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Experimental methods and thermodynamic simulations were employed in this study to assess the H 2 production potential of a CaFe 2 O 4 based blue H 2 production process from natural gas (NG, >90 vol%CH 4). Fixed bed reactor testing was used to verify the product outcomes. Syngas production from methane using CaFe 2 O 4 was demonstrated. Stable H 2 production with high steam conversion was demonstrated with the CaFe 2 O 4 when reduced with methane. The thermodynamic integrated process simulation enabled simulation of the process in two and three reactor configurations to understand the feasibility of a heat integrated system. The 2-reactor process used the generation of syngas as the prevailing mode for H 2 generation while the 3-reactor system utilized steam water splitting in a dedicated reactor as the prevailing mode to generate H 2. Simulation of the 2-reactor process's FR showed syngas generation similar to the products from fixed bed demonstrations, establishing a connection between thermodynamic simulation and experimental data. The H 2 yield potentials of the various configurations were determined and compared to steam methane reforming with capture (SMR-CCS) and a Fe 2 O 3 based system from the literature. The 2-reactor process has the potential to generate 1.6–2.1 mol of H 2 /mole of NG fed to the system. Three reactor configurations showed the highest potential for H 2 yield with a range of 2.2–2.56 mol of H 2 /mole NG but with the need for additional CCS at the highest yield. A thermal management approach was introduced that combined the chemistries of CaFe 2 O 4 and CuFeAlO 4 which enabled increasing the potential yield to 2.66 mol H 2 /mol NG and enabling a system without the need for addition carbon capture to meet 90% threshold targets. The three reactor cases showed the most competitive performance in comparison to SMR-CCS with up to a 14.6% improvement in H 2 yield. [Display omitted] • Bench scale fixed bed reactor demonstrations for H 2 productions with CaFe 2 O 4. • Thermally integrated H 2 production process screening analysis of 2 and 3 reactor concepts. • 2 reactor concept H 2 yield range: 1.6–1.9 mol H 2 /mol CH 4. • 3 reactor concept H 2 yield range: 2.51–2.66 mol H 2 /mol CH 4. • Novel thermal management strategy using exothermic oxygen carrier (CuFeAlO 4) co-mixing with CaFe 2 O 4. [ABSTRACT FROM AUTHOR]

In this work, a synthesis technique for highly homogeneous PVDF-CaFe2O4 polymer films direct from solution was developed. The structural characterizations were conducted using XRD, FTIR, and ESEM experimental techniques. The XRD characteristic peaks of CaFe2O4 nanoparticles revealed a polycrystalline structure. The average crystallite size for CaFe2O4 was calculated to be 17.0 nm. ESEM micrographs of PVDF nanocomposites containing 0.0, 0.25, 0.75, and 1.0 wt% of CaFe2O4 showed smooth surface topography. The direct Edir and indirect Eind band gap energies for the PVDF-CaFe2O4 nanocomposites were decreased with the additions of 0.0–1.0 wt% CaFe2O4. In addition, the refractive index (n0) increased from 3.38 to 10.36, and energy gaps (Eg) decreased from 5.50 to 4.95 eV. The nonlinear refractive index (n2) for the PVDF-CaFe2O4 nanocomposites was improved with the addition of CaFe2O4 nanoparticles, exceeding those reported in the literature for PVC, PVA, and PMMA nanocomposites. Therefore, the PVDF-CaFe2O4 nanocomposites are expected to take the lead in optoelectronic applications because of their unusual optical properties. [ABSTRACT FROM AUTHOR]

In the current work, we provide the electrochemical (EC) characteristics and considerable size of Ca-doped ZnFe 2 O 4 nanoparticles. Mixed transition metal oxides are widely used as excellent electrode materials in superior supercapacitors because of their superior capacitance, low cost, and environmental friendliness. The prepared nanoparticles were characterized by X-ray diffraction (XRD), Field-emission scanning electron microscope (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and EC methods. The results exhibited that the as-synthesized nanoparticles had a cubic spinel crystal structure and efficient EC properties. The EC properties of the prepared electrodes were explored by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) studies. The Ca 0.1 Zn 0.9 Fe 2 O 4 electrode demonstrated a specific capacitance (SC) ∼208 Fg-1 at a 2 mV/s scan rate due to significant morphological behavior. Therefore may be the prepared materials are the finest electrodes for supercapacitor applications. [ABSTRACT FROM AUTHOR]

Rare-earth doped calcium nano ferrites (CaFe2−x−yGdxSmyO4; x = y = 0.0; x = 0.025, y = 0.05) are synthesized by citrate nitrate auto combustion method. The prepared samples are characterized using X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), high-resolution transmission electron microscope (HRTEM), and vibrating sample magnetometer (VSM) analyses. After that, the samples were examined for Pb (II) ions removal from the water solution. The orthorhombic structure with space group Pnma is ratified from XRD. The average crystallite size of the synthesized powders is estimated from the broadening of the XRD lines in the range of 18–21 nm. FTIR confirms the molecular signature of the samples. The magnetic property is discussed based on super exchange interactions. The hysteresis loop shows two different magnetic behaviors of the investigated sample as well as increasing the coercivity from 14 to 240 G due to the doping. The electrical parameters are discussed in the basic hopping charge model, while the conduction mechanism of the samples will be discussed based on de-Boer and Verwey. The experimental results are subjected to kinetic and isotherm analyses. The Langmuir and Freundlich isotherms describe the adsorption of heavy metal ions as designated by the high correlation coefficient (R2). The use only 4 mg of the doped sample with low price ferrite nanoparticles has been utilized for 99.31% removal of Pb (II) from wastewater at a contact time of 10 min at room temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mn@CaFe 2 O 4 anchored on citrus fruit peel-derived biochar was fabricated using the hydrothermal method. • The fabricated photocatalyst can remove 97.31 % of RhB dye within 70 min of solar light irradiation. • Mechanistic pathway of photodegradation process is elucidated, including identification of key reactive species. • Fabricated photocatalyst showed enhanced photodegradation ability of RhB compared to reported photocatalysts. This report details the synthesis of Mn-doped CaFe 2 O 4 nanoparticles supported on biochar. We evaluate the photocatalytic activity of the resulting nanocomposite towards the degradation of Rhodamine B (RhB) dye, a well-known environmental pollutant. Doping CaFe 2 O 4 with Mn and subsequently incorporating it onto biochar results in enhanced visible light absorption and diminished photoluminescence (PL) intensity. The photocatalyst Mn-doped CaFe 2 O 4 /Biochar was synthesized via a hydrothermal method. The morphology and photocatalytic characteristics of the synthesized catalyst were comprehensively scrutinized through powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV-DRS), and photoluminescence (PL) investigations. The photocatalytic properties of the catalyst were further assessed through the degradation of RhB dye, revealing a degradation efficiency of 97.31% within a 70-minute timeframe. The photocatalyst exhibits reusability for up to five cycles while maintaining a photocatalytic efficiency of more than 80%. [ABSTRACT FROM AUTHOR]

