Effect of SrO promoter on the activity and thermal-stability of wüstite based catalyst for ammonia synthesis

doi:10.16085/j.issn.1000-6613.2018-0382

Abstract

Abstract:

Wüstite based catalysts for ammonia synthesis promoted with different amounts of SrO were prepared by fusing method．Under industrial condition (15MPa、30000h^-1)， the activity tests on a fixed-bed show that the catalytic performance of the catalysts (catalytic hydrogenation for nitrogen) increases at first and then decreases with SrO content， presenting a typical volcano curve．The optimal catalyst was obtained when 0.4% SrO promoter was added． The characterizations including EDS， H₂-TPR， N₂-TPD and XRD indicate that strontium segregates on the surface of catalysts．However， no obvious influence on the reduction of the catalysts was observed． Small amounts of SrO improve the catalytic performance due to the decrease in the N₂ dissociation activation energy or destabilization of NH _x ．However， the addition of excessive SrO covers part of active sites，and they can react with Al which is an important structural promoter for ammonia synthesis catalysts． As a result， the activity and thermal-stability was decreased．

Key words: ammonia synthesis, catalyst, fixed-bed, hydrogenation, SrO promoter, segregation, volcano curve

摘要：

采用高温熔融法制备了不同SrO含量的Fe_1- _x O基氨合成催化剂。在类似工业条件下（15MPa、30000h^-1）使用固定床反应器对催化剂进行了氨合成（氮的催化加氢）活性评价，并采用X射线能谱仪（EDS）、程序升温还原（H₂-TPR）、N₂程序升温脱附（N₂-TPD）、X射线粉末衍射（XRD）对样品进行表征。结果表明，随着SrO含量的增加，催化剂的活性先增加后降低，呈现出火山型活性变化趋势。当SrO质量分数为0.4%时，催化剂活性最高。SrO存在表面偏析现象，且SrO的添加并没有促进或明显抑制催化剂的还原。少量SrO的添加降低了N₂的解离活化能或降低了NH _x 物种的稳定性，提高了催化剂的活性；SrO含量进一步增加，覆盖了表面部分活性位，消耗了体相的重要结构助催化剂Al，降低了催化剂的活性和耐热性能。

关键词: 氨合成, 催化剂, 固定床, 加氢, SrO助催化剂, 偏析, 火山型活性曲线

CLC Number:

O643

Linhui LI,Huazhang LIU,Wenfeng HAN,Zhi LI,Lei LI,Zhikun WANG. Effect of SrO promoter on the activity and thermal-stability of wüstite based catalyst for ammonia synthesis[J]. Chemical Industry and Engineering Progress, 2019, 38(03): 1371-1376.

李林辉,刘化章,韩文锋,黎志,李蕾,王志昆. SrO助催化剂对Fe_1- _x O基熔铁氨合成催化剂性能的影响[J]. 化工进展, 2019, 38(03): 1371-1376.

Figures/Tables 8

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SrO质量分数/%	耐热系数α _450℃	耐热系数α _425℃	耐热系数α _400℃
0	0.97	–0.30	–0.45
0.2	0.96	0.97	0.51
0.3	1.08	–0.57	0.18
0.4	0.91	–0.24	–1.63
0.5	–0.69	–2.27	–1.23
0.8	–3.16	–4.24	–5.21

SrO质量分数/%	耐热系数α _450℃	耐热系数α _425℃	耐热系数α _400℃
0	0.97	–0.30	–0.45
0.2	0.96	0.97	0.51
0.3	1.08	–0.57	0.18
0.4	0.91	–0.24	–1.63
0.5	–0.69	–2.27	–1.23
0.8	–3.16	–4.24	–5.21

SrO质量分数/%	元素质量分数/%
SrO质量分数/%	Fe	O	Sr
0	78.73	16.01	0
0.2	77.66	16.91	0.40
0.3	77.47	17.63	0.51
0.4	77.55	12.81	0.72
0.5	71.52	19.66	0.99
0.8	70.93	21.10	1.46

SrO质量分数/%	元素质量分数/%
SrO质量分数/%	Fe	O	Sr
0	78.73	16.01	0
0.2	77.66	16.91	0.40
0.3	77.47	17.63	0.51
0.4	77.55	12.81	0.72
0.5	71.52	19.66	0.99
0.8	70.93	21.10	1.46

SrO 质量分数/%	含Al物相		Ca₂Fe₂O₄ 质量分数/%	FeO 质量分数/%
SrO 质量分数/%	物相	质量分数/%	Ca₂Fe₂O₄ 质量分数/%	FeO 质量分数/%
0	3CaO·Al₂O₃	4.07	8.04	87.89
0.2	3CaO·Al₂O₃	4.17	8.56	87.27
0.4	3CaO·Al₂O₃	6.67	9.23	84.10
0.8	SrO·2Al₂O₃	6.37	8.25	85.37
2	12SrO·7Al₂O₃	11.53	11.23	77.24
4	3SrO·Al₂O₃	12.11	10.60	77.28