Dendrimer decomposition

Catalyst Activation Gas phase activation of supported DENs was examined using in-situ FTIR spectroscopy and FTIR spectroscopy of adsorbed CO. For in-situ dendrimer decomposition studies, the spectra were collected under a gas flow composed of 20% 02/He or 20% H2/He. The supported DEN sample was pressed into a self-supporting wafer, loaded into a controlled atmosphere IR cell, and collected as the sample background. The temperature was raised stepwise and spectra were collected at each temperature until little or no change was observed. After oxidation, the sample was reduced in 20% H2/He flow with various time/temperature combinations. The sample was then flushed with He for lhr at the reduction temperature. After cooling under He flow, a background spectrum was collected at room temperature. A 5% CO/He mixture was flowed over the sample for 15 minutes, followed by pure He. IR spectra of CO adsorbed on the catalyst surface were collected after the gas phase CO had been purged from the cell. 

FIGURE 7.2. Typical in-situ dendrimer decomposition experiment in 20% O2. The supported DENs were pressed into a self-supporting wafer, loaded into the IR cell, and temperature was increased at approximately 5°C/min. The first 5 spectra shown are at intervals of approximately 30°C the bottom three spectra was collected after soaking at 300°C for 2 h. 

In-situ infrared spectroscopy has been used in much the same fashion at TGA, but temperature profiles have been combined with monitoring changes at constant temperature. " IR spectroscopy does not yield the same direct information about the complete removal of organic residues that TGA provides. On the other hand, CO adsorption experiments performed along with dendrimer decomposition experiments provide direct information regarding metal availability. Further, IR experiments provide... 

Crooks and coworkers, who studied Pd and Au DENs immobilized in sol-gel titania, similarly reported the onset of dendrimer mass loss at relatively low temperatures (ca. 150 °C). Pd helped to catalyze dendrimer decomposition in their system, as well. Temperatures of 500 °C or greater were required to completely remove organic residues from their materials. (10) This treatment resulted in... 

In order to better understand changes to the catalyst surface during activation, infrared spectroscopy was used to monitor the dendrimer decomposition process. IR spectra of intact Au-DENs/TiOz are... 

Dendrimer decomposition was also monitored during activation. 

The persistence of the dendrimer decomposition products is the likely cause of the catalyst deactivation over time. The presence of dendrimer and dendrimer byproducts indicates that even the more active catalysts are not particularly clean. It is difficult to distinguish between species adsorbed on the NPs from those primarily on the support however, it is likely that the location of the dendrimer decomposition varies widely along the surface of the catalyst. The dendrimer fragments present on the support could migrate over time and poison the metal active sites, resulting in the lower catalytic activity over time. It is also possible that the residual dendrimer undergoes some slower oxidation processes that result in a stronger, unobservable poison. 

Several studies have shown that the amide bonds that comprise the PAM AM dendrimer backbone are relatively unstable and begin decomposing at temperatures as low as 75 °C [45,50,52,56-58]. The low onset temperature of dendrimer decomposition is not surprising given that PAMAM den-drimers can undergo retro-Michael addition reactions at temperatures above 100 °C [16]. Far more forcing conditions are required to fully activate the catalysts, which suggests that the dendrimer decomposes into various surface species that continue to poison the nanoparticle surfaces. 

This low temperature CO/O2 treatment was also used for Au/Ti02 DENs for some pretreatment temperatures, the Au/Ti02 catalysts were more than an order of magnitude more active for CO oxidation than Pt/Si02 [88], The Au catalysts also showed substantial room temperature activity that was extremely reproducible. However, the catalysts were not stable over time, presumably due to dendrimer decomposition products migrating onto the Au nanoparticles and poisoning the catalysis. 

Thermogravimetric analysis measurements have also provided information regarding dendrimer decomposition. Thermolysis of G40H dendrimers on a gold surface in both air and argon environments has shown that the greatest weight loss... 

Dendrimer thermolysis experiments conducted on Pt/AljOj catalysts show that 400°C is the lowest temperature that will result in the removal of adsorbed carbox-ylates. It is not until these carboxylates are removed from the surface that the platinum nanoparticles exhibit significant CO uptake, indicating that the dendrimer decomposition products, possibly including undetectable nitrogen moieties, interfere with the metal active sites (Figure 9.8). ... 

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