The mechanism affecting CO yield during gasification of brown coal in the CO 2 atmosphere by studying single and bimineral mixtures in brown coal. Fe3+ and Ca2+ were found to form CaFe 2 O 4 , which adsorbs CO 2 and promotes its fracture, thus maximizing CO yield. [Display omitted] • The synergistic effect of dual minerals on the generation of CO. • Reaction of Fe3+ and Ca2+ in coal to form CaFe 2 O 4 promotes CO production. • CO 2 adsorbs on CaFe 2 O 4 , which then breaks off to form CO. To investigate the effect of minerals in brown coal on CO yield in a CO 2 atmosphere, we impregnated pickled coal with nitrate and quartz to simulate the mineral content and assessed their impact on CO yield during gasification. It demonstrated that the reaction of Ca2+ and Fe3+ forming CaFe 2 O 4 enhanced the CO yield. In the 100 % CO 2 atmosphere, pickled coal containing CaFe 2 O 4 produced five fold the amount of CO as after-pickling coal. The effect of minerals on CO generation under the CO 2 atmosphere was explored by adding specific amounts of 1.96 wt% Ca, 1.80 wt% Fe, 6.83 wt% Al, 0.53 wt% Mg, and 11.64 wt% Si to acid-washed brown coal. The results show that the addition of Ca2+ contributes to the enhancement of CO, whereas the combined addition of Ca2+ and Fe3+ increases CO yield. CaFe 2 O 4 enhances CO yield for two reasons: firstly, it is oxidized by CO 2 to produce CO; secondly, CaFe 2 O 4 can further react with CO 2 to form Ca 2 Fe 2 O 5 , Ca 2 Fe 2 O 5 then reacts with carbon in the brown coal to generate CO. This study has significant implications for the utilization of brown coal resources. [ABSTRACT FROM AUTHOR]

Fuel cell device efficiently can convert chemical-energy (CE) of hydrogen/hydrocarbon fuels to electrical-energy. From the various types of fuel cells, solid oxide fuel cell (SOFC) unifies the advantages of combined heat and power-output with multi-fuel-flexibility. Even so, the high operating temperatures (800-1,000 °C) leads to materials compatibility and high costs challenges. However, much progress has been made to develop low-temperature SOFCs. But poor oxygen reduction reaction (ORR) activity of traditional cathode materials at low temperatures is a key-bottleneck to reduce operating-temperature of SOFCs. Therefore, fundamental understanding of oxygen reduction reaction (ORR) is important for low-temperature solid oxide fuel cells (LT-SOFCs). Herein, we present a simple but very effective way to improve the ORR activity of an orthorhombic nanostructured CaFe 2 O 4 embedded on porous Ni-foam for low temperature for SOFCs cathode. The CaFe 2 O 4 embedded on Ni-foam exhibits a very low area-specific resistance (ASR) and excellent power output of 0.532 Wcm−2 at low operating temperature of 500 °C. The excellent ORR activity is mainly supported by the superficial release of oxygen ions with enhanced gas diffusion abilities of CaFe 2 O 4 by embedding on Ni-foam due to its highly porous structure. The electrical conductivity of CaFe 2 O 4 embedded on Ni-foam is also found to be increased dramatically. More importantly, formation of complex oxidation states (Fe2+/Ni3+ and Fe3+/Ni2+) between CaFe 2 O 4 and Ni-foam plays an important role on narrows the bandgap to improve the electrical conductivity and produce more oxygen vacancies to enhance the ionic transport. High electrical conductivity and electrocatalytic functionality of reported nanostructure insights new avenues for LT-SOFCs. In addition, various spectroscopies, such as UV-visible, Raman, and X-ray photoelectron spectroscopy are employed to catch the understandings of CaFe 2 O 4 embedded Ni-foam as new functional ORR electrocatalyst for advanced LT-SOFCs. [Display omitted] • Ni foam embedded CaFe 2 O 4 spinel ferrite is developed for LT-SOFCs cathode. • Ni-foam embedded CaFe 2 O 4 cathode shows highly porous structure. • The ORR activity of CaFe 2 O 4 is greatly improved by embedding it on Ni foam. • The excellent electrochemical performance of the of 0.532 W cm−2 at 500 °C was achieved. • It show low charge and mass transfer losses. [ABSTRACT FROM AUTHOR]

[Display omitted] • Z-type heterojunction CaFe 2 O 4 /Ag/ZnO composite was fabricated by a microwave-assisted hydrothermal method. • Ag doping can both reduce the band gap of ZnO and enhance its absorption of visible light. • Ag/ZnO forms a Z-type heterojunction with CaFe 2 O 4 , which can significantly promote photogenerated carrier separation. • The degradation effect of the CaFe 2 O 4 /Ag/ZnO on ciprofloxacin was significantly better than pure ZnO and CaFe 2 O 4. The Z-type heterojunction CaFe 2 O 4 /Ag/ZnO composite photocatalyst was successfully synthesized by the microwave-assisted hydrothermal methods. The photocatalytic activity of the CaFe 2 O 4 /Ag/ZnO was evaluated by the degradation of ciprofloxacin (CIP). The characterization results indicated that Ag doping can decrease the band gap (Eg) of ZnO and enhance its visible light absorption capacity. Ag/ZnO forms a Z-type heterojunction with CaFe 2 O 4 , which can significantly promote effective photogenerated carrier separation and enhance the visible light catalytic activity of the CaFe 2 O 4 /Ag/ZnO composite catalyst. It was observed that the degradation effect of the CaFe 2 O 4 /Ag/ZnO on CIP was significantly better than pure ZnO and CaFe 2 O 4. [ABSTRACT FROM AUTHOR]

