Characterization of aerogel Ni/Al 2 O 3 catalysts and investigation on their stability for CH 4 -CO 2 reforming in a fluidized bed

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1 available at Characterization of aerogel Ni/Al 2 O 3 catalysts and investigation on their stability for CH 4 -CO 2 reforming in a fluidized bed Zhigang Hao a,b,c, Qingshan Zhu a,, Zheng Jiang a, Baolin Hou a,b, Hongzhong Li a a State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing , China b Graduate School of Chinese Academy of Sciences, Beijing , China c China National Bluestar (Group) Corporation, Beijing , China ARTICLE DATA Article history: Received 21 March 2008 Received in revised form 3 August 2008 Accepted 7 August 2008 Keywords: CH 4 -CO 2 reforming Aerogel Ni/Al 2 O 3 catalyst Fluidized bed Carbon deposition ABSTRACT The CH 4 -CO 2 reforming was investigated in a fluidized bed reactor using nano-sized aerogel Ni/Al 2 O 3 catalysts, which were prepared via a sol gel method combined with a supercritical drying process. The catalysts were characterized with BET, XRD, H 2 -TPR and H 2 -TPD techniques. Compared with the impregnation catalyst, aerogel catalysts exhibited higher specific surface areas, lower bulk density, smaller Ni particle sizes, stronger metal-support interaction and higher Ni dispersion degrees. All tested aerogel catalysts showed better catalytic activities and stability than the impregnation catalyst. Their catalytic stability tested during 48 h reforming was dependent on their Ni loadings. Characterizations of spent catalysts indicated that only limited graphitic carbon formed on the aerogel catalyst, while massive graphitic carbon with filamentous morphology was observed for the impregnation catalyst, leading to significant catalytic activity degradation. An aerogel catalyst containing 10% Ni showed the best catalytic stability and the lowest rate of carbon deposition among the aerogel catalysts due to its small Ni particle size and strong metal-support interaction Elsevier B.V. All rights reserved. 1. Introduction Methane reforming with CO 2 has attracted much attention in recent years, as the process is able to convert the two greenhouse gases of CH 4 and CO 2 into valuable syngas [1 3]. So far, the CH 4 -CO 2 reforming has been studied over numerous supported metal catalysts including Ni-based catalysts as well as noble metal-based ones [4 7]. The latter has been reported to be more active and less sensitive to carbon deposition than the former. However, considering the aspects of high cost and limited availability of noble metals, it is more practical, from an industrial viewpoint, to develop Ni-based catalysts which exhibit high activity in CH 4 -CO 2 reforming. The main problem for Ni-based catalysts to be applied to industrial CH 4 -CO 2 reforming is the significant carbon deposition, which leads to quick catalyst deactivation and/or plugging of the reactor. It has been demonstrated that carbon deposition is dependent on the physicochemical properties of the catalysts, like the particle sizes of the active metal, types of support and promoter, etc. [5 8]. Much effort has therefore been devoted to developing carbon resistant catalysts through optimizing these parameters. However, effective catalysts with long-term stability have not yet been established up to now. Recent investigations on the other hand revealed that the amount of carbon deposition was strongly influenced by the operation mode, where fluidized bed reforming led to significant enhancement in CH 4 conversion and induced less carbon deposition compared with those of fixed-bed reforming, using the same catalyst [9 15]. These investigations demonstrated that the lifetime of Ni-based catalysts might be much prolonged through further optimizing operating parameters. Nano-sized particle catalysts showed better catalytic activity as compared with conventional catalysts and are Corresponding author. Tel./fax: address: qszhu@home.ipe.ac.cn (Q. Zhu) /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.fuproc

