Deactivation redispersion

Thermal Degradation and Sintering Thermally iaduced deactivation of catalysts may result from redispersion, ie, loss of catalytic surface area because of crystal growth ia the catalyst phase (21,24,33) or from sintering, ie, loss of catalyst-support area because of support coUapse (18). Sintering processes generally take... 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

Catalyst redispersion, 5 230-231 Catalyst regeneration, 5 202, 230, 255-322 catalyst deactivated by coke or carbon, 5 304, 309... 

It is worth mentioning that spontaneous monolayer dispersion is also a very useful scientific basis underlying the process of regeneration of deactivated metal catalysts. Supported metal catalysts may sinter during use at elevated temperatures. Sintering will cause the metal catalyst to lose initial activity, and in order to recover it one has to find an effective way to redisperse the metal on the catalyst support. Applying what we have learned from our studies on spontaneous monolayer dispersion to... 

Redispersion through an oxidation-reduction cycle as described previously is, indeed, an effective way to regenerate supported metal catalysts that have been deactivated because of sintering, and the underlying principle is spontaneous monolayer dispersion. 

Nevertheless efforts to understand, treat and model sintering/thermal-deactivation phenomena are easily justified. Indeed deactivation considerations greatly influence research development, design and operation of commercial processes. While catalyst deactivation by sintering is inevitable for many processes, some of its immediate drastic consequences may be avoided or postponed. If sintering rates and mechanisms are known even approximately, it may be possible to find conditions or catalyst formulations that minimize thermal deactivation. Moreover it may be possible under selected circumstances to reverse the sintering process through redispersion (the increase in catalytic surface area due to crystallite division or vapor transport followed by redeposition). 

Since the atomic migration and crystallite migration mechanisms have been amply discussed in the previous proceedings on Catalyst Deactivation, the emphasis of the present paper is on the wetting and spreading phenomena, which appear to play a major role in sintering and redispersion. 

Regeneration consists of (i) removal of inhibiting substances IS sometimes poisons, most often inhibitors or fouling agents, c.g., coke (ii) redispersion of the active species, or (iii) both. Regeneration procedures are often specific to the catalyst and the species causing deactivation. With respect to regeneration, procedures arc described precisely by catalyst manufacturers, for each type of important industrial catalyst. Procedures for... 

On the other hand, the possibility of reactivating aged Cu-ZnO catalyst must be considered. To restore the activity and stability of the catalyst the regeneration process must redisperse the metal while ensuring appropriate thermal stability. The most suitable method to produce these changes in the deactivated cat yst is to oxidize the copper at high temperature (623 K). 

The overall deactivation due to the HTR is reversed by oxygen treatment at 673 K. Hence sintering of the platinum particles is not responsible for the deactivation since redispersion is not likely to... 

The redispersion is carried out industrially treating the deactivated catalyst, after the coke has been burnt off, with a gas containing oxygen and chlorine at high temperatures. This process is called oxychlorination. The catalyst is treated with HCl or CCI4 at 450-500°C in a carrier gas with 2-10 wt% oxygen. This treatment is carried out during 1 to 4 h. 

Deactivated catalysts are reused after adequate regeneration processes either on-site or off-site, when possible and economical. Important examples are catalytic cracking (FCC), naphtha-reforming and HDS catalysts. The generation process to be employed differs depending on the kind of catalyst and cause of deactivation. In general carbon deposits are burnt off by air and metal deposits are removed by chemical treatment such as washing with sulfuric acid. Redispersion of sintered metal particles on support is sometimes possible. 

Deactivation of reforming catalysts derives mainly from carbon deposits and sintering of metal particles. So burn-off of the coke and treatments to redisperse the sintered metal are necessary. Temperature-programmed oxidation of coke formed on Pt/AhOs shows two peaks at 470 and 650K, which are respectively ascribed to cokes... 

The physical deactivation processes can be reversed to a certain extent, but only after the damage has been done. Cyclic treatment of crystallites in an oxidizing atmosphere and then a reducing atmosphere has been found to redisperse large crystallites. Dissolved metal atoms can be made to migrate back to the support surface by proper heat treatment. The deposited particulates can be removed either by combustion or by dissolving them with a suitable solvent. 

These studies focused on inverting only thermal deactivation in TWCs. Nevertheless, access of Cl to the washcoat components might be hindered by the accumulated contaminants deposited on aged TWCs. In this case, the redispersion process might be inhibited by... 

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