Elucidating Synergistic Effects of Different Metal Ratios in Bimetallic Fe/Co-N-C Catalysts for Oxygen Reduction Reaction

Lowering or eliminating the noble-metal content in oxygen reduction fuel cell catalysts could propel the large-scale introduction of commercial fuel cell systems. Several noble-metal free catalysts are already under investigation with the metal-nitrogen-carbon (Me-N-C) system being one of the most promising. In this study, a systematic approach to investigate the influence of metal ratios in bimetallic Me-N-C fuel cells oxygen reduction reaction (ORR) catalysts has been taken. Different catalysts with varying ratios of Fe and Co have been synthesized and characterized both physically and electrochemically in terms of activity, selectivity and stability with the addition of an accelerated stress test (AST). The catalysts show different electrochemical properties depending on the metal ratio such as a high electrochemical mass activity with increasing Fe ratio. Properties do not change linearly with the metal ratio, with a Fe/Co ratio of 5:3 showing a higher mass activity with simultaneous higher stability. Selectivity indicators plateau for catalysts with a Co content of 50% metal ratio and less, showing the same values as a monometallic Co catalyst. These findings indicate a deeper relationship between the ratio of different metals and physical and electrochemical properties in bimetallic Me-N-C catalysts.

The Role of the Disulfide Bridge in the Copper (II) Binding by the Cyclic His4-Peptide

In this paper, we present studies on the influence of the disulfide bridge on the copper (II) ions’ binding abilities by the cyclic His4-peptide. The studied ligand HKHPHRHC-S-S-C consists of nine amino acids. The cyclic structure was obtained through a disulfide bridge between two cysteinyl groups. Moreover, this peptide is characterized by the presence of four His residues in the sequence, which makes it an interesting ligand for transition metal ions. The potentiometric and spectroscopic (UV-Vis spectroscopy and circular dichroism spectroscopy (CD)) studies were carried out in various molar ligand to metal ratios: 2:1, 1:1, and 1:2, in the pH range of 2.5–11 at 25 °C. The results showed that the cyclic His4-peptide promotes dinuclear complexes in each of these systems and forms the final dinuclear species with the {NIm, 3N-amide}{NIm, 3N-amide} coordination mode. The obtained data shows that cyclization by the formation of the disulfide bond has an impact on the peptide chain flexibility and appearance of additional potential donors for metal ions and influences the copper (II) ions’ coordination.

Revealing the etching process of water-soluble Au25 nanoclusters at the molecular level

Etching (often considered as decomposition) is one of the key considerations in the synthesis, storage, and application of metal nanoparticles. However, the underlying chemistry of their etching process still remains elusive. Here, we use real-time electrospray ionization mass spectrometry to study the reaction dynamics and size/structure evolution of all the stable intermediates during the etching of water-soluble thiolate-protected gold nanoclusters (Au NCs), which reveal an unusual “recombination” process in the oxidative reaction environment after the initial decomposition process. Interestingly, the sizes of NC species grow larger and their ligand-to-metal ratios become higher during this recombination process, which are distinctly different from that observed in the reductive growth of Au NCs (e.g., lower ligand-to-metal ratios with increasing sizes). The etching chemistry revealed in this study provides molecular-level understandings on how metal nanoparticles transform under the oxidative reaction environment, providing efficient synthetic strategies for new NC species through the etching reactions.

Are Vapor-Like Fluids Viable Ore Fluids for Cu-Au-Mo Porphyry Ore Formation?

Ore formation in porphyry Cu-Au-(Mo) systems involves the exsolution of metal-bearing fluids from magmas and the transport of the metals in magmatic-hydrothermal plumes that are subject to pressure fluctuations. Deposition of ore minerals occurs as a result of cooling and decompression of the hydrothermal fluids in partly overlapping ore shells. In this study, we address the role of vapor-like fluids in porphyry ore formation through numerical simulations of metal transport using the Gibbs energy minimization software, GEM-Selektor. The thermodynamic properties of the hydrated gaseous metallic species necessary for modeling metal solubility in fluids of moderate density (100–300 kg/m3) were derived from the results of experiments that investigated the solubility of metals in aqueous HCl- and H2S-bearing vapors. Metal transport and precipitation were simulated numerically as a function of temperature, pressure, and fluid composition (S, Cl, and redox). The simulated metal concentrations and ratios are compared to those observed in vapor-like and intermediate-density fluid inclusions from porphyry ore deposits, as well as gas condensates from active volcanoes. The thermodynamically predicted solubility of Cu, Au, Ag, and Mo decreases during isothermal decompression. At elevated pressure, the simulated metal solubility is similar to the metal content measured in vapor-like and intermediate-density fluid inclusions from porphyry deposits (at ~200–1,800 bar). At ambient pressure, the metal solubility approaches the metal content measured in gas condensates from active volcanoes (at ~1 bar), which is several orders of magnitude lower than that in the high-pressure environment. During isochoric cooling, the simulated solubility of Cu, Ag, and Mo decreases, whereas that of Au reaches a maximum between 35 ppb and 2.6 ppm depending on fluid density and composition. Similar observations are made from a compilation of vapor-like and intermediate-density fluid inclusion data showing that Cu, Ag, and Mo contents decrease with decreasing P and T. Increasing the Cl concentration of the simulated fluid promotes the solubility of Cu, Ag, and Au chloride species. Molybdenum solubility is highest under oxidizing conditions and low S content, and gold solubility is elevated at intermediate redox conditions and elevated S content. The S content of the vapor-like fluid strongly affects metal ratios. Thus, there is a decrease in the Cu/Au ratio as the S content increases from 0.1 to 1 wt %, whereas the opposite is the case for the Mo/Ag ratio; at S contents of >1 wt %, the Mo/Ag ratio also decreases. In summary, thermodynamic calculations based on experiments involving gaseous metallic species predict that vapor-like fluids may transport and efficiently precipitate metals in concentrations sufficient to form porphyry ore deposits. Finally, the fluid composition and pressure-temperature evolution paths of vapor-like and intermediate-density fluids have a strong effect on metal solubility in porphyry systems and potentially exert an important control on their metal ratios and zoning.

