| Emission of volatile organic compounds (VOCs) from industrial and automobile sources has caused serious environmental problems, since it contributes significantly to the formation of photochemical smog and secondary aerosol in the urban areas. As a result, research on the abatement of VOCs has been attracting increasingly public attention. Although traditional methods such as adsorption, thermal oxidation, and catalytic combustion have been adopted to remove VOCs from exhaust gases, these techniques often encounter efficient and economical problems when treating the exhaust gas with a concentration of VOCs below 100 ppm. Moreover, the global COX selectivity also needs to be further improved, especially for industrial applications. A promising approach to tackle this matter is to combine non-thermal plasmas (NTP) with catalysts. NTP generated at atmospheric pressure and room temperature is considered to be energy efficient because of fast ignition response and generation of highly energetic radicals which contribute to plasma chemistry reactions. On the other hand, a synergy effect is possible to overcome the low selectivity problem through suitable catalysts integration with plasmas.In this work, a surface acidification method was employed to modify the TiO2 catalyst for the enhancement of its surface activity, which was tested by degrading gaseous xylene in a dielectric barrier discharges (DBD) reactor with a spiral high-voltage electrode inside. The main research conclusions are as follows:(1) Both Lewis and Bronsted acid sites on the surface of the catalyst had been increased, Lewis acidity increasing from 0.018 to 0.045 mmol/g and Bronsted acidity from 0.002 to 0.015 mmol/g, which probably led to the activity promotion of TiO2 catalyst in the degradation process, since Bronsted sites destroyed the stable construction of xylene and Lewis sites facilitated the ozone decomposition reaction. At the peak voltage of 14 kV and specific input energy (SIE) of 828 J/L, xylene conversion ratios achieved by the untreated and acidized TiO2 catalysts were 86.00% and 92.48%; the COx selectivity were 95.16% and 98.53%. FTIR results show that formic acid (HCOOH) was formed as a byproduct from xylene, without any other ring-retaining products being observed compared with the photocatalytic process.(2) SIE and the residence time of VOCs in the DBD reactor decreased while the entering flow rate increased, which resulted in the sharp decline of xylene conversion ratios achieved by the catalysts. At the voltage of 14 kV, conversion ratios achieved by untreated TiO2 catalyst were 85.97%,79.12% and 68.85% when the residence time were 9.04,6.46 and 5.02 s respectively, and they were 92.48%,87.45 and 81.30% with the acidized catalysts. The electrons generated by NTP would be more likely to react with water molecules than oxygen in the carrier gas so that OH radicals were preferentially formed and the oxidations of VOCs and intermediates would be accelerated. Otherwise, OH radicals would also be formed by the consumption of O3 under the humid condition and it was found that aerosol formed in the reactor would negatively influence on the electric discharge and activity of the catalysts. |
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