First Principles Insight into H₂ Activation and Hydride Species on TiO₂ Surfaces

Hydrogen interaction with the TiO₂ surfaces is important in many catalytic and photocatalytic reactions. However, the mechanisms of H₂ activation on TiO₂ surfaces remain unclear. Here, we study H₂ activation on the most commonly examined and stable surfaces of different TiO₂ polymorphs, including rutile TiO₂(110), anatase TiO₂(101), and brookite TiO₂(210), by first-principles density functional theory. We find that for all three surfaces, the heterolytic pathway is kinetically more favorable (leading to a hydride and a hydroxyl), even though the homolytic dissociation of H₂ is thermodynamically more favorable (leading to two hydroxyls). For rutile TiO₂(110), the hydride produced by the heterolytic dissociation of H₂ can transfer from Ti to O with an activation energy of 0.99 eV, yielding the homolytic products. For anatase TiO₂(101) and brookite TiO₂(210), the barrier of hydrogen transfer from Ti to O is higher (1.48-1.68 eV). This indicates that hydrides on can be kinetically stabilized on TiO₂ surfaces. Our study sheds light on the mechanisms of H₂ dissociation on TiO₂ and provides rational for the existence of hydrides on TiO₂ or other reducible metal oxides which may facilitate hydrogenation reactions.