H2O Content Research Articles

Study on the phase transformation mechanism and influencing factors of inorganic condensable particulate matter from coal-fired power plants

In this study, the concentration of inorganic ions (SO42−, NH4+, NO3− and NO2−) and morphological characteristics of condensable particulate matter (CPM) were investigated to elucidate the formation mechanism of inorganic CPM from ultra-low emission coal-fired power plants. The concentration of inorganic ions increased with the increase of H2O content and concentration of inorganic gaseous contaminants (SO2, NOX and NH3), and decrease of condensation temperature, indicating the enhancement of heterogenous reaction in the saturated flue gas. Furthermore, NOX and SO2 could undergo redox reactions, leading to an elevation in the concentration of SO42− and NO3−. Additionally, the introduction of NH3 resulted in increased concentrations of SO42−, NO3−, and NO2−, highlighting the significant role of NH3 neutralization in CPM nucleation. The condensation of SO3/sulfuric acid aerosols was enhanced under saturation conditions, and SO2 and SO3/sulfuric acid aerosols could contribute synergistically to the formation of SO42−. Moreover, morphological analysis revealed the presence of both well-aggregated solid CPM and dispersed liquid CPM, confirming the formation of inorganic CPM during fast condensation. Furthermore, the detected CPM were composed of S and O, which identified the significant role of sulfates in the inorganic CPM. These findings provide valuable insights for the control of inorganic CPM in flue gas systems.

Electrochemical Hydrogen Pump Using CsH2PO4/SnO2 Composite Proton Conducting Membrane

Liquid organic hydrogen carriers (LOHCs) are molecular compounds that have potential to be a promising class of hydrogen storage materials. Upon thermal treatment and in the presence of catalysts, H2-rich LOHCs can give off H2 without phase change. One immediate example is methylcyclohexane (C7H14) (MCH) conversion to toluene (C7H8) (TOL). The rate of H2 production of LOHC’s dehydrogenation reaction is kinetically limited. Therefore, adequate catalysts can help boost kinetics. An alternative approach to overcome the kinetic limitation is to shift the dehydrogenation reaction by removing H2 in the product via either a chemical or electrochemical route. Here we show an electrochemical H2 pump concept to promote the dehydrogenation reaction of MCH to TOL by pumping out H2 from the product line. We selected the super-protonic conductor CsH2PO4 (CDP) as the electrolyte for the pump due to the operating temperature of the dehydrogenation reaction (250-350 oC). However, CDP would not be stable if there is an insufficient amount of H2O (<38%) in the working environment, irreversibly dehydrated into a low-conductivity phase. On the other hand, too much H2O in MCH stream would negatively affect the catalytic performance of MCH-to-TOL conversion. Therefore, it is critical to limit the H2O content in MCH stream without affecting H2 pump performance.Here, we report the use of SnO2 to stabilize the super-protonic conducting phase in CDP. SnO2 is a super-hydrophilic material. We first show the ionic conductivity stabilization over a broad H2O content to demonstrate the effectiveness of SnO2 in stabilizing CDP conductivity. We then show the performance of H2 pump based on the composite CDP-SnO2 protonic conductor. We also show materials characterization results including XRD, SEM and Raman to facilitate our understanding of the observed improvement and degradation. Overall, this study provides a supporting method to accelerate the production of green hydrogen with LOHCs.

Experimental study on the ash deposition and NO emission of high-alkali coal under the staged O2/CO2 and O2/RFG conditions

