Role of chromium: Iron ratio and oxygen partial pressure on the processing and chemical stability of iron doped lanthanum strontium chromite based OTM

The present research, as a part of the OTM materials development and testing effort, examines the combined effect of the Cr: Fe ratio (7:3, 8:2 and 9:1) and oxygen partial pressure (PO₂) on the densification, microstructural development, and chemical stability of lanthanum chromite (La_0.8Sr_0.2)_0.95Cr_1-xFe_xO₃ (LSCrF) for application in oxygen transport membrane and solid oxide fuel cell electrode. While highest density of 96.3 (±0.5) % is achieved for LSCrF with Cr: Fe ratio of 7:3 at 1400 °C and PO₂ ～ 10^?10 atm., the relative density decreases with increase in Cr: Fe ratio and PO₂. LSCrF perovskite stability increases with increase in Cr: Fe ratio in reducing gas atmosphere. LSCrF (7:3) dissociates into FeO_x and Fe_1+xCr_2-xO₄ under reducing gas atmosphere (Ar-3%H₂-3%H₂O). LSCrF (9:1) perovskite does not show any evidence of second phase (FeO_x and Fe_1+xCr_2-xO₄) formation with decrease in PO₂ unlike LSCrF (7:3 and 8:2). Defect chemistry and mechanism for FeO_x and Fe_1+x Cr_2?x O₄ formation in reducing atmosphere is described. LSCrF decomposition and the formation of the secondary phases are in agreement with the thermodynamic simulation results obtained with the La-Sr-Cr-Fe-O thermodynamic database.     

Transport and electrochemical properties of the LiyCrxMn2−xO4 (0 < x < 0.5) cathode material

In this paper structural, electrical, electrochemical and thermal (DSC) characterization of series of manganese spinel samples with manganese substituted to different degree (x = 0–0.5) with chromium are presented. The conductivity and thermoelectric power measurements were performed in wide temperature range also versus oxygen partial pressure and for deintercalated samples. Electrochemical studies of these cathode materials were conducted in Li/Li⁺/Li_yCr_xMn_2−xO₄ type cells. Substitution of manganese with chromium causes disappearance of the phase transition characteristic of LiMn₂O₄ spinel. Studies of electrical properties reveal that Cr ions do not participate in charge transport at low temperatures. In the charge curves of Li/Li⁺/Li_yCr_xMn_2−xO₄ cells there are two visible plateaux, separated with distinct potential jump (∼0.5 V), which position on Li content perfectly matches the Mn³⁺ content in the doped cathode material. The lower plateau is related to the Mn³⁺ → Mn⁴⁺ oxidation, while the next of higher voltage, of the dopant Cr³⁺ → Cr⁴⁺ oxidation. The schematic diagrams of relative Mn–Cr electronic levels alignment are proposed.     

Effects of F for enhancement of H2 evolution on visible-light-driven photocatalyst of SrTiO3:Cr/Ta/F prepared by spray pyrolysis

Both anion (F⁻) and cation (Cr³⁺/Ta⁵⁺) doping were conducted simultaneously to prepare tri-doped photocatalyst, SrTiO₃:Cr/Ta/F, by spray pyrolysis from aqueous precursor solutions. The ternary doping of F⁻ ion into SrTiO₃:Cr/Ta increased the hydrogen evolution rate up to 3887.9 μmol g⁻¹ h⁻¹ (quantum efficiency: 6.22%) under visible light irradiation, which was 1.84 times higher than that over SrTiO₃:Cr/Ta. The optimum contents of dopants for the maximum hydrogen evolution rate were Cr (0.1 mol%), Ta (0.1 mol%) and F (6.0 mol%), respectively. The ternary doping of F⁻ ion contributed to the reduction of Ti⁴⁺ and Cr⁶⁺ ions to Ti³⁺ and Cr³⁺ ions, respectively, by the generation of extra electrons in the lattice. The partially substituted structure of the photocatalyst could be expressed as Sr(Ti⁴⁺_1−2x−zTi³⁺_zCr³⁺_x−yCr⁶⁺_yTa⁵⁺_x)(O²⁻_3−wF⁻_w).     

