无氧条件下钯催化剂上氢气还原氧化氮的微观反应动力学研究

Microkinetic Analysis for the Reduction of Nitric Oxide over Palladium in the Absence of Oxygen

通过单位键指标-二次指数势方法(UBI-QEP)进行能学数据计算以及随机模拟,并结合实验结果,研究了无氧条件下反应温度对Pd基催化剂上H2还原NO的反应产物分布的影响. 结果表明, NO以(NO)*2(*表示活性位, (NO)*2表示吸附的(NO)2分子)的形式吸附在Pd催化剂表面参与反应. 还原产物N2O来自两条途径,分别是(NO)*2的分解和相邻NO*分子的结合; N2来自N2O*的分解; NH3则由(NO)*2与H*反应形成中间产物HNO*, 继而逐步加氢产生. 随着反应温度的升高,Pd催化剂表面H*的覆盖度随之下降, H2还原NO反应的控制步骤相应改变,产物的选择性也随之变化. 在70～310 ℃的反应温度区间,模拟数据和实验结果非常吻合,低温下主要进行(NO)*2分解产生N2O*的反应,还原产物以N2O为主; 高温下主要进行(NO)*2加氢产生HNO*的反应,还原产物以NH3为主; 受此影响, N2的选择性先增加后减少,峰值小于50%. 据此提出了反应机理的网络模型图.

Utilizing the stochastic simulation, a microkinetic model that quantitatively describes the reduction of NO by H2 in the absence of O2 is developed. The parameters of elementary reaction steps are calculated by the transition state theory along with the unity bond index-quadratic exponential potential method. It is shown that NO adsorbed in a dimer form （NO）2^＊ on the Pd（100） surface. N2O can be produced via two routes, one is the decomposition of （NO）2^＊ and another is the combination of adjacent NO^＊ N2 is formed from the decomposition of N2O^＊. While NH3 comes from the hydrogenation of HNO^＊. With the increase of reaction temperature, the coverage of H^＊ on the Pd surface decreases, and the rate-limiting step also changes, which has obvious influence on the selectivity for different reduction products. From 70℃ to 310℃, the simulated results are in accordance with the experimental data. N2O is the major reduction product via the decomposition of （NO）2^＊ towards N2O^＊at low temperature, and NH3 is obtained via the hydrogenation of （NO）2^＊ towards HNO^＊ at high temperature. So the selectivity for N2 increases at first and then decreases with a maximum below 50 %. According to the experimental and simulated results the reaction mechanism is suggested.