While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe-N-C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe-N-C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe-N-C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe-N-C materials. These new insights open the path to bottom-up synthesis approaches and studies on site-support interactions.

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Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co–N–C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoN4C12, CoN3C10,porp and CoN2C5. The O2 electro-reduction and operando X-ray absorption response are measured in acidic medium on Co–N–C and compared to those of a Fe–N–C catalyst prepared similarly. We show that cobalt moieties are unmodified from 0.0 to 1.0 V versus a reversible hydrogen electrode, while Fe-based moieties experience structural and electronic-state changes. On the basis of density functional theory analysis and established relationships between redox potential and O2-adsorption strength, we conclude that cobalt-based moieties bind O2 too weakly for efficient O2 reduction.

This contribution reports the discovery and analysis of the first PGM-free, p-block Sn-based single metal and nitrogen-doped carbon (MNC) catalysts for the electroreduction of molecular oxygen (ORR) in acidic conditions at fuel cell cathodes. The prepared SnNC catalysts meet and exceed state of art FeNC catalysts in terms of intrinsic catalytic turn over frequency (TOF) and hydrogenair fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Added benefits include a high favorable selectivity for the 4-electron reduction pathway and a Fenton-inactive character of Sn.A range of analytical techniques, combined with DFT calculations indicate that stannic Sn(IV)-Nx single metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytic active moieties. The superior PEMFC performance of SnNC cathode catalysts under realistic, hydrogen-air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising replacement of today's state-of-art Fe-based catalysts. 4Growing concerns over fossil energy and the environment are incentives to develop new energy technologies. Low-temperature hydrogen/air proton-exchange membrane fuel cell (PEMFC) is one such technology, converting hydrogen into electrical energy 1, 2 . For catalyzing the oxygen reduction reaction (ORR) and hydrogen oxidation reaction at the electrodes, PEMFCs rely however on precious, in particular platinum-based catalysts 3, 4 , a scarce and expensive metal.Research to replace precious group metals (PGMs) has led to a class of bio-inspired catalysts, labelled MNC, that involve non-precious 3d transition metal cations stabilized by nitrogen atoms (Metal-Nx moieties), themselves incorporated in conductive carbon matrices. Fe, Co and Mn are hitherto the only three metals that result in ORR-active Metal-Nx moieties in acidic reaction environments 5,6,7,8,9,10 . While the number and utilization of such moieties embedded in carbon are being improved 9 , the fundamental nature of such sites is not so new. Indeed, the large body of experimental research on pyrolyzed FeNC and CoNC materials identifies Metal-N4 motifs as the most active sites for catalyzing ORR in acid 11,12,13 . Such sites are akin to square-planar Metal-N4 sites in Fe or Co macrocycles, identified in 1964 to be ORR active 14 Here, we report the discovery of the first p-block single metal site catalyst, SnNC, exhibiting catalytic ORR reactivities in acidic environments that meet and exceed all state-of-art PGM-free catalyst concepts, while adding important benefits in terms of catalyst stability. The catalytically active single-metal SnNx moieties embedded in the surface of the SnNC catalyst were characterized by high-resolution scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), extended X-ray absorption fine structure (EXAFS), Xray photoelectron spectroscopy (XPS) and 119 Sn Mössbauer spectroscopy, com...

Pyrolyzed Fe-N-C materials are promising platinum-group-metal free catalysts for protonexchange membrane fuel cell cathodes. However, the detailed structure, oxidation and spin states of their active sites is still undetermined. 57 Fe Mössbauer spectroscopy has identified FeN x moieties as the most active sites, with their fingerprint being a doublet in lowtemperature Mössbauer spectra. However, the interpretation of the doublets for such materials has lacked theoretical basis. Here, we applied density functional theory to calculate the quadrupole splitting energy of doublets (E QS) for a range of FeN x structures in different oxidation and spin states. The calculated and experimental values are then compared for a reference Fe-N-C catalyst, while further information on the Fe oxidation and spin states was obtained from electron paramagnetic resonance, superconducting quantum interference device and 57 Fe Mössbauer spectroscopy under external magnetic field. The combined theoretical and experimental results identify the main presence of FeN x moieties in Fe(II) low-spin and Fe(III) high-spin states while a minor fraction of sites could exist in Fe(II) S = 1 state. From the analysis of the 57 Fe Mössbauer spectrum under external magnetic field and the comparison of calculated and measured E QS values, we assign the experimental doublet D1 with mean E QS value around 0.9 mm•s-1 to Fe(III)N 4 C 12 moieties in high spin and the experimental doublet D2 with mean E QS value around 2.3 mm•s-1 to Fe(II)N 4 C 10 moieties in low and medium spin. These conclusions indicate that D1 corresponds to surface-exposed sites while D2 may correspond either to bulk sites that are inaccessible to O 2 or to surface sites that bind O 2 weaker than D1.

