Polytetrafluoroethylene catalyst

Oxygen reduction can be accelerated by an application of electrodes with high surface area, e.g. the porous electrodes [9, 13]. The porous electrodes usually consist of catalysts, hydrophobic agent (polytetrafluoroethylene-PTFE) and conductive additive. Electrode kinetics on the porous electrodes is complicated by the mass and charge transfer in the pores and is called the macrokinetics of electrode processes . 

Fluorinated polymers, especially polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP) and perfluorinated alkyl vinyl ethers (PFAVE) as well as other fluorine-containing polymers are well known as materials with unique inertness. However, fluorinated polymers with functional groups are of much more interest because they combine the merits of pefluorinated materials and functional polymers (the terms functional monomer/ polymer will be used in this chapter to mean monomer/polymer containing functional groups, respectively). Such materials can be used, e.g., as ion exchange membranes for chlorine-alkali and fuel cells, gas separation membranes, solid polymeric superacid catalysts and polymeric reagents for various organic reactions, and chemical sensors. Of course, fully fluorinated materials are exceptionally inert, but at the same time are the most complicated to produce. 

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

PS PSF PSU PTFE PU PUR PVA PVAL PVB PVC PVCA PVDA PVDC PVDF PVF PVOH SAN SB SBC SBR SMA SMC TA TDI TEFE TPA UF ULDPE UP UR VLDPE ZNC Polystyrene Polysulfone (also PSU) Polysulfone (also PSF) Polytetrafluoroethylene Polyurethane Polyurethane Poly(vinyl acetate) Poly(vinyl alcohol) poly(vinyl butyrate) Poly(vinyl chloride) Poly(vinyl chloride-acetate) Poly(vinylidene acetate) Poly(vinylidene chloride) Poly(vinylidene fluoride) Poly(vinyl fluoride) Poly(vinyl alcohol) Styrene-acrylonitrile copolymer Styrene-butadiene copolymer Styrene block copolymer Styrene butadiene rubber Styrene-maleic anhydride (also SMC) Styrene-maleic anhydride (also SMA) Terephthalic acid (also TPA) Toluene diisocyanate Ethylene-tetrafluoroethylene copolymer Terephthalic acid (also TA) Urea formaldehyde Ultralow-density polyethylene Unsaturated polyester resin Urethane Very low-density polyethylene Ziegler-Natta catalyst... 

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

Alkaline fuel cells have been used extensively on early spacecraft imtil they were superseded by more reliable solar cells. The high cost of the space cells and the use of corrosive compoxmds requiring special care in handling have been held against AFCs. Current AFC development employs multi-component electrodes using Ni for structural stability and as catalyst, carbon black as electron conductor and polytetrafluoroethylene (PTFE) pore-forming... 

One of the most successful industrial applications of polymeric catalytic membranes is the Remedia Catalytic Filter System to destroy toxic gaseous dioxins and furans from stationary industrial combustion sources by converting them into water, CO2, and HCl. The system consists of an expanded polytetrafluoroethylene (PTFE) microporous membrane, needle-punched into a scrim with a catalytically active PTFE felt. The catalyst is a V2O5 on a Ti02 support. The microporous membrane captures the dust but allows gases to pass to the catalyst where they are converted at temperatures as high as 260° C. 

Precursors and catalysts were characterized in ambient conditions by X-ray diffraction (XRD) on a Rigaku Powder Diffractometer using CuK radiation with a Ni filter. LiF was used as an internal standard for the activated catalysts. Laser Raman spectra (LRS) were collected using Ar ion laser excitation (514.5 nm) at a power of 25 mW at the sample. Spectra for the precursors were collected in ambient conditions, and reaction-used catalysts were characterized in-situ at 400°C in a 70 ml/min flow of C4H,(/02/He (0.99/10.2/88.81). Phosphorus to vanadium ratios (molar) were determined by inductively coupled plasma (ICP). Diffuse reflectance spectra (DRS) were collected in ambient conditions using polytetrafluoroethylene as a reference. 

Significantly stronger Lewis acid aluminum chlorofluoride (ACF, AlCLF ) was shown to be advantageous in promoting electrophilic [2 + 2] cycloadditions of hexafluoroacetone. ACF, known to be an effective catalyst for the condensation of halomethanes and fluoroolefins " with fluoroethylenes, was also found to catalyze the reaction of HFA with fluoroethylenes. For example, the reaction of HFA and TFE gives the corresponding F-(2,2-dimethyloxetane) 11 in moderate yield along with some polytetrafluoroethylene (Scheme 2.1) ... 

The reaction between hydrogen and oxygen produces water. The latter covers the surface of the catalyst and thus retards the reaction of water recombination. To avoid the formation of a water film on the catalyst surface, the catalyst carrier is treated with a hydrophobic substance, e.g. PTFE (polytetrafluoroethylene or teflon). The condensed water runs down draining paths back into the cell. The plug may be fitted with a stopper as well to prevent the H2SO4 electroljrte to get into the plug and to deactivate the catalyst. 

Arenes from Aliphatic and Allcyclic Precursors. —The formation of perfiuoromesityl-ene and perfluoropseudocumene by pyrolysis of polytetrafluoroethylene at 530— 560 °C over metal catalysts (Vol. 2, p. 359) has been patented,and improved yields of cyclic aromatic trimers have been obtained by coupling reactions of perfluoro-(l,2-di-iodocyclo-butene and -pentene) (see p. 74). i The formation of polyfluoronaphthalene derivatives from one of the bicyclic compounds (24) obtained during the reaction of carbon tetrachloride with 3,3,4,4-tetrafluorohexa-... 

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