Perovskite oxides of the type of LaxCa1-xMyAl1-yO3-δ (M = Co, Cr, Fe, Mn; x = 0.5; y = 0.7–1.0) were prepared using the polymerization methods and evaluated via N2 adsorption, X-ray diffraction ...(XRD), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy dispersive X-ray (EDX) spectroscopy, temperature-programmed reduction by hydrogen (TPR-H2) and temperature-programmed oxidation by oxygen (TPO-O2). Catalytic behaviour of the perovskite oxides during methane oxidation was studied using a tubular fixed-bed reactor. In a partial oxidation, which proceeded in two steps, there was total oxidation in the first step and CO2 and H2O were formed; in the second step, the total oxidation products oxidized methane by (dry and wet) reforming reactions to yield CO and H2. Total oxidation and the two reforming reactions proceeded on two types of an active centre formed by transition metal ions, oxygen vacancies and oxide ions. The catalytic system La-Ca-Co-Al-O which contained aluminium, decomposed in partial oxidation of methane (POM) into a composite that contained firmly bonded cobalt nanoparticles in the surface of a substrate made up of La2O3, CaO and Al2O3 which catalysed POM with a high methane conversion and hydrogen selectivity.
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•Redox stability of the La-Ca-M-(Al)-O (M = Co, Cr, Fe and Mn) perovskites.•Partial oxidation of methane by perovskites takes place in two steps.•Decomposition products of La-Ca-M-(Al)-O (M = Co, Fe) show highest catalytic activity.•Aluminium doping of perovskites in most cases increases their catalytic activity.
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•ZrO2 and YSZ aerogels derived by combining sol-gel method and supercritical drying.•Evolution of crystallinity was observed by in-situ XRD measurements up to 1200 °C.•Phase ...transformation of zirconia (ZrO2) is affected by crystallite size growth.•Yttria-stabilized zirconia (YSZ) aerogel forms stable single-phase t-ZrO2 at 455 °C.
Monolithic, structurally stable zirconia (ZrO2) aerogels can be used in high temperature applications and as medical implants. The macroscopic properties of these solids can be fine-tuned by the appropriate thermal treatment of the amorphous aerogels. Herein, we investigate the thermally induced phase transitions of ZrO2 and yttria-stabilized zirconia (YSZ) monolithic aerogels. All aerogels were produced by an acid-catalyzed sol-gel technique and subsequent supercritical drying (SCD). A complete reaction mechanism is proposed for the formation of the wet gel network. Also, the phase transformations taking place during calcination were followed as function of temperature by in-situ X-ray diffraction measurements. Composition and size of the forming crystallites were calculated from the XRD data. Phase transition is controlled by the temperature-dependent growth of crystallite size during calcination up to 1200 °C. Both tetragonal and monoclinic zirconia form in pure ZrO2 aerogels, and a single tetragonal phase forms in YSZ aerogels.
Graphene oxide (GO) sheets have highly tunable electronic properties because of their unique 2D carbon structure, which allows extensive modification with surface functionalities. Photo-driven water ...splitting uses semiconducting materials that have electronic structures suitable for electron and hole injection for H2 and O2 evolution from water decomposition. GO is an ideal material to mediate photogenerated charges for water decomposition. This paper introduces strategies for tuning the electronic structure of GO and presents GO, alone and with other materials, as a mediator for photocatalytic water splitting.
The physicochemical properties of rare-earth zirconates can be tuned by the rational modification of their structures and phase compositions. In the present work, La3+-, Nd3+-, Gd3+-, and ...Dy3+-zirconate nanostructured materials were prepared by different synthetic protocols, leading to powders, xerogels, and, for the first time, monolithic aerogels. Powders were synthesized by the co-precipitation method, while xerogels and aerogels were synthesized by the sol–gel technique, followed by ambient and supercritical drying, respectively. Their microstructures, thermogravimetric profiles, textural properties, and crystallographic structures are reported. The co-precipitation method led to dense powders (S BET < 1 m2 g–1), while the sol–gel technique resulted in large surface area xerogels (S BET = 144 m2 g–1) and aerogels (S BET = 168 m2 g–1). In addition, the incorporation of lanthanide ions into the zirconia lattice altered the crystal structures of the powders, xerogels, and aerogels. Single-phase pyrochlores were obtained for La2Zr2O7 and Nd2Zr2O7 powders and xerogels, while defect fluorite structures formed in the case of Gd2Zr2O7 and Dy2Zr2O7. All aerogels contain a mixture of cubic and tetragonal ZrO2 phases. Thus, a direct effect is shown between the drying conditions and the resulting crystalline phases of the nanostructured rare-earth zirconates.
