Pristine and modified titanium dioxide (TiO$_2$) photocatalysts are extensively used for photocatalytic activation of the stable and inert nitrogen molecule¹. The non-toxicity, high photocatalytic activity and stability make TiO$_2$ photocatalysts a benchmark catalyst for nitrogen activation^2,3. In this study, we investigated the activation of the nitrogen molecule on pristine and ruthenium (Ru)-loaded TiO$_2$ clusters in the ground state and excited states, respectively.

Density functional theory (DFT) was used to obtain all the desired results. The adsorption modes of nitrogen (N$_2$), hydrogen (H) and ammonia (NH$_3$) were determined on pristine and Ru-loaded TiO$_2$ clusters. The latter was performed on (TiO$_2$)$_n$ and Ru-(TiO$_2$)$_n$, n=3, 6, 12 selected cluster sizes. Adsorption was studied both in the ground state and excited states. Excited state calculations provide important insight into the properties associated with photocatalysis.

Theoretical calculations were performed in GPAW with the projector-augmented wave method (PAW)⁴ using the plane-wave basis set, the PBE functional, and the Grimme-D3 correction to account for van der Waals interactions. All calculations of the ground and excited states were performed as implemented in GPAW.

The adsorption of the reactants and the expected products is the first step in elucidating the excited-state reaction mechanism and thus explaining the photocatalytic properties, which are crucial for understanding ammonia synthesis under photocatalytic conditions.

(1) Chen, S.; Liu, D.; Peng, T. Fundamentals and Recent Progress of Photocatalytic Nitrogen-Fixation Reaction over Semiconductors. \textit{Sol. RRL} \textbf{2021}, 5 (2), 1–31. https://doi.org/10.1002/solr.202000487.

(2) Ali, I.; Suhail, M.; Alothman, Z. A.; Alwarthan, A. Recent Advances in Syntheses, Properties and Applications of TiO$_2$ nanostructures. \textit{RSC Adv.} \textbf{2018}, 8 (53), 30125–30147. https://doi.org/10.1039/c8ra06517a.

(3) Kovačič, Ž.; Likozar, B.; Huš, M. Photocatalytic CO$_2$ Reduction: A Review of Ab Initio Mechanism, Kinetics, and Multiscale Modeling Simulations. \textit{ACS Catal.} \textbf{2020}, 10 (24), 14984–15007. https://doi.org/10.1021/acscatal.0c02557.

(4) Mortensen, J. J.; Hansen, L. B.; Jacobsen, K. W. Real-Space Grid Implementation of the Projector Augmented Wave Method. \textit{Phys. Rev. B - Condens. Matter Mater. Phys.} \textbf{2005}, 71 (3), 1–11. https://doi.org/10.1103/PhysRevB.71.035109.