The oxy-fuel combustion contributes to carbon capture, while the recirculation of flue gas brings about high concentrations of SO2 and H2O, which can affect the transformation of minerals in high-alkali coal. The staged oxy-fuel combustion, as an effective method for NOx reduction, can also change the ash deposition behavior of high-alkali coal. Two kinds of diluting agents, including pure CO2 for O2/CO2 combustion and simulated “recycled flue gas” (CO2, SO2, and H2O) for O2/RFG combustion, were employed in the present work. The ash deposition and NO emission of high-alkali coal during the staged oxy-fuel combustion were simultaneously studied under O2/CO2 and O2/RFG conditions. The conversion ratios of fuel-nitrogen to NO (CNO) and ash deposition efficiencies (Ed) at different stoichiometric ratios in primary combustion zone (SR1) and different oxygen concentrations were obtained. Afterwards, a series of tests were performed to further analyze the ash deposits. The experimental results show that as SR1 increases from 0.6 to 1.2, CNO jumps from 2.0 % to 23.5 % (O2/CO2 combustion) and from 1.9 % to 19.9 % (O2/RFG combustion). The additions of SO2 and H2O can reduce the NO emission. With the rising SR1, Ed under the O2/CO2 and O2/RFG conditions decreases from 4.0 % to 2.6 % and from 4.8 % to 2.1 %, respectively. At high SR1, the CaSO4 amount declines and the iron contributes less to the ash deposition. In O2/RFG combustion, the small sticky particles of sodium aluminosilicates on large particle surfaces reduce, and the large particles of calcium aluminosilicates shrink because some calcium produces CaSO4. Moreover, the exposure of ferrous iron to H2O helps its oxidization so iron is harder to cause severe adhesion. As O2 concentration rises from 21 % to 40 %, CNO shows an upward trend. Meanwhile, Ed under the O2/CO2 and O2/RFG conditions increases from 2.6 % to 3.7 % and from 2.3 % to 2.7 %, respectively. The present work is expected to provide some conducive information for the clean utilization of high-alkali coal and secure operation of boiler, as well as large-scale CO2 capture.

Liquid-liquid addition reaction of ethylene oxide with hydrazine hydrate in microreactors: Kinetics and process optimization

The addition reaction of ethylene oxide (EO) and hydrazine hydrate (N2H4·H2O) to β-hydroxyethyl hydrazine (HEH) is an important process for the synthesis of hydrazine-derived monopropellant in the aerospace industry. Due to the complexity of this reaction mechanism, it is difficult to reduce the formation of byproducts and efficiently obtain pure HEH. Hence it is necessary to investigate process behavior and establish kinetic network for further optimization. Here a continuous microflow system was developed to systematically investigate the synthesis of HEH under high pressure in the liquid–liquid form. A simple and effective pre-column derivatization method was applied to characterize and quantify the product. Subsequently, a thorough optimization of parameters, including N2H4·H2O concentration, molar ratio (N2H4·H2O/EO), temperature and reaction time, was conducted. Additionally, response surface methodology was employed to optimize the reaction conditions, illuminating the intricate interactions among the specified variables and achieving a high HEH yield of 96.97 %. Finally, a kinetic model was established, and the activation energies for the main reaction and side reaction were measured to be 38.31 kJ·mol−1 and 44.94 kJ·mol−1, respectively. This kinetic model can enable the accurate determination of kinetic parameters and provide guidance for the process intensification of this EO addition reaction.

The rheology of rhyolite magma from the IDDP-1 borehole and Hrafntinnuhryggur (Krafla, Iceland) with implications for geothermal drilling

Changes in rhyolite melt viscosity during magma decompression and degassing exert a first order control on ascent through the crust and volcanic eruption style. These changes have as yet unknown hazard implications for geothermal drilling in pursuit of particularly hot fluids close to magma storage regions. Here, we exploit the situation at Krafla volcano in which rhyolite has both erupted at Earth's surface and been sampled at shallow storage depths via drilling of the 2009 IDDP-1 and 2008 KJ-39 boreholes. We use differential scanning calorimetry to constrain that the IDDP-1 magma quenched to glass at ∼ 700 K, at a rate of between 7 and 80 K.min−1. We measure the equilibrium viscosity of the IDDP-1 rhyolite at temperatures close to the glass transition interval and show that the rhyolite viscosity is consistent with generalized viscosity models assuming a dissolved H2O concentration of 2.12 wt%. We couple these results with micro-penetration and concentric cylinder rheometry over a range of potential magma storage temperatures to constrain the response of surficial Krafla rhyolites to stress. The surficial rhyolites at Krafla match the same viscosity model, assuming a lower dissolved H2O concentration of 0.12 wt%. Our results show that at a storage temperature of 1123–1193 K, the viscosity of the stored magma is ∼ 3×105 Pa.s. At the same temperature, the viscosity following degassing during ascent to the surface rises to ∼ 2×109 Pa.s. Finally, we use high-stress compression tests on the Hrafntinnuhryggur surface obsidian to determine the onset of unrelaxed behavior and viscoelastic melt rupture or fragmentation pertinent to understanding the melt response to rapid pressure changes that may be associated with further (near-) magma exploration at Krafla. Taken together, we characterize the relaxation and viscosity of these magmas from source-to-surface.