Trace metal variability in Puerto Rican speleothems and drip waters: Indicators for (past) tropical cyclone activity?

<p>In the tropical Americas, extreme precipitation events such as hurricanes are responsible for enormous damage and numerous fatalities each year. However, projections of hydro-climatic change and tropical cyclone (TC) activity in Central America and the Caribbean for the next decades are still challenging, requiring more reconstructions of past precipitation and TC activity. In tropical speleothems, stable oxygen isotope values (δ<sup>18</sup>O) are an often used proxy for precipitation amount, and in some cases TC activity, but may be masked by various effects such as evaporation or kinetic effects inside the cave, temperature, or variable moisture sources and trajectories.</p><p>Here we investigate the potential of trace metals in speleothems and drip waters from Larga Cave, Puerto Rico, as complementary proxies for past effective infiltration, and hence precipitation amount. The analysis of transition metal ratios in drip waters from 2014 to 2019 reveal a seasonal variation, with peaks in the Cu/Ni (and Cu/Co) ratios potentially reflecting the intensity of the prior wet season. The suggested imprint of Hurricanes Bertha (2014) and Maria (2017) in the drip water suggests that transition metal ratios might be even indicators of (past) tropical cyclone activity.</p><p>Laser ablation ICPMS analyses of speleothems from the same cave support the interpretation of a potential climate signal in the transition metal ratios. Both higher Cu/Ni and Cu/Co values are found during presumably warmer and wetter phases, such as e.g. during the late Holocene, as well as at the onsets of Dansgaard/Oeschger interstadials including the Bolling/Allerod (14.6-12.8 ka BP). Replicated records of the past 400 years combined with stable isotope values of oxygen and carbon (δ<sup>13</sup>C) will provide a test of the underlying mechanisms driving the observed variability on different timescales. Comparison with other reconstructions highlights the potential of Cu/Ni (and Cu/Co) ratios in speleothems for hydro-climate and past precipitation variability reconstruction.</p>

Tuning dimensionality between 2D and 1D MOFs by lanthanide contraction and ligand-to-metal ratio

2D/1D dimensionality tuning in LnMOFs is related to both (i) ligand-to-metal ratio and (ii) lanthanide contraction, this is only possible with Er/Tm, lighter lanthanides e.g. Pr only produced 2D MOFs, despite different ligand-to-metal ratios were used.

Kinetic Modeling of Ammonia Decomposition at CVD Conditions

Kinetic modeling has been used to study the decomposition chemistry of ammonia at a wide range of temperatures, pressures, concentrations, and carrier gases mimicking the conditions in chemical vapor deposition (CVD) of metal nitrides. The modeling show that only a small fraction of the ammonia molecules will decompose at most conditions studied. This suggests that the high NH₃ to metal ratios often employed in CVD is due to the very low amount of reactive decomposition products rather than due to rapid decomposition of ammonia into stable dinitrogen and dihydrogen as suggested by purely thermodynamic models.

Kinetic Modeling of Ammonia Decomposition at CVD Conditions

Kinetic modeling has been used to study the decomposition chemistry of ammonia at a wide range of temperatures, pressures, concentrations, and carrier gases mimicking the conditions in chemical vapor deposition (CVD) of metal nitrides. The modeling show that only a small fraction of the ammonia molecules will decompose at most conditions studied. This suggests that the high NH₃ to metal ratios often employed in CVD is due to the very low amount of reactive decomposition products rather than due to rapid decomposition of ammonia into stable dinitrogen and dihydrogen as suggested by purely thermodynamic models.