Proton exchange membrane water electrolysis (PEMWE) is a promising technology for electricity-to-fuel conversion which allows for direct production of hydrogen from water. One of the key problems limiting widespread implementation of PEMWE into energy systems is the sluggish kinetics of the anodic process: the oxygen evolution reaction (OER). Additionally, state-of-the-art OER materials contain large amounts of low abundant noble metals (Ru, Ir), and therefore, development of low-cost, highly active and stable OER catalysts remains an important challenge. We developed a synthetic approach to the iridium pyrochlores−complex oxides of iridium with reduced content of the noble metal as compared to IrO 2 . The materials were synthesized from molten sodium nitrate (Adams fusion method) at moderate temperatures (500−575 °C) and consist of highly crystalline iridium pyrochlore nanoparticles with surface areas of up to 40 m 2 g −1 , which is a significant improvement compared to the traditional high temperature solid-state synthesis. Electrochemical measurements in acidic media showed that yttrium and bismuth pyrochlore catalysts possess high OER activity approaching the activity of state-of-the-art IrO 2 nanoparticles. High intrinsic activities and stability behavior of yttrium iridium catalysts were correlated with the formation of the highly active IrO x surface layer due to leaching of the Y 3+ cations into the electrolyte solution, revealed both experimentally and computationally using density functional theory calculations.

We report herein quantitative in situ monitoring by simultaneous PXRD and Raman spectroscopy of the mechanochemical reaction between benzoic acid and nicotinamide, affording a rich polymorphic system with four new cocrystal polymorphs, multiple phase transformations, and a variety of reaction pathways. After observing polymorphs by in situ monitoring, we were able to isolate and characterize three of the four polymorphs, most of which are not accessible from solution. Relative stabilities among the isolated polymorphs at ambient conditions were established by slurry experiments. Using two complementary methods for in situ monitoring enabled quantitative assessment and kinetic analysis of each studied mechanochemical reaction, even when involving unknown crystal structures, and short-lived intermediates. In situ Raman monitoring was introduced here also as a standalone laboratory technique for quantitative assessment of mechanochemical reactions and understanding of mechanochemical reactivity. Our results provide an important step toward a complete and high-throughput quantitative approach to mechanochemical reaction kinetics and mechanisms, necessary for the development of the mechanistic framework of milling reactions.

Resonant acoustic mixing (RAM) is a new technology designed for intensive mixing of powders that offers the capability to process powders with minimal damage to particles. This feature is particularly important for mixing impact-sensitive materials such as explosives and propellants. While the RAM technique has been extensively employed for the mixing of powders and viscous polymers, comparatively little is known about its use for mechanosynthesis. We present here the first in situ study of RAM-induced co-crystallisation monitored using synchrotron X-ray powder diffraction. The phase profile of the reaction between nicotinamide and carbamazepine in the presence of a small amount of water was monitored at two different relative accelerations of the mixer. In marked contrast to ball-milling techniques, the lack of milling bodies in the RAM experiment does not hinder co-crystallisation of the two starting materials, which occurred readily and was independent of the frequency of oscillation. The reaction could be optimised by enhancing the number of reactive contacts through mixing and comminution. These observations provide new insight into the role of various experimental parameters in conventional mechanochemistry using liquid-assisted grinding techniques.

Metastable γ-Ga 2 O 3 nanocrystals have gained growing interest for a broad range of technological applications. However, a precise description of their atomic structure and changes thereof during thermally induced transformations that is required to understand and fully exploit their physical and chemical properties is still lacking. In this work, we investigate the structure of γ-Ga 2 O 3 nanocrystals (2.5 nm in diameter) obtained via a colloidal synthesis route and their evolution with thermal treatment. To this end, we have applied synchrotron X-ray atomic pair distribution function (PDF) analysis, complemented by 71 Ga solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), X-ray absorption spectroscopy (XAS), and electron microscopy. The local structure of the γ-Ga 2 O 3 nanocrystals deviates from the average cubic spinel-type structure, revealing a high degree of structural disorder. The average structure of the γ-Ga 2 O 3 nanocrystals is described as a defective spinel with gallium sites in tetrahedral and octahedral (Ga IV and Ga VI ) coordination with oxygen atoms. The modeling of the local structure revealed a lowsymmetry distortion of the polyhedra, which are disorderly oriented. The surface structure of the γ-Ga 2 O 3 nanocrystals is different from their bulk, whereby Ga VI sites at the outermost layers of the nanocrystals are found in a nonperiodical stacking arrangement with a higher occupancy than in the core, as revealed by high-angle annular dark field imaging scanning transmission electron microscopy (HAADF-STEM). The structural evolution of γ-Ga 2 O 3 nanocrystals upon thermal treatment in air was probed by in situ time-resolved PDF. A gradual transformation of the γ-Ga 2 O 3 nanocrystals toward the thermodynamically stable β-Ga 2 O 3 polymorph occurs at different structural domains at different temperatures. At ca. 300 °C, changes in the local structure showed an increased distortion of the polyhedral units and revealed the appearance of small β-Ga 2 O 3 domains (ca. <1 nm), while the bulk phase transformation took place between 550 and 750 °C and was associated with an increase in the coherence length of the β-Ga 2 O 3 phase.

Wide angle x-ray scattering of supercooled water down to 234.8 K was studied using high energy x rays at the European Synchrotron Radiation Facility. The oxygen-oxygen pair distribution function (PDF) was calculated from the scattering pattern out to the 5th peak at an intermolecular distance, r ≈ 11 Å. We observe that the 4th peak and the 5th peak in the PDF increase in height upon supercooling. We also observe that the 4th peak position (r4) shifts to shorter distances upon supercooling consistent with previous studies, but we see a more rapid change at the lowest temperature. The running oxygen-oxygen coordination number is calculated for 5 different temperatures, and an isosbestic point at riso = 3.31 ± 0.05 Å was found corresponding to a coordination number of 4.39 ± 0.15. The comparison of the PDF of the coldest water with that of amorphous ice shows distinct differences. We propose that there are 5-member pentamer rings in low density liquid-like structures giving rise to the sharp correlations at r ≈ 9 Å and r ≈ 11 Å.