Exxon Research and Engineering
Fabricating nanostructured materials and characterizing the effect of nanostructure on the properties of such materials are areas of increasing importance in understanding, creating, and improving materials for applications that require high surface areas. High surface areas can be attained (1) by creating materials such as small particles or clusters where the surface-to-volume ratio of each particle is high, or (2) by creating materials where the void (pores) surface area is high compared to the amount of bulk support material. Materials such as highly dispersed supported metal catalysts and gas phase clusters tend to fall into the first category, whereas microporous (nanometer-pored) materials such as zeolites, high surface area inorganic oxides, porous carbons, and amorphous silicas fall into the second category.
Industrial areas where high surface area materials are critical include the following (Aksay, p. 79; Friedlander, p. 83; Schwartz, p. 93; Ying, p. 96; Brotzman, p. 122; Chianelli, p. 133; Stucky, p. 140; Dresselhaus, p. 169):1
It is difficult to predict the areas where nanoscale high surface area materials may have the greatest future impact, but some signs point to possibilities of substantial advancement in the areas of adsorption/separations, particularly in gas storage (Schwartz, p. 93; Dresselhaus, p. 169; Baker, p. 172) and separations, and also in novel chemical catalysis using nanoscale catalyst particles (Brus, p. 89; Chianelli, p. 133; Stucky, p. 140; Goddard, p. 197).
One typical objective of nanoscale catalyst research is to produce a material with exceedingly high selectivity at high yield in the reaction product or product slate, that is, chemicals by design, with the option of altering the product by changing the surface functionality or composition at the nanoscale. For instance, new catalysts with increasing specificity are now being fabricated in which only one or two spatial dimensions are of nanometer size (Chianelli, p. 133; Stuckey, p. 140). A second objective is to discover nanoscale materials or structures with exceedingly high storage capacity per unit volume and weight for gases such as H2 or CH4, which would then be more economic for use either as a combustion fuel or as the means to power fuel cells for ultralow-emission vehicles or for electric power generation (Schwartz, p. 93; Dresselhaus, p. 169; Baker, p. 172).
A third objective is to fabricate molecular sieving membranes using inorganic crystalline materials such as zeolites (Aksay, p. 79; Stuckey, p. 140). For molecular sieving membranes, one critical challenge rests on discovering ways to create large-scale, thin, nearly defect-free membranes.
The choice of approach is strongly dependent on the particular application or process of interest, whether it be highly specific catalytic chemistry, sorption for storage or trapping of gaseous species, or nanoscale control of one or more dimensions. Many different approaches are now being investigated for synthesizing nanoscale particles (Whitesides, p. 76; Aksay, p. 79; Friedlander, p. 83; Schwartz, p. 93; Ying, p. 96; Froes, p. 105; Brotzman, p. 122; Chianelli, p. 133). Some approaches start by attempting to tailor the structural composition and size by building up materials atom-by-atom; some use controlled crystal growth; some start from bulk materials and use procedures such as cryomilling to produce nanoscale particles that can then be examined.
Synthesis techniques include the following, many of which are described or alluded to in this proceedings report: aerosol reactor synthesis of metal oxides; metal vaporization and condensation reactors; micelle template synthesis of crystalline structures such as zeolites; arc vaporization of graphite for producing fullerenes, carbon nanotubes, and carbon nanoparticles; "graphite"-coated magnetic materials; catalyzed hydrocarbon decomposition to produce highly porous high surface area carbon such as activated aerosols and graphite fibers; and self-assembly of nanoscale structures.
Before generation and utilization of high surface area nanoscale materials can become commonplace, at least two major challenges must be faced. One challenge is to control the critical dimensions of the nanoscale structure over long times and varying conditions. In nanoscale catalyst materials the critical chemical selectivity is likely to be intimately associated with the local environment around what may be called the active site. This suggests that the size, type, and geometry of the atoms making up the active site will play a critical role in defining the conditions under which this active site will be able to carry out its designed function. Fabrication of materials with exactly the same structure and composition at each active site has been and will continue to be a major challenge to materials and catalytic scientists.
Another challenge is to control the thermal and chemical stability of the fabricated nanostructure. It is generally accepted that the smaller the nanostructure (active site), the more likely the structure is to move, aggregate, decompose, or in some way change its shape, composition, or morphology upon exposure to thermal and/or chemical cycling. Identifying windows of stable operation in which the specific structure or material will be able to retain the desired (and designed) behavior is critical for commercial applications. On the other hand, the driving force for investigating nanostructured materials is the fact that they typically exhibit unique properties that are expected to open windows of opportunity previously inaccessible with existing materials.
It should be pointed out that biophysics and biological sciences are having an increasingly important impact in nanoscience and nanotechnology themes (Jelinski, p. 161). For example, atmospheric nanoparticles may play an important role in photocatalytic and thermal production of atmospheric pollutants (Chianelli, p. 133). Atmospheric aerosols in heavily polluted areas have the potential to accelerate ozone formation reactions. Furthermore, because they are respirable, they could represent a health hazard.
Finally, studies of biomineralization, in which an organic substance (usually protein or peptide or lipid) interacts with an inorganic phase (e.g., calcium carbonate or hydroxyapatite) have led to the bioinspired synthesis of composite materials. The structure and porosity of the inorganic phase is controlled by templating with an organic surfactant, vesicular arrays, or liquid crystalline materials. Micelle-templated synthesis has produced ceramics with 20-100 Å pore dimensions (Ying, p. 96), which may be useful as catalysts, absorbents, for gas/liquid separations, and thermal and acoustic insulation. Their high selectivity makes them valuable for biochemical and pharmaceutical separations.