Surface Chemistry and Decarbonylation of Molybdenum Hexacarbonyl on Thin Alumina Films

The adsorption of molybdenum hexacarbonyl is studied on thin hydroxylated and partially hydroxylated alumina films using reflection–absorption infrared, X-ray photoelectron, and temperature-programmed desorption spectroscopies. The majority of the Mo(CO)6 adsorbed on hydroxylated alumina at 80 K desorbs at ∼200 K; the remainder decarbonylates leading to a molybdenum coverage of ∼2% of a monolayer. Subcarbonyl species are detected as the sample is heated to ∼200 K and, at higher temperatures, the molybdenum is oxidized to an ∼+4 oxidation state and deposits primarily oxalate species on the surface. The adsorbed oxalates thermally decompose at ∼300 K to evolve CO to form adsorbed bidentate carbonate species. These are stable to ∼560 K and react to evolve CO at this temperature. The amount of molybdenum absorbed onto the surface can be increased by repeatedly dosing Mo(CO)6 at 80 K and annealing to ∼200 K to desorb molecular molybdenum hexacarbonyl where the molybdenum coverage increases by ∼2% of a monolayer for each cycle. It is also found that the extent of decarbonylation depends on the degree of alumina hydroxylation so that heating hydroxylated alumina to 900 K, which removes ∼50% of the surface hydroxyls, decreases both the CO desorption yield and the oxalate coverage by 50%.