No abstract

The widespread use of fuel cells is currently limited by the lack of efficient and cost-effective catalysts for the oxygen reduction reaction. Iron-based non-precious metal catalysts exhibit promising activity and stability, as an alternative to state-of-the-art platinum catalysts. However, the identity of the active species in non-precious metal catalysts remains elusive, impeding the development of new catalysts. Here we demonstrate the reversible deactivation and reactivation of an iron-based non-precious metal oxygen reduction catalyst achieved using high-temperature gas-phase chlorine and hydrogen treatments. In addition, we observe a decrease in catalyst heterogeneity following treatment with chlorine and hydrogen, using Mössbauer and X-ray absorption spectroscopy. Our study reveals that protected sites adjacent to iron nanoparticles are responsible for the observed activity and stability of the catalyst. These findings may allow for the design and synthesis of enhanced non-precious metal oxygen reduction catalysts with a higher density of active sites.

We report Mössbauer and EPR measurements on horseradish peroxidase in the native state and the reaction intermediates with peroxide and chlorite. A detailed analysis of the electronic state of the heme iron is given, and comparisons are drawn with related systems. The native enzyme is high-spin ferric and thus has three Kramers doublets. The unusual magnetic properties of the ground doublet and the large energy of the second, (E2-E1)/k approximately equal to 41 K, and third doublet, (E3-E1)/k greater than or equal to 170 K, can be modeled with a quartet admixture of approximately 11% to the spin sextet. All evidence suggests a ferryl, OFeIV, state of the heme iron in compounds I and II and related complexes. The small isomer shift, delta Fe approximately equal to 0.06 mm/s, the (positive) quadrupole splitting, delta EQ approximately equal to 1.4 mm/s, the spin S = 1, and the large positive zero field splitting, D/k approximately equal to 35 K, are all characteristic of the ferryl state. In the green compound I the iron weakly couples to a porphyrin radical with spin S' = 1/2. A phenomenological model with a weak exchange interaction S . J . S', magnitude of less than or equal to 0.1 D, reproduces all Mössbauer and EPR data of compound I, but the structural origin of the exchange and its apparent distribution require further study. Reaction of horseradish peroxidase with chlorite leads to compound X with delta Fe = 0.07 mm/s and delta EQ = 1.53 mm/s, values that are closest to those of compound II. The diamagnetism of compound III and its Mössbauer parameters delta Fe = 0.23 mm/s and delta EQ = -2.31 mm/s at 4.2 K clearly identify it as an oxyheme adduct.

We report structural and spectroscopic data for a series of six-coordinate (nitrosyl)iron(II) porphyrinates. The structures of three tetraphenylporphyrin complexes [Fe(TPP)(NO)(L)], where L = 4-(dimethylamino)pyridine, 1-methylimidazole, 4-methylpiperidine, are reported here to a high degree of precision and allow observation of several previously unobserved structural features. The tight range of bonding parameters for the [FeNO] moiety for these three complexes suggests a canonical representation for six-coordinate systems (Fe-N(p) = 2.007 A, Fe-N(NO) = 1.753 A, angle FeNO = 138.5 degrees ). Comparison of these data with those obtained previously for five-coordinate systems allows the precise determination of the structural effects of binding a sixth ligand. These include lengthening of the Fe-N(NO) bond and a decrease in the Fe-N-O angle. Several other aspects of the geometry of these systems are also discussed, including the first examples of off-axis tilting of a nitrosyl ligand in a six-coordinate [FeNO](7) heme system. We also report the first examples of Mössbauer studies for these complexes. Measurements have been made in several applied magnetic fields as well as in zero field. The spectra differ from those of their five-coordinate analogues. To obtain reasonable fits to applied magnetic field data, rotation of the electrical field gradient is required, consistent with differing g-tensor orientations in the five- vs six-coordinate species.