INDEX potential

This is also apparent from the relative melt index potential (RMIP) of the catalyst, which is plotted in Fig. 9. Melt index increases with calcining temperature up to 925°C, indicating that the rate of termination relative to propagation varies in the same manner. 

Fig. 16. The relative melt index potential (RMIP) of a series of cogelled Cr/silica titania catalysts rises and then falls with calcining temperature, indicating first dehydroxylation then sintering. However, the more titania in the catalyst, the more easily it sinters and therefore the lower the temperature at which peak RMIP develops.

As noted in Section III,E the hydroxyl population on the catalyst seems to exert a powerful influence on the activity and melt index potential, which are... 

Table IX shows the effect of dehydration by chemical means. Silica samples were calcined at 870°C in various gases, impregnated anhydrously with 0.5% Cr, and finally calcined in air at 650°C, producing Cr(VI). Only the silica (not the chromium) was exposed to the reducing treatment. Even so, CO more than doubled the melt index potential and COS improved it by a factor of 60. Activity was also increased.

Fig. 19. The termination rate, plotted here as relative melt index potential (RMIP), reflects the extent of surface dehydroxylation in two series of Cr/silica-titania catalysts, calcined in (Y) air or ( ) CO and then air to reoxidize the chromium, both at the temperatures shown. The third series ( ) shows the additional benefit of low-temperature attachment. It was calcined in CO at the temperatures shown, then air at a lower temperature (760°C).

Note that, in agreement with Eq. (7.5), the index potential defined by Eq. (7.7) for a cathodic scan (upper sign) is identical in this case to that given by Eq. (7.3). 

Although the usual way of analyzing the influence of the kinetics of the electron transfer on the SWV response is based on the variation of the frequency at fixed values of the staircase and square wave amplitude, a new approach for carrying out this analysis has been proposed based on the study of the influence of the square wave amplitude sw on the current potential curves at a fixed value of the frequency (or the time pulse) [19, 33, 34], The square wave amplitude has been used rarely as a tool in mechanistic and kinetic studies. One of the main reason is that, as stated in Sect. 7.1, in SWV the current is plotted versus an index potential which is an average potential between the forward and reverse potentials (see Eq. (7.7)) and leads to a discrepancy between the plotted and actual potentials at which the current is sampled. Therefore, the role played by Esw in the process is complex. 

In the case of DMPV (see Scheme 7.2), the treatment followed in Sect. 4.2.4.1 for Differential Double Pulse Voltammetry (DDPV) for one and two polarizable interfaces can be used because the equilibrium is quickly reestablished during the longer period Therefore, the peak potential of the DMPV curves when the current is plotted versus the index potential is... 

Scheme 7.5 Potential-time waveform of SWV obtained from Eq. (7.5) ( , red line), and its distribution between the outer interface ( °ul, dark blue line) and the inner interface ( "", green line). The three index potentials (the outer index potential, out,mdex, the inner index potential, """ index, and the membrane index potential, mdex) are also included (blue line, dark green line, and black line, respectively). Inset figure Distribution of the applied potential red line), between the outer and the inner interfaces (dark blue line and green line, respectively). jnitiai = —450mV,...

The second procedure is based on the effect of the square wave amplitude on the peak potential separation between the anodic and cathodic components of the SWV response. This separation depends on both the reversibility of the surface charge transfer (through co and Sw- Thus, by plotting the differences AEp = Epc — E pl>, with Ep c and EpA being the peak potentials of the forward and reverse currents measured versus the index potential, or AE p = Ef c — E p a with h p c and h p a being the peak potentials of the forward and reverse currents measured versus the real potential that is applied in each case (potential-corrected voltammograms), it is possible to obtain linear dependences between the peak potentials separation and... 

PEEP index Potential Ecotoxic Effects Probe index Purpose... 

Results indicate that, far from being a necessary part of the active center as has sometimes been proposed, these hydroxyls may actually interfeiewith the active site. Hydroxyls could be removed by chemical as well as by thermal means to improve activity and melt index potential, such as by treating the catalyst in carbon monoxide, sulfur, or halides. 

Relative Melt Index Potential. Melt indices were obtained from polymer samples by the standard test (ASTM D 1238-73) at 190 C using a weight of 2160 grams. However, melt index values are not just affected by activation parameters, but also by reactor conditions, such as the temperature, monomer concentration, and residence time. Therefore for clarity in this report we have normalized melt index values against those of a reference catalyst run under the same reactor conditions. We call the normalized value the relative melt index potential (RMIP) because it is... 

Figure 2 also plots the melt index (RMIP = relative melt index potential) of these same polymers. Since a high RMIP indicates a high termination rate, it also (like the activity) increased with increasing activation temperature up to the point of sintering. Other measures of the termination rate, such as the vinyl content of the polymer, also displayed this same pattern. 

R/R Activation. Figure 5 shows that the enhanced dehydroxyl-ation by carbon monoxide also had a pronounced effect on the termination rate during polymerization. In these experiments, two series of Cr/silica catalyst samples were activated and allowed to polymerize ethylene to a yield of about 5000g PE/g. In one series the catalyst samples were simply calcined five hours in air as usual at the temperatures shown. The relative melt index potential (RMIP) has been plotted against activation temperature and the expected increase up to the point of sintering was observed. 

Melt index potential means the maximum MI that can be obtained with a catalyst activated at maximum temperature (871 °C), and making homopolymer at maximum reaction temperature (110 °C). Catalystshaving a high-MI potential need not be used to make high MI resins, but they have the capability. Low-MI polymers are typically manufactured more easily with catalysts having a high-MI potential. 

FIGURE 123 Melt index potentials of Cr/silica-titania catalysts that were activated in various gases by the R R process. Catalysts were reduced 3 h in the gas shown at the temperature shown, then reoxidized 2 h in air at the same temperature. Reduction in CS2 produced polymers of the highest melt index. 

FIGURE 125 Melt index potentials of two series of R R-activated catalysts. Amorphous Cr(lIl)/silica—titania was heated to 870 °C in N2. Crystalline Cr(lll)/silica—titania was heated to 650 °C in air, then to 870 °C in N2 to form a-Cr203. Both catalysts were then treated in CO at 870 °C for 3 h, followed by 2 h in dry air at the temperature shown. The crystalline catalyst was more difficult to reoxidize, and therefore it produced polymers of lower melt index. 

Figure 3.12 Relative Melt Index potential (RMIP) vs secondary catalyst activation temperature. RMIP is the melt index of the polyethylene sample normalized by the Melt Index of the standard Phillips catalyst containing 1 wt% Cr and activated with one thermal treatment in air at 870°C. Melt Index is inversely proportional to polymer MW. Reprinted from [12] with permission from Elsevier Publishing.

Thomas-Fermi screening length electron density/number of oscillators complex refractive index refractive index potential of zero charge reflectivity... 

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