2 114 FUEL PROCESSING TECHNOLOGY 90 (2009) promising for the CH 4 -CO 2 reforming [8,16 18]. Oneofthe main problems for fluidized bed reactors to handle nanoparticles lies in the fact that most nano-sized powders are difficult to be fluidized due to strong cohesive forces among nanoparticles. Extensive investigations have revealed that some nanopowders such as aerogels can be smoothly fluidized as a result of self-agglomeration to suitable sizes [19 21]. Besides, external excitations, such as the applications of the acoustic field, the vibration field and the magnetic field, etc., are essential to realize smooth fluidization for other nanopowders [22 27]. To the best of our knowledge, CH 4 -CO 2 reforming over nanoparticle catalysts in a fluidized bed has been seldom reported previously. Recently, we have proved that aerogel catalyst in a fluidized bed showed better catalytic activity and stability than those in a fixed-bed [28,29], but the effect of Ni loadings on the physicochemical properties and long-term stability need to be further investigated. In the present work, a series of aerogel Ni/Al 2 O 3 catalysts were prepared using a sol gel method combined with supercritical drying. In comparison with Ni/Al 2 O 3 impregnation catalyst, the physicochemical properties of the aerogel catalysts were characterized by means of XRD, BET, H 2 -TPR and H 2 -TPD. The catalytic stability of the aerogel catalysts with different Ni loadings and the impregnation catalyst was investigated for CH 4 -CO 2 reforming in the fluidized bed reactor, and the catalyst deactivation was also discussed. 2. Experimental 2.1. Catalyst preparation The aerogel NiO/Al 2 O 3 catalysts were synthesized using raw materials of Al(NO 3 ) 3 9H 2 O, Ni(NO 3 ) 2 6H 2 O and NH 3 H 2 O (chemical-reagent grade, Beijing Yili Fine Chem. Co., Ltd., China). Al(NO 3 ) 3 9H 2 O and Ni(NO 3 ) 2 6H 2 O were separately dissolved into deionized water to form two starting solutions with the concentrations of 0.18 and 0.10 mol/l, respectively. A 2.5 wt.% NH 3 H 2 O solution was then added dropwise to the Al(NO 3 ) 3 solution under continuously vigorous stirring at room temperature up to the ph value of ~7.5. Subsequently, the appropriate amount of the Ni(NO 3 ) 2 solution was added dropwise to the above solution and the ph value of the mixed solution was adjusted to ~9 with 2.5 wt.% NH 3 H 2 O solution. The resultant hydrogel was aged for 2 h at room temperature, followed by filtering and repeatedly washing with deionized water and absolute ethanol to remove the free water involved in the hydrogel. The as-obtained gel was treated in an autoclave at the supercritical drying (SCD) condition of 260 C and 8.0 MPa for 2 h. After the release of the ethanol vapor at 260 C, the powder was cooled down to room temperature with a continuous nitrogen flow. The resultant catalysts were further calcined at 650 C for 4 h in air. Catalysts with Ni loadings ranging from 2.5 wt.% to 20 wt.% were prepared. The aerogel Ni-based catalysts were designated as xanc, where x was the weight percent of Ni in the catalysts. For comparison purpose, a 10 wt.