1 is the molar volume of CO at STP; R H 2 /Ni = 2 is the stoichiometric factor of the H2 : Ni ratio in the H2 chemisorption; mCat-TPD 10


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is the mass of the catalysts used in the H2-TPD experiment, g; mCat-Reaction is the mass of the catalysts used in the CO methanation reaction, g; ACuO-TPR is the peak area of the H2-TPR curve of standard CuO; ACat-TPD is the peak area of the H2-TPD curve of the catalyst; MCuO = 79.54 g mol–1 is the mole mass of CuO; mCuO-TPR is the mass of the CuO used in the H2-TPR experiment, g.

3. RESULTS AND DISCUSSION 3.1. Characterization of the catalysts 3.1.1. Nitrogen physisorption analysis The nitrogen physisorption isotherms and the pore size distribution (PSD) curves of the 700oC-reduced samples are displayed in Figure 1. For all the reduced 10NxFOMA samples, the isotherms reveal type IV isotherms with H1-type hysteresis loops, which suggest the presence of the uniform cylindrically shaped channels.19 All the capillary condensation steps of the hysteresis loops are steep, indicating the excellent uniform mesopores among the frameworks. In contrast to 10N1FA, it exhibits type IV isotherms with a H2-typed hysteresis loop, which is the typical feature of mesopores with ‘ink bottle’ shapes. In addition, the PSD curves of the reduced 10NxFOMA samples have only one narrow peak around 12 nm, confirming the high uniformity of the channels. However, the broad PSD curve of 10N1FA reveals its wider pore size distribution. The textural properties of the reduced samples are summarized in Table 1. All the 10NxFOMA samples possess relatively high specific surface areas up to 194 m2 g–1 and large pore volume up to 0.50 cm3 g–1. The Fe species content (except for 10N1FOMA) has little influence on the above parameters. It is worthwhile to noting that the specific surface area and pore volume of the reduced 10NxFOMA are a little lower than those of the calcined Ni-OMA samples reported in the literature,15 which is related to the partial collapsion of channels during the high temperature 11



reduction. Moreover, the 10N1FA sample has lower surface area and pore volume than the 10NxFOMA samples due to the textural properties of γ-Al2O3 support.45 Overall, the ordered mesoporous 10NxFOMA samples have uniform mesopores, high specific surface areas and large pore volumes, which promise their potential applications as catalysts.

(a)

1050

Adsorbed volume (cm /g,STP)

(b) 10NOMA

dV/(dlogD)

850 10NOMA

3

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650 10N1FOMA 450 10N2FOMA

10N1FOMA

10N2FOMA 10N4FOMA

250 10N4FOMA

10N1FA

10N1FA 50

0.0

0.2

0.4

0.6

0.8

1.0

3

80

10

Relative Pressure (P/P0)

Pore Size(nm)

Figure 1. N2 physisorption isotherms (a) and PSD curves (b) of the 700oC-reduced catalysts (For clarity, the isotherms of 10NOMA, 10N1FOMA, 10N2FOMA, and 10N4FOMA were vertically shifted for 700, 470, 300, and 150 cm3 g–1, respectively.)

Table 1. Textural and chemical properties of the 700oC-reduced samples SBETa 2

–1

Ni crystallite size (nm)c

Vpb 3

–1

e

H2 uptake –1

D (%)f

TOFCO, 220 oC (s–1)g

(m g )

(cm g )

Reduced

10NOMA

209.9

0.63

< 5.0 (3.7)

–

57.1

8.6

0.550×10–3

10N1FOMA

239.9

0.60

< 5.0 (3.3)

< 5.0 (3.6)

91.8

13.8

0.792×10–3

10N2FOMA

195.0

0.64

< 5.0 (4.4)

–

86.3

13.0

0.710×10–3

10N4FOMA

194.1

0.50

< 5.0 (4.0)

–

80.7

12.2

0.736×10–3

Spent

(µmol g )

12


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10N1FA

132.5

5.3 d

0.41

7.8 d

51.0

7.7

0.502×10–3

a

SBET: Surface area of the reduced sample derived from BET equation.

b

Vp: Pore volume of the reduced sample obtained from the volume of nitrogen adsorbed at the relative pressure of 0.97.

c

Estimated from the XRD diffraction peak (2θ = 44.6) using the Debye-Scherrer equation. Considering the limitation of this calculation method, the Ni crystallite size was denoted as ‘< 5.0’ when the calculated value was smaller than 5.0 nm. Meanwhile, the calculated values were also described in the brackets because of the sufficient line broadening of the Ni diffraction peaks of 10NxFOMA.

d

Ni crystallite size of 10N1FA was calculated after peak fitting at 44.6o because of the overlapping of Ni peak with alumina peak at 45.5o.

e

Catalysts recovered after 120 h lifetime test.

f

Ni dispersion calculated based on the H2-TPR and H2-TPD results.

g

Calculated based on the CO conversion at 220 oC and H2-TPD results.

