Shivangi Singh , Ton V. W. Janssens , Henrik Grönbeck
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For H–CHA, we find that dealumination leads to the formation of mobile Al(OH)<sub>3</sub>H<sub>2</sub>O (extra-framework aluminum) species, whereas for Cu–CHA, formation of framework bound Cu–Al species is thermodynamically preferred over Al(OH)<sub>3</sub>H<sub>2</sub>O, which results in the increased stability of Cu–CHA. The formation of mobile Al(OH)<sub>3</sub>H<sub>2</sub>O in Cu–CHA is, moreover, associated with a high energy barrier. The phase diagrams show the formation of Al(OH)<sub>3</sub>H<sub>2</sub>O and Al<sub>2</sub>O<sub>3</sub> from H–CHA and that high temperatures favor the formation of Al<sub>2</sub>O<sub>3</sub>. For Cu–CHA, high temperatures lead to the formation of CuO and Al<sub>2</sub>O<sub>3</sub>, which is favored over Al(OH)<sub>3</sub>H<sub>2</sub>O + CuO. The microkinetic model shows that the formation of Al(OH)<sub>3</sub>H<sub>2</sub>O in the presence of water starts at 380 K and 800 K in H–CHA and Cu–CHA, respectively. Additionally, the time evolution of the Al(OH)<sub>3</sub>H<sub>2</sub>O coverage at 923 K reveals that the process of dealumination is significantly faster for H–CHA as compared to Cu–CHA, which is in accordance with the measured increased stability.</p></div>","PeriodicalId":66,"journal":{"name":"Catalysis Science & Technology","volume":null,"pages":null},"PeriodicalIF":4.4000,"publicationDate":"2024-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Mechanism for Cu-enhanced hydrothermal stability of Cu–CHA for NH3-SCR†\",\"authors\":\"Shivangi Singh , Ton V. W. Janssens , Henrik Grönbeck\",\"doi\":\"10.1039/d4cy00373j\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><p>Exposure of acidic zeolite-based catalysts to water at high temperatures generally leads to deactivation due to dealumination. In Cu–CHA zeolite, which is a preferred catalyst for the selective catalytic reduction of NO by NH<sub>3</sub> (NH<sub>3</sub>-SCR), the acidic protons in the zeolite are partially exchanged by Cu ions. The presence of Cu has been measured to reduce the rate of dealumination, thus stabilizing the catalyst. To understand the stabilizing effect of Cu, density functional theory calculations, <em>ab initio</em> thermodynamics and microkinetic modeling are used to compare the reaction mechanism for the dealumination of H–CHA to Cu–CHA. For H–CHA, we find that dealumination leads to the formation of mobile Al(OH)<sub>3</sub>H<sub>2</sub>O (extra-framework aluminum) species, whereas for Cu–CHA, formation of framework bound Cu–Al species is thermodynamically preferred over Al(OH)<sub>3</sub>H<sub>2</sub>O, which results in the increased stability of Cu–CHA. The formation of mobile Al(OH)<sub>3</sub>H<sub>2</sub>O in Cu–CHA is, moreover, associated with a high energy barrier. The phase diagrams show the formation of Al(OH)<sub>3</sub>H<sub>2</sub>O and Al<sub>2</sub>O<sub>3</sub> from H–CHA and that high temperatures favor the formation of Al<sub>2</sub>O<sub>3</sub>. For Cu–CHA, high temperatures lead to the formation of CuO and Al<sub>2</sub>O<sub>3</sub>, which is favored over Al(OH)<sub>3</sub>H<sub>2</sub>O + CuO. The microkinetic model shows that the formation of Al(OH)<sub>3</sub>H<sub>2</sub>O in the presence of water starts at 380 K and 800 K in H–CHA and Cu–CHA, respectively. Additionally, the time evolution of the Al(OH)<sub>3</sub>H<sub>2</sub>O coverage at 923 K reveals that the process of dealumination is significantly faster for H–CHA as compared to Cu–CHA, which is in accordance with the measured increased stability.