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Abstract: Methanol oxidative carbonylation is a highly desired reaction for the industrial production of dimethyl carbonate (DMC), offering a more sustainable alternative to the conventional, environmentally unfriendly phosgene-methanol process. Although supported copper nanoparticle catalysts can facilitate this reaction, their rapid deactivation due to Cu sintering and overoxidation limits their industrial applicability. In this study, we present robust and reusable single-atoms and cluster-like Cu catalysts (catalyst loading up to 6.5 wt %) supported on N-doped carbon, synthesized via pyrolysis of Cu-doped ZIF-8 precursors. The formation and stability of highly dispersed Cu species during the reaction was confirmed using a comprehensive suite of characterization techniques, including X-ray diffraction (XRD), Fourier transform infrared (FT-IR), high-resolution transmission electron microscopy (HR-TEM), aberration corrected high angle annular dark field-scanning transmission electron microscopy (AC-HAADF-STEM), X-ray photoelectron spectroscopy (XPS), NH3-TPD, H2-TPR, and X-ray absorption spectroscopy (XAS). This unique copper architecture achieved an exceptional DMC selectivity of 99.4% and a space-time yield of 3249 mg DMC·g–1·h–1 (TOFDMC,B = 34.4 h–1; TOFDMC,S = 294 h–1) at 120 °C for 2 h. The catalysts demonstrated excellent reusability, maintaining their performance over at least seven consecutive runs without deactivation. Postreaction analysis of the spent catalyst after seven runs revealed that Cu was largely free of leaching, sintering, and overoxidation.

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Abstract: Arsenic immobilization in soils and sediments is primarily controlled by its sorption onto or incorporation into reactive soil minerals, such as iron (oxyhydr)oxides. However, coexisting ions (e.g., dissolved bicarbonate, phosphate, silica, and organic matter) can negatively impact the interaction of the toxic arsenate species with iron (oxy)hydroxides. Of special note is inorganic phosphate, which is a strong competitor for sorption sites due to its analogous chemical and structural nature to inorganic arsenate. Much of our understanding of this competing nature between phosphate and arsenate focuses on the impact on mineral sorption capacities and kinetics. However, we know very little about how coexisting phosphate will alter the stability and transformation pathways of arsenate-bearing Fe (oxyhydr)oxides. In particular, the long-term fate and behavior regarding arsenate immobilization are unknown under anoxic conditions. Here, we document, through mineral transformation reactions, the immobilization of both phosphate (P) and arsenate [As(V)] in secondary mineral products and characterize their changing compositions during the transformations. We did this while controlling the initial P/As(V) ratios. Our results document that, in the absence or at low P/As(V) ratios, the initial ferrihydrite rapidly transforms to green rust sulfate (GRSO4), which further transforms into magnetite after 180 days. Meanwhile, high P/As(V) ratios resulted in a mixture of GRSO4 and vivianite, with magnetite as a minor fraction. Invariably, the speciation and partitioning of As(V) were also affected by the P/As(V) ratio. A higher P/As(V) ratio also led to a faster partial reduction of mineral-bound As(V) to As(III). The most important finding is that the initial ferrihydrite-bound As(V) became structurally incorporated into magnetite [low P/As(V) ratio] or vivianite [high P/As(V) ratio] and was thus immobilized and not labile. Overall, our results highlight the influence of coexisting phosphate in controlling the toxicity and mobility in anoxic, Fe2+-rich subsurface settings, such as contaminated aquifers.

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Abstract: Development of sustainable hydrogen-based fuel systems requires both efficient ways to produce hydrogen and extract energy from it. In nature, this reaction is performed efficiently by hydrogenase enzymes. Studying the mechanism of hydrogenase enzymes is difficult due to their exceptionally high turnover frequencies. As a result, new techniques are required to probe their catalytic cycle. X-ray Absorption Spectroscopy is a promising technique, due to the structural and electronic information obtainable, but requirements to utilize cryogenic samples to avoid photodamage hinder its applicability. To avoid this, a specially designed cell was developed to allow room-temperature X-ray spectroscopy to be performed on hydrogenase samples under electrochemical control. This cell was successfully trialed using HERFD-XAS measurements of both [NiFe] and [FeFe] hydrogenases and demonstrated promise in mitigating photodamage via novel electrochemical protective methods. Preliminary work using model nickel complexes also demonstrated the feasibility of accessing VtC transitions during X-ray Emission Spectroscopy, allowing access to new probes of the active site. The feasibility of Time-Resolved Multiple Probe spectroscopy, an ultrafast infrared spectroscopy method, was also tested in order to investigate sub-turnover kinetics of [NiFe] hydrogenases. This demonstrated the existence of intermediates with lifetimes below that accessible by more conventional methods. This work provides multiple avenues to investigate the mechanism of [NiFe] hydrogenases.

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Abstract: We successfully synthesised ceria and Cu-doped ceria thin films on fluorine-doped tin oxide (FTO) glass using electrochemical deposition methods. Although X-ray diffraction characterisation did not confirm the presence of pure CeO2 phase, XPS and high-resolution fluorescence detection method to collect XANES data at Ce L3 edge confirmed the presence of only Ce4+ and the spectral features resembled that of CeO2 in all the as-synthesised samples. In situ reactivity studies in the H2 atmosphere of these samples employing XANES data, recorded during the heating and cooling cycle, revealed the extent of Ce4+ conversion to Ce3+. Most importantly the stability of converted Ce3+ appears to depend on the presence of reducible transition metal, where the presence of copper ions indeed decreased the reduction temperature of Ce4+ and upon cooling the formed Ce3+ appears to be stable in a reducing atmosphere. In the absence of copper, not only is the extent of reduction to Ce3+ much less compared with the copper-containing samples but also, the reduced Ce3+ ions appear to reoxidise to Ce4+ upon cooling to room temperature. These results are in line with observed results of powder CeO2 samples typically used as catalyst supports.

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Abstract: Some of the largest Mexican uranium (U) deposits are located in Chihuahua. The most important is in Sierra Peña Blanca, northwest of the capital, which was explored and partially exploited in the 1980s. After the closure of activities, the mining projects were left exposed to weathering. To characterize the spread of U minerals towards the neighboring Laguna del Cuervo, sediment samples were collected in the main streams of the drainage pattern of the largest deposits. The U mineral fragments from the fine sand portion were extracted using fluorescence light at 365 nm. The morphology and elemental composition of these particles were analyzed by focused ion beam microscopy (FIB) and scanning transmission electron microscopy (STEM). The particle density in samples close to the U sources was quantified using gamma spectrometry. The highest density was 2500 part./g, and the lowest was 124 part./g. X-ray absorption spectroscopy (XAS) allowed us to establish via XANES the speciation of U in the U particles, confirming the U(VI) oxidation state, while the exploitation of the EXAFS spectrum put in evidence of the presence of uranophane. Finally, the Fe, Sr, and U distributions in the particle and its matrix were obtained via X-ray fluorescence microtomography (XRF-µCT). It was concluded that the particle is composed of uranophane, imbricated with quartz and other oxides.