Chromium Oxide Species Supported on Silica: A Representative Periodic DFT Model

The Cr/SiO 2 system is investigated using periodic DFT. The model represents the amorphous character of the silica surface and allows the investigation of the effect of hydration on the Cr(VI) monomers. First, the geometry and energetics are discussed and compared with experimental data. The phase diagram plotted from an atomistic thermodynamics model confirms the higher stability of mono-oxo and dioxo chromium, in comparison with species containing Cr–OH groups. In addition, the effect of the siloxane ring size on the spectroscopic signature of chromium is analyzed. A preliminary study is presented on the surface doping effect by Ti on the structure and stability of chromium species. The results reveal that the charge transfer process between Ti and Cr can explain the observed change in the reactivity of chromium species.