Ai-Min Zhu

Enhanced effect of water vapor on complete oxidation of formaldehyde in air with ozone over MnOx catalysts at room temperature.

At room temperature, the enhanced effect of water vapor on ozone catalytic oxidation (OZCO) of formaldehyde to CO2 over MnOx catalysts and the reaction stability was reported. In a dry air stream, only below 20% of formaldehyde could be oxidized into CO2 by O3. In humid air streams (RH≥55%), ∼100% of formaldehyde were oxidized into CO2 by O3 and the reaction stability was significantly enhanced. Meanwhile, in situ Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectra of OZCO of HCHO demonstrate that the amount of both monodentate and bidentate carbonate species on MnOx, in the dry stream, increased gradually with time on stream (TOS). However, in the humid stream, almost no accumulation of carbonate species on the catalysts was observed. To clarify the enhanced mechanism, formaldehyde surface reactions and CO2 adsorption/desorption on the fresh, O3 and O3+H2O treated MnOx catalysts were examined comparatively.

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.

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.

Atmospheric-pressure plasma CVD of TiO 2 photocatalytic films using surface dielectric barrier discharge

Surface dielectric barrier discharge (DBD) was used for atmospheric-pressure plasma CVD of TiO2 films from TiCl4 and O2 for the first time. Under this experiment, the deposition rate was estimated at 22 nm min −1 by scanning electron microscope observation and the as-deposited TiO2 films were amorphous as evidenced by Raman analysis. The photocatalytic application of TiO2 films in removing HCHO from simulated air was examined in a continuous flow reactor. The TiO2 films after calcination at 350 or 450 ◦ C were notably photocatalytically active for complete oxidation of formaldehyde to an innocuous product (CO2), which was consistent with the results of Raman analysis. Using the TiO2 films, an extremely harmful by-product, CO, was not detected from photocatalytic oxidation of HCHO in a simulated air stream. (Some figures in this article are in colour only in the electronic version)

Modulating effects of the low-frequency source on ion energy distributions in a dual frequency capacitively coupled plasma

With the energy resolved quadrupole mass spectrometer and hybrid simulation, the influence of low-frequency (LF) source parameters on the ion energy distributions (IEDs) of argon ions impinging on the grounded electrode was studied, both experimentally and numerically, in a dual frequency capacitively coupled plasma. It was shown that for decreasing LF or increasing LF power, the high energy peak in IEDs shifts toward the high energy region significantly. The simulation results were in general agreement with the experimental data.

Determination of vibrational and rotational temperatures in a gliding arc discharge by using overlapped molecular emission spectra

Vibrational and rotational temperatures were simultaneously determined in a kilohertz alternating current (ac) gliding arc discharge by using overlapped emission spectra of N2(C 3Πu–B 3Πg) with OH(A 2Σ+–X 2Πi) and with . The simulated emission spectra of OH(A 2Σ+–X 2Πi) and were largely overlapped by radiation transition bands of N2(C 3Πu–B 3Πg) when the rotational temperature was elevated from 500 K to 2500 K. The temporally resolved vibrational and rotational temperatures in a discharge voltage period suggested that the rotational temperature from OH(A 2Σ+–X 2Πi) was remarkably larger than that from N2(C 3Πu–B 3Πg) and . The ratio of number densities of excited electronic states was also determined based on the overlapped emission spectra.

NO Reduction with Hydrogen over Cobalt Molybdenum Nitride and Molybdenum Nitride: A Comparison Study

Cobalt molybdenum nitride (Co3Mo3N) and molybdenum nitride (Mo2N) were investigated for the catalytic reduction of NO with H2. The latter deactivated rapidly with time on stream, whereas the former remained active and stable over a test period of 30 h. The results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and H2-temprature-programmed reduction (H2-TPR) characterization indicated that the deactivation of Mo2N was due to the bulk oxidation of Mo2N to MoO2. As for the Co3Mo3N catalyst, despite partial decomposition into Mo2N and Co, it remained resistant to oxidation. The results suggest that compared to the monometallic nitride, the bimetallic one is more suitable for NO reduction with H2.

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.

Ozone Catalytic Oxidation of HCHO in Air over MnOx at Room Temperature

Abstract The complete oxidation of HCHO in air with O 3 over MnO x catalysts at room temperature was studied. The MnO x catalysts were prepared by a redox method. The catalysts showed amorphous patterns in X-ray diffraction characterization. Formaldehyde in simulated air containing 137 mg/m 3 HCHO with 56% relative humidity (RH, 25 °C) was totally oxidized to CO 2 by 642 mg/m 3 O 3 over the MnO x catalysts at GHSV (gas hourly space velocity) = 2 × 10 5 h −1 . The formaldehyde conversion and CO 2 selectivity were maintained at ∼ 100% during 150 min time-on-stream. The effect of the molar ratio of O 3 to HCHO was also investigated. At a O 3 to HCHO molar ratio of 2:3, which was significantly lower than the stoichiometric ratio, a CO 2 selectivity of 100% was still achieved. No byproduct was detected during HCHO oxidation with O 3 over the MnO x catalysts using an online Fourier transform infrared spectrometer.