Ludwig Jeffery, Vlachos Dionisios G, van Duin Adri C T, Goddard William A
Department of Chemical Engineering and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716-3110, USA.
J Phys Chem B. 2006 Mar 9;110(9):4274-82. doi: 10.1021/jp0561064.
The dissociation of hydrogen on eight platinum surfaces, Pt(111), Pt(100), Pt(110), Pt(211), Pt(311), Pt(331), Pt(332), and Pt(533), has been studied using molecular dynamics and the reactive force field, ReaxFF. The force field, which includes the degrees of freedom of the atoms in the platinum substrate, was used unmodified with potential parameters determined from previous calculations performed on a training set exclusive of the surfaces considered in this work. The energetics of the eight surfaces in the absence of hydrogen at 0 K were first compared to previous density functional theory (DFT) calculations and found to underestimate excess surface energy. However, taking Pt(111) as a reference state, we found that the trend between surfaces was adequately predicted to justify a relative comparison between the various stepped surfaces. To assess the strengths and weaknesses of the force field, we performed detailed simulations on two stepped surfaces, Pt(533) and Pt(211), and compared our findings to published experimental and theoretical results. In general, the absolute magnitude of reaction rate predictions was low, a result of the force field's tendency to underpredict surface energy. However, when normalized, the simulations show the correct linear scaling with incident energy and angular dependence at collision energies where a direct dissociation mechanism is observed. Because ReaxFF includes all degrees of freedom in the substrate, we carried out simulations aimed at understanding surface-temperature effects on Pt(533). On the basis of the results on Pt(533)/Pt(211), we studied the reaction of hydrogen at normal incidence on all eight surfaces in a range of energies where we anticipated the force field to give reasonable qualitative trends. These results were subsequently fit to a simple linear model that predicts the enhanced reactivity of surfaces containing 111-type atomic steps versus 100-type atomic steps. This model provides a simple framework for predicting high-energy/high-temperature kinetics of complex surfaces not vicinal to Pt(111).
利用分子动力学和反应力场ReaxFF研究了氢在八个铂表面Pt(111)、Pt(100)、Pt(110)、Pt(211)、Pt(311)、Pt(331)、Pt(332)和Pt(533)上的解离。该力场包含铂基底中原子的自由度,在使用时未作修改,其势参数由先前对一个训练集进行的计算确定,该训练集不包括本工作中所考虑的表面。首先将0 K下无氢时八个表面的能量学与先前的密度泛函理论(DFT)计算结果进行比较,发现该力场低估了表面过剩能量。然而,以Pt(111)作为参考态,我们发现表面之间的趋势得到了充分预测,足以证明对各种阶梯表面进行相对比较是合理的。为了评估该力场的优缺点,我们在两个阶梯表面Pt(533)和Pt(211)上进行了详细模拟,并将我们的结果与已发表的实验和理论结果进行比较。总体而言,反应速率预测的绝对值较低,这是力场倾向于低估表面能量的结果。然而,当进行归一化处理时,模拟结果显示在观察到直接解离机制的碰撞能量下,反应速率与入射能量和角度依赖性呈正确的线性标度关系。由于ReaxFF包含了基底中的所有自由度,我们进行了旨在了解表面温度对Pt(533)影响的模拟。基于Pt(533)/Pt(211)的结果,我们研究了在一系列能量下氢垂直入射到所有八个表面上的反应,在这些能量范围内,我们预期该力场能给出合理的定性趋势。随后将这些结果拟合到一个简单的线性模型中,该模型预测了含有111型原子台阶的表面相对于100型原子台阶表面的反应活性增强。该模型为预测非Pt(111)邻域复杂表面的高能/高温动力学提供了一个简单框架。