Active Sites in Supported Single-Site Catalysts: An NMR Perspective

Stationary wavelet denoising of solid-state NMR spectra using multiple similar measurements

Accumulating several scans of free induction decays is always needed to improve the signal-to-noise ratio of NMR spectra, especially for the low gyromagnetic ratio solid-state NMR. In this study, we present a new denoising approach based on the correlations between multiple similar NMR spectra. Contrary to the simple averaging of multiple scans or denoising the final averaged spectrum, we propose a Wavelet-based Denoising technique for Multiple Similar scans(WDMS). Firstly, the stationary wavelet transform is applied to decompose every spectrum into approximation coefficients and detail coefficients. Then, the detail coefficients are multiplied by weights calculated based on Pearson's correlation coefficient and structural similarity index between approximation coefficients of different spectra. Finally, the average of these detailed components is used to denoise the spectra. The proposed method is carried on the assumption that noise between multiple spectra is uncorrelated while peak signal information is similar between different spectra, thus preserving the possibility of applying further processing to the data. As a demonstration, the standard wavelet denoise is applied to the WDMS-processed spectra, achieving a further increase in the S/N ratio. We confirm the reliability of the denoising approach based on multiple scans on 1D/2D solid-state MAS/static NMR spectra. In addition, we also show that this method can be used to deal with a single Car-Purcell-Meiboom-Gill (CPMG) echo train.

Pushing the Detection Limit of Static Wideline NMR Spectroscopy Using Ultrafast Frequency-Swept Pulses

We report a simple design strategy for wideband uniform-rate smooth truncation (WURST) pulses that enables ultrafast frequency sweeps to maximize the sensitivity of Carr-Purcell-Meiboom-Gill (CPMG) acquisition in static wideline nuclear magnetic resonance (NMR). Three compelling examples showcase the advantage of ultrafast frequency sweeps over currently employed WURST-CPMG protocols, demonstrating the potential of investigating materials that are typically inaccessible to static wideline NMR techniques, e.g., paramagnetic solids with short homogeneous transverse relaxation times.

On the use of NMR distance measurements for assessing surface site homogeneity

The past few decades have seen tremendous growth in the area of single-site heterogeneous catalysis, which aims to combine the best aspects of homogeneous and heterogeneous catalysis, namely molecular-level site control and ease of separation/recycling. Despite this, we still do not have a means of assessing site homogeneity and whether the produced catalyst is indeed a "single-site". Recent developments have enabled the use of NMR-based distance measurements to determine the conformations and configurations of surface sites, leading to the question whether such measurements can be used to distinguish materials containing either single or multiple surface sites with otherwise indistinguishable NMR properties. We describe a Monte Carlo-based multi-structure search algorithm and its application to the determination of multi-site structures from supported metal complexes. The sensitivity of REDOR data to the existence of multiple sites is assessed using synthetic data and prior literature examples are revisited to determine whether the single-site approximation was indeed appropriate. We lastly apply this new methodology to differentiate the configurations of zirconocene complexes grafted onto alumina supports that were thermally treated at different temperatures.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Diamond surface engineering for molecular sensing with nitrogen-vacancy centers

Quantum sensing using optically addressable atomic-scale defects, such as the nitrogen-vacancy (NV) center in diamond, provides new opportunities for sensitive and highly localized characterization of chemical functionality. Notably, near-surface defects facilitate detection of the minute magnetic fields generated by nuclear or electron spins outside of the diamond crystal, such as those in chemisorbed and physisorbed molecules. However, the promise of NV centers is hindered by a severe degradation of critical sensor properties, namely charge stability and spin coherence, near surfaces (< ca. 10 nm deep). Moreover, applications in the chemical sciences require methods for covalent bonding of target molecules to diamond with robust control over density, orientation, and binding configuration. This forward-looking Review provides a survey of the rapidly converging fields of diamond surface science and NV-center physics, highlighting their combined potential for quantum sensing of molecules. We outline the diamond surface properties that are advantageous for NV-sensing applications, and discuss strategies to mitigate deleterious effects while simultaneously providing avenues for chemical attachment. Finally, we present an outlook on emerging applications in which the unprecedented sensitivity and spatial resolution of NV-based sensing could provide unique insight into chemically functionalized surfaces at the single-molecule level.

Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of 195Pt Chemical Shift Anisotropy

Techniques that can characterize the molecular structures of dilute surface species are required to facilitate the rational synthesis and improvement of Pt-based heterogeneous catalysts. ¹⁹⁵Pt solid-state NMR spectroscopy could be an ideal tool for this task because ¹⁹⁵Pt isotropic chemical shifts and chemical shift anisotropy (CSA) are highly sensitive probes of the local chemical environment and electronic structure. However, the characterization of Pt surface-sites is complicated by the typical low Pt loadings that are between 0.2 and 5 wt% and broadening of ¹⁹⁵Pt solid-state NMR spectra by CSA. Here, we introduce a set of solid-state NMR methods that exploit fast MAS and indirect detection using a sensitive spy nucleus (¹H or ³¹P) to enable the rapid acquisition of ¹⁹⁵Pt MAS NMR spectra. We demonstrate that high-resolution wideline ¹⁹⁵Pt MAS NMR spectra can be acquired in minutes to a few hours for a series of molecular and single-site Pt species grafted on silica with Pt loading of only 3-5 wt%. Low-power, long-duration, sideband-selective excitation, and saturation pulses are incorporated into t₁-noise eliminated dipolar heteronuclear multiple quantum coherence, perfect echo resonance echo saturation pulse double resonance, or J-resolved pulse sequences. The complete ¹⁹⁵Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the ¹⁹⁵Pt pulses is varied in increments of the MAS frequency. Analysis of the ¹⁹⁵Pt MAS NMR spectra yields the ¹⁹⁵Pt chemical shift tensor parameters. Zeroth order approximation density functional theory calculations accurately predict ¹⁹⁵Pt CS tensor parameters. Simple and predictive orbital models relate the CS tensor parameters to the Pt electronic structure and coordination environment. The methodology developed here paves the way for the detailed structural and electronic analysis of dilute platinum surface-sites.

Multiple Surface Site Three-Dimensional Structure Determination of a Supported Molecular Catalyst

The structural characterization of supported molecular catalysts is challenging due to the low density of active sites and the presence of several organic/organometallic surface groups resulting from the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in an N-heterocyclic carbene based on iridium and supported on silica, at all stages of its synthesis. By combining a suitable isotope labeling strategy with the implementation of multinuclear dipolar recoupling DNP-enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multistep synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting EXAFS spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multisite environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC-supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances.

Methyl-Driven Overhauser Dynamic Nuclear Polarization

The Overhauser effect is unique among DNP mechanisms in that it requires the modulation of the electron-nuclear hyperfine interactions. While it dominates DNP in liquids and metals, where unpaired electrons are highly mobile, Overhauser DNP is possible in insulating solids if rapid structural modulations are linked to a modulation in hyperfine coupling. Herein, we report that Overhauser DNP can be triggered by the strategic addition of a methyl group, demonstrated here in a Blatter's radical. The rotation of the methyl group leads to a modulation of the hyperfine coupling to its protons, which in turn facilitates electron-nuclear cross-relaxation. Removal of the methyl protons, through deuteration, quenches the process, as does the reduction of the hyperfine coupling strength. This result suggests the possibility for the design of tailor-made Overhauser DNP polarizing agents for high-field MAS-DNP.

Dynamic Nuclear Polarization in battery materials

The increasing need for portable and large-scale energy storage systems requires development of new, long lasting and highly efficient battery systems. Solid state NMR spectroscopy has emerged as an excellent method for characterizing battery materials. Yet, it is limited when it comes to probing thin interfacial layers which play a central role in the performance and lifetime of battery cells. Here we review how Dynamic Nuclear Polarization (DNP) can lift the sensitivity limitation and enable detection of the electrode-electrolyte interface, as well as the bulk of some electrode and electrolyte systems. We describe the current challenges from the point of view of materials development; considering how the unique electronic, magnetic and chemical properties differentiate battery materials from other applications of DNP in materials science. We review the current applications of exogenous and endogenous DNP from radicals, conduction electrons and paramagnetic metal ions. Finally, we provide our perspective on the opportunities and directions where battery materials can benefit from current DNP methodologies as well as project on future developments that will enable NMR investigation of battery materials with sensitivity and selectivity under ambient conditions.

Surface NMR using quantum sensors in diamond

NMR is a noninvasive, molecular-level spectroscopic technique widely used for chemical characterization. However, it lacks the sensitivity to probe the small number of spins at surfaces and interfaces. Here, we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect NMR signals from chemically modified thin films. To demonstrate the method's capabilities, aluminum oxide layers, common supports in catalysis and materials science, are prepared by atomic layer deposition and are subsequently functionalized by phosphonate chemistry to form self-assembled monolayers. The surface NV-NMR technique detects spatially resolved NMR signals from the monolayer, indicates chemical binding, and quantifies molecular coverage. In addition, it can monitor in real time the formation kinetics at the solid-liquid interface. With our approach, we show that NV quantum sensors are a surface-sensitive NMR tool with femtomole sensitivity for in situ analysis in catalysis, materials, and biological research.

