Effect of Citric Acid on the Synthesis of CO Methanation Catalysts with

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Effect of citric acid on the synthesis of CO methanation catalysts with high activity and excellent stability Zhicheng Bian, Zhong Xin, Xin Meng, Miao Tao, Yuhao Lv, and Jia Gu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04027 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 15, 2017

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Industrial & Engineering Chemistry Research

Effect of citric acid on the synthesis of CO methanation catalysts with high activity and excellent stability Zhicheng Bian1, Zhong Xin1,2*, Xin Meng1,2, Miao Tao1, YuHao Lv1 and Jia Gu1 1

Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, 2State Key Laboratory of Chemical

Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai, People Republic of China

Abstract: Several Ni-based SBA-16 catalysts were synthesized by citric acid (CA) assisted impregnation method and applied for syngas methanation in a fix-bed reactor. Various characterization techniques (N2 physical-adsorption, powder XRD, TPR, TPD, XPS, TEM, SEM, UV-vis) were used to characterize the catalysts. The amount of citric acid has significant influence on the properties of the catalysts. A small amount of CA could improve the dispersion of nickel species and refine the particle size of the catalysts. The refined nickel particles (3-5nm) could anchor into the pores (6-7nm) of the silica, which lead to the confinement of nickel particles. The best activity was obtained on Ni/SBA-16-15%CA catalysts with 100% CO conversion and 99.9% CH4 selectivity at 350 °C, 0.1 MPa and 15, 000 mL/g/h. The catalysts prepared with CA exhibited excellent stability after calcined at high temperature, which could be attributed to the refined nickel particles anchored into the pores of the silica.

Keywords: SBA-16, citric acid, nickel, confinement, CO methanation catalysts 1. Introduction In recent years, the depletion of oil fuel and the ecology environment problems caused by industrial emission (CO2) have become increasingly serious. Especially in China, the energy structure is “rich coal, lack oil and gas” and 80% of the coal was directly converted by combusting. The direct conversion has many significant problems such as the large pollution emission, waste of coal resources and low utilization of heat energy. Therefore, development of high 1

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efficient, low carbon and clean energy is of vital importance. The coal could transform to syngas by gasification and further syngas could convert to natural gas by methanation. The synthetic natural gas (SNG) process has higher utilization heat energy (53%). In the production of SNG, the CO methanation is a key reaction: CO+3H2= CH4 +H2O, ∆H298K=－206.1kJ/mol 1,2. Owing to the strongly exothermic and carbon deposition of the reaction, the catalysts should be high active, anti-sinter and resistant to carbon deposition 2. Among the active metals (Ru

3,4

, Rb, Co 5, Fe, Pt, Ni

6-8

) for methanation, nickel-based catalysts exhibited

superiority owing to its cheap price and high activity. Nickel-based catalysts has been used in many areas such as: Fischer-Tropsch synthesis, syngas methanation and some other hydrocracking in the industry. However, the nickel-based catalysts existed many problems in the industrial application such as sintering 9, carbon deposition 10 and sulfur poison 11. As known to all, nickel based catalysts easily become deactivated at high temperature. Lots of methods have been investigated to improve the situations. Zhang et al

12

has introduced MoO3 into the catalysts and found that

the addition could effectively inhibit the agglomeration of nickel species in CO methanation reaction. The addition of ceria could also suppress the deactivation of nickel catalysts and the collapse of the SBA-16 framework 13. Hu et al

14

found that the addition of MgO could improve stability of the nickel catalysts by suppressing the carbon deposition. Although the introduction of second metal may increase the activity of the nickel-based catalysts, the bimetallic catalysts would increase the cost and cause some environment problems because of the hazardous property. Furthermore, some investigations focused on the preparation methods such as pH adjust methods

15,16

, microwave

heating 17, modifying the support 18 and vary the impregnation solvent 19. These methods may more or less improve the catalytic performance and the stability of the catalysts. However, these methods seemed to be more complexed than conventional impregnation methods and consume more energy and materials. Some methods used the poisonous solvent, which would cause pollution to the environment. Recent research 2


20-25

has selected the citric acid as the

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promotion of the catalysts and prepared highly active HDS catalysts. Citric acid, an important organic acid, is cheap and innocuous and nontoxic and widely used in industry. Citric acid would decompose at 175°C and release gases, which would not change the component of the catalysts. In this work, mesoporous material SBA-16 was used to prepare catalysts for syngas methanation. In the preparation step, citric acid was added in the impregnation system. The catalysts were characterized by physical adsorption-desorption, X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), hydrogen pulse chemisorption, transmission electron microscopy (TEM) and scanning electron microscope (SEM). The results indicated that the addition of citric acid could effectively refine the nickel particle size and anchor the particle into the pores of mesoporous material, which would increase the active nickel content surface area and inhibit the agglomeration of nickel particles.

