Deactivated surfaces

Exhaustive silanization with the most active of silanizing reagents cannot eliminate completely all silanol groups on the i silica surface. Encapsulating the deactivated surface by, ... 

Pt-Rh/AROs catalysts are widely used in automotive-exhaust emission control. In these systems, Pt is generally used for the oxidation of CO and hydrocarbons and Rh is active for the reduction of nitric oxide to N2. HRTEM and AEM show two discrete particle morphologies and Pt-Rh alloy particles (Lakis et al 1995). EM studies aimed at understanding the factors leading to deactivation, surface segregation of one metal over the other and SMSI are limited. There are great opportunities for EM studies, in particular, of surface enrichment, and defects and dislocations in the complex alloy catalysts as sites for SMSI. 

The presence of a large fraction of adsorbed species on the catalyst surface thtis not only decreases the number of available active sites for alkane selective oxidation, but also decreases their s >ecific turnover number, thus increasing the surface lifeiime of adsorbed intermediates on the PVO deactivated surface. This increases the probability of parallel unselective reactions with a decrease in the selectivity to anhydrides. 

The strong adsorption of the intermediates/products increases their surface lifetime and decreases the surface availability of oxygen sites for anhydride synthesis. Both aspects concur in the decrease in selectivity to anhydrides on deactivated surfaces. 

Returning to Fig. 15, the sympathetic structure sensitivity of curves 1 and 2 is slightly enhanced for the partly deactivated surface (curve 2), in agreement with the higher resistance to carbon deposition of the smaller particles for this system. 

Certain basic materials have been used to deactivate surface acidity. By modifying the surface acidity of HZSM-5 with 4-methylquinoline or hexamethyl-disilazane more linear products have been obtained in propene oligomerization, and, by modifying the surface acidity of HZSM-5 with 2,6-di-/ r/-butylpyridine,... 

The surface sites, represented by S, will be considered to be at an activity level s, lower than that of the undeactivated surface. Indeed, we will let. s represent a scaled activity variable, with 5=1 for the fresh surface and 5 = 0 for the completely deactivated surface. In this case the kinetics of the three steps of the reaction are... 

Size exclusion chromatography (SEC) with columns of 3mm i.d. was used for the isolation of pesticides from edible oils or fatty food extracts. Reversed-phase LC with water-containing eluents was used with various (special) transfer techniques, but has probably never been used for routine applications. Water causes several problems, attack of deactivated surfaces being the most severe. 

EOF can be minimized by using capillaries with a deactivated surface. Deactivation is obtained by chemically binding a ligand to the silica surface or by adding compounds (additives) to the electrolyte solution. Such deactivation will also aid in preventing unwanted adsorption of analytes to the capillary wall (which may be a problem with basic analytes.)... 

In a series of papers, Lee and coworkers described the use of pentafluorobenzyl bromide as derivatization agent for the determination of 22 phenols in water samples [83,84]. Before derivatization, the phenols were extracted from the water sample into dichloromethane. In the first paper, six different columns were tested, and the OV-101 fused silica capillary column with Carbowax-deactivated surface was found to give the most efficient separation [83]. Detection was carried out using both the BCD and MS. A similar approach, using derivatization with pentafluorobenzyl chloride and BCD, was described for the analysis of monochlorinated and brominated phenols in aqueous samples [85]. 

The active surface area can be reduced, not only by chemical deactivation, but also by sintering. While only the active surface area is affected by chemical deactivation, active as well as deactivated surface area is affected by sintering. In... 

An example of structure sensitivity in deactivation is found in the work of Ostermaier et al. (3A) for 2-15 nm Pt/Al203 and Pt black in ammonia oxidation at 368-473 K. The effects of crystallite size and temperature in deactivation were investigated it was found that the extent of deactivation increased with decreasing temperature and there was a difference in the Arrhenius behavior between sintered and unsintered materials. Deactivation was more severe with smaller crystallites, but the surface could be completely reactivated by H2 at 673 K. It was suggested that PtO was the deactivated surface, and an excellent correlation of activity was provided by ... 

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