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Effect of metal particle size on coking during CO2 reforming of CH4 over Ni–alumina aerogel catalysts
Applied Catalysis A: General 197 (2000) 191–200
Effect of metal particle size on coking during CO2 reforming of CH4 over Ni–alumina aerogel catalysts Jin-Hong Kim a , Dong Jin Suh b,∗ , Tae-Jin Park b , Kyung-Lim Kim a a
b
Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, South Korea Clean Technology Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-791, South Korea Received 21 January 1999; received in revised form 24 August 1999; accepted 31 August 1999
Abstract CO2 reforming of CH4 was carried out over Ni–alumina aerogel catalysts prepared with various Ni loadings. The preparation of alumina supported Ni catalysts via sol–gel synthesis and subsequent supercritical drying led to the formation of very small metal particles, which are evenly distributed over the alumina support. The activity of the aerogel catalysts increased along with increasing metal loading, and eventually, the SAA25 (0.25 in Ni/Al mole ratio) catalyst exhibited the high activity comparable to that of a 5 wt.% Ru/alumina catalyst (ESCAT44, Engelhard). Compared to the alumina-supported Ni catalyst prepared by conventional impregnation method, Ni–alumina aerogel catalysts showed a remarkably low coking rate due to highly dispersed metal particles. From TEM micrograph studies, it was observed that the formation of filamentous carbon was significantly influenced by the metal particle size and proceeded mostly over the metal particles larger than 7 nm. The loss of catalytic activity at 973 K was mainly caused by coke deposition and sintering. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Methane; Carbon dioxide; Reforming; Sol–gel; Aerogel; Nickel; Alumina; Metal particle size; Deactivation; Filamentous carbon
1. Introduction During the past decades, CO2 reforming of CH4 has attracted interest from both industrial and environmental perspectives. In the environmental aspect, both CO2 and CH4 are recognized as undesirable greenhouse gases, and hence, the reaction provides a method of consuming CO2 effectively. From the industrial viewpoint, conversion of CH4 and CO2 into useful products is an important area of current catalytic research, especially C1 chemistry, because these two gases are the cheapest and most abundant carbon-containing mate∗ Corresponding author. Fax: +82-29585199. E-mail address: [email protected] (D.J. Suh)
rials and are convertible to synthesis gas with H2 /CO ratio lower than that of steam reforming [1,2]. The CO2 reforming reaction has been studied over numerous supported metal catalysts including Ni-based catalysts as well as noble metal-based ones [3–8]. The latter have been reported to be more active and less sensitive to coking than the former. However, considering the aspects of high cost and limited availability of noble metals, it is more practical, from the industrial standpoint, to develop Ni-based catalysts which are resistant to carbon deposition, while exhibiting high activity for this reaction [4,9]. Recently, we developed a method to prepare alumina aerogels having specific surface areas in excess of 700 m2 /g and high thermal stability and success-
0926-860X/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 4 8 7 - 1
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fully applied this method to prepare stable Ni–alumina aerogel catalysts for CH4 reforming with CO2 [10,11]. The result of catalytic tests showed that these catalysts were suitable for the CO2 reforming reaction due to good activity and remarkable resistance to coke formation [11]. In this study, we tried to find the optimum level of metal loading which could provide Ni–alumina aerogel catalysts with high activity and coke resistance, by testing and characterizing the aerogel catalysts prepared with various metal loading amounts. Especially, our work has been mainly focused on elucidating the causes of deactivation and offering some criteria of filamentous carbon formation during the reforming reaction, in relation to the morphology of metal particles. In some studies published recently, much effort has been devoted to clarify the relationship between the carbon-forming behavior of supported Ni catalysts and the morphology of metal particles [12,13–16]. According to these studies, the carbon-forming tendency of supported Ni catalysts may have a close relationship with the metal particle size. However, unfortunately, the supported Ni catalysts prepared by conventional impregnation method could not offer a clear explanation about the effect of metal particle size on coke formation because the morphological control of metal particles was quite limited. On the other hand, the metal particle size could be properly controlled by varying Ni loading in the preparation step of sol–gel process and subsequent supercritical drying and thermal treatment. In this work, we showed the presence of a minimum particle size to initiate the growth of filamentous carbon and we found that the sintering of metal particles was another major factor to deactivate supported Ni catalysts.
2. Experimental 2.1. Catalyst preparation Ni–alumina aerogel catalysts were prepared by the sol–gel processing of nickel acetate and aluminum sec-butoxide (ASB) in ethanol and subsequent supercritical drying with CO2 at 333 K and 24 MPa (designated as SAAx, where x means the Ni/Al mole ratio multiplied by 100). The dried aerogel was subjected to
our standard calcination procedure, which consisted of heating in helium at 573 K and in oxygen at 773 K [17]. Further details of the procedure have been described previously [10,11]. For comparison purposes, alumina-supported 10 wt.% Ni catalyst was prepared by impregnating a commercial ␥-alumina (supplied by Strem Chemicals, 362 m2 /g) with aqueous nickel nitrate solution (designated as ICN). 2.2. Catalyst characterization BET surface areas, pore volumes, and pore size distributions were determined by nitrogen adsorption–desorption at 77 K using a Micromeritics ASAP 2000 instrument. The mesopore size distributions were calculated from the desorption isotherms. Prior to the measurement, all the samples were outgassed in vacuum at 383 K overnight. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku D/MAX-IIIA diffractometer using Cu K␣ radiation. The surface composition of Ni–alumina aerogel catalysts was estimated via X-ray photoelectron spectroscopy (XPS) using an SSI 2803-S spectrometer. The samples for XRS measurements were pressed into self-supported discs. Before pressing, they were heat-treated at 973 K under H2 flowing for 2 h and then passivated by 1 vol% O2 /N2 at room temperature for 2 h. TEM images of the catalysts before and after reaction were taken with a Philips CM 30 microscope. Total carbon contents were determined with a Perkin–Elmer 240DS elemental analyzer in order to quantify the amount of carbonaceous deposits on the catalysts after reaction. 2.3. Catalytic tests The catalytic tests were carried out in a fixed-bed flow reactor made of a 4 mm ID quartz tube at atmospheric pressure. All the catalysts were pretreated in situ at 973 K for 2 h in H2 flow prior to the catalytic measurements. The standard reaction conditions were as follows: temperature, 973 K; catalyst amount, 0.05 g; feed composition, CH4 /CO2 /N2 = 10/10/20 (ml/min). The flow rates of the gases were controlled by mass flow controllers (Bronkhorst). During the reaction, the gaseous product coming out of the reactor, after water condensation, was directly sent to a gas
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chromatograph through a multiport sampling valve. The concentrations of reactants and products were determined using the on-line gas chromatograph (Varian 3700) equipped with a TCD and a Carbosphere packed column.
