Catalyst deactivation thermal aging

With amorphous silica-alumina catalysts [5,6], the primary mode of aging involves steam-induced loss of surface area by the growth of the ultimate gel particles, resulting also In loss of porosity. While amorphous catalysts deactivate thermally as well as hydrothermally, thermal deactivation is a significantly slower process. 

Besides the prediction of calcination temperatures during catalyst preparation, thermal analysis is also used to determine the composition of catalysts based on weight changes and thermal behavior during thermal decomposition and reduction, to characterize the aging and deactivation mechanisms of catalysts, and to investigate the acid-base properties of solid catalysts using probe molecules. However, these techniques lack chemical specificity, and require corroboration by other characterization methods. 

Recent evaluations of S02 oxidation over noble metal catalysts (Pt, Pd, and Rh) have given some information on one particular secondary reaction. It was observed in car tests that S03 formation under the conditions of automobile exhaust is highly vulnerable to catalyst deactivation either by thermal sintering or by poisoning (78, 79). At the same time, the data indicated a lesser sensitivity of CO and hydrocarbon oxidation to catalyst aging. The results were confirmed in laboratory experiments (80). This is one example of preferential suppression of an undesirable side reaction. Obviously, the importance of a given poison on the different secondary reactions will vary widely with catalyst formulation and operating conditions. 

In order to suggest an efficient catalyst system for automotive emission control, Pd-W03 and Pd-La203 catalysts were characterized before and after thermal aging and phosphorous contact, li was found that deactivation of Pd-W03 involves severe metal vaporization during ihermal aging. On the other hand, Pd-La203 has exceptional thermal stability while it is poisoned by 1-5 wi. phosphorous,... 

Environmental catalysis is required for cleaner air, soil, and water. Various catalysts are in use to improve and/or protect our environment. Catalysts are used in environmental technologies to convert environmentally hazardous materials into harmless compounds. Deactivation of the environmental catalysts occurs as a result of thermal aging, physical and chemical poisoning, and masking mechanisms. Regeneration procedures, which include thermal, physical, and chemical treatment have been developed in order to extend catalyst life. 

Thermal aging, S02 poisoning, and reaction of the active phase with the support are factors that are known to be important in the deactivation of solid catalysts. Noble metals are usually poisoned by lead, whereas base metal oxide catalysts are more susceptible to poisoning by sulfur (88,286-289). Indeed, the deactivation of oxides when used in oxidation or reduction processes and particularly as catalysts for exhaust gas purification has been attributed to a large extent to S02 (14,174,290). In this section, some aspects of the S02 poisoning effect and the nature of the interactions of S02 with perovskite oxides are reviewed. 

Fresh and thermally aged catalysts containing mixtures of platinum and palladium were laboratory tested for the oxidation of carbon monoxide, propane, and propylene. For both monolithic and particulate catalysts, resistance to thermal deactivation was optimum when palladium content was 80%. Full-scale vehicle tests confirmed these findings. Catalysts of this composition were developed which, on the basis of durability tests at Universal Oil Products and General Motors, appeared capable of meeting the 1977 Federal Emissions Standards with as little as 0.56 g noble metal per vehicle. The catalyst support was thermally-stabilized, low density particulate. 

Although the in-situ XRD study suggests that the Cu/beta SCR catalyst is thermally stable up to 800 °C, it deactivates noticeably when the catalyst is hydrothermally treated at lower temperatures but for a longer period of time. As shown in Fig. 5.3, the Cu/beta catalyst can achieve >90 % NOx reduction efficiency under typical diesel application conditions with a temperature between 200 and 300 °C and a Space Velocity (SV) of 30,000 h when it is relatively fresh (labeled as degreened in Rg. 5.3). After being hydrothermally treated at 670 °C for 16 h in a flow of 4.5 % H20/air mixture, the catalyst starts to lose its NOx conversion efficiency at temperatures below 350 °C. Further extending the aging time to 64 h, which could be considered as the end of useftd life of the catalyst from a hydrothermal durability requirement point of view, the catalyst can barely reach 90 % NOx conversion. Such an extent of catalyst deactivation is not acceptable for real-world applications. 

Finally, certain aspects that have not been covered in this chapter include coupled NH3 and hydrocarbon SCR and SCR catalyst poisoning and aging/ deactivation. Understanding and hopefully predicting the useful life of these catalysts is paramount. A molecular-level understanding of the mechanisms of hydrocarbon and sulfur poisoning and thermal degradation relies on mechanistic-based kinetic models. 

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). 

---