Pt-specific activity

Electrocatalysts for the Oxygen Reaction, Core-Shell Electrocatalysts, Fig. 4 Pt specific activities ofPt(lll),PtML/Pcl(lll) and PtML/Pd/armealed/ PdsFefl 11) at 0.9 V versus RHE (Reprinted with permission from Ref. [28] (Copyright 2009 American Chemical Society))... 

Pt monolayer catalysts show a promising pathway toward solving one of the major problems facing PEM fuel cells by enhancing the Pt-specific activity and the utilization of Pt atoms and therefore reducing the cost of the cathode catalyst, although more fuel cell tests of durability are needed before the monolayer catalysts can be put in fuel cell vehicles. There is still a need for a reduction in the total noble metals in these catalysts. 

There are two ways one might think of that could help to reach that goal further increasing the platinum dispersion (defined as the ratio of surface metal atoms to total number of atoms) by making finer platinum particles, if there is no decrease of Pt-specific activity (activity per tmit Pt surface area) or alternatively, increasing the Pt-specific activity. One could also seek a combination of these two approaches. 

Nevertheless, facilitating the ORR kinetics by augmenting the Pt specific activity is important for the future of fuel cells. A recent perspective in Science by Gasteiger and Markovic [51] showed some promising advances in this aspect. 

Specific Activity (SA) and Mass Activity (MA) of Pt Electrocatalysts Supported on Different Carbon Powders Characterized by Specific Surface Area (S) and Particle Size (d)... 

This allows a direct influence of the alloying component on the electronic properties of these unique Pt near-surface formations from subsurface layers, which is the crucial difference in these materials. In addition, the electronic and geometric structures of skin and skeleton were found to be different for example, the skin surface is smoother and the band center position with respect to the metallic Fermi level is downshifted for skin surfaces (Fig. 8.12) [Stamenkovic et al., 2006a] owing to the higher content of non-Pt atoms in the second layer. On both types of surface, the relationship between the specific activity for the oxygen reduction reaction (ORR) and the tf-band center position exhibits a volcano-shape, with the maximum... 

Figure 8.12 Relationships between the catalytic properties and electronic structure of Pt3M alloys correlation between the specific activity for the oxygen reduction reaction measured experimentally by a rotating disk electrode on Pt3M surfaces in 0.1 M HCIO4 at 333 K and 1600 lev/min versus the li-band center position for (a) Pt-skin and (b) Pt-skeleton surfaces. (Reprinted with permission from Stamenkovic et al. [2007b]. Copyright 2007. Nature Pubhshing Group.)...

The mass activity MA (in A g ) of the Pt catalyst is, of course, the product of the specific activity js (in A m ) and the specitic surface area 5mass (in ni g ) MA = js mass- Because S ass is inversely proportional to the particle diameter dpt, the use of supported Pt nanoparticles is effective for increasing MA, if js is a constant independent of dpt- However, even at pure Pt, conflicting results on the values of js and P(H202) have been reported, suggesting the presence of differences in electrochemical properties between bulk and supported nanoparticles. For example, Bregoli [1978]... 

Figure 15.11 Specific activities for the MOR on Pt/Vulcan XC-72 (1) [Guerin et al., 2004] and Pt/GC (2) [Takasu et al., 2000] in aqueous electrolytes as functions of particle size. For curve 1, currents were measured after 1 minute at 0.6 V vs. RHE in 1 M CH3OH + 0.5 M H2SO4 at room temperature for curve 2, they were measured after 10 minutes at 0.6 V vs. RHE in 0.1 M CH3OH + 0.02 M HCIO4 at 298K. (Curves have been leplotted from Guerin et al. [2004], Copyright 2004 American Chemical Society and from Takasu et al. [2000], Copyright 2000, with permission from Elsevier.)...

II Umbilical cord, joint, and mucosal tract bleeding Prolonged prothrombin (PT) and activated partial thromboplastin time (aPTT) Specific assay for prothrombin... 

The enantioselective hydrogenation of a,p-unsaturated acids or esters, using 5wt% Pt/Al203 or Pd/Al203 commercial catalysts doped with cinchonidine (CD), was deeply investigated to evidence the specific activity of Pd or Pt and the role of the reaction parameters and solvent polarity. Finally, the steric and electronic effects of different substituent groups were also studied. 

Kubicka further reported that the specific activities of the metals for benzene hydrogenation fell in the sequence Ru > Pt > Tc Pd > Re. We note that, for the elements of the second transition series, the maximum activity was observed for the element of group VIIIi (group VIII2 was not studied). This should be compared with the results in Fig. 4 which show that the activities for the exchange reaction pass through a maximum at... 

Separation and Quantitation.—The specific radioactivity of [y-32P]ATP of very high activity (up to 240 Ci per millimole) may be measured by using it to phosphorylate [dT(pT)10] quantitatively, using polynucleotide kinase, isolating the labelled undecanucleotide, and measuring its activity.170 Isotope dilution is used to confirm the values. An alternative method of measuring specific radioactivities of ribo-nucleoside triphosphates utilizes a 3H-labelled nucleoside triphosphate (e.g. UTP) of unknown specific activity, a 14C-labelled nucleoside (e.g. ATP), a suitable primer in... 

The specific activation conditions required for an individual catalyst likely depend on the metal and support, but 300°C appears to be somewhat of a watershed temperature. Activation at temperatures above 300°C generally coincides with loss of Pt metal surface area due to sintering. The metal loading, dendrimer loading, and metahdendrimer ratios also impact activation conditions, suggesting that it may be necessary to optimize activation conditions for individual catalysts. Using temperatures at or near 300°C, supported pt,45,53,58,6i 45 pt Au, Pt-Cu,. and Ru ... 

Oxygen reduction specific activities of the low index planes of Pt and PtjNi in 0.1 M HCIO4 at 0.9 V versus RHE at 60°C. (From Science, 315, 493 (2007). Reproduced with permission from AAAS.)... 

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