3. Results and discussion 3.1. Properties of Ni–alumina aerogel catalysts The textural properties of Ni–alumina aerogel catalysts (SAA series) are compared in Table 1 with those of 10 wt.% Ni supported on alumina catalyst (ICN, 0.096 in Ni/Al mole ratio) prepared by conventional impregnation method. Aerogel-based catalysts via sol–gel synthesis and subsequent supercritical drying exhibited high surface area, large pore volume, and pronounced mesoporosity. These catalytically favorable textural properties were well preserved even
Table 1 Textural properties of various Ni–alumina catalysts treated under different conditions Treatment
SBET a
VP b
DP c
383 K, vacuum 773 K, 2 h, O2 973 K, 2 h, H2
846 466 436
5.43 3.85 3.37
25.7 33.0 30.9
SAA15
383 K, vacuum 773 K, 2 h, O2 973 K, 2 h, H2
933 549 448
5.86 3.67 3.21
25.2 26.7 30.4
SAA20
383 K, vacuum 773 K, 2 h, O2 973 K, 2 h, H2
841 609 439
4.93 4.08 3.02
23.5 26.8 27.5
SAA25
383 K, vacuum 773 K, 2 h, O2 973 K, 2 h, H2
862 559 406
5.09 3.38 2.33
23.6 24.2 23.0
Alumina aerogel
773 K, 2 h, O2
456
2.61
22.9
773 K, 2 h, O2 973 K, 2 h, H2
261 145
0.27 0.25
4.1 6.9
362
0.26
2.9
Catalyst Ni–alumina aerogel SAA10
Impregnated catalyst ICN Alumina support a
–
SBET , BET surface area (m2 /g). VP , total pore volume (cm3 /g). c D , average pore diameter (nm). P b
Fig. 1. XRD patterns of Ni–alumina aerogel catalysts compared to impregnated Ni–alumina catalyst (ICN): (a) SAA10; (b) SAA15; (c) SAA20; (d) SAA25. All the samples were reduced in H2 flowing at 973 K for 2 h and then passivated with 1 vol% O2 /N2 before obtaining the spectra.
when the materials were treated at high temperatures. After thermal treatment at 973 K in hydrogen, none of the aerogel catalysts showed any significant change in the specific surface area with increasing metal loading. On the other hand, the corresponding total pore volume and average pore diameter decreased. A sharp decline in pore volume and pore diameter was observed at an Ni/Al mole ratio in excess of 0.2. This may be due to the existence of redispersion or aggregation of metallic Ni to form large metal particles during the thermal treatment, as reported by Zou and Gonzalez [18]. The XRD patterns of Ni–alumina aerogel catalysts after reduction at 973 K are depicted in Fig. 1. Compared to the ICN catalyst, which showed the presence of well-developed metallic Ni crystalline phase (fcc), there was no noticeable sign of forming X-ray detectable Ni crystalline particles in SAA series aerogel catalysts at low metal loadings. Even in the case of the SAA25 catalyst prepared with the highest metal loading (0.25 in Ni/Al mole ratio) among SAA series catalysts, only a small amount of crystalline Ni was detected by XRD. From this result, we concluded that a large fraction of Ni forms either very small Ni clusters undetectable by XRD or is imbedded in the network of alumina, as reported by Lopez et al. [19].
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Fig. 2. TEM micrographs of (a) impregnated Ni–alumina catalyst (ICN) and (b) Ni–alumina aerogel catalyst (SAA15) after reduction at 973 K.
From the TEM micrographs shown in Fig. 2, we confirmed that the preparation methods influence the morphology and size of the metal particles distributed over the support material. The micrograph of the ICN catalyst shows a substantial number of large Ni particles with different sizes formed irregularly over the alumina support, resulting in a broad size distribution (Fig. 3(a)). The formation of small Ni particles in the aerogel catalysts, on the other hand, could be confirmed clearly by the TEM micrograph of SAA15 having a metal concentration to comparable to that of ICN. As presented in Fig. 2(b), the SAA15 catalyst
Fig. 3. Ni particle size distributions derived from TEM for (a) impregnated Ni–alumina catalyst (ICN) and (b) Ni–alumina aerogel catalyst (SAA15) after reduction at 973 K.
consists of small Ni particles of about 3.3 nm mean size. The metal particles are evenly distributed over the amorphous alumina support and exhibit a narrow size distribution (Fig. 3(b)). It is likely that such good textural properties and excellent thermal stability of Ni–alumina aerogel catalysts play an important role in forming small metal particles distributed uniformly over the support. Thermal stability of the support may influence the morphology of the metal particles. If the support was transformed into a certain crystalline phase during the thermal treatment process, both the pore volume and the diameter would decrease abruptly
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and the changes in pore structure could affect the process of metal sintering, eventually resulting in the formation of large metal particles with low dispersion [20]. According to our recent study [10], we could prepare polymeric gels using substoichiometric amounts of water. Alumina aerogels obtained after subsequent CO2 supercritical drying exhibited good textural properties and remained amorphous up to 1073 K. Their superior thermal stability seemed to be successfully delivered to the Ni–alumina aerogel catalysts prepared from mixed gels. From the results of both XRD and TEM, we observed that the alumina support did not undergo substantial transformation into crystalline phases such as ␦- or ␥-Al2 O3 even after the thermal treatment at 973 K in flowing H2 .
