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Hyun-Seog Roh b. , Yu Taek Seo c. , Dong Joo Seo c. ,. Wang Lai Yoon c,*, Seung Bin Park a a Department of Chemical and Biomolecular Engineering, KAIST, ...
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Applied Catalysis A: General 340 (2008) 183–190 www.elsevier.com/locate/apcata

Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process Kee Young Koo a, Hyun-Seog Roh b, Yu Taek Seo c, Dong Joo Seo c, Wang Lai Yoon c,*, Seung Bin Park a a

Department of Chemical and Biomolecular Engineering, KAIST, 373-1, Guseong-dong, Yuseong, Daejeon 305-701, Republic of Korea b Department of Environmental Engineering, Yonsei University, 234, Maeji, Heungeop, Gwangwon-do 220-710, Republic of Korea c Korea Institute of Energy Research, 71-2, Jang-dong, Yuseong, Daejeon 305-343, Republic of Korea Received 12 September 2007; received in revised form 2 February 2008; accepted 13 February 2008 Available online 17 February 2008

Abstract MgO-promoted Ni/Al2O3 catalysts have been investigated with respect to catalytic activity and coke formation in combined steam and carbon dioxide reforming of methane (CSCRM) to develop a highly active and stable catalyst for gas to liquid (GTL) processes. Ni/Al2O3 catalysts were promoted through varying the MgO content by the incipient wetness method. X-ray diffraction (XRD), BET surface area, H2-temperature programmed reduction (TPR), H2-chemisorption and CO2-temperature programmed desorption (TPD) were used to observe the characteristics of the prepared catalysts. The coke formation and amount in used catalysts were examined by SEM and TGA, respectively. H2/CO ratio of 2 was achieved in CSCRM by controlling the feed H2O/CO2 ratio. The catalysts prepared with 20 wt.% MgO exhibit the highest catalytic performance and have high coke resistance in CSCRM. MgO promotion forms MgAl2O4 spinel phase, which is stable at high temperatures and effectively prevents coke formation by increasing the CO2 adsorption due to the increase in base strength on the surface of catalyst. # 2008 Elsevier B.V. All rights reserved. Keywords: Gas to liquid (GTL); Combined reforming; MgO-promoted Ni/Al2O3; Coke resistance

1. Introduction There has been interest in gas to liquid (GTL) processes which consist of synthesis gas production and Fischer–Tropsch process with relation to synfuel and substitute energy [1–4]. In these processes, there are some key issues to produce syngas with adequate H2/CO ratio required for Fischer–Tropsch synthesis and to develop the catalyst with high activity as well as stability. Combined steam and carbon dioxide reforming of methane (CSCRM) has thus attracted much attention from industry for syngas production [5–7]. In general, representative reforming processes such as steam reforming of methane (SRM) and carbon dioxide reforming of methane (CRM) have some disadvantages because both reforming reactions require an additional process to adjust the H2/CO ratio [8–11]. In addition, partial oxidation of methane (POM) which meets the

* Corresponding author. Fax: +82 42 860 3309. E-mail address: [email protected] (W.L. Yoon). 0926-860X/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.02.009

H2/CO ratio of 2 creates difficulties in controlling the process because of the hot spots and explosion dangers [12–17]. On the contrary, CSCRM is an available process for the direct control of the H2/CO ratio which would be suitable for the Fischer– Tropsch process by adjusting the feed ratio of steam and carbon dioxide [18,19]. The supported Ni catalysts have been used for the industrial reforming processes because they are economical compared to noble metal-based catalysts. Commercial Ni-based catalysts have been optimized for SRM with excess steam (H2O/ CH4 > 2.5). However, commercial Ni-based catalysts are not suitable in CSCRM because of coke formation [20–22]. In other words, commercial catalysts are not suitable under severe reaction conditions ((H2O + CO2)/CH4 < 1.5) [23–25]. Therefore, it is necessary to develop the catalysts with high activity as well as good stability for CSCRM. It has been reported that the addition of a basic metal oxide into the catalyst improved the coke resistance. Choudhary et al. [26] reported that the catalyst precoated with MgO and CaO showed high activity compared to that of catalyst without any precoating in methane-to-syngas

