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.
probst@qmc.ufsc.br
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. CeO_2. Carbon Formation. Syngas.

I. INTRODUCTION

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 (>800^oC) 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 100^oC and calcined at 450^oC 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 100^oC and calcined at 650^oC during 3h.

B. Characterization

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 10^oC/min of heating rate and monitoring the hydrogen consumption from a 5%H₂/N₂ mixture at the 50-930^oC temperature range. The surface area and porosity measurements following the nitrogen adsorption method (-196^oC), the metallic accessibility using H₂ chemisorption at 27^oC and the CO₂ chemisorption at 27 and 625^oC 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 650^oC 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 625^oC 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 (>900^oC). However, in this work the samples have higher then 5 wt% of CeO₂ and were heattreated at 650^oC, 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 -196^oC; # - 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 600^oC and a second one at 770^oC. The peak at 600^oC is likely due to the more accessible Ni and the peak at 770^oC 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 600^oC to a lower temperature value, pointing to the changes in the catalysts properties promoted by CeO₂. The H₂ consumption around 900^oC 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 270^oC and 400^oC 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 650^oC/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 650^oC/1h, 625^oC 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 625^oC 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/C_i) 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 650^oC/1h, 625^oC 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 625^oC, the reaction temperature, and at 27^oC. 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 650^oC/1h, 625^oC 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%.

IV. CONCLUSIONS

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.

ACKNOWLEDGEMENT
The authors acknowledge the financial support from the Brazilian agencies CNPq and FAPESP/CEPID.

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