Leszek Kępiński

Carbon deposition on Ni/Al2O3 catalysts doped with small amounts of molybdenum

High resolution transmission electron microscopy (HRTEM) was used in the study of the structure of carbon deposits obtained on Ni/Al2O3 and Ni–Mo/Al2O3 catalysts during steam reforming of n-butane. The introduction of small amounts of molybdenum compounds (≤ 1 wt% of Mo) into the nickel catalysts greatly improved their resistance to coking. The morphology of carbon is similar on both catalysts. On the Ni–Mo/Al2O3 catalyst much lower numbers of Ni particles were observed that are active in the carbon filaments growth.

Effect of chlorine on microstructure and activity of Pd/CeO2 catalysts

The evolution of microstructure and activity in benzene hydrogenation of Pd/CeO2 catalysts, prepared from Pd chloride or Pd nitrate, upon reduction at 573–973 K has been studied by X-ray diffraction (XRD), selected-area electron diffraction (SAD), high-resolution transmission electron microscopy (HRTEM) and gas chromatography. The crucial role of chloride from the metal precursor in structural transformations of the ceria support has been established and attributed to its ability to form stable cerium oxychloride at temperatures as low as 573 K. A. rapid decline to zero of the activity of Pd/CeO2 catalysts in benzene hydrogenation was observed when the temperature of reduction was increased from 573 to 773 K. This effect could not be explained by sintering, but XRD data indicated that a metal–support reaction leading to Pd–Ce alloy formation was likely to be the cause. At higher temperature (873 K) the coverage of Pd particles by a thin overlayer (probably CeOx) was observed by HRTEM.

Rietveld refinement of the structure of CeOCI formed in Pd/CeO2 catalyst: Notes on the existence of a stabilized tetragonal phase of La2O3 in LaPdO system

Abstract A structure of CeOCl formed during the synthesis of Pd/CeO 2 catalyst from PdCl 2 and CeO 2 was refined by the Rietveld method from the X-ray powder diffraction data. CeOCl has a PbFCl-type structure with tetragonal space group P4nmm; a = 4.0785(1)A, c = 6.8346(3)A. It is shown that a phase found and refined by Guru Row et al. ( J. Solid State Chem. 95, 224, 1991) described as a palladium stabilized tetragonal phase of La 2 O 3 seems to the LaOCl, the compound with a structure isomorphous with CeOCl.

Reduction study of Co3O4 model catalyst by electron microscopy

Evolution of microstructure and morphology of Co3O4 particles in the model systems during reduction in hydrogen was studied by transmission electron microscopy methods. Based on SAED and HRTEM results we found that the degree of reduction of Co3O4 strongly depends on the particle size and morphology, which are determined by the pretreatment conditions. Preferential epitaxial growth of CoO and Co phases on Co3O4 during reduction was deduced from HRTEM images.

Microstructure of Pd/CeO2 catalyst: Effect of high temperature reduction in hydrogen

Abstract The effect of high temperature reduction (HTR) in hydrogen (up to 1180 K) on the microstructure of 9 wt.-% Pd CeO 2 catalyst was studied by HRTEM and XRD methods. Reduction of the catalyst at or above 973 K caused severe recrystallization of CeO2 and Pd with simultaneous strong interaction between the two components appearing as three phenomena: epitaxial growth of small Pd particles on CeO2 (most frequently with [111]Pd∥[111]CeO2); decoration of large Pd particles with ordered CeO2 overlayer and expansion of the lattice parameter of Pd (by 2.1%). The origin of the Pd lattice expansion is discussed and diffusion of Ce species into the Pd lattice seems to be the most probable one. HTR caused also phase transformations in the ceria support. At 973 K and 1100 K, whole CeO2 was transformed into oxygen deficient CeOx phase exhibiting the same or similar structure but with expanded lattice parameter (by 2.8%). At 1180 K most ceria was transformed into hexagonal A-Ce23. The CeOx phase appeared to be stable in hydrogen and in vacuum at room temperature, but upon exposure to air at room temperature it rapidly reoxidised to CeO2. Ce2O3 also reoxidised to CeO2 but much slower. Another consequence of HTR at or above 773 K was formation of pits in CeO2 crystallites, mainly on (112)-type crystal faces. The pits (1–10 nm) exhibited well defined walls parallel to CeO2 lattice fringes and they could possibly constitute nucleation sites for strongly bonded, epitaxial oriented Pd particles.

Giant enhancement of upconversion in ultra-small Er3+/Yb3+:NaYF4 nanoparticles via laser annealing

Most of the synthesis routes of lanthanide-doped phosphors involve thermal processing which results in nanocrystallite growth, stabilization of the crystal structure and augmentation of luminescence intensity. It is of great interest to be able to transform the sample in a spatially localized manner, which may lead to many applications like 2D and 3D data storage, anti-counterfeiting protection, novel design bio-sensors and, potentially, to fabrication of metamaterials, 3D photonic crystals or plasmonic devices. Here we demonstrate irreversible spatially confined infrared-laser-induced annealing (LIA) achieved in a thin layer of dried colloidal solution of ultra-small ∼8 nm NaYF₄ nanocrystals (NCs) co-doped with 2% Er³⁺ and 20% Yb³⁺ ions under a localized tightly focused beam from a continuous wave 976 nm medium power laser diode excitation. The LIA results from self-heating due to non-radiative relaxation accompanying the NIR laser energy upconversion in lanthanide ions. We notice that localized LIA appears at optical power densities as low as 15.5 kW cm⁻² (∼354 ± 29 mW) threshold in spots of 54 ± 3 µm diameter obtained with a 10 × microscope objective. In the course of detailed studies, a complete recrystallization to different phases and giant 2-3 order enhancement in luminescence yield is found. Our results are highly encouraging and let us conclude that the upconverting ultra-small lanthanide-doped nanophosphors are particularly promising for direct laser writing applications.