Volatiles and Redox Along the East African Rift

AbstractThe upper mantle under the Afar Depression in the East African Rift displays some of the slowest seismic wave speeds observed globally. Despite the extreme nature of the geophysical anomaly, lavas that erupted along the East African Rift record modest thermal anomalies. We present measurements of major elements, H2O, S, and CO2, and Fe3+/ΣFe and S6+/ΣS in submarine glasses from the Gulf of Aden seafloor spreading center and olivine‐, plagioclase‐, and pyroxene‐hosted melt inclusions from Erta Ale volcano in the Afar Depression. We combine these measurements with literature data to place constraints on the temperature, H2O, and fO2 of the mantle sources of these lavas as well as the initial and final pressures of melting. The Afar mantle plume is C/FOZO/PHEM in isotopic composition, and we suggest that this mantle component is damp, with 852 ± 167 ppm H2O, not elevated in fO2 compared to the depleted MORB mantle, and has temperatures of ∼1401–1458°C. This is similar in fO2 and H2O to the estimates of C/FOZO/PHEM in other locations. Using the moderate H2O contents of the mantle together with the moderate thermal anomaly, we find that melting begins at around 93 km depth and ceases at around 63 km depth under the Afar Depression and at around 37 km depth under the Gulf of Aden, and that ∼1%–29% partial melts of the mantle can be generated under these conditions. We speculate that the presence of melt, and not elevated temperatures or high H2O contents, are the cause for the prominent geophysical anomaly observed in this region.

Quantification of organic and inorganic hydrogen in mudstones: a novel approach using the difference between organic-rich and organic-free mudstones during pyrolysis process

Whether mudstone is rich in or free of organic matter has a great influence on the occurrence of water. Comparing different types of water in organic-rich and organic-free mudstones is helpful for further understanding the role of water in hydrocarbon generation. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) combined with mass spectrometry (MS) afford the opportunity to identify the mass change, reactions and products of the sample in a real-time monitored heating process. This study compared the pyrolysis characteristics of an organic-rich mudstone (CN1) and an organic-free mudstone (CW1) by using the TGA/DTA-MS method to estimate the content of different types of H2O and CO2 in organic-rich mudstones. The results show that the mass changes in CN1 and CW1 can be divided into the three thermogravimetric (TG) stages of 0°C–200°C, 200°C–650°C, and 650°C–900°C, while the peak temperatures of H2O and CO2 obtained through MS are different for CN1 and CW1. The differences in mineral components and organic matter between CN1 and CW1 suggest that the MS peaks of H2O and CO2 in CW1 are mainly influenced by clay and carbonate minerals, and that those of CN1 are also influenced by organic matter. In addition, quantification equations for CO2 and H2O contents from both the organic and inorganic origin of the organic-rich mudstone can be established by using the MS peak area of CO2 and H2O, mass loss in TGA and the mineral composition of the organic-free mudstone. This work provides useful insights for further understanding the hydrocarbon generation mechanism, as well as quantifying different types of water in organic-rich mudstones.

Mechanical Impact of Heterogeneously Distributed H2O on Quartz Deformation

AbstractIn order to identify relations between mechanical behavior, deformation mechanisms, microstructural properties, and H2O distribution, Tana‐quartzite samples with added H2O ranging from 0 to 0.5 wt.% were deformed by axial shortening at constant displacement rates, at 900°C and 1 GPa, reaching up to ∼30% bulk strain. Samples with lower quantities of added H2O (0.1 and 0.2 wt.%) were in average ∼30 MPa weaker than the as‐is samples with no added H2O. In contrast, samples with more than 0.2 wt.% added H2O revealed more variable mechanical behavior, showing either weaker or stronger trend. The weaker samples showed strain localization in their central parts in the vicinity of the thermocouple, that is, the hottest parts of the samples, whereas the stronger samples showed localization in their upper, slightly colder parts. Bulk deformation is accommodated by crystal plasticity and dissolution‐precipitation processes. Distribution of H2O in our samples revealed systematic decrease of H2O content in the interiors of original grains, caused by increasing strain and H2O draining into grain boundary regions. With increasing content of added H2O, the quartz recrystallization gradually changes from subgrain‐rotation‐dominated to crack‐induced nucleation, along with increasing quantity of melt/fluid pockets. The unexpected strain localization in the upper parts of stronger samples most likely results from mode‐1‐cracking that led to drainage of grain boundaries (GB) due to the crack dilatancy effect, and inhibited dissolution‐precipitation in the hottest part of the samples next to the thermocouple. The locus of deformation is then shifted to colder regions where more H2O is available along GB.

