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SiO2 catalysts using fluidized-bed reactor
ARTICLE IN PRESS
Energy 31 (2006) 2184–2192 www.elsevier.com/locate/energy
Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modified Ni/SiO2 catalysts using fluidized-bed reactor Q.S. Jinga,, X.M. Zhengb a
Department of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, PR China b Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China Received 9 September 2003
Abstract Nickel on zirconium-modified silica was prepared and tested as a catalyst for reforming methane with CO2 and O2 in a fluidized-bed reactor. A conversion of CH4 near thermodynamic equilibrium and low H2/CO ratio (1oH2/COo2) were obtained without catalyst deactivation during 10 h, in a most energy efficient and safe manner. A weight loading of 5 wt% zirconium was found to be the optimum. The catalysts were characterized using X-ray diffraction (XRD), H2-temperature reaction (H2-TPR), CO2-temperature desorption (CO2-TPD) and transmission election microscope (TEM) techniques. Ni sintering was a major reason for the deactivation of pure Ni/SiO2 catalysts, while Ni dispersed highly on a zirconiumpromoted Ni/SiO2 catalyst. The different kinds of surface Ni species formed on ZrO2-promoted catalysts might be responsible for its high activity and good resistance to Ni sintering. r 2005 Elsevier Ltd. All rights reserved. Keywords: Methane; Partial oxidation; CO2 reforming; Ni/ZrO2-SiO2; Fluidized-bed reactor
1. Introduction There has recently been a revival of interest in the reforming of methane with CO2 and O2 to produce synthesis gas [1–3]. An advantage of this combined process, instead of processes such as stream reforming and partial oxidation, is the suitable H2/CO ratio obtained, which is of particular interest to the synthesis of valuable oxygenated chemicals. Furthermore, this combined process is autothermic in the most energy efficient and safe manner. Coking formation on the catalyst is a serious problem in CO2 reforming alone but not in the combined process. Ross et al. [4] have reported that Pt/ZrO2 is an active and stable catalyst for combined partial oxidation and CO2 reforming of CH4 in a fixed-bed reactor. A small amount of carbon deposition occurred in parts of the catalyst bed that was not exposed to oxygen. Hot spots in the catalyst bed were reduced significantly. Corresponding author. Tel.: +86 376 6390702; fax: +86 376 6390957.
E-mail address: [email protected] (Q.S. Jing). 0360-5442/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.07.005
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It has been reported [5,6] that methane combustion is followed by CO2 and H2O reforming of methane and water-gas shift reaction. In that case, highly exothermic combustion and considerably endothermic reforming caused significant temperature gradient in the catalyst bed; therefore, hot spots at the beginning part of the catalyst bed and carbon deposition at the rear part of the catalyst bed were observed in the fixed-bed reactor. Several studies on the reforming of methane with CO2 and O2 using the fluidized-bed reactor have been reported [7,8]. It has been insisted that the high rates of heat transfer and stability of the operation were obtained by using the fluidized-bed reactor. Combustion proceeded in the front part of the catalyst bed and reforming did in the rear part both cases of the fixed and fluidized bed. It is thought that both combustion and reforming proceeded simultaneously in fluidized-bed reactor. Furthermore, fluidization of the catalyst particles between the oxidizing and reducing zones can effectively limit the amount of carbon formation. Previous reports from our laboratory have shown that the Ni/SiO2 catalyst was active for the combined reaction in fluidized-bed reactor [9]. Nevertheless, the Ni/SiO2 catalyst was prone to lose activity due to nickel sintering. In this paper, we report syngas production from the combined reaction in a fluidized-bed reactor, using Ni/SiO2 catalysts that are promoted by ZrO2. 2. Experimental 2.1. Catalyst preparation Ni/SiO2 and Ni–ZrO2/SiO2 catalysts were prepared by the incipient-wetness impregnation method. Nitrate salts were used as precursors, and commercial microspherical silica (SiO2, from Qingdao Oceanic Chemical Co, China; Brunauer-Emmett-Teller (BET) surface area (SBET) of 374 m2/g, particle diameter of 0.28–0.45 mm) was used as support. After impregnation, the samples were dried at 393 K for 12 h and subsequently calcined at 773 K for 4 h in air. The prepared catalysts were designated as Ni/xZrO2–SiO2. The x in the terminology of Ni/xZrO2–SiO2 catalysts represents the zirconium loading (x wt%) over Ni/xZrO2–SiO2 catalysts. The loading of nickel on the tested catalysts was always 5 wt%. 2.2. Catalytic reaction The catalytic reaction was performed in a fluidized-bed reactor that was comprised of a quartz tube (inner diameter (ID) of 12 mm, height (H) of 750 mm) under atmospheric pressure. A thermocouple that was connected to a PID temperature controller was placed outside the quartz tube and in the middle of the catalyst bed. With a given molar ratio, CH4 (99.9%), CO2 (99.9%) and O2 (99.9%) were fed to the reactor controlled by mass flow controller (Brooks, 5850E, 71%.FS). Prior to the reaction, 2 cm3 catalysts were used and pretreated at 923 K for 1 h under hydrogen flow at atmospheric pressure. Effluent gas of reaction was cooled in an ice-water bath and analyzed with an online gas chromatograph (Shimadzu, GC-8A) equipped with a TCD detector (column packing: TDX-01). The conversion and the selectivity were calculated as given in the literature [10]: X ðCH4 Þð%Þ ¼ ðF CH4 ;in F CH4 ;out Þ=F CH4 ;in 100,
(1)
X ðCO2 Þð%Þ ¼ ðF CO2 ;in ; F CO2 ;out Þ=F CO2 ;in 100,
(2)
SðH2 Þð%Þ ¼ F H2 ;out =2ðF CH4 ;in F CH4 ;out Þ 100,
(3)
SðCOÞð%Þ ¼ F CO;out =½ðF CH4 ;in F out;CH4 Þ þ ðF CO2 ;in F CO2 ;out Þ 100,
(4)
F i ¼ C i F total ,
(5)
where X, S and F are the conversion, selectivity and gas flow rate, respectively; F total is the total feed gas flow rate or reaction effluent gas rate, and C i is the molar fraction of component i in the feed gas or reaction effluent gas. In our reaction conditions, the detected oxygen conversion was 100%.
