TiO2 and Al2O3 are universally employed supports in heterogeneous catalysis. Depending on the support, however, there are sensible performance differences, the origin of which, at present, is poorly understood. In addition, reduced oxides, in particular TiO2 , hold great promise as a support for efficient photocatalytic devices[1]. Hydrogen spillover from a supproted metal cluster is widely employed in hydrogenation reactions and could also be adopted to introduce ”engineered disorder” in the titania structure, thus enhancing its light adsorption properties and electrical conductivity[2]. Here, the interface reactivity and electronic structure of hydrogen-reduced TiO2 are investigated with GGA and hybrid DFT. We demonstrate that, upon H2 adsorption, two protons are bound to the surface, while two electrons are transferred to the oxide, creating electronic trap sites (T i(III) centers) that strongly interact with the surface protons. Even though surface protons are highly mobile on the titania interface, the same can not be said for the Ti(III) centers, and thus the combined migration rate (e-/H+) of adsorbed H atoms is slower. Defect concentration effects[3] are also relevant: stronger H-oxide interaction is obtained at lower concentrations, as modelled with a realistic titania slab (∼ 1000 atoms) and hybrid functionals. The case of gamma-Al2O3 is instead different: on Al2O3 the hydrogen molecule is split in a heterolytic manner, with formation of a proton, bound to a surface oxide anion, and of an hydride moiety, strongly bound to particularly reactive surface Al tricoordinated (Al3c) site and with limited mobility. The electronic structure of the oxide is only marginally altered, without formation of defect states in the oxide band gap. Hydrogen spillover from a Pt cluster to TiO2 has also been modelled, demonstrating that the spillover rate depends on the cluster hydrogen load and on the thickness of the oxide acceptor layer. To conclude, even if hydrogen molecules can exothermically adsorb on both TiO2 and Al2O3 surfaces, only in the first case oxide reduction, with formation of polaronic states, is achieved, while for alumina reducing species are confined at the interface, and demonstrate a lower mobility. [1] Ni, M., Leung, M., Leung, D., Sumathy., K., Ren. Sust. En. Rev., 11(3):401–425, 2007. [2] Chen, X., Liu, L., Yu, Y., and Mao.,S., Science, 331(6018):746–750, 2011. [3] Spreafico,C. and VandeVondele.,J. Phys. Chem. Chem. Phys., 16:26144–26152, 2014

Is oxide hydrogenation equivalent to reduction? Fundamental differences between TiO2 and Al2O3 from DFT

251st National Meeting (2016)

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