Ultramafic melt viscosity: A model

A non-Arrhenian model for the Newtonian viscosity (η) of ultramafic melts is presented. The model predicts the viscosity of ultramafic melts as a function of temperature (T), pressure (P), H2O content and for a range of melt compositions (70 < Mg# < 100). The calibration consists of 63 viscosity measurements at ambient pressure for 20 individual melt compositions and 5 high-P measurements on a single melt composition, all drawn from the literature. The data span 14 orders of magnitude of η (10−2 to 1011.8 Pa s), a T range of 880 to 2700 K, pressures from 1 atm to 25 GPa, and include measurements on hydrous melts containing 0.2 to 4.4 wt.% H2O. The T-dependence of viscosity is modelled with the VFT equation [log η = A+B/(T(K) − C)] whereby A is assumed to be a common, high-T limit for these melt compositions (i.e. log η∞ = -5.4). The pressure and composition effects are parameterised in terms of 5 adjustable parameters in expanded forms of B and C. The viscosity model is continuous across T-P-composition space and can predict ancillary transport properties including glass transition temperatures (Tg) and melt fragility (m). Melt viscosity decreases markedly with increasing H2O content but increases significantly with increasing pressure and decreasing Mg# (i.e. higher Fe-content). We show strong systematic decreases in Tg and m with increasing H2O content whereas an increase in P causes a rise in Tg and decrease in m. The predictive capacity of this model for ultramafic melt viscosity makes it pertinent to the fields of volcanology, geophysics, petrology, and the material sciences. Moreover, it provides constraints on models of magma oceans on terrestrial planets and, the evolution of planetary atmospheres via magmatic degassing on exoplanets.

An investigation on the H2O, unburned H2 and NO emission characteristics from a direct injection hydrogen engine

Hydrogen engines show great potential as a power source with good combustion performance, zero carbon emissions, and diverse sources. However, the exhaust emission characteristics with high water vapor (H2O) content have not yet been clarified. This study presents the exhaust emissions of a heavy-duty direct-injection hydrogen engine under various operational parameters, with a focus on the H2O content. The results reveal important exhaust emissions characteristics of the hydrogen engine, including lower exhaust temperature (200 °C–400 °C), higher unburned H2 emissions (up to 17100 ppm), less nitrogen oxide (NO) generation (<1000 ppm), and higher H2O content (6%–40%). Additionally, unburned H2 rises, and NO and H2O decrease as the excess air ratio (λ) increases. When λ is greater than 3, NO levels fall sharply to below 100 ppm. Optimal emissions performance is achieved with a 5 °CA BTDC ignition timing where low levels of H2, H2O, and NO are maintained. Increasing hydrogen injection pressure decreases H2 but raises exhaust temperature, H2O, and NO emissions. Excessively delaying hydrogen injection timing adversely affects the concentration gradient in the cylinder, increasing unburned H2 emissions. The best emissions performance is observed at a hydrogen injection timing of 110 °CA BTDC.

ND70 Series Basaltic Glass Reference Materials for Volatile Element (H2O, CO2, S, Cl, F) Measurement and the C Ionisation Efficiency Suppression Effect of Water in Silicate Glasses in SIMS

We present a new set of reference materials, the ND70‐series, for in situ measurement of volatile elements (H2O, CO2, S, Cl, F) in silicate glass of basaltic composition. The materials were synthesised in piston cylinders at pressures of 1 to 1.5 GPa under volatile‐undersaturated conditions. They span mass fractions from 0 to 6% m/m H2O, from 0 to 1.6% m/m CO2 and from 0 to 1% m/m S, Cl and F. The materials were characterised by elastic recoil detection analysis for H2O, by nuclear reaction analysis for CO2, by elemental analyser for CO2, by Fourier transform infrared spectroscopy for H2O and CO2, by secondary ion mass spectrometry for H2O, CO2, S, Cl and F, and by electron probe microanalysis for CO2, S, Cl and major elements. Comparison between expected and measured volatile amounts across techniques and institutions is excellent. It was found however that SIMS measurements of CO2 mass fractions using either Cs+ or O− primary beams are strongly affected by the glass H2O content. Reference materials have been made available to users at ion probe facilities in the US, Europe and Japan. Remaining reference materials are preserved at the Smithsonian National Museum of Natural History where they are freely available on loan to any researcher.

