Gauging Iron-Sulfur Cubane Reactivity from Covalency: Trends with Oxidation State.

We investigated room-temperature metal and ligand K-edge X-ray absorption (XAS) spectra of a complete redox series of cubane-type iron-sulfur clusters. The Fe K-edge position provides a qualitative but convenient alternative to the traditional spectroscopic descriptors used to identify oxidation states in these systems, which we demonstrate by providing a calibration curve based on two analytic methods. Furthermore, high energy resolution fluorescence detected XAS (HERFD-XAS) at the S K-edge was used to measure Fe-S bond covalencies and record their variation with the average valence of the Fe atoms. While the Fe-S(thiolate) covalency evolves linearly, gaining 11 ± 0.4% per bond and hole, the Fe-S(µ3) covalency evolves asystematically, reflecting changes in the magnetic exchange mechanism. A strong discontinuity manifested for superoxidation to the all-ferric state, distinguishing its electronic structure and its potential (bio)chemical role from those of its redox congeners. We highlight the functional implications of these trends for the reactivity of iron-sulfur cubanes.

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst.

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3 RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo2 CTx : Fe (Tx are oxo, hydroxy and fluoro surface termination groups). Mo2 CTx : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2 CTx ) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 µmol h-1 mg-1 , respectively, for the reduction of NO3 - to NH4 + in acidic media and 70 % and 12.9 µmol h-1 mg-1 in neutral media. Regardless of the media, Mo2 CTx : Fe outperforms monometallic Mo2 CTx owing to a more facile reductive defunctionalization of Tx groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a Tx vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3 RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.

Mixed-metal Ionothermal Synthesis of Metallophthalocyanine Covalent Organic Frameworks for CO2 Capture and Conversion.

Phthalocyanines (PCs) are intriguing building blocks owing to their stability, physicochemical and catalytic properties. Although PC-based polymers have been reported before, many suffer from relatively low stability, crystallinity, and low surface areas. Utilizing a mixed-metal salt ionothermal approach, we report the synthesis of a series of metallophthalocyanine-based covalent organic frameworks (COFs) starting from 1,2,4,5-tetracyanobenzene and 2,3,6,7-tetracyanoanthracene to form the corresponding COFs named M-pPPCs and M-anPPCs, respectively. The obtained COFs followed the Irving-Williams series in their metal contents, surface areas, and pore volume and featured excellent CO2 uptake capacities up to 7.6 mmol g-1 at 273 K, 1.1 bar. We also investigated the growth of the Co-pPPC and Co-anPPC on a highly conductive carbon nanofiber and demonstrated their high catalytic activity in the electrochemical CO2 reduction, which showed Faradaic efficiencies towards CO up to 74 % at -0.64 V vs. RHE.

Co-Electrospun Poly(ε-Caprolactone)/Zein Articular Cartilage Scaffolds.

Osteoarthritis scaffold-based grafts fail because of poor integration with the surrounding soft tissue and inadequate tribological properties. To circumvent this, we propose electrospun poly(ε-caprolactone)/zein-based scaffolds owing to their biomimetic capabilities. The scaffold surfaces were characterized using Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, static water contact angles, and profilometry. Scaffold biocompatibility properties were assessed by measuring protein adsorption (Bicinchoninic Acid Assay), cell spreading (stained F-actin), and metabolic activity (PrestoBlue™ Cell Viability Reagent) of primary bovine chondrocytes. The data show that zein surface segregation in the membranes not only completely changed the hydrophobic behavior of the materials, but also increased the cell yield and metabolic activity on the scaffolds. The surface segregation is verified by the infrared peak at 1658 cm-1, along with the presence and increase in N1 content in the survey XPS. This observation could explain the decrease in the water contact angles from 125° to approximately 60° in zein-comprised materials and the decrease in the protein adsorption of both bovine serum albumin and synovial fluid by half. Surface nano roughness in the PCL/zein samples additionally benefited the radial spreading of bovine chondrocytes. This study showed that co-electrospun PCL/zein scaffolds have promising surface and biocompatibility properties for use in articular-tissue-engineering applications.

Probing the Role of Environmental and Sample Characteristics in Gap Mode Tip-Enhanced Raman Spectroscopy.

Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful analytical tool for nondestructive and label-free molecular characterization at the nanoscale. However, the influence of environmental factors and sample characteristics on the occurrence of spurious signals, enhancement of TERS signals, and longevity of TERS probes is not well understood yet. Herein, we present a detailed investigation of the influence of oxygen, humidity, and atmospheric carbon contaminants on scanning tunneling microscopy-TERS (STM-TERS) measurements of self-assembled monolayer systems in ambient and inert environments. Our results reveal a consistent increase of TERS signals, significant reduction of spurious signals, and drastically improved longevity of TERS probes in the inert environment. Additionally, sample characteristics such as molecular packing, chemisorption behavior, and hydrophilicity are found to have a direct impact on signal enhancement in the TERS measurements of molecular self-assembled monolayers (SAMs). The novel insights gained in this study are expected to pave the way for a more robust data analysis and improved experimental design in the future gap mode STM- and atomic force microscopy-TERS (AFM-TERS) studies.

Copper(II) defect-cubane water oxidation electrocatalysts: from molecular tetramers to oxidic nanostructures.

We report on the synthesis and spectroscopic evidence for a sequence of structural transformations of a new defect-cubane type copper complex, [Cu4(pyalk)4(OAc)4](ClO4)(HNEt3), which acts as a pre-catalyst for water oxidation. In situ and post-catalytic studies showed that the tetrameric complex undergoes a structural transformation into dimeric and monomeric species, induced by water molecules and carbonate anions, respectively. Further, the observed electrocatalytic water oxidation activity has been confirmed to arise from in situ-generated Cu(II) oxidic nanostructures at the electrode interface.

Highly dispersed silica-supported iridium and iridium-aluminium catalysts for methane activation prepared via surface organometallic chemistry.

The grafting of an iridium-aluminium precursor onto silica followed by thermal treatment under H2 yields small (<2 nm), narrowly distributed nanoparticles used as catalysts for methane H/D exchange. This Ir-Al/SiO2 catalyst demonstrated enhanced catalytic performances in comparison with the monometallic Ir/SiO2 analogue (TOFs of 339 h-1versus 117 h-1 respectively), highlighting the promoting effect of aluminium. TON up to 900 is obtained after 9 hours, without evidence of catalyst deactivation, and identical performances are achieved after air exposure, underlining the good robustness of both Ir-Al/SiO2 and Ir/SiO2 catalytic materials.

Design and Synthesis of Ir/Ru Pyrochlore Catalysts for the Oxygen Evolution Reaction Based on Their Bulk Thermodynamic Properties.

Density functional theory (DFT) has proven to be an invaluable and effective tool for identifying highly active electrocatalysts for the oxygen evolution reaction (OER). Herein, we take a computational approach to first identify a series of rare-earth pyrochlore oxides based on Ir and Ru as potential OER catalysts. The DFT-based phase diagrams, Pourbaix diagrams (E vs pH), projected density of states, and band energy diagrams were used to identify prospective OER catalysts based on rare-earth Ir and Ru pyrochlores. The predicted materials were synthesized using the spray-freeze freeze-drying approach to afford nanoparticulate oxides conforming to the pyrochlore structural type A2B2O7 where A = Nd, Gd, or Yb and B = Ir or Ru. In agreement with the computed Pourbaix diagrams, the materials were found to be moderately stable under OER conditions. All prepared materials show higher stability as compared to the benchmark IrO2 catalyst, and the OER mass activity of Yb2Ir2O7 and the ruthenate pyrochlores (Nd2Ru2O7, Gd2Ru2O7, and Yb2Ru2O7) were also found to exceed those of the benchmark IrO2 catalyst. We find that the OER activity of each pyrochlore series (i.e., iridate or ruthenate) generally improves as the size of the A-site cation decreases, indicating that maintaining control over the structure can be used to influence the electrocatalytic properties.

Functional Role of Fe-Doping in Co-Based Perovskite Oxide Catalysts for Oxygen Evolution Reaction.

Perovskite oxides have been at the forefront among catalysts for the oxygen evolution reaction (OER) in alkaline media offering a higher degree of freedom in cation arrangement. Several highly OER active Co-based perovskites have been known to show extraordinary activities and stabilities when the B-site is partially occupied by Fe. At the current stage, the role of Fe in enhancing the OER activity and stability is still unclear. In order to elucidate the roles of Co and Fe in the OER mechanism of cubic perovskites, two prospective perovskite oxides, La0.2Sr0.8Co1- xFe xO3-δ and Ba0.5Sr0.5Co1- xFe xO3-δ with x = 0 and 0.2, were prepared by flame spray synthesis as nanoparticles. This study highlights the importance of Fe in order to achieve high OER activity and stability by drawing relations between their physicochemical and electrochemical properties. Ex situ and operando X-ray absorption spectroscopy (XAS) was used to study the local electronic and geometric structure under oxygen evolving conditions. In parallel, density function theory computational studies were conducted to provide theoretical insights into our findings. Our findings show that the incorporation of Fe into Co-based perovskite oxides alters intrinsic properties rendering efficient OER activity and prolonged stability.