Chemisorption of isobutane and neopentane on Ir(110)

Molecular beam techniques have been employed to investigate the chemisorption of isobutane and neopentane on Ir(110) at surface temperatures, Ts, from 85 to 1000 K. The beam translational energies, Ei, varied from 3.1 to 31 kcal/mol for isobutane, and 1.8 to 48 kcal/mol for neopentane. For Ts<100 K, isobutane formed molecular multilayers, as did neopentane for Ts<110 K. The initial adsorption probabilities, Pa, remained nearly independent of surface temperature between 100 and 300 K for isobutane, and 110 and 350 K for neopentane for different beam impact energies. For Ts=100 K, isobutane adsorbs molecularly with an intrinsic trapping probability, ξ, equal to 0.96 at Ei=3.1 kcal/mol while neopentane adsorbs molecularly with ξ=0.94 at Ei=1.8 kcal/mol and Ts=110 K. In contrast, for isobutane with Ts between 300 and 1000 K, and for neopentane with Ts between 350 and 1000 K, the initial adsorption probabilities decrease with increasing Ts. For both molecules, the adsorption probabilities show characteristics of both trapping-mediated and direct dissociative chemisorption which dominate the kinetics for low and high impact energies, respectively. A kinetic model is proposed, which includes CH bond cleavage reaction pathways.