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γ-Al2O3 catalyst
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Hydrogen production from carbon dioxide reforming of methane over highly active and stable MgO promoted CoeNi/g-Al2O3 catalyst In Hyuk Son a,**, Seung Jae Lee a, Hyun-Seog Roh b,* a
Environment Group, Energy & Environment Research Center, Samsung Advanced Institute of Technology (SAIT), Samsung Electronics Co. LTD, Gyounggi-Do 446-712, Republic of Korea b Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon 220-710, Republic of Korea
article info
abstract
Article history:
CoNi/Al2O3 and MgCoNi/Al2O3 catalysts are investigated for hydrogen production from CO2
Received 5 July 2013
reforming of CH4 reaction at the gas hourly space velocity of 40,000 mL g1 h1. The MgO
Received in revised form
promoted CoNi/Al2O3 catalyst shows much higher conversions (97% for CO2 and 95% for
18 December 2013
CH4 at 850 C) than the CoNi/Al2O3 catalyst. In addition, the stability is maintained for 200 h
Accepted 24 December 2013
in CO2 reforming of CH4. The outstanding catalytic activity and stability of the MgO pro-
Available online 25 January 2014
moted CoNi/Al2O3 catalyst is mainly due to the basic nature of MgO, an intimate interaction between Ni and the support, and rapid decomposition/dissociation of CH4 and CO2,
Keywords:
resulting in preventing coke formation in CO2 reforming of CH4.
Carbon dioxide
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Reforming
reserved.
Methane Magnesia Intimate interaction
1.
Introduction
The production of H2 or synthesis gas from carbon dioxide reforming of methane (CDR: CH4 þ CO2 / 2H2 þ 2CO) is an emerging and challenging issue for the chemical utilization of green house gases such as carbon dioxide and methane [1e4]. The CDR reaction becomes industrially advantageous compared to steam reforming of methane (SRM: CH4 þ H2O / 3H2 þ CO) in synthesis gas production since H2/ CO ratio of product is close to 1.0. The synthesis gas with low H2/CO ratio is suitable for direct use in FeT synthesis or oxo-
synthesis, and both of which require lower H2/CO ratio than that obtained from conventional SRM. Supported Ni catalysts have been employed for the target reaction from an economical point of view [5e8]. However, supported Ni catalysts easily deactivate on account of either coke formation and/or sintering of Ni metal. Therefore, the primary difficulty in CDR is developing nano-sized Ni catalysts with high activity and stability under severe conditions. According to the literature, the nature of supports affects the catalytic performance of supported Ni catalysts in CDR [9e11]. Many commercial Ni/Al2O3-based catalysts are available for fuel reforming [12]. These catalysts are more economical
* Corresponding author. Tel.: þ82 33 760 2834; fax: þ82 33 760 2571. ** Corresponding author. Tel.: þ82 31 280 1852; fax: þ82 31 280 9359. E-mail addresses: [email protected] (I.H. Son), [email protected] (H.-S. Roh). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.141
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than noble metals and can operate stably with high activity under excess steam. However, severe carbon deposition is observed on the Ni/Al2O3 catalyst in CDR, which does not use excess steam [13]. Recently, extensive research has been performed to develop a highly active and stable catalyst. A Ni alloy with precious metals such as Pt, Rh, Ru, and Pd showed excellent performance in CDR resulting from preventing coke formation [14e18]. In addition, it was reported that the strong interaction between Ni and supports such as TiO2, CeO2, and ZrO2 enhanced the resistance against sintering of Ni and coke formation [19e24]. However, the typical catalyst for reforming reaction is composed of Ni particles deposited on an alumina support [25e29]. This is due to the lower cost and availability of Al2O3. Likewise, many commercial catalysts for reforming are Ni/ Al2O3-based catalysts, such as Ni-0309S (Engelhard Company), ICI-46-1 (Imperial Chemical Industries), and FCR-4 (Su¨dChemie). However, the problem of coke formation and sintering could not be addressed completely. For this reason, Ni/Al2O3 catalyst compositions have been intensively investigated to minimize coke formation by preparing a Ni alloy with other transition metals such as Cu, Co, and Sn [30e34]. Especially, researchers have been interested in bimetallic NieCo systems due to the fact that the addition of cobalt to nickel catalysts reduces coke formation in CDR [35e39]. They ascribed the enhanced performance to the improved catalyst resistance to metal oxidation. Therefore, it is possible that bimetallic catalysts may exhibit superior performance for CDR compared with the corresponding monometallic catalysts [40]. Considerable studies also have been conducted on the modification of catalyst supports using promoters to overcome the catalyst deactivation [40e47]. Choudhary et al. [41] reported that the catalyst precoated with MgO and CaO showed high activity compared to that of catalyst without precoating. Koo et al. [42] reported that the MgO promoted Ni/ Al2O3 catalyst forms MgAl2O4 spinel phase, which is stable at high temperature and effectively prevents coke formation by increasing the CO2 adsorption due to the increase in base strength on the surface of catalyst. In this study, we have designed a MgO promoted CoNi/gAl2O3 catalyst to enhance coke resistance and have found that this catalyst showed much higher activity and stability than CoNi/g-Al2O3 catalyst at 800 C for 200 h in CDR. Detailed studies on the characterization and catalytic activity for CDR are furnished in this study. Namely, the beneficial effects of MgO on the performance over CoNi/g-Al2O3 catalyst for CDR were systematically investigated. Especially, we tried to explain excellent catalytic performance of MgCoNi/g-Al2O3 catalyst with the location of each element by using the SEM-EDS elemental mapping technique for the first time. Moreover, we confirmed that the adsorbed chemical species strongly affects on catalytic performance using the in situ DRIFTS analysis.
2.
Experimental procedures
2.1.
Catalyst preparation
Magnesium (MgN2O6$6H2O, Aldrich), Cobalt (Co(NO3)2$6H2O, Aldrich) and Nickel (Ni(NO3)2$6H2O, Aldrich) were co-
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impregnated over g-Al2O3 (SBET ¼ 150 m2/g, w3 mm 4; Aldrich) using the incipient wetness method with quantitative loading (Mg ¼ 3 wt.%, Ni ¼ 3 wt.%, and Co ¼ 3 wt.%) [48e51]. The prepared catalysts were dried at 120 C for 24 h and calcined in air (300 ml/min) at 500 C for 5 h. The calcined catalyst was reduced in pure H2 with increasing temperature (10 C/min) and maintained at 850 C for 1 h.
