Nanocatalysts are catalysts made up of nanoparticles or some other nanostructure such as a nanofoam. A catalyst is a material which increases the rate of a chemical reaction, without itself being altered or changed by the reaction. Nanostructures are engineered structures with features at the nanoscale — between one and 100 nanometers. An obvious advantage of nanocatalysts over conventional materials is that the surface area is much greater, increasing the area over which reactions can be catalyzed. The use of nanostructures may also enable control of surface strain and the arrangement of surface atoms.
Catalysts are extremely important within industry, where they are used to accelerate the overwhelming majority of chemical processes. There are two fundamental types of catalysis: Homogeneous catalysis involves the catalyst being dispersed through reactants that are in the same phase (generally gas or liquid), while heterogeneous catalysis involves different phases.
Homogeneous catalysis usually involves solid particles of the catalyst suspended in reactants that are either gas or liquid. In this case the speed of the reaction is directly related to the surface area of the catalyst in contact with the reactants. Nanoparticles created by relatively simple bulk processes which successively break down particles can therefore be effective homogeneous catalysts.
The effect of surface area is less straightforward for heterogeneous catalysts as reactions only occur at active sites such as crystal faces, which may be a small percentage of the total exposed surface. More precisely controlled nanostructures may therefore be beneficial, with precisely controlled arrangements of surface atoms designed to increase the number of reaction sites.
The Terrace Step Kink model (TSK) classifies surface atoms according to at atoms position within a crystal structure and the way it is bonded to its neighbors. Atoms which are less coordinated such as kinks and defects tend to be more reactive. Increasing these sites often improve catalyst performance although the ideal surface configuration is highly reaction specific.
Electrocatalysts are an important class of catalyst that form the active materials of electrodes in fuel cells and batteries. Nanocatalysts are being widely studied and applied as electrocatalysts. For example, typical Proton Exchange Membrane (PEM) fuel cells use a polymer membrane charged in platinum nanoparticles. Carbon nanotubes and graphene have many applications as electrocatalysts due the to combination of extremely high surface area and electrical conductivity.
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