Surface and Interfacial Chemistry

Based on a molecular approach combining controlled surface chemistry, advanced spectroscopic methods, in particular solid-state NMR, and computational chemistry, it is possible to develop single-site species and to control the growth of nanoparticles on supports. This allows the generation of highly efficient catalysts combining the advantages of both homogeneous and heterogeneous catalysts and function materials with defined properties

Solid-state NMR: An EYE Opener in Surface Chemistry

Solid-state NMR plays a critical role in establishing the atomic structure of surface species, obtained by the controlled grafting of organometallic complexes onto amorphous oxide supports, a promising strategy towards molecularly defined heterogeneous catalysts. Using one-dimensional or multi-dimensional NMR analysis allows us to map the structure of organometallic residues on surfaces, in a similar fashion that the structure of homogeneous catalysts can be determined using solution NMR techniques. In addition, chemical shift anisotropy analysis can be used as a tool to obtain detail structures and to determine the dynamics of surface species. In combination with DFT calculations we have also shown that the structure of aluminum species can be determined using high-field and ultrafast 27Al NMR. Finally, we discussed the latest development in Dynamic Nuclear Polarization, which allows the selective enhancements of the NMR signals of surface species, thus reducing the NMR acquisition time by factors up to 10,000. This makes solid-state NMR an indispensible tool in determining structure-property relationship and in the development of advanced materials including catalysts through more rational approach

The Role of Proton Transfer in Heterogeneous Transformations of Hydrocarbons

Heterogeneous catalysis is essential for the transformation of light hydrocarbons into chemical feedstocks. Many of the catalysts that mediate these transformations consist of isolated metal ions on the surface of a metal oxide support, such as silica or alumina. Due to the complexity of these catalysts, studying the active site and mechanism of these reactions is difficult. Surface organometallic chemistry (SOMC) could offer a solution to this problem by allowing the synthesis of well-defined surface organometallic species. This approach has been used to study the reactions of light hydrocarbons with isolated metal species on silica and alumina surfaces. These studies showed that proton transfers play a key role in the reactions of many hydrocarbons. The mechanisms of these reactions and their role in some common catalytic cycles are discussed

CO2 Hydrogenation: Supported Nanoparticles vs. Immobilized Catalysts

The conversion of CO2 to more valuable chemicals has been the focus of intense research over the past decades, and this field has become particularly important in view of the continuous increase of CO2 levels in our atmosphere and the need to find alternative ways to store excess energy into fuels. In this review we will discuss different strategies for CO2 conversion with heterogeneous and homogeneous catalysts. In addition, we will introduce some promising research concerning the immobilization of homogeneous catalysts on heterogeneous supports, as a hybrid of hetero- and homogeneous catalysts

Classifying and Understanding the Reactivities of Mo-Based Alkyne Metathesis Catalysts from ⁹⁵Mo NMR Chemical Shift Descriptors

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X3Mo≡CR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the context of alkyne metathesis that 95Mo chemical shift tensors encode detailed information on the electronic structure of these catalysts. Analysis by first-principles calculations of 95Mo chemical shift tensors extracted from solid-state 95Mo NMR spectra shows a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2 + 2]-cycloaddition step, thus linking 95Mo chemical shifts to reactivity. In particular, the 95Mo chemical shifts are driven by ligand electronegativity (σ-donation) and electron delocalization through Mo–O π interactions, thus explaining the reactivity patterns of the silanolate canopy catalysts. These results further motivate exploration of transition metal NMR signatures and their relationships to electronic structure and reactivity

State of the Art and Perspectives in the "Molecular Approach” Towards Well-Defined Heterogeneous Catalysts

Molecular understanding of heterogeneous catalysts is a key step towards their rational development since catalysis is a molecular phenomenon. Here we describe our efforts towards molecularly defined heterogeneous catalysts through the anchoring of molecular precursors on solid supports and our approaches towards bridging the gap between well-defined and industrial catalysts

