Abstract
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.
