A coaxial nonthermal plasma (NTP) reactor was used for the removal of nitric oxide in mist. The effects of discharge gap, thorn number, tooth slice number as well as discharge polarity on NO oxidation and NOx removal were investigated. Decreasing the discharge gap is favorable for the NO oxidation and NOx removal. The mist is almost captured completely even under a relative low input energy. Increasing the thorn number of discharge slice or tooth slice number facilitates the energy input in the plasma reactor and thus enhances the NO oxidation and NOx removal under a fixed applied voltage. The energy efficiency for the positive DC (4.5 g NO/kWh) is three times as much as that for the negative DC (1.5 g NO/kWh), corresponding to NO oxidation efficiency of around 80%.

The effect of Mn on the catalytic performance of V2O5/TiO2 catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR) has been investigated in this study. It was found that the added Mn significantly enhanced the activity of V2O5/TiO2 catalyst for NH3-SCR below 400 °C. The redox cycle (V4+ + Mn4+ ↔ V5+ + Mn3+) over Mn-promoted V2O5/TiO2 catalyst plays a key role for the high catalytic deNOx performance. The redox cycle promotes the adsorption and activation of NH3 and NO, forming more reactive intermediates (NH4+, coordinated NH3, NO2, and monodentate nitrate species), thus promoting the NH3-SCR to proceed.

•PAHs concentrations decreasing order: winter > spring > autumn > summer•Vehicle emission, coal combustion and biomass burning are dominant sources of indoor/outdoor PAHs•Indoor sources are found, including camphor pollution in dormitory while camphor pollution and cooking in residential home•The annual averages of BaPeq in four sites > the annual limit of 1 ng/m3•The Lifetime Lung Cancer Risk of PAHs in four sites > the acceptable levelThree indoor (residential home, dormitory, and office) and one outdoor concentrations of PM2.5-bound Polycyclic aromatic hydrocarbons (PAHs) were analyzed in Beijing across four seasons. The highest and lowest concentration of total PAHs for outdoor appeared in winter and in summer with averages of 200.1 and 9.1 ng/m3 respectively. The seasonal variations of total PAHs in three indoor sites were the same as outdoor. The correlation analysis between the indoor and outdoor samples showed that the annual mean I/O ratios of total PAHs in the three sites were lower than 1. Source apportionment showed vehicle exhaust, coal combustion, and biomass burning were the major contributors of indoor and outdoor PM2.5-bound PAHs. Indoor source, such as camphor pollution, was identified in the dormitory, while camphor pollution and cooking sources were identified in the residential home. The annual averages of Benzo[a]pyrene equivalent concentration (BaPeq) were 7.6, 7.8, 7.7 and 12.7 ng/m3 for the dormitory, office, residential home and outdoor samples respectively, far higher than the annual limit of 1 ng/m3 regulated by European Commission. Life lung cancer risk (LLCR) in four sites across four seasons were over the acceptable cancer risk level, showing the cancer risk were at a high level in both indoor and outdoor sites in Beijing, and its level in indoor sites was much lower than in the outdoor site. The health risk assessment indicated the level of PAHs cancer risk on human for three indoor sites were similar. The results call for the development of more stringent control measures to reduce PAHs emissions.Download high-res image (271KB)Download full-size image

A series of Pt–Au/CeO2 catalysts were prepared via the impregnation deposition–precipitation (IDP) and reduction–deposition precipitation (RDP) methods. The performances of the catalysts for the simultaneous removal of carbon monoxide (CO) and formaldehyde (HCHO) at room temperature were evaluated. The results show that the Pt–Au/CeO2 catalyst, which was prepared via the RDP method, exhibited higher catalytic activity. The catalyst characterization results reveal that two factors accounted for this phenomenon. The first factor is that more negatively charged metallic Pt nanoparticles were obtained during the liquid phase NaBH4 reduction treatment preparation process and the second factor is that more Au+ species were formed using urea as the precipitant in the Au deposition–precipitation. The larger number of negatively charged metallic Pt nanoparticles and Au+ species resulted in abundant chemisorbed oxygen, which contributed to the co-oxidation of HCHO and CO. In addition, water exhibited a negative effect on the simultaneous removal of CO and HCHO. Based upon these results, a possible mechanism for the simultaneous removal of CO and HCHO at room temperature is also proposed.

An environmentally benign Fe–Ce–Ti mixed oxide catalyst, which was prepared via the hydrothermal method, has been investigated for the selective catalytic reduction of NOx with NH3 (NH3-SCR). It was found that the Fe–Ce–Ti catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a wide operation temperature window. XRD and Raman spectra suggest that the Fe–Ce–Ti catalyst has an amorphous structure. The co-presence of Fe and Ce induced the formation of a redox cycle (Ce4+ + Fe2+ ↔ Ce3+ + Fe3+), which promotes the activation of NO and NH3. In situ DRIFTS studies demonstrate that the synergetic effect between Fe and Ce contributes to the formation of reactive intermediate species, thus leading to the high catalytic deNOx performance of the Fe–Ce–Ti mixed oxide catalyst.

