Mixed Metal Oxide Clusters: Model Systems for Catalytically Active Materials

我们的初始项目（2020年1月至2022年12月）将最先进的质谱和红外光谱实验与量子化学计算相结合，以获得由负载金属氧化物催化剂激活C-H键的分子水平见解。我们确定了Co-， Fe-和ni -取代的Al8O12+和Al3O4+簇作为这些催化剂活性位点的气相模型体系，并研究了它们的结构和对甲烷的反应性。Al8O12+和Al3O4+簇在结构和反应性方面表现出相反的Fe取代Al行为。在Al8O12+中，取代是同构的，但伴随着Fe氧化态的变化，将活性端的Al−O•−键转变为非活性端的Fe=O键。相反，在Al3O4+中，Fe氧化态的变化引起结构变化，并产生高活性的末端Al - O•−自由基位点。我们证明了Kohn-Sham密度泛函理论（DFT）不能预测这些价异构体的相对稳定性，甚至不能预测单取代体系，如Al2FeO4+。多参考波函数计算需要正确预测最低能量结构。在这里，我们建议继续这一富有成果的研究路线，将反应性重点从第一阶段的甲烷转移到水。除了甲烷和水的C-H键和O-H键分别发生均裂外，水还可能发生异裂。我们研究了混合金属氧化物团簇的反应性，它们是(i)固体简单金属氧化物，如掺杂过渡金属原子（Fe, Co, Ni, Cu, Zn）的Al2O3，和（ii）掺杂铁的羟基氧化镍（NiOOH）的模型。特别地，我们想找出O•−自由基位点在这些簇中起什么作用。我们将质谱实验与振动作用谱的结构测定结合起来，并将这些实验与量子化学反应性计算以及结构和振动谱的量子化学预测结合起来。

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.