I. A Nuclear Magnetic Resonance Study of Metal Carbonlys in the Solid State, and II. Studies of the Surface Chemistry of Rhodium Supported on Alumina

Citation

Gleeson, James William (1982) I. A Nuclear Magnetic Resonance Study of Metal Carbonlys in the Solid State, and II. Studies of the Surface Chemistry of Rhodium Supported on Alumina. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/vmgw-qf86. https://resolver.caltech.edu/CaltechTHESIS:05152018-092853858

Abstract

Part I

The principal components of the ¹³C nuclear magnetic resonance chemical shift tensors of metal carbonyls containing between one and six metal atoms were determined from the powder patterns of the solid compounds. The tensors of terminally-bound CO groups are highly anisotropic (380 ± 60 ppm) and nearly axially symmetric. The tensors of bridging CO groups are much less anisotropic, due to significant asymmetry in the electron orbitals about the CO internuclear axis. The tensors vary only slightly for different transition metals. There is no intramolecular rearrangement of the metal carbonyls in the solid state at frequencies ≳ 10kHz, except in Fe₃(CO)₁₂.

Part II

The surface chemistry of rhodium supported on alumina was studied using infrared spectroscopy and quantitative measurements of the gases adsorbed and evolved during various procedures. First, the behavior of alumina-supported Rh upon heating in the presence of CO, CO₂, O₂ and H₂ was studied. The loss in the capacity to adsorb CO after heating to 525 K increases in the order O₂, H₂, vacuum < CO₂ < CO. Upon heating in CO, some CO is oxidized to CO₂ with oxygen from the surface, while the dicarbonyl-forming Rh^I is reduced to Rh°. The Rh° agglomerates, accounting for the substantial loss in capacity to adsorb CO. Upon heating in CO₂, the dicarbonyl-forming Rh^I is also deactivated. There is little loss in the capacity to adsorb CO upon heating in H₂, O₂ or vacuum.

Second, the adsorption of H₂S and its interaction with CO on Rh supported on alumina was studied. The dissociation of H₂S on the Rh at 300 K produces H₂ and is inhibited by preadsorbed CO. Rh also facilitates the reaction of H₂S with surface oxygen below or at 373 K, in which water is produced. After exposure of the Rh to H₂S, CO adsorbs in the linear, but not in the dicarbonyl or bridging modes. Exposure of a CO-precovered surface to H₂S displaces much of the bridging CO, but only slowly removes the dicarbonyl and linear CO. Exposure to H₂S strongly inhibits the removal of adsorbed CO by O₂, but exchange of adsorbed and gas phase CO occurs readily.

Thesis Files