Characterization of Ultrathin Films of γ-Al2O3and the Chemistry of 1,3-Butadiene on NiAl(001) and γ-Al2O3

Abstract

Ultrathin films of γ-Al2O3 grown on NiAl(001) were studied using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Growth of the ultrathin oxide films with water produces a hydroxylated surface, as confirmed by vibrational spectroscopy. Also, exposure of a film grown with O2, to H2O following growth results in OH groups on the surface. Following adsorption at 170 K, the OH-stretching mode is observed (HREELS) as a relatively narrow band at 3690 cm-1 with a broad, low-intensity shoulder to lower frequencies, indicative of isolated OH groups bridge-bonded to aluminum sites and a small degree of OH hydrogen-bonding. The hydrogen-bonded species are removed by warming above 210 K. Adsorption and reaction of 1,3-butadiene on NiAl(001), and thin films of γ-Al2O3, have been studied using HREELS, laser-induced desorption coupled with Fourier transform mass spectroscopy (LID-FTMS), AES, and LEED. We find that at 170 K, the 1,3-butadiene is irreversibly π-bonded to NiAl(001). Upon warming the surface to 300 K, the adsorbate is efficiently converted into σ-bonded species without undergoing decomposition, and is stable within the 300−400 K temperature range. Heating the surface above 400 K causes decomposition of the adsorbate. In contrast, the 1,3-butadiene adsorption on thin films of both hydroxylated and non-hydroxylated γ-Al2O3 is largely reversible. Dimerization of 1,3-butadiene to 4-vinylcyclohexene was observed on hydroxylated γ-Al2O3. Some decomposition of the 1,3-butadiene takes place on both oxide surfaces at temperatures as low as 170 K.