High-Entropy Oxides: A New Frontier in Photocatalytic CO2 Hydrogenation

Herein, we investigate the potential of nanostructured high-entropy oxides (HEOs) for photocatalytic CO2 hydrogenation, a process with significant implications for environmental sustainability and energy production. Several cerium-oxide-based rare-earth HEOs with fluorite structures were prepared for UV-light driven photocatalytic CO2 hydrogenation toward valuable fuels and petrochemical precursors. The cationic composition profoundly influences the selectivity and activity of the HEOs, where the Ce0.2Zr0.2La0.2Nd0.2Sm0.2O2-δ catalyst showed outstanding CO2 activation (14.4 molCO kgcat-1 h-1 and 1.27 mol CH 3 OH kgcat-1 h-1) and high methanol and CO selectivity (7.84% CH3OH and 89.26% CO) under ambient conditions with 4 times better performance in comparison to pristine CeO2. Systematic tests showed the effect of a high-entropy system compared to midentropy oxides. XPS, in situ DRIFTS, as well as DFT calculation elucidate the synergistic impact of Ce, Zr, La, Nd, and Sm, resulting in an optimal Ce3+/Ce4+ ratio. The observed formate-routed mechanism and a surface with high affinity to CO2 reduction offer insights into the photocatalytic enhancement. While our findings lay a solid foundation, further research is needed to optimize these catalysts and expand their applications.

Electron crystallography and dedicated electron-diffraction instrumentation

Electron diffraction (known also as ED, 3D ED or microED) is gaining momentum in science and industry. The application of electron diffraction in performing nano-crystallography on crystals smaller than 1 µm is a disruptive technology that is opening up fascinating new perspectives for a wide variety of compounds required in the fields of chemical, pharmaceutical and advanced materials research. Electron diffraction enables the characterization of solid compounds complementary to neutron, powder X-ray and single-crystal X-ray diffraction, as it has the unique capability to measure nanometre-sized crystals. The recent introduction of dedicated instrumentation to perform ED experiments is a key aspect of the continued growth and success of this technology. In addition to the ultra-high-speed hybrid-pixel detectors enabling ED data collection in continuous rotation mode, a high-precision goniometer and horizontal layout have been determined as essential features of an electron diffractometer, both of which are embodied in the Eldico ED-1. Four examples of data collected on an Eldico ED-1 are showcased to demonstrate the potential and advantages of a dedicated electron diffractometer, covering selected applications and challenges of electron diffraction: (i) multiple reciprocal lattices, (ii) absolute structure of a chiral compound, and (iii) R-values achieved by kinematic refinement comparable to X-ray data.

Rapid Structure Determination of Microcrystalline Molecular Compounds Using Electron Diffraction

Chemists of all fields currently publish about 50 000 crystal structures per year, the vast majority of which are X-ray structures. We determined two molecular structures by employing electron rather than X-ray diffraction. For this purpose, an EIGER hybrid pixel detector was fitted to a transmission electron microscope, yielding an electron diffractometer. The structure of a new methylene blue derivative was determined at 0.9 Å resolution from a crystal smaller than 1×2 μm2 . Several thousand active pharmaceutical ingredients (APIs) are only available as submicrocrystalline powders. To illustrate the potential of electron crystallography for the pharmaceutical industry, we also determined the structure of an API from its pill. We demonstrate that electron crystallography complements X-ray crystallography and is the technique of choice for all unsolved cases in which submicrometer-sized crystals were the limiting factor.

Structural Fingerprinting of Nanocrystals: Advantages of Precession Electron Diffraction, Automated Crystallite Orientation and Phase Maps

Strategies for the structurally identification of nanocrystals from Precession Electron Diffraction (PED) patterns in a Transmission Electron Microscope (TEM) are outlined. A single-crystal PED pattern may be utilized for the structural identification of an individual nanocrystal. Ensembles of nanocrystals may be fingerprinted structurally from “powder PED patterns”. Highly reliable “crystal orientation & structure” maps may be obtained from automatically recorded and processed scanning-PED patterns at spatial resolutions that are superior to those of the competing electron backscattering diffraction technique of scanning electron microscopy. The analysis procedure of that automated technique has recently been extended to Fourier transforms of high resolution TEM images, resulting in similarly effective mappings. Open-access crystallographic databases are mentioned as they may be utilized in support of our structural fingerprinting strategies.

Precession electron diffraction and its advantages for structural fingerprinting in the transmission electron microscope

The foundations of precession electron diffraction in a transmission electron microscope are outlined. A brief illustration of the fact that laboratory-based powder X-ray diffraction fingerprinting is not feasible for nanocrystals is given. A procedure for structural fingerprinting of nanocrystals on the basis of structural data that can be extracted from precession electron diffraction spot patterns is proposed.

Correlations, convolutions and the validity of electron crystallography

Although often an object of controversy, electron crystallography has emerged as a useful technique for characterization of the microcrystalline state, capable of elucidating crystal structures of unknown substances. Despite the complicated multiple scattering perturbations to diffracted intensities, experimental conditions can be adjusted to favor data collection where the experimental Patterson function still resembles the autocorrelation function of the actual crystal structure. Satisfying this condition is often sufficient to permit structure solution from such data by direct methods. While the application to organic structures may seem obvious, there are surprising successes with data sets from inorganic materials. The account given in this paper, in part, portrays work leading to the A. L. Patterson Award to the author from the American Crystallographic Association.