% Ni/Al 2 O 3 catalyst was prepared by impregnating a commercial γ-al 2 O 3 support (particle sizes of μm, surface area of 178 m 2 /g, bulk density of g/ml, Tianjin Research Institute of Chemical Industry, Tianjin, China) with Ni(NO 3 ) 2 solution, followed by overnight drying at 110 C and calcination at 650 C for 4 h in air. After milling and sieving, the impregnation catalyst with particle sizes of μm was designated as 10INC and was applied to reforming reaction Catalyst characterization The phase structure of the catalysts was analyzed by using X- ray diffractometry (XRD, X'Pert MPD Pro, Panalytical, Netherlands) with Cu Kα radiation (λ= Å). The specific surface area of the samples was measured by N 2 adsorption at 196 C in a sorptomatic apparatus (Autosorb-1, Quantachrom, USA). Temperature programmed reduction of hydrogen (H 2 -TPR) was performed on a chemisorption apparatus (ChemBet 3000, Quantachrom, USA). The sample (30 mg) was pretreated with Ar (99.99%) flow at 150 C for 30 min, followed by cooling it to room temperature. Then the sample was heated to 1000 C at a constant heating rate of 10 C/min using a flow of H 2 /Ar mixture (volume ratio, 5:95) under a flow rate of 30 ml/min. The signal of hydrogen consumption was detected by a thermal conduction detector (TCD). Temperature programmed desorption of hydrogen (H 2 -TPD) was performed on a chemisorption apparatus (ChemBet 3000, Quantachrom, USA). The reduced catalyst sample (30 mg) was allowed adsorption of H 2 (99.99%) under a flow rate of 15 ml/min for 30 min at room temperature. Then the flow was switched from H 2 to Ar (99.99%) with a flow rate of 30 ml/min, and the temperature was elevated from room temperature to 900 C at a constant heating rate of 10 C/min. The signal of hydrogen was monitored by a TCD. The amount of deposited carbon on the spent catalysts was measured by thermogravimetry (TG, STA 449C, Netzsch, Germany). Before TG was carried out, the sample (about 10 mg) was pretreated by N 2 (99.99%) at 300 C for 30 min, cooled down to room temperature, then introduced O 2 to the sample. The temperature was raised to 800 C with a rate of 10 C/min. Temperature programmed oxidation (TPO) was performed under a flow of O 2 /Ar mixture by heating the spent sample from room temperature to 800 C with a rate of 10 C/ min. The amount of CO 2 evolution was analyzed by a gas chromatographer equipped with GDC-104 column every 2 min, and then relationship between the CO 2 signal and reaction temperature was simulated and was plotted in the figure. The microstructure of the catalysts was studied on fieldemission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan). The sample was dispersed in ethanol in an ultrasonic bath carefully and then deposited on aluminum foil Catalytic reaction CH 4 -CO 2 reforming was carried out over various catalysts in a fluidized bed quartz micro-reactor with the inner diameter of m at atmospheric pressure. Prior to the reforming reaction, the catalyst was reduced in-situ in a H 2 /N 2 mixture of volumetric ratio of 20:80 at a total flow rate of 150 ml/min at 800 C for 1 h. The CH 4 -CO 2 reforming was performed at 800 C

3 115 Fig. 1 XRD patterns for (a) and (b) of the catalysts after being calcined at 650 C for 4 h. (1) support Al 2 O 3 (2) 2.5ANC (3) 5ANC (4) 10ANC (5) 15ANC (6) 20ANC (7) 10INC. with the catalyst of 0.2 g and gas flow rate of 300 ml/min with a CH 4 /CO 2 /N 2 molar ratio of 1:1:1. The reactants and products were analyzed with an on-line gas chromatographer (SP3420, BEIFEN, China) equipped with 13X and GDC-104 columns. An ice-cold trap was set between the reactor exit and the gas sampling valve to remove the water formed during the reaction. The selectivity of H 2 and CO was calculated as follows. clearly distinguish Al 2 O 3 and NiAl 2 O 4 phases in the 2 theta range of 10 80, so the diffraction peaks from 60 to 70 are enlarged to show the difference as illustrated in Fig. 1(b). For all the catalysts, the main diffraction peak shifts left to lower angle due to the incorporation of NiO phase to the Al 2 O 3 phase to form the NiAl 2 O 4 phase, the shift of the peak is dependent on the amount of NiO in the Al 2 O 3 support. Only diffraction peaks of the NiAl 2 O 4 and the Al 2 O 3 phases are observed for the Ni loadings up to 15 wt.