Long-range poisoning of D2 dissociative chemisorption on Pt(111) by coadsorbed K

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<jats:p>We report detailed molecular beam studies of the effects of K adsorption on the dissociative chemisorption probabilities S0 for D2 on Pt(111). In contrast to conventional wisdom for many other molecular systems, we find that K is a very strong poison rather than promoter for H2 dissociation. S0 decreases roughly exponentially with K coverage ΘK. The effective cross section for poisoning per adsorbed K varies between 70 and 430 Å2, depending upon incident energy Ei. This suggests that an extremely long-range electronic perturbation is responsible for the poisoning. A theoretical model is developed to describe these sticking measurements. It is based on the fact that the adsorbed K lowers the work function of the surface. This enhances Pauli repulsion for the molecule-surface interaction which, in turn, increases the barrier to dissociation. When the model is generalized to include inhomogeneous effects through a local work function, excellent agreement is obtained between the model and experiments. This model also naturally accounts for the increased associative desorption temperatures for H2 when coadsorbed K is present.</jats:p>

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