The structure and surface acidity of thin water-oxidized alumina films on Al( 110) have been studied as a function of film treatment temperature and ethylene adsorption using X-ray photoelectron spectroscopy. The O H content of the oxides decreases monotonically with temperature while the crystallinity increases uniformly to form a polycrystalline y-alumina at 500 'C. Ethylene adsorbed on the oxides identifies the presence of acid sites with a maximum spectral intensity for alumina films pretreated between 350 and 400 OC. An interpretation is proposed in terms of a molecular orbital adsorbate-surface model based on the core level and valence level photoemission data to explain how chemisorbed molecular ethylene adsorbate may form from a ?r-complex adduct on the acid sites. The results are consistent with the formation of a +C*H5 carbenium ion.
IntroductionMarked structural and compositional variations in metal oxides make them particularly useful and important as catalysts and catalyst supports. Of their many interesting surface properties, the acid/base character is of particular significance in understanding the catalytic chemistry of these systems. It is also important for improving their effectiveness in practical applications. Theobjectiveof this paper is to investigate the structural, electronic, and chemical characteristics of acid-base sites on alumina films as a function of treatment temperature. These insights were extended where appropriate to the molecular orbital interpretation of ethylene chemisorption behavior on a characterized and reproducibly prepared oxide surface. This paper describes the use of X-ray and ultraviolet photoelectron spectroscopies (XPS, UPS) to study the effect of thermal pretreatment of the alumina on the dehydroxylation process, the development of surface acidity, and the degree of crystallinity of the alumina films. As documented by Pines,l skeletal isomerizations of olefins is a standard technique for identifying the number and acid strengths of sites which produce carbenium ions on alumina surfaces. Adsorption of ethylene, a weak Lewis base, may be used therefore to identify the presence of acid sites and to elucidate the electronic interactions of the acid-base adduct. Complementary X-ray absorption spectroscopic results on surface defect structure and related effects are reported separately.2 Many techniques have been used to determine the nature, strength, and distribution of acid sites on alumina surface^.^ Some specific published information relevant to the present soft X-ray study is reviewed here. Infrared spectroscopy (IRS) and highresolution electron loss spectroscopy (HREELS) are particularly useful because, on many oxide surfaces, the acid/base sites consist of hydroxyl groups which have readily identifiable stretching modes in the 3400-3700-cm-1 range. For example, on aluminum oxides five resolvable absorption bands in this range have been attributed to hydroxyl groups of various Bronsted acid strengths.& To our knowledge, one of the more definitive XPS studies ...
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
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.