H2O CO2 Research Articles

Direct Measurement of the Four-Phase Equilibrium Coexistence Vapor-Aqueous Solution-Ice-Gas Hydrate in Water-Carbon Dioxide System.

Precise data on the non-variant equilibrium of the four phases (vapor-aqueous solution-ice-gas hydrate) in P-T coordinates are highly desired for developing accurate thermodynamic models and can be used as reference points (similar to the triple point of water). Using the two-component hydrate-forming system CO2-H2O, we have proposed and validated a new express procedure for determining the temperature and pressure of the lower quadruple point Q1. The essence of the method is the direct measurement of these parameters after the successive formation of the gas hydrate and ice phases in the initial two-phase gas-water solution system under intense agitation of the fluids. After relaxation, the system occurs in the same equilibrium state (T = 271.60 K, P = 1.044 MPa), regardless of the initial parameters and the order of crystallization of the CO2 hydrate and ice phases. Considering the combined standard uncertainties (±0.023 K, ±0.021 MPa), the determined P and T values agree with the results of other authors obtained by a more sophisticated indirect method. Validating the developed approach for systems with other hydrate-forming gases is of great interest.

A Comparative Study of the Performance and Electrode Processes of Solid Oxide H2O Electrolysis and CO2 Electrolysis

Solid oxide CO2 electrolysis was less studied compared with H2O electrolysis and the relevant reaction mechanism and electrode processes were poorly understood. This paper investigated the performance and electrode processes differences between CO2 electrolysis and H2O electrolysis. The shape of j-V curves, electrochemical impedance spectroscopy data and calculated distribution relaxation time results were analyzed. It was found that the performance of CO2 electrolysis degraded the most compared with that in SOFC mode and for H2O electrolysis after a 65 h’ operation. Meanwhile, j-V curve of CO2 electrolysis changed a lot and finally showed a convex total resistance pattern with applied current. It was also found that the characteristic frequency of charge transfer reaction and diffusion process for CO2 electrolysis was much lower than that of H2O electrolysis. A thermal activated peak at 10-100 Hz which was not noted for H2O electrolysis was considered to be CO2 adsorption and dissociation process in fuel electrode. Besides, diffusion process for H2O-H2 and CO2-CO mixture also differed. Finally, it was indicated that the different performance degradation may be due to different suppressed degree of charge transfer reaction in fuel electrode for H2O electrolysis and CO2 electrolysis and additional suppressed adsorption or dissociation process of CO2 for CO2 electrolysis.

Interactions of CO2–H2O-coal and its impact on micro mechanical strength of coal

High temperature flue gas enhanced coalbed methane (CBM) recovery is a potential technique for improving CBM recovery efficiency and carbon sequestration, and it is of significance to carbon emission reduction in coal mine area. Previous research has demonstrated that the injection of CO2 could markedly alter the mechanical properties of coal seam, which further changes the gas flow and long-term safety of CO2 storage. Aiming at this issue, we studied the coupling effect of CO2–H2O on micro mechanical properties of coal with nanoindentation method. And the pore-fracture structure and minerals of the target sample was systematically characterized with LF-NMR and SEM to explore the influence mechanism of CO2–H2O on coal. The results show that (1) the micro strength of coal is obviously weakened after the treatment of CO2–H2O, and with an increase of the time, both the hardness and reduced modulus decreases, while the indentation depth shows an opposite trend. (2) Compared with the control group, the volumes of micropore, minipore, mesopore and macropore/microfracture in coal treated by CO2–H2O all increase in varying degrees. The increase of micropore volume is generally less than 20%. While the increases of the volumes of mesopore and macropore/microfractures are significant, with the maximum increase of 44.13% for the mesopore, and 299.58% for the macropore/microfracture. (3) Ulan coal contains a large number of calcareous minerals, which undergo chemical reaction after encountering the acid solution formed by CO2–H2O, resulting in dissolution of part of the minerals, which can be evidenced by the corrosion pits observed on the coal surface. (4) The injection of CO2 results in nonuniform swelling of the coal, which further leads to the formation of new cracks and the resultant coal strength deterioration. In addition, the acid solution formed by CO2–H2O reacts with the minerals in coal, resulting in dissolution of carbonate and other minerals in the coal, thus weakening the coal strength. The research results in this work provide basic support for the evaluation of the effectiveness of high temperature flue gas enhanced CBM and the safety of long-term CO2 storage.

