Ce0.5Zr0.5O2 catalysts and their catalytic performance in methane partial oxidation to produce synthesis gas

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 40, Issue 4, April 2012 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2012, 40(4), 418423

RESEARCH PAPER

Preparation of Co/Ce0.5Zr0.5O2 catalysts and their catalytic performance in methane partial oxidation to produce synthesis gas YU Chang-lin1,*, HU Jiu-biao1, WENG Wei-zheng2, ZHOU Xiao-chun1, CHEN Xi-rong1 1

Scholl of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China

2

State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China

Abstract: Porous Ce0.5Zr0.5O2 solid solution support was prepared by co-precipitation and then different contents of Co (10%, 20%, and 30 %) were loaded over the Ce0.5Zr0.5O2 support by impregnation; the Co/Ce0.5Zr0.5O2 catalysts obtained in this way were used in partial oxidation of methane (POM) to produce synthesis gas. The fresh catalysts were characterized by N2 physical sorption (BET), X-ray diffraction (XRD), temperature-programmed reduction by hydrogen (H2-TPR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The amount of deposited carbon over the spent catalysts after POM tests was determined by temperature-programmed oxidation (O2-TPO) and thermogravimetric (TG) analysis. The results showed that cobalt oxide over Ce0.5Zr0.5O2 is easy to be reduced to metal cobalt. The Co/Ce0.5Zr0.5O2 catalysts show high activity and selectivity to H2 and CO in methane partial oxidation; higher Co loading is beneficial to enhance their catalytic activity and resistance to catalyst deactivation from coke deposition. Key words: Ce0.5Zr0.5O2 solid solution; cobalt; partial oxidation of methane; syngas

There is plenty of nature gas resource in China and the economic application of nature gas has attracted the researchers’ extensive attention[1–3]. The main composition of nature gas is methane. Catalytic partial oxidation of methane (POM) to synthesis gas (syngas) is a mild exothermic process and can produce the syngas with a H2/CO molar ratio of 2, which can be directly used for Fischer-Tropsch synthesis or methanol synthesis. Therefore, the process of POM to produce syngas is one of the most important application routes of nature gas[4–7]. The active components of the catalysts for POM are nickel and noble metals, such as Ru[8], Rh[9], Pt[10] and cobalt. The noble metal catalyst has comparatively high activity, but suffers from a serious deactivation and high price, which limits its commercial application. Nickel-based catalyst has the advantage of low price and high activity; however, its deactivation is also a serious issue. The deactivation of nickel-based catalysts is often caused by the sintering of nickel metal particles, deposition of carbons, and the loss of active component. Compared to nickel, cobalt-based catalysts have low activity in POM; moreover, cobalt is apt to result in a

complete oxidation of methane, leading to the low selectivity to H2 and CO. However, cobalt has a high melting point which benefits its resistance to the loss of active component[11] at high temperature. As a result, when using cobalt-based catalysts in POM, it is essential to enhance their activity and selectivity to CO and H2. The addition of appropriate promoters may promote the reducibility of cobalt and effectively enhance the activity and selectivity to syngas in POM[12–14]. Moreover, the performance of cobalt-based catalysts is closely related to their support properties and the interaction between the support and active component. The supports reported for cobalt-based catalysts are -Al2O3, SiO2, etc. CeO2-based materials are often used as the catalyst support and promoter due to their unique physical and chemical properties. Noronha et al[15] reported that the CeO2-ZrO2 support can effectively improve the transfer of oxygen in the catalyst surface, which benefits the contact between coke and oxygen and then promotes the removal of coke deposited on the catalyst. In this work, porous Ce0.5Zr0.5O2 solid solution support was prepared by co-precipitation and then different contents of Co

Received: 26-Oct-2011; Revised: 21-Dec-2011 * Corresponding author. E-mail: [email protected]. Foundation item: Open Foundation of State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University (200906); the National Natural Science Foundation of China (21067004); Natural Science Foundation of Jiangxi Province (2010GZH0048); Technological Project of Jiangxi Province Education Office (GJJ12344). Copyright ” 2012, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

YU Chang-lin et al. / Journal of Fuel Chemistry and Technology, 2012, 40(4): 418423

were loaded over the Ce0.5Zr0.5O2 support by impregnation. The effect of Co loading on the performance of Co/Ce0.5Zr0.5O2 catalysts in POM to produce syngas was then investigated.