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severe coke formation problem appeared to be unavoidable with the impregnated catalyst. As reported in our previous study [11], the ICN catalyst deactivated almost immediately due to coke formation and no further reaction could be carried out due to operational problems such as the blockage of the pore mouth and eventual clogging of the reactor tube. On the other hand, all the Ni–alumina aerogel catalysts, regardless of the Ni loadings, exhibited remarkable stability during the reaction for 30 h. The excellent stability of the Ni–alumina aerogel catalysts during the reforming reaction can be explained mainly by the existence of small metal particles distributed evenly over the support. The relationship between metal particle size and catalytic stability will be discussed in the later part of this section. From the results in Table 2, it can be seen that not only the catalytic activity but also the rate of coke formation over Ni–alumina aerogel catalyst are greatly influenced by metal loading. Although it was not unusual that the activity was a function of the overall metal loading, there seemed to be an optimum level of Ni loading for obtaining the best catalytic performance with respect to activity and stability. In this study, the optimum level of Ni loading in the Ni–alumina aerogel catalysts was found to be approximately 0.2 in Ni/Al mole ratio. Above this level, the catalytic activity did not increase noticeably while the rate of carbon formation increased abruptly, as shown in Fig. 4(a). It has been well known that coke formation is the major factor for deactivating supported Ni catalyst during the reforming reaction [4,9,21]. Therefore, it is of industrial importance, in particular, for the develop-
3.2. Activities of Ni–alumina aerogel catalysts CO2 reforming of CH4 was carried out over Ni–alumina aerogel catalysts at the temperature of 973 K. For the purpose of comparison, the ICN catalyst and a 5 wt.% Ru–alumina catalyst (ESCAT44 from Engelhard) were also tested. The results of the catalytic tests are summarized in Table 2. Ni–alumina aerogel catalysts were highly active for the reforming of CH4 with CO2 . With the increase in Ni loading, the activities of Ni–alumina aerogel catalysts increased, and eventually, the SAA25 catalyst showed high activity that was comparable to that of ESCAT44. As shown in some other studies on the supported Ni catalyst [4,6], the ICN exhibited high catalytic activity in the earlier period of the reaction. However, a
Table 2 Activities and coke formation rates of various catalystsa Catalysts
SAA10 SAA15 SAA20 SAA25 ICNc ESCAT44
CH4 conversion (%)
CO2 conversion (%)
1h
30 h
1h
30 h
61 64 69 70 66 72
47 55 66 66 – 71
66 69 73 75 68 74
54 62 67 71 – 72
Reaction conditions: 973 K, 1 atm, CH4 /CO2 /N2 = 1/1/2, W/F = 1.25 × 10−3 g-cat s/cm3 . Calculated from total carbon content in the used catalysts (mol C/g-cat h). c Reaction was terminated within 4–5 h. a
b
Rate of coke formationb
1.53 × 10 6.81 × 10 1.07 × 102 2.50 × 102 3.11 × 103 6.84
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particle size of the aerogel catalysts measured directly from TEM images is shown in Fig. 4(b) as a function of metal loading. The average metal particle size increased quite linearly with increasing Ni loading from 0.1 to 0.2 in Ni/Al mole ratio. Compared to the results in Fig. 4(a), the increase in the metal particle size was slow enough to have linearity with the slow increase in the rate of coke formation. However, as the metal loading reached 0.25 in Ni/Al mole ratio, the average metal particle size increased abruptly from about 4 to about 8 nm and coke formation was extremely accelerated in this range of metal loading. Unless there is a noticeable improvement in textural properties such as specific surface area, pore volume, and pore diameter, the drastic increase in the metal particle size means significant loss of dispersion, as discussed by Meijers et al. [22]. In this study, although the metal loading exceeded the level of 0.2 in Ni/Al mole ratio, the surface composition did not increase any more as shown in Fig. 4(b). This observation indicates that the formation of large Ni particles on the surface of the support and the corresponding loss of dispersion took place due to redispersion or aggregation of metallic Ni particles during the thermal treatment, as discussed in Section 3.1. Indeed, from the results shown in Fig. 4, it can be easily found that carbon deposition was greatly influenced by the metal particle size, and in particular, it was favored on large metal particles due to its structure-sensitive nature. Fig. 4. Effect of metal loading (a) on the catalytic activity and the rate of coke formation; (b) on the particle size and the surface composition of Ni–alumina aerogel catalysts. The average metal particle size was determined from TEM and surface composition was calculated from XPS. The rate of coke formation was calculated from total carbon contents in the used catalysts.
ment of efficient catalysts, not only to accelerate the overall reaction but also to adjust the appropriate elementary step in a way which minimizes the net rate of carbon deposition. From this point of view, it would be worthwhile to elucidate the reason of the sudden increase in carbon deposition observed during the reforming reaction over the aerogel catalyst prepared with the metal loading of 0.25 in Ni/Al mole ratio. We observed that the metal particle size of our aerogel catalysts was under the influence of the metal loading after thermal treatment. The average metal
3.3. Formation of filamentous carbon It has been recognized that carbon deposition is the main cause of the deactivation of supported Ni catalysts in steam reforming. This phenomenon may be more pronounced in the CO2 reforming of CH4 due to the lower H/C ratio of this system [21]. Ni–alumina aerogel catalysts prepared in this study showed remarkably lower coke-forming rate than the ICN catalyst to the extent of one or two orders of magnitude. In the case of ICN, the rate of carbon formation via reaction (1) and/or (2) was likely to be faster than the carbon removal rate, giving rise to rapid deactivation: CH4 → C + 2H2
1H = 75 kJ/mol
(1)
2CO → C + CO2
1H = −171 kJ/mol
(2)
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Fig. 5. TEM micrograph of impregnated Ni–alumina catalysts (ICN) after reaction illustrating fractured Ni particles and filamentous carbon whiskers.
In the reforming reaction, the deposition of carbon could occur on Ni particles in the various forms of carbon such as adsorbed atomic carbon, amorphous carbon, bulk Ni carbide and crystalline graphitic carbon [21]. TEM micrograph of the ICN catalyst (Fig. 5) provided clear evidence of whisker carbon formation during the reforming reaction. It is also noted that the morphology of surface whisker carbon formed during the reaction, in the case of Ni–alumina aerogel catalysts, was seriously affected by metal particle size determined by metal loading. As shown in Fig. 6, no whisker carbon formation was observed on the surface of the SAA10 catalyst. The SAA15 catalyst with a mean metal particle size of 3.3 nm could form very small amounts of filamentous carbon, but the formation of carbon filament was accelerated with an Ni/Al mole ratio in excess of 0.2. It seems, therefore, that a minimum diameter is required for Ni particles to generate carbon filaments. In the previous study on hydrogenolysis of cyclopentane over Ni/Al2 O3 catalysts, Duprez et al. [23] reported that metal particle size could influence both catalytic activity and coke formation. They pointed out that a minimum metal particle diameter was required to form whisker carbon and this value was 6 nm. Kroll et al. [13] also showed that the enhancement of stability of supported Ni catalyst could be achieved by controlling the metal
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particle size by varying the pretreatment condition. Although their results could not fully explain the relation of metal particle size with either the catalyst deactivation or the growth of whisker carbon, they pointed out that sintering occurred continuously during the reforming reaction and that various forms of coke accumulated outside the sintered particles. In our study, similar results were obtained. As listed in Table 3, after thermal treatment in hydrogen at 973 K, the SAA20 catalyst had very small Ni particles of about 4 nm, which is smaller than the minimum value proposed by Duprez et al. However, some sintered metal particles of above 7 nm in diameter were observed after the reaction. They were likely to offer the sites for filamentous carbon growth. In our study, carbon filament began to form as the metal loading reached the level of 0.15 in Ni/Al mole ratio, where the average metal particle size was about 7.1 nm after the reaction as listed in Table 3. Therefore, we could draw a conclusion that the average particle size of no less than 7 nm would be required in order to form filamentous carbon. As reported in our previous study [11], if severely formed, filamentous carbon could affect the pore size distributions of the catalysts after the reaction. In the case of the ICN catalyst, new pores, which did not exist before the reaction, were observed after the reaction. These newly developed pores of above 30 nm were attributed to the formation of fabric which consisted of whisker carbon and fractured Ni particles. Although we could also observe filamentous carbon in the SAA25 catalyst after reaction, the fibers were not densely formed and had the radius of 5–6 nm and were about half in thickness as regards those of the ICN catalyst. On account of this difference, the SAA25 catalyst did not show any substantial changes in pore size distribution after the reaction, as in the case of the other Ni–alumina aerogel catalysts prepared in this study. In conclusion, our results demonstrated that the formation of large metal particles in catalyst preparation and/or pretreatment steps and additional sintering during the reaction may be two important factors for inducing filamentous carbon formation. Goula et al. [12] also reported that the Ni particle morphology and its size distribution influenced the origin, the kinetics, and the reactivity of carbon deposition under reforming reaction conditions. Therefore, in order to reduce deactivation of supported Ni catalysts, proper
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Fig. 6. TEM micrographs showing the effect of metal loading on filamentous carbon formation for (a) SAA10, (b) SAA15, (c) SAA20, and (d) SAA25 after the reaction at 973 K for 30 h.