3.1.2. XRD analysis Small-angle XRD patterns of the calcined samples are shown in Figure 2A, which can give further evidence of the presence of ordered mesopores structure. For 10NxFOMA samples, there is a strong peak at around 0.75o, which can be indexed as the reflection of (100) plane in the p6mm hexagonally ordered mesoporous structure.15, 16 As expected, no diffraction peak was observed over 10N1FA. Wide-angle XRD patterns of the calcined samples are displayed in Figure 2B. In order to obtain the real state of both Ni and Fe species in the samples, 10N4F was prepared (seen in Experimental 13



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section) and measured as a comparison. Only the characteristic diffraction peaks of NiO and Fe2O3 can be observed over its XRD pattern, indicating that 10N4F is the complex of NiO and Fe2O3. For 10N1FA, the peaks at 37.3, 45.5, and 67.2o are attributed to the characteristic diffraction peaks of γ-Al2O3 (JCPDS 00-004-0880), while the weak diffraction peak located at 43.3o, corresponding to (202) plane of NiO (JCPDS 01-089-3080). In contrast, there is no apparent diffraction peak of NiO, Fe2O3, and Al2O3 over the patterns of 10NxFOMA, which may be related to their poor crystallinity or high dispersion in the OMA skeleton. This observation is consistent with the results reported by Morris et al..47 Figures 2C and 2D show the wide-angle XRD patterns of the 700oC-reduced catalysts as well as the reduced 10N4F, Fe2O3 and 10FOMA were also measured as the reference. For 10N4F, three diffraction peaks at 44.1, 51.5 and 75.6o are observed, which attribute to (111), (200) and (220) planes of NiFe alloy (JCPDS 00-003-1175). Besides, Fe2O3 was reduced to metallic Fe (44.7, 65.0 and 82.3o, JCPDS 01-087-0721) after 700oC-reduction. While for 10NxFOMA, the OMA framework retains amorphous even after the high-temperature reduction, and there are three new diffraction peaks at 44.6, 51.8 and 76.6o corresponding to (111), (200) and (220) planes of metallic Ni (JCPDS 01-070-1849). It is noteworthy that there was no the occurrence of metallic Fe and the formation of NiFe alloy over 10NxFOMA, especially for 10N4FOMA, whose Ni and Fe compositions were identical with 10N4F. In other words, the Fe oxide species in 10NxFOMA cannot be reduced to metallic Fe even after the 700oC-high-tempertaure reduction. Similarly, for 10N1FA, only diffraction peaks corresponding to metallic Ni and γ-Al2O3 were observed. Wan et al.48 reported that the Fe-based catalyst incorporated with Al2O3 exhibited a strong Fe–Al2O3 interaction, in which the reduction of Fe2O3 ended at FeO phase rather than Fe. In addition, no diffraction peak for the 700oC-reduced 10FOMA is observed, which further confirms that the 14


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reduction of Fe2O3 to crystalline Fe0 is quite difficult, even the Fe2O3 loading is as high as 10 wt%. Thus, the strong interaction of Fe–Al2O3 leads to the absence of NiFe alloy over 10NxFOMA. However, it is hard to ascertain the final reduction form of the Fe oxide species by XRD analysis due to their low loadings and high dispersion on the support, and more evidences are given in the XPS analysis section. The Ni crystallite sizes of the catalysts are estimated from the XRD patterns using the Debye-Scherrer equation and listed in Table 1. The Ni crystallites in the 10NxFOMA catalysts are all < 5.0 nm, which is smaller than that in 10N1FA (5.3 nm) prepared by the traditional impregnation method. In short, the 10NxFOMA catalysts can control the Ni crystallites in small size due to the confinement effect of the ordered mesoporous structure.

(B)

(A)

α−Fe2O3

NiO

γ−Al2O3

(a)

(a)

(b)

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) (c) (d)

(c) (d)

(e)

(e)

1

2 3 4 2 Theta (degree)

5

20

40 60 2 Theta (degree)

15


80

(f)


(C)

NiFe alloy Ni

Fe γ−Al O 2 3

(D)

Intensity (a.u.)

(e)

alloy Ni

Fe γ−Al O 2 3

(d)

NiFe

(a) (b) (c)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 42

(f) (g)

(b) (c) (d)

(e)

(f)

(g) (h)

(h)

20

(a)

42


44

46 48 50 52 2 Theta (degree)

54

Figure 2. Small-angle XRD (A) and wide-angle XRD (B) patterns of the calcined samples; wide-angle XRD patterns of the 700oC-reduced samples (C) and enlarged view (D): (a) 10NOMA, (b) 10N1FOMA, (c) 10N2FOMA, (d) 10N4FOMA, (e) 10N1FA, (f) 10N4F, (g) Fe2O3, and (h) 10FOMA.

3.1.3. TEM and SEM observations The TEM images and the particle statistical distribution images of the 700oC-reduced catalysts are shown in Figure 3. The typical ordered mesostructure with regular alignment of cylindrical pores along the (100) direction is observed in Figures 3a–d, which is in good agreement with the results of N2 physisorption and small-angle XRD results. In addition, the small Ni nanoparticles (dark spots) are uniformly dispersed in the framework over the 10NxFOMA catalysts. According to the particle size statistical analysis, the average Ni particle sizes of 10NOMA, 10N1FOMA, 10N2FOMA and 10N4FOMA were 3.8±2.5, 3.5±1.5, 3.9±2.0 and 4.2±1.8 nm, respectively, which is consistent with the results of WXRD and indicates that the addition of Fe species affects the dispersion of Ni particles unobviously. At the same time, the irregular Ni particles (6.3±5.1 nm) are spotted in 10N1FA, which is larger in size compared with that of 10NxFOMA prepared by the EISA 16


