Fraction exposed

FIGURE 1.2 Estimation of particle size from the fraction exposed (FE). Dispersion (D) = percentage of atoms exposed, i.e., number of surface atoms/total number of atoms. Usually designated by %D, but sometimes by FE or %FE. 

FIGURE 1.6 Correlation of the ratio of isomerization to the addition of apopinenes with the fraction exposed (%D) Pt/SiO, and Pt/Al203. 

X-ray diffraction, 27 152 fee clusters, 34 249-250 FE, see Fraction exposed, of total metal atoms... 

CO adsorbed Pd/Al Oj, 37 69-70 diffuse reflectance spectroscopy, 34 14 Fowler-Nordheim equation, 21 72 Fraction exposed, of total metal atoms definition, 36 72-73... 

The degree of dispersion is defined as the fraction (percentage) of the total metal atoms which are surface atoms. (The term fraction exposed (percentage exposed) is synonymous with the degree of dispersion). 

Fig. 6.3 Binding energy shifts, SBE, of metal core levels versus fraction exposed (FE) and mean particle size, d, for various supported metal systems (data 6 and 8 are Pt-alumina). (Taken from Ref. 28.)...

Fig. 1. Some of the ways in which turnover frequency has been found to vary with fraction exposed and particle size (see text for a discussion of the curves).

We prefer the term fraction-exposed (96) to dispersion, since the latter conveys an idea of distribution in space that is not directly related to particle size and shape. The quantity FE is commonly measured by hydrogen chemisorption at room temperature, using the equation... 

Fig. 2. Binding energy shifts ABE of metal core levels measured by photoelectron spectroscopy (XPS) versus fraction exposed FE and mean particle size d for various supported metal systems (see Table II for details of the studies).

Fic. II. Turnover frequency TOF versus fraction exposed FE and mean particle size d for the hydrogenation of 1-butyne on Pd/Al203 at 20 °C and 20 atm in liquid n-heptane. From Ref. 266. 

Fig. 12. Turnover frequency TOF versus fraction exposed FE and mean particle size d for the hydrogenation of olefins on Pd/Al203 at 20°C and 20 atm in liquid zi-heptane. The triangles represent the isomerization of l-butene to 2-butenes (cis and trans), whereas the circles refer to the sum of isomerization and hydrogenation reactions. The squares refer to the partial hydrogenation of 1,3-butadiene to butenes. From Ref. 265.

The hydrogenolysis of propane has been studied over Ni/Si02 for a particle size range of 2.5 to 21 nm (276). The curve for TOF versus FE is quite similar to curve 6 of Fig. 14, for ethane hydrogenolysis on the same catalyst (275). The selectivities toward ethane or methane did not change with fraction exposed. 

Results for the turnover frequency for the production of i-butane from this reaction at 0°C have been reported as a function of fraction exposed over both Pt/Si02 and Pt/Al203 (277). For the alumina support the reaction is structure insensitive, whereas for the silica support the TOF for FE = 0.9 is about four times that at low dispersion. For both catalysts the activity is reduced as the solid is treated at higher temperatures (370°C, 480°C) in H2, followed by cooling in H2 to 0°C. Such differences arising from varying a pretreatment probably persist at 0°C but are less likely to do so for other reactions that occur at 200-400°C. 

Fig. 16. Hydrogenolysis of methylcyclopentane. Selectivity and mechanistic effects as a function of fraction exposed FE and particle size d (see Table IX for details of the studies).

The influence of the fraction exposed on the turnover frequency for the oxidation of H2 on Pt/Si02 catalyst is shown in Fig. 19. Hanson and Boudart (313) found structure-sensitive behavior when the reaction mixture contained excess H2 and structure insensitivity in mixtures with excess 02. This reaction (TOF = 10 s l) is very fast and must be studied at... 

Another support (0.97 m2/g) shows the TOF going through a minimum at about 70 nm, and there is some evidence that the selectivity shows a maximum at a similar fraction exposed. Note that Jaijoui et al. 

We summarize here what seem at this moment to be the ways in which turnover frequency may vary with fraction exposed. We note, however, that in the figures presenting the results there is often disagreement between studies on a given system concerning the shapes of the curves. Perhaps this discussion will encourage the future experimental resolution of some of these problems. 

In the present work we study the reactions of CO with preadsorbed oxygen and also the subsequent reaction of oxygen with preadsorbed CO. The catalyst is 0.12% Pt/Si O2 (Cabosil) which has a fraction exposed of 0.47. As the temperature is changed from 25°C to 192°C the surface C0/Pts ratio decreases from 0.86 to 0.68. However, at the same time the ratio of 0 atoms to Pt surface atoms changes from about 0.53 to 1.62. Thus at 192°C to ratio of 0 to total Pt atoms is 0.76. 

This work has been undertaken as part of a program to add to our knowledge of the oxidation of CO over Pt via models based on elementary steps (refs.1,2,3,4). Here we measure the reaction of C0(g) with adsorbed oxygen, and 0 (g) with adsorbed CO. These processes, although not elementary steps, are simpler to analyze than the full reaction. In the present study we are concerned only with the amounts of adsorbed CO and oxygen. As such, the measurements are equivalent to the titration of one adsorbed species by the other gaseous species. We are particlarly interested in the effect of temperature. The fraction exposed (FE) of the Pt particles supported on Si0 (Cabosil) may have an effect on the titrations also. 

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