Highly platinum deposition

The platinum electrode is also very convenient for investigating various adsorption phenomena in electrochemical systems. The surface of platinum is very stable and reproducible. As will be shown in what follows, the true working area can be determined with high accuracy for platinum surfaces with appreciable roughness and even for electrodes with highly dispersed platinum deposits. It is comparatively easy to clean the surface of adsorbed impurities and to control the state of the surface. 

Recent studies performed with deactivated anodes show [55] that electroless or electrolytic platinum deposition on failed anodes, not only lowered the polarisation behaviour of these anodes (see Fig. 5.20), but also demonstrated an equivalent lifetime as that of a new anode in accelerated life tests in the sulphuric acid solution (see Fig. 5.21). These results unequivocally demonstrate that the deactivation of anodes, for which the Ru loading is still high, is a direct consequence of the depletion of Ru from the outer region of the anode coating. Note that this process of surface enrichment by conducting electroactive species will not lead to reactivating a failed anode, if there is a TiC>2 build-up at the Ti substrate/coating interface. 

The catalyst contained 10% platinum deposited on activated carbon. This high platinum content (20 times more than in the commercial catalyst) made the activity of the catalyst more stable, thus facilitating the studies of kinetics. The surface of platinum in 1 g of the catalyst measured by hydrogen chemisorption was 2 m2. Since the concentrations of H2 and NO in the... 

Sudi a Catalytic system has also been developed by Mobil with its MHTT process (MobO High Temperature IsomcrizationX It uses platinum deposited over a low add ZSM5 zeolite and is well adapted to feedstock having a high paraffins and ethylbenzene content. Paraffins are cracked and ethylbenzene hydrodealkylated. 

Similar results were found by Bozo [44]. Palladium deposited onto ceria-zirconia Ceo67Zro3302 solid solution showed very high activity in methane combustion (T50 close to 300 C) but similar to that of palladium deposited onto alumina. Like for the case of platinum a deactivation is observed during tests at temperatures comprised between 200°C and 400 C (Fig. 13.3). However when aged at 1000 C under an air+water mixture this catalysts showed superior resistance compared to classical catalysts as far as activity is considered. Despite a severe sintering of both metal (dispersion is now 1%) and support, whose surface area is close to 4 mVg, T50 was shifted to 420 C, i.e. 120°C only, still much lower for platinum deposited on the same support which showed a TSO close to 620°C. Calculation of specific activities in the 200-300°C range have clearly evidenced that ceria-zirconia support does not have any influence upon performance of PdO in... 

Figure 6.3 shows crystal-shaped platinum deposits juxtaposed to the computer simulations. Note that the computer simulation were calculated for a minority phase volume fraction of 12%, while depending on the assumed polymer densities, the copolymer PLA volume fraction lies between 36 and 39 %. As discussed previously, for these high volume fractions the computer model predicts that the < 110> directions form vertices and tlius, increase the number of faces surrounding the < 100 > vertices by a factor of two to a total of 8 facets. 

Basically, the construction of phosphoric acid fuel cells differs little from what was said in Section 20.4 about fuel cells with a liquid acidic electrolyte. In the development of phosphoric acid fuel cells and, two decades later, in the development of polymer electrolyte membrane fuel cells many similar steps can be distinguished, such as the change from pure platinum catalysts to catalysts consisting of highly disperse platinum deposited on a carbon support with a simultaneous gradual reduction... 

With in situ fabrication, the electrocatalytic material is created directly at the surface of the perfluorinated membrane. By the electrodeless method, platinum deposits are obtained after chemical reduction of chloroplatinate ions dissolved in a solution which is in contact with the membrane. A highly reducing substance, such as NaBH4 or N2H4, which is contained in the second aqueous side, diffuses through the membrane and reacts with the chloroplatinate ions. The concentrations of reagents are chosen so that the reduction takes place at the surface of the membrane . Aldebert coworkers obtained a localized precipitation of metal from a cationic platinum species, Pt(NH4)4, this cation being present in the membrane as a counter-ion. The chemical precipitation of reduced, metallic platinum occurs in the close vicinity of the membrane surface and not in the bulk. The reduction of cations which are present in the membrane can also be performed by electrochemical and radiochemical methods . 

Carbon black is favorable as a support material not only because of its high surface area and electronic properties, but it is also abundant, chemically inert, and environmental friendly (Bleda-Martinez et al., 2007). Carbon blacks are typically used as supports which are manufactured by the pyrolysis of hydrocarbons or oil fractions using oil furnaces or acetylene processes. Some of the most common carbon blacks used for platinum deposition in PEMFC catalysts are synthesized using the furnace method where the input materials are burned with hmited air at about 1400°C (Dowlapalli et al., 2006). Vulcan XC-72 and Black Pearl 2000 represent these types of carbon blacks. These carbon blacks are easily made and abundant making them popular choices for carbon black supports for Pt/C catalysts (Cameron et al., 1990). 

To increase the surface area of conductive diamond supports, a technique called vacuum annealing is utilized in place of doping that anneals un-doped nanocrystalline diamonds to make a conductive diamond. These diamonds, also termed nanodiamonds, are advantageous as catalyst supports because they have high surface areas created by the crevices and surface boundaries between the nanocrystallites. These surface defects acting in favor of platinum deposition however cripple the stability of the material compared to pure diamond. 

Much of the current research with conductive diamonds in PEMFC research is to develop platinum deposition techniques that can result in more uniform and smaller particle sizes on the conductive surfaces of diamond. However, conductive diamonds for catalyst support applications have often been used to examine the intrinsic properties of catalytic metals because of their inertness to electrochemical processes, lack of surface corrosion, and oxide formation. They remain strong candidates for fuel cell apphcations where catalyst integrity and durability are high priorities, and where typically carbon supports may fail due to the harsh operating conditions and high operating voltages. 

It was soon found, however, that catalysts in the form of platinum deposited onto carbon black (Pt-C) have a rather important defect, inasmuch as during long-term use in a fuel ceU, the surface of the carbon-black particles becomes oxidized, particularly so at the positive electrode, which upsets the contact between the platinum catalyst and the carbon black (in some cases the crystallites slip away from the carbon black). For this reason it became urgent to search for other supports for the highly disperse platinum catalysts. 

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