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method. Also, the HRTEM image of the reduced 10N4FOMA catalyst was measured and the observed lattice spacing of ca. 0.20 nm corresponds to the Ni (111) plane. However, no lattice corresponding to Fe oxide species can be observed, because of its high dispersion in the framework of OMA or poor crystallinity. In order to obtain more information about the real state of Ni species in the catalysts, both calcined and reduced 10N2FOMA samples were characterized by TEM (Figure S1). None NiO particles can be observed over the calcined sample (Figure S1a), indicating the high dispersion of NiO species and the low contrast ratio between NiO and OMA framework. After H2 reduction, the dark spots (metallic Ni particles) are clearly observed over the catalysts (Figures S1b and c), which are mainly embedded in the skeleton rather than in the channel or on the surface (Figure S1d). The above results illustrate that the Ni species was anchored in the channels in OMA after reduction from the framework, and the confinement effect of OMA could control the Ni particles in a much narrow range with small size. To analyze the element distribution in OMA, the elemental mapping of the reduced 10N4FOMA is shown in Figure S2, which further confirms the homogeneous dispersion of Ni and Fe species over the whole OMA-based catalyst (Figures S2a–d). Moreover, the EDS measurement in Figure S2e shows clear peaks of Ni, Fe, Al, and O elements as well as their contents, implying that all elements have been successfully introduced into the ordered mesoporous 10N4FOMA catalyst with the required contents.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b)

(a)

1

2 3 4 5 6 Particle Size (nm)

(d)

(c)

2

7

3 4 5 Particle Size (nm)

(e)

3

2

3 4 5 6 Particle Size (nm)

(f)

4

4 5 6 Particle Size (nm)

Page 18 of 42

6 8 10 12 Particle Size (nm)

Figure 3. TEM images of the reduced samples: (a) 10NOMA, (b) 10N1FOMA, (c) 10N2FOMA, (d and f) 10N4FOMA, and (e) 10N1FA.

3.1.4. H2-TPR, H2-TPD and NH3-TPD analyses H2-TPR profiles of the catalysts are presented in Figures 4a–b; and to obtain insight into the redox properties of the catalysts, the bulk NiO, Fe2O3, 10N4F as well as 10FOMA are also characterized as the references. In Figure 4a, there is only a strong peak at 462 oC for the bulk NiO, corresponding to the reduction of NiO to metallic Ni.49 For the bulk Fe2O3, two H2 reduction peaks at around 462 and 682 oC are observed. The first peak at low temperature is attributed to the reduction of Fe2O3 to Fe3O4 with H2 consumption of 1.99 mmol H2 g–1, and the second peak located at 682 oC can be derived from the reduction of Fe3O4 to Fe with H2 consumption of 16.21 mmol H2 g–1, which is consistent with the result in the literature.50 As seen from the reduction profile of 10N4F, two reduction peaks at 436 and 526 oC can be observed. Apparently, the reduction peaks 18


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shift to low temperatures compared with those of pure phase NiO and Fe2O3, which is the typical characteristic of reduction of alloy.51 Meanwhile, there is only a broad peak at around 470 oC for 10FOMA (10 wt% Fe2O3) with H2 consumption of 2.4 mmol H2 g–1, which is a little larger than the stoichiometric value for the reduction of Fe2O3 to Fe3O4 (2.09 mmol g–1, 3Fe2O3 + H2→2Fe3O4 + H2O) while much smaller than that of the reduction of Fe2O3 to FeO (6.26 mmol g–1, Fe2O3 + H2→2FeO + H2O). Thus this peak may be ascribed to the partial reduction of Fe2O3 to FeOy (4/3>y>1). This finding provides the further evidence for absence of metallic Fe over the 700oC-reduced 10FOMA, which is consistent with the XRD result in Figure 2C(h). Additionally, for 10NOMA, a broad reduction peak appears at about 618 oC, assigning to the reduction of NiO species with medium strength of interaction with the support.15, 17 Compared with 10NOMA, a new weak peak appears at around 412 oC over the H2-TPR profile of 10N1FOMA, whose integral area is about 1/10 of that of 10FOMA; at the same time, for 10NxFOMA, the integral area of the peak at 412 oC increases linearly with the increase of Fe2O3 loading. Therefore, the peak at around 412 oC should be the reduction peak of Fe2O3 to low valence (> 0) Fe oxides species rather than the metallic Fe0.48 It is worth noting that both Fe and Ni species reduction peaks of 10NxFOMA shift to lower temperatures at around 412 and 585 oC compared with the corresponding peak of 10FOMA and 10NOMA, which may because the coexistence of Fe and Ni species can weaken their interaction with the OMA framework compared with the samples with single Fe or Ni component. For 10N1FA, there is a broad reduction peak in the temperature range of 350–700 oC, which is mainly assigned to the reduction of NiO species with medium and strong interactions with the γ-Al2O3 support. It is noteworthy that the peak value in 10N1FA profile (568 oC) is a little lower than that of 10NxFOMA (585 oC), which suggests that the one-pot EISA method promotes the interaction between NiO species and the OMA support. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Furthermore, the H2-TPD measurement can provide useful information on the interactions between the metallic active sites and adsorbed H2 species. Consequently, the H2-TPD characterization is preformed and the related profiles are displayed in Figure 4c. For 10NOMA, there are three H2 desorption peaks at around 175, 282 and 440 oC, respectively. The first peak located at 175 oC is derived from the chemisorbed hydrogen on the highly dispersed Ni particles, which has a large number surface defects with the high capacity of hydrogen dissociation.15 The second peak at 282 oC should be attributed to the H2 adsorbed on the surface of the bulk Ni particles, which also refer to the exposed fraction of Ni atoms.16, 52 For the last peak at 440 oC, it can be assigned to the H2 adsorbed in the subsurface layers of Ni atoms and/or support from the spillovered H2,53 which cannot be considered for the determination of monolayer coverage of chemisorbed H2. For 10NxFOMA, a significant increment in the integrated area of the three H2 desorption peaks can be observed after Fe species addition, and the integral peak areas of different catalysts decease in the order 10N1FOMA > 10N2FOMA > 10N4FOMA > 10NOMA, indicating that the catalyst with proper amounts of Fe species can have the largest H2 chemisorption, while the excess Fe species has the adverse effect. Meanwhile, there is a weak peak at around 385 oC on the profile of the 700oC-reduced 10FOMA, indicating the H2 chemisorption of FeOx species is negligible. Also, Li et al. found the Fe oxides species had no H2 adsorption capacity below 300 oC,48 so the enlarged H2 desorption peaks at 175 and 282 oC over 10NxFOMA (x=1, 2 and 4) catalysts are corresponded to the H2 adsorbed on the surface of Ni particles. As mentioned above, although the Ni particles in the 700oC-reduced 10NxFOMA catalysts are all smaller than 5.0 nm calculated by the Debye-Scherrer equation, the larger Ni particles (5–7 nm) are also observed by TEM observation. Therefore, it is reasonable that the 10NxFOMA catalysts have larger integral peak areas of H2-TPD profiles than 10NOMA owing to its relatively lower Ni dispersion. For 10N1FA, unlike 10NxFOMA, only two 20