Proton-phosphorous connectivities revealed by high-resolution proton-detected solid-state NMR

Proton-detected solid-state NMR enables atomic-level insight in solid-state reactions, for instance in heterogeneous catalysis, which is fundamental for deciphering chemical reaction mechanisms. We herein introduce a phosphorus-31 radiofrequency channel in...

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Propane to olefins tandem catalysis: a selective route towards light olefins production

On-purpose synthetic routes for propylene production have emerged in the last couple of decades in response to the increasing demand for plastics and a shift to shale gas feedstocks for ethylene production. Propane dehydrogenation (PDH), an efficient and selective route to produce propylene, saw booming investments to fill the so-called propylene gap. In the coming years, however, a fluctuating light olefins market will call for flexibility in end-product of PDH plants. This can be achieved by combining PDH with propylene metathesis in a single step, propane to olefins (PTO), which allows production of mixtures of propylene, ethylene and butenes, which are important chemical building blocks for a.o. thermoplastics. The metathesis technology introduced by Phillips in the 1960s and mostly operated in reverse to produce propylene, is thus undergoing a renaissance of scientific and technological interest in the context of the PTO reaction. In this review, we will describe the state-of-the-art of PDH, propylene metathesis and PTO reactions, highlighting the open challenges and opportunities in the field. While the separate PDH and metathesis reactions have been extensively studied in the literature, understanding the whole PTO tandem-catalysis system will require new efforts in theoretical modelling and operando spectroscopy experiments, to gain mechanistic insights into the combined reactions and finally improve catalytic selectivity and stability for on-purpose olefins production.

NMR Crystallography Enhanced by Quantum Chemical Calculations and Liquid State NMR Spectroscopy for the Investigation of Se-NHC Adducts*

N-heterocyclic carbene ligands (NHC) are widely utilized in catalysis and material science. They are characterized by their steric and electronic properties. Steric properties are usually quantified on the basis of their static structure, which can be determined by X-ray diffraction. The electronic properties are estimated in the liquid state; for example, via the ⁷⁷ Se liquid state NMR of Se-NHC adducts. We demonstrate that ⁷⁷ Se NMR crystallography can contribute to the characterization of the structural and electronic properties of NHC in solid and liquid states. Selected Se-NHC adducts are investigated via ⁷⁷ Se solid state NMR and X-ray crystallography, supported by quantum chemical calculations. This investigation reveals a correlation between the molecular structure of adducts and NMR parameters, including not only isotropic chemical shifts but also the other chemical shift tensor components. Afterwards, the liquid state ⁷⁷ Se NMR data is presented and interpreted in terms of the quantum chemistry modelling. The discrepancy between the structural and electronic properties, and in particular the π-accepting abilities of adducts in the solid and liquid states is discussed. Finally, the ¹³ C isotropic chemical shift from the liquid state NMR and the ¹³ C tensor components are also discussed, and compared with their ⁷⁷ Se counterparts. ⁷⁷ Se NMR crystallography can deliver valuable information about NHC ligands, and together with liquid state ⁷⁷ Se NMR can provide an in-depth outlook on the properties of NHC ligands.

Probing 29Si-17O connectivities and proximities by solid-state NMR

The measurement of dipolar and J- couplings between ²⁹Si and ¹⁷O isotopes is challenging owing to (i) the low abundance of both isotopes and (ii) their close Larmor frequencies, which only differ by 19%. These issues are circumvented here by the use of isotopic enrichment and dedicated triple-resonance magic-angle spinning NMR probe. The surface of ²⁹Si-enriched silica was labelled with ¹⁷O isotope and heated at 80 and 200 °C. ²⁹Si-¹⁷O connectivities and proximities were probed using two-dimensional (2D) through-bond and through-space heteronuclear multiple-quantum coherences (J- and D-HMQC) experiments between ¹⁷O and ²⁹Si nuclei. The simulation of the build-up of the J- and D-HMQC signals allowed the first experimental measurement of J- and dipolar coupling constants between ¹⁷O and ²⁹Si nuclei. These HMQC experiments allow distinguishing two distinct siloxane (SiOSi) oxygen sites: (i) those covalently bonded to Q³ and Q⁴ groups, having a hydroxyl group as a second neighbour and (ii) those covalently bonded to two Q⁴ groups. The measured J- and dipolar coupling constants of siloxane ¹⁷O nucleus with Q^{4 29}Si nuclei differ from those with Q^{3 29}Si nuclei. These results indicate that the ²⁹Si-¹⁷O one-bond J-coupling and Si-O bond length depend on the second neighbours of the Si atoms.

