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SBA-15 catalysts in dry reforming of methane
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CCLET 3049 1–5 Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet 1 2 3 4 5 6 7
Original article
Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane Q1 Wen-Jia
Cai, Lin-Ping Qian, Bin Yue *, He-Yong He *
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 May 2014 Received in revised form 9 June 2014 Accepted 11 June 2014 Available online xxx
A series of SBA-15 supported bimetallic Rh–Ni catalysts with different weight ratio of Rh/Ni in the range of 0–0.04 were prepared for carbon dioxide reforming of methane. The doping effect of Rh on catalytic activity as well as carbon accumulation and removal over the catalysts was studied. The characterization results indicated that the addition of a small amount of Rh promoted the reducibility of Ni particles and decreased the Ni particle size. During the dry reforming reaction, the carbon deposition was originated from CH4 decomposition and CO disproportionation. The Rh–Ni catalyst with small metallic particle size inhibited the carbon formation and exhibited high efficiency in the removal of coke. In comparison with bare Ni-based catalyst, the Rh–Ni bimetallic catalysts showed high activity and stability in the dry reforming of methane. ß 2014 Bin Yue and He-Yong He. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Methane reforming Carbon dioxide Rh–Ni/SBA-15 Carbon deposition
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1. Introduction
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In the development of Ni-based catalysts for carbon dioxide reforming of methane (DRM), a major focus is directed on the effective suppression of carbon deposition on catalyst surface during the reaction [1]. Although tremendous attempts have been made, carbon deposition is still a great challenge for Ni-based catalyst, preventing DRM from commercialization. In fact, DRM is a promising way to utilize natural gas and to reduce greenhouse gases [2]. Also, the synthetic gas with a low H2/CO ratio formed in DRM is suitable in the synthesis of valuable long chain hydrocarbons and oxygenated chemicals through the Fischer–Tropsch process [3,4]. It has been pointed out that the origin of carbon deposition during DRM mainly occurs via either methane decomposition (CH4 ! C(s) + 2H2) or carbon monoxide disproportionation (2CO ! C(s) + CO2). The former is endothermic and favored at higher temperatures, while the latter is exothermic and favored at lower temperatures [5–7]. Promoter and catalyst particle size are
* Corresponding authors. E-mail addresses: [email protected] (B. Yue), [email protected] (H.-Y. He).
very important in controlling carbon deposition on nickel-based catalysts [1]. Bychkov et al. [8] reported that the rate of carbon accumulation depends on the Ni particle size in the catalyst. Guczi et al. [9] found that the addition of 0.5 wt% Au to 8.8 wt% Ni/ MgAl2O4 improved the catalytic activity and decreased the formation of coke. Pt impregnated Ni-MCM-41 also exhibited the high conversion and stability. The effect of Pt was suggested to be due to its low tendency to carbon deposition [10]. For Ni-MCM41 catalyst, the addition of Pd leads to formation of well dispersed small nickel particles on surface which accounts for the good activity and stability of the catalyst [2]. Wu and Chou [11] studied bimetallic Rh–Ni/BN catalyst, and they found that the addition of Rh increased the activity of dry reforming as well as the stability of the catalysts. Hou and Yashima [12] also found that small amount of Rh–promoted Ni/a-Al2O3 combined the advantages of both Ni and Rh, and exhibited excellent performance for its higher activity and lower cokeformation rate. The present work focuses on the effects of Rh addition on catalyst properties and catalytic performance with an attempt to avoid carbon formation in dry reforming of methane over Rh–Ni bimetallic catalysts. Catalysts with different weight ratio of Rh/Ni are comparatively investigated. In addition, the carbonaceous species formed via either CH4 decomposition or CO disproportionation and removed by CO2 oxidation are discussed.
http://dx.doi.org/10.1016/j.cclet.2014.06.016 1001-8417/ß 2014 Bin Yue and He-Yong He. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: W.-J. Cai, et al., Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.016
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2. Experimental
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2.1. Catalyst preparation
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SBA-15 was synthesized according to the literature method [13]. Nickel nitrate and rhodium chloride were dissolved in deionized water to form solution A, in which Ni content (10 wt%) was kept constant while the Rh content was 0, 0.2 and 0.4 wt%, respectively. Solution A with a volume identical to the pore volume of SBA-15 was added dropwise into the dispersion of 20 mL nhexane containing 0.5 g SBA-15. The mixture was stirred continually for one day. After removal of the solvent by volatilization, the samples were calcined in air from room temperature to 550 8C with a ramp rate of 2 8C min1 and then calcined for further 6 h at 550 8C. The samples prepared were denoted as Ni/SBA-15, 0.02Rh–Ni/SBA-15 and 0.04Rh–Ni/SBA-15, where the value is the weight ratio of Rh/Ni.
