AN UNUSUAL ADSORPTION STATE OF HYDROGEN ON THE PD(100)-P(2X2)-P4G-AL BIMETALLIC SURFACE

A Pd(100) surface modified by aluminum was investigated with AES, LEED and H-2-TPD. The Al/Pd(100) surface gives an ordered surface alloy with p(2 x 2)-p4g symmetry upon annealing at 700 K. Molecular hydrogen dissociates spontaneously on the p(2 x 2)-p4g surface alloy, whereas the Al monolayer deposited at 300 K prevents the adsorption of H-2. A recombinative desorption (delta-H-2) process starts on the alloy at 360 K with first-order kinetics. The activation energy and the pre-exponential factor for the delta desorption state are evaluated to be 144 +/- 10 kJ/mol and 4 x 10(20+/-1) s-1, respectively. These unusually large values are ascribed to bimetallic ensembles on the ordered surface alloy.