Catalysis of Organic..

Auten, Crump, Singh and Chandler 317substantial particle agglomeration, particularly for Au-based materials, although poretemplating by the dendrimer mitigated the particle growth.Using Pt-DENs/SiO 2 , we showed that activation temperatures could be reduced to aslow as 150 °C by using CO oxidation reaction conditions. Supported Pt catalystspretreated in 1% CO & 25% O 2 at 150 °C for 16 hours had essentially the same COoxidation activity as catalysts oxidized at 300 °C for 16 hours. In this activationprotocol, CO essentially acts as a protecting group: strong CO adsorption preventsPt from participating in dendrimer oxidation and prevents dendrimer fragments fromfouling the nanoparticle catalyst.Results and DiscussionBased on our previous work developing activation conditions for supported PtDENs, we began investigating dendrimer removal from supported Au DENs underCO oxidation catalysis conditions (1% CO, 25% O 2 ). Au based catalysts areextremely sensitive to preparation techniques and sintering, so minimizing activationtemperatures is in important consideration for this system. Additionally, in spite ofthe potential for high CO oxidation activity, it is unclear whether Au NPs will becapable of participating in dendrimeroxidation. Consequently, we began our40study by directly monitoring CO35oxidation activity during catalyst150 ºC30pretreatment.In these experiments, shown inFigure 1, the CO oxidation reactorsystem was charged with supported, intact Au-DENs/TiO 2 , and CO oxidationactivity was monitored as a function oftime on stream at various temperatures(100, 125, and 150 ºC). The timerequired to reach maximum activityvaried from 8 hours at 150 °C to 24hours at 100 °C. We sought to keepactivation temperatures as low asTime (hours)Figure 1. Au-DENs/TiO 2 activationunder CO/O 2 atmosphere.possible to minimize sintering, so higher temperatures were not investigated.Additionally, activation under CO oxidation conditions relies on adsorbed CO to“protect” the metal particle surface from poisoning by dendrimer decompositionproducts. The “protective” properties of CO diminish as adsorption equilibira favorfree CO at higher temperatures.Rate (1/ min)2520151050125 ºC100 ºC0 5 10 15 20 25