Thermodynamic Properties of LaCrO4, La2CrO6, and La2Cr3O12, and Subsolidus Phase Relations in the System Lanthanum–Chromium–Oxygen

In the system La–Cr–O, there are three ternary oxides (LaCrO₄, La₂Cr₃O₁₂, and La₂CrO₆) that contain Cr in higher valence states (V or VI). On heating, LaCrO₄ decomposes to LaCrO₃, La₂Cr₃O₁₂ to a mixture of LaCrO₄ and Cr₂O₃, and La₂CrO₆ to LaCrO₃ and La₂O₃ with loss of oxygen. The oxygen potentials corresponding to these decomposition reactions are determined as a function of temperature using solid‐state cells incorporating yttria‐stabilized zirconia as the electrolyte. Measurements are made from 840 K to the decomposition temperature of the ternary oxides in pure oxygen. The standard Gibbs energies of formation of the three ternary oxides are derived from the reversible electromotive force (EMF) of the three cells. The standard enthalpy of formation and standard entropy of the three ternary oxides at 298.15 K are estimated. Subsolidus phase relations in the system La–Cr–O are computed from thermodynamic data and displayed as isothermal sections at several temperature intervals. The decomposition temperatures in air are 880 (±3) K for La₂Cr₃O₁₂, 936 (±3) K for LaCrO₄, and 1056 (±4) K for La₂CrO₆.     

Studies on the redox properties of chromite perovskite catalysts for soot combustion

This paper deals with the preparation (by combustion synthesis), the characterization (by XRD, AAS, BET, SEM, TEM, TPD/R, and XPS analyses), the catalytic activity testing (in a temperature-programmed combustion microreactor and in a DSC analyzer), and the assessment of the reaction mechanism of a series of nanostructured soot combustion catalysts based on La–Cr substoichiometric or alkali-metal-substituted perovskites (La_0.9CrO₃, La_0.8CrO₃, La_0.9Na_0.1CrO₃, La_0.9K_0.1CrO₃, La_0.9Rb_0.1CrO₃, La_0.8Cr_0.9Li_0.1O₃), whose performance is compared with that of the standard LaCrO₃. Some conclusions are drawn concerning the role of each single constituting element on the activity of the most promising catalyst, La_0.8Cr_0.9Li_0.1O₃, which is already active well below 400 °C. The role of weakly chemisorbed O⁻ surface species in particular is pointed out as crucial for the soot combustion process. This indicates the way for the development of new, more active catalysts, possibly capable of delivering amounts of these oxygen species even higher than those obtained (about 700 μmol / g) for the most active Li-substituted lanthanum chromite catalyst developed.     

On the La–Cr–O Phase Change During Methane Catalytic Combustion Investigated with in situ HT-XRD

La₂CrO₆ (Cr6), LaCrO₃ (Cr3), LaCrO₃–La₂CrO₆ (Cr6–Cr3) and LaCrO₃–La₂O₃ (Cr3-L) catalysts were synthesised and investigated with in situ X-ray diffraction (ISXRD) during methane catalytic combustion in order to characterise the solid phases present under reactants and to determine the effect of chromium oxidation state on the catalytic activity. Methane conversion was evaluated over a temperature range of 300–800 °C, using oxygen-to-methane ratio of 4 and GHSV of 8,000 h^?1. The TPR provided information about the oxygen depletion temperatures characteristic of lattice oxygen mobility in the samples and ISXRD results evidenced a cooperative effect of Cr3 and Cr6 phases at low temperatures (<725 °C) and of Cr3 and LaCrO₄ (Cr4) phases at high temperatures (>750 °C). The relative phase composition determined the oxygen activation capability and hence the corresponding activity for the oxidation of methane. It was observed that while direct and back Cr6 ? Cr4 transition temperatures were unaffected by Cr6 content in the samples, the methane conversion was strongly modified. This suggests that Cr³⁺/Cr⁶⁺ and Cr³⁺/Cr⁵⁺ species involved substantial modification of the surface chemistry which affected the catalytic activity. These results provide the first direct evidence of the presence of Cr4 metastable phase during methane combustion over La–Cr–O catalysts.     