2. Experimental 2.1 Preparation of catalysts The mesoporous molecular sieve SBA-16 silica was synthesized according to the literature

13,26

. Firstly, 2.5 g

Pluronic F127 (Sigma) and 15 g K2SO4 were dissolved in 150mL distilled water with 2.0 M HCl. After F127 and K2SO4 were completely dissolved, a certain amount of TEOS was added into the mixture. The molar ratio of TEOS, F127, HCl, K2SO4 and H2O is 1, 0.0035, 1.5, 6 and 166. After the mixture was stirred at 38 °C for 24 h, the resulting gel undertook hydrothermal treatment at 100°C for 24h. The obtained solid was filtered, washed with deionized water and dried at 80 °C for 12 h. Finally, the obtained white powder was calcined under air at 550 °C for 6h (heating ramp 1°C/min) to remove the template. Ni-based SBA-16 catalysts were prepared by citric acid assisted impregnation method. The mass of nickel content was controlled at 10 wt % and the amount of citric acid added in the catalysts was 4 wt %, 8 wt %, 15 wt %, 25 3



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wt %, respectively. The as-prepared catalysts was calcined at 500°C for 5 h. The obtained catalysts were denoted as Ni/SBA-16-x%CA, which x indicated the amount of citric acid. Also the catalyst Ni/SBA-16 (10 wt% nickel content) prepared without citric acid was selected as a reference.

2.2 Characterization The crystallographic structure of the catalysts were characterized by a Bruker advanced D8 powder X-ray diffractometer. The test conditions was selected as follows: Cu Kα, λ=1.5418 Å, 40KV, 40mA. Physical adsorption properties of the catalysts were measured at 77 K using a physisorption analyzer (ASAP 2020, Micromeritics). Prior to test, samples were degassed at 473 K for 2 h. Specific surface area (SBET) were calculated Brunauer−Emmett−Teller (BET) method, while pore volume (Vt) was obtained from nitrogen adsorption data in the relative pressure range from 0.05 to 0.20. UV-vis electronic spectra of the impregnation solutions were recorded using a UV-vis spectrophotometer (Unico UV-2102PCS). The reduction properties of the catalysts were measured by hydrogen-temperature-programmed reduction (H2-TPR) on an Auto ChemⅡ2920 instrument with a GC-4000A chromatograph thermal conductivity detector. The catalyst was heating from room temperature to 900°C with a heating rate of 10°C min−1 in a 10 vol% H2/Ar gas flow. The temperature-programmed desorption (TPD) of CO was conducted using a Micromeritics AutoChem II 2920 equipped with a thermal conductivity detector (TCD). The catalyst sample was pretreated in He for 1h at 200°C. After cooling to room temperature in Ar, 10% CO/Ar adsorption was performed at 40°C for 1h. The catalysts were then purged with 30mL/min He for 1h to remove the physical-adsorbed CO. The temperature was increased from 100 to 500°C under pure Ar flow. The dispersion of the catalysts were examined by H2 pulse chemisorption. Prior to the test, he samples was treated at 200°C in He for 1h to remove the impurities and absorbed steams. After cooling to room temperature in Ar, 10 % H2/Ar pulse adsorption was performed at 40°C for 10 times. Thermogravimetric analysis (TG) was used to measure the amounts of carbon deposited on the spent catalysts and was 4


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performed with a Netzsch Model STA 409 PC instrument under an air flow from room temperature to 800°C at a heating rate of 10°C/min.

