Thermal surface enrichment

Table 7.1 Surface enrichment in mixed thermal oxides.

Following a thermal pulse in the hehum shell, convective dredge-up of matter from the hehum shell brings helium, s-process elements, and a significant mass of to the surface. It is the surface enrichment associated with this source of that leads to the condition that the envelope i2c/16o ratio exceeds 1, such that carbon star is bom. Calculations of galactic chemical evolution indicate that this source of carbon is sufficient to account for the level of in galactic matter. The levels of production of and other CNO... 

TPR and WAXS data showed that, when Pd and Au have been coimpregnated on active carbon, almost complete alloying occurs. When impregnation of the two metals is consecutive, partial alloying, with the formation of different Au-rich alloys, occurs only after suitable thermal treatment. CO chemisorption data showed that Pd dispersion does not change after the addition of Au, whose surface enrichment is therefore to be excluded. Au proved to have no effect on the Pd activity (per gram) in benzaldehyde hydrogenation, but the increase of turnover frequency with Pd particle size has been confirmed. 

ESCA and XRD characterizations carried out after the catalytic experiments reveal a surface enrichment in molybdenum with a specific (010) orientation of crystal faces of M0O3. The cristalline structure of the solids however is not very much modified so that the most significant factor of the partial covering of NiMo04 with M0O3 could be the thermal treatment. 

The fragile element fluorine can be produced inside the He-intershell of AGB stars. Jorissen, Smith Lambert [130] discovered that the [F/O] abundance correlates with the C/O ratio in AGB stars, and that some N-type carbon stars show surface enrichments a factor of 30 above the solar ratio. The increase in the C/O ratio is clearly a result of the third dredge-up and thermal pulses, hence it was concluded that the carbon and fluorine are produced in the same region in the star and mixed together to the surface. Jorissen et al. [130] examined the many pathways that fluorine, or more precisely the isotope 19F, could be produced and concluded that the most likely chain is... 

Previous work has shown that oxidizing thermal treatment at high temperature (800-900°C) of bimetallic Pt-Rh catalysts prepared by coimpregnation would lead to the formation of Pt-Rh alloys with surface enrichment in rhodium oxides (3-7). In order to verify this hypothesis in our case, the coimpregnated Pt-Rh catalyst was characterized by temperature programmed reduction in hydrogen and by measure of the activity for the oxidation of a propane-propene mixture under lean conditions. 

A microemulsion is a thermodynamically stable, optically clear dispersion of two immiscible liquids such as water and oil, stabilized by the presence of a surfactant and, in some cases, a co-surfac-tant.i i7i,265-267 synthesis of nanoparticles by microemulsions has two main advantages. On the one hand, particle size can be controlled by adjusting the size of the micelle containing the metal precursors. Therefore, thermal treatments for particle size control can be avoided. On the other hand, since the micelles have the same composition, i.e. metal precursors are distributed homogeneously the nucleation of metallic particles renders particles of the same composition. This latter feature is very important for the synthesis of bimetallic (or ternary) catalysts. The main drawback of the microemulsion, or any other approach using surfactants, is surfactant removal. Severe thermal treatments are required in order to achieve complete removal of the surfactant which may result in particle aggregation and/or surface enrichment, or complete phase segregation of the components of the bimetallic samples. ... 

Another reason of the surface enrichments or loss is the formation of volatile compounds of a certain component of catalysts with reactants. For example, nickel can react with carbon monoxide in reactants to form the volatile and thermally unstable nickel carbonyl, which escapes gradually from the catalyst surface. The activated carbon supported ruthenium-based catalysts also loses obviously part of itself due to the volatilization of ruthenium oxides or the occurrence of methanation reactions of the activated carbon. 

Atoms at a metal surface exhibit unsatuiated bonds that are available for fixing reactive species, atoms or molecules, present in the gas or liquid surrounding the surface. Such a reaction, when limited to one monolayer or a fraction of a mono-layer, is known as an adsorption phenomenon or chemisorption. Similarly, atoms present in the bulk metal may diffuse toward and enrich the surface by so-called thermal segregation. Segregation may also occur by selective evaporation of the metal in vacuum or in an inert gas or by selective dissolution of the metal in a liquid phase (anodic segregation). Whatever the mechanism of surface enrichment, there is strong experimental evidence that the same structural and chemical states can be achieved by adsorption or segregation. 

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