Effects of Catalyst Properties on Thermal Stability

22 Effects ci support Principles associated with metal-support effects and their relationships to sintoing and redispersion phenomena are addressed by Bartholomew. An important and peiht obvious concept in regard to studies of model catalysts is that investigatitni of model catalysts, because of thdr lack rtf porosity and surface roughness, provides a more definitive measure of the relative strmgths of metal-suf rt interactions than study conventional supported metals. 

On the other hand, the oder of thermal stability for model Ag catalysts in vacuum is somewhat different than for Pt, namely Ag/silica Ag/alumina Ag/carbon but in agreement with the findings of Seyedmonir et al. who found that thermal stability of conventional supported Ag catalysts in Oj decreases in the order Ag/silica Ag/titania Ag/alumina.  

The effects of support on the breakup of metal films and aggregation of metal particles are also measures of thermal stability for supported metal systems. Two previous studies of model catalysts provide data in this regard. 

Aral and Nishiyama report that the pretreatment of carbon sipports significantly affects the temperature at which Pt films break up in vacuum to fc m metal crystallites. Their data summarized in Table 1 indicate that Pt films deposited on air- and H2-treated carbons break up at significantly higher temperatures (923 and 823 K) compared to that for untreated carbon (773 K). These results are consistent with those obtained for conventional Pt/caibcm catalysts showing that carbon pietreatirrent affects their sintering behavior.  

EssoitiaUy all of the model catalyst studies discussed thus far were conducted using smooth, low surface area supports, with one exception. Rai studied sintraing and redispersicm of a model Pt/alumina catalyst prepared using a ptxous anodized alumina with a surface area of 8 to 10 The model catalyst was sintered in vacuum by electrcMi beam heating during which 

---