2.3 Activity Evaluation CO methanation was performed in a continuous flow fixed-bed reactor. The catalyst loading was 0.4g and sandwiched by two quartz layer. Before the test, the catalysts were reduced in flow of pure H2 (50mL/min) at 500°C for 2 h, and then cooled down to 250°C in N2 atmosphere. After that, the reactant gases of CO/H2/N2 with a molar of 3:1:1 were introduced in the reactor. The reaction temperature were varied in the range of 250-500°C. The pressure and gas hourly space velocity (GHSV, mL (gas)/g (catalyst) · h) were selected to be 0.1 MPa and 15000, respectively. The product gases were first cooled to room temperature and examined online by using gas chromatography (Techcomp, GC7890T). A thermal conductivity detector (TCD) was used to detect he amounts of H2, N2, CH4, and CO. The flame ionization detector (FID) was used to detect the amounts of unreacted CO and byproduct CO2. The calculation formulas were described as equations (1)-(2): CO conversion: Cେ୓ (%) =

୚ిో,౟౤ ି୚ిో,౥౫౪ ୚ిో,౟౤

(1)

× 100

CH4 selectivity: Sେୌర (%) = ୚

୚ిౄర ,౥౫౪ ిో,౟౤ ି୚ిో,౥౫౪

(2)

× 100

3 Results and discussions 3.1 Characterization The influence of citric acid on the structure of SBA-16 were investigated by physical adsorption instrument. The N2 adsorption-desorption isotherms were shown in Fig. 1. It can be clearly seen that all catalysts exhibited a typical IV isotherms with a H2 hysteresis loop. The H2 hysteresis loop proved the existence of ink-bottle model in the 5



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materials 27. Comparing with the reference catalyst SBA-16, the addition of citric acid didn’t destroy the mesoporous structure of the support. The detailed textural properties of the catalysts were listed in Table 1. It can be concluded that the surface area and pore volume firstly decreased and then increased with the increasing amount of citric acid. Ni/SBA-16-25%CA exhibited a similar BET surface area and pore volume with Ni/SBA-16. The addition of CA could significantly affect the distribution of metal in the channel of the support, which would cause a difference of the physical properties of the supports. And this need to be identified in further section. In addition, the average pore size just maintained at 6.0 nm, indicating the metals didn’t block the pores of the silica.

Figure 1. N2 adsorption-desorption isotherms of the catalysts

6


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Table 1 Textural characteristics of catalysts Surface

Pore

Pore sizec

Nickel content

areaa(m2/g)

Volumeb(cm3/g)

(nm)

Surface aread(m2/g)

SBA-16

827.7

0.532

6.9

Ni/SBA-16

541.0

0.328

6.0

1.03

Ni/SBA-16-4%CA

491.7

0.308

6.1

1.48

Ni/SBA-16-8%CA

511.8

0.313

6.0

3.89

Ni/SBA-16-15%CA

520.6

0.323

5.9

4.80

Ni/SBA-16-25%CA

534.8

0.329

6.0

3.06

Catalyst

a Calculated by the BET equation. b Accumulated pore volume with P/P0 lower than 0.99 from N2 adsorption branch of the isotherm by BJH method. c Obtained from N2 adsorption isotherm by the BJH method. d Calculated by H2 pulse chemisorption. The X-ray diffraction was used to determine the mesostructure of the support and the crystalline structure of the metal. Fig.2a showed the small-angle x-ray diffraction of the prepared catalysts. SBA-16 exhibited a strong diffraction peak at 0.82° (110) and a weak intensity peak at 1.2° (200). These two diffraction peaks were the characteristic reflection of cubic Im3m space group

28,29

. After the loading of nickel species, the intensity of 1.2° decreased，

meaning that the regularity of the structure has decreased. However, this also proved that the catalysts retained the structure of the support with the addition of CA. The crystalline structure of the catalysts before and after reduction were shown in Fig.2b-c. In Fig 2b, the unreduced catalysts exhibited the diffraction peaks at 37.3°, 43.2°, 62.9°and 75.3°, which were attributed to distinct peaks of cubic NiO 6 (JCPDS card NO.73-1519). However, the intensity of NiO peaks in the catalysts Ni/SBA-16-x%CA were too weak to identify, while that can be clearly seen in Ni/SBA-16. This

7



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significant difference between the catalysts can be attributed to the different morphology of NiO. Relatively small particle size of NiO or the amorphous NiO on the support may also lead to the weak intensity of diffraction peaks 17,30,31. Fig.2c showed the XRD patterns of the catalysts reduced at 500 °C. The diffraction peaks at 44.3°, 52.5° and 72.8° were attributed to distinct peaks of crystalline Ni 32. Similar to Fig.2b, the intensity of catalysts prepared with CA were much weaker than Ni/SBA-16 without CA. It was suggested that the catalysts after reduction didn’t change the particle size or the crystal morphology of the nickel species. In the preparation of catalysts, the calcination step plays a vital role in the crystalline or morphology of the nickel species. For Ni/SBA-16, the precursor Ni (NO3)2 decomposed and form crystalline NiO species after calcination. With the addition of CA, the precursor Ni(NO3)2 and citric acid decomposed at the same time and the gases released by citric acid could hinder the NiO species to form well-organized crystalline. It can also be speculated that the gases released by citric acid could inhibit the accumulation of NiO, which could be proved by the particle size calculated by Scherrer equation. The average particle size in Ni/SBA-16 was calculated to be 23.1 nm, while the particle size of the catalysts Ni/SBA-16-x%CA were identified just 3-5 nm from TEM images.