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conversion. Roh et al. [27] also reported that Ni catalyst supported on Ce-ZrO2-modified Al2O3 exhibited high activity as well as good stability in CRM. This is due to the fact that a precoated support prevents the formation of inactive NiAl2O4 and increases the dispersion of nickel oxide. There are some reports on the suppression of coke formation which results from the enhanced base strength of support due to the increase of CO2 adsorption and decrease of CH4 decomposition [28–32]. However, there are few papers that have focused on the optimization of basic metal oxide in the promoted catalysts in relation to coke resistance in CSCRM. The goals of this work are to study the MgO effect on coke formation over MgO-promoted Ni/Al2O3 by monitoring the SEM image and analyzing the TG data of used catalysts and to determine the adequate MgO content in MgO-promoted Ni/Al2O3 catalysts with high activity and coke resistance in CSCRM. 2. Experimental 2.1. Catalyst preparation MgO-promoted Ni/Al2O3 catalysts were prepared by the incipient wetness method using aqueous solutions of Mg (NO3)26H2O (99%, Kanto Chem.) and Ni(NO3)26H2O (97%, JUNSEI) on g-Al2O3 (SASOL). Supports were loaded with varying MgO contents from 0 to 28 wt.% and then pre-calcined in air at 900 8C for 12 h. The loading amount of Ni was fixed at 12 wt.%. The catalysts were calcined in air at 800 8C for 6 h. 2.2. Characterization The crystal structure and compositional homogeneity of the prepared catalysts were examined by X-ray diffraction (XRD, Rigaku D/Max-III C, CuKa radiation). The surface area of each catalyst was measured by BET (Micromeritics, ASAP 2000) through the nitrogen adsorption at 196 8C. H2-chemisorption was performed to identify the metallic dispersion and metal surface area in ASAP 2010 (Micromeritics). The sample of 0.2 g was reduced in H2 flow at 800 8C for 1 h and then analyzed in He flow at 50 8C. The adsorbed H2 amount obtained by assuming the adsorption stoichiometry of one hydrogen atom per nickel surface atom (H/Nisurface = 1) was used to estimate Ni dispersion and surface area. Temperature programmed reduction (TPR, Autochem 2910) was carried out to identify the reduction temperature and H2 consumption of catalysts. The sample of 0.1 g in a quartz reactor was pre-treated with He gas at 500 8C for 1 h, then cooled down to 50 8C, and then re-heated by an electrical furnace at the heating rate of 20 8C/min from 50 to 1000 8C under 10% H2 in Ar gas. The sensitivity of the detector was calibrated by reducing a known weight of NiO. H2 consumption was estimated by integrating the peak area of the reduction profile relative to the calibration curve. CO2-temperature programmed desorption (CO2-TPD, BelMetal) was performed to identify the basic strength distribution of the support. The sample of 50 mg in a quartz reactor was pre-

