Mixed Metal Oxide Clusters: Model Systems for Catalytically Active Materials

我们的初始项目（1/2020-12/2022）将最先进的质谱和红外光谱实验与量子化学计算相结合，以获得对负载型金属氧化物催化剂的C-H键活化的分子水平的见解。我们确定了Co-，Fe-和Ni-取代的Al_8 O_（12+）和Al_3 O_（4+）簇作为这类催化剂中活性中心的气相模型系统，并研究了它们的结构和对甲烷的反应性。Al_8 O_（12+）和Al_3 O_（4+）团簇在Fe取代Al时，无论是结构还是反应活性都表现出相反的行为。在Al 8 O 12+中，取代是同晶的，但伴随着Fe氧化态的变化，将反应性末端Al−O·−键转化为非反应性末端Fe=O键。相反，在Al 3 O 4+中，Fe氧化态的变化引起结构变化，并产生高度反应性的末端Al−O·−自由基位点。我们证明，Kohn-Sham密度泛函理论（DFT）是不预测的相对稳定性，这样的价异构体，即使不是单取代的系统，如Al 2FeO 4+。需要多参考波函数计算来正确预测最低能量结构。在这里，我们建议继续这一富有成效的研究路线，将反应重点从第一阶段的甲烷转移到本提案的水。除了甲烷和水的C-H和O-H键分别发生均裂外，水也可能发生异裂。我们研究了混合金属氧化物团簇的反应性，这些团簇是（i）固体简单金属氧化物如掺杂有过渡金属原子（Fe，Co，Ni，Cu，Zn）的Al 2 O3和（ii）Fe掺杂的羟基氧化镍（NiOOH）的模型。特别是，我们想知道O·−自由基位点在这些簇中扮演什么角色。我们使我们的质谱实验与结构测定使用振动作用光谱，和联合收割机这些实验与量子化学反应性计算和量子化学预测的结构和振动光谱。

Our initial project (1/2020-12/2022) combines state-of-the-art mass spectrometric and IR spectroscopic experiments with quantum chemical calculations to gain molecular-level insights into C–H bond activation by supported metal oxide catalysts. We identified Co-, Fe- and Ni-substituted Al8O12+ and Al3O4+ clusters as gas-phase model systems for active sites in such catalysts and studied their structure and reactivity towards methane. The Al8O12+ and Al3O4+ clusters showed an opposite behavior on substitution of Fe for Al, both with respect to structure and reactivity. In Al8O12+, substitution is isomorphous, but accompanied by a change of the Fe oxidation state, which converts the reactive terminal Al−O•− bond into a non-reactive terminal Fe=O bond. In Al3O4+, on the contrary, a change of the Fe oxidation state induces a structural change and creates a highly reactive terminal Al−O•− radical site. We demonstrated that Kohn-Sham density functional theory (DFT) is not predictive with regard to the relative stability of such valence isomers, even not for singly substituted systems, like Al2FeO4+. Multi-reference wave-function calculations are required to correctly predict the lowest energy structure. Here, we propose to continue this fruitful line of research with the shift of the reactivity focus from methane in the first period to water for the present proposal. In addition to the homolytic splitting of the C-H and O-H bonds of methane and water, respectively, water may also undergo heterolytic splitting. We study the reactivity of mixed metal oxide clusters that are models for (i) solid simple metal oxides such as Al2O3 doped with transition metal atoms (Fe, Co, Ni, Cu, Zn), and (ii) Fe-doped nickel oxyhydroxide (NiOOH). In particular, we would like to find out, which role O•− radical sites in these clusters play. We make us of mass spectrometric experiments together with structure determination using vibrational action spectroscopy, and combine these experiments with quantum chemical reactivity calculations and quantum chemical predictions of structures and vibrational spectra.