8


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Figure 2. XRD patterns of prepared catalysts: (a) small angle of catalysts after reduction (b) wide-angle of catalysts before reduced(c) wide-angle of catalysts after reduction (d) wide-angle of catalysts after high temperature treatment Transmission electron microscope was used to reveal the dispersion and particle size of the catalysts. Fig.3 showed the TEM images of the catalysts prepared with or without citric acid. In Fig. 3a, SBA-16 exhibited an orderly three-dimensional channel, while nickel particles in Ni/SBA-16 scattered on the support irregularly (Fig.3b). For the catalyst prepared with citric acid, it was observed that the regular channel in the support was well retained (Fig.3f) and the nickel particles arranged regularly in the order of the channel of the support. The particle size distribution of the catalysts with or without citric acid were showed in Fig.3e and Fig.3h. It was apparently observed that the nickel 9



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particle size was in a non-uniform distribution from 15nm to 35 nm. In contrast, Ni particles was in a much narrower size distribution (average 3-4 nm, which is within the pore size of the SBA-16 support of 6.9 nm) for Ni/SBA-16-15%CA. This phenomenon could also prove that the nickel particle entered the pores of SBA-16 with the addition of citric acid 20.

Figure 3. TEM images of prepared catalysts: (a) SBA-16 (b) Ni/SBA-16 (c) Ni/SBA-16 after calcination (e) SBA-16-CA (f)

Ni/SBA-16-15%CA(g) Ni/SBA-16-15%CA after calcination, with nickel particle distribution for (d) Ni/SBA-16(h)

Ni/SBA-16-15%CA

In recent literatures, it was suggested that the citric acid could form metal-complex in the aqueous solution and stabilize the metal

23,33,34

. In order to investigate this influence, the UV-vis spectra was used to detect the metal-CA

complex in the aqueous solution and the results were shown in Fig. 4. For nickel nitrate, the spectra showed two sharp bands at 300 nm and 389 nm and one broad band in the 640-700 nm region. The first absorption band is assigned to the 10


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π-π* transition of un-complexed nitrate counterion. Two other bands are attributed to the two spin-allowed d-d transition of octahedrally coordinated Ni2+ ions. In all solution with or without citric acid, the same bands corresponding to octahedral nickel species Ni (H2O)2+ were observed. There was no ever band shift among the solutions. This indicated that no Ni-CA complex was formed in the aqueous solution under the situation. Tatiana E. Klimova

21

has proposed a speciation diagrams for Ni species present in aqueous solutions at different pH values in

absence and presence of citric acid. From the diagram, it was suggested that the nickel species in our work couldn’t form complex with citric acid. Therefore, nickel-CA complex could be excluded the possibility to cause the difference in crystalline and particle size.

Figure 4. UV-vis spectra of the solution of nickel nitrate with different citric acid content

The surface nickel species, reducibility and metal-support interaction can be characterized by H2-TPR technique. Fig. 5 showed the TPR profiles of different catalysts. The H2 consumption peaks appeared at 310°C and 470°C in Ni/SBA-16. The first peak was assigned to the reduction of NiO crystalline freed on the support, while the second peak at higher temperature was attributed to the reduction of NiO dispersed on the support

7,13

. Similar to Ni/SBA-16,

Ni/SBA-16-x%CA also exhibited two peaks at the same temperature. However, the proportion of the higher 11



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temperature peaks increased, which indicated more nickel species were highly dispersed on the support than Ni/SBA-16. Also, the highly dispersed NiO species possessed stronger interaction towards the support. Therefore, the addition of citric acid could effectively improve the dispersion of nickel species on the support. From the XRD and TEM results, it was concluded that citric acid could refine the nickel particle. Especially in mesoporous support, the refined particles could enter into the mesopores of the supports. Interestingly, with the content of CA increased to 25 wt %, a peak at low temperature 240 °C existed in Ni/SBA-16-25%CA. Qin 17 attributed this low temperature reduction peaks to the surface amorphous NiO species.