Operando Surface Spectroscopy and Microscopy during Catalytic Reactions: From Clusters via Nanoparticles to Meso-Scale Aggregates

Operando characterization of working catalysts, requiring per definitionem the simultaneous measurement of catalytic performance, is crucial to identify the relevant catalyst structure, composition and adsorbed species. Frequently applied operando techniques are discussed, including X-ray absorption spectroscopy, near ambient pressure X-ray photoelectron spectroscopy and infrared spectroscopy. In contrast to these area-averaging spectroscopies, operando surface microscopy by photoemission electron microscopy delivers spatially-resolved data, directly visualizing catalyst heterogeneity. For thorough interpretation, the experimental results should be complemented by density functional theory. The operando approach enables to identify changes of cluster/nanoparticle structure and composition during ongoing catalytic reactions and reveal how molecules interact with surfaces and interfaces. The case studies cover the length-scales from clusters via nanoparticles to meso-scale aggregates, and demonstrate the benefits of specific operando methods. Restructuring, ligand/atom mobility, and surface composition alterations during the reaction may have pronounced effects on activity and selectivity. The nanoscale metal/oxide interface steers catalytic performance via a long ranging effect. Combining operando spectroscopy with switching gas feeds or concentration-modulation provides further mechanistic insights. The obtained fundamental understanding is a prerequisite for improving catalytic performance and for rational design.

Dual Active Sites on Molybdenum/ZSM-5 Catalyst for Methane Dehydroaromatization: Insights from Solid-State NMR Spectroscopy

Methane dehydroaromatization (MDA) on Mo/ZSM-5 zeolite catalyst is promising for direct transformation of natural gas. Understanding the nature of active sites on Mo/ZSM-5 is a challenge for applications. Herein, using 1 H{95 Mo} double-resonance solid-state NMR spectroscopy, we identify proximate dual active sites on Mo/ZSM-5 catalyst by direct observation of internuclear spatial interaction between Brønsted acid site and Mo species in zeolite channels. The acidic proton-Mo spatial interaction is correlated with methane conversion and aromatics formation in the MDA process, an important factor in determining the catalyst activity and lifetime. The evolution of olefins and aromatics in Mo/ZSM-5 channels is monitored by detecting their host-guest interactions with both active Mo sites and Brønsted acid sites via 1 H{95 Mo} double-resonance and two-dimensional 1 H-1 H correlation NMR spectroscopy, revealing the intermediate role of olefins in hydrocarbon pool process during the MDA reaction.

Structure evolution of single-site Pt in a metal-organic framework

Heterogeneous single-metal-site catalyst or single-atom catalyst research has grown rapidly due to the accessibility of modern characterization techniques that can provide invaluable information at the atomic-scale. Herein, we study the structural evolution of isolated single Pt sites incorporated in a metal-organic framework containing bipyridine functional groups using in situ diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule. The structure and electronic properties of the isolated Pt sites are further corroborated by x-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy. We find the prerequisite of high temperature He treatment for Pt activation and CO insertion and inquire into the structural transformation of Pt site process by dynamic nuclear polarization-enhanced solid-state nuclear magnetic resonance spectroscopy.

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

Since its early days, olefin metathesis has been in the focus of scientific discussions and technology development. While heterogeneous olefin metathesis catalysts based on supported group 6 metal oxides have been used for decades in the petrochemical industry, detailed mechanistic studies and the development of molecular organometallic chemistry have led to the development of robust and widely used homogeneous catalysts based on well-defined alkylidenes that have found applications for the synthesis of fine and bulk chemicals and are also used in the polymer industry. The development of the chemistry of high-oxidation group 5-7 alkylidenes and the use of surface organometallic chemistry (SOMC) principles unlocked the preparation of so-called well-defined supported olefin metathesis catalysts. The high activity and stability (often superior to their molecular analogues) and molecular-level characterisation of these systems, that were first reported in 2001, opened the possibility for the first direct structure-activity relationships for supported metathesis catalysts. This review describes first the history of SOMC in the field of olefin metathesis, and then focuses on what has happened since 2007, the date of our last comprehensive reviews in this field.

