One of the key challenges in developing quantum networks is to generate single photons with high brightness, purity, and long temporal coherence. Semiconductor quantum dots potentially satisfy these ...requirements; however, due to imperfections in the surrounding material, the coherence generally degrades with increasing excitation power yielding a broader emission spectrum. Here we overcome this power-broadening regime and demonstrate an enhanced coherence at exciton saturation where the detected count rates are highest. We detect single-photon count rates of 460 000 counts per second under pulsed laser excitation while maintaining a single-photon purity greater than 99%. Importantly, the enhanced coherence is attained with quantum dots in ultraclean wurtzite InP nanowires, where the surrounding charge traps are filled by exciting above the wurtzite InP nanowire band gap. By raising the excitation intensity, the number of possible charge configurations in the quantum dot environment is reduced, resulting in a narrower emission spectrum. Via Monte Carlo simulations we explain the observed narrowing of the emission spectrum with increasing power. Cooling down the sample to 300 mK, we further enhance the single-photon coherence twofold as compared to operation at 4.5 K, resulting in a homogeneous coherence time, T2, of 1.2 ns, and two-photon interference visibility as high as 83% under strong temporal postselection (~5% without temporal postselection).
The hydrogenation of a dichloromethane soluble fraction of flash pyrolysis oil (bio-oil, BO), obtained by treatment of BO with a water–dichloromethane solvent mixture, was investigated using a ...water-soluble homogeneous ruthenium catalyst (RuCl
3·3H
2O/tris(
m-sulfonatophenyl)phosphine, TPPTS). The catalyst is active at mild conditions (<70
°C, 45
bar hydrogen) and particularly the levels of aldehydes in the BO fraction are reduced considerably. Model studies using vanillin (
1), acetoguaiacone (
2) and
iso-eugenol (
3) (45
bar hydrogen and 45–70
°C) showed that vanillin is the most reactive and unexpectedly forms creosol (
6) instead of the vanillylalcohol (
5). An optimisation study was performed on the latter reaction and the highest TOF (36.4
mol/(mol
h)) was obtained at 60
°C, 45
bar hydrogen and a NaI concentration of 0.08
M. Kinetic studies imply that the formation of
6 proceeds via the intermediate alcohol
5 in a series type of mechanism. TEM-EDX measurements suggest that the reactions are most likely catalyzed by single metal homogeneous Ru complexes and not by Ru nanoparticles/colloids.
The use of water-soluble homogeneous ruthenium catalysts (Ru–TPPTS, prepared
in situ by reacting RuCl
3·3H
2O and tris(
m-sulfonatophenyl)phosphine, TPPTS) to hydrogenate the dichloromethane soluble fraction of fast pyrolysis oil is reported. Significant reductions in the aldehyde content were observed at mild conditions (<70
°C, 45
bar hydrogen). Model studies with typical lignin derived compounds were performed and it was shown that vanillin (
1) is hydrogenated to creosol (
6) via the intermediate vanillylalcohol (
5).
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A comprehensive experimental and modelling study on the acid-catalysed hydrolysis of the water hyacinth plant (
Eichhornia crassipes) to optimise the yield of levulinic acid (LA) is reported (
T
=
...150–175
°C,
C
H
2
SO
4
=
0.1
–
1
M
, water hyacinth intake
=
1–5
wt%). At high acid concentrations (>0.5
M), LA was the major organic acid whereas at low acid concentrations (<0.1
M) and high initial intakes of water hyacinth, the formation of propionic acid instead of LA was favoured. The highest yield of LA was 53
mol% (35
wt%) based on the amount of C6-sugars in the water hyacinth (
T
=
175
°C,
C
H
2
SO
4
=
1
M
, water hyacinth intake
=
1
wt%). The LA yield as a function of the process conditions was modelled using a kinetic model originally developed for the acid-catalysed hydrolysis of cellulose and good agreement between the experimental and modelled data was obtained.
