Mo-based catalyst supported on binary ceria-lanthanum solid solution

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Kinetics, Catalysis, and Reaction Engineering

Mo-based catalyst supported on binary ceria-lanthanum solid solution for sulfur-resistant methanation: effect of La dopant Jiaming Cheng, Yan Xu, Zhaopeng Liu, Zhenhua Li, Baowei Wang, Yujun Zhao, and Xinbin Ma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04296 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Industrial & Engineering Chemistry Research

Mo-based catalyst supported on binary ceria-lanthanum solid solution for sulfur-resistant methanation: effect of La dopant

Jiaming Cheng, Yan Xu, Zhaopeng Liu, Zhenhua Li, Baowei Wang, Yujun Zhao,* Xinbin Ma

Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.

Corresponding Author *E-mail: [email protected]; Tel: +86-022-87401818

1

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Industrial & Engineering Chemistry Research 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

ABSTRACT: A series of Mo-based catalysts supported on Ce1-xLaxO2-δ solid solution were prepared and performed in sulfur-resistant methanation. Characterization results suggested the formation of Ce1-xLaxO2-δ solid solution with a cubic fluorite structure. The correlation between the surface concentration of Ce3+ and oxygen mobility was well revealed. The higher oxygen storage capacity induced by La dopant enhanced the reducibility of the MoO3/Ce1-xLaxO2-δ during the sulfidation. Meanwhile, the presence of oxygen vacancy induced by Ce3+ prompted the sulfidation of Mo species, resulting in a higher dispersion of MoS2. The coupling between the CO hydrogenation and water-gas-shift (WGS) reaction is important for the sulfur-resistant methanation. The apparent methanation rate is easily determined by CO hydrogenation, and WGS reaction could provide more H2 for the CO hydrogenation. MoO3/Ce0.8La0.2O2-δ catalyst exhibited the highest activity in a broad temperature regime of 450-550 °C, due to its high MoS2 dispersion and surface concentration of Ce3+.

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1. INTRODUCTION In response to national environmental policies, the development of clean energy synthetic natural gas (SNG) has attracted wide attention.1 Methanation technology is one of the essential method for producing SNG from syngas.2 Due to high CO conversion and excellent CH4 selectivity, Ni catalyst was regarded as an excellent methanation catalyst.3-5 However, it needs a desulfurization process in case of sulfur poisoning. To simplify the process, a sulfur-resistant methanation process on basis of MoS2 catalyst has been proposed. It is widely considered that the sulfur-resistant methanation on the MoS2 catalyst occurs as follows: CO + 3H2 → CH4 + H2O

ѳ Δ𝐻298𝑘 = −206.1 kJ/mol

(1)

CO + H2O ⇋ CO2 + H2

ѳ Δ𝐻298𝑘 = −41.2 kJ/mol

(2)

It is well known that support has the function of increasing the dispersion and stability of active components. CeO2 has been often used in various catalytic system as support.6, 7 Owing to Ce4+/Ce3+ redox couple,8-10 CeO2-based catalyst has excellent oxygen storage capacity (OSC) as well as redox capacity. But small surface area and weak resistance to high temperature may limit its application. Hence, modification of ceria oxide has attracted much attention. In our previous research,11 the effect of cubic and tetragonal CexZr1-xO2 solid solution support on activity of Mo catalyst in the sulfurresistant methanation has been studied. The cubic Ce0.8Zr0.2O2 solid solution supported Mo catalyst exhibited excellent catalytic performance at reaction temperature ~450 °C because of well dispersed active MoS2 species. Meanwhile, the higher OSC of the Ce0.8Zr0.2O2 was also a main reason for the good activity of the MoO3/Ce0.8Zr0.2O2 3



