The Identification of Stable Reaction Intermediates on Aluminum Oxide Surfaces with Inelastic Electron Tunneling Spectroscopy (2007)

The stable surface intermediates that were formed in several heterogeneous reactions on aluminum oxide films were identified with inelastic electron tunneling spectroscopy. The heterogeneous reactions included associative and dissociative adsorption, nucleophilic substitution, surface catalyzed elimination, hydrolysis and hydrogenolysis. Primarily two chemisorption systems are discussed. These are cyclopropane carboxylic acid on aluminum oxide and phosphonate esters on aluminum oxide. In conjunction with the temperature and exposure dependence of the stable reaction intermediates, the nature of these reactions and the aluminum oxide surface are discussed. The temperature and exposure dependent interaction of cyclopropane carboxylic acid with alumina surfaces has been studied with inelastic electron tunneling spectroscopy. Cyclopropane carboxylate and n-butane carboxylate were the only adsorbed species formed, as determined by a comparison with tunneling spectra of n-butane carboxylic acid and cyclopropane carboxylic acid coadsorbed on alumina. The n-butane carboxylate results from hydrogenolysis of the cyclopropyl ring of the adsorbed cyclopropane carboxylate with hydrogen supplied by surface hydroxyl groups. The relative populations of the two surface species are strongly dependent upon coverage and temperature. The adsorption of gaseous dimethyl methyl phosphonate (DMMP) on aluminum oxide film surfaces has been investigated with inelastic electron tunneling spectroscopy. Surface temperatures ranged between 200 K and 673 K, and exposures ranged between 3 x 10-4 and 10 Torr-s. Tunneling spectra of deuterium labeled DMMP, perdeutero methyl alcohol, methyl methyl phosphonic acid, methyl phosphonic acid and trimethyl phosphine oxide, all adsorbed on aluminum oxide surfaces, were used to clarify the structures of the species resulting from the adsorption and decomposition of DMMP. At 200 K, DMMP is adsorbed molecularly with high surface coverages. At surface temperatures above 295 K, DMMP is adsorbed dissociatively in low coverages. Surface temperatures above 473 K lead to the dealkylation of the dissociatively adsorbed adspecies, which results in the formation of adsorbed methyl phosphonate. Inelastic electron tunneling spectroscopy was used to examine comparatively the adsorption and reaction of three phosphonate esters on aluminum oxide surfaces which were synthesized by the plasma oxidation of aluminum metal films. The phosphonate esters were diisopropyl methyl phosphonate (DIMP), dimethyl methyl phosphonate (DMMP) and diphenyl methyl phosphonate (DPMP). The adsorption temperatures ranged from 200 to 673 K. The exposures of gaseous DIMP and DMMP were 1.0 Torr-s, while DPMP was exposed to the surface as a 0.025 M solution in hexane. DIMP was found to adsorb associatively in low coverages at 295 K, whereas at 373 K, DIMP was found to adsorb dissociatively in low coverages as isopropyl methyl phosphonate. Above 373 K the isopropyl methyl phosphonate decomposed to the metastable hydroxy methyl phosphonate, which, in turn, decomposed completely to the methy phosphonate above 573 K. Although the dissociative adsorption temperature was lower (295 K), adsorbed DMMP decomposed similarly through the hydroxy methyl phosphonate to form the methyl phosphonate. However, in this case the hydroxy methyl phosphonate was a short-lived intermediate. DPMP was found to adsorb dissociatively at 295 K, and the adsorbed species formed is stable through 673 K. Consistent with these observations, mechanisms for the adsorption and decomposition of phosphonate esters on aluminum oxide are proposed that involve P-O bond cleavage upon adsorption but O-C bond cleavage upon decomposition of the adspecies. A versatile tunnel junction fabrication system is described. A temperature controller which resistively heats the aluminum oxide films by passing a current through the underlying aluminum film is presented. Computer software for spectral analysis is discussed.