Search Results (34)

In order to reduce the sintering temperature, MnO₂ was used as a sintering aid to prepare [(Ba_0.85Ca_0.15)_0.999(Dy_0.5Tb_0.5)_0.001](Zr_0.1Ti_0.9)O₃-x mol% MnO₂ (BCDTZT-x mol% MnO₂, x = 0.05, 0.2, 0.4, 0.6, 0.8, 1, 1.5, 3) lead-free piezoelectric ceramics in which the effects of the MnO₂ doping amount and sintering temperature on the phase structure, sintering behavior, and electrical properties of the BCDTZT-x mol% MnO₂ ceramics were systematically analyzed. All ceramics have a single perovskite structure and coexist in multiple phases. The optimal sintering temperature was reduced from 1515 °C to 1425 °C, and the density of all ceramics was increased as compared with the undoped ceramic, reaching a maximum of 5.38 g/cm³ at x = 0.8 mol%. An appropriate MnO₂ doping amount of 0.4 mol% could effectively suppress oxygen vacancies and improve electrical properties, resulting in the best comprehensive performance of the ceramics, with a dielectric constant maximum of 12,817, a high piezoelectric constant of 330 pC/N, and good strain value (S_max = 0.118%) and low strain hysteresis (Hys = 2.66%). The calculation of activation energy indicated that the high-temperature conductivity was dominated by oxygen vacancies in all ceramics. The results showed that the appropriate introduction of MnO₂ as a sintering aid could improve the performance of BCZT-based ceramics while reducing the sintering temperature, presenting high practical application value in the fields of low electric field sensors and actuators. Full article

In this work, a series of Ba_xMn_0.7Cu_0.3O₃ samples (x: 1, 0.9, 0.8, and 0.7, BxMC) was synthesized, characterized, and used as catalysts for CO oxidation reaction. All formulations were active for CO oxidation in the tested conditions. A correlation between the electrical conductivity, obtained by impedance spectroscopy, and the reducibility of the samples, obtained by H₂-TPR, was observed. The Ba_0.8Mn_0.7Cu_0.3O₃ composition (B0.8MC) showed the best catalytic performance (comparable to that of the 1% Pt/Al₂O₃ reference sample) during tests conducted under conditions similar to those found in the exhaust gases of current gasoline engines. The characterization data suggest the simultaneous presence of a high Mn(IV)/Mn(III) surface ratio, oxygen vacancies, and reduced copper species, these two latter being key properties for ensuring a high CO conversion percentage as both are active sites for CO oxidation. The reaction temperature and the reactant atmosphere composition seem to be the most important factors for achieving a good catalytic performance, as they strongly determine the location and stability of the reduced copper species. Full article

Solid Oxide Electrolysis Cells (SOECs) can electro-reduce carbon dioxide to carbon monoxide, which not only effectively utilizes greenhouse gases, but also converts excess electrical energy into chemical energy. Perovskite-based oxides with exsolved metal nanoparticles are promising cathode materials for direct electrocatalytic reduction of CO₂ through SOECs, and have thus received increasing attention. In this work, we doped Pr_0.7Ba_0.3MnO_3−δ at the B site, and after reduction treatment, metal nanoparticles exsolved and precipitated on the surface of the cathode material, thereby establishing a stable metal–oxide interface structure and significantly improving the electrocatalytic activity of the SOEC cathode materials. Through research, among the Pr_0.7Ba_0.3Mn_1−xNi_xO_3−δ (PBMN_x = 0–1) cathode materials, it has been found that the Pr_0.7Ba_0.3Mn_0.9Ni_0.1O_3−δ (PBMN_0.1) electrode material exhibits greater catalytic activity, with a CO yield of 5.36 mL min⁻¹ cm⁻² and a Faraday current efficiency of ~99%. After 100 h of long-term testing, the current can still remain stable and there is no significant change in performance. Therefore, the design of this interface has increasing potential for development. Full article

