Preparation of carbon-supported Pt catalysts covered with microporous silica layers using organosilanes: Sintering resistance and superior catalytic performance for cyclohexane dehydrogenation
Graphical abstract
Highlights
► Pt particles are covered with microporous silica layers using different organosilanes. ► Sintering resistance of Pt particles at high-temperature thermal treatment. ► Pt catalysts are used for cyclohexane dehydrogenation. ► Higher conversion of cyclohexane compared with Pt catalysts without silica coating.
Introduction
Highly dispersed precious metal particles on supports have been shown to be active for various catalytic applications such as automotive catalysts [1], the hydrogenation or dehydrogenation of organic compounds [2], [3] and electrocatalysts for fuel cells [4]. The efficiency and lifetime of these supported precious metal catalysts are influenced by the stability of the highly dispersed metal particles. However, it has been reported that supported precious metal catalysts are easily deactivated for reasons like the sintering of metal particles because of high reaction temperatures, the surrounding atmosphere [5], [6] and the dissolution of metal particles in the polymer electrolyte fuel cell electrodes under severe conditions [7]. The deactivation of supported metal catalysts has been recognized as a big problem in industrial catalysis. Thus, the development of supported metal catalysts with high resistance toward deactivation is required.
The silica coating technique is a promising method to improve the durability of metal particles toward deactivation. In previous work, we have developed supported metal catalysts covered with silica layers by the hydrolysis and condensation of tetraethoxysilane (TEOS) [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. We have demonstrated that the metal particles in these silica-coated metal catalysts have good resistance to sintering even at high temperatures because the metal nanoparticles were physically covered with silica layers. In addition, the metal particles can react with reactant molecules through the silica layers since the silica layers that are wrapped around the metal particles have a porous structure, therefore, the silica-coated metal catalysts can be applied to catalytic reaction. For example, the coverage of Co, Ni and Pt metal particles with silica layers prevents the sintering of their metal particles during hydrocarbon decomposition, which results in the preferential formation of carbon nanotubes or nanofibers with uniform diameters, while metal catalysts that are not covered with silica layers form carbon nanotubes or nanofibers with various diameters because these metal particles are severely aggregated during hydrocarbon decomposition [10], [11], [12], [13], [14]. In addition, we prepared the carbon nanotube-supported Pt or Pd nanoparticles covered with silica layers [15], [16], [17], [18], [19] and this coverage of Pt or Pd nanoparticles with silica layers prevents the dissolution of Pt or Pd particles from the supports, resulting in the excellent durability for the oxygen reduction reaction under severe cathode conditions in proton-exchange-membrane fuel cells [15], [18], [19]. Thus, the coverage of metal particles with silica layers is an effective method to enhance the stability of catalysts. Based on our previous studies, we expect that the development of metal catalysts covered with silica layers with larger pores, pore volumes or additional functionality would lead to an increase in their catalytic application. For example, organically functionalized silica structures have advantages for the porosity of silica structures, which allows for the easier adsorption and diffusion of reactants into the silica structure, and ultimately these advantages improve their catalytic activity [20], [21], [22], [23], [24], [25], [26]. Recently, only a few studies that focus on the porous silica structure in the preparation of silica-coated metal particles have been reported [24], [25], [26] and they showed the high sintering resistance of metal particles and their possible application to catalytic reactions associated with light hydrocarbons at high temperatures [25], [26]. We believe that the effective structural design of silica layers in silica-coated metal particles would lead to their wider application as larger reactant molecules may be used. Among the silica-coated metal catalysts, different from the metal catalysts enclosed within spherical silica particles (so-called core-shell structures), the silica-coated structure of the supported metal catalysts would be appropriate for the control of the thickness of silica layers, which are often related to catalytic performance. This silica-coated structure can possibly be applied to various supported metal catalysts.
Consequently, in this study, carbon black (CB)-supported Pt metal nanoparticles (Pt/CB) were covered with microporous silica layers using organosilanes. Methyl and phenyl groups were introduced to the silica layers that wrapped around the Pt metal particles by the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and methyltriethoxysilane (MTES) or phenyltriethoxysilane (PhTES). The Pt/CB that were covered with microporous silica layers were used as catalysts for the dehydrogenation of cyclohexane in a fixed bed flow reactor as it is a potential system for the storage and transport of hydrogen as an organic hydride [27], [28], [29]. This was done to demonstrate and compare their catalytic activity with that of Pt/CB prepared by conventional impregnation.
Section snippets
Preparation of CB-supported Pt nanoparticles covered with microporous silica layers
Commercially available CB (Vulcan XC-72 supplied by the Cabot Co.) was used as a support for the Pt particles. The coverage of Pt/CB with silica was performed by the successive hydrolysis of APTES and other organosilane [16], [17]. CB (1.00 g) was immersed in 220 ml of an aqueous solution containing H2PtCl6 (0.55 mmol). The pH of the solution was adjusted to ca. 11.8 by the addition of aqueous NH3 and the Pt metal precursors were then deposited onto the CB surfaces. After this solution was
Coverage of Pt/CB with microporous silica layers
Pt/CB catalysts that were covered with silica layers were prepared by the hydrolysis of organosilanes such as APTES and TEOS, MTES or PhTES followed by the reduction of the resulting products with hydrogen at 623 K. At first, we will begin by considering the sintering resistance of Pt particles in the silica-coated Pt catalysts. Table 1 lists the SiO2, Pt and carbon content of the Pt/CB as well as the silica-coated Pt catalysts upon reduction and thermal treatment at 973 K. The Pt loading was
Conclusion
Pt/CB were covered with microporous silica layers using the successive hydrolysis of APTES and other organosilanes (TEOS, MTES or PhTES). The microporous silica-coated Pt catalysts have remarkable thermal stability of the Pt metal without major sintering. Micropores were effectively formed in the silica layers upon thermal treatment. Higher cyclohexane diffusion was achieved by the formation of microporous silica layers that covered the Pt particles. As a result, the microporous silica-coated
Acknowledgments
This work was funded by a Grant-in-Aid for JST Research for Promoting Technological Seeds (2008) and a Grant-in-Aid for Young Scientists (B) (no. 22760599) for the Ministry of Education, Culture, Sports, Science and Technology of Japan to KN. The authors gratefully acknowledge Dr. M. Tagami and Mr. T. Ueki (Center for Technical Support, Institute of Technology and Science, The University of Tokushima) for their assistance with the TEM experiments.
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