Copper nanoparticles supported on zirconia (Cu/ZrO2) or related supported oxides (Cu/ZrO2/SiO2) show promising activity and selectivity for the hydrogenation of CO2 to CH3OH. However, the role of the ...support remains controversial because most spectroscopic techniques provide information dominated by the bulk, making interpretation and formulation of structure–activity relationships challenging. In order to understand the role of the support and in particular of the Zr surface species at a molecular level, a surface organometallic chemistry approach has been used to tailor a silica support containing isolated Zr(IV) surface sites, on which copper nanoparticles (∼3 nm) are generated. These supported Cu nanoparticles exhibit increased CH3OH activity and selectivity compared to those supported on SiO2, reaching catalytic performances comparable to those of the corresponding Cu/ZrO2. Ex situ and in situ X-ray absorption spectroscopy reveals that the Zr sites on silica remain isolated and in their +4 oxidation state, while ex situ solid-state nuclear magnetic resonance spectroscopy and catalytic performances show that similar mechanisms are involved with the single-site support and ZrO2. These observations imply that Zr(IV) surface sites at the periphery of Cu particles are responsible for promoting CH3OH formation on Cu–Zr-based catalysts and provide a guideline to develop selective CH3OH synthesis catalysts.
Transition metal nanoparticles (NPs) are typically supported on oxides to ensure their stability, which may result in modification of the original NP catalyst reactivity. In a number of cases, this ...is related to the formation of NP/support interface sites that play a role in catalysis. The metal/support interface effect verified experimentally is commonly ascribed to stronger reactants adsorption or their facile activation on such sites compared to bare NPs, as indicated by DFT-derived potential energy surfaces (PESs). However, the relevance of specific reaction elementary steps to the overall reaction rate depends on the preferred reaction pathways at reaction conditions, which usually cannot be inferred based solely on PES. Hereby, we use a multiscale (DFT/microkinetic) modeling approach and experiments to investigate the reactivity of the Ni/Al2O3 interface toward water–gas shift (WGS) and dry reforming of methane (DRM), two key industrial reactions with common elementary steps and intermediates, but held at significantly different temperatures: 300 vs 650 °C, respectively. Our model shows that despite the more energetically favorable reaction pathways provided by the Ni/Al2O3 interface, such sites may or may not impact the overall reaction rate depending on reaction conditions: the metal/support interface provides the active site for WGS reaction, acting as a reservoir for oxygenated species, while all Ni surface atoms are active for DRM. This is in contrast to what PESs alone indicate. The different active site requirement for WGS and DRM is confirmed by the experimental evaluation of the activity of a series of Al2O3-supported Ni NP catalysts with different NP sizes (2–16 nm) toward both reactions.
Many of the functions and applications of advanced materials result from their interfacial structures and properties. However, the difficulty in characterizing the surface structure of these ...materials at an atomic level can often slow their further development. Solid-state NMR can probe surface structure and complement established surface science techniques, but its low sensitivity often limits its application. Many materials have low surface areas and/or low concentrations of active/surface sites. Dynamic nuclear polarization (DNP) is one intriguing method to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude. In a DNP experiment, the large polarization of unpaired electrons is transferred to surrounding nuclei, which provides a maximum theoretical DNP enhancement of ∼658 for 1H NMR. In this Account, we discuss the application of DNP to enhance surface NMR signals, an approach known as DNP surface enhanced NMR spectroscopy (DNP SENS). Enabling DNP for these systems requires bringing an exogeneous radical solution into contact with surfaces without diluting the sample. We proposed the incipient wetness impregnation technique (IWI), a well-known method in materials science, to impregnate porous and particulate materials with just enough radical containing solution to fill the porous volume. IWI offers several advantages: it is extremely simple, provides a uniform wetting of the surface, and does not increase the sample volume or substantially reduce the concentration of the sample. This Account describes the basic principles behind DNP SENS through results obtained for mesoporous and nanoparticulate samples impregnated with radical solutions. We also discuss the quantification of the overall sensitivity enhancements obtained with DNP SENS and compare that with ordinary room temperature NMR spectroscopy. We then review the development of radicals and solvents that give the best possible enhancements today. With the best polarizing mixtures, DNP SENS enhances sensitivity by a factor of up to 100, which decreases acquisition time by five orders of magnitude. Such enhancement enables the detailed and expedient atomic level characterization of the surfaces of complex materials at natural isotopic abundance and opens new avenues for NMR. To illustrate these improvements, we describe the successful application of DNP SENS to characterize hybrid materials, organometallic surface species, and metal–organic frameworks.
Surface hydrides are ubiquitous in catalysis. However, their structures and properties are not as well-understood as those of their molecular counterparts, which have been extensively studied for the ...past 70 years. Hydrides isolated on surfaces have been characterized as stable entities on oxide surfaces or in zeolites. They have also been proposed as reaction intermediates in numerous catalytic processes (hydrogenation, hydrogenolysis, etc.). They have also been prepared via surface organometallic chemistry. In this review, we describe their key structural features and spectroscopic signatures. We discuss their reactivity and stability and also point out unexplored areas.
