Table 1.
Mössbauer parameters and assignment of the six components featured in the deconvoluted Mössbauer spectra of Figure 1 and Figure 5. The second column features the color code of each Fespecies in these spectra.
| Component | δiso / mm●s−1 | ΔEQ / mm●s−1 | fwhm / mm● s−1 | Assignment* | References | |
|---|---|---|---|---|---|---|
| Doublet Dl | 0.39 (± 0.04) | 1.16 (± 0.06) | 0.63 (± 0.07) | FeIIN4/C, low spin | 36, 37, 41–43, 46 | |
| Doublet D2 | 0.36 (± 0.13) | 2.40 (± 0.28) | 0.61 (± 0.04) | FeIIN2+2/C, intermediate spin | 36, 37, 46, 48 | |
| Doublet D3 | 1.01 (± 0.13) | 2.32 (± 0.17) | 0.63 (± 0.07) | N-FeIIN2+2…Nprot/C, high spin** | 49–52, this work | |
| Doublet D4 | 0.43 (± 0.03) | 0.30 (± 0.03) | 0.28 (fixed) | FexN with x≤ 2.1 | 53,55 | |
| Doublet D5 | 0.29 (± 0.04) | 0.31 (± 0.02) | 0.28 (fixed) | |||
| Singlet | 0.36 (fixed) | - | 3.3 (± 0.2) | disordered FexNy | this work | |
For ferrous iron (FeII, 3d6) the low, intermediate and high spin states correspond to zero, two and four unpaired spins, as schematized in Figures 2a–2c.
According to Ref.28, this site is characterized by two different ORR-activities: (i) a low-activity one in which the basic N-group ('…Nprot') remains unprotonated (N-FeN2+2…Nprot/C), or has uptaken a proton and has already been neutralized by an anion, [N-FeN2+2…NH+/C] HSO4−; or (ii) a highly active, composite site in which the N-group is protonated and not neutralized yet, N-FeN2+2…NH+/C.