Søren Kegnæs

Bio: Søren Kegnæs is an academic researcher from Technical University of Denmark. The author has contributed to research in topics: Catalysis & Heterogeneous catalysis. The author has an hindex of 20, co-authored 76 publications receiving 2669 citations. Previous affiliations of Søren Kegnæs include University of Copenhagen.

Molybdenum sulfides—efficient and viable materials for electro - and photoelectrocatalytic hydrogen evolution

Abstract: This perspective covers the use of molybdenum disulfide and related compounds, generally termed MoSx, as electro- or photoelectrocatalysts for the hydrogen evolution reaction (HER). State of the art solutions as well as the most illustrative results from the extensive electro- and photoelectrocatalytic literature are given. The research strategies currently employed in the field are outlined and future challenges pointed out. We suggest that the key to optimising the HER activity of MoS2 is divided into (1) increasing the catalytic activity of the active site, (2) increasing the number of active sites of the catalyst, and (3) improving the electrical contact to these sites. These postulations are substantiated by examples from the existing literature and some new results. To demonstrate the electrocatalytic properties of a highly conductive MoS2 hybrid material, we present the HER activity data for multi-wall MoS2 nanotubes on multi-wall carbon nanotubes (MWMoS2@MWCNTs). This exemplifies the typical data collected for the electrochemical HER. In addition, it demonstrates that the origin of the activity is closely related to the amount of edges in the layered MoS2. The photoelectrocatalytic HER is also discussed, based on examples from literature, with an emphasis on the use of MoSx as either (1) the co-catalyst providing the HER activity for a semiconductor, e.g. Mo3S+4on Si or (2) MoS2 as the semiconductor with an intrinsic HER activity. Finally, suggestions for future catalyst designs are given.

Substrate Size-Selective Catalysis with Zeolite-Encapsulated Gold Nanoparticles

Abstract: Over the years, many strategies have been developed to address the problem of sintering of nanoparticle catalysts, including encapsulating metal nanoparticles in protective shells, and trapping nanoparticles in the cavities of certain zeolites in post-synthesis steps. In general, materials that contain metal nanoparticles that are only accessible via zeolite micropores are intriguing, specifically, but not exclusively, for catalytic applications. The encapsulation of carbon nanoparticles during zeolite crystallization is a well-known approach for making carbon–zeolite composites that afford mesoporous zeolites after combustion. Herein, we show that metal nanoparticles can also be encapsulated during zeolite crystallization, as exemplified by silicalite-1 crystals that are embedded with circa 1–2 nm-sized gold nanoparticles that remain stable and catalytically active after calcination in air at 550 8C. Moreover, we show that the encapsulated gold nanoparticles are only are accessible through the micropores of the zeolite, which makes this material a substrate-size selective oxidation catalyst. Currently, more than 175 different zeolite structures have been reported, and these can be tuned according to the desired acidity and/or redox properties. Expanding the scope from pure zeolites to hybrid materials, by combining the properties of zeolites with other components, significantly widens the field of zeolite materials design. Aside from posttreatment methods, two types of approaches have been pursued for preparing hybrid zeolite–nanoparticle materials. The first type of approach involves crystallization of the zeolite from a gel that contains metal ions that are immobilized in the zeolite during crystallization. With this kind of approach, it is very difficult to control the properties of the non-zeolite component in terms of, for example, particle size. The other type of approach is to first synthesize the nonzeolite component and subsequently encapsulate this in the individual zeolite crystals during crystallization. Indeed, this strategy is also well-known and an entire family of materials, known as mesoporous or hierarchical zeolite crystals, are based on the embedding of carbon nanoparticles, nanofibers, nanotubes, or other nanostructures during zeolite crystallization (and subsequent combustion) in a process known as carbon templating. 15, 16] Concerning the embedding of metal nanoparticles in zeolites, Hashimoto et al. reported a top down approach that features downsizing gold flakes to approximately 40 nm particles by laser ablation, and subsequent encapsulation of these particles during crystallization. A reduction in particle size by one order of magnitude is necessary for an efficient use of costly noble metals in catalytic applications. However, a reduction of the particle size enhances the tendency for sintering, owing to the increase in surface free energy. To mitigate this problem, we report herein a bottom-up approach for the preparation of hybrid zeolite-nanoparticle materials that contain small metal nanoparticles, dispersed throughout the zeolite crystals. This synthetic approach comprises three steps (Figure 1): First, a metal nanoparticle colloid is prepared with suitable anchor points for the generation of a silica shell. Second, the particles are encapsulated in an amorphous silica matrix. Third, the silica nanoparticle precursor is subjected to hydrothermal conditions in order for zeolite crystallization to take place. Using this approach, we successfully prepared a material that consisted predominantly of circa 1–2 nm sized gold particles that were embedded in silicalite-1 crystals. X-ray diffraction revealed that the material contained exclusively gold as well as MFI-structured material (generalized silicalite-1 crystal structure type). Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the hybrid material that consists of gold nanoparticles embedded in silicalite-1 crystals. The SEM image reveals that the material is mainly composed of circa 1–2 mm long coffinshaped crystals, with a minor fraction of intergrown coffin[*] A. B. Laursen, K. T. Højholt, L. F. Lundegaard, S. B. Simonsen, S. Helveg, Prof. C. H. Christensen, K. Egeblad Haldor Topsøe A/S Nymøllevej 55, 2800 Kgs. Lyngby (Denmark) E-mail: chc@topsoe.dk kreg@topsoe.dk

