Key processing of porous and fibrous LaCoO3 nanostructures for successful CO and propane sensing

Currently, perovskite structures have had an important impact in the development of gas sensors. In this work, perovskite LaCoO₃ nanoparticles were synthesized by a simple, economic and reproducible processing by the solution method. The reactive precursors were nitrates of lanthanum and cobalt, using ethylenediamine as a chelating agent and distilled water as solvent. The gel formed by the solvent evaporation (through microwave radiation) was dried at 200?°C and later calcined at 300, 400, 500, 600, and 700?°C for 5?h. The samples were analyzed by X-ray diffraction, infrared spectroscopy, thermogravimetry, scanning, transmission, and atomic force microscopies, and nitrogen physisorption. These analyses confirmed the formation of LaCoO₃ nanoparticles (size ~ 47?nm) at relatively low temperatures. The particles showed a continuous connectivity, generating a porous surface with a fibrous appearance. Starting with the synthesized powders, pellets were made and tested as gas sensors in carbon monoxide and propane atmospheres (at concentrations of 0–300?ppm) at different temperatures (25, 150, 250, and 350?°C). The nanoparticles presented high sensitivity, with a greater response in the propane atmosphere.     

High temperature stiffening of ferroelastic LaCoO3

The cyclic ferroelastic hysteretic behavior of pure LaCoO₃ perovskite ceramic has been studied at different temperatures in four point bending. The stress-strain deformation behavior of LaCoO₃ was analyzed both in the term of the maximum stress in the cycle and in terms of the temperature used when the cyclic testing was performed. The characteristics of the stress-strain hysteresis loops, such as hysteresis loop area and irreversible strain, as well as effective Young’s modulus, were analyzed, and it was established that both the loading and the temperature history have a significant influence on the mechanical behavior of LaCoO₃. Young’s modulus values are reported to be much higher in the 700–900 °C temperature range as compared to the measurements performed in the RT-400 °C temperature range.     

Direct synthesis of PMN samples by spray-drying

The processing and characterisation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) materials, obtained either by spray-drying the solution of the precursors or by the conventional “columbite” method, were investigated and the morphological and micro-structural characteristics were compared. The acid solution of ammonium-peroxo-niobium complex, magnesium and lead nitrates was spray-dried and the precursor powder obtained was calcined at different temperatures ranging from 350 to 900 °C. The morphologies and the XRD patterns of the powders were compared. The calcined powders exhibited a pyrochlore phase above 400 °C converting into an almost pure perovskite phase at 800 °C. The powder calcined at 350, 500 and 800 °C were sintered at different temperatures, ranging from 950 to 1150 °C, always resulting in a pure perovskite PMN material. The XRD patterns of as-fired surfaces of samples sintered at 950 and 1050 °C showed an unwanted PbO phase together with the main PMN, nevertheless this secondary phase is not present in the ground surfaces. The high reactivity of sprayed powder is reflected in the formation and densification of pure perovskite PMN material with a faster process as regards the conventional one; in particular samples of about 96% theoretical density were obtained starting from the amorphous powder calcined at low temperature (350 °C) through a reaction sintering process. Furthermore, due to the better flowability of the spray-dried powder, the cold consolidation process is highly improved and no binder addition to powder is necessary.     

Nanostructured perovskite oxides – LaMO3 (M=Al,Co, Fe) prepared by co-precipitation method and their ethanol-sensing characteristics

Nanostructured La-based perovskite oxides − LaMO₃ (M=Al, Co, Fe) were synthesized by a new co-precipitation procedure using metal nitrate and carbonate salts as starting materials. X-ray diffraction and energy dispersive X-ray spectroscopic results confirmed the formation of single-phase nanocrystalline perovskite oxides with high purity. Characterizations by scanning/transmission electron microscopy and nitrogen adsorption revealed that LaAlO₃ was produced in the form of rectangular porous nanorods exhibiting much larger surface area and porosity compared with densely aggregated LaCoO₃ particles and loosely clustered LaFeO₃ nanoparticles with cracked-egg morphologies. The materials were characterized for gas sensing towards ethanol at 200–350 °C. From gas-sensing results, the LaAlO₃ sensor displayed n-type gas-sensing behaviors with considerably higher ethanol response than p-type LaFeO₃ and LaCoO₃ sensors, respectively. In particular, the LaAlO₃ sensor exhibited a high response of 16.45–1000 ppm ethanol and excellent ethanol selectivity against NO₂, SO₂, CO and H₂ at 350 °C. The superior gas-sensing performances could be attributed to the effective receptor function, transducer function and utility factor of LaAlO₃ nanorod structures prepared by the co-precipitation method.     