Comparison of density functionals for energy and structural differences between the high- † 5 T 2g : "t 2g … 2ϩ . Since very little experimental results are available ͑except for crystal structures involving the cation in its high-spin state͒, the primary comparison is with our own complete active-space self-consistent field ͑CASSCF͒, second-order perturbation theory-corrected complete active-space self-consistent field ͑CASPT2͒, and spectroscopy-oriented configuration interaction ͑SORCI͒ calculations. We find that generalized gradient approximations ͑GGAs͒ and the B3LYP hybrid functional provide geometries in good agreement with experiment and with our CASSCF calculations provided sufficiently extended basis sets are used ͑i.e., polarization functions on the iron and polarization and diffuse functions on the water molecules͒. In contrast, CASPT2 calculations of the low-spin-high-spin energy difference ⌬E LH ϭE LS ϪE HS appear to be significantly overestimated due to basis set limitations in the sense that the energy difference of the atomic asymptotes ( 5 D→ 1 I excitation of Fe 2ϩ ) are overestimated by about 3000 cm Ϫ1 . An empirical shift of the molecular ⌬E LH based upon atomic calculations provides a best estimate of 12 000-13 000 cm Ϫ1 . Our unshifted SORCI result is 13 300 cm Ϫ1, consistent with previous comparisons between SORCI and experimental excitation energies which suggest that no such empirical shift is needed in conjunction with this method. In contrast, after estimation of incomplete basis set effects, GGAs with one exception underestimate this value by 3000-4000 cm Ϫ1 while the B3LYP functional underestimates it by only about 1000 cm Ϫ1 . The exception is the GGA functional RPBE which appears to perform as well as or better than the B3LYP functional for the properties studied here. In order to obtain a best estimate of the molecular ⌬E LH within the context of density functional theory ͑DFT͒ calculations we have also performed atomic excitation energy calculations using the multiplet sum method. These atomic DFT calculations suggest that no empirical correction is needed for the DFT calculations. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1710046͔ I. INTRODUCTIONA well-known feature of d 6 Tanabe-Sugano ligand field theory ͑LFT͒ diagrams for octahedral complexes is the reversal of the ordering of the low-spin 1 A and high-spin 5 T in the spin-crossover region of ligand field strength.1 For compounds in this region, spin crossover may be either thermally or optically induced, 2 leading to possible applications in storage and display devices. [3][4][5] We are particularly interested in the phenomenon of light-induced excited spin-state trapping ͑LIESST͒ in octahedral iron II compounds, which involves the optical interconversion of the high-spin ͑HS͒ 5 T 2g and low-spin ͑LS͒ 1 A g electronic states. While this can be understood at a qualitative level using LFT, 1,2 it is also known that the e g orbitals, populated in going from the LS to the HS state, are antibonding, so that b...

International audienceThis article provides a brief overview of the quantum chemical auxiliary density functional theory program deMon2k. A basic introduction into its key computational features is given. By selected examples, it is shown how deMon2k can contribute to the elucidation of problems in chemistry, biology, and materials science such as finite temperature effects, nuclear magnetic resonance studies, structure determinations, heterogeneous, and enzymatic catalysi

The complexation of (1→4) linked α-L-guluronate (G) and β-D-mannuronate (M) disaccharides with Mg(2+), Ca(2+), Sr(2+), Mn(2+), Co(2+), Cu(2+), and Zn(2+) cations have been studied with quantum chemical density functional theory (DFT)-based method. A large number of possible cation-diuronate complexes, with one and two GG or MM disaccharide units and with or without water molecules in the inner coordination shells have been considered. The computed bond distances, cation interaction energies, and molecular orbital composition analysis revealed that the complexation of the transition metal (TM) ions to the disaccharides occurs via the formation of strong coordination-covalent bonds. On the contrary, the alkaline earth cations form ionic bonds with the uronates. The unidentate binding is found to be the most favored one in the TM hydrated and water-free complexes. By removing water molecules, the bidentate chelating binding also occurs, although it is found to be energetically less favored by 1 to 1.5 eV than the unidentate one. A good correlation is obtained between the alginate affinity trend toward TM cations and the interaction energies of the TM cations in all studied complexes, which suggests that the alginate affinities are strongly related to the chemical interaction strength of TM cations-uronate complexes. The trend of the interaction energies of the alkaline earth cations in the ionic complexes is opposite to the alginate affinity order. The binding strength is thus not a limiting factor in the alginate gelation in the presence of alkaline earth cations at variance with the TM cations.