HCl dissociation on Pt/Au(111) and Au/Pt(111): Contrasting barrier characters and rotational dynamics.

The dissociative chemisorption of HCl on Pt/Au(111) and Au/Pt(111) bimetallic surfaces is investigated by six-dimensional quantum dynamics calculations on accurate neural network potential energy surfaces. The two systems exhibit sharply contrasting barrier character, with Pt/Au(111) in a central-barrier regime (transition state H-Cl bond length of 2.92 Bohr) and Au/Pt(111) in a late-barrier regime (bond length of 3.89 Bohr). As a result, on Pt/Au(111), translational energy is more effective than vibrational excitation in promoting the reaction, and all impact sites exhibit comparable reactivity. On Au/Pt(111), by contrast, vibrational excitation is more effective than translational energy, and the top-site reactivity is three orders of magnitude lower than at the other sites. The rotational dynamics of HCl dissociation are equally distinct. Pt/Au(111) displays a monotonic angular minimum energy path with a persistent helicopter alignment preference. Au/Pt(111) exhibits a non-monotonic angular dependence induced by weak interfacial charge transfer, leading to a j-dependent inversion of alignment preference and modest rotational enhancement only at high rotational states. The opposite directions of interfacial charge transfer and surface strain in the two systems are responsible for their contrasting dynamical behavior.