Cu supported on various oxides as a candidate catalyst for dry methane reforming in DIR-SOFCs systems

A series of Cu-support systems were tested as potential candidates for DIR-SOFC (Direct Internal Reforming SOFC) catalysts towards a dry reforming of methane (DRM). The various supports (α-Al₂O₃, CeO₂, ZrO₂, SrTiO₃) with comparable specific surface area (SSA), and additionally γ-Al₂O₃ with SSA an order of magnitude larger than that of the other supports has been applied. The obtained Cu-support systems were characterized in terms of structure (XRD, XPS), microstructure (SEM), redox properties (TPR/TPOx), and next their catalytic activity and selectivity in DRM reaction were tested. All Cu-support materials show catalytic activity in DRM reaction, but only activity of Cu–SrTiO₃ is high (due to the incorporation of Cu into SrTiO₃ structure). The catalytic activity of other materials depends on the copper oxidation state (Cu²⁺ and Cu⁺). The highest catalytic activity in DRM process was obtained for Cu–AlO(OH) catalyst thanks to an order of greater SSA than in the case of other systems.     

Effects of synthesis temperature and precursor composition on the crystal structure, morphology, and electrode activity of 1D nanostructured manganese oxides

1D nanostructured manganese oxides are prepared by oxidation reaction of precursor LiMn_2−xCr_xO₄ microcrystals under hydrothermal condition. The crystal structure and morphology of the obtained manganese oxides are strongly dependent on the reaction condition and the chemical composition of the precursors. The α-MnO₂ nanowires are prepared by reaction at 120 °C, and their aspect ratios decrease with the Cr content in the precursor. Treating precursors with persulfate ions at 160-180 °C yields the β-MnO₂ nanorods for the precursors LiMn_2−xCr_xO₄ with lower Cr content and the α-MnO₂ nanowires for the precursors with higher Cr content. The structure dependence of the products on the Cr content in the precursors is related to the high octahedral site stabilization energy of Cr³⁺ ions and/or to the increase of Mn valence state upon Cr substitution. The increase of Cr content in the precursors degrades the electrode performance for the manganates prepared at 160 °C but improves electrode activity for those prepared at 180 °C. This observation can be explained by the structural variation and chromium substitution of the hydrothermally treated manganates. We conclude that the use of spinel LiMn_2−xCr_xO₄ as precursors provides an effective way to synthesize 1D nanostructured manganate with tailored crystal structure and morphology.     

Highly dense and chemically stable proton conducting electrolyte sintered at 1200 °C

The BaCe_0.7Zr_0.1Y_0.2?xZn_xO_3?δ (x = 0.05, 0.10, 0.15, 0.20) has been synthesized by the conventional solid state reaction method for application in protonic solid oxide fuel cell. The phase purity and lattice parameters of the materials have been studied by the room temperature X-ray diffraction (XRD). Scanning electron microscopy (SEM) has been done for check the morphology and grain growth of the samples. The chemical and mechanical stabilities have been done using thermogravimetric analysis (TGA) in pure CO₂ environment and thermomechanical analysis (TMA) in Argon atmosphere. The XRD of the materials show the orthorhombic crystal symmetry with Pbnm space group. The SEM images of the pellets show that the samples sintered at 1200 °C are highly dense. The XRD after TGA in CO₂ and thermal expansion measurements confirm the stability. The particles of the samples are in micrometer ranges and increasing Zn content decreases the size. The conductivity measurements have been done in 5% H₂ with Ar in dry and wet atmospheres. All the materials show high proton conductivity in the intermediate temperature range (400–700 °C). The maximum proton conductivity was found to be 1.0 × 10^?2 S cm^?1 at 700 °C in wet atmosphere for x = 0.10. From our study, 10 wt % of Zn seems to be optimum at the B-site of the perovskite structure. All the properties studied here suggest it can be a promising candidate of electrolyte for IT-SOFCs.     

Selective substitution of Ni by Ti in LaNiO3 perovskites: A parameter governing the oxy-carbon dioxide reforming of methane

A series of LaNi_1?xTi_xO₃ perovskite catalysts varying titanium (x = 0.0, 0.2, 0.4, 0.5, 0.6 and 1.0) are synthesized and investigated using BET, XRD, TPR, TEM, FT-IR and XPS. The catalysts were evaluated for oxy-carbon dioxide reforming of methane at 800 °C under atmospheric pressure maintaining CO₂/CH₄/O₂ ratio 0.8/1.0/0.2. LaNi_0.5Ti_0.5O₃ is showing typical stability with gradual H₂ consumption in TPR. The stability of these catalysts is supported by O 1s binding energies wherein it is clearly evident that incorporation of Ti stabilized LaNiO₃ generating suitable catalysts in the range of x = 0.4–0.6 with high performance.     