A coaxial nonthermal plasma (NTP) reactor was used for oxidizing and removing NO in mist. The effects of gas/mist components on O3 formation, NO oxidation, and NOx removal by NTP in simulated flue gas after wet desulfurization were investigated. The presence of mist results in the significant decrease of O3 concentration. Formation of active species, such as OH and HO2, would promote the reactions between NO and OH/HO2. Thus, the NO oxidation, NOx removal, and energy efficiency are significantly improved in the presence of mist. The promotion effects of mist component on the NO oxidation and NOx removal follow the order: Na2SO3 mist > FeSO4 mist > Na2SO4 mist. Increase of the mist pH can enhance the absorption of nitrogen oxides with the mist. Adding SO2 to the gas mixtures or raising the NO2/NOx ratio of simulated gas increases the NOx removal efficiency. This process demonstrates that NO oxidation by NTP and subsequently absorption by mist are feasible and have great value to practical application.

Experiments were conducted using a bubbling reactor to investigate nitrogen oxide absorption in the calcium sulfite slurry. The effects of CaSO3 concentration, NO2/NO mole ratio and O2 concentrations on NO2 and SO2 absorption efficiencies were investigated. Five types of additives, including MgSO4, Na2SO4, FeSO4, MgSO4/Na2SO4 and FeSO4/Na2SO4, had been evaluated for enhancing NO2 absorption in CaSO3 slurry. Results showed that CaSO3 concentration had significant impact on NO2 and SO2 absorption efficiencies, and the highest absorption efficiencies of SO2 and NO2 could reach about 99.5 and 75.0 %, respectively. Furthermore, the NO2 absorption was closely related to the NO2/NO mole ratio, and the existence of NO2 in flue gas may promote NO absorption. The presence of O2 in simulated flue gas was disadvantage for NOx removal because it can oxidize sulfite to sulfate. It was worth pointing out that FeSO4/Na2SO4 was the best additive among those investigated additives, as the NO2 removal efficiency was significantly increased from 74.8 to 95.0 %. IC and in situ FTIR results suggest that the main products were NO3− and NO2− in liquid phase and N2O, N2O5 and HNO3 in gas phase during the CaSO3 absorption process.

Carbide-derived carbon (CDC) was prepared by selective extraction of titanium from titanium carbide in a flow of freshly prepared chlorine. The dynamic adsorption and desorption performance of CDC of small molecule volatile organic compounds (VOCs) including methanol, acetaldehyde and acetone, was investigated and compared with that of two types of commercial activated carbons. The physicochemical properties of carbons were characterized by nitrogen adsorption, temperature programmed desorption, Raman spectroscopy and transmission electron microscopy. It was observed that the CDC could adsorb much more VOCs than commercial activated carbons (especially for the less polar methanol). The desorption behavior of VOCs from the saturated CDC was similar to that of commercial activated carbons, with adsorbed VOCs desorbed in the maximum degree at 110–150 °C, which indicated that the adsorption sites for the VOCs on the three carbon adsorbents were similar and the saturated CDC could be effectively regenerated by simple heat treatment just like commercial activated carbons. Based on the characterizations, the large adsorption capacity of CDC was attributed to its larger micropore volume, narrower pore distributions (0.7–1.5 nm), as well as higher specific surface area than those of two commercial activated carbons.

A series of TiO2 supported Pt–Au bimetallic catalysts were prepared by impregnation, deposition–precipitation and impregnation–deposition–precipitation, and their catalytic activities for the co-oxidation of formaldehyde (HCHO) and carbon monoxide (CO) were evaluated at room temperature. The results show that the Pt–Au/TiO2 catalyst prepared via introducing Pt by impregnation and subsequently introducing Au by deposition–precipitation, exhibits excellent catalytic performance for the co-oxidation of HCHO and CO. The characterizations of the catalyst by means of Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Temperature Programmed Reduction (TPR) and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS) revealed that the isolated Pt and Au sites are essential to the co-oxidation of HCHO and CO, because the HCHO oxidation occurring over Pt active sites while CO oxidation occurring over Au active sites can be conducted without mutual interference, thus simultaneously achieving both the high oxidation activity of HCHO and CO.

The effect of Zr on the catalytic performance of Pd/γ-Al2O3 for the methane combustion was investigated. The results show that the addition of Zr can improve the activity and stability of Pd/γ-Al2O3 catalyst, which, based on the catalyst characterization (N2 adsorption, XRD, CO-Chemisorption, XPS, CH4-TPR and O2-TPO), is ascribed to the interaction between Pd and Zr. The active phase of methane combustion over supported palladium catalyst is the Pd0/Pd2+ mixture. Zr addition inhibits Pd aggregation and enhances the redox properties of active phase Pd0/Pd2+. H2 reduction could effectively reduce the oxidation degree of Pd species and regenerate the active sites (Pd0/Pd2+).

