Catalysis of Organic..

318 Dendrimer TemplatesThe differences between catalysts treated at 125 and 100 ºC suggest that loweractivation temperatures might produce catalysts that are more active. This data iscollected with different conversions, at different temperatures, and with small massesof catalyst, so observed rates in these experiments are only qualitative activitymeasures. Variances in surface water content, which might be substantial at thesetemperatures, also can dramatically affect the activity of supported Au catalysts.(11)Consequently, it is difficult to draw meaningful conclusions regarding catalystactivity from the low temperature activation data.Rate (1/ min)9.08.07.06.05.04.03.02.01.00.0AA lowox A redA high ox20 40 60 80 100 120 140Temperature (°C)ln (Rate)2.52.01.51.00.50.0-0.5-1.0-1.5BA high oxA lowoxA red-2.02.4 2.6 2.8 3.0 3.21000/KFigure 2. CO oxidation catalysis by Au/TiO 2 for various pretreatments. (A)rate vs. temperature plots and (B) Arrhenius plots.For further studies, we focused on catalysts activated at 150 °C as a model forthe low temperature activation conditions. Figure 2a shows carefully determinedcatalytic activity data for Au/TiO 2 catalysts activated with the low temperatureprotocol (16 hours at 150 ºC in 1% CO and 25% O 2 ) versus activation treatmentspreviously developed in our lab using other supported DENs. These previouslydetermined conditions included a 6 hour oxidation with 25% O 2 at 300 ºC (A high-ox )and a 4 hour oxidation with 25% O 2 plus two hour reduction with 25% H 2 at 300 ºC(A red ). For the Au/TiO 2 catalysts, activated with these protocols, only minimaldifferences in reactivity were observed.Catalyst A low-ox was substantially more reactive than catalysts prepared withshorter, high temperature treatments (rates were roughly 6 times faster with the A lowoxpretreatment at about 80 ºC). Although the Au/TiO 2 catalysts prepared fromDENs are more active than the supported Pt catalysts we have previouslyexamined,(6) 1 they are not as active as the best literature reports for CO oxidation bysupported Au catalysts prepared by other means. Arrhenius plots using rate databetween 2 and 12% conversion for the Au/TiO 2 catalysts activated with these three