Xue-Feng Yang

Removal of formaldehyde from gas streams via packed-bed dielectric barrier discharge plasmas

Formaldehyde is a major indoor air pollutant and can cause serious health disorders in residents. This work reports the removal of formaldehyde from gas streams via alumina-pellet-filled dielectric barrier discharge plasmas at atmospheric pressure and 70 °C. With a feed gas mixture of 140 ppm HCHO, 21.0% O2, 1.0% H2O in N2, ~92% of formaldehyde can be effectively destructed at GHSV (gas flow volume per hour per discharge volume) of 16 500 h−1 and Ein = 108 J l−1. An increase in the specific surface area of the alumina pellets enhances the HCHO removal, and this indicates that the adsorbed HCHO species may have a lower C–H bond breakage energy. Based on an examination of the influence of gas composition on the removal efficiency, the primary destruction pathways, besides the reactions initiated by discharge-generated radicals, such as O, H, OH and HO2, may include the consecutive dissociations of HCHO molecules and HCO radicals through their collisions with vibrationally- and electronically-excited metastable N2 species. The increase of O2 content in the inlet gas stream is able to diminish the CO production and to promote the formation of CO2 via O-atom or HO2-radical involved reactions.

Plasma-assisted CVD of hydrogenated diamond-like carbon films by low-pressure dielectric barrier discharges

Hydrogenated diamond-like carbon (DLC) films have been deposited on silicon substrates from dielectric barrier discharge (DBD) plasmas of CH4 at room temperature under a pressure of 0.4-4.0 Torr. The effects of discharge gas pressure (P), the applied peak voltage and the distance of the discharge gas spacing (d) on the film quality have been systematically investigated. The film hardness is mainly dependent on the Pd value and applied peak voltage. The best films of 40-70 mm with Knoop hardness up to 20 GPa can be deposited at 30 kV peak voltage with a Pd value of about 2 Torr mm. The deposited films were characterized by scanning electron microscopy, Raman and FTIR spectroscopy. These analyses show that the deposited films are homogeneous hydrogenated amorphous carbon films with very smooth surfaces containing significant amounts of sp3 C-C bonding. The high-voltage and current waveform measurements of the discharge indicate that the low-pressure DBD consists of uniform (along the whole electrode) glow-like single breakdowns with half-widths of several microseconds. The DBD-induced deposition technique used in this work has many advantages, including the simplicity of the experimental set-up, large area deposition of DLC films and a lower consumption of feed gas and electric power.

Low-temperature plasma-catalytic oxidation of formaldehyde in atmospheric pressure gas streams

Formaldehyde (HCHO) is a typical air pollutant capable of causing serious health disorders in human beings. This work reports plasma-catalytic oxidation of formaldehyde in gas streams via dielectric barrier discharges over Ag/CeO2 pellets at atmospheric pressure and 70 °C. With a feed gas mixture of 276 ppm HCHO, 21.0% O2, 1.0% H2O in N2, ~99% of formaldehyde can be effectively destructed with an 86% oxidative conversion into CO2 at GHSV of 16500 h−1 and input discharge energy density of 108 J l−1. At the same experimental conditions, the conversion percentages of HCHO to CO2 from pure plasma-induced oxidation (discharges over fused silica pellets) and from pure catalytic oxidation over Ag/CeO2 (without discharges) are 6% and 33% only. The above results and the CO plasma-catalytic oxidation experiments imply that the plasma-generated short-lived gas phase radicals, such as O and HO2, play important roles in the catalytic redox circles of Ag/CeO2 to oxidize HCHO and CO to CO2.

Observations of H3− and D3− from dielectric barrier discharge plasmas

Abstract This work reports the reliable experimental observations of the simplest negative triatomic ions, H 3 − and D 3 − anions, the stability of which has been debated for several decades, from dielectric barrier discharge hydrogen and deuterium plasmas. The observed H 3 − and D 3 − mass signals with widely distributed kinetic energies, from a few eV to ∼100 eV, are contributed from their ground states with lifetimes greater than 8–35 μs. The agreement between theoretical and experimental results for H 3 − has been reached. The three-body collision process is proposed to be the dominant formation mechanism for the H 3 − and D 3 − anions.