2.2.
CDR reaction test unit
The reaction temperature was changed from 700 to 850 C. The reaction pressure was fixed at 1 atm. CDR reaction was conducted in a micro-tubular quartz reactor. Prior to each catalytic measurement, the catalyst was reduced in pure H2 at 850 C for 1 h. The detailed procedure for the CDR reaction was explained in the literature [12]. The reactant feed comprised a gaseous mixture of CH4:CO2:N2 (1:1:1). N2 was employed as a reference for calculating CH4 and CO2 conversions. To screen the catalysts effectively, a gas hourly space velocity (GHSV) of 40,000 mL g1 h1 was used in this study. The conversions of CH4 and CO2 were calculated using the following formulas. CH4 conversionð%Þ ¼
½CH4 in ½CH4 out 100 ½CH4 in
CO2 conversionð%Þ ¼
½CO2 in ½CO2 out 100 ½CO2 in
2.3.
Characterization
The BET surface area of support and catalyst was measured by N2 adsorption at 196 C using a BET instrument (BELsorp, BEL, Japan). X-ray diffraction (XRD) patterns were obtained by a Philips X’pert Pro X-ray diffractometer. The crystalline size of the Ni particle was estimated using the DebyeeScherrer’s equation [52]. CO2 temperature-programmed desorption (TPD) was performed on a Chemisorption Analyzer (Micrometrics ASAP 2010). The detailed procedure for CO2-TPD was described earlier [12]. To check coke amount of the used samples, TGA study was carried out from 30 to 850 C in air using a METTLER TOLEDO TGA/DSC1 with a heating rate of 5 C/min. Temperature-programmed reduction (TPR) experiments were conducted in a Chemisorption Analyzer (AutoChem II 2920). The detailed procedure for TPR was described in the previous paper [12,53]. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study was carried out on a Nicolet 5700 FTIR spectrometer with a MCT detector. A powder catalyst sample was put into a reaction cell (Harricks, Praying Mantis) and reduced in situ at 800 C for 30 min in H2 flow. For in situ reaction monitoring, the reaction temperature was set to 300 C. A mixture flow of 100 ml/min (CH4:CO2:N2 ¼ 1:1:1) was introduced at that temperature. For all the spectra recorded, a 32-scan data accumulation was carried out at a resolution of 4 cm1. The catalysts were also examined using the ultra-high-resolution field emission scanning electron microscopy (UHR-FE-SEM; Hitachi S-5500, resolution 0.4 nm) with transmission electron microscopy operating at 30 kV. Elemental composition was assessed using an energy dispersive X-ray spectroscopy (EDS) in conjunction with UHR-FE-SEM.
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3.
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Results and discussion
Table 1 summarizes physical properties of CoNi/Al2O3 (marked as CoNi in figures) and MgCoNi/Al2O3 (marked as MgCoNi in figures) catalysts (Mg ¼ 3 wt.%, Ni ¼ 3 wt.%, and Co ¼ 3 wt.%). The BET surface area of reduced CoNi/Al2O3 is relatively lower than that of reduced MgCoNi/Al2O3. However, the decrease of the BET surface area is ca. 50% (from 156.3 m2/ g to 78.8 m2/g) for used CoNi/Al2O3 catalyst after the CDR reaction, while only ca. 30% decrease (from 167.3 m2/g to 117.4 m2/g) is measured for the catalyst with MgO. Likewise, the pore volume of CoNi/Al2O3 decreased from 0.44 m2/g to 0.32 m2/g. On the contrary, there was no change of the pore volume for the MgCoNi/Al2O3 catalyst. The crystallite size of Ni was estimated using XRD. The crystallite size of Ni increased significantly after the CDR reaction. In the case of MgCoNi/Al2O3, the Ni crystallite size of used MgCoNi/Al2O3 (9.5 nm) is slightly bigger than that of reduced MgCoNi/Al2O3 (6.9 nm). In the case of CoNi/Al2O3, however, the reduced catalyst shows 5.8 nm but the used catalyst 18.9 nm. Thus, it has been confirmed that sintering took place severely for the CoNi/Al2O3 catalyst. On the contrary, sintering of Ni crystallite could be prevented effectively for the MgCoNi/Al2O3 catalyst due to the beneficial role of MgO. XRD patterns of reduced catalysts are shown in Fig. 1. XRD patterns of both CoNi/Al2O3 and MgCoNi/Al2O3 show diffraction lines associated with pseudo-amorphous gamma alumina (JCPDS 750921) with a slight shift to lower angle. This indicates the presence of Ni aluminate (JCPDS 100339) [54]. The XRD patterns of MgCoNi/Al2O3 show the formation of Mg spinel (MgAl2O4, JCPDS 705187) because a shift in the diffraction lines of alumina is observed. The MgAl2O4 is stable and efficient support, due to its spinel structure and basic property [32]. In addition, both metallic Co and Ni peaks appear at 51.4 and 76.0 . Other peaks originate from the support [54]. Fig. 2 shows H2-TPR patterns of CoNi/Al2O3 and MgCoNi/ Al2O3 catalysts. The CoNi/Al2O3 catalyst shows two major peaks. The first peak appears at 360 C and the other at 750 C. The first reduction peak is due to the reduction of relatively free NiO species, which have weak interaction with the support [55]. The peak appearing at 750 C can be assigned to the reduction of NiAl2O4 [33]. In addition, the reduction of Co oxides occurs with the two-step process: Co3O4 / CoO / Co0. The reduction peaks of Co3O4 and CoO appear at ca. 400 C and at ca. 700 C, respectively [56]. However, it is hard to distinguish the Co3O4 peak from the relatively free NiO peak and the CoO peak from the NiAl2O4 peak because peaks overlap each other.