Mixed oxides featuring perovskite-type structures (ABO₃) offer promising catalytic properties for applications focused on the control of atmospheric pollution. In this work, a series of Ba_xMnO₃ (x = 1, 0.9, 0.8 and 0.7) samples have been synthesized, characterized and tested as catalysts for CO oxidation reaction in conditions close to that found in the exhausts of last-generation automotive internal combustion engines. All samples were observed to be active as catalysts for CO oxidation during CO-TPRe tests, with Ba_0.7MnO₃ (B0.7M) being the most active one, as it presents the highest amount of oxygen vacancies (which act as active sites for CO oxidation) and Mn (IV), which features the highest levels of reducibility and the best redox properties. B0.7M has also showcased a high stability during reactions at 300 °C, even though a slightly lower CO conversion is achieved during the second consecutive reaction cycle. This performance appears to be related to the decrease in the Mn (IV)/Mn (III) ratio. Full article

Ba_1−xCe_xMnO₃ (BM-Ce_x) and Ba_1−xLa_xMn_0.7Cu_0.3O₃ (BMC-La_x) perovskite-type mixed oxides were synthesized using the sol–gel method adapted for aqueous media with different values of x (0, 0.1, 0.3, 0.6) to estimate the effect of the degree of the partial substitution of Ba by Ce or La on the structure and properties that are relevant for their use as catalysts for gasoline direct injection (GDI) soot oxidation. The samples were deeply characterized by ICP-OES, XRD, XPS, N₂ adsorption, H₂-TPR, and O₂-TPD, and their potential as catalysts for soot oxidation has been analyzed in various scenarios that replicate the exhaust conditions of a GDI engine. By comparing the catalytic performance for soot oxidation of the two tested series (BM-Cex and BMC-Lax) and in the two conditions used (100% He and 1% O₂ in He), it could be concluded that (i) in the absence of oxygen in the reaction atmosphere (100% He), BMC-La_0.1 is the best catalyst, as copper is also able to catalyze the soot oxidation; and (ii) if oxygen is present in the reaction atmosphere (1% O₂/He), BM-Ce_0.1 is the most-active catalyst as it presents a higher proportion of Mn(IV) than BMC-La_0.1. Thus, it seems that the addition of an amount of Ce or La higher than that corresponding to x = 0.1 in Ba_1−xCe_xMnO₃ and Ba_1−xLa_xCu_0.3Mn_0.7O₃ does not allow us to improve the catalytic performance of BM-Ce_0.1 and BMC-La_0.1 for soot oxidation in the tested conditions. Full article

The sol–gel method, adapted to aqueous media, was used for the synthesis of BaMn_0.7Cu_0.3O₃ (BMC) and Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A, A = Ce, La or Mg) perovskite-type mixed oxides. These samples were fully characterized by ICP-OES, XRD, XPS, H₂-TPR, BET, and O₂–TPD and, subsequently, they were evaluated as catalysts for CO oxidation under different conditions simulating that found in cars exhaust. The characterization results show that after the partial replacement of Ba by A metal in BMC perovskite: (i) a fraction of the polytype structure was converted to the hexagonal BaMnO₃ perovskite structure, (ii) A metal used as dopant was incorporated into the lattice of the perovskite, (iii) oxygen vacancies existed on the surface of samples, and iv) Mn(IV) and Mn(III) coexisted on the surface and in the bulk, with Mn(IV) being the main oxidation state on the surface. In the three reactant atmospheres used, all samples catalysed the CO to CO₂ oxidation reaction, showing better performances after the addition of A metal and for reactant mixtures with low CO/O₂ ratios. BMC-Ce was the most active catalyst because it combined the highest reducibility and oxygen mobility, the presence of copper and of oxygen vacancies on the surface, the contribution of the Ce(IV)/Ce(III) redox pair, and a high proportion of surface and bulk Mn(IV). At 200 °C and in the 0.1% CO + 10% O₂ reactant gas mixture, the CO conversion using BMC-Ce was very similar to the achieved with a 1% Pt/Al₂O₃ (Pt-Al) reference catalyst. Full article

