Oxidation-resistant supports

The employment of suitable organic solvents, such as acetonitrile and acetic acid, with oxidation-resistant supporting electrolytes permits the anodic formation of reactive radical cations from many organic materials. Most aromatic compounds and olefins, as well as those alkanes which have particularly weak C—H bonds, are oxidised in acetonitrile containing fluoroborate or hexafluorophosphate electro-lytes. °" 2 Some aromatic radical cations can be further oxidised to dications within the available potential range. Radical cations in general either deprotonate or attack nucleophiles present in the medium reactions with pyridine, methanol, water, cyanide ion, acetate ion or acetonitrile itself produce addition or substitution products. The complete reactions involve a second electron transfer and coupled chemical... 

For a low-temperature PEMFC, the most common problems for reliability and durability are degradation of catalysts and catalyst support oxidation. A number of papers have been published in this area, the details of which will be given in Section 18.2.2. It is expected that at high temperatures, these problems will be more pronounced. Recently, however, high-temperature PEMFC catalysts have begun to attract researchers attention, and several papers have been published. Liu et al. [24, 25] developed a Pt4Zr02/C catalyst and found that it was more durable than Pt/C in PEMFCs operated at 150 °C. Other research activities have mainly focused on the development of oxidation-resistant supports (Section 18.2.3). 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

The photovoltaic effects found in model arrangements of sensitized systems support this mechanism 40,90,101,106). Moreover, measurements of the Dember effect 103- 5) an increase of zinc oxide resistance by low coverages i°2> — and other observations 107> confirm this mechanism. Therefore, we can assume a correlation between the semiconductivity and sensitizing behavior of dyes, according to which the sensitizing behavior of dyes depends upon... 

As can be seen by inspection of Table 1, the anodic and cathodic limit of a particular SSE depends on an electrochemical process involving either solvent or supporting electrolyte. In for example acetonitrile the anodic limit is dependent on the nature of the anion and the cathodic limit on the nature of the cation, whereas in dimethyl sulfoxide (see Table 2) the anodic limit is due to oxidation of solvent in cases where an oxidation-resistant anion is present, and the cathodic limit is dependent on a process involving reduction of the cation. Thus, one can order anions in a series of increasing resistance towards anodic oxidation ... 

These various possibilities for oxidation-resistant 1,2-diols demonstrate the need for caution in the interpretation of the results of periodate and lead tetraacetate oxidation. As in the studies of D-glucosan < 1,4 > 0-< 1,6> and D-galactosana, supporting evidence based on other chemical reactions and the formation of derivatives should be obtained for the choice between absence of adjacent hydroxyl groups and presence of resistant 1,2-diols. 

Figure 19 Schematic Bode plots from EIS measurements and equivalent circuits that could be used to fit them for various possible corrosion product deposit structures (A) nonporous deposit (passive film) (B) deposit with minor narrow faults such as grain boundaries or minor fractures (C) deposit with discrete narrow pores (D) deposit with discrete pores wide enough to support a diffusive response (to the a.c. perturbation) within the deposit (E) deposit with partial pore blockage by a hydrated deposit (1) oxide capacitance (2) oxide resistance (3) bulk solution resistance (4) interfacial capacitance (5) polarization resistance (6) pore resistance (7) Warburg impedance (8) capacitance of a hydrated deposit.

Alloy 602 CA a modification of other 600 alloys is used in heat-treating, annealing furnaces, furnace rolls, high-temperature calcination application, catalytic support systems, and glow plugs in automobiles, vitrification of nuclear waste 800, 45 TM Have high creep and rupture strength and oxidation resistance used... 

Non-oxide ceramic materials such as silicon carbide has been used commercially as a membrane support material and studied as a potential membrane material. Silicon nitride has also the potential of being a ceramic membrane material. In fact, both materials have been used in other high-temperature structural ceramic applications. Oxidation resistance of these non-oxide ceramics as membrane materials for membrane reactor applications is obviously very important. The oxidation rate is related to the reactive surface area thus oxidation of porous non-oxide ceramics depends on their open porosity. The generally accepted oxidation mechanism of porous silicon nitride materials consists of two... 

CH4 oxidation has been experienced for ceria supported on a barium hexaaluminate, an heat resistant support. Preparation by a new reverse microemulsion method leads to ceria nanoparticles deposited on support and having a BET area close to 100 mVg after calcination at 1000 0 [72]. Such ultrahigh disperse nanoparticles show exceptional thermal resistance the authors mentioned that ceria particles prepared with a size of 6 nm sinters only to 18 nm after a calcination at 1IOO°C under a water containing atmosphere. Of course excellent activity in methane combustion has been observed. According to their experimental conditions calculated specific activity expressed as mol(CH4).h. m was estimated to 6.4x10 at 500°C whereas Bozo [44J reported a value of 1.5x1 O at the same temperature both values look similar. Thus the difference in methane conversion may be related to BET area only which is spectacularly preserved using the reverse micro-emulsion method for synthesis. 

Also, Marsh and co-workers [145] showed that gold on cobalt oxide particles, supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania remains active in a gas stream containing 15 ppm SO2. Haruta and co-workers [207] found that although the low-temperature CO oxidation activity of Ti02-supported Au can be inhibited by exposure to SO2, the effect on the activity for the oxidation of H2 or propane is quite small. Venezia and co-workers [208] reported that bimetallic Pd-Au catalysts supported on silica/alumina are resistant to sulphur poisoning (up to 113 ppm S in the form of dibenzothiophene) in the simultaneous hydrogenation of toluene and naphthalene at 523 K. 

The initial microporous support films used in the work were made from cellulose acetate by a modification of the Loeb-Sourirajan procedure. Later work showed that several types of the membrane filters manufactured by Millipore Corporation and Gelman Sciences, Inc., performed as well and allowed higher flux. A continued search for a more compression-resistant support film led to the development of polycarbonate, polyphenylene oxide and polysulfone microporous films in 1966 to 1967 (8). Of these, microporous polysulfone film proved to have the best properties. The polysulfone support was made by casting a liquid layer of a 12.5 to 15 percent solution of Union Carbide Udel P35OO polysulfone in dimethylformamide onto a glass plate at 4 to 7 mils (100-175 pm) thickness, then coagulating the film in water. 

The mechanical strength of the fluidized catalyst is a significant process issue that must be solved. To address this operational problem, a two-step approach was taken to (1) identify and develop economical and attrition resistant support materials that could withstand high-temperature fluidization, and (2) prepare reforming catalysts from the best supports. The catalysts, containing nickel oxide (NiO), manganese oxide (MgO) and/or potassium oxide (K2O), were then evaluated for attrition resistance and activity in a fluid bed system. 

On the other hand, Kato et al. (1988) studied the addition of different rare-earth oxides to V-AI2O3 in a 5/95 ratio. They found that in those systems where (1-aluminates were detected after firing at 1473 K the surface area was always higher than in those cases where only (X-AIO3 was detected. They concluded that the common feature of thermally resistant supports is that they have a layered aluminate structure which retards support sintering. 

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