Increase In Collision Frequency Research Articles

Promotional effect of short‐chain saturated alcohols on Fe3O4‐catalyzed decomposition of H2O2 and its application in selective oxidation of benzyl alcohol

AbstractBACKGROUNDHydrogen peroxide (H2O2) has been used widely in diverse areas of biotechnology, environmental remediation and organic synthesis wherein its catalytic activation to generate reactive oxygen species (e.g. hydroxyl radicals) is of vital importance. Magnetite (Fe3O4) microspheres are attractive heterogeneous peroxidase‐like catalysts due to many technological advantages (e.g. biocompatible, stable and magnetically recoverable). However, its peroxidase‐like activity has to be improved for practical applications.RESULTSSmall amounts of short‐chain saturated alcohols (e.g. methanol, ethanol, iso‐propanol, tert‐butanol, mono‐ethylene glycol and glycerol) increased collision frequency between H2O2 and Fe3O4 (e.g. by about two orders of magnitude with the addition of 1 mol% glycerol) via facilitating the nucleophilic addition of H2O2 to surface Fe sites and promoted Fe3O4‐catalyzed decomposition of H2O2 and selective oxidation of benzyl alcohol with H2O2 in water. In addition, Fe3O4 microspheres were magnetically recoverable and reusable for at least five times without loss of catalytic activity and selectivity.CONCLUSIONSThe Fe3O4/trace alcohol catalytic system decomposed H2O2 effectively. Its potential application was demonstrated by selective oxidation of benzyl alcohol with H2O2 in water in this work. This simple and efficient catalytic system may be applied also to other oxidative transformations in organic syntheses, or to oxidative degradation of organic pollutants in environmental remediation. © 2019 Society of Chemical Industry

Electrostatic simulations of the radio frequency heating of a non‐neutral plasma in an electron trap

The electrostatic simulations of the radio frequency (RF) heating mechanism, excitations, and ionization process of an electron plasma are carried out using a two‐dimensional (2D) particle‐in‐cell (PIC) code. RF drives with excitation frequencies of 1–15 MHz and amplitudes of 5 and 10 V were applied at two different axial positions, to the centre and to one end on the electrode stack of the ELTRAP device, at ultra‐high vacuum conditions. It is observed that the axial kinetic energy (eV) profile of the confined electrons increases with an increase of the RF excitation amplitudes, and densities from 5 × 107 to 1012 m−3 for all cases under consideration. The simulation results indicate that with continuous RF excitations, the electron heating in the beginning is higher at the trap wall of the device and extends towards the central region of the trap over a simulation time of up to 100 µs. These results on the electron heating are in good agreement with the experimental findings (optical diagnostics of ELTRAP). The heating effect is larger when the RF power is applied from the position close to one end of the trap in comparison to the central position. Monte–Carlo PIC simulations with hydrogen as a background gas are also performed to evaluate the ionization process at pressures of 10−8, 10−7, and 10−6 torr using the same electron plasma densities. The results show that at increasing pressures, the electron‐neutral collisions rate increases linearly with the background gas pressure. Increased collision frequency is obtained at higher RF drive amplitudes, which proportionally increases electron temperature, so that more ionization and secondary electrons are generated.

Effects of Plasma Sheath on the Signal Detection of Narrowband Receiver

When vehicle travels sufficiently quickly, a plasma sheath forms around the vehicle. The plasma sheath strongly affects signal detection by a receiver on the vehicle. This paper mainly investigates the effect of the plasma sheath on the detection of the single-frequency pulse signal. Using an analytical technique referred to the wave impedance calculation method, the transmission coefficient is obtained. The detection probability of transmission signal which is received by a narrowband receiver is then derived. Effects of the incident angle, signal frequency, and parameters of the plasma sheath including electron density and collision frequency on the detection probability are analyzed. Numerical results indicate that the detection probability decreases with increasing electron density and incident angle, while the detection probability first decreases and then increases with increasing collision frequency. Moreover, for high collision frequency, an interesting phenomenon that the detection probability is higher at lower signal frequency than at higher signal frequency occurs in the low-frequency signal range. This is because there is a turning point in the transmission coefficient as well as amplitude attenuation in the low-frequency range.

