Mechanistic understanding of N2 activation: a comparison of unsupported and supported Ru catalysts

Yves Ira A Reyes; Kai-Shiang Yang; Ho Viet Thang; Carmine Coluccini; Shih-Yuan Chen; Hsin-Yi Tiffany Chen

doi:10.1039/d2fd00172a

Mechanistic understanding of N₂ activation: a comparison of unsupported and supported Ru catalysts

Faraday Discuss. 2023 Jul 19;243(0):148-163. doi: 10.1039/d2fd00172a.

Authors

Yves Ira A Reyes¹, Kai-Shiang Yang¹, Ho Viet Thang², Carmine Coluccini³, Shih-Yuan Chen⁴, Hsin-Yi Tiffany Chen^{1

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Affiliations

¹ Department of Engineering and System Science, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu 300044, Taiwan. hsinyi.chen@mx.nthu.edu.tw.
² The University of Danang, University of Science and Technology, 54 Nguyen Luong Bang, Danang 550000, Vietnam.
³ Institute of New Drug Development, China Medical University, No. 91 Hsueh-Shih Road, Taichung 40402, Taiwan.
⁴ Energy Catalyst Technology Group, Energy Process Research Institute (EPRI), National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.
⁵ College of Semiconductor Research, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu 300044, Taiwan.
⁶ Department of Material Science and Engineering, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu 300044, Taiwan.

PMID: 37057657
DOI: 10.1039/d2fd00172a

Abstract

N₂ dissociative adsorption is commonly the rate-determining step in thermal ammonia synthesis. Herein, we performed density functional theory (DFT) calculations to understand the N₂ dissociation mechanism on models of unsupported Ru(0001) terraces, Ru B5 sites, and polar MgO(111)-supported Ru₈ cluster mimicking a B5 site geometry, denoted (Ru₈(B5-like)/MgO(111)). The activation energy of N₂ dissociative adsorption on the Ru₈(B5-like)/MgO(111) model (E_a = 0.33 eV) is much lower than that on the unsupported Ru(0001) terrace (E_a = 1.74 eV) and Ru B5 (E_a = 0.62 eV) models. The lower N₂ dissociation barrier on Ru B5 sites is facilitated by the enhanced σ donation and π* back-donation between N₂(σ, π*) and Ru(d) orbitals resulting in the stronger activation of the molecular side-on N₂* dissociation precursor. The Ru₈(B5-like)/MgO(111) also exhibits enhanced σ donation because of the B5-like cluster geometry. Furthermore, the Ru cluster of the bare Ru₈(B5-like)/MgO(111) model is positively charged. This induced an unusual π donation from N₂(π) to Ru(d) orbitals as revealed by analyses of the density of states and partial charge densities. The combined σ and π donation resulted in an increased synergistic π* back-donation. The total interactions between N₂(σ, π, π*) and Ru(d) resulted in an overall electron transfer to the adsorbed N₂ from the Ru atoms in the B5-like site with no direct involvement of the MgO(111) substrate. Analyses of bond stretching vibrations and bond lengths show that the N₂(σ, π, π*) and Ru(d) interactions lead to a weaker N-N bond and stronger Ru-N bonds. These correspond to a lower barrier of N₂ dissociation on the Ru₈(B5-like)/MgO(111) model, where the highest red-shift of N-N vibration and the longest N-N bond length were observed after side-on N₂* adsorption. These results demonstrate that an electron-deficient Ru catalyst are not always inhibited from donating electrons to adsorbed N₂. Rather, this study shows that the electron deficiency of Ru can promote π* back-donation and N₂ activation. These new insights may therefore open new avenues to design supported Ru catalysts for nitrogen activation.