Nanostructured Catalytic Materials: Design and Synthesis
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Natural and manmade catalytic materials are among the oldest “nanostructured materials” known long before the era of nanoscience and nanotechnology. The classic heterogeneous catalysts consist of active nanoscale metal component(s) and solid carriers, namely, “inert” oxide supports such as alumina, silicates, or magnesium oxide, to increase the reaction surface area and metal utilization. Conventional processing techniques for fabrication of supported catalysts contain one or more of the following steps: impregnation, precipitation, coating, rewashing, ion-exchange, pulverization, dying, and calcination, etc. Nanoscale metal clusters or particles can be thus formed on the oxide carriers after these processes. It is now well known that the particle size, local composition, and structure (shape) of nanoscale catalysts determine the ultimate catalytic activity and selectivity. For example, it has been demonstrated that the activity of TiO2-supported gold particles is very sensitive to their size (2 to 3 nm) in the CO oxidation reaction with oxygen at ambient conditions. It is also well known that the structure and composition of nanocatalysts may change under the reaction conditions, and thus their performance could be time-dependent. Nonetheless, prevailing catalyst preparation still remains largely as a technological art rather than a science, although surface science has significantly deepened our general understanding of heterogeneous catalysis using single-crystal model catalysts.
Over the past 15 to 20 years, we have witnessed exciting advances in the design and synthesis of low-dimensional nanostructured materials, ranging from fullerenes, carbon nanotubes, supramolecular assemblies, mesoporous structures, to various organic–inorganic hybrid materials. Taking advantage of the rapid development in nanoscience and nanotechnology, a wide range of synthetic techniques are now in place. For example, nanostructured materials can be prepared with constrained or unconstrained synthetic methods, in which inorganic or organic templates (e.g., porous oxides, organic ligands, well-oriented crystal planes, and supramolecule-directing agents) are commonly employed. With these newer approaches and knowledge, a huge variety of nanostructured catalytic materials have been designed and synthesized, which will be the main review topic of the present article. With the emphasis on catalytic prospect, the objective of this article thus aims at introducing various design and synthesis strategies for this new class of materials. Future challenges and research directions in this area will also be addressed.