Paper HC-ThP6
Methane Dissociation on Ni(111) at High Surface Temperatures: The Observed role of Surface and Subsurface C on Reactivity

Thursday, November 2, 2017, 6:30 pm, Room Central Hall

Steam reforming methane over a Ni catalyst is the chief industrial process for H₂ gas production, and activation of methane’s C-H bond to form surface-bound CH₃ and H is rate limiting. Conventional and vibrational state-selected molecular beam studies have highlighted the importance of translational (E_trans) and vibrational (E_vib) energy in promoting this rate limiting step on clean, well-ordered Ni single crystal surfaces. Nearly all of these studies have been performed at low to moderate surface temperatures (T_surf< 700K), where carbonaceous reaction products remain bound to the surface throughout the measurement.

Here, we describe experiments that extend these state-resolved measurements to the high surface temperatures typically used in the steam reforming process. Under these conditions,the methyl product promptly dehydrogenates to form surface-bound C and H, and H atoms recombinatively desorb, leaving C on the surface. The remaining carbon can dissolve into the nickel subsurface or bulk during the molecular beam dose, with a T_surf-dependent dissolution rate. We measured methane uptake onto, and into, a Ni(111) single crystal in situ across a range of surface temperatures from T_surf = 680 – 850 K. We varied incident translational energies and incident methane flux, and measured S(θ) for both laser-off and state resolved (v=1, v₃ antisymmetric C-H stretch) methane. A unique molecular beam reflectivity method allowed us to quantify the initial S₀ as well as S(t) in real time during the dose. Integrating S(t) yielded the integrated amount of C deposited during the dose, (θ) and allowed us to calculate S(θ).

Over the T_surf range studied, we observed drastic differences in carbon dissolution during deposition. At T_surf = 680K, carbon uptake into the nickel lattice was minimal and about 0.5 ML of C was deposited before the surface became deactivated due to site-blocking. This situation changed dramatically at temperatures above T_surf = 750 K. At intermediate temperatures, we observed an induction period prior to the onset of site blocking and surface passivation, and, at T_surf = 850 K, deposition of more than 50 ML of C did not completely passivate the surface. Furthermore, we observed that under some conditions, S(θ) increased with increasing C concentration beneath the surface. A simple two-step dissolution process that includes T-dependent rate constants for C transport between the surface, subsurface, and bulk qualitatively describes our data. We will also describe our most recent efforts to refine this model to more quantitatively describe our experimental measurements to better understand the role of dissolved C on methane activation.