% and the intensity of the NiAl 2 O 4 phase peaks increases with increasing the Ni loadings, suggesting that the addition of Ni species was combined with the Al 2 O 3 support to form the spinel NiAl 2 O 4 phase. With further increasing the Ni loadings to 20 wt.%, diffraction peaks attributed to the NiO phase have been observed, indicating that the Ni loadings have exceeded the limit of the NiAl 2 O 4 phase formation [30]. In order to study the effect of the preparation method on the phase structure, the XRD pattern of the 10INC was also depicted in the Fig. 1, which shows that the NiO phase peak at 43.3 has already been detected for the 10INC prepared by the impregnation method. The reason for the NiO formation at lower Ni loading as compared with those of the aerogel catalysts might be due to the inhomogeneous distribution of the Ni species, thus the locations with NiO phase may exceed the limit of the NiAl 2 O 4 formation. The XRD patterns of the reduced catalysts are depicted in Fig. 2. For all the catalysts, most of Ni species were reduced at 800 C as compared with unreduced catalysts in Fig. 1. For the 2.5ANC, Ni phase has not been observed possibly because the amount of Ni in the 2.5ANC is too low to be detected by the XRD technique [8]. When the Ni loadings over 5 wt.%, obvious diffraction peaks attributed to Ni phase (44.5, 51.7 and 76.2 ) are observed, and their intensities increase with increasing Ni loadings. Moreover, compared with the diffraction peaks of the 10INC, the 10ANC shows much weaker and broader diffraction peaks due to smaller Ni particle sizes. The particle sizes of metallic Ni were calculated from the (2 0 0) peak using the Scherrer equation and are shown in Table 1. Although Ni particle sizes of the aerogel catalysts increase with increasing the Ni loadings, the Ni particle sizes are much smaller for the aerogel catalysts as compared with the impregnated catalyst moles of H 2 produced Selectivity of H 2 ¼ 100k 2 moles of CH 4;in moles of CH 4;out ð1þ Selectivity of CO moles of CO produced ¼ ðmoles of CH 4 þ moles of CO 2 Þ in ðmoles of CH 4 þ moles of CO 2 100k Þ out ð2þ 3. Results and discussion 3.1. Physicochemical properties of the catalysts Fig. 1(a) shows the XRD patterns of various catalysts after being calcined at 650 C. Due to the peak broadening and superimposition of Al 2 O 3 and NiAl 2 O 4 phases, it is difficult to Fig. 2 XRD patterns of the catalysts after reduction at 800 C for 1 h. (1) support Al 2 O 3 (2) 2.5ANC (3) 5ANC (4) 10ANC (5) 15ANC (6) 20ANC (7) 10INC.

4 116 FUEL PROCESSING TECHNOLOGY 90 (2009) Table 1 Physicochemical properties of the aerogel catalysts and the impregnation catalyst Catalyst samples Specific surface area (m 2 /g) Bulk density (g/ml) Ni particle size a (nm) Ni reduction degree b (%) Ni dispersion degree c (%) 2.5ANC d ANC ANC ANC ANC INC a Ni particle size: calculated from the (2 0 0) peak using the Scherrer equation. b Ni reduction degree: the amount of reduced Ni/the amount of Ni in the catalysts, assuming that NiO+H 2 Ni+H 2 O. c Ni dispersion degree: the amount of exposed Ni on the surface of the catalysts/the amount of Ni in the catalysts, assuming Ni/H=1. d Not calculated because the diffraction peaks of Ni phase were not observed for 2.5ANC. (10INC) and the smaller Ni particle sizes would be beneficial for suppression of carbon deposition in the CH 4 -CO 2 reforming [8]. The reducibility of the catalysts is necessary to obtain information about the metal-support interaction. Previous studies have indicated that strong metal-support interaction can effectively suppress the formation of deposited carbon [16 18]. The H 2 -TPR profiles of all the catalysts after being calcined at 650 C are shown in Fig. 3. For the aerogel catalysts with Ni loadings range from 2.5 to 15 wt.