Thermodynamic properties of the La2 O3 -ZrO2 system by Knudsen effusion mass spectrometry at high temperature.
RATIONALE
Zirconia doped with a lanthanum oxide system is of high interest due to its exceptional thermal stability for the development of high-performance ceramics. It possesses the beneficial properties of pure zirconia, such as heat resistance, mechanical strength and inertness, but also eliminates its main disadvantage, i.e. brittleness. At high temperatures, components of such systems may vaporize selectively, leading to significant change in composition, and hence, the thermal resistance, phase stability and performance of the ceramic materials. Therefore, information on the vaporization processes and thermodynamic properties of the La2 O3 -ZrO2 system is of great importance.


METHODS
Knudsen effusion mass spectrometry was used to study the vaporization processes and thermodynamic properties of solid solutions in the La2 O3 -ZrO2 system at 2110 K. Pure La2 O3 was used as a reference substance. Comprehensive characterization of the precursors and ceramics was performed via SEM, STA, XRD and PSD analysis.


RESULTS
Zirconia-doped lanthania precursor powders and ceramics with La2 O3 of 33.3, 50, 70 and 90 mol.% content were manufactured by solid-state synthesis and the original cryochemical technique. La, LaO, ZrO and ZrO2 were found to be the main vapor species over the samples studied. The activities and thermodynamic properties of La2 O3 were calculated. Via XRD analysis it was shown that the phase composition of xLa2 O3 -(100-x)ZrO2 powders (x = 0.33, 0.5, 0.7 and 0.9 mol. fraction) significantly depends on the synthesis technique chosen.


CONCLUSIONS
According to the XRD results combined with Rietveld refinement of the patterns, 33.3 La2 O3 -66.7 ZrO2 ceramics after solid-state synthesis are composed of well-formed cubic pyrochlore-type La2 Zr2 O7 with 5 wt.% admixture of monoclinic and cubic ZrO2 whereas 33.3 La2 O3 -66.7 ZrO2 precursor powders after cryochemical synthesis correspond to low-crystalline La(OH)3 . The components of the La2 O3 -ZrO2 system evaporate separately: there is no temperature range where lanthanum and zirconium gaseous species are present together. It was found that the activities of lanthania have low negative deviation from the ideal case.
La2O3 nanoplate: An efficient electrocatalyst for artificial N2 fixation to NH3 with excellent selectivity at ambient condition
Abstract Identification of new effective and selective N2 reduction electrocatalyst is still of critical importance for fundamental and application research. Here, we report La2O3 nanoplate performing as an efficient metal oxide catalyst for electrochemical N2 fixation to NH3, which exhibits excellent selectivity and stability with a high Faradaic efficiency of 4.76% and a NH3 yield rate of 17.04 μg h−1 mg−1cat. at −0.8 V vs. reversible hydrogen electrode (RHE) in 0.1 M Na2SO4 solution under atmospheric pressure. Density functional theory (DFT) calculation results show that La atoms in La2O3 act as the active sites for efficiently adsorbing N2. The optimal adsorption approach of N2 molecule on the (011) face of La2O3 is end-on-two configuration which is highly activated. Thus La2O3 exhibits high performance for electrochemical reduction of N2.
Mass Spectrometric Determination of the Heats of Atomization of NdO2, NdBO, and NdBO2 and Upper Values for the Dissociation Energies of NdAg and Nd2
During evaporation of Nd and Ag from a boron nitride Knudsen cell the new molecules NdO2(g), NdBO(g), and NdBO2(g) were detected by mass spectrometric analysis. The enthalpies of the following reactions were determined by a third‐law method: NdO2(g)+Nd(g)→2NdO(g), ΔH0° = − 10.5 ± 1.2 kcal mol−1, NdBO(g)→Nd(g)+BO(g), ΔH0° = 77.4 ± 1.9 kcal mol−1, Nd(g)+NdBO2(g)→NdBO(g)+NdO(g), ΔH0° = 10.5 ± 4. kcal mol−1. From these reaction enthalpies the heats of atomization of NdO2, NdBO, and NdBO2 were calculated: NdO2(g): ΔH0°atom = 323.5 ± 12.5 kcal mol−1; NdBO(g): ΔH0°atom = 268.7 ± 10 kcal mol−1; NdBO2(g): ΔH0°atom = 446.2 ± 13 kcal mol−1. A discussion of the possible geometric arrangement of these molecules on the basis of thermochemical data is given. Upper limits for the bond energies of Nd2(g) and NdAg(g) are given as 39 and 50 kcal mol−1, respectively.