Nanosized Pt-Perovskite Catalyst for the Regeneration of a Wall-Flow Filter for Soot Removal from Diesel Exhaust Gases

Perovskite-type catalysts have been investigated for diesel soot combustion: (i) the LaCr_0.9O_3– substoichiometric perovskite, (ii) K–La partially substituted chromites; (iii) Pt added ii-type perovskites. The catalysts prepared showed a progressively higher activity and potential for practical application in diesel particulate traps. Engine bench tests performed on a SiC wall-flow trap (Ibiden) lined with the La_0.9K_0.1Cr_0.9O_3– + 1 wt%Pt catalyst showed that the catalyst not only speeds up soot combustion on occasional trap heating (regeneration phase) but also prolongs the trap loading phase (soot accumulation during normal operation) as Pt active sites promote NO–NO₂ oxidation, followed by the non-catalytic reaction of NO₂ with the trapped soot.     

Effect of strontium and cerium doping on the structural characteristics and catalytic activity for C3H6 combustion of perovskite LaCrO3prepared by sol–gel

Two series of Sr- or Ce-doped La_1−xM_xCrO₃ (x = 0.0, 0.1, 0.2 and 0.3) catalysts were prepared by thermal decomposition of amorphous citrate precursors followed by annealing at 800 °C in air atmosphere. The effect of Ce and Sr on the morphological/structural properties of LaCrO₃ was investigated by means of thermogravimetric/differential thermal analysis (TG/DTA) of the precursors decomposition under air, X-ray diffraction (XRD), electron paramagnetic resonance (EPR), transmission electron microscopy–X-ray energy dispersive spectroscopy (TEM–XEDS), S_BET determination, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. The characterization results are employed to explain catalytic activity results for C₃H₆ combustion. It is shown that the lanthanum chromite perovskite structure is obtained upon thermal treatment of the sol–gel derived precursors at T > ca. 800 °C. The presence of the dopant generally induces the formation of segregated oxide phases in the samples calcined at 800 °C although some introduction of the Sr in the perovskite structure is inferred from EPR measurements. The oxidation activity becomes maximised upon formation of such doped perovskite structure.     

Study on an iron–manganese Fischer–Tropsch synthesis catalyst prepared from ferrous sulfate

A systematic study was undertaken to investigate the effects of the initial oxidation degree of iron on the bulk phase composition and reduction/carburization behaviors of a Fe–Mn–K/SiO₂ catalyst prepared from ferrous sulfate. The catalyst samples were characterized by powder X-ray diffraction (XRD), Mössbauer spectroscopy, X-ray photoelectron spectroscopy (XPS) and H₂ (or CO) temperature-programmed reduction (TPR). The Fischer–Tropsch synthesis (FTS) performance of the catalysts was studied in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that the fresh catalysts are mainly composed of α-Fe₂O₃ and Fe₃O₄, and the crystallite size of iron oxides is decreased with the increase of the initial oxidation degree of iron. The catalyst with high content of α-Fe₂O₃ in its as-prepared state has high content of iron carbides after being reduced in syngas. However, the catalyst with high content of Fe₃O₄ in its as-prepared state cannot be easily carburized in CO and syngas. FTS reaction study indicates that Fe-05 (Fe³⁺/Fe_total = 1.0) has the highest CO conversion, whereas Fe-03 (Fe³⁺/Fe_total = 0.55) has the lowest activity. The catalyst with high CO conversion has a high selectivity to gaseous hydrocarbons (C₁–C₄) and low selectivity to heavy hydrocarbons (C₅₊).     