catalyst. As reported by Kalamaras et al.,12 lanthanum as a homologous element is beneficial to modify the physico-chemical properties of mixed oxides. It was well established that ceria-lanthanum solid solution could increase dispersion of ceria so as to reducibility of catalyst.13 Benjaram et al.14 clearly insighted into the formation of ceria-lanthanum solid solution. They found that one oxygen vacancy is introduced into the lattice when every two La3+ ions substitute two Ce4+ ions, promoting oxygen mobility in bulk catalyst. Therefore, a higher OSC of the solid solution support is ensured by the introduction of La species. Furthermore, La has a positive effect on increasing surface area and decreasing crystallite size of catalyst. Besides, La could effectively inhibit the sintering of CeO2 under high temperature condition (1100 °C),15 which strengthens the stability of catalyst. Hence, it is expected that the Ce1-xLaxO2-δ solid solution could be an advantageous support in sulfur-resistant methanation, which has never mentioned before. In this paper, ceria-lanthanum solid solutions were prepared by co-precipitation and then impregnated with 10 wt.% MoO3. All the catalysts were examined in sulfurresistant methanation and the effect of La species was discussed carefully.

2. EXPERIMENTAL SECTION Catalyst Preparation. A series of Ce1-xLaxO2-δ (x=0, 0.2, 0.5, 0.8 and 1.0, δ=x/2) supports was synthesized by co-precipitation method, as was reported previously.16 For Ce0.8La0.2O2-δ, the following is the preparation procedure: Add Ce(NO)3∙6H2O (99 wt.% 4


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Kermel) and La(NO)3∙6H2O (99 wt.% Kermel) (Ce/La=4:1, mole ratio) into deionized water to make 1 M mixed salt solution, and an aqueous solution of KOH (95 wt.% Aladdin) was employed as the precipitant. Drop the mixed salt solution to KOH aqueous solution until the pH reached ~10. Subsequently, the mixture was heated to 80 °C with stirring and lasted for 6 h. A white suspension was acquired, and the solids were washed and filtered to remove potassium impurity. Then it was dried 24 h at 120 °C in air dry oven, and finally heated to 600 °C with 5 °C/min to calcinate 4 h in muffle furnace. The as-prepared Ce0.8La0.2O2-δ support was impregnated with a certain amount of ammonium heptamolybdate solution to form 10 wt.% MoO3/ Ce0.8La0.2O2-δ catalyst. The obtained catalyst was dried overnight at room temperature, air-dried 24 h at 120 °C, and finally calcined at 600 °C for 4 h (heating rate was 5 °C/min). The preparation of other catalysts followed the same procedure. Catalyst Characterization. The X-ray diffraction (XRD) data were collected on D/MAX-2500 X-ray diffraction meter (Rigaku, Japan). It uses Ni-filtered Cu-Kα radiation (λ=1.54056 nm). The 2θ range of diffraction patterns were recorded at 5°-90° with 5 °/min. The crystalline phases could be determined according to data from Joint Committee on Powder Diffraction Standards (JCPDS). The average crystallite sizes of catalysts could be calculated using FWHM from the Scherrer equation. The Raman spectroscopy (RS) result was obtained on inVia reflex (Renishaw, England) Raman spectrometer. The Ar ion laser (532 nm) was used as excitation light source, and 6 mV laser beam was focused on the catalysts. 5



N2-physisorption measurement of catalysts were performed at -196 °C on Tristar3000 apparatus. Before analysis, the catalysts were put under vacuum to degas 4 h at 300 °C. The specific surface area of catalysts was determined by Brunauer-EmmettTeller method, and the pore size of the catalysts was calculated using Barrett-JoynerHalenda method. Temperature-programmed reduction (TPR) patterns of catalysts were recorded on AutoChem 2910 apparatus (Micromeritics, America). To remove trace of water, about 100 mg sample was blown with argon for 2 h under 200 °C. Subsequently it was cooled down to 60 °C and heated to 800 °C in a flowing mixed gas (30 ml/min, 10% H2 in Ar) at 10 °C/min. A TCD detector was used to monitor the consumption of hydrogen. X-ray photoelectron spectroscopy (XPS) was recorded on Thermo (ESCALAB 250Xi) spectrometer with Al-Kα (1486.6 eV) radiation. The binding energies were calibrated by C1s at 284.8 eV. The morphology of catalysts was detected using Tecnai G2 F20 transmission electron microscope (TEM) (FEI, Holland). The maximum resolution could reach to 0.15 nm/200 kV. The catalyst was dispersed in ethanol and then deposited on ultra-thin carbon film. Evaluation of Catalytic Activity. A continuous-flow fixed-bed reactor was used to evaluate the catalytic performance. The details of the reactor tube are as follows: internal diameter = 12 mm, length = 700 mm.17 Three thermocouples were set at upper, medium and lower regime of furnace to control the temperature of reactor. One thermocouple was set inside the reactor to monitor the reaction temperature of the 6


