Effect of surface structure on the photoreactivity of TiO₂

Although it has been widely accepted that the crystal phase, morphology, and facet significantly influence the catalytic and photocatalytic activity of TiO₂, establishing the correlation between structure and activity of heterogeneous reactions is very difficult because of the complexity of the structure. Utilizing ultrahigh vacuum (UHV) based temperature-programmed desorption (TPD) and density functional theory (DFT) calculations, we have successfully assessed the photoreactivity of two well characterized rutile surfaces ((011)-(2×1) and (110)-(1×1)) through examining the photocatalyzed oxidation of methanol. The photocatalytic products, such as formaldehyde and methyl formate, are the same on both surfaces under UV illumination. However, the reaction rate on (011)-(2×1) is only 42% of that on (110)-(1×1), which contradicts previous reports in aqueous environments where characterization of TiO₂ structure is difficult. The discrepancy probably comes from the differences of the TiO₂ structure in these studies. Our DFT calculations reveal that the rate-determining step of methanol dissociation on both surfaces is C-H scission,; however, the barrier of this elementary step on (011)-(2×1) is about 0.2 eV higher than that on (110)-(1×1) because of their distinct surface atomic configurations. The present work not only demonstrates the importance of surface structure in the photoreactivity of TiO₂, but also provides an example for building the correlation between structure and activity using surface science techniques and DFT calculations.