Glycerol steam reforming for hydrogen production: Design of Ni supported catalysts
Graphical abstract
Highlights
► Ni catalysts have been tested in glycerol steam reforming for H2 production. ► A strong effect of the support on the catalytic activity has been detected. ► TiO2 cannot stabilize metallic Ni species on the surface. ► SBA-15 collapses in the reaction conditions because of its low hydrothermal stability. ► ZrO2 exhibits the best performance and stabilizes Ni nanoparticles to a high degree.
Introduction
The last century witnessed the rise of the petroleum-based chemistry and the exploitation of fossil resources for the production of energy and chemicals. Nevertheless the diminishing availability of these resources, together with the environmental issues related to greenhouse gas emissions, renders the birth of a new chemical industry essential.
Hydrogen is considered as the future energy vector [1], because it is clean and carbon-free and it can be used directly by either thermal combustion or converted into electrical energy by means of fuel cells [2]. Currently hydrogen is produced from fossil fuels, so the amount of carbon dioxide formed during its production is the same as that formed by direct combustion of these fuels [3]. To reduce effectively the greenhouse effect and the global warming, hydrogen should be produced from renewable resources. In this context glycerol has emerged as a promising source of hydrogen, because it has a high hydrogen content and it is safe and non toxic [4]; moreover glycerol is the main by-product (approximately 10 wt%) in biodiesel production from transesterification of vegetable oils extracted from biomass [5], so its employ would be highly desirable for several reasons. First of all, the expected increase in biodiesel production will cause a glut of waste glycerol, whose disposal will rise even further the price of biodiesel itself; it is then essential to find useful applications for this by-product. Besides that, glycerol is a cheap and renewable source of hydrogen, so its employ for hydrogen production would be advantageous from both the economical and environmental point of view.
The steam reforming (SR) of oxygenated compounds is usually affected by the formation of several by-products, thus reducing the selectivity to hydrogen and leading to the formation of coke. The design of a highly selective catalyst is then fundamental.
Many investigations have been published on glycerol reforming, both in the aqueous [6], [7], [8] and in the gas phase [9], [10], [11], and several metal-supported catalysts have been tested. The most studied metals are Ir [10], Ru [9], Pt [12] and Pd [8]. Nickel is also both highly active and selective in the steam reforming reactions, because of its high capability to break C–C bonds and also to promote the water–gas shift reactions, thus increasing hydrogen production [13], [14]. We then decided to use nickel in the preparation of our catalysts, because it is cheaper and more available than noble metals.
As for the support, it plays a key role in the steam reforming reactions, affecting in particular the selectivity of the catalyst [15], [16]. The ideal support should possess a good chemical and mechanical resistance and a high surface area, in order to favour the dispersion of the active phase [17], [18].
Therefore, the aim of the present work is to investigate the effect of the nature of the support on the catalytic performance of nickel in terms of both activity and selectivity in the steam reforming of glycerol.
Section snippets
Supports synthesis
TiO2 was prepared by a conventional precipitation method. 20 g of TiOSO4·xH2SO4·xH2O (Aldrich, synthesis grade) were dissolved in 300 mL of distilled water at room temperature, then NaOH (Carlo Erba, 9 M) was added dropwise until the system reached a pH of 5.5. The precipitate was aged at 60 °C for 20 h, then repeatedly washed with distilled water and finally dried overnight at 110 °C.
SBA-15 was synthesized as previously reported [19], in the presence of Pluronic 123 (P123, Aldrich) as structure
Results and discussion
Before discussing the experimental results, it can be useful to see in detail how hydrogen production through glycerol steam reforming takes place.Eq. (4) represents the overall steam reforming reaction, an endothermic transformation favoured at low pressure which is due to the contribution of two reactions, namely glycerol decomposition (Eq. (5)) and water gas shift (WGS, Eq. (6)). The reaction pathway is quite complex and many other reactions can
Conclusions
In this work we demonstrated the suitability of Ni-based catalysts to be used in glycerol steam reforming, due to Ni activity in breaking C–C bonds and also in promoting the water–gas shift reaction. In particular, the present work highlights the importance of the nature of the support, because it has been demonstrated to play a key role in designing the catalytic performance. According to our results, the support affects the stability of the active phase above all: as a consequence, the
Acknowledgements
The financial support of Regione Lombardia (project “M4H2 – Materiali innovativi per la produzione di H2 da fonti rinnovabili”), Regione Lombardia – INSTM (RU of Venice) and CNR Milano; Italian MIUR (Project “ItalNanoNet”) is gratefully acknowledged.
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