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2.3. Catalyst characterization X-ray diffraction (XRD) patterns were obtained on a Rigaku-D/max-B (Rigaku, Japan) automated power X-ray diffractometer using a CuKa radiation (45 kV, 40 mA). The samples for XRD measurement were at oxide state. The average NiO particle sizes were calculated using the half-width at half-height of the most intense peak of the diffraction pattern and the Scherrer equation. And the catalysts were examined at a scanning rate of 0.11/min. Transmission election microscope (TEM) images were taken by means of JEM-2020F (JEOL) operated at 200 kV. Samples were dispersed in tetrachloromethane by supersonic waves and put on Cu grids for the TEM observation in the air. 3. Results and discussion 3.1. Catalytic performance The stability of 5 wt%Ni/ZrO2–SiO2 catalyst is conducted at 1023 K and GHSV ¼ 9000 h1. The effect of zirconium loading on catalytic activity and stability is shown in Fig. 1. The pure Ni/SiO2 catalyst exhibited same performance as the modified catalysts, and there is no evident effect on the initial activity for different zirconium content. However, the stability of catalyst was significantly enhanced when the support was promoted with ZrO2. When the Zr loading is below 2 wt%, especially for unmodified catalyst, the catalysts deactivated rapidly during 6 h on stream. The catalytic stability of 5Ni/ZrO2–SiO2 is remarkably enhanced when zirconium loading is above 2 wt%. However, the catalytic activity of 5Ni/10ZrO2–SiO2 is not good as that of 5Ni/5ZrO2–SiO2 catalyst, which is near the thermodynamic equilibrium. It is suggested that the nickel active sites be embedded by superfluous zirconium oxide on support. Fig. 2 shows the dependence of the conversion, selectivity and H2/CO ratio on the space velocity over 5Ni/ 5ZrO2–SiO2 in a fluidized-bed reactor. The space velocity is varied by changing the amount of catalyst while maintaining the total flow rate at 300 cm3/min, the catalysts are diluted to 2 cm3 with SiO2 support. As the space velocity increases, conversion of methane monotonically decreases. This is because the contact time becomes shorter, methane combustion can process quickly, while methane reforming with CO2 and/or H2O
90
Conversion of CH4 /%
80
70
60 5Ni/5ZrO2-SiO2
50
5Ni/10ZrO2-SiO2 5Ni/2ZrO2-SiO2
40
5Ni/0.5ZrO2-SiO2 5Ni/SiO2
30
0
2
4
6 Time on stream /h
8
10
Fig. 1. Effect of ZrO2 loading on the conversion of CH4 (reaction conditions: prereduction temperature 923 K; reaction temperature:1023 K; feed gas ratio CH4:CO2:O2 ¼ 1:0.4:0.3, GHSV ¼ 9000 h1).
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80 2.4 2.2
60 50
2.0
40
1.8
30
H2/CO ratio
CH4 or CO2 conversion/ %
70
1.6 20 CH4 conversion CO2 conversion
10
1.4 1.2
0 5
10
15 20 25 30 Space velocity (103 h-1)
35
40
Fig. 2. CH4 conversion, CO2 conversion and H2/CO ratio as a function of space velocity over the 5Ni/5ZrO2–SiO2 catalyst (T ¼ 1023 K; CH4:CO2:O2 ¼ 1:0.4:0.3).
Fig. 3. Influence of reaction temperature on catalytic activity of 5Ni/ZrO2–SiO2 (feed gas ratio: FCH4/FCO2/FO2 ¼ 176/71/53; 2 cm3 catalyst).
could not complete under this reaction condition. In the partial oxidation of methane, the CH4 conversion increases with increasing space velocity because of the presence of hot spots [11]. For the endothermic CO2 reforming of methane, the reaction had to be operated at a low space velocity to reach high H2 yield. Consequently, the autothermal reforming methane can be operated in a safer manner than the partial oxidation and in a more efficient manner than the CO2 reforming. The influence of reaction temperature on the steady-state activity of 5Ni/5ZrO2–SiO2 can be seen in Fig. 3. The conversions of CH4 and CO2 are near the equilibrium at every reaction temperature, this suggests that nickel is a suitable catalyst for reforming of methane with CO2 and O2. Because the reforming of CH4 with H2O and CO2 is an endothermic reaction, increasing reaction temperature is favorable to these
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2.0
100
1.8
80 1.6 70 1.4
60 50
H2/CO ratio
CH4 or CO2 conversion (%)
90
1.2 conv. of CH4 conv. of CO2 ratio. of H2/CO
40 30
1.0
0.8
20 0.0
0.5
1.0 O2/CO2 ratio
1.5
2.0
Fig. 4. Influence of O2/CO2 ratio in the feed on the conversion, selectivity and H2/CO product ratio in the combining process at 1023 K (GHSV ¼ 9000 h1).
