Formation of Active Cu-oxo Clusters for Methane Oxidation in Cu-Exchanged Mordenite
Formation of Active Cu-oxo Clusters for Methane Oxidation in Cu-Exchanged Mordenite
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DOI：
10.1021/acs.jpcc.8b10293
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发表时间：
2019-04-11
影响因子：
3.7
通讯作者：
Lercher, Johannes A.
中科院分区：
文献类型：
--
作者：
Ikuno, Takaaki;Grundner, Sebastian;Lercher, Johannes A.
已知的是，在中等温度下，铜交换沸石对甲烷选择氧化制甲醇具有很高的活性。其中，铜交换丝光沸石(MOR)是迄今为止铜原子甲醇产率最高的体系。这种高效率归因于MOR在高温下被O-2激活时选择性地稳定具有[Cu3(Mu-O)(3)](2+)结构的活性三元簇的能力。在本研究中，我们用原位X射线吸收光谱和紫外可见光谱研究了[Cu3(Mu-O)(3)](2+)形成的基本步骤。我们证明了在活性氧化物种形成之前，铜离子在活化过程中经历了一系列热驱动步骤。我们推测，在惰性气体中，通过Cu2+的自动还原，高流动性的铜物种的热形成是使MoR中的铜离子能够重组的关键，而重组是形成[Cu3(Mu-O)(3)](2+)还原前体所必需的。这种前体可以在强氧化剂--如O-2和N2O--的存在下，在低至50℃的温度下被氧化，生成活性的[Cu-3(Mu-O)(3)](2+)。
Cu-exchanged zeolites are known to be active in the selective oxidation of methane to methanol at moderate temperatures. Among them, Cu-exchanged mordenite (MOR) is the system that has so far shown the highest methanol yield per Cu atom. This high efficiency is attributed to the ability of MOR to selectively stabilize an active tricopper cluster with a [Cu-3(mu-O)(3)](2+) structure when activated in the presence of O-2 at high temperatures. In this study, we investigate the elementary steps in the formation of [Cu-3(mu-O)(3)](2+) by in situ X-ray absorption spectroscopy and ultraviolet-visible spectroscopy. We demonstrate that the Cu cations undergo a series of thermally driven steps during activation that precede the formation of the active oxidizing species. We hypothesize that the thermal formation of highly mobile Cu+ species by autoreduction of Cu2+ in an inert gas is essential to enable the reorganization of Cu ions in MOR, which is necessary for the formation of a reduced precursor of [Cu-3(mu-O)(3)](2+). Such a precursor can be oxidized in the presence of strong oxidants-such as O-2 and N2O-to form active [Cu-3(mu-O)(3)](2+) at temperatures as low as 50 degrees C.