Introduction
Mössbauer spectroscopy is of central importance for studying the
electronic structures of iron in diverse environments: from mononuclear
coordination complexes in frozen solution, to inorganic cofactors in
enzymes, to iron sites in bulk materials and catalytically active sites
on surfaces or amorphous materials.1-9 The
experimental study of iron in such systems is often complemented with a
computational analysis, chiefly via density functional theory (DFT). As
a result, the oxidation and spin states of the iron ion, its
coordination number and the composition and symmetry of its immediate
environment can be pinpointed. To translate the parameters predicted by
DFT into those derived experimentally and assess the reliability of the
DFT prediction, calibration studies for various combinations of density
functionals, basis sets, grid sizes, relativistic options and solvation
and dispersion corrections exist alongside very early work based on
Hartree–Fock theory.10-22,23 Given this wealth of
information, the reasons for presenting another calibration study may
not be obvious. Our motivation to calibrate computational Mössbauer
spectroscopy is threefold: (i) several technical advances have been made
that are not included in previous calibration studies, e.g. newer basis
sets, approximations and corrections; (ii) some of the more recent
studies do not report on the quadrupole
splitting;19,24 (iii) the emergence of single-atom
catalysts, where the catalytically active iron site is embedded in an
ill-defined carbonaceous environment with significant
π-character,8 presents a challenging problem for
computational Mössbauer spectroscopy and hence warrants a dedicated
assessment of its predictive power for this specific coordination
environment.
Single-atom catalysts (SACs) are materials synthesized from metal,
nitrogen and carbon precursors with at least one pyrolysis step.
Typically, the resulting MeNC catalysts are amorphous, carbonaceous
materials with multiple phases and contain only small amounts of metal
(< 5 wt%) in various forms. The synthesis is thought to
result in MeNC active sites that contain individual metal ions
coordinated by nitrogen.25-32 The catalytic
transformations these SACs achieve include CO2conversion,33-35 selective C-H
oxidation,36 hydrogen evolution and water oxidation as
half-reactions of the water splitting reaction,37 and
oxygen reduction.38-41 A specific subclass are
so-called FeNC catalysts, where an iron center is most likely
coordinated by several nitrogen donor atoms embedded in a graphene-like
matrix.8,42-44 FeNC catalysts show high activity in
the oxygen reduction reaction (ORR), a key reaction for fuel cells that
are of central importance for green mobility applications. In fact, the
ORR activity of recent FeNC catalysts is on par with that of
low-platinum content electrodes, the current state of the
art.45 However, the stability and hence the long-term
usability of FeNC materials is still lacking.46-48 An
important step towards a better understanding of this promising
substitution material for platinum electrodes is to clarify the
structure and electronic properties of the active
site.39,43,49,50
The current consensus in the literature is that the catalytically active
iron ion is surrounded by two to four nitrogen donor atoms embedded in a
graphene sheet that may have structural or electronic defects and
possibly an additional axial ligand.39,49,51,52 The
sketch shown in Figure 1A attempts to summarize the types of environment
discussed in the literature. It can be seen that several aspects are
unclear: the electronic character of the N-donors, the presence and
nature of axial ligands, the type and abundance of defects close to the
active site, and the position of the active site within a sheet, at the
edge or positioned between two sheets.39,44,53-58While other spectroscopies fall short due to the amorphous nature of the
SAC material, Mössbauer spectroscopy is ideally equipped to study the
coordination environments and electronic structures of iron sites within
the amorphous material. However, the definitive assignment of structural
characteristics proves difficult purely by comparison with reference
data from model complexes. Therefore, a broad variety of active site
candidates derived from density functional theory are needed to develop
a deeper understanding of the influence that specific structural and
electronic aspects of the active site may
have.43,44,58-60 The long-term goal is to decipher the
composition of the active site in its resting state and the changes it
undergoes during catalysis in a joint effort of experimental and
computational Mössbauer spectroscopy.