Improving rate capability and decelerating voltage decay of Li-rich layered oxide cathodes by chromium doping

In order to improve the rate capability and mitigate the voltage decay of lithium rich layered oxides, chromium doped cathode materials Li_1.2[Mn_0.54Ni_0.13Co_0.13]_1?xCr_xO₂ (x = 0,0.003,0.005,0.007) is synthesized via co-precipitation method followed by calcination. As a result, Cr-doped samples present shortened voltage platform at 4.5 V, indicating that the escape of lattice oxygen is suppressed by Cr doping. Among all samples, Li_1.2[Mn_0.54Ni_0.13Co_0.13]_1?xCr_xO₂ (x = 0.005) sample shows the greatest rate capability (119.3 mAh g^?1 at 10 C) and the minimum voltage drop (0.6167 V) after 200 cycles at 1.0 C. The reasons for the improved electrochemical performance are the enhanced structural stability, the stronger CrO bond than MnO band and the decreased metal–oxygen covalency induced by Cr doping.     

CO2 reforming of methane over NixMg6−xAl2 catalysts: Effect of lanthanum doping on catalytic activity and stability

Ni_xMg_6?xAl₂ and Ni_xMg_6?xAl_1.8La_0.2 (x = 2, 4 or 6) catalysts were prepared via a co-precipitation method and calcined under an air flow at 800 °C. X-ray diffraction (XRD) results showed that the Ni_xMg_6?xAl_1.8La_0.2 catalysts contained different lanthanum oxide species after calcination. Fourier Transform Infrared Spectroscopy (FTIR) spectra demonstrated that the lanthanum doped catalysts adsorbed more CO₂ compared to the lanthanum free solids. This improved basicity was verified in the CO₂-TPD profiles. Temperature programmed reduction (TPR) analyses proved that the addition of lanthanum affected nickel species distribution in the catalysts and strengthened NiO-MgO interactions inside the solid matrix. The CO₂ reforming of methane reaction (Ar/CO₂/CH₄:60/20/20; GHSV 60000 mL g^?1 h^?1) was carried out over the different catalysts in the temperature range of 600 °C–800 °C. Lanthanum addition improved the catalytic activity particularly by favoring the methane dry reforming reaction over all the other secondary reactions in addition to the creation of more basic sites that enhanced CO₂ adsorption and contributed to the removal of carbon deposits. The most active lanthanum containing catalyst kept a constant catalytic performance for 14 h on stream despite the formation of carbon deposits. These carbon deposits can be removed under an oxidative atmosphere at moderate temperature due to the presence of lanthanum oxide species in the catalyst.     

La0.8Sr0.2Cr0.95Ru0.05O3−x and Sm0.8Ba0.2Cr0.95Ru0.05O3−x as partial oxidation catalysts for diesel

La_0.8Sr_0.2Cr_0.95Ru_0.05O_3−x (LSCR) and Sm_0.8Ba_0.2Cr_0.95Ru_0.05O_3−x (SBCR), Ru-substituted perovskite catalysts, are investigated for the partial oxidation (POX) of diesel to produce hydrogen-rich gases for fuel cell applications. Metal-substituted perovskite materials have been investigated as reforming catalysts because the metal atoms are well-dispersed in the perovskite structure. However, Ru de-mixing and a secondary phase of LSCR and SBCR are observed after reduction at high temperature. The thermal stability, sulfur tolerance and aromatic decomposition over LSCR and SBCR are compared to those over Ru on Ce_0.9Gd_0.1O_2−x (CGO). During the thermal stability test, the catalytic activities of LSCR and SBCR improve after operating at 1000 °C. The sulfur tolerance and aromatic decomposition activity of LSCR and SBCR improve when the temperature increases to 950 °C. The improvements are attributed to the de-mixed Ru from the perovskite structure at high temperatures under a reductive atmosphere.     