BaMnO₃ (BM) and Ba_0.9A_0.1MnO₃ (BM-A) (A = Ce, La or Mg) perovskite-type mixed oxides were synthesized by the aqueous sol–gel method; thoroughly characterized by ICP-OES, XRD, H₂-TPR, BET, and O₂-TPD; and tested as catalysts for CO oxidation under simulated automobile exhaust conditions. The characterization results indicate that the main effects of the partial substitution of Ba with A-metal in BM perovskite are the maintenance of the hexagonal structure of the perovskite and the increase in reducibility and oxygen mobility. All samples catalyze the CO to CO₂ oxidation reaction in the different reactant mixtures employed, showing the best performance for the mixture with the lowest CO/O₂ ratio and in the presence of a dopant in the BM perovskite formulation. BM-La is the most active catalyst for improving CO oxidation, as it is the most reducible, and because is able to evolve oxygen at intermediate temperatures. Full article

The oxygen vacancy formation behavior and electrochemical and thermal properties of Ba_0.5Sr_0.5Fe_1−xMn_xO_3−δ (BSFMnx, x = 0–0.15) cathode materials were investigated. For thermogravimetric analysis, the weight decreased from 1.98% (x = 0) to 1.81% (x = 0.15) in the 400–950 °C range, which was due to oxygen loss from the lattice. The average oxidation state of the B-site increased, the O_ads/O_lat ratio decreased, and the binding energy of the O_lat peak increased with Mn doping. These results indicate that Mn doping increases the strength of the metal–oxygen bond and decreases the amount of oxygen vacancies in the lattice. The electrical conductivity of BSFMnx increased with the temperature due to the thermally activated small-polaron hopping mechanism showing a maximum value of 10.4 S cm⁻¹ (x = 0.15) at 450 °C. The area-specific resistance of BSFMn0.15 was 0.14 Ω cm² at 700 °C and the thermal expansion coefficient (TEC) gradually decreased to 12.7 × 10⁻⁶ K⁻¹, which is similar to that of Ce_0.8Sm_0.2O₂ (SDC) (12.2 × 10⁻⁶ K⁻¹). Mn doping increased the metal–oxygen bonding energy, which reduced the oxygen reduction reaction activity but improved the electrical conductivity and thermal stability with SDC. Full article

In this study, we analyzed the role of a series of BaMn_1−xNi_xO₃ (x = 0, 0.2, and 0.4) mixed oxide catalysts, synthesized using the sol–gel method, in NO_x-assisted diesel soot oxidation. ICP-OES, XRD, XPS, and H₂-TPR techniques were used for characterization and Temperature-Programmed Reaction experiments (NO_x-TPR and Soot-NO_x-TPR), and isothermal reactions at 450 °C (for the most active sample) were carried out to determine the catalytic activity. All samples catalyzed NO and soot oxidation at temperatures below 400 °C, presenting nickel-containing catalysts with the highest soot conversion and selectivity to CO₂. However, the nickel content did not significantly modify the catalytic performance, and in order to improve it, two catalysts (5 wt % in Ni) were synthesized via the hydrothermal method (BMN2H) and the impregnation of nickel on a BaMnO₃ perovskite as support (M5). The two samples presented higher activity for NO and soot oxidation than BMN2E (obtained via the sol–gel method) as they presented more nickel on the surface (as determined via XPS). BMN2H was more active than M5 as it presented (i) more surface oxygen vacancies, which are active sites for oxidation reactions; (ii) improved redox properties; and (iii) a lower average crystal size for nickel (as NiO). As a consequence of these properties, BMN2H featured a high soot oxidation rate at 450 °C, which hindered the accumulation of soot during the reaction and, thus, the deactivation of the catalyst. Full article

