Structural Characterization of Supported RhI(CO)2/γ-Al2O3 Catalysts by Periodic DFT Calculations

Microscopic structures of monodispersed rhodium dicarbonyl species chemisorbed on a ceramic metal-oxide support (γ-alumina) have been obtained by density functional theory (DFT) calculations with periodic boundary conditions applied. Several minimum energy structures of species were obtained and their relative energies indicate that, in the most energetically stable geometry, the rhodium atom is coordinated in a square-planar environment and forms a four-membered Rh–O–Al–O ring, with one Al atom octahedrally coordinated. Another docking geometry, close lying in energy, also has a square-planar coordination for the rhodium atom and involves a six-membered Rh–O–Al–O–Al–O ring with one Al octahedrally coordinated and one Al tetrahedrally coordinated. Computed bond lengths were found to be in reasonable agreement with experimental bond lengths as determined by EXAFS spectroscopy. Theoretical Rh K-edge XANES spectra suggest that the pre-edge region probes electronic states localized on the RhI(CO)2 unit, while postedge features probe the electronic states arising from the RhI(CO)2 interaction with the support, which partly depends on the docking geometry of the RhI(CO)2 units.通过采用周期性边界条件的密度泛函理论（DFT）计算，获得了单分散二羰基铑物种化学吸附于陶瓷金属氧化物载体（γ-氧化铝）上的微观结构。研究得到了该物种的多种最低能量结构，其相对能量结果显示：在能量最稳定的几何构型中，铑原子呈平面正方形配位环境，并形成四元Rh–O–Al–O环，其中一个铝原子为八面体配位。另一种能量相近的吸附构型中，铑原子同样采用平面正方形配位，且形成六元Rh–O–Al–O–Al–O环，该构型中一个铝原子为八面体配位，另一个为四面体配位。计算所得的键长与通过扩展X射线吸收精细结构（EXAFS）光谱测定的实验键长具有较好的一致性。理论计算得到的铑K边X射线吸收近边结构（XANES）光谱表明，预边区域探测的是定域于RhI(CO)₂单元上的电子态，而后边特征峰则探测源于RhI(CO)₂与载体相互作用产生的电子态，该相互作用在一定程度上取决于RhI(CO)₂单元的吸附构型。