| 1. |
- Beye, Martin, et al.
(author)
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Chemical Bond Activation Observed with an X-ray Laser
- 2016
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In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:18, s. 3647-3651
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Journal article (peer-reviewed)abstract
- The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free electron laser to directly observe the decreased bonding antibonding splitting following bond-activation using an ultrashort optical laser pulse.
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| 2. |
- Bligaard, T., et al.
(author)
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Understanding Heterogeneous Catalysis from the Fundamentals
- 2008
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In: Handbook of Surface Science. - 1573-4331. - 9780444520562 ; 3, s. 269-340
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Journal article (peer-reviewed)abstract
- Catalysis describes the acceleration of a chemical reaction by means of a substance that is itself not consumed by the overall reaction. It is not only important for numerous human activities, but it has also always been a major spur for the development of surface science. Today there is an extensive surface-science heritage of understanding, and there are examples of catalysis phenomena that are now understood from the fundamentals. For instance, modern-day theory is able to predict the turnover frequency of an industrially relevant catalytic reaction in a semi-quantitative way. A major part of this chapter sums up such a successful surface-scientific development, pointing out descriptors for metal catalysts and identifying trends in adsorption energies and activation energies for surface reactions on transition metal surfaces by extensive computations. This is done using the density-functional theory (DFT), whose accuracy in this context is secured, and analyzed in electron-structural terms, in particular the d-band model. Via correlations determined from DFT calculations, universal relationships in heterogeneous catalysis are built up, including variations in catalytic rates, volcano relations. The optimization and design of catalysts through modeling is within reach. For instance, experimental verification for pure CO methanation, for CO2 methanation, and for simultaneous CO and CO2 methanation means that a technical methanation catalyst is discovered on the basis of computational screening. To further detail the surface-science approach to catalysis it is natural to supplement this presentation with some other examples of recent work on some catalytic reactions from the fundamentals. Oxidation of some monoxides illustrates the use of kinetic Monte Carlo simulations. The successful prediction of the outcome of the ammonia synthesis from first-principles supports the view that in the future theory will be a fully integrated tool in the search for the next generation of catalysts. The hydrogen evolution reaction on MoS2 is given as an example of successful interplay between theory and experiment. It is concluded that, thanks to the strong development in surface science, the understanding of heterogeneous catalysis from the fundamentals is approaching an advanced stage. Design of new catalyst on the basis of computational screening is today a realistic perspective. The list of issues that need further considerations includes nonadiabaticity, complex reactions, and other classes of catalyst materials than transition metals.
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| 3. |
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| 4. |
- Moses, PG, et al.
(author)
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Density functional study of the adsorbtion and van der Waals binding of aromatic and conjugated compounds on the basal plane of MoS2
- 2009
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In: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 130:10, s. 104709-
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Journal article (peer-reviewed)abstract
- Accurate calculations of adsorption energies of cyclic molecules are of key importance in investigations of, e.g., hydrodesulfurization (HDS) catalysis. The present density functional theory (DFT) study of a set of important reactants, products, and inhibitors in HDS catalysis demonstrates that van der Waals interactions are essential for binding energies on MoS2 surfaces and that DFT with a recently developed exchange-correlation functional (vdW-DF) accurately calculates the van der Waals energy. Values are calculated for the adsorption energies of butadiene, thiophene, benzothiophene, pyridine, quinoline, benzene, and naphthalene on the basal plane of MoS2, showing good agreement with available experimental data, and the equilibrium geometry is found as flat at a separation of about 3.5 Å for all studied molecules. This adsorption is found to be due to mainly van der Waals interactions. Furthermore, the manifold of adsorption-energy values allows trend analyses to be made, and they are found to have a linear correlation with the number of main atoms. © 2009 American Institute of Physics.
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| 5. |
- Park, Jihye, et al.
