| 1. |
- Hellman, Anders, 1974, et al.
(författare)
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Ammonia Synthesis: State of the Bellwether Reaction
- 2013
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Ingår i: Comprehensive Inorganic Chemistry II (Second Edition): From Elements to Applications. - 9780080965291 ; 7, s. 459-474
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic ammonia synthesis has been judged to be one of mankind's greatest scientific achievements during the twentieth century. The socioeconomic implications of producing ammonia industrially have been a strong driving force, and this development has spurred a range of new discoveries within physics, chemistry, and chemical engineering. In this chapter, we describe how it has been possible in recent years to provide a full understanding of the catalytic ammonia synthesis reaction at the atomic level through the combined use of experiments and quantum mechanical electronic structure calculations, thus clearly showing many of the reasons why ammonia synthesis has been, and still is, the bellwether reaction in heterogeneous catalysis.
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| 2. |
- Hellman, A, et al.
(författare)
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Predicting catalysis : understanding ammonia synthesis from first-principles calculations
- 2006
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Ingår i: Journal of Physical Chemistry B. - 1520-6106 .- 1520-5207. ; 110, s. 17719-17735
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Tidskriftsartikel (refereegranskat)abstract
- Here, we give a full account of a large collaborative effort toward an atomic-scale understanding of modern industrial ammonia production over ruthenium catalysts. We show that overall rates of ammonia production can be determined by applying various levels of theory (including transition state theory with or without tunneling corrections, and quantum dynamics) to a range of relevant elementary reaction steps, such as N(2) dissociation, H(2) dissociation, and hydrogenation of the intermediate reactants. A complete kinetic model based on the most relevant elementary steps can be established for any given point along an industrial reactor, and the kinetic results can be integrated over the catalyst bed to determine the industrial reactor yield. We find that, given the present uncertainties, the rate of ammonia production is well-determined directly from our atomic-scale calculations. Furthermore, our studies provide new insight into several related fields, for instance, gas-phase and electrochemical ammonia synthesis. The success of predicting the outcome of a catalytic reaction from first-principles calculations supports our point of view that, in the future, theory will be a fully integrated tool in the search for the next generation of catalysts.
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| 3. |
- Vang, Ronnie T., et al.
(författare)
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Ethylene dissociation on flat and stepped Ni(111): A combined STM and DFT study
- 2006
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 600, s. 66-77
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Tidskriftsartikel (refereegranskat)abstract
- The dissociative adsorption of ethylene (C2H4) on Ni(1 1 1) was studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM studies reveal that ethylene decomposes exclusively at the step edges at room temperature. However, the step edge sites are poisoned by the reaction products and thus only a small brim of decomposed ethylene is formed. At 500 K decomposition on the (1 1 1) facets leads to a continuous growth of carbidic islands, which nucleate along the step edges. DFT calculations were performed for several intermediate steps in the decomposition of ethylene on both Ni(1 1 1) and the stepped Ni(2 1 1) surface. In general the Ni(2 1 1) surface is found to have a higher reactivity than the Ni(1 1 1) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the different reaction pathways. In particular the barrier for dissociation is lowered significantly more than the barrier for dehydrogenation, and this is of great importance for the bond-breaking selectivity of Ni surfaces. The influence of step edges was also probed by evaporating Ag onto the Ni(1 1 1) surface. STM shows that the room temperature evaporation leads to a step flow growth of Ag islands, and a subsequent annealing at 800 K causes the Ag atoms to completely wet the step edges of Ni(1 1 1). The blocking of the step edges is shown to prevent all decomposition of ethylene at room temperature, whereas the terrace site decomposition at 500 K is confirmed to be unaffected by the Ag atoms. Finally a high surface area NiAg alloy catalyst supported on MgAl2O4 was synthesized and tested in flow reactor measurements. The NiAg catalyst has a much lower activity for ethane hydrogenolysis than a similar Ni catalyst, which can be rationalized by the STM and DFT results.
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| 4. |
- Diaz, C, et al.
(författare)
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Multidimensional effects on dissociation of N2 on Ru(0001)
- 2006
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Ingår i: Physical Review Letters. - 0031-9007. ; 96:9, s. 096102-
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Tidskriftsartikel (refereegranskat)abstract
- The applicability of the Born-Oppenheimer approximation to molecule-metal surface reactions is presently a topic of intense debate. We have performed classical trajectory calculations on a prototype activated dissociation reaction, of N2 on Ru(0001), using a potential energy surface based on density functional theory. The computed reaction probabilities are in good agreement with molecular beam experiments. Comparison to previous calculations shows that the rotation of N2 and its motion along the surface affect the reactivity of N2 much more than nonadiabatic effects.
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| 5. |
- Frondelius, P., et al.
(författare)
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Charging of atoms, clusters, and molecules on metal-supported oxides: A general and long-ranged phenomenon
- 2008
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 78:8, s. 7-
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Tidskriftsartikel (refereegranskat)abstract
- The density-functional theory is used to investigate the adsorption of Au atoms, Au clusters, and NO2 molecules on transition-metal-supported oxides. As compared to unsupported oxides, the adsorbates on supported oxide films are charged and experience a higher adsorption energy. The origin of the effect is explored by considering two different oxides (MgO and Al2O3) and a range of supporting metals. Moreover, the limits of the enhancement are probed by explicit calculations for thick MgO films and low coverage. The long-range character of the phenomenon is attributed to electrostatic polarization. The absolute strength depends on several contributions and their relative importance changes with system composition.