MgFe and Mg–Co–Fe mixed oxides derived from hydrotalcites: Highly efficient catalysts for COx free hydrogen production from NH3

Mg–Fe Layered Double Hydroxide (LDH) with M²⁺: M³⁺ 3:1 stoichiometric ratio was synthesized and employed as catalyst precursor for CO_x-free hydrogen production from ammonia. The resulting catalyst showed good catalytic activity. A series of Mg/Co–Fe layered double hydroxides were synthesized by replacing Mg²⁺ with Co²⁺ without disturbing M²⁺:M³⁺ ratio. The influence of nature and extent of Co(II) substitution on structure, morphology and surface properties were studied. A systematic study was carried out using these materials as catalyst precursors for ammonia decomposition. BET, XRD, TPR, XPS, CO₂-TPD and TEM techniques were used to characterize the synthesized catalysts. These Fe-based catalysts are highly active, highly stable and not promoting any stable surface nitridation during the ammonia decomposition reaction. Among all catalysts, the Mg₃Co₃Fe₂ catalyst showed the highest activity i.e. 100% conversion at 6,000 h⁻¹ and 60% at 50,000 h⁻¹ space velocities at 550 °C. The registered superior catalytic activity was result of the formed specific catalyst's properties like high surface area, high surface Co and Fe atomic concentration and suitable basicity. These Fe-based materials are, cost-effective, easily synthesize and highly stable, thus attractive for large-scale operation.     

Dy doped SrTiO3: A promising anodic material in solid oxide fuel cells

The perovskite-type oxides, having a general formula ABO₃, are promising candidates for anode materials in solid oxide fuel cells. In particular, doped SrTiO₃ based perovskites are potential mixed ionic-electronic conductors and they are known to have excellent thermal and chemical stability along with carbon and sulfur tolerance. In this work, Dy_xSr_1-xTiO_3-δ system with x = 0.03, 0.05, 0.08 and 0.10 is studied to understand the influence of Dy content on its structural and electrical behavior. Electrochemical properties are measured, both in air and hydrogen atmosphere, and structural characterizations are performed before and after electrochemical tests and compared each other to study the stability. Results show that Dy_xSr_1-xTiO_3-δ powders with x ≤ 0.05, are single phase, while for x ≥ 0.08 a small amount of secondary phases is formed. In air, the conductivity is predominantly mixed ionic-electronic type for x ≤ 0.05, becoming ionic for x ≥ 0.08. It is observed that conductivity, for each composition, increases passing from air to hydrogen and activation energy decreases. Dy_0.05Sr_0.95TiO_3-δ shows the highest conductivity in air whereas Dy_0.08Sr_0.92TiO_3-δ in H₂ atmosphere. Degradation observed by XRD is negligible for x ≤ 0.05 but increases with higher Dy content.     

Novel SrTixCo1−xO3−δ cathodes for low-temperature solid oxide fuel cells

The SrTi_xCo_1−xO_3−δ (STC, x = 0.05, 0.1, 0.15, 0.2) perovskite-type oxides synthesized by the polymerized complex (PC) method have been investigated as cathode materials for low-temperature solid oxide fuel cells (SOFCs) with composite electrolyte for the first time. Thermogravimetry-differential thermal analysis (TG-DTA) shows the crystallization of SrTi_0.1Co_0.9O_3−δ occurs at 780 °C. The oxides have been stabilized to be a cubic perovskite phase after the B-site is doped with Ti ion. The maximum power density reaches as high as 613 mW cm⁻² at 600 °C for SOFC with SrTi_0.2Co_0.8O_3−δ cathode. The maximum power densities increase with the increasing Ti content in the cathode, which can be attributed to the enhancement of conductivity and electrocatalytic activity. The stability of the fuel cell with SrTi_0.1Co_0.9O_3−δ cathode has been examined for 18 h at 600 °C. Only a slight decline in the cell performance can be observed with increasing time. The high performance cathodes together with the low-cost fabrication technology are highly encouraging for development of low-temperature SOFCs.     