Figure 5. H2-TPR profiles of the catalysts

The chemisorption of CO molecular on the catalysts were obtained by CO-TPD and the results were showed in Fig.6. For the catalysts prepared with or without citric acid, one desorption peak at around 100°C was assigned to the desorption of physical adsorbed CO molecular on the catalysts. A desorption peak at 300°C were observed on Ni/SBA-16, which could be attributed to CO molecular dissociated on nickel species 35. However, another desorption peak around 500°C was exhibited in catalysts prepared with citric acid. Meanwhile, the low temperature desorption peak shifted to higher temperature for Ni/SBA-16-x%CA, meaning that CO molecular strongly adsorbed on the nickel species of Ni/SBA-16-x%CA. The 500°C peak desorption in CO-TPD mean that the CO absorbed on nickel species in 12


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Ni/SBA-16-xCA possessed a relatively stronger interaction with nickel. For Ni/SBA-16-Xca, the stronger interaction between CO and Ni species could offer more reactant molecular for methanation at high temperature, while most CO molecular dissociated for Ni/SBA-16 below 400°C.

Figure 6. CO-TPD results of the catalysts.

Also in order to know the chemical state of nickel species in the catalysts, a XPS measurement was provided. Fig.7a showed the XPS spectra of the catalysts before reaction in the Ni 2p region. The Ni 2p3/2 cove level spectra exhibited two main peaks at 855.7 eV and 856.0 eV. In Ni/SBA-16, the Ni(2p 3/2) binding energy at 855.7 eV along with the broad satellite at around 860 Ev was characteristic of NiO species 36. In the catalysts prepared with citric acid, another peak around 855.7 eV shifted to the low binding energy with the increasing amount of citric acid, which was attributed to the nickel-silicon phase. According to some literature, the peak at 853.8 eV was due to the nickel-silicon phase, which are slighter higher than Ni0 (852.6 eV), indicating that the covalent interaction between Ni and Si lead to a slight transfer of electron density from Si to Ni. Zhang et al

12

discussed that the increase of electron cloud density of

Ni is conductive to enhance Ni-C-O bond, which makes CO dissociation easier.

13



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Figure 7. XPS results of the catalysts: (a)samples before reaction(b)samples after reaction

In general, calcination step plays a vital role in the preparation of the catalysts. In order to clarify the effect of citric acid on the properties of the catalysts, two comparative samples were used for comparison. The nickel precursor with and without citric acid were calcined at 500°C and scanning electron microscope was used to detect the obtained samples. Fig. 8 showed SEM images of the nickel precursor calcined at 500°C. In Fig. 8, it can be seen that NiO without CA exhibited the well-organized crystalline structure. When calcined with citric acid together, many porous holes existed on the NiO surface and the regularity of crystalline was broke. As mentioned before, citric acid decomposed and release lots of gases at high temperature calcination. These released gases may inhibit the nickel species to form perfect crystalline or destroy the crystalline structure. The nickel content surface area was obtained by H2 pulse chemisorption and the results were showed in Table 1. It was clearly seen that nickel content surface area of Ni/SBA-16-x%CA was much larger than that of Ni/SBA-16. Also in H2-TPR results, a relatively low temperature (240°C) reduction peak was observed in Ni/SBA-16-25%CA. Qin attributed this low temperature reduction to amorphous NiO on the support. So the H2-TPR results could indirectly prove that addition of citric acid destroy the perfect crystalline structure. The nickel content surface area firstly increased and then decreased with the increasing 14


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content of citric acid. Ni/SBA-16-15%CA obtained the largest nickel content surface area of 4.8 m2/g, which was approximately 4.8 times higher than that of Ni/SBA-16 (1.03 m2/g). For the same nickel content, Ni/SBA-16-x%CA had larger nickel content surface area, which could be attributed to the smaller particles in the catalysts. However, a decrease in the nickel surface area was observed with the amount of CA further increased. This contrary effect can be attributed to the different interaction between citric acid and the support. It has been reported that the citric acid could react with basic and neutral OH groups of alumina surface and moderate the interaction between tungsten species23,37 and the support. Similarly, the interaction of citric acid and SBA-16 surface groups can result in a decrease of a number of anchoring points for deposited Ni species.

