versión impresa ISSN 0327-0793
Lat. Am. appl. res. v.34 n.3 Bahía Blanca jul./sept. 2004
Improved activity and stability of Ce-promoted Ni/γ -Al2O3 catalysts for carbon dioxide reforming of methane
A. Valentini1, N. L. V. Carreño2, E. R. Leite2, R. F. Gonçalves2, L. E. B. Soledade2, Y. Maniette3, E. Longo2 and L. F. D. Probst1
1 Dto de Química, Univ. Federal de Santa Catarina, CP 476, 88040-900 Florianópolis, SC, Brazil.
2 Centro Multidisciplinar de Desenvolvimento de Materiais Cerâmicos (CMDMC), Dto de Química, Univ. Federal de São Carlos, CP 676, 13560-905 São Carlos, SP, Brazil
3 Instituto de Química, Univ. Estadual de São Paulo, CP 355, 14801-970 Araraquara, SP, Brazil
Abstract The CO2 reforming of CH4 was carried out over Ni catalysts supported on γ -Al2O3 and CeO2-promoted γ -Al2O3. The catalysts were characterized by means of surface area measurements, TPR, CO2 and H2 chemisorption, XRD, SEM, and TEM. The CeO2 addition promoted an increase of catalytic activity and stability. The improvement in the resistance to carbon deposition is attributed to the highest CO2 adsorption presented by the CeO2 addition. The catalytic behavior presented by the samples, with a different CH4/CO2 ratio used, points to the CH4 decomposition reaction as the main source of carbon deposition.
Keywords Methane. Dry Reforming. CeO2. Carbon Formation. Syngas.
Oil is the main source of chemical products and fuels. In spite of this, the natural gas explored in many countries is a good alternative (Dry, 2002). The natural gas abundance, with CH4 as the main component, consists of an interesting H2 and synthesis gas source (Hu and Ruckenstein, 2002). This can furnish a fraction of the energetic and chemical products demands through the Fisher-Tropsch process (Hu and Ruckenstein, 2002; Rostrup-Nielsen, 2000).
The catalytic reforming of CH4 with CO2 (Eq. 1) for the production of synthesis gas is an interesting process. Besides the production of high-value compounds it is followed by the consumption of greenhouse gases (Tang et al., 1995; Kroll et al., 1996; Nichio, 2000).
CH4 + CO2 2CO + 2H2 (1)
Numerous authors (Nichio, 2000; Tomishige et al., 2000; Tomishige et al., 2001; Wang and Ruckenstein, 2001; Takeguchi et al., 2001; Frusteri et al., 2001; Xu et al., 2001) have carried out the CO2 reforming of CH4 on different catalysts, and the major problem is the catalyst deactivation, induced by carbon deposition (Hu and Ruckenstein, 2002; Kroll et al., 1996). Noble metal catalysts supported on different carriers exhibit better activity and high stability (Zhu and Stephanopoulos, 2001), but they are expensive. In spite of the carbon deposition, the Ni-based catalysts have high activity, stability and selectivity and are cheap. Therefore, the development of such catalysts is an attractive challenge (Crisafulli et al., 2002).
Several processes have been used for reducing the coke deposition on the catalysts. Recently, Leite et al. (2002) described a route to synthesize nanometric Ni particles embedded in a mesoporous silica material. This method showed promising results in the catalytic carbon dioxide reforming of methane, with low coke deposition (Probst et al., 2002).
There are several publications reporting the CeO2 application and properties (Trovarelli, 1996; Probst and Valentini, 2001; Piras et al., 2000; Rossignol and Kappenstein, 2001). But there is only a limited amount of work devoted to study the CeO2 application as the metal support in the CO2 reforming of CH4 (Wang et al., 2001), due to the low CH4 conversion observed (Montoya et al., 2000; Wang and Lu, 1998; Noronha et al., 2001). On the other hand, CeO2 is an effective promoter for the Ni/Al2O3 catalysts in the suppression of carbon deposition (Montoya et al., 2000; Wang and Lu, 1998; Xu et al., 1999). The CeO2 enhancement in the catalytic properties like stability against coke deposition is attributed to the ability of Ce to reversibly change oxidation states between Ce4+ and Ce3+ (Noronha et al., 2001). The Ce2O3 oxide present on the catalyst surface promotes the process of transferring oxygen (Xu et al., 1999).
