COx free hydrogen production over cobalt incorporated silicate structured mesoporous catalysts
Graphical abstract
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Highlights
► Synthesis solution pH had significant effects on the structure of the final product. ► KOH greatly enhanced the incorporation of cobalt into the mesoporous lattice of amorphous silica. ► Catalysts prepared with KOH promoter gave the highest activity with 70% conversion at 600 °C. ► Ammonia conversion was increased by increasing the space velocity over the same catalyst. ► The texture and porosity of SiO2 is associated to the availability of Co species that affect activity.
Introduction
Hydrogen is considered to be one of the most important clean energy sources and it can be utilized by means of fuel cells. Due to the limitations in storage and delivery of hydrogen, its on-site generation has attracted the attention of researchers [1]. For the on-board production of hydrogen, reforming of alcohols (methanol or ethanol) was proposed in some of the recent studies [2]. However, formation of COx byproducts, especially CO, resulted in a decrease in cell performance even at extremely small concentrations [3], [4], [5]. More recently, ammonia (NH3) has been considered as an important alternative feedstock to produce COx free hydrogen [1]. It has high energy density (3000 Wh/kg) and higher hydrogen storage capacity (17.7 wt%) in comparison to methanol. Consequently it is considered to be one of the best alternates for hydrogen storage in fuel cell derived transportation vehicles [6]. Moreover, it is clean, its storage or transportation is not a problem since it is in liquid form at room temperature under the pressure of 10 bar [7]. In addition, the only side product is nitrogen in the case of its decomposition [5], [8].
Different metals, such as Pt [9], Ru [4], [8], [10], [11], [12], [13], [14], [15], Rh [9], Fe [5], [7], [16], [17], [18], [19], [20], Pd [9], W [21], Ni [1], [4], [10], [22], [23], Ir [10] have been tested in ammonia decomposition reaction. Utilization of different supports such as Al2O3 [1], [10], [23], ordinary silica SiO2 [10], carbon [7], [24], carbon nanotubes [11], zeolite [13] was also mentioned for the synthesis of metal impregnated catalysts for ammonia decomposition reaction. Carbon based supports, such as carbon nanotubes, have been frequently used due to their good electron conductivity properties. However, undesired methanation reaction may take place at high temperature in the presence of hydrogen [4], [8].
MCM-41 and SBA-15 like mesoporous silicate structured materials have high surface area with well-ordered pore structures and high thermal stability. Due to their larger pore sizes than zeolites, these mesoporous materials also create much less diffusional resistances in catalytic reactions. These properties make these materials attractive catalyst supports. There are very few studies published in the literature for ammonia synthesis over such mesoporous catalytic materials. In study of Li et al. [4] it was indicated that Ni and Ru supported mesoporous silicate materials, especially MCM-41, were very active in ammonia decomposition reaction. In another study, the feasibility of silica-coated Fe nanoparticles compared to the naked Fe nanoparticles was investigated and the iron nanoparticles encapsulated by microporous and mesoporous silica shells were found to be highly stable and active [5].
In the literature, alkali or alkaline earth metal ions were reported as efficient promoters for supported Ru and Fe catalysts for ammonia decomposition reaction [3], [4], [8], [17]. They were found to be good at preventing sintering of Ru or Fe [4] and in some cases their usage resulted in the modification on the basicity of the support, which was in turn resulted in an enhancement of the catalytic activity of the catalyst [3]. Li et al. [4] indicated that the effect of potassium as a promoter highly dependent on the metal as well as the support used in the synthesis of the catalyst.
Cobalt-silicate catalysts were studied by different researchers [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40] for different reactions and in these studies catalysts were mostly prepared by impregnation procedures. These catalysts were generally utilized as Fischer–Tropsch catalysts. Their application to ammonia decomposition reaction is very rare. Lendzion-Bielun et al. [41] investigated catalytic decomposition of ammonia over cobalt oxide catalysts and they reported that Co was more effective than the iron catalyst while in ammonia synthesis reaction cobalt supported catalyst seemed to be less active than the iron catalysts.
The main objectives of the present study are the synthesis of Co incorporated silicate structured mesoporous materials, by following a one-pot hydrothermal procedure and testing of catalytic performances of these materials in COx free hydrogen production by ammonia decomposition. These catalysts were synthesized with different Co/Si ratios in the synthesis solution using different silicate sources. Different reduction temperatures and the addition of KOH as a promoter were also tested. Activity comparison of the synthesized materials was made at different temperatures and space velocities in a fixed bed flow reactor.
Section snippets
Experimental
Cobalt based mesoporous silicate catalysts were prepared by using a one-pot hydrothermal synthesis procedure. The procedure used in this work is a modified version of the procedure used in our recent work for the synthesis of heteropolyacid incorporated mesoporous materials [42]. According to this procedure, cethyltrimethyl ammonium bromide (Merck) was used as the surfactant and 87 ml of deionized water was used to solve 13.2 g of surfactant. After a clear solution was obtained, a silicate source
Catalyst characterization results
In order to get a porous structure, the surfactant should be removed from the synthesized material by a calcination process. The calcination temperature was determined using thermal analysis (TGA/DTA) of the uncalcined material. TGA/DTA analysis result of an uncalcined cobalt based mesoporous silicate catalyst is presented in Fig. 1. The weight loss observed at a temperature interval of 25–115 °C was due to the removal of physisorbed water molecules. Two other weight loss ranges were observed at
Conclusions
Cobalt based mesoporous silicate catalysts were prepared by using one-pot hydrothermal synthesis procedure. Catalysts prepared by using sodium silicate had a well ordered mesoporous structure with a higher surface area and narrower pore size distribution compared to the catalysts prepared by using TEOS. These results proved the importance of synthesis solution pH on the structure of the final product. Incorporation of cobalt was better for the former case; this could be explained with the basic
Acknowledgment
This study was financially supported by TUBITAK through Project No. 109M560 Project and Gazi University Research Fund through Project No.18-2010/07. They are gratefully acknowledged.
References (54)
- et al.
Applied Catalysis A
(2002) - et al.
Journal of Molecular Catalysis A: Chemical
(2010) - et al.
Journal of Catalysis
(2004) - et al.
Journal of Catalysis
(2005) - et al.
Catalysis Communications
(2010) - et al.
Journal of Power Sources
(1994) - et al.
Catalysis Communications
(2006) - et al.
Journal of Molecular Catalysis A: Chemical
(1997) - et al.
Catalysis Today
(2004) - et al.
Journal of Catalysis
(1997)
Journal of Molecular Catalysis A: Chemical
Journal of Catalysis
Applied Catalysis A-General
Catalysis Today
Applied Catalysis A
Applied Catalysis A
Applied Catalysis A
Catalysis Today
Applied Catalysis A
Applied Catalysis A
Journal of Catalysis
Studies in Surface Science and Catalysis
Studies in Surface Science and Catalysis
Applied Catalysis A
Journal of Catalysis
Studies in Surface Science and Catalysis
Catalysis Today
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