Thermal behaviour of Cu–Mg–Mn and Ni–Mg–Mn layered double hydroxides and characterization of formed oxides

Thermal behaviour of synthetic Cu–Mg–Mn and Ni–Mg–Mn layered double hydroxides (LDHs) with M^II/Mg/Mn molar ratio of 1:1:1 was studied in the temperature range 200–1100 °C by thermal analysis (TG/DTA/EGA), powder X-ray diffraction (XRD), Raman spectroscopy, and voltammetry of microparticles. Powder XRD patterns of prepared LDHs showed characteristic hydrotalcite-like phases, but further phases were indirectly found as admixtures. The Cu–Mg–Mn precipitate was decomposed at temperatures up to ca. 200 °C to form an XRD-amorphous mixture of oxides. The crystallization of CuO (tenorite) and a spinel type mixed oxide of varying composition Cu_xMg_yMn_zO₄ with Mn⁴⁺ was detected at 300–500 °C. At high temperatures (900–1000 °C), tenorite disappeared and a consecutive crystallization of 2CuO·MgO (gueggonite) was observed. The high-temperature transformation of oxide phases led to a formation of Cu^I oxides accompanied by oxygen evolution. The DTA curve of Ni–Mg–Mn sample exhibited two endothermic effects characteristic for hydrotalcite-like compounds. The first one with minimum at 190 °C can be ascribed to a loss of interlayer water, the second one with minimum at 305 °C to the sample decomposition. Heating of the Ni–Mg–Mn sample at 300 °C led to the onset of crystallization of oxide phases identified as Ni_xMg_yMn_zO₄ spinel, (Ni,Mg)O oxide containing Mn⁴⁺ cations, and easily reducible XRD-amorphous species, probably free Mn^III,IV oxides. At 600 °C (Raman spectroscopy) and 700 °C (XRD), the (Ni,Mg)₆MnO₈ oxide with murdochite structure together with spinel phase were detected. Only spinel and (Ni,Mg)O were found after heating at 900 °C and higher temperatures. Temperature-programmed reduction (TPR) profiles of calcined Cu–Mg–Mn samples exhibited a single reduction peak with maximum around 250 °C. The highest H₂ consumption was observed for the sample calcined at 800 °C. The reduction of Ni–Mg–Mn samples proceeded by a more complex way and the TPR profiles reflected the phase composition changing depending on the calcination temperature.     

Synthesis and characterization of doped nano-sized ceria–zirconia solid solutions

Two compositions Ce_0.50Zr_0.39La_0.04Y_0.07O_2−δ and Ce_0.25Zr_0.65La_0.04Y_0.06O_2−δ based on ceria-zirconia solid solutions were prepared as nanopowders using a continuous hydrothermal flow synthesis reactor, followed by either freeze-drying or hotplate-drying of the slurry. Each dried nanopowder was then subjected to 10 h heat-treatment at 1000 °C, 1100 °C or 1200 °C in air (to simulate accelerated ageing). The reducibility and hydrogen consumption of the oxidised samples were measured using temperature programmed reduction (TPR) up to 1000 °C. The effects of composition, drying method and heat-treatment temperature were evaluated on the TPR profiles of the materials. The powders were further investigated using a range of analytical methods including UV/Vis spectroscopy (which yielded colour data), Raman spectroscopy, powder X-ray diffraction, BET surface area measurements and X-ray photoelectron spectroscopy (XPS). Chemometric methods were used to investigate relationships between the spectroscopic and total oxygen storage capacity (OSC) data. Principal component analysis (PCA) was used to provide a simple interpretation of the effects of various synthesis and treatment parameters on Raman spectra. Principal component regression (PCR) was used to build regression models relating the Raman spectra and the temperature of hydrogen consumption peak at several set temperatures in the TPR. The total hydrogen consumption of the materials was generally high, while the drying and heat-treatment conditions appeared to have a significant effect on the final properties of the resulting powders, such as the surface area and total oxygen storage capacity.     

Hydrogen production by steam reforming of methanol over Cu–CeZrYOx-based catalysts

Active and selective Cu_x(CeZrY)_1−xO_y catalysts (pure and with addition of Al₂O₃ and Cr) for the steam reforming of methanol were synthesized via the urea–nitrate combustion method. Structural, surface and redox characteristics of these catalysts were investigated by XRD, BET, IR spectroscopy, differential dissolution (DD), H₂-TPR and XPS methods. It was shown that addition of alumina and Cr leads to the steep increase in H₂ production due to appearance of highly dispersed copper species and stabilizes their activity. The parallel change of SRM rate constants and maximal rates of reduction with hydrogen characterizing mobility of lattice oxygen at variation of the catalyst composition was revealed that shows the importance of lattice oxygen mobility for steam reforming of methanol.     

