Inelastic Neutron Scattering Spectroscopy Research Articles

Supported Metal Nanoparticles‐Based Catalysts and their Interactions with H2: Insights from Inelastic Neutron Scattering Spectroscopy

AbstractThe investigation of supported metal nanoparticle in the presence of H2 is fundamental to better understand their behavior as hydrogenation catalysts. In this respect, this review will focus on the main recent results obtained by Inelastic Neutron Scattering (INS) spectroscopy, which is of particular interest thanks to its ability to provide very detailed vibrational spectra of the H‐containing species in the sample. The review will cover the main results concerning the characterization of the H‐species already present in the pristine catalyst, with a particular focus on carbon‐based materials and their modification upon the interaction with metal nanoparticles. Successively, the forms in which H2 interacts with the catalysts will be examined, starting from H2 physisorption, to then pass to the metal hydride species formed through the splitting of dihydrogen upon the active metallic phase, and finally to the migration of atomic hydrogen from the metal nanoparticles onto the support as a result of hydrogen spillover. The aim of the review is to present an overview of all the hydrogenous species identifiable by INS spectroscopy in hydrogenation catalysts, focusing on the specific case of supported metal nanoparticles systems.

Inelastic neutron scattering study of the H2 interaction with carbon-supported Pt and Pd catalysts

Carbon-supported Pt and Pd catalysts are largely employed as heterogeneous catalysts for hydrogenation reactions. Albeit their routine use in industry, many details about their interaction with H2 are still not completely understood. In this article, we propose a detailed investigation of the behavior in the presence of H2 of an activated carbon and of two catalysts obtained by depositing on it Pt and Pd nanoparticles by means of Inelastic Neutron Scattering (INS) spectroscopy. The high-quality INS spectra of the three samples were measured in absence and in presence of different amounts of H2, providing very detailed information about H2 physisorption on each system and about the nature of the metal hydrides formed upon hydrogenation of the catalysts. In addition, we were also able to obtain a direct observation of the occurrence of hydrogen spillover from the metal nanoparticles onto the activated carbon, as revealed by the increase of the number of C-H species at the borders of the graphenic domains. Overall, this work provides important insights for better understanding the complex nature of the interaction between H2 and Pt and Pd-loaded activated carbon catalysts.

Counting the Acid Sites in a Commercial ZSM-5 Zeolite Catalyst.

This work investigates the acid sites in a commercial ZSM-5 zeolite catalyst by a combination of spectroscopic and physical methods. The Brønsted acid sites in such catalysts are associated with the aluminum substituted into the zeolite lattice, which may not be identical to the total aluminum content of the zeolite. Inelastic neutron scattering spectroscopy (INS) directly quantifies the concentrations of Brønsted acid protons, silanol groups, and hydroxyl groups associated with extra-framework aluminum species. The INS measurements show that ∼50% of the total aluminum content of this particular zeolite is extra framework, a conclusion supported by solid-state NMR and ammonia temperature-programmed desorption (TPD) measurements. Evidence for the presence of extra-framework aluminum oxide species is also seen in neutron powder diffraction data from proton- and deuterium-exchanged samples. The differences between results from the different analytical methods are discussed, and the novelty of direct proton counting by INS in this typical commercial catalyst is emphasized.

A Spectroscopic Paradox: The Interaction of Methanol with ZSM-5 at Room Temperature

The adsorption of methanol in HZSM-5 at low temperatures has long been regarded as an associative process involving hydrogen bonding to the acidic zeolite hydroxyl groups. Recent studies employing inelastic neutron scattering spectroscopy (INS) have reported that complete dissociation to methoxylate the zeolite occurs at 298 K, and infrared evidence for a partial dissociation at 298 K has also been described. Here we investigate the apparent contradictions between different techniques, using a combination of INS, infrared spectroscopy and solid-state NMR spectroscopy, including isotopic substitution experiments. Different possible explanations are proposed and considered; we conclude that at room temperature methanol is very largely associatively adsorbed, although the presence of some small extent (>1%) of methoxylation cannot be ruled out.

