Abstract
Single atom catalysts (SACs) are emergent catalytic materials that have the promise of merging the scalability of heterogeneous catalysts with the high activity and atom economy of homogeneous catalysts. Computational, first-principles modeling can provide essential insight into SAC mechanism and active site configuration, where the sub-nm-scale environment can challenge even the highest-resolution experimental spectroscopic techniques. Nevertheless, the very properties that make SACs attractive in catalysis, such as localized d electrons of the isolated transition metal center, make them challenging to study with conventional computational modeling using density functional theory (DFT). For example, Fe/N-doped graphitic SACs have exhibited spin-state dependent reactivity that remains poorly understood. However, spin-state ordering in DFT is very sensitive to the nature of the functional approximation chosen. In this work, we develop accurate benchmarks from correlated wavefunction theory (WFT) for relevant octahedral complexes. We use those benchmarks to evaluate optimal DFT functional choice for predicting spin state ordering in small octahedral complexes as well as models of pyridinic and pyrrolic nitrogen environments expected in larger SACs. Using these guidelines, we determine Fe/N-doped graphene SAC model properties and reactivity as well as their sensitivities to DFT functional choice. Finally, we conclude with broad recommendations for computational modeling of open-shell transition metal single-atom catalysts.
Highlights
Single atom catalysts (SACs) (Yang et al, 2013) are emergent catalytic materials (Yang et al, 2013; Liang et al, 2015, 2017) that have the promise of merging the scalability of heterogeneous catalysts with the high activity and atom economy of homogeneous catalysts, but the reactivity of single atom catalyst (SAC) is poorly understood (Figure 1)
In previous work (Gani and Kulik, 2016), we found tuning range-separation parameters in range-corrected hybrids to have a comparable effect on density and energetics of transition metal complexes to global exchange tuning, and we focus on only global exchange tuning in this work
We have presented an overview of the effect of computational model choice on the properties of octahedral transition metal complexes and emergent single atom catalyst (SAC) materials made from Fe centers in N-doped graphene
Summary
Single atom catalysts (SACs) (Yang et al, 2013) are emergent catalytic materials (Yang et al, 2013; Liang et al, 2015, 2017) that have the promise of merging the scalability of heterogeneous catalysts with the high activity and atom economy of homogeneous catalysts, but the reactivity of SACs is poorly understood (Figure 1). SAC active sites that are fundamentally sub-nm-scale challenge the resolution of spectroscopic techniques (Fei et al, 2015; Wang and Zhang, 2016), making first-principles modeling essential to mechanistic study For these emerging catalysts, changing synthesis (Liu et al, 2017) or reaction (Li et al, 2016; Zitolo et al, 2017) conditions changes the distribution of SAC coordination geometries, and the most reactive species for key reactions (e.g., ORR or selective partial hydrocarbon oxidation) remain under debate (Zitolo et al, 2015; Zhu et al, 2017; Yang et al, 2018). In SAC electrocatalysts (Fei et al, 2015; Qiu et al, 2015; Zitolo et al, 2015, 2017; Back et al, 2017; Chen et al, 2017; Cheng et al, 2017; Zhang et al, 2017a,b; Zhu et al, 2017; Gao et al, 2018; Jiang et al, 2018; Wang et al, 2018; Zhang et al, 2018), changes in applied potential (e.g., in ORR) have been suggested to change the Fe SAC active site, possibly through a change in spin state (Zitolo et al, 2017)
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