Studies of skeletal rearrangements of labeled hexanes on iridium and iridium-cobalt catalysts: Correlations between the product distributions and some structural information on the catalysts given by EXAFS

Isomerization of hydrocarbons using 13C-labeled molecules over both iridium and iridium-cobalt catalysts proceeds via a selective cyclic mechanism. By decreasing the iridium loading from 10 to 0.25 wt% or by adding cobalt to iridium, isomerization via bond-shift intermediates becomes more important. For CC bond rupture a methyl migration mechanism is favored, whereas the tertiary carbon atom always seems to be nonreactive. To explain the change in the reaction pathway, an alkyne mechanism was postulated in which the alkyne species are in equilibrium with the surface carbynes. The former species are favored when adsorbed hydrogen is less available, which is the case with cobalt as seen by TPR measurements on bimetallic cobalt-iridium catalysts. On the other hand, carbynes are the precursor species for the selective cyclic mechanism or for selective demethylation. The catalytic results are well supported by EXAFS measurements. The multiple scission of the CC bonds characteristic of cobalt is suppressed by the presence in the topmost layer of iridium atoms surrounded by cobalt atoms. Total surface iridium concentration is constant irrespective of the iridium loading as also seen by TPR measurements. From EXAFS data, it is shown that iridium atoms (i) are involved in a bimetallic phase very diluted in iridium having a unit mesh identical to that of hexagonal cobalt; (ii) are involved in very small aggregates of iridium embedded in the matrix of cobalt with iridium-iridium distances of 0.265 nm (these aggregates are insensitive to oxidation passivated by cobalt and then catalytically inactive); and (iii) are in some cases making large fcc particles of iridium, particularly when the catalyst is heated at 1273 K in helium. The catalytic results are discussed via two hypotheses: (i) an electronic interaction between iridium and cobalt and (ii) the availability of surface hydrogen. Both hypotheses are directly correlated to the product distribution observed during the surface rearrangement.