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desorption peaks at around 132 and 490 oC are observed, which is consistent with the result in the literature.15 The integral area of the former peak at 132 oC decrease obviously in comparison with that of 10NxFOMA owing to its larger Ni particle size (3–15 nm by TEM). On the contrary, the latter peak at 490 oC has the comparable intensity with others. Since only the H2 species desorbed at relatively lower temperature can be considered for the determination of monolayer coverage of chemisorbed H2, the hydrogen uptakes and the Ni dispersions of the catalysts are calculated based on the H2-TPD profiles below 365 oC and the results are listed in Table 1. As seen from Table 1, 10N1FOMA has the highest total H2 uptake of 91.8 µmol g–1 and Ni dispersion of 13.8% among all the catalysts. In other words, 10N1FOMA can generate more surface-dissociated hydrogen, which is beneficial for the enhanced catalytic activity. In order to investigate the acidity property of the catalysts, NH3-TPD measurements are conducted and the results are shown in Figure 4d. Overall, the NH3-TPD profiles of all the 10NxFOMA catalysts look similar with two main NH3 desorption peaks located at around 262 and 406 oC. The former peak at 262 oC is ascribed to medium acid sites, while the latter one is assigned to strong acid sites.54, 55 After addition of Fe species, both the acidity and acid strength of the 10NxFOMA catalysts are all changed compared with those of 10NOMA. The integral peak areas decrease as the increase of Fe species loading (except for 10N1FOMA), indicating that their acidity is slightly decreased. On the other hand, the maximum desorption peaks of 10NxFOMA shift to higher temperatures along with the increase of Fe species loading. This may because the Fe species tends to form basic oxide particles which improve the interaction between NH3 and the catalysts, resulting in their enhanced acid strength.48 For 10N1FA, both the acidity and the acid strength increase obviously compared with those of 10NxFOMA owing to the support effect; in detail, γ-Al2O3 support itself has more medium and strong acidic sites than the amorphous OMA support. 21



However, it is unfavorable to suppress the formation of deposited coke for 10N1FA, because of the absence of basic centers for CO2 chemisorption, which can promote the elimination of coke through the reverse CO Boudouard reaction (CO2 + C → 2CO).

o

H2-TPR Signal (a.u.)

H2-TPR Signal (a.u.)

(b)

462 C

(a) NiO

o

682 C

Fe O 2

o

436 C

3

o

526 C

o

470 C o

10FOMA

618 C

o

585 C

10NOMA o

412 C

10N1FOMA 10N2FOMA o

568 C

10N4FOMA

10N4F 10N1FA

300 600 o 900 Temperature ( C)


10N1FOMA 10N4FOMA 10FOMA

(d) NH3-TPD Signal (a.u.)

10NOMA 10N2FOMA 10N1FA

(c) H2-TPD Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 42

o

282 C o

490 C o

175 C

o

440 C o 365 C

100 200 300 400 o 500 600 Temperature ( C)

10N1FA 10NOMA o

326 C

10N4FOMA 10N1FOMA 10N2FOMA o

436 C

o

262 C o

406 C

100 200 300 400 500 600 o Temperature ( C)

Figure 4. H2-TPR profiles (a and b), H2-TPD profiles (c) and NH3-TPD profiles (d) of the catalysts.