reforming reactions; therefore, CH4 and CO2 conversions monotonically increases with increasing reaction temperature. Fig. 4 shows the influence of feed gas composition on the catalytic activity of 5Ni/5ZrO2–SiO2. At C/O ratio of 1 and a total flow rate of 300 cm3/min, the effect of the O2/CO2 ratio has been investigated. CH4 conversion increases with the ratio increasing of O2/CO2; in this process, the CO2 conversion increases with increasing O2 concentration, this result is attributed to the CO2 formation via the combustion of CH4. The selectivity of H2 decrease slightly as part amount of H2 would be oxidized with increasing O2 content in feed gas. The H2/CO ratios obtained in single CO2 reforming of methane (in the absence of O2) and in single partial oxidation of methane (in the absence of CO2) are around 1.0 and 2.0, respectively. That is, the H2/CO ratio can be adjusted between 1 and 2 for various Fischer–Tropsch synthesis products by controlling the feed gas composition. 3.2. Results of characterizations The XRD patterns of the NiO/ZrO2–SiO2 and NiO/SiO2 catalysts are shown in Fig. 5. It is observed that the intensities of nickel peaks are influenced by the amount of ZrO2 additive. As the amount of ZrO2 loading increases on catalyst, diffraction peak of Ni becomes weaker and broader. These results show that nickel dispersion on higher ZrO2 loading catalyst is better. There is also no evidence of ZrO2 in 5Ni/ZrO2–SiO2 XRD patterns. This indicates that the ZrO2 is well dispersed on the support of SiO2 and it can effectively promote the dispersion of NiO or inhibit metal nickel particles aggregation during the reaction. During CO2 reforming methane, sintering and graphitic carbon deposition mainly demonstrated the Ni/SiO2 rapid deactivation [12]. Several studies [13,14] have reported that the carbon formation is strongly diminished in catalysts with Ni particles as small as possible. High nickel loading may result in the formation of NiO crystalline phase on the support. The NiO crystalline phase submits little contribution to nickel dispersion in the reduction or reaction process. The large Ni particles provide less active metal surface area and cause rapid deactivation. Furthermore, addition of ZrO2 to SiO2 significantly affects the acidity of the support, leading to a profound change in the bulk phase composition. TEM micrograph of the 5Ni/SiO2 and 5Ni/5ZrO2–SiO2 catalysts (Fig. 6) provided clear evidence of nickel oxide particles distribution on support. In the case of Ni/SiO2 catalyst, nickel particles are not uniformly distributed on the surfaces, with a diameter range of 40–60 nm (Fig. 6(b)), while the nickel particles are segregated in the 5Ni/5ZrO2–SiO2 catalysts with a crystallite size range of 20–30 nm (Fig. 6(a)). Because the strong metal–support interaction (SMSI) is weak between nickel and SiO2 support, the active metal
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Intensity (a.u)
NiO
(a)
(b) (c) (d) 20
40
60
80
2θ/degree Fig. 5. XRD spectra of ZrO2-modified Ni/SiO2 catalysts ((a) 5Ni/SiO2; (b) 5Ni/2ZrO2–SiO2; (c) 5Ni/5ZrO2–SiO2; (d) 5Ni/10ZrO2–SiO2).
Fig. 6. TEM images of 5Ni/5ZrO2–SiO2 (a) and 5Ni/SiO2 (b) catalysts.
component did not directly stick to the support, but to the surface of zirconium oxide, therefore, as a promoter, high dispersion of ZrO2 can enhance the dispersion of nickel on support. The interaction between them occurred or the specific structure with filling was formed on the support, this structure inhibited the migration and enrichment of Ni and improved the nickel dispersion in the catalyst preparation and reaction processes. This result was consistent with the XRD results. The CO2-temperature desorption (CO2-TPD) patterns of promoted and pure Ni/SiO2 catalysts are compared in Fig. 7. Addition of zirconium oxide increased the amount of adsorbed CO2 and the desorption peak of CO2 in effluent shifted to higher temperatures as the amount of zirconium added. That is, the addition of ZrO2 increased the basicity of SiO2, and thus enhanced the adsorption and activation of CO2. These increased CO2 would be helpful for the CO2 reforming methane. Ross et al. [15,16] have reported Pt/ZrO2 is an active catalyst for CO2 reforming of methane; however, some deactivation of the catalyst occurred during the first 300 h of 1000 h test. Park [17] claimed carbon as an intermediate during the carbon dioxide reforming of methane over ZrO2-supported high nickel loading catalysts. The deactivation of Ni/ZrO2 catalysts during these processes is mainly due to the formation of carbonaceous deposits and to active metal sintering. Carbon formation is limited by oxygen input in feed gas, forms the TGA and TEM results, and no carbon was observed in all the spent catalysts used in the
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Desorption rate of CO2 (a.u)
2190
(d)
(c)
(b) (a)
323
373
473
573 673 Temperature / K
773
873
Fig. 7. CO2-TPD profiles of 5Ni/ZrO2–SiO2 catalysts ((a) 5Ni/SiO2; (b)5Ni/2ZrO2–SiO2; (c) 5Ni/5ZrO2–SiO2; (d) 5Ni/8ZrO2–SiO2).
