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We have examined the electrocatalytic activity of iron phthalocyanine (FePc) and perchlorinated iron phthalocyanine 16(Cl)FePc for the oxygen reduction reaction (ORR) in alkaline medium with the two molecules either adsorbed on the external surface of single-wall carbon nanotubes (SWCNT) or covalently anchored via an axial pyridine ligand on pyridine-functionalized single-wall carbon nanotubes (py-SWCNT). Regardless of the particular phthalocyanine type, the ORR activity is higher when the substrate is py-SWCNT rather than SWCNT. The Tafel slopes for ORR are very similar for the two Fe macrocyclic complexes attached to SWCNTs in the two different configurations, suggesting a common rate-determining step for the ORR for all four catalysts. It is also observed that, for both the SWCNT and py-SWCNT supports, the ORR activity is higher for 16(Cl)FePc than for FePc. This is attributed to the electron-withdrawing effect of the peripheral and non-peripheral chlorine atoms in the macrocyclic ligand. While FeN4 macrocycles are known to be located on the strong binding side of a volcano correlation including several MN4 species, the chlorine substituents decrease the binding energy of O2 on the central Fe cation, thereby moving up the macrocycle catalyst towards the apex of the volcano correlation. Both the carbon surface and macrocyclic ligand effects were optimized with 16(Cl)FePc attached to py-SWCNT, which is more active than both FePc on SWCNT and FePc on py-SWCNT. Here we show that both the axial ligand and the electron withdrawing groups (-Cl) have a combined collaborative effect in increasing the catalytic activity for ORR of Fe-phthalocyanines confined on the external walls of single-wall carbon nanotubes.

Pyrolysis is indispensable for synthesizing highly active Fe-N-C catalysts for the oxygen reduction reaction (ORR) in acid, but how Fe, N, and C precursors transform to ORR-active sites during pyrolysis remains unclear. This knowledge gap obscures the connections between the input precursors and the output products, clouding the pathway toward Fe-N-C catalyst improvement. Herein, we unravel the evolution pathway of precursors to ORR-active catalyst comprised exclusively of single-atom Fe1(II)-N4 sites via in-temperature X-ray absorption spectroscopy. The Fe precursor transforms to Fe oxides below 300 °C and then to tetrahedral Fe1(II)-O4 via a crystal-to-melt-like transformation below 600 °C. The Fe1(II)-O4 releases a single Fe atom that diffuses into the N-doped carbon defect forming Fe1(II)-N4 above 600 °C. This vapor-phase single Fe atom transport mechanism is verified by synthesizing Fe1(II)-N4 sites via "noncontact pyrolysis" wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.

The development of catalysts for electrochemical reduction of carbon dioxide (eCO2RR) with high activity and selectivity remains a grand challenge to render the technology useable. As promising candidates, metal−nitrogen−carbon (MNC) catalysts with metal atoms present as atomically dispersed metal−Nx moieties (MNx, M = Mn, Fe, Co, Ni, and Cu) were investigated as model catalysts. The distinct activity for CO formation observed along the series of catalysts is attributed to the nature of the transition metal in MNx moieties because of otherwise similar composition, structure, and morphology of the carbon matrix. We identify a volcano trend between their activity toward CO formation and the nature of the transition metal in MNx sites, with Fe and/or Co at the top of the volcano, depending on the electrochemical potential. Regarding selectivity, FeNC, NiNC, and MnNC had Faradaic efficiency for CO >80%. To correctly model the active sites in operando conditions, experimental operando X-ray absorption near edge structure spectroscopy was performed to follow changes in the metal oxidation state with electrochemical potential. Co and Mn did not change the oxidation state with potential, whereas Fe and Ni were partially reduced and Cu largely reduced to Cu(0). Computational models then led to the identification of M2+N4−H2O as the most active centers in FeNC and CoNC, whereas Ni1+N4 was predicted as the most active one in NiNC at the considered potentials of −0.5 and −0.6 V versus the reversible hydrogen electrode. The experimental activity and selectivity could be rationalized from our density functional theory results, identifying in particular the difference between the binding energies for CO2*− and H* as a descriptor of selectivity toward CO. This indepth understanding of the activity and selectivity based on the speciation of the metals for eCO2RR over atomically dispersed MNx sites provides guidelines for the rational design of MNC catalysts toward eCO2RR for their application in highperformance devices.