ZnxZrO2+x-Based Photocatalyst with Dual Active Sites for Tunable Syngas Production from CO2 Photoreduction

Photocatalytic reduction of CO2 in H2O to CO and H2 for syngas production is an attractive option to solve global warming and generate a feedstock for the Fischer–Tropsch process. The syngas generated from CO2 photoreduction with a precise and controllable ratio of H2/CO remains a major challenge to date. In this work, CdS-sensitized nanoscale ZnxZrO2+x solid solutions with dual active sites were synthesized as photocatalysts for tunable syngas H2 and CO production from CO2 photoreduction. The staggered band energy alignments, electron paramagnetic resonance results, and the site of palladium/gold nanoparticles by photodeposition indicate that the charge transfer in the CdS/ZnxZrO2+x heterojunction mainly follows the conventional type II mechanism. Density functional theoretical calculation results reveal that Zn and Zr sites on ZnxZrO2+x are favorable for the adsorption of H2O and CO2 molecules and generation of H2 and CO, respectively. The H2/CO ratio can be controlled in the range from 0.9 to 25.0 by adjusting the content of Zn in ZnxZrO2+x. Moreover, syngas production can be further improved by the modification of Pd nanoparticles. For the first time, this work proposes a strategy to construct a photocatalyst with dual active sites for effectively controlling the H2/CO ratio in syngas synthesis through a fundamental understanding of adsorption and catalytic reaction sites in both H2 evolution and CO2 reduction reactions.

Dew benefits on alpine grasslands are cancelled out by combined heatwave and drought stress.

Increasing frequencies of heatwaves combined with simultaneous drought stress in Europe threaten the ecosystem water and carbon budgets of alpine grasslands. Dew as an additional water source can promote ecosystem carbon assimilation. It is known that grassland ecosystems keep high evapotranspiration as long as soil water is available. However, it is rarely being investigated whether dew can mitigate the impact of such extreme climatic events on grassland ecosystem carbon and water exchange. Here we use stable isotopes in meteoric waters and leaf sugars, eddy covariance fluxes for H2O vapor and CO2, in combination with meteorological and plant physiological measurements, to investigate the combined effect of dew and heat-drought stress on plant water status and net ecosystem production (NEP) in an alpine grassland (2000m elevation) during the June 2019 European heatwave. Before the heatwave, enhanced NEP in the early morning hours can be attributed to leaf wetting by dew. However, dew benefits on NEP were cancelled out by the heatwave, due to the minor contribution of dew in leaf water. Heat-induced reduction in NEP was intensified by the combined effect of drought stress. The recovery of NEP after the peak of the heatwave could be linked to the refilling of plant tissues during nighttime. Among-genera differences of plant water status affected by dew and heat-drought stress can be attributed to differences in their foliar dew water uptake, and their reliance on soil moisture or the impact of the atmospheric evaporative demand. Our results indicate that dew influence on alpine grassland ecosystems varies according to the environmental stress and plant physiology.

Enhanced Adsorption and Dissociation of H2O Facilitates Photocatalytic CO2 Reduction on Defective BiSbO4

The dissociation of H2O molecules plays an important role in producing protons for promoting hydrogenation reduction of CO2. Therefore, developing excellent photocatalysts to promote the dissociation of H2O molecules and reveal the effect mechanism is the key research direction to enhance CO2 reduction efficiency. In this work, we synthesized BiSbO4 with oxygen vacancies (OVs) to investigate the photocatalytic mechanism of CO2 conversion and H2O dissociation. Experimental results indicate that the introduction of OVs into BiSbO4 can enhance the absorption capacity of light, narrowing band gaps and impeding the recombination of photogenerated carriers. Importantly, the existence of OVs in BiSbO4 can promote the adsorption and activation of H2O molecules via the stronger covalent interaction, which facilitates the dissociation of H2O molecules to generate more protons and lowers the energy barriers of •COOH to improve the efficiency of CO2 reduction. This study reinforces our understanding of the principle of photocatalytic CO2 reduction and the important role of H2O dissociation in photocatalytic CO2 conversion.

