Small Pt particles

In a sample of bulk Pt metal, all of the nuclei have the same interaction with the conduction electrons and thus see the same local field. The resulting NMR line is quite narrow. However, in our samples of small Pt particles, many of the nuclei are near a surface where the state of the conduction electron is disturbed. This tends to reduce the Knight shift for these nuclei. Since the Pt particles in our samples are of many different sizes and shapes, this reduction in the Knight shift is not the same for every nuclear spin near a surface. Thus, we obtain a broad "smear" of Knight shifts resulting in the lineshapes of Figure 5. 

The sfabilify of Pf particles during the 1.2 V hold has also been investigated. At 1.2 V and 80°C in 1 M H2SO4, up to 35% of the ECA was lost after 24 h. Transmission electron microscopy analysis of the tested catalysts found a growth in the Pt particle size distribution, suggesting that small Pt particles (-2 nm) are particularly susceptible to dissolution/agglomeration xmder steady-state voltage holds at 1.2 V. 

The catalysts used in low temperature fuel cells are usually based on small Pt particles dispersed on a carbon support with typical particle sizes in the range 1 — 10 nm in diameter. The XAS provides a measure of the average electronic state and local coordination of the absorbing atom, for example, Pt, on a per-atom basis, as described above. Thus, the XAS, for both the XANES and EXAFS regions, of such Pt/C catalysts reflects the size of the particles. 

On different metals the dominant mechanism may change. Thus, for Ir the selective cyclic mechanism (SCM) operates exclusively irrespective of particle size for Pd (particle size 9nm) there is 88% NSCM and 12% BS and for large Pt particles there is 9% NSCM, 7% SCM, and 84% BS, whereas for small Pt particles there is 83% NSCM and 17% BS. 

This paper provides a theoretical basis for understanding the metal support interaction. X-ray absorption spectroscopy (XAS) experiments are carried out in order to probe the empty part of the density of d states (DOS) of supported Pt particles. Small Pt particles on 3 different... 

After impregnation a Pt/A C catalyst is dried and calcined to produce supported Pt-oxide species (some Cl is retained on the alumina), which can then be reduced, usually at relatively mild temperatures (about 300-350°C) in hydrogen to produce small Pt particles (about 10 A). 

The term electron deficiency was introduced by Dalla Betta and Bou-dart to account for the anomalously high hydrogenation activity of small Pt particles in zeolite Y (50). The electron deficiency was ascribed to an electron transfer from small Pt particles to the zeolite. X-Ray absorption has been applied to measure the Pt Lm white line area as an indication of the electron deficiency because the white line is related to the number of unoccupied electronic states in the 5d and the 6j bands (273). For reduced Pt/NaHY it appeared that the white line area, and hence the electron deficiency of Pt particles, are closely related to the proton concentration of the zeolites. For example, the relative white line areas for Pt/H4gY, Pt/ H19Y, and Pt foil are 1.6, 1.2, and 1, respectively. White line areas at the Liii X-ray absorption threshold to determine the if-band occupancy of supported metal catalysts were first reported by Lytle 274). The use of the white line area as an indication for electron deficiency has been questioned by Lewis, who argues that a decrease of the metal particle size will also lead to an increase of the white line area (275). 

Previously Lankhorst et al. and Barbler et al. had found structure sensitivity i.e. small Pt particles are less coked than large Pt partlclos(lL),(12). In this context Che finding by Somorjai et al.(13),(14) is relevant that comer and edge atoms are less coked than metal atoms Inside crystal planes. 

The electron deficiency of small particles has been discussed by Boudart,33 and Sachtler.34 XPS measurements of the high electron binding energies in small Pt particles indicate that they are highly electron-deficient. In addition to this intrinsic character, there are strong interactions between metal particles and positively charged Lewis sites leading to the polarisation of Pt clusters to form local stable Pt Al(0)3 entities.35... 

Ad (a), as has been found by Bernard [39], platinum on the nonacidic zeolite KL is superior to conventional reforming catalysts in the dehydrocyclization of hexane to benzene (460 °C, molar ratio Ih hexane 6). It is proposed that zeolite KL is unique in its ability to prevent agglomeration of the small Pt particles required for the reaction. Chevron workers partially exchanged KL towards a Pt-BaKL catalyst [40]. The aromatization selectivity over Pt-BaKL was high and nearly constant (between 90 and 82 %) for n-alkanes having six to nine carbons. A process was developed by Chevron under the name AROMAX. 

Equilibria to methylcyclopentane and the methylpentanes play a role in the reaction network together with a route to benzene via cyclohexane. Initially shape selectivity exerted by the zeolite was assumed, but equally small Pt particles on magnesia gave similarly good results [41],... 

The change in morphology that small Pt particles supported on Ti02 undergo may also explain the apparent contradiction in the findings of... 

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