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It has been of great interest to understand relationships between the potential of a metal catalyst and its thermochemical catalytic activity, especially in an aqueous phase environment. In the literature, there are correlations of open circuit potential with reaction rates or surface concentrations of reaction intermediates. In this study, we measured the open circuit potential (OCP) of a Au gauze during selective oxidation of glycerol to glyceric acid and hydrogen peroxide in a basic solution, as a function of hydroxide and alcoholate concentrations. It is found that applying a potential to the Au catalyst has no influence on the reaction rate. Although a rough correlation appears to exist between OCP and reaction rate, the data are better fit to an equation which assumes that the potential of the metal (i.e. the OCP) is in equilibrium with the electrochemical potential of the solution, which is defined by the thermodynamic activities of the oxidizing and reducing species. The equation: OCP = constant + Σ_ox[(RT/FZ_ox)ln[Ox]] - Σ_red[(RT/FZ_red)ln[Red]] implicitly assumes that the Au metal functions as a probing electrode. It is further found that this equation also applies to the Au-catalyzed H₂O₂ oxidation/decomposition reaction in a basic medium, and possibly to formic acid dehydrogenation. Here, we postulate that the apparent correlation between OCP and reaction rate is due to the fact that the reaction rate is proportional to the concentrations of the reaction products and/or reactants that define the electrochemical potential of the solution.

Heterogeneous NO₂-mediated oxidation of uranium, also known as advanced voloxidation, is a proposed head-end reprocessing method for used nuclear fuel. An advantage of advanced voloxidation is the removal of volatile fission products, which complicate downstream separation and containment challenges leading to increased processing economics. Iodine, one of the volatile species of interest, has exhibited varied results in this process. Using CsI as a surrogate material, this work mimics the effect of NO₂-based voloxidation on iodine and sheds light on the factors that influence the solid–gas phase reaction. Solid-state analysis using Fourier transform infrared attenuated total reflectance spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy confirmed the conversion of CsI to CsNO₃. Iodine separation ranged from 46% to 100% across multiple tests. Iodine separation was most effective when multiple recharges of NO₂ were administered. In conclusion, at the bench scale, liberating iodine from CsI appears to occur within 1 h, but the presence of surface H₂O and the composition of the NO_x reagent mixtures greatly influence its success.

As newer cells structures come online, the pressing need to replace silver in the metallization pastes has renewed interest in alternative technologies employing base metals. Copper typically leads the charge with its abundance and lower cost but has faced numerous obstacles from relatively higher oxidation and diffusion rates which can damage the lifetime of the devices. In this study, a low-cost alternative to silver metallization pastes has been shown on PERC cells. The screen printable copper paste can be fired in air at temperatures >500 degrees C, and the impact of processing conditions and equipment on the performance and reliability of 274 cm2 cells have been evaluated. Through damp heat testing of micro-modules using 16 cm2 cells, routes that can lead to both the failure and success of durable contacts have been demonstrated.

Reprocessing is considered a competent strategy for spent nuclear fuel management, yet radioiodine (¹²⁹I) is emitted in reprocessing off-gas as a hazardous byproduct. Silver functionalized silica aerogel (Ag⁰-aerogel), a promising iodine capture material, experiences a reduction in its capacity after prolonged exposure to off-gas components at elevated temperatures, a phenomenon termed as aging. To fully understand this process, we isolated the contribution of each aging factor, exposing Ag⁰-aerogel samples to N₂ and dry air gas streams, respectively, at 150 °C for different time periods. Aged samples were loaded with I₂ to examine the capacity change and comprehensively characterized to investigate the evolution of their properties. Results show that temperature alone did not alter Ag⁰-aerogel's capacity but triggered Ag⁰ nanoparticles sintering and generated organic sulfur species. The presence of O₂ reduced the capacity by ~20 %, causing (i) formation of silver sulfide (Ag₂S) crystals and (ii) oxidation of Ag-thiolate (Ag-S-r) to Ag sulfonate (Ag-SO₃-r). Given that Ag₂S readily adsorbs I₂, the formation of Ag-SO₃-r is the major inhibitor for iodine adsorption. This hypothesis was supported by density functional theory (DFT) simulations. These findings unraveled key mechanisms of Ag₀-aerogel aging, which are useful in the development of materials that withstand realistic spent-nuclear-fuel-reprocessing off-gas conditions.

