Chemisorption of isobutane and neopentane on Ir(110)

Molecular beam techniques have been employed to investigate the chemisorpti on of isobutane and neopentane on Ir(110) at surface temperatures, T-s, fro m 85 to 1000 K. The beam translational energies, E-i, varied from 3.1 to 31 kcal/mol for isobutane, and 1.8 to 48 kcal/mol for neopentane. For T-s < 1 00 K, isobutane formed molecular multilayers, as did neopentane for T-s < 1 10 K. The initial adsorption probabilities. P-a, remained nearly independen t of surface temperature between 100 and 300 K for isobutane, and 110 and 3 50 K for neopentane for different beam impact energies. For T-s = 100 K, is obutane adsorbs molecularly with an intrinsic trapping probability, xi, equ al to 0.96 at E-i = 3.1 kcal/mol while neopentane adsorbs molecularly with xi = 0.94 at E-i = 1.8 kcal/mol and T-s = 110 K. In contrast, for isobutane with T-s between 300 and 1000 K, and for neopentane with T-s between 350 a nd 1000 K, the initial adsorption probabilities decrease with increasing T- s. For both molecules, the adsorption probabilities show characteristics of both trapping-mediated and direct dissociative chemisorption which dominat e the kinetics for low and high impact energies, respectively. A kinetic mo del is proposed, which includes C-H bond cleavage reaction pathways. (C) 20 00 Published by Elsevier Science B.V. All rights reserved.