The dry reforming of methane (DRM), i.e., the reaction of methane and CO2 to form a synthesis gas, converts two major greenhouse gases into a useful chemical feedstock. In this work, we probe the ...effect and role of Fe in bimetallic NiFe dry reforming catalysts. To this end, monometallic Ni, Fe, and bimetallic Ni-Fe catalysts supported on a Mg x Al y O z matrix derived via a hydrotalcite-like precursor were synthesized. Importantly, the textural features of the catalysts, i.e., the specific surface area (172–178 m2/gcat), pore volume (0.51–0.66 cm3/gcat), and particle size (5.4–5.8 nm) were kept constant. Bimetallic, Ni4Fe1 with Ni/(Ni + Fe) = 0.8, showed the highest activity and stability, whereas rapid deactivation and a low catalytic activity were observed for monometallic Ni and Fe catalysts, respectively. XRD, Raman, TPO, and TEM analysis confirmed that the deactivation of monometallic Ni catalysts was in large due to the formation of graphitic carbon. The promoting effect of Fe in bimetallic Ni-Fe was elucidated by combining operando XRD and XAS analyses and energy-dispersive X-ray spectroscopy complemented with density functional theory calculations. Under dry reforming conditions, Fe is oxidized partially to FeO leading to a partial dealloying and formation of a Ni-richer NiFe alloy. Fe migrates leading to the formation of FeO preferentially at the surface. Experiments in an inert helium atmosphere confirm that FeO reacts via a redox mechanism with carbon deposits forming CO, whereby the reduced Fe restores the original Ni-Fe alloy. Owing to the high activity of the material and the absence of any XRD signature of FeO, it is very likely that FeO is formed as small domains of a few atom layer thickness covering a fraction of the surface of the Ni-rich particles, ensuring a close proximity of the carbon removal (FeO) and methane activation (Ni) sites.
A current challenge faced in water electrolysis is the development of structure–activity relationships for understanding and improving IrOx-based catalysts for the oxygen evolution reaction (OER). We ...report a simple and scalable modified Adams fusion method for preparing highly OER active, chlorine–free iridium oxide nanoparticles of various size and shape. The applied approach allows for the effects of particle size, morphology, and the nature of the surface species on the OER activity of IrO2 to be investigated. Iridium oxide synthesized at 350 °C from Ir(acac)3, consisting of 1.7 ± 0.4 nm particles with a specific surface area of 150 m2 g–1, shows the highest OER activity (E = 1.499 ± 0.003 V at 10 A gox –1). Operando X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) studies indicate the presence of iridium hydroxo (Ir–OH) surface species, which are strongly linked to the OER activity. Preparation of larger IrO2 particles using higher temperatures results in a change of the particle morphology from spherical to rod-shaped particles. A decrease of the intrinsic OER activity was associated with the predominant termination of the rod-shape particles by highly ordered (110) facets in addition to limited diffusion within mesoporous features.
Conspectus 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 energythe frontier molecular orbitalsunder the effect of the external magnetic field (B 0). 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 σM–C orbital and low-lying vacant orbitals of πMC * character. The deshielding at the α-carbon hence probes the extent of σM–C and πMC * 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 < δiso < 400 ppm). Fischer carbenes are generally more deshielded than Schrock or Grubbs alkylidenes due to their low-lying πMC * 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.
Processes that rely on heterogeneous catalysts underpin the production of bulk chemicals and fuels. In spite of this, understanding of the interplay between the structure and reactivity of these ...complex materials remains elusiverendering rational improvement of existing systems challenging. Herein, we describe efforts to understand complex materials capable of selective thermochemical conversion of CO2 to methanol using a surface organometallic chemistry (SOMC) approach. In particular, we focus on the remarkable, but often subtle, roles of metal–metal synergy and metal–support interfaces in determining the reactivity of many different systems for the conversion of CO2 to methanol. Specifically, we explore synthetic and analytical strategies for the systematic study of synergistic behaviors of multi-component catalytic systems in the context of CO2 hydrogenation, and we discuss how the insights obtained can inform the design of materials. We also address limitations of the approach employed and opportunities to expand upon the observations emerging from this work, before attempting to establish transposable and generalizable trends for Cu-based catalysts and beyond.
Carbon monoxide is a ubiquitous molecule, a key feedstock and intermediate in chemical processes. Its adsorption and activation, typically carried out on metallic nanoparticles (NPs), are strongly ...dependent on the particle size. In particular, small NPs, which in principle contain more corner and step-edge atoms, are surprisingly less reactive than larger ones. Hereby, first-principles calculations on explicit Ru NP models (1–2 nm) show that both small and large NPs can present step-edge sites (e.g., B5 and B6 sites). However, such sites display strong particle-size-dependent reactivity because of very subtle differences in local chemical bonding. State-of-the-art crystal orbital Hamilton population analysis allows a detailed molecular orbital picture of adsorbed CO on step-edges, which can be classified as flat (η1 coordination) and concave (η2 coordination) sites. Our analysis shows that the CO π-metal d π hybrid band responsible for the electron back-donation is better represented by an oxygen lone pair on flat sites, whereas it is delocalized on both C and O atoms on concave sites, increasing the back-bonding on these sites compared to flat step-edges or low-index surface sites. The bonding analysis also rationalizes why CO cleavage is easier on step-edge sites of large NPs compared to small ones irrespective of the site geometry. The lower reactivity of small NPs is due to the smaller extent of the Ru–O interaction in the η2 adsorption mode, which destabilizes the η2 transition-state structure for CO direct cleavage. Our findings provide a molecular understanding of the reactivity of CO on NPs, which is consistent with the observed particle size effect.
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