Crystallization subsurface formation

It is interesting to compare our results on single crystal surfaces with those of Turner and coworkers for Pt, Pd, and Ir. In this study wires of Pt formed less than one layer of oxide under CO oxidation conditions. Considering that the Pt wires were known to have substantial Si impurities, which form subsurface oxides , it is not surprising that some oxide was formed. The absence of impurities on the rigorously cleaned, Pt single crystal surface used in this study precluded the formation of any oxides during CO oxidation. 

Reich et al. (2008) measured CI in atacamite from Mantos Blancos, Spence and three other deposits. These show low CI-to-CI ratios (11xi0 to 28x10 ), comparable to previously reported ratios of deep formation waters. Further, CI-to-Cl ratios in atacamite correlate with U and Th concentrations in host rocks. This suggests that subsurface production of fissiogenic CI was in secular equilibrium with waters involved in atacamite formation. Because atacamite does not contain U or Th, production of CI is not continued once chlorine has entered the crystal structure from that time the CI-to-Cl ratio decreases with age. The fact that measurable Cl is present indicates that atacamite formation occurred less than 1.5 Ma ago (five times half-life of CI). 

Computer simulations were also used to show that the crystallization nucleus is more likely to form in the subsurface than in the bulk phase of the water slab. This result can have far reaching atmospheric implications. It has been suggested that formation of an ice nucleus at the interface would be hampered by contamination of the surface by organic surfactants. The effect of the adsorbed material will surely propagate towards the subsurface as well, however it will be smaller than in the topmost layer. Therefore, the anthropogenic emissions should have an effect on the radiative balance of the Earth atmosphere. This effect should, however, be smaller than predicted using the assumption of surface nucleation. 

Detailed studies of the coadsorption of oxygen and carbon monoxide, hysteresis phenomena, and oscillatory reaction of CO oxidation on Pt(l 0 0) and Pd(l 1 0) single crystals, Pt- and Pd-tip surfaces have been carried out with the MB, FEM, TPR, XPS, and HREELS techniques. It has been found that the Pt(l 0 0) nanoplane under self-osciUation conditions passes reversibly from a catalytically inactive state (hex) into ahighly active state (1 x 1). The occurrence of kinetic oscillations over Pd nanosurfaces is associated with periodic formation and depletion of subsurface oxygen (Osub)- Transient kinetic experiments show that CO does not react chemically with subsurface oxygen to form CO2 below 300 K. It has been found that CO reacts with an atomic Oads/Osub state beginning at temperature 150 K. Analysis of Pd- and Pt-tip surfaces with a local resolution of 20 A shows the availability of a sharp boundary between the mobile COads and Oads fronts. The study of CO oxidation on Pt(l 0 0) and Pd(l 1 0) nanosurfaces by FEM has shown that the surface phase transition and oxygen penetration into the subsurface can lead to critical phenomena such as hysteresis, self-oscillations, and chemical waves. 

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