Origins of the High Thermal Stability of Alkylidene Groups on the Surface of β-Mo2C

Surface alkylidenes can be formed on β-Mo2C through the selective carbonyl bond scission of ketones and aldehydes. Spectroscopic studies show that alkylidene groups remain intact on the carbide surface to above 900 K under ultrahigh vacuum conditions. A rationalization for the anomalously high thermal stability is presented on the basis of surface analysis studies. It is shown that the relatively high reactivity of the clean carbide surface permits both carbonyl bond scission and other less selective processes involving CC and CH bond cleavage. The combined decomposition channels lead to the deposition of excess carbon and high-coordination oxygen, resulting in an inert surface on which alkylidenes are thermally stable. Removal of surface carbon, through CO desorption and carbon diffusion, occurs at high temperatures leading to a newly reactive surface. Alkylidenes trapped at low temperatures on the intrinsically passivated surface can survive until the clean surface is partially restored at high temperatures.