Ni/ZrO2 catalysts in ethanol steam reforming: Inhibition of coke formation by CaO-doping
Graphical abstract
Introduction
One of the main challenges for scientists today is to reduce the dependence from fossil fuels. Hydrogen can be the solution to meet the ever growing world energy demand in a clean and sustainable way. It is the ideal candidate for both energy and transport sectors because its combustion does not emit environmental pollutants in the atmosphere. Unfortunately hydrogen is still produced from fossil fuels, so increasing the concerns about global warming. Clean hydrogen can be produced from renewable sources and ethanol emerged as a good candidate for hydrogen production because it has high hydrogen content, it is renewable, easy to store, handle and transport because of its low volatility and non toxicity.
Ethanol steam reforming (ESR) is promising to produce hydrogen in a sustainable way [1], [2], [3]. It is an endothermic process which takes place according to the following stoichiometric reaction, Eq. (1):including the water-gas shift of the intermediate CO (Eq. (2)), which further increases the hydrogen yield:
Nevertheless the overall process is a complex network of reactions, such as ethanol dehydrogenation, dehydration or decomposition, which can lead to the formation of several byproducts (acetaldehyde, ethylene and methane respectively) [4], [5]. The formation of coke should also be considered, through the Boudouard reaction (2CO ⇌ CO2 + C), the decomposition of methane (CH4 ⇌ C + 2H2) or the polymerization of ethylene [6]. Coke deposition can be controlled by properly tuning the operating conditions (i.e. steam-to-ethanol ratio, reaction temperature) [7], but the formulation of the catalyst plays a key role as well [6]. Coking is particularly significant around 500 °C, where coke forms but is not effectively gasified by steam. Nevertheless, it would be very advantageous being able to operate around this temperature, because some catalyst formulations demonstrated sufficient activity and selectivity, and this relatively low temperature would limit the energy input to the process with respect to common reaction conditions (T > 650 °C). Therefore, improving catalyst resistance to coking represents a milestone for the development of low temperature ESR.
Nickel is a highly active and selective active phase for ESR, comparable to noble metals, because of its high capability to break CC bonds and also to promote the water-gas shift reactions, thus increasing hydrogen yield [8], [9]. It is cheaper and more available than noble metals, although it may be quickly deactivated by coking and sintering [10]. In particular, coking is thought to occur more promptly over large Ni particles and aggregates [11], [12], [13], [14] than over very dispersed crystallites.
With regard to the metal catalyst support, it should possess a good chemical and mechanical resistance and a high surface area, in order to favour the dispersion of the active phase [15], [16]. Our recent results highlighted, on one hand, the importance of the stability of the support in the reaction conditions, on the other the key role of the metal–support interaction in determining both activity and stability of the catalyst [17], [18], [19]. In particular, a strong interaction stabilizes the active phase, preserving it from sintering and, thus, from coking phenomena.
Another important property that should be controlled is the acidity of the support. In fact it is well known that the side reactions leading to the formation of coke occur mainly on the acid sites of the support [20], [21], [22]. This means that the support plays a key role in determining the reaction pathway and, as a consequence, the selectivity of the process [23], [24].
Ni/ZrO2 proved to be highly active in steam reforming reactions [17], [18], [19]. Its performance was ascribed to some of its features, such as: (i) high surface area; (ii) high stability under the reaction conditions; (iii) strong interactions with the active phase; (vi) ability to first adsorb and then dissociate water, thus enhancing the adsorption of steam on its surface and activating the gasification of hydrocarbons [23] and the water-gas shift [25]. Nevertheless, in some conditions, a slight deactivation due to coke deposition was detected [17], probably related to the presence of acid sites on the surface of the support. Zirconia is known to be a solid acid: both acidic OH groups and Lewis acid sites (coordinatively unsaturated, cus, Zr4+ ions) can be detected on the surface, depending on the synthesis conditions [26], [27], [28].
The addition of oxides of alkaline earth metals (i.e. CaO, MgO, BaO), which are strong Lewis bases, can decrease the acidity of the support [29], [30], [31], thus inhibiting the side reactions responsible for coke deposition. It should be considered that doping can modify also other properties, of both the support (oxygen transport, redox properties) [32], [33] and the catalyst (dispersion of the active phase, interactions between metal and support) [34], [35].
The aim of this work was to properly modify the zirconia support in order to improve the catalytic performance of the catalyst in the steam reforming of ethanol, in particular with regard to resistance to coking. Ni/ZrO2 catalysts doped with various amounts of CaO were synthesized and characterized. The effect of CaO-doping on both the physico-chemical properties of the materials and the catalytic performance of the samples was evaluated.
Section snippets
Catalysts preparation
Zr(OH)4 was prepared by a precipitation method [36] at a constant pH of 10. ZrOCl2*8H2O (Sigma–Aldrich, purity ≥99.5%) was dissolved in distilled water and added with a peristaltic pump under vigorous stirring to an ammonia (33%, Riedel-de Haën) solution. During the precipitation, the pH value was kept constant at 10.0 ± 0.1 by the continuous addition of a 33% ammonia solution. After the complete addition of the salt solution, the hydroxide suspension was aged for 20 h at 90 °C, then filtered and
Catalysts characterization
The catalysts were characterized by means of N2 physisorption, in order to evaluate the effect of CaO-doping on the textural properties of the support. According to IUPAC classification [41], the non-modified sample ZNi exhibits a IV-type isotherm containing a H3-type hysteresis, typical of materials that do not possess a well-defined mesoporous structure, with a surface area of approximately 140 m2/g (see Table 1). The isotherms of the doped samples present the same features, thus indicating
Conclusions
Ni/ZrO2 proved to be highly active in ethanol steam reforming. Nevertheless, the presence of Lewis acid sites on the surface of the support, due to coordinatively unsaturated Zr4+ ions, was probably related to the coke formation which slightly deactivated the catalyst. The addition of CaO to the support was effective in reducing the Lewis acidity of zirconia and improving catalyst resistance towards coking. Moreover, CaO addition was responsible for the formation of oxygen vacancies, that can
Acknowledgements
The authors are indebted to Agnieszka Iwanska for the excellent technical assistance. The valuable help of Giacomo Mariani, Alessio Sozzi and Matteo Compagnoni in collecting activity data and for the characterization of spent catalysts is gratefully acknowledged. The work was partly supported by H2FC European Infrastructure Project (Integrating European Infrastructure to support science and development of Hydrogen and Fuel Cell Technologies towards European Strategy for Sustainable Competitive
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