Porous PdRh nanobowls: facile synthesis and activity for alkaline ethanol oxidation.

Optimizing structure and composition with respect to electrocatalytic performance is critical to achieve outstanding Pd-based electrocatalysts. Herein, we have successfully developed a novel electrocatalyst of hollow and porous PdRh nanobowls (PdRh NBs) for the ethanol oxidation reaction (EOR) by using urea as a guiding surfactant. Under alkaline hydrothermal conditions, urea molecules can release bubbles (NH3 and CO2) that in turn guide the formation of PdRh nanobowls. The porous bowl-like structures of PdRh NBs expose abundant surface sites, which allows for increased collision frequency via confining reactants within open spaces. In regards to composition, the reason for introducing Rh is that not only is the redox potential of Rh approximate with that of Pd (beneficial to the formation of high PdRh alloy phase), but also it can effectively facilitate the breakage of C-C bond on the electrode surface (enhancing the total oxidation of ethanol to CO2). Benefiting from the compositional and structural advantages, the newly developed PdRh NBs exhibit significantly improved electrocatalytic activity for the EOR compared with those of the pure Pd NBs, PdRh nanoparticles (PdRh NPs) and commercial Pd black. These attributes might make them good anodic candidates for application in direct ethanol fuel cells.

Strong decay modes of warm space-charge waves in thermal plasma waveguides

The strong collisional decay modes of warm space-charge waves are derived in a thermal plasma waveguide composed of warm collisional electrons and cold ions. The kinetic theory is applied to obtain the dispersion relation of the space-charge wave. In addition, the analytic expressions of the real part and the damping rate of the wave frequency are obtained for a plasma waveguide including the influence of collision and geometric configuration. It is found that the space-charge wave has the strong decay mode in small wave numbers. In these decay modes, the damping rate increases with an increase of the collision frequency and decreases with an increase of the order of the root of the Bessel function. It is found that the damping rate of the space-charge wave increases with an increase of the radius of the plasma waveguide. It is also found that the space-charge wave is always highly damped in the propagation domain. In addition, the influence of geometric configuration on the damping rate is found to be much significant in high-harmonic cases.

Transient Electrocatalytic Water Oxidation in Single-Nanoparticle Collision

We report the observation of electrocatalytic water oxidation during transient collision and oxidation of single-Ag nanoparticles (NPs) on a gold ultra-microelectrode (UME). Collision and oxidation of single-Ag NPs are studied under higher positive potentials (≥1 V versus Ag/AgCl electrode) on a gold UME than previously used. The use of higher potentials allows us to observe several unique features not seen at lower potentials (0.6–0.8 V), including a greatly increased collision frequency and the appearance of subcollision peaks with a long current tail. Moreover, the average charge transferred in a collision event is 3–5 times more than that estimated from a complete NP oxidation. Our results suggest that the enhanced charge transfer is due to coupled water oxidation catalyzed by Ag oxide transiently formed upon NP collision. A pH-dependence study was further performed to reveal a characteristic feature of water oxidation, and the formation of silver oxide was directly confirmed by single-NP fluorescence...

Confinement of Reactive Oxygen Species in an Artificial-Enzyme-Based Hollow Structure To Eliminate Adverse Effects of Photocatalysis on UV Filters.

Skin cancers caused by UV irradiation have been a major public health problem. One simple and effective way to avoid the above detrimental effects is the use of UV-protective sunscreens. However, there has been considerable concern with the issue of the production of reactive oxygen species (ROS) through the photodegradation of commercial UV filters. Herein, for the first time, it is reported that the integration of ZnO nanoparticles and CeOx nanoparticles into hollow microspheres (ZnO/CeOx HMS) could provide broad-spectrum UV protection and scavenge generated ROS under UV irradiation. Benefiting from the cooperative effect of the hollow structure and the antioxidative activity of CeOx , ROS generated under UV irradiation could be confined to a limited space and effectively conversion into nontoxic molecules is catalyzed as a consequence of increased collision frequency. Therefore, both primary, direct UV-induced damage and secondary ROS toxicity could be greatly reduced.