methods seem to be required to control the morphology, dispersion, and thermal stability of the catalysts. The preparation of catalytic materials via sol–gel process may be a suitable way to satisfy this demand. In particular, aerogel catalysts, which have better textural properties and thermal stability than xerogel, can offer a promise in this catalytic application due to their unique morphological and chemical properties.
Table 3 Effect of sintering on the average metal particle size during the reactiona Catalysts
SAA10 SAA15 SAA20 SAA25
Average Ni particle sizeb (nm) Before the reaction
After the reaction
2.1 3.3 3.9 7.9
6.3 7.1 7.4 8.3
Reaction conditions: 973 K, 1 atm, CH4 /CO2 /N2 = 1/1/2, W/F = 1.25 × 10−3 g-cat s/cm3 . b Measured directly from TEM images; 100 particles were measured per fixed area for taking an average. a
3.4. Temperature dependency of deactivation process Gadalla and Sommer [24] indicated that the deactivation of supported Ni catalysts may be influenced by either carbon deposition, metal sintering, or formation of inactive species such as NiAl2 O4 during the reform-
ing reaction. In the previous section, we discussed the deactivation process during the reforming reaction at 973 K, mainly focusing on coke formation and sinter-
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ing as a function of metal loading. However, the main cause of deactivation could have been different when the reaction was carried out under the different conditions, especially at different temperatures. Therefore, we examined the activity loss of the Ni–alumina aerogel catalysts during the reforming reaction at different temperatures. The deactivation process was investigated by the stability tests carried out at 873 and 1073 K. Fig. 7 shows the loss of catalytic activity in terms of CH4 or
Fig. 7. Activity changes as a function of time-on-stream for Ni–alumina aerogel catalysts under different reaction temperatures: 1 atm, CH4 /CO2 /N2 = 1/1/2, W/F = 1.25 × 10−3 g-cat s/cm3 ; 䊊, SAA25; 䊉, SAA10; }, stopping the reaction by clogging the reactor tube.
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CO2 conversion as a function of time. At 1073 K, the activity decline of each catalyst showed quite a different tendency. Only slight loss of initial activity was observed in the case of the SAA25 catalyst during the reforming reaction for 30 h, while the SAA10 catalyst lost its activity more rapidly in the earlier period of the reaction. This result hints that the SAA25 catalyst will have better stability than the SAA10 catalyst. However, a completely different result was observed in the reaction at the lower temperature of 873 K. During the reaction at this temperature, in thermodynamically favorable conditions to form coke, the activity of the SAA25 catalyst lasted for no more than 10 h and there existed a minimum point of CH4 conversion at the reaction time of about 4 h. After the reaction passed this minimum point, the conversion of CO2 decreased continuously while CH4 consumption was accelerated, followed by sudden stopping of the reaction due to clogging of the reactor tube. The result indicates that the growth of whisker carbon is mainly dominated by dissociation of CH4 , the following elementary reaction steps being well known in the literature [25–27]: CH4 (s) → CHx (s) + (4 − x)H(s)
(3)
CHx (s) → C(s) + xH(s)
(4)
where (s) is a site on the reduced Ni surface. Even though the initial activity of the SAA10 catalyst was lower than that of SAA25, SAA10 maintained its activity quite well during the reaction. In the case of our Ni–alumina aerogel catalysts, the influence of metal loading on the catalytic deactivation can be more clearly observed, by means of comparing CO yields obtained during the reaction at the different temperatures. Fig. 8 shows the changes in CO yields. At 873 K, with the increase in metal loading, the decrease in CO yield was accelerated during the reaction. This sudden deactivation, shown especially at higher metal loadings, can be explained by rapid coke formation on the catalyst surface at the low temperatures thermodynamically favorable for coking. However, in the case of the reaction at above 973 K, the CO yield decreased dramatically with decreasing metal loading. As shown in Table 2, at low Ni loading, the activity loss was not fully explained by carbon deposition. This deactivation may be caused by thermal degradation of active sites at high temperature, related to sintering or
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Large Ni particles formed in the catalyst preparation and/or the reaction steps are susceptible to the growth of whisker carbon. It seems that a minimum diameter of about 7 nm is required for the Ni particles to generate filamentous carbon. References [1] [2] [3] [4] [5] Fig. 8. Changes in CO yield during the reaction at different temperatures: (䊊) at 1073 K; (䊐) at 973 K; (䉫) at 873 K; open symbol, after 1 h; closed symbol, after 30 h (at 873 K, after 5 h).
[6] [7] [8]
the interaction of small Ni particles with neighboring Al atoms.
[9] [10] [11] [12]
4. Conclusions [13]
The present study demonstrates that the high-surface area Ni–alumina aerogel catalysts with mesoporosity could be used as suitable catalysts for CO2 reforming of CH4 . Good textural properties and stability during the thermal treatment up to 973 K led to the formation of small Ni particles dispersed evenly over alumina support. By varying the Ni loading, the control of metal particle size can be achieved. The reaction tests showed that the activities became higher with the increase in Ni loading, and eventually, the SAA25 catalyst (Ni/Al mole ratio = 0.25) had high activity that was comparable to that of alumina-supported ruthenium catalyst. Catalyst deactivation may be caused by both carbon formation and sintering of Ni particles. Even though aerogel catalysts showed good catalytic performances with respect to activity and stability, it seemed to be difficult to completely avoid carbon formation during the reaction at the temperature of 973 K.
