All Publications

  • Tracking Active Phase Behavior on Boron Nitride during the Oxidative Dehydrogenation of Propane Using Operando X-ray Raman Spectroscopy. Journal of the American Chemical Society Cendejas, M. C., Paredes Mellone, O. A., Kurumbail, U., Zhang, Z., Jansen, J. H., Ibrahim, F., Dong, S., Vinson, J., Alexandrova, A. N., Sokaras, D., Bare, S. R., Hermans, I. 2023


    Hexagonal boron nitride (hBN) is a highly selective catalyst for the oxidative dehydrogenation of propane (ODHP) to propylene. Using a variety of ex situ characterization techniques, the activity of the catalyst has been attributed to the formation of an amorphous boron oxyhydroxide surface layer. The ODHP reaction mechanism proceeds via a combination of surface mediated and gas phase propagated radical reactions with the relative importance of both depending on the surface-to-void-volume ratio. Here we demonstrate the unique capability of operando X-ray Raman spectroscopy (XRS) to investigate the oxyfunctionalization of the catalyst under reaction conditions (1 mm outer diameter reactor, 500 to 550 °C, P = 30 kPa C3H8, 15 kPa O2, 56 kPa He). We probe the effect of a water cofeed on the surface of the activated catalyst and find that water removes boron oxyhydroxide from the surface, resulting in a lower reaction rate when the surface reaction dominates and an enhanced reaction rate when the gas phase contribution dominates. Computational description of the surface transformations at an atomic-level combined with high precision XRS spectra simulations with the OCEAN code rationalize the experimental observations. This work establishes XRS as a powerful technique for the investigation of light element-containing catalysts under working conditions.

    View details for DOI 10.1021/jacs.3c08679

    View details for PubMedID 37931025

  • The Local Electronic Structure of Supercritical CO2 from X-ray Raman Spectroscopy and Atomistic-Scale Modeling. The journal of physical chemistry letters Muhunthan, P., Paredes Mellone, O., Kroll, T., Sokaras, D., Ihme, M. 2023: 4955-4961


    Supercritical CO2 is encountered in several technical and natural systems related to biology, geophysics, and engineering. While the structure of gaseous CO2 has been studied extensively, the properties of supercritical CO2, particularly close to the critical point, are not well-known. In this work, we combine X-ray Raman spectroscopy, molecular dynamics simulations, and first-principles density functional theory (DFT) calculations to characterize the local electronic structure of supercritical CO2 at conditions around the critical point. The X-ray Raman oxygen K-edge spectra manifest systematic trends associated with the phase change of CO2 and the intermolecular distance. Extensive first-principles DFT calculations rationalize these observations on the basis of the 4ssigma Rydberg state hybridization. X-ray Raman spectroscopy is found to be a sensitive tool for characterizing electronic properties of CO2 under challenging experimental conditions and is demonstrated to be a unique probe for studying the electronic structure of supercritical fluids.

    View details for DOI 10.1021/acs.jpclett.3c00668

    View details for PubMedID 37216638

  • Investigating the electronic structure of high explosives with X-ray Raman spectroscopy. Scientific reports Paredes-Mellone, O. A., Nielsen, M. H., Vinson, J., Moua, K., Skoien, K. D., Sokaras, D., Willey, T. M. 2022; 12 (1): 19460


    We investigate the sensitivity and potential of a synergistic experiment-theory X-ray Raman spectroscopy (XRS) methodology on revealing and following the static and dynamic electronic structure of high explosive molecular materials. We show that advanced ab-initio theoretical calculations accounting for the core-hole effect based on the Bethe-Salpeter Equation (BSE) approximation are critical for accurately predicting the shape and the energy position of the spectral features of C and N core-level spectra. Moreover, the incident X-ray dose typical XRS experiments require can induce, in certain unstable structures, a prominent radiation damage at room temperature. Upon developing a compatible cryostat module for enabling cryogenic temperatures ([Formula: see text] 10 K) we suppress the radiation damage and enable the acquisition of reliable experimental spectra in excellent agreement with the theory. Overall, we demonstrate the high sensitivity of the recently available state-of-the-art X-ray Raman spectroscopy capabilities in characterizing the electronic structure of high explosives. At the same time, the high accuracy of the theoretical approach may enable reliable identification of intermediate structures upon rapid chemical decomposition during detonation. Considering the increasing availability of X-ray free-electron lasers, such a combined experiment-theory approach paves the way for time-resolved dynamic studies of high explosives under detonation conditions.

