Modeling Single‐Atom Catalysis

Abstract

Electronic structure calculations represent an essential complement of experiments to characterize single‐atom catalysts (SACs), consisting of isolated metal atoms stabilized on a support, but also to predict new catalysts. However, simulating SACs with quantum chemistry approaches is not as simple as often assumed. In this work, the essential factors that characterize a reliable simulation of SACs activity are examined. The Perspective focuses on the importance of precise atomistic characterization of the active site, since even small changes in the metal atom's surroundings can result in large changes in reactivity. The dynamical behavior and stability of SACs under working conditions, as well as the importance of adopting appropriate methods to solve the Schrödinger equation for a quantitative evaluation of reaction energies are addressed. The Perspective also focuses on the relevance of the model adopted. For electrocatalysis this must include the effects of the solvent, the presence of electrolytes, the pH, and the external potential. Finally, it is discussed how the similarities between SACs and coordination compounds may result in reaction intermediates that usually are not observed on metal electrodes. When these aspects are not adequately considered, the predictive power of electronic structure calculations is quite limited.

Exploring the Inner- and Outer-Sphere Mechanistic Pathways of ORR on M–N–Cs with Pyrrolic MN4 Motifs

“…11−13 Due to the highly complex structures of these amorphous carbon materials, density functional theory (DFT) is a crucial tool for extracting atomic-level insights on the single atom sites. 14 While the specific coordination can vary across different materials, there is a general consensus that the natureinspired MN 4 sites are responsible for the observed electrochemical activities. 7 For the modeling of the MN 4 sites, most studies assume the pyridinic MN 4 coordination embedded in a graphene-like lattice.…”

“…32 Unfortunately, as pointed out by Di Liberto et al, this nontrivial source of error is frequently ignored or mishandled, possibly leading to exaggerated results with regard to innersphere reactivity. 14,28 For periodic calculations where hybrid functionals are less practical due to computational costs, the Hubbard correction (DFT + U) could be invoked to mitigate the self-interaction error through calibration with an intuitively "similar" reference material, but it can still cause issues pertaining the transferability of Hubbard parameters and reproducibility across systems. 28 In the same publication, it was also reported that even with a reliable GGA + U that matched the results from hybrid functionals, the reactivity on periodic systems can be artifacts of suboptimal lattice parameters, which are not typically optimized during the reaction studies.…”

“…Recently, metal- and nitrogen-doped carbon materials (M–N–Cs) have been widely studied as potential alternatives to PGMs for ORR electrocatalysis, especially with Fe–N–Cs and Co–N–Cs exhibiting ORR currents that are comparable to those of PGMs in alkaline medium. − Less commonly investigated, yet significant to this study, Cr–N–C and Mn–N–Cs have also been shown to offer promising ORR activity. − Due to the highly complex structures of these amorphous carbon materials, density functional theory (DFT) is a crucial tool for extracting atomic-level insights on the single atom sites . While the specific coordination can vary across different materials, there is a general consensus that the nature-inspired MN 4 sites are responsible for the observed electrochemical activities .…”

“…Conventionally, periodic boundary conditions are still imposed for the modeling of pyrrolic MN 4 motifs, leading to structures with hole defects relatively close to the active site, high dopant concentrations, or structures with indications of ring strain. ,− Furthermore, imposing the periodic boundary conditions often entails several technical challenges, namely, the high computational cost of hybrid functionals and the influence of lattice strain on the reactivity. , For transition metal complexes, GGA functionals can produce computational artifacts arising from the self-interaction error and the associated delocalization error. − In addition, Kirchhoff et al showed that the self-interaction error of GGA and meta-GGA functionals can manifest even on nitrogen-doped graphene without transition metals, further emphasizing the importance of hybrid functionals for carbon-based catalysis . Unfortunately, as pointed out by Di Liberto et al, this nontrivial source of error is frequently ignored or mishandled, possibly leading to exaggerated results with regard to inner-sphere reactivity. , For periodic calculations where hybrid functionals are less practical due to computational costs, the Hubbard correction (DFT + U ) could be invoked to mitigate the self-interaction error through calibration with an intuitively “similar” reference material, but it can still cause issues pertaining the transferability of Hubbard parameters and reproducibility across systems . In the same publication, it was also reported that even with a reliable GGA + U that matched the results from hybrid functionals, the reactivity on periodic systems can be artifacts of suboptimal lattice parameters, which are not typically optimized during the reaction studies.…”

Predicting the Stability of Single-Atom Catalysts in Electrochemical Reactions

“…Catalysis with single metal atoms is a potentially relevant new frontier with an expected impact on the incoming energy and environmental transition. − A single-atom catalyst (SAC) is made by a transition metal (TM) atom embedded in a solid matrix. − SACs bridge the two worlds of homogeneous and heterogeneous catalysts. − The former are often more active but the latter are more stable and for this reason are involved in the vast majority of technological applications of catalysis . Indeed, an SAC is a heterogeneous catalyst with atomic dispersion and a well-defined local environment.…”