Effects of the preparation methods on the performance of the Cu–Cr–Fe/γ-Al2O3 catalysts for the synthesis of 2-methylpiperazine

Co-precipitation, impregnation and ultrasonic sol–gel (USG) methods have been used to prepare Cu–Cr–Fe/γ-Al₂O₃ catalysts, which were further used to synthesize 2-methylpiperazine. The catalysts were characterized by XRD, XPS, TG/DSC, BET, TPR, AAS and TEM. It is found that preparation method can greatly impact the catalytic performance of the catalysts, the Cu–Cr–Fe/γ-Al₂O₃ catalyst prepared by the ultrasonic sol–gel method proved to be the most active and stable for this reaction. The dispersion and stabilization of Cu⁰ in the reduced catalysts are attributed to the existence of CuCr₂O₄ and Fe₂O₃. A surprising copper migration was detected by XPS analysis for the Cu–Cr–Fe/γ-Al₂O₃-USG catalyst after the calcination process, which may be crucial to the high activity and stability of this catalyst.     

Selective oxidation of propane and ethane on diluted Mo–V–Nb–Te mixed-oxide catalysts

Te-free and Te-containing Mo–V–Nb mixed oxide catalysts were diluted with several metal oxides (SiO₂, γ-Al₂O₃, α-Al₂O₃, Nb₂O₅, or ZrO₂), characterized, and tested in the oxidation of ethane and propane. Bulk and diluted Mo–V–Nb–Te catalysts exhibited high selectivity to ethylene (up to 96%) at ethane conversions <10%, whereas the corresponding Te-free catalysts exhibited lower selectivity to ethylene. The selectivity to ethylene decreased with the ethane conversion, with this effect depending strongly on the diluter and the catalyst composition. For propane oxidation, the presence of diluter exerted a negative effect on catalytic performance (decreasing the formation of acrylic acid), and α-Al₂O₃ can be considered only a relatively efficient diluter. The higher or lower interaction between diluter and active-phase precursors, promoting or hindering an unfavorable formation of the active and selective crystalline phase [i.e., Te₂M₂₀O₅₇ (M = Mo, V, and Nb)], determines the catalytic performance of these materials.     

W-promoted Co–Mo–K/γ–Al2O3 catalysts for water–gas shift reaction

The effects of incorporating tungsten into the traditional Co–Mo–K/γ–Al₂O₃ catalysts on the catalytic performances for water–gas shift reaction were investigated. Activity tests showed that W-promoted Co–Mo–K/γ–Al₂O₃ catalysts exhibited higher activity than W-free Co–Mo–K/γ–Al₂O₃ catalyst. Raman and H₂-TPR studies indicated that part of the octahedrally coordinated Mo–O species on Co–Mo–K catalysts transformed into tetrahedrally coordinated Mo–O species in the presence of W promoter.     

Selective oxidation of ethane: Developing an orthorhombic phase in Mo–V–X (X = Nb, Sb, Te) mixed oxides

Mo–V–X (X = Nb, Sb and/or Te) mixed oxides have been prepared by hydrothermal synthesis and heat-treated in N₂ at 450 °C or 600 °C for 2 h. The calcination temperature and the presence or absence of Nb determines the nature of crystalline phases in the catalyst. Nb-containing catalysts heat-treated at 450 °C are mostly amorphous solids, while Nb-free catalysts heat-treated at 450 °C and samples treated at 600 °C clearly contain crystalline phases. TPR-H₂ experiments show higher H₂-consumption on catalysts with amorphous phases. Catalytic results in the oxidative dehydrogenation of ethane indicate that the selective production of the olefin is strongly related to the development of the orthorhombic Te₂M₂₀O₅₇ or (SbO)₂M₂₀O₅₆ (M = Mo, V, Nb) phase (the so-called M1 phase), which is mainly formed at 600 °C. This active and selective crystalline phase is characterized to show moderate reducibility and active centers enough for the selective oxidative activation of ethane with the minimum quantity possible of active centers for ethylene activation. In this sense, the best yield to ethylene has been achieved on a Mo–V–Te–Nb mixed oxide.     

