Drying supported catalyst diffusion, effect

The use of supported liquid-phase catalysts (SLPCs) in organic reactions was mentioned in Chapter 6. The diffusion-reaction problem in such catalysts is considerably more complicated than that for solid catalysts (Rony, 1969 Abed and Rinker, 1973 Livbjerget al., 1974, 1976). In a more recent analysis of SLPC systems, it has been shown (Datta and Rinker, 1985 Datta et al., 1985) that a critical parameter is the ratio of the effective diffusivity of a liquid-loaded pellet to that of a dry pellet ( JcA.l/ ca)- This ratio is a strong function of the liquid loading q and the gas-liquid partition coefficient of reactant A, as shown in Figure 7.8. Thus the effectiveness factor is a function of liquid loading. 

This reaction is currently unavoidable and appears to be favored at hot and dry operating conditions of the fuel cell. The peroxide decomposition forms reactive radials such as hydroxyl, OH, and peroxyl, OOH, that cause oxidative degradation of both the fuel cell membrane and catalyst support [67]. Both electrodes currently use Pt or Pt alloys to catalyze both the HOR and ORR reactions. The catalyst particles are typically supported on a high surface area, heat-treated carbon to both increase the effectiveness of the catalyst and to provide a path for the electrons to pass through to the external circuit via the gas diffusion media (which is typically also made of carbon) and the current collecting bipolar plates. In addition, the catalyst particles are coated in ionomer to facilitate proton transport however, the electrode structure must also be porous to facilitate reactant gas transport. A schematic of a typical PEM MEA is shown in Fig. 17.1. A boundary condition exists at the catalyst particle where protons from the ionomer, electrons from the electrically conducting Pt and carbon, and reactant gases meet. This is usually referred to as the three-phase boundary. The transport of reactants, electrons, and protons must be carefully balanced in terms of the properties, volume, and distribution of each media in order to optimize operation of the fuel cell. 

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