Thokozile. A. Kathyola

Controlling lattice-oxygen reactivity in earth-abundant OER catalysts requires precise tuning of defect chemistry in the oxide lattice. Here, we combine DFT + U calculations with plasma-assisted synthesis to show how O₂ and H₂O in the discharge govern vacancy formation, electronic structure, and catalytic predisposition in NiO thin films. Oxygen-rich plasmas generate isolated and clustered Ni vacancies that stabilize oxygen-ligand-hole states and produce shallow O 2p-Ni 3d hybrid levels, enhancing Ni-O covalency. In contrast, introducing H₂O during growth drives local hydroxylation that compensates vacancy-induced Ni³⁺ centers, restoring Ni²⁺-like coordination, suppressing deep divacancy-derived in-gap states, and introducing shallow Ni-O-H-derived valence-band tails. EXAFS confirms that hydroxylation perturbs only the local environment while preserving the medium-range NiO lattice, and Ni L-edge spectroscopy shows a persistent but redistributed ligand-hole population. These complementary vacancy- and hydroxylation-driven pathways provide a plasma-controlled route to predefine electronic defect landscapes in NiO and to tune its activation toward OER-relevant NiOOH formation.

Time-resolved structural changes from Ca(OH)₂ to different CaCO₃ polymorphs, in aqueous and non-aqueous environments, were monitored operando using mid-infrared (mid-IR) and X-ray absorption near edge structure (XANES) spectroscopy.

A free-standing and compact reaction cell for combined in situ/operando x-ray spectroscopy, scattering, and imaging measurements at high pressures and high temperatures is described. The cell permits measurements under realistic operating conditions (up to 50 bar and 1000 °C), under static and flow conditions (up to 100 ml/min), over a wide range of hard x-ray energies, variable detection modes (transmission, fluorescence, and scattering), and at all angles of rotation. An operando XAS, x-ray fluorescence, x-ray computed tomography, and x-ray diffraction computed tomography case study on the reduction of a heterogeneous catalyst is presented to illustrate the performance of the reaction cell.

A combination of ex situ and in situ characterization techniques was used to determine the mechanism of calcium carbonate (CaCO₃) formation from calcium hydroxide (Ca(OH)₂) dispersions in methanol/water (CH₃OH/H₂O) systems. Mid-infrared (mid-IR) analysis shows that in the absence of carbon dioxide (CO₂) Ca(OH)₂ establishes a reaction equilibrium with CH₃OH, forming calcium hydroxide methoxide (Ca(OH)(OCH₃)) and calcium methoxide (Ca(OCH₃)₂). Combined ex situ mid-IR, thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray absorption spectroscopy and scanning electron microscopy examination of the reaction product formed in the presence of CO₂ reveals the formation of calcium dimethylcarbonate (Ca(OCOOCH₃)₂). This strongly suggests that carbonation takes place by reaction with the Ca(OCH₃)₂ formed from a Ca(OH)₂ and CH₃OH reaction. Time-resolved XRD indicates that in the presence of H₂O the Ca(OCOOCH₃)₂ ester releases CH₃OH and CO₂, forming ACC, which subsequently transforms into vaterite and then calcite. TGA reveals that thermal decomposition of Ca(OCOOCH₃)₂ in the absence of H₂O mainly leads to the reformation of Ca(OCH₃)₂, but this is accompanied by a significant parallel reaction that releases dimethylether (CH₃OCH₃) and CO₂. CaCO₃ is the final product in both decomposition pathways. For CH₃OH/H₂O mixtures containing more than 50 mol % H₂O, direct formation of calcite from Ca(OH)₂ becomes the dominant pathway, although the formation of some Ca(OCOOCH₃)₂ was still evident in the in situ mid-IR spectra of 20 and 40 mol % CH₃OH systems. In the presence of ≤20 mol % H₂O, hydrolysis of the ester led to the formation of an ACC sol-gel. In both the 90 and 100 mol % CH₃OH systems, diffusion-limited ACC → vaterite → calcite transformations were observed. Traces of aragonite were also detected. We believe that this is the first time that these reaction pathways during the carbonation of Ca(OH)₂ in a methanolic phase have been systematically and experimentally characterized.

A functionalized vanadyl(IV) acetylacetonate (acac) complex has been found to be a superior and highly effective antiwear agent, affording remarkable wear protection, compared to the current industry standard, zinc dialkyldithiophosphates (ZDDPs). Analysis of vanadium speciation and the depth profile of the active tribofilms by a combination of X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) analyses indicated a mixed-valence oxide composite, comprising V(III), V(IV), and V(V) species. A marked difference in composition between the bulk and the surfaces of the tribofilms was found. The vanadyl(VI) acac precursor has the potential to reduce or even replace ZDDP, which would represent a paradigm shift in the antiwear agent design. A major benefit relative to ZDDPs is the absence of S and P moieties, eliminating the potential for forming noxious and environmentally harmful byproducts of these elements.

Process analytical technologies are widely used to inform process control by identifying relationships between reagents and products. Here, we present a novel process analytical technology system for operando XAS on multiphase multicomponent synthesis processes based on the combination of a conventional lab-scale agitated reactor with a liquid-jet cell. The preparation of sulfonate-stabilized CaCO₃ particles from polyphasic Ca(OH)₂ dispersions was monitored in real time by Ca K-edge XAS to identify changes in Ca speciation in the bulk solution/dispersion as a function of time and process conditions. Linear combination fitting of the spectra quantitatively resolved composition changes from the initial conversion of Ca(OH)₂ to the Ca(R-SO₃)₂ surfactant to the ultimate formation of nCaCO₃·mCa(R- SO₃)₂ particles. The system provides a novel tool with strong chemical specificity for probing multiphase synthesis processes at a molecular level, providing an avenue to establishing the relationships between critical quality attributes of a process and the quality and performance of the product.

Herein, we designed a versatile cell for operando X-ray absorption spectroscopy (XAS) monitoring of chemical and physical transformations in liquid solutions and dispersions by fluorescence-yield detection. The cell operates under atmospheric pressure, and the composition of the gas phase can be varied, allowing monitoring of gas–liquid reactions. Samples can be examined as a flowing liquid-jet or as a sessile droplet. Both liquid-jet and droplet operations of the cell were demonstrated at the Ca K-edge through time-resolved studies of CaCO₃ formation from CO₂ and solid Ca(OH)₂ in an aqueous suspension. The liquid-jet reaction was performed at 28 °C, and the droplet reaction was performed at ambient temperature, but both configurations had the scope to be modified for operations at higher temperatures. The liquid-jet volumes were recycled, permitting the use of the cell in a continuous sampling loop for process studies in larger reactor vessels. The jet supply tube was interchangeable, permitting adjustment of the jet size through the tubing bore size. Larger bore sizes minimized the pressure drop at the nozzle and thereby the risk of blockage of the liquid supply by the suspended particulates or growth of solid deposits. Sessile droplet operation enabled studies with minimal sample volumes and mechanical disturbances under controlled environmental conditions. The cell is portable and modular and is based on readily available standard vacuum equipment parts. Furthermore, the operation under a He atmosphere allows measurements above 4 keV, covering part of the tender X-ray range, and it is also applicable to the hard X-ray range.