Pd-doped LaCoO3 regenerative catalyst for automotive emissions control

The effect of partial substitution of Co by Pd in LaCoO₃ perovskite structure (i.e., LaCo_0.95Pd_0.05O₃) and the reductive diffusion of Pd from the bulk of perovskite to its surface, thus forming Pd nanoparticles, on CO and C₃H₈ oxidation present in air (simulated exhaust gas) are reported. X-ray powder diffraction (XRD) analyses confirm the perovskite structure for the catalysts. Scanning electron microscopy (SEM) and BET surface area measurements show that partial substitution of Co by Pd decreases the crystallite size of the perovskite and therefore increases its surface area. H₂-temperature programmed reduction (TPR) experiments reveal that Pd reduces at 135 °C and facilitates the reduction of Co in the perovskite structure. By partial reduction of the Pd containing catalyst at 180 °C for 30 min, the complete oxidation temperatures of CO and C₃H₈ decrease by about 70 and 50 °C, respectively.The reduction duration of the Pd containing catalyst strongly affects the T₅₀ and T₉₀ temperatures (temperatures at which 50 and 90% conversion occurs, respectively) and has an optimum, where it decreases by increasing the reduction temperature of the catalyst.     

Structural regeneration of LaCoO3 perovskite-based catalysts during the NO + H2 + O2 reactions

In situ X-ray diffraction (XRD) analysis was used to investigate structural evolutions of LaCoO₃ catalysts and then further modified by palladium (Pd) addition, under various controlled atmospheres, particularly during the reduction of NO by hydrogen in lean conditions. Complementary, XPS measurements provided information about changes in the chemical environments of Pd, Co and nitrogen during sequential temperature-programmed reactions. A preactivation thermal treatment under hydrogen led to the destruction of the perovskite structure while in the course of the NO + H₂ + O₂ reactions, the regeneration of the perovskite structure evidenced by XRD at 873 K started at lower temperature (573 K) at the surface. Palladium has been incorporated in order to evidence its effective role in the surface modifications of LaCoO₃ and its consequence on the catalytic activity.     

Catalytic Post-Treatment of Automotive Exhaust Gas from Natural Gas Combustion Engines: Potential Interest of Perovskite Materials

This study reports an investigation of the surface properties of Pd-modified perovskite catalysts for NO _x removal from Natural Gas engines. H₂ production from reforming and water-gas-shift reactions can be profitably used for the reduction of NO _x . Particular attention has been paid to the nature of interactions between Pd and LaCoO₃ according to in situ reductive thermal treatments. The catalytic properties have been investigated by temperature-programmed NO desorption and temperature-programmed NO/H₂ reaction. Different experiments performed on partially and extensively reduced catalysts lead to changes in surface reactivity. Interestingly, beneficial interactions with a significant selectivity enhancement to the production of nitrogen is observed on pre-reduced Pd/LaCoO₃ under smooth conditions at 250 °C particularly in the presence of oxygen. More extensive reduction at 450 °C leads to the loss of the structural properties of LaCoO₃ accompanied with partial segregation of CoO _x and La₂O₃. Such structural changes lead to a detrimental effect on the catalytic performances.     