treated with He gas at 700 8C for 1 h and then cooled down. CO2 was adsorbed at 50 8C for 1 h and weakly adsorbed CO2 was removed by purging with He. The sample was heated from 50 to 700 8C at a heating rate of 10 8C/min in He flow and the desorbed amount of CO2 was detected with a thermal conductivity detector (TCD). The desorbed CO2 amount was obtained by integrating the TPD profile. 2.3. Catalytic test The combined steam and carbon dioxide reforming of methane was carried out from 750 to 650 8C over 20 h under atmospheric pressure. The prepared catalyst (5 mg) diluted with MgAl2O4 (100 mg) was charged in a fixed-bed quartz reactor (I.D. = 4 mm). Before the reaction, the catalyst was reduced at 800 8C for 1 h under a mixture of 10% H2 in N2. The reactant feed ratio of (H2O + CO2)/CH4 was 1.2 and the space velocity was 530,000 ml/h gcat. To get syngas with H2/CO ratio of 2, we fixed the ratio of H2O/CO2 as oxidative species at 2. The effect of H2O/CO2 ratio on the produced H2/CO ratio was investigated through our previous woks, thus the reforming test was carried out in this condition [33]. The effluent was passed through a trap to condense residual water and then analyzed with an on-line micro gas chromatograph (Agilent 3000) equipped with a TCD detector. 2.4. Coke study To study the formation of coke on the catalyst related with the effect of MgO promotion, we tested the samples under severe conditions resulting in the deactivation of the catalyst. The catalyst (15 mg) was charged in a fixed-bed reactor without diluents. Before the reaction, the catalyst was reduced at 700 8C for 1 h under a mixture of 10% H2 in N2. The reactant feed ratio of (H2O + CO2)/CH4 was the same as that of the catalytic test and the space velocity was 530,000 ml/h gcat. CSCRM was carried out at 650 8C over 17 h under atmospheric pressure. The morphologies of used catalyst and of deposited coke were examined by scanning electron microscopy (SEM, Philips XL30SREG). To confirm the morphology of coke deposited on the surface clearly, we examined fresh catalyst reduced at 700 8C. The quantitative analysis of coke amount on the used catalyst was performed with a thermogravimetry analyzer (TGA, Netzsch TG209F3). The sample of 50–70 mg was heated from 30 to 800 8C with a heating rate of 10 8C/min in air. The electron state of used catalysts was measured with an X-ray photoelectron spectroscope (XPS, VG Scientifics ESCALAB250) equipped with a hemispherical electron analyzer and an AlKa X-ray source. The binding energy (BE) was calibrated by using the C 1s peak at 284.6 eV as a reference. 3. Results and discussion 3.1. Catalyst characterization Fig. 1 shows the XRD patterns of MgO-promoted Ni/Al2O3 catalysts with various MgO contents. After being pre-calcined

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Fig. 2. H2-TPR patterns of Ni/MgO/Al2O3 catalysts with various MgO contents.

Fig. 1. XRD patterns of (a) MgO/Al2O3 supports and (b) Ni/MgO/Al2O3 catalysts with various MgO contents.

at 900 8C for 12 h, the Al2O3 support without MgO content (0 wt.%) shows all characteristic peaks, which can be assigned to the d-Al2O3 phase that is present between g-Al2O3 and uAl2O3 in Fig. 1(a). As the MgO content increases, Al2O3 is transformed into MgAl2O4 spinel phase and the intensity of MgAl2O4 peak increases. In the case of 28 wt.% MgO, MgO phase co-exists with MgAl2O4. After Ni (12 wt.%) was loaded and then calcined at 800 8C for 6 h, Al2O3 peaks disappeared in Fig. 1(b). It is confirmed that MgO-promoted Al2O3 was

transformed into MgAl2O4 spinel, while NiAl2O4 spinel was formed without the addition of MgO (0 wt.%). However, it is hard to distinguish the MgAl2O4 peak from the NiAl2O4 peak and the MgO peak from the NiO peak because the peaks overlap each other. H2-TPR patterns of MgO-promoted Ni/Al2O3 catalyst are shown in Fig. 2. It is known that above 800 8C the peak can be assigned to the reduction peak of complex NiOx species, which have strong interaction with support [34]. TPR peak of 0 wt.% MgO is present over 900 8C resulting from the reduction of NiAl2O4. Ni/Al2O3 catalyst with the addition of 3–10 wt.% MgO shows similar TPR patterns to Ni/Al2O3 catalyst without MgO. As the MgO content increases, NiAl2O4 formation decreases and the strong interactions between complex NiOx species and support exist at 20 and 28 wt.% MgO. From TPR results, it is difficult to fully reduce the Ni/Al2O3 catalysts with 3–10 wt.% MgO under reduction condition at 800 8C for 1 h due to the high reduction temperature over 900 8C [35]. In the cases of 20 and 28 wt.% MgO, we can observe that the peak shifts toward the low temperature and that a shoulder peak coexists. Especially, free NiO species are present at about 400 8C with 28 wt.% MgO. Table 1 summarizes the BET surface area, reduction degree, Ni surface area, metal dispersion and Ni crystallite size of the prepared catalysts. Ni/Al2O3 catalyst without MgO has the highest BET surface area. As MgO content increases to