Green rusts (GR) are mixed-valence iron (Fe) hydroxides which form in reducing redox environments like riparian and wetland soils and shallow groundwater. In these environments, silicon (Si) can influence Fe oxides’ chemical and physical properties but its role in GR formation and subsequent oxidative transformation have not been studied starting at initial nucleation. Green rust sulfate [GR(SO₄)] and green rust carbonate [GR(CO₃)] were both coprecipitated from salts by base titration in increasing % mol Si (0, 1, 10, and 50). The minerals were characterized before and after rapid (24 h) aqueous air-oxidation by x-ray diffraction (XRD), scanning electron microscopy (SEM), Fe extended x-ray absorption fine structure spectroscopy (EXAFS), and N₂-BET surface area. Results showed that only GR(SO4) or GR(CO3) was formed at every tested Si concentration. Increasing % mol Si caused decreased plate size and increased surface area in GR(CO3) but not GR(SO4). GR plate basal thickness was not changed at any condition indicating a lack of Si interlayering. Air oxidation of GR(SO4) at all % mol Si contents transformed by dissolution and reprecipitation into lepidocrocite and goethite, favoring ferrihydrite with higher % Si content. Air oxidation of GR(CO3) transformed into magnetite and goethite but increasing Si caused GR to oxidize while retaining its hexagonal plate structure via solid-state oxidation. Our results indicate that Si has the potential to cause GR to form in smaller particles and upon air oxidation, Si can either stabilize the plate structure or alter transformation to ferrihydrite.

Chemical weathering over geological timescales acts as a source or sink of atmospheric carbon dioxide (CO₂), while influencing long-term redox cycling and atmospheric oxygen (O₂) at Earth's surface. There is a growing recognition that the oxidative weathering of rock organic carbon (OC_petro) can release more CO₂ than is locally drawn down by silicate weathering, and may vary due to changes in erosion and climate. The element rhenium (Re) has emerged as a proxy to track the oxidative weathering of OC_petro, yet uncertainties in its application remain namely that we lack a systematic assessment of the comparative mobility of Re and OC_petro during sedimentary rock weathering. Here we measure Re and OC_petro loss across gradients in rock weathering at 9 global sites, spanning a range of initial OC_petro values from ~0.2 % to >10 %. We use titanium to account for volume changes during weathering and assess Re and OC_petro loss alongside major elements that reflect silicate (Na, Mg), carbonate (Ca, Mg) and sulfide (S) weathering. Across the dataset, Re loss is correlated with OC_petro loss but not with loss of any other major element. Further, across the weathering profiles, the average molar ratio of OC_petro to Re loss was 0.84 ± 0.15, with 8 out of 9 sites having a ratio >0.74. At one site (Marcellus Shale), the average ratio was lower at 0.58 ± 0.11. The excess loss of Re matches expectations that, typically, between ~0 and 20 % of the Re liberated by sedimentary rock weathering derives from silicate or sulfide phases, while some OC_petro may be physically or chemically protected from weathering. Overall, our measurements provide validation for the Re proxy of OC_petro oxidation and allow future work to further improve our knowledge of regional and global-scale rates of this important source of CO₂ in the geochemical carbon cycle.

Lakes are important sentinels of climate change and may contribute over 30% of natural methane (CH₄) emissions; however, no earth system model (ESM) has represented lake CH₄ dynamics. To fill this gap, we refined a process-based lake biogeochemical model to simulate global lake CH₄ emissions, including representation of lake bathymetry, oxic methane production (OMP), the effect of water level on ebullition, new non-linear CH₄ oxidation kinetics, and the coupling of sediment carbon pools with in-lake primary production and terrigenous carbon loadings. We compiled a lake CH₄ data set for model validation. The model shows promising performance in capturing the seasonal and inter-annual variabilities of CH₄ emissions at 10 representative lakes for different lake types and the variations in mean annual CH₄ emissions among 106 lakes across the globe. The model reproduces the variations of the observed surface CH₄ diffusion and ebullition along the gradients of lake latitude, depth, and surface area. The results suggest that OMP could play an important role in surface CH₄ diffusion, and its relative importance is higher in less productive and/or deeper lakes. The model performance is improved for capturing CH₄ outgassing events in non-floodplain lakes and the seasonal variability of CH₄ ebullition in floodplain lakes by representing the effect of water level on ebullition. The model can be integrated into ESMs to constrain global lake CH₄ emissions and climate-CH₄ feedback.