It is known that the coke deposition during dry reforming is a function of operating conditions. Operations at high temperature (>800oC) and at high CO2/CH4 ratios (>1) avoid carbon deposition (Reitmeier et al., 1948; Gadalla and Bower, 1988). However, lower temperatures and a CO2/CH4 ratio near unity are more interesting.
The aim of the present study is to obtain a better understanding of the nature of the carbon deposition on CeO2-promoted Ni:Al2O3 catalysts prepared by the impregnation method. Are investigated the catalytic performance and carbon deposition behavior in the CO2 reforming of CH4 under atmospheric pressure and with different CH4/CO2 ratios operating at moderate temperature. The sample characterization was performed by means of X-ray diffraction (XRD), H2 and CO2 chemisorption, specific surface area measurement (BET), elemental analysis (CHN) and transmission electron microscopy (TEM).
II. MATERIALS AND METHODS
A. Catalyst preparation
The Ce-doping supports were prepared by the impregnation of an aqueous solution of Ce(NO3)3 (Aldrich, 99,99 %) on γ -Al2O3 (AL-3996R, 200 m2/g, Engelhard Exceptional Technologies). The CeO2 content was (0, 5, 10, 15 and 20 wt%). After impregnation the supports were dried for 24 h at 100oC and calcined at 450oC during 2h. The supports were denominated AlCe-X (with X = CeO2 wt%).
The Ni addition was carried out by impregnation of an aqueous solution of Ni(NO3)2 (Fluka, 98 %) on γ -Al2O3 and on AlCe-X supports. The samples with 10 wt% of Ni were dried for 24h at 100oC and calcined at 650oC during 3h.
The amount of metal (Ni) in the solids was determined by atomic absorption spectrometry, using an HITACHI Z8230 equipment.
The TPR profiles were taken in the Micromeritics TPD/TPR 2900 model equipment, using 10oC/min of heating rate and monitoring the hydrogen consumption from a 5%H2/N2 mixture at the 50-930oC temperature range. The surface area and porosity measurements following the nitrogen adsorption method (-196oC), the metallic accessibility using H2 chemisorption at 27oC and the CO2 chemisorption at 27 and 625oC were determined in an Autosorb-1C (Quantachrome Instruments). The amount of irreversible H2 uptake was obtained from the difference between the total adsorption of H2 on the catalyst and a second adsorption series of H2 determined after evacuation of the catalyst sample for 30 minutes at the same temperature.
Elementary chemical analysis was employed to estimate the total coke content after the catalytic reaction (Carlo Erba EA 1110 CHNS-O). The XRD spectra were acquired by the use of a Siemens D-5000 diffractometer with Cu-Kα radiation and a graphite crystal monochromator.
For the microstructure characterization were used the transmission electron microscope CM200 200 kV and the scanning electron microscope DSM940A.
C. Catalytic activity
Reactions were performed in a tubular fixed-bed flow reactor made of a stainless steel tube of 9.5 mm inner diameter. The catalyst (0.050 g) was in situ pretreated in a H2 stream at 650oC for 1h. The reaction gas was composed of carbon dioxide, methane and nitrogen in the (CO2:CH4:N2) ratios of 1:1:4, 1:2:6 and 2:1:6 with the total flow rate of 35 cm3/min. All catalytic tests were performed at 625oC under atmospheric pressure. Nitrogen was used as a diluent and an internal standard for the analysis. The reactant and the product gases were analyzed with an on-stream gas chromatograph (GC) SHIMADZU GC 8A, equipped with a thermal conductivity detector (TCD), Porapak-Q and a 5A molecular sieve column (with Ar as the carrier gas). The systematic errors in the conversions are ±1%.
III. RESULTS AND DISCUSSION
The results of chemical analysis, H2 chemisorption and specific surface area for the catalysts are summarized in Table 1.
A gradual surface area decreasing is observed with the CeO2 loading. It is known that low CeO2 loading could stabilize γ -Al2O3 against surface area loss (Piras et al., 2000; Ozawa and Kimura, 1990) when calcined at high temperature (>900oC). However, in this work the samples have higher then 5 wt% of CeO2 and were heattreated at 650oC, a low temperature to promote a γ -Al2O3 surface area loss.
Table 1: Chemical analysis and surface properties measured by N2 physisorption and H2 chemisorption.
‡ - Measured by N2 adsorption at -196oC; # - Based on total H2 adsorption at 300 K; * - Based on irreversible H2 adsorption at 300 K
The surface area decreasing promoted by the CeO2 addition points to a partial pore obstruction. In the impregnation process, the Ce is mainly deposited inside the γ -Al2O3 pores. The profile behaviors observed in Fig. 1 confirm that the pore volume decreasing follows the surface area loss.