New Insights into the Internal Structure of GJ 1214 b Informed by JWST

Recent JWST observations of the sub-Neptune GJ 1214 b suggest that it hosts a high-metallicity (≳100× solar), hazy atmosphere. Emission spectra of the planet show molecular absorption features, most likely due to atmospheric H2O. In light of this new information, we conduct a thorough reevaluation of the planet’s internal structure. We consider interior models with mixed H/He/H2O envelopes of varying composition, informed by atmospheric constraints from the JWST phase curve, in order to determine possible bulk compositions and internal structures. Self-consistent atmospheric models consistent with the JWST observations are used to set boundary conditions for the interior. We find that a total envelope mass fraction of at least 8.1% is required to explain the planet’s mass and radius. Regardless of H2O content, the maximum H/He mass fraction of the planet is 5.8%. We find that a 1:1 ice-to-rock ratio along with 3.4%–4.8% H/He is also a permissible solution. In addition, we consider a pure H2O (steam) envelope and find that such a scenario is possible, albeit with a high ice-to-rock ratio of at least 3.76:1, which may be unrealistic from a planet formation standpoint. We discuss possible formation pathways for the different internal structures that are consistent with observations. Since our results depend strongly on the atmospheric composition and haze properties, more precise observations of the planet’s atmosphere would allow for further constraints on its internal structure. This type of analysis can be applied to any sub-Neptune with atmospheric constraints to better understand its interior.

Fast, furious, and gassy: Etna's explosive eruption from the mantle

The 3930 BP Fall Stratified (FS) eruption at Mt. Etna is a rare example of a highly explosive eruption of primitive (picritic) magma directly from the mantle. The eruption produced ash plumes up to an estimated 20 km height, leading to a volcanic explosivity index (VEI) 4 (subplinian). Given its volatile-rich and primitive nature, the FS magma may have ascended rapidly from great depths to avoid fractionation and mixing within the extensive plumbing system beneath Etna. To determine the pressures from which the FS magma derived, we perform rehomogenization experiments on melt inclusions hosted in Fo90–91 olivines to resorb shrinkage bubbles and determine the initial H2O and CO2 in the melt. With measured CO2 concentrations of up to 9600 ppm, volatile solubility models yield magma storage pressures of 630–800 MPa. These correspond to depths of 24–30 km, which are comparable to the seismologically estimated Moho. Therefore, the magma's high CO2 concentration must come from carbon in the mantle (likely from subducted carbonates), as opposed to assimilation of shallow (<10 km) crustal carbonates.Diffusion modeling of H2O and forsterite zonation profiles in clear, euhedral, and crystallographically oriented olivines indicates rapid ascent of magma directly from its source region to the surface. Forsterite profiles exhibit a narrow rim of growth zoning but no detectable diffusional zoning, reflecting maximum ascent times of 1–5 days. Eighteen measured H2O profiles result in remarkably uniform decompression rates of 0.47 MPa/s (95% confidence interval of 0.16–1.28 MPa/s), which is among the fastest measured for basaltic-intermediate magmas. These decompression rates indicate that the final stage of magma ascent over the region in which H2O degasses (between the surface and ∼ 15 km) occurred extremely fast at ∼ 17.5 m/s. This eruption may provide a link between primary magma composition and eruption intensity: we propose that the unusually explosive nature of this picritic eruption was driven by high H2O and CO2 concentrations, which led to continuously rapid ascent without stalling, all the way from the Moho.

Experimental constraints on iron and sulfur redox equilibria and kinetics in basaltic melt inclusions

