Fundamentals of Aerosol Reactor Design for Catalysts

Aerosol manufacturing routes are advantageous over wet chemistry ones as the former offer fewer process steps, easier particle collection and no liquid by-products requiring costly cleaning¹. In addition, aerosol-made particles and films have unique morphology and high purity (e.g. optical fibers) and can even form metastable phases (e.g. low temperature BaCO₃ for NOx storage-reduction catalysts²). So they easily form mixed oxides, salts and even pure metals and highly porous (98%) films resulting in novel catalysts, micropatterned sensors, phosphors, battery electrodes, dental prosthetics and even nutritional supplements¹.

The emphasis then shifts is on the fundamentals of flame synthesis of particles. In principle, clusters are formed by oxidation of appropriate precursors and grow by coagulation and sintering to ramified fractal-like aggregates and eventually agglomerates of nanosize primary particles. Primary particles in an aggregate may have a narrow size distribution as it is predicted by theory³ and verified by experiments at Cabot⁴.

Recent understanding of the dynamics of these non-spherical particles shows that at high particle concentrations, classic coagulation theory may not be sufficient to describe the ensuing aerosol dynamics, especially if fractal-like particles are formed⁵. In fact, a transition takes place from dilute to concentrated aerosol dynamics as there is less “free gas” volume per agglomerate. They deviate from the kinetic theory of gases especially during instantaneous coalescence: completely inelastic particle-particle collisions, the so-called cooling  of granular gases⁶. In the transition regime, the coagulation rate of highly concentrated aerosols is progressively higher than that for dilute aerosols as growing particles enter the continuum regime where coagulation rates are 2 - 30 times higher than that of Smoluchowski theory⁵. Then fractal-like particles are likely to coagulate till gelation as has been observed in batch formation of soot. In aerosol flow reactors, however, shear forces such agglomerates to restructure or break-up given the employed rather high (10⁵-10⁶) Reynolds numbers⁷.

1. B. Schimmoeller, S.E. Pratsinis, A. Baiker, “Flame Aerosol Synthesis of Metal Oxide Catalysts with Unprecedented Structural and Catalytic Properties”, ChemCatChem in press (2011).

2. R. Buchel, S.E. Pratsinis, A. Baiker “Mono- and bimetallic Rh and Pt NSR-catalysts prepared by controlled deposition of noble metals on support or storage component“, J. Catal., in review (2011).

3. M.C. Heine, S.E. Pratsinis, "Polydispersity of Primary Particles in Agglomerates made    by Coagulation and Sintering" J. Aerosol Sci., 38, 17-38 (2007).

4. D. Boldridge, Morphological Characterization of Fumed Silica Aggregates. Aerosol Sci. Technol. 44, 182-186 (2010).

5. M.C. Heine, S.E. Pratsinis, "High Concentration Agglomerate Dynamics at High Temperatures", Langmuir, 22, 10238-10245 (2006).

6. B. Buesser, M.C. Heine, S.E. Pratsinis, "Coagulation of highly concentrated aerosols” J. Aerosol Sci., 40, 89-100 (2009).

7. Y. Xiong, S.E. Pratsinis, "Gas Phase Production of Particles in Reactive Turbulent Flows", J. Aerosol Sci., 22, 637-655 (1991).