Mechanism of the poisoning action of sulfur on catalytically active Pd(100)

The influence of sulfur adsorbates on the stereodynamics of surface reactions is evaluated for the associative desorption of D-2 from Pd(100). The experimental results show that translational and rotational steering is more effective than at the clean surface, which implies that the potential energy surface (PES) for the reaction shows a larger spatial extension into the vacuum than for the clean surface. A significant polar corrugation of the PES becomes evident at high J states. At J = 8 a very strong coverage dependence of the alignment with A(0)((2)) = 0.07 +/- 0.06 at the clean surface and A(0)((2)) = 0.60 +/- 0.11 at Theta (S) = 0.5 monolayer is observed. This coverage dependence indicates a shift of the reaction site from the bridge to the c-type hollow position at a sulfur coverage of more than Theta (S) = 0.25 monolayer.