26 research outputs found
Extended Wiener-Khinchin theorem for quantum spectral analysis
The classical Wiener-Khinchin theorem (WKT), which can extract spectral
information by classical interferometers through Fourier transform, is a
fundamental theorem used in many disciplines. However, there is still need for
a quantum version of WKT, which could connect correlated biphoton spectral
information by quantum interferometers. Here, we extend the classical WKT to
its quantum counterpart, i.e., extended WKT (e-WKT), which is based on
two-photon quantum interferometry. According to the e-WKT, the
difference-frequency distribution of the biphoton wavefunctions can be
extracted by applying a Fourier transform on the time-domain Hong-Ou-Mandel
interference (HOMI) patterns, while the sum-frequency distribution can be
extracted by applying a Fourier transform on the time-domain NOON state
interference (NOONI) patterns. We also experimentally verified the WKT and
e-WKT in a Mach-Zehnder interference (MZI), a HOMI and a NOONI. This theorem
can be directly applied to quantum spectroscopy, where the spectral correlation
information of biphotons can be obtained from time-domain quantum interferences
by Fourier transform. This may open a new pathway for the study of light-matter
interaction at the single photon level.Comment: 13 pages, 5 figure
Optimization of Zn<sub><i>x</i></sub>Fe<sub>3ā<i>x</i></sub>O<sub>4</sub> Hollow Spheres for Enhanced Microwave Attenuation
We report here the composition optimization
of Zn<sub><i>x</i></sub>Fe<sub>3ā<i>x</i></sub>O<sub>4</sub> hollow nanospheres for enhancing microwave attenuation.
Zn<sub><i>x</i></sub>Fe<sub>3ā<i>x</i></sub>O<sub>4</sub> hollow nanospheres were synthesized through a simple
solvothermal process. The maximum magnetization moment of 91.9 emu/g
can be obtained at <i>x</i> = 0.6. The composite filled
with Zn<sub>0.6</sub>Fe<sub>2.4</sub>O<sub>4</sub> exhibited the bandwidth
of 3.21ā8.33 GHz for RL < ā10 dB and a maximum relative
bandwidth (<i>W</i><sub>p,max</sub>) of 88.6% at optimized
thickness <i>t</i><sub>0</sub> = 0.34 cm. The enhancement
should be attributed to the enhanced permeability resonance at high
frequency. This optimized hollow material is very promising to be
used as a mass efficient and broadband microwave attenuation material
High-Performance NiāFe Redox Catalysts for Selective CH<sub>4</sub> to Syngas Conversion via Chemical Looping
In
traditional steam reforming of CH<sub>4</sub>, the CH<sub>4</sub> conversion
and its selectivity to CO and H<sub>2</sub> are thermodynamically
limited. In this work, we designed a series of NiāFe redox
catalysts with varying Ni/Fe ratios. The NiāFe redox catalysts
could function as oxygen carriers to selectively convert CH<sub>4</sub> to syngas via chemical looping. The selectivity to CO was dramatically
enhanced via a selective conversion route of CH<sub>4</sub> to C and
H<sub>2</sub> in the reduction, followed by C gasification to syngas
with hot steam. Taking the advantages of the highly reactive Ni species
for CH<sub>4</sub> activation and Fe species for water splitting,
together with the resulting NiFe alloy in the reduced catalyst for
catalytic CH<sub>4</sub> decomposition, high CH<sub>4</sub> conversion
up to 97.5% and CO selectivity up to 92.9% were achieved at 900 Ā°C
with productivity of CO and H<sub>2</sub> of 9.6 and 29.0 mol kg<sub>catalyst</sub><sup>ā1</sup>, respectively, on equimolar NiāFe
catalyst
Sub-Surface Boron-Doped Copper for Methane Activation and Coupling: First-Principles Investigation of the Structure, Activity, and Selectivity of the Catalyst
Copper (Cu) is a
commercial catalyst for the synthesis of methanol
from syngas, low-temperature water gas shift reaction, oleo-chemical
processing, and for the fabrication of graphene by chemical vapor
deposition. However, high barriers for CāH bond activation
and the ease of formation of carbon/graphene on its surface limits
its application in the utilization and conversion of methane to bulk
chemicals. In the present paper, using first-principles calculations,
we predict that Cu catalyst doped with a monolayer of sub-surface
boron (BāCu) can efficiently activate the CāH bond of
methane and can selectively facilitate the CāC coupling reaction.