Diagnosis of dielectric barrier discharge CH4 plasmas for diamond-like carbon film deposition

Abstract Dielectric barrier discharge (DBD) CH4 plasmas during diamond-like carbon (DLC) film deposition have been characterized in-situ by means of optical emission spectrometry (OES), the Langmuir double probe method and molecular beam mass spectrometry (MBMS). With a 1.4 kHz, 30-kV peak voltage DBD power source, while the Pd-value (the product of CH4 pressure P, and discharge gas spacing d) decreases from ∼14 to 4 torr mm, the measured electron temperature and the hydrogen atom excitation temperature of the CH4 plasmas rise from ∼3.0 to 5.8 eV, and from ∼6.3×103 to 7.8×103 K, respectively. The higher electron temperature and H excitation temperature of the plasmas at smaller Pd, imply the generation of more energetic ions in the plasma sheath near the film surface, that is confirmed by the MBMS observations. The MBMS experiments also show that the major ions near the coating are CH3+, CH2+, CH+ and C+, which are mainly produced through the collisions between fast CHx+ (x=1–4) and neutral CH4 molecules in the plasma sheath region.

Diamond-like Carbon Films Deposited in the Plasma of Dielectric Barrier Discharge at Atmospheric Pressure

Dielectric barrier discharge at atmospheric pressure has been applied to prepare diamond-like hydrogenated amorphous carbon films on glass and silicon substrates from CH4–H2 mixtures. The coating with Knoop hardness up to 10 GPa can be deposited under CH4-to-H2 ratio of 1:2 and substrate temperature of 300°C. The IR absorption analysis of the coating shows, that its hydrocarbon group ratio [CH3/(CH2+CH)] and C–C bond type ratio (sp3C/sp2C for CH and CH2 groups) are 20%:80% and 15:1, respectively.

Atomic hydrogen determination in medium-pressure microwave discharge hydrogen plasmas via emission actinometry

Atomic hydrogen plays an important role in the chemical vapour deposition of functional materials, plasma etching and new approaches to the chemical synthesis of hydrogen-containing compounds. This work reports experimental determinations of atomic hydrogen in microwave discharge hydrogen plasmas formed from the TM01 microwave mode in an ASTeX-type reactor, via optical emission spectroscopy using Ar as an actinometer. The relative intensities of the H atom Balmer lines and Ar-750.4 nm emissions as functions of input power and gas pressure have been investigated. At an input microwave power density of 13.5 W cm−3, the approximate hydrogen dissociation fractions calculated from electron-impact excitation and quenching cross sections in the literature, decreased from ~0.08 to ~0.03 as the gas pressure was increased from 5 to 25 Torr. The influences of the above cross sections, and the electron and gas temperatures of the plasmas on the determination of the hydrogen dissociation fraction data have been discussed.

Formation of NOx from N2 and O2 in catalyst-pellet filled dielectric barrier discharges at atmospheric pressure

At temperatures above 350 degrees C, significant amounts of NOx formed from N2 and O2 have been observed in Cu-ZSM-5 catalyst-pellet filled dielectric barrier discharges, indicating the necessity of using low-temperature performance in all plasma-catalytic processes for removal of air pollutants.

The reactions and composition of the surface intermediate species in the selective catalytic reduction of NOx with ethylene over Co-ZSM-5

A pretreatment-transient reaction product analysis method was applied to study the reactions and average composition of the possible surface intermediate species in selective catalytic reduction with ethylene of NOx over Co-ZSM-5. The reactions of the surface species, formed by the pretreatment of Co-ZSM-5 in a NO/C2H4/O2 mixture at 275°C, with the NO/O2 flow produced much more N2 than that with the individual NO or O2 flow. The similarity of N2/COx/H2O product distribution generated from the above surface species-NO/O2 reactions and that from the normal NO/C2H4/O2 flow reactions implies that the surface species NCaObHc formed in the three-component pretreatment process is very likely the primary intermediate surface species generated during the real flow reactions. The in situ FT-IR (DRIFT) spectroscopy measurements of the surface species support the above conclusion.

Observations of long-lived H−2 and D−2 ions from non-thermal plasmas

Strong mass signals of H−2 and D−2 ions have been observed from low-pressure dielectric barrier discharge hydrogen and deuterium plasmas via molecular beam mass spectrometry. The observed H−2/H− and D−2/D− ratios (~0.35–0.4) are over five orders of magnitude higher than those observed by other techniques. The kinetic energy of H−2 and D−2 ions sampled from the plasmas was determined to be widely distributed, from a few eV to >100 eV, giving lifetimes greater than ~40 µs for H−2 and ~55 µs for D−2. The highest vib-rotational excitation of neutral H2 species in the plasma was determined to be about J = 0, v = 5 or J = 19, v = 0 via threshold ionization mass spectrometry. The possible pumping mechanisms for generating H−2 with further high J, required by the current high-rotation model, have been proposed. Similar to the lifetime of D−2 determined recently by another group, the H−2 lifetime observed in this work is about two orders of magnitude longer than that predicted by the current theoretical model. To explain these experimental observations regarding the meta-stability of long-lived H−2 and D−2 ions, the improved current high-rotation model or other new models, including the possible existence of some long-lived electronically excited states of H−2/D−2, need to be developed.