Hydrogen activation competition between

It is noteworthy that the relative proportion of amine 44 and bicumyl (43) which reflects the ratio of the rate of electronation to the rate of reaction with M(H) (the competition between electronation and reaction with M(H)), varies with the Raney metal (compare entries 1 and 3 of Table 1, and entries 2 and 4) and with the electrode potential (compare entries 1 and 2). The more negative is the potential, the faster is the rate of electronation and the higher should be the proportion of bicumyl (43) as observed (entries 1 and 2). The less active the Raney metal as hydrogenation catalyst, the slower is the rate of reaction with M(H) (the lower is the amount of M(H) at the surface of the electrode) and the lower is the amount of aminocumene (44). RCu is the least active catalyst and the proportion of aminocumene (44) is indeed the lowest at the RCu cathode (entry 4). 

Fig. 2. Plot of normalized rate vs. the activity of silicic acid for the LAWABP1 (see Table 1) glass composition at two temperatures (26 and 40 °C). Rates are all computed at steady-state conditions. Boron and Na release rates are identical at low silica activities, then decrease, and become constant at or near saturation with respect to amorphous silica (vertical dot-dashed line). Note that the B rate decreases more than the Na rate. This behaviour can be rationalized as competition between two concurrent reactions alkali-hydrogen exchange and matrix dissolution (see text). Error bars represent 2-

In this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

For reactions carried out in homogeneous solution or under solid-phase conditions the use of Fmoc amino acid chlorides is limited by the competition between their aminolysis and the formation of the less reactive oxazol-5(4//)-ones in the presence of tertiary amines, which are essential components of such reaction systems. To improve the results under these conditions a hindered base, e.g. 2,6-di-/er/-butylpyridine, can be used as a hydrogen chloride acceptor since conversion to oxazol-5(4//)-one is slow with such bases. Although shown to be advantageous in certain cases, Fmoc amino acid chlorides are used in homogeneous solution synthesis only in particular cases. They react efficiently in the presence of pyridine with weak nucleophiles such as imine 2P l (Scheme 2) where other activated species such as an active ester, anhydride, acyl fluoride, and acyl imidazolide fail. 

As a conclusion, the adsorption competition between butadiene and butene is actually in favour of butadiene on Pd(lll), making this catalyst highly selective in butenes for this hydrogenation reaction. This is not true for Pt(l 11) which is poorly selective. Moreover, one can remark that the more open (110) faces of fee metals are more active for the butadiene conversion into butenes than the close packed (111) faces [29, 33]. Some striking results are given in Table 3. 

On balance then the evidence favours the hypothesis that the selectivity in ammonia oxidation is determined by competition between NH 3 and O2 molecules for active step sites and not by the relative rates of NO desorption and reaction with ammonia. Conditions can be found favouring complete coverage of active sites by N atoms (<200 °C, desorption rate limited), or by NH3 molecules (200—500 °C, Eley-Rideal reaction with gaseous oxygen) or by O atoms (500— 1000 °C, Eley-Rideal reaction with gaseous ammonia). Above 1000 °C, simple NH3 decomposition supervenes, perhaps with oxidation of the hydrogen thus liberated. 

The data presented above indicate that, although the metal component is not able to activate saturated hydrocarbon molecules, it is very active as a trap for free radicals generated by the oxide catalyst. As a result, reaction (5) competing with (4) leads to the apparent increase in activity of the binary catalyst, and the selectivity of the overall process is determined by the competition between reactions (3) and (5). On the other hand, the treatment in hydrogen flow causes exhaustion of this oxygen buffer , which cannot be restored in the presence of both reactants in the reaction mixture due to the high reducing activity of methyl radicals. 

The second mechanism involves the reaction between one chemisorbed molecule (hydrocarbon or hydrogen) with the other molecule (hydrogen or hydrocarbon) in the gas phase. This mechanism does not involve competition between the hydrogen and the hydrocarbon molecules for the active sites and therefore does not give non-monotonic kinetics. This is a CSD mechanism usually referred to as Eley-Rideal mechanism (Thompson and Webb, 1968 Merangozis et al., 1979). 

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