Fuel Gas Upgrading Over La1−xCexCoO3 Mixed Oxide with Toluene as Model Compound

Perovskite-type mixed oxides La_1?xCe_xCoO₃ (x = 0, 0.2, 0.4) were synthesized by sol–gel method via polyvinyl alcohol (PVA) as gelating agent. LaCoO₃ and CeO₂ phases were presented while intermediate phase, La(OH)₃, disappeared during LaCoO₃ transformation. Introduction of Ce decreased crystal size of catalyst from 22.18 to 13.38 nm. Particle size and specific surface area were in the range of 9.58–13.72 μm and 6.03–9.23 m²/g, respectively. The addition of Ce increased the reduction temperature which indicated the strong interaction between Co and perovskite structure. Catalytic activity was investigated by steam reforming of toluene at 500–800 °C. Conversions of carbon and hydrogen to CO, H₂, and CH₄ at steam per carbon ratio (S/C) of 2:1 were clearly higher than 4:1. Increasing S/C ratio to 4:1 inhibited syngas production efficiency by combustion and water–gas shift reaction. The presence of Ce in the catalyst did not improve activity of catalyst significantly. However the metal enhanced stability by promoting the formation of filamentous carbon on the surface such that the active sites are still accessible to the active gas during the experiment. After reforming, catalysts were transformed to La(OH)₃, Co⁰, and Ce₄O₇ phases and no significant deterioration in catalytic performance was detected after 6 h. In this study steam reforming of toluene over La_0.6Ce_0.4CoO₃ at 800 °C with S/C 2:1 yielded highest carbon conversion as CO and hydrogen conversion as H₂ of 64.42 and 63.23 %. The LHV and H₂/CO of produced gas at this optimum condition are 4.22 MJ/N m² and 2.91, respectively.     

Redox Isotherms for Vanadia Supported on Zirconia

Calcination of a Pt/Ba/CeO₂ catalyst at 700 °C and subsequent reduction in hydrogen, carbon monoxide or propene at 350–550 °C resulted in a considerable improvement of its NO _x storage-reduction (NSR) properties compared to those of a freshly prepared Pt/Ba/CeO₂ catalyst. This behavior is traced back to the temporary formation of BaPtO₃ perovskite which leads after reduction to well-distributed Pt particles in intimate contact with the barium-containing phases. The oxidation and reduction of platinum is reversible which can be exploited for the design of “self-regenerating” NSR-catalysts under lean (>600 °C) and rich (>400 °C) reaction conditions. The formation of the BaPtO₃-perovskite may not only be interesting for NSR-catalysis, but generally for Pt-based catalysts where a high dispersion of Pt is important.     

Synthesis and Characterization of Perovskite-Type Oxides La1−xMxCoO3 (M = Ce,Sr) for the Selective CO Oxidation (SELOX)

Perovskite-type oxides La_1?xM_xCoO₃ (M = Ce, Sr) were prepared by citrate method, characterized and evaluated in the selective CO oxidation (SELOX-CO). The insertion of low Cerium or Strontium content generated solids with a single phase related LaCoO₃ perovskite. For higher contents we observed segregation of CeO₂ and SrCO₃. The iso-structural substitution favors the formation of vacancies. The SELOX-CO showed 100 % CO conversion at 200 °C. Higher temperatures favored hydrogen oxidation and methanation.     

INFLUENCE OF GLASS COMPOSITION ON REDUCIBILITY OF Ag+, Bi3+, Sb3+, AND PP+*

The reducibility of a number of ions by dry hydrogen was studied in various glass compositions. Reduction temperatures with dry hydrogen were about 75° C, lower than with wet hydrogen. The glasses contained 5.0% of Sb₂O₃, Bi₂O₃, or PbO or 0.1% of Ag₂O. In all of the glasses, silver ions were reduced at lower temperatures as basicity was increased, while an increased ratio of B³⁺ to O^2- or Si⁴⁺ to O^- increased stability. The reducibility of Bi³⁺, Sb³⁺, and Pb²⁺ are not easily interpreted. Silver showed reduction at approximately 130°C., bismuth at 200°C., lead at 350°C., and antimony at 400°C.     

Oxygen transport,thermal and electrochemical properties of NdBa0.5Sr0.5Co2O5+δ cathode for SOFCs

In this study, the crystal structure, thermal, oxygen transport, electrical conductivity and electrochemical properties of the perovskite NdBa_0.5Sr_0.5Co₂O_5+δ (NBSC55) are investigated. In the temperature range of 250 °C–350 °C, the weight loss upon heating was due to a partial loss of lattice oxygen and along with a reduction of Co⁴⁺ to Co³⁺. The tend of weight-loss slows down as temperature increased above 350 °C indicating a reduction of Co³⁺ to Co²⁺ during this stage. The oxygen migration is dominated by surface exchange process at high temperature range (650-800 °C); however, the bulk diffusion process prevails at low temperature range (500–600 °C). For long-term testing, the polarization resistance of NBSC55 increases gradually form 3.13 Ω cm² for 2 h to 3.34 Ω cm² for 96 h at 600 °C and an increasing-rate for polarization resistance is around 0.22% h^?1. The power density of the single cell with NBSC55 cathode reached 341 mW cm^?2 at 800 °C.     

