Development and Validation of ReaxFF Reactive Force Field for Hydrocarbon Chemistry Catalyzed by Nickel

To enable the study of hydrocarbon reactions catalyzed by nickel surfaces and particles using reactive molecular dynamics on thousands of atoms as a function of temperature and pressure, we have developed the ReaxFF reactive force field to describe adsorption, decomposition, reformation and desorption of hydrocarbons as they interact with the nickel surface. The ReaxFF parameters were determined by fitting to the geometries and energy surfaces from quantum mechanics (QM) calculations for a large number of reaction pathways for hydrocarbon molecules chemisorbed onto nickel (111), (100) and (110) surfaces, supplemented with QM equations of state for nickel and nickel carbides. We demonstrate the validity and accuracy of ReaxFF by applying it to study the reaction dynamics of hydrocarbons as catalyzed by nickel particles and surfaces. For the dissociation of methyl on the (111), (100), and stepped (111) surfaces of nickel, we observe the formation of chemisorbed CH plus subsurface carbide. We observe that the (111) surface is the least reactive, the (100) surface has the fastest reaction rates, and the stepped (111) surface has an intermediate reaction rate. The importance of surface defects in accelerating reaction rates is highlighted by these results.

为了借助基于数千原子的反应分子动力学方法，开展温度与压力依赖的镍基表面及颗粒催化烃类反应研究，我们开发了ReaxFF反应力场（ReaxFF reactive force field），用于描述烃类与镍表面相互作用时的吸附、分解、重整与脱附过程。该力场的参数通过拟合大量吸附于镍(111)、(100)及(110)晶面的烃类分子化学吸附反应路径的量子力学（QM）计算几何结构与能量表面确定，并辅以镍与碳化镍的QM状态方程进行补充。我们通过将该力场应用于镍基颗粒与表面催化的烃类反应动力学研究，验证了其有效性与准确性。针对镍(111)、(100)及阶梯型(111)晶面上甲基的解离过程，我们观测到化学吸附态CH与表面下碳化物种的生成。观测结果显示，镍(111)晶面反应活性最低，(100)晶面反应速率最快，而阶梯型(111)晶面的反应速率处于中等水平。本研究结果凸显了表面缺陷对反应速率的加速作用。