Restructuring surfaces

These phenomena have been studied in three papers by L. D. Schmidt and his co-workers. They first report on the morphology and surface composition of gauzes after typical use in the nitric acid and Andrussow processes. 

They conclude that surface diffusion and/or gaseous PtOa transport in a boundary layer can best explain their observations. Thus loss of metal in the oxygen-rich regime is consistent with Pt02(g) formation. It also provides an explanation of catalyst activation by facetting since the oxygen pressure adjacent to active zones will be low relative to that adjacent to inactive zones Pt will then preferentially evaporate from the inactive zones. 

In ammonia-rich conditions surface diffusion due to differences in surface energies between planes is suggested. The origin of such surface energy differences is uncertain. Etch pit formation correlates reasonably with dislocation density, but this hypothesis does not explain the lack of attack at grain boundaries. 

Finally, Schmidt et have examined faceting of microspherical single crystals of platinum in reaction mixtures. At 720 °C in the oxygen-rich regime stable planes formed were (111), (100), (110), and (421). Ledges and pyramids, whose faces are made of these planes, form adjacent to the flat planes. In 20% NHs/air (210) planes also become stable. 

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Van der Waals (physical Surface restructuring on No change in surface... 

Similar disproportionation is likely to occur during catalytic hydrocarbon oxidation since the Bl2Mo20g catalyst is subjected to continuous redox cycling under such conditions. Therefore, any kinetic or catalytic information about Bi2Mo20n is suspect unless the absence of surface restructuring can be confirmed. 

Since the most active catalytic sites are usually steps, kinks, and surface defects, atomically resolved structural information including atomic distribution and surface structure at low pressure, possible surface restructuring, and the mobility of adsorbate molecules and of the atoms of the catalyst surface at high temperature and high pressure is crucial to understanding catalytic mechanisms on transition metal surfaces. The importance of studying the structural evolution ofboth adsorbates... 

The use of neutron reflectivity at liquid interfaces, which is a method sensitive to both surface roughness and surfactant layer thickness, was reviewed with the examples of polydimethylsiloxane-surfactant layers.633 Sum-frequency generation (SFG) vibrational spectroscopy was applied to study surface restructuring behavior of PDMS in water in an attempt to understand antifouling properties of silicones.6 ... 

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

The pitting of the membrane surface is interesting since the micrographs shown in Figure 4 provide evidence for removal of metal, but no indications of its accumulation or redistribution on the surface. Thus, it is conceivable that the surface restructuring observed... 

This is different at semiconductor surfaces where the covalent bonds between the substrate atoms are often strongly perturbed by the presence of adsorbates. This can result in a significant surface restructuring. Hence the dynamics of the substrate atoms has to be explicitly taken into account which of course increases the complexity of the modelling of the adsorption/desorption dynamics, as will be shown below for the H2/Si system. 

Rajh T, Chen LX, Lukas K, Liu T, Thumauer MC, Tiede DM. Surface restructuring of nanoparticles An efficient route for hgand-metal oxide crosstalk. J Phys Chem B 2002 106 10543-52. 

The limitation of the ex situ approach is that the composition and structure of the catalyst surface are not investigated under reaction conditions. This limitation prevents the post-mortem analysis of the catalyst under UHV to determine an assessment of the surface restructuring, intermediate species, segregation of specific components, etc., that are characteristic of reaction conditions. 

Kaszkur (2001) Pd/Si02 Surface restructuring + + n.a. Adsorption of oxygen, subsurface carbon... 

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