Relationships among the Catalytic Performance, Redox Activity, and Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Investigated Using In Situ XAFS and UV-Vis Spectroscopies
Relationships among the Catalytic Performance, Redox Activity, and Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Investigated Using In Situ XAFS and UV-Vis Spectroscopies
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DOI：
10.1021/acscatal.1c05559
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发表时间：
2022-02-18
期刊：
影响因子：
12.9
通讯作者：
Takahashi, Keisuke
中科院分区：
文献类型：
--
作者：
Ohyama, Junya;Tsuchimura, Yuka;Takahashi, Keisuke
用Cu沸石直接将甲烷转化为甲醇的催化反应系统已经被开发出来。在这些材料中，Cu-CHA已被报道在CH4-O-2-H2O混合物的反应中表现出相对较高的催化性能，尽管其催化活性随组成的不同而不同。本研究制备了4种不同成分和催化活性水平的Cu-CHA催化剂，并结合CH4-O-2-H2O反应混合物，利用原位x射线吸收精细结构和紫外-可见漫反射光谱分析了这些样品的氧化还原性能和局部结构。对材料的氧化还原速率与催化活性（反映在CH4氧化过程中的周转频率（TOF））之间的关系进行了评估，分析表明Cu-CHA还原速率与活性高度相关。数据还表明，Cu2+的还原与CH4中C-H键的激活有关，这是整个反应的速率决定步骤。研究了Cu-CHA的局部结构对CH3OH选择性、TOF值和Cu2+还原速率的影响。在CHA框架中Cu2+离子与六元环（Z(2)Cu）配位比例高的Cu-CHA样品比[CuOH]（+）离子与八元环（ZCuOH）配位比例高的Cu-CHA样品表现出更高的选择性，尽管前者Cu-CHA样品的TOF值并不高于后者，因为Cu2+的还原速率较慢。富Z(2) cu样品和富zcuoh样品中形成的cu2o活性结构不同，这是催化作用不同的原因。
Catalytic reaction systems for the direct conversion of methane to methanol have been previously developed using Cu zeolites. Among these materials, the Cu-CHA has been reported to show relatively high catalytic performance during the reaction of CH4-O-2-H2O mixtures, although this catalytic activity varies with composition. In the present study, four Cu-CHA catalysts having different compositions and catalytic activity levels were prepared, and the redox properties and local structures of these specimens were analyzed using in situ X-ray absorption fine structure and UV-vis diffuse reflectance spectroscopies in conjunction with a CH4-O-2-H2O reaction mixture. The relationships between the redox rates of the materials and catalytic activities (as reflected in the turnover frequency (TOF) during CH4 oxidation) were assessed, and this analysis showed that the Cu-CHA reduction rate was highly correlated with activity. The data also suggested that the Cu2+ reduction is associated with the activation of C-H bonds in CH4, which is the rate-determining step for the overall reaction. The effects of the local structure of the Cu-CHA on selectivity for CH3OH, the TOF value, and the Cu2+ reduction rate were studied. Those Cu-CHA samples having high proportions of Cu2+ ions coordinated to six-membered rings (Z(2)Cu) in the CHA framework exhibited higher selectivity than specimens having high proportions of [CuOH](+) ions coordinated to eight-membered rings (ZCuOH), although the TOF values of the former Cu-CHA specimens were not higher than those of the latter because of the slower reduction rates of Cu2+ species. The different catalyses can be attributed to the difference in Cu2Ox active structures formed in the Z(2)Cu-rich samples and the ZCuOH-rich samples.