Long-term real-time monitoring catalytic synthesis of ammonia in a microreactor by VUV-lamp-based charge-transfer ionization time-of-flight mass spectrometry.
Long-term real-time monitoring catalytic synthesis of ammonia in a microreactor by VUV-lamp-based charge-transfer ionization time-of-flight mass spectrometry.
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DOI：
10.1021/ac501576f
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发表时间：
2014-07
影响因子：
7.4
通讯作者：
Yuanyuan Xie;Lei Hua;Keyong Hou;Ping Chen;Wuduo Zhao;Wendong Chen;Bangyu Ju;Haiyang Li
中科院分区：
文献类型：
--
作者：
Yuanyuan Xie;Lei Hua;Keyong Hou;Ping Chen;Wuduo Zhao;Wendong Chen;Bangyu Ju;Haiyang Li
针对氨的大量消耗和苛刻的工业合成条件，许多研究致力于在温和条件下开发更环保的氨合成催化剂。然而，用于分析合成氨的传统方法（例如，离线离子色谱法（IC）和化学滴定）存在灵敏度差、时间分辨率低和样品操作的问题。本文采用真空紫外（VUV）灯为光源，以O2（+）为反应离子，在飞行时间质谱仪（CTI-TOFMS）上进行电荷转移电离（CTI），对微反应器中的氨合成过程进行实时监测。为满足长期稳定监测的需要，提出了一种稳定O2（+）离子强度的自调节算法，以自动补偿由于真空紫外灯光电极氧化和MgF 2窗口污染而导致的离子源中O2（+）离子产额的衰减。在0.2-1000 ppmv浓度范围内获得了宽线性校准曲线，相关系数（R（2））为0.9986，NH3的定量限（LOQ）为ppbv。采用过渡金属/碳纳米管制备的3种催化剂进行了微催化合成氨实验，定量测定了氨转化率随反应温度的快速变化，时间分辨率为30 s。高时间分辨率的CTI-TOFMS不仅可以快速获得氨的平衡转化率，而且还可以监测氨合成反应中所研究催化剂的活性变化。
With respect to massive consumption of ammonia and rigorous industrial synthesis conditions, many studies have been devoted to investigating more environmentally benign catalysts for ammonia synthesis under moderate conditions. However, traditional methods for analysis of synthesized ammonia (e.g., off-line ion chromatography (IC) and chemical titration) suffer from poor sensitivity, low time resolution, and sample manipulations. In this work, charge-transfer ionization (CTI) with O2(+) as the reagent ion based on a vacuum ultraviolet (VUV) lamp in a time-of-flight mass spectrometer (CTI-TOFMS) has been applied for real-time monitoring of the ammonia synthesis in a microreactor. For the necessity of long-term stable monitoring, a self-adjustment algorithm for stabilizing O2(+) ion intensity was developed to automatically compensate the attenuation of the O2(+) ion yield in the ion source as a result of the oxidation of the photoelectric electrode and contamination on the MgF2 window of the VUV lamp. A wide linear calibration curve in the concentration range of 0.2-1000 ppmv with a correlation coefficient (R(2)) of 0.9986 was achieved, and the limit of quantification (LOQ) for NH3 was in ppbv. Microcatalytic synthesis of ammonia with three catalysts prepared by transition-metal/carbon nanotubes was tested, and the rapid changes of NH3 conversion rates with the reaction temperatures were quantitatively measured with a time resolution of 30 s. The high-time-resolution CTI-TOFMS could not only achieve the equilibrium conversion rates of NH3 rapidly but also monitor the activity variations with respect to investigated catalysts during ammonia synthesis reactions.