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., Nitrogen-doped carbon materials with atomically dispersed iron or cobalt are promising for catalytic use. Here, the authors show that cobalt moieties have a higher redox potential, bind oxygen more weakly and are less active toward oxygen reduction than their iron counterpart, despite similar coordination.

The structure and dynamics of water in ionic solutions at high pressure have been investigated using a combined approach based on extended X-ray absorption fine structure (EXAFS) spectroscopy and Molecular Dynamics (MD) simulations. Modification of the hydration properties of the Zn(2+) ion induced by a pressure increase from ambient condition up to ∼6.4 GPa has been revealed and accurately analyzed. With increasing pressure the first hydration shell of the Zn(2+) ion has been found to retain an octahedral symmetry with a shortening of the Zn-O distance up to 0.09 Å and an increased width associated with thermal motion, as compared to the ambient condition hydration complex. A very interesting picture of the dynamic behavior of the first hydration shell has emerged from the analysis of the simulations: up to 2.5 GPa no exchange events between first and second shell water molecules occurred, while above this pressure value several exchange events take place in the solution following an associative interchange mechanism. This result can be explained by the very high compression and packing of the solvent which force second shell water molecules to enter the Zn(2+) first hydration shell. MD simulations indicate a strong pressure effect also on the structure of the second coordination shell which is compressed and becomes more disordered and less structured with increasing pressure. The water mobility and the ion diffusion coefficient have been found to increase in the high density conditions, as a consequence of the rupture of the hydrogen bond network caused by pressure.

The combination of multiwalled carbon nanotubes (MWCNTs) with undoped CeO2 nanoparticles (NPs) is effective for the direct electrocatalytic reduction of CO2 to formic acid (FA) at acidic pH (0.1 M HNO3), at overpotential as low as eta = -0.02 V (vs RHE) with Faradic efficiency (FE) up to 65%. Ex situ and operando evidence identifies nonstoichiometric Ce4+/3+O2-x reduced sites as essential for the selective CO2 reduction reaction (CO2RR). The MWCNT-mediated electrochemical reduction of the CeO2 NPs offers a definite advantage with respect to the generally adopted thermochemical cycles (800-1500 degrees C) or deep hydrogenation pretreatments, thus presenting an interesting perspective for the engineering of CeO2 electrocatalysts.

Prion diseases are a class of fatal neurodegenerative disorders characterized by brain spongiosis, synaptic degeneration, microglia and astrocytes activation, neuronal loss and altered redox control. These maladies can be sporadic, iatrogenic and genetic. The etiological agent is the prion, a misfolded form of the cellular prion protein, PrP(C). PrP(C) interacts with metal ions, in particular copper and zinc, through the octarepeat and non-octarepeat binding sites. The physiological implication of this interaction is still unclear, as is the role of metals in the conversion. Since prion diseases present metal dyshomeostasis and increased oxidative stress, we described the copper-binding site located in the human C-terminal domain of PrP-HuPrP(90-231), both in the wild-type protein and in the protein carrying the pathological mutation Q212P. We used the synchrotron-based X-ray absorption fine structure technique to study the Cu(II) and Cu(I) coordination geometries in the mutant, and we compared them with those obtained using the wild-type protein. By analyzing the extended X-ray absorption fine structure and the X-ray absorption near-edge structure, we highlighted changes in copper coordination induced by the point mutation Q212P in both oxidation states. While in the wild-type protein the copper-binding site has the same structure for both Cu(II) and Cu(I), in the mutant the coordination site changes drastically from the oxidized to the reduced form of the copper ion. Copper-binding sites in the mutant resemble those obtained using peptides, confirming the loss of short- and long-range interactions. These changes probably cause alterations in copper homeostasis and, consequently, in redox control.