%, a single and broad reduction peak has been observed around C, which should be attributed to the reduction of NiAl 2 O 4 phase, according to the results of XRD (in Fig. 1) and the reported [16,17]. The temperature of reduction maximum peak decreases with increasing the Ni loadings, i.e., the temperature of reduction maximum decreases from 909 to 857 C when the Ni loadings increase from 2.5 to 15 wt.%. As the Ni loadings further increase to 20 wt.% (20ANC), there are two reduction peaks centered at 727 and 872 C, which correspond to the reduction of fixed NiO phase on the support and NiAl 2 O 4 phase, respectively [30]. But most of Ni species in the 20ANC is reduced under lower temperature. From the relationship between the Ni particle sizes in the aerogel catalysts and the results of the H 2 -TPR, it can be found that the larger the Ni particle sizes in the catalysts, the easier they could be reduced to metallic nickel [31], suggesting that the metal-support interaction becomes much weaker with increasing the Ni loadings. On the other hand, the reduction behavior of the 10INC is quite different from that of the 10ANC. The 10INC with three reduction peaks centered at 512, 621 and 800 C are attributed to the reduction of free NiO phase, fixed NiO phase and NiAl 2 O 4 phase, respectively [30,32], which is related to the inhomogeneous distribution of the Ni species in the Al 2 O 3 support. The Ni reduction degrees and Ni dispersion degrees of the catalysts derived from H 2 -TPR and H 2 -TPD are listed in Table 1.It can be seen that the Ni reduction degree of the 10ANC is lower than that of the 10INC, which is consistent with the H 2 -TPR result that the 10ANC is difficult to be reduced under the same reduction conditions. On the other hand, the aerogel catalyst shows higher Ni dispersion degree than that of impregnation catalyst, i.e., the Ni dispersion degree of the 10ANC is estimated at 2.1 times of the 10INC. This result suggests that Ni species on the aerogel catalyst is much more homogeneously dispersed than that on the 10INC, which is helpful to obtain the small Ni particle sizes in the aerogel catalysts and then enhance the reaction efficiency in CH 4 -CO 2 reforming. Physicochemical properties of the catalysts are shown in Table 1. Compared with the 10INC prepared by impregnation method, all aerogel catalysts show higher specific surface areas and lower bulk density, which should be attributed to the differences of preparation method. In the sol gel process, Ni species can be uniformly dispersed and reached to atomic or molecular level interaction with support. The skeleton of the original sol gel was almost kept intact in the aerogel, as in the supercritical drying process, the influence of surface tension and capillary force on the gel skeleton can be neglected [33]. While in the case of the conventional preparation process, the dispersion of nickel species could block part of pores on support and resulted in the lower specific surface area. Thus, the aerogel catalysts exhibit high specific surface areas, low bulk density, small Ni particle sizes, strong metal- Fig. 3 H 2 -TPR profiles of the aerogel catalysts and the impregnation catalyst. (1) 2.5ANC (2) 5ANC (3) 10ANC (4) 15ANC (5) 20ANC (6) 10INC. Fig. 4 Effect of nickel loadings on conversions of CH 4,CO 2 and selectivity of H 2, CO. Reaction conditions: 800 C, 0.1 MPa, 90, 000 ml/(h g), n(ch 4 ):n(co 2 ):n(n 2 )=1:1:1.