The oxidation states of dissolved iron (Fe) and sulfur (S) in silicate melts were studied experimentally by heating Hawaiian olivines containing S-bearing melt inclusions (MIs) in a 1 atm gas-mixing furnace. The MIs are closed with respect to S and C, but equilibrate with the oxygen fugacity (ƒO2) and water fugacity (ƒH2O) of the furnace. We also conducted experiments using a S-free synthetic Hawaiian MI composition held on Pt-loops. Experiments were run at 1225 °C for 8–96 hr in H2-CO2 gas mixtures corresponding to ƒO2 values that varied over seven orders of magnitude and then drop-quenched into water. Time-series experiments show that MI Fe3+/Fe2+ and S6+/S2− ratios reached constant values within ∼24 hr. The H2O contents of the Pt-loop glasses and the dehydrated MIs overlap and are proportional to the fH2O in the furnace gas. The slope of log10(Fe3+/Fe2+) vs. log10ƒO2 is 0.25±0.02(2σ) for the Pt-loop glasses and 0.23±0.01(2σ) for the quenched MIs; the slopes of the two sets of experiments are consistent with equilibrium having been reached via the reaction FeO + ¼ O2 = FeO1.5. S-bearing quenched MI glasses have Fe3+/Fe2+ ratios that are systematically low compared to ratios measured in the S-free Pt-loop glasses run at the same T and ƒO2; this may reflect an effect of S on Fe3+/Fe2+, although further experiments are needed to explore this possibility. S6+/S2− ratios in the experimental melt inclusions vary systematically as a function of ƒO2, and the measured ratios agree with independent calculations of S oxidation state in basaltic melts.After being held at 1225 °C for 24 or 48 hr, a subset of olivine-hosted MIs were cooled at rates of 1700–40,000 °C/hr and quenched at temperatures between 900 °C and 1150 °C to explore whether Fe and S speciation are modified during cooling. Fe and S speciation in isothermal and cooled MIs held initially at the same ƒO2 overlap within uncertainty, suggesting that the T dependence of the homogeneous reaction 8Fe3+ + S2− = 8Fe2+ + S6+ is small and/or its kinetics are sluggish relative to cooling rates required to quench basaltic liquids to glasses. The centers of unheated, naturally quenched MIs from Papakōlea, Hawaii with syneruptive cooling rates ranging from ∼50 to 12,000 °C/hr were also measured for Fe and S speciation. MIs that experienced the lowest cooling rates have higher Fe3+/Fe2+ and S6+/S2− ratios relative to rapidly cooled inclusions of comparable size. The increase in Fe3+/Fe2+ is due to olivine crystallization on the inclusion walls during cooling and diffusion of the resulting oxidized boundary layer into MI interiors. Diffusive modification is minimized in MIs that have been rapidly cooled and/or are sufficiently large such that the oxidized boundary layer is restricted to a narrow region adjacent to the inclusion walls. Our experiments and their comparison to naturally quenched MIs show that measurements of Fe3+/Fe2+ and S6+/S2− ratios in quenched basaltic glasses and in the centers of rapidly cooled and/or large MIs are likely to be representative of the redox state in the melt at magmatic temperatures prior to quenching to glass.

Separation and concentration of CO2 from air using a humidity-driven molten-carbonate membrane

Separation processes are substantially more difficult when the species to be separated is highly dilute. To perform any dilute separation, thermodynamic and kinetic limitations must be overcome. Here we report a molten-carbonate membrane that can ‘pump’ CO2 from a 400 ppm input stream (representative of air) to an output stream with a higher concentration of CO2, by exploiting ambient energy in the form of a humidity difference. The substantial H2O concentration difference across the membrane drives CO2 permeation ‘uphill’ against its own concentration difference, analogous to active transport in biological membranes. The introduction of this H2O concentration difference also results in a kinetic enhancement that boosts the CO2 flux by an order of magnitude even as the CO2 input stream concentration is decreased by three orders of magnitude from 50% to 400 ppm. Computational modelling shows that this enhancement is due to the H2O-mediated formation of carriers within the molten salt that facilitate rapid CO2 transport.

CO2 Fixation to Prebiotic Intermediates over Heterogeneous Catalysts.

