Bulk and surface compositions

One of the features of liquid as well as solid alloys is that their bulk and surface compositions are as a rule substantially different because one of the components is more surface active than the other. In the... 

Various pc electrode models have been tested.827 Using the independent diffuse layer electrode model74,262 the value of E n = -0.88 V (SCE) can be simulated for Cd + Pb alloys with 63% Pb if bulk and surface compositions coincide. However, large deviations of calculated and experimental C,E curves are observed at a 0. Better correspondence between experimental and calculated C,E curves was obtained with the common diffuse-layer electrode model,262 if the Pb percentage in the solid phase is taken as 20%. However, the calculated C, at a Ois noticeably lower than the experimental one. It has been concluded that Pb is the surface-active component in Cd + Pb alloys, but there are noticeable deviations from electrical double-layer models for composite electrodes.827... 

Further progress in the study of the Cu-Ni system awaited the preparation and careful characterization of alloy films of known bulk and surface composition. The essential step was taken by Sachtler and his co-workers 28, 88, 114) who prepared Cu-Ni alloy films by successive evaporation of the component metals in UHV. After evaporation the films were homogenized by heating in vacuum at 200°C. The bulk composition of the alloys was derived from X-ray diffraction, and the photoelectric work function of the films was also measured. A thermodynamic analysis, summarized by Fig. 13, indicated that alloy films sintered at 200°C should consist, at equilibrium, of two phases, viz., phase I containing 80% Cu and phase II containing 2% Cu. Evidence was presented that alloys within the... 

Table 18.3 Bulk and surface compositions of supported tungsten carbides...

Chemical analysis of the supported tungsten carbides allows quantification of the amount of carbidic and amorphous carbon,8 and in addition XPS measurements permit calculation of the carbide stoichiometry before and after FT reaction. This determination is not very accurate but it allows general trends to be discerned and correlations with catalytic properties. The bulk and surface compositions of supported tungsten carbides are shown in Table 18.3 in addition to their BET surface areas. 

In the case of TiC, further work on substrates with well characterised bulk and surface compositions is needed to elucidate the diverging results of the literature. Whatever the explanation, it has to be recognized that the wettability of TiC and TiN, and probably of other stable and oxidizable carbides and nitrides such as ZrC, ZrN, HfC and HfN, is very sensitive to the furnace atmosphere. 

Bulk and Surface Compositions. The chemical compositions of the molecular sieves used in this study are given in Table I in terms of tetrahedral atom (T-atom) fractions, and are grouped according to structure type. The bulk compositions of AIPO4-5, AlPO -20 and VPI-5 show the ideal 1 1 ratio of A1 and P characteristic of aluminophosphate molecular sieves. The SAPO materials have frameworks consisting of Si, A1 and P T-atoms. 

The results of recent studies summarized in Table 3 and briefly discussed in this section demonstrate the beneficial effects of promoting the VPO system for the partial oxidation of n-butane to maleic anhydride. However, the specific roles of promoters in modifying the morphology, phase and elemental (bulk and surface) compositions, structures and redox properties of the VPO catalysts at present are poorly understood. Improved fundamental understanding of the VPO promoter effects will enable rational design VPO catalysts with enhanced catalytic performance in n-butane oxidation to maleic anhydride. Therefore, detailed studies of several classes of well-defined promoted VPO catalysts containing promoters (1) in solid solution with the VPO lattice, (2) as surface species and (3) nanosized oxides or phosphates, etc., are expected to provide critical fundamental insights into the specific roles of key promoter species in selective oxidation of n-butane. 

Conventional synthesis methods offer limited control over desirable phase and elemental (bulk and surface) compositions, preferential exposure of active and selective surface planes, surface areas and pore structures of VPO catalysts, which define their catalytic performance in selective oxidation of n-butane. For instance, conventional methods produce VPO catalysts with relatively low surface areas (< 20 m7g) and a limited choice of crystal morphologies. 

The bulk and surface compositions of the samples as well as their structural and textural features have been determined by chemical analysis, BET, XRD, TEM and XPS measurements. 

Actual metal contents were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Metal particles were examined by X-ray diffraction, transmission electron microscopy and CO chemisorption. Details about the procedures used can be found elsewhere [9]. In the case of Pd-Ag/C catalysts, the combination of these three techniques enabled us to obtain the metal particles size and their bulk and surface composition [9, 13]. Finally, the Pt/C catalysts were tested for benzene hydrogenation, and the Pd-Ag/C catalysts were used to study mass transfer in the support during a well-known reaction the selective hydrodechlorination of 1,2-diehloroethane into ethylene. 

This type of reaction requires a complex, multifunctional catalyst with acid and hydrogenating properties. In a previous work we showed that these properties and hence the activity and the selectivity of the catalyst depended very much on the support and on the various promoters of copper which was chosen as a basic metallic element (1, 2). Furthermore the stability of the catalysts also depends largely on their composition and of certain products of the reaction such as water and ammonia (3). We have therefore studied the modifications of the bulk and surface compositions as well as those of the adsorption properties of supported (alumina or graphite) or promoted (barium) copper-chromium... 

The bulk and surface compositions of tungstates and molybdates can be modified through ionic exchange process, such as those used to obtain transition metal molybdate from sodium molybdate [5]. In this topic, as a specific example of preparation and uses of modified tungstates and molybdates, will be presented the preparation of Eu(III) compounds for optical purposes. 

While the above discussion has dealt with hquid systems, it is noteworthy that surface solution theory is also of interest for sohds. For example, the surface composition of metal alloy catalyst particles can differ significantly from the bulk composition. The proper design of the catalyst thus requires knowledge of how bulk and surface compositions are related. 

EDX analysis (1 pm depth) of the bulk and surface composition of the monolith and XPS analysis (4-5 atomic layers) of the monolith surface. 

Electrode Catalysts for Direct Methanol Fuel Cells, Table 1 Pt-Ru catalysts Pairing factors, bulk, and surface composition of ... 

So we compute maximum correction values for surface tension (DS) and ketone vapor pressure (PVAP) corresponding to a non equilibrium state where bulk and surface composition are equal. The adjusted corrections (Table III)amount to about 15% of their maximum values. 

Rogynskaya, Y.E., Morozova, O.V., Lubnin, E.N., Ulitina, Y.E., Lopukhova, G.V., and Trasatti, S. Characterization of bulk and surface composition of CojcNii mixed oxides for electrocatalysis. Langmuir 1997,13,4621-4627. 

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