5 117 support interaction and high Ni dispersion degrees, which make they an ideal material for catalytic applications Catalytic performance The effect of Ni loadings on CH 4,CO 2 conversions and H 2,CO selectivity over the aerogel catalysts in CH 4 -CO 2 reforming is shown in Fig. 4. It is observed that the conversions and the selectivity increase with increasing the Ni loadings for the Ni loadings up to 10 wt.%, after which the influence of the Ni loadings on the conversions and selectivity is marginal, indicating that there are enough Ni active sites for CH 4 -CO 2 reforming reaction under the employed conditions when the Ni loadings are greater than 10 wt.%. Moreover, the selectivity of CO is obvious higher than the selectivity of H 2, and the ratios of H 2 /CO obtained under these conditions are found to vary between 0.90 and The deviation from the stoichiometric ratio may be attributed to the occurrence of the reverse water gas shift reaction (RWGS) (Eq. (3)) simultaneously with reforming, as observed by others [14,31]. The detection of water in the outlet was a clear indication of the occurrence of the RWGS reaction. CO 2 þ H 2 ¼ CO þ H 2 O D r H 298 ¼ 41kJ=mol ð3þ To investigate the stability of the aerogel catalysts in the fluidized bed reactor, the CH 4 -CO 2 reforming over the 10ANC, the 15ANC and the 20ANC was carried out up to 48 h, and compared with the 10INC under the same reaction conditions. The conversions and selectivity over the different catalysts are illustrated in Fig. 5. The figure shows two distinct features. Firstly, the conversions of the 10ANC are higher than those of the 10INC for the same weight catalysts in the fluidized bed, i.e., the conversions of CH 4 and CO 2 for the 10ANC are 95.2% and 96.1%, respectively, while those obtained by the 10INC are 64.5% and 66.2%, respectively. The gap is mainly attributed to the difference of the number of the active sites on the surface of the two kinds of catalysts. Characterizations reveal that the Ni dispersion degree of the 10ANC is 2.1 times greater than that of the 10INC. Thus, the aerogel catalysts with much more active sites could promote the reaction efficiency in CH 4 -CO 2 reforming reaction. Moreover, the better fluidization quality of the aerogel particles is beneficial to obtain high catalytic activity. The aerogel powders can be fluidized in the form of porous agglomerates, with a high bed expansion ratio. While infrequent particles circulation and low bed expansion are observed in the case of the fluidization of 10INC particles. Secondly, the catalytic stability of the aerogel catalysts is superior to that of the 10INC. The conversions and selectivity of the 10INC rapidly decrease in the fluidized bed, which was caused by catalyst deactivation [14,15]. On the contrary, all the aerogel catalysts reveal the similar catalytic activities during the tested period up to 30 h, after when the catalytic activity degradations of the 15ANC and the 20ANC start to accelerate with further increasing the reaction time. The 10ANC reveals the best catalytic stability among the tested aerogel catalysts during 48 h reforming reaction. Fig. 5 Conversions and selectivity of different Ni/Al 2 O 3 catalysts in the fluidized bed reactor. Reaction conditions: 800 C, 0.1 MPa, 90, 000 ml/(h g), n(ch 4 ):n(co 2 ):n(n 2 )=1:1:1.

6 118 FUEL PROCESSING TECHNOLOGY 90 (2009) CH 4 -CO 2 reforming over impregnation catalysts has been reported previously in a fluidized bed reactor. Effendi et al. [14] proposed that the better catalytic stability of 30 h can be obtained over impregnation Ni/Al 2 O 3 catalyst in a fluidized bed under the condition of CO 2 /CH 4 =1.5. Whereas, the conversions of CH 4 and CO 2 over impregnation Ni/SiO 2 catalyst decreased about 12 17% in a fluidized bed under the condition of CO 2 /CH 4 =1 during 12 h reforming [9]. Therefore, long-term catalytic stability has not been attained for impregnation catalyst even in a fluidized bed reactor when the CO 2 / CH 4 is stoichiometrically fed. In the present study, the aerogel catalysts exhibited excellent catalytic stability under the fluidized bed operation, as compared with the impregnation catalyst. The different catalytic stability observed in the present study may be due to different behaviors of catalyst deactivation. To verify the presumption, the characterization of the spent catalysts was investigated and will be discussed below Characterization