Effect of CaO on Phase Composition and Properties of Aluminates for Barium Tungsten Cathode

Neodymium-Doped Novel Barium Tungstate Nanospindles for the Enhanced Oxygen Evolution Reaction

In this work, pristine, 0.02, 0.04, and 0.06 M neodymium (Nd)-doped barium tungstate nanostructures were synthesized via a simple co-precipitation method for the water oxidation process. The obtained X-ray diffraction high-intensity peak at a 2θ value of 26.4° corresponding to the (112) lattice plane confirmed the formation of a tetragonal structure of BaWO₄. Moreover, the BaWO₄ morphology was examined by scanning electron microscopy, which showed the existence of nanospindles. An energy-dispersive X-ray spectrum confirmed the subsistence of the produced materials, for example, barium (Ba), tungsten (W), oxide (O), and neodymium (Nd), with weight percentages of 28.58, 46.63, 16.64, and 8.16%, respectively. The 0.04 M Nd-doped BaWO₄ product was explored to attain a high surface area of 18.18 m²/g, a pore volume of 0.079 cm³/g, and a pore diameter of 2.215 nm. Compared to the other prepared electrodes, the 0.04 M Nd-doped BaWO₄ product exhibited low overpotential values of 330 mV and 450 mV to deliver current densities of 10 mA/cm² and 50 mA/cm², respectively. In addition, the optimized electrode achieved a small Tafel slope value of 158 mV dec^-1 and followed the Volmer-Heyrovsky mechanism. Moreover, the electrical conductivity of BaWO₄ was tuned due to the addition of a rare-earth metal dopant, and it exhibited the charge-transfer resistance and solution resistance values of 0.98 and 1.01 Ω, respectively. The prepared electrocatalyst was further studied by using cyclic voltammetry, and it exhibited a high double-layer capacitance value of 29.3 mF/cm² and high electrochemically active surface areas of 1.465 cm². The electrochemical performance was greatly improved depending on the concentration of the doping agent, and it was well consistent with the obtained results. The best electrocatalyst was subjected to a chronoamperometry test, which exhibited excellent stability even after 20 h. Hence, this work suggests that alkaline metal tungstates have a cost-effective, efficient, and promising electrocatalyst, and it is a new approach for the water oxidation process.

Near-Surface Material Phases and Microstructure of Scandate Cathodes

Scandate cathodes that were fabricated using the liquid-solid process and that exhibited excellent emission performance were characterized using complementary state-of-the-art electron microscopy techniques. Sub-micron BaAl₂O₄ particles were observed on the surfaces and edges of tungsten particles, as seen in cross-section samples extracted from the scandate cathode surface regions. Although several BaAl₂O₄ particles were observed to surround smaller Sc₂O₃ nanoparticles, no chemical mixing of the two oxides was detected, and in fact the distinct oxide phases were separately verified by chemical analysis and also by 3D elemental tomography. Nanobeam electron diffraction confirmed that the crystal structure throughout W grains is body-centered cubic, indicating that they are metallic W and did not experience noticeable changes, even near the grain surfaces, as a result of the numerous complex chemical reactions that occur during cathode impregnation and activation. 3D reconstruction further revealed that internal Sc/Sc₂O₃ particles tend to exhibit a degree of correlated arrangement within a given W particle, rather than being distributed uniformly throughout. Moreover, the formation of Sc/Sc₂O₃ particles within W grains may arise from W surface roughening that occurs during the liquid-solid synthesis process.

Efficient Catalytic Production of Biodiesel with Acid-Base Bifunctional Rod-Like Ca-B Oxides by the Sol-Gel Approach

The search for acid-base bifunctional catalysts has become a hot topic in the preparation of biofuels from renewable resources. In the present work, a series of novel acid-base bifunctional metal-boron catalysts were successfully prepared by a sol-gel method and characterized by XRD, IR, SEM, TEM, TGA, BET, and TPD. Among those bifunctional solid materials, the Ca-B(700) catalyst had the highest density of both acid and base sites and showed excellent catalytic performance in the production of biodiesel from nonedible oils with high acid value. Under the optimal reaction conditions of 20/1 methanol/oil mole ratio and 4 wt % catalyst dosage at 105 °C for 2 h, a high biodiesel yield of 96.0% could be obtained from Jatropha curcas oil in one-pot. In addition, Ca-B(700) was also applicable to producing biodiesel from Firmiana platanifolia L.f. oil in a relatively low acid value, with an almost quantitative yield (98.5%) at 65 °C after 2 h. The Ca-B(700) catalyst had good stability and reusability, which is a promising acid-base bifunctional catalytic material for the preparation of biodiesel.