Titania selective poisoning

The influence of titania can be isolated by selectively poisoning the titania-associated sites. When the Cr(VI)/silica-titania is treated with CO at 100-150 °C, the more reactive, low-MW producing sites are preferentially reduced first (i.e., those associated with titania). If the catalyst is cooled in CO, those same reduced sites adsorb CO and are deactivated, while other sites are unaffected (Scheme 15). By analysis of the polymer from such partially poisoned catalysts, the influence of titania becomes more apparent. 

A similar experiment was conducted without 1-hexene added to the reactor and those results are also listed in Table 36. The same effect on MW distribution was observed. The incorporation of 1-hexene increased significantly when the titania-associated sites were selectively poisoned. This is another indication that titania inhibits branching in the low-MW side. Table 36 shows the drop in the polymer density, and the rise in the amount of 1-hexene incorporated, when the catalyst was treated in CO at only 100 °C. 

As noted in Section 11.11, the presence of titania on the catalyst can increase the elasticity of the polymer. The JC-ot values characterizing the polymers from these two series of experiments are also listed in Table 36. There was a steep drop in the polymer LCB level when the catalyst was partially poisoned, for both copolymers and homopolymers. Further reduction and poisoning continued to lower catalyst activity, but had no further effect on LCB levels in the polymer. The CY-a parameter, another indicator of LCB levels (Section 9), was measured for these polymers and these values are also presented in Table 36. The CY-a values increased sharply with the first selective poisoning treatment, which is another indication of diminished elasticity. 

Titania as with the ceria, led to Pd promotion in terms of activity for nitrate degradation but yielded catalyst which exhibited poorer selectivity than bimetallic catalysts. However, unlike ceria based systems, these catalysts did not suifer from poisoning when CO2, was used as a pH buifer. Nitrate reduction over Pd/Ti02 catalysts was associated with partially reduced titania species which migrated over the Pd particles during the reductive pre-treatment. The latter also led to the formation of a Pd (3-hydride phase which was also thought to play a role in the reduction process. 

Vanadia supported on silica(with titania) promoted by Fe and CTu oxides was studied for SCR of NOx by Bjorklund et al.28 Both Fe and Cu (as oxides) enhanced the activity however, the resistance to deactivation by SO2 differed as the Fe-promoted catalyst became slightly more active with time on stream and the Cu-promoted catalyst decreased in activity to less than half the initial activity with time on stream. The activity for SCR was related to the concentration of as inferred from the solid electrical conductivity. In this case different promoters were shown to change dramatically the ability of a potential poison to deactivate the SCR catalyst Nikolov, Klissurski, and Hadjiivanov29 also studied the deactivation of vanadia/titania. A combination of ESCA, XRD, and IR were employed to characterize the surface and bulk compositions. They concluded that deactivation involved the transformation of the active anatase titania to inactive rutile. Further, there was a concomitant decrease in total surface area and a loss of phosphate promoters for the selective oxidation of xylenes to phthalic anhydride. 

Titanium dioxide is an n-type semiconductor that is used in thin-film oxygen and humidity sensors [155, 156]. Doping with other metal oxides (e.g., iron oxides) increases the sensitivity and selectivity of the titania oxygen sensors [157, 158]. In comparison to zirconia, titania sensors have a better resistance against lead poisoning. 

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