1

Experimental section

1.1

Catalyst preparation

Ce0.5Zr0.5O2 solid solution was prepared by co-precipitation method. Stoichiometric amounts of Ce(NO3)3·6H2O and Zr(NO3)4·5H2O were dissolved in deionised (DI) water. Meanwhile, a given amount of polyvinylpyrrolidone (PVP) was dissolved in another DI water; it was stirred for 3 h and then mixed with the former solution of Ce(NO3)3 and Zr(NO3)4. The mixed solution was further stirred 1 h. Under stirring, ammonia water was added dropwise to the mixed solution until the pH value of the solution reached 11. The obtained slurry was refluxed at 80°C for 24 h. The sediment product was filtered under vacuum and washed with water. After drying at 120°C for 10 h, the powder was calcined at 700°C for 4 h in a furnace to obtain the Ce0.5Zr0.5O2 solid solution. Co/Ce0.5Zr0.5O2 catalysts were prepared by impregnation method. Ce0.5Zr0.5O2 was impregnated with Co(NO3)2 solution; during impregnation, the mixture was stirred slowly. After impregnation for 24 h, the obtained solid was dried at 100°C for 24 h and then calcined at 700°C for 4 h. Finally, the obtained Co/Ce0.5Zr0.5O2 catalysts was pressed and sieved to get a sieving of 40–60 mesh particles. The loadings of cobalt considered were 10%, 20% and 30%. 1.2

Catalytic test

The catalytic POM tests were carried out in a continuous flow fixed-bed quartz tube (inner diameter 6 mm) reactor (WFSM-3060, provided by Tianjin Xianqian Company). About 150 mg of catalyst was loaded in the quartz tube. The reaction mixture was composed of CH4 and O2 (CH4/O2 molar ratio of 2) and the total gas hourly space velocity (GHSV) was 9000 h–1. Before each test, the catalyst was first reduced on-line under pure hydrogen flow (11.28 mL/min) at 750°C for 1 h; the reduced catalyst was then purged with a high-purity nitrogen flow (30 mL/min) for 30 min. After reaction started for 10 min, the effluent containing CO, H2, CO2, unreacted CH4, etc. was sampled periodically and analyzed with an online chromatograph equipped with a TCD and a TDX-101 packed column; the catalytic reaction generally lasted for 3 h. 1.3

Catalyst characterization

N2 physical sorption was measured at liquid nitrogen

temperature on an automatic analyzer (V-Sorb 2800P, Beijin Jinanpu Company of Science and Technology). Prior to the measurement, the catalyst samples were degassed for 1 h under vacuum at 200°C. The surface area was obtained by BET equation from N2 adsorption/desorption isotherms. Temperature-programmed reduction by hydrogen (H2-TPR) was carried out in a chemical adsorption instrument (TP-5080, provided by Tianjin Xianqian Company). 0.05 g catalyst sample was filled in the quartz reactor. Prior to TPR, the samples were treated in situ by purging with nitrogen at 300°C for 0.5 h and then cooled down to room temperature. After that, a flow of 5 % hydrogen in nitrogen (30 mL/min) used as the reducing gas was introduced. After the TCD baseline was stabilized, the reactor was heated from room temperature to 800°C at a ramp of 10°C/min to get the TPR profiles. Temperature-programmed oxidation by oxygen (TPO) was carried out in the same instrument as TPR and 0.05 g sample was used for each measurement. Prior to TPO analysis, the catalyst sample was treated in situ by purging with nitrogen at 300°C for 0.5 h and then cooled down to room temperature. After that, a flow of 10 vol. % oxygen in nitrogen was introduced as the oxidation gas and the reactor was heated from room temperature to 800°C at a ramp of 10°C/min to get the TPO profiles. CO2 produced by the oxidation of coke was also identified by mass spectrometry (Agilent 6890N-5973i). Thermogravimetry (TG) test of the spent catalyst after POM reaction was performed on thermogravimetric analyzer (Diamond TG/DTA 6000, PE) in air. The amount of the catalyst sample used in this test was 0.045 g and the heating rate is 10°C/min. Powder X-ray diffraction (XRD) patterns were obtained on a Bruker D8-advance X-ray diffractometer (Cu KD, O = 0.154 nm). The X-ray tube was operated at 30 kV and 15 mA. Spectra were scanned in the range from 10° to 80°. Morphology of the catalyst sample was observed by scanning electron microscopy (SEM) with a XL30 spectrometer (Philips). Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2000 (120 kV) spectrometer. The reduced samples were deposited on thin amorphous carbon films supported by copper grids from their ultrasonically processed ethanol solutions.

2 2.1

Results and discussion N2 physical sorption

The porous structure and surface area were analyzed by N2 physical sorption. Fig. 1(a) shows nitrogen adsorption-desorption isotherms and Fig. 1(b) gives the pore size distribution plots for the synthesized Ce0.5Zr0.5O2 solid solution and Co/Ce0.5Zr0.5O2 catalysts.

