Relative surface concentrations

XPS spectra were obtained for the catalysts in the calcined, sulfided, and sometimes in the reduced state, as described before. Table III gives the binding energies and relative surface concentrations for the M0/AI2O3 and C0-M0/AI2O3 catalysts, with and without arsenic. Data for the used catalysts, which are not listed in the table, are similar to those for the catalysts prepared in the laboratory. 

When a steady state condition has been achieved. Equation 21 implies that the relative surface concentrations are only functions of the bulk concentrations and the sputtering coefficients. This point cannot be overemphasized. Many authors have misinterpreted their data because they did not understand the consequences of this result. Once the sputtering coefficients are known, then thermodynamic properties, such as a tendency towards surface segregation, do not affect the surface concentration. However, the sputtering yields themselves are partially determined by binding energies and the type of compounds which are present in the surface region. These parameters are, of course, influenced by thermodynamic considerations. 

Mechanism A is a generalised mechanism which was proposed for those metals where the frans-but-2-ene cis-but-2-ene ratio was around unity. This mechanism contains a variety of reversible steps which permit the conformational interconversion of the diadsorbed buta-1 3-diene. Consequently, the trans cis ratio will depend upon the relative rates of these reversible steps and the ratio may be much lower than would be expected if the relative surface concentrations of anti- and syn-diadsorbed buta-1 3-diene, species I and III, respectively, in Fig. 37, were similar to the relative amounts of anti- and syn-buta-1 3-diene in the gas phase. It was also suggested that the relative importance of the various steps in mechanism A may be different for different metals. Thus, for example, the type A behaviour of nickel and cobalt catalysts, as deduced from the butene distributions and a detailed examination of the butene AAprofiles [166], was... 

Mechanism B was originally proposed for the palladium-catalysed reaction [208], which is characterised by a high trans cis ratio. In this mechanism, conformational interconversion of adsorbed species does not occur and the trans cis ratio is a direct reflection of the relative surface concentrations of the anti- and syn-diadsorbed buta-1 3-diene, which are dependent upon the nature of the sites available at the surface and upon the relative stabilities of the two conformers. In addition to palladium, mechanism B has been proposed, along with a contribution from mechanism A,... 

Figure 1. Plots of the relative surface concentrations (solid curves) and relative peak intensities (points, dashed curve) of the 500 cm-- - [Ru(III)-NHj stretch] and 460 cm l [Ru(II)-NH3 stretchJ as a function of electrode potential for Ru(NH3)g +/2+ electrostatically adsorbed at chloride-coated silver. Ru(III) and Ru(II) surface concentrations determined by integrating cyclic voltammogram obtained for conditions [50 ttM Ru(NH3)g3+ in 0.1 M KC1, 50 V sec l sweep rate] where faradaic response dominated by adsorbed redox couple. Both surface concentration and SERS intensities for Ru(III) and Ru(II) normalized to values at -100 and -500 mV vs SCE. SER spectra obtained using 647.1 irradiation.

Here denotes the catalytic Pt-site, CPt the concentration of the catalytic sites, yk the molar fractions of the gaseous components (CO, 02), and 0k the relative surface concentrations of the deposited components (CO, O ). The values of kinetic parameters kj are taken from Nibbelke et al. (1998), where the complete microkinetic scheme for CO oxidation on P t/ R h /CeO 2/ y - A1 zO 3 catalyst is given. In Fig. 26 we can observe that steep concentration gradients may occur in the system. There are even regions with zero CO concentration inside the sintered 7-Al203 particles, and the reaction takes place only on the surface of the macropores. The Pt distributed deeper is not utilized in the reaction. 

According to Table VIII, P-OH groups (Fig. 21b) represent the most stable Bronsted acid sites on the aluminophosphate surfaces. At the same time, P-OH-A1 sites (bridged hydroxyl groups) exhibit the strongest acidity. Their relative surface concentration with respect to P-OH could be estimated... 

In a LEISS spectrum vide infra), the backscattered intensity is plotted against Ei/Eq a high-intensity peak corresponds to a topmost atom of a given mass, and the relative intensities provide a measure of the relative surface concentrations. Since the atoms scatter with different intensities, sensitivity factors / for each element must be obtained by calibration. For example, for an alloy film consisting of Pt and Co, the surface concentrations Cp and Cco can be extracted from the following equations ... 

The relative surface concentrations of CgHg, CgHD, and CjDj have been determined from the steady state treatment referred to above typical values which represent the mean in each case of values obtained under a wide variety of conditions, are given in Table XXV. An... 

Figure 1. Spectral dependencies of transmission coefficients for a monolayer of surface oxidized Cu nanogranules with the diameter of 8 nm deposited on the quartz substrate. Curves 1, 2 correspond to the measured values of direct (Tdi,) and diffuse (Tjif) transmission, respectively. Curves 3, 4, 5 depict calculated coherent transmission for a monolayer of homogeneous Cu spherical nanoparticles (3) and monolayers of two-layered particles with a Cu core covered with CU2O (4) or CuO (5) shell (shell thickness is 1 nm) surrounded by Si02 (20 nm), Relative surface concentration of particles is 0.72.

