A CLUSTER MODEL STUDY OF HYDROGEN ADSORPTION ON AL(100)

Quantum-chemical cluster calculations are used to model H adsorption on the Al(100) surface. At low H surface coverage, the bridging and on-top site vibrational frequencies are computed to be 1091 and 1767 cm-1 and are in excellent agreement with the 1125 and 1750 cm-1 vibrations observed in electron energy loss spectra (EELS) experiments. Calculations on the Al14H2 cluster provide an explanation for the simultaneous growth of the 2 EELS peaks at 1750 and 750 cm-1 when the H/Al (100) surface is annealed. In the optimal Al14H2 structure the two H atoms are adsorbed at neighboring on-top sites with the computed 1776 cm-1 Al-H stretch and 784 cm-1 H-H vibration. The cluster calculations which simulate H monolayer coverage indicate the H adsorption energy to be relatively insensitive to surface coverage and are consistent with the zeroth-order kinetics found in the thermal desorption spectra of H2 from Al surfaces.