In this study, the potential of dry reforming reaction over CaFe2O4 supported Ni and Co catalysts were investigated. The Co/CaFe2O4 and Ni/CaFe2O4 catalysts were synthesized using wet impregnation method by varying the metal loading from 5-15 %. The synthesized catalysts were tested in methane dry reforming reaction at atmospheric pressure and reaction temperature ranged 700-800 oC. The catalytic performance of the catalysts based on the initial screening is ranked as 5%Co/CaFe2O4 < 10%Co/CaFe2O4 < 5%Ni/CaFe2O4 < 10%Ni/CaFe2O4 according to their performance. The Ni/CaFe2O4 catalyst was selected for further investigation using full factorial design of experiment. The interaction effects of three factors namely metal loading (5-15 %), feed ratio (0.4-1.0), and reaction temperature (700-800 oC) were evaluated on the catalytic activity in terms of CH4 and CO2 conversion as well as H2 and CO yield. The interaction between the factors showed significant effects on the catalyst performance at metal loading, feed ratio and reaction temperature of 15 %, 1.0, and 800 oC. respectively. The 15 wt% Ni/CaFe2O4 was subsequently characterized by Thermogravimetric (TGA), X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS), N2-physisorption, Temperature Programmed Desorption (TPD)-NH3, TPD-CO2, and Fourier Transform Infra Red (FTIR) to ascertain its physiochemical properties. This study demonstrated that the CaFe2O4 supported Ni catalyst has a good potential to be used for syngas production via methane dry reforming.

Phase‐pure and highly crystalline CaFe2O4 with a sponge‐like macroporous structure is synthesized for the first time via facile solution‐based microwave reaction and subsequent short thermal treatment. The formation mechanism of the orthorhombic phase (Pnma) and the pore network is investigated in detail and reveals a complex formation of this p‐type semiconductor. Superconducting quantum interference device (SQUID) magnetometry and Mössbauer spectroscopy confirm the phase changes by altered magnetic properties. Furthermore, the optical and photoelectrochemical properties of the material were investigated. The material has a bandgap of 1.9 eV and sufficient band positions for water splitting. Valence‐to‐core X‐ray emission spectroscopy is used to support the band positions obtained from Mott–Schottky analysis. Photocathodes prepared from macroporous CaFe2O4 generate photocurrents under illumination with light of up to 600 nm. [ABSTRACT FROM AUTHOR]

[Display omitted] • CaFe 2 O 4 nanoparticles show superparamagnetic behavior at 300 K. • CaFe 2 O 4 nanoparticles could be useful for biomedical applications. • Magnetic and electronic properties of CaFe 2 O 4 nanoparticles contribute to breakthroughs in electronics and spintronic applications. • CaFe 2 O 4 electrodes improve efficiency useful in batteries and supercapacitors for renewable energy applications. • CaFe 2 O 4 nanoparticles show cutting-edge advancements for catalytic applications. Over the last decade, nanotechnology has emerged as a game-changing field, with transition metal nanoparticles, notably those consisting of CaFe 2 O 4 , standing out as a key component. These nanoparticles have gotten a lot of interest because of their great potential in a variety of biological applications. Scientists all around the world are working hard to create new calcium-based nanoparticles in a variety of forms and sizes to improve their functioning. While there is a substantial corpus of research on CaFe 2 O 4 nanoparticles, there is a significant void in comprehensive reviews that properly elucidate their current advances and future applications in biological sciences. This review study attempts to give in-depth insights into the most recent advances in CaFe 2 O 4 nanoparticle research, with a particular emphasis on electrochemical, environmental, and biological nanotechnology. The research digs into the finer points of the most recent breakthroughs in the synthesis of CaFe 2 O 4 nanoparticles, with a focus on enhancing their imaging capability. Their applications in electrochemical processes, photocatalysis, sensing technologies, and mechanical functionalities receive special attention. The paper highlights the superparamagnetic response of CaFe 2 O 4 nanoparticles to room-temperature oscillating magnetic fields, which opens up new possibilities for investigation. The study not only elucidates the potential benefits of using these nanoparticles but also addresses the limitations connected with their larger application. The magnetic properties of CaFe 2 O 4 nanoparticles are given special attention, particularly in the context of data storage and magnetic sensors. The paper finishes with a complete summary of current developments in several disciplines utilizing CaFe 2 O 4 nanoparticles, highlighting specific applications and exploring their distinctive features. The review emphasizes the importance of future research into ways to improve the magnetic characteristics of CaFe 2 O 4 materials, particularly in the realms of data storage and magnetic sensor technologies. [ABSTRACT FROM AUTHOR]

The relationship between local structure and visible-light activated photocatalytic effect of simulated domestic waste slag glass–ceramics (R-NaWSFe) was investigated. The largest pseudo-first-order rate constant of 9.75 × 10−3 min−1 was estimated for methylene blue decomposition test under the visible-light irradiation using R-NaWSFe with additional 30 mass% of Fe2O3 heat-treated at 900 °C for 100 min. The reason for the high photoactivity of this sample was mainly due to nanoparticles of CaFe2O4 and α-Fe2O3 confirmed by the Mössbauer spectrum measured at 77 K. It is concluded that the nanoparticles of magnetic components in silica are essential for exhibiting visible-light activated catalytic effect. [ABSTRACT FROM AUTHOR]

The performance of existing lithium-ion batteries (LIBs) is greatly hindered by the low specific capacity of graphite-based anodes (372 mAh g−1). Therefore, development of suitable anode materials that exhibit higher and stable capacity is necessary to improve performance. Transition metal oxides have attracted tremendous attention as next-generation anode materials for LIBs due to their high theoretical capacity. Herein, we report the synthesis of a porous CaFe 2 O 4 by a facile and time efficient solution combustion synthesis technique for use in an anode for LIBs. The as-prepared material exhibited improved electrochemical performance with specific discharge capacities of 441 mAh g−1, 518 mAh g−1, and 516 mAh g−1 for the initial three cycles. It achieved stability with a deliverable specific discharge capacity of 551 mAh g−1 after 150 cycles at a current density of 200 mA g−1. The porous CaFe 2 O 4 exhibits improved cyclic performance and rate capability. These improvements are attributed to the porous nature of active material and, more importantly, the presence of CaO, which effectively alleviates the volume changes by acting as a buffer matrix. Unlabelled Image • Porous CaFe 2 O 4 has been synthesized by a facile solution combustion synthesis technique. • Mixed-metal oxides possessing electrochemically active-inactive constituents are reported. • The porous CaFe 2 O 4 as an anode for LIBs exhibited improved electrochemical performances. • The porous structure and the buffer matrices of CaO are owed for their improved activity. [ABSTRACT FROM AUTHOR]

