Platinum particle isolation

Directly applying Gubkin s concept of a plasma cathode, Koo et al. produced isolated metal nanoparticles by reduction of a platinum salt at the free surface of its aqueous solution [39]. The authors used an AC discharge as cathode over the surface of an aqueous solution of ITPtCk. Platinum particles with a diameter of about 2 nm were deposited at the plasma liquid electrolyte interface by reduction with free electrons from the discharge. 

The linear CO stretching frequency for the carbonylated platinum colloid while lower than that found for surface bound CO, is in the range reported for the platinum carbonyl clusters [Pt 3 (CO) 6 ] n / sind we find that the carbonylated colloid is easily transformed into the molecular cluster [Pt 12 (CO) 24 ] (10) reaction with water. The cluster was isolated in 50 yield based on platinum content of the precipitate by extraction with tetraethylammonium bromide in methanol from the aluminum hydroxide precipitated when water is added to the aluminoxane solution. The isolation of the platinum carbonyl cluster reveals nothing about the size or structure of the colloidal platinum particles, but merely emphasizes the high reactivity of metals in this highly dispersed state. The cluster isolated is presumably more a reflection of the stability of the [Pt3(CO)6]n family of clusters than a clue to the nuclearity of the colloidal metal particles - in a similar series of experiments with colloidal cobalt with a mean particle size of 20A carbonylation results in the direct formation of Co2(CO)8. 

We have observed that carbon-supported nanoscale platinum electrodes retain quite closely the Pt NMR spectral characteristics of isolated small platinum particles supported on oxides [4, 7, 25, 30], with the clean-surface platinum atoms also resonating at 1.100 G kHz with respect to the value for bulk atoms, 1.138 G kHz . We show in Fig. 4(a-c), Pt NMR spectra... 

Furthermore, alkane hydrogenolysis, an undesired side reaction, is often encountered over pure platinum particles. It is known that hydrogenolysis is a demanding reaction requiring at least two adjacent platinum atoms. Therefore, alloying the noble metal by an inactive metal such as copper appeared as an appropriate solution to this problem. In addition, because alloying did not mo dify the size of the bimetallic particles, it increased the total number of those particles and, therefore, the overall number of isolated siuface platinum atoms. 

Current-voltage curves of this MEA before and after long-term operation are shown in Fig. 3. The slope in the Tafel region seems to be unchanged and to slide downward by 50 mV. Since the Tafel slope is approximately -87 mV per decade, the voltage drop of 50 mV corresponds to the decrease in effective surface area by approximately 1/3.8. Various factors are considered as a reason for the decrease in effective surface area sintering of platinnm particles, dissolntion of platinum, electrical isolation, and isolation from reactants. In Fig. 3, the cnrrent density at which the... 

Th was isolated with a minimum amount of carrier from a solution containing 100,000 c.p.m. of U2 0 (which had been produced by the irradiation of thorium with 100 Mev a particles in a cyclotron) and mounted on a platinum disc. By inserting this disc In a standard a-eountlng chamber for a few minutes,... 

The existence of these highly mobile quasi-isolated atoms (Fig. 10) could provide new possibilities for catalytic reactions, favoring, for instance, the occurrence of the nonselective cyclic mechanism. Although the potentials used in Hoare and Pal s calculations (Lennard-Jones and Morse-Mye) may be considered as unsuitable for metal clusters, recent calculations (181), made with more realistic potentials, indicate that below 15 A, the clusters with icosahedral symmetry are more stable than the fee cubooctahedra. Unfortunately, the size range in which the polyhedral metal clusters are supposed to be stable does not allow microdiffraction studies. Moreover, when platinum is deposited on an oriented rock-salt face, pseudocrystals with uncommon symmetry are present only in very small amounts, and in a particle size range of 80-120 A, where they obviously result from multiple twinning of fee tetrahedra (182). 

A good approximation to the system might be to consider an array of stationary platinum microelectrodes surrounded by spherical diffusion fields. If we neglect the interaction of these diffusion fields for the moment and begin by considering the flux of MV+ to an isolated spherical metal particle of radius r ... 

Extended X-ray absorption fine structure (EXAFS) measurements for Pd-Pt systems prepared either by direct redox or refilling method indicated the presence of platinum in the vicinity of palladium for both preparations [13]. Transmission electron microscopy (TEM) coupled with energy dispersive X-ray (EDX) microanalysis was carried out on Pd or Pt modified by Cu, Ag and Au [35], Pd-Sn [38], Pd-Cu [36], Rh-Ge [41], Pt-Ru-Sn and Pt-Ru-Mo (Fig. 9.5) [76], Pd-Sn-Au and Pt-Sn-Au [46] to collect more information on metallic particles in terms of particle size and composition. The catalysts prepared by redox reactions showed that the parent metal is always associated to the additive, but can be found isolated for supports which are able to adsorb the modifier. 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

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