Nanoporous Materials: Science and Engineering
In the last decade, we have witnessed a rapid growth in research and development of nanotechnology, especially nanostructured materials. Nanoporous materials as an important class of nanostructured materials possess high specific surface area, large pore volume, uniform pore size, and rich surface chemistry. These materials present great promises and opportunities for a new generation of functional materials with improved and tailorable properties for applications in adsorption, membranes, sensors, energy storage, catalysis and photocatalysis, and biotechnology, etc.
Interest in making materials from nanoscale building blocks arose from discoveries that by controlling the size in the range of 1-100 nm and the assembly of such constituents, one could alter and prescribe the properties of the assembled nanostructures. Nanoscale phenomena and objects have been around for some time. Catalysts, for example, are mostly nanoscale particles, and catalysis is a nanoscale phenomenon. What is new and different now is the degree of understanding and deliberate control and precision that the new nanoscale techniques afford. Instead of discovering new materials by random search (trial-and-error), we can now design them systematically. Nanoporous materials can have long-range structural order or disordered structure and contain pores of the dimension of a few nanometers to tens of nanometers. Some applications such as catalysis take advantage of high surface area and pore confinement effects. Synthesis and processing of nanoporous materials with controllable structures and properties require new approaches such as molecular templating and intercalation in a bottom-up manner.
From a practical standpoint, a large specific surface for nanoparticles is most desired for catalysis. However, fine powder catalysts can cause serious operational problems such as agglomeration, difficulties in loading, pressure drop, and separation of catalyst from the reaction products. A feasible approach to generating a large and accessible surface area of catalyst but avoiding the morphology of fine powder is to create a composite or immobilized structure. One can disperse nanoparticles of metals or oxides in an inorganic support to stabilize the discrete nanoparticles, meanwhile maintaining most of their surface accessible to reactant molecules. However, the conventional methods of preparing the catalysts such as impregnation often result in agglomerated catalyst particles in the support, thus decreasing the active surface area, and uniformity of the active centers. With nanostructuring techniques, active metal or oxide precursors can be incorporated or grafted on the nanoporous support during synthesis thus not only increase the control in catalyst particle size, surface area and dispersion, but also eliminating the cost and problems associated with impregnation.
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