New Spectroscopic Insight into the Deactivation of a ZSM‐5 Methanol‐to‐Hydrocarbons Catalyst

AbstractThe nature of the hydrocarbon pool at different stages of the methanol‐to‐hydrocarbons reaction over ZSM‐5 is examined. A combination of reaction testing, analytical and spectroscopic techniques is employed to investigate changes in the nature and form of the hydrocarbon pool as a function of reaction conditions and reaction time. It is shown that inelastic neutron scattering spectroscopy (INS) complements other spectroscopic methods for observing molecular components in the hydrocarbon pool of working catalysts. INS is uniquely able to spectroscopically identify the form of coke species present in deactivated catalysts.

Looking for Minor Phenolic Compounds in Extra Virgin Olive Oils Using Neutron and Raman Spectroscopies.

Extra virgin olive oil (EVOO) is defined as a functional food as it contains numerous phenolic components with well-recognized health-beneficial properties, such as high antioxidant and anti-inflammatory capacity. These characteristics depend on their structural/conformational behavior, which is largely determined by intra- and intermolecular H-bond interactions. While the vibrational dynamics of isolated compounds have been studied in a number of recent investigations, their signal in a real-life sample of EVOO is overwhelmed by the major constituent acids. Here, we provide a full characterization of the vibrational spectroscopic signal from commercially available EVOO samples using Inelastic Neutron Scattering (INS) and Raman spectroscopies. The spectra are dominated by CH2 vibrations, especially at about 750 cm−1 and 1300 cm−1. By comparison with the spectra from hydroxytyrosol and other minor phenolic compounds, we show that the best regions in which to look for the structure–activity information related to the minor polar compounds is at 675 and 1200 cm−1 for hydroxytyrosol, and around 450 cm−1 for all minor polar compounds used as reference, especially if a selectively deuterated sample is available. The regional origin of the EVOO samples investigated appears to be related to the different amount of phenolic esters versus acids as reflected by the relative intensities of the peaks at 1655 and 1747 cm−1.

Inter- vs. Intramolecular Hydrogen Bond Patterns and Proton Dynamics in Nitrophthalic Acid Associates.

Noncovalent interactions are among the main tools of molecular engineering. Rational molecular design requires knowledge about a result of interplay between given structural moieties within a given phase state. We herein report a study of intra- and intermolecular interactions of 3-nitrophthalic and 4-nitrophthalic acids in the gas, liquid, and solid phases. A combination of the Infrared, Raman, Nuclear Magnetic Resonance, and Incoherent Inelastic Neutron Scattering spectroscopies and the Car–Parrinello Molecular Dynamics and Density Functional Theory calculations was used. This integrated approach made it possible to assess the balance of repulsive and attractive intramolecular interactions between adjacent carboxyl groups as well as to study the dependence of this balance on steric confinement and the effect of this balance on intermolecular interactions of the carboxyl groups.

How do the graphenic domains terminate in activated carbons and carbon-supported metal catalysts?

This work is focused on the determination of the most likely border morphologies in the H-terminated graphenic domains that constitute activated carbons, focusing on four different carbons and six supported-metal catalysts. Among the available characterization techniques, Inelastic Neutron Scattering spectroscopy provides detailed vibrational spectra containing the fingerprint of the terminal C-H groups of carbon materials. From the experimental spectra, we observed clear differences among carbons having different origin, and a systematic decrease in the integrated area upon metal deposition that linearly scales with the nanoparticles dispersion. Density Functional Theory simulations are fundamental to assign the experimental bands to specific species. Thus, an extended simulation work on both regular and defective aromatic models was carried out, providing the inelastic neutron scattering fingerprint of a large number of different C-H terminations. By fitting the experimental spectra with a linear combination of the simulated spectra, it was possible to quantify the concentration of each terminal C-H geometry in the samples, and to identify the species most affected by the metal nanoparticles deposition. Specific benzene rings exposing a single C-H group appear to have a decisive role in the interaction with the metal nanoparticles and their deposition procedure.

Onset of Propene Oligomerization Reactivity in ZSM-5 Studied by Inelastic Neutron Scattering Spectroscopy.

