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| 2. |
- Benavidez, A. D., et al.
(författare)
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Environmental Transmission Electron Microscopy Study of the Origins of Anomalous Particle Size Distributions in Supported Metal Catalysts
- 2012
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 2:11, s. 2349-2356
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Tidskriftsartikel (refereegranskat)abstract
- In this Environmental Transmission Electron Microscopy (ETEM) study we examined the growth patterns' of uniform distributions of nanoparticles (NPs) using model catalysts. Pt/SiO2 was heated at 550 degrees C in 560 Pa of O-2 while Pd/carbon was heated in vacuum at 500 degrees C and in 300 Pa of 5%H-2 in Argon at temperatures up to 600 degrees C. Individual NPs of Pd were tracked to determine the operative sintering mechanisms. We found anomalous growth Of NPs occurred during the early stages of catalyst sintering wherein some particles started to grow significantly larger than the mean, resulting in a broadening of the particle size distribution (PSD). The abundance of the larger particles did not fit the log normal distribution. We can rule out sample nonuniformity as a cause for the growth of these large particles, since images were recorded prior to heat treatments. The anomalous growth of these particles may help explain PSDs in heterogeneous catalysts which often show particles that are significantly larger than the mean, resulting in a long tail to the right. It has been suggested previously that particle migration and coalescence could be the likely cause for such broad size distributions. We did not detect any random migration of the NPs leading to coalescence. A directed migration process was seen to occur at elevated temperatures for Pd/carbon under H-2. This study shows that anomalous growth of NPs can occur under conditions where Ostwald ripening is the primary sintering mechanism.
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| 3. |
- Blikstad, Cecilia, et al.
(författare)
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Stereoselective oxidation of aryl-substituted vicinal diols into chiral α-hydroxy aldehydes by re-engineered propanediol oxidoreductase
- 2013
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 3:12, s. 3016-3025
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Tidskriftsartikel (refereegranskat)abstract
- α-Hydroxy aldehydes are chiral building blocks used in synthesis of natural products and synthetic drugs. One route to their production is by regioselective oxidation of vicinal diols and, in this work, we aimed to perform the oxidation of 3-phenyl-1,2-propanediol into the corresponding α‑hydroxy aldehyde applying enzyme catalysis. Propanediol oxidoreductase from E. coli efficiently catalyzes the stereoselective oxidation of S-1,2-propanediol into S-lactaldehyde. The enzyme, however, shows no detectable activity with aryl-substituted or other bulky alcohols. We conducted ISM-driven directed evolution on FucO and were able to isolate several mutants that were active with S-3-phenyl-1,2-propanediol. The most efficient variant displayed a kcat/KM of 40 s-1M-1 and the most enantioselective variant an E-value (S/R) of 80. Furthermore, other isolated variants showed up to 4400-fold increased activity with another bulky substrate, phenylacetaldehyde. The results with engineered propanediol oxidoreductases identified amino acids important for substrate selectivity and asymmetric synthesis of aryl-substituted α-hydroxy aldehydes. In conclusion, our study demonstrates the feasibility of tailoring the catalytic properties of propanediol oxidoreductase for biocatalytic properties.
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| 4. |
- Börner, Tim, et al.
(författare)
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Hydrophobic Complexation Promotes Enzymatic Surfactant Synthesis from Alkyl Glucoside/Cyclodextrin Mixtures
- 2014
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 4:8, s. 2623-2634
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Tidskriftsartikel (refereegranskat)abstract
- The unique ability of cyclodextrin glycosyltransferase to form and utilize the cyclic maltooligosaccharide cyclodextrin (CD) makes this enzyme an attractive catalyst for the synthesis of alkyl glycosides. Here, we characterize the sugar headgroup elongation of alkyl glucosides (acceptor) via two transglycosylation reactions from either a linear (maltohexose) or a cyclic (CD) glycosyl donor. Inclusion complex formation overcomes both poor substrate solubility and aggregation. We have used pure alkyl glucosides and alpha CD as model compounds. The complex between CD and alkyl glucoside was efficiently used as a substrate. Kinetic and thermodynamic measurements allow the prediction of the optimal synthesis conditions. This optimum corresponds to the transition between a donor-limiting and an acceptor-limiting regime. The resulting rational design should lead to the practical development of a cost-efficient industrial synthesis. Our findings with respect to the importance of complexation should also readily apply to other enzymatic systems.
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| 5. |
- Cassimjee, Karim Engelmark, et al.