Synthesis and characterization of mesoporous Ta2O5–TiO2 photocatalysts for water splitting

Two series of Ta₂O₅–TiO₂ photocatalysts (Ta:Ti = 4:1, 1:1 and 1:4) were prepared by sol–gel technique applying triblock copolymer of Pluronic P123 and were tested in platinized form (0.3 wt.%) in photodecomposition of water under ultraviolet and visible light (λ > 300 nm). It was found the mesoporous character of tantalum containing catalysts with relatively high surface area (100–130 m² g⁻¹) of these samples. However, higher concentration of TiO₂ in mixed oxides leads to the destruction of mesoporous character of synthesized photocatalysts. All samples were characterized with thermogravimetry, XRD, N₂ physisorption, DR-UV–vis and FTIR spectroscopy. The mixed oxides of Ta₂O₅–TiO₂ system showed much lower band-gap than pure Ta₂O₅ and relatively high activity in platinized state in photocatalytic hydrogen generation under visible. Doping of pure oxides and mixed systems with sulfur resulted in lowering of the band-gap values below 3 eV and much better activity in H₂ evolution reaction. Non-platinized photocatalysts showed activity in liquid phase cyclohexene photooxidation at 305 K.     

Low-temperature catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

MnO_x–CeO₂ mixed oxide catalysts prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of chlorinated aromatic volatile organic compounds (CVOCs). MnO_x–CeO₂ catalysts with the different ratio of Mn/Ce + Mn were found to possess high catalytic activity for catalytic combustion of CB, and MnO_x(0.86)–CeO₂ was the most active catalyst, on which the complete combustion temperature (T_90%) of chlorobenzene was 236 °C. The stability of MnO_x–CeO₂ catalysts in the CB combustion was investigated. MnO_x–CeO₂ catalysts with high Mn/Ce + Mn ratios present high stable activity, which is related to their high ability to remove Cl species adsorbed and a large amount of active surface oxygen.     

Pt–Ru–Co catalysts for PEMFC synthesized by combustion

In this work the novel use of urea combustion synthesis as a straightforward method for the preparation of a trimetallic alloy, 60 Pt–30 Ru–10 Co (mol%) is described. A novel composition in the Pt–Ru–Co ternary system has been considered for a possible substitution of commercial anode catalysts in PEMFC. The specific surface area (102 m²/g) of the carbon supported catalysts produces anodes which gave power density stable values of 260 mW/cm². The XPS (X-ray Photoelectron Spectrometry) data showed that combustion synthesis produces Platinum and Cobalt segregated in the catalyst’s surface, these metals are layered structured and exhibit different oxidation states.     

Ca2+‐Doped LaCrO3: A Novel Energy‐Saving Material with High Infrared Emissivity

The present study describes the successful synthesis of a Ca²⁺‐doped LaCrO₃ ceramic with high infrared (IR) emissivity, which is important for high‐temperature applications for significant energy saving. It is demonstrated that 20 mol% Ca²⁺‐doped LaCrO₃, i.e., La_0.8Ca_0.2CrO₃, exhibited an IR emissivity as high as 0.95 in the spectral region of 3–5 μm, which was 33.8% higher than that of LaCrO₃. By using La_0.8Ca_0.2CrO₃ as IR radiation agent in surface coating of heating unit, the radiative heat transfer could be enhanced significantly. The mechanism of the high IR emissivity of La_0.8Ca_0.2CrO₃ was attributed to the following aspects: Ca²⁺ doping introduced an impurity energy level of Cr⁴⁺ into LaCrO₃ and increased the hole carrier concentration, enhancing both impurity absorption and hole carrier absorption in the IR region; moreover, the doping caused lattice distortion enhanced the lattice vibration absorption. This novel high IR emissivity ceramic shows a promising future in high‐temperature applications for the purpose of energy‐saving.     