of the spent catalysts Most of investigations revealed that the catalyst deactivation in CH 4 -CO 2 reforming mainly attributed to the sintering of Ni particles and the carbon deposition on the catalysts [8 10,14 17]. To study the sintering of active metal, the mean sizes of Ni particles were calculated from the (2 0 0) peak using Scherrer equation [17]. As listed in Table 2, it can be found that the metal sintering is unavoidable under the reaction conditions. The growth rates of Ni particles on the aerogel catalysts are slower than that of the 10INC, however, the effect of the Ni loadings on the growth rates of Ni particles over the aerogel catalysts is marginal. These results demonstrated that the Ni particles on the aerogel catalysts showed better resistance to sintering by taking the advantage of higher dispersivity and homogeneity. According our previous result [28], the similar sintering behaviors of the aerogel catalysts should not be responsible for the significant difference in their deactivation rates. Therefore, the metal sintering is not the main reason for the catalyst deactivation, but large Ni particles can accelerate the rate of carbon deposition on the catalysts [6]. The deposited carbon on the spent samples was evaluated by the TG analysis. From Table 2, it is clear that the rates of deposited carbon on the aerogel catalysts are much lower than that on the 10INC, indicating that aerogel catalysts are more resistant to carbon deposition [17,18]. Additionally, the rates of Table 2 The Ni particle size and the rate of deposited carbon in spent catalyst samples Catalyst sample Reaction time (h) Ni particle size a (nm) Growth rate of Ni particle (nm/h) Rate of deposited carbon (wt. %/h) 10ANC ANC ANC INC a Calculated from the (2 0 0) diffraction peak using the Scherrer equation. Fig. 6 TPO patterns of (1) 10ANC, (2) 15ANC, (3) 20ANC after 48 h reforming and (4) 10INC after 30 h reforming in the fluidized bed. deposited carbon over the aerogel catalysts mainly depend on the Ni loadings, i.e., the 10ANC shows a lower rate of deposited carbon than that of the 20ANC during 48 h reforming. Generally, the carbon deposition could occur on Ni-based catalysts in various forms such as atomic carbon, amorphous carbon and graphitic carbon, which can be gasified to CO 2 in O 2 atmosphere at the temperature ranges of b250 C, 250~600 C and N600 C, respectively [34,35]. TPO profiles of spent catalysts were applied to characterize the reactivity of carbon species, as shown in Fig. 6. The TPO curves of all spent aerogel catalysts indicate the formation of two kinds of carbon deposition, which correspond to the much reactive amorphous carbon (C β ) at low temperature and the less reactive graphitic carbon (C γ ) at high temperature [35,36]. The peak area of the amorphous carbon is bigger than that of the graphitic carbon, indicating that most of deposited carbon can be gasified at lower reaction temperature. Whereas, most of deposited carbon on the 10INC is the graphitic carbon, which is the main reason for catalyst deactivation [14,15]. Investigations also revealed that the morphologies of deposited carbon on the aerogel catalysts are quite different from the 10INC, as illustrated by the FESEM images in Fig. 7. It is observed that limited filamentous carbon is formed besides the nano-sized catalyst particles for the aerogel catalysts. While in the case of the 10INC, almost all active sites are covered by the long filamentous morphology, which is the main characteristic of the graphitic carbon [36]. Therefore, massive graphitic carbon deposition on the 10INC is mainly reason for catalyst deactivation due to its high gasification temperature, however, only limited graphitic carbon formed on the aerogel catalysts after 48 h reforming, which would be the one of main reasons for the much better long-term catalytic stability. Carbon deposition mechanism has been previously investigated for the CH 4 -CO 2 reforming [2,9,37,38]. It has been proposed that the CO 2 -CH 4 reforming reaction involves two steps: the decomposition of CH 4 to form carbon species and hydrogen (Eq. (4)), and gasification of the carbon species with CO 2 (Eq. (5)) [2,38]. The reaction between surface carbon species and the oxidant CO 2 is the rate-determining step in the CH 4 - CO 2 reforming [37]. It has been proved that fluidized bed is suitable reactor to promoting reaction rate of C-CO 2 (Eq. (3))

