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Instruments / Technical Area	Publication Details	Published	Actions
B18-Core EXAFS I15-1-X-ray Pair Distribution Function (XPDF)	Combined EXAFS and XPDF analysis of calcium methoxide short-range structure Thokozile Kathyola , Elizabet Willneff , Sven Schroeder , Giannantonio Cibin , Philip Chater XAFS2021 Diamond Proposal Number(s): [14673, 17686, 17391] Show Abstract ▼ Abstract: Extended X-ray absorption fine structure (EXAFS) and X-ray Pair Distribution Function (XPDF) analysis are versatile complementary techniques that provide information about local structure and bonding in crystalline and non-crystalline materials. Both techniques provide information about the coordination number of neighbouring atoms, bond distances and the degree of static/thermal disorder. EXAFS is a well-established element-specific method that provides short-range order information around the X-ray absorber of interest. XPDF is not element-specific, but provides short-range order information similar to EXAFS, alongside long- range structure. Calcium methoxide (Ca(OCH3)2) is one of the prominent heterogeneous catalysts that are being used in the formation of sustainable biolubricants and biodiesel from natural/synthetic oils via transesterification. Other compounds that have been considered as viable catalysts include calcium oxide (CaO) and calcium hydroxide (Ca(OH)2). Unlike these two bases, which have been comprehensively characterised using EXAFS, the short-range crystalline structure of Ca(OCH3)2 is yet to be fully determined. Not much is known about its structure beyond the indication that it has the same hexagonal space group (P-3m1) as Ca(OH)2. This was determined over 40 years ago, using powder X-ray diffraction and mid infrared spectroscopy, and has not been investigated since. Hence, we present a comparative study on the local structure of CaO, Ca(OH)2 and Ca(OCH3)2 using EXAFS and XPDF analysis. Our results demonstrate how a new/refined model crystal Ca(OCH3)2 structure was obtained using XPDF data, crystal symmetry and unit cell dimensions. This model was subsequently used in EXAFS simulations to determine atom coordination numbers, bond distances and static disorder in Ca(OCH3)2. To the best of our knowledge, this is the first time EXAFS and XPDF data has been presented on a calcium alkoxide. The structural information obtained from this study could benefit development of more sophisticated alkoxide-based heterogenous catalysts. This multi-technique ex situ analysis of the three basic calcium catalysts could also inform operando studies of alkaline-earth metal catalysis with these techniques.	Jul 2021
B18-Core EXAFS	A versatile liquid-jet/sessile droplet system for operando studies of reactions in liquid dispersions and solutions by X-ray absorption spectroscopy S.-Y. Chang , T. A. Kathyola , E. A. Willneff , C.. J. Willis , P. Wilson , P. J. Dowding , G. Cibin , A. B. Kroner , E. J. Shotton , S. L. M. Schroeder Reaction Chemistry & Engineering 4, 679 - 687 DOI: 10.1039/C8RE00207J Diamond Proposal Number(s): [15850, 14673] Open Access Show Abstract ▼ Abstract: 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.	Mar 2019

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