Promotional effect of La2O3 and CeO2 on Ni/γ-Al2O3 catalysts for CO2 reforming of CH4

https://doi.org/10.1016/j.apcata.2010.06.050Get rights and content

Abstract

The CO2 reforming reaction of CH4 was conducted by using a Ni-based catalyst prepared by the co-impregnation method. The properties of the catalysts and the carbon deposition performance on the catalyst surface were surveyed by XRD, TPR, TG-DTA, and SEM. The results showed that the activity of the Ni/γ-Al2O3 catalyst with Ni loading of 10% was higher than that of those with other Ni loadings. However, the Ni/γ-Al2O3 catalyst was not stable in the continuous reaction due to a large amount of carbon deposition on the catalyst surface. Although the activity of Ni/γ-Al2O3 with the La2O3 and CeO2 as promoters was not obviously increased, the carbon deposition on Ni/γ-Al2O3 with La2O3 and CeO2 as promoters was greatly suppressed in the CO2 reforming reaction of CH4. The amount of carbon deposition on the 10%Ni/3%CeO2–3%La2O3–γ-Al2O3 catalyst was reduced 76.2% compared with the 10%Ni/γ-Al2O3 catalyst, due to alkaline function and dispersion enhancement of La2O3–CeO2, as well as the electronic interactions between CeO2 and Ni. The results of SEM showed that the moss-like carbon on the 10%Ni/γ-Al2O3 catalyst was not easily gasified with CO2 and Ni particles, which would result in the catalyst deactivation. On the other hand, the filamentous carbon on the 10%Ni/3%CeO2–3%La2O3–γ-Al2O3 catalysts could be eliminated readily by gasifying with CO2. Because of the suppression in the sintering of Ni particles and the dominant formation of the reactive filamentous carbon on the Ni-based catalyst that included La2O3 and CeO2 promoters, 10%Ni/3%CeO2–3%La2O3–γ-Al2O3 catalyst showed stable activity in the continuous CO2 reforming reaction of CH4 at 1073 K and atmospheric pressure.

Research highlights

▶ Carbon deposition on Ni/γ-Al2O3 with La2O3–CeO2 as promoter was greatly suppressed. ▶ Filamentous carbon was formed on the 10%Ni/3%CeO2–3%La2O3–γ-Al2O3 catalyst. ▶ Filamentous carbon could be eliminated readily by gasifying with CO2. ▶ 10%Ni/3%CeO2–3%La2O3–γ-Al2O3 catalyst showed stable activity in reaction.

Introduction

In recent years, the catalytic process of CO2 reforming of CH4 (CDRM) (also called dry reforming) into synthesis gas (CH4 + CO2  2H2 + 2CO) has become an attractive but challenging subject for the chemical utilization of carbon-containing “greenhouse” gases (CH4 and CO2) [1]. This process generates synthesis gas (CO + H2) with a H2/CO ratio of 1:1 which is suitable for some industrial processes such as DME (dimethyl ether) synthesis, oxo synthesis and Fischer–Tropsch synthesis on Fe-based catalysts [2], [3]. Extensive efforts have thus been made to develop novel and practical catalysts for this process [4], [5].

Various catalysts, including Ni-based catalysts and supported noble metal catalysts, have been investigated for the CO2 reforming of CH4 [6], [7], [8], [9], [10]. The cheap Ni-based catalysts have high activity and selectivity. However, thermodynamic studies indicated that the Ni-containing catalysts are prone to cause carbon deposition by the side reactions in the CDRM, leading to the deactivation of the catalyst [11]. The side reactions to cause carbon deposition include the methane decomposition (1) and the CO disproportionation (2) [9]:CH4  C + 2H22CO  C + CO2

Compared to Ni-based catalysts, the noble metal-based catalysts are less sensitive to carbon deposition [9]. However, their limited availability and expensive prices make them less favored for practical applications [12], [13]. Therefore the development of carbon deposition resistant Ni-based catalysts for CO2 reforming of CH4 is important.

In recent years, a number of studies on CO2 reforming of CH4 over supported Ni catalysts [14] have indicated that support may play an important role in the activity and resistance of carbon deposition on the catalysts. The modified supports have been studied in order to improve the stability and the carbon resistance of nickel catalysts, mainly by the addition of alkali and alkaline-earth metal oxides as well as rare earth oxides [15], [16], [17], [18]. The rare earth oxides have a high oxygen storage capacity and can absorb or release oxygen reversibly in responding to the oxygen concentration in the gas-phase [19]. Their presence shows beneficial effects on the catalyst performance, such as improving the dispersion of the active species and delaying the transition of alumina support from γ-Al2O3 to low-surface-area phase α-Al2O3 [20], [21]. It has been demonstrated that the addition of ceria as a promoter in the Ni/Al2O3 catalyst can enhance the activity, stability and resistance of carbon deposition in CH4 reforming with CO2 [22], [23]. Lanthanum oxide has been previously used in catalysts for CDRM, mainly as the sole support component [24], [25]. Studies conducted by Zhang et al. [26] with Ni/Al2O3 and Ni/La2O3 catalysts in this reaction at 1023 K show that Ni/Al2O3 was active but deactivated soon, while Ni/La2O3 was not active but was stable.

Although the importance of support and promoter effect to improve the performance of the nickel based catalysts has been recognized, there have been very few investigations on the CO2 reforming of CH4 over Ni-based catalysts with lanthanum/cerium oxide binary promoters, as well as a lack of study on the effect of binary promoters on the stability and the carbon deposition behaviors of the catalysts.

In the present work, a series of supported Ni/γ-Al2O3 catalysts and La2O3–CeO2 binary promoted Ni-based catalysts were prepared with the impregnation method for investigating their performances and reactivity in the continuous reaction CO2 reforming of CH4. In particular, the carbon deposition and the stability of different catalysts during the reaction were evaluated.

Section snippets

Catalyst preparation

Ni/γ-Al2O3 catalyst was prepared by incipient wetness impregnation of γ-Al2O3 (Catalysis Society of Japan, SN2 = 151.2 m2/g, total pore volume 0.53 ml/g, average pore diameter 14.08 nm) with the aqueous solution of nickel nitrate. The impregnated solid was subsequently dried at 393 K for 12 h and calcined in air at 773 K for 2 h.

The La2O3 and/or CeO2 promoted Ni-based catalysts were prepared by incipient wetness impregnation of the γ-Al2O3 with the aqueous solution of Ni(NO3)2·6H2O and La(NO3)3·6H2O or

Influence of Ni loading in Ni/γ-Al2O3 catalysts on the reaction

Table 1 shows the results of the CO2 reforming of CH4 at 973 K and atmospheric pressure for 8 h by using the Ni/γ-Al2O3 catalysts with different Ni loading, which were calcined at 773 K for 2 h, and reduced in situ by 99.999% H2 at 773 K for 1 h. Fig. 2 shows the influence of Ni loading on the carbon deposition on Ni/Al2O3 catalysts.

Table 1 shows that the CH4 conversion increased, and the CO2 conversion did not obviously change, and the total carbon conversion increased with Ni loading increasing

Conclusions

The CO2 reforming reaction of CH4 was investigated by using different Ni-based catalysts prepared by co-impregnation method. The properties of the catalysts and the carbon deposition performance on the catalyst surface were surveyed by XRD, TPR, TG-DTA, and SEM. The results showed that the activity of the Ni/γ-Al2O3 catalyst with Ni loading of 10% was higher than those with other Ni loading amounts. However, the Ni/γ-Al2O3 catalyst was not stable in the continuous reaction of the CO2 reforming

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