Mapping the oxygen structure of γ-Al2O3 by high-field solid-state NMR spectroscopy

γ-Al₂O₃ is one of the most widely used catalysts or catalyst supports in numerous industrial catalytic processes. Understanding the structure of γ-Al₂O₃ is essential to tuning its physicochemical property, which still remains a great challenge. We report a strategy for the observation and determination of oxygen structure of γ-Al₂O₃ by using two-dimensional (2D) solid-state NMR spectroscopy at high field. 2D ¹⁷O double-quantum single-quantum homonuclear correlation NMR experiment is conducted at an ultra-high magnetic field of 35.2 T to reveal the spatial proximities between different oxygen species from the bulk to surface. Furthermore, 2D proton-detected ¹H-¹⁷O heteronuclear correlation NMR experiments allow for a rapid identification and differentiation of surface hydroxyl groups and (sub-)surface oxygen species. Our experimental results demonstrate a non-random distribution of oxygen species in γ-Al₂O₃.

γ-Al₂O₃ is widely used in catalytic processes, but understanding its detailed structure remains a challenge. The authors, using two-dimensional solid-state NMR spectroscopy at a high magnetic field, characterize the spatial proximity and connectivity between oxygen species from the bulk to the surface.

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS-PIETA NMR Spectroscopy

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands-phase incremented echo-train acquisition (PASS-PIETA). The improved sensitivity and resolution of DNP enhanced PASS-PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the ^111/113Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin-orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.

A Formulation Protocol with Pyridine to Enable Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy on Reactive Surface Sites: Case Study with Olefin Polymerization and Metathesis Catalysts

Dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS) has emerged as a powerful characterization tool in material chemistry and heterogeneous catalysis by dramatically increasing, by up to 2 orders of magnitude, the NMR signals associated with surface sites. DNP-SENS mostly relies on using exogenous polarizing agents (PAs), typically dinitroxyl radicals, to boost the NMR signals. However, the PAs may interact with the surface or even react with surface sites, thus leading to loss or quenching of DNP enhancements. Herein, we describe the development of a DNP-SENS formulation that allows broadening the application of DNP-SENS to samples containing highly reactive surface sites, namely a Ziegler-Natta propylene polymerization catalyst, a sulfated zirconia-supported metallocene, and a silica-supported cationic Mo alkylidene. The protocol consists of adsorbing pyridine prior to the DNP formulation (TEKPol/TCE). The addition of pyridine not only preserves the PAs and thereby restores the DNP enhancement but also allows probing Lewis/Brønsted acid surface sites that are often present on these catalysts.

Proton-Detected Multidimensional Solid-State NMR Enables Precise Characterization of Vanadium Surface Species at Natural Abundance

Heterogeneous catalysts fulfill vital roles in industrial processes; however, the nature of the catalytic surfaces, typically either containing a low abundance of active sites or being amorphous in nature, leads to difficulties when attempting to study the structure of the active sites. In this work, we show how making use of fast MAS ssNMR allows one to efficiently detect well-resolved ¹H-detected spectra of heterogeneous catalysts. This approach was applied to study the structure of surface species resulting from the grafting of VO(OⁱPr)₃ onto a partially dehydroxylated silica using the surface organometallic chemistry approach. The use of ¹H sensitivity enabled detection of various hetero- and homonuclear correlation spectra in order to study the structure of this system and to resolve the structure of the grafted vanadium complex. More specifically, VO(OⁱPr)₃ grafts through both protonolysis and opening of siloxane bridges to generate a bis-grafted species.

The Comparison between Single Atom Catalysis and Surface Organometallic Catalysis

Single atom catalysis (SAC) is a recent discipline of heterogeneous catalysis for which a single atom on a surface is able to carry out various catalytic reactions. A kind of revolution in heterogeneous catalysis by metals for which it was assumed that specific sites or defects of a nanoparticle were necessary to activate substrates in catalytic reactions. In another extreme of the spectrum, surface organometallic chemistry (SOMC), and, by extension, surface organometallic catalysis (SOMCat), have demonstrated that single atoms on a surface, but this time with specific ligands, could lead to a more predictive approach in heterogeneous catalysis. The predictive character of SOMCat was just the result of intuitive mechanisms derived from the elementary steps of molecular chemistry. This review article will compare the aspects of single atom catalysis and surface organometallic catalysis by considering several specific catalytic reactions, some of which exist for both fields, whereas others might see mutual overlap in the future. After a definition of both domains, a detailed approach of the methods, mostly modeling and spectroscopy, will be followed by a detailed analysis of catalytic reactions: hydrogenation, dehydrogenation, hydrogenolysis, oxidative dehydrogenation, alkane and cycloalkane metathesis, methane activation, metathetic oxidation, CO₂ activation to cyclic carbonates, imine metathesis, and selective catalytic reduction (SCR) reactions. A prospective resulting from present knowledge is showing the emergence of a new discipline from the overlap between the two areas.