In the present contribution, we used the spray drying method for the preparation of molybdenum-based composite powders, which belong to an important class of high-temperature structural materials – ...oxide dispersion strengthened molybdenum alloys (ODS-Mo). Three types of materials were prepared via spray drying and subsequent steps of calcination and reduction, including undoped reference Mo, ODS-Mo doped with 1 wt% of La2O3, and ODS-Mo doped with 1 wt% of La2O3 and 1 wt% of ZrO2. The particular focus was given to precursor solution; specifically, the effects of type (aqueous or water-ethanol mixture) and chemical composition (the addition of La and Zr precursors) were studied in detail, which was rarely given attention in previous research. The morphological and microstructural changes of the particles during oxidation and reduction of sprayed-dried powders were extensively analysed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), X-ray diffractometry (XRD), and X-ray photoelectron spectroscopy (XPS). The morphology and crystallinity of spray dried powders were controlled by the state of spraying precursor solution, i.e. homogeneous solution or particle suspension. Calcination at the temperatures of 500 °C and 650 °C produced spherical polycrystalline and slab-shaped monocrystalline MoO3 oxide particles. Subsequent reductive treatment of transformed calcined powders into metal-based powders took place with efficiency that depended mainly on the type of spraying solution (aqueous solution or water-ethanol suspension) and its chemical composition, in which case, the addition of La and Zr precursors resulted in lower efficiency than the addition of La alone. In terms of powder particle size, homogeneity of alloying elements, and reduction efficiency, the powders produced from water-ethanol suspensions were clearly superior to those produced from aqueous solutions.
•La and La-Zr doped Mo powders were produced by spray drying/calcination/reduction.•Spraying solution (aqueous vs. water-ethanol) affected microstructure of powder.•Calcination at 500/650 °C produced spherical/slab-shaped particle morphology.•Undoped and La-doped powders reduced more efficiently than La-Zr doped powder.•Powders dried from water-ethanol and calcined at 500 °C have optimal properties.
Base catalyzed hydrolysis and condensation of Ti isopropoxide with water at ambient temperature and molar ratio H
2
O/Ti(i-PrOH)
4
varied from 1 to 100 was studied. It was found that molar ratio H
2
...O/Ti(i-PrOH)
4
= 100 originated amorphous TiO
2
-precursor of summary composition TiO
1.9
(OH)
0.2
with a specific surface area of 354 m
2
/g. Low-temperature crystallization of amorphous TiO
2
-precursor to nanostructured anatase at 80 °C in a slightly reducing environment of
d
-glucose was studied. It was found that the low-temperature nucleation and crystallization of anatase was initiated (activated) by combined effects of
d
-glucose and Au
0
, Pd
0
and Pt
0
nanoparticles, generated in situ by slow reduction of Au
3+
, Pd
2+
and Pt
4+
ions using
d
-glucose. Considerable photocatalytic activity of Ti-hydroxide-oxide/Au, Pd, Pt catalysts prepared at low-temperature was associated with high content of nanostructured anatase and low content of Au, Pd and Pt nanoparticles (0.02 wt.%) effectively deposited on the surface of titania particles. The maximum hydrogen evolution rates 3.4 μmol/min g at Ti-hydroxide-oxide/Au, 4.0 μmol/min g at Ti-hydroxide-oxide/Pd and 4.1 μmol/min g at Ti-hydroxide-oxide/Pt were found. The activity of all TiO
2
/Au, Pd and Pt catalysts increased by calcination at 600 °C from 50 to 100 %.
Sintering of Ce, Sm, and Pr Oxide Nanorods Castkova, Klara; Matousek, Ales; Bartonickova, Eva ...