3.1.5. XPS analysis To reveal the real state of the surface element species in the 700oC-reduced catalysts, the XPS measurements were carried out and the results were shown in Figure 5. As seen in Figure 5a, the Ni0 22


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peaks of the three different reduced catalysts are at the same position (852.9 eV), suggesting that the addition of different amount of Fe species as well as the different supports do not alter the binding energy of metallic Ni.15 Meanwhile, the peaks at 856.5 and 862.2 eV assigning to the Ni2+ can also be observed in the Ni 2p3/2 spectra, which is derived from the quick oxidation of the active Ni0 in air during sample transfer.15 Moreover, it is obviously that the peak area ratio of Ni0/Ni2+ over the reduced 10N1FA is much smaller than those in the reduced 10N1FOMA and 10N4FOMA. This may because the Ni0 particles of the 10NxFOMA samples are anchored in the frameworks of the OMA, which can suppress the degree of oxidation and retain the state of Ni0. The Fe 2p3/2 XPS spectra of the reduced catalysts are shown in Figure 5b and can be fitted to two peaks. The peak with the binding energy value at around 710.9 eV is derived from to Fe species in FeO form, while that at 712.4 eV can be attributed to Fe3O4 species.51 At the same time, the integral peak area ratios of Fe3O4 to FeO are significantly larger in 10N1FOMA and 10N1FA than that in 10N4FOMA. This may because the interaction between Fe species and the OMA support decreases when the amount of Fe species increases, resulting in the higher reduction degree in 10N4FOMA. Furthermore, it is noteworthy that there is the absence of peaks corresponding to Fe0 species in the total Fe XPS spectra, suggesting no metallic Fe is formed on the surface of all the 700oC-reduced catalysts. Considering the aforementioned XRD and H2-TPR results, it is believed that the Fe species in the 700oC-reduced Ni-Fe-Al catalysts is mostly in the oxidized form FeOy

(4/3>y>1)

rather

than the Fe0 form. In addition, the surface metal atomic ratios calculated from the XPS spectra are listed in Table 2. The Ni/Al atomic ratios in 10N1FOMA and 10N4FOMA are lower than that of 10N1FA, which is consistent with the fact that the metallic Ni particles are mainly embedded in the OMA frameworks in the ordered mesoporous catalysts rather than dispersed on the surface of the support.16 23



Meanwhile, the Ni/Fe atomic ratios over the surfaces of the reduced 10N1FOMA, 10N4FOMA, and 10N1FA catalysts are 1.57, 0.82, and 1.95 respectively, which is much lower than their theoretical values, suggesting that Fe species is enriched on the surface the reduced catalyst compared with the Ni species. This unique distribution of Ni and Fe species can provide further evidence for the chemisorption capacity order of H2 and NH3 over the different catalysts in Figures 4c and 4d.

2+

Ni

Ni

0

Fe3O4

Intensity (a.u.)

Ni

2+

Fe 2p3/2

(b)

Ni 2p3/2

(a) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 42

10N1FOMA

10N4FOMA

FeO 10N1FOMA

10N4FOMA

10N1FA

10N1FA

864 860 856 852 Binding Energy (eV)

718

715 712 709 706 Binding Energy (eV)

Figure 5. Ni 2p (a) and Fe 2p (b) XPS spectra of the 700oC-reduced catalysts.

Table 2. Surface atomic ratios of the metal elements in the reduced catalysts measured by XPS Sample

Ni/Al (%)

Fe/Al (%)

Ni/Fe

10N1FOMA

2.7

1.7

1.57

10N4FOMA

4.0

4.8

0.82

10N1FA

5.9

3.0

1.95

3.2. Catalytic performances of the catalysts 3.2.1. Catalytic activity in CO methanation In order to investigate the effect of Fe species addition on the catalytic activity for CO 24


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methanation and highlight the difference of different catalysts, the experiments were carried out in the temperature range 300–500 oC at 0.1 MPa using a high weight hourly space velocity (WHSV) of 60000 mL g–1 h–1, and the results are shown in Figure 6. Also, the CO thermodynamic equilibrium data calculated by the Gibbs free energy minimization method were given in Figure 6a as the reference.56 For 10NOMA, the CO conversion increase with the increase of the reaction temperature (except for 500 oC) with the maximum of 97% at 450 oC; however, CH4 selectivity has a declined trend with the increase of temperature, and CH4 yield can reach the maximum value of 79% at 450 oC. As expected, both CO conversion and CH4 selectivity of the 10N1FOMA catalysts are obviously improved. The CO conversion can reach the equilibrium data56 above 400 oC with the maximum CO conversion and CH4 selectivity of 100 and 96% respectively at 400 oC. For 10N2FOMA and 10N4FOMA, the CO conversion is similar with that of 10N1FOMA in the total temperature range due to the similar Ni particle size and H2 uptakes. Unlike CO conversion, their CH4 selectivity decreases with the increase of the Fe species loading (except for 10N1FOMA) with the by-product detected by GC of CO2 as well as C2H6. CO2 may be the product of the water-gas shift reaction, CO Boudouard reaction, and the reversed methane reforming reaction;3, 56 whereas, C2H6 should be the product of Fischer-Tropsch synthesis (FTS).57 Also, the C2H6 selectivity and C2H6 yield were calculated in the temperature range of 300–400 oC, and the results were listed in Figure S3. Although both the C2H6 selectivity and C2H6 yield are very low, the C2H6 yield increases obviously with the increase of the Fe species loading especially in the range of 300–375 oC. As we know, magnetite can co-exist with other iron phases such as metallic Fe and iron carbides in the high-temperature FTS (300–350 oC),57, 58 which plays an important role in determining the overall activity and selectivity of the catalyst. As mentioned above, the real state of Fe species in the reduced 10NxFOMA is in the form of FeOy(4/3>y>1), thus it is reasonable that 10N4FOMA has C2H6 25



