DFT Studies of the Structure and Vibrational Spectra of Isolated Molybdena Species Supported on Silica

Different cluster models of isolated MoVI molybdate species anchored on silica surface were investigated
using density functional theory (DFT). Isolated molybdate centers were modeled as either a penta-coordinated
mono-oxo species or a tetra-coordinated di-oxo species. Standard free energy changes for interconversion
between mono-oxo and di-oxo species indicate that these two species can coexist in equilibrium on the surface
of amorphous silica with di-oxo species being the favored species at high temperature and low partial pressures
of water. Comparison of Raman-spectra from experiment and DFT suggests that di-oxo species might be the
prominent species responsible for the peak at 988 cm-1. This conclusion is strongly supported by the similarity
in the EXAFS spectra obtained from experiment and theory. The thermodynamics for H2 reduction of isolated
molybdate species were determined and found to be in reasonable agreement with experimental observation.
DFT calculations of the structure and properties of the reduced MoIV centers and comparison of these with
experimental results suggest that the reduced centers are present as mono-oxo species.

本研究采用密度泛函理论（Density Functional Theory, DFT），对锚定在二氧化硅表面的孤立六价钼（MoVI）钼酸盐物种的各类团簇模型开展了系统研究。孤立钼酸盐中心的模型分为两类：五配位单氧物种与四配位双氧物种。单氧与双氧物种间相互转化的标准吉布斯自由能变计算结果表明，两类物种可在无定形二氧化硅表面达到平衡共存；在高温、低水汽分压条件下，双氧物种为优势存在物种。实验与DFT计算所得拉曼光谱的对比分析显示，988 cm⁻¹处的特征峰主要由双氧物种贡献。实验与理论获得的扩展X射线吸收精细结构（Extended X-ray Absorption Fine Structure, EXAFS）光谱具有高度相似性，为上述结论提供了强有力的支撑。本研究测定了孤立钼酸盐物种的氢气还原热力学过程，计算结果与实验观测值吻合良好。针对还原态四价钼（MoIV）中心的结构与性质开展DFT计算，并将计算结果与实验数据对比后发现，还原态中心以单氧物种形式存在。