Surface properties of a coke-free Sn doped nickel catalyst for the CO2 reforming of methane

https://doi.org/10.1016/j.apsusc.2004.03.223Get rights and content

Abstract

Small amounts Sn doped Ni catalysts exhibited excellent coke resistance ability with minor loss of reforming activity in CH4 reforming with CO2. Temperature-programmed reduction with H2 (H2-TPR) showed that the addition of Sn increased the reducibility of Ni. X-ray photoelectron spectroscopy (XPS) analysis of Ni/α-Al2O3 (I), Sn0.02Ni/α-Al2O3 (II), indicated that the binding energy of the Ni 2p level was decreased from 855.7 (I) to 855.2 eV (II) by the addition of Sn. Field emission scanning electron microscopy (FE-SEM) and field emission transmission electron microscopy (FE-TEM) analysis of the fresh and used catalysts (I, II) revealed that the added Sn increased the dispersion of Ni and retarded sintering of Ni. XPS and EDX also disclosed that Sn was aggregated on the surface of Ni, and the coverage of Sn on the surface of Ni depressed the reforming activity.

Introduction

CH4 reforming with CO2 is of great industrial interest because of the lower H2/CO ratio in the product gas, which is suitable for the synthesis of oxygenated derivatives:CH4+CO2→2CO+2H2(ΔG=−150.56kJ/mol,1100K)It was found that the product gas is close to the thermodynamic equilibrium, and nickel was identified as the preferred catalyst [1], [2]. Thermodynamic calculations indicate that the formation of carbon is also favorable in the same reaction conditions [3]:CH4C+2H2(ΔG=−127.66kJ/mol,1100K)During the past 10 years, many researchers attracted their attentions to this reaction because of abundant amount of methane and CO2, both of them are known as greenhouse gases and their utilizations are environmental friendly. Noble metals such as Rh, Ru, and Ir exhibit high stability and less sensitive to coke deposition [4], [5], [6], [7], [8], [9], [10], but high cost and restricted availability limited the use of noble metals as a catalyst. Ni is another well-known catalyst for this reaction and popular reported for its low price [4], [5], [6], [7], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Ni catalyst possesses high activity for this reforming reaction, while coke deposition, which deactivates Ni catalyst and blocks the reactor, is the main problem. A lot of promoters have been reported for the supported Ni catalyst, in order to depress coke deposition [17], [21], [22], [23], [24], [25].

Coke deposition has been retarded in some extent with the above promoters, but they lowered the reforming activity of the catalyst at the same time. Recently, the important effects of tin in the PtSn catalyst [26] and SnNi catalyst [27], [28] on the stability of Pt and Ni have been clarified in CO2 reforming process. In this paper, Ni were doped with different amounts of Sn and used in methane reforming with CO2. The reducibility and characteristics of Sn doped sample were examined by means of H2-temperature-programmed reduction (H2-TPR), field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscopy (FE-TEM) and X-ray photon electron spectrum (XPS).

Section snippets

Catalyst preparation and test

Catalysts used in this research were prepared by co-impregnation method. Ni(NO3)2·6H2O and SnCl2·2H2O (Wako Pure Chemicals, Japan) were dissolved in corresponding amount of ethanol, then α-Al2O3 was added into the solution under stirring. The slurry was dried at 120 °C for overnight, and finally calcined at 800 °C for 3 h. The loading amount of Ni is 10 wt.% of the support and the amount of Sn were defined as the atomic ratio between Sn and Ni.

Experiments were carried out in a continuous flow

The activity of Sn doped Ni catalysts

Fig. 1 shows the activity of Sn doped Ni/α-Al2O3 catalysts for methane reforming with CO2. It was found that coke formation rate on Sn doped Ni catalysts decreased continuously with the added amount of Sn, and coke deposition was completely suppressed when the amount of Sn was higher enough (Sn/Ni>0.02). On the other hand, the reforming activities of Sn doped Ni/α-Al2O3 catalysts decreased gradually and attained to constant level.

H2-TPR analysis

Fig. 2 shows the H2-TPR profiles of Sn doped Ni/α-Al2O3

Discussions

It was confirmed that the reforming reaction requires the dissociative adsorption of CH4 to form a carbonaceous intermediate, while coke formation (at least on the surface of catalyst) also originates from this carbonaceous intermediate [38]. In this case, controlled transform of the carbonaceous intermediate is very important in order to diminish the coke formation. As the coke formation would require a bigger ensemble sites and mainly on the surface of Ni, researchers on the process of steam

Conclusions

A coke-free CH4 dry reforming process was attained on small amount of Sn doped Ni catalysts. H2-TPR, FE-SEM, FE-TEM analysis indicated that Sn improved the dispersion of Ni and retarded the sintering of active Ni particles during the reaction. Surface enrichment of Sn made a large portion of the Ni surface covered by these promoters and hindered the access of CH4 and/or CO2 on the surface of Ni particles.

Acknowledgements

This work was supported by New Energy and Industrial Technology Development Organization (NEDO, Japan).

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