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2.2. Characterization
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Temperature-programmed reduction (TPR) studies were performed on a Micromeritics ChemiSorb 2720 apparatus. TPR profiles were obtained by passing a 10% H2/Ar flow (50 mL min1) through the samples. Temperature was increased at a rate of 10 8C min1 from room temperature to 1000 8C and the amount of H2 consumed was determined with a thermal conductivity detector (TCD). XRD patterns were recorded on a Bruker D8 Advances X-ray diffractometer using Cu-Ka1 radiation (l = 0.15406 nm) with a voltage of 40 kV and a current of 40 mA. TEM images were obtained from a JEOL JEM2011 microscope operated at 200 kV. In order to obtain statistically reliable information, the size of ca. 200 particles from at least 10 representative images for each sample was measured to calculate the average particle size. The amounts of carbon formed on the catalysts were determined by a Perkin– Elmer TGA7/DTA7 thermal analyzer in air with a flow rate of 50 mL min1 and a heating rate of 10 8C min1.
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2.3. Reforming of CH4 with CO2
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The dry reforming experiments were conducted at atmospheric pressure in a conventional flow apparatus using a stainless steel fixed bed reactor with quartz lining (i.d. = 5 mm). Typically, 40 mg of the catalyst with 40–60 mesh was reduced in hydrogen at 650 8C for 2 h before reaction. The CO2/CH4 reforming was carried out at 650 8C with a gas hourly space velocity (GHSV) of 12,000 mL h1 g1 (CH4:CO2:N2 = 1:1:3). The effluent was analyzed using on-line gas chromatography with a TCD. A TDX-01 column was used for the separation of H2, CO, CH4 and CO2.
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3. Results and discussion
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TPR experiments were performed to investigate the reducibility of the Ni/SBA-15 and Rh–Ni/SBA-15 catalysts and the interaction between the metal species and the support (Fig. 1). For 0.02Rh–Ni/ SBA-15, the small well defined TPR peak at 165 8C is caused by the reduction of rhodium oxide, implying that there is only one kind of rhodium oxide on SBA-15. However, the reduction temperature is higher than that observed on the supported Rh/SiO2 catalyst at 80 8C [14], indicating that either Rh is better dispersed in bimetallic catalyst than in the monometallic one, or is in strong interaction with nickel [15]. The profile of 0.04Rh–Ni/SBA-15 shows two clear peaks, one at 178 8C and another at 212 8C. Both can be attributed to the reduction of Rh, indicating that there are at least two kinds of rhodium oxide species [16]. For all catalysts, one main reduction peak centered at around 340–400 8C is observed. This can be explained by the reduction of supported Ni oxide species in
Fig. 1. TPR profiles of Ni/SBA-15 and Rh–Ni/SBA-15 catalysts.
different interaction with the support surface. The Rh–free Ni/SBA15 catalyst shows a strong peak at 391 8C and a shoulder at 489 8C. This means an uneven distribution of nickel oxide species interacting differently with the support [17]. The main peak is shifted to 369 8C in 0.02Rh–Ni/SBA-15, indicating that the reduction of NiO is promoted in the presence of the noble metal. Rhodium oxide precursor, which is easily reduced to Rh0 at lower temperatures, is able to activate the hydrogen, thus favoring the reduction of nickel oxide species by H2 spill-over at lower temperature [15,18,19]. Meanwhile, the main peak of 0.04Rh– Ni/SBA-15 shifted to an even lower temperature of 349 8C due to a stronger interaction between Rh and Ni. The catalyst promoted with rhodium behaves more reducible than the monometallic Ni catalyst, ascribing to the increase of the interaction between Rh and Ni and higher dispersion of nickel [19]. Damyanova et al. [2] reported that the catalysts with synergy effect between Pd and Ni provided good metal dispersion and high degree of reduction. The small-angle XRD patterns of the catalysts are shown in Fig. 2A. Three well-resolved peaks of SBA-15 