Structural and spectroscopic characterization of Lu2O3:Eu nanocrystalline spherical particles

Spherical particles of Lu2O3:x% Eu, with x varying from 0% to 10% with respect to Lu, were prepared by precipitating hydroxides with urea at 80 °C and subsequently decomposing these hydroxides to oxides at 650 °C. TEM pictures revealed that the spherical particles were very uniform in size and their diameters were about 130 nm. Each of the particles consisted of crystallites about 20 nm in diameter. Luminescence and excitation spectra contained all the characteristic features of the Eu3+ ion. The most intense line in the emission was located around 611 nm. Energy transfer was observed from the Eu3+ ions occupying the S6(C3i) centrosymmetric site to the Eu3+ located in the non-centrosymmetric position of C2 symmetry. The decay kinetics were slightly non-exponential, especially for the lowest dopant concentrations. At liquid nitrogen temperature the average decay time for the 0.2% powder was shorter by about 40% compared to the 3–10% materials. At room temperature the average decay time varies only slightly. Rise times were observed for all concentrations at room temperature but only for higher concentrations at liquid nitrogen temperature. This effect is in contrast to that of nanoparticles of Lu2O3:Eu prepared using different synthesis procedures.

Nature and optical behaviour of heavily europium-doped silica glasses obtained by the sol–gel method

Abstract Silica glasses doped with Eu 3+ were prepared by the sol–gel method. The effects of the synthetic procedure on optical properties of the europium heavily doped glasses have been investigated. The fluorescence characteristics of Eu 3+ have been measured for several thermal stages of the sol–gel process. In particular, we have studied aggregation of the Eu ions leading to creation of nanocrystallites during the thermal treatment of silica glasses. The size of the nanocrystalites and their structure have been determined by means of high-resolution transmission electron microscopy and X-ray diffraction. The influence of the OH − groups on the luminescence lifetimes of Eu 3+ has been studied. A broad luminescence band in the blue region has been observed for the silica glasses possessing silanol (Si–OH) groups. Its intensity decreases with the increasing temperature of sintering for the samples treated in the presence of NH 4 Cl. In the case of those heated at high temperatures, doubly doped with Eu and Al silica glasses, a blue emission assigned to the Eu 2+ ions has been observed. Its intensity increases with the Al 3+ ions concentration.

Nanocrystalline rare earth silicates: structure and properties

Formation of nanocrystalline rare earth (RE) silicates (RE = Y, Nd) inside or at the surface of amorphous SiO 2 matrix upon heal treatment in air was studied by transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). RE-doped SiO 2 samples were prepared by the sol-gel or impregnation method. The structure evolution of the silicate phase depends on the preparation method and RE 2 O 3 :SiO 2 molar ratio. For a Nd 2 O 3 :SiO 2 molar ratio below 1:2, the formation of a superficial silicate, similar to Ce 6 [Si 4 O 13 ][SiO 4 ] 2 reported recently, was observed in the temperature range 900-1100 C.

Crystal Structure and Morphology Evolution in the LaXO3, X = Al, Ga, In Nano-Oxide Series. Consequences for the Synthesis of Luminescent Phosphors

The LaXO(3):Tb(3+) (X = Al(3+), Ga(3+), In(3+)) perovskite nanoparticles were obtained using the nonhydrolytic treatment (Bradley reaction) of the molecular precursors of the La(O(i)Pr)(3), Al(O(i)Pr)(3), Ga(O(i)Pr)(3), In(5)O(O(i)Pr)(13), and Tb(acac)(3), respectively. It was shown that crystal structure and morphology evolution in the LaXO(3), X = Al, Ga, In nano-oxide series depended on the size and chemical properties of the X-metal atom. Formation of the LaInO(3):Tb(3+) nanoparticles is distinctly less thermodynamically demanding on contrary to the LaAlO(3):Tb(3+) and LaGaO(3):Tb(3+) since it provided crystalline product directly in the solution synthesis at 202 °C, which is the lowest reported synthesis temperature for this compound up-to-date. This behavior was ascribed to the effects directly connected with the dopant substitution (exchange of bigger La(3+) cation with smaller Tb(3+)) as well as reduction of the particle size. The size effects are mostly reflected in the expansion of the cell volume, changes of the cell parameters as well as shifting and broadening of the Raman bands. Indirectly, size reduction has also an effect on the luminescence properties through the higher probability of presence of surface and net defects as well as heterogeneous distribution of the Tb(3+) ions caused by high surface-to-volume ratio. The prepared nanophosphors show basically green emission with exception of white-green in case of the LaInO(3):Tb(3+). Strong emission quenching was found in the latter case being most likely a consequence of the nonradiative energy transfer between Tb(3+) and In(3+) as well as the presence of defects. In comparison to the Pechinis method, the LaXO(3) nanoparticles required significantly lower annealing temperature (700 °C) necessary for complete crystallization. Generally the resulting particles are distinctly smaller (5 to 25 nm) and less agglomerated (50-100 nm) depending on the reaction conditions as well as thermal treatment. For the first time, it was shown that the LaGaO(3):Tb(3+) nanopowder has crystallized in the high-temperature rhombohedral R3c phase.