Paper SS2-ThA10
The Role of Vibrationally Hot Precursors in Methane Activation on Ir(111) and Ir(110)

Thursday, October 31, 2013, 5:00 pm, Room 202 A

Vibrationally excited molecules can play an important role in methane activation on transition metal catalysts. This talk will focus on the reactivity of vibrationally excited methane molecules physisorbed to the surface. Under thermal reaction conditions typical in a steam reforming reactor, over half of the methane molecules are vibrationally excited, but their average translational energy is low enough that their trapping probability is significant. We use seeding techniques and molecular beam scattering methods to control the translational energy of the methane molecules incident on the surface, and infrared laser excitation of molecules in the molecular beam to control the amount and type of vibrational excitation. We find that vibrationally hot molecules can undergo reaction on low-index Ir surfaces via a precursor-mediated mechanism even in the presence of efficient vibrational quenching channels on the metal surface. In another set of experiments we explore how methane molecules in the ground vibrational state are activated while physisorbed to a hot surface.

In contrast to prior suggestions, this work suggests that an increase in reactivity with gas temperature may not be an automatic signature for a direct reaction mechanism where methane dissociates in a single collision. High gas temperatures also result in significant populations of vibrationally excited molecules whose additional internal energy may enhance their precursor-mediated reactivity. Our ability to control independently the translational and vibrational energy of the methane reagent and the surface temperature allows us to deconvolute these contributions to reactivity and more clearly reveal the mechanistic basis for methane activation under conditions typical in thermal processing.