Catalytic Performance of Zeolite‐Supported Vanadia in the Aerobic Oxidation of 5‐hydroxymethylfurfural to 2,5‐diformylfuran

Abstract: The catalytic performance of zeolite-supported vanadia catalysts was examined for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) in organic solvents such as N,N-dimethylformamide (DMF), methyl isobutyl ketone, toluene, trifluorotoluene and DMSO. Catalysts based on the four different zeolite supports H-beta, H-Y, H-mordenite, and H-ZSM-5 with 1–10 wt % vanadia loading were prepared and characterized by nitrogen physisorption, X-ray powder diffraction, scanning electron microscopy, ammonia temperature-programmed desorption, Raman spectroscopy and UV/Vis spectrophotometry. The H-beta zeolite catalysts were found to contain highly dispersed vanadium oxide species at all loadings, and provided the highest reaction selectivity towards DFF and the lowest metal leaching of the examined systems. In particular, 1w t % V 2O5/H-beta was found to be a stable, recyclable, and non-leaching catalyst for the production of DFF under mild conditions in DMF as solvent, although with low DFF yield. To increase the yield, oxidation of HMF at elevated pressures was also investigated with this catalyst. Under optimized conditions, a reaction selectivity towards DFF of > 99 % at 84 % HMF conversion was obtained, albeit with some contribution from lixiviated species to the total catalyst activity.

Porous Organic Polymers Containing Active Metal Centers as Catalysts for Synthetic Organic Chemistry

Abstract: Porous organic polymers (POPs) containing catalytically active mononuclear metal centers can bridge the gap between homogeneous and heterogeneous catalysis. These materials offer catalysts with straightforward control over the active site analogously to homogeneous organometallic catalysts; however, just like classical heterogeneous catalysts, they are easy to separate from reaction mixtures and recycle. The main objective of this Review is to provide an overview of the different types of reactions for synthetic organic chemistry where metal-POP catalysts have been utilized. In addition, a brief description of different synthesis strategies for accessing metal-POPs is included. We also propose a uniform naming system for metal-POP catalysts. Finally, current challenges that could advance the field and facilitate industrial application are discussed.

Effect of Support in Heterogeneous Ruthenium Catalysts Used for the Selective Aerobic Oxidation of HMF in Water

Abstract: Heterogeneous ruthenium-based catalysts were applied in the selective, aerobic oxidation of 5-hydroxymethylfurfural, a versatile biomass-derived chemical, to form 2,5-furandicarboxylic acid. The oxidation reactions were performed in water with dioxygen as the oxidant at different pressures without added base. Catalysts were prepared by depositing catalytically active Ru(OH)x species on a number of different supports, such as titanium-, aluminum-, cerium-, zirconium-, magnesium- and lanthanum oxides, magnetite, spinel, hydrotalcite and hydroxyapatite. All the catalysts were found to be active in the oxidation reactions, and the choice of support was demonstrated to be important for the catalytic performance.