by-product derived from the FTS in the temperature range of 300–400 oC. In addition, there is no catalytic activity over the 10FOMA sample (not shown here), indicating that FeOy cannot be the active phase for CO methanation. In short, FeOy is an efficient promoter for the Ni-based catalyst in the CO methanation reaction, in which the CH4 selectivity is more sensitive with the amount of FeOy species compared with the CO conversion. Furthermore, 10N1FA shows the poorest performance, coinciding with its lowest H2 uptakes and Ni dispersion. This result indicates that the catalyst prepared by the one-pot EISA method is more active than that prepared through the impregnation method. At the same time, a reported 10Ni/CaTiO3 catalyst for CO methanation is selected as the reference to compare with the 10NxFOMA catalysts.42 Although, the WHSV is only 10000 mL g–1 h–1, the maximum CO conversion and CH4 selectivity of 10Ni/CaTiO3 is 100% and 85% respectively at 400 oC. Meanwhile, a commercial methanation catalyst CMC is selected as the reference to compare with the 10NxFOMA catalysts. It can be seen from Figure 6, CMC is less active than all the 10NxFOMA (x=1, 2 and 4) catalysts, although the former has the much higher Ni loading of 18 wt%. Apparently, 10N1FOMA shows the higher catalytic activity than the catalyst with similar NiO loading in the literature45 as well as other 10NxFOMA in this work, indicating that it is a promising catalyst for CO methanation.

80

CH4 Selectivity (%)

80

80

60 CMC Thermodynamics 10N1FA 10N4FOMA 10N2FOMA 10N1FOMA 10NOMA

40 20

-1

0

100

100

300

350 400 450 o Temperature ( C)

60

CMC 10N1FA 10N4FOMA 10N2FOMA 10N1FOMA 10NOMA

60 40

-1

0.1 MPa, 60000 mL g h

500

20

CH4 Yield (%)

100 CO Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

0.1 MPa, 60000 mL g-1 h-1

0.1 MPa, 60000 mL g-1 h-1

300

CMC 10N1FA 10N4FOMA 10N2FOMA 10N1FOMA 10NOMA

40

500

0

300


500

Figure 6. Catalytic properties of the catalysts at 0.1 MPa, 60000 mL g–1 h–1: (a) CO conversion, (b) 26


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CH4 selectivity, and (c) CH4 yield.

Additionally, the TOFCO values of the catalysts were calculated based on the results of CO conversion at 220 oC as well as the corresponding H2-TPD results and listed in Table 1. It can be seen that 10N1FOMA has the highest TOFCO value, while the excess FeOx species has no significant effect on their TOFCO values for 10N2FOMA and 10N4FOMA. The hydrogenation of *CHx species has been identified to be the rate-controlling step in the CO methanation reaction15. The fast removal of these surface CHx species by surface dissociated hydrogen is crucial for promotion of catalytic activity. Therefore the high H2 uptakes and Ni dispersion of 10NxFOMA (x=1, 2 and 4) catalysts results in their high TOFCO values. This finding further confirms that the appropriate amount of FeOy species can significantly improve the catalytic performance of 10NxFOMA for CO methanation.

3.2.2 Stability of the catalysts To investigate the catalytic stability of the best catalyst 10N1FOMA, especially for the coke and sintering resistance of the catalyst, a lifetime test was carried out under harsh reaction conditions at 550 oC, 0.1 MPa using different WHSVs of 60000 and 120000 mL g–1 h–1 for 120 h. Meanwhile, the Ni-Fe bimetallic catalysts have shown higher stability compared with the monometallic Ni or Fe based catalyst in open reported literature,27 hence only 10N1FA was also tested as the reference, and the related results are shown in Figure 7. For 10N1FA, the CO conversion remains stable in the first 30 h using 60000 mL g–1 h–1, but the slight decline is observed during the next 90 h using 120000 mL g–1 h–1. In contrast, 10N1FOMA exhibits better long-term stability than 10N1FA with highly stable CO conversion during the whole lifetime test. Although the CH4 selectivity of the both 27



catalysts can be almost constant in the whole test, 10N1FOMA exhibits much higher CH4 yield than 10N1FA after 120 h. Apparently, 10N1FOMA can show a better stability at high temperature and WHSVs, which is crucial for the industrial process.

