Energy-level alignment in the adsorption of phosphonyl reagents on γ-Al2O3

Density functional theory is applied to the computation of the adsorption energy (ΔEads) for a series of molecules on γ-Al2O3. Three different cluster models are used to represent the γ-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 [Al(Td)] 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 Al(Td) site constitutes a surface state. A linear relationship is found between ΔEads and the difference between the orbital energies of the molecular HOMO (εH) and the surface state (εS). Trends in ΔEads for different molecules can then be understood in terms of variations in εH. Likewise, differences in ΔEads for various cluster models can be explained by considering the differences in the predicted εS 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 εH and εS 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 ΔEads values for different molecules and cluster models.