Catalysts as nanomaterials
Although the report rightly includes catalysts as an example of existing nanomaterials, there is no discussion of future directions for catalysis as a branch of nanotechnology.

The active sites in homogeneous catalysis are often discrete metal-ligand complexes. Although their design is largely based on traditional organometallic preparative chemistry, there is an increasing demand for the active sites to be immobilised on a support material. A major challenge lies in nanoengineering the individual active sites, so that they are tethered to the support, while still retaining their catalytic properties.

In heterogeneous catalysis, where metals are dispersed on high surface area support materials, exposed metal atoms often provide the active sites. Standard preparative routes generate nanoparticles of metal, but not within a narrow size distribution range. Here, the challenges are

(a) to develop preparative methods that allow complete control over the size, morphology and distribution of metallic nanoparticles;

(b) to scale up these new methods into large scale catalyst manufacturing processes.
Controlled formation of specific nanoparticles will allow more effective utilisation of the metal, not just because the number of exposed sites can be increased, but because some catalytic reactions are structure sensitive or occur at the interface between the metal and the support.

Nanoengineering of catalyst surfaces is also likely to provide step-change improvements in reactant specificity and product selectivity. By altering the physical pathways to and from the active sites, the access of reactants can be controlled, while the formation of undesired products can be suppressed. In practice, this means either designing a ‘hierarchical pore structure’ within which the active sites are located, or by applying ultra-thin selective membranes over the active sites.

Apart from conventional homogeneous catalysis (eg fine chemicals production) and heterogeneous catalysis (eg bulk chemical manufacture, emission control), advances in nanomaterial design will also impact on the newer catalytic fields – such as electrocatalysis (including fuel cells) and biocatalysis.

Nanomaterials for microengines
In the UK, the EPSRC is sponsoring studies into the miniaturisation of combustion engines, as potential alternatives to batteries. Most of the required components are functionally very conventional, but their construction will probably force the development of new materials and nanofabrication techniques.

Catalysis is also likely to play a role. It is unlikely that the fuel/air mixture fed to the microengines will be spark ignited or compression ignited, but will probably be catalytically combusted.

Health, safety and environment
As an indirect outcome of nanotechnology developments, the health and environmental effects of all nanoparticles (including those formed as unwanted byproducts) will be more widely appreciated. For example, current vehicle-emission legislation is based on the mass of emitted material, and therefore preferentially regulates the emission of larger particulate (ie within the PM 10 range). Improvements in the detection of airborne nanoparticles, and an understanding of their effects, could prompt emission legislation that targets ultrafine particulate (< 100 nm).

Education
The RS/RAE report captures many aspects of a vibrant field that is attracting widespread interest, extending beyond the scientific and engineering communities. This should be exploited, particularly in education (not included in the terms of reference for the report), where new technologies can revitalise traditional subjects and improve their uptake.


Stan Golunski at Johnson Matthey