Microkinetic Mechanisms for Partial Oxidation of Methane over Platinum and Rhodium

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 that
of 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.