Digging Ti interstitials at the r-TiO2(1 1 0) surface: Mechanism of porphyrin Ti sequestration by iminic N nucleophilic attack

Ti interstitials are key actors in the reduction/re-oxidation processes of TiO2. The charge redistribution originated by these point defects is acknowledged to trigger the dissociation of small molecules at the surface, while the direct involvement of Ti interstitials in reactions with adsorbates is an open question. From a combined experimental and theoretical investigation, we show how the potential energy surface for the outdiffusion of Ti interstitials at rutile-TiO2(1 1 0) is strongly lowered by adsorbed porphyrins, independently of their peripheral terminations. The process is activated by a nucleophilic attack of an iminic N to a surface 6-fold coordinated Ti atom, which is sequestrated by the porphyrin and replaced by a subsurface interstitial. From comparison with acidic porphyrins, we show that Ti extraction is specifically driven by the Lewis basicity of iminic N, which opens the pathway to the chemical reaction of Ti interstitials with a larger class of molecular systems.