TiO2-supported catalysts for the steam reforming of ethanol
Graphical abstract
Introduction
Ethanol steam reforming (ESR) is a promising reaction to produce H2 from a renewable and non-toxic source [1], [2], [3], [4], [5]. It is an endothermal reaction, usually taking place around 700–800 °C.
A detailed description of reaction mechanism may be found elsewhere [6]. Along with the desired steam reforming and water gas shift (WGS) reactions, some competitive pathways may occur, depending on temperature and steam/ethanol ratio. Those particularly challenging are ethanol dehydration to ethylene, precursor of coke deposition due to its polymerisation over acidic sites, the disproportion of CO through the Boudouard reaction and the decomposition of CH4 (formed by decomposition of ethanol) at high temperature [7], [8]. Therefore, in the whole temperature range some reactions leading to catalyst coking may be active. At high temperature, carbon gasification by steam may keep under control the phenomenon, particularly at high steam/ethanol ratio. Below 600 °C coke may accumulate due to the coexistence of different coke forming reactions and slow gasification [9]. Nevertheless, some catalyst formulations are active at relatively low temperature, raising the interest in operating around 500 °C to limit the heat input to the reactor and to improve H2 yield by favouring the exothermal WGS reaction. Under such operation conditions catalyst resistance to coking is very critical. Therefore, in this work we focused our attention to operation at 500 °C, trying to understand the limits for applicability in such conditions and the catalyst requirements to exploit high activity [10]. Additional testing at higher temperature also allowed to further explore the reactivity of the proposed catalysts from a more applicative point of view.
Among non-noble metals, Ni demonstrated a very promising active phase due to its ability to activate the CC bond and Co was suggested to improve the WGS activity and to operate at lower temperature [1], [11], [12], [13]. Unfortunately, both Ni and Co may easily form carbon deposits in the form of filaments, especially when poorly dispersed in form of big particles [14], [15], [16], [17].
In our previous investigations we focused on some Ni-, Co- and Cu-based catalysts evidencing the effect of catalyst formulation and of the preparation method on its performance [12], [13], [18]. We found that in the case of Ni the metal–support interaction plays a key role in keeping the active phase dispersed, thus avoiding extensive coking [12]. The strength of the metal–support interaction also showed important for Pt-based catalysts [19]. However, also the redox properties of the catalytic system may influence activity [13].
In the present work we compared Ni, Co and Cu as active phases, supported over TiO2. This support, which was scarcely investigated for this application, may exhibit different acidic and redox properties with respect to other supports, such as silica or zirconia. Furthermore, it may be prepared in different crystal habits, which in principle may differently host the selected active phases and its ability to develop strong metal–support interaction with some metals is well known [20], [21], [22]. The pivotal effects of the nature of the active phase and of its interaction with the support have been taken into account here to interpret the catalyst behaviour.
Section snippets
Catalyst preparation
A summary of the preparation conditions and of the main physical chemical properties of each sample is reported in Table 1, Table 2.
Results and discussion
According to our previous investigations, Ni usually showed very active, selective and stable when tested at temperature higher than 600 °C. On the contrary, it proved sufficiently resistant to coking when operating at 500 °C provided that very small particle size was achieved during preparation and kept stable by establishing a sufficiently strong interaction with the support [10], [12], [13], [18], [27]. TiO2 was reported as a support able to develop a strong metal–support interaction (SMSI)
Conclusions
TiO2 was explored as possible support for Ni-, Co- and Cu-based catalysts for the steam reforming of ethanol, due to the documented possibility to establish a strong metal–support interaction with some active phases. Because of possible accumulation of carbon deposits around 500 °C, due to both support acidity and the tendency of some active phases to form filaments, the catalytic activity was explored at 500 °C, focusing not only on ethanol conversion and H2 productivity, but mostly on C balance
Acknowledgements
The authors are indebted with Regione Lombardia and the Consortium for Material Science and Technology (INSTM) for financial support. The valuable help of the PhD student Cesare Biffi and of the MoS graduating student Mauro Castellana is gratefully acknowledged. The authors are indebted to Prof. Giuseppe Cruciani for XRD analyses and their elaboration. The characterisation was partly supported by H2FC European Infrastructure Project (Integrating European Infrastructure to support science and
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