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Pyridine adsorption on NiAl(100) and ultrathin films of γ-Al 2 O 3 was studied using high-resolution electron energy loss spectroscopy (HREELS). Pyridine adsorbs on NiAl(100) with its molecular axis parallel to the surface plane at low coverages and with its molecular plane inclined more toward the surface normal at higher coverages. On the hydroxylated and nonhydroxylated thin films of γ-Al 2 O 3 , pyridine interacts with the coordinatively unsaturated Al 3+ cations via its nitrogen lone-pair electrons. Pyridine was also found to interact with the surface hydroxyl groups on the hydroxylated γ-Al 2 O 3 thin films, forming C 5 H 5 N-HO complexes. Complex formation causes the OH bond strength to decrease and the OH stretch vibration to shift from 3711 cm -1 , which is characteristic of uncomplexed, isolated OH, to lower frequency. At low coverages, pyridine only interacts with the more acidic surface OH groups, those located on 3-fold Al 3+ cation sites. This interaction forms a C 5 H 5 N-HO complex, which has an O-H stretch at 2920 cm -1 . As the coverage is increased, an additional C 5 H 5 N-HO complex is formed from an interaction between pyridine and the less acidic OH groups, which are bonded to 2 Al 3+ cations. The vibrational frequency for O-H stretch of this C 5 H 5 N-HO complex is 3150 cm -1 . As the intensities for the O-H stretches of the C 5 H 5 N-HO complexes increase, the intensity for the free O-H stretch at 3711 cm -1 decreases. The interaction with the surface hydroxyl groups is reversible, confirming that the observed shifts in the O-H stretching frequency result from the formation of weakly bonded acid-base complexes. While most of the pyridine desorbs from the γ-Al 2 O 3 thin films by 290 K, annealing the pyridine-dosed γ-Al 2 O 3 thin films to temperatures above 290 K, results in a small amount of pyridine dehydrogenation and the formation of surface OH groups with an O-H stretch at 3742 cm -1 .

The adsorption of CO on hydrated 5 wt % Ru/Al 2 O 3 produced ν CO absorbance features at ∼2048, 1992, and 1924 cm -1 that are red-shifted by 50-116 cm -1 from those seen in the absence of water (2020-2040, 2080, and 2140 cm -1 ). This red-shift most likely arises from dipole-dipole interaction between coadsorbed CO and water molecules since (1) the exact frequency of the ν CO absorbance feature depends upon the amount of coadsorbed water and (2) the presence of flowing liquid water further red-shifts the frequencies. These ν CO absorbance features are uncorrelated, since the relative intensities of the ν CO absorbances at 2049, 1992, and 1924 cm -1 depend on the amount of coadsorbed water and CO on the surface. Temperature programmed desorption done with TGA-MS indicated three different high-temperature CO 2 desorption peaks. These CO 2 peaks (T ≈ 350, 400, and 550 °C) are most likely the result of the oxidation of adsorbed CO reacting with surface adsorbed water (CO ads + H 2 O ads f H 2 + CO 2 ) and/or the disproportionation of CO (2CO f C ads + CO 2 ). These high-temperature CO 2 desorption peaks suggest that CO strongly adsorbs to hydrated 5 wt % Ru/Al 2 O 3 catalysts. This is corroborated by the fact that intensities of the ν CO absorbance features do not decrease in the presence of flowing liquid water.

The surface chemistry of thiophene on reduced and sulfided Ni/SiO 2 and Ni 2 P/SiO 2 catalysts has been investigated by using infrared (IR) spectroscopy. Thiophene is quite reactive on Ni 2 P/SiO 2 catalysts; even at 190 K, cleavage of some C-S bonds was observed, forming 1,3-butadiene-like species on the surface. Annealing this thiophene-dosed Ni 2 P/SiO 2 catalyst to 250 K resulted in the formation of adsorbed butenes and other thiophene decomposition products on the surface. In the presence of H 2 and elevated temperatures, the reactivity of thiophene on Ni 2 P/SiO 2 catalysts increased, producing adsorbed butenes and saturated hydrocarbon fragments. Coadsorption experiments indicate that CO and thiophene compete for the same sites on the Ni 2 P/SiO 2 catalysts, as preadsorbed CO blocked the adsorption of thiophene on Ni 2 P sites. The IR spectroscopic data for the adsorption and reactivity of thiophene on reduced and sulfided Ni/SiO 2 and Ni 2 P/SiO 2 catalysts correlate well with the trend of HDS activity of the catalysts. Specifically, the Ni 2 P/SiO 2 catalysts are more reactive toward thiophene than reduced and sulfided Ni/SiO 2 catalysts in UHV and in an atmospheric pressure flow reactor. The increased reactivity of thiophene on Ni 2 P/SiO 2 catalysts in UHV may explain the high turnover frequency previously reported for Ni 2 P/SiO 2 catalysts relative to sulfided Ni/SiO 2 , Mo/SiO 2 , and Ni-Mo/SiO 2 catalysts.

Ultrathin films of γ-Al 2 O 3 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 O 2 , to H 2 O 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-Al 2 O 3 , 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-Al 2 O 3 is largely reversible. Dimerization of 1,3-butadiene to 4-vinylcyclohexene was observed on hydroxylated γ-Al 2 O 3 . Some decomposition of the 1,3-butadiene takes place on both oxide surfaces at temperatures as low as 170 K.

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