The techniques of quasi-elastic and inelastic neutron scattering (QENS and INS) are applied to investigate the oligomerization of propene over a ZSM-5 zeolite. Investigations are performed at low temperatures, allowing identification of the onset of the oligomerization reaction and observation of the low-energy spectral changes due to intermediate formation that are difficult to observe by optical methods. Oligomerization proceeds via formation of a hydrogen-bonded precursor by an interaction of the propene with an internal acid site followed by protonation and chain growth with protonation being the rate-limiting step. The use of quasi-elastic neutron scattering to observe changes in system mobility with temperature via the elastic window scan technique allows identification of the active temperature range where catalyst activity commences and permits targeting of the more time-consuming INS investigations to conditions of interest. From examination of the product’s spectrum, the structure of the resulting oligomer is deduced to be primarily linear.

New Insights about CuO Nanoparticles from Inelastic Neutron Scattering.

Inelastic Neutron Scattering (INS) spectroscopy has provided a unique insight into the magnetodymanics of nanoscale copper (II) oxide (CuO). We present evidence for the propagation of magnons in the directions of the ordering vectors of both the commensurate and helically modulated incommensurate antiferromagnetic phases of CuO. The temperature dependency of the magnon spin-wave intensity (in the accessible energy-range of the experiment) conforms to the Bose population of states at low temperatures (T ≤ 100 K), as expected for bosons, then intensity significantly increases, with maximum at about 225 K (close to TN), and decreases at higher temperatures. The obtained results can be related to gradual softening of the dispersion curves of magnon spin-waves and decreasing the spin gap with temperature approaching TN on heating, and slow dissipation of the short-range dynamic spin correlations at higher temperatures. However, the intensity of the magnon signal was found to be particle size dependent, and increases with decreasing particle size. This “reverse size effect” is believed to be related to either creation of single-domain particles at the nanoscale, or “superferromagnetism effect” and the formation of collective particle states.

Synthesis, Computational Studies, Inelastic Neutron Scattering, Infrared and Raman Spectroscopy of Ruthenocene

In this work we report a new and simplified synthesis of ruthenocene that prevents the formation of unwanted by‐products. We have also revisited the vibrational spectroscopy of this iconic molecule and in addition to infrared and variable temperature Raman spectra, we present the first inelastic neutron scattering (INS) spectrum, a technique that is little used in inorganic chemistry. The Raman spectra in the low energy range (< 200 cm–1) clearly show that the mode assigned as the ring–Ru–ring torsion is a librational mode. By generating the INS spectra predicted by previous assignment schemes we are able to show that they are all, at least partially, wrong because they fail to correctly predict the experimental INS spectrum. In the case of ruthenocene, the addition of INS data, in combination with periodic‐DFT calculations, has enabled the first correct assignment of the internal modes of ruthenocene. This straightforward means to test proposed assignments is one of the great strengths of vibrational spectroscopy with neutrons.

Hydrogen Bond Dynamics of Cellulose through Inelastic Neutron Scattering Spectroscopy.

This work explores the dynamics of hydrogen bond networks in cellulose through inelastic neutron scattering (INS) and periodic CASTEP calculations. Estimated spectra were based on the crystal structure of cellulose Iα and Iβ and replicate the INS spectrum of cellulose samples with remarkable similarity, allowing a reliable assignment of INS bands to vibrational modes of cellulose. Comparison of cellulose samples from varied sources, from bacterial to kraft pulp, allows the identification of characteristic INS bands, arising from C2-OH torsional motions, which easily identify which allomorph-Iα or Iβ-is prevalent. A high crystallinity index is revealed by the presence of well-defined INS bands associated with highly cooperative CH bending modes along the chain. Hydrating kraft cellulose samples clearly affects those INS bands related with the hydroxymethyl group, identified as the preferred binding site for water molecules. At high humidity content level, a significant proportion of the water molecules is aggregated in clusters within the amorphous cellulose domains. The formation of ice microcrystals leads to a partial disruption of the hydrogen-bond network, as can be concluded from the observed red-shift of the torsional OH vibrational modes. The full assignment and interpretation of cellulose's INS spectra herein provided is a sound basis for future use of INS spectroscopy in the characterization of functionalized cellulose fibers and composite materials.

