Isotopic oxygen exchange reactions on magnesium oxide

Mellors, I, 1999. Isotopic oxygen exchange reactions on magnesium oxide. PhD, Nottingham Trent University.

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Abstract

Isotopic oxygen exchange is a very useful tool in probing the reactivity of the surface oxygen of metal oxide catalysts. From this it is possible to obtain information regarding the mechanism of oxygen activation, obtaining a fundamental understanding of how catalysts operate. This is of importance since, for example, oxygen activation may be the rate determining step in oxidation catalysis, where the Mars-van Krevelen mechanism occurs.

In this thesis isotopic oxygen exchange reactions have been performed on magnesium oxide (MgO). This material was chosen as its defect chemistry is well understood, and it can be prepared with well defined surfaces exhibiting variations in the relative numbers of co-ordinatively unsaturated ions present at the surface. Each of these may differ in their reactivity towards exchange, hence measuring the activity of a series of MgO samples exposing different populations of co-ordinatively unsaturated surface ions, can lead to the elucidation of the most reactive site.

Three sets of MgO catalysts have been tested in this study. Initially the exchange characteristics of commercial MgO samples donated by Ube Industries of Japan were determined. These catalysts were observed to expose the (100) face and comparison of specific rates of exchange indicated structure insensitivity. MgO was also prepared from magnesium ribbon, magnesium hydroxide and magnesium basic carbonate precursors, looking at the influence of starting material upon isotopic oxygen exchange properties. The ex basic carbonate showed an activity five times that of the other two materials, which were found to be very similar. Finally an attempt was made to produce (111) terminated MgO surfaces, which are generally not observed due to their relative instability. Upon stabilisation of these planes rates of isotopic oxygen exchange were subsequently measured, comparing them to those obtained previously. Suprisingly the (111) terminated surface showed a higher specific rate than expected, as it exposed predominantly low co-ordinated ions.

Each of the MgO catalysts used has been extensively characterised using techniques such as surface ai'ea determination, TEM, powder XRD and XPS. Also the relative populations of 3, 4 and 5 co-ordinated sites have been determined for the commercial Ube samples from UV/VIS/NIR diffuse reflectance, photoluminescence and FTIR spectroscopies.

Item Type:	Thesis
Creators:	Mellors, I.
Date:	1999
ISBN:	9781369325447
Identifiers:	Number Type PQ10290295 Other
Divisions:	Schools > School of Science and Technology
Record created by:	Linda Sullivan
Date Added:	05 Jul 2021 14:51
Last Modified:	20 Mar 2024 15:16
URI:	https://irep.ntu.ac.uk/id/eprint/43320

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