Particle reshaping

Heat treatments of nanofabricated catalyst particles in different reactant mixtures can result in restructuring and eventually crystallization. Figure4.36 shows two differently sized Pt/ceria samples made by EBL, before and after reaction in a lean H2 + O2 gas mixture for 10 h at 1,000 K. The smaller 130 nm particles reshaped and formed crystallites. In contrast, the larger 500-nm Pt particles (and 20 nm in height), were stable at 623 K, but were found to disintegrate and form clusters of smaller crystalline particles at 773 K [29]. The particle disintegration was also found to be dependent on the support material, occurring more readily on ceria than on silica [72,77]. 

Fig. 4.36. Particle size effect of particle reshaping and disintegration for EBL-fabricated Pt/ceria (a) As-prepared SEM images of 130-nm Pt particles supported on ceria (top) and after 10 min of H2 oxidation (Ph2/(Fh2 + P02) = 0.33) at 1 atm in a flow reactor (bottom), (b) SEM images 500-nm Pt/ceria particles before (top) and after identical treatment as above (bottom) (from [82])...

Fig. 4.38. The effects of various pretreatments (oxidative and reductive) on CO oxidation on a 40-nm Pt/ceria model catalyst prepared by colloidal lithography as measured by the temperature of 50% of CO conversion and the apparent activation energy from the Arrhenius plot. CO reduction was made in 0.5% CO for Ih at 573K, H2 oxidation (a-treatment) was done at a = Ph2/(.Ph.2 + P02) = 0.33 at 573 K for 1 h, and finally /3 = CO oxidation (/3-treatment) was done in the O-rich regime (oxidative conditions), /3 = Pco/ Pco + P02) = 0.2 with 0.3% CO and 1.2% O2 at temperatures between 300 and 673 K. It is seen that reduction leads to a lower Tbo and activation energy, while sustained CO oxidation leads to an increase of the activation energy, which is not recovered by reductive treatments. The latter is explained in terms of strong-metal-support interactions (SMSI) and particle reshaping (see text)...

Treatments to prepare the site for seeding or planting are determined after evaluation of site data and establishment of the land-management options and objectives. Grass and legumes often require more surface preparation than trees and shrubs. Uncompacted fill slopes and freshly reshaped surfaces may make acceptable seedbeds. Hard crusts form on many spoils fine clay-size particles are consolidated by the drying action of wind and sun. This crust may be broken by rainfall, frost, or mechanical scarification. In many cases, ground cover will be denser if it is seeded into fresh spoil or where spoil surf.ices have been broken by natural or mechanical scarification. 

Figure 35. Mode of operation for the removal of a melt particle from an oxidic support by growing a carbon filament. Stage (I) initial saturation of the particle with carbon atoms from dissociation of a hydrocarbon and subsurface dissolution of the resulting free atoms. Stage (2) is after about 1 h on stream the particle exsolutes at the most active faces carbon which grow in a concentric set of graphene bands and remove the particle from the support. Stage is (3) after some time on stream the particle has reshaped such that surfaces within the carbon tube also become active for graphene formation and are deposited as little flakes inside the tube.

Van de Vyver S, Geboers J, Dusselier M, Schepers H, Vosch T, Zhang L, Van Tendeloo G, Jacobs PA, Sels BE (2010) Selective bifunctional catalytic conversion of cellulose over reshaped Ni particles at the tip of carbon nanofibers. ChemSusChem 3(6) 698-701... 

The particles must have a floatable size and shape larger particles have to be reduced in size, too small particles have to be agglomerated into larger ones, flat particles should be reshaped to cubic ones. 

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