Quantum twin spectra in nanocrystalline silicon

Using inelastic neutron-scattering spectroscopy, we have identified twin-split scattering spectra in hydrogen-terminated nanocrystalline silicon. We show that this duality originates from the cooperative motion of hydrogen and silicon. Our formalism for the inelastic neutron-scattering spectrum, which is derived from the Hermite orthogonal wave functions in terms of the normal coordinates, elucidates the physical origin of the observed quantum twin spectra and predicts the possible occurrence of this behavior in other materials such as metal hydrides.

Computational Modeling and Inelastic Neutron Scattering Contributions to the Study of Methyl-silica Xerogels: A Combined Theoretical and Experimental Analysis

In amorphous materials, such as xerogels, the properties are the key to most of their applications. Therefore, it is of great importance to get insight into the xerogel structure. In order to achieve a better understanding of structure formation in methyl-silica xerogels and thus be able to better influence the properties of these materials, we have chosen the molecular approach to model the hybrid–xerogel structure using the density functional theory. A theoretical cage model for hybrid xerogels was performed from the PBEPBE/6-31G(d,p) method. The model could explain the presence of ordered domains in the structure. A comprehensive listing of all IR and 29Si NMR assignments is provided. The effects in the structure of the organic functional groups incorporated on the host structure network are investigated using inelastic neutron scattering spectroscopy (INS). The combination of experiments and modeling permitted an analysis of the INS studies, including unique INS assignments, contributing to the unders...

General Selection Rule in the Inelastic Neutron Scattering Spectroscopy of a Diatomic Molecule Confined Inside a Near-Spherical Nanocavity.

Knowledge of the relevant selection rules is crucial for the accurate interpretation of experimental spectra in general. There has been a consensus for a long time that the incoherent inelastic neutron scattering (INS) spectroscopy of the vibrations of discrete molecular compunds is free from any selection rules. We contradict this widely held view by presenting an analytical derivation of the general selection rule for the INS spectroscopy of a hydrogen molecule inside a near-spherical nanocavity. It defines all forbidden transitions, originating in a range of initial translation-rotation (TR) states, ground and excited, of the caged para- and ortho-H2, as well as HD, that are unobservable in the INS spectra. These predictions are amenable to experimental verification. In addition, we demonstrate that the general selection rule applies to the INS spectroscopy of any diatomic molecule in a nanocavity with near-spherical symmetry, which exhibits strong TR coupling. Its existence strongly suggests that similar selection rules apply to the INS spectra of other molecular and supramolecular systems, and need to be identified.

Assignment of the Internal Vibrational Modes of C70 by Inelastic Neutron Scattering Spectroscopy and Periodic-DFT.

The fullerene C70 may be considered as the shortest possible nanotube capped by a hemisphere of C60 at each end. Vibrational spectroscopy is a key tool in characterising fullerenes, and C70 has been studied several times and spectral assignments proposed. Unfortunately, many of the modes are either forbidden or have very low infrared or Raman intensity, even if allowed. Inelastic neutron scattering (INS) spectroscopy is not subject to selection rules, and all the modes are allowed. We have obtained a new INS spectrum from a large sample recorded at the highest resolution available. An advantage of INS spectroscopy is that it is straightforward to calculate the spectral intensity from a model. We demonstrate that all previous assignments are incorrect in at least some respects and propose a new assignment based on periodic density functional theory (DFT) that successfully reproduces the INS, infrared, and Raman spectra.

