Microkinetic Mechanisms for Partial Oxidation of Methane over Platinum and Rhodium

Abstract

A systematic approach for the development of heterogeneous mechanisms is applied and evaluated for the catalytic partial oxidation of methane over platinum (Pt) and rhodium (Rh). The derived mechanisms are self-consistent and based on a reaction class-based framework comprising variational transition state theory (VTST) and two-dimensional collision theory for the calculation of pre-exponential factors with barrier heights obtained using the unity bond index-quadratic exponential potential (UBI-QEP) method. The surface chemistry is combined with a detailed chemistry for the gas phase and the accuracy of the approach is evaluated over Pt for a wide range of stoichiometries (0.3 ≤ φ ≤ 4.0), pressures (2 ≤ P (bar) ≤ 16) and residence times. It is shown that the derived mechanism can reproduce experimental data with an accuracy comparable to the prevalent collision theory approach and without the reliance on experimental data for sticking coefficients. The derived mechanism for Rh shows encouraging agreement for a similar set of conditions and the robustness of the approach is further evaluated by incorporating partial updates via more accurate DFT determined barrier heights. Substantial differences are noted for some channels (e.g. where reaction progress is strongly influenced by early transition states) though the impact on the overall agreement with experimental data is moderate for the current systems. Remaining discrepancies are explored using sensitivity analyses to establish key parameters. The study suggests that the overall framework is well-suited for the efficient generation of heterogeneous reaction mechanisms, that it can serve to identify key parameters where high accuracy ab initio methods are required and that it permits the inclusion of such updates as part of a gradual refinement process.

Constrained Chemical Dynamics of CO Dissociation/Hydrogenation on Rh Surfaces

“…The discrepancy in the available rate constants is large. For the CO dissociation, the Rh(111) value obtained in current work is in good agreement with the semiempirical value of Kraus and Lindstedt . The fit to indirect experimental data results in a significantly higher rate constant, comparable with dissociation on the Rh(211) surfaces .…”

“…The value of Yang and Liu for the Fe‐doped Rh(111) surface is also included for comparison. On the contrary, the semiempirical rate constant of Kraus and Lindstedt is significantly slower than all other data because of the overpredicted barrier height. Finally, the results for CO(s) hydrogenation to COH(s) are much less consistent, with the two literature values enveloped by the current Rh(111) and Rh(211) results.…”

“…The vibrational temperatures are evaluated from the power spectra of the stable species and of the constrained transition‐state‐like structures; the power spectra are calculated using Travis . The pre‐exponential factors evaluated on the Rh(111) surface are consistent with the collision theory values of Kraus and Lindstedt, within a factor of five for all processes, with the collision theory values providing the upper limit. The notably higher pre‐exponential factors for processes involving H(s) are attributed to the considerably lower translational wavelength λ T of H(s), leading to a much smaller product of the reactant partition functions.…”

“…Calculated rate constants on the Rh(111) (black solid line) and Rh(211) (black dashed line) surfaces. Literature values of Kraus and Lindstedt (red), Filot et al . (blue), Choi and Liu (yellow), Yang and Liu (gray), and Karakaya et al .…”

RETRACTED: Computational Fluid Dynamics Modeling of the Catalytic Partial Oxidation of Methane in Microchannel Reactors for Synthesis Gas Production

“…The reaction proceeds at a lower temperature than the gas-phase partial oxidation route [17][18][19][20], thus offering many advantages such as a reduction of undesired by-products and the ability to control the process temperature [21][22][23][24]. The catalysts used for this reaction usually contain group VIII transition metals, such as rhodium [25,26], ruthenium [27,28], platinum [29,30], palladium [31,32], nickel [33,34], iridium [35,36], and cobalt [37,38].…”