are indexed as (100), (200) and (210) reflections associated with p6mm hexagonal symmetry. In the large-angle XRD patterns (Fig. 2B), the reduced Ni/SBA-15 and Rh–Ni/SBA-15 samples show reflections at 44.58, 51.88 and 76.48 assigned to Fm3m Ni phases with the lattice constant a of 3.523 A˚ (JCPDS 87-0712). Furthermore, no peak associated with Rh was observed, suggesting that Rh is well dispersed or the content of Rh is too low to be detected by XRD [15,19,20]. Fig. 3 shows the TEM images of the reduced Ni/SBA-15 and Rh– Ni/SBA-15 samples. Ni particle sizes of average 6.5, 6.2 and 6.1 nm can be observed on the reduced Ni/SBA-15, 0.02Rh–Ni/SBA-15 and 0.04Rh–Ni/SBA-15 with narrow size distribution, respectively, which are smaller than the pore size of SBA-15 of 6.6 nm. It can be concluded that the presence of Rh in Ni catalysts leads to a decrease of the Ni particle size. Besides, it is to be noted that large Ni particles with an average particle size of 22.4, 17.2 and 16.9 nm were also observed by TEM on the reduced Ni/SBA-15, 0.02Rh–Ni/ SBA-15 and 0.04Rh–Ni/SBA-15, respectively. These Ni particles were located on the external surface of the SBA-15 framework, which could not be confined within the pore channels of mesoporous support. The initial catalytic activity of the catalysts for the DRM reaction at 650 8C with GHSV of 48,000 mL h1 g1 was studied. For Ni/SBA15, the yields of H2 and CO were 49% and 67%, respectively. Meanwhile, the yields of H2 and CO were increased to 58% and 73%
Please cite this article in press as: W.-J. Cai, et al., Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.016
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CCLET 3049 1–5 W.-J. Cai et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
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Fig. 2. XRD patterns of reduced Ni/SBA-15 and Rh–Ni/SBA-15 catalysts. (A) Small-angle XRD in the range of 0.5–5.08 and (B) large-angle XRD in the range of 30–808.
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over 0.02Rh–Ni/SBA-15. Addition of small amount of Rh to monometallic Ni catalyst leads to an increase of the initial activity. The yields of H2 and CO as a function of the time on stream for the first 30 h are presented in Fig. 4. The apparent deactivation was clearly observed over the monometallic Ni catalyst. During the 30 h time-on-stream, the yields of H2 and CO over Ni/SBA-15 catalyst dropped from 66% to 60% and from 79% to 75%, respectively. For 0.04Rh–Ni/SBA-15, the yield of H2 decreased from 66% to 63% while the yield of CO decreased from 79% to 78% within 30 h. It is important to point out that the Rh promoted 0.02Rh–Ni/SBA-15 catalyst is quite stable for DRM reaction. After 30 h reaction the yield of CO was nearly the same whereas the yield of H2 was only about 2% lower than that at beginning of the
reaction. It can be observed that even adding small amount of Rh the stability of bimetallic Rh–Ni catalyst for the DRM is considerably improved with respect to the monometallic Ni catalyst. The carbonaceous deposits on the used Ni/SBA-15 and Rh–Ni/ SBA-15 catalysts after stability test were investigated by TG/DTA under oxidative atmosphere (not shown). The total amount of deposited carbon over the used catalysts increases in the following sequence: 0.02Rh–Ni/SBA-15 (14.7%) < 0.04Rh–Ni/SBA-15 (20.7%) < Ni/SBA-15 (27.3%). This sequence is consistent with the trend of catalytic activity in the long-term stability test shown in Fig. 4. According to the DTA results, the exothermal peak for 0.02Rh–Ni/SBA-15 catalyst is at 560 8C. This low temperature peak
Fig. 3. TEM images of (A) Ni/SBA-15, (B) 0.02Rh–Ni/SBA-15 and (C) 0.04Rh–Ni/SBA-15. Ni particle size distributions of (D) Ni/SBA-15, (E) 0.02Rh–Ni/SBA-15 and (F) 0.04Rh– Ni/SBA-15. Catalysts were reduced with H2 at 650 8C for 2 h.
Please cite this article in press as: W.-J. Cai, et al., Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.016
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Fig. 4. Long-term stability test over Ni/SBA-15 and Rh–Ni/SBA-15 catalysts. Reaction conditions: CH4:CO2:N2 = 1:1:3, GHSV = 12,000 mL h1 g1, 650 8C, 1 atm.
