无氧条件下Pd/CeO2催化剂表面NH3还原NO过程中N2O形成机理研究

doi:10.1016/S1872-2067(19)63328-0

催化学报 ›› 2019, Vol. 40 ›› Issue (7): 1070-1077.DOI: 10.1016/S1872-2067(19)63328-0

无氧条件下Pd/CeO₂催化剂表面NH₃还原NO过程中N₂O形成机理研究

盛丽萍, 马朝霞, 陈思园, 楼锦泽, 李成业, 李松达, 张泽, 王勇, 杨杭生

浙江大学材料科学与工程学院, 硅材料国家重点实验室, 浙江大学电子显微镜中心, 浙江杭州 310027

收稿日期:2019-01-14 修回日期:2019-02-19 出版日期:2019-07-18 发布日期:2019-05-24
通讯作者: 王勇, 杨杭生
基金资助:
国家重点研发计划（2017YFB0310403）；国家自然科学基金（51872260，51390474，91645103）；国家科技部基金（2016YFE0105700）；中国医疗废物可持续管理基金（C/V/S/10/251）；浙江省自然科学基金（Z4080070，LD19B030001）.

Mechanistic insight into N₂O formation during NO reduction by NH₃ over Pd/CeO₂ catalyst in the absence of O₂

Liping Sheng, Zhaoxia Ma, Shiyuan Chen, Jinze Lou, Chengye Li, Songda Li, Ze Zhang, Yong Wang, Hangsheng Yang

State Key Laboratory of Silicon Materials and Center of Electron Microscopy, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China

Received:2019-01-14 Revised:2019-02-19 Online:2019-07-18 Published:2019-05-24
Supported by:
The authors acknowledge the support of the National Key Research and Development Program of China (2017YFB0310403), the National Natural Science Foundation of China (51872260, 51390474, 91645103), the Ministry of Science and Technology of China (2016YFE0105700), the Environmentally Sustainable Management of Medical Wastes in China (C/V/S/10/251), and the Zhejiang Provincial Natural Science Foundation of China (Z4080070, LD19B030001).

摘要/Abstract

摘要：

氮氧化物（NO_x，主要包括NO和NO₂）是主要的大气污染物之一，造成酸雨，光化学烟雾和臭氧层破坏等环境问题，甚至直接危害人体健康.化石燃料燃烧和汽车尾气排放是NO_x的主要来源，严格控制火力发电厂，大型锅炉，汽车尾气等污染源中NO_x的排放刻不容缓.以NH₃为还原剂选择性催化还原NO_x（NH₃-SCR）是目前公认的最有效的NO_x脱除技术，然而在催化NO_x还原为N₂的过程中往往伴随着副产物N₂O的生成，降低了催化剂的选择性，造成温室气体效应和破坏臭氧层等环境问题.因此充分理解NH₃-SCR过程中N₂O的形成机理对于抑制N₂O的产生、提高催化剂的选择性十分重要.本文将高度分散的Pd纳米团簇负载在CeO₂纳米棒上制成Pd/CeO₂催化剂，结合NH₃-TPD，NO-TPD和原位傅里叶转换红外光谱等表征手段研究了无氧条件下该催化剂上利用NH₃催化还原NO过程中N₂O的产生路径.结果表明，N₂O的形成途径与反应温度和反应气体的浓度相关.当反应气体中NH₃含量大于化学计量比时，在反应温度低于200℃时，由NH₃活化产生的吸附态H·自由基与催化剂表面吸附的NO反应先生成中间产物HON，两个HON分子进一步反应生成N₂O；过量的吸附态的H·自由基也可以与HON反应生成N₂，所以低温下（<200℃）随着反应气氛中NH₃的增加，解离生成的H·也随之增加，促进反应向着生成N₂的方向进行，从而抑制了N₂O的产生.随着反应温度增加，NH₃解离产生的H·被CeO₂表面的O捕获形成羟基，中间产物HON的生成被切断，从而阻断了N₂O的生成.同时由于体系中含有大量的NH₃，吸附态的NO会优先与活化态的NH₃物种反应生成N₂，阻碍了NO解离生成N₂O这一过程的发生，因此NH₃过量情况下在高温下观察不到N₂O的产生，可获得100%的N₂选择性.但是当反应气体中的NH₃含量不足时，即体系中含有过量的NO，当反应温度高于250℃，NO可在催化剂表面解离生成吸附态的N·自由基和O·自由基，N·自由基可进一步与吸附态的NO反应生成N₂O，NO的解离是N₂O生成的速控步，还原性吸附物种对O·自由基的捕捉将有利于N₂O的生成.当反应温度介于200-250℃，NH₃解离产生的H·自由基既可以与NO结合生成HON中间产物，又能被CeO₂表面的O捕获形成羟基，两个反应之间存在竞争，此时N₂O产生与反应气体浓度之间的关系不再呈单调变化.

关键词: N₂O形成, NO还原, Pd/CeO₂催化剂, 原位红外, 机理研究

Abstract:

N₂O is a major by-product emitted during low-temperature selective catalytic reduction of NO with NH₃ (NH₃-SCR), which causes a series of serious environmental problems. A full understanding of the N₂O formation mechanism is essential to suppress the N₂O emission during the low-temperature NH₃-SCR, and requires an intensive study of this heterogeneous catalysis process. In this study, we investigated the reaction between NH₃ and NO over a Pd/CeO₂ catalyst in the absence of O₂, using X-ray photoelectron spectroscopy, NH₃-temperature-programmed desorption, NO-temperature-programmed desorption, and in-situ Fourier-transform infrared spectroscopy. Our results indicate that the N₂O formation mechanism is reaction-temperature-dependent. At temperatures below 250℃, the dissociation of HON, which is produced from the reaction between surface H·adatoms and adsorbed NO, is the key process for N₂O formation. At temperatures above 250℃, the reaction between NO and surface N·, which is produced by NO dissociation, is the only route for N₂O formation, and the dissociation of NO is the rate-determining step. Under optimal reaction conditions, a high performance with nearly 100% NO conversion and 100% N₂ selectivity could be achieved. These results provide important information to clarify the mechanism of N₂O formation and possible suppression of N₂O emission during low-temperature NH₃-SCR.

Key words: N₂O formation, NO reduction, Pd/CeO₂ catalyst, in-situ IR spectroscopy, Mechanism

盛丽萍, 马朝霞, 陈思园, 楼锦泽, 李成业, 李松达, 张泽, 王勇, 杨杭生. 无氧条件下Pd/CeO₂催化剂表面NH₃还原NO过程中N₂O形成机理研究[J]. 催化学报, 2019, 40(7): 1070-1077.

Liping Sheng, Zhaoxia Ma, Shiyuan Chen, Jinze Lou, Chengye Li, Songda Li, Ze Zhang, Yong Wang, Hangsheng Yang. Mechanistic insight into N₂O formation during NO reduction by NH₃ over Pd/CeO₂ catalyst in the absence of O₂[J]. Chinese Journal of Catalysis, 2019, 40(7): 1070-1077.

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无氧条件下Pd/CeO₂催化剂表面NH₃还原NO过程中N₂O形成机理研究

Mechanistic insight into N₂O formation during NO reduction by NH₃ over Pd/CeO₂ catalyst in the absence of O₂

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