International Congress on Nanoscience & Nanotechnology (ICNN2012) 8-10 September 2012, Kashan, I. R. Iran
A Survey on Recent Developments in Catalysis Using Nanostructured Materials Soudabeh Rahmani1, Mehran Rezaei1, 2 1
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran 2 Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran E-mail:
[email protected]
Abstract This review describes that nanomaterials have received more attention by virtue of their excellent properties suited for applications in various fields such as electronic, pharmaceutical, biomedical, cosmetic, energy, and catalysis. Nanomaterial-based catalysts are usually heterogeneous catalysts. The extremely small size of the particles maximizes surface area exposed to the reactant, allowing more reactions to occur. However, thermal stability of these nanomaterials is limited by their critical sizes; the smaller the crystallite size, the lower thermal stability. The majority of industrial catalysts contain an active component in the form of nanoparticles smaller than 20nm in size that are dispersed onto high-surface-area supports. The importance of nanoparticles and nanostructure to the performance of catalysts has stimulated wide efforts to develop methods for their synthesis and characterization. Nanoparticles offer higher catalytic efficiency per gram than larger size materials due to their large surface-to-volume ratio. This makes them an attractive choice to use as catalysts. Indeed, catalysts are undoubtedly “the most successful current application of nanotechnology”. In the first part of the paper, application of catalysis using nanostructured material in some reactions including alkylation, dehydrogenation, hydrogenation, Steam reforming of methane (SRM), carbon dioxide reforming (DRM), epoxidation of alkenes and oxidative coupling of methane to ethylene (OCM) are investigated. In the second part of the paper, properties of some chemical elements such as gold, silver, platinum, palladium, nickel and rhodium as nanocatalyst are studied. Our investigation shows that the metal nanoparticles can act as best catalyst for industrial application as these have large surface to volume ratio and have unique quantum size effect. Moreover, their homogeneous size distribution with the mean particle size is optimal for catalytic properties and also they increase conversion reactions. Finally, it is shown that despite some disadvantages in using nanostructured materials in catalysts such as high synthesis precise of them and also restriction in industrial usages, they have several prominent advantages which convince everyone to use nanomaterials as catalysts. In addition, this area of study is still interesting for researcher all over the world to extend application of nanostructured catalysis to all reactions which needs catalysts and also reduce synthesis price and solve other problems of using these kinds of catalysts in industrial applications. Keywords: Catalysis, Nanomaterial, Nanocatalyst
1
International Congress on Nanoscience & Nanotechnology (ICNN2012) 8-10 September 2012, Kashan, I. R. Iran Introduction: The science and technology of catalysis have played a critical role in improving our standard of living over the last century [1, 2] and an increasingly important role in the production of industrial chemicals and derivatives, and currently well in excess of 90 % of all chemical production [3, 4]. Materials produced by catalytic technology are responsible for modern medicines, new fibers for clothing and construction, a wide variety of consumer products, cleaner-burning fuels, and environmental protection [1, 4-6]. The science and technology of catalysis is more important today than at any other time in our history. Since extraction of the earth‟s fossil fuel resources will reach peak production in the coming century [1, 5, 7-8], the cost of energy and chemicals from hydrocarbon resources will continue to increase in the near future [1, 9]. Many industrially relevant catalysts are heterogeneous, or solid-phase, which allows for very large scale continuous operation at elevated temperatures and pressures [1, 10-11]. Reduction of the size parameters of a substance to nanometer scale causes appearance of unique properties which can be used in practice for development of novel materials and technologies. This manifestation has not only reflected in physical properties like melting point, optical absorption, electrical and magnetic properties but also in the reactivity of the substance. This has direct consequence in generation of new functional nanomaterials especially in catalysis and adsorbents [12]. Nanoscience today gives a large contribution to the fundamental understanding and also for designing and fabricating catalytic systems with optimization of the performance. It is a misconception that the prefix „Nano‟ implies the dimensionality of the materials; it means the new state of matter which has completely altered behavior as compared to bulk or molecular state materials [12, 13]. Nanomaterials have received more attention by virtue of their excellent properties suited for applications in various fields such as electronic, pharmaceutical, biomedical, cosmetic, energy, and catalysis. Physical, chemical and biological properties of materials in nano-scale differ in fundamental from those of individual atoms and molecules or bulk materials. Nanomaterial-based catalysts are usually heterogeneous catalysts. The extremely small size of the particles maximizes surface area exposed to the reactants, allowing more reactions to occur. However, thermal stability of these nanomaterials is limited by their critical sizes; the smaller the crystallite size, the lower thermal stability [13]. The majority of industrial catalysts contain an active component in the form of nanoparticles 900oC) and high energy input for both the reaction and product-separation processes. A potentially promising reaction for direct natural gas activation is the oxidative coupling of methane (OCM) to ethylene: 2CH4+O2→C2H4+2H2O. The reaction is exothermic (LH=-67 kcal\mole) and to date has only 1
Dry reforming of methane
7
International Congress on Nanoscience & Nanotechnology (ICNN2012) 8-10 September 2012, Kashan, I. R. Iran been shown to occur at very high temperatures (>700oC). In the reaction, methane (CH4) is activated on the catalyst surface, forming methyl radicals which then couple to ethane (C 2H6), followed by dehydrogenation to ethylene (C2H4). One catalyst with a specific activity for C: H4 bond activation at lower temperatures (
