Diffusion and Surface Segregation of Interstitial Ti Defects Induced by Electronic Metal-Support Interactions on a Au/TiO2Nanocatalyst

Both the O vacancy and interstitial Ti are ubiquitous defect types found in reduced TiO2-x catalytic support materials. Without a metal cluster, the interstitial Ti defect is more stable in bulk of TiO2-x rather than becoming the Ti adatom on the reduced surface. Meanwhile, it can slowly diffuse out to regrow TiO2 islands after surface oxidation. However, for the widely used Au/TiO2 nanocatalyst, ab initio molecular dynamics simulation and density functional theory calculation found, even under the reducing condition with O vacancy, that the Au cluster could induce the outward diffusion and surface segregation of interstitial Ti defects. The Au cluster, which acts as an electron reservoir, enables the interstitial Ti to shed excess electrons and stabilize itself at the Au/TiO2 interface. The electronic metal-support interaction not only makes this process exothermic but also reduces its diffusion barrier. When excess O2 is further introduced, the TiO2 islands can regrow at the metal-support interface to form the inverse supporting local sites. Comparatively speaking, the Au cluster plays a critical role over interstitial Ti diffusion rates, while the additional O2 stabilizes the regrowth of TiO2 islands. Generally, the surface segregation of interstitial Ti defects could be more favorable for the metals with higher work function, such as Pt and Au, but less for those electron-rich metals with a lower work function, such as Zn. Considering the ubiquity of support defects and the fact that the surface segregation is so facile under not only oxidating but also reducing conditions, we postulate that, in addition to O vacancies, interstitial Ti defects are also crucial for fully understanding the catalytic role of TiO2-supported metal catalysts.