fluidized-bed reactor. so we expected little activity loss due to carbon deposition. In this case, nickel sintering appears to be the cause of the deactivation process. Thus, SMSI effects play a very important role in the formation of the active phase of nickel and are responsible for the performance of nickel-based catalysts in the reaction of CO2 and CH4. Kroll et al. [18] have reported that Ni/ZrO2 presented a low dispersion but a high degree of reduction, and Ni/SiO2 appeared partially reduced with a high dispersion, while these two catalysts showed same initial activity in CO2 reforming methane. This feature tends to indicate that the active Ni phase is actually the metal and not the oxide. When prepared by successive wet impregnation, these Ni species provide large surface areas which increase the reactivity and reduce the coking rate, after reduction on stream at high temperature, these ‘‘bound-state’’ Ni species remain highly dispersed due to their very low mobility; therefore, they have long life and stability. In addition, this ‘‘bound-state’’ Ni species is favorable to inhibit carbon deposition deriving from CH4 dehydrogenation, which usually requires more Ni atoms as ‘‘ensemble sites’’ [19]. It is well known that, as far as CH4 conversion reaction is concerned, coke deposition and nickel sintering are responsible for the catalyst deactivation. Much lower amounts of coke were measured on the used catalyst, when compared to the fixed-bed operation, especially in the case of supported nickel catalysts. This is a consequence of the permanent circulation of the solid particles, which are periodically transported to the oxygen-rich entrance zero of the bed, where gasification process involves oxygen and possibly steam (from the combustion of methane). The average NiO crystallite size was calculated as 46.1 and 82.9 nm of fresh and used catalyst, respectively. The large metallic particles generated during reduction and reaction, it suggests that Ni sintering dominate the catalytic activity and stability. 3.3. Effect of fluidized-bed reactor on the combined process It has been suggested [3,7] that the high rates of heat transfer and stability of the operation were obtained using fluidized-bed reactor. A small amount of coke was measured on the used catalyst, when compared to the fixed-bed operation, especially in the case of supported nickel catalysts. This is a consequence of the permanent circulation of the solid particles, which are periodically transported to the oxygen-rich entrance zero of the bed, where gasification process involves oxygen and possibly steam (from the combustion of methane). The fluidized-bed reactor is in principle a suitable device to handle the hot spot generally occurring in fixedbed reactor, which often leads to nickel sintering. The solid catalyst as an internal heat carrier within the bed
ARTICLE IN PRESS Q.S. Jing, X.M. Zheng / Energy 31 (2006) 2184–2192
CO/H2
Heater
Heater
CO/H2
quartz wool Ni(0) Ni(ll) CH4/CO2/O2
2191
Ni(0) Ni(ll)
CH4/CO2/O2
Fig. 8. Models of fluidized-bed and fixed-bed reactors in methane reforming with CO2 and O2.
can resolve the hot spot problem and provide a more homogeneous reactor temperature profile. The heat produced in the exothermic reactions of methane with O2 is used instantly in the endothermic CO2 reforming, thus making the process most energy efficient, and also avoiding hot spot formation and consequently avoiding reaction runaway conditions and thereby making the process very safe to operate. Tomishige [7] has claimed that the catalyst in the fluidized-bed reactor is in the cycle of reduction–oxidation. Models of a fluidized and a fixed-bed reactor are depicted in Fig. 8. In a fixed-bed reactor, combustion proceeds on the catalyst in the front part of the catalyst bed, followed by the reforming of methane. In a fluidized-bed reactor, the catalyst oxidized by oxygen and exhibits reforming activity, this decreases the amount of oxidized catalyst, and increases the amount of reduced catalyst.
4. Conclusion The following conclusions can be drawn regarding the combined catalytic partial oxidation and CO2 reforming of methane over promoted Ni/SiO2 catalysts: 1. After high-temperature calcinations, ZrO2 could promote the dispersion of Ni on SiO2 support, which had a significant effect on its reduction behavior, CO2 adsorption and catalytic performance. 2. The reduced Ni/ZrO2–SiO2 catalysts exhibited a high and stable activity for the reforming of methane with CO2 and O2, showing stronger resistance to Ni aggregation, which seems to be a direct consequence of the high dispersion of nickel on the catalyst surface, while the 5Ni/SiO2 deactivated rapidly for the nickel particles sintering. 3. By changing the O2 input in the feed gas, the H2/CO ratio in the product can be controlled between 1 and 2. 4. The fluidized-bed reactor can effectively allay hot spots in catalyst bed, and the carbon deposition is also suppressed. The promoting effect of fluidized-bed reactor is mainly caused by the cycle of reduction–oxidation for Ni/ZrO2–SiO2 catalysts.
References [1] Ross JRH, Keulen ANJ, Hegarty MES, Seshan K. The catalytic conversion of natural gas to useful products. Catal Today 1996;30:193–9. [2] Souzaa MVM, Schmal M. Methane conversion to synthesis gas by partial oxidation and CO2 reforming over supported platinum catalysts. Catal Lett 2003;91:11–7. [3] Tomoshige K, Matsuo Y, Sekine Y, Fujimoto K. Effective of reforming with CO2 and O2 under pressurized condition using NiO–MgO and fluidized bed reactor. Catal Commun 2001(2):11–5. [4] O’Connor AM, Ross JRH. The effect of O2 addition on the carbon dioxide reforming of methane over Pt/ZrO2 catalysts. Catal Today 1998;46:203–10.