Fig. 1- Pore volume and surface area change promoted by the CeO2 loading increase.
On the other hand, the CeO2 loading promoted an increase in the total H2 chemisorption (Table 1). This points to a higher metal (Ni) dispersion, the best result is observed for the sample with the lowest CeO2 loading (NiAlCe-5). Similar results have been reported (Wang and Lu, 1998; Montoya et al., 2000). It suggests that low CeO2 loading actuates as a textural promoter. In the reduction treatments, the Ni particles can be partially decorated by the CeO2 (Bernal et al., 2003). With this, the metal particles show a better resistance against sintering. However, the occurrence of spillover phenomena should be considered when CeO2 is present (Bernal et al., 2003). The irreversible H2 chemisorption corroborates to that. With the Ni/AlCe-5 sample exception, the CeO2-doping promoted a decreasing in the irreversible H2 adsorption.
Temperature programmed reduction (TPR) profiles for the catalysts are presented in Figure 2. The Ni/Al2O3 exhibited one peak at 600oC and a second one at 770oC. The peak at 600oC is likely due to the more accessible Ni and the peak at 770oC due to the Ni inside the pore material, what can present a strong support influence on the Ni reduction (Molina and Poncelet, 1998; Chen et al., 1991). It is seen that CeO2 loading shifts the peak around 600oC to a lower temperature value, pointing to the changes in the catalysts properties promoted by CeO2. The H2 consumption around 900oC is attributed to the CeAlO3 formation (Piras et al., 2000; Damyanova et al., 2002) what was confirmed by XRD. A broad reduction feature ranging from 270oC and 400oC that increases with the CeO2 loading is attributed to the partial CeO2 surface reduction (Trovarelli, 1996; Damyanova et al., 2002; Perrichon et al., 1994; Fajardie et al., 1998); however, likewise it can be related to the Ni present in CeO2 rich areas (Wang et al., 2001).
Fig. 2- Temperature programmed reduction (TPR) profiles of Ni/Al2O3 and CeO2 loading catalysts.
By the scanning electron microscopy (SEM) images illustrated in Fig. 3 for Ni/AlCe-15 and Ni/AlCe-20 samples, it is possible to see high CeO2 concentration areas on the catalyst surfaces.
In the Ni/AlCe-20 sample it is possible to see the typical spherical shape of the CeO2 particles.
Fig. 3- Scanning electron microscopy (SEM) images of Ni/AlCe-15 and Ni/AlCe-20 samples after activation at 650oC/1h.
The catalytic performances in the CO2 reforming of CH4 for the catalysts are presented in Fig. 4. With the exception of the Ni/AlCe-5 (5 wt% of CeO2), the catalytic CH4 conversion shows a gradual increase with the CeO2 loading. The particular behavior for the Ni/AlCe-5 sample can be attributed to the higher H2 chemisorption, which points to a higher Ni dispersion (Table 1).
A CH4 conversion increase promoted by the CeO2 loading is followed by the catalytic stability increase. The stability improvement can be estimated by the ratios between the CH4 conversion after 15h and the CH4 conversion after 2h of time-on-stream (C15/C2). The C15/C2 ratios were: 0.91, 0.92, 0.95, 0.96 and 0.98 for Ni/Al2O3, Ni/AlCe-5, Ni/AlCe-10, Ni/AlCe-15 and Ni/AlCe-20, respectively. It is shown that the CeO2 addition favors the enhancement of the Ni/Al2O3 catalytic properties in the CO2 reforming of CH4.
Fig. 4- Profiles of CH4 conversion with time-on-stream for the catalysts. Activation at 650oC/1h, 625oC of reaction temperature, wt = 50 mg
In order to confirm the stability increase promoted by the CeO2 loading, a catalytic test was performed during 50h with a CH4/CO2 ratio of 1:1 at 625oC for the Ni/Al2O3 and Ni/AlCe-20 samples. Figure 5 shows that the deactivation rate of Ni/Al2O3 is higher than Ni/AlCe-20. The ratios between the CH4 conversion after 50h and its initial conversion (C50/Ci) are 0.55 and 0.94 for Ni/Al2O3 and Ni/AlCe-20, respectively. The CH4 conversion decreases linearly with time-on-stream for Ni/AlCe-20, however the Ni/Al2O3 catalyst showed the same behavior only in the initial test. After 25h of catalytic reaction a non-linear deactivation is considerable for the Ni/Al2O3 catalyst. This behavior can be related to the coke and carbon filament formation that promotes a blocking of the active surface. This suggests that with the coke and carbon filaments continuous grow with the time-on-stream, the metal particles are encapsulated in the carbon filaments and there is a critical point in the carbon nanotube growth above which the catalyst deactivation is more pronounced. The limit of reagent diffusion to the metal particles should be considered, in the case of coke deposition, even with no particle encapsulation.