The conversion of hydrogen to power via combined external reforming of liquid alcohol and solid oxide fuel cell (SOFC) technology is an effective approach to address future energy challenges. In this study, an La_0.8Ba_0.1Mn_0.8Ni_0.1Cu_0.1O₃ (LBMNCu) perovskite nanofiber with high porosity was synthesized with a modified electrostatic spinning method, which acted as an efficient catalyst for steam reforming of liquid alcohols (methanol and ethanol). After reduction, fine metallic Ni-Cu was uniformly distributed throughout the perovskite nanofiber surface. The obtained composite displayed a methanol conversion above 99.9% at 450 °C and an ethanol conversion above 99% at 600 °C, which was highly superior to the common Ni-Cu/Al₂O₃ catalyst. The catalytic performance of our assembled catalysts also remained stable in methanol and ethanol atmospheres for 50 h and no coking was detected. Furthermore, when the reformed gas was fed into a Y_0.08Zr_0.92O₂ (YSZ)-based SOFC system, the open circuit voltage remained around 1.1 V at 700 °C for 50 h accordingly, without coking, and the voltage remained virtually unchanged at 0.7 V for 50 h at 700 °C and 400 mA cm⁻² during galvanostatic discharge mode, indicating that using LBMNCu nanofiber as a catalyst for hydrogen production and utilization is an efficient strategy. The interaction of the in situ exsolved metallic nanoparticles and nanofibrous perovskite could also be a promising approach for designing a highly active catalyst for H₂ generation. Full article

Ba_0.9A_0.1MnO₃ (BM-A) and Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A) (A = Mg, Ca, Sr, Ce, La) perovskite-type mixed oxides were synthesised, characterised, and used for soot oxidation in simulated Gasoline Direct Injection (GDI) engine exhaust conditions. The samples have been obtained by the sol-gel method in an aqueous medium and deeply characterised. The characterization results indicate that the partial substitution of Ba by A metal in BaMnO₃ (BM) and BaMn_0.7Cu_0.3O₃ (BMC) perovskites: (i) favours the hexagonal structure of perovskite; (ii) improves the reducibility and the oxygen desorption during Temperature-Programmed Desorption (O₂-TPD) tests and, consequently, the oxygen mobility; (iii) mantains the amount of oxygen vacancies and of Mn(IV) and Mn(III) oxidation states, being Mn(IV) the main one; and (iv) for Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A) series, copper is partially incorporated into the structure. The soot conversion data reveal that Ba_0.9La_0.1Mn_0.7Cu_0.3O₃ (BMC-La) is the most active catalyst in an inert (100% He) reaction atmosphere, as it presents the highest amount of copper on the surface, and that Ba_0.9Ce_0.1MnO₃ (BM-Ce) is the best one if a low amount of O₂ (1% O₂ in He) is present, as it combines the highest emission of oxygen with the good redox properties of Ce(IV)/Ce(III) and Mn(IV)/Mn(III) pairs. Full article

Ba_0.5Sr_0.5FeO_3−δ (BSF) nanofibers are prepared via electrospinning and sintering at a temperature of 800 °C for 2 h to produce a pure cubic perovskite phase, with the aim of finding a high-performance cathode material suitable for proton-conducting solid oxide fuel cells (PCFCs) operating at medium temperatures. The results of SEM analysis showed that the BSF nanofibers are alternately stacked, presenting fluffy and uniform high porosity, which is favorable to the cathode polarization reaction of PCFCs. To balance the excessive coefficient of thermal expansion of the BSF cathode, it is combined with a negative thermal expansion (NTE) material NdMnO_3−δ (NM). At 700 °C, the peak power density (PPD) of the BSF–NM cathode cell is 1170 mW cm⁻², and the open-circuit voltage (OCV) of the single cell is 1.04 V. The corresponding polarization impedance (R_p) of the BSF–NM is 0.037 Ω cm². This result is significantly better than the previously reported powder cathode BSF, indicating that the performance of PCFC can be further optimized by transforming the BSF cathode into a nanofiber state and compounding it with the NTE material NM. Full article