(author)
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Two-Dimensional Conductive Ni-HAB as a Catalyst for the Electrochemical Oxygen Reduction Reaction
- 2020
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In: ACS Applied Materials and Interfaces. - : AMER CHEMICAL SOC. - 1944-8244 .- 1944-8252. ; 12:35, s. 39074-39081
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Journal article (peer-reviewed)abstract
- Catalytic systems whose properties can be systematically tuned via changes in synthesis conditions are highly desirable for the next-generation catalyst design and optimization. Herein, we present a two-dimensional (2D) conductive metal-organic framework consisting of M-N-4 units (M = Ni, Cu) and a hexaaminobenzene (HAB) linker as a catalyst for the oxygen reduction reaction. By varying synthetic conditions, we prepared two Ni-HAB catalysts with different crystallinities, resulting in catalytic systems with different electric conductivities, electrochemical activity, and stability. We show that crystallinity has a positive impact on conductivity and demonstrate that this improved crystallinity/conductivity improves the catalytic performance of our model system. Additionally, density functional theory simulations were performed to probe the origin of M-HAB's catalytic activity, and they suggest that M-HAB's organic linker acts as the active site with the role of the metal being to modulate the linker sites' binding strength.
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| 6. |
- Vojvodic, Aleksandra, 1981, et al.
(author)
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Chracterization of MoS2 Magnetic States Using Density-Functional Theory
- 2009
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In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 80, s. 125416-
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Journal article (peer-reviewed)abstract
- It is known that the edges of a two-dimensional slab of insulating MoS2 exhibit one-dimensional metallic edge states, the so-called “brim states.” Here, we find from density-functional theory calculations that several edge structures, which are relevant for the hydrodesulfurization process, are magnetic. The magnetism is an edge phenomenon associated with certain metallic edge states. Interestingly, we find that among the two low-index edges, only the S edge displays magnetism under hydrodesulfurization conditions. In addition, the implications of this on the catalytic activity are investigated. Despite large changes in the magnetic moments, a small influence on the adsorption energies is observed. This has implications on the suitability of magnetic measurements for monitoring the catalytic properties.
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| 7. |
- Wang, Ying, et al.
(author)
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Scaling Relationships for Binding Energies of Transition Metal Complexes
- 2016
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In: Catalysis Letters. - : Springer. - 1011-372X .- 1572-879X. ; 146:2, s. 304-308
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Journal article (peer-reviewed)abstract
- In this work, we investigate the trends in binding energies of different reaction intermediates on transition metal complexes for homogeneous catalysis. We find that linear scaling relationships exist between the binding energies of N and NHx (x = 1, 2), analogous to the bond-order formulation well-known in heterogeneous catalysis. This approach provides a new perspective of reactivity trends in transition metal complexes and opens up the possibility of using a descriptor-based approach for investigating homogeneous catalysis. We show the first steps towards this analysis by analyzing a wide range of modified "Schrock-type" catalysts.
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| 8. |
- Zaborskyte, Greta, et al.
(author)
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Real-Time Monitoring of nfxB Mutant Occurrence and Dynamics in Pseudomonas aeruginosa Biofilm Exposed to Subinhibitory Concentrations of Ciprofloxacin
- 2017
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In: Antimicrobial Agents and Chemotherapy. - : AMER SOC MICROBIOLOGY. - 0066-4804 .- 1098-6596. ; 61:3
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Journal article (peer-reviewed)abstract
- Biofilm infections caused by Pseudomonas aeruginosa are frequently treated with ciprofloxacin (CIP); however, resistance rapidly develops. One of the primary resistance mechanisms is the overexpression of the MexCD-OprJ pump due to a mutation in nfxB, encoding the transcriptional repressor of this pump. The aim of this study was to investigate the effect of subinhibitory concentrations of CIP on the occurrence of nfxB mutants in the wild-type PAO1 flow cell biofilm model. For this purpose, we constructed fluorescent reporter strains (PAO1 background) with an mCherry tag for constitutive red fluorescence and chromosomal transcriptional fusion between the P-mexCD promoter and gfp leading to green fluorescence upon mutation of nfxB. We observed a rapid development of nfxB mutants by live confocal laser scanning microscopy (CLSM) imaging of the flow cell biofilm (reaching 80 to 90% of the whole population) when treated with 1/10 minimal biofilm inhibitory concentration of CIP for 24 h and 96 h. Based on the observed developmental stages, we propose that nfxB mutants emerged de novo in the biofilm during CIP treatment from filamentous cells, which might have arisen due to the stress responses induced by CIP. Identical nfxB mutations were found in fluorescent colonies from the same flow cell biofilm, especially in 24-h biofilms, suggesting selection and clonal expansion of the mutants during biofilm growth. Our findings point at the significant role of high-enough antibiotic dosages or appropriate combination therapy to avoid the emergence of resistant mutants in biofilms.
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