Interaction of molybdenum hexacarbonyl with hydroxylated alumina thin films at high temperatures: Formation and removal of surface carbides

The chemistry of Mo(CO)6 is studied on a thin hydroxylated alumina (HA) film grown in ultrahigh vacuum on a Mo(1 0 0) substrate using temperature-programmed desorption, and X-ray and Auger spectroscopies. The uptake of molybdenum on HA at ∼700 K is about six times larger than on a dehydroxylated surface and this is ascribed to a combination of a higher surface area for the hydroxylated alumina film and the presence of surface hydroxyl groups. This reaction results in the formation of molybdenum carbides where the stoichiometry is close to MoC at low exposures, while it approaches Mo2C as the exposure increases. The carbide incorporates a small amount of oxygen and measuring the Mo 3d5/2 chemical shifts suggests that the carbides are present as small particles at low Mo(CO)6 exposures. With increasing exposure, the surface carbide forms a thin film. Heating the surface evolves predominantly carbon monoxide due to alumina reduction to form metallic aluminum and, at high Mo(CO)6 exposures, the molybdenum diffuses into the bulk while, at lower Mo(CO)6 exposures, a surface MoAl alloy is formed.

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