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catalyst bed. Before the evaluation, about 3 mL catalyst was packed between two layers of quartz fibers inside the reactor, and then sulfided at 300 °C for 4 h in a 100 mL/min flow of 3 vol.% H2S/H2 gas mixture. The reaction conditions as follows: CO/H2 =1.0, 3 MPa, 450 °C or 550 °C. The composition of the feeding gas is: 50 vol.% CO =125 mL/min, 0.6 vol.% H2S/H2 =125 mL/min. The catalytic activity of all the catalysts were performed at the same gas hourly space velocity (GHSV) of about 5000 mL/mL Cat.·h, simply named as 5000 h-1. An online gas chromatograph was used to analyze the composition of outlet gases. The conversion of CO and selectivity for products were obtained by the formulas as follows: Xco =

n(COin )−n(COout ) n(COin )

SCO2 =

× 100%

n(CO2 out)

(3)

× 100%

(4)

n(CH4 out) in )−n(COout )

× 100%

(5)

2n(C2 H6 out) in )−n(COout )

× 100%

(6)

3n(C3 H8 out) in )−n(COout )

× 100%

(7)

n(COin )−n(COout )

SCH4 = n(CO SC2 H6 = n(CO SC3 H8 = n(CO

Where Xco , SCO2 , SCH4 , SC2H6 , SC3 H8 refer to CO conversion, CO2 selectivity, CH4 selectivity, C2H6 selectivity and C3H8 selectivity, and n refers to the mole flow rate.

3. RESULTS AND DISCUSSION XRD. The XRD profiles obtained for 10% MoO3/CeO2 and 10% MoO3/Ce1-xLaxO2δ

are shown in Figure 1. Clearly, all the catalysts have no characteristic peaks of Mo

species, meaning that MoO3 is well dispersed on the prepared catalysts.18 As is shown, the standard characteristic peaks of CeO2 with cubic fluorite structure (JCPDS No. 437



1002) is prominent in MoO3/CeO2 catalyst, and the 2θ values of diffraction peaks are 28.5, 33.1, 47.5, 56.3, 59.1, 69.4, 76.7, 79.1 and 88.4. The introduction of La leads to an obvious shift of the diffraction peaks of CeO2 toward lower angles. Further, no any peak corresponding to single La2O3 phase is observed. These results indicate the formation of ceria-lanthanum solid solutions.19

Figure 1. XRD patterns of catalysts. (a) MoO3/CeO2 (b) MoO3/Ce0.8La0.2O2-δ (c) MoO3/Ce0.5La0.5O2-δ (d) MoO3/Ce0.2La0.8O2-δ

It is obvious the average crystallite size of catalysts decreases when the amount of La increases (Table 1) because of La incorporation. Meanwhile, the diffraction peaks for the cubic fluorite structure of CeO2 gradually disappear when La/(Ce+La) mole ratio is larger than 0.5. This result is consistent with earlier report, in which CeO2 can change 8


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from fluorite structure to pyrochloric structure when the ratio of La/(Ce+La) was increased to 0.5. It was believed that fluorite structure of CeO2 can be maintained up to 40% La substitution, and the structure can be stabilized by thermal treatment or doping Ti4+ ions.20

Table 1. 2θ values and average crystallite size of catalysts determined by XRD. Sample

2θ (d111)

Average crystallite size (nm)