Table 1 BET surface area and H2-chemisorption results of 12 wt.% Ni/MgO/Al2O3 catalyst MgO (wt.%)

Surface area (m2/g) a

Reduction degree (%) b

Ni surface area (m2/g)

Metal dispersion (%)

Ni size (nm)

0 3 5 10 20 28

107 101 99 92 70 58

62.5 62.2 60.1 57.1 75.7 78.5

3.3 2.8 2.5 2.8 4.6 4.0

6.3 5.4 5.0 5.9 7.3 6.1

15 18 19 16 13 16

a b

Estimated from N2 adsorption at 196 8C. Calculated from reduction of the catalysts at 800 8C for 1 h.

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K.Y. Koo et al. / Applied Catalysis A: General 340 (2008) 183–190 Table 2 CO2-TPD results of 12 wt.% Ni/MgO/Al2O3 catalyst MgO (wt.%)

Total desorbed CO2 (mmol/g) a

Fig. 3. CO2-TPD patterns of Ni/MgO/Al2O3 catalysts with MgO contents of (a) 0 wt.%, (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.% and (e) 28 wt.%.

10 wt.% MgO, BET surface area, Ni surface area and metal dispersion decrease due to the decrease of pores in support. As expected from the TPR results, at 20 and 28 wt.% MgO, reduction degree is larger compared to those of catalysts with 0–10 wt.% MgO. It is known that the formation of MgAl2O4 spinel phase in the MgO-promoted Ni/Al2O3 catalysts improves the Ni dispersion on the support [36,37]. The Ni surface area of catalyst with 20 wt.% MgO is higher than that of catalyst with 28 wt.% MgO. It is possible that, at 28 wt.% MgO, the Ni surface area decreases due to the agglomeration of free Ni species, which is confirmed by TPR. Generally, highly dispersed and small Ni crystallites have a strong metal to support interaction (SMSI) [38]. In this table, the Ni/Al2O3 catalyst with 20 wt.% MgO has the highest metal dispersion (7.3%) and the smallest Ni crystallite size (13 nm) resulting from a strong metal to support interaction. Fig. 3 shows the CO2-TPD patterns of MgO-promoted Ni/ Al2O3 catalyst. A TPD peak of catalyst without MgO is present at low temperature of 120 8C due to the weak basicity. As the MgO content increases, TPD peaks shift toward the high temperature. In addition, TPD peaks show higher peak intensity with a long tail attributed to the increase of basicity. Table 2 summarizes the total amount of desorbed CO2 in prepared catalysts. This result demonstrates that the basicity of prepared catalysts increases obviously with the addition of MgO. It has been reported that the basic catalysts could improve the adsorption of acidic CO2 gas which supplies the surface oxygen to prevent the coke deposition [29,30,39,40]. Choudhary et al. [41] reported that the catalytic performance correlates with the basicity and strength of basic sites of the catalyst. It is expected that the difference of basicity with MgO content may have some effect on the catalytic activity and coke formation in CSCRM. 3.2. Catalytic test in CSCRM First, the prepared catalysts were applied for CSCRM at the GHSV of 265,000 ml/h gcat. All the catalysts reached the thermodynamic equilibrium state from 750 to 650 8C (not

a

0

5

10

20

28

0.57

0.61

0.73

0.84

1.01

Estimated from the integration of CO2-TPD peaks.

shown here). To see the MgO effect on Ni/Al2O3 catalyst clearly, we doubled the GHSV to 530,000 ml/h gcat. Fig. 4 shows the comparison results, depending on the MgO content. CH4 conversion of each catalyst drops with decreasing the reaction temperature from 750 to 650 8C. There is little difference of the catalytic performance depending on the MgO content at 700 and 750 8C, while the catalytic activity shows some difference at 650 8C. Table 3 summarizes the CH4 conversion, CO2 conversion and H2/CO ratio over MgOpromoted Ni/Al2O3 catalysts at 700 8C in CSCRM. 0 wt.% MgO shows the lowest CH4 and CO2 conversion among the catalysts. It is most likely that the addition of MgO is favorable to CO2 adsorption, resulting in the increase of CO2 conversion [27–29]. In general, the adsorption of H2O and CO2 takes place competitively on the support in CSCRM. Song and Pan [42] reported that basic supports prefer to interact with CO2 than with H2O and O2 in tri-reforming of methane, because CO2 is more acidic compared with H2O and O2. Thus it is likely that the adsorption of CO2 is dominant in CSCRM.