Table 1 TG data of Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalysts. Sample
Ni/SBA-15 0.02Rh–Ni/SBA-15
The content of deposited carbon (wt%)
The efficiency of removal of deposited carbon (%)
a1
a2
b1
b2
ra
rb
17.8 23.5
2.3 1.3
18.7 17.4
4.6 1.8
87 94
75 90
a1: CH4/N2 (9600 mL h1 g1, CH4:N2 = 1:3), 650 8C, 10 h. a2: CH4/N2 (9600 mL h1 g1, CH4:N2 = 1:3), 650 8C, 10 h; then CO2/N2 (9600 mL h1 g1, CO2:N2 = 1:3), 650 8C, 10 h. b1: CO/N2 (10,800 mL h1 g1, CO:N2 = 1:2), 650 8C, 10 h. b2: CO/N2 (10,800 mL h1 g1, CO:N2 = 1:2), 650 8C, 10 h; then CO2/N2 (9600 mL h1 g1, CO2:N2 = 1:3), 650 8C, 10 h. ra = (1 a2/a1) 100%. rb = (1 b2/b1) 100%.
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is due to the oxidation of amorphous carbon [21]. In contrast, 0.04Rh–Ni/SBA-15 shows only a weak peak around 560 8C and a strong peak above 600 8C, and Ni/SBA-15 shows only a sharp peak above 600 8C. The peaks above 600 8C are normally considered from graphitic carbon [22]. The improved stability exhibited by 0.02Rh–Ni/SBA-15 catalyst could be attributed to the more reactive carbon produced as compared with that formed over 0.04Rh–Ni/SBA-15 catalyst. It has been proved that both CH4 and CO contribute to the accumulation of deposited carbon [23,24]. To explain the role of Rh on the suppression of carbon formation, other experiments have been performed. Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst were first exposed to the flow of methane or carbon monoxide at 650 8C for 10 h, then switched to the flow of CO2 at the identical condition. The amounts of carbon deposition over both Ni/SBA-15 and 0.02Rh–Ni/SBA-15 after the reaction were investigated by means of TG as shown in Table 1. After being exposed in the flow of methane, the content of carbon deposition can be estimated as ca. 17.8 wt% and 23.5 wt% for Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst, respectively. This is due to the high efficiency of methane decomposition over 0.02Rh–Ni/SBA-15 than that over Ni/SBA-15. It has been reported that the rate of methane activation increased along with the Ni dispersion [12], suggesting that more active sites for CH4 dissociation can be obtained by decreasing the metal particle size [25]. XRD, TEM and TPR results indicate that when Rh is added into the Ni catalyst, a synergic effect is created between Rh and Ni, which is capable of decreasing the Ni particle size and increasing the dispersion. After further treatment with CO2, the residual carbon was ca. 2.3 wt% and 1.3 wt% for Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst, indicative of the enhancing removal of
carbon over 0.02Rh–Ni/SBA-15 (94%), compared to Ni/SBA-15 (87%). The amount of carbon deposited on Ni/SBA-15 and 0.02Rh–Ni/ SBA-15 catalyst after exposure in the flow of carbon monoxide was ca 18.7 wt% and 17.4 wt%, respectively, in accordance with the observation reported by Goula et al. [25] that the amount of carbon derived from CO disproportionation was larger on the catalyst with the bigger nickel particle size. After further treatment with CO2, the residual carbon was ca. 4.6 wt% and 1.8 wt% for Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst, respectively, corresponding to the efficiencies of removal of deposited carbon of 75% and 90%. It is clear that Rh–Ni species activates CO2 and releases active oxygen atom for removal of deposited carbon and, in particular, the 0.02Rh–Ni/SBA-15 catalyst shows higher efficiency than Ni/SBA15 catalyst (Table 1). It was reported [2] that the small particle size can accelerate the activation of CO2. Comparing the Ni metal particle size of reduced Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst it may be concluded that the small metal particle size for bimetallic catalyst enhances the activation of the CO2 dissociation, which leads to the low carbon deposition in the DRM reaction.
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4. Conclusion
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The effect of Rh on Ni/SBA-15 catalyst was investigated for carbon dioxide reforming of methane. Comparative studies between supported monometallic Ni and bimetallic Rh–Ni catalyst show that the high catalytic performance of Rh promoted Ni catalysts arises from the interaction between Ni and Rh in the bimetallic system, which provides higher metal dispersion and improves reducibility of Ni and coke resistance.
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Please cite this article in press as: W.-J. Cai, et al., Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.016
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Acknowledgment Q2
This work was supported by the National Natural Science Foundation of China (Nos. 21173050, 21371035).
