Dioxygen Activation Kinetics over Distinct Cu Site Types in Cu-Chabazite Zeolites
Dioxygen Activation Kinetics over Distinct Cu Site Types in Cu-Chabazite Zeolites
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DOI：
10.1021/acscatal.1c03471
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发表时间：
2021-09-10
期刊：
影响因子：
12.9
通讯作者：
Flaherty, David W.
中科院分区：
文献类型：
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作者：
Bregante, Daniel T.;Wilcox, Laura N.;Flaherty, David W.
Cu交换沸石活化分子氧以形成用于部分甲烷氧化（PMO）、氮氧化物分解和一氧化碳氧化的活性位点。O-2活化的表观速率取决于不同Cu位点类型的本征动力学和给定沸石内的此类位点的分布，这取决于骨架Al原子的密度和排列。在这里，我们使用水热合成方法来控制菱沸石（CHA）中的骨架Al位点的排列，进而形成不同的Cu位点类型。时间分辨原位拉曼光谱揭示了这些定义明确的Cu-CHA材料内的O-2吸附和活化的动力学以及伴随的铜-氧（CuxOy）络合物的结构演变，这与从原位X射线吸收光谱（XAS）提取的Cu（I）氧化动力学一起解释。拉曼光谱的几个合理的CuxOy物种模拟密度泛函理论表明，实验光谱（λ（ex）= 532 nm）捕获的单（μ-氧代）二铜物种（ZCuOCuZ）的形成。瞬态实验表明，形成不再改变拉曼光谱中的CuxOy结构所需的时间尺度对应于在化学计量PMO循环中使CH 3OH产率最大化的氧化处理的持续时间（约2小时）。然而，这些周期远远超出了初始Cu（I）中间体完全转化为Cu（II）状态的时间尺度（
Cu-exchanged zeolites activate dioxygen to form active sites for partial methane oxidation (PMO), nitrogen oxide decomposition, and carbon monoxide oxidation. Apparent rates of O-2 activation depend both on the intrinsic kinetics of distinct Cu site types and the distributions of such sites within a given zeolite, which depend on the density and arrangement of the framework Al atoms. Here, we use hydrothermal synthesis methods to control the arrangement of framework Al sites in chabazite (CHA) zeolites and, in turn, the distinct Cu site types formed. Time-resolved in situ Raman spectroscopy reveals the kinetics of O-2 adsorption and activation within these well-defined Cu-CHA materials and the concomitant structural evolution of copper-oxygen (CuxOy) complexes, which are interpreted alongside Cu(I) oxidation kinetics extracted from in situ X-ray absorption spectroscopy (XAS). Raman spectra of several plausible CuxOy species simulated using density functional theory suggest that experimental spectra (lambda(ex) = 532 nm) capture the formation of mono(mu-oxo)dicopper species (ZCuOCuZ). Transient experiments show that the timescales required to form CuxOy structures that no longer change in Raman spectra correspond to the durations of oxidative treatments that maximize CH3OH yields in stoichiometric PMO cycles (approximately 2 h). Yet, these periods extend well beyond the timescales for the complete conversion of the initial Cu(I) intermediates to their Cu(II) states (