Journal of the American Ceramic Society,
April 2016, Letnik:
99, Številka:
4
Journal Article
Recenzirano
Synthesis of CeO2, Pr2O3, and Sm2O3 nanorods and their sintering have been investigated. In a strongly alkaline medium, nanorods of CeO2, Pr2O3, and Sm2O3 were prepared from trivalent salts of rare ...earths (Ce, Pr, Sm) via precipitation synthesis. Nanorods were formed by nanocrystallites of fibrous structure, which were produced by the mechanism of self‐arrangement of hexagonal particles of Re(III) hydroxides. The subsequent transformation of hydroxide into oxide proceeded via self‐preservation of the rod‐like structure. In CeO2, the fibrous structure was noncohesive during thermal treatment at temperature of 500°C and higher. Regardless of the shape of the CeO2 particles (spherical versus rod‐like), sintered ceramic was formed by equiaxial grains. The cohesion of the fibrous structure of Pr and Sm oxides was higher than in CeO2. The rod‐like shape of the particles of Pr and Sm oxides was (partially) preserved during sintering.
Dense multi-cationic Sm-Co-O, Sm-Ni-O, Al-Co-O, Al-Ni-O, and Al-Ni-Co-O oxide aerogels were prepared by epoxide-driven sol-gel synthesis. Catalysts for dry reformation of methane, Sm
O
/Co, Sm
O
/Ni, ...Al
O
/Co, Al
O
/Ni, Al
O
/Co, and Ni were prepared by reduction of aerogels with hydrogen and their catalytic activities and C-deposition during dry reformation of methane were tested. Catalytic tests showed high methane conversion (93-98%) and C-deposition (0.01-4.35 mg C/g
h). The highest content of C-deposits after catalytic tests was determined for Al
O
/Co and Al
O
/Ni catalysts, which was related to the formation of Al alloys with Co and Ni. A uniform distribution of Co
and Ni
nanoparticles (in the form of a CoNi alloy) was found only for the Al
O
/Co and Ni catalysts, which showed the highest activity as well as low C deposition.
TiO2 aerogels doped with Ni, Co, Cu, and Fe were prepared, and their structure and photocatalytic activity during the decomposition of a model pollutant, acid orange (AO7), were studied. After ...calcination at 500 °C and 900 °C, the structure and composition of the doped aerogels were evaluated and analyzed. XRD analysis revealed the presence of anatase/brookite and rutile phases in the aerogels along with other oxide phases from the dopants. SEM and TEM microscopy showed the nanostructure of the aerogels, and BET analysis showed their mesoporosity and high specific surface area of 130 to 160 m2·g−1. SEM–EDS, STEM–EDS, XPS, EPR methods and FTIR analysis evaluated the presence of dopants and their chemical state. The concentration of doped metals in aerogels varied from 1 to 5 wt.%. The photocatalytic activity was evaluated using UV spectrophotometry and photodegradation of the AO7 pollutant. Ni–TiO2 and Cu–TiO2 aerogels calcined at 500 °C showed higher photoactivity coefficients (kaap) than aerogels calcined at 900 °C, which were ten times less active due to the transformation of anatase and brookite to the rutile phase and the loss of textural properties of the aerogels.
A mechanochemical method is reported for the synthesis of Au(diphos)X complexes of diphosphine (diphos = XantPhos and
N
-XantPhos) ligands and halide ions (X = Cl and I). The Au(XantPhos)X (
1
: X = ...Cl;
2
: X = I) and Au(
N
-XantPhos)Cl (
3
) complexes exhibited either yellowish green (
1
) or bluish green (
2
) emission, whereas
3
was seemingly non-emissive in the solid state at room temperature. Blue- (
2B
) and bluish green (
2G
) luminescent concomitant solvates of
2
were obtained by recrystallization. Luminescent colour changes from blue (
2B
) or bluish green (
2G
) to yellow were observed when these forms were subjected to mechanical stimulus, while the original emission colour can be recovered in the presence of solvent vapours. Moreover, the luminescence of
2B
can be reversibly altered between blue and yellow by heating/cooling-cycles. These results demonstrate the power of mechanochemistry in the rapid (4 min reaction time), efficient (up to 98% yield) and greener synthesis of luminescent and stimuli-responsive gold(
i
) complexes.
A mechanochemical method was developed for the synthesis of Au(XantPhos)X (
1
: X = Cl;
2
: X = I) and Au(
N
-XantPhos)Cl (
3
) complexes.
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