Paper HC+SS-ThM11
State-resolved Reactivity of Methane on Ir(110)-(1x2)

Thursday, November 10, 2016, 11:20 am, Room 103A

The rate-limiting step in the steam reforming reaction, in which methane and water react to form hydrogen gas and carbon monoxide (syngas), is the initial cleavage of a C-H bond in the methane molecule. Methane’s dissociative chemisorption is highly activated on catalytically active transition metal surfaces. To date, experimental measurements have focused on CH₄ molecules whose internal (vibrational) energy is less, and frequently much less than the threshold energy for reaction. Under those conditions, significant incident translational energy (TE) or energy transfer from the surface is required to activate dissociation, and reactions most often proceed via a direct dissociative chemisorption mechanism. Using a molecular beam in conjunction with an OPO-OPA continuous-wave IR laser, we are able to prepare methane molecules with a sharply defined kinetic energy and 36 kJ/mol of internal vibrational energy, which approaches or even exceeds the threshold energy for dissociative chemisorption. These molecules possess sufficient energy to react via direct or precursor-mediated mechanisms. The direct channel is characterized by an increase in reactivity with increasing TE, and is dominant for molecules with >10 kJ/mol of TE. Molecules with <10 kJ/mol of energy react through precursor-mediated channel, in which reactivity decreases with increasing TE. This low-TE precursor channel is especially interesting in a catalytic context, as most molecules under typical industrial reactor conditions have TEs where trapping, and therefore physisorption probabilities are high.

In studies of CH₄ dissociation on Ir single crystal surfaces, we observe both the precursor and direct channels for reaction. On Ir(111) we observe that 36 kJ/mol of E_vib in the n₃ C-H stretch enhances reactivity in both channels at a surface temperature of 1000K. On the corrugated Ir(110)-(1x2) surface, we still observe vibrational-energy enhancement in both channels, but the TE dependence of S₀ differs for the vibrationally hot and ground state CH₄ molecules at T_surf = 1000K. Upon lowering T_surf to 500K, vibrational ground state molecules no longer have a pronounced precursor-mediated reaction channel, but the vibrationally excited molecules do. We will discuss the origin of these similarities and differences. The observed reactivity of vibrationally hot methane molecules with thermal TE points to the potentially important role that vibrationally hot precursor molecules may play in industrially catalyzed reactors.