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CCLET 3049 1–5 Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet 1 2 3 4 5 6 7
Original article
Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane Q1 Wen-Jia
Cai, Lin-Ping Qian, Bin Yue *, He-Yong He *
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 May 2014 Received in revised form 9 June 2014 Accepted 11 June 2014 Available online xxx
A series of SBA-15 supported bimetallic Rh–Ni catalysts with different weight ratio of Rh/Ni in the range of 0–0.04 were prepared for carbon dioxide reforming of methane. The doping effect of Rh on catalytic activity as well as carbon accumulation and removal over the catalysts was studied. The characterization results indicated that the addition of a small amount of Rh promoted the reducibility of Ni particles and decreased the Ni particle size. During the dry reforming reaction, the carbon deposition was originated from CH4 decomposition and CO disproportionation. The Rh–Ni catalyst with small metallic particle size inhibited the carbon formation and exhibited high efficiency in the removal of coke. In comparison with bare Ni-based catalyst, the Rh–Ni bimetallic catalysts showed high activity and stability in the dry reforming of methane. ß 2014 Bin Yue and He-Yong He. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Methane reforming Carbon dioxide Rh–Ni/SBA-15 Carbon deposition
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1. Introduction
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In the development of Ni-based catalysts for carbon dioxide reforming of methane (DRM), a major focus is directed on the effective suppression of carbon deposition on catalyst surface during the reaction [1]. Although tremendous attempts have been made, carbon deposition is still a great challenge for Ni-based catalyst, preventing DRM from commercialization. In fact, DRM is a promising way to utilize natural gas and to reduce greenhouse gases [2]. Also, the synthetic gas with a low H2/CO ratio formed in DRM is suitable in the synthesis of valuable long chain hydrocarbons and oxygenated chemicals through the Fischer–Tropsch process [3,4]. It has been pointed out that the origin of carbon deposition during DRM mainly occurs via either methane decomposition (CH4 ! C(s) + 2H2) or carbon monoxide disproportionation (2CO ! C(s) + CO2). The former is endothermic and favored at higher temperatures, while the latter is exothermic and favored at lower temperatures [5–7]. Promoter and catalyst particle size are
* Corresponding authors. E-mail addresses: [email protected] (B. Yue), [email protected] (H.-Y. He).
very important in controlling carbon deposition on nickel-based catalysts [1]. Bychkov et al. [8] reported that the rate of carbon accumulation depends on the Ni particle size in the catalyst. Guczi et al. [9] found that the addition of 0.5 wt% Au to 8.8 wt% Ni/ MgAl2O4 improved the catalytic activity and decreased the formation of coke. Pt impregnated Ni-MCM-41 also exhibited the high conversion and stability. The effect of Pt was suggested to be due to its low tendency to carbon deposition [10]. For Ni-MCM41 catalyst, the addition of Pd leads to formation of well dispersed small nickel particles on surface which accounts for the good activity and stability of the catalyst [2]. Wu and Chou [11] studied bimetallic Rh–Ni/BN catalyst, and they found that the addition of Rh increased the activity of dry reforming as well as the stability of the catalysts. Hou and Yashima [12] also found that small amount of Rh–promoted Ni/a-Al2O3 combined the advantages of both Ni and Rh, and exhibited excellent performance for its higher activity and lower cokeformation rate. The present work focuses on the effects of Rh addition on catalyst properties and catalytic performance with an attempt to avoid carbon formation in dry reforming of methane over Rh–Ni bimetallic catalysts. Catalysts with different weight ratio of Rh/Ni are comparatively investigated. In addition, the carbonaceous species formed via either CH4 decomposition or CO disproportionation and removed by CO2 oxidation are discussed.
http://dx.doi.org/10.1016/j.cclet.2014.06.016 1001-8417/ß 2014 Bin Yue and He-Yong He. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: W.-J. Cai, et al., Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.016
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2. Experimental
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2.1. Catalyst preparation
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SBA-15 was synthesized according to the literature method [13]. Nickel nitrate and rhodium chloride were dissolved in deionized water to form solution A, in which Ni content (10 wt%) was kept constant while the Rh content was 0, 0.2 and 0.4 wt%, respectively. Solution A with a volume identical to the pore volume of SBA-15 was added dropwise into the dispersion of 20 mL nhexane containing 0.5 g SBA-15. The mixture was stirred continually for one day. After removal of the solvent by volatilization, the samples were calcined in air from room temperature to 550 8C with a ramp rate of 2 8C min1 and then calcined for further 6 h at 550 8C. The samples prepared were denoted as Ni/SBA-15, 0.02Rh–Ni/SBA-15 and 0.04Rh–Ni/SBA-15, where the value is the weight ratio of Rh/Ni.
