CO2 reduction over NaNbO3 and NaTaO3 perovskite photocatalysts

Both NaNbO₃ and NaTaO₃ exhibit interesting intrinsic photocatalytic activities for CO₂ reduction in terms of conversion and selectivity.

Revisiting the BaO2/BaO redox cycle for solar thermochemical energy storage

The barium peroxide-based redox cycle was proposed in the late 1970s as a thermochemical energy storage system. Since then, very little attention has been paid to such redox couples. In this paper, we have revisited the use of reduction-oxidation reactions of the BaO2/BaO system for thermochemical heat storage at high temperatures. Using thermogravimetric analysis, reduction and oxidation reactions were studied in order to find the main limitations associated with each process. Furthermore, the system was evaluated through several charge-discharge stages in order to analyse its possible degradation after repeated cycling. Through differential scanning calorimetry the heat stored and released were also determined. Oxidation reaction, which was found to be slower than reduction, was studied in more detail using isothermal tests. It was observed that the rate-controlling step of BaO oxidation follows zero-order kinetics, although at high temperatures a deviation from Arrhenius behaviour was observed probably due to hindrances to anionic oxygen diffusion caused by the formation of an external layer of BaO2. This redox couple was able to withstand several redox cycles without deactivation, showing reaction conversions close to 100% provided that impurities are previously eliminated through thermal pre-treatment, demonstrating the feasibility of this system for solar thermochemical heat storage.

Assessing biomass catalytic pyrolysis in terms of deoxygenation pathways and energy yields for the efficient production of advanced biofuels

The present work systematically studies the effect of the operation conditions of biomass catalytic pyrolysis on parameters like bio-oil oxygen content and mass yield, but also on additional indicators, such as the distribution of oxygen and chemical energy among the products.

Revisiting the hydrothermal synthesis of titanate nanotubes: new insights on the key factors affecting the morphology

Recently, titanate nanotubes have attracted a significant deal of interest, but the mechanism of formation as well as some structural aspects of these materials is still under debate. In this context, the main goal of the present study is to identify the key factors affecting the morphology of the titanate nanostructures during hydrothermal synthesis in order to increase the homogeneity and control, as finely as possible, the dimensions of the nanotubes. Previous studies also analyzed the influence of synthesis conditions but, unlike other works, the simultaneous assessment of the influence of parameters such as the phase and crystal size of the TiO(2) used as precursor, and the time and temperature of the hydrothermal treatment has been now analyzed under the same experimental context. The characterization of the prepared materials by means of TEM, XRD, N(2) adsorption isotherms and Raman spectroscopy was performed to establish correlations between the structural features and the preparation conditions. In this research, special attention has been paid to study the non-washed materials, which most of the previous studies have excluded from characterization. After hydrothermal treatment amorphous particles with different morphologies are isolated before acidification, which seems to be a crucial stage in controlling nanotube formation. Besides, the obtained data indicate that some characteristics of the nanotubes, such as the degree of conversion, the homogeneity and crystallinity of the structures or the aspect ratio, can be easily selected by adjusting the synthesis parameters.

X-ray diffraction, and Raman scattering study of nanostructured ZrO2-TiO2 oxides prepared by sol-gel

In the present work, we study the phase composition of ZrO2-TiO2 system by means of XRD and Raman spectroscopy, using also TG-ATD, and N2 adsorption isotherms as complementary characterization techniques. TiO2-ZrO2 samples of selected compositions (0, 10, 90, 50 and 100% in weight of TiO2) were prepared by sol-gel method and annealed at three different temperatures (400, 600 and 800 degrees C). Structural characterization reveals that only the pure oxides are crystalline at 400 degrees C: TiO2 as anatasa with a minor brookite component, and ZrO2 as a mixture of tetragonal (majority) and monoclinic phases. Following the 600 degrees C calcination, the TiO2-ZrO2 50-50% sample forms the ZrTiO4 mixed oxide, although this materials remains partly amorphous. In contrast, samples with higher and lower TiO2 content form solid solutions with, respectively, anatasa and tetragonal ZrO2 structures. Zirconium incorporation into the TiO2 lattice leads to the expansion of the unit cell parameters, and it stabilizes the anatase phase, hindering its transformation into rutile. Similarly, dissolving titanium atoms into the ZrO2 structure delays the transformation from the tetragonal to the monoclinic polymorph.