Single particle deposition

For the analytical characterization of single particles, deposition of the particle on very flat surfaces is necessary (e.g., Nucleopore filters or organic foils) (McCrone and Delly 1973 Grasserbauer 1978a Spumy etal. 1979 Chatfield 1984). 

Therefore, in many fundamentally oriented studies the complex catalyst is replaced by a simplified model, which is better defined. Such models range from supported particles from which all promoters have been removed, via well-defined particles deposited on planar substrates, to single crystals (Fig. 4.1). With the latter we are in the domain of surface science, where a wealth of informative techniques is available that do not work on technical catalysts. 

Vented Underground Burst. An underground detonation which produces no visible fireball, but which results in the release of volatile radionuclides through fissures or other vents, produces a single particle class— atmospheric aerosol particles with condensable radionuclides deposited on their surface. Radionuclide abundance is independent of particle size. 

A number of elegant studies over the past few years have also addressed the need to minimize particle size distributions through the use of segmented flow microfluidic systems. Such an approach removes the possibility of particle deposition on channels and eliminates the problems of residence time distributions that occur in single phase systems (where drag at the channel walls sets up a velocity distribution inside the channel). For example, Shestopalov et al. reported the two-step synthesis... 

The rate of deposition of Brownian particles is predicted by taking into account the effects of diffusion and convection of single particles and interaction forces between particles and collector [2.1] -[2.6]. It is demonstrated that the interaction forces can be incorporated into a boundary condition that has the form of a first order chemical reaction which takes place on the collector [2.1], and an expression is derived for the rate constant The rate of deposition is obtained by solving the convective diffusion equation subject to that boundary condition. The procedure developed for deposition is extended to the case when both deposition and desorption occur. In the latter case, the interaction potential contains the Bom repulsion, in addition to the London and double-layer interactions [2.2]-[2.7]. Paper [2.7] differs from [2.2] because it considers the deposition at both primary and secondary minima. Papers [2.8], [2.9] and [2.10] treat the deposition of cancer cells or platelets on surfaces. 

Calculations of the deposition rate show a dramatic dependence on the mechanism by which the surface charge is generated, in addition to the dependence on the charge itself. This reflects the importance of surface chemistry in particle deposition. Characterization of the electrostatic interactions, involving a given surface, require electrokinetic measurements on that surface under a broad spectrum of electrolyte conditions so that the number density and dissociation constant of ionizable surface groups can be deduced a single electrokinetic measurement is not sufficient. 

The adsoqjtion of NO on metal loaded ceria has been examined for Pt, and Pd, As known from work on single crystals, NO dissociates to some extent on each of these metals. The amount of dissociation is dependent upon the structure of the metal surface. Gorte considered Pt and Pd particles deposited on rough, poly crystal line ceria films grown by spray pyrolysis.For Pt they found variation in the TPD results (amount of NO uptake and shape of N2 desorption profile) that varied with the size of the Pt particles. However, the results were comparable to NO TPD results from Pt grown on sapphire. It was concluded that no unusual interaction existed between Pt and the (oxidized) ceria. For Pd it was found that a pronounced difference in the TPD product ratio, NO/N2, occurred for Pd on ceria compared to Pd on sapphire. They attributed the difference to NO adsorption on reduced ceria. 

Panda, R.C. Zank, J. Martin, H. Modelling the droplet deposition behaviour on a single particle in fluidized bed spray granulation process. Powder Technol. 2001, 115, 51-57. 

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