Kinetic modelling of NH3 production in N2-H2 non-equilibrium atmospheric-pressure plasma catalysis
Kinetic modelling of NH3 production in N2-H2 non-equilibrium atmospheric-pressure plasma catalysis
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DOI：
10.1088/1361-6463/aa6229
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发表时间：
2017-04-20
影响因子：
3.4
通讯作者：
Murphy, Anthony B.
中科院分区：
文献类型：
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作者：
Hong, Jungmi;Pancheshnyi, Sergey;Murphy, Anthony B.
提出了用于氨合成的低电子能 N-2-H-2 大气压放电的详细等离子体动力学模型。该模型考虑了电子和振动动力学，包括激发的 N-2(X,V) 和 H-2(X,V) 物质，以及表面反应，例如由 Eley-Rideal 和 Langmuir-Hinshelwood 机制以及分子解离吸附发生的反应。将模型的预测结果与填充床介质阻挡放电反应器中产生的测量的 NH3 浓度进行比较，该浓度是工艺参数（例如输入气体成分和施加电压）的函数。与典型的低压等离子体工艺不同，在此处考虑的等离子体条件下（电场 E/N 在 30-50 Td 范围内减小，电子密度为 10(8) cm(-3) 量级），离子的影响并不显着。相反，自由基和振动激发分子之间的反应更为重要。表面反应中的活性物质，例如表面吸附的原子氮 N(s) 或氢 H(s)，被发现主要是通过分子的解离吸附产生的，这与之前提出的低压、高 E/N 条件下的等离子体催化机制相反。研究发现NH自由基在NH3生成过程的早期阶段发挥着重要作用，NH又由N和H-2(V)生成。电子动力学被证明在产生这些前体的分子解离和振动激发反应中发挥着关键作用。进一步发现，表面吸附的原子氢H(s)在NH3的形成中起主导作用，这是与传统热化学过程和低压等离子体中的机制的另一个显着区别。研究发现，施加的电压、气体温度、输入气体混合物中的 N-2: H-2 比例以及表面材料的反应性都会影响氨的产生。计算结果再现了观察到的 NH3 产量对输入气体混合物中 N-2/H-2 比率和施加电压的依赖性趋势。
Detailed plasma kinetics modelling is presented of a low electron energy N-2-H-2 atmospheric-pressure discharge for ammonia synthesis. The model considers both electron and vibrational kinetics, including excited N-2(X,V) and H-2( X,V) species, and surface reactions such as those occurring by the Eley-Rideal and Langmuir-Hinshelwood mechanisms and dissociative adsorption of molecules. The predictions of the model are compared to the measured NH3 concentration produced in a packed-bed dielectric barrier discharge reactor as a function of process parameters such as input gas composition and applied voltage. Unlike typical low-pressure plasma processes, under the plasma conditions considered here (reduced electric field E/N in the range 30-50 Td, electron density of the order 10(8) cm(-3)), the influence of ions is not significant. Instead, the reactions between radicals and vibrationally-excited molecules are more important. The active species in surface reactions, such as surface-adsorbed atomic nitrogen N(s) or hydrogen H(s), are found to be predominantly generated through the dissociative adsorption of molecules, in contrast to previously proposed mechanisms for plasma catalysis under low-pressure, high-E/N conditions. It is found that NH radicals play an important role at the early stages of the NH3-generation process, NH in turn is produced from N and H-2(V). Electron kinetics is shown to play a critical role in the molecular dissociation and vibrational excitation reactions that produce these precursors. It is further found that surface-adsorbed atomic hydrogen H(s) takes a leading role in the formation of NH3, which is another significant difference from the mechanisms in conventional thermo-chemical processes and low-pressure plasmas. The applied voltage, the gas temperature, the N-2: H-2 ratio in the input gas mixture and the reactivity of the surface material are all found to influence the ammonia production. The calculated results reproduce the observed trends in the dependence of NH3 production on the N-2/H-2 ratio in the input gas mixture and the applied voltage.