MoO3/CeO2

28.47

17.7

MoO3/Ce0.8La0.2O2-δ

28.18

7.3


27.80

4.8


27.35

3.8

Raman. As a useful technique, Raman spectroscopy is often applied to analyze lattice defect and M-O bond in catalyst.21 To obtain more structural information, Raman spectroscopy investigation was conducted on the as-prepared catalysts. Figure 2 shows Raman spectra of MoO3/Ce1-xLaxO2-δ catalysts before reaction. The spectra of MoO3/CeO2 exhibits a sharp F2g band of fluorite structure at 465 cm-1, which is related to symmetric breathing mode of oxygen atoms around Ce4+ ions.22, 23 As stated, the F2g band shifts to lower wavenumber and becomes broad because of La incorporation. These changes can be attributed to the lattice expansion, which account for difference in ionic radius (Ce4+: 0.97 Å, La3+: 1.10 Å).24 Similarly, the cubic fluorite structure is fading while La/(Ce+La) ratio is over 0.5, which agrees well with XRD analysis. According to M.Reddy et al.,14 the cubic fluorite structure is favorable for the 9



oxygen migration due to sufficient channel size. The presence of weak peak at 560-600 cm-1 region is ascribed to oxygen vacancy formed by replacing Ce4+ with La3+, which can compensate for negative charge.25 From the intensity of the bands shown in the Figure 2, excessive amount of La results in peak vanishing. It states that a larger amount of oxygen vacancy is formed when the amount of La dopant is 20 at.%.

Figure 2. Raman patterns of catalysts before reaction. (a) MoO3/CeO2 (b) MoO3/Ce0.8La0.2O2-δ (c) MoO3/Ce0.5La0.5O2-δ (d) MoO3/Ce0.2La0.8O2-δ Figure 3 shows the Raman spectra of MoO3/Ce1-xLaxO2-δ catalysts after reaction. The bands observed at 226, 380 and 405 cm-1 are attributed to MoS2 species,26, 27 while the bands of 810 and 920 cm-1 are respectively ascribed to crystalline MoO3 and amorphous MoOx species.28 Compared to the unreacted samples, the obvious characteristic bands for MoS2 illustrate part of the MoOx species was sulfided during the sulfidation and 10


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reaction process. Particularly, the disappear of the bands for MoOx species of the Ladoped catalysts reacted further evidence the sulfidation of MoOx species, indicating that the doping of La species can significantly enhance MoS2 dispersion in the final Mobased catalyst. It is speculated that La has a positive effect on Mo species sulfidation.

Figure 3. Raman patterns of the spent catalysts. (a) MoO3/CeO2 (b) MoO3/Ce0.8La0.2O2-δ (c) MoO3/Ce0.5La0.5O2-δ (d) MoO3/Ce0.2La0.8O2-δ

N2-physisorption analysis. The detailed results of BET surface area and pore structure of catalysts are given in Table 2. It is obvious that specific surface area of supports and catalysts increases with La incorporation, suggesting La3+ is beneficial to specific surface area of Ce1-xLaxO2-δ support and the corresponding MoO3/Ce1-xLaxO2δ

catalyst. For the Ce1-xLaxO2-δ support, a suitable amount of La can enhance the

formation of ceria-lanthanum solid solution, thus the aggregation of the support was 11



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inhibited and higher specific surface area could be achieved. However, excess amount of La in the binary support can only result in a big decrease in specific surface area, since no more solid solution of ceria-lanthanum can be formed. It is often believed that pore volume and specific surface area of catalyst are lower than corresponding support, resulting from the blocking effect of the MoO3 on the pores of the support during the loading process.29 But while the ratio of La/(Ce+La) is up to 0.5, the specific surface area of final catalyst was increased by the loading of MoO3, which is higher than their supports. We speculate that MoO3 could have certain interaction with La2O3 or Ce1xLaxO2-δ

solid solution, which may inhibit support aggregation during the high-

temperature calcination, leading to a small increase in their specific surface area in comparison with their origin support.

