Mechanochemical Activation of Cu–CeO<Subscript>2</Subscript> Mixture as a Promising Technique for the Solid-State Synthesis of Catalysts for the Selective Oxidation of CO in the Presence of H<Subscript>2</Subscript>

A new ecologically clean method for the solid-phase synthesis of oxide copper–ceria catalysts with the use of the mechanochemical activation of a mixture of Cu powder (8 wt %) with CeO₂ was developed. It was established that metallic copper was oxidized by oxygen from CeO₂ in the course of mechanochemical activation. The intensity of a signal due to metallic Cu in the X-ray diffraction analysis spectra decreased with the duration of mechanochemical activation. The Cu¹⁺, Cu²⁺, and Ce³⁺ ions were detected on the sample surface by X-ray photoelectron spectroscopy. The application of temperature-programmed reduction (TPR) made it possible to detect two active oxygen species in the reaction of CO oxidation in the regions of 190 and 210–220°C by a TPR-H₂ method and in the regions of 150 and 180–190°C by a TPR-CO method. It is likely that the former species occurred in the catalytically active nanocomposite surface structures containing Cu–O–Ce bonds, whereas the latter occurred in the finely dispersed particles of CuO on the surface of CeO₂. The maximum conversion of CO (98%, 165°C) reached by the mechanochemical activation of the sample for 60 min was almost the same as conversion on a supported CuO/CeO₂ catalyst.     

Trivalent zirconium and hafnium ions in yttrium oxide ceramics

An analysis of the electron spin resonance (ESR) spectrum of transparent ceramics composed of yttrium oxide with zirconium and hafnium additives has revealed the presence of signals (with similar parameters) from Zr³⁺ and Hf³⁺ ions, which have a similar electron configurations of the ground states: [Kr]4d ¹ and [Xe]5d ¹, respectively. It is shown that the pulsed cathodoluminescence spectra of these ions consist of two bands peaking at λ ≈ 818 and 900 nm.     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Cu–CeO<Subscript>2</Subscript> nanocomposites: mechanochemical synthesis,physico-chemical properties,CO-PROX activity

Catalytic systems designated for preferential oxidation of CO in the presence of H₂ are prepared by ball milling of Cu and CeO₂, a simple and cheap one-step process to synthesize such catalysts. It is found that after 60 min of milling, a mixture of 8 wt.% Cu–CeO₂ powders exhibits CO conversion of 96% and CO selectivity of about 65% at 438 K. Two active oxygen states, which are not observed in case of pure CeO2, were detected in the nanocomposite lattice and attributed to the presence of Cu in surface sites as well as in subsurface bulk sites. Correspondingly, oxidation of CO to CO₂ was found to occur in a two-stage process with T _max ≈ 395/460 K, and oxidation of H₂ to H₂O likewise in a two-stage process with T _max ≈ 465/490 K. The milled powder consists of CeO₂ crystallites sized 8–10 nm agglomerated to somewhat larger aggregates, with Cu dispersed on the surface of the CeO₂ crystallites, and to a lesser extent present as Cu₂O.

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Time-resolved luminescence spectroscopy of pure and doped with Ce3+ ions SrAlF5 crystals

The results of a study of time-resolved photoluminescence (PL) and energy transfer in both pure and doped with Ce³⁺ ions SrAlF₅ (SAF) single crystals are presented. The time-resolved and steady-state PL spectra in the energy range of 1.5–6.0 eV, the PL excitation spectra and the reflectivity in the energy range of 3.7–21 eV, as well as the PL decay kinetics were measured at 8.8 and 295 K. The lattice defects were revealed in the low temperature PL spectra (emission bands at 2.9 and 4.5 eV) in the undoped SAF crystals. The luminescence spectra of the doped Ce³⁺:SAF crystals demonstrate a new selective emission bands in the range of 3.7–4.5 eV with the exponential decay kinetics (τ ≈ 60 ns at X-ray excitation). These bands correspond to the d-f transitions in Ce³⁺ ions, which occupy nonequivalent sites in the crystal lattice.     

Influence of halide mixing on thermal and photochemical stability of hybrid perovskites: XPS studies

The effect of I^–/Cl^–and I^–/Br^–mixing on the thermal and photochemical degradation of organometallic perovskite MeNH₃PbI₃ (MAPbI₃) was studied by X-ray photoelectron spectroscopy, which revealed the opportunity to essentially increase the photo and thermal stabilities of the material depending on the level and position of halide-mixing. The largest positive effect was observed for the small concentration of chloride substituent (MAPbI_2.7Cl_0.3), while the full halide substitution (MAPbBr₃) had a negative effect on the stability of hybrid perovskite.     

Spectroscopy of defects in irradiated alpo 4 and GaPO 4 crystals

The time-resolved luminescence spectra in energy region of 2.0-6.0 v eV, as well as the excitation spectra (4-35 v eV), reflectivity and the decay kinetics were studied at T =10 v K and 295 v K using selective vacuum ultraviolet excitation in nominally pure crystals as well as crystals with intrinsic defects and radiation defects induced by fast electrons.     

Electronic structure of the Si-C-N amorphous films

The results of the investigation of amorphous a-SiC _x N _y films of various compositions by the ultra-soft X-ray emission spectroscopy are presented. The Si L _{2, 3}, C K _α, and N K _α emission spectra, which reflect the partial densities of states Si 3s3d, C 2p, and N 2p, respectively, have been measured and analyzed. It has been established that the Si-N chemical bond is similar in type to the Si-C chemical bond and completely substitutes for the Si-C bonds in thin a-SiC _x N _y films as the oxygen concentration increases. A similar effect is associated with the local agglomeration and subsequent clusterization of carbon atoms in the SiN-enriched regions of the internal volume of the film. A high ability of the a-SiC _x N _y films to oxidation in air has been established. This has been confirmed by a satisfactory approximation of the Si L _{2, 3} X-ray emission spectra of a-SiCN with the use of the superposition of the spectra of γ-Si₃N₄ and Si₂N₂O. The relative weight coefficients of the spectra of γ-Si₃N₄, which have been used to approximate the spectra of thin a-SiC _x N _y films, are proportional to the values of the Young’s modulus for different values of compositions (different values of x and y).