Figure 7,16. Variation of the relative surface concentration, y, of atomic nitrogen as a function of N2 exposure [33]. 1 L (Langmuir) = 10 torr-sec.

Three basic approaches have been developed In this study to eliminate these complications, each one being suited for a particular type of stationary phase. First, If only the surface concentration of the primary ligand Is of Interest, simple acid hydrolysis with dimerization Is used. This approach works best for determination of the primary ligand, e.g. Cg, and for the estimation of impurities present. The formation of TMS-Cg dimers will be minimal, provided the relative surface concentration of capping agent Is low. 

The mechanism of the reaction could be clarified by means of the surface science approach. Earlier investigations with the "real" doubly promoted catalyst had revealed that the probability for adsorption (i.e., the sticking coefficient) is only of the order 10 and that this process is activated [9]. In Fig. 2.2, the variation of the relative surface concentration of adsorbed N atoms with exposure to gaseous N2 at 693K on the Fe(l 10), (100), and (111) surfaces is shown. From these data initial sticking coefficients of 7 X 10 2 X 10 , and 4 x 10 were derived for the respective... 

Figure 6.11. Relative surface concentration of surfactant as a function of drying time, as determined by ESCA. The NP ethoxylate is nonylphenol ethoxylate (12 EO) and the amide ethoxylate is linseed oil fatty acid monoethanolamide ethoxylate (14 EO). (From K. Holmberg, Prog. Colloid Polym. Sci., 101, 69 (1996))...

In this model system, we may change the relative surface concentration by preparing the samples with different silane solution concentrations. At low concentration, there may be less surface density and the surface may be nonuniform of an "island type" [26]. The predominance of the hydrophilic silanol phase causes low... 

Fig. 6. Variation in the relative surface concentration y of atomic nitrogen with N2 exposure of different single-crystal surfaces at 693 K. (1 L = 10 Torr-s corresponds to the exposure equivalent to one monolayer). From Ref (21).

Figure 6 shows the variation in the relative surface concentration of adsorbed N atoms with N2 exposure at 693 K for the Fe(llO), Fe(lOO), and Fe(lll) surfaces from which data for the initial (i.e., extrapolated to zero coverage) sticking coefficient was found to vary from 7 x 10 to 2 x 10 to 4 x 10 for the respective surfaces (21). The value for Fe(lll) is of the same order of magnitude as that derived by Emmett and Brunauer (22) for the doubly promoted catalyst. Even more remarkably, these numbers are also of the same order as the reaction probabilities derived in Somoijai and co-workers work (26). This agreement shows that kinetic parameters derived from single-crystal studies are transferable across the pressure gap (rate measurements were performed at 20 bar, whereas that on adsorption kinetics at 10 mbar ) and are also consistent with the behavior of a real catalyst. 

The surface mass spectrum characterises the surface chemical structure. The spectral intensities can be used to determine the relative surface concentrations of the different surface species. Both positive and negative ion detection modes are possible in SIMS, as in all mass spectrometry techniques. A comparison of the positive and negative ion spectra can often substantially improve the analysis of the results. In SIMS, the charged fraction of the secondary particle flux is very small (10 ). Moreover, the number of sputtered ions per incident primary ion (i.e. the secondary-ion yield) is matrix dependent. With such yield variations direct quantification of surface species based on the number of desorbed secondary ions (i.e. from the SIMS data) is generally impossible [123]. Wucher et al. [143] have recently described a method to determine the secondary ion formation probability, i.e. the ionisation probability of sputtered particles in a direct and quantitative manner. 

Figure 5.4 Evolution of the relative surface concentration of hydroxyl groups as a function of degassing temperature...

Figure 3.8. Variation in the relative surface concentration y of Nad determined by Auger electron spectroscopy) with N2 exposure of different Fe single-crystal planes. 1 L (Langmuir) = 10" torrs is approximately the exposure required to saturate an adlayer if the sticking coefficient is unity.

In reacting systems, under special conditions the composition of the surface phase may also be controlled by long-range correlation effects. Due to differences in adsorption energies the relative surface concentration of different components is a function of temperature. Usually the surface concentration of one component, the man dominates, but the surface composition is a function of temperature. At a transition temperature (often the maximum in the rate versus temperature curve (see Figure 2.9), the mari starts to change. At this transition temperature several components will be coadsorbed in comparable quantities. 

The TiO configuration predominates in the whole range of the surface concentration studied. The contribution of the TiO-TiO and Ti20-TiO configurations is also important at too low Ni(II) surface concentrations. But this contribution drastically decreases as the Ni(II) surface concentration increases. The relative surface concentrations of the Ti20-... 

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