Abstract In this paper, a facile synthesis method for CaFe 2 O 4 is introduced that produces a catalyst capable of significant photocatalytic degradation of POME under visible light irradiation. The co-precipitation method was used to produce two catalysts at calcination temperatures of 550 °C and 700 °C dubbed CP550 and CP700. CP550 demonstrated the maximum COD removal of 69.0% at 0.75 g/L catalyst loading after 8 h of visible light irradiation which dropped to 61.0% after three consecutive cycles. SEM images indicated that the higher calcination temperature of CP700 led to annealing which reduced the pore volume (0.025 cm3/g) and pore diameter (10.3 nm) while simultaneously creating a smoother and more spherical surface with lower S BET (9.73 m2/g). In comparison, CP550 had a rough hair-like surface with higher S BET (27.28 m2/g) and pore volume (0.077 cm3/g) as evidenced by BET analysis. XRD data indicated the presence of CaFe 5 O 7 in the CP550 composition which was not present in CP700. The presence of Wustite-like FeO structures in CaFe 5 O 7 are likely the cause for lower photoluminescence intensity profile and hence better charge separation of CP550 as these structures in CaFe 2 O 4 have been known to increase resistivity and electron localization. The COD removal of CP550 dropped from 69.0% to just 7.0% upon adding a small quantity of isopropanol into the reaction mixture indicating hydroxyl radicals as the primary reactive oxidative species. Graphical abstract Unlabelled Image Highlights • CaFe 2 O 4 photocatalyst was synthesized to degrade POME under visible light. • A maximum COD removal of 69.0% at 0.75 g/L catalyst loading was achieved. • Hydroxyl radicals were the primary reactive oxidative species. • Photodegradation efficiency dropped only 8% over 3 cycles from 69% to 61%. [ABSTRACT FROM AUTHOR]

In this work, we present conductometric gas sensors based on p-type calcium iron oxide (CaFe2O4) nanoparticles. CaFe2O4 is a metal oxide (MOx) with a bandgap around 1.9 eV making it a suitable candidate for visible light-activated gas sensors. Our gas sensors were tested under a reducing gas (i.e., ethanol) by illuminating them with different light-emitting diode (LED) wavelengths (i.e., 465−640 nm). Regardless of their inferior response compared to the thermally activated counterparts, the developed sensors have shown their ability to detect ethanol down to 100 ppm in a reversible way and solely with the energy provided by an LED. The highest response was reached using a blue LED (465 nm) activation. Despite some responses found even in dark conditions, it was demonstrated that upon illumination the recovery after the ethanol exposure was improved, showing that the energy provided by the LEDs is sufficient to activate the desorption process between the ethanol and the CaFe2O4 surface.

In this study, visible-light active photocatalyst based on TiO2 was designed using CaFe2O4 nanoparticle as an effective sensitizer. CaFe2O4 nanoparticles (CFO) were synthesized by Pechini method using calcium nitrate and iron nitrate. Then, the high crystalline CFO calcined at 800 oC was combined with TiO2 in a simple sol-gel aging. In order to form coupled structure, CFO suspension and TiO2-precursor solution were mixed in ethanol solution, and then heated at 80 °C while vigorously stirring. After 6 hrs, the coupled particles were filtered and dried, followed by calcination at 400 °C. The prepared composites were tested for photocatalytic oxidation of Salicylic acid, a model organic compound under visible-light (μ ≥ 420 nm) as monitored by UV-Visible Spectrophotometer. The origin of photoactivity of the heterojunction was found mainly due to the superoxide radical and generation of hydroxyl radical was negligible. The average photo-conversion performance was obtained because of unique energy band matching between CFO and TiO2. [ABSTRACT FROM AUTHOR]

In this study, the potential of dry reforming reaction over CaFe2O4 supported Ni and Co catalysts were investigated. The Co/CaFe2O4 and Ni/CaFe2O4 catalysts were synthesized using wet impregnation method by varying the metal loading from 5-15 %. The synthesized catalysts were tested in methane dry reforming reaction at atmospheric pressure and reaction temperature ranged 700-800 oC. The catalytic performance of the catalysts based on the initial screening is ranked as 5%Co/CaFe2O4 < 10%Co/CaFe2O4 < 5%Ni/CaFe2O4 < 10%Ni/CaFe2O4 according to their performance. The Ni/CaFe2O4 catalyst was selected for further investigation using full factorial design of experiment. The interaction effects of three factors namely metal loading (5-15 %), feed ratio (0.4-1.0), and reaction temperature (700-800 °C) were evaluated on the catalytic activity in terms of CH4 and CO2 conversion as well as H2 and CO yield. The interaction between the factors showed significant effects on the catalyst performance at metal loading, feed ratio and reaction temperature of 15 %, 1.0, and 800 °C. respectively. The 15 wt% Ni/CaFe2O4 was subsequently characterized by Thermogravimetric (TGA), X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS), N2-physisorption, Temperature Programmed Desorption (TPD)-NH3, TPD-CO2, and Fourier Transform Infra Red (FTIR) to ascertain its physiochemical properties. This study demonstrated that the CaFe2O4 supported Ni catalyst has a good potential to be used for syngas production via methane dry reforming. [ABSTRACT FROM AUTHOR]

