Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Supported and unsupported molybdate films have served as the backbone as of the hydrodesulphurisation catalysts used for removing organo-sulphur compounds from light petroleum feedstocks. The catalyst material starts as one or more molybdenum (VI) oxide(s), but the oxidation state reduces to Mo(IV) during sulphidation. Despite the extensive analytical studies carried out on these systems over the past 25 years, the fundamental questions still remain--what is the nature of the catalytic site on the molybdate hydrodesulphurisation catalyst, and is the site identical on promoted and unpromoted catalysts?;A new approach to catalyst film preparation was developed using thin supported molybdate films. The films could be easily prepared and their compositions could be easily changed, and they could be readily studied using modern surface analytical equipment. X-ray photoelectron spectroscopy (XPS) was chosen as the primary tool to characterise the molybdate films, while laser Raman spectroscopy (LRS) was used to complement the information gained from XPS.;The surfaces of supported molybdenum oxide thin films were characterised following calcination, reduction, and sulphidation treatments. Special peak broadening of Mo(VI) oxide films have been observed on electrically conducting thin films deposited on planar alumina and graphite substrates. This peak broadening decreases as the film thickness increases. This phenomenon may be influenced either by aggregate sizes and the local electronic environment of the molybdate clusters, or it may be indicative of multiple, unresolved Mo(VI) species. Thermal reduction of supported MoO{dollar}\sb3{dollar} in H{dollar}\sb2,{dollar} N{dollar}\sb2,{dollar} or in vacuo at 500{dollar}\sp\circ{dollar}C for extended periods yields a mixture of MoO{dollar}\sb2{dollar} and higher Mo(3d) oxidation states, including a non-octahedral Mo(VI) species. The diversity of reduction products observed under these conditions can be adequately explained using crystallographic shear mechanisms. Sulphidation of thin film molybdenum oxides yields a variety of Mo(3d) species including oxide and sulphide components. Two sulphide species are observed on thin {dollar}({dollar}100 nm) yield only MoS{dollar}\sb2.{dollar} The reduced sulphide species may be a thiomolybdate or polysulphide component which forms at coordinatively unsaturated positions on the MoS{dollar}\sb2{dollar} slabs such as corner and edge sites. This Mo(3d) sulphide species has been observed previously on Co-Mo-{dollar}\rm Al\sb2O\sb3{dollar} films exhibiting measurable catalytic activity for thiophene hydrodesulphurisation.



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