Dense Iodoapatite Ceramics Consolidated by Low‐Temperature Spark Plasma Sintering

Pb_9.85(VO₄)₆I_1.7, a potential waste form for long‐lived I‐129 immobilization, experiences phase decomposition and thus iodine loss at an elevated temperature above 400°C, presenting a significant challenge for effective management of radioactive iodine. In this work, we report low‐temperature consolidation of dense iodoapatite pellets with above 95% theoretical density by spark plasma sintering (SPS) at temperatures as low as 350°C for 20 min without iodine loss. Microstructure analysis indicates a nanocrystalline ceramic with an average grain size less than 100 nm. Grain growth dominates the sintered microstructure at higher temperatures and longer durations. The dense nanoceramics have significantly‐improved fracture toughness as compared with bulk coarsened grain structures. The effects of sintering temperatures (350°C, 400°C, 500°C, and 700°C) and durations (0–20 min) on microstructure, density, fracture morphology, and mechanical properties including Young's modulus and hardness of bulk samples were investigated. Low temperature densified iodoapatites suggest immense potential of SPS as an advanced materials fabrication technology for the development of waste forms for immobilization of volatile radionuclides including radioactive iodine.     

Spray pyrolysis synthesis of mesoporous TiO<Subscript>2</Subscript> microspheres and their post modification for improved photocatalytic activity

Mesoporous TiO₂ microspheres were prepared by spray pyrolysis for photocatalysis. Post modification of TiO₂ by heat treatment was performed to optimize its photocatalytic performance. First, spherical TiO₂ particles with mesoporous structure were synthesized at pyrolysis temperatures of 500, 600, and 700 °C. After characterization by XRD, SEM, and N₂ adsorption, a sample prepared at 500 °C was found to possess desirable properties for photocatalytic performance through post-modification. In methylene blue degradation, mesoporous TiO₂ microspheres synthesized at 500 °C outperformed other microspheres. Furthermore, samples obtained by spray pyrolysis at 500 °C were calcined at various temperatures as a post-modification process. The sample calcined at 350 °C showed improved photocatalytic activity due to optimal anatase crystallinity and surface area.     

The Chemical Evolution of the La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>CoO<Subscript>3−δ</Subscript> Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity

Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO₃ (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O₂ reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.     

Synthesis of LaCoO3 nanoparticles by microwave process and their photocatalytic activity under visible light irradiation

We have investigated the photocatalytic activity for the decomposition of methyl orange on the LaCoO₃ perovskite-type oxides prepared at different conditions using microwave process. In the case of LaCoO₃ catalysts calcined above 500 °C, the formation of the perovskite crystalline phase was confirmed. From the results of UV–Vis DRS, all the catalysts have the similar absorption spectrum up to visible region. The chemisorbed oxygen plays an important role on the photocatalytic decomposition of methyl orange and the higher the contents of chemisorbed oxygen, the better the performance of photocatalyst.     

Selective catalytic reduction of NO x over Ag/Al2O3 using various bio-diesels as reducing agents

The effect of bio-diesel compounds (vegetable methyl and ethyl laurate and hexadecane) as reducing agents on the selective catalytic reduction of NO _x over a 2 wt.% Ag/Al₂O₃ was investigated. These components were found to have a two-fold effect on the SCR over Ag/Al₂O₃. First, the reduction activity below 400 °C was higher with bio-diesel than with n-octane, which is a representative compound for fossil fuels. This effect is attributed to the presence of the ester group in these molecules. However, the conversion above 400 °C decreased sharply and was considerable lower than with n-octane. The most interesting observation was found when the reduction efficiency of bio-diesel components was tested in the presence of hydrogen. The well known low temperature boosting effect of hydrogen was visible not only at lower temperatures, but also above 400 °C. Mechanistically the observation is extremely interesting and indicates that hydrogen effect cannot directly be connected to reduction of surface nitrates, which can be operative only at low temperature domain.     

