球形表面富锰MnxCo3−xO4−ƞ尖晶石型催化剂选择性催化还原NOx研究

doi:10.3866/PKU.WHXB202212003

摘要/Abstract

摘要：

采用共沉淀法制备了高比表面积的Mn_xCo_3−xO₄球形催化剂，研究了NH₃选择性催化还原NO_x性能。Mn-Co金属氧化物具有尖晶石结构，随着Co含量的增加，晶体结构由四方相转变为立方相。高浓度的表面活性氧物种和变价元素的强有效电子转移(Co³⁺ + Mn³⁺ ↔ Co²⁺ + Mn⁴⁺)有利于提高Mn_xCo_3−xO₄ (x = 1.0、1.5、2.0)尖晶型石催化剂的氧化还原能力，催化剂表面的Mn富集作用形成了氧缺陷结构和丰富的表面活性位点，进一步促进SCR脱硝反应，呈现出优异的催化性能。Co_tet(CoMn)_octO₄晶体结构中，Mn离子(Mn³⁺和Mn⁴⁺，以三价锰为主)和部分Co离子被配置到八面体中心，这些物种作为活性位点存在着较强的电子转移交互作用，该构型对促进低温脱硝活性和保护活性位点耐受SO₂毒害具有重要的意义。Mn-Co尖晶石表面的NH₃-SCR脱硝反应过程主要遵循Eley-Rideal反应机理，即吸附态NH₃与气态NO (或NO₂)的反应路径。随着反应温度的增加，反应生成的NH₄NO₃中间体很可能转化为NH₄NO₂物种，进而分解为N₂，提高了催化剂的氮气选择性。

关键词: Mn-Co复合氧化物, 尖晶石结构, 富锰表面, 选择性催化还原, 协同作用, 反应机理

Abstract:

Currently, there is an urgent need to develop an efficient, non-toxic, and stable catalyst for the removal of NO_x via selective catalytic reduction using NH₃ (NH₃-SCR) that is effective at low temperatures. Mn-based catalysts are particularly representative and have been widely studied. An investigation of the collaborative participation of Mn and Co can be of great importance for improving the catalytic activity and SO₂ resistance of Mn-Co oxides with a spinel structure. Therefore, in this study, we prepared Mn_xCo_3−xO₄ spherical particles with high surface area using a co-precipitation method and investigated their ability to remove NO_x via NH₃-SCR. Mn-Co bimetal oxides mainly possess a spinel structure and undergo a tetragonal-to-cubic phase transformation with increasing Co-content. A high concentration of surface oxygen and strong effective electron transfer between the variable valence elements (Co³⁺ + Mn³⁺ ↔ Co²⁺ + Mn⁴⁺) improves the redox ability of typical Mn_xCo_3−xO₄ (x = 1.0, 1.5, 2.0) spinel catalysts. In addition, Mn-enrichment leads to more oxygen vacancies and abundant surface-active sites, which further promotes the SCR catalytic performance. The investigated Mn_xCo_3−xO₄ catalysts exhibit > 91% NO_x conversion at 75 ℃, almost reaching 100% conversion with increasing reaction temperature. Notably, the NO_x conversion rate remained above 80% during the test time of 15 h under 150 × 10⁻⁶ SO₂ at 175 ℃. It was found that the coordination structure likely formed into a Co_tet(CoMn)_octO₄ spinel structure in which Mn ions (Mn³⁺ and Mn⁴⁺, mainly in trivalent manganese) and partial Co ions are configured into octahedral sites. These species were identified as the activity descriptor for probably owing to their strong electronic transfer interactions that were directly correlated with SCR activity. Furthermore, the Co_tet(CoMn)_octO₄ configuration was important for promoting low-temperature de-NO_x activity and highly conducive to protecting Mn active sites from poisoning by SO₂. The active sites in this particular spinel structure with the micro-coordination structure were effectively built and maintained to ensure the smooth circulation of electronic interactions in the core octahedron. The reaction of adsorbed NH₃ and gaseous NO (or NO₂) mainly occurred on the surface of Mn-Co spinel following the Eley-Rideal mechanism. Additionally, the NH₄NO₃ intermediate was likely first transformed into NH₄NO₂ and then to N₂ with increasing reaction temperature. Herein, we successfully synthesized a spinel-structured Mn-Co oxide catalyst comprising a Mn-enriched surface of (MnCo)₃O_4−ƞ spinel oxides that exhibited high NH₃-SCR catalytic activity and good resistance to SO₂ poisoning.