    View details for DOI 10.1038/s41598-022-24066-z

    View details for PubMedID 36376464

  • Local Structure of Sulfur Vacancies on the Basal Plane of Monolayer MoS2. ACS nano Garcia-Esparza, A. T., Park, S., Abroshan, H., Paredes Mellone, O. A., Vinson, J., Abraham, B., Kim, T. R., Nordlund, D., Gallo, A., Alonso-Mori, R., Zheng, X., Sokaras, D. 2022


    The nature of the S-vacancy is central to controlling the electronic properties of monolayer MoS2. Understanding the geometric and electronic structures of the S-vacancy on the basal plane of monolayer MoS2 remains elusive. Here, operando S K-edge X-ray absorption spectroscopy shows the formation of clustered S-vacancies on the basal plane of monolayer MoS2 under reaction conditions (H2 atmosphere, 100-600 °C). First-principles calculations predict spectral fingerprints consistent with the experimental results. The Mo K-edge extended X-ray absorption fine structure shows the local structure as coordinatively unsaturated Mo with 4.1 ± 0.4 S atoms as nearest neighbors (above 400 °C in an H2 atmosphere). Conversely, the 6-fold Mo-Mo coordination in the crystal remains unchanged. Electrochemistry confirms similar active sites for hydrogen evolution. The identity of the S-vacancy defect on the basal plane of monolayer MoS2 is herein elucidated for applications in optoelectronics and catalysis.

    View details for DOI 10.1021/acsnano.2c01388

    View details for PubMedID 35380038

  • X-ray spectroscopic identification of strain and structure-based resonances in a series of saturated carbon-cage molecules: Adamantane, twistane, octahedrane, and cubane JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A Willey, T. M., Lee, J. R., Brehmer, D., Mellone, O., Landt, L., Schreiner, P. R., Fokin, A. A., Tkachenko, B. A., de Meijere, A., Kozhushkov, S., van Buuren, A. W. 2021; 39 (5)

    View details for DOI 10.1116/6.0001150

    View details for Web of Science ID 000685058100001

  • Dynamic Structure Factor and Dielectric Function of Valence Electrons in Lithium Hydride: An Inelastic X-Ray Scattering Study at Finite Momentum Transfer PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS Paredes-Mellone, O. A., Koskelo, J., Ceppi, S. A., Stutz, G. E. 2020; 257 (6)
  • Li 1s core exciton in LiH studied by x-ray Raman scattering spectroscopy JOURNAL OF PHYSICS-CONDENSED MATTER Paredes-Mellone, O. A., Stutz, G. E., Ceppi, S. A., Arneodo Larochette, P., Huotari, S., Gilmore, K. 2019; 31 (5): 055501


    The Li 1s core excitation spectra in LiH was studied by means of x-ray Raman scattering (XRS) spectroscopy in a wide range of momentum transfers q. The analysis of the near-edge region of the measured spectra in combination with q-dependent ab initio calculations of XRS spectra based on the Bethe-Salpeter equation (BSE) reveals that the prominent peak at the excitation onset arises from two main contributions, namely a pre-edge peak associated to a p-type core exciton and strong transitions to empty states near the bottom of the conduction band, which is in contrast to previous experimental studies that attributed that feature to a single excitonic peak. The p-like angular symmetry of the core exciton is supported by BSE calculations of the relative contributions to the XRS spectra from monopole and dipole transitions and by the observed decrease of its normalised intensity for increasing momentum transfers. Higher energy spectral features in the measured XRS spectra are well reproduced by BSE, as well as by real-space multiple-scattering calculations.

    View details for DOI 10.1088/1361-648X/aaf24c

    View details for Web of Science ID 000454109900001

    View details for PubMedID 30523931

  • Background radiation in inelastic X-ray scattering and X-ray emission spectroscopy. A study for Johann-type spectrometers NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT Paredes Mellone, O. A., Bianco, L. M., Ceppi, S. A., Goncalves Honnicke, M., Stutz, G. E. 2018; 894: 119-128