Degradation of catalyst

In general, the stability of a PT catalyst is a function of cation structure, presence of anions, type of solvent, concentration, and temperature. Degradation of catalysts under PTC conditions may occur. For instance, ammonium and pho.sphonium salts may be subject to decomposition by internal displacement (usually at temperatures of 100 - 200 °C) ... 

A lot of attempts have been made to describe the time dependence of the attrition rate in batch fluidized bed processes. Gwyn (1969) studied the degradation of catalysts in a small-scale test apparatus and defined the elutriated particles as the only attrition product. He described the increase of the elutriated mass, Wel, with time, t, based on the initial solid bed mass, Wbed 0, by the now widely known Gwyn equation ... 

Minisci and coworkers followed Ishii s procedure, and implemented it in the oxidation of benzyhc alcohols to benzaldehydes in almost quantitative yields" (Table 12). A,Af-dimethylbenzylamines were converted into aldehydes in good yields, by using catalytic amounts of either HPI or A-hydroxysuccinimide (HSI) for the formation of the corresponding aminoxyl radical intermediates. Because the attempted oxidation of primary and secondary amines caused the degradation of catalyst HPI, protection of the amino group in those substrates by acetylation was considered. This led one to develop... 

A sometimes nagging aspect of dioxirane-based oxidations is the degradation of catalyst. In this regard, Camell and co-workers <99TL8029> have reported on the use of A//-dialkyl-alloxan 163 as a particularly robust dioxirane precursor, which can be recovered in high yield with no evidence of catalyst decomposition. Attempts thus far to parlay this catalyst into an asymmetric induction paradigm (e.g., via 164) have been unsuccessful. 

The use of numerous polymer-supported optically active phase transfer catalysts was further extended by Kelly and Sherrington11351 in a range of phase transfer reactions including a variety of displacement reactions, such as sodium borohydride reductions of prochiral ketones, epoxidation of chalcone, addition of nitromethane to chalcone and the addition of thiophenol to cyclohexanone. Except in the chalcone epoxidation, all the examined resin catalysts proved to be very effective. However, with none of the chiral catalyst system examined was any significant ee achieved. The absence of chiral induction is a matter of debate, in particular over the possible reversibility of a step and the minimal interaction within an ion pair capable of acting as chiral entities in the transition state and/or the possible degradation of catalysts and leaching. 

First, this reserve activity could be utilized to offset thermal and contaminant degradation of catalyst activity. In simplistic terms, the 4300-cm3 production prototype converter could lose 70% of its catalytic activity (with a remaining catalyst volume equivalent to 1300 cm3) without suffering degradation in the overall control of exhaust HC and CO emissions. 

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). 

Research on the degradation of catalysts has mainly focused on low-temperature PEMFCs (< 90 °C) [28-42]. For high-temperature operation, studies on eatalyst degradation have been in the areas of phosphoric acid fuel cells (PAFCs) and PBI-based MEAs [41-43]. Since the catalysts used in PAFCs are the same as those in PEMFCs, the degradation mechanisms should be applicable to high-temperature PEMFCs. Normally, catalyst degradation includes two parts Pt catalyst degradation and carbon support oxidation. 

Water management, which can significantly influence the degradation of catalyst and membrane, is of vital importance for a PEMFC s lifetime. 

Physical Properties. The catalyst support material must be stable under process conditions and under the conditions used during start-up and shut-down of the plant. In particular, resistance to conditions during upsets may become critical. Degradation of catalyst may cause partial or total blockage of some tubes, resulting in the development of "hot spots , "hot bands" or totally hot tubes. Coking may cause similar problems. 

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