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Effect of metal particle size on coking during CO2 reforming of CH4 over Ni–alumina aerogel catalysts Jin-Hong Kim a , Dong Jin Suh b,∗ , Tae-Jin Park b , Kyung-Lim Kim a a
b
Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, South Korea Clean Technology Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-791, South Korea Received 21 January 1999; received in revised form 24 August 1999; accepted 31 August 1999
Abstract CO2 reforming of CH4 was carried out over Ni–alumina aerogel catalysts prepared with various Ni loadings. The preparation of alumina supported Ni catalysts via sol–gel synthesis and subsequent supercritical drying led to the formation of very small metal particles, which are evenly distributed over the alumina support. The activity of the aerogel catalysts increased along with increasing metal loading, and eventually, the SAA25 (0.25 in Ni/Al mole ratio) catalyst exhibited the high activity comparable to that of a 5 wt.% Ru/alumina catalyst (ESCAT44, Engelhard). Compared to the alumina-supported Ni catalyst prepared by conventional impregnation method, Ni–alumina aerogel catalysts showed a remarkably low coking rate due to highly dispersed metal particles. From TEM micrograph studies, it was observed that the formation of filamentous carbon was significantly influenced by the metal particle size and proceeded mostly over the metal particles larger than 7 nm. The loss of catalytic activity at 973 K was mainly caused by coke deposition and sintering. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Methane; Carbon dioxide; Reforming; Sol–gel; Aerogel; Nickel; Alumina; Metal particle size; Deactivation; Filamentous carbon
1. Introduction During the past decades, CO2 reforming of CH4 has attracted interest from both industrial and environmental perspectives. In the environmental aspect, both CO2 and CH4 are recognized as undesirable greenhouse gases, and hence, the reaction provides a method of consuming CO2 effectively. From the industrial viewpoint, conversion of CH4 and CO2 into useful products is an important area of current catalytic research, especially C1 chemistry, because these two gases are the cheapest and most abundant carbon-containing mate∗ Corresponding author. Fax: +82-29585199. E-mail address: [email protected] (D.J. Suh)
rials and are convertible to synthesis gas with H2 /CO ratio lower than that of steam reforming [1,2]. The CO2 reforming reaction has been studied over numerous supported metal catalysts including Ni-based catalysts as well as noble metal-based ones [3–8]. The latter have been reported to be more active and less sensitive to coking than the former. However, considering the aspects of high cost and limited availability of noble metals, it is more practical, from the industrial standpoint, to develop Ni-based catalysts which are resistant to carbon deposition, while exhibiting high activity for this reaction [4,9]. Recently, we developed a method to prepare alumina aerogels having specific surface areas in excess of 700 m2 /g and high thermal stability and success-
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fully applied this method to prepare stable Ni–alumina aerogel catalysts for CH4 reforming with CO2 [10,11]. The result of catalytic tests showed that these catalysts were suitable for the CO2 reforming reaction due to good activity and remarkable resistance to coke formation [11]. In this study, we tried to find the optimum level of metal loading which could provide Ni–alumina aerogel catalysts with high activity and coke resistance, by testing and characterizing the aerogel catalysts prepared with various metal loading amounts. Especially, our work has been mainly focused on elucidating the causes of deactivation and offering some criteria of filamentous carbon formation during the reforming reaction, in relation to the morphology of metal particles. In some studies published recently, much effort has been devoted to clarify the relationship between the carbon-forming behavior of supported Ni catalysts and the morphology of metal particles [12,13–16]. According to these studies, the carbon-forming tendency of supported Ni catalysts may have a close relationship with the metal particle size. However, unfortunately, the supported Ni catalysts prepared by conventional impregnation method could not offer a clear explanation about the effect of metal particle size on coke formation because the morphological control of metal particles was quite limited. On the other hand, the metal particle size could be properly controlled by varying Ni loading in the preparation step of sol–gel process and subsequent supercritical drying and thermal treatment. In this work, we showed the presence of a minimum particle size to initiate the growth of filamentous carbon and we found that the sintering of metal particles was another major factor to deactivate supported Ni catalysts.
2. Experimental 2.1. Catalyst preparation Ni–alumina aerogel catalysts were prepared by the sol–gel processing of nickel acetate and aluminum sec-butoxide (ASB) in ethanol and subsequent supercritical drying with CO2 at 333 K and 24 MPa (designated as SAAx, where x means the Ni/Al mole ratio multiplied by 100). The dried aerogel was subjected to
our standard calcination procedure, which consisted of heating in helium at 573 K and in oxygen at 773 K [17]. Further details of the procedure have been described previously [10,11]. For comparison purposes, alumina-supported 10 wt.% Ni catalyst was prepared by impregnating a commercial ␥-alumina (supplied by Strem Chemicals, 362 m2 /g) with aqueous nickel nitrate solution (designated as ICN). 2.2. Catalyst characterization BET surface areas, pore volumes, and pore size distributions were determined by nitrogen adsorption–desorption at 77 K using a Micromeritics ASAP 2000 instrument. The mesopore size distributions were calculated from the desorption isotherms. Prior to the measurement, all the samples were outgassed in vacuum at 383 K overnight. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku D/MAX-IIIA diffractometer using Cu K␣ radiation. The surface composition of Ni–alumina aerogel catalysts was estimated via X-ray photoelectron spectroscopy (XPS) using an SSI 2803-S spectrometer. The samples for XRS measurements were pressed into self-supported discs. Before pressing, they were heat-treated at 973 K under H2 flowing for 2 h and then passivated by 1 vol% O2 /N2 at room temperature for 2 h. TEM images of the catalysts before and after reaction were taken with a Philips CM 30 microscope. Total carbon contents were determined with a Perkin–Elmer 240DS elemental analyzer in order to quantify the amount of carbonaceous deposits on the catalysts after reaction. 2.3. Catalytic tests The catalytic tests were carried out in a fixed-bed flow reactor made of a 4 mm ID quartz tube at atmospheric pressure. All the catalysts were pretreated in situ at 973 K for 2 h in H2 flow prior to the catalytic measurements. The standard reaction conditions were as follows: temperature, 973 K; catalyst amount, 0.05 g; feed composition, CH4 /CO2 /N2 = 10/10/20 (ml/min). The flow rates of the gases were controlled by mass flow controllers (Bronkhorst). During the reaction, the gaseous product coming out of the reactor, after water condensation, was directly sent to a gas
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chromatograph through a multiport sampling valve. The concentrations of reactants and products were determined using the on-line gas chromatograph (Varian 3700) equipped with a TCD and a Carbosphere packed column.
