| Plasma-catalytic deep oxidation of methane?CH4?is a potential technology for CH4 emission control at low temperature.However,the mechanism of CH4 plasma-catalytic oxidation is still unidentified,which prevents further optimization of deep-oxidation process.Therefore,a mechanism study on plasma-catalytic oxidation of CH4 is urgently required.The aim of this study is to propose a detailed reaction mechanism for CH4 oxidation over Au/?-Al2O3 catalyst,and give out the characteristics of OH production in a dielectric barrier discharge?DBD?reactor.In this study,decomposition of CH4 in 10%O2?He balance?in a tubular DBD reactor was firstly carried out.Influences of Au loadings and temperature on its decomposition were investigated.Products of CH4 decomposition were analyzed by using in-situ mass spectrometry?MS?.The catalysts used in this study were characterized by N2-sorption,X-ray diffraction?XRD?,X-ray photoelectron spectra?XPS?,inductively coupled plasma-optical emission spectrometer?ICP-OES?,transmission electron microscopy?TEM?and Fourier-transform infrared?FT-IR?spectroscopy.The mechanism on plasma-catalytic oxidation of CH4 was proposed.OH group plays a major role in plasma-catalytic oxidation of CH4.The characterization of OH group?309 nm?in O2/H2O gas mixture was also investigated in a planar DBD reactor.Influences of applied peak voltage,the frequency,the discharge gap distance,and the relative humidity on OH group production were also discussed.The spatial distribution and intensity of OH?A-X?were investigated by using a digital single-lens reflex camera and Image-Pro+ software.OH production and consumption pathways were discussed based on the results of a numerical analysis of O2/H2O discharge.The main results are summarized as follows:?1?When the temperature was in the range of 25-150 ?,CH4 conversions increased with the increasing temperature;CH4 conversions with catalyst balls were almost twice to that without catalyst balls.At room temperature,CH4 adsorption decreased with the increasing gold loading;when gold loading was 2wt.%,catalysts lost adsorption capacity of CH4.Besides,CH4 conversions were almost constant whatever Au loadings changed,while CO2 selectivity changed greatly.When using?-Al2O3 as a catalyst,CO2 selectivity was only 15%.In contrast,when using Au/?-Al2O3,CO2 selectivity was as high as 80%and almost constant with the increasing gold loading.It can be inferred that CH4 is firstly decomposed to CH3 and H by discharges which are then mainly adsorbed on ?-Al2O3 surface.?2?From the BETs,TEM and XPS characterization analysis results,the BET surface areas and pore volumes of ?-Al2O3 balls and Au/?-Al2O3 balls did not significantly change with the increasing Au loadings.The decreased CH4 adsorption could not be explained by the change of physical properties of the ?-Al2O3 balls.Meanwhile,from the TEM results,the surface of ?-Al2O3 balls was not completely covered by Au clusters,indicating the adsorption sites on the surface of ?-Al2O3 balls still existed.XPS results showed that the decreasing CH4 adsorption may be caused by the electric charges attraction between lattice O sites and Au on ?-Al2O3.The electric charge reducing led to CH4 adsorption decreasing.?3?From the MS analysis for CH4 decomposition products,CH4 were mainly decomposed to CO2 and H2O.With discharges,the H2O adsorbed on catalyst surface was replaced by reactive oxygen species;when the temperature was more than 100 ?,the amount of H2O was reduced.Oxygen atoms produced by discharges could react with H2O to yield surface reactive OH group,those contribute CH3 oxidation.XPS results showed 5%of Au0 could be converted to Au+ with discharge,which should be in the form of Au+-OH-.?4?The results of FT-IR spectrum analysis and temperature-programmed CO2 desorption experiment revealed that Auwas beneficial to the decomposition of surface carbonate species into gaseous CO2,where the carbonate species accumulated on the y-Al2O3 when Au was absent.?5?The mechanism of CH4 plasma-catalytic oxidation is as follow:CH4 is firstly converted into CH3 and H by discharges.For the DBD reactor coupled with y-Al2O3 balls,CH3 and H are adsorbed on alumina surface to form CH3?ad?most posibly on lattice O atoms,and H2O?ad?on Al sites.The desorption of H2O?ad?from Al sites can be attributed to the replacement of H2O?ad?by plasma-produced O atoms.CH3?ad?on lattice O sites is step-by-step dehydrated and finally form C?ad?on lattice O sites.The C?ad?is then further oxidized to CO?ad?which is chelating with O atoms on Al sites to yield bidentate carbonate species,where the bidentate carbonate species is confirmed from FT-IR analysis results.This bidentate carbonate species can not be readily converted to gaseous CO2.This mechanism indicates that CH3 is terminated at bidentate carbonate species,and thus prevent the deep oxidation into CO2.By loading Au on alumina surface,deep oxidation to CO2 is found.It is speculated that he bidentate carbonate species on Au and Al2O3 can migrate to Au cluster.Bidentate carbonate species on Au cluster are then desorbed by plasma-produced 0 atoms.CO2 desorption from Au sufficiently improves Au/alumina's capability to complete the catalysis cycle for CH3 and H oxidation to CO2 and H2O.?6?The OH?A-X?intensity and discharge power were linearly dependent on the tested parameters.It increased with increasing the frequency and peak voltage but decreased with increasing the relative humidity and discharge gap.Besides,the OH?A-X?intensity linearly increased with the discharge power.When the discharge powers were at a similar level,increasing the applied voltage resulted in the largest OH?A-X?intensity,indicating it was the most effective way for the OH production.?7?O?1D?radical was responsible for OH production.The OH was mainly produced by a reaction between O?1D?and H2O?O?1D?+H2O?20H?.While,OH radical itself and O?3P?radical contributed mostly to the OH consumption?2OH?H2O2;O?3P?+OH?O2+H?.Therefore,increasing the concentration of O?1D?radical or decreasing the concentration of O?3P?radical was an alternative way to enrich the OH radical in a plasma process. |
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