Energy-Level Alignment in the Adsorption of Phosphonyl Reagents on Gamma-Al2O3
NAVAL RESEARCH LAB WASHINGTON DC ELECTRONICS SCIENCE AND TECHNOLOGY DIV
Pagination or Media Count:
Density functional theory is applied to the computation of the adsorption energy DeltaEsub ads for a series of molecules on gamma-Al2O3. Three different cluster models are used to represent the gamma-Al2O3 surface. The molecules of interest all contain a phosphonyl PO functional group and adsorb via formation of a donor bond between the O atom and a threefold-coordinated tetrahedral Al AlTd surface site. The highest occupied molecular orbital HOMO of the free molecule is, in all cases, composed largely of non-bonding orbitals on the O atom of the PO group. The empty dangling orbital on the coordinatively-unsaturated AlTd site constitutes a surface state. A linear relationship is found between DEads and the difference between the orbital energies of the molecular HOMO eH and the surface state eS. Trends in DeltaEsub ads for different molecules can then be understood in terms of variations in eH. Likewise, differences in DeltaEsub ads for various cluster models can be explained by considering the differences in the predicted eS values. A further evaluation of the cluster models is presented by comparing results for the physisorption of H2O or CO with those obtained from two-dimensionally-periodic slab models. When differences in eH and eS are accounted for, the various models and computational procedures are seen to yield essentially equivalent results for adsorption of the molecules considered. These results are thought to constitute a useful conceptual tool for rationalizing DeltaEsub ads values for different molecules and cluster models.
- Inorganic Chemistry
- Physical Chemistry