非热等离子体强化TiO2催化尿素分解副产物水解性能的研究

doi:10.11949/0438-1157.20210060

摘要/Abstract

摘要：

尿素-选择性催化还原技术低温下运行时尿素分解不彻底，易形成缩二脲、三聚氰酸和三聚氰胺等副产物。本研究将TiO₂催化剂与介质阻挡放电等离子体相结合，在程序升温条件下考察了载气中有无O₂时引入等离子体前后TiO₂催化尿素分解副产物水解的性能。结果表明：TiO₂表面缩二脲、三聚氰酸和三聚氰胺分别在43~261℃、217~300℃和199~300℃水解生成NH₃和CO₂，载气中有无O₂对催化水解过程几乎无影响。引入等离子体后缩二脲、三聚氰酸和三聚氰胺水解所需温度显著降低，载气中无O₂时引入等离子体NH₃产率变化不大，副产物仅有少量N₂O和NO，有O₂时NH₃产率显著降低，且生成较多N₂O、NO、NO₂及少量NH₄NO₂和NH₄NO₃。未来需从优化放电条件和催化剂组成等方面解决引入等离子体导致副产物形成等问题。

关键词: 尿素分解副产物, 非热等离子体, 催化, 水解, 选择性催化还原

Abstract:

When the urea-selective catalytic reduction technology runs at low temperatures, urea is not completely decomposed, and it is easy to form by-products such as biuret, cyanuric acid and melamine. In this study, TiO₂ catalyst was combined with dielectric barrier discharge plasma for hydrolysis of urea decomposition by-products with or without O₂ in the carrier gas under temperature-programmed conditions. Experimental results show that biuret, cyanuric acid and melamine over TiO₂ could be hydrolyzed to NH₃ and CO₂ at 43—261℃, 217—300℃ and 199—300℃, respectively. The presence or absence of O₂ in the carrier gas hardly affects the catalytic hydrolysis process. Introducing plasma significantly decreases the hydrolysis temperatures of biuret, cyanuric acid and melamine. Insignificant variation of NH₃ yield and small amounts of N₂O and NO by-products are observed when plasma is applied in the O₂-free carrier gas. In the O₂-containing case, NH₃ yield obviously decreases while more N₂O, NO, NO₂ and small amounts of NH₄NO₂ and NH₄NO₃ are generated by applying plasma. To overcome drawbacks of the plasma-enhanced process, mainly the formation of by-products, further work is required in the future, such as optimizing discharge conditions and catalyst compositions.

Key words: urea decomposition by-products, non-thermal plasma, catalysis, hydrolysis, SCR

中图分类号:

X 511

李泽严, 樊星, 李坚. 非热等离子体强化TiO₂催化尿素分解副产物水解性能的研究[J]. 化工学报, 2021, 72(9): 4698-4707.

Zeyan LI, Xing FAN, Jian LI. Non-thermal plasma enhanced hydrolysis of urea decomposition by-products over TiO₂[J]. CIESC Journal, 2021, 72(9): 4698-4707.

图/表 9

图1 实验装置示意图1—气瓶；2—减压阀；3—质量流量控制器；4—鼓泡塔（水）；5—温度控制仪；6—接地极；7—管式炉；8—DBD反应器；9—催化剂；10—交流高压电源；11—冷凝塔；12—FT-IR光谱仪；13—NOx分析仪

Fig.1 Schematic diagram of the experimental set-up

图2 N2+H2O和5%O2+N2+H2O气氛下催化和等离子体强化催化缩二脲水解的曲线(平均放电功率5.6 W；放电时间段：0~85 min)

Fig.2 Hydrolysis curves of biuret by catalysis and plasma-enhanced catalysis in N2+H2O and 5%O2+N2+H2O (average discharge power: 5.6 W; discharge period: 0—85 min)