ConspectusThe study of the origin of life requires a multifaceted approach to understanding where and how life arose on Earth. One of the most compelling hypotheses is the chemosynthetic origin of life at hydrothermal vents, as this condition has been considered viable for early forms of life. The continuous production of H2 and heat by serpentinization generates reductive conditions at hydrothermal vents, in which CO2 can be used to build large biomolecules. Although this involves surface catalysis and an autocatalytic process, in which solid minerals act as catalysts in the conversion of CO2 to metabolically important organic molecules, the systematic investigation of heterogeneous catalysis to comprehend prebiotic chemistry at hydrothermal vents has not been undertaken.In this Account, we discuss geochemical CO2 fixation to metabolic intermediates by synthetic minerals at hydrothermal vents from the perspective of heterogeneous catalysis. Ni and Fe are the most abundant transition metals at hydrothermal vents and occur in the active site of the enzymes carbon monoxide dehydrogenases/acetyl coenzyme A synthases (CODH/ACS). Synthetic free-standing NiFe alloy nanoparticles can convert CO2 to acetyl coenzyme A pathway intermediates such as formate, acetate, and pyruvate. The same alloy can further convert pyruvate to citramalate, which is essential in the biological citramalate pathway. Thermal treatment of Ni3Fe nanoparticles under NH3, which can occur in hydrothermal vents, results in Ni3FeN/Ni3Fe heterostructures. This catalyst has been demonstrated to produce prebiotic formamide and acetamide from CO2 and H2O using Ni3FeN/Ni3Fe as both substrate and catalyst. In the process of serpentinization, Co can be reduced in the vicinity of olivine, a Mg-Fe silicate mineral. This produces CoFe and CoFe2 with serpentine in nature, representing SiO2-supported CoFe alloys. In mimicking these natural minerals, synthetic SiO2-supported CoFe alloys demonstrate the same liquid products as NiFe alloys, namely, formate, acetate, and pyruvate under mild hydrothermal vent conditions. In contrast to the NiFe system, hydrocarbons up to C6 were detected in the gas phase, which is also present in hydrothermal vents. The addition of alkali and alkaline-earth metals to the catalysts results in enhanced formate concentration, playing a promotional role in CO2 reduction. Finally, Co was loaded onto ordered mesoporous SiO2 after modification with cations to simulate the minerals found in hydrothermal vents. These catalysts were then investigated under diminished H2O concentration, revealing the conversion of CO2 to CO, CH4, methanol, and acetate. Notably, the selectivity to metabolically relevant methanol was enhanced in the presence of cations that could generate and stabilize the methoxy intermediate. Calculation using the machine learning approach revealed the possibility of predicting the selectivity of CO2 fixation when modifying mesoporous SiO2 supports with heterocations. Our research demonstrates that minerals at hydrothermal vents can convert CO2 into metabolites under a variety of prebiotic conditions, potentially paving the way for modern biological CO2 fixation processes.

Inhibited hydrogen uptake in metasomatised cratonic eclogite

Water occurs in Earth’s interior mostly as trace hydroxyl in nominally anhydrous minerals. Clinopyroxene is known to be an important water carrier in the uppermost mantle, and eclogite, which forms a subordinate part of the cratonic lithosphere, contains some 50% of jadeite-rich clinopyroxene, making this potentially a significant H2O reservoir in the bulk lithospheric mantle. Mantle metasomatism, in particular by small-volume melts like kimberlite, is known to enrich the lithosphere in highly incompatible components, but its effect on H2O contents in cratonic eclogite remains unclear. We report H2O concentrations for clinopyroxene and garnet in eclogite and pyroxenite xenoliths from several African kimberlites, obtained by Fourier-Transform Infrared Spectroscopy (FTIR). Except one sample showing evidence for minor within-grain variability of H2O concentrations (< 15%), FTIR images demonstrate that H2O is homogeneously distributed in optically clear areas of clinopyroxene fragments mounted for this study. The samples were variably metasomatised by a kimberlite-like melt, as evidenced by elevated MgO contents and abundances of highly incompatible elements (e.g., Sr, Ce, Th). Although metasomatised eclogites and pyroxenites on average show higher H2O abundances than pristine ones, mantle metasomatism decreases the Al2O3 content in clinopyroxene, which is known to enhance hydrogen incorporation in this mineral. As a consequence, hydrogen incorporation is inhibited, and c(H2O) becomes increasingly decoupled from other highly incompatible components, such as LREE. Thus, eclogite – metasomatised or not - does not significantly contribute to the H2O inventory in the bulk cratonic mantle.

Selective and stable CO2 electroreduction at high rates via control of local H2O/CO2 ratio

Controlling the concentrations of H2O and CO2 at the reaction interface is crucial for achieving efficient electrochemical CO2 reduction. However, precise control of these variables during catalysis remains challenging, and the underlying mechanisms are not fully understood. Herein, guided by a multi-physics model, we demonstrate that tuning the local H2O/CO2 concentrations is achievable by thin polymer coatings on the catalyst surface. Beyond the often-explored hydrophobicity, polymer properties of gas permeability and water-uptake ability are even more critical for this purpose. With these insights, we achieve CO2 reduction on copper with Faradaic efficiency exceeding 87% towards multi-carbon products at a high current density of −2 A cm−2. Encouraging cathodic energy efficiency (>50%) is also observed at this high current density due to the substantially reduced cathodic potential. Additionally, we demonstrate stable CO2 reduction for over 150 h at practically relevant current densities owning to the robust reaction interface. Moreover, this strategy has been extended to membrane electrode assemblies and other catalysts for CO2 reduction. Our findings underscore the significance of fine-tuning the local H2O/CO2 balance for future CO2 reduction applications.

