Ab Initio Molecular Dynamics Spectra for Characterization of Hydrated Al2O3 Supported MoO x

The structure of supported MoOx/Al2O3 catalysts is investigated using ab initio molecular dynamics (AIMD) and density-functional theory (DFT). Phase diagrams were computed to understand the hydroxylation coverage as a function of the temperature, H2O partial pressure, and MoOx loading. We relate the shifts in experimental Raman vibrational frequencies under hydrated conditions to hydrogen bonding interactions and the MoOx anchoring location. We showcase that the use of AIMD as a benchmark for DFT can provide insight into how, under certain hydrated conditions, DFT excels as an inexpensive method for computing vibrational frequencies, while, under dehydrated conditions, it is susceptible to the largest errors. Additionally, to facilitate the analysis of the hydroxyl region of experimental infrared spectra, we compute the power spectra of individual hydroxyls and see a strong relationship between OH stretching frequencies and the surrounding coordination environment, which are also impacted by anchored MoOx. We compare computational and experimental IR and Raman spectra of catalysts synthesized herein under the same conditions.

Automated summary

The structure of supported MoOx/Al2O3 catalysts was investigated using AIMD and DFT. Phase diagrams were computed to understand the hydroxylation coverage under different conditions. The results showed that DFT is a cost-effective method for computing vibrational frequencies under certain hydrated conditions, but is susceptible to errors under dehydrated conditions. The power spectra of individual hydroxyls revealed a strong relationship between OH stretching frequencies and the surrounding coordination environment and MoOx anchoring location.