(författare)
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Active Site Quantification of an omega-Transaminase by Performing a Half Transamination Reaction
- 2011
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 1:9, s. 1051-1055
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Tidskriftsartikel (refereegranskat)abstract
- Measurement of the active enzyme fraction in a given enzyme preparation is a requirement for accurate kinetic measurements and activity comparisons of, for example, engineered mutants. omega-Transaminases, enzymes capable of interconverting ketones and amines by use of pyridoxal-5'-phosphate (PIP), can be used for the production of pharmaceutically important chiral amines but are subject to engineering to meet the practical requirements in synthesis reactions. Therefore, an active site quantification method is needed. Such a method was developed by quantifying the amount of consumed substrate in a virtually irreversible half transamination reaction. (S)-1-phenylethylamine was converted to acetophenone, while the holo enzyme (E-PLP) was converted to apo enzyme with bound pyridoxamine-5'-phosphate (E:PMP). Further, the mass of active enzyme was correlated to the absorbance of the holo enzyme to achieve a direct measurement method. The active Chromobacterium violaceum omega-transaminase with bound PLP can be quantified at 395 nm with an apparent extinction coefficient of 8.1 mM(-1) cm(-1).
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| 6. |
- Ding, Xin, et al.
(författare)
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Visible Light-Driven Water Splitting in Photoelectrochemical Cells with Supramolecular Catalysts on Photoanodes
- 2014
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 4:7, s. 2347-2350
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Tidskriftsartikel (refereegranskat)abstract
- By using a supramolecular self-assembly method, a functional water splitting device based on a photoactive anode TiO2(1+2) has been successfully assembled with a molecular photosensitizer 1 and a molecular catalyst 2 connected by coordination of 1 and 2 with Zr4+ ions on the surface of nanostructured TiO2. On the basis of this photoanode in a three-electrode photoelectrochemical cell, a maximal incident photon to current conversion efficiency of 4.1% at similar to 450 nm and a photocurrent density of similar to 0.48 mA cm(-2) were successfully obtained.
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| 7. |
- Fu, Qiang, et al.
(författare)
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Active sites of Pd-doped flat and stepped Cu(111) surfaces for H 2 dissociation in heterogeneous catalytic hydrogenation
- 2013
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 3:6, s. 1245-1252
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Tidskriftsartikel (refereegranskat)abstract
- It has been shown in recent experiments that the Cu(111) surface doped by a small amount of Pd atoms can exhibit excellent catalytic performance toward the dissociation of H2 molecules. Here we performed systematic first-principles calculations to investigate the corresponding mechanism. Our results clearly demonstrate that a very small number of Pd atoms in the subsurface layer can effectively reduce the energy barrier of H2 dissociation, making the ensembles composed of the surface and contiguous subsurface Pd atoms as the active sites. The catalytic activity can be further improved if the Pd atoms are doped in the stepped Cu surfaces. The impact of the subsurface Pd atoms comes from an enhanced surface-adsorbate interaction caused by adjusting the electronic structure of the substrate. The important role played by the subsurface atoms offers an efficient approach to finely tune the surface activity by a very limited number of atoms. Our findings should be very useful for understanding and improving the catalytic properties of alloy systems for the industrially important hydrogenation reactions.
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| 8. |
- Härelind, Hanna, 1973, et al.
(författare)
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Influence of the Carbon–Carbon Bond Order and Silver Loading on the Formation of Surface Species and Gas Phase Oxidation Products in Absence and Presence of NOx over Silver-Alumina Catalysts
- 2012
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 2:8, s. 1615-1623
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Tidskriftsartikel (refereegranskat)abstract
- The influence of carbon–carbon bond order, here systematically represented by prototypical C2H6, C2H4, and C2H2, on the formation of oxidation products and surface species in the absence and presence of NO has been studied for silver-alumina catalysts with different silver loadings (2 and 6 wt %). The catalysts were prepared with a sol–gel method including freeze-drying, which results in small silver species uniformly distributed throughout the alumina matrix. The performance of the catalysts was investigated by temperature programmed extinction-ignition experiments using a continuous gas flow reactor. The evolution of surface species during reactant step-response experiments was studied in situ by diffuse reflection Fourier transform infrared spectroscopy. The results show that activation for oxidation generally proceeds more easily with increasing bond order of the hydrocarbon. For example, C2H2 shows the highest conversion at low temperatures. Furthermore, the use of hydrocarbons with high bond order, that is, C2H2, as reductant for lean NOx reduction results not only in the highest peak activity but also in considerable high activity in a wide temperature range mainly thanks to high activity at low temperature. With increasing silver loading, the oxidation reactions are favored such that both the hydrocarbon and the NO activation occur at lower temperatures. Several types of adsorbates, for example, carbonate, acetate, formate, enolic and isocyanate/cyanide surface species, are present on the catalyst during reaction. Generally the nature of the surface species and likely also the surface processes are more influenced by the carbon–carbon bond order than the silver loading. This needs to be considered when designing catalysts for emission control systems. Especially for applications using homogeneous fuels with short hydrocarbons, this may provide opportunities to tailor the catalyst functionality for the needs at hand.
