Pt particle size distribution

Figure 6. Pt particle size distributions measured from TEM images.

Pt particle size distributions (histograms) measured directly from TEM images are shown in Figure 6. 

Pt particle coalescence is due to migration. This mechanism is supported by observations that, upon cycling, Pt particle size distributions are shifted toward larger sizes, indicating that smaller particles are more mobile. It is noted that this observation could also result from the effects of Ostwald ripening. 

It has been well established that Pt can dissolve under oxidizing conditions, although the exact manner of how the species formed is a matter of debate at present. The formation of Pt crystallites in the membrane (or at the anode if no H2 is present) would indicate that micrometer transport of soluble Pt occurs. However, careful analysis of the Pt particle size distributions in the cathode after testing suggested that purely Ostwald ripening could not explain the observed distributions. Therefore, at present, it is concluded that a mixture of Pt dissolution/reprecipitation and Pt particle coalescence is responsible for Pt ECA loss. 

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. 

Figure 7. Pt particles size distribution obtained from TEM on AI2O3 support with a TEM image of the catalyst. (The researchers wish to express their gratitude to DARPA for funding Grant N66001-02-1-8942.)...

Preparation and characterization of the metal deposits. Pt deposit was made by impregnation with PtClg and reduction in H2at 753 K. The Pt particle size distribution was determined by transmission electron microscopy (TEM) (8, 9 ) (Figure 3) and H2, O2 chemisorptions and titrations (43). The Pt particle size distribution was narrow with a surface weighted mean diameter of ca. 2 mn, almost independent of the Pt content between 0.5 and 10 wt (9 ), provided the preparation method, which includes a treatment in O2 before the reduction, was thoroughly followed. 

Figure 2 Typical TEM images of (A) Silogel 5%Pt, scale bar= 5nm (B) A size histogram of the Silogel 5%Pt (acid pre-treated) showing the Pt particle size distribution of 3.42 0.43 nm...

Solvent-stabilized Pt and Pd nanoparticles, of sizes 2.3-2.8 and 2.7-3.8 nm, respectively, have been prepared by metal vapor synthesis routes and modified with Cnd. These catalysts were used in the hydrogenation of EtPy, and the Pd catalysts produced EtLa of inverse configuration, that is, instead of the expected ( )-EtLa, the (S)-EtLa was obtained. The Pt particle size distribution showed a higher degree of monodispersity after use in catalysis (Collier et al. 

Pt Particle Size, Distribution, Hydrogen Desorption Charge, and Roughness Factor for Pt/Diamond Composite Electrodes Before and After Secondary Diamond Growth... 

Figure 23.21. TEM image of membrane close to membrane/cathode interface (top) and Pt particle size distribution (bottom) from a MEA cross-section after AST voltage cycling xmder air flowing in cathode (bottom right) and N2 flowing in cathode (bottom left) [37]. (Reprinted by permission of ECS—The Electrochemical Society, from Li J, He P, Wang K, Davis M, Ye S. Characterization of catalyst layer structural changes in PEMFC as a function of durability testing.)...

Only a few publications have reported the stabihty of Pt/C catalysts for HT-PEM fuel cells, especially PA-doped PBl -membrane-based HT-PEM fuel cells. For example, Liu et al. [52] conducted a 600-hour lifetime test of a PA-doped PBI -membrane-based HT-PEM fuel cell by using commercially available Pt/C as both anode and cathode catalysts. The Pt particle sizes before and after the lifetime test were evaluated by transmission electron microscopy (TEM) measurements. Figure 10.10 shows the TEM images and Pt particle size distribution histograms of the Pt/C catalysts before and after the test. The TEM results reveal that the Pt particle agglomeration occurred at both the anode and the cathode, but more severely on the latter. For the fresh Pt/C catalyst, the Pt particle size distribution was relatively narrow, with a range of 2-5 nm and an... 

FIGURE 10.10 TEM images and Pt particle size distribution histograms of Pt/C catalysts before and after lifetime testing [52]. (For color version of this figure, the reader is referred to the online... 

Equation 2.101 enables calculation of local average quantities such as moments of the particle size distribution. Baldyaga and Orciuch (2001) review expressions for local instantaneous values of particle velocity and diffusivity of particles, Z)pT, required for its solution and recover the distribution using the method of Pope (1979). 

Figure 9. Schematic representation of the polyol process exemplified with Pt. TEM (left) shows a narrow particle size distribution (ca. 3 nm). (Reproduced from [223], 2000, with permission from Elsevier Science.) Experimental XPS curves (right) fit sufficiently well with the Pt(0) standard. (Reprinted from Ref [53], 2007, with permission from Wiley-VCH.)...

Pt/MWNT) [20,21], fine and homogeneous Pt nanoparticles deposited on MWNTs were obtained when pure EG was used as the solvent or less water (<5vol.%) was introduced. With the increase in water content, aggregation of the metal nanoparticles occurred, the average particle size increased and the particle size distribution became wider. 

These particle size distributions reveal that the average Pt particle size for all three samples prepared by sputtering was about 1.6 nm and independent of weight loading. 

Figure 7. Electron micrographs and relative histograms of the particle size distribution of 5% (w/w) of Pt/y-Al203 systems obtained (a) from the solution containing Pt particles with hydrodynamic diameter of 0.9 nm (b) from the solution containing Pt particles with hydrodynamic diameter of 1.5 nm.

Some of the reports are as follows. Mizukoshi et al. [31] reported ultrasound assisted reduction processes of Pt(IV) ions in the presence of anionic, cationic and non-ionic surfactant. They found that radicals formed from the reaction of the surfactants with primary radicals sonolysis of water and direct thermal decomposition of surfactants during collapsing of cavities contribute to reduction of metal ions. Fujimoto et al. [32] reported metal and alloy nanoparticles of Au, Pd and ft, and Mn02 prepared by reduction method in presence of surfactant and sonication environment. They found that surfactant shows stabilization of metal particles and has impact on narrow particle size distribution during sonication process. Abbas et al. [33] carried out the effects of different operational parameters in sodium chloride sonocrystallisation, namely temperature, ultrasonic power and concentration sodium. They found that the sonocrystallization is effective method for preparation of small NaCl crystals for pharmaceutical aerosol preparation. The crystal growth then occurs in supersaturated solution. Mersmann et al. (2001) [21] and Guo et al. [34] reported that the relative supersaturation in reactive crystallization is decisive for the crystal size and depends on the following factors. 

Figure 3. TEM images of Pt/H-MCM-22 calcined at 500 (A) and 300°C (B) after activation at 800°C in air flow. The particle size distributions for the two samples are reported in panels A and B , respectively.

Spray pyrolysis routes have been extensively investigated to prepare Pt-based catalysts. Typically, a liquid feed of metal precursor and carbon is atomized into an aerosol and fed into a continuous furnace to evaporate and heat-treat to form a collectable powder. The method has good control over final aggregate particle size and metal particle size distributions, as well as producing powder without further isolation or separation. Hampton-Smith et al. have reviewed efforts of Superior MicroPowder (now Cabot Fuel Cells) in this area. ... 

Kubelkova and coworkers studied the interaction of CO with platinum in a NaX zeolite [141]. They decomposed PtlNHj I]] on the zeolite under various conditions and observed distinct CO vibrational spectra for PtO-CO, Pt -CO, Pti -CO and [Pt(CO)2] complexes after CO adsorption at room temperature. Subsequent reduction with hydrogen resulted in characteristic particle size distributions with distinct CO vibrational frequencies depending on whether the clusters (-1.2 nm) were inside the supercages or outside the cages (-4.0nm). 

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