Fig. 1 e XRD patterns of reduced CoNi/Al2O3 and MgCoNi/ Al2O3 catalysts. The TPR pattern of MgCoNi/Al2O3 is different from that of CoNi/Al2O3. The reduction peak of NiAl2O4 is remarkably decreased. This result indicates that NiAl2O4 formation is effectively prevented due to the addition of MgO. In addition, the first peak shifts toward lower temperature. It is known that the addition of MgO has a beneficial effect in the reduction of free NiO species [57e59]. Fig. 3 shows CO2-TPD patterns of reduced CoNi/Al2O3 and MgCoNi/Al2O3 catalysts. Compared with CoNi/Al2O3, MgCoNi/ Al2O3 shows higher peak intensity from 150 to 400 C. This is mainly due to the increase of basicity resulting from the addition of MgO. It has been reported that the basic catalyst supplies the surface oxygen by acidic CO2 gas to prevent coke formation [60e63]. Choudhary et al. [64] reported that the catalytic performance depends significantly on the basicity and strength of basic sites of the catalyst. Thus, it is expected that the MgCoNi/Al2O3 catalyst can have strong resistance against coke formation due to the beneficial effect of MgO. Fig. 4 illustrates the results of ultra-high-resolution SEM/ TEM dual mode microscopy showing images in both the scanning and transmission modes and energy-dispersive Xray analysis in the same position. MgCoNi particles are visible both in the SEM image and in the TEM image. According to Fig. 4(c), Mg is present on CoNi particles. Due to this structure, it is expected that MgCoNi/Al2O3 can have enhanced activity and strong resistance against coke formation in CDR. In addition, the surface of the MgCoNi particles is partially covered by alumina. Alumina is located at the same position as the MgCoNi particles. This result indicates that MgCoNi particles are not isolated, but surrounded with alumina supports.
Table 1 e Physical properties of the catalysts. The used catalysts indicate after 200 h of CDR reaction (Reaction conditions: T [ 850 C, P [ 1 atm, GHSV [ 40,000 mL gL1 hL1, CH4:CO2:N2 [ 1:1:1). Catalyst description
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Ni crystallite diameter (nm)a
Reduced CoNi/Al2O3 Used CoNi/Al2O3 Reduced MgCoNi/Al2O3 Used MgCoNi/Al2O3
156.3 78.8 167.3 117.4
0.44 0.32 0.40 0.40
11.18 16.15 9.48 13.49
5.8 18.9 6.9 9.5
a
Data obtained from Ni(220) diffraction peak broadening using the Scherrer equation.
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Fig. 2 e TPR patterns of CoNi/Al2O3 and MgCoNi/Al2O3 catalysts.
Fig. 5 depicts the in-situ DRIFT spectra of CO2 reforming over CoNi/Al2O3 and MgCoNi/Al2O3 catalysts. In CH4eCO2 cofeeding conditions, large amount of formate (nɑS, 1 1 C]O ¼ 1588 cm ) and carbonate (nS, C]O ¼ 1431 cm ) species were formed. It was reported that the adsorption of CO2 on basic support forms carbonate ðCO2 3 Þ/hydrocarbonate ðHCO3 Þ species which react with H atoms produced from CH4 decomposition to form formate (HCOO) intermediates [65], which are subsequently decomposed into CO and adsorbed OH groups. Efstathiou et al. [66] found that the formate species on Al2O3 surface was stable under CH4/CO2 reforming conditions. Generally, the spectra of CoNi/Al2O3 are similar to those of MgCoNi/Al2O3. Interestingly, with the addition of MgO, the adsorption of formate species was clearly increased. This result indicates that the addition of MgO helps to accelerate the decomposition/dissociation of CH4 and CO2, resulting in increasing the formation of formate intermediate.
Fig. 4 e (a) SEM, (b) TEM and (c) line-scan EDX analysis for reduced MgCoNi/Al2O3 catalyst at the same position.
Fig. 3 e CO2-TPD patterns of reduced CoNi/Al2O3 and MgCoNi/Al2O3 catalysts.
The CO2 reforming reaction was performed with the feed ratio of CH4:CO2:N2 ¼ 1:1:1 at gas hourly space velocity (GHSV) ¼ 40,000 mL g1 h1 from 700 to 850 C. CDR reaction data with reaction temperature over CoNi/Al2O3 and MgCoNi/
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Fig. 5 e In-situ DRIFT spectra of CO2 reforming on CoNi/Al2O3 and MgCoNi/Al2O3 catalysts at 300 C.
Al2O3 catalysts are presented in Fig. 6. MgCoNi/Al2O3 catalyst exhibited much higher CH4 and CO2 conversion than CoNi/ Al2O3 catalyst within the temperature range between 700 and 850 C. It is very interesting to see that CO2 conversion is relatively higher than CH4 conversion for both catalysts. This is due to the reverse water gas shift reaction. As a result, the H2/CO ratio is less than unity for both catalysts. However, with increasing the temperature, CH4 conversion is close to CO2 conversion. To check the stability of CoNi/Al2O3 and MgCoNi/Al2O3 catalysts with time on stream, CO2 reforming reaction data were collected for 200 h. Fig. 7 shows CH4 and CO2 conversion and H2/CO ratio with time on stream over CoNi/Al2O3 and MgCoNi/Al2O3 catalysts. MgCoNi/Al2O3 catalyst revealed very high CH4 and CO2 conversion. Surprisingly, the stability was maintained for 200 h without detectable catalyst deactivation. On the contrary, CoNi/Al2O3 catalyst continuously deactivated with time on stream. Both CH4 and CO2 conversion dropped with time on stream. Table 2 summarizes CH4 and CO2 conversion, H2 and CO yield, H2/CO ratio and coking rate. In the case of initial data, CO2 conversion of CoNi/Al2O3 was lower than that of MgCoNi/ Al2O3. After 200 h of CDR, however, the former showed 89.3% CO2 conversion and the latter 96.7%. These data indicate that MgCoNi/Al2O3 is more active and stable than CoNi/Al2O3. The trend of CH4 conversion was similar to that of CO2 conversion. In the case of CoNi/Al2O3, CH4 conversion was decreased from 88.3 to 85.5%. On the contrary, the MgCoNi/Al2O3 catalyst exhibited still high CH4 conversion (95.1%) at the GHSV of 40,000 mL g1 h1. It is a rare case that a supported Ni catalyst shows very high activity as well as stability at the GHSV of 40,000 mL g1 h1 in CDR for 200 h. H2 yield was lower than CO yield due to the reverse water gas shift reaction (RWGS: H2 þ CO2 / H2O þ CO). As a result, the H2/CO ratio value was lower than 1.0. To evaluate the effect of coke, used catalysts were characterized by TGA in air. Fig. 8 depicts the Thermo-gravimetric (TG) profiles for the quantitative analysis of carbonaceous
species in the used catalysts (CoNi/Al2O3 and MgCoNi/Al2O3). According to previous reports [67e70], up to 300 C, the initial weight loss is assigned to the thermal desorption of water and removal of easily oxidizable carbonaceous species. Above 500 C, the large weight loss can be assigned to the oxidation of coke to CO and CO2. In other words, the oxidation of amorphous carbonaceous species occurs at low temperature, while graphitic carbon is oxidized at high temperature [61,71]. In the
Fig. 6 e CH4 and CO2 conversion and H2/CO ratio of CoNi/ Al2O3 and MgCoNi/Al2O3 catalysts with various temperatures (Reaction conditions: P [ 1 atm, GHSV [ 40,000 mL gL1 hL1, CH4:CO2:N2 [ 1:1:1).
