Surface enrichment, calculation

Williams and Boudart (29) measured the surface composition of Ni-Au alloys that had been preequilibrated above the miscibility gap and subsequently generated. They found large enrichments in gold. The surface composition measurements fit the surface enrichment calculated according to Eq. (8) rather well. They report enhancements of the nickel concentration by treatment with oxygen. 

For ail samples, both a.p. and s.o., irrespective of the preparation method, the experimental intensity ratios, V2p/Zr3d, increased proportionally to the V-content up to 3 atoms nm 2 (pjg 2). The ratio approaches those calculated with the spherical model proposed recently by Cimino et al. [27] (full line in Fig. 2). For ZV samples with V-content < 3 atoms nm 2, this finding shows that vanadium species are uniformly spread on the Zr02 surface. On ZV catalysts with a larger V content (not shown in Fig. 2), the intensity ratios were markedly larger than the corresponding values yielded by the spherical model. The results obtained on samples with V-content > 3 atoms nm 2 point therefore to a V surface enrichment. 

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

For the actual Pt3Sn compound, the surface enrichment of (111) and (100) planes has been computed (45) by assuming that it only takes place by inversion between surface atoms and neighboring atoms. Since in this calculation only interaction between nearest neighbors was taken into account, this assumption was justified. However, if interactions are of longer range, the layers that interchange atoms with atoms from the surface can extend deeper into the solid. 

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... 

Chemisorption data are reported in Table 1. The average Pd particle size has been calculated by the procedure recently developed by our group [7] for the monometallic Pd sample. For bimetallic samples, in the absence of any information about a possible influence of Au on Pd/CO chemisorption stoichiometry, only the experimental CO/Pd values have been reported. As expected, Pd dispersion decreases with the increase of treatment temperature, but no evident role seems to be played by Au neither on Pd sintering nor on Pd dispersion. This allows to exclude the occurrence of Au surface enrichment and also strong electronic interactions in the alloy. 

For monometallic Pd samples, the specific surface activity, or turnover frequency (TOF), roughly calculated as ratio between Kobs and CO/Pd, appears to increase with Pd particle size, a behavior that was suggested in our previous paper on the effect of different carriers [9]. When Au is present, this behavior seems to disappear (TOF roughly independent on Pd dispersion), but presently no straightforward explanation can be suggested. As a possible one, a progressive Au surface enrichment on heating could occur. 

In Table 1, the surface free energies of several important oxides are listed. As may be seen in the table, the differences between the various Materials are remarkable. In single and multicomponent. Materials the tendency towards the lowest possible value of surface free energy is the reason for recrystallization effects and for sorption, phase separation, and segregation phenomena. Surface enrichment of the component with the lowest surface-tension value is generally observed. Calculations of the surface-tension and surface-enrichment effects were made for alloys (real... 

Daumas et al. (1976), within their diverse set of analyses, made measurements of proteins Emd total carbohydrate in the particulate phase for microlayer and subsurface samples from the Etang de Berre (the polluted site) and Brusc in the north Mediterranean. Their results are given in Table IX together with some quantities calculated with a view to assessing the importance of proteins and total carbohydrates in the surface enrichment of POC and PON. For the Brusc region, where there were no visible slicks, it can be seen that there is an enrichment of proteins and carbohydrates by factors of about 1.3 for the screen and 3.3 for the drum device, i.e., approximately comparable with the results for POC and PON given in Table V, and for fatty acids (Table VII, screen). The POC content of the proteins and carbohydrates combined (P + CHO) has been calculated by assuming 60% carbon by... 

The results obtained for the fresh and aged commercial Pt/Rh and Pd/Rh TWCs are shown in Table 2. The first column contains the dispersions and calculated spherical particle sizes (in parentheses) derived from the CO methanation technique based on an assumed adsorption stoichiometry of 1 CO per exposed noble metal atom. The arbitrary choice of a stoichiometric factor of 1, rather than the value of 0.7 suggested by the EmoPt-l catalyst, was made on the basis of several factors. The main reason is that the presence of Rh in these catalysts (16% and 10% of the noble metal weight in the Pt/Rh and Pd/Rh catalysts, respectively) is likely to increase the average stoichiometric factor above 0.7 due to the presence of gem-dicarbonyl species on Rh. Bimetallic Pt/Rh particles have been found in automotive catalysts, sometimes with surface enrichment by Rh [20,21] or even bulk enrichment of selected particles as... 

Surface enrichment of the proteins, calculated as the ratio of the weight fraction of each protein in the surface and bulk, varied with copolymer composition, indicating substantial differences in the composition of the surface and bulk phases. 

The surface enrichment for the three elements (As, Cu, V) is summarised in Table 7, giving an enrichment factor, expressed as a percentage based on bulk levels, and calculated depths over which the enrichment is observed. Arsenic has a similar level of enrichment for both the recirculated and non-rednculated material. However, the thickness of the As layer in the recirculated material is three times that of the non-recirculated. This indicates that As probably diffuses into the matrix during the much longer residence in the hot zone in the run with recinculatioa For the other elements the layer thickness remains at a similar level but there is an appreciable difference in the level of surface enrichment. 

All the bimetallic samples prepared in this study contained 5 wt.% of Pd and 5 wt.% of Bi, while the surface Pd and Bi atomic % (XPS) decreased with the degree of functionalization when grafting both metals (Table 2). Pd/Bi XPS ratios reached a unique value of 1 for activated PdBi samples whereas this ratio increased with the degree of functionalization for activated BiPd samples. These experimental surface Pd/Bi atomic ratios appear to be systematically lower than the theoretical value of 1.96 calculated from the bulk composition, independently from the order of incorporation. This reflects the lower surface energy of Bi with respect to Pd, leading to surface enrichment in this element, as always observed [3]. Various PdxBiy intermetallics were suspected by XRD. The activity decreased with the functionalization in PdBi samples while it increased in BiPd samples (Table 2 and Table 3). These catalytic results are linked to the amount of Pd and Bi on the surface and also to their surface ratio. 

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