Termination site density

In the case ofn-Ge(lll) substrates, surface states affect electrochemical deposition of Pb [319]. At high cathodic potentials, the deposition occurs by instantaneous nucleation and diffusion-controlled three-dimensional growth of lead clusters. Comparing H- and OH-terminated n-Ge(lll) surfaces, the nucleation is more inhibited at n-Ge(lll)-OH, which can be explained by the different densities of Ge surface free radicals, being nucleation sites. In this case, nucleation site density is about 1 order of magnitude lower than that for n-Ge(lll)-H. 

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

Relative rate constants for a-olefin readsorption decrease as follows kr c0>kr Ru> r Fe (7)- Although kr on Fe catalysts is smaller than on Ru or Co, the other parameters in Eq. (2), such as the low diffusivity of large hydrocarbon and the high site density on unsupported Fe catalysts, ultimately increase the probability of a-olefin readsorption therefore, pore diffusion effects also play a crucial role in Fe-catalyzed FT synthesis (Figures 3 and 4). Fe catalysts, however, give lower C20+ selectivity because of lower intrinsic values of kr- even though asymptotic chain termination probabilites are lower on Fe. 

Fig. 11.11. Surface coverage with dangling bonds (6>db), hydrogen-terminated (cross linked) sites (6 n) and methyl-terminated sites (6 113) as a function of the atomic hydrogen flux density...

Non-Flory molecular weight distributions have also been attributed to the presence of several types of active sites with different probabilities for chain growth and for chain termination to olefins and paraffins (45). Two-site models have been used to explain the sharp changes in chain growth probability that occur for intermediate-size hydrocarbons on Fe-based catalysts (46,47). Many of these reports of non-Flory distributions may instead reflect ineffective dispersal of alkali promoters on Fe catalysts or inadequate mass balances and product collection protocols. Recently, we have shown that multisite models alone cannot explain the selectivity changes that occur with increasing chain size, bed residence time, and site density on Ru and Co catalysts (4,5,40,44). 

Carbon number distributions are similar on all Co catalysts. As on Ru catalysts, termination probabilities decrease with increasing chain size, leading to non-Flory product distributions. The modest effects of support and dispersion on product molecular weight and C5+ selectivity (Table III) reflect differences in readsorption site density and in support pore structure (4,5,14,40,41), which control the contributions of olefin readsorption to chain growth. Carbon number distributions obey Flory kinetics for C30+ hydrocarbons the chain growth probability reaches a constant value (a ) as olefins disappear from the product stream. This constant value reflects the intrinsic probability of chain termination to paraffins by hydrogen addition it is independent of support and metal dispersion in the crystallite size range studied. 

Fro. 13. Site density effects on selectivity and chain termination probability Co/TiOa (catalyst A 11.7 wt% Co, 1.5% Co dispersion, 79 h site-time yield, site density I.O /ig-atom surface Co m catalyst B 12.1 wt% Co, 5.8% Co dispersion, 82 h site-time yield, site density 3.3 /cg-atom surface Co m" 473 K, 2000 kPa, H2/CO = 2.1, <10% CO conversion). (a) C5+ selectivity (b) olefin chain termination probability (c) paraffin chain termination probability. 

At higher Co site densities, the probability of chain termination to olefins is lower and decreases somewhat faster with hydrocarbon chain size (Fig. 13b). Thus, the heavier product obtained on materials with high site densities reflects a lower probability of chain termination to olefins, an effect that arises from the enhanced readsorption of such olefins within transport-limited pellets with high site density. In contrast, chain termination to paraffins is not influenced by Co site density (Fig. 13c). Site density does not affect primary chain growth and termination chemistry or inhibit secondary olefin hydrogenation pathways, effects that could otherwise account for the higher Cs+ selectivity observed on higher site density pellets. 

H-covered surfaces. TPD studies [15,18,20,22] of CO and H2 co-adsorption on Ni(lOO) concluded that CO+H interactions in the co-adsorbed layer lead to formation of a new surface entity exhibiting new low-temperature desorption states of CO and H2. Relevant LEED investigations [22,25,26] brought about consistent results, by characterizing formation of CO+H interaction species. HREELS studies [18,25,26] of a similar system proved the existence of a strong link between the CO+H interaction species and a terminal-CO stretching vibration at 2100 cm , i.e. close to that observed here at 2095 cm for the terminal-CO species (spectrum (a). Fig. 3). These features were observed [18] only for CO and H2 coadsorption on low site-density surfaces ofNi°, namely, Ni(lOO). 

According to another avenue of speculation, it is possible that a portion of the LCB can arise from incorporation of macromers generated on neighboring sites, when they are in close proximity to each other. All of the variables associated with LCB noted above are known to affect active-site density and thus the distance between sites. Once desorbed, the terminated vinyl end-group may have some limited distance of mobility because most of the chain is frozen into a solid matrix. Thus it may have... 

Schematic representations of two mesoporous xerogels. (A) Randonutlose packing of low-density spheres. (B) Hierarchical random-close packing of dense spheres. (C) Schematic representation of microporous xerogei illustrating uniform distribution of terminal sites. (D) Schematic representation of the particulate xerogei illustrating the dense oxide core and nonuniform distribution of terminal sites.

In molecular orbital terms, the stability of the allyl radical is due to the fact that the unpaired electron is delocalized, or spread out, over an extended 7T orbital network rather than localized at only one site, as shown by the computer-generated MO in Fig 10.3. This delocalization is particularly apparent in the so-called spin density surface in Figure 10.4, which shows the calculated location, of the unpaired electron. The two terminal carbons share the unpaired electron equally. 

---