Periodically Ordered Mesoporous Co 3 O 4 /Heteropoly Acid Composite Frameworks for Catalytic Applications
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Periodically Ordered Mesoporous Co3O4/Heteropoly Acid Composite Frameworks for Catalytic Applications
Gerasimos S. Armatas*1, Ioannis Tamiolakis1 and Dimitris E. Petrakis2 1
Department of Materials Science and Technology, University of Crete, Heraklion 71003, Greece.
2
Department of Chemistry, University of Ioannina, Ioannina 45110, Greece.
ABSTRACT
Mesoporous cobalt oxide-tungstophosphoric acid composite frameworks have been synthesized by structure replication from cubic mesoporous KIT-6 silica. The products possess a regular structure with uniform wall thickness (~7 nm) and large internal BET surface area (~87-141 m2/g). The pore walls of these materials consist of nanocrystalline Co3O4 and 12-tungstophosphoric acid (HPW) components with different HPW content, i.e. 6, 15 and 36 wt. %. Total X-ray scattering analysis and UV/vis spectroscopy confirmed the Keggin structure of HPW into the mesoporous frameworks. Preliminary catalytic experiments found that Co3O4-HPW composites exhibited a remarkable activity in the direct decomposition of nitrous oxide into N2 and O2.
INTRODUCTION
The metal-oxygen compounds of the early transition metals (V, Nb, Ta, Mo, W) in their highest oxidation state, also known as polyoxometalates (POMs), are interesting supramolecular complexes with fascinating properties, especially those in redox and photo-catalysis [1]. The plethora of structural and compositional types in these clusters makes them suitable building blocks for the construction of novel materials with specific functionalities. For example, heteropoly acids of tungsten and molybdenum have been widely used in the catalytic oxidation of hydrocarbons (e.g. alkenes, alcohols) and decomposition of pollutant gases (e.g. de-NOx, CO) [2]. However, bulk POMs have a low surface area (less than 10 m2/g) and exhibit difficulties in separation from reaction media, which significantly restrict their application in many catalytic reactions. To overcome these limitations and, therefore, to improve the catalytic performance, the POM clusters are dispersed on high-surface-area inorganic supports [3]. Nanoporous POM-based materials are usually prepared by wet impregnation or post-synthesis grafting of soluble POM anions onto the surface of inorganic hosts such as porous silica, alumina, titania and zirconia [4]. The sol-gel inclusion of POMs in a mesoporous framework, such as silica, alumina, and recently tantalum oxide, seems to be a promising route for producing stable and high active catalysts [5].
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These materials feature a solid-solution structure with a large internal surface area and uniform mesopores. Herein, we report the synthesis of well-ordered mesoporous cobalt oxide-tungstophosphoric acid (H3PW12O40, hereafter HPW) composites through a hard-templating technique. The mesoporous replicas consisted of nanocrystalline Co3O4 and HPW clusters in different composition (i.e. 6-36 wt.% in HPW). Preliminary catalytic studies showed that the mesoporous Co3O4/HPWcomposites can decompose N2O into N2 and O2 with an exceptional act
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