Fig. 5- Profiles of stability performance for the catalysts on CH4 conversion with time-on-stream. Activation at 650oC/1h, 625oC of reaction temperature, wt = 50 mg
The condensation of carbon over the Ni crystals exposed on the surface of the catalysts allows the formation of a sheet around the metal particle surface (Hester and Louchev, 2002) with subsequent growth of the nanotubes following the detachment of Ni from the support (Tsang et al., 1995). This leads to the encapsulation of the metal particles that causes the activity loss. Bright-Field (BF) TEM images of Ni/Al2O3 sample after catalytic test (Fig. 6) evidences an abundant carbon nanotube growing out of the Ni particles.
Fig. 6- BF-TEM images of the catalysts. Images of the Ni/Al2O3, (a) 15h of time-on-stream and (b) 50h of time-on-stream. Ni/AlCe-20, (c) 50h of time-on-stream, (d) transversal image of a carbon tube formation with a metal particle in the center.
Therefore, there is a close relation between catalytic activity decreasing and the metal particle blocking by the carbon filaments, as illustrated in Fig. 6. Otherwise, the CeO2-doped catalysts do not have the tendency of carbonaceous structure formation. The TEM analysis of Ni/Al2O3 after 50h of catalytic reaction showed a carbon tube formation higher than 10 mm, Fig. 6b. On the other hand, the Ni/AlCe-20 presented carbon tubes lower than 1 mm. Figure 6d shows the thick carbon tube formation from the carbon deposition on the metal particles. This carbon structure will promote the metal particles disintegration or abstraction from the support.
The ordered carbon structure is detected also by XRD. The powder diffraction patterns of the fresh samples and of the samples after the catalytic reaction are presented in Fig. 7. Both, fresh and spent CeO2-doped catalysts presented a broader and weaker intensity of Ni peaks. This suggests that CeO2 promotes a higher dispersion of Ni or there is an interface region between metal and support, associated to the interaction. This second point is in agreement with Ni dispersion obtained by the irreversible H2 adsorption. Assuming that only Ni0 is responsible for the irreversible H2 adsorption, data from Table 1 points to a decrease in the metal surface area or an increase in the interfacial region, with the CeO2 loading, what is unable to adsorb irreversible H2.
Fig. 7- X-ray diffraction patterns of the samples (a) activated at 650 oC for 1 h in H2 flow, (b) after catalytic reaction at 625 oC for 15h. ◊ = CeO2; # = Al2O3; ♣ = Ni; * = coke deposition.
This distinct interfacial region promoted by CeO2 addition may be responsible by the lower carbon deposition, as indicated by the decreasing in the ordered carbon peak in the samples after the catalytic test (Fig. 7b). A decrease in the carbon deposition for all Ni/AlCe samples was confirmed by CHN analysis (Table 2).
CO2 adsorption isotherms uptake were performed at 625oC, the reaction temperature, and at 27oC. The results (Table 2) showed an increase in the CO2 adsorption with CeO2 loading. This suggests that the principal CeO2 contribution is to improve the CO2 adsorption in the interfacial region that leads to a lower carbon deposition, via CH4 decomposition reaction (Eq. 2), as well as via CO disproportionation (Eq. 3), by shifting the equilibrium concentrations (Bradford and Vannice, 1999).
CH4 C + 2H2 (2)
2CO C + CO2 (3)
Table 2: CO2 chemisorption and elementary chemical analysis of catalysts after CO2 reforming of CH4.
* - CH4/CO2 = 0.5; ♦ - CH4/CO2 = 2.0;
In order to support this conclusion, the Ni/Al2O3 and Ni/AlCe-20 catalysts were tested with different gas compositions, CH4/CO2 = 0.5 and 2.0. The CH4 conversions for these new CH4/CO2 ratios are plotted in Fig. 8.
