Small iron particles, high selectivity

Mechanistically it is logical to observe high selectivity for propylene if we assume that coupling of methylene to ethylene as well as ethylene coordination to surface carbene are fast reactions. Propylene for steric hindrance would react more slowly than ethylene with surface carbene reaction (3) whereas reaction (4) would be less favored for electronic reasons. It is difficult at this point to speculate why the selectivity for propylene is associated with small iron particles. One possibility is that the small iron particles displace the equilibrium olefin(acarbene mechanism besides these small Fe particles would have small hydrogenation properties which thus avoid methane formation from the carbene and saturated hydrocarbon formation from the olefin. 

In summary it can be concluded that catalysts containing small particles of iron oxide highly dispersed on the silica surface show the best performance in the selective oxidation of H2S. Impregnation with solutions of iron chelates, especially of iron citrate, provides the best results. 

The reducing side of photosystem I (PS I) reaction center of oxygenic photosynthetic organisms is known to consist of five different acceptors (1,2). The primary electron acceptor, called AO, is assumed to be a monomeric chlorophyll molecule. An electron is passed from AO to a secondary acceptor A1 which is believed to be phylloquinone. The electron transfer involves three different iron-sulfur centers, called FX, FB and FA. PS I high-molecular-mass subunits (about 82 kDa) are known to contain AO, Al, and FX as well as P700 (1,2). FA and FB are believed to be located in a small polypeptide of about 9 kDa (3,4). As we demonstrated elsewhere (5), heat treatment of spinach PS I particles in the presence of ethylene glycol (EG) caused the selective destruction of the iron-sulfur centers and led to the dissociation of polypeptides from the particles. A small subunit of about 5 kDa was closely associated with large subunits under this treatment. In this paper, we present the N-terminal amino acid sequence of the 5 kDa polypeptide. 

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