QuantumATK Case Study: Unraveling Controversy over an Electrocatalytic Reaction Mechanism

Sang Uck Lee’s lab from Hanyang University, South Korea in collaboration with the Synopsys QuantumATK Team published a study elucidating the longstanding controversial issue of the iodine reduction reaction (IRR) mechanism on a surface [1]. Calculations were performed with the one probe non-equilibrium surface Green’s function methodology (surface NEGF) under an applied external electric field implemented in QuantumATK [2], as depicted in Figure 1. This study is a good inspiration for studying other catalytic reaction mechanisms with the surface NEGF method in the R&D of new catalysts.

The study validated a consecutive mechanism presented in Figure 3, which is initiated by I₂ molecule in a vertical position (I₂V) approaching the Pt(111) surface and an efficient electron transfer from surface to I₂ molecule governed by a negative electric field. The detailed investigation of this mechanism was carried out for the first time as the applied methodology in QuantumATK overcame the limitations of the conventional approach of a slab model based on the free energy diagram (FED) without a proper description of electron transfer and any external electric field environment. Importantly, this study verified the reliability of the surface NEGF methodology by examining how IRR activity depends on the surface structure and metal species, Pt surface having the best IRR activity, in a good agreement with experimental results [3,4].

Surface NEGF methodology in QuantumATK

The one probe non-equilibrium surface Green’s function methodology (surface NEGF) in QuantumATK simulates the physically correct surface structure under an external electric field. Surface is represented as a truly semi-infinite system, which is divided into a finite surface region and a semi-infinite bulk region, as shown in Figure 1. Thus, the surface NEGF method naturally takes into account the charge transfer effects, ensuring that adsorbed species may be charged with charges from the bulk electrode region without changing the chemical potential of the surface. When applying an electric field, the Dirichlet boundary condition is used to shift the electrostatic potential value in the vacuum, without changing the chemical potential of the semi-infinite bulk region. These surface NEGF method features ensure precise studies of electrocatalytic reactions on surfaces.

Consecutive IRR mechanism

The study first investigated if I₂ molecule approaching the Pt surface under electric field prefers a vertical (I₂V) or a parallel (I₂P) orientation. Calculated relative stabilities of I₂V and I₂P configurations under negative and positive electric fields, depicted in Figure 2, revealed the I₂V configurational preference in a reductive environment for IRR dictated by a negative electric field. Then the study validated a consecutive IRR mechanism, which is initiated by I₂V approaching the Pt(111) surface and then I atoms are sequentially reduced and desorbed in the reductive environment dictated by a negative electric field, as shown in Figure 3. Without applying an electric field, the reaction is initiated by dissociative adsorption on the Pt(111) surface, i.e. concerted mechanism, which has been proposed and studied before using the FED and conventional slab approach without applying an electric field [1,3].  

As shown in Figure 3, the strength of the external electric field impacts the degree of the reductive process.  The impact can be well described by investigating the plots shown in Figure 4: interatomic distance of I₂V molecule and adsorption distance of I(1)* on the Pt(111) surface as a function of the electric field. Authors suggest that the slope of the linear relation (0.45 and 0.36 for I₂ on the Pt(111) surface) in these plots can be used as descriptors for the evaluation of IRR activity. Thus, the calculated slope values can be used to compare IRR activity of various catalysts. This approach was employed to showcase, that the Pt(111) surface has a steepest slope and thus higher IRR activity than the Pt(100) surface or (111) surfaces of Ag, Au, Co, Ir, Pd and Rh, in a good agreement with experiments [3,4].

 Relevant resources

• Tutorial on how to perform Surface NEGF simulations
• Paper on the surface NEGF methodology in QuantumATK and these application examples:
  • Simulation of thin film/surface heterostructures
  • Simulation of electronic surface states in external electric fields
  • Investigation of surface chemistry in external electric fields
• Paper on discovering magnetic states on the edges of 1T´ TMDs (news item)

References

[1] C. H. Lee, E. B. Nam, M. E. Lee, S. U. Lee, “Unraveling the controversy over a catalytic reaction mechanism using a new theoretical methodology: One probe and non-equilibrium surface Green’s function”, Nano Energy 63, 103863 (2019).

[2] S. Smidstrup, D. Stradi, J. Wellendorff, P. Khomyakov, U. Vej-Hansen, M. E. Lee, T. Ghosh, E. Jonsson, H. Jonsson and K. Stokbro, "First-principles Green’s-function method for surface calculations: a pseudopotential localized basis set approach", Phys. Rev. B 96, 195309  (2017), arXiv:1707.02141 [cond-mat.mtrl-sci]

[3] B. Zhang, D. Wang, Y. Hou, S. Yang, X.H. Yang, J.H. Zhong, J. Liu, H.F. Wang, P. Hu, H.J. Zhao, H.G. Yang, “Facet-dependent catalytic activity of platinum nanocrystals for triiodide reduction in dye-sensitized solar cells”, Sci. Rep. 3, 1836 (2013).

[4] J. H. Wu, Z. Lan, J. M. Lin, M. L. Huang, Y. F. Huang, L. Q. Fan, G. G. Luo, Y. Lin, Y. M. Xie, Y. L. Wei, “Counter electrodes in dye-sensitized solar cells”, Chem. Soc. Rev. 46, 5975 (2017).