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Fig. 8 e Thermogravimetric and differential thermogravimetric profile of CoNi/Al2O3 and MgCoNi/Al2O3 catalysts after reaction for 200 h (Reaction conditions: T [ 850 C, P [ 1 atm, GHSV [ 40,000 mL gL1 hL1, CH4:CO2:N2 [ 1:1:1). latter 9.5 nm (Table 1). Thus, it is confirmed that the addition of MgO is highly effective in preventing the sintering of Ni in CDR. Nitrogen adsorption/desorption isotherms were used to confirm the structural properties of CoNi/Al2O3 and MgCoNi/ Al2O3 catalysts and the results are shown in Fig. 10. All isotherms present a sharp step at intermediate relative pressure (P/P0) of 0.5e0.9. All the samples give type IV adsorption isotherms with a hysteresis loop, indicating that the mesoporous framework of Al2O3 can be well-retained after the impregnation of Mg, Co, and Ni [72]. It is clear that the pore volume of CoNi/Al2O3 was significantly decreased, which is due to coke formation and sintering, while the pore volume of MgCoNi/ Al2O3 did not change significantly.
Fig. 7 e CH4 and CO2 conversion and H2/CO ratio of CoNi/ Al2O3 and MgCoNi/Al2O3 catalysts over reaction for 200 h (Reaction conditions: T [ 850 C, P [ 1 atm, GHSV [ 40,000 mL gL1 hL1, CH4:CO2:N2 [ 1:1:1).
case of MgCoNi/Al2O3, about 5% weight loss was measured due to the removal of both amorphous and graphitic carbon. On the contrary, CoNi/Al2O3 showed the drastic weight loss (16%) between 450 and 660 C. As a consequence, CoNi/Al2O3 showed the coking rate of 0.80 mgc/gcat h, while MgCoNi/Al2O3 revealed the coking rate of 0.25 mgc/gcat h. It should be noted that the coking rate of MgCoNi/Al2O3 is 3 times lower than that of CoNi/ Al2O3. This clearly confirms that MgCoNi/Al2O3 has much stronger coke resistance than CoNi/Al2O3 in CDR. It suggests that the addition of MgO effectively stabilizes the catalyst in CDR. This is possibly related to a strong interaction between Ni and MgO, confirmed from TPR. As a result, the MgCoNi/Al2O3 catalyst exhibited the outstanding performance under severe condition (GHSV of 40,000 mL g1 h1 for 200 h). In addition, to elucidate the beneficial effect of MgO in preventing sintering of Ni, used catalysts were characterized by XRD (Fig. 9). At the initial stage, the Ni crystallite size of CoNi/Al2O3 is slightly smaller than that of MgCoNi/Al2O3. After the reaction for 200 h, however, the former shows 18.9 nm Ni crystallite size and the
4.
Conclusions
MgCoNi/Al2O3 exhibited higher CH4 and CO2 conversion than CoNi/Al2O3 with the temperature range from 700 to 850 C at the GHSV of 40,000 mL g1 h1 in CDR. In addition, the MgCoNi/Al2O3 catalyst showed stable activity at 850 C for 200 h, while the CoNi/Al2O3 catalyst deactivated with time on stream. The remarkable catalytic performance of the MgCoNi/ Al2O3 catalyst is mainly ascribed to the beneficial effect of MgO, resulting from the basic nature of MgO, a strong interaction between Ni and MgO, and rapid decomposition/dissociation of CH4 and CO2. As a result, the MgCoNi/Al2O3 catalyst can possess stronger resistance against coke formation and Ni
Table 2 e Reaction data of the catalysts. Final data were obtained after 200 h of CDR reaction. Catalyst description
Initial CoNi/Al2O3 Final CoNi/Al2O3 Initial MgCoNi/Al2O3 Final MgCoNi/Al2O3
Conversion (%)
Yield (%)
CH4
CO2
H2
CO
88.3 85.5 95.0 95.1
92.1 89.3 96.6 96.7
88.9 86.6 94.2 94.1
92.4 89.9 96.9 97.0
(Reaction conditions: T ¼ 850 C, P ¼ 1 atm, GHSV ¼ 40,000 mL g1 h1, CH4:CO2:N2 ¼ 1:1:1).
H2/CO ratio
Coking rate (mgC/gcat h)
0.92 0.90 0.97 0.96
e 0.80 e 0.25
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Fig. 9 e XRD patterns of used CoNi/Al2O3 and MgCoNi/Al2O3 catalysts (Reaction conditions: T [ 850 C, P [ 1 atm, TOS [ 200 h).
Fig. 10 e Nitrogen adsorption/desorption isotherms of reduced (filled symbol) and used (open symbol) catalysts after reaction for 200 h.
sintering than the CoNi/Al2O3 catalyst. Therefore, MgCoNi/ Al2O3 catalyst can be a promising catalyst for CDR, which utilizes two major green house gases (CO2 and CH4).
Acknowledgments The authors gratefully acknowledge financial support from the Samsung Advanced Institute of Technology (SAIT), Samsung Electronics Co. Ltd.