3. Results and discussion 3.1. Properties of Ni–alumina aerogel catalysts The textural properties of Ni–alumina aerogel catalysts (SAA series) are compared in Table 1 with those of 10 wt.% Ni supported on alumina catalyst (ICN, 0.096 in Ni/Al mole ratio) prepared by conventional impregnation method. Aerogel-based catalysts via sol–gel synthesis and subsequent supercritical drying exhibited high surface area, large pore volume, and pronounced mesoporosity. These catalytically favorable textural properties were well preserved even
Table 1 Textural properties of various Ni–alumina catalysts treated under different conditions Treatment
SBET a
VP b
DP c
383 K, vacuum 773 K, 2 h, O2 973 K, 2 h, H2
846 466 436
5.43 3.85 3.37
25.7 33.0 30.9
SAA15
383 K, vacuum 773 K, 2 h, O2 973 K, 2 h, H2
933 549 448
5.86 3.67 3.21
25.2 26.7 30.4
SAA20
383 K, vacuum 773 K, 2 h, O2 973 K, 2 h, H2
841 609 439
4.93 4.08 3.02
23.5 26.8 27.5
SAA25
383 K, vacuum 773 K, 2 h, O2 973 K, 2 h, H2
862 559 406
5.09 3.38 2.33
23.6 24.2 23.0
Alumina aerogel
773 K, 2 h, O2
456
2.61
22.9
773 K, 2 h, O2 973 K, 2 h, H2
261 145
0.27 0.25
4.1 6.9
362
0.26
2.9
Catalyst Ni–alumina aerogel SAA10
Impregnated catalyst ICN Alumina support a
–
SBET , BET surface area (m2 /g). VP , total pore volume (cm3 /g). c D , average pore diameter (nm). P b
Fig. 1. XRD patterns of Ni–alumina aerogel catalysts compared to impregnated Ni–alumina catalyst (ICN): (a) SAA10; (b) SAA15; (c) SAA20; (d) SAA25. All the samples were reduced in H2 flowing at 973 K for 2 h and then passivated with 1 vol% O2 /N2 before obtaining the spectra.
when the materials were treated at high temperatures. After thermal treatment at 973 K in hydrogen, none of the aerogel catalysts showed any significant change in the specific surface area with increasing metal loading. On the other hand, the corresponding total pore volume and average pore diameter decreased. A sharp decline in pore volume and pore diameter was observed at an Ni/Al mole ratio in excess of 0.2. This may be due to the existence of redispersion or aggregation of metallic Ni to form large metal particles during the thermal treatment, as reported by Zou and Gonzalez [18]. The XRD patterns of Ni–alumina aerogel catalysts after reduction at 973 K are depicted in Fig. 1. Compared to the ICN catalyst, which showed the presence of well-developed metallic Ni crystalline phase (fcc), there was no noticeable sign of forming X-ray detectable Ni crystalline particles in SAA series aerogel catalysts at low metal loadings. Even in the case of the SAA25 catalyst prepared with the highest metal loading (0.25 in Ni/Al mole ratio) among SAA series catalysts, only a small amount of crystalline Ni was detected by XRD. From this result, we concluded that a large fraction of Ni forms either very small Ni clusters undetectable by XRD or is imbedded in the network of alumina, as reported by Lopez et al. [19].
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Fig. 2. TEM micrographs of (a) impregnated Ni–alumina catalyst (ICN) and (b) Ni–alumina aerogel catalyst (SAA15) after reduction at 973 K.
From the TEM micrographs shown in Fig. 2, we confirmed that the preparation methods influence the morphology and size of the metal particles distributed over the support material. The micrograph of the ICN catalyst shows a substantial number of large Ni particles with different sizes formed irregularly over the alumina support, resulting in a broad size distribution (Fig. 3(a)). The formation of small Ni particles in the aerogel catalysts, on the other hand, could be confirmed clearly by the TEM micrograph of SAA15 having a metal concentration to comparable to that of ICN. As presented in Fig. 2(b), the SAA15 catalyst
Fig. 3. Ni particle size distributions derived from TEM for (a) impregnated Ni–alumina catalyst (ICN) and (b) Ni–alumina aerogel catalyst (SAA15) after reduction at 973 K.
consists of small Ni particles of about 3.3 nm mean size. The metal particles are evenly distributed over the amorphous alumina support and exhibit a narrow size distribution (Fig. 3(b)). It is likely that such good textural properties and excellent thermal stability of Ni–alumina aerogel catalysts play an important role in forming small metal particles distributed uniformly over the support. Thermal stability of the support may influence the morphology of the metal particles. If the support was transformed into a certain crystalline phase during the thermal treatment process, both the pore volume and the diameter would decrease abruptly
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and the changes in pore structure could affect the process of metal sintering, eventually resulting in the formation of large metal particles with low dispersion [20]. According to our recent study [10], we could prepare polymeric gels using substoichiometric amounts of water. Alumina aerogels obtained after subsequent CO2 supercritical drying exhibited good textural properties and remained amorphous up to 1073 K. Their superior thermal stability seemed to be successfully delivered to the Ni–alumina aerogel catalysts prepared from mixed gels. From the results of both XRD and TEM, we observed that the alumina support did not undergo substantial transformation into crystalline phases such as ␦- or ␥-Al2 O3 even after the thermal treatment at 973 K in flowing H2 .