表1 催化和等离子体强化催化作用下缩二脲水解的结果

Table 1 Results of biuret hydrolysis by catalysis and plasma-enhanced catalysis

平均放电功率/W	载气气氛	CO₂开始出现的温度/℃	CO₂达到峰值浓度时的温度/℃	CO₂开始消失的温度/℃	NH₃产率/%	CO₂产率/%
0	N₂+H₂O	43	94/161/200	261	82.1	71.0
0	5%O₂+N₂+H₂O	47	77/165/202	255	79.7	62.4
5.6	N₂+H₂O	36	61/136/176	221	70.3	59.8
5.6	5%O₂+N₂+H₂O	33	54/142/183	223	47.4	56.3

图3 N2+H2O和5%O2+N2+H2O气氛下催化和等离子体强化催化三聚氰酸水解的曲线(平均放电功率：无O2时6.2 W，有O2时5.9 W；放电时间段：0~85 min)

Fig.3 Hydrolysis curves of cyanuric acid by catalysis and plasma-enhanced catalysis in N2+H2O and 5%O2+N2+H2O (average discharge power: 6.2 W and 5.9 W without and with O2, respectively; discharge period: 0—85 min)

表2 催化和等离子体强化催化作用下三聚氰酸水解的结果

Table 2 Results of cyanuric acid hydrolysis by catalysis and plasma-enhanced catalysis

平均放电功率/W	载气气氛	CO₂开始出现的温度/℃	CO₂达到峰值浓度时的温度/℃	CO₂开始消失的温度/℃	NH₃产率/%	CO₂产率/%
0	N₂+H₂O	217	282	300	54.5	57.3
0	5%O₂+N₂+H₂O	218	282	300	54.8	57.3
6.2	N₂+H₂O	162	253	300	52.6	64.0
5.9	5%O₂+N₂+H₂O	160	253	300	34.1	66.8

图4 N2+H2O和5%O2+N2+H2O气氛下催化和等离子体强化催化三聚氰胺水解的曲线(平均放电功率：无O2时5.5 W，有O2时5.3 W；放电时间段：0~85 min)

Fig.4 Hydrolysis curves of melamine by catalysis and plasma-enhanced catalysis in N2+H2O and 5%O2+N2+H2O (average discharge power: 5.5 W and 5.3 W without and with O2, respectively; discharge period: 0—85 min)

表3 催化和等离子体强化催化作用下三聚氰胺水解的结果

Table 3 Results of melamine hydrolysis by catalysis and plasma-enhanced catalysis

平均放电功率/W	载气气氛	CO₂开始出现的温度/℃	CO₂达到峰值浓度时的温度/℃	CO₂开始消失的温度/℃	NH₃产率/%	CO₂产率/%
0	N₂+H₂O	199	263	300	68.1	73.0
0	5%O₂+N₂+H₂O	201	262	300	66.7	74.9
5.5	N₂+H₂O	171	235	300	60.7	71.5
5.3	5%O₂+N₂+H₂O	166	242	300	50.9	77.1

图5 空白放电以及催化和等离子体强化催化作用下缩二脲水解时反应器出口气体典型红外光谱图i—空白放电（N2+H2O）；ii—空白放电（5%O2+N2+H2O）；iii—催化水解（N2+H2O）；iv—催化水解（5%O2+N2+H2O）；v—等离子体强化催化水解（N2+H2O）；vi—等离子体强化催化水解（5%O2+N2+H2O）

Fig.5 Typical infrared spectra of the outlet gas from discharge with TiO2 alone and from catalytic and plasma-enhanced catalytic hydrolysis of biuret

图6 空白放电以及等离子体强化缩二脲、三聚氰酸和三聚氰胺催化水解生成的N2O、NO和NO2浓度随时间的变化(放电时间段：0~85 min)

Fig.6 Concentration curves of N2O, NO and NO2 generated during discharge with TiO2 alone and during plasma-enhanced catalytic hydrolysis of biuret, cyanuric acid and melamine (discharge period: 0—85 min)

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非热等离子体强化TiO₂催化尿素分解副产物水解性能的研究

Non-thermal plasma enhanced hydrolysis of urea decomposition by-products over TiO₂

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