• CaFe 2 O 4 can effectively improve the integrated photocatalytic technology with persulfate for tartrazine removal. • SO 4 •− are •OH are mostly responsible for tartrazine degradation. • RSM and BBD were used to study the interaction between variables and detect the best conditions for TZ degradation. • Tartrazine degradation pathway was proposed. CaFe 2 O 4 (CFO) prepared by a hydrothermal method was used as a catalyst to generate radicals via peroxymonosulfate (PMS) activation for excellent tartrazine (TZ) degradation. Response surface methodology (RSM) with Box-Behnken design (BBD) was used to evaluate the effects of catalyst dosage, PMS and initial dye concentration on the removal. The removal was improved with increasing dosage of PMS or catalyst but declined with increasing initial concentration. The optimum conditions were determined as 0.51 g/L catalyst dosage, 2.4 mM PMS concentration and 10 mg/L initial TZ concentration, achieving with 98.1% degradation of TZ (98.8%). Radical trapping tests showed that the oxidation reactions were mainly directed by sulfate (SO 4 •−) and hydroxyl radicals (OH•). Furthermore, CFO/PMS/vis system had no obvious deactivation after five cycles, which implied stability of the catalyst. Thus, the CFO/PMS/vis system was found to be promising approach for wastewater treatment. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Bi- self doped Bi 2 MoO 6 with nanoplate morphology prepared by reflux route. • Bi self-doping induces part reduction of Mo6+ to Mo5+ and creation of Mo vacancy. • CaFe 2 O 4 /Bi 2 MoO 6 0D-2D p-n heterojunction fabricated by integrating CaFe 2 O 4 QDs. • Improved activity for bisphenol A (k app = 0.0132 min−1) degradation and Cr(VI) (k app = 0.15 min−1) reduction. • Reaction rates for p-n heterojunction 5–9 times higher than pure semiconductors. In this study, a mild reflux route was developed for facile synthesis of Bi-self doped Bi 2 MoO 6 (BMOR) with nanoplate morphology. Microstructural study revealed substitution of Bi5+ ions in the molybdate layer which resulted in partial reduction of Mo6+ to Mo5+ ions and creation of Mo vacancy. The defect engineered BMOR material exhibited improved optical and photoelectrochemical properties compared to its undoped analogue. The BMOR material was subsequently employed as host lattice for construction of CaFe 2 O 4 /Bi 2 MoO 6 0D-2D p-n heterojunctions by using an in situ preparative strategy. Well dispersed CaFe 2 O 4 quantum dots over BMOR nanoplates provide a strong interfacial contact conducive for fast charge mobilization. Comprehensive characterization of the composites revealed extended UV–vis response, improved charge carrier separation, prolonged excited state life time and robust radical generation efficiency compared to pure components. The CaFe 2 O 4 /Bi 2 MoO 6 composites displayed improved photocatalytic performance for bisphenol A (BPA) degradation (k app = 0.0132 min−1) and Cr(VI) reduction (k app = 0.15 min−1) with reaction rates 5–9 times higher than pure components. The involvement of •OH and •O 2 − radicals in the photodegradation of BPA was confirmed from radical trapping study. The formation of an interfacial p-n heterojunction and a double charge migration mechanism accounted for the improved photocatalytic efficacy of the composite. [ABSTRACT FROM AUTHOR]

The gas sensing properties of Zr-added and pure CaFe2O4 powders for CO2 in air were examined in the temperature range of 250-450 °C. The semiconductor-type gas sensor made from pure CaFe2O4 powder showed a fairly good response to CO2. Furthermore, the addition of a small amount of Zr into CaFe2O4 powder was found to be effective for enhancing the CO2 response of the present gas sensor. It was also found that the gas response, defined by the ratio of the resistance in air and that of the target gas reached maximum at the operating temperature of 300 °C. The gas response of the Zr-added CaFe2O4-based sensor at 300 °C was estimated to be 2.5 times higher than that of the sensor made from pure CaFe2O4 powder. However, the 90% response time of the Zr-added CaFe2O4-based sensor was much quicker at 350 °C than that at 300 °C. Thus, the optimal gas sensing performance of the Zr-added CaFe2O4-based sensor is expected to be obtained at the operating temperature of 350 °C, considering the still higher response to CO2 gas at this temperature. It is noted that the present CaFe2O4-based sensor responded reversibly as well as continuously to CO2 gas. Infrared analysis revealed that the sensing mechanism of the present CaFe2O4-based sensor is the change in the electric resistance of CaFe2O4 caused by reactive CO2 adsorption with negatively charged oxide ions (O-) resulting in the increase in the hole concentration in the base material of CaFe2O4. [ABSTRACT FROM AUTHOR]

Herein, n-type CaTiO 3 was constructed with p-type CaFe 2 O 4 through in-situ growth method and the as-prepared heterostructure (CTO@CFO) was tested in photocatalytic degradation of persistent pharmaceutical under simulated sunlight. The heterojunction could harvest more visible light, suppress recombination owing to new oxygen vacancies and strong interfacial interaction. SEM and TEM images verified the attachment of tiny-rod CaFe 2 O 4 around CaTiO 3 nanoplates while the CaFe 2 O 4 growth was observed by XRD and HRTEM analyses. Benefiting from p-n heterojunction, the optimal CTO@CFO(0.50) catalyst displayed effective photocatalytic degradation of tetracycline antibiotic (80.81%) which was superior to that of pristine CaTiO 3 (20.8%) and CaFe 2 O 4 (45.1%) catalysts. Radical scavenging tests showed that the photo-induced holes were the main active species in the reaction and the underlying degradation mechanism was discussed. This study examines a novel p-n heterojunction between titanate and ferrite type materials that contribute a scientific basis for design of efficient and cost-effective photocatalysts. [Display omitted] • p-n heterojunction was constructed with CaTiO 3 and CaFe 2 O 4 via in-situ growth • Strong interaction between CaTiO 3 and CaFe 2 O 4 was observed • p-n junction displayed highly efficient TC degradation performance • The heterojunction could harvest more visible light and suppressed recombination • The TC degradation was found efficient in different water matrices [ABSTRACT FROM AUTHOR]

Hierarchically porous CaFe2O4/carbon fiber hybrids with enhanced microwave induced cat-alytic activity for the degradation of methyl violet (MV) from water were synthesized from kapok by a novel two-step process coupling pore-fabricating and nanoparticles assembling. The as-prepared samples exhibited characteristic hollow fiber morphology, CaFe2O4 nanoparticles dispersed uniformly on the surface of hollow carbon fibers (HCF). The effects of various factors such as CaFe2O4 loading, microwave power, catalyst doses, initial concentration of MV solution and pH value on the microwave induced degradation of MV over CaFe2O4/HCF were evaluated. It was found that the microwave induced degradation of MV over CaFe2O4/HCF had high reaction rate and short process time. The kinetic study indicated that the degradation of MV over CaFe2O4/HCF followed pseudo-first-order kinetics model. The high catalytic activity of CaFe2O4/HCF was facilitated by the synergistic relationship between microwave induced catalytic reaction and adsorption characteristics. [ABSTRACT FROM AUTHOR]