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Available online at www.sciencedirect.com
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Hydrogen production from carbon dioxide reforming of methane over highly active and stable MgO promoted CoeNi/g-Al2O3 catalyst In Hyuk Son a,**, Seung Jae Lee a, Hyun-Seog Roh b,* a
Environment Group, Energy & Environment Research Center, Samsung Advanced Institute of Technology (SAIT), Samsung Electronics Co. LTD, Gyounggi-Do 446-712, Republic of Korea b Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon 220-710, Republic of Korea
article info
abstract
Article history:
CoNi/Al2O3 and MgCoNi/Al2O3 catalysts are investigated for hydrogen production from CO2
Received 5 July 2013
reforming of CH4 reaction at the gas hourly space velocity of 40,000 mL g1 h1. The MgO
Received in revised form
promoted CoNi/Al2O3 catalyst shows much higher conversions (97% for CO2 and 95% for
18 December 2013
CH4 at 850 C) than the CoNi/Al2O3 catalyst. In addition, the stability is maintained for 200 h
Accepted 24 December 2013
in CO2 reforming of CH4. The outstanding catalytic activity and stability of the MgO pro-
Available online 25 January 2014
moted CoNi/Al2O3 catalyst is mainly due to the basic nature of MgO, an intimate interaction between Ni and the support, and rapid decomposition/dissociation of CH4 and CO2,
Keywords:
resulting in preventing coke formation in CO2 reforming of CH4.
Carbon dioxide
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Reforming
reserved.
Methane Magnesia Intimate interaction
1.
Introduction
The production of H2 or synthesis gas from carbon dioxide reforming of methane (CDR: CH4 þ CO2 / 2H2 þ 2CO) is an emerging and challenging issue for the chemical utilization of green house gases such as carbon dioxide and methane [1e4]. The CDR reaction becomes industrially advantageous compared to steam reforming of methane (SRM: CH4 þ H2O / 3H2 þ CO) in synthesis gas production since H2/ CO ratio of product is close to 1.0. The synthesis gas with low H2/CO ratio is suitable for direct use in FeT synthesis or oxo-
synthesis, and both of which require lower H2/CO ratio than that obtained from conventional SRM. Supported Ni catalysts have been employed for the target reaction from an economical point of view [5e8]. However, supported Ni catalysts easily deactivate on account of either coke formation and/or sintering of Ni metal. Therefore, the primary difficulty in CDR is developing nano-sized Ni catalysts with high activity and stability under severe conditions. According to the literature, the nature of supports affects the catalytic performance of supported Ni catalysts in CDR [9e11]. Many commercial Ni/Al2O3-based catalysts are available for fuel reforming [12]. These catalysts are more economical
* Corresponding author. Tel.: þ82 33 760 2834; fax: þ82 33 760 2571. ** Corresponding author. Tel.: þ82 31 280 1852; fax: þ82 31 280 9359. E-mail addresses: [email protected] (I.H. Son), [email protected] (H.-S. Roh). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.141
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than noble metals and can operate stably with high activity under excess steam. However, severe carbon deposition is observed on the Ni/Al2O3 catalyst in CDR, which does not use excess steam [13]. Recently, extensive research has been performed to develop a highly active and stable catalyst. A Ni alloy with precious metals such as Pt, Rh, Ru, and Pd showed excellent performance in CDR resulting from preventing coke formation [14e18]. In addition, it was reported that the strong interaction between Ni and supports such as TiO2, CeO2, and ZrO2 enhanced the resistance against sintering of Ni and coke formation [19e24]. However, the typical catalyst for reforming reaction is composed of Ni particles deposited on an alumina support [25e29]. This is due to the lower cost and availability of Al2O3. Likewise, many commercial catalysts for reforming are Ni/ Al2O3-based catalysts, such as Ni-0309S (Engelhard Company), ICI-46-1 (Imperial Chemical Industries), and FCR-4 (Su¨dChemie). However, the problem of coke formation and sintering could not be addressed completely. For this reason, Ni/Al2O3 catalyst compositions have been intensively investigated to minimize coke formation by preparing a Ni alloy with other transition metals such as Cu, Co, and Sn [30e34]. Especially, researchers have been interested in bimetallic NieCo systems due to the fact that the addition of cobalt to nickel catalysts reduces coke formation in CDR [35e39]. They ascribed the enhanced performance to the improved catalyst resistance to metal oxidation. Therefore, it is possible that bimetallic catalysts may exhibit superior performance for CDR compared with the corresponding monometallic catalysts [40]. Considerable studies also have been conducted on the modification of catalyst supports using promoters to overcome the catalyst deactivation [40e47]. Choudhary et al. [41] reported that the catalyst precoated with MgO and CaO showed high activity compared to that of catalyst without precoating. Koo et al. [42] reported that the MgO promoted Ni/ Al2O3 catalyst forms MgAl2O4 spinel phase, which is stable at high temperature and effectively prevents coke formation by increasing the CO2 adsorption due to the increase in base strength on the surface of catalyst. In this study, we have designed a MgO promoted CoNi/gAl2O3 catalyst to enhance coke resistance and have found that this catalyst showed much higher activity and stability than CoNi/g-Al2O3 catalyst at 800 C for 200 h in CDR. Detailed studies on the characterization and catalytic activity for CDR are furnished in this study. Namely, the beneficial effects of MgO on the performance over CoNi/g-Al2O3 catalyst for CDR were systematically investigated. Especially, we tried to explain excellent catalytic performance of MgCoNi/g-Al2O3 catalyst with the location of each element by using the SEM-EDS elemental mapping technique for the first time. Moreover, we confirmed that the adsorbed chemical species strongly affects on catalytic performance using the in situ DRIFTS analysis.
2.
Experimental procedures
2.1.
Catalyst preparation
Magnesium (MgN2O6$6H2O, Aldrich), Cobalt (Co(NO3)2$6H2O, Aldrich) and Nickel (Ni(NO3)2$6H2O, Aldrich) were co-
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impregnated over g-Al2O3 (SBET ¼ 150 m2/g, w3 mm 4; Aldrich) using the incipient wetness method with quantitative loading (Mg ¼ 3 wt.%, Ni ¼ 3 wt.%, and Co ¼ 3 wt.%) [48e51]. The prepared catalysts were dried at 120 C for 24 h and calcined in air (300 ml/min) at 500 C for 5 h. The calcined catalyst was reduced in pure H2 with increasing temperature (10 C/min) and maintained at 850 C for 1 h.