7 119 Fig. 7 FESEM images of (1) 10ANC, (2) 15ANC, (3) 20ANC after 48 h reforming and (4) 10INC after 30 h reforming in the fluidized bed. and then reduces the amount of carbon deposition due to circulation of catalyst particles between CO 2 -rich and CO 2 - deficient zones [9,14,28]. In this study, the 10ANC and the 10INC in the same fluidized bed showed different behaviors of carbon deposition, which was mainly attributed to the different fluidization quality of two kinds catalysts. The aerogel catalysts can be well fluidized through these porous agglomerates and show a high bed expansion ratio, while the fluidization of the 10INC with large particle sizes reveals little particles back-mixing and low bed expansion. Since the gas solid contact efficiency is better for the aerogel catalysts in the fluidized bed, the gas concentration would be more homogeneously distributed in the reactor, and then the rate of deposited carbon decreases as compared with the 10INC in the fluidized bed. On the other hand, small Ni particles on the aerogel catalysts are also a secondary factor for suppression of carbon deposition. Generally, carbon adsorbed on the small Ni particles diffuses with more difficulty than that on the large Ni particles based on the mechanism of carbon formation [39]. Characterizations have revealed that Ni particle size on the 10ANC is lower than that of the 10INC, which is helpful to reduce the rate of deposited carbon. In this study, the aerogel catalysts show similar fluidization quality in the fluidized bed, however, there are large differences in the catalytic stability and the rate of deposited carbon, which depend on the Ni loadings in the catalysts. Firstly, smaller Ni particle sizes on the 10ANC is helpful to decrease the rate of deposited carbon, as compared with the 15ANC and the 20ANC. Secondly, strong metal-support interaction is also key factor to suppression of carbon deposition because active carbon species is not easy to transform to the graphitic carbon on the interface between active metal and support [40]. The 10ANC shows stronger metal-support interaction than the 15ANC and the 20ANC according to the results of the H 2 -TPR, thus lower rate of deposited carbon is obtained for 10ANC during 48 h reforming. Although much lower Ni loadings catalysts such as 2.5ANC or 5ANC could obtain better catalytic stability due to strong metal-support interaction, the lower catalytic activity is mainly disadvantage for two catalysts. In a word, strong metal-support interaction and small Ni particle contribute the excellent catalytic stability and the low deposited carbon of the 10ANC for the CH 4 -CO 2 reforming in a fluidized bed. CH 4 ¼ C þ 2H 2 D r H 298 ¼ 75 kj=mol ð4þ C þ CO 2 ¼ 2CO D r H 298 ¼ 171 kj=mol ð5þ 4. Conclusions Aerogel Ni/Al 2 O 3 nanoparticle catalysts, which were prepared by a sol gel method combined with a supercritical drying route, exhibited higher specific surface areas, lower bulk density, smaller Ni particle sizes, stronger metal-support interaction and higher Ni dispersion degrees than those of the impregnation catalyst. All tested aerogel catalysts showed better catalytic activities and stability than the impregnation catalyst, and catalytic stability of aerogel catalysts was dependent on the Ni loadings in the aerogel catalysts. Characterization of spent catalysts revealed that only limited graphitic carbon with filamentous was found on the surface of

8 120 FUEL PROCESSING TECHNOLOGY 90 (2009) the aerogel catalysts, however, massive graphitic carbon with filamentous morphology covered most of the surface of the impregnation catalyst and resulted in the catalyst deactivation. The comparative study demonstrated that better catalytic stability and lower rate of deposited carbon for the aerogel catalysts attributed to their better fluidization quality and excellent physicochemical properties. The 10ANC showed best catalytic stability and lowest rate of deposited carbon among the aerogel catalysts due to its small Ni particle size and strong metal-support interaction. Acknowledgment We would like to express our gratitude for financial support from the National Natural Science Foundation of China (grant No and ). REFERENCES [1] A.T. Ashcroft, A.K. Cheetham, M.L.H. Green, P.D.F. Vernon, Partial oxidation of methane to synthesis gas using carbon dioxide, Nature 52 (1991) [2] M.C.J. 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