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

The reaction of Mo(CO)6 with thin films of dehydroxylated alumina grown on a Mo(1 0 0) substrate above 500 K is studied in ultrahigh vacuum using temperature-programmed desorption and Auger and X-ray photoelectron spectroscopies. This results in the formation of a molybdenum carbide that incorporates a small amount of oxygen, where MoC is formed initially, becoming closer to Mo2C as the Mo(CO)6 exposure increases. Heating the (oxy)carbide-covered surface desorbs CO in two states at ∼1130 and 1320 K. Forming the alumina films with H218O shows that the majority of the oxygen in the CO derives from the substrate. The lower-temperature desorption state arises from both oxycarbide decomposition and alumina reduction by the carbide, while the high-temperature state results only from reduction of the alumina, finally resulting in the formation of a MoAl alloy.