The dissociative adsorption of hydrogen on Pt(111): Actuation and acceleration by atomic defects

The dissociation of hydrogen at atomic surface defects is the strongly dominant, if not the decisive, step in the chain of events eventually leading to chemisorbed H-atoms on Pt(111). This holds for perpendicular kinetic energies of the gas phase molecules from 8 to 60 meV, i.e., covering the range relevant to hydrogenation reactions. This insight has been gained in the present study in which we reversibly varied the defect density on one and the same crystal in a controlled way. Information has been derived from measuring the adsorption kinetics as a function of coverage. Two distinct adsorption channels are distinguished. The first, indirect one, prevails at lower H-coverage and involves capture into a non-accommodated molecular precursor state followed by dissociation at step sites as described in our recent paper. The second one, dominant at higher coverage and non-negligible defect densities, obeys second order Langmuir kinetics. Here the dissociative adsorption takes place directly at step sites with a cross section of 0.24 unit cells (initial sticking probability 24% of the step density). These results are consistent with thermally programmed desorption data: the direct channel is responsible for the emergence of the low temperature peak in thermal desorption spectroscopy, usually denoted with β(1), while the indirect channel is represented by the β(2) state. The dependence on the perpendicular component of the hydrogen kinetic energy is distinctly different for the two channels: the indirect one shows power law behavior with an exponent 1.9 ± 0.1, while the direct one shows no perpendicular energy dependence at all.