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2.2. Characterization
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Temperature-programmed reduction (TPR) studies were performed on a Micromeritics ChemiSorb 2720 apparatus. TPR profiles were obtained by passing a 10% H2/Ar flow (50 mL min1) through the samples. Temperature was increased at a rate of 10 8C min1 from room temperature to 1000 8C and the amount of H2 consumed was determined with a thermal conductivity detector (TCD). XRD patterns were recorded on a Bruker D8 Advances X-ray diffractometer using Cu-Ka1 radiation (l = 0.15406 nm) with a voltage of 40 kV and a current of 40 mA. TEM images were obtained from a JEOL JEM2011 microscope operated at 200 kV. In order to obtain statistically reliable information, the size of ca. 200 particles from at least 10 representative images for each sample was measured to calculate the average particle size. The amounts of carbon formed on the catalysts were determined by a Perkin– Elmer TGA7/DTA7 thermal analyzer in air with a flow rate of 50 mL min1 and a heating rate of 10 8C min1.
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2.3. Reforming of CH4 with CO2
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The dry reforming experiments were conducted at atmospheric pressure in a conventional flow apparatus using a stainless steel fixed bed reactor with quartz lining (i.d. = 5 mm). Typically, 40 mg of the catalyst with 40–60 mesh was reduced in hydrogen at 650 8C for 2 h before reaction. The CO2/CH4 reforming was carried out at 650 8C with a gas hourly space velocity (GHSV) of 12,000 mL h1 g1 (CH4:CO2:N2 = 1:1:3). The effluent was analyzed using on-line gas chromatography with a TCD. A TDX-01 column was used for the separation of H2, CO, CH4 and CO2.
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3. Results and discussion
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TPR experiments were performed to investigate the reducibility of the Ni/SBA-15 and Rh–Ni/SBA-15 catalysts and the interaction between the metal species and the support (Fig. 1). For 0.02Rh–Ni/ SBA-15, the small well defined TPR peak at 165 8C is caused by the reduction of rhodium oxide, implying that there is only one kind of rhodium oxide on SBA-15. However, the reduction temperature is higher than that observed on the supported Rh/SiO2 catalyst at 80 8C [14], indicating that either Rh is better dispersed in bimetallic catalyst than in the monometallic one, or is in strong interaction with nickel [15]. The profile of 0.04Rh–Ni/SBA-15 shows two clear peaks, one at 178 8C and another at 212 8C. Both can be attributed to the reduction of Rh, indicating that there are at least two kinds of rhodium oxide species [16]. For all catalysts, one main reduction peak centered at around 340–400 8C is observed. This can be explained by the reduction of supported Ni oxide species in
Fig. 1. TPR profiles of Ni/SBA-15 and Rh–Ni/SBA-15 catalysts.
different interaction with the support surface. The Rh–free Ni/SBA15 catalyst shows a strong peak at 391 8C and a shoulder at 489 8C. This means an uneven distribution of nickel oxide species interacting differently with the support [17]. The main peak is shifted to 369 8C in 0.02Rh–Ni/SBA-15, indicating that the reduction of NiO is promoted in the presence of the noble metal. Rhodium oxide precursor, which is easily reduced to Rh0 at lower temperatures, is able to activate the hydrogen, thus favoring the reduction of nickel oxide species by H2 spill-over at lower temperature [15,18,19]. Meanwhile, the main peak of 0.04Rh– Ni/SBA-15 shifted to an even lower temperature of 349 8C due to a stronger interaction between Rh and Ni. The catalyst promoted with rhodium behaves more reducible than the monometallic Ni catalyst, ascribing to the increase of the interaction between Rh and Ni and higher dispersion of nickel [19]. Damyanova et al. [2] reported that the catalysts with synergy effect between Pd and Ni provided good metal dispersion and high degree of reduction. The small-angle XRD patterns of the catalysts are shown in Fig. 2A. Three well-resolved