Vibrational Spectroscopy with Neutrons: A Review of New Directions

Inelastic neutron scattering spectroscopy (INS) has enabled vibrational spectra to be measured for over 50 years. Most studies have used a type of spectrometer that is straightforward to build and use and that provides spectra that are not dissimilar to infrared and Raman spectra. In this Focal Point Review Article we show the advantages of a type of neutron vibrational spectrometer that has been largely unknown to the spectroscopy community. These instruments are able to access regions of low momentum transfer at relatively large energy transfer. This means that the C–H, N–H, and O–H stretch regions can be exploited by INS spectroscopy for the first time. The instruments generally have very large detector area, which means that they are significantly more sensitive than the more commonly used instruments. They also allow the energy transfer as a function of momentum transfer to be examined. After briefly outlining the basics of INS spectroscopy, we describe the operational principles of the instruments and show how flux and resolution can be traded. We then review how the advantages of the instruments can be used to gain understanding of molecular systems in areas as diverse as hydrogen storage, hydrogen bonding, and fullerenes. The instruments are starting to have a significant impact in studies of catalysts and this is illustrated with recent studies of hydrogen on fuel cell catalysts, methyl chloride synthesis, the deactivation of methane reforming catalysts, and a model carbon monoxide oxidation catalyst.

Formation of a non-classical “η2-H2–Ag” hydride in zeolitic materials

Quantum chemical calculations including DFT, MP2, MP4 and QCISD predict the formation of non-classical η2-hydride of Ag+ coordinated to two oxygen atoms, in the frameworks of various zeolites. These model calculations are compared with existing zeolite structural data, other modelling works, and Inelastic Neutron Scattering (INS) spectroscopy data of dihydrogen adsorbed in a Ag-exchanged zeolite with LTA topology. It is also found that, although a little less stable, the corresponding η2-H2–Ag+ exist as well in the gas phase.

The Vibrational Spectrum of Parabanic Acid by Inelastic Neutron Scattering Spectroscopy and Simulation by Solid-State DFT

The incoherent inelastic neutron scattering spectrum of parabanic acid was measured and simulated using solid-state density functional theory (DFT). This molecule was previously the subject of low-temperature X-ray and neutron diffraction studies. While the simulated spectra from several density functionals account for relative intensities and factor group splitting regardless of functional choice, the hydrogen-bending vibrational energies for the out-of-plane modes are poorly described by all methods. The disagreement between calculated and observed out-of-plane hydrogen bending mode energies is examined along with geometry optimization differences of bond lengths, bond angles, and hydrogen-bonding interactions for different functionals. Neutron diffraction suggests nearly symmetric hydrogen atom positions in the crystalline solid for both heavy-atom and N-H bond distances but different hydrogen-bonding angles. The spectroscopic results suggest a significant factor group splitting for the out-of-plane bending motions associated with the hydrogen atoms (N-H) for both the symmetric and asymmetric bending modes, as is also supported by DFT simulations. The differences between the quality of the crystallographic and spectroscopic simulations by isolated-molecule DFT, cluster-based DFT (that account for only the hydrogen-bonding interactions around a single molecule), and solid-state DFT are considered in detail, with parabanic acid serving as an excellent case study due to its small size and the availability of high-quality structure data. These calculations show that hydrogen bonding results in a change in the bond distances and bond angles of parabanic acid from the free molecule values.

Interaction of Hydrogen with Extraframework Cations in Zeolite Hosts Probed by Inelastic Neutron Scattering Spectroscopy

The hindered rotations of molecular hydrogen adsorbed at low loadings into a number of partially ion-exchanged zeolites A, Y and X have been studied at low temperatures with the use of inelastic neutron scattering (INS) techniques. The factors that determine the sorption sites and strength of the interaction with the host material are found to be a complex combination of the type, charge and size of the cations, their coordination to the host framework, and accessibility to the hydrogen molecule as well as the relative acidity of the framework, and lead to important criteria for the development of more effective hybrid materials for hydrogen storage. The highest barriers to rotation were found for the undercoordinated, exposed Li+ cations in LiA and in LiX. Interaction with the extra framework Cu2+ and Zn2+ cations in zeolite A is found to be noticeably stronger than with the neutral Zn- or Cu- containing clusters in metal-organic framework compounds. Our observation that binding of hydrogen in these charged frameworks is strongly enhanced relative to those that are neutral suggests an important approach to improvement of porous materials as ambient temperature hydrogen storage media.