Table 2. Structural property of the catalysts. SBET(m2/g)

Pore volume(cm3/g)

Pore size (nm)

Sample Support Catalyst

Support

Catalyst

Support Catalyst

MoO3/CeO2

52.2

20.8

0.16

0.15

10.8

22.1


89.3

84.0

0.19

0.18

6.9

7.2


81.6

89.1

0.23

0.25

9.0

11.2


55.9

60.9

0.17

0.27

8.8

13.7

H2-TPR. The results of H2-TPR for MoO3/Ce1-xLaxO2-δ catalysts are presented in Figure 4. For MoO3/CeO2 catalyst, two peaks are seen in temperature regions: 350560 °C and 560-760 °C. The areas of the reduction peaks at 350-560 °C for Mo/Ce0.8La0.2O2-δ and Mo/Ce0.2La0.8O2-δ catalysts were integrated respectively. The 12


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results are as follows: S(Ce/La=4:1)= 4.3516, S(Ce/La=1:4)= 0.9254. Obviously, the peak area of Mo/Ce0.8La0.2O2-δ catalyst is more than four times that of Mo/Ce0.2La0.8O2-δ catalyst, which is nearly the same with the ratio of Ce species between the two catalysts. It suggests that the peak is mainly for the reduction of Ce4+. According to reference,30 the reduction temperature of surface Mo species is 450 °C, which is close to that of Ce4+. Therefore, it is difficult to identify which is the reduction of Mo species from the broad peak in TPR patterns. Thus, the main reduction peak (350-560 °C) should be attributed to the reduction of both the surface Ce4+ to Ce3+ and well dispersed Mo6+ to Mo4+ species,31 and another peak at 560-760 °C could be due to the reduction of crystalline MoO3.17 It is clear that main reduction peak exhibits a lower temperature when the amount of La3+ increases, indicating that La3+ could promote the reduction rate of CeO2 because of the formation of ceria-lanthanum solid solution, where the oxygen mobility is enhanced. However, the peak becomes flat gradually when the La3+ molar ratio is larger than 0.5, which suggests excessive La3+ could not form more ceria-lanthanum solid solution. Therefore, an appropriate amount of La3+ could benefit the reducibility of catalyst.

13



Figure 4. H2-TPR profiles of the catalysts. (a) MoO3/CeO2 (b) MoO3/Ce0.8La0.2O2-δ (c) MoO3/Ce0.5La0.5O2-δ (d) MoO3/Ce0.2La0.8O2-δ

XPS. XPS measurement is carried out to determine the surface elemental composition on all spent catalysts. O 1s spectra. The O 1s spectra shown in Figure 5 presents three peaks: the lowest binding energy peak at 529.8 eV is attributed to lattice oxygen (Oα); the peak at 532 eV is assigned to hydroxyl groups, carbonates and surface adsorbed oxygen (Oβ); and the peak observed at 533.8 eV is related to adsorbed molecular water (Oγ).32 Clearly, the peak area of Oα or Oβ is much larger than Oγ, indicating that lattice oxygen and surface adsorbed oxygen are crucial in sulfur-resistant methanation, which coincides with previous study.33 As listed in Table 3, when the amount of La increases, the distribution proportion of lattice oxygen decreases accompanied by the increase of adsorbed oxygen. Chen et al.

34

approved that surface oxygen has greater mobility than lattice oxygen, 14


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which correlates with higher activity in phenol catalytic wet oxidation. The solid solution with cubic fluorite structure was formed by introduction of La, which could enhance the mobility of adsorbed oxygen by providing enough channel size. However, the solid solution with cubic fluorite structure could be difficult formed when excess amount of La was introduced in the binary support, limiting the mobilization of oxygen.

Figure 5. O 1s spectra of the catalysts. (a) MoO3/CeO2 (b) MoO3/Ce0.8La0.2O2-δ (c) MoO3/Ce0.5La0.5O2-δ (d) MoO3/Ce0.2La0.8O2-δ

Ce 3d spectra. As presented in Figure 6, the Ce 3d spectra of catalysts could be deconvoluted into ten peaks.10, 35 The Ce 3d5/2 and Ce 3d3/2 are labeled as v and u, respectively. Six peaks observed as v (882.5 eV), v’’ (888.9 eV), v’’’ (898.2 eV), u (901.1 eV), u’’ (908.1 eV) and u’’’ (916.2 eV) are assigned to Ce4+, while four peaks denoted as v0 (880.6 eV), v’ (884.5 eV), u0 (899.3 eV) and u’ (904.1 eV) are attributed to Ce3+. 15



