Lenka Matějová

XRD analysis of nanocrystalline anatase powders prepared by various chemical routes: correlations between micro-structure and crystal structure parameters

Nanocrystalline anatase powders synthesised by various chemical processes as super/subcritical fluid extraction, sol-gel technique and hydrolysis of titanium alkoxides in hydrogen peroxide were studied by X-ray diffractionxa0(XRD) whole profile modelling methodxa0(WPPM) in order to reveal correlations between structural and micro-structural parameters as well as sample treatment conditions. Anisotropy of the diffraction line broadening due to truncated bipyramidal shape of anatase crystals was discussed. The hkl -anisotropy can be very strong but also almost negligible in dependence on relative ratio of the crystallite dimensions. The latter was the case for the studied samples. The size of synthesised anatase nanoparticles was within the range 3–25xa0nm. The theoretical total surface area of crystallites calculated from XRD was in a good correlation with the surface area measured by the nitrogen physisorption up to the temperature 400–450xa0°C, when the particles started to agglomerate. At atomic scale a unit cell volume contraction with decreasing crystallite size and a significant deficiency in the Ti-site occupancy was observed. Both effects were attributed to the presence of Ti-vacancies and a linear coefficient between the relative cell volume contraction and the fraction of Ti-vacancies was estimated to (–0.017xa0±xa00.003).

Preparation, characterization and photocatalytic performance of TiO2 prepared by using pressurized fluids in CO2 reduction and N2O decomposition

AbstractnTiO2 photocatalyst was prepared unconventionally, using the sol–gel process controlled within the reverse micelles and the processing by pressurized hot fluids as an alternative to standard calcination. Conventional calcined TiO2 was prepared as well. Textural, microstructural and optical properties of prepared photocatalysts were characterized by using nitrogen physisorption, powder X-ray diffraction and DR UV–Vis spectroscopy. The photocatalytic properties of developed TiO2 catalysts were investigated in the photocatalytic reduction of CO2 and photocatalytic decomposition of N2O. It was revealed that TiO2 processed by pressurized hot fluids shows significantly improved textural properties and different crystallinity compared to its calcined analog. Yields of both reaction products (H2 and CH4) of CO2 photoreduction were higher for the extracted TiO2 photocatalyst. The same result was achieved in N2O photodecomposition. The maximum N2O conversion (83xa0% after 20xa0h of illumination) in inert gas was reached also over the TiO2 extracted photocatalyst, and it can be attributed to simultaneous N2O photocatalytic decomposition and N2O photolysis. Reaction kinetics of N2O-photoinduced decomposition was described well by the pseudo-first-rate law. The surface heterojunction of bicrystalline anatase–brookite phase corresponded to better catalytic activity of TiO2 processed by pressurized hot fluids in both reactions, in a consequence of reduced electron–hole pair recombination.Graphical Abstract

Refining bimodal microstructure of materials with MSTRUCT

The possibilities of modelling the diffraction profiles from bimodal microstructure in computer program MSTRUCT are demonstrated on two examples. A special “Double Component” profile effect can be utilized for such problems. At first it was applied to an analysis of a mixture of two nanocrystalline anatase powders with different crystallite sizes and the relative ratio of both components was determined from X-ray diffraction data. In the second case study, diffraction peaks from a pure polycrystalline copper sample treated by equal channel angular pressing were fitted using a two-phase model of large recrystallized defect-free grains and ultrafine crystallites with high dislocation density. The method is shown to be suitable for determination of the relative fraction of the microstructural components as well as other parameters (e.g. dislocation density).

Microstructure-performance study of cerium-doped TiO2 prepared by using pressurized fluids in photocatalytic mitigation of N2O

Parent TiO2 and cerium-doped TiO2 photocatalysts with various Ce molar loadings (Ce0.05Ti0.95O2, Ce0.30Ti0.70O2) were prepared unconventionally using the sol–gel process controlled within the reverse micelles and the processing by pressurized hot fluids as an alternative to standard calcination. Purity, textural, micro/structural and optical properties of prepared photocatalysts were characterized by organic elementary analysis, nitrogen physisorption, powder X-ray diffraction, and DRS UV–Vis spectroscopy. The activity of developed cerium-doped TiO2 catalysts was investigated in the photocatalytic decomposition of N2O for the first time. It was revealed that photocatalysts processed by pressurized hot fluids show significantly improved textural properties and different crystallinity compared to their calcined analogues. Ce loading as well as the processing procedure had a key effect on the crystallization of CexTi1−xO2 materials. The maximum N2O conversion (77xa0% after 20xa0h of illumination) in inert gas was reached over the Ce0.05Ti0.95O2 photocatalyst and it can be attributed to the simultaneous N2O photocatalytic decomposition and N2O photolysis. Reaction kinetics of N2O photocatalytic decomposition was described well by the pseudo-first-rate law. The lower photocatalytic activity of the Ce0.30Ti0.70O2 photocatalyst compared to the Ce0.05Ti0.95O2 photocatalyst can be correlated with two factors; the lower disorder within anatase crystal structure and possible presence of some cerium-containing amorphous phase (e.g., CeO2 or CeTi2O6), which can partially block the surface active sites within the anatase crystal structure.