Boron binds strongest at the sub-surface octahedral site of Cu and
the thermodynamic driving force for the diffusion of B from an on-surface
to the sub-surface position in Cu is stronger than that for the experimentally
synthesizable BāNi (sub-surface boron in nickel) catalyst,
providing a proof of concept for the experimental synthesis of this
novel catalyst. Additionally, the first-principles computed free energy
of the reaction to form BāCu from boron precursor and Cu is
also favorable. The presence of the monolayer sub-surface B in Cu
creates a corrugated step-like structure on the Cu surface and significantly
brings down the methane CāH activation barrier from 174 kJ/mol
on Cu(111) to only 75 kJ/mol on BāCu. The subsequent dehydrogenation
of the adsorbed CH<sub>3</sub>* to CH<sub>2</sub>* is also kinetically
and thermodynamically feasible. Our calculations also suggest that,
unlike most of the transition metals, complete decomposition of methane
to carbon would not be favored on BāCu. The dissociation of
the surface CH<sub>2</sub>* moiety on BāCu is limited due to
the high activation barrier of 161 kJ/mol and lower relative stability
of the resultant CH* species, under reaction conditions. The coupling
of CH<sub>2</sub>* fragments however is kinetically and thermodynamically
favorable, with an activation barrier of only 92 kJ/mol; suggesting
that BāCu catalyst would have higher selectivity toward C<sub>2</sub> hydrocarbons. Furthermore, the formation of carbon from the
adsorbed CH* moiety has a very high activation barrier of 197 kJ/mol
and the completely dehydrogenated C* is relatively much less stable
than CH*, under reaction conditions; predicting that coking might
not be an issue on the BāCu catalyst. Evaluation of CāH
activation on Cu(110) surface, which has a similar step-like surface
structure as BāCu, and Bader charge and density of states analyses
of BāCu reveal that the geometrical/corrugation effect and
the charge transfer from B to Cu synergistically promote the CāH
activation on BāCu, making it as active as other expensive
transition metals like Rh, Ru, Ir, and Pt
Hydrogenation of Furfural as Model Reaction of Bio-Oil Stabilization under Mild Conditions Using Multiwalled Carbon Nanotube (MWNT)-Supported Pt Catalysts
Pt nanoparticles supported on multiwalled
carbon nanotubes (MWNT)
were synthesized and applied in the catalytic hydrogenation of furfural,
the model reaction of mild stabilization of reactive bio-oil. The
catalytic properties of the catalysts were modified by changing the
surface concentration of oxygen-containing groups (OCGs) of MWNT.
Various characterization techniques, including fluorescent labeling,
nitrogen physisorption, powder X-ray diffraction, transmission electron
microscopy, and in situ DRIFTS analysis, were employed to study both
the supports and catalysts. Pt nanoparticles supported on MWNT with
high OCGs show high activity and selectivity, attributed to the formation
of small Pt particles as a result of the high concentration of OCGs
Charge Transfer between Metal Clusters and Growing Carbon Structures in Chirality-Controlled Single-Walled Carbon Nanotube Growth
Synthesis of single-walled carbon nanotubes (SWCNTs) with specific chirality has been a great challenge. The detailed role of catalyst clusters in chirality-selective growth of SWCNTs is still unclear. We studied armchair (5,5), chiral (6,5), and zigzag (9,0) nanotube growths on a relaxed Ni<sub>55</sub> cluster. Although adhesion energies and chemical potentials of growing carbon structures only show small differences, charges are evidently transferred (or redistributed) from Ni atoms to the growing end edges of nanotubes, which enhance the reactivity of carbon edges. Different chiral nanotubes exhibit distinct reaction active sites. (5,5) has five identical double-carbon active sites, while (9,0) has nine single-carbon active sites. (6,5) has a kink site with the highest reaction activity. These findings imply that the structures of metal clusters strongly correlate with nanotube growth sites through charge transfer (or redistribution). Potential opportunities exist in enabling (<i>n</i>,<i>m</i>) selective growth by engineering charge transfer between metal clusters and growing carbon structures