Stability of Carbonates during Subduction: Influence of the Dehydration Regime of Chlorine-Bearing Metapelite

It was shown that, at a pressure of 3.0, 5.5, and 7.8 GPa and a temperature of 750–1030°C, a set of reactions occurred in carbonate and Cl-bearing pelite that is finally converted into an eclogite-like assemblage and formed a H2O–CO2 Cl-bearing fluid. The eclogite-like assemblage remains stable when the P–T conditions change concordantly with hot subduction geotherms, whereas carbonate is completely dissolved in the fluid already at ≥5.5 GPa. The CO2 content in the quenched fluid reaches 20–30 wt %. However, preliminary defluidization of pelite at 3.0 GPa and 750°С leads to chlorine removal and carbonate stabilization at 5.5 GPa and at 7.8 GPa in equilibrium with the next chlorine-free portions of the fluid. Comparison of the data available for simplified model systems and new data on carbonate and Cl-bearing pelite indicate that chlorine fluid essentially contributes to carbonate dissolution in the fluid. Thus, the stability of carbonates under P–T conditions typical of subduction zones is dependent on the behavior of chlorine during defluidization of marine sediments.

C–O–H fluid-melt-rock interaction in graphitic granulites and problems of quantifying carbon budget in the lower continental crust

Estimates on the geological carbon cycle are subject to large uncertainties that can be reduced by thorough observation of rocks. In this contribution, we focus specifically on C–O–H fluid-melt-rock interactions in graphitic metapelitic granulites and on their bearing to the carbon budget of granulitic roots of continents. We provide robust microstructural and thermometric constraints on the coexistence of anatectic silicate melts and C–O–H fluids up to near ultrahigh temperature conditions in the archetypal crustal section of Ivrea-Verbano Zone (IVZ, Italian Alps). Fluid inclusions in garnet are investigated before and after high-temperature experiments, and contain considerable proportions of CO2, CH4, N2, but lower H2O than predicted for graphitic systems at granulite facies. When comparing and contrasting the melt compositions obtained by Perple_X and rhyolite-MELTS with natural melts from IVZ, a much better match is obtained by the former, questioning the choice of rhyolite-MELTS for modelling melting equilibria of metasedimentary rocks and for quantifying carbon budget of the lower crust. Overall, data show that assuming only a limited extent of fluid-melt immiscibility in the deep crust contradicts the evidence from natural rocks and prompts to an incomplete view of actual carbon behavior and carbonic fluids. The available experimental dataset on CO2 solubility in felsic melts cannot be used to interpret the volatile budget of melt inclusions in graphitic migmatites and granulites, as most solubility experiments were conducted under carbonate-saturated (i.e. highly oxidizing) conditions which maximize CO2 content of melt, compared to graphitic (i.e. more reducing) protoliths. As a consequence, thermodynamic models still cannot account for all the complexities related with interactions among H2O–CO2–CH4 ternary fluids, H2O- and CO2-bearing anatectic melts and graphite-bearing residues in graphitic metapelites. Targeted experimental studies are therefore crucial to boost substantial computational efforts, before any precise estimates on carbon budget and fluxes in the lower anatectic crust can be made.

G‐C3N4 Nanosheet Nanoarchitectonics: H2 Generation and CO2 Reduction

AbstractFew‐layered graphitic carbon nitride (g‐C3N4) has been considered a popular base for constructing composite materials in photocatalytic CO2 reduction and H2 generation applications, particularly for constructing g‐C3N4 based composites based on the idea of nanoarchitectonics. The photocatalytic activity of these constructed g‐C3N4 composites relies on the composition, morphology, microstructure of the material. This review aims to focus on the summary of latest progress in the development of few‐layered g‐C3N4 nanosheets based noble metal clusters/nanoparticles and other semiconductor nanomaterials modified composite materials constructed using the technology of nanoarchitectonics for photocatalytic H2 generation and CO2 reduction. This review covers a brief introduction of the enhanced photogenerated carrier separation and transfer as well as extended light absorption of superior thin g‐C3N4 nanosheets based photocatalysts; discussions on the mechanism of g‐C3N4 based photocatalyst involved photocatalytic H2O splitting and CO2 reduction system; the latest progress in the study of nanoarchitectonics of a variety of g‐C3N4 based heterostructures including type II, Z‐scheme, S‐scheme, and Schottky junction. Major challenges on nanoarchitectonics of superior thin g‐C3N4 based heterostructures, as well as the recommendations for future studies of these materials to attain higher efficiency in photocatalytic H2 generation and CO2 reduction, are also concluded.