Precursors of active Ni species in Ni/Al2O3 catalysts for oxidative dehydrogenation of ethane

Ni/Al2O3 catalysts for oxidative dehydrogenation (ODH) of ethane were prepared by impregnation of Al2O3 with nickel acetate or nickel nitrate, and by mechanical mixing of NiO and Al2O3. The Ni-based catalysts were characterized by N2 adsorption-desorption, X-ray diffraction, diffuse reflectance UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction of hydrogen. The results showed that formation of crystalline NiO particles with a size of < 8 nm and/or non-stoichiometric NiO species in the Ni/Al2O3 catalysts led to more active species in ODH of ethane under the investigated reaction conditions. In contrast, tetrahedral Ni species present in the catalysts led to higher selectivity for ethene. Formation of large crystalline NiO particles (22–32 nm) over Ni/Al2O3 catalysts decreased the selectivity for ethene.

Optimization of cerium doping of TiO2 for photocatalytic reduction of CO2 and photocatalytic decomposition of N2O

The cerium-doped TiO2 photocatalysts (0–0.8xa0mol% Ce) were prepared by using a sol–gel method. Textural, structural, optical and electronic properties of Ce/TiO2 photocatalysts were characterized in detail by using nitrogen physisorption, powder X-ray diffraction, diffuse reflectance UV–Vis spectroscopy and contact potential difference measurements. It was proved that increasing amount of cerium ions in TiO2 (1) decreased the anatase crystallite size, which corresponded to the increase in specific surface area of the photocatalysts, and (2) decreased the absorption edge (shifting the spectral response toward the visible light region). The prepared photocatalysts were tested for CO2 photocatalytic reduction in a stirred batch annular reactor, and methane was a main product. The photocatalytic decomposition of nitrous oxide was carried out in batch reactor with circulation, and only oxygen and nitrogen were detected as reaction products. It was found out that the energies of electrons and holes played the key role in both photocatalytic reactions and can be markedly affected by doping of TiO2 by cerium.Graphical Abstract

TiO2–CeO2 prepared by using pressurized and supercritical fluids: effect of processing parameters and cerium amount on (micro)structural and morphological properties

CexTi1−xOn composites with different Ti:Ce molar ratios (from 95:05 up to 70:30) were prepared unconventionally, via the sol–gel process controlled within reverse micelles of nonionic surfactant and processing by pressurized hot and supercritical fluids in a flow regime as an alternative to common thermal treatment. Nitrogen physisorption, powder X-ray diffraction (XRD) combined with Rietveld/whole powder pattern modeling (WPPM) and organic elemental analysis (OEA) were used as tools for characterization of the porous structure morphology, structural and microstructural properties, and purity of the prepared composites. All prepared CexTi1−xOn composites possessed well-developed mesoporous structure with minimum portion of micropores, showing specific surface area in the range of 203–256xa0m2/g. The experimental conditions during pressurized hot and supercritical processing as well as the Ce loading played a key role in crystallization of individual CexTi1−xOn composites. With increasing Ce loading, crystallization of anatase at the expense of brookite was promoted, accompanied with decreasing microstrain in anatase crystallites. The elevated processing temperature (250xa0°C) led to crystallization of CeO2 cubic beside TiO2 anatase. As a consequence of the different solubility of the used surfactant in pressurized hot and supercritical solvents under pressures of 10 and 30xa0MPa, cubic CeO2 crystallites of different sizes were formed. This property of CeO2 crystallites crucially affected the recrystallization of Ce0.30Ti0.70On-S composites at elevated temperatures; small and uniform CeO2 crystallites stabilized the anatase–cerianite phase mixture, giving rise to minor brookite phase.

