Effect of the Chemical and Phase Composition of Nanocrystalline Gadolinium Complex Oxides on the Propane Cracking Process

Abstract: Effect of the chemical composition and crystal structure type of gadolinium complex oxides on their catalytic activity is studied. It is shown that nanocrystalline powders Gd2Ti2O7, Gd2Zr2O7, and Gd2Hf2O7 form highly symmetrical face-centered сubic crystal structures having localized (in Gd2Ti2O7) and delocalized (in Gd2Zr2O7 and Gd2Hf2O7) oxygen vacancies. At the same time, low-symmetrical crystal structures are formed in Gd2(WO4)3 (the monoclinic structure) and Gd2(МоO4)3 (a mixture of monoclinic and rhombic structures). Catalytic runs show that formation of the cubic structure contributes to an increase in the degree of conversion of propane and causes a shift in cracking temperatures to lower values. Formation of this type of nanocrystalline oxides facilitates the dehydrogenation reaction with propylene selectivity up to 80% at temperatures up to 700 К. Formation of the mixture of monoclinic and rhombic structures in Gd2(МоO4)3 leads to a shift in the degree of conversion to the catalytic temperature range of 700–900 К, in which the dehydrogenation reaction predominates (80%). In the case of the monoclinic structure of Gd2(WO4)3 in the same cracking temperature ranges the process of degradation occurs in parallel; this decreases propylene selectivity to 50% and promotes an increase in the yield of ethylene to 30%. © 2020, Pleiades Publishing, Ltd

Rearrangement in the local, electronic and crystal structure of europium titanates under reduction and oxidation

We studied powder samples of europium titanates Eu1−x2+Eux3+TiO3+x/2 prepared by sol-gel and coprecipitation methods with subsequent thermal treatment of precursors at different conditions (reducing or oxidizing atmosphere, temperature, exposure time). We have revealed a radical rearrangement in the local, electronic and crystal structure of europium titanates depending on synthesis conditions. The combination of synchrotron X-ray diffraction (s-XRD), X-ray absorption (XANES and EXAFS), infrared and Raman spectroscopies supplemented by photoluminescence and simultaneous thermal analysis was used in our study. It is shown that the combined XANES and thermogravimetric analysis gives clear evidence of a change in the oxidation state of europium cations from Eu2+ to Eu3+. The comparison of s-XRD, XANES, and EXAFS data allowed us to explain the local structure rearrangement during the phase transition from cubic perovskite Eu2+TiO3 (space group Pm3¯m) to pyrochlore phase Eu23+Ti2O7 (space group Fd3¯m) through intermediate layered perovskite Eu23+Ti2O7 with monoclinic structure (space group P21) under annealing in oxidizing atmosphere. The Raman spectroscopy study pointed to significant changes in the local structure of the anionic sublattice upon the observed phase transitions. The analysis of luminescence spectra has shown that Eu3+ cations occupy positions with low local site symmetry in all synthesized compounds regardless of the type of the crystal structure. © 2020 Elsevier B.V