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| 6. |
- Hellman, Anders, 1974, et al.
(författare)
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Including lateral interactions into microkinetic models of catalytic reactions
- 2007
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 127:19, s. 6-
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Tidskriftsartikel (refereegranskat)abstract
- In many catalytic reactions lateral interactions between adsorbates are believed to have a strong influence on the reaction rates. We apply a microkinetic model to explore the effect of lateral interactions and how to efficiently take them into account in a simple catalytic reaction. Three different approximations are investigated: site, mean-field, and quasichemical approximations. The obtained results are compared to accurate Monte Carlo numbers. In the end, we apply the approximations to a real catalytic reaction, namely, ammonia synthesis. (c) 2007 American Institute of Physics.
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| 7. |
- Honkala, K., et al.
(författare)
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Water Dissociation on MgO/Ag(100): Support Induced Stabilization or Electron Pairing?
- 2010
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:15, s. 7070-7075
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory is used to compare water splitting on MgO(100) and MgO/Ag(100). Adsorption is considered on terraces and step edges. On the terrace, the limits of low and monolayer coverage are explored. The metal support enhances the adsorption energies of isolated OH and H through electron exchange with the oxide/metal interface, forming either OH- or H+. As a consequence, the nature of the bonding of dissociated H2O on MgO(100) is different as compared to that on MgO/Ag(100). The bonding on MgO(100) is governed by electron pairing in the oxide whereas bonding to MgO/Ag(100) is determined by the interaction with the oxide/metal interface. The difference in bond character is manifested in a red shift (similar to 200 cm(-1)) of hydrogen-bonded O-H stretching vibrations when MgO is supported.
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| 8. |
- Kauppinen, Minttu Maria, 1991, et al.
(författare)
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The role of polaronic states in the enhancement of CO oxidation by single-atom Pt/CeO 2
- 2023
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Ingår i: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 423, s. 26-33
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Tidskriftsartikel (refereegranskat)abstract
- Single Atom Catalysts (SACs) have shown that the miniaturization of the active site implies new phenomena like dynamic charge transfer between isolated metal atoms and the oxide. To obtain direct proof of this character is challenging, as many experimental techniques provide averaged properties or have limitations with poorly conductive materials, leaving kinetic measurements from catalytic testing as the only reliable reference. Here we present an integrated Density Functional Theory-Microkinetic model including ground and metastable states to address the reactivity of Pt1/CeO2 for CO oxidation. Our results agree with experimentally available kinetic data in the literature and show that CO oxidation activity of Pt1/CeO2 is tunable via the electronic properties of the support. Particularly, samples with higher n-doping via oxygen depletion should be better in CO oxidation, as they help maintain the active state Pt0 of the catalyst. This provides an alternative strategy for tuning the performance of low-temperature oxidations in single-atom catalysts via charge transfer control.
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| 9. |
- Motiani, P., et al.
(författare)
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Exercise training alters lipoprotein particles independent of brown adipose tissue metabolic activity
- 2019
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Ingår i: Obesity science & practice. - : WILEY. - 2055-2238. ; 5:3, s. 258-272
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Tidskriftsartikel (refereegranskat)abstract
- Introduction New strategies for weight loss and weight maintenance in humans are needed. Human brown adipose tissue (BAT) can stimulate energy expenditure and may be a potential therapeutic target for obesity and type 2 diabetes. However, whether exercise training is an efficient stimulus to activate and recruit BAT remains to be explored. This study aimed to evaluate whether regular exercise training affects cold-stimulated BAT metabolism and, if so, whether this was associated with changes in plasma metabolites. Methods Healthy sedentary men (n = 11; aged 31 [SD 7] years; body mass index 23 [0.9] kg m(-2); VO2 max 39 [7.6] mL min(-1) kg(-1)) participated in a 6-week exercise training intervention. Fasting BAT and neck muscle glucose uptake (GU) were measured using quantitative [F-18]fluorodeoxyglucose positron emission tomography-magnetic resonance imaging three times: (1) before training at room temperature and (2) before and (3) after the training period during cold stimulation. Cervico-thoracic BAT mass was measured using MRI signal fat fraction maps. Plasma metabolites were analysed using nuclear magnetic resonance spectroscopy. Results Cold exposure increased supraclavicular BAT GU by threefold (p < 0.001), energy expenditure by 59% (p < 0.001) and plasma fatty acids (p < 0.01). Exercise training had no effect on cold-induced GU in BAT or neck muscles. Training increased aerobic capacity (p = 0.01) and decreased visceral fat (p = 0.02) and cervico-thoracic BAT mass (p = 0.003). Additionally, training decreased very low-density lipoprotein particle size (p = 0.04), triglycerides within chylomicrons (p = 0.04) and small high-density lipoprotein (p = 0.04). Conclusions Although exercise training plays an important role for metabolic health, its beneficial effects on whole body metabolism through physiological adaptations seem to be independent of BAT activation in young, sedentary men.
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