Hydration of mafic crustal rocks at high temperature during brittle‐viscous deformation along a strike‐slip plate boundary

AbstractThe Poroshiri ophiolite (Hidaka metamorphic belt, Japan) occurs within a crustal‐scale network of high‐temperature, dextral shear zones that accommodated hundreds of kilometres of displacement due to the opening of the Japan Sea in the Neogene. The opholitic rocks comprise ultramafic, mafic and sedimentary protoliths that have been variably hydrated and metamorphosed. This work investigates the mechanisms and timing of fluid influx relative to viscous deformation in metagabbros and amphibolites deformed during exhumation from granulite‐ to amphibolite‐facies conditions. We consider a range of microstructures, from low strain domains and 1–2 mm thick shear bands to mylonites with a thickness of a few meters. Low strain domains of metagabbros exhibit corona textures with symplectites consisting of pargasitic amphibole (Ed0.7) ‐ anorthitic plagioclase (An80–92) ± orthopyroxene ± clinopyroxene forming around olivine and igneous pyroxene and with similar plagioclase–amphibole‐orthopyroxene ± clinopyroxene granoblastic aggregates in micrometric‐thick fractures. These textures result from hydration under low fluid‐rock ratio, with elevated H2O content only occurring locally (H2O &gt; 1–1.2 wt%). Igneous mineral replacement leads to grain size reduction from 1 mm to ~10 μm. Amphibole exhibits a strong core‐rim zonation primarily controlled by the high diffusivity of Fe, Mg and OH and the low diffusivity of Al. Mineral compositional equilibrium is achieved at the scale of 100–200 μm. In mm‐thick localized shear bands and in metric‐scale mylonitic amphibolites, the heterogeneous mineral composition of amphibole (Ed0.2–0.5) and plagioclase (An40−An80) indicates only partial re‐equilibration (at the scale of 200–500 μm) despite higher fluid‐rock ratios and more pervasive fluid percolation than in metagabbros. Plagioclase–amphibole thermometry and equilibrium phase diagrams indicate that the initial fluid infiltration and corona formation occurred at 800–850°C by fracturing and percolation along grain boundaries. This was followed by the main fluid percolation and mylonitization event, which occurred during exhumation and cooling at conditions of 720–580°C, ~4 kbar. Continuous hydration during retrogression was achieved by the influx of dominantly seawater‐derived fluid, as attested by the high chlorine (Cl) contents (&gt;300–400 ppm) of amphiboles in fractures. The heterogeneous distribution of fractures controls the distribution of fluid from the earliest stages of hydration, creating positive feedback where the growth of hydrous minerals as amphibole (up to +300 vol% of amphibole in high strain areas relative to the low‐strain ones) and the formation of fine‐grained mixed domains that led to further localization of viscous strain and mass transfer (variations of ± 30–40% in major elements). The Poroshiri ophiolite therefore represents a good fossil example of a former transpressional plate boundary where coupled metamorphic and deformation processes were triggered by seawater‐derived fluid that percolated to depths of ~15 km.

Following fuel conversion during biomass gasification using tunable diode laser absorption spectroscopy diagnostics

The efficiency of the gasification process and product quality largely depend on the degree of fuel conversion. We present the real-time in-situ tunable diode laser measurements of main carbon and oxygen-containing species in the hot reactor core of a pilot-scale entrained flow biomass gasifier (EFG). The concentrations of CO, CO2, CH4, C2H2, H2O, soot, and gas temperature were measured during the air and oxygen-enriched gasification of stem wood at varying equivalence ratios. The experiments were made at the upper and lower optical ports inside a 4 m long, ceramic-lined, atmospheric EFG, allowing to access the degree of the fuel conversion inside the reactor. The exhaust composition was measured by micro-GC, FTIR, and low-pressure impactor. There was a good agreement between the data measured inside the reactor and at the exhaust for oxygen-enriched gasification implying that the chemical reactions are practically frozen downstream the optical ports. For air, the data indicated that the gasification reactions are still active at the measurement locations. Significant concentrations of C2H2, up to 5000 ppm, were found inside the reactor.