The hydrogenolysis of isobutane on iridium

Klima, Rudolph
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The hydrogenolysis of isobutane on iridium has been investigated over the pressure range 0-100 torr. The iridium catalyst is a 48.5 (ANGSTROM) thick film vapor deposited on the inside of a round bottom flask under ultra high vacuum conditions. Methane was observed to be the only reaction product. Adsorption experiments indicate competitive adsorption to occur for hydrogen and hydrocarbon with respect to the iridium;Thermal desorption studies show hydrocarbon adsorption to be irreversible. These studies also show only monocarbon fragments desorb from iridium when the film is exposed to hydrogen and heat. There appears to be three adsorption states on polycrystalline iridium for hydrogen. The two states desorbing at 110(DEGREES)C and 160(DEGREES)C are due to adsorption of atomic hydrogen. The state appearing at 320(DEGREES)C is due to hydrogen bound to carbonaceous fragments chemisorbed to the surface. Hydrocarbon adsorption increases as the catalyst's temperature is increased, while hydrogen adsorption decreases. Dependence of reaction rate on hydrogen and isobutane partial pressures was established by measurements of initial rates of methane production. The following rate law fit the experimental data very well:;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI);(UNFORMATTED TABLE FOLLOWS)Temperature 209(DEGREES)C 175(DEGREES)C;A(molecules/site sec torr) 1.75 x 10('-2) 1.46 x 10('-2);B(torr('- 1/2)) .25 1.90;C(torr('- 1/2)) .81 .81;D(dimensionless) 6.34 6.12(TABLE ENDS);This rate equation was derived via a Langmuir Hinshelwood reaction mechanism. The hydrogen order varies from +2 to -1. The isobutane order varies from +1 to -1. The hydrogen adsorption equilibrium constant decreases as temperature increases, indicating that hydrogen is more mobile on the surface at high temperatures. The activation energy for isobutane hydrogenolysis is 24.20 kcal/mole.