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[5] Tomoshige K, Knanazawa S, Suzuki K, Asadullah M, Sato M, Ikushima K, et al. Effective heat supply from combustion to reforming in methane reforming with CO2 and O2: comparison between Ni and Pt catalysts. Appl Catal A: Gen 2002;233:35–44. [6] Choudhary VR, Rajput AM, Prahabker B. Energy efficient methane-to-syngas conversion with low H2/CO ratio by simultaneous catalytic reactions of methane with carbon dioxide and oxygen. Catal Lett 1995;32:391–6. [7] Matsuo Y, Yoshinaga Y, Sekine Y, Tomoshige K. Autothermal CO2 reforming of methane over NiO–MgO solid solution catalysts under pressurized condition. Effect of fluidized bed reactor and its promoting mechanism. Catal Today 2000;63:439–45. [8] Tomishige K, Matsuo Y, Yoshinaga Y, Sekine Y. Comparative study between fluidized bed and fixed bed reactors in methane reforming combined with methane combustion for the internal heat supply under pressurized condition. Appl Catal A: Gen 2002;223:225–38. [9] Jing QS, Fei JH, Mo LY, Lou H, Zheng XM. Effective reforming of methane with CO2 and O2 to low H2/CO ratio syngas over Ni/MgO–SiO2 using fluidized bed reactor. Energy Convers Manage 2004;45:3127–37. [10] Jing QS, Fei JH, Lou H, Zheng XM. Syngas production from reforming of methane with CO2 and O2 over Ni/SrO–SiO2 catalysts in a fluidized bed reactor. Int J Hydrogen Energy 2004;29:1245–51. [11] Matsumura Y, Moffat JB. Oxidative methane conversion to carbon monoxide and hydrogen at low reactor wall temperatures over ruthenium supported on silica. Catal Lett 1994;24:59–64. [12] Wang SB, Lu GQM. CO2 reforming of methane on Ni catalysts: effects of the support phase and preparation technique. Appl Catal B: Environ 1998;16:269–77. [13] Kroll VCH, Swaan HM, Mirodatos C. Methane reforming reaction with carbon dioxide over Ni/SiO2 catalyst: I. Deactivation Studies. J Catal 1996;161:409–22. [14] Zhang ZL, Tsipouriari VA, Efstathiou AM, Verykios XE. Reforming of methane with carbon dioxide to synthesis gas over supported rhodium catalysts: I. Effects of support and metal crystallite size on reaction activity and deactivation characteristics. J Catal 1996;158:51–63. [15] Ross JRH, van Keulen ANJ, Hegarty MES, Seshan K. The catalytic conversion of natural gas to useful products. Catal Today 1996;30:193–9. [16] van Keulen ANJ, Seshan K, Hoebink JHBJ, Ross JRH. TAP investigations of the CO2 reforming of CH4 over Pt/ZrO2. J Catal 1997;166:306–14. [17] Li XS, Chang JS, Park SE. Carbon as an intermediate during the carbon dioxide reforming of methane over zirconium-supported high nickel loading catalysts. Chem Lett 1999:1099–104. [18] Swaan HM, Kroll VCH, Martin GA, Mirodatos C. Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide. Catal Today 1994;21:571–8. [19] Bond GC. The role of carbon deposits in metal-catalysed reactions of hydrocarbons. Appl Catal A: Gen 1997;149:3–25.
Energy 31 (2006) 2184–2192 www.elsevier.com/locate/energy
Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modified Ni/SiO2 catalysts using fluidized-bed reactor Q.S. Jinga,, X.M. Zhengb a
Department of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, PR China b Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China Received 9 September 2003
Abstract Nickel on zirconium-modified silica was prepared and tested as a catalyst for reforming methane with CO2 and O2 in a fluidized-bed reactor. A conversion of CH4 near thermodynamic equilibrium and low H2/CO ratio (1oH2/COo2) were obtained without catalyst deactivation during 10 h, in a most energy efficient and safe manner. A weight loading of 5 wt% zirconium was found to be the optimum. The catalysts were characterized using X-ray diffraction (XRD), H2-temperature reaction (H2-TPR), CO2-temperature desorption (CO2-TPD) and transmission election microscope (TEM) techniques. Ni sintering was a major reason for the deactivation of pure Ni/SiO2 catalysts, while Ni dispersed highly on a zirconiumpromoted Ni/SiO2 catalyst. The different kinds of surface Ni species formed on ZrO2-promoted catalysts might be responsible for its high activity and good resistance to Ni sintering. r 2005 Elsevier Ltd. All rights reserved. Keywords: Methane; Partial oxidation; CO2 reforming; Ni/ZrO2-SiO2; Fluidized-bed reactor
1. Introduction There has recently been a revival of interest in the reforming of methane with CO2 and O2 to produce synthesis gas [1–3]. An advantage of this combined process, instead of processes such as stream reforming and partial oxidation, is the suitable H2/CO ratio obtained, which is of particular interest to the synthesis of valuable oxygenated chemicals. Furthermore, this combined process is autothermic in the most energy efficient and safe manner. Coking formation on the catalyst is a serious problem in CO2 reforming alone but not in the combined process. Ross et al. [4] have reported that Pt/ZrO2 is an active and stable catalyst for combined partial oxidation and CO2 reforming of CH4 in a fixed-bed reactor. A small amount of carbon deposition occurred in parts of the catalyst bed that was not exposed to oxygen. Hot spots in the catalyst bed were reduced significantly. Corresponding author. Tel.: +86 376 6390702; fax: +86 376 6390957.
