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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• 作者：Yuanyuan Xie ; Lei Hua ; Keyong Hou ; Ping Chen ; Wuduo Zhao ; Wendong Chen ; Bangyu Ju ; Haiyang Li
• 刊名：Analytical Chemistry
• 出版年：2014
• 出版时间：August 5, 2014
• 年：2014
• 卷：86
• 期：15
• 页码：7681-7687
• 全文大小：434K
• ISSN：1520-6882

文摘

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鈥?000 ppmv with a correlation coefficient (R²) 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.