Understanding Atomic-Scale Compositions, Structures, and Properties of Semi-Crystalline Inorganic Nanomaterials

Many important material properties are determined by the surface layer (0.5–100 nm); these include optoelectronic properties like conductivity, absorptivity, and reflectance as well as physiochemical properties such as molecular adsorption, hydrophobicity, and surface diffusivity. Furthermore, the crystallization or assembly processes of technologically-important nanomaterials are mediated by surface interactions and influence the surface compositions and properties of the resultant material, including zeolites, colloidal semiconductors, carbon-based electrocatalysts, and mesoporous inorganic oxides. Catalytic reaction properties of diverse porous heterogeneous materials are determined by molecular interactions of adsorbates, reactants, or product species at pore or exterior surface sites. Despite the broad importance of surface interactions in determining material properties, fundamental questions remain regarding the physiochemical interactions at surfaces that determine crystallization, adsorption, optoelectronic, and/or reaction properties. This is because such properties often depend on dilute surface moieties, defect species, and/or molecular adsorbates that occupy distributions that are partially- or non-ordered and are therefore challenging or impossible to characterize by conventional scattering techniques. Developing atomic-level insights into the types, interactions, and distributions of such dilute non-ordered species is crucial to elucidate the molecular-level origins of the properties and synthesis pathways of materials such as zeolites, semiconductor nanoparticles, and mesoporous electrocatalysts. By understanding the crystallization, synthesis, and assembly processes of these materials, as well as the resulting structures and active species, the resulting insights can be applied to develop new synthetic or post-synthetic treatments to generate more effective, stable, and/or active materials with desirable properties. The objective of this dissertation is to measure, understand, and correlate the atomic-scale compositions, structures, and properties of heterogenous materials with diverse applications, including heterogeneous catalysis and solid-state lighting. Recently-developed solid-state nuclear magnetic resonance (NMR) techniques with complementary X-ray diffraction and electron microscopy analyses are applied to elucidate the structures and compositions of dilute surface, defect, or heteroatom species, which are correlated to the macroscopic properties of interest. These techniques are applied to analyze diverse heterogeneous inorganic nanomaterials, including aluminosilicate zeolites, cementitious solids, precious-metal-free electrocatalysts, and nanocrystalline semiconductors. Though the material systems vary in composition and application, in each case the important optical, electronic, and/or catalytic properties arise from dilute partially- or non-ordered defect or heteroatom species in a semi-crystalline lattice. The overall unifying themes are: (1) analysis of order and disorder in semi-crystalline inorganic solids using state-of-the-art diffraction and spectroscopic characterization techniques; (2) determining the distributions and structures of non-stoichiometric species, particularly at surfaces and interfaces; and (3) correlating atomic-level structures and compositions with macroscopic material properties. The insights provided are of broad importance and relevance to diverse material systems of technological interest for sustainable energy storage, conversion, and utilization

Leveraging Surface Siloxide Electronics to Enhance the Relaxation Properties of a Single-Molecule Magnet

International audienceSingle-molecule magnets (SMMs) hold promise for unmatched information storage density as well as for applications in quantum computing and spintronics. To date, the most successful SMMs have been organometallic lanthanide complexes. However, their surface immobilization, one of the requirements for device fabrication and commercial application, remains challenging due to the sensitivity of the magnetic properties to small changes in the electronic structure of the parent SMM. Thus, finding controlled approaches to SMM surface deposition is a timely challenge. In this contribution we apply the concept of isolobality to identify siloxides present at the surface of partially dehydroxylated silica as a suitable replacement for archetypal ligand architectures in organometallic SMMs. We demonstrate theoretically and experimentally that isolated siloxide anchoring sites not only enable successful immobilization but also lead to a 2 orders of magnitude increase in magnetization relaxation times

Assigning H-1 chemical shifts in paramagnetic mono- and bimetallic surface sites using DFT: a case study on the Union Carbide polymerization catalyst

The Union Carbide (UC) ethylene polymerization catalyst, based on silica-supported chromocene, is one of the first industrial catalysts prepared by surface organometallic chemistry, though the structure of the surface sites remains elusive. Recently, our group reported that monomeric and dimeric Cr(ii) sites, as well as Cr(iii) hydride sites, are present and that their proportion varies as a function of the Cr loading. While H-1 chemical shifts extracted from solid-state H-1 NMR spectra should be diagnostic of the structure of such surface sites, unpaired electrons centered on Cr atoms induce large paramagnetic H-1 shifts that complicate their NMR analysis. Here, we implement a cost-efficient DFT methodology to calculate H-1 chemical shifts for antiferromagnetically coupled metal dimeric sites using a Boltzmann-averaged Fermi contact term over the population of the different spin states. This method allowed us to assign the H-1 chemical shifts observed for the industrial-like UC catalyst. The presence of monomeric and dimeric Cr(ii) sites, as well as a dimeric Cr(iii)-hydride sites, was confirmed and their structure was clarified.ISSN:2041-6520ISSN:2041-653

Assigning 1H Chemical Shifts in Paramagnetic Mono and Bimetallic Surface Sites using DFT: a Case Study on the Union Carbide Polymerization Catalyst

