The activated, dissociative chemisorption of methane on tungsten

The dissociative chemisorption of methane on tungsten has been investigated for temperatures between 600 and 2600°K and it is shown that at least two processes are operative. The apparent activation energies are 10.2 kcal/mole (1200°K T 2600°K) and 2.7 kcal/mole (600°K T 1000°K). The sticking probability varies from 3×10−3 at 2600°K to ∼10−4 at 600°K. The sticking probabilities are independent of pressure for the clean surface but are shown to decrease as the surface concentration of carbon or nitrogen increases. Above ∼1900°K the sticking probability was independent of the number of molecules previously reacted, indicating that carbon was diffusing into the bulk of being desorbed. A large kinetic isotope effect with rather unique features is observed. Tentative explanations of these phenomena are suggested.