Small Addition Of Cu Research Articles

Surface reactions of alkanes and organometallic chemistry applied to the study of pure and bimetallic catalysts

A series of Rh and Ru catalysts of widely varying dispersion has been prepared, using acetylacetonate precursors. A homogeneous distribution of particle sizes was obtained, with average sizes ranging from 1 nm to 10 nm. The conversion of 2,2,3,3-tetramethylbutane (TeMB) on these catalysts consists of two different reactions: well-dispersed catalysts mainly catalyze demethylation to iC 7 and poorly dispersed metals favour splitting of the central bond into 2iC 4. Taking into account the results reported for the TeMB-D 2 exchange, and the stoichiometric reactions of metallacyclopentanes, it is concluded that the selectivity of TeMB hydrogenolysis is controlled by coordination on active sites. A small addition of Cu to Rh, or Sn to Ru, increases the selectivity to iC 4; a further addition decreases the selectivity to iC 4 and increases the selectivity to iC 7. Whereas the results obtained for large additions of Cu or Sn fit with the ensemble theory of catalysis by alloys, the initial shift of the catalytic properties towards those of large particles supports the proposal of topological segregation of these metals at the edge and corner sites on the surfaces of small particles. Supported Ru/Al 2O 3 can be alloyed with Sn, Pb or Ge using surface reactions with organometallic compounds. Adding Pb or Sn results in similar catalytic properties and hence it is concluded that both additives segregate at low coordination sites. Germanium, however, shifts the catalytic properties in the direction of small particles, even at low contents. It is thus assumed that Ge is either randomly distributed or located on dense planes, at the surface of the Ru metallic particles.