This work reports our discoveries from the first exploration in microwave pyrolysis of a metal−organic framework. A time-and energy-efficient approach was developed for direct fabrication of electrochemical single-atom catalysts (E-SACs) without the requirement of post-treatment. The most unique structure of the fabricated E-SAC is that the Cu catalytic sites were not in the amorphous carbon matrix as those achieved via traditional pyrolysis but in the basal planes of pristine holey graphene nanoplatelets. The as-prepared Cu-E-SAC exhibits excellent catalytic activity and selectivity in reducing oxygen to water in both acidic and alkaline media. The desired direct 4e − pathway is more favorable in acidic versus alkaline media, which is different from all the Cu-E-SACs reported so far and most transition-metal-based E-SACs. The superior performance is attributed to the unique structure of the catalytic sites. The large graphene domains in the holey graphene materials provide higher delocalized electron-rich π band and increase the d-orbital energy level of the Cu centers. Consequently, their binding strength for molecular oxygen is largely enhanced, improving the oxygen reduction reaction and likely promoting a direct 4e − pathway with minimized generation of a peroxide byproduct. Considering the high conductivity and excellent stability against oxidation of the holey graphene material, this work, for the first time, suggests that the contradictory structural requirement of a carbon matrix for high catalytic activity and long-term durability can be unified and simultaneously satisfied. Combined with the merits of simplicity and rapidness for fabricating both holey graphene and E-SACs, this work provides a possible strategy to address the critical challenges of precious metal-free singleatom catalysts.

The porous Hoffman-type 3D lattice Fe­(pz)­[NiII(CN)4] exhibits thermally induced spin-crossover (SCO) behavior that is dependent on the solvent guest species occupying the pores. Here, in situ Fe K-edge X-ray absorption spectroscopy (XAS) and both non-resonant and resonant Kβ X-ray emission spectroscopy (XES) methods are used to probe this framework under two solvent environments that yield different extremes of spin crossover temperature: acetonitrile and toluene. While the acetonitrile pore environment engenders an SCO response around room temperature, toluene guests stabilize the high spin state and effectively suppress SCO behavior throughout the ambient temperature range. The multipronged X-ray spectroscopy approach simultaneously confirmed this spin crossover behavior and provided new local coordination and electronic structural insights of the framework under these two solvent environments. Extended X-ray absorption fine structure analysis revealed spin state and solvent guest-dependent differences in coordination bond lengths and structural disorder. Resonant XES measurements produced high-resolution XAS spectra with distinct pre-edge and edge features, whose assignment was established using both simple ligand field theory and time-dependent density-functional theory calculations and further supported by their observed resonance behavior in the 2D RXES plane. Edge feature variation with the Fe spin state was interpreted to reveal changes in specific metal-linker bond covalency.

The Argonne X-ray Emission Analysis Package (AXEAP) has been developed to calibrate and process X-ray emission spectroscopy (XES) data collected with a two-dimensional (2D) position-sensitive detector. AXEAP is designed to convert a 2D XES image into an XES spectrum in real time using both calculations and unsupervised machine learning. AXEAP is capable of making this transformation at a rate similar to data collection, allowing real-time comparisons during data collection, reducing the amount of data stored from gigabyte-sized image files to kilobyte-sized text files. With a user-friendly interface, AXEAP includes data processing for non-resonant and resonant XES images from multiple edges and elements. AXEAP is written in MATLAB and can run on common operating systems, including Linux, Windows, and MacOS.

We present a compact 3D printed x-ray emission spectrometer based on the von Hamos geometry that represents a significant upgrade to the existing von Hamos geometry-based miniature x-ray emission spectrometer (miniXES) [Mattern et al., Rev. Sci. Instrum. 83(2), 023901 (2012)]. The upgrades include the incorporation of a higher pixel density 500K detector for improved energy resolution and an enlarged sample area to accommodate a wider range of sample formats. The versatile spectrometer houses removable crystal holders that can be easily exchanged, as well as movable alignment eyelets that give flexibility in Bragg angle selection. Designed for ease of manufacture, all the components, except for the apertures, can be 3D printed and readily assembled. We describe its implementation in measurements of resonant and non-resonant Cu Kα and Kβ x-ray emission and report the theoretical and measured energy resolution and collected solid angle of the emission.