Hydrogen or hydrogen blend fuels are expected to replace natural gas in land-based industrial gas turbines (IGTs) to support a greener power economy. Silicon carbide (SiC) base ceramic matrix composites (CMCs) are considered for replacement of Ni-based superalloys to facilitate future efficiency improvements. SiC CMCs require environmental barrier coatings (EBCs) to mitigate volatilization from high-temperature steam, thus making the EBC lifetime critical information for identifying CMC component lifetimes. Here, the goal of this project is to determine the maximum bond coating temperature underneath the EBC for achieving an IGT component lifetime goal of 25,000 h, which is far greater than current CMC component lifetime requirements for aeroturbine applications. To provide data for the lifetime model, laboratory testing used atmospheric plasma-sprayed rare-earth silicate EBCs on monolithic SiC substrates with an intermediate Si bond coating. Specimens exposed to 1-h thermal cycles in flowing air–steam environments and reaction kinetics were assessed from 700 °C to 1350 °C by measuring the thickness of the thermally grown silica scales. The silica growth and phase transformation appear critical in predicting EBC lifetime and several strategies have been explored to reduce the oxide growth rate and improve EBC durability at elevated temperatures. Advanced characterization using Raman spectroscopy has helped clarify this system.

PdO/γ-Al₂O₃ catalysts suffer from gradual and irreversible catalyst deactivation under lean CH₄ oxidation conditions, especially in a wet feed. Here, time-resolved CO chemisorption DRIFTS measurements are conducted systematically on a series of PdO/γ-Al₂O₃ catalysts to probe the surface reactivity of PdO nanoparticles after various in situ pretreatments. At 80 °C, CO barely adsorbs on fully oxidized PdO surfaces but interacts with coordinatively unsaturated Pd sites, causing gradual reduction of the PdO surfaces. This results in the formation of characteristic IR bands on various metallic Pd⁰ sites. By monitoring and comparing the formation kinetics of these IR bands on samples before and after CH₄ oxidation, we theorize that the irreversible catalyst deactivation during CH₄ oxidation is caused by PdO surface reconstruction, in which coordinatively unsaturated Pd sites gradually become fully coordinated by oxygen. Effectively, the surface reconstruction leads to the formation of a passivation layer on the PdO nanoparticles, which hinders their ability in activating CH₄, and hence the subsequent oxidation reaction. Temperature-programmed reduction with CO as the reductant (CO-TPR) reveals that the passivation layer formed during CH₄ oxidation is significant enough to increase the reduction temperature of PdO nanoparticles of the 3.0% PdO/γ-Al₂O₃ samples, although such an effect is less obvious for the 0.4% PdO/γ-Al₂O₃ samples. On the other hand, it is also discovered that the passivation layer is not completely inert. Under certain reaction conditions, with some being relatively mild, such as low-temperature CO oxidation in a net lean atmosphere and in the presence of H₂O, the passivation layer can undergo structure change which results in regeneration or even activation of CH₄ oxidation activity of an already deactivated catalyst. Additionally, it is discovered that the fully coordinated Pd–O surface is a metastable phase under CH₄ oxidation conditions. In the presence of H₂O and at ambient temperatures, surfaces with coordinatively unsaturated Pd sites are thermodynamically more favorable.

Palladium (Pd) catalysts have been extensively studied for the direct synthesis of H₂O through the hydrogen oxidation reaction at ambient conditions. This heterogeneous catalytic reaction not only holds considerable practical significance but also serves as a classical model for investigating fundamental mechanisms, including adsorption and reactions between adsorbates. Nonetheless, the governing mechanisms and kinetics of its intermediate reaction stages under varying gas conditions remain elusive. This is attributed to the intricate interplay between adsorption, atomic diffusion, and concurrent phase transformation of catalyst. Herein, the Pd-catalyzed, water-forming hydrogen oxidation is studied in situ, to investigate intermediate reaction stages via gas cell transmission electron microscopy. The dynamic behaviors of water generation, associated with reversible palladium hydride formation, are captured in real time with a nanoscale spatial resolution. Our findings suggest that the hydrogen oxidation rate catalyzed by Pd is significantly affected by the sequence in which gases are introduced. Through direct evidence of electron diffraction and density functional theory calculation, we demonstrate that the hydrogen oxidation rate is limited by precursors’ adsorption. These nanoscale insights help identify the optimal reaction conditions for Pd-catalyzed hydrogen oxidation, which has substantial implications for water production technologies. The developed understanding also advocates a broader exploration of analogous mechanisms in other metal-catalyzed reactions.