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

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Abstract
We report detailed molecular beam studies of the effects of K adsorption on the dissociativechemisorption probabilities S 0 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 H2dissociation.S 0 decreases roughly exponentially with K coverage Θ K . The effective cross section for poisoning per adsorbed K varies between 70 and 430 Å2, depending upon incident energy E i . 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.