E-mail address: [email protected] (Q.S. Jing). 0360-5442/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.07.005
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It has been reported [5,6] that methane combustion is followed by CO2 and H2O reforming of methane and water-gas shift reaction. In that case, highly exothermic combustion and considerably endothermic reforming caused significant temperature gradient in the catalyst bed; therefore, hot spots at the beginning part of the catalyst bed and carbon deposition at the rear part of the catalyst bed were observed in the fixed-bed reactor. Several studies on the reforming of methane with CO2 and O2 using the fluidized-bed reactor have been reported [7,8]. It has been insisted that the high rates of heat transfer and stability of the operation were obtained by using the fluidized-bed reactor. Combustion proceeded in the front part of the catalyst bed and reforming did in the rear part both cases of the fixed and fluidized bed. It is thought that both combustion and reforming proceeded simultaneously in fluidized-bed reactor. Furthermore, fluidization of the catalyst particles between the oxidizing and reducing zones can effectively limit the amount of carbon formation. Previous reports from our laboratory have shown that the Ni/SiO2 catalyst was active for the combined reaction in fluidized-bed reactor [9]. Nevertheless, the Ni/SiO2 catalyst was prone to lose activity due to nickel sintering. In this paper, we report syngas production from the combined reaction in a fluidized-bed reactor, using Ni/SiO2 catalysts that are promoted by ZrO2. 2. Experimental 2.1. Catalyst preparation Ni/SiO2 and Ni–ZrO2/SiO2 catalysts were prepared by the incipient-wetness impregnation method. Nitrate salts were used as precursors, and commercial microspherical silica (SiO2, from Qingdao Oceanic Chemical Co, China; Brunauer-Emmett-Teller (BET) surface area (SBET) of 374 m2/g, particle diameter of 0.28–0.45 mm) was used as support. After impregnation, the samples were dried at 393 K for 12 h and subsequently calcined at 773 K for 4 h in air. The prepared catalysts were designated as Ni/xZrO2–SiO2. The x in the terminology of Ni/xZrO2–SiO2 catalysts represents the zirconium loading (x wt%) over Ni/xZrO2–SiO2 catalysts. The loading of nickel on the tested catalysts was always 5 wt%. 2.2. Catalytic reaction The catalytic reaction was performed in a fluidized-bed reactor that was comprised of a quartz tube (inner diameter (ID) of 12 mm, height (H) of 750 mm) under atmospheric pressure. A thermocouple that was connected to a PID temperature controller was placed outside the quartz tube and in the middle of the catalyst bed. With a given molar ratio, CH4 (99.9%), CO2 (99.9%) and O2 (99.9%) were fed to the reactor controlled by mass flow controller (Brooks, 5850E, 71%.FS). Prior to the reaction, 2 cm3 catalysts were used and pretreated at 923 K for 1 h under hydrogen flow at atmospheric pressure. Effluent gas of reaction was cooled in an ice-water bath and analyzed with an online gas chromatograph (Shimadzu, GC-8A) equipped with a TCD detector (column packing: TDX-01). The conversion and the selectivity were calculated as given in the literature [10]: X ðCH4 Þð%Þ ¼ ðF CH4 ;in F CH4 ;out Þ=F CH4 ;in 100,
(1)
X ðCO2 Þð%Þ ¼ ðF CO2 ;in ; F CO2 ;out Þ=F CO2 ;in 100,
(2)
SðH2 Þð%Þ ¼ F H2 ;out =2ðF CH4 ;in F CH4 ;out Þ 100,
(3)
SðCOÞð%Þ ¼ F CO;out =½ðF CH4 ;in F out;CH4 Þ þ ðF CO2 ;in F CO2 ;out Þ 100,
(4)
F i ¼ C i F total ,
(5)
where X, S and F are the conversion, selectivity and gas flow rate, respectively; F total is the total feed gas flow rate or reaction effluent gas rate, and C i is the molar fraction of component i in the feed gas or reaction effluent gas. In our reaction conditions, the detected oxygen conversion was 100%.
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2.3. Catalyst characterization X-ray diffraction (XRD) patterns were obtained on a Rigaku-D/max-B (Rigaku, Japan) automated power X-ray diffractometer using a CuKa radiation (45 kV, 40 mA). The samples for XRD measurement were at oxide state. The average NiO particle sizes were calculated using the half-width at half-height of the most intense peak of the diffraction pattern and the Scherrer equation. And the catalysts were examined at a scanning rate of 0.11/min. Transmission election microscope (TEM) images were taken by means of JEM-2020F (JEOL) operated at 200 kV. Samples were dispersed in tetrachloromethane by supersonic waves and put on Cu grids for the TEM observation in the air. 3. Results and discussion 3.1. Catalytic performance The stability of 5 wt%Ni/ZrO2–SiO2 catalyst is conducted at 1023 K and GHSV ¼ 9000 h1. The effect of zirconium loading on catalytic activity and stability is shown in Fig. 1. The pure Ni/SiO2 catalyst exhibited same performance as the modified catalysts, and there is no evident effect on the initial activity for different zirconium content. However, the stability of catalyst was significantly enhanced when the support was promoted with ZrO2. When the Zr loading is below 2 wt%, especially for unmodified catalyst, the catalysts deactivated rapidly during 6 h on stream. The catalytic stability of 5Ni/ZrO2–SiO2 is remarkably enhanced when zirconium loading is above 2 wt%. However, the catalytic activity of 5Ni/10ZrO2–SiO2 is not good as that of 5Ni/5ZrO2–SiO2 catalyst, which is near the thermodynamic equilibrium. It is suggested that the nickel active sites be embedded by superfluous zirconium oxide on support. Fig. 2 shows the dependence of the conversion, selectivity and H2/CO ratio on the space velocity over 5Ni/ 5ZrO2–SiO2 in a fluidized-bed reactor. The space velocity is varied by changing the amount of catalyst while maintaining the total flow rate at 300 cm3/min, the catalysts are diluted to 2 cm3 with SiO2 support. As the space velocity increases, conversion of methane monotonically decreases. This is because the contact time becomes shorter, methane combustion can process quickly, while methane reforming with CO2 and/or H2O
90
Conversion of CH4 /%
80
70
60 5Ni/5ZrO2-SiO2
50
5Ni/10ZrO2-SiO2 5Ni/2ZrO2-SiO2
40
5Ni/0.5ZrO2-SiO2 5Ni/SiO2
30
0
2
4
6 Time on stream /h
8
10
Fig. 1. Effect of ZrO2 loading on the conversion of CH4 (reaction conditions: prereduction temperature 923 K; reaction temperature:1023 K; feed gas ratio CH4:CO2:O2 ¼ 1:0.4:0.3, GHSV ¼ 9000 h1).
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80 2.4 2.2
60 50
2.0
40
1.8
30
H2/CO ratio
CH4 or CO2 conversion/ %
70
1.6 20 CH4 conversion CO2 conversion
10
1.4 1.2
0 5
10
15 20 25 30 Space velocity (103 h-1)
35
40
Fig. 2. CH4 conversion, CO2 conversion and H2/CO ratio as a function of space velocity over the 5Ni/5ZrO2–SiO2 catalyst (T ¼ 1023 K; CH4:CO2:O2 ¼ 1:0.4:0.3).