</p></div>\",\"PeriodicalId\":66,\"journal\":{\"name\":\"Catalysis Science & Technology\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":4.4000,\"publicationDate\":\"2024-06-17\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Catalysis Science & Technology\",\"FirstCategoryId\":\"92\",\"ListUrlMain\":\"https://www.sciencedirect.com/org/science/article/pii/S2044475324002995\",\"RegionNum\":3,\"RegionCategory\":\"化学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q2\",\"JCRName\":\"CHEMISTRY, PHYSICAL\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Catalysis Science & Technology","FirstCategoryId":"92","ListUrlMain":"https://www.sciencedirect.com/org/science/article/pii/S2044475324002995","RegionNum":3,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"CHEMISTRY, PHYSICAL","Score":null,"Total":0}
引用次数: 0
摘要
酸性沸石基催化剂在高温下与水接触,通常会由于脱铝而导致失活。Cu-CHA 沸石是选择性催化还原 NH3(NH3-SCR)中 NO 的首选催化剂,沸石中的酸性质子部分被 Cu 离子交换。据测定,Cu 的存在可降低脱铝速率,从而稳定催化剂。为了解 Cu 的稳定作用,我们利用密度泛函理论计算、ab initio 热力学和微动力学模型来比较 H-CHA 与 Cu-CHA 脱胶的反应机理。我们发现,对于 H-CHA 来说,脱铝会导致形成流动的 Al(OH)3H2O(框架外铝)物种,而对于 Cu-CHA 来说,热力学上框架结合的 Cu-Al 物种的形成比 Al(OH)3H2O 更优先,这导致 Cu-CHA 的稳定性增加。此外,在 Cu-CHA 中形成流动的 Al(OH)3H2O 与高能垒有关。相图显示 H-CHA 中形成了 Al(OH)3H2O 和 Al2O3,高温有利于 Al2O3 的形成。对于 Cu-CHA 来说,高温导致形成 CuO 和 Al2O3,而 Al2O3 比 Al(OH)3H2O + CuO 更有利。微动力学模型显示,在 H-CHA 和 Cu-CHA 中,水分别在 380 K 和 800 K 时开始形成 Al(OH)3H2O。此外,923 K 时 Al(OH)3H2O 覆盖率的时间演变表明,与 Cu-CHA 相比,H-CHA 的脱胶过程明显更快,这与测量到的更高稳定性相符。
Mechanism for Cu-enhanced hydrothermal stability of Cu–CHA for NH3-SCR†
Exposure of acidic zeolite-based catalysts to water at high temperatures generally leads to deactivation due to dealumination. In Cu–CHA zeolite, which is a preferred catalyst for the selective catalytic reduction of NO by NH3 (NH3-SCR), the acidic protons in the zeolite are partially exchanged by Cu ions. The presence of Cu has been measured to reduce the rate of dealumination, thus stabilizing the catalyst. To understand the stabilizing effect of Cu, density functional theory calculations, ab initio thermodynamics and microkinetic modeling are used to compare the reaction mechanism for the dealumination of H–CHA to Cu–CHA. For H–CHA, we find that dealumination leads to the formation of mobile Al(OH)3H2O (extra-framework aluminum) species, whereas for Cu–CHA, formation of framework bound Cu–Al species is thermodynamically preferred over Al(OH)3H2O, which results in the increased stability of Cu–CHA. The formation of mobile Al(OH)3H2O in Cu–CHA is, moreover, associated with a high energy barrier. The phase diagrams show the formation of Al(OH)3H2O and Al2O3 from H–CHA and that high temperatures favor the formation of Al2O3. For Cu–CHA, high temperatures lead to the formation of CuO and Al2O3, which is favored over Al(OH)3H2O + CuO. The microkinetic model shows that the formation of Al(OH)3H2O in the presence of water starts at 380 K and 800 K in H–CHA and Cu–CHA, respectively. Additionally, the time evolution of the Al(OH)3H2O coverage at 923 K reveals that the process of dealumination is significantly faster for H–CHA as compared to Cu–CHA, which is in accordance with the measured increased stability.
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