The Union Carbide (UC) ethylene polymerization catalyst, based on silica-supported chromocene, is one of the first industrial catalysts prepared by surface organometallic chemistry, though the structure of the surface sites remains elusive. Recently, our group reported that monomeric and dimeric Cr (II) sites as well as Cr(III) hydride sites are present and that their proportion varies as a function of the Cr loading. While 1H chemical shifts extracted from solid-state 1H NMR spectra should be diagnostic of the structure of such surface sites, unpaired electrons centered on Cr atoms induce large paramagnetic 1H shifts that complicate NMR analysis. Here, we implement a cost-efficient DFT methodology to calculate 1H chemical shifts for antiferromagnetically coupled metal dimeric sites using a Boltzmann-averaged Fermi contact term over the population of the different spin states. This method allowed us to assign the 1H chemical shifts observed for the industrial-like UC catalyst. The presence of monomeric and dimeric Cr(II) sites as well as a dimeric Cr(III)-hydride site was confirmed and their structure was clarified

Solid-state NMR spectra of protons and quadrupolar nuclei at 28.2 T: Resolving signatures of surface sites with fast magic angle spinning

Advances in solid-state nuclear magnetic resonance (NMR) methods and hardware offer expanding opportunities for analysis of materials, interfaces, and surfaces. Here, we demonstrate the application of a very high magnetic field strength of 28.2 T and fast magic-angle-spinning rates (MAS, >40 kHz) to surface species relevant to catalysis. Specifically, we present as case studies the 1D and 2D solid-state NMR spectra of important catalyst and support materials, ranging from a well-defined silica-supported organometallic catalyst to dehydroxylated γ-alumina and zeolite solid acids. The high field and fast-MAS measurement conditions substantially improve spectral resolution and narrow NMR signals, which is particularly beneficial for solid-state 1D and 2D NMR analysis of 1H and quadrupolar nuclei such as 27Al at surfaces.ISSN:2691-370

Tailored Lewis Acid Sites for High-Temperature Supported Single-Molecule Magnetism

Generating or even retaining slow magnetic relaxationin surfaceimmobilized single-molecule magnets (SMMs) from promising molecularprecursors remains a great challenge. Illustrative examples are organolanthanidecompounds that show promising SMM properties in molecular systems,though surface immobilization generally diminishes their magneticperformance. Here, we show how tailored Lewis acidic Al-(III) siteson a silica surface enable generation of a material with SMM characteristicsvia chemisorption of (Cp-ttt)(2)DyCl ((Cp-ttt)(-) = 1,2,4-tri-(tert-butyl)-cyclopentadienide).Detailed studies of this system and its diamagnetic Y analogue indicatethat the interaction of the metal chloride with surface Al sites resultsin a change of the coordination sphere around the metal center inducingfor the dysprosium-containing material slow magnetic relaxation upto 51 K with hysteresis up to 8 K and an effective energy barrier(U (eff)) of 449 cm(-1),the highest reported thus far for a supported SMM.ISSN:0002-7863ISSN:1520-512

Tailored Lewis Acid Sites for High-Temperature Supported Single-Molecule Magnetism

International audienceGenerating or even retaining slow magnetic relaxationin surfaceimmobilized single-molecule magnets (SMMs) from promising molecularprecursors remains a great challenge. Illustrative examples are organolanthanidecompounds that show promising SMM properties in molecular systems,though surface immobilization generally diminishes their magneticperformance. Here, we show how tailored Lewis acidic Al-(III) siteson a silica surface enable generation of a material with SMM characteristicsvia chemisorption of (Cp-ttt)(2)DyCl ((Cp-ttt)(-) = 1,2,4-tri-(tert-butyl)-cyclopentadienide).Detailed studies of this system and its diamagnetic Y analogue indicatethat the interaction of the metal chloride with surface Al sites resultsin a change of the coordination sphere around the metal center inducingfor the dysprosium-containing material slow magnetic relaxation upto 51 K with hysteresis up to 8 K and an effective energy barrier(U (eff)) of 449 cm(-1),the highest reported thus far for a supported SMM

Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si–OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.ISSN:2691-370

Lithium Promotes Acetylide Formation on MgO During Methane Coupling Under Non-Oxidative Conditions

A prototypical material for the oxidative coupling of methane (OCM) is Li/MgO, for which Li is known to be essential as a dopant to obtain high C₂ selectivities. Herein, Li/MgO is demonstrated to be an effective catalyst for non-oxidative coupling of methane (NOCM). Moreover, the presence of Li is shown to favor the formation of magnesium acetylide (MgC₂), while pure MgO promotes coke formation as evidenced by solid-state ¹³C NMR, thus indicating that Li promotes C−C bond formation. Metadynamic simulations of the carbon mobility in MgC₂ and Li₂C₂ at the density functional theory (DFT) level show that carbon easily diffuses as a C₂ unit at 1000 °C. These insights suggest that the enhanced C₂ selectivity for Li-doped MgO is related to the formation of Li and Mg acetylides.ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

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.ISSN:2041-6520ISSN:2041-653