Diffusion and Surface Segregation of Interstitial Ti Defects Induced by Electronic Metal-Support Interactions on a Au/TiO2 Nanocatalyst

Both the O vacancy and interstitial Ti are ubiquitous defect types found inreduced TiO2-xcatalytic support materials. Without a metal cluster, the interstitial Ti defect ismore stable in bulk of TiO2-xrather than becoming the Ti adatom on the reduced surface.Meanwhile, it can slowly diffuse out to regrow TiO2islands after surface oxidation. However, forthe widely used Au/TiO2nanocatalyst, ab initio molecular dynamics simulation and densityfunctional theory calculation found, even under the reducing condition with O vacancy, that theAu 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 excesselectrons and stabilize itself at the Au/TiO2interface. The electronic metal-support interaction not only makes this processexothermic but also reduces its diffusion barrier. When excess O2is further introduced, the TiO2islands can regrow at the metal-support interface to form the inverse supporting local sites. Comparatively speaking, the Au cluster plays a critical role overinterstitial Ti diffusion rates, while the additional O2stabilizes the regrowth of TiO2islands. Generally, the surface segregation ofinterstitial Ti defects could be more favorable for the metals with higher work function, such as Pt and Au, but less for thoseelectron-rich metals with a lower work function, such as Zn. Considering the ubiquity of support defects and the fact that the surfacesegregation 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.

Automated summary

Interstitial Ti defects and O vacancies are common in TiO2-x catalytic support materials and play significant roles in catalytic performance. Au clusters on TiO2-x surface can induce outward diffusion and surface segregation of interstitial Ti defects, promoting the regrowth of TiO2 islands. The interaction between different metal clusters and interstitial Ti affects the diffusion rates and provides insights into reaction mechanisms.