Figure 8. SEM images of the calcined precursor with nickel with or without citric acid: (a) Ni (b) Ni-8%CA (c) Ni-15%CA

3.2 Catalytic performance 3.2.1 Effect of temperature The catalytic performance of five catalysts was tested from 250 °C to 500 °C. The reaction conditions was 15000 mL/g/h and 0.1 MPa. The conversion of CO and selectivity of CH4 were plotted as a function of reaction temperature and the results were shown in Fig.9. Fig. 9a showed that all catalysts exhibit the same CO conversion of 100% in the temperature range of 250-450 °C. However, the apparent difference existed in the selectivity of CH4 in Fig. 9b. The 15



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catalysts prepared with citric acid (4-15%) exhibited better selectivity of methane than Ni/SBA-16. Especially at lower temperature(250-300°C) and high temperature(450-500°C), the gap of selectivity between the catalysts was larger. If the addition of citric acid increased to 25%, it was found that the activity performance of the catalyst declined. This may be due to the different morphology formed in Ni/SBA-16-25%CA. In previous characterization, it was discussed that the nickel species existed in an amorphous state. Combining the activity test and characterization, it proved that the amorphous nickel species was inferior to the methanation reaction. Also it was observed that the effect of using different amount of CA over the activity reflected in the difference of nickel content surface area. The trend of the activity has super consistency with the trend in the difference of the nickel content surface area.

Figure 9. Catalytic performance of prepared catalysts: (a)(b) under 15,000 mL/g/h (c)(d) under 200,000 mL/g/h. Conditions: n(H2):n(CO):n(N2)=3:1:1, Temperature:250-500°C, WHSV:15,000mL/g/h and 120, 000 mL/g/h，，Pressure: 0.1MPa.

16


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From CO-TPD results, it could be concluded that interaction between CO molecular and nickel species in Ni/SBA-16-x%CA was much stronger than that in Ni/SBA-16. Most CO molecular desorbed at 300°C for Ni/SBA-16, while for Ni/SBA-16-x%CA another portion of CO desorbed at higher temperature. The different desorption temperature could lead to different activity performance at higher temperature. Another proper reasons can attributed to the difference in particle size and nickel content surface area. At low temperature the reaction rate was relatively low and higher nickel content surface area means more active site for the reaction. Higher temperature is inferior to methanation reaction and nickel particle size could sintering at high temperature, which lead to the decrease of activity. To investigate the stability of the surface properties of the catalysts, XPS analysis was performed for the spent catalyst. After reaction, the signals of Ni 2p3/2 do not show significant variation. From Ni 2p spectra in Fig.7b, all catalysts exhibited a peak around 853.4 eV, which are higher than that of metallic Ni (853.2eV)38.

It was suggested that the

nickel species transferred from oxidation state to the metallic nickel after reaction, which was consistent with the results of XRD patterns. 3.2.2 Effect of space velocity In previous section, it was referred that under lower WHSV of 15, 000 mL/g/h the methanation reaction reached 100% CO conversion, which couldn’t discern the difference of the activity among these catalysts. Therefore, the catalytic performance of the catalysts under higher WHSV should be investigated. Fig. 9c-d displayed the CO conversion and CH4 selectivity of the catalysts at WHSV of 200,000 mL/g/h. As can be seen in Fig. 9c, Ni/SBA-16-x%CA showed significantly higher CO conversion compared with Ni/SBA-16 at 300 °C, which could be attributed to the higher nickel content surface area in Ni/SBA-16-x%CA. The number of available sites will decrease with the increase of reactant molecular. In Table 1, the nickel content surface area of Ni/SBA-16-x%CA was much higher than that in Ni/SBA-16, which offered more active sites for the reaction and obtained better activity. 17



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3.2.3 Heat-resistant performance In this section, the thermo-stability of the catalysts were investigated. As discussed before, the methanation reaction is a strongly exothermic reaction and higher temperature is inferior for the reaction. The heat released by the reaction could lead to the temperature rise in the reaction system. In addition, the nickel species were easily sintering at high temperature. Therefore, the thermo-stability is of vital importance in methanation. In order to exam the thermal-stability of the catalysts, a calcination step was chosen to simulate the condition of the reaction. Firstly, the catalysts were tested for activity at 350 °C under 0.1 MPa, and then the catalysts were calcined at 700°C in gases atmosphere. The WHSV was controlled at 120, 000 mL/g/h. Finally, the catalysts were cooled down to 350°C and tested the activity again. The comparison of the activities before and after high temperature treatment could demonstrate the thermal-stability of the catalysts and the results were listed in Table 2. Table 2 Heat-resistant performance of the catalysts