A variety of interesting bulk chemicals is accessible by the acid-catalyzed hydrolysis of cellulose. An interesting example is levulinic acid, a versatile precursor for fuel additives, polymers, and ...resins. A detailed kinetic study on the acid-catalyzed hydrolysis of cellulose to levulinic acid is reported in this paper. The kinetic experiments were performed in a temperature window of 150−200 °C, sulfuric acid concentrations between 0.05 and 1 M, and initial cellulose intakes between 1.7 and 14 wt %. The highest yield of levulinic was 60 mol %, obtained at a temperature of 150 °C, an initial cellulose intake of 1.7 wt %, and a sulfuric acid concentration of 1 M. A full kinetic model covering a broad range of reaction conditions was developed using the power-law approach. Agreement between the experimental data and the kinetic model is good. The kinetic expressions were used to gain insights into the optimum process conditions for the conversion of cellulose to levulinic acid in continuous-reactor configurations. The model predicts that the highest obtainable levulinic acid yield in continuous-reactor configurations is about 76 mol %, which was obtained when using reactors with a large extent of backmixing.
Lignocellulosic biomass is a key feedstock for the sustainable production of biofuels, biobased chemicals and performance materials. Biomass can be efficiently converted into pyrolysis liquids (also ...known as bio-oils) by the well-established fast pyrolysis technology. Currently, there is significant interest in the application of fast pyrolysis technology as principle biomass conversion technology due to its feedstock flexibility, low cost and high energy conversion efficiency, with many emerging commercial enterprises being established around the globe. Upgrading of the bio-oils is a requisite, and is complicated by its complex and heterogeneous organic nature. Pyrolysis liquids may be further separated by a simple water fractionation, yielding an aqueous sugar-rich phase and a water-insoluble pyrolytic lignin (PL) fraction. This separation step allows the use of dedicated conversion strategies for each fraction, which can be highly advantageous due to their differences in composition and reactivity. For example, the sugar-rich fractions can be used for fermentation, while the phenolic-rich PL is a particularly promising feedstock for the production of a wide range of platform chemicals and energy-dense streams upon depolymerization. To aid the emerging use of PL, novel characterization techniques and valorization strategies are being explored. In this review, the fast pyrolysis process and PL characterization efforts are discussed in detail, followed by the state-of-the-art regarding PL processing using both oxidative and reductive (catalytic) strategies, as well as a combination thereof. Possible applications are discussed and recommendations for future research are provided.
Pyrolysis oil from lignocellulosic biomass can be fractionated into a lignin and sugar fraction. We here provide a review on the structure, properties, depolymerisation strategies and applications for pyrolytic lignin in the framework of a biorefinery.
•Cassava starch grafted with acrylic acid shows superabsorbent properties.•More than half of the absorbed water is retained under severe suction.•A mathematical analysis was made to assess the ...mechanisms of water absorption.•Polymer chain relaxations are the governing factor at the most important conditions.
An important application of starch grafted with copolymers from unsaturated organic acids is the use as water absorbent. Although much research has been published in recent years, the kinetics of water absorption and the swelling behavior of starch based superabsorbents are relatively unexplored. Also, water retention under mechanical strain is usually not reported. Cassava starch was used since it has considerable economic potential in Asia. The gelatinized starch was grafted with acrylic acid and Fenton's initiator and crosslinked with N,N′-methylenebisacrylamide (MBAM). Besides a good initial absorption capacity, the product could retain up to 63g H2O/g under severe suction. The material thus combines a good absorption capacity with sufficient gel strength. The mathematical analysis of the absorption kinetics shows that at conditions of practical interest, the rate of water penetration into the gel is determined by polymer chain relaxations and not by osmotic driven diffusion.
Quantum states can be stabilized in the presence of intrinsic and environmental losses by either applying an active feedback condition on an ancillary system or through reservoir engineering. ...Reservoir engineering maintains a desired quantum state through a combination of drives and designed entropy evacuation. We propose and implement a quantum-reservoir engineering protocol that stabilizes Fock states in a microwave cavity. This protocol is realized with a circuit quantum electrodynamics platform where a Josephson junction provides direct, nonlinear coupling between two superconducting waveguide cavities. The nonlinear coupling results in a single-photon-resolved cross-Kerr effect between the two cavities enabling a photon-number-dependent coupling to a lossy environment. The quantum state of the microwave cavity is discussed in terms of a net polarization and is analyzed by a measurement of its steady state Wigner function.
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