La1−xSrxFe0.7Ni0.3O3−δ as both cathode and anode materials for Solid Oxide Fuel Cells

La_1?xSr_xFe_0.7Ni_0.3O_3?δ (x = 0, 0.1 and 0.2, LSFNx) are investigated as both cathode and anode materials for Solid Oxide Fuel Cells (SOFCs). The structure, microstructure and electrochemical properties of these materials are studied under oxidizing and reducing atmospheres. In air, the electrodes exhibit polarization resistance of 0.1 Ωcm² at 800 °C under open circuit voltage. In a H₂ atmosphere, Ni nanoparticles are exsolved on the surface, leading to a polarization resistance as low as 0.06 Ωcm². A cell with a 350 μm thick La_0.9Sr_0.1Ga_0.8Mg_0.2O_3?δ electrolyte and LSFN electrodes generates a power output of 540 mWcm^?2 at 800 °C. Moreover, stable values of power density are obtained after successive oxidation/reduction cycles, confirming the reversibility of the electrodes.     

Synthesis, characterization and evaluation of PrBaCo2−xFexO5+δ as cathodes for intermediate-temperature solid oxide fuel cells

Iron doped layered structured perovskites, PrBaCo_2−xFe_xO_5+δ (x = 0, 0.5, 1.0, 1.5 and 2.0), are evaluated as cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The effects of dopant content are investigated on their structural and electrochemical properties including crystalline structure, oxygen nonstoichiometry, stability in presence of CO₂, compatibility with electrolytes, thermal expansion coefficient, electrical conductivity, and cathodic interfacial polarization resistance. The lattice parameter and oxygen nonstoichiometry content, δ, at room temperature increase, whereas the conductivity, thermal expansion coefficient, and cathodic performance decrease with increasing iron content, x. PrBaCo_2−xFe_xO_5+δ exhibit excellent stability at 700 °C in atmosphere consisting of 3% CO₂ and 97% air, show good chemical compatibility with doped ceria electrolytes at 1000 °C, but react readily with yttria-stabilized zirconia at 700 °C. Even with a Co-free PrBaFe₂O_5+δ as the electrode, a symmetrical cell demonstrates area specific resistance of 0.18 Ω cm² at 700 °C with samaria-doped ceria electrolyte. The resistance is lower than those for typical Co-free electrodes reported in the literatures, suggesting that PrBaCo_2−xFe_xO_5+δ are potential promising cathode materials for IT-SOFCs.     

Electrical conductivity and cell performance of La0.3Sr0.7Ti1−xCrxO3−δ perovskite oxides used as anode and interconnect material for SOFCs

The anode materials La_0.3Sr_0.7Ti_1−xCr_xO_3−δ (LSTC, x = 0, 0.1, 0.2) with cubic structure were prepared via solid state reaction route. The influence of Cr content on the properties of LSTC as anode and interconnect materials for solid oxide fuel cells (SOFCs) was investigated. The Cr-doping decreased the lattice parameter while increased the sinterability of LSTC materials. The total electrical conductivity decreased with Cr doping level, from 230 S cm⁻¹ for x = 0 to 53 S cm⁻¹ for x = 0.2. The total electrical conductivity exhibited good stability and recoverability in alternative atmospheres of air and 5% H₂/Ar, showing excellent redox stability. The cell testing showed that the anode performance of LSTC was enhanced somewhat by Cr doping. The present results indicated that the prepared La_0.3Sr_0.7Ti_1−xCr_xO_3−δ can be potential anode and interconnect materials for SOFCs.     

Ba-doped Pr2NiO4+δ electrodes for proton-conducting electrochemical cells. Part 1: Structure,mechanical, and chemical properties

Materials based on Pr₂NiO_4+δ have attracted widespread research attention as air electrodes for proton conducting electrochemical devices due to a wide range of promising properties, such as low values of polarization resistance and high conductivities. However, problems of their chemical interaction with electrolyte materials are insufficiently investigated. In the present work, we optimize the thermomechanical and chemical properties of Pr₂NiO_4+δ-based electrode materials via a Ba-doping strategy (Pr_2–xBa_xNiO_4+δ) to reduce their chemical interaction with the-state-of-the-art Ba(Ce,Zr)O₃-based electrolytes. A decrease in the chemical interaction degree between the Ba-doped nickelites and Ba(Ce,Zr)O₃-based oxide was confirmed experimentally. The average values of the thermal expansion coefficients were found to decrease from 13.9·10⁻⁶ К⁻¹ for the undoped material to 13.5·10⁻⁶ К⁻¹ for the material with x = 0.2. The barium-doped materials were found to have good thermal stability and acceptable stability in a CO₂ environment. This work, being the first part of a comprehensive analysis, reports advantages in chemical stability and mechanical properties of the developed Pr_2–xBa_xNiO_4+δ materials.     

