Trapping-mediated dissociative chemisorption of C{sub 3}H{sub 8} and C{sub 3}D{sub 8} on Ir(110)
- Department of Chemical Engineering, University of California, Santa Barbara, California 93106 (United States)
Molecular beam techniques were used to investigate the molecular trapping and trapping-mediated dissociative chemisorption of C{sub 3}H{sub 8} and C{sub 3}D{sub 8} on Ir(110) at low beam translational energies, E{sub {ital i}}{le}5 kcal/mol, and surface temperatures, T{sub {ital s}}, from 85 to 1200 K. For T{sub {ital s}}=85 K, C{sub 3}H{sub 8} is molecularly adsorbed on Ir(110) with a trapping probability, {xi}, equal to 0.94 at E{sub {ital i}}=1.6 kcal/mol and {xi}=0.86 at E{sub {ital i}}=5 kcal/mol. At {ital E}{sub {ital i}}=1.9 kcal/mol and {ital T}{sub {ital s}}=85 K, {xi} of C{sub 3}D{sub 8} is equal to 0.93. From 150 K to approximately 700 K, the initial probabilities of dissociative chemisorption of propane decrease with increasing {ital T}{sub {ital s}}. For {ital T}{sub {ital s}} from 700 to 1200 K, however, the initial probability of dissociative chemisorption maintains the essentially constant value of 0.16. These observations are explained within the context of a kinetic model which includes both C{endash}H (C{endash}D) and C{endash}C bond cleavage. Below 450 K propane chemisorption on Ir(110) arises essentially solely from C{endash}H (C{endash}D) bond cleavage, an unactivated mechanism (with respect to a gas-phase energy zero) for this system, which accounts for the decrease in initial probabilities of chemisorption with increasing {ital T}{sub {ital s}}. With increasing {ital T}{sub {ital s}}, however, C{endash}C bond cleavage, the activation energy of which is greater than the desorption energy of physically adsorbed propane, increasingly contributes to the measured probability of dissociative chemisorption. The activation energies, referenced to the bottom of the physically adsorbed molecular well, for C{endash}H and C{endash}C bond cleavage for C{sub 3}H{sub 8} on Ir(110) are found to be {ital E}{sub {ital r},CH}=5.3{plus_minus}0.3 kcal/mol and {ital E}{sub {ital r},CC}=9.9{plus_minus}0.6 kcal/mol, respectively. (Abstract Truncated)
- DOE Contract Number:
- FG03-89ER14048
- OSTI ID:
- 367218
- Journal Information:
- Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 1 Vol. 105; ISSN JCPSA6; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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