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severe coke formation problem appeared to be unavoidable with the impregnated catalyst. As reported in our previous study [11], the ICN catalyst deactivated almost immediately due to coke formation and no further reaction could be carried out due to operational problems such as the blockage of the pore mouth and eventual clogging of the reactor tube. On the other hand, all the Ni–alumina aerogel catalysts, regardless of the Ni loadings, exhibited remarkable stability during the reaction for 30 h. The excellent stability of the Ni–alumina aerogel catalysts during the reforming reaction can be explained mainly by the existence of small metal particles distributed evenly over the support. The relationship between metal particle size and catalytic stability will be discussed in the later part of this section. From the results in Table 2, it can be seen that not only the catalytic activity but also the rate of coke formation over Ni–alumina aerogel catalyst are greatly influenced by metal loading. Although it was not unusual that the activity was a function of the overall metal loading, there seemed to be an optimum level of Ni loading for obtaining the best catalytic performance with respect to activity and stability. In this study, the optimum level of Ni loading in the Ni–alumina aerogel catalysts was found to be approximately 0.2 in Ni/Al mole ratio. Above this level, the catalytic activity did not increase noticeably while the rate of carbon formation increased abruptly, as shown in Fig. 4(a). It has been well known that coke formation is the major factor for deactivating supported Ni catalyst during the reforming reaction [4,9,21]. Therefore, it is of industrial importance, in particular, for the develop-
3.2. Activities of Ni–alumina aerogel catalysts CO2 reforming of CH4 was carried out over Ni–alumina aerogel catalysts at the temperature of 973 K. For the purpose of comparison, the ICN catalyst and a 5 wt.% Ru–alumina catalyst (ESCAT44 from Engelhard) were also tested. The results of the catalytic tests are summarized in Table 2. Ni–alumina aerogel catalysts were highly active for the reforming of CH4 with CO2 . With the increase in Ni loading, the activities of Ni–alumina aerogel catalysts increased, and eventually, the SAA25 catalyst showed high activity that was comparable to that of ESCAT44. As shown in some other studies on the supported Ni catalyst [4,6], the ICN exhibited high catalytic activity in the earlier period of the reaction. However, a
Table 2 Activities and coke formation rates of various catalystsa Catalysts
SAA10 SAA15 SAA20 SAA25 ICNc ESCAT44
CH4 conversion (%)
CO2 conversion (%)
1h
30 h
1h
30 h
61 64 69 70 66 72
47 55 66 66 – 71
66 69 73 75 68 74
54 62 67 71 – 72
Reaction conditions: 973 K, 1 atm, CH4 /CO2 /N2 = 1/1/2, W/F = 1.25 × 10−3 g-cat s/cm3 . Calculated from total carbon content in the used catalysts (mol C/g-cat h). c Reaction was terminated within 4–5 h. a
b
Rate of coke formationb
1.53 × 10 6.81 × 10 1.07 × 102 2.50 × 102 3.11 × 103 6.84
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particle size of the aerogel catalysts measured directly from TEM images is shown in Fig. 4(b) as a function of metal loading. The average metal particle size increased quite linearly with increasing Ni loading from 0.1 to 0.2 in Ni/Al mole ratio. Compared to the results in Fig. 4(a), the increase in the metal particle size was slow enough to have linearity with the slow increase in the rate of coke formation. However, as the metal loading reached 0.25 in Ni/Al mole ratio, the average metal particle size increased abruptly from about 4 to about 8 nm and coke formation was extremely accelerated in this range of metal loading. Unless there is a noticeable improvement in textural properties such as specific surface area, pore volume, and pore diameter, the drastic increase in the metal particle size means significant loss of dispersion, as discussed by Meijers et al. [22]. In this study, although the metal loading exceeded the level of 0.2 in Ni/Al mole ratio, the surface composition did not increase any more as shown in Fig. 4(b). This observation indicates that the formation of large Ni particles on the surface of the support and the corresponding loss of dispersion took place due to redispersion or aggregation of metallic Ni particles during the thermal treatment, as discussed in Section 3.1. Indeed, from the results shown in Fig. 4, it can be easily found that carbon deposition was greatly influenced by the metal particle size, and in particular, it was favored on large metal particles due to its structure-sensitive nature. Fig. 4. Effect of metal loading (a) on the catalytic activity and the rate of coke formation; (b) on the particle size and the surface composition of Ni–alumina aerogel catalysts. The average metal particle size was determined from TEM and surface composition was calculated from XPS. The rate of coke formation was calculated from total carbon contents in the used catalysts.
ment of efficient catalysts, not only to accelerate the overall reaction but also to adjust the appropriate elementary step in a way which minimizes the net rate of carbon deposition. From this point of view, it would be worthwhile to elucidate the reason of the sudden increase in carbon deposition observed during the reforming reaction over the aerogel catalyst prepared with the metal loading of 0.25 in Ni/Al mole ratio. We observed that the metal particle size of our aerogel catalysts was under the influence of the metal loading after thermal treatment. The average metal
3.3. Formation of filamentous carbon It has been recognized that carbon deposition is the main cause of the deactivation of supported Ni catalysts in steam reforming. This phenomenon may be more pronounced in the CO2 reforming of CH4 due to the lower H/C ratio of this system [21]. Ni–alumina aerogel catalysts prepared in this study showed remarkably lower coke-forming rate than the ICN catalyst to the extent of one or two orders of magnitude. In the case of ICN, the rate of carbon formation via reaction (1) and/or (2) was likely to be faster than the carbon removal rate, giving rise to rapid deactivation: CH4 → C + 2H2
1H = 75 kJ/mol
(1)
2CO → C + CO2
1H = −171 kJ/mol
(2)
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Fig. 5. TEM micrograph of impregnated Ni–alumina catalysts (ICN) after reaction illustrating fractured Ni particles and filamentous carbon whiskers.
In the reforming reaction, the deposition of carbon could occur on Ni particles in the various forms of carbon such as adsorbed atomic carbon, amorphous carbon, bulk Ni carbide and crystalline graphitic carbon [21]. TEM micrograph of the ICN catalyst (Fig. 5) provided clear evidence of whisker carbon formation during the reforming reaction. It is also noted that the morphology of surface whisker carbon formed during the reaction, in the case of Ni–alumina aerogel catalysts, was seriously affected by metal particle size determined by metal loading. As shown in Fig. 6, no whisker carbon formation was observed on the surface of the SAA10 catalyst. The SAA15 catalyst with a mean metal particle size of 3.3 nm could form very small amounts of filamentous carbon, but the formation of carbon filament was accelerated with an Ni/Al mole ratio in excess of 0.2. It seems, therefore, that a minimum diameter is required for Ni particles to generate carbon filaments. In the previous study on hydrogenolysis of cyclopentane over Ni/Al2 O3 catalysts, Duprez et al. [23] reported that metal particle size could influence both catalytic activity and coke formation. They pointed out that a minimum metal particle diameter was required to form whisker carbon and this value was 6 nm. Kroll et al. [13] also showed that the enhancement of stability of supported Ni catalyst could be achieved by controlling the metal
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particle size by varying the pretreatment condition. Although their results could not fully explain the relation of metal particle size with either the catalyst deactivation or the growth of whisker carbon, they pointed out that sintering occurred continuously during the reforming reaction and that various forms of coke accumulated outside the sintered particles. In our study, similar results were obtained. As listed in Table 3, after thermal treatment in hydrogen at 973 K, the SAA20 catalyst had very small Ni particles of about 4 nm, which is smaller than the minimum value proposed by Duprez et al. However, some sintered metal particles of above 7 nm in diameter were observed after the reaction. They were likely to offer the sites for filamentous carbon growth. In our study, carbon filament began to form as the metal loading reached the level of 0.15 in Ni/Al mole ratio, where the average metal particle size was about 7.1 nm after the reaction as listed in Table 3. Therefore, we could draw a conclusion that the average particle size of no less than 7 nm would be required in order to form filamentous carbon. As reported in our previous study [11], if severely formed, filamentous carbon could affect the pore size distributions of the catalysts after the reaction. In the case of the ICN catalyst, new pores, which did not exist before the reaction, were observed after the reaction. These newly developed pores of above 30 nm were attributed to the formation of fabric which consisted of whisker carbon and fractured Ni particles. Although we could also observe filamentous carbon in the SAA25 catalyst after reaction, the fibers were not densely formed and had the radius of 5–6 nm and were about half in thickness as regards those of the ICN catalyst. On account of this difference, the SAA25 catalyst did not show any substantial changes in pore size distribution after the reaction, as in the case of the other Ni–alumina aerogel catalysts prepared in this study. In conclusion, our results demonstrated that the formation of large metal particles in catalyst preparation and/or pretreatment steps and additional sintering during the reaction may be two important factors for inducing filamentous carbon formation. Goula et al. [12] also reported that the Ni particle morphology and its size distribution influenced the origin, the kinetics, and the reactivity of carbon deposition under reforming reaction conditions. Therefore, in order to reduce deactivation of supported Ni catalysts, proper
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Fig. 6. TEM micrographs showing the effect of metal loading on filamentous carbon formation for (a) SAA10, (b) SAA15, (c) SAA20, and (d) SAA25 after the reaction at 973 K for 30 h.
