Acid-Site Characterization of Water-Oxidized Alumina Films by Near-Edge X-Ray Absorption and Soft X-Ray Photoemission

Abstract

Hydroxylated alumina films have been synthesized by water oxidation of single crystal Al( 110) surfaces. Thermal dehydroxylation results in anion vacancies which produce an Al(3s) defect state 3.5 eV below the conduction band edge. A maximum in the defect-DOS occurs for c oxides heated to 350 to 400 "C, which is where the materials exhibit maximum Lewis acidity With respect to GH,. Adsorbed GH, produces thermally reactive G species which interact covalently with the defect-DOS and nonbonding O(2p) from the top of the valence band. C(1s) binding energies suggest significant charge transfer which is consistent with a carbenium ion. Ni evaporated onto the surface, however, transfers charge directly to Al species and does not interact with 0 atoms at the defect site. The defect-DOS is regenerated when the C, species decomposes or when Ni migrates thermally through the oxide layer.It has been suggested that surface acidiv in aluminas results primarily from coordinatively unsaturated aluminum sites. Since some, If not the majority, of these acid sites function as Lewis acids, i e electron acceptors, it is possible that, electronically, they would produce empty