Fig. 3. Influence of reaction temperature on catalytic activity of 5Ni/ZrO2–SiO2 (feed gas ratio: FCH4/FCO2/FO2 ¼ 176/71/53; 2 cm3 catalyst).
could not complete under this reaction condition. In the partial oxidation of methane, the CH4 conversion increases with increasing space velocity because of the presence of hot spots [11]. For the endothermic CO2 reforming of methane, the reaction had to be operated at a low space velocity to reach high H2 yield. Consequently, the autothermal reforming methane can be operated in a safer manner than the partial oxidation and in a more efficient manner than the CO2 reforming. The influence of reaction temperature on the steady-state activity of 5Ni/5ZrO2–SiO2 can be seen in Fig. 3. The conversions of CH4 and CO2 are near the equilibrium at every reaction temperature, this suggests that nickel is a suitable catalyst for reforming of methane with CO2 and O2. Because the reforming of CH4 with H2O and CO2 is an endothermic reaction, increasing reaction temperature is favorable to these
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2.0
100
1.8
80 1.6 70 1.4
60 50
H2/CO ratio
CH4 or CO2 conversion (%)
90
1.2 conv. of CH4 conv. of CO2 ratio. of H2/CO
40 30
1.0
0.8
20 0.0
0.5
1.0 O2/CO2 ratio
1.5
2.0
Fig. 4. Influence of O2/CO2 ratio in the feed on the conversion, selectivity and H2/CO product ratio in the combining process at 1023 K (GHSV ¼ 9000 h1).
reforming reactions; therefore, CH4 and CO2 conversions monotonically increases with increasing reaction temperature. Fig. 4 shows the influence of feed gas composition on the catalytic activity of 5Ni/5ZrO2–SiO2. At C/O ratio of 1 and a total flow rate of 300 cm3/min, the effect of the O2/CO2 ratio has been investigated. CH4 conversion increases with the ratio increasing of O2/CO2; in this process, the CO2 conversion increases with increasing O2 concentration, this result is attributed to the CO2 formation via the combustion of CH4. The selectivity of H2 decrease slightly as part amount of H2 would be oxidized with increasing O2 content in feed gas. The H2/CO ratios obtained in single CO2 reforming of methane (in the absence of O2) and in single partial oxidation of methane (in the absence of CO2) are around 1.0 and 2.0, respectively. That is, the H2/CO ratio can be adjusted between 1 and 2 for various Fischer–Tropsch synthesis products by controlling the feed gas composition. 3.2. Results of characterizations The XRD patterns of the NiO/ZrO2–SiO2 and NiO/SiO2 catalysts are shown in Fig. 5. It is observed that the intensities of nickel peaks are influenced by the amount of ZrO2 additive. As the amount of ZrO2 loading increases on catalyst, diffraction peak of Ni becomes weaker and broader. These results show that nickel dispersion on higher ZrO2 loading catalyst is better. There is also no evidence of ZrO2 in 5Ni/ZrO2–SiO2 XRD patterns. This indicates that the ZrO2 is well dispersed on the support of SiO2 and it can effectively promote the dispersion of NiO or inhibit metal nickel particles aggregation during the reaction. During CO2 reforming methane, sintering and graphitic carbon deposition mainly demonstrated the Ni/SiO2 rapid deactivation [12]. Several studies [13,14] have reported that the carbon formation is strongly diminished in catalysts with Ni particles as small as possible. High nickel loading may result in the formation of NiO crystalline phase on the support. The NiO crystalline phase submits little contribution to nickel dispersion in the reduction or reaction process. The large Ni particles provide less active metal surface area and cause rapid deactivation. Furthermore, addition of ZrO2 to SiO2 significantly affects the acidity of the support, leading to a profound change in the bulk phase composition. TEM micrograph of the 5Ni/SiO2 and 5Ni/5ZrO2–SiO2 catalysts (Fig. 6) provided clear evidence of nickel oxide particles distribution on support. In the case of Ni/SiO2 catalyst, nickel particles are not uniformly distributed on the surfaces, with a diameter range of 40–60 nm (Fig. 6(b)), while the nickel particles are segregated in the 5Ni/5ZrO2–SiO2 catalysts with a crystallite size range of 20–30 nm (Fig. 6(a)). Because the strong metal–support interaction (SMSI) is weak between nickel and SiO2 support, the active metal
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Intensity (a.u)
NiO
(a)
(b) (c) (d) 20
40
60
80
2θ/degree Fig. 5. XRD spectra of ZrO2-modified Ni/SiO2 catalysts ((a) 5Ni/SiO2; (b) 5Ni/2ZrO2–SiO2; (c) 5Ni/5ZrO2–SiO2; (d) 5Ni/10ZrO2–SiO2).
Fig. 6. TEM images of 5Ni/5ZrO2–SiO2 (a) and 5Ni/SiO2 (b) catalysts.
