Dispersion above surface measurement

As described above, XAS measurements can provide a wealth of information regarding the local structure and electronic state of the dispersed metal particles that form the active sites in low temperature fuel cell catalysts. The catalysts most widely studied using XAS have been Pt nanoparticles supported on high surface area carbon powders,2 -27,29,so,32,33,38-52 represented as Pt/C. The XAS literature related to Pt/C has been reviewed previ-ously. In this section of the review presented here, the Pt/C system will be used to illustrate the use of XAS in characterizing fuel cell catalysts. 

Datametrics Model lOOVT Air Flow Meter. Measurements were taken at various locations above the soil or water surface at a height of 0.8 cm, where the laminar air flow velocity was greatest. Depending on the probe location relative to the air dispersion tube, the measured wind speed varied from 0.5 to 1.5 m/s, with an average of 1 m/s. At greater heights above the surface, the air flow rate was much lower and the air flow patterns were unknown. 

Boudart and Cheng 1 concur this view in that cyclohexene hydrogenation 16 structure insensitive when carried out in the gas or liquid phase over Pt and Ni of widely differing dispersions (above say 14% or 29%) in that turnover numbers do not vary greatly over Ft (or Pd and Nl), This was even true when measured per free surface Pt atoms when the metal was progressively poisoned by sulphur in the sense that N was independent of the free Pt surface area provided this was correctly estimated after partial poisorting. This view is again consistent with the theme developed by the authors previously... 

With a supported catalyst system, the information derived by the above methods relates to the composition as a whole. A number of methods have been applied to measure the sizes of metal crystallites on the support surface, and hence estimate the dispersion or surface area of the metal itself. These include electron microscopy. X-ray diffraction and Mdssbauer spectroscopy. More direct measurements of metal surface area have been obtained from adsorption isotherms for hydrogen, carbon monoxide and other adsorbates, by assuming that all the exposed metal surface adsorbs such gases. GSC methods can again be applied. 

In conclusion to this section let us discuss the problem of interaction between a dispersion medium and filler. It was noted above that by varying intermoecular interaction on the filler s surface the yield stress may be varied over a wide range as a measure of strength of the structure being formed. 

Modification of filler s surface by active media leads to the same strong variation in viscosity. We can point out as an example the results of work [8], in which the values of the viscosity of dispersions of CaC03 in polystyrene melt were compared. For q> = 0.3 and the diameter of particles equal to 0.07 nm a treatment of the filler s surface by stearic acid caused a decrease in viscosity in the region of low shear rates as compared to the viscosity of nontreated particles more than by ten times. This very strong result, however, should not possibly be understood only from the point of view of viscometric measurements. The point is that, as stated above, a treatment of the filler particles affects its ability to netformation. Therefore for one and the same conditions of measuring viscosity, the dispersions being compared are not in equivalent positions with respect to yield stress. Thus, their viscosities become different. 

The solids analysis described above can be taken to yet another level by correlating the color measurement to chemical properties. An excellent model system is vanadium pyrophosphate (VPO), which is a well-known catalyst for butane oxidation to maleic anhydride. During the synthesis of the catalyst precursor, solid V2O5 particles are dispersed in a mixture of benzyl alcohol and i-butanol. In this slurry phase, the vanadium is partly reduced. Addition of phosphoric acid leads to a further reduction and the formation of the VPO structure. With a diffuse reflectance (DR) UV-vis probe by Fiberguide Ind., the surface of the suspended solid particles could be monitored during this slurry reaction. Four points can be noted from Figure 4.4 ... 

As has been discussed above, knowledge of the dispersion of the catalyst is extremely difficult to obtain, especially in the promoted systems. This is the main obstacle to clear differentiation between the promoter models proposed and remains so today. Chemisorption techniques to count the active sites present on the sulfided surfaces have had limited success. 02 chemisorption has been associated with edge vacancies (active sites) on pure M0S2 (100). However, the same authors showed that it was impossible to correlate activity and amount of 02 chemisorbed on Co- or Ni-promoted molybdenum sulfide (101). The use of other test molecules was disputable, particularly NO, which can strongly modify the structure of the surface during the measurement (102). 

Oxidation states of palladium-loaded Y zeolites were measured by ESR and IR spectrometry. After treatment by oxygen at 500°C the Pd is almost in the Pd(II) form, and few Pd (1%) are found in the Pd(III) form. After reduction by hydrogen at room temperature the Pd at zero oxidation state is almost atomically dispersed. The electron density of the Pd(0) is low because of its strong interaction with Lewis acid sites of the zeolite network it could even form Pd(I) (8%) (detected by ESR). This species is easily reoxidizable to Pd(II) by treatment in oxygen at 800°C. For reduction temperatures above 250°C, crystallites of metallic palladium are dispersed on the surface. 

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