(a)

(b)

-1

-1

60000 mL g h

-1

-1

120000 mL g h

70

65

10N1FA 10N1FOMA

65

0

30

60 90 Time (h)

60

120

-1

60000 mL g h

70

75

60

60 -1

-1

CH4 Yield (%)

80

(c)

65

75 CH4 Selectivity (%)

85 CO Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1

120000 mL g h

10N1FA 10N1FOMA

0

30

60 90 Time (h)

55

-1

-1

-1

120000 mL g h

50 10N1FA 10N1FOMA

45

120

-1

60000 mL g h

40

0

30

60 90 Time (h)

120

Figure 7. Lifetime test of 10N1FA and 10N1FOMA at 550 oC and 0.1 MPa: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

3.3. Characterization of the spent catalysts 3.3.1. Deposited carbon analysis Deactivation of supported Ni catalysts by deposited carbon is a problem of serious magnitude in CO methanation reaction.6 As seen in Figure 7, the loss of catalytic activity of the 10N1FA catalyst can occur rapidly (within 120 h), thus understanding the formation of deposited carbon and suppression of these effects are of great importance. In this work, various characterizations such as SEM, TEM, TG, XRD and CO2-TPO/O2-TPO were used to analyze the coke on the spent catalysts. First of all, SEM images of the spent 10N1FOMA and 10N1FA catalysts are shown in Figures 8a–b. There is no carbon filament on the surface of the spent 10N1FOMA (Figure 8a), on the contrary, some filamentous carbon can be observed over the spent 10N1FA catalyst (Figure 8b). Similarly with SEM observation, carbon filaments can be clearly figured out only in the TEM image of the 28


Page 29 of 42

spent 10N1FA catalyst (Figure 8d) rather than that of the spent 10N1FOMA catalyst (Figure 8c). Then, the amount of deposited carbon on the spent catalysts is further measured by TG analysis following the method in the literatures.15, 46 As seen from Figure 8e, the carbon content on the spent 10N1FOMA and 10N1FA is estimated to be 1.1 and 3.0 wt%, respectively. Although, the total deposited carbon amount of both catalysts is not huge, while the spent 10N1FA has three times as much coke as the spent 10N1FOMA. Moreover, the graphitic and amorphous carbons in the spent catalysts can be estimated by XRD analysis. However, no new diffraction peak attributing to graphitic carbon in the XRD patterns is observed (Figure 8f), indicating that the deposited carbon is amorphous form both catalysts, or the amount of graphitic carbon is below the detection limit of XRD.30, 43

(b)

(d)

102

(c)

(e)

100 3.0% 10N1FA-spent 10N1FA-reduced 10N1FOMA-spent 10N1FOMA-reduced

98

0

(f)

1.1%


Intensity (a.u.)

(a)

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ni

γ−Al2O3

10N1FOMA-spent

10N1FA-spent

20


Figure 8. SEM images of the spent catalysts: (a) 10N1FOMA and (b) 10N1FA, TEM images of the 29



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spent catalysts: (c) 10N1FOMA and (d) 10N1FA, TG curves of the reduced and spent catalysts in air (e), and XRD patterns of the spent catalysts (f).

In order to further investigate the deposited carbon types, the direct O2-TPO and sequential CO2-TPO/O2-TPO measurements were carried out on the spent catalysts in the temperature range of 200–900 oC due to the relative high stability of deposited carbon in CO2 or O2 flow,43 and the related profiles are shown in Figure 9. It is seen from Figure 9a that there is a strong asymmetric peak in the direct O2-TPO profile of the spent 10N1FA owing to the peak overlap of both filamentous carbon and amorphous carbon.43 In contrast, a negligible peak is observed in the direct O2-TPO profile as well as the sequential CO2-TPO/O2-TPO profiles of the spent 10N1FOMA, because its trace amount of deposited carbon is below the detection limit of TPO analysis. As seen from Figure 9b, the CO2-oxidizable deposited carbon with two peaks at 650 and 750 oC (shoulder peaks) could be clearly detected in CO2-TPO profiles of the spent 10N1FA catalyst. Chen et al.43 reported that the filamentous carbon could be gasified by CO2 especially for that with Ni particles, which can play an important part in catalyzing this carbon elimination reaction. Combining with the above SEM and TEM images (Figures 8a‒d), the carbon in Figure 9b should be assigned to the carbon filaments. At the same time, unlike the asymmetric peaks in Figures 9a–b, nearly symmetrical peak was presented in the subsequent O2-TPO profile after the above CO2-TPO measurements of the spent 10N1FA catalyst (Figure 9c). Some authors found that the amorphous carbon on the nickel sites or the carbon encapsulating Ni crystallite could not be removed by oxidation with CO2 below 800 oC,43, 59 which is the main reason for the coke deactivation. Hence, the carbon detected by sequential O2-TPO in Figure 9c is the amorphous carbon encapsulating Ni crystallite or deposited on the support, which cannot be oxidized by CO2 even at 900 oC. 30


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Furthermore, the amorphous carbon is around 23 wt% of the total deposited carbon calculated by their integral peak areas in Figures 9a and c. It can be rationally inferred that the amorphous carbon in Figure 9c is one of the reason for the deactivation of 10N1FA in the 120h-lifetime test.

(b)

(a)

(c)

o

660 C

10N1FA

200


10N1FOMA

o

750 C

o

650 C

10N1FOMA

o

604 C

10N1FA

10N1FA

200

Intensity (a.u.)