The acidic nature of "NMR-invisible" tri-coordinated framework aluminum species in zeolites

The unambiguous characterization of different acid sites in zeolites is of great importance for understanding their catalytic performance and the rational design of highly efficient zeolite catalysts. In addition to various well-characterized extra-framework Al species, a tri-coordinated framework aluminum species can also serve as a Lewis acid site in zeolites, which is "NMR-invisible" owing to its extremely distorted local environment. Here we provide a feasible and reliable approach to elucidate the acidic nature of the tri-coordinated framework Al in dehydrated H-ZSM-5 zeolites via sensitivity-enhanced two-dimensional (2D) multiple nuclear correlation NMR experiments coupled with trimethylphosphine oxide (TMPO) probe molecules. Two types of tri-coordinated framework Al sites have been unambiguously identified, which amount to 11.6% of the total Brønsted and Lewis acid sites. Furthermore, it was found that synergistic effects arising from the close spatial proximity between the tri-coordinated framework Al site and the Brønsted acid site lead to the generation of superacidity (with an acid strength stronger than 100% H₂SO₄) in the zeolite.

Dynamic nuclear polarization on a hybridized hammerhead ribozyme: An explorative study of RNA folding and direct DNP with a paramagnetic metal ion cofactor

While uniform isotope labeling of ribonucleic acids (RNA) can simply and efficiently be achieved by in-vitro transcription, the specific introduction of nucleotides in larger constructs is non-trivial and often ineffective. Here, we demonstrate how a medium-sized (67-mer), biocatalytically relevant RNA (hammerhead ribozyme, HHRz) can be formed by spontaneous hybridization of two differently isotope-labeled strands, each individually synthesized by in-vitro transcription. This allows on the one hand for a significant reduction in the number of isotope-labeled nucleotides and thus spectral overlap particularly under magic-angle spinning (MAS) dynamic nuclear polarization (DNP) NMR conditions, on the other hand for orthogonal ¹³C/¹⁵N-labeling of complementary strands and thus for specific investigation of structurally or functionally relevant inter-strand and/or inter-stem contacts. By this method, we are able to confirm a non-canonical interaction due to single-site resolution and unique spectral assignments by two-dimensional ¹³C-¹³C (PDSD) as well as ¹⁵N-¹³C (TEDOR) correlation spectroscopy under "conventional" DNP enhancement. This contact is indicative of the ribozyme's functional conformation, and is present in frozen solution irrespective of the presence or absence of a Mg²⁺ co-factor. Finally, we use different isotope-labeling schemes in order to investigate the distance dependence of paramagnetic interactions and direct metal-ion DNP if the diamagnetic Mg²⁺ is substituted by paramagnetic Mn²⁺.

Recent developments in MAS DNP-NMR of materials

Solid-state NMR spectroscopy is a powerful technique for the characterization of the atomic-level structure and dynamics of materials. Nevertheless, the use of this technique is often limited by its lack of sensitivity, which can prevent the observation of surfaces, defects or insensitive isotopes. Dynamic Nuclear Polarization (DNP) has been shown to improve by one to three orders of magnitude the sensitivity of NMR experiments on materials under Magic-Angle Spinning (MAS), at static magnetic field B₀ ≥ 5 T, conditions allowing for the acquisition of high-resolution spectra. The field of DNP-NMR spectroscopy of materials has undergone a rapid development in the last ten years, spurred notably by the availability of commercial DNP-NMR systems. We provide here an in-depth overview of MAS DNP-NMR studies of materials at high B₀ field. After a historical perspective of DNP of materials, we describe the DNP transfers under MAS, the transport of polarization by spin diffusion and the various contributions to the overall sensitivity of DNP-NMR experiments. We discuss the design of tailored polarizing agents and the sample preparation in the case of materials. We present the DNP-NMR hardware and the influence of key experimental parameters, such as microwave power, magnetic field, temperature and MAS frequency. We give an overview of the isotopes that have been detected by this technique, and the NMR methods that have been combined with DNP. Finally, we show how MAS DNP-NMR has been applied to gain new insights into the structure of organic, hybrid and inorganic materials with applications in fields, such as health, energy, catalysis, optoelectronics etc.