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| 9. |
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| 10. |
- Langhammer, Christoph, 1978, et al.
(författare)
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Nanoplasmonic In Situ Spectroscopy for Catalysis Applications
- 2012
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 2:9, s. 2036-2045
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Forskningsöversikt (refereegranskat)abstract
- Indirect nanoplasmonic sensing, INPS, as the key step forward, facilitates the use of nanoplasmonic sensor technology in highly demanding environments in terms of temperature (up to 850 °C, so far), chemical harshness (strongly oxidizing and reducing atmospheres), and pressure for in situ and real time probing of catalyst and other functional nanomaterials. Furthermore, INPS allows for almost infinite material combinations. We also note that the pressure range within INPS can be used is not limited by the sensor or readout principle itself, but rather, by the design of the measurement cell; hence, experiments above atmospheric pressure should be straightforward. The INPS sensor chip features a dielectric spacer layer physically separating the nanoplasmonic sensors from the probed nanomaterial and serving several additional key functions, including (i) protection of the Au nanosensors from the environment and from structural reshaping at high temperature, (ii) providing tailored surface chemistry (support mateial) for the nanomaterial/catalyst to be studied, (iii) being chemically inert or (iv) participating actively in the process under study, e.g., in spillover effects during a catalytic reaction. In principle, any other dielectric material (oxides, nitrides, carbides) that can be deposited as a thin flat or porous film-but also polymers-can be used as the spacer layer/support material for an INPS experiment, depending on the needs of the specific probed system. To date, we have successfully applied the INPS sensing platform to investigate structural and chemical changes of nanomaterials, such as in catalyst sintering processes,19 the oxidation/reduction of Pd nanoparticles, or the storage of NOx species in BaO. We have also applied INPS to scrutinize size effects in the hydride formation process in nanoparticles in the sub-10 nm size range1,16 or to measure in situ changes in adsorbate surface coverage on heterogeneous catalysts at atmospheric pressure.2 Optical nanocalorimetry has been used to measure local temperature changes at the nanolevel and relate the latter, for example, to the activity of a catalyst.1 Furthermore, we have recently applied INPS to study dye molecule adsorption/impregnation of 10-μm-thick mesoporous TiO2 photoanodes in dye-sensitized solar cells by placing the INPS sensor at the hidden, internal interface between the support and the mesoporous TiO2.18 This approach provides a unique opportunity to selectively follow dye adsorption locally in the hidden interface region inside the material and inspires a generic and new type of nanoplasmonic hidden interface spectroscopy that makes highly time-resolved measurements inside a material possible. This first application of hidden interface INPS has thus also prepared grounds for studies of even more realistic catalyst structures comprising a micrometers-thick mesoporous washcoat-like support structure on the INSP chip, loaded with "real" catalyst nanoparticles. Finally, we have also demonstrated first experiments toward single particle INPS spectroscopy in the example of hydride formation in individual Pd and Mg nanoparticles. In summary, owing to its sensitivity, versatility, robustness, compatibility with harsh environments and high temporal resolution in the millisecond range, INPS constitutes a very promising novel experimental platform for the in situ spectroscopy of functional nanomaterials such as catalysts under close-to or real application conditions. The lack of specificity of the readout signal, that is, shifts in the spectral position of the localized plasmon peak of the INPS sensor, requires careful design of experiments and, in some cases, combinations with complementary techniques, such as AFM/ SEM/TEM, or other spectroscopic techniques, such as XPS. Hence, as one important future direction for further development, we identify the direct integration of the INPS function on a sample compatible with simultaneous additional readouts (such as the aforementioned ones but also others, such as quartz crystal microbalance,42 or nonlinear optical spectroscopies, such as SFG) as a high priority. Furthermore, we believe that more efforts directed toward the probing of individual catalyst nanoparticles during a catalytic reaction are well motivated, because of both the promising first proof-ofprinciple experiments already presented and the potential to efficiently circumvent inhomogeneous sample material artifacts. As the main challenges, here, we identify on one hand the optimization of the utilized microspectroscopy for compatibility with high temperatures and, on the other hand, the further optimization of sensitivity and geometrical arrangement of sensor and probed nanoparticle to ultimately be able to probe individual nanoparticles in the sub-10 nm size range under realistic application conditions. © 2012 American Chemical Society.
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