Effect of reduction temperature on the preparation of zero-valent iron aerogels for trichloroethylene dechlorination

Zero-valent iron (ZVI) aerogels have been synthesized by sol-gel method and supercritical CO₂ drying, followed by H₂ reduction in the temperature range of 350–500 °C. When applied to trichloroethylene (TCE) dechlorination, the ZVI aerogel reduced at 370 °C showed the highest performance in the conditions employed in this study. Thus, the effect of reduction temperature in preparing ZVI aerogels has been investigated by several characterizations such as BET, XRD, TPR, and TEM analyses. As the reduction temperature decreased from 500 to 350 °C, the BET surface area of the resulting aerogels increased from 6 to 30 m²/g, whereas their Fe⁰ content decreased up to 64%. It was also found that H₂ reduction at low temperatures such as 350 and 370 °C leads to the formation of ZVI aerogel particles consisting of both Fe⁰ and FeO _x in the particle cores with a different amount ratio, where FeO _x is a mixture of maghemite and magnetite. It is, therefore, suggested that reduction at 370 °C for ZVI aerogel preparation yielded particles homogeneously composed of Fe⁰ and FeO _x in the amount ratio of 87/13, resulting in high TCE dechlorination rate. On the other hand, when Pd- and Ni-ZVI aerogels were prepared via cogellation and then applied for TCE dechlorination, we also observed a similar effect of reduction temperature. However, the reduction at 350 or 370 °C produced Pd- or Ni-ZVI aerogel particles in which Fe⁰ and Fe₃O₄ co-exist homogeneously. Since both Fe⁰ and Fe₃O₄ are advantageous in TCE dechlorination, the activities of Pd- and Ni-ZVI aerogels reduced at 350 °C were comparable to those of both aerogels reduced at 370 °C, although the former aerogels have less Fe⁰ content.     

Non-congruence of high-temperature mechanical and structural behaviors of LaCoO3 based perovskites

This paper presents the mechanical behavior of LaCoO₃ and La_0.8Ca_0.2CoO₃ ceramics under four-point bending in which the two cobaltites are subjected to a low stress of ∼8 MPa at temperatures ranging from room temperature to 1000 °C. Unexpected stiffening is observed in pure LaCoO₃ in the 700–900 °C temperature range, leading to a significant increase in the measured Young’s modulus, whereas La_0.8Ca_0.2CoO₃ exhibits softening from 100 °C to 1000 °C, as expected for most materials upon heating. Neutron diffraction, X-ray diffraction and micro-Raman spectroscopy are used to study the crystal structure of the two materials in the RT–1000 °C temperature range. Despite a detailed study, there is no conclusive evidence to explain the stiffening behavior observed in pure LaCoO₃ as opposed to the softening behavior in La_0.8Ca_0.2CoO₃ at high temperatures (above 500 °C).     

Characterization of hydrogenation active sites on LaCoO3 perovskite

LaCoO₃ becomes active for hydrogenation of ethene upon reduction in hydrogen at temperatures between 300 and 490 °C. Several aspects of the reacting system were studied in order to ascertain the nature of the active sites generated in this manner. Catalyst deactivation was evaluated by comparing rates between two successive experiments. An upper limit was estimated for the amount of polymeric residues formed after a single run: 1.1 ± 0.5 × 10¹⁴ molecules of C₂ per square centimeter. Reduced LaCoO₃ also catalyzed the self-hydrogenation of ethene. When a mixture of C₂H₄:D₂ = 1:1 was reacted over LaCoO₃ reduced to varying extents multiple-exchanged ethenes and ethanes were formed. The exchange patterns were almost unaffected by the extent of reduction. The effect of pretreatment temperatures was also evaluated. The solid in its reduced form was particularly sensitive to high-temperature treatments. The amounts of CO chemisorbed when plotted vs extent of reduction gave curves that were almost identical to the activity plots. The results reported here, discussed in terms of the current literature, are consistent with a model in which finely dispersed Co⁰, formed in the oxide matrix upon reduction, is the locus of hydrogenation activity.