HT peak

In contrast to the results on smaller particles, larger (3.6 nm) particles on 0001 a-A O gave only one peak in the TPD curve which corresponded to the HT peak for 1.7 nm particles and the area under the curve decreased only slightly (less than 5%), even after five cycles. The amount of methane detected per CO adsorption site by TPR-H was two orders of magnitude less than that observed on 1.7 nm particles. 

It is noteworthy that surface carbon did not come from those CO molecules responsible for the HT peak but from sites that are able to disproportionate CO and correspond to the LT peak. Because the latter sites are important only on quite small particles, it is tempting to associate them with low coordination number surface metal atoms, the relative concentration of which increases rapidly as the particle size decreases below 2 nm (8). Thus, these atoms may be the sites responsible for the relatively weakly adsorbed state of CO. Results similar to our work were found on other Group VIII metals. In the case of a Ru/Si02 sample, Yamasaki et al. (9) have shown by infrared spectroscopy that the deposition of carbon occurs rapidly by CO disproportionation on the sites for weakly held CO. The disproportionation also occurred on a Rh/Al20 sample with 66% metal exposed so that appreciable concentrations of low coordination atoms are expected (10). 

Fig. 2.42 DSC desorption temperature from Table 2.17 versus powder particle size (BCD) of synthesized MgHj powders, (a) Onset temperature (T ) and (b) low-temperature (LT) and high-temperature (HT) peaks in a doublet. Numbers beside data points indicate the grain size of the MgH phases [63]...

Fig.2. PXRO patterns of (a) MgAI-HT and (b) CuAl-HT (Peaks marked as arc due to gcrhardilc phase)...

Some monomer/catalyst systems give highly tactic polymers and we shall consider later in more detail the various extreme cases. Figure 11.10a shows an example of the spectrum of an all-c/s syndiotactic polymer, which has an HH, TT structure when made from a single enantiomer. (The small HT peaks arise from the presence in the monomer of a small amount of the minor enantiomer.) When the polymer is made from racemic monomer, Figure 11.10b, the polymer may still be... 

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