2.2.
CDR reaction test unit
The reaction temperature was changed from 700 to 850 C. The reaction pressure was fixed at 1 atm. CDR reaction was conducted in a micro-tubular quartz reactor. Prior to each catalytic measurement, the catalyst was reduced in pure H2 at 850 C for 1 h. The detailed procedure for the CDR reaction was explained in the literature [12]. The reactant feed comprised a gaseous mixture of CH4:CO2:N2 (1:1:1). N2 was employed as a reference for calculating CH4 and CO2 conversions. To screen the catalysts effectively, a gas hourly space velocity (GHSV) of 40,000 mL g1 h1 was used in this study. The conversions of CH4 and CO2 were calculated using the following formulas. CH4 conversionð%Þ ¼
½CH4 in ½CH4 out 100 ½CH4 in
CO2 conversionð%Þ ¼
½CO2 in ½CO2 out 100 ½CO2 in
2.3.
Characterization
The BET surface area of support and catalyst was measured by N2 adsorption at 196 C using a BET instrument (BELsorp, BEL, Japan). X-ray diffraction (XRD) patterns were obtained by a Philips X’pert Pro X-ray diffractometer. The crystalline size of the Ni particle was estimated using the DebyeeScherrer’s equation [52]. CO2 temperature-programmed desorption (TPD) was performed on a Chemisorption Analyzer (Micrometrics ASAP 2010). The detailed procedure for CO2-TPD was described earlier [12]. To check coke amount of the used samples, TGA study was carried out from 30 to 850 C in air using a METTLER TOLEDO TGA/DSC1 with a heating rate of 5 C/min. Temperature-programmed reduction (TPR) experiments were conducted in a Chemisorption Analyzer (AutoChem II 2920). The detailed procedure for TPR was described in the previous paper [12,53]. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study was carried out on a Nicolet 5700 FTIR spectrometer with a MCT detector. A powder catalyst sample was put into a reaction cell (Harricks, Praying Mantis) and reduced in situ at 800 C for 30 min in H2 flow. For in situ reaction monitoring, the reaction temperature was set to 300 C. A mixture flow of 100 ml/min (CH4:CO2:N2 ¼ 1:1:1) was introduced at that temperature. For all the spectra recorded, a 32-scan data accumulation was carried out at a resolution of 4 cm1. The catalysts were also examined using the ultra-high-resolution field emission scanning electron microscopy (UHR-FE-SEM; Hitachi S-5500, resolution 0.4 nm) with transmission electron microscopy operating at 30 kV. Elemental composition was assessed using an energy dispersive X-ray spectroscopy (EDS) in conjunction with UHR-FE-SEM.
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3.
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Results and discussion
Table 1 summarizes physical properties of CoNi/Al2O3 (marked as CoNi in figures) and MgCoNi/Al2O3 (marked as MgCoNi in figures) catalysts (Mg ¼ 3 wt.%, Ni ¼ 3 wt.%, and Co ¼ 3 wt.%). The BET surface area of reduced CoNi/Al2O3 is relatively lower than that of reduced MgCoNi/Al2O3. However, the decrease of the BET surface area is ca. 50% (from 156.3 m2/ g to 78.8 m2/g) for used CoNi/Al2O3 catalyst after the CDR reaction, while only ca. 30% decrease (from 167.3 m2/g to 117.4 m2/g) is measured for the catalyst with MgO. Likewise, the pore volume of CoNi/Al2O3 decreased from 0.44 m2/g to 0.32 m2/g. On the contrary, there was no change of the pore volume for the MgCoNi/Al2O3 catalyst. The crystallite size of Ni was estimated using XRD. The crystallite size of Ni increased significantly after the CDR reaction. In the case of MgCoNi/Al2O3, the Ni crystallite size of used MgCoNi/Al2O3 (9.5 nm) is slightly bigger than that of reduced MgCoNi/Al2O3 (6.9 nm). In the case of CoNi/Al2O3, however, the reduced catalyst shows 5.8 nm but the used catalyst 18.9 nm. Thus, it has been confirmed that sintering took place severely for the CoNi/Al2O3 catalyst. On the contrary, sintering of Ni crystallite could be prevented effectively for the MgCoNi/Al2O3 catalyst due to the beneficial role of MgO. XRD patterns of reduced catalysts are shown in Fig. 1. XRD patterns of both CoNi/Al2O3 and MgCoNi/Al2O3 show diffraction lines associated with pseudo-amorphous gamma alumina (JCPDS 750921) with a slight shift to lower angle. This indicates the presence of Ni aluminate (JCPDS 100339) [54]. The XRD patterns of MgCoNi/Al2O3 show the formation of Mg spinel (MgAl2O4, JCPDS 705187) because a shift in the diffraction lines of alumina is observed. The MgAl2O4 is stable and efficient support, due to its spinel structure and basic property [32]. In addition, both metallic Co and Ni peaks appear at 51.4 and 76.0 . Other peaks originate from the support [54]. Fig. 2 shows H2-TPR patterns of CoNi/Al2O3 and MgCoNi/ Al2O3 catalysts. The CoNi/Al2O3 catalyst shows two major peaks. The first peak appears at 360 C and the other at 750 C. The first reduction peak is due to the reduction of relatively free NiO species, which have weak interaction with the support [55]. The peak appearing at 750 C can be assigned to the reduction of NiAl2O4 [33]. In addition, the reduction of Co oxides occurs with the two-step process: Co3O4 / CoO / Co0. The reduction peaks of Co3O4 and CoO appear at ca. 400 C and at ca. 700 C, respectively [56]. However, it is hard to distinguish the Co3O4 peak from the relatively free NiO peak and the CoO peak from the NiAl2O4 peak because peaks overlap each other.