Solar photo degradation of the Rose Bengal (RB), a recalcitrant dye in water is carried out in the presence of the spinel CaFe 2 O 4 synthetized by sol-gel route. Its structural characterization and FTIR spectroscopy were undertaken. The BET analysis measured is A specific surface area of 29.5 m2 g−1 is determined from the BET analysis. The diffuse reflectance of the normal spinel CaFe 2 O 4 is used to determine a band gap of 1.83 eV. This optical transition directly allowed, is assigned to the crystal field splitting of Fe3+ octahedrally coordinated. The oxide is characterized photo-electrochemically. The absorption of solar light increases the presence of electrons in the conduction band (E CB = −1.45 V SCE). The chrono-amperometry curve indicates that CaFe 2 O 4 exhibits p -type semiconductor. A flat band potential of 0.24 V SCE is determined from the capacitance measurement. The p -type character is corroborated by chrono-photo amperometry. The values of the conductivity increases with raising temperature attesting a semi-conducting character of CaFe 2 O 4 with an activation energy (E a) of 0.14 eV. As application, CaFe 2 O 4 was successfully used as an efficient photocatalyst for the RB photo-degradation. Due to the good charge separation of photogenerated electron-hole pairs, a quasi-complete degradation of RB occurred within 2 h under sunlight for an initial concentration of 10 mg/L, a CaFe 2 O 4 dose of 1 mg/mL of catalyst and an environmental pH ~ 8. The degradation kinetics are well illustrated by the Langmuir-Hinshelwood (L-H) model, follows pseudo-first order with an apparent constant of 0.016 min−1, corresponding to a half photocatalytic life of 43 min. • CaFe 2 O 4 was used for Rose Bengal (RB) dye degradation under solar light irradiation. • The degradation of RB is quasi total after 120 min for an initial concentration of 10 mg/L. • The mechanism for the enhanced photocatalytic performance was investigated. • RB degradation obeys a pseudo-first-order kinetics [ABSTRACT FROM AUTHOR]

Magnetic CaFe2O4 photocatalyst was successfully synthesized via a solution combustion synthesis (SCS) method, using glycine as the fuel. The as-prepared CaFe2O4 nanoparticles had an average size of 100 nm and broad photo-absorption range. Its band gap calculated from the adsorption edge was fixed at 1.82 eV, which suggested that it could be excited by visible-light. The photo-degradation of methylene blue (MB) under visible light, demonstrated that the CaFe2O4 nanocrystals exhibited high photocatalytic activity. The enhanced photocatalytic activity could be attributed to the small particle size of the CaFe2O4 nanocrystals. Moreover, the product could be easily recycled from the solution by applying an external magnetic field. [ABSTRACT FROM AUTHOR]

Abstract: Heterogeneous CaFe2O4–Ca2Fe2O5-based catalyst with weak magnetism was prepared by co-precipitation and calcination. It was characterized by various techniques including X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed desorption method. Its active components were identified as mainly Ca–Fe composite oxides such as CaFe2O4 for transesterification. The magnetism was further strengthened by reducing its component of Fe2O3 to Fe3O4–Fe under H2 atmosphere for better magnetic separation. Both catalysts were used for the catalytic transesterification of soybean and Jatropha oils to biodiesel. The highest biodiesel yields for soybean oil of 85.4% and 83.5% were obtained over the weak and strong magnetic catalysts, respectively under the optimized conditions (373 K, 30 min, 15/1 methanol/oil molar ratio and 4 wt% catalyst). The catalysts could be recycled three times. Biodiesel production from pretreated Jatropha oil was tested with the magnetic CaFe2O4–Ca2Fe2O5–Fe3O4–Fe catalyst, and 78.2% biodiesel yield was obtained. The magnetic CaFe2O4–Ca2Fe2O5-based catalyst shows a potential application for the green production of biodiesel. [Copyright &y& Elsevier]

For the first time, cubic shape calcium ferrite (CaFe 2 O 4) nanoparticles were used for drug delivery. Specifically, CaFe 2 O 4 nanocubes with hydrophilic surface were synthesized by thermal decomposition and ligand exchange, and the surface was modified with polyvinylpyrrolidone (PVP) for drug loading. Finally, doxorubicin hydrochloride (DOX) was loaded to the CaFe 2 O 4 /PVP composite. The DOX release from CaFe 2 O 4 /PVP/DOX was studied under various pH environments. The results indicated a lower pH could result in more DOX release due to enhanced DOX solubility in the more acidic environment. The cytotoxicity study showed that CaFe 2 O 4 /PVP had low toxicity to the cells, and the DOX released from the carriers could kill the tumor cells as effectively as the pure DOX, but has higher release selectivity at tumor sites. This research proposed a promising platform for delivering DOX to the tumor sites by pH-regulated drug release. • Monodispersed cubic shape calcium ferrite (CaFe 2 O 4) nanoparticles were synthesized. • The surface of CaFe 2 O 4 nanoparticles was converted from hydrophobic to hydrophilic. • DOX was loaded to CaFe 2 O 4 nanoparticle surface for cancer treatment. [ABSTRACT FROM AUTHOR]

The immobilization of powder catalysts on substrates has been the subject of increasing interest in the field of photocatalysis. For the first time, highly efficient LaFeO 3 and CaFe 2 O 4 perovskites were hydrothermally deposited on polyurethane filters (LFO/PU, CFO/PU). Furthermore, the photo-Fenton catalytic decomposition of tetracycline antibiotic by the new catalyst filters was explored under visible light irradiation. The structural, optical, and morphological characteristics of the photo-active filters were investigated using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM-EDS), X-ray diffraction (XRD), UV–visible diffusion spectra (UV–vis DRS), X-Ray fluorescence (XRF), and thermogravimetric/dynamic thermal analyses (TG-DTG). The decomposition studies were conducted in both batch and micro-flow reactor systems. The effects of solution pH, reactive species, and water type on the reaction mechanism were examined. It was found that the photo-Fenton degradation of tetracycline improved when the perovskite proportions on the filter were increased; the removal rates reached 94% and 80% forLFO/PU and CFO/PU, respectively. The high degradation performance obtained in lake (∼71%) and seawater (∼76%) demonstrated the great potential of photo-active catalyst filters, and their excellent stability was confirmed by reusability tests. In the continuous flow system, the photocatalytic filters kept the degradation rate stable at 83% and 44% after 7 h of examination forLFO/PU and CFO/PU, respectively. These results suggest that the as-prepared catalytic filters may be suitable for industrial photocatalytic applications. • The photocatalytic filters kept the degradation capacity stable after 7 h. • LaFeO3 and CaFe2O4 were deposited on polyurethane filter for the first time. • The filters demonstrated high efficiency in lake and sea water medium. • The reusability tests confirmed excellent with insignificant losses was obtained. [ABSTRACT FROM AUTHOR]

