Can Density Functional Theory Predict Mössbauer Spectra in Pyrolyzed Fe-N-C Catalysts?

Abstract

57Fe Mössbauer spectroscopy, based on the recoil-free emission and resonant absorption of γ-rays, allows an easier separation of the signals arising from the different Fe environments. This is in the basis of the growing interest of using 57Mössbauer spectroscopy for identification of Fe-based species present in pyrolyzed Fe-N-C catalysts. Because of their high temperature synthesis, these catalysts often comprise different coordinations of atomically-dispersed Fe ions covalently bound to the nitrogen-doped carbon matrix (FeNxCy moieties), but also metallic iron and iron carbides with long-range order.1-3 In the 57Fe Mössbauer spectra (recorded in the absence of a strong external magnetic field) metallic iron and iron carbides result in sextets (except for the paramagnetic γ-Fe structure, leading to singlet spectral component) while the fingerprint of FeNxCy moieties is a doublet spectral component. Precise interpretation of the doublets (often labeled as D1 and D2) allowing to distinguish between iron coordination geometries, spin- and oxidation state remains however challenging and has so far lacked strong theoretical or experimental reference basis for their unambiguous assignments. D1 doublet has an isomer value ranging from 0.33 to 0.4 mm/s and quadrupole splitting (QS) ranging from 0.7 to 1.2 mm/s, which is usually attributed to Fe-N4 defects with Fe2+ in its low spin state. The assignment of the D2 doublet with isomer shifts ranging from 0.36 to 0.52 mm/s and with quadrupole splitting ranging from 2.4 to 2.7 mm/s is more ambiguous. It is attributed to Fe-N4 defects similar to iron phthalocyanine with Fe2+ in its intermediate spin state or to a Fe-N2+2 defect in which Fe binds to four pyridinic nitrogen atoms from two adjacent nitrogen-doped graphene layers. However, these assignments have been based on experimental records of carbon-supported FeN4 macrocycles, which differ greatly from the case in which the Fe-Nx moieties are integrated into the graphene matrix. It appears, therefore, obvious that theoretical Mössbauer spectra should be highly beneficial to rationally assign the D1 and D2 spectra of Fe-N-C materials. Theoretical studies (mostly obtained with density-functional-theory, DFT, based approaches) of 57Fe Mössbauer spectra, have evidenced that the choice of the exchange-correlation functional and atomic basis sets has a strong impact on the accuracy of QS values, in particular for intermediate and high spin iron complexes. It is therefore necessary to examine for every new class of Fe-containing compounds the variation of QS with the DFT functional and other computational details. Theoretical 57Fe Mössbauer spectra in pyrolyzed Fe-N-C catalysts have not yet been made available, which prompted us toward the present DFT study of the quadrupole splitting energies in Fe-N4 defects that are identified as the active sites in the pyrolyzed Fe-N-C materials. Extensive computations of QS in Fe-N4 models with Fe in various coordinations and with different oxidation/spin states have been carried out. The use of several DFT exchange-correlation functionals and basis sets in cluster and periodic approaches are compared and will be reported. The analysis of the DFT calculated values in comparison with experimental data are used to interpret quadrupole doublets ubiquitous in the Mössbauer spectra of pyrolyzed Fe-N-C catalysts. Acknowledgement: This work is supported by the LabEx CheMISyst ANR-10-LABX-05-01.