Far-red (FR) emitting LEDs are known as a promising supplement light source for photo-morphogenesis of plants, in which FR emitting phosphors are indispensable components. However, mostly reported FR emitting phosphors are suffering from problems of wavelength mismatch with LED chips or low quantum efficiency, which are still far from practical applications. Here, a new efficient FR emitting double-perovskite phosphor BaLaMgTaO₆:Mn⁴⁺ (BLMT:Mn⁴⁺) has been prepared by sol-gel method. The crystal structure, morphology and photoluminescence properties have been investigated in detail. BLMT:Mn⁴⁺ phosphor has two strong and wide excitation bands in the range of 250–600 nm, which matches well with a near-UV or blue chip. Under 365 nm or 460 nm excitation, BLMT:Mn⁴⁺ emits an intense FR light ranging from 650 to 780 nm with maximum emission at 704 nm due to ²E_g → ⁴A_2g forbidden transition of Mn⁴⁺ ion. The critical quenching concentration of Mn⁴⁺ in BLMT is 0.6 mol%, and its corresponding internal quantum efficiency is as high as 61%. Moreover, BLMT:Mn⁴⁺ phosphor has good thermal stability, with emission intensity at 423 K keeping 40% of the room temperature value. The LED devices fabricated with BLMT:Mn⁴⁺ sample exhibit bright FR emission, which greatly overlaps with the absorption curve of FR absorbing phytochrome, indicating that BLMT:Mn⁴⁺ is a promising FR emitting phosphor for plant growth LEDs. Full article

Hexagonal perovskite-related oxides Ba₇Ta_3.7Mo_1.3O_20.15 (BTM) have recently been reported as promising electrolyte materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this work, sintering properties, thermal expansion coefficient, and chemical stability of BTM were studied. In particular, the chemical compatibilities of (La_0.75Sr_0.25)_0.95MnO_{3±$\mathsf{\delta }$} (LSM), La_0.6Sr_0.4CoO₃ (LSC), La_0.6Sr_0.4Co_0.2Fe_0.8O_{3+$\mathsf{\delta }$} (LSCF), PrBaMn₂O_{5+$\mathsf{\delta }$} (PBM), Sr₂Fe_1.5Mo_0.5O_{6-$\mathsf{\delta }$} (SFM), BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_{3-$\mathsf{\delta }$} (BCFZY), and NiO electrode materials with the BTM electrolyte were evaluated. The results show that BTM is highly reactive with these electrodes, in particular, BTM tends to react with Ni, Co, Fe, Mn, Pr, Sr, and La elements in the electrodes to form resistive phases, thus deteriorating the electrochemical properties, which has not been reported before. Full article

Mixed oxides with perovskite-type structure (ABO₃) are promising catalysts for atmospheric pollution control due to their interesting and tunable physicochemical properties. In this work, two series of Ba_xMnO₃ and Ba_xFeO₃ (x = 1 and 0.7) catalysts were synthesized using the sol–gel method adapted to aqueous medium. The samples were characterized by μ-XRF, XRD, FT-IR, XPS, H₂-TPR, and O₂-TPD. The catalytic activity for CO and GDI soot oxidation was determined by temperature-programmed reaction experiments (CO-TPR and soot-TPR, respectively). The results reveal that a decrease in the Ba content improved the catalytic performance of both catalysts, as B0.7M-E is more active than BM-E for CO oxidation, and B0.7F-E presents higher activity than BF for soot conversion in simulated GDI engine exhaust conditions. Manganese-based perovskites (BM-E and B0.7M-E) achieve better catalytic performance than iron-based perovskite (BF) for CO oxidation reaction due to the higher generation of actives sites. Full article