Solid‐State NMR of Heterogeneous Catalysts

Recent advances in solid‐state nuclear magnetic resonance (NMR) spectroscopy, combined with dynamic nuclear polarization (DNP), quantum chemical DFT calculations, and gas‐phase NMR spectroscopy investigating the structure and reactivity of heterogeneous catalysts are reviewed. The investigated catalysts range from classical mononuclear catalysts, like immobilized derivates of Wilkinson's catalysts over binuclear catalysts such as the dirhodium paddlewheel catalyst to catalytic nanoparticles, employing various support materials, such as mesoporous silica gels, coordination polymers, and biomaterials such as cellulose.

Interrogating site dependent kinetics over SiO2-supported Pt nanoparticles

A detailed knowledge of reaction kinetics is key to the development of new more efficient heterogeneous catalytic processes. However, the ability to resolve site dependent kinetics has been largely limited to surface science experiments on model systems. Herein, we can bypass the pressure, materials, and temperature gaps, resolving and quantifying two distinct pathways for CO oxidation over SiO2-supported 2 nm Pt nanoparticles using transient pressure pulse experiments. We find that the pathway distribution directly correlates with the distribution of well-coordinated (e.g., terrace) and under-coordinated (e.g., edge, vertex) CO adsorption sites on the 2 nm Pt nanoparticles as measured by in situ DRIFTS. We conclude that well-coordinated sites follow classic Langmuir-Hinshelwood kinetics, but under-coordinated sites follow non-standard kinetics with CO oxidation being barrierless but conversely also slow. This fundamental method of kinetic site deconvolution is broadly applicable to other catalytic systems, affording bridging of the complexity gap in heterogeneous catalysis.

Metal Nanoclusters Synthesized in Alkaline Ethylene Glycol: Mechanism and Application

The "unprotected" metal and alloy nanoclusters (UMCs) prepared by the alkaline ethylene glycol method, which are stabilized with simple ions and solvent molecules, have the advantages of a small particle size, a narrow size distribution, good stability, highly efficient preparation, easy separation, surface modification and transfer between different phases. They can be composited with diverse materials to prepare catalytic systems with controllable structures, providing an effective means of studying the different factors' effects on the catalytic properties separately. UMCs have been widely used in the development of high-performance catalysts for a variety of functional systems. This paper will review the research progress on the formation mechanism of the unprotected metal nanoclusters, exploring the structure-function relationship of metal nanocluster catalysts and the preparation of excellent metal catalysts using the unprotected metal nanoclusters as building blocks or starting materials. A principle of the influence of carriers, ligands and modifiers in metal nanocluster catalysts on the catalytic properties is proposed.

Systematic assessment of DFT methods for geometry optimization of mononuclear platinum-containing complexes

Platinum is used extensively as a catalyst for a wide variety of chemical reactions, though its scarcity and price present limitations to expansions of its use. To understand the origin of platinum's versatility-with the goals of both improving the efficiency of existing catalysts and mimicking its reactivity with more abundant metals-the mechanisms of platinum-catalyzed chemical reactions must be understood via structural and spectroscopic characterization of these catalysts under operando conditions. Such data, typically consisting of complex mixtures of species, often prove challenging to interpret, inviting the aid of chemical theory. DFT calculations in particular have proven successful at predicting structural and spectroscopic parameters of transition metal species, though a thorough investigation of how these methods perform for platinum-based complexes has yet to be undertaken. Herein, we evaluated the performance of geometry optimization for five commonly used functionals (BP86, PBE, B3LYP, PBE0, and TPSSh) in combination with various ligand basis sets, relativistic approximations, and solvation and dispersion models. We applied these DFT methods to a training set of 14 platinum-containing complexes with varying sizes, oxidation states, and number and type of ligands and determined that the best-performing method was the PBE0 functional together with the def2-TZVP basis set for the ligand atoms, the ZORA relativistic approximation, and solvation and dispersion corrections. The ability of this DFT methodology to accurately predict metrical parameters was confirmed using two case studies, most notably by comparing the DFT optimized geometry of a previously uncharacterized complex to newly collected EXAFS data, which showed excellent agreement.

Solid-state nuclear magnetic resonance studies of nanoparticles

In this trends article, we review seminal and recent studies using static and magic-angle spinning solid-state NMR to study the structure of nanoparticles and ligands attached to nanoparticles. Solid-state NMR techniques including one-dimensional multinuclear NMR, cross-polarization, techniques for measuring dipolar coupling and internuclear distances, and multidimensional NMR have provided insight into the core-shell structure of nanoparticles as well as the structure of ligands on the nanoparticle surface. Hyperpolarization techniques, in particular solid-state dynamic nuclear polarization (DNP), have enabled detailed studies of nanoparticle core-shell structure and surface chemistry, by allowing unprecedented levels of sensitivity to be achieved. The high signal-to-noise afforded by DNP has allowed homonuclear and heteronuclear correlation experiments involving nuclei with low natural abundance to be performed in reasonable experimental times, which previously would not have been possible. The use of DNP to study nanoparticles and their applications will be a fruitful area of study in the coming years as well.

Non-oxidative dehydrogenation of isobutane over supported vanadium oxide: nature of the active sites and coke formation

We combine Raman spectroscopy, EPR, XPS, temperature programmed reduction, XRD, ⁵¹V MAS ssNMR, TEM and N₂-physisorption to unravel structure–activity relationships during the non-oxidative dehydrogenation of isobutane over a V based catalyst.