Surface atom ratio

The surface atomic ratios (Cd/F, S/F and C(naflon)/F) calculated from the XPS peak areas are listed In Table 1. The results Indicate that the cublc-CdS film has a higher surface concentration of CdS than the hexagonal film. In both, the C(ls)/F(ls) ratio Is similar to Naflon Itself. 

With the BiPt/C catalyst with a Pt/Bi surface atom ratio of 3, in acidic medium (2 < pH < 4) glycerol could be oxidized with a selectivity of 80% into dihydroxyacetone. It is proposed that the Bi adatoms function as blockers of the Pt(lll) surface, controlling the glycerol surface orientation. 

The influence of the modification on the surface atomic ratios of Si Al as measured by ESCA is shown in Table 2. The enrichment of alumina was observed for modified HZSH-5. The surface Si Al ratio was decreased to about one half its original value. It seemed impossible for diazomethane modification to remove the framework alumina of the zeolite. Ve assumed that the enrichment was caused by the migration of the amorphous aluminum oxide to the surface of the zeolite. The mechanism should be studied further in detail. 

W Na AI Surface Atomic Ratio Agrees with Bulk Ratio No Deposition or Migration to the Surface... 

Fig. 31. Surface atomic ratio vs bulk atomic ratio for a series of NiV intermetallic alloys, (t) Untreated surfaces (2) HF-treated surfaces. (------) Straight line of unit slope. After ref. 542, by...

Combining the NMR and TG data and knowing the size of monodispersed NCs in a given sample, it is possible to calculate the number of their surface atoms and, in the next step, to find the degree of their surface complexation or, in other words, to find the surface ligand to NCs surface atoms ratio. This is also an important parameter characterizing the hybrid. 

Figure 5 Surface atomic ratios of terrace and edge together with metal dispersion as a function of particle diameter. 

Unlike Ti, A1 demonstrates very pronounced tendency to segregate on the surface of mixed oxide. This tendency becomes particularly distinct with decreasing A1 content. Thus, for the sample containing 15 and 3 mol % AI2O3 (the Ti02 content is 85 and 97 mol %, respectively), the Al/Ti surface atomic ratio exceeds the bulk value by a factor of 5-6 (Pai 4-5). 

The material, calcined at different temperatures and under different atmospheres was studied by XPS analysis in order to determine the surface composition and the oxidation state of tin, vanadium and antimony in each of the samples. Table 4 reports the atomic percentage of tin, vanadium and antimony in the various ca ysts the bulk atomic ratio analysis is made by atomic absorption and the surface atomic ratio by XPS spectroscopy. 

Characterization data evidenced that the prepared NiMo04 is stoichiometric and that Cs is deposited only on the catalyst surface (atomic ratio Cs/Mo = 0.03) not affecting the molybdate structure. However, Cs doping causes a decrease of the catalyst surface area Sbet (NiMo04) = 44.1 m /g and Sbet (3% Cs-NiMo04) = 28.7 mVg. Moreover, the promoted sample exhibits a higher surface basicity, electrical conductivity and also a larger resistance to reduction [4,5,12]. 

The results of surface Si/Ti compositions of titania/silicas characterized by XPS are shown in figure 3. In order to calculate the surface atomic ratios, n(Si)/n(Ti), the following equation was used [5] ... 

The surface atomic ratios of n(Si)/n(Ti) of the sol-gel titania/silicas increase with increasing Si/Ti ratio of 1 to 5. However, those of the precipitated titania/silicas are almost constant and their values are low. These results mean that the distribution of titanium and silicon around the surface of catalyst depends on the preparation method. Therefore, there is a good correlation between the surface and bulk Si/Ti ratios in the sol-gel catalyst. This suggests that Si and Ti components are homogeneously dispersed at both the surface and inside of the sol-gel titania/silica. On the other hand, the content of titanium at the surface of precipitated titania/silica was much higher compared to the corresponding sol-gel titania/silica and did not depend on the bulk titanium content. These results mean that the surface of precipitated titania/silica was covered with titania as is expected from the preparation procedure. 