Hydrothermal Reduction of CO2 to Value-Added Products by In Situ Generated Metal Hydrides.

An integrated process by coupling hydrothermal reactions, including CO2 reduction and H2O dissociation with metals, is proposed. The hydrogen could be rapidly produced under hydrothermal conditions, owing to the special characteristics of high temperature water, generating metal hydrides as intermediates. Hydrogen production from the H2O dissociation under hydrothermal conditions is one of the most ideal processes due to its environmentally friendly impact. Recent experimental and theoretical studies on the hydrothermal reduction of CO2 to value-added products by in situ generated metal hydrides are introduced, including the production of formic acid, methanol, methane, and long-chain hydrocarbons. These results indicate that this process holds promise in respect to the conversion of CO2 to useful chemicals and fuels, and for hydrogen storage, which could help alleviate the problems of climate change and energy shortage.

Evaluation of two plumes jet plasma reactor for plasmolysis of H2O vapor and CO2 combinations – Optimization study

AbstractA custom design multi‐flying jet plasma torches (MFJPT) reactor was tested for plasmolysis of water vapor and mixtures of water vapor‐carbon dioxide in a series of experimental investigations at various reactor operational parameters. Experimentation plans were applied within the range of induced power (100–300 watts) and various vapor/gas throughputs. The produced gases were analyzed through online gas chromatography. The results of water vapor plasmolysis in two schemes demonstrated the production of 1337 ppm of hydrogen from water vapor/argon and 1665 ppm from applying a water vapor/argon/CO2 combination. Valuable hydrocarbon gases (e.g., Ethane, Ethylene/Acetylene) were generated and detected at higher conversions upon introducing H2O vapor, argon, and CO2 mixtures. The experimental data were trained through machine learning and a Gaussian Process Regression (GPR) model has fitted the data quite well. Ultimately, optimization study outcomes are presented through a color heat‐map for system scaling‐up purposes.

Origin and evolution of the ore-forming fluids of the Bajiazi gold deposit in the Jiapigou gold ore belt, NE China: Constraints from fluid inclusions, H-O-S isotopes, and pyrite trace element concentrations

The Bajiazi gold deposit, located within the southeastern Jiapigou gold ore belt (JGB) along the northeastern corner of the North China Craton (NCC), contains > 6 tons of gold at an average grade of 24 g/t. More than 20 total auriferous quartz veins are hosted in the Neoarchean basement gneiss and amphibolite and are spatially and temporally associated with Mesozoic intermediate-acid dikes. The distribution of the gold orebodies is mainly controlled by NNE-trending brittle–ductile shear zones and faults. Three hydrothermal stages were identified, namely, quartz–pyrite (stage I), quartz–gold–polymetallic sulfide (stage II), and quartz–carbonate (stage III). Three types of fluid inclusions (FIs) were identified: H2O–NaCl (W-type), CO2–H2O (C-type), and pure CO2 (PC-type) FIs. From stage I to stage III, the FI suite changes from W-type and C-type in stage I to all three types in stage II and to only W-type in stage III. The FIs in the quartz of stages I to III mainly homogenized at temperatures (Th) of 283–395 °C, 183–287 °C and 116–186 °C, respectively, with salinities of 3.87 to 13.01 wt% NaCl equiv, 4.65 to 18.55 wt% NaCl equiv, and 3.83 to 9.68 wt% NaCl equiv, respectively. The ore-forming fluids of the Bajiazi gold deposit evolved from a H2O–NaCl–CO2 system in stages I and II to a H2O–NaCl system in stage III. Fluid immiscibility is an important mechanism leading to rapid precipitation of sulfides and gold. H–O–S isotope signatures indicate that the initial ore-forming fluids and materials were originally derived from magmatic sources. Electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) data on pyrite grains show that abundant gold mineralization occurred in the middle hydrothermal stage. The high Co/Ni ratios of pyrite are consistent with a magmatic–hydrothermal origin. The results, combined with previous regional studies, indicate that the Bajiazi gold deposit is a mesothermal magmatic–hydrothermal lode gold deposit.