Density gradient driven microinstabilities and turbulence in ASDEX Upgrade pellet fuelled plasmas

ASDEX Upgrade plasmas fuelled by pellets in the H-mode confinement regime are analyzed. The gyrokinetic code GKW is applied to calculate the microinstabilities which are predicted to be unstable in these plasmas. Two types of density gradient driven modes are found, outside and inside the pellet deposition location. The first mode is driven by a negative radial density gradient, and corresponds to the usual density gradient driven trapped electron mode instability, producing a large diffusive particle flux directed outwards, and becomes more unstable with increasing trapped particle fraction and with decreasing collisionality. The second is driven by a positive radial density gradient (that is, a locally hollow density profile) and is identified for the first time in this work. The instability is located in the proximity of the high field side of the poloidal cross section, and drives a diffusive particle flux directed inward. It is mainly produced by the non-adiabatic response of passing particles with low parallel velocities at high collisionality and it becomes more unstable with increasing passing particle fraction and increasing collision frequency. Nonlinear gyrokinetic turbulence simulations show that these instabilities can lead to saturated turbulence and produce particle diffusion at experimentally relevant levels. In contrast to the usual behavior of the turbulent fields in tokamak plasmas, which have largest fluctuations on the low field side, locally hollow density profiles are prediced to lead to turbulent electrostatic potential and density fluctuations which are maximum on the high field side of the torus.

Whimsicality of multi-mode Hasegawa space-charge waves in a complex plasma containing collision-dominated electrons and streaming ions

The influence of collision-dominated electrons on multi-mode Hasegawa space-charge waves are investigated in a complex plasma containing streaming ions. The dispersion relation for the multi-mode Hasegawa space-charge wave propagating in a cylindrical waveguide filled with dusty plasma containing collision-dominated electrons and streaming ions is derived by using the fluid equations and Poisson’s equation which lead to a Bessel equation. By the boundary condition, the roots of the Bessel function would characterize the property of space-charge wave propagation. It is found that two solutions exist for wave frequency, which are affected by the radius of waveguide and the roots of the Bessel function. The damping and growing modes are found to be enhanced by an increase of the radius. However, an increase of electron collision frequency would suppress the damping and the growing modes of the propagating space-charge wave in a cylindrical waveguide plasma.

Partial coalescence in double (W1/O/W2) emulsions prepared with skimmed milk, polyglycerol polyricinoleate, and different fats

Partial coalescence was studied in double (W1/O/W2) emulsions prepared with skimmed milk, polyglycerol polyricinoleate (PGPR) as lipophilic emulsifier, and different fats. Microstructural and rheological analyses were performed. Encapsulation efficiency (as a parameter of inner water retention) and solid fat content were estimated by differential scanning calorimetry. The presence of PGPR in dispersed lipid phase promoted partial coalescence especially at higher concentration. This promotion of partial coalescence may have been produced by the increase of collision frequency due to protein displacement by PGPR at the outer interface and/or the increase of capture efficiency due to the modification of fat crystals by PGPR. Partial coalescence was also favored when inner water droplets were released as a consequence of an osmotic unbalance between inner and outer aqueous phases. Thus, the addition of glucose in dispersed aqueous phase decreased the partial coalescence degree due to the higher encapsulation efficiency given by the balanced osmotic pressures. With respect to the effect of the employed fat on partial coalescence, the obtained data indicate that the phenomenon was favored at higher solid fat content in lipid phase. Results led to the conclusion that inner osmolality, encapsulation efficiency, and inhibition of partial coalescence were correlated.Practical applications: The originality of this work resides on boarding the subject of partial coalescence in W1/O/W2 emulsions. The studied systems are proposed as potential lipid‐reduced substitutes of dairy creams, with the employment of alternative non‐dairy fats such as low trans vegetable fat. The combined analysis of partial coalescence and encapsulation efficiency allowed studying their reciprocal effects and evaluating the potentiality of the systems for the encapsulation of hydrophilic compounds. The variations of relative osmotic pressures in inner and outer aqueous phases, lipophilic emulsifier concentration, and solid fat content may lead to desirable or undesirable rheological properties depending on the required texture of the food emulsion. The findings of this work could be an important step pointing to control the factors leading to partial coalescence in W1/O/W2 emulsions for food applications.The stability against partial coalescence in W1/O/W2 emulsions containing polyglycerol polyricinoleate (PGPR) and solid fat in lipid phase is linked to the retention of inner water droplets or the encapsulation efficiency of the system. The higher the inner osmolality the higher the encapsulation efficiency; and the higher the encapsulation efficiency the lower the partial coalescence degree. This aggregation of fat globules is attributed to the increase of collision frequency due to protein displacement by PGPR at the outer interface and/or the increase of capture efficiency due to the modification of fat crystals by PGPR.

