Dissociative chemisorption of methane on Ir(111): Evidence for direct and trapping-mediated mechanisms

Abstract

Molecular beam and bulb gas techniques were employed to study dissociative chemisorption of methane on Ir(111). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence (θi). As the incident kinetic energy increases, the value of S0 first decreases and then increases with Ei indicating that a trapping-mediated chemisorption mechanism dominates methane dissociation at low kinetic energy, and a direct mechanism dominates at higher kinetic energies. The values of the reaction probability determined from molecular beam experiments of methane on Ir(111) are modeled as a function of Ei, θi, and surface temperature. These fits are then integrated over a Maxwell–Boltzmann energy distribution to calculate the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. The calculations are in excellent agreement with results obtained from bulb experiments conducted with room-temperature methane gas over Ir(111) and indicate that a trapping-mediated pathway governs dissociation at low gas temperatures. At the high gas temperatures characteristic of catalytic conditions, however, these calculations indicate that a direct mechanism dominates methane dissociation over Ir(111). These dynamical results are qualitatively similar to the results of a previous study of methane dissociation on Ir(110), although the reactivity of thermalized methane is approximately an order of magnitude higher on the (110) surface of iridium.

Theoretical Study of the Dissociative Adsorption of Methane on Ir(111): The Role of Steps and Surface Distortions at High Temperatures

“…The E b values obtained for the Ir(111) terraces are consistent with the apparent activation energy obtained experimentally for the high energy part of the sticking curve 13,14 . Nevertheless, the lowest E b value found is considerably higher than the apparent activation energy extracted from experiments corresponding to low-energy molecules impinging Ir(111).…”

“…In such a case, trapping acts as a precursor for dissociation 15 . For CH 4 /Ir(111), two experimental groups independently obtained very similar sticking curves S 0 (E i ) pointing to precursor mediated dissociative adsorption at low impact energies 13,14 . Combining molecular beam and bulb experiments, Seets et al determined an apparent activation energy of 0.28 eV for the low kinetic energy regime E i 0.15 eV 13 .…”

“…For E i 0.3 eV, the sticking probabilities reported (for T s = 1000 K) are ∼ 10 −4 and the fraction of defect sites is estimated to be less than 5×10 −313 . In addition, trapping of low energy molecules has been identified both experimentally, 13 and in MD simulations ? that predicted, e.g., a trapping probability of ∼ 0.4 in a wide range of temperatures (between 50 K and 1500 K) and a lifetime of the trapped molecules as long as ∼ 9 ps for E i = 50 meV.…”

Methane Activation by Heterogeneous Catalysis

“…2, the rate constant for PMD follows a Brønsted-Evans-Polanyi relation with the total adsorption energy of a CH 3 fragment and an H atom as catalyst descriptors. At high kinetic energies (high temperatures) the precursor mediated dissociation shifts gradually to a direct dissociation (DD) mechanism as indicated by molecular beam and thermal bulb experiments of Seets et al [26,27]. Strong evidence exists that the reaction is structure sensitive [28].…”

Energy dissipation and scattering angle distribution analysis of the classical trajectory calculations of methane scattering from a Ni(111) surface

“…1.͒ Molecular beam experiments in which the dissociation probability was measured as a function of translational energy have observed that the dissociation probability is enhanced by the normal incidence component of the incidence translational energy. [2][3][4][5][6][7][8][9][10][11][12] This suggests that the reaction occurs primarily through a direct dissociation mechanism at least for high translational kinetic energies. Some experiments have also observed that vibrationally hot CH 4 dissociates more readily than cold CH 4 , with the energy in the internal vibrations being about as effective as the translational energy in inducing dissociation.…”