Interactions of hydrogen with alkali promoted Ru/SiO2 catalysts: a proton NMR study
The work completed for this dissertation has endeavored to characterize alkali promoted Ru/SiO[subscript]2 catalysts. Initial work investigated the role of hydrogen spillover to the silica support on the characterization of the active metal sites via a hydrogen chemisorption technique. A strongly bound component of spilled over hydrogen was found in the silica support which interfered with accurate measurements of active metal sites via volumetric strong hydrogen chemisorption techniques. Based on the results of this work, the volumetric chemisorption technique was modified such that typical measurement times were reduced from 12-36 hours to 1 hour for silica supported ruthenium catalysts;The active Ru surface was characterized by observing the changes in the proton spin counts and NMR Knight shifts as a function of alkali loading. Na and K promoters blocked the active surface of the Ru metal. However, Cs was pushed off the metal surface upon hydrogen chemisorption. Invariance of proton Knight shifts as a function of alkali coverage suggested the absence of electron transfer from the alkali promoter to ruthenium metal. Dynamic proton NMR studies indicated that the presence of the alkali promoters restricted hydrogen mobility on both the metal surface and at the metal support interfaces (spillover). This finding was consistent with the previously reported effects of the alkali promoters on the Fischer Tropsch synthesis;The effect of the active metal and the promoter on the support hydroxyl groups were characterized via proton nuclear magnetic resonance ([superscript]1H NMR) spectroscopy. The characterization of the support at different Ru loadings indicated that the OH group density in the silica support decreased with increasing metal and or promoter loading but not on a one-to-one basis. The exchange efficiency of the hydroxyls decreased with increasing atomic size of the alkali metal (Na > K > Cs). An additional downfield proton resonance was detected for all the alkali promoted catalysts which was assigned to the alkali hydroxide species present in the support.