Hydrogenolysis of n-butane has been studied over the (1 10) and ( 1 11) surfaces of Rhodium. On Rh( 1 lo), the products of the hydrogenolysis consist of methane > ethane > propane. The hydrogenolysis reaction exhibits a good fit to Arrhenius behaviour for reaction temperatures up to 500 K. At higher temperatures the reaction rate tends to 'roll over' due to insufficient surface coverage of hydrogen. The 'roll over' affects product distribution, yielding more complete hydrogenolysis to methane. The behaviour is qualitatively similar on Rh( 11 l ) , with 'roll over' occurring at a lower temperature, namely 475 K. However, the hydrogenolysis is more selective on Rh( 11 l), yielding 50 mol O/O ethane. The high ethane selectivity seen in previous work on Ir( 110) is not seen on the Rh( 110) surface presumably because the Rh surface does not exhibit the (1 x 2) 'missing-row' reconstruction that is stable on Ir( 110) surfaces under reaction conditions. The hydrogenolysis selectivity of the Rh single-crystal surfaces correlates well with supported Rh metal particles subjected to oxidation-reduction cycling.
Reactions of 1,1,3,3-tetramethylcyclobutane (TMCBf-hydrogen mixtures on evaporated metal films have shown that both ring scission to 2,2,4trimethylpentane (TMP) and ring enlargement to 1,1,3-trirnethylcyclopentane (TMCP) are dominant on sintered platinum films, while ring scission to TMP predominates on sintered palladium and on unsintered molybdenum films. Unsintered tantalum and molybdenum films gave a large production of iC, hydrocarbon, particularly above ca. 500 K : ring enlargement was a minor reaction found particularly with tantalum. The homogeneous reaction of TMCB giving a large iC, production sets in at 600 K. The possibility, suggested by the product distribution, that the reaction of TMCB on Mo and Ta is a metal-assisted free-radical reaction is examined. While it may not be completely excluded that the catalysed ring enlargement on Pt and Pd is a free-radical reaction it is argued that only the previously proposed Rooney-Samman bond-shift mechanism accommodates without added qualifications published facts on bond shifts, including those at quaternary carbon atoms, and also ring enlargements.
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