Carbon, surface, over reduced

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. 

The pulse-flow reaction study performed at 298 K with a propyne dihydrogen ratio of 1 3, confirmed many of the infra-red spectroscopy results even though there was a considerable difference in residence time between the systems. No propane was detected in the reactor eluant and, apart from 10 % of the first pulse, only C-3 hydrocarbons were observed in the gas phase, supporting the infra-red results which indicated only C-3 species on the surface. It is also clear from Table 2 that the effect of the higher hydrogen concentration is to reduce the extent of carbon deposition over the first four pulses. The results at the lower temperature and dihydrogen propyne ratio, show that as the surface is covered with retained species the rate of hydrogenation decreases, the amount of carbon deposition decreases, and... 

This reductive deammination [43] can be attributed to the more complex functionality of the carbon surface, which besides various types of oxygen-containing groups, contains pi bonds and pyridine-N-oxide groups [44] that give rise to the oxidation-reduction reaction wherein the metal complex is reduced and the carbon surface is oxidized. Carbon has also been shown to reduce Pt+ chloride and oxychloride complexes to Pt+ at acidic conditions, where in the same pH range the Pf is completely stable over alumina [45, 46]. 

The pyrolysis of benzene over the active carbon surface results in the deposition of the carbon on the surface of the substrate carbon as weU as in the microporous system and at some preferred sites. The adsorption isotherms of organic molecules of varying sizes and shapes indicated that the carbon gets deposited preferentially in the pore entrances reducing entrance diameter resulting in the formation of carbon molecular sieves. Pore-size distribution curves indicated that the treatment with benzene between 3 and 6 hrs reduces the mean pore dimensions to 0.6 nm, and a larger time of treatment reduces pore entrances to less than 0.6 nm. 

Ethylene conversion to acetic acid can be increased in at least two ways. One is making the catalyst surface more acidic, which will strengthen ethylene adsorption on the surface to give more time for its oxidation to acetic acid. A more acidic surface will also facilitate desorption of acetic acid thus reducing its surface over-oxidation to carbon oxides. This approach has been realized by adding small amounts of P, B and Te to the MoVNb oxide catalyst. Table 11.2 presents the results of adding phosphorus. 

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