A higher CH4 conversion is observed with the increase in the CO2 concentration as a consequence of the equilibrium shifting (Eq. 1). In this condition (CH4/CO2 = 0.5) the Ni/Al2O3 sample presented better initial catalytic performance with higher CH4 conversion (Fig. 8a). However, with the CH4 concentration increase (CH4/CO2 ratio of 2:1), the better catalytic performance, CH4 conversion and stability, is presented by the Ni/AlCe-20 sample (Fig. 8b).
Fig. 8- Profile performance for the catalysts on CH4 conversion with time-on-stream at different CH4/CO2 ratios. Activation at 650oC/1h, 625oC of reaction temperature, wt = 50 mg
With the CH4/CO2 ratio of 2:1, the catalysts showed a high initial deactivation rate and an apparent stabilization after 3h of time-on-stream. This behavior is attributed to the coke deposition as a consequence of the favorable coking reaction condition, low temperature and high CH4/CO2 ratio (Gadalla and Bower, 1988; Bradford and Vannice, 1999). With a high CH4 concentration, the metal particles that have a higher tendency to promote the carbon deposition via CH4 decomposition are deactivated faster.
In spite of the higher initial CH4 conversion for Ni/Al2O3 with CH4/CO2 ratio of 1:2, it is seen that Ni/AlCe-20 has a superior performance on the catalytic stability in all reaction composition.
The results presented in Fig. 8 show that an increase in the CO2 adsorption promoted by the CeO2 loading is very important to improve the catalytic performance.
On the other hand, increases in the CO2 concentration promote a lower H2/CO ratio, mainly through the reverse water gas shift (RWGS) reaction (Eq 4), that leads to the H2 consumption and CO production.
H2 + CO2 CO + H2O (4)
The H2/CO ratios presented in Fig. 9 are in agreement with this, in spite of the higher CH4 conversion.
Experimental observations reported in the literature (Richardson and Paripatyadar, 1990) showed that the main contributor to carbon deposition is the CO disproportionation. A low CO2 concentration (CH4:CO2 ratio higher than unity) favors the Boudouard reaction (Eq. 3), in agreement with thermodynamic calculations (Reitmeier et al., 1948; Gadalla and Bower, 1988).
The profile of H2/CO ratio presented in the CH4/CO2 ratio of 2:1 (Fig. 9b) suggests that the carbon deposition is promoted mainly by the CH4 decomposition reaction (Eq. 2). It is observed a high H2/CO ratio (>1.0) in the initial reaction. These behaviors suggest a fast deactivation of the main active sites that promotes the CH4 decomposition. This signs that after 3h of time-on-stream only the active sites with lower deactivation affinity, or coke generation, are actives. It is known that CH4 decomposition is a structure sensitive reaction (Beebe et al., 1987), therefore this site deactivation promotes a decrease in the CH4 conversion. With decreasing of the CH4 decomposition, the H2 production drastically diminished, the same is not observed for the CO formation.
Fig. 9- Profiles of H2/CO performance for the catalysts on CH4 conversion with time-on-stream at different CH4/CO2 ratios
In a simple form, CO2 plays an important role in the coke elimination by reacting with the carbon (Eq. 5) deposited by CH4 decomposition (Eq. 2). If the equilibrium reaction presented in Eq. 5 is not fast enough to simultaneously eliminate the carbon generated by the CH4 decomposition reaction, as it is deposited, the coke is accumulated.
CO2 + C 2CO (5)
The reactions of Eq. 2 and Eq. 5 are endothermic and the equilibrium constants increase with the increase of the temperature, promoting the CH4 decomposition and the CO2 reaction with the deposited carbon (Eq. 5). This is in agreement with thermodynamic calculations (Reitmeier et al., 1948; Gadalla and Bower, 1988) that point to a lower carbon deposition with high reaction temperature and high CO2/CH4 ratio.
In addition, the CH4 decomposition reaction (without CO2) carried out for the Ni/Al2O3 and Ni/AlCe-20 samples, presented a CH4 conversion of 25 and 16% at 2min of reaction time, respectively, and at 7min the CH4 conversion was near 1%.
The activity and stability of Ni/Al2O3 catalysts are improved by CeO2 addition. The catalyst deactivation is promoted by carbon deposition, which is mainly due to the CH4 decomposition reaction.
The main CeO2 contribution is on the CO2 adsorption increase, which plays an important role on the coke elimination.
The authors acknowledge the financial support from the Brazilian agencies CNPq and FAPESP/CEPID.
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