peaks of SBA-15 are indexed as (100), (200) and (210) reflections associated with p6mm hexagonal symmetry. In the large-angle XRD patterns (Fig. 2B), the reduced Ni/SBA-15 and Rh–Ni/SBA-15 samples show reflections at 44.58, 51.88 and 76.48 assigned to Fm3m Ni phases with the lattice constant a of 3.523 A˚ (JCPDS 87-0712). Furthermore, no peak associated with Rh was observed, suggesting that Rh is well dispersed or the content of Rh is too low to be detected by XRD [15,19,20]. Fig. 3 shows the TEM images of the reduced Ni/SBA-15 and Rh– Ni/SBA-15 samples. Ni particle sizes of average 6.5, 6.2 and 6.1 nm can be observed on the reduced Ni/SBA-15, 0.02Rh–Ni/SBA-15 and 0.04Rh–Ni/SBA-15 with narrow size distribution, respectively, which are smaller than the pore size of SBA-15 of 6.6 nm. It can be concluded that the presence of Rh in Ni catalysts leads to a decrease of the Ni particle size. Besides, it is to be noted that large Ni particles with an average particle size of 22.4, 17.2 and 16.9 nm were also observed by TEM on the reduced Ni/SBA-15, 0.02Rh–Ni/ SBA-15 and 0.04Rh–Ni/SBA-15, respectively. These Ni particles were located on the external surface of the SBA-15 framework, which could not be confined within the pore channels of mesoporous support. The initial catalytic activity of the catalysts for the DRM reaction at 650 8C with GHSV of 48,000 mL h1 g1 was studied. For Ni/SBA15, the yields of H2 and CO were 49% and 67%, respectively. Meanwhile, the yields of H2 and CO were increased to 58% and 73%
Please cite this article in press as: W.-J. Cai, et al., Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.016
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Fig. 2. XRD patterns of reduced Ni/SBA-15 and Rh–Ni/SBA-15 catalysts. (A) Small-angle XRD in the range of 0.5–5.08 and (B) large-angle XRD in the range of 30–808.
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over 0.02Rh–Ni/SBA-15. Addition of small amount of Rh to monometallic Ni catalyst leads to an increase of the initial activity. The yields of H2 and CO as a function of the time on stream for the first 30 h are presented in Fig. 4. The apparent deactivation was clearly observed over the monometallic Ni catalyst. During the 30 h time-on-stream, the yields of H2 and CO over Ni/SBA-15 catalyst dropped from 66% to 60% and from 79% to 75%, respectively. For 0.04Rh–Ni/SBA-15, the yield of H2 decreased from 66% to 63% while the yield of CO decreased from 79% to 78% within 30 h. It is important to point out that the Rh promoted 0.02Rh–Ni/SBA-15 catalyst is quite stable for DRM reaction. After 30 h reaction the yield of CO was nearly the same whereas the yield of H2 was only about 2% lower than that at beginning of the
reaction. It can be observed that even adding small amount of Rh the stability of bimetallic Rh–Ni catalyst for the DRM is considerably improved with respect to the monometallic Ni catalyst. The carbonaceous deposits on the used Ni/SBA-15 and Rh–Ni/ SBA-15 catalysts after stability test were investigated by TG/DTA under oxidative atmosphere (not shown). The total amount of deposited carbon over the used catalysts increases in the following sequence: 0.02Rh–Ni/SBA-15 (14.7%) < 0.04Rh–Ni/SBA-15 (20.7%) < Ni/SBA-15 (27.3%). This sequence is consistent with the trend of catalytic activity in the long-term stability test shown in Fig. 4. According to the DTA results, the exothermal peak for 0.02Rh–Ni/SBA-15 catalyst is at 560 8C. This low temperature peak
Fig. 3. TEM images of (A) Ni/SBA-15, (B) 0.02Rh–Ni/SBA-15 and (C) 0.04Rh–Ni/SBA-15. Ni particle size distributions of (D) Ni/SBA-15, (E) 0.02Rh–Ni/SBA-15 and (F) 0.04Rh– Ni/SBA-15. Catalysts were reduced with H2 at 650 8C for 2 h.
Please cite this article in press as: W.-J. Cai, et al., Rh doping effect on coking resistance of Ni/SBA-15 catalysts in dry reforming of methane, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.016
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Fig. 4. Long-term stability test over Ni/SBA-15 and Rh–Ni/SBA-15 catalysts. Reaction conditions: CH4:CO2:N2 = 1:1:3, GHSV = 12,000 mL h1 g1, 650 8C, 1 atm.