Key words: Mn-Co oxides, Spinel structure, Mn-enriched surface, Selective catalytic reduction, Synergistic effect, Reaction mechanism

高凤雨, 刘恒恒, 姚小龙, Sani Zaharaddeen, 唐晓龙, 罗宁, 易红宏, 赵顺征, 于庆君, 周远松. 球形表面富锰Mn_xCo_3−xO_4−ƞ尖晶石型催化剂选择性催化还原NO_x研究[J]. 物理化学学报, 2023, 39(9), 2212003. doi: 10.3866/PKU.WHXB202212003

Fengyu Gao, Hengheng Liu, Xiaolong Yao, Zaharaddeen Sani, Xiaolong Tang, Ning Luo, Honghong Yi, Shunzheng Zhao, Qingjun Yu, Yuansong Zhou. Spherical Mn_xCo_3−xO_4−ƞ Spinel with Mn-Enriched Surface as High-Efficiency Catalysts for Low-Temperature Selective Catalytic Reduction of NO_x by NH₃[J]. Acta Phys. -Chim. Sin. 2023, 39(9), 2212003. doi: 10.3866/PKU.WHXB202212003

链接本文: https://www.whxb.pku.edu.cn/CN/10.3866/PKU.WHXB202212003

https://www.whxb.pku.edu.cn/CN/Y2023/V39/I9/2212003

图/表 11

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Samples (designed ratio)	Mn content (mg∙g⁻¹)	Co content (mg∙g⁻¹)	Mn : Co (molar ratio)	BET surface area (m²∙g⁻¹)	Pore volume (cm³∙g⁻¹)	Pore diameter (nm)
MnO_x	–	–	–	26.5	0.08	11.9
Mn_2.5Co_0.5O₄	572.22	129.47	4.74	64.1	0.21	13.1
Mn_2.0Co_1.0O₄	483.40	269.54	1.92	83.9	0.32	15.4
Mn_1.5Co_1.5O₄	331.38	366.06	0.97	112.3	0.35	12.5
Mn_1.0Co_2.0O₄	228.29	490.99	0.50	155.7	0.25	6.4
Mn_0.5Co_2.5O₄	132.56	691.06	0.21	120.6	0.32	10.6
CoO_x	–	–	–	13.2	0.04	12.2

Samples (designed ratio)	Surface content (atomic ratio)			Mn/Co ratio	Surface O_L (atomic ratio)	Surface O_S (atomic ratio)	Mn³⁺/(Mn³⁺ + Mn⁴⁺)	Co³⁺/(Co³⁺ + Co²⁺)
Samples (designed ratio)	Mn	Co	O	Mn/Co ratio	Surface O_L (atomic ratio)	Surface O_S (atomic ratio)	Mn³⁺/(Mn³⁺ + Mn⁴⁺)	Co³⁺/(Co³⁺ + Co²⁺)
Mn_2.0Co_1.0O₄	18.87	7.85	50.08	2.40	26.69 (53.3%)	23.39 (46.7%)	64.1%	51.5%
Mn_1.5Co_1.5O₄	18.46	12.94	48.95	1.43	31.87 (65.1%)	17.08 (34.9%)	60.1%	44.6%
Mn_1.0Co_2.0O₄	11.17	15.50	45.77	0.72	26.68 (58.3%)	19.09 (41.7%)	62.5%	45.5%

Samples (designed ratio)	Normalized surface (atomic ratio)			Mn/O_L ratio	Co/O_L ratio	Oxygen vacancies (%)	Mn-enrichment (%)	Normalized surface formulas
Samples (designed ratio)	Mn	Co	O_L	Mn/O_L ratio	Co/O_L ratio	Oxygen vacancies (%)	Mn-enrichment (%)	Normalized surface formulas
Mn_2.0Co_1.0O₄	35.33	14.70	49.97	0.71	0.29	16.73	10.34	Mn_2.12Co_0.88O_3.33
Mn_1.5Co_1.5O₄	29.18	20.45	50.37	0.58	0.41	15.80	10.29	Mn_1.77Co_1.23O_3.37
Mn_1.0Co_2.0O₄	20.94	29.05	50.01	0.42	0.58	16.64	8.50	Mn_1.26Co_1.74O_3.33

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