757 C

10N1FOMA

Intensity (a.u.)

o

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



200


Figure 9. Direct O2-TPO (a) and sequential CO2-TPO/O2-TPO (b and c) profiles of the spent catalysts (Note: The Y axis scale of (a) is the same with that of (c)).

In all, combined with the results of SEM (Figures 8a and b), TEM (Figures 8c and d), TG (Figure 8e), XRD (Figure 8f) and TPO (Figure 9), it can be concluded that 10N1FOMA shows superior anti-coking property compared with 10N1FA, which can be attributed to the following reasons. First, the confinement effect of the mesopore channels of 10N1FOMA, in detail the small pore size and strong interaction between Ni and the support, can inhibit the growth of carbon filament and lifting the Ni particle from the frameworks;16 Second, the acidity of 10N1FOMA is much weaker than that of 10N1FA, which is beneficial for the elimination of deposited carbon through the reverse CO Boudouard reaction;60 Finally, the Ni particles size is very small (< 5.0 nm) after addition of Fe species in 10N1FOMA, and the rate of accumulation of coke on the surface of Ni 31



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particles can be significantly reduced.19, 61

3.3.2. Ni particle sintering analysis Besides coking issue, metal nanoparticle and/or the support sintering is another reason for the deactivation of the supported metal catalysts. In this work, TEM observation and XRD analysis were used to analyze the Ni sintering on the spent catalysts. TEM images of the spent 10N1FOMA and 10N1FA catalysts are shown in Figures 8c–d. For the spent 10N1FOMA, the ordered mesostructure remains unchanged, and the highly dispersed Ni nanoparticles are embedded in the cylindrical channels with the particle size of 3‒6 nm (Figure 8c). However, some Ni particles as large as 22 nm in size can also be identified in the spent 10N1FA catalyst, indicating occurrence of the Ni sintering in this catalyst during the lifetime test (Figure 8d). Additionally, based on the XRD results in Figure 8f, the calculated average Ni crystallite size of the spent 10N1FOMA and 10N1FA catalysts is estimated to be < 5.0 and 7.8 nm, respectively, indicating that the sintering degree of Ni particle in the ordered mesoporous catalyst is much less severe than that in the 10N1FA catalyst prepared by impregnation method owing to the confinement effect of the ordered mesostructure.

4. CONCLUSIONS Ordered mesoporous NiO-Fe2O3-Al2O3 ternary oxide composites have been synthesized as the novel catalysts for CO methanation via the one-pot EISA method. The ordered mesoporous structure can be well remained after addition of 10 wt% NiO and 1 to 4 wt% Fe2O3 species. Both CO conversion and CH4 selectivity can be enhanced after addition of a proper amount of Fe species into the ordered mesoporous Ni-Al catalyst due to increased Ni dispersion and H2 uptake as well as the synergistic effect between Fe and Ni species, while excess Fe species is adverse for the 32


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enhancement of CH4 selectivity especially in the temperature range of 300–400 oC. 10N1FOMA shows the highest CO conversion and CH4 selectivity among all the ordered and unordered mesoporous catalysts. It is found that the Fe species is in the state of FeOy (4/3>y>0) over the 700oC-reduced Ni-Fe-Al catalysts, which is an efficient promoter for the Ni-based catalyst in the CO methanation reaction, and also the active species for Fischer-Tropsch synthesis. This results in poor CH4 selectivity over the Ni-Fe-Al catalysts with high Fe species content due to enhanced Fischer-Tropsch synthesis. In a 120h-lifetime test, two Ni-Fe-Al catalysts with the identical component and different structure were evaluated. The ordered mesoporous 10N1FOMA catalyst exhibits better long-term stability in comparison with the unordered mesoporous 10N1FA catalyst prepared by impregnation method. 10N1FOMA shows superior anti-sintering and anti-coking properties, which is related to the confinement effect of the mesoporous channels, the weak acidity of the OMA support and the small Ni particle size (< 5.0 nm). As analyzed, both Ni sintering and deposited carbon with amorphous form are the main reasons for deactivation of 10N1FA. In short, this work demonstrates that 10N1FOMA is a promising catalyst for CO methanation, which can simultaneously inhibit the Ni sintering and coke formation while maintain high catalytic activity.

ASSOCIATED CONTENT Supporting Information Further details are given in Figures S1–3 and Table S1.

AUTHOR INFORMATION Corresponding Author *Tel.: +86-10-82544850; Fax: +86-10-82544851. E-mail addresses: [email protected] & 33



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[email protected] (Q. Liu); [email protected] (F. Gu); [email protected] (F. Su)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the supports from the National Natural Science Foundation of China (Nos. 21476238 and 21606146), the National Basic Research Program (No 2014CB744306), the National High Technology Research and Development Program 863 (No. SS2015AA050502), Natural Science Foundation of Shandong Province (No. 2016ZRB01037), ‘Strategic Priority Research Program’ of the Chinese Academy of Sciences (Nos. XDA07010100 and XDA07010200), the Open Research Fund of State Key Laboratory of Multiphase Complex Systems (No. MPCS-2014-D-03), the Fund of State Key Laboratory of Multiphase complex systems (No. MPCS-2015-A-06), Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (No. 2016RCJJ005), and China National Coal Association Science and Technology Research Program (No. MTKJ2016-266).

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