Table 1 TG data of Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalysts. Sample
Ni/SBA-15 0.02Rh–Ni/SBA-15
The content of deposited carbon (wt%)
The efficiency of removal of deposited carbon (%)
a1
a2
b1
b2
ra
rb
17.8 23.5
2.3 1.3
18.7 17.4
4.6 1.8
87 94
75 90
a1: CH4/N2 (9600 mL h1 g1, CH4:N2 = 1:3), 650 8C, 10 h. a2: CH4/N2 (9600 mL h1 g1, CH4:N2 = 1:3), 650 8C, 10 h; then CO2/N2 (9600 mL h1 g1, CO2:N2 = 1:3), 650 8C, 10 h. b1: CO/N2 (10,800 mL h1 g1, CO:N2 = 1:2), 650 8C, 10 h. b2: CO/N2 (10,800 mL h1 g1, CO:N2 = 1:2), 650 8C, 10 h; then CO2/N2 (9600 mL h1 g1, CO2:N2 = 1:3), 650 8C, 10 h. ra = (1 a2/a1) 100%. rb = (1 b2/b1) 100%.
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is due to the oxidation of amorphous carbon [21]. In contrast, 0.04Rh–Ni/SBA-15 shows only a weak peak around 560 8C and a strong peak above 600 8C, and Ni/SBA-15 shows only a sharp peak above 600 8C. The peaks above 600 8C are normally considered from graphitic carbon [22]. The improved stability exhibited by 0.02Rh–Ni/SBA-15 catalyst could be attributed to the more reactive carbon produced as compared with that formed over 0.04Rh–Ni/SBA-15 catalyst. It has been proved that both CH4 and CO contribute to the accumulation of deposited carbon [23,24]. To explain the role of Rh on the suppression of carbon formation, other experiments have been performed. Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst were first exposed to the flow of methane or carbon monoxide at 650 8C for 10 h, then switched to the flow of CO2 at the identical condition. The amounts of carbon deposition over both Ni/SBA-15 and 0.02Rh–Ni/SBA-15 after the reaction were investigated by means of TG as shown in Table 1. After being exposed in the flow of methane, the content of carbon deposition can be estimated as ca. 17.8 wt% and 23.5 wt% for Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst, respectively. This is due to the high efficiency of methane decomposition over 0.02Rh–Ni/SBA-15 than that over Ni/SBA-15. It has been reported that the rate of methane activation increased along with the Ni dispersion [12], suggesting that more active sites for CH4 dissociation can be obtained by decreasing the metal particle size [25]. XRD, TEM and TPR results indicate that when Rh is added into the Ni catalyst, a synergic effect is created between Rh and Ni, which is capable of decreasing the Ni particle size and increasing the dispersion. After further treatment with CO2, the residual carbon was ca. 2.3 wt% and 1.3 wt% for Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst, indicative of the enhancing removal of
carbon over 0.02Rh–Ni/SBA-15 (94%), compared to Ni/SBA-15 (87%). The amount of carbon deposited on Ni/SBA-15 and 0.02Rh–Ni/ SBA-15 catalyst after exposure in the flow of carbon monoxide was ca 18.7 wt% and 17.4 wt%, respectively, in accordance with the observation reported by Goula et al. [25] that the amount of carbon derived from CO disproportionation was larger on the catalyst with the bigger nickel particle size. After further treatment with CO2, the residual carbon was ca. 4.6 wt% and 1.8 wt% for Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst, respectively, corresponding to the efficiencies of removal of deposited carbon of 75% and 90%. It is clear that Rh–Ni species activates CO2 and releases active oxygen atom for removal of deposited carbon and, in particular, the 0.02Rh–Ni/SBA-15 catalyst shows higher efficiency than Ni/SBA15 catalyst (Table 1). It was reported [2] that the small particle size can accelerate the activation of CO2. Comparing the Ni metal particle size of reduced Ni/SBA-15 and 0.02Rh–Ni/SBA-15 catalyst it may be concluded that the small metal particle size for bimetallic catalyst enhances the activation of the CO2 dissociation, which leads to the low carbon deposition in the DRM reaction.
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4. Conclusion
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The effect of Rh on Ni/SBA-15 catalyst was investigated for carbon dioxide reforming of methane. Comparative studies between supported monometallic Ni and bimetallic Rh–Ni catalyst show that the high catalytic performance of Rh promoted Ni catalysts arises from the interaction between Ni and Rh in the bimetallic system, which provides higher metal dispersion and improves reducibility of Ni and coke resistance.
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Acknowledgment Q2
This work was supported by the National Natural Science Foundation of China (Nos. 21173050, 21371035).
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