Influence of the Morphological Characteristics of Separator Membranes on Ionic Mobility in Lithium Secondary Batteries

Microviscosity components influencing the ionic mobility of lithium electrolyte solution in separator membranes were evaluated and the ionic mobility was correlated with membrane morphology. The microviscosity component responsible for the anion–cation Coulombic interactions (α) was anomalously large in low porosity membranes compared to that in the free electrolyte solution, owing to restricted ionic motion in the narrow pore spaces within the membrane. With increase in the membrane porosity, α diminished owing to the enlarged space and motional freedom for the ionic species, whereas βcation, which is responsible for the cation/membrane interactions, appeared. This behavior may be due to the specific charge on the walls of the pore spaces and the increased collision frequency for the ions. Consequently, the cationic mobility (Dcation) selectively decreased, whereas the anionic mobility (Danion) increased in high porosity membranes. Despite the positive correlation between the diffusion coefficient distribution width (σ) and averaged diffusion value for the anion and solvent species, a clear correlation between σ and Dcation was not observed, which is attributed to the fact that the cation is specifically affected by the membrane, perhaps via Coulombic interactions, as evidenced by the appearance of βcation. Controlling the microviscosities of the ions in the electrolyte by controlling the porous morphology of the separator membrane is significant for designing high-performing battery systems.

Mobility shifts when buffer gas temperature increases in ion mobility spectrometry are affected by intramolecular bonds

When buffer gas temperatures increase in ion mobility spectrometry (IMS), ions lose hydration molecules and their mobilities increase. Also, shift reagents (SRs, dopants or modifiers) introduced in the buffer gas, form adducts with the ions and their ion mobilities decrease. Some ions, such as methionine and diamines, are not largely influenced by the introduction of SRs or the buffer gas temperatures because they form intramolecular hydrogen bonds (IHBs). We used electrospray ionization IMS coupled to quadrupole mass spectrometry to study the drift behavior of these ions using data obtained at different temperatures or with different SRs. In this study, we explain the unusually small mobility shifts in methionine and diamine ions when buffer gas temperature increases in IMS based on the breaking of IHBs that increase the ions’ collision cross-sections. We also explain the small mobility shifts in these ions when buffer gas SRs are introduced in the drift tube based on the formation of these IHBs, This study is important because when temperature increases in IMS, mobility shifts had been considered to be affected exclusively by declustering and the increase in collision frequency. This information will help understand the behavior of IHB-forming ions in IMS at different temperatures and when buffer gas SRs are introduced in the drift tube, and could be used to demonstrate the presence of these bonds and measure their binding energy.

Comparison study of electromagnetic wave propagation in high and low pressure Ar inductively coupled plasma

The electromagnetic wave propagation in plasmas has been investigated based on the hypothetical parameters in a number of previous literature, but the results can't mirror the feature of real plasma sources for waves. In this paper, the two-dimension parameters of Ar inductively coupled plasma (ICP) are investigated under high and low pressure by multi-physics coupling analysis method. Meanwhile, the profiles of electron density are diagnosed by the Langmuir probe for comparison with the simulated. The increasing pressure induces the significant increase of oscillation frequency and collision frequency, while a large density gradient causes the more nonuniform distribution. The wave propagation in ICPs is calculated with Z-transform finite-different time-domain method. The curves of attenuation, transmission and reflection coefficient are obtained versus pressure, frequency and power. The low pressure ICP induces the relative more attenuation of wave in the frequency band (1 GHz–6 GHz), while the high pressure ICP is more effective to the frequency band (6 GHz–17 GHz). The increasing power is conductive to the attenuation within limits. The reflection is complicated due to the high degree of wave vector gradient over a wavelength. Moreover, the pseudo secondary wave source is created inside high electron density region of ICP.

Correcting the fundamental ion mobility equation for field effects.

