Short communicationHydrogen production from glycerol steam reforming over molybdena–alumina catalysts
Graphical abstract
Introduction
Biomass is an important source of energy and provides raw materials for the chemical industry. The advantage for the energy obtained from biomass, compared with fossil fuels which are the traditional sources of energy, consists in reduction of greenhouse gas emissions [1].
Lately, a special importance was given to the use of compounds obtained from renewable biomass sources for hydrogen production. The glycerol valorification is of particular interest since it comes from renewable resources and is a suitable bio-renewable substrate for hydrogen production being preferred over the fossil fuels [2]. The hydrogen is an energy carrier, a renewable and clean fuel, and a raw material for ammonia production [3] and Fischer–Tropsch synthesis [4].
The usual methods for hydrogen production are based on catalytic reforming of hydrocarbons. In the past years, the hydrogen obtained by glycerol reforming has been widely studied. The convenience of using glycerol for H2 production comparatively with ethanol, another source of hydrogen, consists in the fact that glycerol contains more number of moles of H in its chemical structure. The glycerol steam reforming was deeply investigated over supported transition metals, such as Ni, Co, Pt, and Pd.
Ni/Al2O3 was proved to be a good catalyst for this reaction but presents some drawbacks: is susceptible to deactivation by carbon deposition; the hydrogen production is strongly affected by reaction temperature, and increases with temperature increasing [5], [6]. The catalytic activity was improved by the addition of Co [7] and Sn as a promoter and CeO2–MgO–Al2O3 as support [8]. Likewise, CeO2 has been used as a promoter as well as a support for Ni-based catalysts as a result of its distinctive oxygen storage capacity, which determines the presence of highly active oxygen, making the catalyst more active for this reaction [9]. Recently, nanomaterial oxides with particular morphology were synthesized and used as catalysts for glycerol steam reforming, as for example noble metals containing materials, Ir/La2O2CO3 [10]. Pt-based catalysts supported on Al2O3, SiO2, MgO, TiO2 [11], Pt/C, and Pt-Re/C [12] were also studied in this reaction and were found to be more effective. Nevertheless, such catalysts are not common in industrial applications because of their cost and limited availability.
Molybdena supported on different substrates such as alumina, silica, titania, and zirconia is used as a catalyst for selective oxidation reactions [13], [14], [15], olefin metathesis [16], [17], and dehydrogenation [18]. The higher reductibility of molybdenum makes this a good catalyst for water–gas shift reaction [19]; MoOx could be reduced by CO (with CO2 generating) and re-oxidized by H2O forming H2. Furthermore, the use of alumina at support provides a high dispersion of molybdena and has an advantage in the prevention of Mo oxo-species aggregation. From the above reasons, in the present work molybdenum was selected as an active phase to be included in catalyst formulations for glycerol steam reforming.
Herein, we report the results obtained for the glycerol steam reforming over molybdena nanomaterials supported on alumina. No references were found in the literature for this reaction with molybdena catalysts. The effects of operating conditions including temperature, steam to glycerol molar ratio, flow rate of liquids and carrier gas on glycerol steam reforming were tested. The material characterization was performed by XRD, N2 adsorption, UV–VIS–NIR spectroscopy, DRIFT spectroscopy, SEM and TEM microscopy.
Section snippets
Catalyst preparation
The catalysts used for glycerol steam reforming, MoO3/Al2O3, were prepared by the sol–gel method and gel combustion. Aluminum nitrate (Al(NO3)3·9H2O from Tunic) was dissolved in water (10 wt.%) and the solution was stirred at 60 °C for 30 min [20]. Citric acid (C6H8O7·H2O, from Silal, 99.5% purity) was added in the solution, with a molar ratio citrate-to-nitrate of 0.5. Then aqueous (NH4)6Mo7O24·4H2O (Fluka Analytical) solutions were added, so as to get a percentage of 2, 5, and 12% MoO3 on
Catalyst characterization
Crystalline phases in the catalysts were characterized by XRD. The diffractograms are presented in SI 1. The diffraction lines corresponding to alumina support were detected for all samples studied. No diffraction lines corresponding to molybdenum were detected, showing that the molybdena species are amorphous and well dispersed on the support surface. From the diffraction patterns, the amorphous character of the powders is evidenced for 5 and respectively 12MoAl, when for 2MoAl appears a
Conclusions
Molybdena catalysts supported on alumina obtained by the sol–gel method are tested in the glycerol steam reforming in the 400–500 °C temperature range, steam to glycerol ratio 9–20 and feed flow rate 0.04–0.08 ml/min. Hydrogen is formed as the major product followed by CO and CO2 while the selectivity from CH4 is very low, below 5%. The turnover frequency depends on a specific surface area, while the space time yield of hydrogen is directly proportional with the Mo surface density. Increasing of
Acknowledgments
This work was supported by a grant of the Romanian National Authority for Scientific Research, CNDI–UEFISCDI, project number PCCA-II-56/2014.
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