Direct dissociative chemisorption of propane on Ir(110)

Kelly, D.; Weinberg, W. H.
December 1996
Journal of Chemical Physics;12/22/1996, Vol. 105 Issue 24, p11313
Academic Journal
We have employed molecular beam techniques to investigate the initial probability of direct dissociative chemisorption, P[SUBd], and the intrinsic trapping probability,&xgr;, of C[SUB3]H[SUB8], C[SUB3]D[SUB8] , and (CH[SUB3])[SUB2]CD[SUB2] on Ir(110) as a function of beam translational energy, E[SUBi], from 1.5 to 59 kcal/mol. For C[SUB3]H[SUB8] and (CH[SUB3])[SUB2]CD[SUB2], a measurable (⩾0.02) initial probability of direct dissociative chemisorption is observed above a beam energy of approximately 7 kcal/mol. For C[SUB3]D[SUB8] this energy is roughly 10 kcal/mol. Above these energies the initial probability of direct chemisorption of each of the isotopomers of propane increases nearly linearly with E[SUBi], approaching a value of approximately P[SUBd] = 0.48 at E[SUBi] = 52 kcal/mol for C[SUB3]H[SUB8] and (CH[SUB3])[SUB2]CD[SUB2], and P[SUBd] = 0.44 at E[SUBi] = 59 kcal/mol for C[SUB3]D[SUB8]. This kinetic isotope effect for the direct chemisorption of C[SUB3]D[SUB8] relative to C[SUB3]H[SUB8] is smaller than that expected for a mechanism of H (or D) abstraction by tunneling through an Eckart barrier, suggesting a contribution of C-C bond cleavage to direct chemisorption. The lack of a kinetic isotope effect for the direct chemisorption of (CH[SUB3])[SUB2]CD[SUB2] relative to C[SUB3]H[SUB8] indicates that 1° C-H bond cleavage dominates over 2° C-H bond cleavage during the direct chemisorption of propane on Ir(110). The trapping behavior of each of these isotopomers of propane is approximately identical as a function of E[SUBi], with &xgr; > 0.9 at E[SUBi] = 1.5 kcal/mol, &xgr; = 0.3 at E[SUBi] = 20 kcal/mol, and &xgr; < 0.1 above E[SUBi] = 40 kcal/mol.


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