A study of the influence of the synthesis conditions upon the catalytic properties of Co/SiO2 or Co/Al2O3 catalysts used for ethanol steam reforming

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Abstract

Oxides of cobalt supported on various supports such as SiO2 and Al2O3 have been prepared by using incipient wetness technique (IMP), the sol–gel (SG) route and the combination of the two methods (ISG). The solids formed, after calcination of the corresponding precursors, were characterised by X-ray diffraction, BET area measurements, metal dispersion, and by X-ray photoelectron spectroscopy. Using metal loading of 8%, a high catalytic activity was demonstrated for ethanol steam reforming. Products distribution of the reaction has been found to be dependent on the nature of the support and of the preparation method of the catalysts.

Introduction

The push for more efficient and clean vehicles fuel and the increasingly stringent environmental regulations have spurred intensive research toward catalytic hydrogen production for fuel cell uses. However, despite great strides made in recent years in order to improve and understand methanol [1], [2], [3], [4], [5], ethanol [6], [7], [8], [9], [10] and 2-propanol [11] steam reforming catalysis, a commercial catalyst has not been developed yet. There are many challenges in the development of a commercial catalyst. A major one is to find a catalyst that will promote hydrogen formation with high selectivity and having only CO2 as side product. An added difficulty is the requirement that the catalyst has to have long-term stability under reaction conditions.

Supported catalysts are frequently used for selective production of hydrogen but the influence of the support on the hydrogen selectivity has not been thoroughly investigated yet. In recent studies the reaction was studied on Rh based catalysts such as Rh/CeO2, Rh/ZrO2, Rh/CeO2–ZrO2[12], [13] and Rh/Al2O3[11], [13], [14]. At 673 K, Rh/Al2O3 catalyst has been reported to be more active and selective than Ru, Pd and Pt supported on Al2O3[13], [14] and, due to its ability to form less amounts of carbon at the surface, had the longest catalytic life [11]. Addition of a little amount of oxygen in the feed gas was found to decrease both the CO selectivity and catalyst deactivation but it also promotes metal sintering [14]. A potential approach for decreasing the CO selectivity in the coupling of the well known properties of Rh catalysts for WGSR with those of the effect of the CeO2 modified ZrO2 support has been proposed [13]. Other studies report CO free hydrogen from ethanol steam reforming over ZnO-supported cobalt catalysts [10].

In a previous study, the vapour phase ethanol steam reforming over Co supported on various supports such as ZrO2, MgO, SiO2, and Al2O3 was reported [15]. The latter system was prepared also by using several cobalt precursors [16] and it was observed that, while the activity for ethanol conversion was independent of the starting materials used to obtain cobalt, the selectivity for steam reforming was closely related to the cobalt crystallite size.

In the present study, we report the preparation of cobalt supported silica and alumina using different methods (impregnation, sol–gel and combination of both), their characterisation and their catalytic properties in the ethanol steam reforming for hydrogen production.

Section snippets

Impregnation (IMP)

The catalysts have been prepared by the incipient wetness method using SiO2 (Roth 0201; 100 mesh) or Al2O3 (Merck 1077; 70–230 mesh) and an aqueous solution of Co(NO3)3 · 6H2O (Analyticals-Carlo Erba 99%), respectively. The impregnated solid has been dried at 100 °C for 60 min, and then at 150 °C overnight under air. Finally, the catalyst has been calcined in air at 550 °C for 6 h.

Sol–gel method

The catalyst has been prepared (see Fig. 1) in one hand by dissolving cobalt carbonate (Strem Chemical 99%),

Characterisation

The BET surface data of the different solids are given in Table 1. Among all the prepared catalysts, 8% Co/SiO2 (SG) has shown the highest BET surface area. These results emphasise the initial preparation step (sol) to determine the distribution of the active phase on the support matrix. The N2-sorption capacities expressed in m2/g agree with those previously reported in the literature [18], [19]. On the contrary, the impregnation of commercial SiO2 (380 m2 g−1) using the IMP or the ISG

Catalytic activity

A variety of cobalt supported catalysts with different properties have been tested under several reaction conditions. To investigate the dependence of hydrogen selectivity in products, activity measurements have been carried out on the catalysts at temperatures ranging from 400 to 480 °C. Having noticed that the catalysts deactivated when catalytic tests were performed first at lower temperatures and then at higher temperatures, measurements have been carried out first at 480 °C. As for Co/Al2O3

Conclusions

Different preparation methods have been examined to develop highly active Co/SiO2 and Co/Al2O3 catalysts for ethanol steam reforming of ethanol into hydrogen.

The hydrogen selectivity and products distribution were found to be dependent on the preparation procedure and on the nature of the support used. A maximum hydrogen selectivity of ca. 62% and 67% was obtained with Co/SiO2 ISG and Co/Al2O3 IMP, respectively. With sol–gel silica based catalysts methane formation was drastically decreased

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