Effective Nitrogen Removal and Recovery from Dewatered Sewage Sludge Using a Novel Integrated System of Accelerated Hydrothermal Deamination and Air Stripping
In
order to reduce considerable emissions of N-containing pollutants
from combustion of sewage sludge derived solid fuel, an integrated
system of hydrothermal deamination and air stripping was developed
to effectively remove and recover nitrogen from dewatered sewage sludge
(DSS). Three characteristic hydrothermal regimes contributing to deamination
were identified. Initial hydrolysis of inorganic-N and labile protein-N
was responsible for ammonium (NH<sub>4</sub><sup>+</sup>-N) released below 300 Ā°C/9.3 MPa, whereas
deamination of pyridine-N dominated when being raised to 340 Ā°C/15.5
MPa. At 380 Ā°C and 22.0 MPa, remarkable deamination of stable
protein-N occurred, which was accompanied by formation of more heterocyclic-N
compounds and resulted in 76.9% N removal from DSS and 7980 mg/L NH<sub>4</sub><sup>+</sup>-N solution. As
a result of catalytic hydrolysis and cracking, calcium oxide additive
not only accelerated deamination of stable protein-N, pyrrole-N, and
pyridine-N, but also favored transformations of protein-N and quaternary-N
to nitrile-N and pyridine-N, respectively, leading to 86.4% total
N removal efficiency. The nitrogen transformation reactions and conversion
pathways during hydrothermal deamination were proposed and elaborated
in detail. Moreover, an efficient air stripping process was coupled
to remove and recover ammonia from liquid fraction via ammonium sulfate.
Consequently, this system achieved an overall N recovery rate of 62%
White and Grey Matter Changes in the Language Network during Healthy Aging
<div><p>Neural structures change with age but there is no consensus on the exact processes involved. This study tested the hypothesis that white and grey matter in the language network changes during aging according to a ālast in, first outā process. The fractional anisotropy (FA) of white matter and cortical thickness of grey matter were measured in 36 participants whose ages ranged from 55 to 79 years. Within the language network, the dorsal pathway connecting the mid-to-posterior superior temporal cortex (STC) and the inferior frontal cortex (IFC) was affected more by aging in both FA and thickness than the other dorsal pathway connecting the STC with the premotor cortex and the ventral pathway connecting the mid-to-anterior STC with the ventral IFC. These results were independently validated in a second group of 20 participants whose ages ranged from 50 to 73 years. The pathway that is most affected during aging matures later than the other two pathways (which are present at birth). The results are interpreted as showing that the neural structures which mature later are affected more than those that mature earlier, supporting the ālast in, first outā theory.</p></div
Neural structural changes with age in Group 1.
<p>(A) shows FA changes. Note that the skeletonized results are āthickenedā to help visualization. Left SLF-IFC/Prg, left superior longitudinal fasciculus underlying the inferior frontal cortex and precentral gyrus; Left SLF-TP, left superior longitudinal fasciculus underlying the temporal-parietal association cortex; Left and right IFOF-IFC, left and right inferior fronto-occipital fasciculus underlying the inferior frontal cortex; Right FM-MeFC, right forceps minor underlying the medial frontal cortex; Left bCC, left body of corpus callosum. (B) shows cortical thickness changes. The colored blobs (blue for cortical thickness, red for FA) indicate brain areas that correlated negatively with age. Left IFC, left inferior frontal cortex; Left Prg, left precentral gyrus. No positive correlations were found.</p
Linear regression fit results.
<p>(A) shows fitting result when using grand-averaged cortical thickness as the dependent variable in Group 1. (B) shows fitting result when using age as the dependent variable in Group 1. (C) Re-examination of Group 1's ROI in Group 2's data. Age is the dependent variable. Note that the red line is the linear fit result, whereas the green lines are the confidence internal (95%). For all three panels, the x-axis corresponds to the actual age or cortical thickness, whereas the y-axis corresponds to the age or cortical thickness produced by the regression model.</p