component did not directly stick to the support, but to the surface of zirconium oxide, therefore, as a promoter, high dispersion of ZrO2 can enhance the dispersion of nickel on support. The interaction between them occurred or the specific structure with filling was formed on the support, this structure inhibited the migration and enrichment of Ni and improved the nickel dispersion in the catalyst preparation and reaction processes. This result was consistent with the XRD results. The CO2-temperature desorption (CO2-TPD) patterns of promoted and pure Ni/SiO2 catalysts are compared in Fig. 7. Addition of zirconium oxide increased the amount of adsorbed CO2 and the desorption peak of CO2 in effluent shifted to higher temperatures as the amount of zirconium added. That is, the addition of ZrO2 increased the basicity of SiO2, and thus enhanced the adsorption and activation of CO2. These increased CO2 would be helpful for the CO2 reforming methane. Ross et al. [15,16] have reported Pt/ZrO2 is an active catalyst for CO2 reforming of methane; however, some deactivation of the catalyst occurred during the first 300 h of 1000 h test. Park [17] claimed carbon as an intermediate during the carbon dioxide reforming of methane over ZrO2-supported high nickel loading catalysts. The deactivation of Ni/ZrO2 catalysts during these processes is mainly due to the formation of carbonaceous deposits and to active metal sintering. Carbon formation is limited by oxygen input in feed gas, forms the TGA and TEM results, and no carbon was observed in all the spent catalysts used in the
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Desorption rate of CO2 (a.u)
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(d)
(c)
(b) (a)
323
373
473
573 673 Temperature / K
773
873
Fig. 7. CO2-TPD profiles of 5Ni/ZrO2–SiO2 catalysts ((a) 5Ni/SiO2; (b)5Ni/2ZrO2–SiO2; (c) 5Ni/5ZrO2–SiO2; (d) 5Ni/8ZrO2–SiO2).
fluidized-bed reactor. so we expected little activity loss due to carbon deposition. In this case, nickel sintering appears to be the cause of the deactivation process. Thus, SMSI effects play a very important role in the formation of the active phase of nickel and are responsible for the performance of nickel-based catalysts in the reaction of CO2 and CH4. Kroll et al. [18] have reported that Ni/ZrO2 presented a low dispersion but a high degree of reduction, and Ni/SiO2 appeared partially reduced with a high dispersion, while these two catalysts showed same initial activity in CO2 reforming methane. This feature tends to indicate that the active Ni phase is actually the metal and not the oxide. When prepared by successive wet impregnation, these Ni species provide large surface areas which increase the reactivity and reduce the coking rate, after reduction on stream at high temperature, these ‘‘bound-state’’ Ni species remain highly dispersed due to their very low mobility; therefore, they have long life and stability. In addition, this ‘‘bound-state’’ Ni species is favorable to inhibit carbon deposition deriving from CH4 dehydrogenation, which usually requires more Ni atoms as ‘‘ensemble sites’’ [19]. It is well known that, as far as CH4 conversion reaction is concerned, coke deposition and nickel sintering are responsible for the catalyst deactivation. Much lower amounts of coke were measured on the used catalyst, when compared to the fixed-bed operation, especially in the case of supported nickel catalysts. This is a consequence of the permanent circulation of the solid particles, which are periodically transported to the oxygen-rich entrance zero of the bed, where gasification process involves oxygen and possibly steam (from the combustion of methane). The average NiO crystallite size was calculated as 46.1 and 82.9 nm of fresh and used catalyst, respectively. The large metallic particles generated during reduction and reaction, it suggests that Ni sintering dominate the catalytic activity and stability. 3.3. Effect of fluidized-bed reactor on the combined process It has been suggested [3,7] that the high rates of heat transfer and stability of the operation were obtained using fluidized-bed reactor. A small amount of coke was measured on the used catalyst, when compared to the fixed-bed operation, especially in the case of supported nickel catalysts. This is a consequence of the permanent circulation of the solid particles, which are periodically transported to the oxygen-rich entrance zero of the bed, where gasification process involves oxygen and possibly steam (from the combustion of methane). The fluidized-bed reactor is in principle a suitable device to handle the hot spot generally occurring in fixedbed reactor, which often leads to nickel sintering. The solid catalyst as an internal heat carrier within the bed
ARTICLE IN PRESS Q.S. Jing, X.M. Zheng / Energy 31 (2006) 2184–2192
CO/H2
Heater
Heater
CO/H2
quartz wool Ni(0) Ni(ll) CH4/CO2/O2
2191
Ni(0) Ni(ll)
CH4/CO2/O2
Fig. 8. Models of fluidized-bed and fixed-bed reactors in methane reforming with CO2 and O2.
can resolve the hot spot problem and provide a more homogeneous reactor temperature profile. The heat produced in the exothermic reactions of methane with O2 is used instantly in the endothermic CO2 reforming, thus making the process most energy efficient, and also avoiding hot spot formation and consequently avoiding reaction runaway conditions and thereby making the process very safe to operate. Tomishige [7] has claimed that the catalyst in the fluidized-bed reactor is in the cycle of reduction–oxidation. Models of a fluidized and a fixed-bed reactor are depicted in Fig. 8. In a fixed-bed reactor, combustion proceeds on the catalyst in the front part of the catalyst bed, followed by the reforming of methane. In a fluidized-bed reactor, the catalyst oxidized by oxygen and exhibits reforming activity, this decreases the amount of oxidized catalyst, and increases the amount of reduced catalyst.
4. Conclusion The following conclusions can be drawn regarding the combined catalytic partial oxidation and CO2 reforming of methane over promoted Ni/SiO2 catalysts: 1. After high-temperature calcinations, ZrO2 could promote the dispersion of Ni on SiO2 support, which had a significant effect on its reduction behavior, CO2 adsorption and catalytic performance. 2. The reduced Ni/ZrO2–SiO2 catalysts exhibited a high and stable activity for the reforming of methane with CO2 and O2, showing stronger resistance to Ni aggregation, which seems to be a direct consequence of the high dispersion of nickel on the catalyst surface, while the 5Ni/SiO2 deactivated rapidly for the nickel particles sintering. 3. By changing the O2 input in the feed gas, the H2/CO ratio in the product can be controlled between 1 and 2. 4. The fluidized-bed reactor can effectively allay hot spots in catalyst bed, and the carbon deposition is also suppressed. The promoting effect of fluidized-bed reactor is mainly caused by the cycle of reduction–oxidation for Ni/ZrO2–SiO2 catalysts.
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