Surface oxidation reactions of cobalt, molybdenum and mixed cobalt-molybdenum metals have been investigated using x-ray pbchelectron spectroscopy (XPS). Tbe oxide stoicbiometries have been determined from XPS intensity measurements Sucb quantification has been important in identifying oxide compositions with cbaracteristic XPS spectra. A number of discrete binary molybdenum oxides have been characterized after reactions at 2OOOC and pressures ranging from 1 atm to Pa. At the lowest pressure, the Mo 3d spectra and O/Mo ratias suggest the formation of a molybdenum-oxygen structure with a stoicbiometry oear unity. By contrast, at bigber presa~ures, oxides that are predominantly MOO, and MOO, appear, but other intermediate oxide structures are also identified. Wellde6ned binary oxides of cobalt can be prepared from cobalt metal after beating in oxygen at merent pressures. New binary oxides of cobalt and molybdenum have been generated by the reaction on MOO, or c0#4 substrates. Oxidation of a tbin film of molybdenum on a Co,O, substrate bas been shown to produce a nearly stoichiometric rrurface layer of CoMd), . The procedure could be wieful for the production of other ternary oxides of possible interest for use as XPS reference materials.

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Laboratory studies of catalysed hydrodesulphurization reactions on surfaces have usually been studied using the same bulk form of catalyst as is used in the field. Possibly the most heavily investigated system is alumina-supported cobalt oxide molybdenum oxide. The surface of this catalyst has been characterized by many techniques, particularly x-ray photoelectron spectroscopy (XPS), during the reduction, sulphidation and reaction stages.While a wealth of experimental data has been produced from this approach, there are several compelling reasons for studying the catalyst in a flat, thin film form, rather than a bulk material. Firstly, if the catalyst structure is layered, covering a macroscopic area, it becomes easier to study the interaction between support and reactive layer using techniques such as non-destructive depth profiling. Secondly. the deposition of catalysts in thin films affords a much better control over the surface composition than by the usual bulk precipitation reactions employed. Finally, studies of such films by electron spectroscopic techniques is rendered much easier. since the films are usually deposited on an electrically conducting substrate and are not therefore subject to significant surface charging effects as bulk catalysts usually are. Therefore, photoelectron line shapes and positions become much easier to identify by XPS.The objective of this particular work therefore, has been to prepare thin film analogues of the oxide precursor of the alumina-supported Co-Mo catalyst used in hydrodesulpherization reactions. Earlier XPS studies of such oxides' have suggested that the maximum efficiency of the catalyst, in its reduced and sulphided form, comes from an oxide precursor with a Co/Mo atomic ration of -1 : 5 , with cobalt present as Co304 and molybdenum oxides present in oxidation states from + 4 to + 6. This paper reports the fabrication and characterization of thin films of cobalt and molybdenum oxides on an equally thin film of alumina grown on an aluminium metat substrate. The alumina film, 10-20 nm in thickness, was grown by thermal oxidation at 200°C. Subsequently, layers of molybdenum and/or cobalt were deposited onto the alumina substrate using ion beam deposition. These metallic layers were oxidized and calcined under a number of temperature (2S°C-500"C) and pressure (760 torr-lop4 torr) conditions. Ultimately, several series of thin film specimens were produced: 1. Cobalt oxides on alumina 2. Molybdenum oxides on alumina 3. Mixed cobalt-molybdenum oxides on alumina 4. Molybdenum oxides on cobalt oxides on alumina 5 . Cobalt oxides on molybdenum oxides on alumina The surfaces of some of these films were intended to correspond to the pre-reduced form of the bulk catalyst.Auger electron spectroscopy and ion bombardment were used to depth profile these films in an effort to measure layer thickness and to determine the degree of interaction of the layers. Prior to calcining, the films were found to be 5-10 nm in thickness while following calcining the distribution of Calcining of...

Alumina-supported cobalt and molybdenum oxides, after sulphiding, have high hydrogenation and hydrodesulphurization (HDS) activities and are therefore widely used as catalysts in the petroleum refining industry. ' Such sulphides have recently been prepared in thin film form, and the preparation methods and advantages of such surfaces for catalysis research have been described in a previous paper in these Proceedings.2 This present paper describes the reaction of some of the prepared films with a model organic sulphide (thiophene) mixed with hydrogen and performed under HDS process conditions.A micro-activity testing unit (MAT) was used to determine the hydrodesulphurization capability of the five thin film types. The amount of injected thiophene (C4H4S) converted to hydrocarbons (butanes and butenes) is monitored by gas chromatography. X-ray photoelectron spectroscopy (XPS) chemically characterized the surface region of the specimens prior to the following reduction (H?), sulphidation (H2S/H2) and thiophene (C4H4S/H2) exposure. A commercial Co-Mo-alumina H D S catalyst was processed identically for reference purposes. Details of the reaction vessel, reaction conditions, specimen transfer procedures and transfer hardware to permit specimen examination by XPS without air exposure will be published elsewhere.A semi-quantiative surface chemical analysis of each sample is performed using the XPS determined elemental peak intensities and empirically derived elemental sensitivity factors. Peak shape and peak binding energy values are used to determine element oxidation states and, where possible, aid in compound identification.Reduction, sulphidation and thiophene HDS steps were done sequentially with XPS analysis performed after each step. Each specimen was reduced in flowing hydrogen for 30 min (120 ml min-') at 623 K, sulphided in flowing 10% H2S/H2 for 60 min (120 ml min-') at 623 K and exposed to thiophene by direct injection of 10 pl of feed into the cool