Co-doping schemes to enhance H2 evolution under visible light irradiation over SrTiO3:Ni/M (M = La or Ta) prepared by spray pyrolysis

Two photocatalysts, SrTiO₃:Ni/La and SrTiO₃:Ni/Ta, were prepared by continuous spray pyrolysis. The effects of the co-dopants on hydrogen evolution over the uncalcined photocatalysts were evaluated under visible light irradiation. The co-doping of La³⁺ into SrTiO₃:Ni transformed the charge structure and increased the presence of Ni²⁺ at the expense of Ni³⁺ in the host lattice structure. The co-doping of Ta⁵⁺ into SrTiO₃:Ni also increased the Ni²⁺/Ni³⁺ ratio around the Ti⁴⁺ ions. Compared with SrTiO₃:Ni, SrTiO₃:Ni/La showed a 3 times greater rate of hydrogen evolution under visible light irradiation and SrTiO₃:Ni/Ta, a 4 times greater rate. The co-doping levels required for optimized hydrogen evolution over SrTiO₃:Ni/La and SrTiO₃:Ni/Ta prepared by spray pyrolysis were smaller than those prepared by other methods. Spray pyrolysis also produced particles with large surface areas and high roughnesses.     

Property evaluation of Sm1‐xSrxFe0.7Cr0.3O3‐δ perovskites as cathodes for intermediate temperature solid oxide fuel cells

Perovskite‐type (ABO₃) complex oxides of Sm_1‐xSr_xFe_0.7Cr_0.3O_3‐δ (x = 0.5‐0.7) series were prepared by a glycine‐nitrate combustion process. The crystal structure, oxygen nonstoichiometry, electrical conducting, thermal expansion, and electrocatalytic properties of Sm_1‐xSr_xFe_0.7Cr_0.3O_3‐δ perovskites were inspected in view of their use as cathode materials for intermediate temperature solid oxide fuel cells (IT‐SOFCs). Changing the content of Sm³⁺ at the A‐site was demonstrated to be effective in tuning the structure and properties. The variation of the various properties with Sm³⁺ content was explained in relation to the corresponding evolution of the crystal structure and oxygen nonstoichiometry. Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3‐δ (x = 0.7) was determined to be the optimal composition in the Sm_1‐xSr_xFe_0.7Cr_0.3O_3‐δ series based on a trade‐off between the thermal expansion and electrocatalytic properties. Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3‐δ ceramic specimen exhibited an electrical conductivity of approximately 40 S·cm^?1 at 800°C and a thermal expansion coefficient of 14.1 × 10^?6 K^?1 averaged in the temperature range from 40°C to 1000°C. At 800°C in air, Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3‐δ electrode showed a cathodic polarization resistance of 0.19 Ω·cm², a cathodic overpotential of 30 mV at current density of 200 mA·cm^?2, and an exchange current density of 257 mA·cm^?2. It is suggested that Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3‐δ is a potential candidate material for cathode of IT‐SOFCs in light of its overall properties.     

Spectrophotometric study of the corrosion behaviour of chromium in molten alkali carbonates

The corrosion of chromium species in molten (Li/K)₂CO₃ and (Li/Na/K)₂CO₃ are investigated by spectrophotometry. A strong charge-transfer absorption band is observed in the ultraviolet region with λ_max=372 nm, and the molar absorptivity of CrO₄²⁻ in molten (Li/K)₂CO₃ is calculated. The corrosion rates are investigated by means of increasing the concentration of CrO₄²⁻ in the melts. Under an air atmosphere, the corrosion rate is slower in molten (Li/K)₂CO₃ than in molten (Li/Na/K)₂CO₃ and Cr₂O₃ corrodes more easily in molten (Li/K)₂CO₃ than Cr. With an atmosphere of CO₂=1 atm, there is no increase in the CrO₄²⁻ concentration in the solution phase during a period of 5 h.