The Interaction of Hydrogen with Titanium Dioxide: A Periodic DFT Study

Baohuan Wei, and Monica Calatayud
Laboratoire de Chimie Théorique, Sorbonne Université & CNRS, Paris, France

Jeudi 9 Septembre 2021, 11h00
VisioConférence ZOOM:
https://zoom.us/j/99315354721?pwd=QVBvSDZER2FrMXBEZDBjbnlUSHlVdz09
ID de réunion : 993 1535 4721
Code secret : 0KJA6Y

Titanium dioxide (TiO2) has been widely used in many fields[1] such as photocatalysis, photovoltaics, catalysis, and sensors. The interaction of hydrogen with TiO2 is important in many catalytic redox processes and involves both electrostatic interactions and electron transfer. In the present work we investigate theoretically the role of surface topology in the H2 adsorption, dissociation (see Figure) and surface to subsurface diffusion. For the TiO2 materials, we consider a set of representative structures including rutile & anatase well-defined slabs[2] and also cluster, amorphous titania nanoparticles. The study was carried out by means of density functional theory (PBE+U) with VASP. The following parts will be presented:

1) Dissociation of hydrogen on surfaces (rutile & anatase)
2) Subsurface diffusion on rutile
3) Nanosized TiO2

The following issues will be addressed:
i) The nature of the mechanisms including thermodynamic energy and kinetic energy barrier, and the stability of hydride intermediates, were characterized.
ii) The structure-properties relationships were obtained.
iii) The effect of surface topology was highlighted.
iv) A characterization by IR spectroscopy is proposed. These data will be used as an aid for the experimental characterization of hydride species on titanium dioxide surfaces.

[Picture]

>br> >B>Figure. The mechanisms of the H2 dissociation on TiO2 surface studied in this work.

For the part of H2 dissociation, our study points out that dissociative H2 adsorption takes place through a heterolytic pathway via a hydride/hydroxyl pair, followed by the transfer of an H atom from a Ti atom to a nearby O atom accompanied by reduction of the substrate. The latter process leads to the homolytic product, with two hydroxyl groups and two Ti3+, and is the product thermodynamically most favorable.

The analysis of electronic structure, and temperature effects to characterize the reactivity of the various TiO2 orientations was carried out to provide a comprehensive understanding of this process. In addition, the frequencies calculated were found to depend on the facet exposed and could be used as a qualitative guideline to identify them experimentally.

Hydrogen is found to diffuse from the surface into the subsurface as a proton in rutile surfaces. The topology plays a key role in kinetic barriers, while slightly changes the thermodynamic energies: the process is quasi-isoenergetic and requires an activation barrier between 0.3-1 eV depending on the surface termination.

Our results indicate that (001) shows the most favorable path for (reversible) diffusion. Once in the bulk, H could diffuse easily inside cavities with low barriers. We observe a linear correlation between the activation energy and the distance Osurf - Osub in the bare slab. For rutile (110), the isotopic substitution by deuterium and the inclusion of thermal effects only modifies slightly the energetic profiles. However, the substrate reduction degree has a moderate impact on the activation barriers, decreasing from 1 eV to 0.8 eV as the surface is more reduced.

The role of irregular surfaces was investigated by considering small (sub)-nanoclusters, where the surface atoms are less coordinated and disordered. Compared to regular surfaces, the reduction by hydrogen adsorption is found to be more favorable in the clusters, as they present singly-coordinated oxygen sites. Moreover, the energy of removing a lattice oxygen is found to be linearly correlated with the energy released in the adsorption of a hydrogen molecule for both slab models and small clusters.

In summary, this thesis studied the interaction between titanium dioxide and hydrogen, while considering the role of surface topology effect and the reduction degree. Our results clearly indicate the complexity of the process, and suggest that controlling the topology and the reduction degree are valid strategies to tune such process.

_________________________________

References :
[1] S. G. Ullattil, S. B. Narendranath, S. C. Pillai, and P. Periyat, Chem. Eng. J., 343 (2018) 708-736.
[2] B. Wei, F. Tielens, and M. Calatayud, Nanomaterials, 9 (2019) 1199.