The Influence of Silica Structure on the Properties of Supported Nickel Catalysts

[摘要] A series of silica-supported nickel catalysts has been prepared under standard conditions by impregnation of a range of silica supports with nickel(II) nitrate solutions. The catalysts, in their unreduced and reduced states, and the supports have been characterised using temperature programmed reduction (T.P.R.), neutron diffraction, small angle neutron scattering (S.A.N.S.), 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, carbon monoxide chemisorption and transmission electron microscopy (T.E.M.). Using these characterisation techniques it was established that, in unreduced calcined catalysts, two distinct forms of nickel oxide can exist on the surface of the support; the ratio of the two forms being dependent on the fundamental structure of the silica. One form, which predominated in calcined catalysts prepared from supports containing a small proportion of strained three-fold siloxane rings in their structures, reduced at low temperature and had a negligible interaction with the silica: a three-fold siloxane ring is a ring structure consisting of three [Si-O-] units. The other form, which was the predominant nickel oxide species in calcined catalysts prepared from supports whose structures contained a large proportion of three-fold siloxane rings, reduced at high temperature and interacted with the silica. The nickel particles formed upon reduction of the more reducible oxide species were large and sintered readily having no anchorage to the support. In contrast, reduction of the less reducible oxide species led to nickel particles which were small, thermally stable and anchored to the support. In consequence, reduced catalysts prepared from silicas with large proportions of three-fold siloxane rings in their structures possessed good and thermally stable nickel dispersions. The observation that the relative amounts of the two forms of nickel oxide were related to the proportion of three-fold siloxane rings in the silica structure has allowed a model for the interaction between nickel oxide and silica in calcined catalysts to be developed. This model can account for observations in the literature which previous models have found difficult to explain. Ethene hydrogenation was used as a test reaction to ensure that the reduced catalysts were behaving in a comparable manner to similar catalysts reported in the literature. It was found, in agreement with other workers, that the reaction was surface insensitive and that deactivation occurred during a series of hydrogenation reactions performed using the catalysts. This deactivation was probably due to the formation and build-up of carbonaceous residues on the metal surfaces of the catalysts. The use of ethene hydrogenation as a test reaction, combined with the T.P.R. characterisation results, demonstrated that the catalysts in their reduced and unreduced states behaved similarly to previously reported analogous studies. Since the catalysts were not anomalous, at least with respect to ethene hydrogenation and their T.P.R. characteristics, the conclusions reached in this dissertation can be extended to silica-supported nickel catalysts prepared by impregnation in general.

[发布机构] University:University of Glasgow

[关键词] Physical chemistry, Inorganic chemistry