Synthesis of β-FeOOH/polyaniline heterogeneous catalyst for efficient photo-Fenton degradation of AOII dye.

Discharge of the untreated dye-containing wastewaters will induce water source pollution and further harm aquatic organisms. In this study, the akaganéite/polyaniline catalyst (β-FeOOH/PANI, about 1.0μm) could be successfully composed by polyaniline (PANI, (C6H7N)n, 200-300nm) and akaganéite (β-FeOOH, FeO(OH)1-xClx, less than 200nm), according to the identification and characterization results of XRD, Ramon, FTIR, XPS, SEAD, EDS, and FESEM (or HRTEM). Due to PANI providing more photogenerated electrons, the β-FeOOH/PANI composite (compared with β-FeOOH) in photo-Fenton system had the more highly catalytic degradation capacity to Acid Orange II (AOII) under an optimal condition (7.5mmol/L of H2O2 oxidant, 40mg/L of AOII, 0.2g/L of catalyst dosage, and pH 4.0). The AOII degradation kinetics could be well fitted by pseudo-first-order model. In photo-Fenton catalytic process of AOII dye, the ∙OH and h+ were the main reaction substances. The AOII in solutions could be gradually mineralized into non-toxic inorganic H2O molecule and CO2. The β-FeOOH/PANI catalyst also had a good reusable ability of about 91.4% AOII degradation after 4 runs. These results can provide a reference for synthesis of catalyst used in photo-Fenton system and the applications in degradation removal of organic dye from wastewaters.

Primary cordierite with > 2.5 wt% CO2 from the UHT Bakhuis Granulite Belt, Surinam: CO2 fluid phase saturation during ultrahigh-temperature metamorphism

The Paleoproterozoic Bakhuis Granulite Belt (BGB) in Surinam, South America, shows ultrahigh-temperature metamorphism (UHTM) at temperatures of around 1000 °C which, unusually, produced peak-to-near-peak cordierite with sillimanite and, in some cases, Al-rich orthopyroxene on a regional scale. Mg-rich cordierite (Mg/(Mg + Fe) = 0.88) in a sillimanite-bearing metapelitic granulite has a maximum birefringence of second-order blue (ca. 0.020) indicative of a considerable amount of CO2 (> 2 wt%) within its structural channels. SIMS microanalysis confirms the presence of 2.57 ± 0.19 wt% CO2, the highest CO2 concentration found in natural cordierite. This high CO2 content has enabled the stability of cordierite to extend into UHT conditions at high pressures and very low to negligible H2O activity. Based on a modified calibration of the H2O–CO2 incorporation model of Harley et al. (J Metamorph Geol 20:71–86, 2002), this cordierite occupies a stability field that extends from 8.8 ± 0.6 kbar at 750 °C to 11.3 ± 0.65 kbar at 1050 °C. Volatile-saturated cordierite with 2.57 wt% CO2 and negligible H2O (0.04 wt%) indicates fluid-present carbonic conditions with a CO2 activity near 1.0 at peak or near-peak pressures of 10.5–11.3 kbar under UHT temperatures of 950–1050 °C. The measured H2O content of the cordierite in the metapelite is far too low to be consistent with partial melting at 1000–1050 °C, implying either that nearly all of any H2O originally in this cordierite under UHT conditions was lost during post-peak cooling or that the cordierite was formed after migmatization. The high level of CO2 required to ensure fluid saturation of the c. 11 kbar UHT cordierite is proposed to have been derived from an external, possibly mantle, source.

Spatially Separated Redox Cocatalysts on Ferroelectric Nanoplates for Improved Piezophotocatalytic CO2 Reduction and H2O Oxidation.

