CO-free hydrogen from steam-reforming of bioethanol over ZnO-supported cobalt catalysts: Effect of the metallic precursor

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Abstract

The ethanol steam-reforming reaction was studied over ZnO-supported cobalt catalysts (10 wt.% Co). Catalysts were prepared by impregnation of nitrate and carbonyl cobalt precursors. Characterization was accomplished by transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), and in situ techniques: magnetic measurements, and diffuse reflectance infrared spectroscopy (DRIFT) coupled to mass spectrometry. The use of Co2(CO)8 as precursor produced a catalyst that was highly stable and selective for the production of CO-free hydrogen at reaction temperature as low as 623 K. The only by-product was methane and selectivity of 73% to H2 and 25% to CO2 was obtained. Under reaction conditions, the catalyst showed 92% of reduced cobalt, mainly as small particles.

Introduction

Hydrogen is considered a potential source of clean energy, mainly through its use as fuel in fuel-cell systems. However, its production is still based on fossil-derived fuels [1], [2], [3]. If a global cycle of clean and sustainable production of energy is envisaged, new eco-friendly reservoirs of hydrogen are needed. In this context, bioethanol satisfies most of the requirements. It is easy to produce from biomass and it is safe to handle, store and transport. Moreover, if the release of CO2 to obtain hydrogen is taken into account, bioethanol can be considered globally neutral. Besides, for small stationary or mobile systems conventional distribution could be used. The transformation of ethanol via the steam-reforming reaction has recently been the focus of several studies [4], [5], [6], [7], [8], [9], [10], [11]. This is an endothermic reaction which produces 6 H2 mol per mol of reacted ethanol:C2H5OH+3H2O→6H2+2CO2

However, in the steam-reforming conditions other ethanol reactions can also take place, such as dehydration, dehydrogenation, decomposition, etc. These reactions give undesirable products such as ethylene, acetaldehyde or CO, which interfere in the process. The development of a catalyst which could operate at low temperatures and avoid the formation of by-products is a current goal of research in this area.

We have recently proposed the use of ZnO-supported cobalt catalysts in the steam-reforming of ethanol after screening a range of doped cobalt systems [9], [12]. Taking into account that the preparation method affects the structural characteristics of catalysts and their performance, here we report a study of the steam-reforming of ethanol over several ZnO-supported cobalt catalysts prepared by impregnation of various precursors. Aqueous solutions of ethanol 20% (v/v) were used for the reaction, in order to gain insight into the potential use of bioethanol without further distillation.

Section snippets

Preparation of catalysts

Three catalysts were prepared by impregnation of Co(NO3)2 (Panreac) or Co2(CO)8 (Strem Chemicals Inc.) on ZnO (100 m2/g). The support was prepared by decomposition of 3ZnO·2ZnCO3·3H2O under Ar at 573 K. Co(NO3)2 was impregnated by the incipient wetness method from an aqueous solution. The sample was then dried at 373 K for 6 h and calcined at 673 K for 6 h, resulting in the catalyst labeled as Co(N)/ZnOcalc. Reduction of Co(N)/ZnOcalc under H2 at 673 K gave the catalyst labeled as Co(N)/ZnOred. When Co

Catalyst characterization

Co(N)/ZnOcalc and Co(N)/ZnOred were characterized before and after the catalytic tests. The characterization by different techniques of Co(N)/ZnOcalc before reaction revealed the sole presence of Co3O4 besides ZnO. Fig. 1a shows its XRD pattern, where only diffraction peaks corresponding to Co3O4 (calculated mean particle size ca. 5 nm) and ZnO appear. Several particles with planes corresponding to the spacing of Co3O4(1 1 1) are visible in the high-resolution image of this catalyst shown in Fig.

Conclusions

The performance of Co/ZnO catalysts for the steam-reforming of ethanol depended on the cobalt-precursor used. Co(CO)/ZnO prepared from Co2(CO)8 showed high performance at 623 K. In a wide range of reaction conditions, only small amounts of CH4 were produced as by-products and no CO was detected suggesting that this catalyst is a good candidate for use in fuel-cell systems, in which the production of CO is undesirable. A long-term catalytic test (75 h) at 623 K showed its stability in terms of

Acknowledgements

We thank CICYT (MAT1999-0477 and MAT2002-01739) and Generalitat de Catalunya (2001SGR 00052) for financial support. J. Ll. is grateful to MCYT for a Ramón y Cajal Research Program grant.

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