Microscopic metals sintering

Sintering is an important mode of deactivation in supported metals that involves complex microscopic physical and chemical phenomena, e.g., dissociation, emission, diffusion, and capture of metal atoms and crystallites. The relative importance of these different processes may change with reaction conditions and catalyst formulation. Modeling and prevention of sintering processes require an understanding of these basic processes as well as quantitative measurements of sintering rates. 

This unusual reaction has not been reported previously, however, a number of studies have demonstrated the formation of analogous structures when platinum/alumina specimens were heated to temperatures in excess of 8009C. Baker and co-workers [23] using the CAEM technique to study the sintering characteristics of platinum on alumina in oxygen observed spectacular transitions in the appearance of the specimens at temperatures in excess of 800°C. The metal particles initially spread on the alumina to from diffuse islands and then quite suddenly reconstructed to produce well defined dense shapes. Sprys and Mencik [24] found the same effect when platinum/alumina specimens were subjected to intense electron beams within the electron microscope and characterized the structures as the intermetallic compound PtaAl from electron diffraction analysis. 

During the past two decades, sophisticated microscopic and spectroscopic techniques have been aj lied to the investigation of sintering and redispersion of model film and single crystal metal-support systems. This review presents and discusses results obtained in these investigations with an emphasis on mechanistic evidence. 

FIGURE 7 Schematic illustration of a sandwich reactor (top) and scanning electron microscopic images of a sintered metal fiber catalyst. 

Fig. 8.5 Scanning Eiectron Microscope image of a perovskite-paliadium cermet (ceramic-mettii) made by sintering together LaFeo.wCro.ioOs-x and Pd powder to form dense continuous matrices of both metal and ceramic. The palladium and ceramic were lattice matched to minimize stiain and interfacial dislocations. (S. Rolfe, Eltron Research) (Copyright Elsevier, 2005. Adapted with permission from [11], Ctubon Dioxide Capture and Storage in Deep Geological Formations.)...

The specific surface area of the activated catalyst was found to increase with alumina content up to about 20 m /g at around 2% AI2O3 and then to remain constant (10), demonstrating the role of this additive as structural promoter that (together with other nonreducible phases such as hercynite and calcium ferrite) prevents sintering of the metallic iron particles into low surface area material. These values are compatible with the mean particle sizes of around 30 nm, as determined by mercury porosimetry and seen directly in the scanning electron microscope (Fig. 2) (11). This agreement further shows that the texture of the catalyst permits the N2 molecules of the BET analysis to reach essentially the whole internal surface. 

Particles may be trapped in frits made of sintered glass or metal. These filters are more effective when the vapor is passed through the frit. N-hexane samples of very low self-conductivity of 10" Qr cmr were prepared this way by Nikuradse in 1931 (Nikuradse, 1931). The influence of microscopic particles on the conductivity of cyclohexane was demonstrated by subjecting the liquid to the force field of an ultracentrifuge (Matuszewski et al., 1975). Removal of dissolved particles from the measurement volume and sedimentation led to a decrease of the initial conductivity. Zone melting has also been applied to the purification of long-chain hydrocarbons (Belmont et al., 1985). 

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