Abstract: Photoelectrochemical hydrogen and oxygen production from water were demonstrated without external voltage using calcium iron oxide (p-type) and TiO2 (n-type) semiconductor electrodes. The calcium iron oxide electrode with the ratio Fe/Ca of 1.9, which consisted of two crystal phases (main phase: CaFe2O4, impurity phase: Ca2Fe2O5), showed the highest photocathodic current in 0.1M NaOH aq. at potentials below +0.30V vs. Ag/AgCl under a 500W Xe lamp illumination, and TiO2 showed photo-anodic current in 0.1M NaOH aq. at potentials over −0.78V vs. Ag/AgCl. In the system where the two electrodes were connected under illumination, the open-circuit voltage was 1.09V and the short-circuit current density was 550μAcm−2. Hydrogen and oxygen were successfully generated from this present system without applying an external voltage. The ratio of hydrogen/oxygen evolved after 12-h reaction with both the electrodes short-circuited was around 3.7. This system is an ultimately artificial photosynthesis system where hydrogen and oxygen are generated separately under illumination. It is believed that this present system can form the basis of the future artificial photosynthesis system where hydrogen and oxygen are produced directly from water and sunlight. [Copyright &y& Elsevier]

Abstract: The p-type CaFe2O4 nanofilms with a thickness of about 100nm on F doped tin-oxide-coated (FTO) glass sheets were fabricated by a pulsed laser deposition (PLD) method at a relatively low temperature. Furthermore, their photoelectrochemical properties for splitting water into hydrogen have also been studied in detail. It has been found that these CaFe2O4 thin films exhibit high photoelectrochemical performances (−117μA/cm2 at −0.3 vs. SCE) for the hydrogen generation reaction even without applying any external voltage. [Copyright &y& Elsevier]

Abstract: In situ X-ray diffraction measurements of CaFe2O4-type MgAl2O4 have been conducted at pressures up to 42GPa and temperatures to 2400K using Kawai-type multianvil apparatus with sintered diamond anvils. Additional measurements have also been conducted at pressures to 12GPa using diamond anvil cell with helium as a pressure medium at room temperature, and at temperatures to 836K at the ambient pressure using a high-temperature X-ray diffractometer. The analysis of room-temperature data yielded V 0 =240.1(2)Å3, K 0 =205(6)GPa, and . A fit of the present data to high-temperature Birch–Murnaghan equation of state (EOS) yielded (∂K 0/∂T) P =−0.030(2)GPa/K and α 0 = a 0 + b 0 T with values of a 0 =1.96(13)×10−5 K−1 and b 0 =1.64(24)×10−8 K−2. The present data set was also fitted to Mie–Grüneisen–Debye (MGD) EOS and we obtained γ 0 =1.73(7), q =2.03(37), and θ 0 =1546(104)K. Density changes of MORB have been estimated using the newly obtained thermoelastic parameters, assuming that the Al-rich phase in this composition possesses the CaFe2O4-type structure under the lower mantle P, T conditions. The calculated densities along geotherms for the normal mantle and subducting cold slabs are both significantly higher than those of typical seismological models, confirming the conclusion of some recent results on MORB by laser-heated diamond anvil cell experiments. [Copyright &y& Elsevier]

Abstract: The present paper reports a novel synthetic procedure of both calcium ferrites (CaFe2O4 and Ca2Fe2O5) starting from mechanically activated mixtures of organic precursors (calcium citrate tetrahydrate and iron(III) oxalate hexahydrate). With the help of TG analysis the different stages of the mass loss process taking place could be individuated and their chemical nature assessed. The DSC signal, recorded on the same sample, revealed that much higher enthalpies are released from the milled mixtures than those expected on the basis of the reactions taking place. From such enthalpic excess the enthalpies of formation of Ca2Fe2O5 and CaFe2O4 have been obtained. X-ray powder diffraction (XRPD) indicated that the mechanically activated mixtures yield, after an 18h-annealing at 800°C (or 750°C), Ca2Fe2O5 (or CaFe2O4). On the contrary, the same compounds could be obtained, when starting from physical mixtures, only by performing thermal treatments (for t >18h) at 1180°C (Ca2Fe2O5) or 1100°C (CaFe2O4). [Copyright &y& Elsevier]

Calcium ferrite oxides were prepared by calcining a mixture powder of iron- and calcium oxide. The 57Fe-Mössbauer spectra of the calcium ferrites oxides were measured, revealing that the products should be Ca2Fe2O5 and CaFe2O4, the ratio of which was dependent of the Fe/Ca atomic ratio of the mixture powder. [ABSTRACT FROM AUTHOR]

The CaFe2O4 composition was studied, in order to analyze its physical and chemical behavior, verifying its stability under industrial conditions, when applied as pigments. The characterization was accomplished using thermal analysis, X-ray diffraction (XRD), nitrogen adsorption, scanning electronic microscopy (SEM) and diffuse reflectance. It observed successions exothermics reactions, adequate in events of thermal decomposition of the organic material, reach your stability in 700 °C. The material became completely crystalline at 800 °C. Between 700 and 1100 °C, the color was stabilized, showing an absorption band in the region of 650 to 750 nm, characteristic of the red color. [Copyright &y& Elsevier]

• We prepare the CaFe 2 O 4 nanocubes by one-step hydrothermal method. • The CaFe 2 O 4 nanocubes exhibit a high gas response to formaldehyde. • The CaFe 2 O 4 nanocubes present a superior humidity resistance. Formaldehyde (HCHO) is a harmful gas to human health. Here, the CaFe 2 O 4 nanocubes have been successfully synthesized via one-step hydrothermal method. The characterizations of the CaFe 2 O 4 have been carried out, such as XRD, BET, TEM and XPS, etc. The gas-sensing performance of the CaFe 2 O 4 to HCHO has been investigated. The CaFe 2 O 4 based sensor displays high gas response (16.50), fast response-recovery time (153 s, 54 s), excellent humidity resistance and longtime stability. This unique CaFe 2 O 4 nanocube can be used as a hopeful HCHO gas sensing material. [ABSTRACT FROM AUTHOR]

