Effect Of Activities Of Manganese, Nickel And Chromium-based Catalysts On The Catalytic Performance Of Non-thermal Plasma Catalysis System For Toluene Oxidation

Non-thermal plasma technique has unique advantage of rapid oxidation of VOCs at roomtemperature and atmospheric pressure. It is a promising solution to treat VOCs pollution.Nowadays, its two challenges are as follows: one is its low energy efficiency, and the other isthat the performance of the plasma catalysis system would be decreased in the presence ofwater vapor in the gas stream. The key to solve these problems is to develop catalysts withhigh activity and durability to water vapor. In this work, a series of manganese, nickel andchromium-based catalysts were prepared, characterized, and evaluated in plasma under bothdry and moist conditions.The effects of preparation conditions for the MnOx/γ-Al2O3catalysts for toluene oxidationwere investigated in a plasma catalysis system (PCS). Results showed that the catalystMnOx/γ-Al2O3with1wt%Mn loading and calcined at500oC had the higher surface area andthe pore volueme compared to the other MnOx/γ-Al2O3catalysts prepared under differentconditions, and the1wt%Mn loading was close to the monolayer dispersion threshold ofMnOx on γ-Al2O3. The toluene conversion of the PCS reached100%at an input energy of21J/l when the MnOx/γ-Al2O3catalyst with1wt%Mn loading and calcined at500oC wasuesed.The performance of the PCSs with a series of transition metal oxides supported on γ-Al2O3was further investigated. The activities of the catalysts decreased in the following order:NiO/γ-Al2O3> MnO2/γ-Al2O3> CeO2/γ-Al2O3> Fe2O3/γ-Al2O3> CuO/γ-Al2O3. The NiOcatalyst exhibited better performance than the other catalysts, which can be attributed to itsgenerating more toluene-destroying oxygen species by decomposing ozone. When the nickelloading of the NiO/γ-Al2O3catalyst was5wt%, which was close to the monolayer dispersionthreshold of NiO on γ-Al2O3, it improved the performance of the PCSs for toluene conversiondue to its high activity.The effect of water vapor on the performance of a PCS with nickel oxide catalysts loadedon different supports (γ-Al2O3、SBA-15、TiO2) was investigated. The activities of catalysts forthe toluene conversion in dry air decreased in the following order: NiO/γ-Al2O3>NiO/SBA-15> NiO/TiO2, which was dependent on the NiO dispersion on the support. Thehigher NiO dispersion led to the higher activity. The presence of water vapor in the feedstream had a significant negative impact on the performance of the PCS. This reduction inperformance was primarily due to the quenching by water vapor of active species in theplasma and the competitive adsorption of water on the catalyst surfaces. The impact of water vapor on the performance of the catalyst decreased in the following order: NiO/γ-Al2O3>NiO/SBA-15> NiO/TiO2. H2O-TPD results indicated that the desorption activation energiesof water vapor on the catalysts decreased in the following order: NiO/γ-Al2O3>NiO/SBA-15> NiO/TiO2. The catalyst with lower water vapor desorption activation energyhad higher resistance to water vapor.A novel in situ FTIR system was constructed and used to obtain in situ FTIR spectra of thereactive surfaces of the catalysts during the plasma catalysis reaction. It revealed that watermolecules adsorbed on the catalyst surfaces came from both water vapor present in the gasstream and from water vapor formed during the oxidation of toluene. The intermediateproducts such as benzaldehyde were formed during the oxidation of toluene in both cases ofthe plasma system and the PCSs. The presence of catalysts MnOx/γ-Al2O3or NiO/γ-Al2O3reduced the formation of these intermediate products. Interestingly, for the PCSs, the loadingof the catalyst MnOx/γ-Al2O3greatly reduced the species of the intermediate products formed,and the loading of the catalyst NiO/γ-Al2O3greatly decreased the amounts of the intermediateproducts formed during the oxidation of toluene.Catalytic performance of MIL-101in the PCS for toluene removal was finally studied.Results showed that MIL-101catalyst had higher performance for toluene decomposition thanγ-Al2O3and CrOx/γ-Al2O3. It was attributed to the higher dispersion of CrOx on the MIL-101than on the γ-Al2O3support, leading to a better performance of decomposing ozone toproduce atom oxygen species. The MIL-101prepared by microwave heating method showedbetter performance compared to hydrothermal synthesis method. The loading of MnOx onMIL-101further promoted its performance. When Ag/γ-Al2O3was placed behind the plasmacatalyst reactor, outlet ozone was nearly eliminated and then the toluene oxidation was furtherimproved.