To quantify distribution of element in different valence states, the Ce3+ distribution is estimated by Ce3+/(Ce4++Ce3+)×100%, where Ce4+= (v + v’’+ v’’’ + u + u’’ + u’’’) and Ce3+= (v’ + v0 + u’ + u0).36 As shown in Table 3, when the amount of La is 20 at.%, the Ce3+ content is the highest among all the catalysts. But when the amount of La is larger than 50 at.%, the Ce3+ content decreases. It is widely accepted that formation of oxygen vacancies on the catalyst was ensured by the presence of Ce3+ ions.37-39 Therefore, appropriate Ce/La ratio benefit the generation of oxygen vacancies and mobility of oxygen species.

Figure 6. Ce 3d spectra of the catalysts. (a) MoO3/CeO2 (b) MoO3/Ce0.8La0.2O2-δ (c) MoO3/Ce0.5La0.5O2-δ (d) MoO3/Ce0.2La0.8O2-δ

Mo 3d spectra. Figure 7 displays the Mo 3d spectra of catalysts. On the basis of literature,40 the binding energy at 229.2 eV and 232.1 eV are associated with Mo4+ that 16


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has much relevance to MoS2 active species. The characteristic peaks of Mo6+ are located at 232.8 eV and 236.1 eV, while Mo5+ corresponds with the peaks at 230.6 eV and 233.8 eV.41 In addition, the peak labeled 226.5 eV refers to S2- 2s. As given in Table 3, the MoO3/Ce0.8La0.2O2-δ catalyst achieves the highest content of MoS2. Combined with Ce 3d spectra analysis, the MoO3/Ce0.8La0.2O2-δ catalyst with high OSC and good reducibility improves the Mo dispersion as well as the MoS2 concentration.

Table 3. The element distribution of the catalysts by XPS analysis.

O species (%)

Mo species (%) Ce3+ species (%)a

Sample Oα

Oβ

MoS2

MoOxSy

MoOx

(Mo4+)

(Mo5+)

(Mo6+)

Oγ

MoO3/CeO2

71.7

25.7

2.6

40.1

47.4

6.5

46.1


63.8

34.2

2.0

48.4

58.5

6.6

34.9


45.6

50.2

4.2

34.0

45.9

7.9

46.2


10.0

86.4

3.6

30.6

42.5

6.9

50.6

a

Ce3+/(Ce4++Ce3+)×100%, Ce4+= (v+ v’’+ v’’’+ u+ u’’+ u’’’), Ce3+= (v’ + v0 + u’ + u0).

17



Figure 7. Mo 3d spectra of the catalysts. (a) MoO3/CeO2 (b) MoO3/Ce0.8La0.2O2-δ (c) MoO3/Ce0.5La0.5O2-δ (d) MoO3/Ce0.2La0.8O2-δ

TEM. To further explore the morphology and dispersion of MoS2 active species, transmission electronic microscopy with high resolution was used. As shown in Figure 8, the layered structure of MoS2 could be clearly observed on all spent catalysts (see the red oval). For MoO3/Ce0.8La0.2O2-δ catalyst, the MoS2 slabs are shorter and more disordered compared to the other catalysts, indicating a higher MoS2 dispersion. As reported in literatures,42-44 Mo edge sites is related to the high catalytic activity of MoS2. The characterization results showed the as-prepared Mo-based catalyst supported on ceria-lanthanum solid solution with Ce/La = 4 exhibited high oxygen vacancy concentration and well dispersed MoS2 species. It was deduced that oxygen vacancy 18


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has positive effect on MoS2 dispersion, which is in accordance with Raman analysis and XPS results.

