| The phenomena of hydrogen spillover on heterogeneous catalysis has been studied extensively. However, questions involving the nature of the spiltover species; the possibility of activated sites on an inert support; the magnitude and importance of spillover in terms of industrial catalysis and kinetics, etc. are still unresolved. Using Proton NMR Spectrometry, Mass Spectrometry, MacBain Balance, and Isotopic studies, these questions are addressed for the system of hydrogen spillover on silica aerosil.;Depending on the activation conditions, one or two types of sites seem to be created. The mechanism of site activation can involve vicinal and/or geminal hydroxyls at specific surface sites on the oxide interacting with spiltover hydrogen to form an F center-type defect and gaseous water. Stressed siloxane bonds could also be involved. The rate of this zero order reaction is approximately 3.3 x 10('11) sites formed per hour with an apparent activation energy of 28 kcal/gmol.;This induced activity is independent of the metal and quite different. The mechanism of ethylene hydrogenation on activated aerosil is similar to that on metal oxides in the sense that the molecular identity of the reacting deuterium is retained and dideuteroethane is the primary product. Differences in activity from both metals and metal oxides are apparent such as partial ethylene exchange and H(,2)-D(,2) exchange. It is not clear whether these differences are due to unique catalytic properties of the activated silica or to the higher reaction temperatures (200(DEGREES)C) involved.;The proton NMR studied indicate that spiltover hydrogen has a noticeable effect on the resonance spectra of hydrogen on aerosil. Three peaks are present, one upfield and one downfield from a central H(,2)-OH resonance peak. The experimental results can be explained most simply by the existance of a radical hydrogen species involved in the formation of a surface complex. Also, a resonance peak associated with spillover-induced active sites is observed. The active sites seem to have a high electron density and are thus electron donor or "metallic" in nature. |