Before high temperature treatment

After high temperature treatment

Catalysts CCO/%

SCH4/%

D* nickel/nm

CCO/%

SCH4/%

D* nickel/nm

Ni/SBA-16

93.4

79.2

23.1

28.9

42.3

30.0

Ni/SBA-16-4%CA

94.0

81.3

4.0

93.8

80.9

4.4

Ni/SBA-16-8%CA

96.5

85.8

3.6

96.5

85.1

3.8

Ni/SBA-16-15%CA

97.8

84.0

3.9

97.5

83.5

4.2

Ni/SBA-16-25%CA

94.1

80.2

4.5

92.1

78.9

4.9

Cacination conditions: gas atmosphere with n(H2):n(CO):n(N2)=3:1:1, Temperature:700°C, WHSV:200, 000mL/g/h. *Calculated by Sherrer equations

As shown in Table 2, a significant deactivation existed in Ni/SBA-16 with the CO conversion and CH4 selectivity 18


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decreased from 93.4 % and 79.2 % to 28.9 % and 42.3 %, respectively. For the catalysts prepared with citric acid, Ni/SBA-16-x%CA exhibited no obvious decrease in CO conversion and CH4 selectivity. In general, the reasons for the deactivation of the catalysts were carbon deposition and sintering of metals. In order to detect the main reasons for deactivation of catalysts, TG and XRD analysis were carried out for the catalysts calcined at 700 °C. The TG results were shown in Fig. 10. With the increase of the temperature, three weight changes were observed in Fig. 10. The weight of the catalysts first decreased at 100-300 °C and increased at 300-500 °C and then decreased after 500 °C. The weight increase at 300-500 °C was due to the oxidation of metal nickel and the carbon deposition over the catalysts was the sum of weight loss at 100-300 °C and 500-700 °C. It can be seen that the total carbon deposition of Ni/SBA-16 was 4 wt %, while the catalyst prepared with citric acid was also 2-3 wt %. Therefore, the amount of carbon deposition among these catalysts were almost the same, indicating that carbon deposition was not the main reason caused the catalysts deactivated. Then XRD was used to tested the catalysts after calcined at 700 °C and the results were shown in Fig. 2d. Comparing with the catalysts without calcination, an obvious characteristic diffraction peaks of carbon appeared at 26.9°, which was consistent with TG results. For Ni/SBA-16, the intensity of nickel diffraction increased, suggesting that the nickel particles on Ni/SBA-16 are partially sintered or aggregated after high temperature treatment at 700 °C. In contrast, no obvious change was seen for the catalysts prepared with citric acid before and after high temperature treatment. In TEM images it was suggested that the nickel species entered into the pores, which were anchored in the cage of SBA-16. The anchored Ni in the cage was difficult to sinter at high temperature. For the Ni/SBA-16-25%CA, it was also exhibited better thermo-stability than Ni/SBA-16.

19



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Figure 10. Thermogravimetric (TG) results of the catalysts after high temperature treatment at 700°C

3.2.4 Life-time Test Catalytic life time is a key consideration in the economical production of SNG from coal-derived gases. As discussed in precious section, sintering of nickel particle size at high temperature would deteriorate the activity of the catalysts. The catalytic stability test of the prepared catalysts were investigated at 350°C under atmosphere pressure with a WHSV of 15,000 mL/g/h, and the results are shown in Figure 11a-b, where CO conversion and CH4 selectivity are plotted as the function of time.

Figure 11.

Catalytic stability of catalysts: (a) CO conversion (b) CH4 selectivity.

20


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From Fig.11a-b, it is observed that there's no obvious decline in CO conversion and CH4 selectivity for Ni/SBA-16-15%CA. In contrast, the CO conversion of Ni/SBA-16 decreased from 100% to 85% and the selectivity of CH4 of Ni/SBA-16 decrease from 97% to 89% after a 100h reaction. In order to detect the reasons of the difference between the two catalysts, the TEM images for these two spent catalysts were provided in Fig.3c and Fig.3g. From Fig.3c, it was observed that the nickel particles in Ni/SBA-16 underwent a significantly agglomeration, while nickel particles still remain the narrow size distribution in Ni/SBA-16-15%CA. Combining the life-time test with the TEM images, the catalysts prepared with citric acid exhibited better long-term stability, which is contributed the suppress of the agglomeration.

