Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
Terry S. King
An in situ [superscript]1H nuclear magnetic resonance (NMR) technique has been employed to study the interaction of hydrogen with silica supported metal and bimetallic catalysts. This technique was applied in the temperature range of 300 to 473 K and at pressures of 10[superscript]-5 to 760 Torr. On ruthenium and rhodium catalysts two distinct adsorbed resonances were detected. We have labelled these resonances as [alpha] and [beta]. The [beta] resonance is associated with a more weakly adsorbed species and was observable only at pressures greater than 100 Torr. Both these resonances represent adsorbed hydrogen interacting with the metal. The delocalized nature of [beta] hydrogen makes it difficult to associate it with any particular adsorption site. The population of [beta] hydrogen was found to be a strong function of the concentration of surface defects. Studies of hydrogen interaction with silica supported Ru-Group Ib bimetallic catalysts revealed several interesting facts. Addition of silver to Ru was found to greatly diminish the relative amount of the [beta] resonance. Since silver preferentially populates edge, corner and other defect-like sites, the weakly bound species associated with the [beta] resonance was correlated with low coordinated metal atoms. The total hydrogen population on the catalyst remained approximately the same with the addition of copper because dissociated hydrogen on ruthenium atoms can spillover to copper. We postulate that the difference in catalytic behavior of Ru, Ru-Cu and Ru-Ag is due to the surface segregation induced structure sensitivity of the weakly bound hydrogen. On silica supported platinum catalyst, no [beta] hydrogen was detected. The availability of the [beta] hydrogen, a weakly adsorbed species associated with the [beta] resonance, on ruthenium and rhodium may open reaction pathways that would not be available on platinum catalysts. Several hydrocarbon reactions involving hydrogen show great sensitivity to hydrogen pressure and the catalyst metal. We postulate that some of these previously well noted hydrogen effects are the manifestation of the availability (or unavailability) of different hydrogen states with variations in pressure.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Bhatia, Sandeep, "Fundamental studies of hydrogen interaction with supported metal and bimetallic catalysts " (1993). Retrospective Theses and Dissertations. 10797.