TiO2 and nitrogen doped TiO2 prepared by different methods; on the (micro)structure and photocatalytic activity in CO2 reduction and N2O decomposition

TiO2 as nanostructured powders were prepared by (1) sol-gel process and (2) hydrothermal method in combination with (A) the processing by pressurized hot water and methanol or (B) calcination. The subsequent synthesis step was the modification of prepared nanostructured TiO2 with nitrogen using commercial urea. Textural, structural, surface and optical properties of prepared TiO2 and N/TiO2 were characterized by nitrogen physisorption, powder X-ray diffraction, X-ray photoelectron spectroscopy and DR UV-vis spectroscopy. It was revealed that TiO2 and N/TiO2 processed by pressurized fluids showed the highest surface areas. Furthermore, all prepared materials were the mixtures of major anatase phase and minor brookite phase, which was in nanocrystalline or amorphous (as nuclei) form depending on the applied preparation method. All the N/TiO2 materials exhibited enhanced crystallinity with a larger anatase crystallite-size than undoped parent TiO2. The photocatalytic activity of the prepared TiO2 and N/TiO2 was tested in the photocatalytic reduction of CO2 and the photocatalytic decomposition of N2O. The key parameters influencing the photocatalytic activity was the ratio of anatase-to-brookite and character of brookite. The optimum ratio of anatase-to-brookite for the CO2 photocatalytic reduction was determined to be about 83 wt.% of anatase and 17 wt.% of brookite (amorphous-like) (TiO2-SG-C). The presence of nitrogen decreased a bit the photocatalytic activity of tested materials. On the other hand, TiO2-SG-C was the least active in the N2O photocatalytic decomposition. In the case of N2O photocatalytic decomposition, the modification of TiO2 crystallites surface by nitrogen increased the photocatalytic activity of all investigated materials. The maximum N2O conversion (about 63 % after 18 h of illumination) in inert gas was reached over all N/TiO2.

Investigation of low Ce amount doped-TiO2 prepared by using pressurized fluids in photocatalytic N2O decomposition and CO2 reduction

Ce doped TiO2 anatase/brookite composites with 0.6–5.5u2009wt% of Ce, as well as parent TiO2 anatase/brookite were synthesized to be investigated in two environmentally-beneficial reactions, the photocatalytic decomposition of N2O and the photocatalytic reduction of CO2. Composites were prepared unconventionally, by using sol–gel method combined with the processing by pressurized hot fluids (in the sequence water/methanol/water). The physicochemical and electronic properties of all synthesized composites were characterized by organic elementary analysis, nitrogen physisorption, powder X-ray diffraction, X-ray fluorescence spectroscopy, diffuse reflectance UV-vis spectroscopy and work function measurements. It was revealed that all composites show comparable textural properties, crystallite size, as well as optical properties, except for the 5.5u2009wt% Ce/TiO2 composite which showed significantly lowered band gap energy due to the significantly higher population of Ce. Concerning the composite structural properties, the addition of different amounts of Ce in the range of 0.6–5.5u2009wt% affected markedly the phase composition of composites, namely the anatase-to-brokite weight ratio. Concerning the photocatalytic tests the 5.5u2009wt% Ce/TiO2 composite showed the highest photocatalytic performance. The highest photocatalytic performance of the 5.5u2009wt% Ce/TiO2 composite can be attributed to the lowest composite work function which is affected by both the amount of Ce, as well as the phase composition.Graphical Abstract

Adsorption of As(V), Cd(II) and Pb(II), in multicomponent aqueous systems using activated carbons

u2003 This paper studies the use of two activated carbon samples made of cocoa pod husk (CPH-AC) and one commercial activated carbon sample in the adsorption of As(V), Cd(II) and Pb(II) from multicomponent synthetic solutions and from the Puyango-Tumbes River water, a river located in northwest Peru. The characterization of the activated carbon samples was conducted. The CPH-AC samples exhibited a specific surface area (SBET) between 709 and 1117 m2/g and a pH point of zero charge (pHPZC) between 4.4 ± 0.2 and 5 ± 0.2, while the commercial material gave an SBET value of 775 m2/g and a pHPZC value of 7.6 ± 0.1. All the evaluated samples displayed the capacity to adsorb As(V), Cd(II) and Pb(II) from both aqueous systems. The adsorption efficiency for Pb was outstanding reaching the value of 89%. A pseudo-second order kinetic model was satisfactorily applied for most of the activated carbon samples.