4. Conclusion In this work, Ni/SBA-16 catalysts were prepared by citric acid assisted impregnation method. The catalysts prepared with or without citric acid were compared. Relatively low diffraction of NiO and Ni crystalline phase were observed in Ni/SBA-16-x%CA (x is from 4%-8%). This was due to the high dispersed nickel species formed in Ni/SBA-16-x%CA. A small amount of citric acid (4-8 wt %) resulted in an apparently increase in the dispersion of NiO species. With the increasing amount of citric acid to 15 wt%, it was clearly seen that the nickel species arranged according to the structure of SBA-16, indicating that the nickel particles (3-5 nm) may enter into the pores (6-7 nm) of the silica. The organic groups in citric acid possessed a relatively strong interaction with nickel ions, which could inhibit the accumulation of the nickel particles in the dry step of preparation catalysts. The catalysts prepared with citric acid exhibited best activity in CO methanation with 100 % CO conversion and 99.9 % CH4 selectivity. Meanwhile, Ni/SBA-16-xCA exhibited better thermos-stability and long-term stability than Ni/SBA-16. The highly dispersion, anchoring into the pores of silica and refined nickel particles 21



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together improved the stability of the catalysts. Acknowledgments This research was financially supported by National Natural Science Funds of China (Grant No. U1203293 and 91434128), The program of Shanghai Subject Chief Scientist (Grant No.10Xd1401500) and the Program of Shanghai Leading Talents (2013). Author Information Corresponding Author *E-mail: [email protected] Reference: (1) Gao, J.; Liu, Q.; Gu, F.; Liu, B.; Zhong, Z.; Su, F. Recent advances in methanation catalysts for the production of synthetic natural gas. RSC Adv. 2015, 5, 22759-22776. (2) Rönsch, S.; Schneider, J.; Matthischke, S.; Schlüter, M.; Götz, M.; Lefebvre, J.; Prabhakaran, P.; Bajohr, S. Review on methanation – From fundamentals to current projects. Fuel 2016, 166, 276-296. (3) Zhang, Y.; Zhang, G.; Wang, L.; Xu, Y.; Sun, Y. Selective methanation of carbon monoxide over Ru-based catalysts in H2-rich gases. Journal of Industrial and Engineering Chemistry 2012, 18, 1590-1597. (4) Tada, S.; Kikuchi, R.; Takagaki, A.; Sugawara, T.; Oyama, S. T.; Urasaki, K.; Satokawa, S. Study of RuNi/TiO2 catalysts for selective CO methanation. Applied Catalysis B: Environmental 2013, 140-141, 258-264. (5) Zhou, G.; Wu, T.; Xie, H.; Zheng, X. Effects of structure on the carbon dioxide methanation performance of Co-based catalysts. International Journal of Hydrogen Energy 2013, 38, 10012-10018. (6) Gao, J.; Jia, C.; Li, J.; Gu, F.; Xu, G.; Zhong, Z.; Su, F. Nickel Catalysts Supported on Barium Hexaaluminate for Enhanced CO Methanation. Industrial & Engineering Chemistry Research 2012, 51, 10345-10353. (7) Zhang, J. Y.; Xin, Z.; Meng, X.; Lv, Y. H.; Tao, M. Effect of MoO3 on the heat resistant performances of nickel based MCM-41 methanation catalysts. Fuel 2014, 116, 25-33. (8) Li, J.; Zhou, L.; Li, P.; Zhu, Q.; Gao, J.; Gu, F.; Su, F. Enhanced fluidized bed methanation over a Ni/Al2O3 catalyst for production of synthetic natural gas. Chemical Engineering Journal 2013, 219, 183-189. (9) Appari, S.; Janardhanan, V. M.; Bauri, R.; Jayanti, S. Deactivation and regeneration of Ni catalyst during steam reforming of model biogas: An experimental investigation. International Journal of Hydrogen Energy 2014, 39, 297-304. (10) Barrientos, J.; Lualdi, M.; Boutonnet, M.; Järås, S. Deactivation of supported nickel catalysts during CO methanation. Applied Catalysis A: General 2014, 486, 143-149. (11) Struis, R. P. W. J.; Schildhauer, T. J.; Czekaj, I.; Janousch, M.; Biollaz, S. M. A.; Ludwig, C. Sulphur poisoning of Ni catalysts in the SNG production from biomass: A TPO/XPS/XAS study. Appl Catal a-Gen 2009, 362, 121-128. (12) Zhang, J.; Xin, Z.; Meng, X.; Lv, Y.; Tao, M. Effect of MoO3on Structures and Properties of Ni-SiO2Methanation 22


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Graphical Abstract

25


Effect of Citric Acid on the Synthesis of CO Methanation Catalysts with

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