Qi Tingting, Bauschlicher Charles W, Lawson John W, Desai Tapan G, Reed Evan J
Department of Materials Science and Engineering, Stanford University , Stanford, California 94305, United States.
J Phys Chem A. 2013 Nov 7;117(44):11115-25. doi: 10.1021/jp4081096. Epub 2013 Oct 28.
A systematic comparison of atomistic modeling methods including density functional theory (DFT), the self-consistent charge density-functional tight-binding (SCC-DFTB), and ReaxFF is presented for simulating the initial stages of phenolic polymer pyrolysis. A phenolic polymer system is simulated for several hundred picoseconds within a temperature range of 2500 to 3500 K. The time evolution of major pyrolysis products including small-molecule species and char is examined. Two temperature zones are observed which demark cross-linking versus fragmentation. The dominant chemical products for all methods are similar, but the yields for each product differ. At 3500 K, DFTB overestimates CO production (300-400%) and underestimates free H (30%) and small C(m)H(n)O molecules (70%) compared with DFT. At 3500 K, ReaxFF underestimates free H (60%) and fused carbon rings (70%) relative to DFT. Heterocyclic oxygen-containing five- and six-membered carbon rings are observed at 2500 K. Formation mechanisms for H2O, CO, and char are discussed. Additional calculations using a semiclassical method for incorporating quantum nuclear energies of molecules were also performed. These results suggest that chemical equilibrium can be affected by quantum nuclear effects at temperatures of 2500 K and below. Pyrolysis reaction mechanisms and energetics are examined in detail in a companion manuscript.
本文对包括密度泛函理论（DFT）、自洽电荷密度泛函紧束缚方法（SCC-DFTB）和反应分子动力学（ReaxFF）在内的原子尺度建模方法进行了系统比较，以模拟酚醛聚合物热解的初始阶段。在2500至3500 K的温度范围内，对酚醛聚合物体系进行了数百皮秒的模拟。研究了包括小分子物种和炭在内的主要热解产物随时间的演变。观察到两个温度区域，分别对应交联和裂解过程。所有方法得到的主要化学产物相似，但每种产物的产率有所不同。在3500 K时，与DFT相比，DFTB高估了CO的生成量（300 - 400%），低估了游离H（约30%）和小分子C(m)H(n)O（约70%）的生成量。在3500 K时，相对于DFT，ReaxFF低估了游离H（约60%）和稠合碳环（约70%）的生成量。在2500 K时观察到了含杂环的五元及六元碳环。讨论了H2O、CO和炭的形成机制。还使用半经典方法进行了额外计算，以纳入分子的量子核能。这些结果表明，在2500 K及以下的温度下，量子核效应会影响化学平衡。热解反应机制和能量学在一篇配套论文中进行了详细研究。