Steam reforming of ethanol over Pt–Ni Catalysts

A parametric study was conducted over Pt–Ni/δ-Al₂O₃ to explore the effect of Pt and Ni contents on the ethanol steam reforming characteristics of the bimetallic catalyst. Experiments with catalysts having 0.2–0.3 wt%Pt and 10–15 wt%Ni contents indicated that the best ethanol steam reforming performance is achieved over 0.3 wt%Pt–15 wt%Ni/δ-Al₂O₃. Kinetics of ethanol steam reforming was studied over this catalyst in the 673–823 K interval using differential and integral methods of data analysis. A power-function rate expression was obtained with reaction orders of 1.01 and −0.09 in ethanol and steam, respectively, and the apparent activation energy of ethanol steam reforming over 0.3 wt%Pt–15 wt%Ni/δ-Al₂O₃ was calculated as 59.3 ± 2.3 kJ mol⁻¹.     

Catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

A series of MnOx–CeO₂ mixed oxide catalysts with different compositions prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of volatile organic compounds of chlorinated aromatics. MnOx–CeO₂ catalysts with different ratios of Mn/Ce + Mn were found to possess high catalytic activity in the catalytic combustion of CB, and MnOx(0.86)–CeO₂ was identified as the most active catalyst, on which the temperature of complete combustion of CB was 254 °C. Effects of systematic variation of reaction conditions, including space velocity and inlet CB concentration on the catalytic combustion of CB were investigated. Additionally, the stability and deactivation of MnOx–CeO₂ catalysts were studied by various characterization methods and other assistant experiments. MnOx–CeO₂ catalysts with high Mn/Ce + Mn ratios present a stable high activity, which is related to their high ability to remove the adsorbed Cl species and a large amount of active surface oxygen.     

La(1−x)SrxCo1−yFeyO3 perovskites prepared by sol–gel method: Characterization and relationships with catalytic properties for total oxidation of toluene

La_(1−x)Sr_xCo_(1−y)Fe_yO₃ samples have been prepared by sol–gel method using EDTA and citric acid as complexing agents. For the first time, Raman mappings were achieved on this type of samples especially to look for traces of Co₃O₄ that can be present as additional phase and not detect by XRD. The prepared samples were pure perovskites with good structural homogeneity. All these perovskites were very active for total oxidation of toluene above 200 °C. The ageing procedure used indicated good thermal stability of the samples. A strong improvement of catalytic properties was obtained substituting 30% of La³⁺ by Sr²⁺ cations and a slight additional improvement was observed substituting 20% of cobalt by iron. Hence, the optimized composition was La_0.7Sr_0.3Co_0.8Fe_0.2O₃. The samples were also characterized by BET measurements, SEM and XRD techniques. Iron oxidation states were determined by Mössbauer spectroscopy. Cobalt oxidation states and the amount of O⁻ electrophilic species were analyzed from XPS achieved after treatment without re-exposition to ambient air. Textural characterization revealed a strong increase in the specific surface area and a complete change of the shape of primary particles substituting La³⁺ by Sr²⁺. The strong lowering of the temperature at conversion 20% for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples can be explained by these changes. X photoelectron spectra obtained with our procedure evidenced very high amount of O⁻ electrophilic species for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples. These species able to activate hydrocarbons could be the active sites. The partial substitution of cobalt by iron has only a limited effect on the textural properties and the amount of O⁻ species. However, Raman spectroscopy revealed a strong dynamic structural distortion by Jahn–Teller effect and Mössbauer spectroscopy evidenced the presence of Fe⁴⁺ cations in the iron containing samples. These structural modifications could improve the reactivity of the active sites explaining the better specific activity rate of the La_0.7Sr_0.3Co_0.8Fe_0.2O₃ sample. Finally, an additional improvement of catalytic properties was obtained by the addition of 5% of cobalt cations in the solution of preparation. As evidenced by Raman mappings and TEM images, this method of preparation allowed to well-dispersed small Co₃O₄ particles that are very efficient for total oxidation of toluene with good thermal stability contrary to bulk Co₃O₄.