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

Abstract: Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Abstract: Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Liquid Exfoliation of Layered Materials

Abstract: Background Since at least 400 C.E., when the Mayans first used layered clays to make dyes, people have been harnessing the properties of layered materials. This gradually developed into scientific research, leading to the elucidation of the laminar structure of layered materials, detailed understanding of their properties, and eventually experiments to exfoliate or delaminate them into individual, atomically thin nanosheets. This culminated in the discovery of graphene, resulting in a new explosion of interest in two-dimensional materials. Layered materials consist of two-dimensional platelets weakly stacked to form three-dimensional structures. The archetypal example is graphite, which consists of stacked graphene monolayers. However, there are many others: from MoS 2 and layered clays to more exotic examples such as MoO 3 , GaTe, and Bi 2 Se 3 . These materials display a wide range of electronic, optical, mechanical, and electrochemical properties. Over the past decade, a number of methods have been developed to exfoliate layered materials in order to produce monolayer nanosheets. Such exfoliation creates extremely high-aspect-ratio nanosheets with enormous surface area, which are ideal for applications that require surface activity. More importantly, however, the two-dimensional confinement of electrons upon exfoliation leads to unprecedented optical and electrical properties. Liquid exfoliation of layered crystals allows the production of suspensions of two-dimensional nanosheets, which can be formed into a range of structures. (A) MoS 2 powder. (B) WS 2 dispersed in surfactant solution. (C) An exfoliated MoS 2 nanosheet. (D) A hybrid material consisting of WS 2 nanosheets embedded in a network of carbon nanotubes. Advances An important advance has been the discovery that layered crystals can be exfoliated in liquids. There are a number of methods to do this that involve oxidation, ion intercalation/exchange, or surface passivation by solvents. However, all result in liquid dispersions containing large quantities of nanosheets. This brings considerable advantages: Liquid exfoliation allows the formation of thin films and composites, is potentially scaleable, and may facilitate processing by using standard technologies such as reel-to-reel manufacturing. Although much work has focused on liquid exfoliation of graphene, such processes have also been demonstrated for a host of other materials, including MoS 2 and related structures, layered oxides, and clays. The resultant liquid dispersions have been formed into films, hybrids, and composites for a range of applications. Outlook There is little doubt that the main advances are in the future. Multifunctional composites based on metal and polymer matrices will be developed that will result in enhanced mechanical, electrical, and barrier properties. Applications in energy generation and storage will abound, with layered materials appearing as electrodes or active elements in devices such as displays, solar cells, and batteries. Particularly important will be the use of MoS 2 for water splitting and metal oxides as hydrogen evolution catalysts. In addition, two-dimensional materials will find important roles in printed electronics as dielectrics, optoelectronic devices, and transistors. To achieve this, much needs to be done. Production rates need to be increased dramatically, the degree of exfoliation improved, and methods to control nanosheet properties developed. The range of layered materials that can be exfoliated must be expanded, even as methods for chemical modification must be developed. Success in these areas will lead to a family of materials that will dominate nanomaterials science in the 21st century.

Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets

Abstract: Promising catalytic activity from molybdenum disulfide (MoS2) in the hydrogen evolution reaction (HER) is attributed to active sites located along the edges of its two-dimensional layered crystal structure, but its performance is currently limited by the density and reactivity of active sites, poor electrical transport, and inefficient electrical contact to the catalyst. Here we report dramatically enhanced HER catalysis (an electrocatalytic current density of 10 mA/cm2 at a low overpotential of −187 mV vs RHE and a Tafel slope of 43 mV/decade) from metallic nanosheets of 1T-MoS2 chemically exfoliated via lithium intercalation from semiconducting 2H-MoS2 nanostructures grown directly on graphite. Structural characterization and electrochemical studies confirmed that the nanosheets of the metallic MoS2 polymorph exhibit facile electrode kinetics and low-loss electrical transport and possess a proliferated density of catalytic active sites. These distinct and previously unexploited features of 1T-MoS2 make ...