The fundamental ion mobility equation computes the energy-averaged collision cross section as a function of measured drift velocity, electric field strength, ion and neutral masses, and drift gas state parameters. As field strength approaches zero, in particular when the drift velocity drops below about 4% of the average ion-neutral thermal speed, the fundamental equation takes on an especially simple form because the collision frequency and average momentum transfer become indistinguishable from their thermal values. However, in modern high-performance IMS-MS instruments, ion drift velocities may be 10-50% or more of thermal speed, and analysis using the zero-field equation gives rise to erroneously large cross sections. We address this problem by developing correction factors for the zero-field equation from an improved momentum-transfer (MT) theory for ion mobility, corrected and completed herein, and from the well-known two-temperature (2T) theory. The corrected and uncorrected equations are compared by their ability to recover known hard-sphere cross sections from accurately-computed mobility data. Both MT and 2T expressions adjust for the field-driven increase in collision frequency and are noticeably superior to zero-field expression whenever the ion drift velocity is greater than ∼4% of thermal speed. The MT expression also adjusts for the mass and field dependent change in average momentum transfer, and is more accurate than the 2T first approximation whenever the mass of the ionic species is greater than about four times that of drift gas molecules, as is the case in most analytical applications of IMS coupled to MS.

The propagation characteristics of electromagnetic waves through plasma in the near-field region of low-frequency loop antenna

A high-speed vehicle flying through the atmosphere between 100 and 20 km may suffer from a “communication blackout.” In this paper, a low frequency system with an on-board loop antenna to receive signals is presented as a potential blackout mitigation method. Because the plasma sheath is in the near-field region of the loop antenna, the traditional scattering matrix method that is developed for the far-field region may overestimate the electromagnetic (EM) wave's attenuation. To estimate the EM wave's attenuation in the near-field region, EM interference (EMI) shielding theory is introduced. Experiments are conducted, and the results verify the EMI shielding theory's effectiveness. Simulations are also conducted with different plasma parameters, and the results obtained show that the EM wave's attenuation in the near-field region is far below than that in the far-field region. The EM wave's attenuation increases with the increase in electron density and decreases with the increase in collision frequency. The higher the frequency, the larger is the EM wave's attenuation. During the entire re-entry phase of a RAM-C module, the EM wave's attenuations are below 10 dB for EM waves with a frequency of 1 MHz and below 1 dB for EM waves with a frequency of 100 kHz. Therefore, the low frequency systems (e.g., Loran-C) may provide a way to transmit some key information to high-speed vehicles even during the communication “blackout” period.

Effects of Pressure Variation on Polarization Properties of Obliquely Incident RF Waves in Re-Entry Plasma Sheath

Synthesizing the compressible turbulent flow theory and plasma electromagnetic (EM) theory together, a novel method connecting pressure variation and relative permittivity of re-entry plasma sheath is developed. Using this method and transmission line analogy, effects of pressure variation on polarization properties of obliquely incident EM waves at GPS frequency and Ka frequency is studied. Numerical results indicate that effects in different conditions are various. When electron density is lower than $10^{18}~\text{m}^{\mathrm {-3}}$ , influences of pressure variation are obvious at GPS frequency, while Ka frequency is feeble. For GPS frequency, effects enlarge with the increase in incident angle, but reduce with the increase in collision frequency and electron density. Ka frequency has more advantages on mitigating the disturbances of pressure variations in this situation. While in the opposite situation, influences are more remarkable at Ka frequency. Considering that GPS wave rapidly attenuates in the sheath when electron density is higher than $10^{18}~\text{m}^{\mathrm {-3}}$ and Ka wave does not, Ka frequency is an effective way to mitigate the blackout during the re-entry process after its shortcomings on polarization alteration have been carefully considered.

