Ni/SiO2 and Ni/ZrO2 catalysts for the steam reforming of ethanol

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Abstract

SiO2 and ZrO2 supported Ni catalysts were prepared for use in the steam reforming of ethanol. The catalytic performances, in terms of both H2 productivity and stability towards coking and sintering, were related to the physico-chemical properties of the catalysts.

The samples were prepared either by synthesis of the support by precipitation and subsequent impregnation with the active phase, or by direct synthesis through flame pyrolysis. The latter has been chosen because it leads to nanostructured oxides, often quenched in very disperse or metastable form, characterised by high thermal resistance, important for this high temperature application.

Many techniques have been used to assess the physico-chemical properties of the catalysts. The samples showed different textural, structural and morphological properties, as well as different reducibility and thermal resistance, depending on the preparation method and support. Therefore, besides evaluating the effect of catalyst formulation and preparation method on the catalytic performance, the influence of all such properties has been considered. The fundamental parameter governing the final catalyst properties was metal–support interaction. In particular, the stronger the latter parameter, the higher was metal dispersion, leading to small and stable Ni clusters. This influenced both activity and the resistance towards coking. Surface acidity was also taken into account considering the effect of the different nature of acid sites (silanols or Lewis a.s.) of both support and metal phase on catalyst deactivation. The best results were obtained with a 10 wt% Ni/SiO2 sample, prepared by FP, when tested at 625 °C. H2 productivity of 1.44 mol H2/min kgcat was reached, corresponding to ca. 80% of the maximum value achievable under the selected conditions. This result was accompanied by the lowest CO/CO2 ratio and 100% carbon balance without by-products in the outflowing gas.

Graphical abstract

Conversion, products distribution and C balance during the steam reforming of bioethanol depended on both surface acidity and on the stability of the Ni clusters. The latter was in turn dependent on the metal–support interaction and tuneable during catalyst synthesis.

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Highlights

► Set up and comparison of different synthesis methods for reforming catalysts. ► Comparison of activity and stability under different reaction conditions. ► Correlation of activity and stability with surface acidity and metal dispersion. ► 80% of equilibrium conversion reached at 625 °C with stable performance.

Introduction

The steam reforming of biofuels, such as ethanol, represents a hot research topic of the last few years. Different metals have been proposed as active phase, e.g. Ni, Co and Cu, to consider just the less expensive non-noble metals, whereas the most used support is alumina, in case doped with alkali or lanthana to limit its acidity [1], [2], [3], [4], [5]. The most interesting results have been obtained with Co and Ni [6], [7], [8]. The latter seems very promising, though some drawbacks remain unsolved due to sintering and coking [6], [9], [10]. Indeed, very dispersed Ni particles tend to agglomerate during high temperature operations and in the presence of water vapour [11], [12], [13], [14]. The loss of exposed active phase influences, activity, selectivity and coke formation, due to the easier formation of carbon filaments over big Ni particles [15], [16], [17]. The possibility to operate at low temperature may be advantageous from this point of view, in order to limit Ni sintering. In addition, lower heat input would be required to sustain this endothermal reaction (the reaction is feasible above ca. 300 °C [18]). Nevertheless, thermodynamic investigations on coke formation routes indicate that coke accumulation may be more severe at 500 °C than at higher temperature [8], [19]. The thermal resistance of the catalyst, as well as Ni interactions with the support, are then essential in determining the catalytic performance.

Of course activity and stability of the catalyst also depend on the nature of the support. The latter should activate both ethanol and water, it may ensure a suitable dispersion of the active phase, possibly stabilising it during the high temperature operation, but it is also responsible of coking if uncontrolled surface acidity is present. Indeed, strong acidity may lead to ethanol dehydration to ethylene, which oligomerises and polymerises. The dehydration activity is competitive with the dehydrogenation/decomposition route, which leads to acetate/glycolate surface intermediates, readily decomposed into products (CO/CO2/H2) or reformable intermediates such as methane or acetaldehyde.

The aim of the work was the design and the characterisation of heterogeneous catalysts to be used for the steam reforming of ethanol. A series of Ni-based catalysts was prepared by using different synthetic procedures. The active phase was supported on SiO2 (mesoporous SBA-15 or amorphous dense nanoparticles) and ZrO2, chosen due to their different acidity and redox properties with respect to the most commonly used alumina. The samples were prepared by (i) synthesis by precipitation of the support, impregnation with the active phase and calcination at 800 °C to impart proper thermal resistance and (ii) by flame pyrolysis (FP), a special technique able to impart high temperature stability and to tune metal dispersion. Indeed, the FP technique allows the continuous and one-step synthesis of oxides, single or mixed, usually showing good phase purity, along with nanometer-size particles and hence very high surface area (up to 250 m2/g). The latter parameter could help in improving low temperature performance in the present case. In addition, the high temperature of the flame in principle should also ensure thermal stability, provided that a solvent with sufficiently high combustion enthalpy is chosen [20], [21].

The catalysts were characterised by different techniques, namely N2 adsorption–desorption, temperature programmed reduction and oxidation (TPR–TPO), X-ray diffraction (XRD), atomic absorption (AA), X-ray photoelectron spectroscopy (XPS) infrared spectroscopy (FT-IR) and scanning or transmission electron microscopy (SEM–TEM). Activity testing data were then collected for the steam reforming of ethanol at different reaction temperatures.

Section snippets

Support synthesis

SBA-15 was synthesised as previously reported [22], in the presence of Pluronic 123 (P123, Aldrich) as structure directing agent. Silicon hydroxide was calcined at 800 °C for 6 h.

ZrO2 was prepared by a conventional precipitation method [23] at a constant pH of 10.

Addition of the active phase

The active phase was added to each support by incipient wetness impregnation with an aqueous solution of the metallic precursor (Ni(NO3)2·6H2O, Sigma–Aldrich, purity ≥98.5%), in the proper concentration in order to obtain the desired Ni

Textural, structural and morphological characterisation

The textural properties of the samples prepared and the actual concentration of Ni are reported in Table 1.

The FP prepared samples were characterised by different surface areas depending on the support. According to [20], [21], this is tightly related to the decomposition mechanism of the oxide precursor in the flame and to the type of solvent used. Sample NiSiL exhibited the highest surface area and retained its mesoporous structure in spite of the high calcination temperature. By contrast,

Conclusions

Silica and zirconia supported catalysts were prepared by different methods, inducing variable specific surface area, metal dispersion and metal/support interaction. All the samples were tested under different conditions for the steam reforming of ethanol. At 625 and 750 °C good catalytic performance was achieved by every sample. The best results were obtained with NiSiF, prepared by FP, when tested at 625 °C, leading to the highest H2 productivity, to the lowest CO/CO2 ratio and to 100% carbon

Acknowledgements

The authors are indebted with Regione Lombardia and the Consortium for Material Science and Technology (INSTM) for financial support.

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