Fig. 4. The effect of temperature on CH4 conversion over Ni/MgO/Al2O3 catalysts with various MgO contents in CSCRM (reaction conditions— CH4:H2O:CO2:N2 = 1:0.8:0.4:1, GHSV = 530,000 ml/h gcat).

Table 3 Comparison of reaction results over Ni/MgO/Al2O3 catalyst at 700 8C MgO (wt.%)

CH4 conversion

CO2 conversion

H2/CO

0 5 10 20 28

87 88 88 91 89

64 77 75 77 77

2.1 2.0 2.0 2.0 2.0

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Fig. 5. The stability over Ni/MgO/Al2O3 catalysts with various MgO contents in CSCRM (reaction conditions—T = 650 8C, CH4:H2O:CO2:N2 = 1:0.8:0.4:1, GHSV = 530,000 ml/h gcat).

Fig. 5 shows the stability of catalysts at 650 8C. It is confirmed that the catalyst with 20 wt.% MgO exhibits the highest CH4 conversion and relatively stable activity for 20 h. This is due to the interaction between NiOx species and support and finely dispersed Ni crystallites. The reaction results of 20 wt.% MgO with different reaction temperatures are shown in Table 4. As reaction temperature decreases from 750 to 650 8C, the CH4 and CO2 conversion decreases while H2/CO ratio increases from 2.0 to 2.1. 3.3. Coke study The coke formation over used catalysts was monitored by SEM analysis (Fig. 6) and the coke amount of used catalysts was measured by TGA (Fig. 7). It has been reported that the addition of basic metal oxide results in the suppression of coke formation, which is attributed to the supply of surface oxygen from CO2 adsorption [30]. The difference of catalytic performance in relation to MgO effect can be demonstrated by a tendency to coke Table 4 Comparison of reaction results over Ni/Al2O3 catalyst with MgO = 20 wt.%

CH4 conversion CO2 conversion H2/CO

750 8C

700 8C

650 8C

97 86 2.0

91 77 2.0

76 54 2.1

187

formation in our study. Fig. 6 shows the SEM images of reduced catalyst and used catalysts. Compared to the surface of reduced catalyst, a lot of filamentous carbon species are present on used catalysts, except for the catalyst prepared with 20 wt.% MgO. Fig. 7 shows the TG profiles for the quantitative analysis of carbonaceous species in the used catalysts. The sample weight behavior shows the weight loss in the sample due to the carbon gasification and indicates different types of carbonaceous species formed on the catalysts. According to some previous reports [37,43–45], up to about 300 8C, the initial weight loss is ascribed to the thermal desorption of H2O and CO2 and removal of easily oxidizable carbonaceous species like an amorphous type. Above 500 8C, the large weight loss can be assigned to the oxidation of coke to CO and CO2 (COx). Between 400 and 450 8C, the increase of weight results from the oxidation of nickel particles on the surface. In the case of catalysts prepared without MgO addition, weight increase is ascribed to the oxidation of nickel particles, while this peak is absent in the TG profile of catalysts promoted with 5 and 10 wt.% MgO. It can be explained that the addition of MgO improves the dispersion of Ni particles, resulting in the strong interaction with support; thus the highly dispersed Ni particles are hard to oxidize. These results are in agreement with the fact that it is difficult to fully reduce the Ni/Al2O3 catalyst with 3–10 wt.% MgO at 800 8C based on the result of H2-TPR (Fig. 2). The quantitative data of deposited carbon for TG analysis are summarized in Table 5. As mentioned previously, the oxidation of amorphous carbonaceous species occurs at low temperature, while graphitic carbon is oxidized at high temperature [30,46]. At low MgO content (MgO < 10 wt.%), it is confirmed that coke formation can be ascribed to acid sites which induce the methane decomposition. As the MgO content increases from 0 to 20 wt.%, the amounts of both amorphous and graphitic carbon decrease. On the contrary, 28 wt.% MgO showed the drastic loss in sample weight at between 500 and 700 8C. As shown in Fig. 6, such a loss is attributed to the gasification of whisker coke into CO and CO2. This is due to the presence of free NiO species, which is confirmed from TPR. It is known that free NiO species are responsible for coke formation [38]. However, it was reported that whisker coke does not affect the catalytic activity but results in a pressure drop due to reactor blockage [46]. This result is in agreement with the results of reforming reaction (Fig. 5). Compared to the catalyst with 0 wt.% MgO, the catalyst with 28 wt.% MgO showed high catalytic activity and stability. The electron states of Ni and carbon on the surface of used catalyst are identified using XPS spectra (Fig. 8). Ni 2p3/2 spectra show the main peak (853 eV) and satellite (860 eV) and