This study reports a facile preparation of black TiO 2-x /CaFe 2 O 4 (Ti3+ self-doped titania coupled with calcium ferrite), an efficient photocatalyst for ibuprofen (IBP) oxidation under visible light irradiation. Compared with TiO 2-x or CaFe 2 O 4 , TiO 2-x /CaFe 2 O 4 heterojunction displayed better photo-oxidation performance, and their immobilization onto diatomaceous earth (DE) further increased the oxidation rate due to the adsorption ability of DE. The morphology, phase composition and other physicochemical properties of the composite were studied using different characterization methods. The effects of several parameters on IBP oxidation efficiency were studied, such as pH, IBP concentration, catalyst dosage, co-existing ions, microplastics and peroxymonosulfate (PMS). Quenching experiment results demonstrated the roles of the following reactive species: hydroxyl radical, photo-induced hole, sulfate radical and superoxide radical. The intermediates during IBP oxidation were determined by HPLC-MS method, and their acute toxicity was studied. Furthermore, the plausible reaction pathway and mechanism were proposed. Unlabelled Image • Black TiO 2-x /CaFe 2 O 4 /diatomaceous earth photocatalyst was prepared. • The photocatalytic activity was evaluated by IBP elimination under visible light. • The magnetic catalyst showed good recyclability and stability. • The effects of various experimental and environmental factors were studied. • The active species, intermediates and mechanism were studied. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic properties of the calcium ferrite-type are reported. The static susceptibility data reveal a cusp at , suggesting a magnetic glassy transition. Although the crystal structure is essentially different from that of the spinel , the compound shows a similarity of the frustrated magnetism of the spinel. The probable magnetic frustration of the calcium ferrite-type compound might be due to a freezing of geometrically frustrated Mn magnetic moments in a triangular-like alignment. [Copyright &y& Elsevier]

The samples with the CaFe 2 O 4 -type crystal structure were obtained by the solid-state reaction method at 1000 °C in the air and the helium atmosphere for the first time. We investigated the modification of the structural and electronic properties of the obtained samples. Mössbauer, XAFS -, XPS -spectroscopies, and dc- , ac- conductivity measurements were carried out. Mössbauer and XAFS -spectroscopies showed that the local environment of Fe and Ca cations does not change in the case of the inert atmosphere synthesis. Nevertheless, a sharp six-order increase in the electrical resistance observed at room temperature for the sample obtained in the in the helium atmosphere. Moreover, calculated from dc -conductivity data activation energy rises from 0.327 for the air-synthesized sample to 0.585 eV for helium-obtained one. This behavior indicates significant modification of in-band-gap energy structure, which correlated with thermally activated charge carriers. Our ac -conductivity measurements in the frequency range of 1 kHz–2 MHz for the CaFe 2 O 4 obtained in the air showed the presence of defect levels in the energy band structure. Oxygen pressure reduction during the synthesis results in levels vanishing. Therefore, we suppose the key role of oxygen atoms in the transport properties of the material, which is indirectly confirmed by XPS data. In prospect, CaFe 2 O 4 can be used in promising gas analyzers. Image 1 • First synthesis of CaFe 2 O 4 in a helium atmosphere with a low oxygen pressure (10−5–10−9 kPa). • It is shown that CaFe 2 O 4 structure is very stable for selected synthesis conditions. • We show a key role of the oxygen atoms in the formation of the additional levels in the bandgap. [ABSTRACT FROM AUTHOR]

Abstract: The metallic compound forms a full range solid solution to the insulating phase . At a Na concentration of 0.25mole per formula unit, we found an unexpected contribution to the specific heat at low temperature [K. Yamaura, et al., Chem. Mater. 17 (2005) 359]. To address this issue, specific heat and AC and DC magnetic susceptibilities were additionally measured under a variety of conditions for the sample. A new set of data clearly indicate the additional specific heat is magnetic in origin; however, the magnetic entropy is fairly small ( of Schottky term for a simple splitting doublet), and there is no other evidence to suggest that a magnetic phase transition is responsible for the anomalous specific heat. [Copyright &y& Elsevier]

Artificial photosynthesis constitutes one of the most promising alternatives for harvesting solar energy in the form of fuels, such as hydrogen. Among the different devices that could be developed to achieve efficient water photosplitting, tandem photoelectrochemical cells show more flexibility and offer high theoretical conversion efficiency. The development of these cells depends on finding efficient and stable photoanodes and, particularly, photocathodes, which requires having reliable information on the mechanism of charge transfer at the semiconductor/solution interface. In this context, this work deals with the preparation of thin film calcium ferrite electrodes and their photoelectrochemical characterization for hydrogen generation by means of electrochemical impedance spectroscopy (EIS). A fully theoretical model that includes elementary steps for electron transfer to the electrolyte and surface recombination with photogenerated holes is presented. The model also takes into account the complexity of the semiconductor/solution interface by including the capacitances of the space charge region, the surface states and the Helmholtz layer (as a constant phase element). After illustrating the predicted Nyquist plots in a general manner, the experimental results for calcium ferrite electrodes at different applied potentials and under different illumination intensities are fitted to the model. The excellent agreement between the model and the experimental results is illustrated by the simultaneous fit of both Nyquist and Bode plots. The concordance between both theory and experiments allows us to conclude that a direct transfer of electrons from the conduction band to water prevails for hydrogen photogeneration on calcium ferrite electrodes and that most of the carrier recombination occurs in the material bulk. In more general vein, this study illustrates how the use of EIS may provide important clues about the behavior of photoelectrodes and the main strategies for their improvement.

Three structures of CaMn2O4, CaFe2O4, and CaTi2O4 have been proposed as post-spinel phases. Because these structures are very similar, several ambiguities and inconsistencies appear in high-pressure studies, leading to many problems that are yet to be solved. Systematic powder diffraction studies related to these three phases were conducted under high pressure using synchrotron radiation. All three samples have further high-pressure polymorphs. CaMn2O4 transforms to the CaTi2O4-type structure at about 30 GPa. The MnO6 octahedron in the lower-pressure structure is distorted by the Jahn-Teller effect. A new phase was observed at pressures above 50 GPa during compression of CaFe2O4. Rietveld profile fitting analysis of diffraction data at 63.3 GPa demonstrated that the high-pressure structure, with space group Pnam, is produced via a martensitic transformation by displacing atoms in every third layer perpendicular to the c axis. CaTi2O4 also has a new high-pressure polymorph above 39 GPa with space group Bbmm. The most probable post-spinel candidate in the mantle is the CaTi2O4-type structure. The CaMn2O4-type structure is only formed at high pressure from spinel phases with atoms susceptible to Jahn-Teller distortion. [ABSTRACT FROM AUTHOR]