Materializing opportunities for NMR of solids

Advancements in sensitivity and resolution of NMR of solids are opening a bonanza of fundamental and technological opportunities in materials science. Many of these are at the boundaries of related disciplines that provide creative inputs to motivate the development of new methodologies and possibilities for new applications. As Boltzmann limitations are surmounted by dynamic-nuclear-polarization- and laser-enhanced hyperpolarization techniques, the correlative benefits of multidimensional NMR are becoming more and more impactful. Nevertheless, there are limits, and the atomic-level information provided by solid-state NMR will be most useful in combination with state-of-the-art diffraction, microscopy, computational, and materials synthesis methods. Collectively these can be expected to lead to design criteria that will promote discovery of new materials, lead to novel or improved material properties, catalyze new applications, and motivate further methodological advancements.

Noble Metal Particles Confined in Zeolites: Synthesis, Characterization, and Applications

Noble metal nanoparticles or subnanometric particles confined in zeolites, that is, metal@zeolite, represent an important type of functional materials with typical core-shell structure. This type of material is known for decades and recently became a research hotspot due to their emerging applications in various fields. Remarkable achievements are made dealing with the synthesis, characterization, and applications of noble metal particles confined in zeolites. Here, the most representative research progress in metal@zeolites is briefly reviewed, aiming to boost further research on this topic. For the synthesis of metal@zeolites, various strategies, such as direct synthesis from inorganic or ligand-assisted noble metal precursors, multistep postsynthesis encapsulation and ion-exchange followed by reduction, are introduced and compared. For the characterization of metal@zeolites, several most useful techniques, such as electron microscopy, X-ray based spectroscopy, infrared and fluorescence emission spectroscopy, are recommended to check the successful confinement of noble metal particles in zeolite matrix and their unique physiochemical properties. For the applications of metal@zeolites, catalysis and optics are involved with an emphasis on catalytic applications including the size-dependent catalytic properties, the sintering-resistance properties, the substrate shape-selective catalysis, and catalysis modulation by zeolite microenvironment.

Carbon-13 NMR Chemical Shift: A Descriptor for Electronic Structure and Reactivity of Organometallic Compounds

Metal-bonded carbon atoms in metal-alkyl, metal-carbene/alkylidene, and metal-carbyne/alkylidyne species often show significantly more deshielded isotropic chemical shifts than their organic counterparts (alkanes, alkenes, and alkynes). While isotropic chemical shift is universally used to characterize a chemical compound in solution, it is an average value of the three principal components of the chemical shift tensor (δ₁₁ > δ₂₂ > δ₃₃). The tensor components, which are accessible by solid-state NMR spectroscopy, can provide detailed information about the electronic structure (frontier molecular orbitals) at the observed nuclei. This information can be accessed in detail by quantum chemical calculations, most notably by an analysis of the paramagnetic contribution to the NMR shielding tensor. The paramagnetic term mainly results from the coupling of occupied and empty molecular orbitals close in energy-the frontier molecular orbitals-under the effect of the external magnetic field (B₀). In organometallic compounds, a large deshielding of the isotropic carbon-13 chemical shift of the metal-bonded carbon atom is commonly related to the coupling between the occupied σ_M-C orbital and low-lying vacant orbitals of π_M═C^* character. The deshielding at the α-carbon hence probes the extent of σ_M-C and π_M═C^* interactions. This molecular orbital view readily explains the strong deshielding and large anisotropy (evidenced by the span Ω = δ₁₁ - δ₃₃) observed in metal alkylidenes and alkylidynes (200 < δ_iso < 400 ppm). Fischer carbenes are generally more deshielded than Schrock or Grubbs alkylidenes due to their low-lying π_M═C^* orbital. Chemical shift hence shows their higher electrophilic character, connecting NMR spectroscopy to reactivity patterns. Similarly, the α-carbon of metal-alkyls display deshielded chemical shifts in specific coordination environments. This deshielding, which is often prominently pronounced for cationic species, indicates the presence of partial π-bond character in the metal-carbon bond, making these bonds topologically equivalent to alkylidene π-bonds. The π-character in metal-alkyl bonds favors (i) α-H abstraction processes in metal bis-alkyl compounds yielding metal alkylidenes, (ii) [2 + 2]-retrocyclization of metallacyclobutanes that participate in olefin metathesis, (iii) olefin insertion in cationic metal alkyls thus explaining polymerization activity trends and the importance of α-H agostic interactions, and (iv) C-H bond activation on metal-alkyls via σ-bond metathesis. The presence of π-character in the metal-carbon bonds involved in these processes rationalizes the parallel reactivity patterns of metal-alkyls toward olefin insertion and σ-bond metathesis and the fact that σ-bond metathesis, olefin insertion, and olefin metathesis are commonly observed with metal atoms in the same ligand field. Because of the similarities in the frontier molecular orbitals involved in these processes, these reactions can be viewed as isolobal. This explains why certain fragments, such as bent metallocenes (d⁰ Cp₂M) or T-shaped L₃M, are ubiquitous in these reactions.

