ABSTRACT
Nanocatalysis is one of the most interesting elds in nanotechnology in which the contact between the catalyst and reactants is surprisingly increased. Nanocatalysts usually have high surface area and the ability to achieve the desired physical and chemical properties. Most of these catalysts are made of metal particles of a few nanometers in size. Metal nanoparticles (NPs) supported on inorganic matrices have shown promising features like higher catalytic activity and/or selectivity in mild conditions than conventional catalysts in many reactions.1-5
Applying the NPs in catalysts leads to unique catalytic properties of the obtained nanocatalysts. Large surface area of the NPs and considerable number of surface atoms lead to an increase in amount of active sites. In other words, the surface of NPs plays an important role in catalysis, being responsible for their selectivity and activity.1-4
Nanoparticles have a high surface-to-volume ratio, which makes them attractive to use compared to bulk catalytic materials. However, their surface atoms are also very active due to their high surface energy. As a result, the surface atoms become so active that their size and shape could change during the course of their catalytic function.1-5 The surface atoms are chemically more active compared to the bulk atoms because they usually have fewer adjacent coordinate atoms and unsaturated sites or more dangling bonds.6,7 The catalytic properties of NPs depend on size, size distribution, concentration, and electronic properties of them within the desired environment.8,9
Nanoparticles with high surface area supported on different support materials are currently used extensively as catalysts for chemical transformations.10 Nanocatalysts can lead
toward chemical reactions with maximum efciency and minimum consumption of material and energy. In order to improve the catalytic activity, several approaches have been taken, of which the search for new active phases, supports, and catalysts synthesis methods has been particularly noticeable. Catalyst performance can be determined by controlling variables such as size, structure, electron and spatial distribution, surface composition, and chemical and thermal stability.5,10
Various catalyst supports are available and applied in catalyst preparation. They can be divided into different groups such as metal oxides, zeolites, microporous and mesoporous materials, nanostructured carbon materials, and polymeric and ceramic compounds. The support has to be stable under the reaction conditions.4 The support plays a crucial role for the catalyst to reach the best performance. First, diffusion of reactants can be determined by the support.11,12 Second, the support may inuence the surface properties of NPs because of the NPs-support interactions.13,14 Third, the support can drive the hydrophilic character of catalyst. The performance of supported catalysts, including the redox catalysts such as supported-gold ones, signicantly depends on the physicochemical properties (surface area, pore diameter, and acidity) of their support. On the other hand, controlling textural properties (surface area, pore volume, pore size distribution), morphology, and particle size of support such as alumina and boehmite material is of the greatest interest to improve their potential in catalysis.15