A self-consistent two-fluid model of a magnetized plasma-wall transition

A self-consistent one-dimensional two-fluid model of the magnetized plasma-wall transition is presented. The model includes magnetic field, elastic collisions between ions and electrons, and creation/annihilation of charged particles. Two systems of differential equations are derived. The first system describes the whole magnetized plasma-wall transition region, which consists of the pre-sheath, the magnetized pre-sheath (Chodura layer), and the sheath, which is not neutral, but contains a positive space charge. The second system of equations describes only the neutral part of the plasma-wall transition region—this means only the pre-sheath and the Chodura layer, but not also the sheath. Both systems are solved numerically. The first system of equations has two singularities. The first occurs when ion velocity in the direction perpendicularly to the wall drops below the ion thermal velocity. The second occurs when the electron velocity in the direction perpendicularly to the wall exceeds the electron thermal velocity. The second system of differential equations only has one singularity, which has also been derived analytically. For finite electron to ion mass ratio, the integration of the second system always breaks down before the Bohm criterion is fulfilled. Some properties of the first system of equations are examined. It is shown that the increased collision frequency demagnetizes the plasma. On the other hand, if the magnetic field is so strong that the ion Larmor radius and the Debye length are comparable, the electron velocity in the direction perpendicularly to the wall reaches the electron thermal velocity before the ion velocity in the direction perpendicularly to the wall reaches the ion sound velocity. In this case, the integration of the model equations breaks down before the Bohm criterion is fulfilled and the sheath is formed.

Using magnetic powder to enhance coagulation membrane filtration for treating micro-polluted surface water

The magnetic enhanced coagulation membrane filtration (MECMF) process was introduced into micro-polluted surface water treatment. The process was conducted by adding magnetic powder (MP) for enhancing coagulation. By contrasting the water quality parameters including dissolved organic carbon (DOC), UV254 and turbidity of permeates of MECMF and coagulation/flocculation membrane filtration (CFMF), results showed that the removal efficiency in the MECMF process was higher than those in the CFMF process. According to normalized flux and resistance analysis, membrane flux declined relatively slow and irreversible fouling resistance was lesser in the MECMF process. By analyzing the three-dimensional fluorescence of permeates and raw water, it was clearly shown that the permeate quality of the MECMF process was superior to that of the CFMF process apparently, which highlighted the removal of protein-like substances was more effectively in MECMF. Furthermore, the addition of MP could provide Lorentz and magnetic dipole forces between colloidal matters during coagulation, leading to the increase of collision frequency and efficiency and the formation of large size flocs with high fractal dimension. Large and high fractal dimension flocs could form a porous cake layer, which could increase water permeability. On the basic of the above findings, it was suggested that magnetic-enhanced coagulation that promoted flocs to develop could regulate cake layer structure and mitigate membrane fouling to some extent.

Neutral particle effects on the spin-dependent electron scattering in dense plasmas

The influence of neutral particle collisions on the spin-channel preference for spin-asymmetry scattering is investigated in dense plasmas. The effective electron-electron interaction potential taking into account the electron-neutral collision effects is employed to obtain the scattering cross sections for the spin-triplet and singlet states and spin-asymmetry scattering parameter. It is found that the electron-neutral collision effect enhances the spin-asymmetry scattering parameter as well as the preference for the spin-singlet scattering channel. It is also shown that the preference for the spin-singlet scattering channel increases with an increase of the thermal energy. In addition, it is found that the angular averaged spin-asymmetry parameter decreases with increasing collision frequency and thermal energy. The variations of the spin-singlet and spin-triplet scattering channels are also discussed.

Nonreciprocal Anderson localization in one-dimensional ternary disordered media containing magnetized plasma layers

This paper proposes a one-dimensional random structure composed of three types of alternating layers of dielectric and magnetized plasma materials. By employing the transfer matrix method, the localization lengths of the waves propagating in opposite directions are calculated. The numerical results demonstrate that nonreciprocal features appear in the averaged localization length and individual transmission resonances. However, in the short wavelength regime, the nonreciprocal behavior of the averaged localization length disappears, and the maximum of differential transmission decreases. The author investigates the effects of the external magnetic field, incident angle, collision frequency, and plasma density of the plasma layer on the reciprocal properties. The frequencies at which nonreciprocity occurs depend on the external magnetic field. Thus, it is possible to realize a photonic diode that is tunable with the external magnetic field. Also found is that for small angles of incidence no significant difference exists between the localization lengths of the forward and backward waves. There is a lower limit for the plasma density of the magnetized plasma layers to obtain nonreciprocal Anderson localization. As the collision frequency increases, the nonreciprocal features of the proposed random system survive.