methods seem to be required to control the morphology, dispersion, and thermal stability of the catalysts. The preparation of catalytic materials via sol–gel process may be a suitable way to satisfy this demand. In particular, aerogel catalysts, which have better textural properties and thermal stability than xerogel, can offer a promise in this catalytic application due to their unique morphological and chemical properties.
Table 3 Effect of sintering on the average metal particle size during the reactiona Catalysts
SAA10 SAA15 SAA20 SAA25
Average Ni particle sizeb (nm) Before the reaction
After the reaction
2.1 3.3 3.9 7.9
6.3 7.1 7.4 8.3
Reaction conditions: 973 K, 1 atm, CH4 /CO2 /N2 = 1/1/2, W/F = 1.25 × 10−3 g-cat s/cm3 . b Measured directly from TEM images; 100 particles were measured per fixed area for taking an average. a
3.4. Temperature dependency of deactivation process Gadalla and Sommer [24] indicated that the deactivation of supported Ni catalysts may be influenced by either carbon deposition, metal sintering, or formation of inactive species such as NiAl2 O4 during the reform-
ing reaction. In the previous section, we discussed the deactivation process during the reforming reaction at 973 K, mainly focusing on coke formation and sinter-
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ing as a function of metal loading. However, the main cause of deactivation could have been different when the reaction was carried out under the different conditions, especially at different temperatures. Therefore, we examined the activity loss of the Ni–alumina aerogel catalysts during the reforming reaction at different temperatures. The deactivation process was investigated by the stability tests carried out at 873 and 1073 K. Fig. 7 shows the loss of catalytic activity in terms of CH4 or
Fig. 7. Activity changes as a function of time-on-stream for Ni–alumina aerogel catalysts under different reaction temperatures: 1 atm, CH4 /CO2 /N2 = 1/1/2, W/F = 1.25 × 10−3 g-cat s/cm3 ; 䊊, SAA25; 䊉, SAA10; }, stopping the reaction by clogging the reactor tube.
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CO2 conversion as a function of time. At 1073 K, the activity decline of each catalyst showed quite a different tendency. Only slight loss of initial activity was observed in the case of the SAA25 catalyst during the reforming reaction for 30 h, while the SAA10 catalyst lost its activity more rapidly in the earlier period of the reaction. This result hints that the SAA25 catalyst will have better stability than the SAA10 catalyst. However, a completely different result was observed in the reaction at the lower temperature of 873 K. During the reaction at this temperature, in thermodynamically favorable conditions to form coke, the activity of the SAA25 catalyst lasted for no more than 10 h and there existed a minimum point of CH4 conversion at the reaction time of about 4 h. After the reaction passed this minimum point, the conversion of CO2 decreased continuously while CH4 consumption was accelerated, followed by sudden stopping of the reaction due to clogging of the reactor tube. The result indicates that the growth of whisker carbon is mainly dominated by dissociation of CH4 , the following elementary reaction steps being well known in the literature [25–27]: CH4 (s) → CHx (s) + (4 − x)H(s)
(3)
CHx (s) → C(s) + xH(s)
(4)
where (s) is a site on the reduced Ni surface. Even though the initial activity of the SAA10 catalyst was lower than that of SAA25, SAA10 maintained its activity quite well during the reaction. In the case of our Ni–alumina aerogel catalysts, the influence of metal loading on the catalytic deactivation can be more clearly observed, by means of comparing CO yields obtained during the reaction at the different temperatures. Fig. 8 shows the changes in CO yields. At 873 K, with the increase in metal loading, the decrease in CO yield was accelerated during the reaction. This sudden deactivation, shown especially at higher metal loadings, can be explained by rapid coke formation on the catalyst surface at the low temperatures thermodynamically favorable for coking. However, in the case of the reaction at above 973 K, the CO yield decreased dramatically with decreasing metal loading. As shown in Table 2, at low Ni loading, the activity loss was not fully explained by carbon deposition. This deactivation may be caused by thermal degradation of active sites at high temperature, related to sintering or
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Large Ni particles formed in the catalyst preparation and/or the reaction steps are susceptible to the growth of whisker carbon. It seems that a minimum diameter of about 7 nm is required for the Ni particles to generate filamentous carbon. References [1] [2] [3] [4] [5] Fig. 8. Changes in CO yield during the reaction at different temperatures: (䊊) at 1073 K; (䊐) at 973 K; (䉫) at 873 K; open symbol, after 1 h; closed symbol, after 30 h (at 873 K, after 5 h).
[6] [7] [8]
the interaction of small Ni particles with neighboring Al atoms.
[9] [10] [11] [12]
4. Conclusions [13]
The present study demonstrates that the high-surface area Ni–alumina aerogel catalysts with mesoporosity could be used as suitable catalysts for CO2 reforming of CH4 . Good textural properties and stability during the thermal treatment up to 973 K led to the formation of small Ni particles dispersed evenly over alumina support. By varying the Ni loading, the control of metal particle size can be achieved. The reaction tests showed that the activities became higher with the increase in Ni loading, and eventually, the SAA25 catalyst (Ni/Al mole ratio = 0.25) had high activity that was comparable to that of alumina-supported ruthenium catalyst. Catalyst deactivation may be caused by both carbon formation and sintering of Ni particles. Even though aerogel catalysts showed good catalytic performances with respect to activity and stability, it seemed to be difficult to completely avoid carbon formation during the reaction at the temperature of 973 K.
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