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.

An experimental study on the conversion of NO in the NO/N2, NO/O2/N2, NO/C2H4/N2 and NO/C2H4/O2/N2 systems has been carried out using dielectric barrier discharge (DBD) plasmas at atmospheric pressure. In the NO/N2 system, NO decomposition to N2 and O2 is the dominating reaction; NO conversion to NO2 is less significant. O2 produced from NO decomposition was detected by an on-line mass spectrometer. With the increase of NO initial concentration, the concentration of O2 produced decreases at 298 K, but slightly increases at 523 K. In the NO/O2/N2 system, NO is mainly oxidized to NO2, but NO conversion becomes very low at 523K and over 1.6% of O2. In the NO/C2H4/N2 system, NO is reduced to N2 with about the same NO conversion as that in the NO/N2 system but without NO2 formation. In the NO/C2H4/O2/N2 system, the oxidation of NO to NO2 is dramatically promoted. At 523 K, with the increase of the energy density, NO conversion increases rapidly first, and then almost stabilizes at 93–91% of NO conversion with 61–55% of NO2 selectivity in the energy density range of 317–550 J L−1. It finally decreases gradually at high energy density. A negligible amount of N2O is formed in the above four systems. Of the four systems studied, NO conversion and NO2 selectivity of the NO/C2H4/O2/N2 system are the highest, and NO/O2/C2H4/N2 system has the lowest electrical energy consumption per NO molecule converted.

Investigations on the combination of the dielectric barrier discharge plasma with Co-HZSM-5 catalyst for the selective catalytic reduction (SCR) of NOx by lower hydrocarbons were presented. Compared with the reductant of CH4 and C2H4, C2H2 showed a broader low temperature activity in the SCR of NOx with significant O2 tolerance, as well as enhanced CO2 selectivity under the combination of plasma with the catalyst. With a reactant gas mixture of 500 ppm NOx, 500 ppm C2H2, 15% oxygen in N2 and space velocity of 12000 h-1. The NOx conversion is higher than 50% in the temperature range of 150–450ºC in this plasma-assisted process. Larger than 90% of NOx conversion was obtained at 300ºC and Ein = 138 J l-1. © 2005 Elsevier B. V. All rights reserved.

Strong mass signals of H2− and D2− ions have been observed from low-pressure dielectric barrier discharge hydrogen and deuterium plasmas via molecular beam mass spectrometry. The observed H2−/H− and D2−/D− ratios (～0.35–0.4) are over five orders of magnitude higher than those observed by other techniques. The kinetic energy of H2− and D2− 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 H2− and ∼55 μs for D2− . The highest vibrotational 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 H2− with further high J, required by the current high-rotation model, have been proposed. Similar to the lifetime of D2− determined recently by another group, the H2− 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-livedH2− andD2− ions, the improved current high-rotation model or other new models, including the possible existence of some long-lived electronically excited states of H2−/D2−, need to be developed.

This article reports observations of significant synergistic effects between dielectric barrier discharge (DBD) plasmas and Cu-ZSM-5 catalysts for C2H4 selective reduction of NOx at 250◦C in the presence of excess oxygen by using a one-stage plasma-over-catalyst (POC) reactor. With a reactant gas mixture of 530 ppm NO, 650 ppm C2H4, 5.8% O2 in N2, GHSV=12 000 h−1 and input discharge energy density of 155 J L−1, the pure catalytic, pure plasma-induced (discharges over fused silica pellets) and plasma-catalytic (in the POC reactor) NOx conversion percentages are 39%, 1.5% and 79%, respectively. A moderate plasma enhancement of NOx reduction by C2H4 was also observed in a two-stage plasma-followed-by-catalyst (PFC) reactor consisting of a discharge stage filled by fused silica pellets and a Cu-ZSM-5 catalyst stage.