Particle geometric

FIGURE 9.28 Size distribution of particle geometric cross section (A) as a function of geometric diameter for a typical rural aerosol (adapted from Hegg et at., f993). 

The conclusion is that particle size effects on catalytic activity or selectivity due to variations in the inherent properties of small metal particles (geometric or electronic) are unlikely to be important for particles larger than about 1.5-2.0 nm. If size effects are observed for larger particles it is necessary to consider the nature and origin of such effects. 

When the catalytic properties of supported clusters are measured by standard electrochemical methods such as cyclic voltammetry or oxidation transient measurement, only the average properties of the entire distribution of active particles on the electrode surface can be measured. A range of properties of supported nanoparticles, e.g., their geometric structure, their electronic and magnetic properties, as well as their catalytic activity, depends on the size of the particles. Geometric as well as electronic effects have been used to explain particle-size effects in electrocatalysis. 

Regardless of the method used to generate the aerosol, efficient pulmonary deposition of the active agent is critically dependent on the aerodynamic diameter of the inhaled particle. Aerodynamic diameter is the physical property of a particle, which defines how it will behave in an airstream, and depends on the particle geometric size, density, and shape. An in-depth discnssion of how particle shape affects aerodynamic diameter is beyond the scope of this review therefore, the cited examples will assume a spherical particle. 

Thus, the positional relations established in a stmcture, between a particle and all the other generically define the " neighborhoodparadigm of the considered particle, geometrizing the notion of chemical bond. 

Most physical properties of a particulate system are ensembles or statistical values of the properties from their individual constituents. Commonly evaluated particle geometrical properties are counts, dimension (size and distribution), shape (or conformation), and surface features (specific area, charge and distribution, porosity and distribution). Of these properties, characterization of particle size and surface features is of key interest. The behavior of a particulate system and many of its physical parameters are highly size-dependent. For example, the viscosity, flow characteristics, filterability of suspensions, reaction rate and chemical activity of a particulate system, the stability of emulsions and suspensions, abrasiveness of dry powders, color and finish of colloidal paints and paper coatings, strength of ceramics, are all dependent on particle size distribution. Out of necessity, there are many... 

For t-values higher than 1 nm, the t-plot is linear again with the same slope for all the samples. This slope allows to calculate a mean value of the particle external specific surface area which ranges about 3.6 m /g. This last value agrees with the particle geometrical surface. Indeed an external specific surface area 5 of 3.5 m /g Is estimated from geometrical data (particle diameter dp = 0.6 im, porosity e = 0.47 for a 800°C calcined slab of cylindrical shape, a theoretical density p= 6.08 g/cm (ref. 10)). 

---