Fig. 1 e XRD patterns of reduced CoNi/Al2O3 and MgCoNi/ Al2O3 catalysts. The TPR pattern of MgCoNi/Al2O3 is different from that of CoNi/Al2O3. The reduction peak of NiAl2O4 is remarkably decreased. This result indicates that NiAl2O4 formation is effectively prevented due to the addition of MgO. In addition, the first peak shifts toward lower temperature. It is known that the addition of MgO has a beneficial effect in the reduction of free NiO species [57e59]. Fig. 3 shows CO2-TPD patterns of reduced CoNi/Al2O3 and MgCoNi/Al2O3 catalysts. Compared with CoNi/Al2O3, MgCoNi/ Al2O3 shows higher peak intensity from 150 to 400 C. This is mainly due to the increase of basicity resulting from the addition of MgO. It has been reported that the basic catalyst supplies the surface oxygen by acidic CO2 gas to prevent coke formation [60e63]. Choudhary et al. [64] reported that the catalytic performance depends significantly on the basicity and strength of basic sites of the catalyst. Thus, it is expected that the MgCoNi/Al2O3 catalyst can have strong resistance against coke formation due to the beneficial effect of MgO. Fig. 4 illustrates the results of ultra-high-resolution SEM/ TEM dual mode microscopy showing images in both the scanning and transmission modes and energy-dispersive Xray analysis in the same position. MgCoNi particles are visible both in the SEM image and in the TEM image. According to Fig. 4(c), Mg is present on CoNi particles. Due to this structure, it is expected that MgCoNi/Al2O3 can have enhanced activity and strong resistance against coke formation in CDR. In addition, the surface of the MgCoNi particles is partially covered by alumina. Alumina is located at the same position as the MgCoNi particles. This result indicates that MgCoNi particles are not isolated, but surrounded with alumina supports.
Table 1 e Physical properties of the catalysts. The used catalysts indicate after 200 h of CDR reaction (Reaction conditions: T [ 850 C, P [ 1 atm, GHSV [ 40,000 mL gL1 hL1, CH4:CO2:N2 [ 1:1:1). Catalyst description
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Ni crystallite diameter (nm)a
Reduced CoNi/Al2O3 Used CoNi/Al2O3 Reduced MgCoNi/Al2O3 Used MgCoNi/Al2O3
156.3 78.8 167.3 117.4
0.44 0.32 0.40 0.40
11.18 16.15 9.48 13.49
5.8 18.9 6.9 9.5
a
Data obtained from Ni(220) diffraction peak broadening using the Scherrer equation.
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Fig. 2 e TPR patterns of CoNi/Al2O3 and MgCoNi/Al2O3 catalysts.
Fig. 5 depicts the in-situ DRIFT spectra of CO2 reforming over CoNi/Al2O3 and MgCoNi/Al2O3 catalysts. In CH4eCO2 cofeeding conditions, large amount of formate (nɑS, 1 1 C]O ¼ 1588 cm ) and carbonate (nS, C]O ¼ 1431 cm ) species were formed. It was reported that the adsorption of CO2 on basic support forms carbonate ðCO2 3 Þ/hydrocarbonate ðHCO3 Þ species which react with H atoms produced from CH4 decomposition to form formate (HCOO) intermediates [65], which are subsequently decomposed into CO and adsorbed OH groups. Efstathiou et al. [66] found that the formate species on Al2O3 surface was stable under CH4/CO2 reforming conditions. Generally, the spectra of CoNi/Al2O3 are similar to those of MgCoNi/Al2O3. Interestingly, with the addition of MgO, the adsorption of formate species was clearly increased. This result indicates that the addition of MgO helps to accelerate the decomposition/dissociation of CH4 and CO2, resulting in increasing the formation of formate intermediate.
Fig. 4 e (a) SEM, (b) TEM and (c) line-scan EDX analysis for reduced MgCoNi/Al2O3 catalyst at the same position.
Fig. 3 e CO2-TPD patterns of reduced CoNi/Al2O3 and MgCoNi/Al2O3 catalysts.
The CO2 reforming reaction was performed with the feed ratio of CH4:CO2:N2 ¼ 1:1:1 at gas hourly space velocity (GHSV) ¼ 40,000 mL g1 h1 from 700 to 850 C. CDR reaction data with reaction temperature over CoNi/Al2O3 and MgCoNi/
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Fig. 5 e In-situ DRIFT spectra of CO2 reforming on CoNi/Al2O3 and MgCoNi/Al2O3 catalysts at 300 C.
Al2O3 catalysts are presented in Fig. 6. MgCoNi/Al2O3 catalyst exhibited much higher CH4 and CO2 conversion than CoNi/ Al2O3 catalyst within the temperature range between 700 and 850 C. It is very interesting to see that CO2 conversion is relatively higher than CH4 conversion for both catalysts. This is due to the reverse water gas shift reaction. As a result, the H2/CO ratio is less than unity for both catalysts. However, with increasing the temperature, CH4 conversion is close to CO2 conversion. To check the stability of CoNi/Al2O3 and MgCoNi/Al2O3 catalysts with time on stream, CO2 reforming reaction data were collected for 200 h. Fig. 7 shows CH4 and CO2 conversion and H2/CO ratio with time on stream over CoNi/Al2O3 and MgCoNi/Al2O3 catalysts. MgCoNi/Al2O3 catalyst revealed very high CH4 and CO2 conversion. Surprisingly, the stability was maintained for 200 h without detectable catalyst deactivation. On the contrary, CoNi/Al2O3 catalyst continuously deactivated with time on stream. Both CH4 and CO2 conversion dropped with time on stream. Table 2 summarizes CH4 and CO2 conversion, H2 and CO yield, H2/CO ratio and coking rate. In the case of initial data, CO2 conversion of CoNi/Al2O3 was lower than that of MgCoNi/ Al2O3. After 200 h of CDR, however, the former showed 89.3% CO2 conversion and the latter 96.7%. These data indicate that MgCoNi/Al2O3 is more active and stable than CoNi/Al2O3. The trend of CH4 conversion was similar to that of CO2 conversion. In the case of CoNi/Al2O3, CH4 conversion was decreased from 88.3 to 85.5%. On the contrary, the MgCoNi/Al2O3 catalyst exhibited still high CH4 conversion (95.1%) at the GHSV of 40,000 mL g1 h1. It is a rare case that a supported Ni catalyst shows very high activity as well as stability at the GHSV of 40,000 mL g1 h1 in CDR for 200 h. H2 yield was lower than CO yield due to the reverse water gas shift reaction (RWGS: H2 þ CO2 / H2O þ CO). As a result, the H2/CO ratio value was lower than 1.0. To evaluate the effect of coke, used catalysts were characterized by TGA in air. Fig. 8 depicts the Thermo-gravimetric (TG) profiles for the quantitative analysis of carbonaceous
species in the used catalysts (CoNi/Al2O3 and MgCoNi/Al2O3). According to previous reports [67e70], up to 300 C, the initial weight loss is assigned to the thermal desorption of water and removal of easily oxidizable carbonaceous species. Above 500 C, the large weight loss can be assigned to the oxidation of coke to CO and CO2. In other words, the oxidation of amorphous carbonaceous species occurs at low temperature, while graphitic carbon is oxidized at high temperature [61,71]. In the
Fig. 6 e CH4 and CO2 conversion and H2/CO ratio of CoNi/ Al2O3 and MgCoNi/Al2O3 catalysts with various temperatures (Reaction conditions: P [ 1 atm, GHSV [ 40,000 mL gL1 hL1, CH4:CO2:N2 [ 1:1:1).