Utilizing solar and mechanical vibration energy for catalytic CO2 reduction and H2O oxidation is emerging as a promising way to simultaneously generate renewable energy and mitigate climate change, making it possible to integrate two energy resources into a reaction system for artificial piezophotosynthesis. However, the practical applications are hindered by undesirable charge recombination and sluggish surface reaction in the photocatalytic and piezocatalytic processes. This study proposes a dual cocatalyst strategy to overcome these obstacles and improve the piezophotocatalytic performance of ferroelectrics in overall redox reactions. With the photodeposition of AuCu reduction and MnOx oxidation cocatalysts on oppositely poled facets of PbTiO3 nanoplates, band bending occurs along with the formation of built-in electric fields on the semiconductor-cocatalyst interfaces, which, together with an intrinsic ferroelectric field, piezoelectric polarization field, and band tilting in the bulk of PbTiO3, provide strong driving forces for the directional drift of piezo- and photogenerated electrons and holes toward AuCu and MnOx, respectively. Besides, AuCu and MnOx enrich the active sites for surface reactions, significantly reducing the rate-determining barrier for CO2-to-CO and H2O-to-O2 transformation, respectively. Benefiting from these features, AuCu/PbTiO3/MnOx delivers remarkably improved charge separation efficiencies and significantly enhanced piezophotocatalytic activities in CO and O2 generation. This strategy opens a door for the better coupling of photocatalysis and piezocatalysis to promote the conversion of CO2 with H2O.

Cryogenic liquid nitrogen stimulation-induced cracking in gas-enriched shale reservoirs: A numerical investigation

Cryogenic shock-induced brittle cracking has promising application prospects in reservoir stimulation. In this paper, a fully coupled thermo-hydro-mechanical (T-H-M) model combined with the statistical meso-damage theory is proposed to investigate the trans-scale progressive cracking process of shale gas reservoirs stimulated by liquid nitrogen (LN2). In this theoretical framework, shale meso-heterogeneity in both thermo-mechanical and hydraulic properties is characterized by two independent statistical distribution functions; temperature-dependent rock properties and temperature- and pressure-dependent fluid thermodynamic properties are described using some experimental and theoretical formulas. The numerical model is implemented in a finite element context and subsequently validated against existing classical coupling results. Numerical results demonstrate that LN2 fracturing accompanied by the unique cryogenic shock effect leads to a complex cracking process involving multiple-point fracture initiation, asymmetric macrofracture propagation, and primary fracture bifurcation. Sensitivity analysis indicates that approximately equidistant or equilength radial fractures may be generated at a lower stress difference, lower permeability, or higher elastic modulus; both fracture initiation pressure and induced fracture morphology are more sensitive to injected fluid temperature relative to initial reservoir temperature; a larger thermal expansion coefficient is beneficial for inducing more fractures, but not applicable to the thermal conductivity. Preferable candidates for LN2 stimulation to generate multiple fractures in shale reservoirs are recommended. It is also found that compared with H2O and supercritical CO2, fracturing using LN2 can generate fractures with lower initiation pressure and greater complexity. Preliminary results confirm the potential of using LN2 fracturing to improve the stimulation effect of shale reservoirs.

STABILITY OF CARBONATES DURING SUBDUCTION: INFLUENCE OF THE DEHYDRATION REGIME OF CHLORINE-BEARING METAPELITE

It was shown that at a pressure of 3.0–7.8 GPa and a temperature of 750–1030°C, a set of reactions occurred in carbonate-bearing and Cl-enriched pelite that is finally converted into eclogite-like assemblage and formed H2O–CO2 Cl-enriched fluid. The eclogite-like assemblage remains stable when P-T conditions change concordantly with hot subduction geotherms, whereas carbonate is completely dissolved in the fluid already at ≥5.5 GPa. In quenched fluid the content of CO2 reaches 30 wt. %. However, preliminary defluidization of pelite at 3.0 GPa and 750°С leads to chlorine removal and carbonate stabilization at 5.5 GPa and at 7.8 GPa in equilibrium with the next chlorine-free portions of fluid. Thus, the stability of carbonates under P-T conditions typical of subduction zones is dependent on the behavior of chlorine during defluidization of marine sediments.