Strain in Silica-Supported Ga(III) Sites : Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Altres ajuts: Acord transformatiu CRUE-CSICWell-defined Ga(III) sites on SiO are highly active, selective, and stable catalysts in the propane dehydrogenation (PDH) reaction. In this contribution, we evaluate the catalytic activity toward PDH of tricoordinated and tetracoordinated Ga(III) sites on SiO by means of first-principles calculations using realistic amorphous periodic SiO models. We evaluated the three reaction steps in PDH, namely, the C-H activation of propane to form propyl, the β-hydride (β-H) transfer to form propene and a gallium hydride, and the H-H coupling to release H, regenerating the initial Ga-O bond and closing the catalytic cycle. Our work shows how Brønsted-Evans-Polanyi relationships are followed to a certain extent for these three reaction steps on Ga(III) sites on SiO and highlights the role of the strain of the reactive Ga-O pairs on such sites of realistic amorphous SiO models. It also shows how transition-state scaling holds very well for the β-H transfer step. While highly strained sites are very reactive sites for the initial C-H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need for the right balance in strain to be an effective site for PDH. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites toward the PDH reaction and paves the way toward the design and prediction of better single-site catalysts on SiO for the PDH reaction. We performed computational calculations of Ga(III) single sites on realistic amorphous models of SiO to evaluate their catalytic activity toward the propane dehydrogenation reaction. Our results show that a balance in strain is key, in which neither too stiff nor too loose Ga−O bonding is needed to obtain the highest catalytic activity

Non-Oxidative Coupling of Methane: Interplay of Catalyst Interface and Gas Phase Mechanisms

Non-oxidative coupling of methane (NOCM) is a sought-after reaction that has been studied for decades. Harsh reaction conditions (T >800°C) in the face of limited catalyst stability lead to rapid catalyst deactivation and strong coking, preventing application thus far. Recent reports have shown the significance of an interplay of catalyst nature and reaction conditions, whereas metal carbides have prevailed to play a crucial role which involves incorporation of carbidic carbon in C2Hx and aromatic products. This perspective gives an overview of proposed mechanistic pathways and considerations about experiment conditions in order to foster a rational catalyst design platform for NOCM

Polymerization on CO-Reduced Phillips Catalyst initiates through the C-H bond Activation of Ethylene on Cr-O Sites

Investigation of the polymerization of ethylene on CO-reduced Phillips catalyst (1wt% chromium) by infrared spectroscopy reveals the presence of new OH bands. In particular, an OH-band appears at 3,605cm−1, consistent with the interaction of the SiOH group with an adjacent Lewis acidic chromium center, Si-(μ-OH)-Cr. Polymerization with d4-ethylene leads to the formation of the isotopically shifted band at 2,580cm−1, consistent with heterolytic C-H activation of ethylene over a Cr-O bond to generate the first Cr-C bond in ethylene polymerization with Phillips catalyst, as recently observed on well-defined Cr(III) silicates. Graphical Abstract

Phase Coexistence and Structural Dynamics of Redox Metal Catalysts Revealed by Operando TEM

Metal catalysts play an important role in industrial redox reactions. Although extensively studied, the state of these catalysts under operating conditions is largely unknown, and assignments of active sites remain speculative. Herein, an operando transmission electron microscopy study is presented, which interrelates the structural dynamics of redox metal catalysts to their activity. Using hydrogen oxidation on copper as an elementary redox reaction, it is revealed how the interaction between metal and the surrounding gas phase induces complex structural transformations and drives the system from a thermodynamic equilibrium toward a state controlled by the chemical dynamics. Direct imaging combined with the simultaneous detection of catalytic activity provides unparalleled structure–activity insights that identify distinct mechanisms for water formation and reveal the means by which the system self-adjusts to changes of the gas-phase chemical potential. Density functional theory calculations show that surface phase transitions are driven by chemical dynamics even when the system is far from a thermodynamic phase boundary. In a bottom-up approach, the dynamic behavior observed here for an elementary reaction is finally extended to more relevant redox reactions and other metal catalysts, which underlines the importance of chemical dynamics for the formation and constant re-generation of transient active sites during catalysis