Coverage effect on reactivity can be more complicated than what you believe: H2 dissociation on H-precovered Pd(111)

A systematic investigation based on density-functional-theory calculations is presented for elucidating the effect of adatoms on the energetics of H2 dissociation on H-precovered Pd(111) surfaces. Surprisingly, we found that the H-adatoms do not only have a poisoning effect but can also promote H2 dissociation when they are adsorbed on sites which are sufficiently far from the dissociating H2 molecule. The presence of these antagonistic effects produces many crossovers of the minimum energy profiles for H2 dissociation when coverage is varied and thus leads to a quite perplexing picture for the adatom effects. We devised a sorting procedure which allows for rationalizing nicely the influence of H-adatoms on the energetics of H2 dissociation on H-precovered Pd(111) surfaces.