YU Chang-lin et al. / Journal of Fuel Chemistry and Technology, 2012, 40(4): 418423

Fig. 1 N2 adsorption-desorption isotherms (a) and pore size distributions derived from desorption branches of N2-sorption isotherms (b) of support and catalysts. : CeZrO2; : Co (10%) CeZrO2; : Co (20%) CeZrO2; &: Co (30%) CeZrO2

Table 1 Textural property of samples by N2 adsorption Surface

Pore

Average pore

areaa

volumeb

diameterc

A/ (m2·g–1)

v/ (cm3·g–1)

d/ (nm)

CeZrO2

50.23

0.23

14.54

Co(10%)/CeZrO2

26.16

0.18

13.25

Co(20%)/CeZrO2

17.23

0.035

5.62

Co(30%)/CeZrO2

11.95

0.032

6.84

Catalyst

a

notes: calculated from the linear portion of the BET plot; b

calculated by means of total amount of adsorbed gas at p/p0 = 0.98;

c

determined by BJH method from desorption isotherms

Fig. 2 XRD patterns of the samples

Although Co/CeZrO2 was calcined at a high temperature as 700°C, it shows a Langmuir type-IV isotherm, which is representative of regular mesoporous materials. The pores in the catalysts are produced by the removal of PVP surfactant which is burned off during the calcination process. As listed in Table 1, the surface areas for CeZrO2, Co(10%)/CeZrO2, Co(20%)/CeZrO2 and Co(30%)/CeZrO2 are 50.23, 26.16, 17.23 and 11.95 m2/g, respectively. The corresponding pore volumes are 0.23, 0.18, 0.035 and 0.032 cm3/g and the average pore diameters are 14.54, 13.25, 5.62 and 6.84 nm,

respectively. The surface area and pore volume of the catalysts decreases with the increase of cobalt loading, especially at high cobalt loading, which may be ascribed to that cobalt loaded on the support can plug up the pores in the support. 2.2

XRD

Figure 2 shows XRD patterns of the catalyst samples. For comparison, the XRD patterns of pure CeZrO2 and CeO2 are also provided. Strong diffraction peaks corresponding to the planes of (111), (200), (220) and (311) are observed for the fluorite structure of CeO2. CeZrO2 and Co/CeZrO2 show similar diffraction peaks over the planes of (111), (200), (220) and (311). However, the diffraction peaks at (111) plane for CeZrO2, Co(10%)/CeZrO2, Co(20%)/CeZrO2 and Co(30%)/CeZrO2 are 29.7°, 29.6°, 29.8° and 29.2°, respectively, which are higher than that of CeO2 (28.7°). These suggest that Zr4+ is doped into the crystal lattice of CeO2 and a CeZrO2 solid solution is formed, which decreases the crystal parameter. Over Co/CeZrO2, the weak diffraction peaks of Co3O4 at 32.9°, 33.4° and 33.6° are also observed, indicating that Co3O4 is highly dispersed over CeZrO2 support in the form of hexagonal closed-packed lattice. 2.3

H2-TPR

H2-TPR profiles of all samples are shown in Fig. 3. CeZrO2 displays one weak reduction peak at around 570 °C, which may be ascribed to the reduction of Ce4+ to Ce3+ in the CeZrO2 solid solution. After loading of Co on the Ce0.5Zr0.5O2 support, the Co/Ce0.5Zr0.5O2 catalysts show reduction peaks at lower temperature. Co(10%)/CeZrO2 shows two reduction peaks at 170–255°C and 255–440°C; similarly, two peaks are also observed for Co(20%)/CeZrO2 (162–245°C and 245–460°C) and Co(30%)/CeZrO2 (150–260°C and 260–420°C). The higher temperature reduction peak of the three catalyst

YU Chang-lin et al. / Journal of Fuel Chemistry and Technology, 2012, 40(4): 418423

samples can be divided to two peaks by Gauss fitting: i.e. low temperature reduction and high temperature reduction peaks. The low temperature reduction peak is from the reduction of Co3O4 to CoO, while the high temperature reduction peak is produced by the reduction of CoO to Co. With the increase of cobalt loading, the area of high temperature reduction peak becomes larger, suggesting that higher amount of hydrogen is consumed by cobalt species. The addition of Zr4+ to CeO2 can increase the number of oxygen vacancy, which may increase the mobility of lattice oxygen and make the reduction cobalt species easier. It was reported that the reduction temperature of the cobalt supported on Al2O3 is around 650°C[16] and it decreases 540°C when using ZrO2 as the corresponding support[17]. When cobalt was supported on CeO2, its reduction temperature is around 500°C[18]. Current results suggest that the active component of cobalt supported on Ce0.5Zr0.5O2 can be much more easily reduced, which could promote the POM to form syngas, according to the literature[14]. 2.4

