Reactions of n-butane and neopentane on titania-supported iridium catalysts

Abstract

The reactions of n-butane and neopentane have been studied over titania-supported iridium catalysts of varying dispersions (0.1 ≤ D ≤ 1.0). The catalysts were characterized by electron microscopy and adsorption and temperature-programmed desorption of hydrogen. After low temperature reduction (473 K) hydrogenolysis of n-butane occurred in the C 2-unit mode and hydrogenolysis of neopentane in the iso-unit mode. Both reaction types are dependent on d ̄ Ir , and highly active sites, especially for the reaction in the iso-unit mode, are present in catalysts with d ̄ Ir ≥ 3.5 nm . Treatment in hydrogen above 600 K caused partial reduction of titania and Ti 3+ centers so produced interact strongly with the iridium particles (strong metal-support interaction, SMSI). Catalysts in the SMSI state are unable to adsorb hydrogen and for hydrocarbon reactions their catalytic activities fall by factors of 10 2 to 10 4. The selectivity changes which are observed may be attributed to morphological differences between SMSI and non-SMSI catalysts. Treatment in 1% oxygen in helium at 473 K followed by reduction in hydrogen at 473 K successfully destroyed the SMSI state in the case of highly dispersed Ir/TiO 2 catalysts ( d ̄ Ir ≤ 1.0 nm ), but higher oxidation temperatures (673 K) are necessary to break the strong metal-support interaction in the case of less highly dispersed iridium catalysts. IrO 2 crystals are formed in oxygen at temperatures above 573 K and the catalysts undergo irreversible structural changes.