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Fig. 8 e Thermogravimetric and differential thermogravimetric profile of CoNi/Al2O3 and MgCoNi/Al2O3 catalysts after reaction for 200 h (Reaction conditions: T [ 850 C, P [ 1 atm, GHSV [ 40,000 mL gL1 hL1, CH4:CO2:N2 [ 1:1:1). latter 9.5 nm (Table 1). Thus, it is confirmed that the addition of MgO is highly effective in preventing the sintering of Ni in CDR. Nitrogen adsorption/desorption isotherms were used to confirm the structural properties of CoNi/Al2O3 and MgCoNi/ Al2O3 catalysts and the results are shown in Fig. 10. All isotherms present a sharp step at intermediate relative pressure (P/P0) of 0.5e0.9. All the samples give type IV adsorption isotherms with a hysteresis loop, indicating that the mesoporous framework of Al2O3 can be well-retained after the impregnation of Mg, Co, and Ni [72]. It is clear that the pore volume of CoNi/Al2O3 was significantly decreased, which is due to coke formation and sintering, while the pore volume of MgCoNi/ Al2O3 did not change significantly.
Fig. 7 e CH4 and CO2 conversion and H2/CO ratio of CoNi/ Al2O3 and MgCoNi/Al2O3 catalysts over reaction for 200 h (Reaction conditions: T [ 850 C, P [ 1 atm, GHSV [ 40,000 mL gL1 hL1, CH4:CO2:N2 [ 1:1:1).
case of MgCoNi/Al2O3, about 5% weight loss was measured due to the removal of both amorphous and graphitic carbon. On the contrary, CoNi/Al2O3 showed the drastic weight loss (16%) between 450 and 660 C. As a consequence, CoNi/Al2O3 showed the coking rate of 0.80 mgc/gcat h, while MgCoNi/Al2O3 revealed the coking rate of 0.25 mgc/gcat h. It should be noted that the coking rate of MgCoNi/Al2O3 is 3 times lower than that of CoNi/ Al2O3. This clearly confirms that MgCoNi/Al2O3 has much stronger coke resistance than CoNi/Al2O3 in CDR. It suggests that the addition of MgO effectively stabilizes the catalyst in CDR. This is possibly related to a strong interaction between Ni and MgO, confirmed from TPR. As a result, the MgCoNi/Al2O3 catalyst exhibited the outstanding performance under severe condition (GHSV of 40,000 mL g1 h1 for 200 h). In addition, to elucidate the beneficial effect of MgO in preventing sintering of Ni, used catalysts were characterized by XRD (Fig. 9). At the initial stage, the Ni crystallite size of CoNi/Al2O3 is slightly smaller than that of MgCoNi/Al2O3. After the reaction for 200 h, however, the former shows 18.9 nm Ni crystallite size and the
4.
Conclusions
MgCoNi/Al2O3 exhibited higher CH4 and CO2 conversion than CoNi/Al2O3 with the temperature range from 700 to 850 C at the GHSV of 40,000 mL g1 h1 in CDR. In addition, the MgCoNi/Al2O3 catalyst showed stable activity at 850 C for 200 h, while the CoNi/Al2O3 catalyst deactivated with time on stream. The remarkable catalytic performance of the MgCoNi/ Al2O3 catalyst is mainly ascribed to the beneficial effect of MgO, resulting from the basic nature of MgO, a strong interaction between Ni and MgO, and rapid decomposition/dissociation of CH4 and CO2. As a result, the MgCoNi/Al2O3 catalyst can possess stronger resistance against coke formation and Ni
Table 2 e Reaction data of the catalysts. Final data were obtained after 200 h of CDR reaction. Catalyst description
Initial CoNi/Al2O3 Final CoNi/Al2O3 Initial MgCoNi/Al2O3 Final MgCoNi/Al2O3
Conversion (%)
Yield (%)
CH4
CO2
H2
CO
88.3 85.5 95.0 95.1
92.1 89.3 96.6 96.7
88.9 86.6 94.2 94.1
92.4 89.9 96.9 97.0
(Reaction conditions: T ¼ 850 C, P ¼ 1 atm, GHSV ¼ 40,000 mL g1 h1, CH4:CO2:N2 ¼ 1:1:1).
H2/CO ratio
Coking rate (mgC/gcat h)
0.92 0.90 0.97 0.96
e 0.80 e 0.25
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Fig. 9 e XRD patterns of used CoNi/Al2O3 and MgCoNi/Al2O3 catalysts (Reaction conditions: T [ 850 C, P [ 1 atm, TOS [ 200 h).
Fig. 10 e Nitrogen adsorption/desorption isotherms of reduced (filled symbol) and used (open symbol) catalysts after reaction for 200 h.
sintering than the CoNi/Al2O3 catalyst. Therefore, MgCoNi/ Al2O3 catalyst can be a promising catalyst for CDR, which utilizes two major green house gases (CO2 and CH4).
Acknowledgments The authors gratefully acknowledge financial support from the Samsung Advanced Institute of Technology (SAIT), Samsung Electronics Co. Ltd.
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