Development of Surface Acidity in Water-Oxidized Alumina Films: Ethylene Adsorption as a Function of Pretreatment Temperature

Abstract

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 ...

Computational Study of Environmental Effects in the Adsorption of DMMP, Sarin, and VX on γ-Al2O3: Photolysis and Surface Hydroxylation

“…In the following, reference will be made to the "valence band" (VB) of the cluster and to the "valence band maximum" (VBM), neglecting the fact that the cluster orbitals constitute a finite (albeit closely spaced) set of discrete states. Soft-X-ray photoemission spectroscopy (SXPS) data 87 for hydroxylated Al 2 O 3 films on an Al (110) surface (Figure 4b) show a bonding state, for an unknown type of OH site, at about 9 eV below the VBM. Figure 4a shows the density of states (DOS) computed for the 1,4 type IIIb configuration of the Al 32 O 48 cluster (Figure 3b).…”

“…The DOS has been broadened with an arbitrarily chosen Gaussian width of 0.27 eV. (b) SXPS data (ref with permission) for an hydroxylated Al 2 O 3 film on Al (110). The energy scale has been shifted by 1.0 eV from that given in the original reference to place the VBM at the foot of the strong peak so as to be consistent with (a).…”

“…The energy scale has been shifted by 1.0 eV from that given in the original reference to place the VBM at the foot of the strong peak so as to be consistent with (a). The feature assigned in ref to OH is marked. “O 2s” and “O 2p” indicate the dominant orbital composition of the two SXPS bands.…”

Interaction of molybdenum hexacarbonyl with metallic aluminum at high temperatures: Carbide and alloy formation

“…There are several advantages to this approach. First, the thin oxide film deposited onto a metal substrate allows electron-based spectroscopies to be used to interrogate the surface since the film is sufficiently thin that it does not charge [17][18][19][20][21][22][23][24][25][26][27]. In addition, other, non-electron-based techniques, such as reflection-absorption infrared spectroscopy (RAIRS) can also be used.…”