Figure 2 Surface atomic ratios of n(Si)/n(Ti) of titania/silicas calculated by XPS Spectra...

Figure 8. Dependence of the XPS Cls/Ols surface atom ratio on exposure time for 6FDA/PDA. VUV alone (A) VUV + atomic oxygen (U) atomic oxygen...

Table 1 Surface atomic ratios evaluated from XPS.

Using the method described in the experimental part, the range of pH where maximum adsorption on carbon occurs has been determined for both Au and Pd precursors in aqueous solution. In the case of palladium, a maximum adsorption range is obtained between pH 2.5 and 12.5 (Fig. 1). In the case of gold, the whole amount of metal introduced in solution was adsorbed on the carbon support over the whole pH range. The solid samples obtained at various pH values were characterized by XPS and it was found that the Au/C surface atomic ratio could be maximized between pH 2 and 9 (Fig. 2). 

Surface atomic ratios of supported copper-chromium catalysts obtained from XPS measurements. 

The liquid phase hydrogenation of benzonitrile had also been investigated over alloy type Sn-Pt/Si02 catalysts prepared by CSRs. ° Prior to reaction the Sn-Pt/Si02 catalysts (0.25 g) were re-reduced in H2 at 300 °C. The hydrogenation of benzonitrile was carried out in ethanol at 60 °C and 4 bar H2 pressure. Tin content of the catalysts ranged from 0.05 to 0.63 wt. %, whereas Sn/Pt surface atomic ratios determined by chemisorption measurements were between 0.1 to 3.5. 

The addition of tin to Pt led to an increase in the turnover frequency (TOF) by a factor of 2. TOF showed maximum at surface atomic ratio of Sn/Pt = 1 as shown in Figure 17. These results are in good agreement with earlier findings obtained in the liquid-phase hydrogenation of benzonitrile on catalysts prepared by the addition of tin chloride to Pt/nylon catalyst. ... 

Figure 16 Relationship between product selectivity and Sn/Pt surface atomic ratio in the hydrogenation of benzonitrile over Sn-Pt/Si02 catalysts (Reproduced from ref. 130 with permission)...

White et al. [45] reported in 1985 on experiments involving the same two sulfide semiconductors, CdS and ZnS. Although the focus of this work was on the photocatalytic hydrogen evolution as a function of shape and composition rather than on the optical properties of the particles, it should be mentioned here since it contains the first x-ray photoelectron spectroscopy (XPS) measurements on spherically layered semiconductor particles. By successive precipitation of cadmium and zinc salts with H2S onto Si02 supports, both composites have been prepared, ZnS on a CdS core and CdS on a ZnS core. The surface composition was analyzed by XPS taking the Cd(3d)/Zn(2p) peak area ratio as the measure of the surface atomic ratio in the composite particles. The results are such that, indeed, spherically layered particles are formed in the preparative procedure. 

The volumetric method has very often been used with platinum catalysts for which quite satisfactory results are generally obtained it is usual to assume that the monolayer volume or amount, obtained as just described or by extrapolation corresponds to an H Ms (hydrogen atom to metal surface atom) ratio of 1 1. Some justification for this assumption is to be found, at least for particles of moderate size, in the adsorption stoichiometry shown by films and single crystals, but for very small particles and at high pressures the H/Mj ratio can exceed unity quite substantially this is especially so with rhodium" and iridium (see below). Care is however needed with palladium " " because of the risk of forming the hydride however, monolayer coverage is obtained at pressures below which dissolution starts. The base metals iron, cobalt and nickel have been... 

For nanometer particles, it has recently been discovered that maximum overlayer adsorbate-surface atom ratios can be very different from those typically measured at extended surfaces [46, 47]. 

Polymer Bulk atomic ratio Surface atomic ratio a, (S. cm... 

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