The step-mediated indirect channel to hydrogen dissociation

The step-mediated indirect channel to hydrogen dissociation 

Further evidence that it is a step mediated channel responsible for the indirect dissociation on the W(1 0 0) surfaces comes from a comparison with the results for the step mediated dissociation on Pt(5 3 3) (Section 3.2) which exhibit very similar dynamical characteristics. A similar mechanism is likely to be responsible for the indirect dissociation channel on Ni(9 9 7) [89] (Fig. 25)., Y0 decays with Ei over the range 0 Tq(meV) 150 on all of the metal surfaces studied where steps are suggested to be responsible for indirect dissociation. S0 is also rather insensitive to 7 s in all cases, and, S (0n) exhibits precursor type dependencies. 

The step mediated indirect dissociation of hydrogen on W(1 00) provides an explanation for the insensitivity of the channel to alloying W(1 00) with an inert diluent on W(1 00)-c(2 x 2)Cu, or decorating the surface with nitrogen on W(1 00)-c(2 x 2)N. Both modifications lead to an expected increase in the activation barrier associated with the direct channel to hydrogen dissociation. The indirect channel for H2 dissociation, however, on both surfaces remains the same as that found on W(1 0 0). This contrasts with the case of N2 dissociation on W(1 0 0) and on the W(1 0 0)-c(2 x 2)Cu alloy surface. The increase in the activation barrier induced by alloying the surface with Cu associated with the direct channel to N2 dissociation is accompanied by the disappearance of the accommodated indirect channel to N2 dissociation observed on W(1 0 0). This difference between N2 and H2 dissociation is accounted for by the mediation of step sites in the indirect dissociation of H2, but dissociation at the W(1 0 0)-c(2 x 2)Cu surface unit cell in the case of N2. 

Chemisorption dynamics in the presence of well defined surface defects 

Istituto Nazionale di Fisica della Materia Unita di Genova, IMEM-CNR Sezione di Genova, and Dipartimento di Fisica dell Universita di Genova, Via Dodecaneso 33, 16146 Genova, Italy 

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