Catalysis of Organic..

320 Dendrimer TemplatesWe encountered substantial difficulties in reproducing the A low-ox activity dataover the course of several weeks using the original batch of catalyst. Subsequentactivations yielded catalysts with lower activities than the freshly prepared andactivated material.Although supported Au catalysts are notorious for their problems withreproducibility, these results were surprising, since our previous work with supportedPt catalysts prepared from DENs was highly reproducible among different membersof the group.To examine the peculiarities associated with the Au/TiO 2 catalysts activated atlower temperatures, a second batch (B) was prepared by a second research studentusing identical procedures as the first. Beyond evaluating the general reproducibilityof the preparation method, the goal of these experiments was to determine if it isnecessary to activate supported Au DENs immediately after catalyst preparation.Additionally, we hoped to begin investigating the cause of the apparent catalystdeactivation.Catalyst batch B was separated into three parts: B low-ox was activated and testedimmediately after preparation, BBlow-ox+1mo was activated immediately afterpreparation but activity tests were performed one month later, and B 1mo+low-ox wasactivated and tested one month after preparation. During the waiting period,catalysts were stored in foil wrapped containers under air in a desiccator. The datafor catalysts A low-ox and B low-ox in Figure 3 and Table 1 show that both the COoxidation rates and E app values were highly reproducible between batches, providedthat the catalysts are activated and tested soon after preparation. Catalyst B low-ox+1mowas a factor of two less active than the freshly prepared and activated B low-ox ; catalystB 1mo+low-oxBshowed similar decreases in activity.The changes in apparent activation energies for the various samples offer initialinsights into possible changes in catalytic active sites. The A low-ox and A high-oxcatalysts had similar E app values of approximately 34 kJ/mole, while the A red catalyst,which underwent a reduction treatment prior to activity measurements, had an E appthat was more than a factor of two larger at 75 kJ/mole (Table 1). In our previouswork with Pt catalysts prepared from DENs, lower E app values for CO oxidationgenerally correlated with cleaner surfaces.(3,7) For the Au/TiO 2 catalysts, however,the higher temperature reduction treatments also reduced the overall catalyst activity.The magnitude of the change in E app for the A red sample, which had been reduced at300 ºC, suggests that the deactivation observed for this sample is likely due tosintering. This catalyst also visibly changes color, consistent with an increase in Aunanoparticle size.The apparent activation energies for the unreduced samples are all very similar.The samples pretreated with low temperature oxidation in the presence of CO all