Figure 8. TEM image of the spent catalysts. (a) MoO3/CeO2 (b) MoO3/Ce0.8La0.2O2-δ (c) MoO3/Ce0.5La0.5O2-δ (d) MoO3/Ce0.2La0.8O2-δ Evaluation of Catalytic Activity. Figure 9 displays the performance of catalysts in sulfur-resistant methanation of CO, and Table 4 presents the detailed CO conversion and selectivity of products. It is clearly that all the reactions are far away from their thermodynamic equilibriums. As is well known, the sulfur-resistant methanation 19



process mainly include two reactions: CO hydrogenation and water-gas-shift (WGS) reaction, in which WGS plays a key role in the total conversion of CO since it can generate H2 necessary for the CO hydrogenation. Therefore, a matching reaction rate between WGS and CO hydrogenation could enhance the total conversion and the formation of CH4. Generally, the selectivity of CH4 should be higher than those of CO2, since it is impossible for all the generated water in CO hydrogenation step to react with CO to form CO2 and H2 because of the equilibrium effect. However, as listed in Table 4, when the reaction was performed at 450 °C, some of the CO2 selectivity is nearly the same as CH4 selectivity. The side reactions such as the formation of C2H6 and C3H8 could be the reasons, since they can produce additional water for WGS reaction, allowing the increase of the CO2 selectivity. Meanwhile, at the given conditions, the WGS nearly approach its equilibrium according to the calculations of the term [(pCO2*pH2)/(KeqWGS*pCO*pH2O)], which is close to 1 for most of the catalysts (Table 5). Obviously, the sulfur-resistant methanation should be determined by the CO hydrogenation rate. When the reaction temperature increases from 450 to 550 °C, CO conversion increases significantly for all the catalysts. Besides, the selectivity of CH 4 is nearly approach equilibrium selectivity, indicating the higher temperature could prompt the CO hydrogenation rate more than WGS reaction. In addition, for all the catalysts, the CO conversion showed slight tendency to lower values, which could be due to the aggregation and sintering of MoS2 species. According to Gao. et al.,45 the sintering of active metal under high temperature is a key factor for the stability of methanation catalyst, while carbon deposition is not the major problem in methanation, 20


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which could be alleviated by controlling reaction conditions.

Figure 9. Catalytic activity of the catalysts.

Table 4. CO conversion and products selectivity of the catalysts. React.Temp/°C

450

550

Selectivity of products/%

Sample

CO Conversion/%

CH4

CO2

C2H6

C3H8

MoO3/CeO2

48.57

45.75

46.72

7.12

0.41


57.06

46.26

46.52

6.77

0.45


43.83

47.09

46.48

5.80

0.63


34.80

48.28

44.85

6.15

0.73

MoO3/La2O3

33.80

47.18

46.47

5.69

0.66

thermodynamic equilibrium

96.76

50.49

49.45

0.09

0.002

MoO3/CeO2

63.70

51.67

45.09

3.00

0.24


72.03

50.24

45.65

4.08

0.03


62.01

51.24

45.30

3.07

0.39


56.10

50.80

45.38

3.40

0.42

MoO3/La2O3

53.17

48.96

47.34

3.11

0.59

thermodynamic equilibrium

90.54

51.71

48.24

0.08

0.003

21



Table 5. The calculation of equilibrium at temperature of 450 oC. WGS

CO hydrogenation

pCO2*pH2/( KeqWGS*pCO*pH2O)

pCH4*pH2O/(KeqMeth*pCO*pH2^3)

MoO3/CeO2

1.132

3×10-4


0.971

9×10-4


1.021

2×10-4


0.668

2×10-4

MoO3/La2O3

1.051

1×10-4

Sample

As listed in Table 4, the conversion of CO exhibits a volcano-like trend with the increment of La dopant and reaches the highest level at 20 at.% La dopant. When the reaction was performed at 450 °C, the best performance of MoO3/Ce0.8La0.2O2-δ catalyst resulted from the well dispersed MoS2 species induced by the ceria-lanthanum solid solution, which promotes CO hydrogenation so as to methanation process,46, 47 because the CO hydrogenation should be the determination step for the methanation system. At 550 °C, similarly, the MoO3/Ce0.8La0.2O2-δ catalyst exhibits the best catalytic performance of CO methanation. According to previous characterization results, appropriate amount of La species could enhance reducibility and surface concentration of Ce3+, which provide oxygen species to oxide CO via transformation between Ce3+ with Ce4+. Then oxygen species from H2O could oxidize Ce3+ again.48, 49 Thus, the WGS reaction is enhanced and it can easily reach its equilibrium at the given conditions. The generated H2 from WGS reaction can prompt the CO hydrogenation and ensure a higher conversion of CO. Specially, the MoO3/Ce0.8La0.2O2-δ catalyst exhibits excellent activity either at 450 °C or 550 °C, suggesting it can be used in a wider temperature regime. Meanwhile, both the highly exposed surface of MoS2 species and surface Ce3+ are the key factors for the 22