Table 5 Quantitative data for the amount of carbon deposited on 12 wt.% Ni/MgO/Al2O3 catalysts MgO (wt.%)

Amorphous carbon (mgC/gcat)

Graphitic carbon (gC/gcat)

Total amount of coke (%)

0 5 10 20 28

0.0100 0.0127 0.0080 0.0039 0.0059

0.0184 0.0084 0.0067 0.0078 0.1071

2.84 2.11 1.47 1.16 11.3

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Fig. 6. SEM images of (a) reduced Ni/MgO/Al2O3 catalysts with 0 wt.% MgO and used catalysts with (b) 0 wt.%, (c) 5 wt.%, (d) 10 wt.%, (e) 20 wt.% and (f) 28 wt.% MgO.

Fig. 7. TG profiles of used 12 wt.% Ni/MgO/Al2O3 catalysts under air.

shift toward the high BE with MgO content due to the electron effect [45,47]. The value of Ni 2p3/2 main peak indicates that Ni electron state is intermediate between bulk Ni metal (852 eV) and NiO (854 eV). In the case of 28 wt.% MgO, the intensity of Ni 2p3/2 spectra is very weak because the Ni surface is enclosed by a large amount of coke based on SEM and XPS analysis. Furthermore the C 1s spectra of 28 wt.% MgO is different from that of other catalysts. The BE of C 1s of the catalyst with 28 wt.% MgO is present at 281 eV and 284 eV which can be assigned to carbides and graphitic carbon, respectively [48,49]. The C 1s spectra of other catalysts show easily oxidizable carbonaceous species and shift to higher BE with MgO content, which is in good agreement with TGA data. These results confirm that 20 wt.% MgO is an optimum content for suppressing coke formation. It is most likely that 20 wt.% MgO enhances the basic strength of support resulting in increasing CO2 adsorption, shows optimum SMSI for the

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Corporation’’ and the ‘‘Ministry of Commerce, Industry and Energy’’. References

Fig. 8. XPS spectra of used 12 wt.% Ni/MgO/Al2O3 catalysts.

highly dispersed Ni crystallites without free NiO species, and forms relatively stable MgAl2O4, which prevents NiAl2O4 formation. As a result, this catalyst exhibits the highest activity with stability resulting from high coke resistance. 4. Conclusion The promotion of MgO on Ni/Al2O3 catalysts for CSCRM effectively suppresses the coke formation and prevents NiAl2O4 formation. With low MgO content (MgO: 0– 10 wt.%), inactive NiAl2O4 forms and surface acid sites are present, which results in coke formation. On the contrary, agglomeration of free Ni species is responsible for coke formation with 28 wt.% MgO. Twenty wt.% MgO content exhibits the highest catalytic activity with stability in CSCRM resulting from high coke resistance. This is mainly due to the enhancement of CO2 adsorption and SMSI. Acknowledgements This study was supported by Energy Resource Technology R&D Program funded by the ‘‘Korea Energy Management

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