SEM and TEM

The morphology of the typical Co(30%)/CeZrO2 sample was analyzed by SEM and its particles, size was determined by TEM. Fig. 4 gives the SEM and TEM images of Co(30%)/CeZrO2. The SEM image in Fig. 4(a) shows that the catalyst sample is composed of small particles and the total morphology is irregular. Fig. 4(b) indicates that the prime particle size of Co(30%)/CeZrO2 is around 7–10 nm. 2.5

content, the oxidation temperature of coke deposited on the catalysts shifts to higher temperature and the peak area become larger, indicating that more coke are produced over the high loading cobalt catalysts for POM. Figure 6 shows the TG and DTG profiles of the spent Co(30%)/CeZrO2 catalyst after POM tests. It was noted that the weight loss begins when the temperature exceeds 200°C and the rate of weight loss increases considerably at a temperature higher than 400°C. DTG curve shows that the fast weight loss occurs at about 570°C, suggesting that at this temperature the coke is quickly burned off, corresponding to the high temperature peak in TPO profile (Fig. 5). The weight loss finishes at 670°C and the total weight loss is around 23%, indicating that the mass percent of coke in the spent Co(30%)/CeZrO2 catalyst after POM test is 23%. However, over the spent Co(20%)/CeZrO2 and Co(10%)/CeZrO2 catalysts, the mass percents of coke are 19% and 15%, respectively. The reason for more coke deposited over the high cobalt loading catalyst is that there are more reactive sites in it; more methane are converted over the catalyst with higher cobalt loading during the same reaction period.

TPO and TG analysis

The coke deposited over the spent catalysts after POM tests was analyzed by TPO and TG analysis. As shown by the TPO profiles of the spent catalysts in Fig. 5, oxygen is consumed at both low and high temperatures. The peak at low temperature is from the oxidation of metal cobalt, while the peak at high temperature is produced by the oxidation of coke which quickly consumes a lot of oxygen. With the increase of cobalt

Fig. 3 H2-TPR profiles of the catalysts

Fig. 4 SEM (a) and TEM (b) images of the Co(30%)/CeZrO2 catalyst

YU Chang-lin et al. / Journal of Fuel Chemistry and Technology, 2012, 40(4): 418423

Fig. 5 O2-TPO profiles of the catalysts

Fig. 6 TG and DTG profiles of the spent Co (30%) /CeZrO2 catalysts after POM test

Fig. 7 Catalytic performance comparison of the catalysts in methane partial oxidation reaction (a) conversion of methane; (b) selectivity to H2; (c) selectivity to CO : Co (10%) CeZrO2; : Co (20%) CeZrO2; : Co (30%) CeZrO2

According to the results of thermodynamic calculation and the experimental tests, there are two reasons for the formation of coke in POM. One is the decomposition of methane and another is the dismutation of carbon monoxide; the former should be the primary one at high temperature. High temperature will not benefit the dismutation of carbon monoxide, since it is a strong exothermal reaction; when the reaction temperature is below 600°C, as a result, the amount of coke produced by dismutation of carbon monoxide is more than that produced by the decomposition of methane. In this work, however, the reaction temperature is as high as 750°C, the decomposition of methane should be the main cause for coke formation.

provides more active centers. Moreover, more coke are formed over the catalysts with higher cobalt content that converts more methane to syngas during the same reaction period, indicating that high cobalt loading catalyst has the resistance to the deactivation from the coke deposition. Therefore, to get high conversion of methane, high cobalt loading over CeZrO2 support is necessary. The three catalysts show stable selectivity to H2 and CO. The selectivity is about 97% to H2 and 95% to CO. The syngas produced from POM can be directly used in methanol synthesis and Fischer-Tropsch synthesis.

2.6

Cobalt catalysts supported on porous Ce0.5Zr0.5O2 solid solution were prepared and used in methane partial oxidation to produce syngas. The effect of Co loading on the performance of Co/Ce0.5Zr0.5O2 catalysts in POM was then investigated. The results showed that cobalt oxide over Ce0.5Zr0.5O2 is easy to be reduced to metal cobalt. The Co/Ce0.5Zr0.5O2 catalysts exhibit high activity and selectivity to H2 and CO in methane partial oxidation; higher Co loading is beneficial to enhance their catalytic activity and resistance to catalyst deactivation from coke deposition.

Catalytic activity test

Figure 7 shows the catalytic performance of the catalysts in POM. When the mass percent of Co in the catalyst is 10%, the methane conversion is only 56%; when Co loadings are increased to 20% and 30%, methane conversions reach 60% and 70%, respectively, showing that high content of cobalt will enhance the methane conversion. Although the increase of cobalt content decreases the surface area and pore volume of the Co/Ce0.5Zr0.5O2 catalyst, the activity of high cobalt loading catalyst is enhanced, possible due to that it also

3

Conclusions

YU Chang-lin et al. / Journal of Fuel Chemistry and Technology, 2012, 40(4): 418423

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