Metal Active Sites and Their Catalytic Functions in Zeolites: Insights from Solid-State NMR Spectroscopy

Zeolites are important heterogeneous catalysts widely used in the modern chemical and petrochemical industries. Metal-containing zeolites show distinct performance in the catalytic processes such as fluid catalytic cracking, activation and conversion of light alkanes, methanol-to-aromatics conversion, biomass transformation, and so on. The metal speciation, distribution, and interactions on zeolites have enormous impact on their property and catalytic performance. Significant efforts have been devoted to the synthesis of more active and selective zeolites by engineering the metal active sites. However, the nature of metal species and their role in the reactions are still poorly understood, which makes it difficult to establish the structure-activity relationship toward the rational design and application of zeolites. For example, synergic active sites are often present on the metal-containing zeolites, but their structure, property and quantification still remain to be resolved. Solid-state NMR is a powerful tool for the characterization of heterogeneous catalysts and catalytic reactions by providing information about both molecular structure and dynamics. The heterogeneity and low concentration of the metal sites on zeolites usually leads to a great challenge for their characterization. In this Account, we will describe our effort to study the metal active sites, host-guest interactions, and reaction intermediates by using solid-state NMR spectroscopy, with the aim to highlight recent advances in solid-state NMR techniques for probing the structure and property of metal-containing zeolites as well as the relevant reaction mechanisms. Using sensitivity-enhanced NMR methods such as ⁶⁷Zn, ⁷¹Ga, and ¹¹⁹Sn, NMR enables the identification of metal speciation on zeolites. The synergic active sites constituted by metal species (as Lewis acid sites) and acidic protons (as Brønsted acid sites) on zeolites that amount to only a small fraction of the whole system can be directly probed and quantified with advanced ¹H-⁶⁷Zn or ¹H-⁷¹Ga double-resonance solid-state NMR. We developed NMR methods to study the host-guest interactions in zeolites by observing the spatial interaction/proximity between aluminum sites (associated with Brønsted or Lewis acid sites) in zeolite host and carbon atoms in organic molecule guest formed during catalytic reaction, which leads to the formation of supramolecular reaction centers in the methanol-to-olefins reaction. The mechanisms underlying the catalytic reactions on metal-modified zeolite are revealed by the identification of key reaction intermediates with in situ ¹³C MAS NMR spectroscopy. Our discussion based on the representative examples shows how the metal species serving as active sites significantly affect the property and activity of zeolites and related reaction pathways. The structural information obtained by the state-of-the-art solid-state NMR techniques provides new insights into the structure-activity relationship of zeolites in heterogeneous catalysis, which should be beneficial for rational design of highly efficient zeolite catalysts.

Nature and Topology of Metal-Oxygen Binding Sites in Zeolite Materials: 17 O High-Resolution EPR Spectroscopy of Metal-Loaded ZSM-5

Determining structural models is pivotal to the rational understanding and development of heterogeneous catalytic systems. A paradigmatic case is represented by open-shell metals supported on oxides, where the catalytic properties crucially depend on the nature of the metal-oxygen bonds and the extent of charge and spin transfer. Through a combination of selective ¹⁷ O isotopic enrichment and the unique properties of open-shell s-state monovalent Group 12 cations, we derive a site-specific topological description of active sites in an MFI zeolite. We show that just a few selected sites out of all possible are populated and that the relative occupancies depend on the specific properties of the metal, and we provide maps of charge and spin transfer at the metal-oxygen interface. This approach is not restricted to zeotype materials, rather it is applicable to any catalysts supported on oxygen-containing materials.