Gallium Oxide Assisting Ag-Loaded Calcium Titanate Photocatalyst for Carbon Dioxide Reduction with Water

An efficient and highly selective photocatalytic conversion of carbon dioxide (CO2) into valuable chemicals such as carbon monoxide (CO) using water (H2O) as an electron donor has been much attractive and deeply desired, which requires the development of advanced photocatalysts based on a functional design. As the Ag-loaded calcium titanate (CaTiO3, CTO) photocatalyst showed a high selectivity to CO2 reduction in aqueous solution and the Ag-loaded gallium oxide (Ga2O3) photocatalyst showed a higher activity for both H2O splitting and CO2 reduction, herein, a series of composite photocatalyst samples consisting of Ga2O3 and CTO were simply fabricated by calcination of the physical mixtures, followed by loading of a Ag cocatalyst with a photodeposition method. The optimized sample with the Ag cocatalyst exhibited both a high CO formation rate of 56.9 μmol h–1 (higher than that of Ag/Ga2O3) and a high selectivity of 95.0% (comparable to Ag/CTO) in the photocatalytic CO2 reduction with H2O. In this composite photocatalyst, most of the electrons generated in the photoexcited Ga2O3 part migrated to the minor CTO particles to contribute to the selective CO2 reduction reaction, which was evidenced by the selective photodeposition of Ag species on the CTO part. The selective CO formation originates from the property of Ag-loaded CTO photocatalyst as the active part in the composite photocatalyst. The Ga2O3 part functions as an antenna to receive the light and donate the photoexcited electrons to the much decorated Ag/CTO part, where the concentrated electrons would promote CO2 reduction with high efficiency.

Flexible BiOI thin films photocatalysts toward renewable solar fuels production

Bismuth oxyiodide (BiOI) is an emerging photocatalyst material with remarkable electrical and optical properties, bringing it to the forefront of materials research. Herein, thin films of BiOI photocatalysts have been successfully synthesized by a one-pot microwave-hydrothermal method. The BiOI films were deposited on earth-abundant mica substrates, which provide a flexible and effective surface for the controlled growth of well-dispersed flowers of high surface area and enhanced light absorption. The films were evaluated as photocatalyst to generate alternative and renewable solar fuels from CO2 reduction, H2O splitting, and N2 photofixation under visible light. The results demonstrated the ability of the flower-shaped BiOI films to generate various value-added compounds, such as HCOOH (4.8 mmoles g-1 h-1), CH3OH (9.5 mmoles g-1 h-1), H2 (0.6 mmoles g-1 h-1), and NH3 (0.11 mmoles g-1 h-1) under visible light irradiation. Mainly, the H2 evolution was promoted (x3) after adding 3% wt. of calcium gluconate as a sacrificial agent, reaching a continuous H2 production (1.8 mmoles g-1 h-1). This enhancement effect was related to the oxidation of the sacrificial agent, favoring a high number of electrons available to reduce H+ to H2. In addition, the recycling of the photocatalyst was evidenced after four cycles, which activity was reactivated after the fifth cycle with a thermal treatment at 160 °C under vacuum conditions.

High-Pressure Synthesis, Synchrotron Single-Crystal XRD and Raman Spectroscopy of Synthetic K–Ba Minerals of Magnetoplumbite, Crichtonite and Hollandite Group Indicatory of Mantle Metasomatism

The paper summarizes the results of an experimental study of the formation of K–Ba high-Ti (and Cr) oxides synthesized in the chromite–rutile/ilmenite–K2CO3/BaCO3–H2O–CO2 systems at 1.8–5.0 GPa. Experiments confirm the conclusion that the formation of K–Ba high-Ti oxides characterizes the most advanced or repeated metasomatic stages in upper mantle peridotites, which lead first to the formation of simple Ti oxides and then to the formation of K–Ba high-Ti and Cr oxides. Relations between the oxides is a function of the activity of the K and Ba components in the fluid. The appearance of priderite corresponds to the highest activity of K in the mineral-forming media. Redledgeite is formed only in the Fe-poor chromite–rutile–H2O–CO2–BaCO3 system, and, in the system with ilmenite, minerals of the magnetoplumbite group preferably crystallize. A direct dependence of the Cr content in oxides on pressure is revealed. Raman spectra of K–Ba high-titanium oxides are presented. The structure of a potassium compound of a magnetoplumbite group with the chemical formula K0.90Ti5.16Cr2.94Fe2.54Mg0.87Al0.22Mn0.30O19 is studied by single-crystal X-ray diffraction using a synchrotron radiation. The obtained data can be used to specify the nomenclature of the magnetoplumbite mineral group.