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.

本研究采用并评估了一套用于多相反应机理（heterogeneous mechanisms）开发的系统方法，将其应用于铂（platinum, Pt）与铑（rhodium, Rh）表面上的甲烷催化部分氧化（catalytic partial oxidation of methane）反应。所推导得到的反应机理具备自洽性，其基于反应类别框架，结合变分过渡态理论（variational transition state theory, VTST）与二维碰撞理论（two-dimensional collision theory）计算指前因子（pre-exponential factors），势垒高度（barrier heights）则通过统一键指数-二次指数势（unity bond index–quadratic exponential potential, UBI–QEP）方法求取。将表面化学（surface chemistry）与详细的气相化学（gas phase chemistry）相结合后，本研究针对Pt体系，在宽范围的化学计量比（0.3 ≤ φ ≤ 4.0）、压力（2 ≤ P(bar) ≤ 16）与停留时间（residence times）条件下评估了该方法的准确性。结果显示，所推导的机理能够以与主流碰撞理论方法相当的精度复现实验数据，且无需依赖附着系数（sticking coefficients）的实验数据。针对Rh体系推导的机理在相似条件下也展现出良好的一致性；进一步通过引入更精确的密度泛函理论（density functional theory, DFT）计算得到的势垒高度进行部分更新，以验证该方法的鲁棒性。尽管部分反应通道存在显著差异（例如反应进程受早期过渡态强烈影响的通道），但该方法对当前体系的整体实验数据拟合精度的影响较为温和。本研究通过敏感性分析（sensitivity analyses）探究了剩余偏差的来源，以确定关键参数。综上，该整体框架非常适合高效生成多相反应机理，可用于识别需要高精度从头算方法（ab initio methods）的关键参数，并支持将此类更新纳入逐步优化过程（gradual refinement process）中。