ReaxFF Force Field Development and Application for Toluene Adsorption on MnMO<i>_x</i> (M = Cu, Fe, Ni) Catalysts

In numerous studies, the application of the molecular dynamics scheme based on the reactive force field (ReaxFF) method has been proven effective in modeling the catalytic behavior of metal–organic compounds. Recently, this method has been successfully applied for MxOy (M = Cu, Fe, Mn, Ni) transition-metal oxides. Yet, bimetallic metal oxides of the type MnMOx (M = Cu, Fe, Ni) were also present in the experimental system but could not be modeled since not all of the force field parameters were available at the time. To bridge this gap, the force field for modeling bimetallic metal oxides had to be developed. Here, we establish the needed force field parameter sets (namely, Cu/Mn/O, Fe/Mn/O, and Ni/Mn/O) and apply them to the problem of toluene adsorption on bimetallic oxide catalyst surfaces to verify their validity. Each training set consisted of at least 10 crystal structures containing at least Cu–Mn–O, Fe–Mn–O, or Ni–Mn–O atoms in contact obtained from the available structure databases. The parameter training has been done using the in-home-compiled version of the ReaxFF code. After training the force fields for geometry reproduction, the parameters were refined using the optimization by atom charges, comparing the ReaxFF values to those obtained for the respective structures using periodic crystal density functional theory (DFT) codes. The as-developed force fields were then applied to the process of toluene adsorption/degradation on MnMOx catalysts. Results obtained show agreement with previous experimental expectations, although some remarks are given since the initially presumed crystal structure of bimetallic oxide Mn1–xMxOy crystallites may still have an impact on theoretical predictions. The presented are, to the best of the authors’ knowledge, the first applications of the ReaxFF approach to the Mn–(Cu|Fe|Ni)–O–C–H interaction.诸多研究已证实，基于反应力场（ReaxFF）方法的分子动力学方案，在模拟金属有机化合物的催化行为方面效果显著。近期，该方法已成功应用于MxOy（M=Cu、Fe、Mn、Ni）型过渡金属氧化物的模拟。然而，实验体系中同时存在MnMOx（M=Cu、Fe、Ni）型双金属氧化物，但由于当时尚未获取全部力场参数，无法对其进行建模。为填补这一研究空白，亟需开发适用于双金属氧化物建模的反应力场参数。本研究构建了所需的三类力场参数集（即Cu/Mn/O、Fe/Mn/O及Ni/Mn/O），并将其应用于双金属氧化物催化剂表面的甲苯吸附问题，以验证参数的有效性。每一组训练集均包含至少10个从公开结构数据库中获取的、含Cu-Mn-O、Fe-Mn-O或Ni-Mn-O接触原子的晶体结构。参数训练过程采用自研的ReaxFF代码版本完成。在完成用于几何结构重现的力场训练后，通过原子电荷优化对参数进行精修：将ReaxFF计算得到的电荷结果，与采用周期性晶体密度泛函理论（DFT）代码对对应结构计算得到的电荷结果进行对比。随后，将所开发的力场应用于MnMOx催化剂表面的甲苯吸附/降解过程。研究结果与既往实验预期相符，但需说明：双金属氧化物Mn₁₋ₓMₓOᵧ微晶的初始推定晶体结构，仍可能对理论预测结果产生影响。据作者所知，本研究为首次将ReaxFF方法应用于Mn-(Cu|Fe|Ni)-O-C-H相互作用体系的模拟。