Silica-rich surfaces

Recent studies have shown the formation of altered surface layers thicker than one or two unit cell layers on feldspar surfaces, in apparent support of the leached layer theory. Thick (> 100 nm), silica-rich surface layers were detected by XPS on feldspar samples weathered in solutions having pH < 3 (Casey et at, 1988b) or >9 (Heilman et al., 1990). At these extreme pH values, the rate of release of Al and charge-balancing cations to solution is much faster than the rate of hydrolysis of silica. Under these conditions, oversaturation with respect to amorphous silica could occur, and a highly hydrated, residual leached or precipitated layer of silica could form. However, this layer is probably too porous and discontinuous to be a diffusion-limiting mechanism, but would still account for incongruence under these conditions (Hellmann et al., 1990). Whether the altered layer formed by a leached layer process or by simple precipitation from oversaturated solutions was not determined. 

For example, continuous multilayered ZSM-5 films were grown on cordierite modules.[59] Similar films were generated on as-prepared and acid-treated honeycomb substrates.[60] The latter treatments led to silica-rich surface layers their composition affected the Si/Al ratios of the zeolites crystallized on the cordierite. Thin, defect-free Mid-type films were also made on porous alpha-alumina and yttria-doped zirconia substrates using tclrapropylammonium hydroxide (TPAOH) as a structure-directing agent.[61]... 

Stage 3 Condensation and repolymerisation of the alkali- and alkaline earth-depleted silica-rich surface layer according to... 

FIGURE 56.12 Cross-section of attrition resistant catalyst showing silica rich surface. 

Smith and coworkers recently proposed a specific and novel mineral-based solution to the problem of dilution and diffusion of prebiotic reactants. They have suggested [132-134] the uptake of organics within the micron-sized three-dimensional cross-linked network of pores found to exist within the top 50 xm, or so, of alumina-depleted, silica-rich weathered feldspar surfaces. These surfaces incorporate cavities typically about 0.5 pm in diameter along with cross inter-connections of about 0.2 pm. The nominal area of the weathered feldspar surface is apparently multiplied by a factor of about 130 arising from this network. The similarity of these pores to the catalytic sites in zeolite-type materials is pointedly mentioned. 

Heat-cleaned E-glass slide surface. Table 1 shows that the heat-cleaned glass surface is confirmed to be silica-rich because of higher Si and O surface concentrations compared to the bulk analysis obtained by inductively coupled plasma (ICP). This is accommodated by a lower surface calcium concentration. The SIMS results given below demonstrate that a significant proportion of the oxygen is probably present as silanol. 

Various surface analysis techniques show that silicate glasses rapidly develop surface compositional profiles when exposed to water. When water is present as a vapor an alkali-rich layer (presumably a hydrated alkali carbonate) forms over the SiOj-rich layer. Water as a liquid dissolves the alkali and leaves the silica-rich film. As long as this SiC -rich film is stable the rate of corrosion due to diffusion is reduced with exposure time. Addition of multi-valent species to the glass or reactant results in formation of a complex protective surface layer in the glass which may be stable over a wide range of environmental conditions. 

Removal of iron from mature teeth by acid treatment resulted in silica-impregnated cusps of marked structural integrity. Blocks of sili-cifed material were found at the junction zone between the cusp and tooth base. The outer surfaces of the teeth were generally smooth and showed little evidence of etch pits due to the removal of iron-containing minerals. However, fractured teeth indicated the presence of localized silica-rich anterior zones within the tooth cusps. 

In conclusion, one may suggest that, in silica and silica-rich supported samples, the pore system permits preferential entrance of some CoPc molecules, affecting sensibly the pore dimensions. In alumina and alumina-rich supported samples, the pore system, with its dimensions less than those of CoPc molecules, does not permit free entrance of complex molecules. Here, CoPc molecules may be forced to occupy positions in the defective structure of alumina. All these findings should be reflected on the mode of surface dispersion of the supported CoPc molecules. 

The characteristic chemisorption data obtained for the various supported catalyst samples are summarized in Table 2. It is evident that, the 02-net adsorption (3n) has in general lower magnitude for catalyst samples supported on pure silica and silica-rich support and higher magnitudes for catalyst samples supported on alumina and alumina-rich supports. However, these adsorption values decrease markedly as the CoPc content increases on one and the same support (97.1 SA) being related most probably to the mode of surface complex dispersion. 

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