An Inelastic Electron Tunneling Spectroscopic Investigation of Inorganic and Organometallic Molybdenum Complexes Adsorbed on Aluminum Oxide Surfaces

Citation

Gajda, Gregory Joseph (1986) An Inelastic Electron Tunneling Spectroscopic Investigation of Inorganic and Organometallic Molybdenum Complexes Adsorbed on Aluminum Oxide Surfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2evk-b965. https://resolver.caltech.edu/CaltechTHESIS:04152019-124552652

Abstract

The design, construction and operation of an integrated glove box-based tunnel junction fabrication system are described. This versatile, all stainless steel, ultrahigh vacuum compatible system provides both vapor and liquid phase adsorption capabilities with a wide variety of molecules. The differences between adsorption from the liquid phase under an inert atmosphere and in the laboratory atmosphere are illustrated by spectra from junctions prepared with ethanolamine (2-amino-ethanol). A rapid (~ 10 s) and accurate (~ 1%) constant modulation current tunnel junction spectrometer displays the obtained spectrum on a storage oscilloscope. The design of these electronics is discussed, and the ability to rapidly optimize the lock-in detector phase setting simplifies the measurement of inelastic electron tunneling spectra. The use of a numerical first derivative routine to remove the linear background slope in tunneling spectra is described, and its ability to determine peak positions in the processed spectra is discussed.

Molybdenum hexacarbonyl adsorbs reversibly on hydroxylated alumina surfaces at 22°C to form a carboxylic acid-type adspecies . An estimate of the heat of adsorption energy, 15 ± 3 kcal/mole, is derived from the desorption kinetics. Heating the alumina surface to 140 ± 2°C during adsorption produces thick (> 500 Å) molybdenum films. Heating the surface to 100°C during adsorption produces decomposition products which consist of molybdenum oxides and subcarbonyls, depending on the decomposition parameters (time and post-heating sequence). The decomposition products show no reaction with 2 Torr of ethylene at 100°C even after one hour.

At 22°C, molybdenum oxytetrachloride adsorbs and reacts with the surface hydroxyl groups on the alumina to form a dioxo-bridged molybdenum dichloro complex, which desorbs slowly at 295 K. Heating during the adsorption produces poorly reproducible molybdenum suboxide compounds. At 22°C molybdenum dioxodichloride adsorbs from the vapor phase and reacts with surface hydroxyl groups on the alumina to form a molybdenum oxide which reacts readily with background water vapor. Heating the surface during exposure produces a partially polymerized oxide which still reacts with background water vapor. Heating the exposed surfaces in vacuum dehydrates the oxide and increases the extent of oligomerization. The oxides show no reaction with 2 Torr of ethylene at 100°C after 30 minutes, or with acetic acid at 10^-1 Torr after five minutes at 22°C. Exposure to 5 x 10^-2 Torr of 4-t-butylpyridine for 100 s at 22°C results in complete desorption of the oxide.

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