Center for Molecular Modeling - E. Campbell

A Critical Assessment on Calculating Vibrational Spectra in Nanostructured Materials

leen — Mon, 08 Jan 2024 11:24:45 +0000

A.E.J. Hoffman, W. Temmerman, E. Campbell, A. A. Damin, I. Lezcano-Gonzalez, A.M. Beale, S. Bordiga, J. Hofkens, V. Van Speybroeck

Journal of Chemical Theory and Computation

Volume: 20, Issue: 2, Pages: 513-531

2023

Abstract

Vibrational spectroscopy is an omnipresent spectroscopic technique to characterize functional nanostructured materials such as zeolites, metal–organic frameworks (MOFs), and metal–halide perovskites (MHPs). The resulting experimental spectra are usually complex, with both low-frequency framework modes and high-frequency functional group vibrations. Therefore, theoretically calculated spectra are often an essential element to elucidate the vibrational fingerprint. In principle, there are two possible approaches to calculate vibrational spectra: (i) a static approach that approximates the potential energy surface (PES) as a set of independent harmonic oscillators and (ii) a dynamic approach that explicitly samples the PES around equilibrium by integrating Newton’s equations of motions. The dynamic approach considers anharmonic and temperature effects and provides a more genuine representation of materials at true operating conditions; however, such simulations come at a substantially increased computational cost. This is certainly true when forces and energy evaluations are performed at the quantum mechanical level. Molecular dynamics (MD) techniques have become more established within the field of computational chemistry. Yet, for the prediction of infrared (IR) and Raman spectra of nanostructured materials, their usage has been less explored and remain restricted to some isolated successes. Therefore, it is currently not a priori clear which methodology should be used to accurately predict vibrational spectra for a given system. A comprehensive comparative study between various theoretical methods and experimental spectra for a broad set of nanostructured materials is so far lacking. To fill this gap, we herein present a concise overview on which methodology is suited to accurately predict vibrational spectra for a broad range of nanostructured materials and formulate a series of theoretical guidelines to this purpose. To this end, four different case studies are considered, each treating a particular material aspect, namely breathing in flexible MOFs, characterization of defects in the rigid MOF UiO-66, anharmonic vibrations in the metal–halide perovskite CsPbBr₃, and guest adsorption on the pores of the zeolite H-SSZ-13. For all four materials, in their guest- and defect-free state and at sufficiently low temperatures, both the static and dynamic approach yield qualitatively similar spectra in agreement with experimental results. When the temperature is increased, the harmonic approximation starts to fail for CsPbBr₃ due to the presence of anharmonic phonon modes. Also, the spectroscopic fingerprints of defects and guest species are insufficiently well predicted by a simple harmonic model. Both phenomena flatten the potential energy surface (PES), which facilitates the transitions between metastable states, necessitating dynamic sampling. On the basis of the four case studies treated in this Review, we can propose the following theoretical guidelines to simulate accurate vibrational spectra of functional solid-state materials: (i) For nanostructured crystalline framework materials at low temperature, insights into the lattice dynamics can be obtained using a static approach relying on a few points on the PES and an independent set of harmonic oscillators. (ii) When the material is evaluated at higher temperatures or when additional complexity enters the system, e.g., strong anharmonicity, defects, or guest species, the harmonic regime breaks down and dynamic sampling is required for a correct prediction of the phonon spectrum. These guidelines and their illustrations for prototype material classes can help experimental and theoretical researchers to enhance the knowledge obtained from a lattice dynamics study.

DOI

http://dx.doi.org/10.1021/acs.jctc.3c00942

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hoffman-et-al-2023-a-critical-assessment-on-calculating-vibrational-spectra-in-nanostructured-materials.pdf

Insight into the effects of confined hydrocarbon species on the lifetime of methanol conversion catalysts

alexander — Tue, 15 Sep 2020 08:47:31 +0000

I. Lezcano-Gonzalez, E. Campbell, A.E.J. Hoffman, M. Bocus, I.V. Sazanovich, M. Towrie, M. Agote-Aran, E.K. Gibson, A. Greenaway, K. De Wispelaere, V. Van Speybroeck, A.M. Beale

Nature Materials

19, 1081–1087

2020

Abstract

The methanol-to-hydrocarbons reaction refers collectively to a series of important industrial catalytic processes to produce either olefins or gasoline. Mechanistically, methanol conversion proceeds through a ‘pool’ of hydrocarbon species. For the methanol-to-olefins process, these species can be delineated broadly into ‘desired’ lighter olefins and ‘undesired’ heavier fractions that cause deactivation in a matter of hours. The crux in further catalyst optimization is the ability to follow the formation of carbonaceous species during operation. Here, we report the combined results of an operando Kerr-gated Raman spectroscopic study with state-of-the-art operando molecular simulations, which allowed us to follow the formation of hydrocarbon species at various stages of methanol conversion. Polyenes are identified as crucial intermediates towards formation of polycyclic aromatic hydrocarbons, with their fate determined largely by the zeolite topology. Notably, we provide the missing link between active and deactivating species, which allows us to propose potential design rules for future-generation catalysts.

DOI

http://dx.doi.org/10.1038/s41563-020-0800-y

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NatMater.pdf

Operando Raman spectroscopy to pinpoint the onset of deactivation in zeolites

alexander — Thu, 17 Mar 2022 13:10:47 +0000

A.E.J. Hoffman, I. Lezcano-Gonzalez, E. Campbell, M. Bocus, I.V. Sazanovich, M. Towrie, A.M. Beale, V. Van Speybroeck

ISBN/ISSN:

Poster

Conference / event / venue

FEZA 2021

online

Monday, 5 July, 2021 to Friday, 9 July, 2021

Operando Raman spectroscopy to pinpoint the onset of deactivation in zeolites

alexander — Mon, 01 Mar 2021 15:14:00 +0000

A.E.J. Hoffman, I. Lezcano-Gonzalez, E. Campbell, M. Bocus, I.V. Sazanovich, M. Towrie, A.M. Beale, V. Van Speybroeck

ISBN/ISSN:

Talk

Conference / event / venue

The Netherlands' Catalysis and Chemistry Conference

online

Monday, 1 March, 2021

Abstract (private)

Abstract_submitted.pdf