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sulfur-resistant methanation process on the as-prepared MoO3/Ce0.8La0.2O2-δ catalyst. Mechanism Analysis. A mechanism model of CO methanation reaction on the asprepared MoO3/Ce0.8La0.2O2-δ catalyst is proposed, the sulfur-resistant CO methanation process is considered as a coupling reaction system between CO hydrogenation and WGS reaction. And WGS reaction could provide more H2 for CO hydrogenation under insufficient H2 condition. At the given conditions, the WGS can easily approach its equilibrium due to the presence of highly exposed Ce3+ on the MoO3/Ce0.8La0.2O2-δ catalyst, which is favorable for the WGS reaction. While CO hydrogenation is the ratedetermining step of the sulfur-resistant methanation system. The MoS2 are always believed to be the active site for the CO hydrogenation,46, 50 so the MoO3/Ce0.8La0.2O2δ

catalyst with well dispersed MoS2 species exhibits the highest CO conversion.

Therefore, both high MoS2 dispersion and surface concentration of Ce3+ benefit the sulfur-resistant CO methanation. While the ceria-lanthanum solid solution could effectively enhance the dispersion of MoS2 and surface Ce3+ species, so it achieves higher catalytic activity in sulfur-resistant CO methanation.

4. CONCLUSIONS The MoO3/Ce1-xLaxO2-δ (x=0, 0.2, 0.5, 0.8 and 1.0, δ=x/2) catalysts were synthesized and applied in sulfur-resistant methanation. The characterization results indicated that a certain amount of La3+ (20 at.%) could stabilize CeO2 and enhance its reducibility as a result of more oxygen vacancy formed via ceria-lanthanum solid solution. The presence of oxygen vacancy prompted the sulfidation of Mo species by 23



achieving fast transformation between Ce3+ and Ce4+, resulting in a higher dispersion of MoS2. Besides, the ceria-lanthanum solid solution with cubic fluorite structure was also found to have smaller crystallite size and larger surface area, which results in larger concentration and better dispersion of active MoS2 for MoO3/Ce0.8La0.2O2-δ catalyst. However, excessive La3+ resulted in insufficient channel size to mobilize oxygen species and lower dispersion of MoS2. Besides, a good coupling between CO hydrogenation and WGS reaction should be a prerequisite for the higher conversion of CO in this reaction system. The methanation rate is determined by CO hydrogenation step since WGS reaction approaches equilibrium. Therefore, improving both the MoS2 dispersion and surface concentration of Ce3+ can effectively enhance the apparent activity of the catalyst in a wide temperature regime.

◼ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: +86-022-87401818

Notes The authors declare no competing financial interest.

◼ ACKNOWLEDGEMENTS The Financial supports from National High Technology Research and Development Program of China (2015AA050504) and National Natural Science Foundation of China (21576203 and 21606167) are gratefully acknowledged.

◼ NOMENCLATURE 24


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SBET = BET surface area (m2/g) pCO = partial pressure of CO (Pa) pH2 = partial pressure of H2 (Pa) pH2O = partial pressure of H2O (Pa) pCO2 = partial pressure of CO2 (Pa) pCH4 = partial pressure of CH4 (Pa) KeqWGS = equilibrium constant of WGS reaction expressed by pressure KeqMeth = equilibrium constant of CO hydrogenation expressed by pressure

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For Table of Contents Only:

Mechanism model of CO methanation.

33


Mo-based catalyst supported on binary ceria-lanthanum solid solution

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