Abstract
Iron-containing zeolites are heterogeneous catalysts that exhibit remarkable activity in the selective oxidation of inert hydrocarbons and catalytic decomposition of nitrous oxide (N2O). The reduction of N2O is critical to both these functions, but experimental data tracking the iron active sites during N2O binding and activation are limited. Here, the N2O-ligated Fe(ii) active site in iron-exchanged zeolite beta is isolated and characterized by variable-temperature Mössbauer, diffuse reflectance UV-vis-NIR and Fourier transform infrared spectroscopy. N2O binds through the terminal nitrogen atom with substantial backbonding from the Fe(ii) centre at low temperature. At higher temperatures, the Fe–N2O interaction is weakened, facilitating isomerization to the O-bound form, which is competent in O-atom transfer. Density functional theory calculations show the geometric and electronic structure requirements for N2O binding and activation. A geometric distortion imposed by the zeolite lattice plays an important role in activating N2O. This highlights a mechanism for structural control over function in Fe-zeolite catalysts.
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Acknowledgements
This investigation has been supported by the Flemish Science Foundation (FWO, grant no. G0A2216N to B.F.S. and R.A.S.) and the National Science Foundation (grant no. CHE-1660611 to E.I.S.). B.E.R.S. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under grant no. DGE-11474 and from the Munger, Pollock, Reynolds, Robinson, Smith & Yoedicke Stanford Graduate Fellowship. M.L.B. acknowledges the Research Foundation–Flanders for funding of his stay at Stanford University (grant no. V417018N). E.I.S. acknowledges support from the Stanford Woods Institute for the Environment. P.C. and V.V.S. acknowledge the Research Board of Ghent University (BOF) and funding from the European Union’s Horizon 2020 research and innovation programme (consolidator ERC grant agreement no. 647755–DYNPOR (2015–2020)). The work by G.W. was supported by the ANR-Total ‘Nanoclean Energy’ chair and Normandy’s RIN program. The computational resources and services for the periodic calculations were provided by Ghent University (Stevin Supercomputer Infrastructure) and the VSC (Flemish Supercomputer Centre), funded by the Research Foundation–Flanders (FWO).
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M.L.B., B.E.R.S., B.F.S., E.I.S. and R.A.S. designed the experiments. M.L.B., B.E.R.S. and H.M.R. performed the experiments. G.F. performed the FTIR experiments. B.E.R.S. performed the cluster DFT calculations. P.C. and V.V.S. performed the periodic DFT calculations. M.L.B., B.E.R.S., H.M.R., P.C., V.V.S., B.F.S. and E.I.S. analysed the data. M.L.B., B.E.R.S. and H.M.R. wrote the manuscript with guidance from R.A.S., E.I.S. and B.F.S.
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Extended data
Extended Data Fig. 1 Comparison of the six possible N2O-bound models.
The models are evaluated by cluster DFT calculations. It is not possible to bind additional N2O ligands to the sterically protected trans axial positions of these sites. In the table, the predicted properties of these models are compared. DFT calculations predict S = 2 ground states for all six models. Based on their predicted quadrupole splittings, the η1-O,O/η1-O,N models are potential structural models of the α-(N2O)2 LT2 species observed at low temperature and high N2O concentrations (see Fig. 1f). Values of ΔHads,298K and ΔEel,ads in this figure are relative to lattice adsorbed N2O.
Extended Data Fig. 2 FTIR spectra of different amounts of N2O loaded onto H-*BEA at 77 K.
The antisymmetric N2O stretch region measured at 77 K through FTIR on H-*BEA in vacuum that is cumulatively loaded with pulses of N2O from the gas phase at 77 K.
Extended Data Fig. 3 FTIR spectra at 77K of Fe-*BEA loaded with 0.9 N2O/Fe and 3.0 N2O/Fe.
a) antisymmetric stretching region, b) symmetric stretching region.
Extended Data Fig. 4 Parameters for different aluminium substitutions in cluster models.
a) η1-N models, b) η1-O models, c) Transition state models. The associated ZPE corrected electronic energies of N2O adsorption and activation included at the right of each table. QTS(N2O) quantifies the extent of charge transfer from the Fe centre into the N2O ligand at the N-O cleavage transition state. Note that the results of the T3/T3′, T5/T5′, and T7/T7′ sites parallel those of the T4/T4′, T6/T6′, and T8/T8′ sites, respectively. A comparison of this is shown in d). Reference level is N2O bound to the zeolite lattice.
Extended Data Fig. 5 B3LYP-D3 optimized models in CP2K.
Data given are Fe-ON2 and O-N2 bond distances, Fe-O-N2 bond angle, electronic energies, and adsorption enthalpies at 298K (with respect to the empty Fe-BEA framework and N2O in gas phase) of the transition state and corresponding reactant and product minima for the N-O cleavage reaction in Fe-BEA, obtained from the geometry optimisations at the B3LYP-D3 level of theory in CP2K. * N2O adsorbed to the lattice without Fe is used as the reference level in the main manuscript, that is −5.4 kcal/mol is subtracted from the ΔEel,B3LYP-D3 values and −4.6 kcal/mol is substracted from the ΔH298K,B3LYP-D3 values.
Extended Data Fig. 6 Large structure optimised cluster models of η1-O bound and η1-N bound.
The same functionals and methods were used as for the smaller cluster models, as described in the methods section. The table shows difference in enthalpy between η1-O bound and η1-N bound using different methods.
Extended Data Fig. 7 Difference UV-Vis spectra upon N2O exposure.
Spectra are relative to the spectrum of Fe(II)-*BEA after He 1,173 K before introduction of a 35 vol.% N2O atmosphere (blue). The orange spectrum is after introduction of the N2O atmosphere, the grey spectrum after subsequent removal of the N2O atmosphere at 308 K.
Extended Data Fig. 8 Effect of Hartree-Fock exchange on the predicted energetics of the N-O cleavage reaction.
ΔE values are given in kcal/mol. The three functionals predict different driving forces for O-atom transfer, but similar intrinsic barriers. The intrinsic barriers are calculated by using a Marcus Theory correction. These calculations were done without dispersion or single point calculations on a higher basis set and cannot be directly compared to the data in Figs 5 and 6.
Extended Data Fig. 9 Unconstrained toy model energies and enthalpies used in figure 6.
Energies are relative to α-Fe(II) + N2 coordinated to lattice. The size of the unconstrained model is a 6MR capped with hydrides. Calculations on the unconstrained model were with dispersion and larger basis set single point calculation as described in the methods.
Extended Data Fig. 10 Small cluster models evaluating the effect of a trans-axial ligand.
a) toy model of the N-bound FeII species, in which the 6-membered ring ligand is capped with hydrogens and allowed to freely geometry optimise, b) geometry optimised structure including an axial amine ligand, and c) structure in which the axial amine ligand is deleted, and the coordinates of the N2O ligand are re-optimised (that is the FeII and 6-membered ring ligand are frozen). For each model, the out-of-plane distortion of the iron atom is quantified by the average distortion of the O-Fe-O angle (δOFeOavg), and the doubly-occupied orbital is indicated.
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Supplementary Information
Supplementary Notes 1–3, methods, Figs. 1–13, Tables 1–5 and references.
Supplementary Data 1
Xyz coordinates.
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Bols, M.L., Snyder, B.E.R., Rhoda, H.M. et al. Coordination and activation of nitrous oxide by iron zeolites. Nat Catal 4, 332–340 (2021). https://doi.org/10.1038/s41929-021-00602-4
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DOI: https://doi.org/10.1038/s41929-021-00602-4
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