Surface species formed during the catalytic combustion of methane over Al2O3 supported Pd and Pd–Zr catalysts have been investigated by in situ DRIFTS techniques. It is found that formate and carbonate ions are formed during the reaction as intermediates, and the further oxidation of formate ion to carbonate ion requires reactive oxygen species supplied by activating gaseous oxygen at oxygen vacancies on the catalyst surface. Modification of Pd/Al2O3 catalyst by Zr addition leads to the increase of oxygen vacancy, which promotes the activation of gaseous oxygen and contributes to the oxidation of formate ion, resulting in a higher catalytic performance for the catalytic combustion of methane.

Activated carbon (AC) was modified by hexamethylene diamine (HMDA) to remove low-concentration formaldehyde in air. Results demonstrate that no ammonia or amine compounds give out from the modified AC up to 423 K. And the modification significantly improves the adsorption performance of AC for formaldehyde. There exists an optimum loading amount, being around 0.04 g HMDA/g AC in this study.

The effects of discharge polarity, discharge electrode configuration, O2/CO2 ratio, and water vapor on the elemental mercury (Hg0) oxidation in simulated flue gas were investigated in a wire-cylinder plasma reactor energized by DC power. The Hg0 oxidation efficiency increases with the increase of specific energy density (SED). Compared with the glow corona discharge energized by negative DC high voltage, the streamer corona discharge induced by positive DC high voltage exhibits a much higher Hg0 oxidation efficiency under identical SED. The discharge electrode configuration significantly influences the energy density in the plasma reactor, but hardly affects Hg0 oxidation for a fixed SED. The increase of O2/CO2 ratio in simulated flue gas obviously enhances Hg0 oxidation. However, with the addition of H2O, Hg0 oxidation is remarkably restrained due to the decrease of O3 formation.

The effects of gas compositions and reaction conditions on NO conversion by positive streamer discharge were experimentally investigated by using a link tooth wheel-cylinder reactor. The results showed that NO conversion increased with increasing O2 concentration and NH3 concentration, but decreased with increasing inlet NO concentration and gas flow rate. The addition of CO2 or H2O to the feed gas promoted NO conversion by increasing the maximum discharge voltage, and NH4NO3 was formed in the presence of NH3. There was a most suitable range interval between discharge tooth wheels if both NO conversion and energy consumption were considered. Increasing applied voltage resulted in the increase in the amount of O3 generated by streamer discharge.

The behavior of non-thermal plasma (NTP) and combined plasma catalysis (CPC) was investigated for removal of low-concentration benzene, toluene and p-xylene (BTX mixture) in air using a link tooth wheel-cylinder plasma reactor. Combining NTP with MnOx/Al2O3 catalyst after the discharge zone (CPC) significantly promoted BTX conversion and improved the energy efficiency. For a specific input energy (SIE) of 10 J L−1, the conversion of benzene, toluene and p-xylene reached 94%, 97% and 95%, respectively. The introduction of MnOx/Al2O3 catalyst also moved the BTX conversion towards total oxidation and reduced the emission of O3 and NO2 as compared to NTP alone. For an SIE of 10 J L−1, the O3 outlet concentration decreased from 46.7 for NTP alone to 1.9 ppm for CPC, while the NO2 emission correspondingly decreased from 1380 to 40 ppb.

A series of Pt–Au/CeO2 catalysts were prepared via the impregnation deposition–precipitation (IDP) and reduction–deposition precipitation (RDP) methods. The performances of the catalysts for the simultaneous removal of carbon monoxide (CO) and formaldehyde (HCHO) at room temperature were evaluated. The results show that the Pt–Au/CeO2 catalyst, which was prepared via the RDP method, exhibited higher catalytic activity. The catalyst characterization results reveal that two factors accounted for this phenomenon. The first factor is that more negatively charged metallic Pt nanoparticles were obtained during the liquid phase NaBH4 reduction treatment preparation process and the second factor is that more Au+ species were formed using urea as the precipitant in the Au deposition–precipitation. The larger number of negatively charged metallic Pt nanoparticles and Au+ species resulted in abundant chemisorbed oxygen, which contributed to the co-oxidation of HCHO and CO. In addition, water exhibited a negative effect on the simultaneous removal of CO and HCHO. Based upon these results, a possible mechanism for the simultaneous removal of CO and HCHO at room temperature is also proposed.

An environmentally benign Fe–Ce–Ti mixed oxide catalyst, which was prepared via the hydrothermal method, has been investigated for the selective catalytic reduction of NOx with NH3 (NH3-SCR). It was found that the Fe–Ce–Ti catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a wide operation temperature window. XRD and Raman spectra suggest that the Fe–Ce–Ti catalyst has an amorphous structure. The co-presence of Fe and Ce induced the formation of a redox cycle (Ce4+ + Fe2+ ↔ Ce3+ + Fe3+), which promotes the activation of NO and NH3. In situ DRIFTS studies demonstrate that the synergetic effect between Fe and Ce contributes to the formation of reactive intermediate species, thus leading to the high catalytic deNOx performance of the Fe–Ce–Ti mixed oxide catalyst.

A series of TiO2 supported Pt–Au bimetallic catalysts were prepared by impregnation, deposition–precipitation and impregnation–deposition–precipitation, and their catalytic activities for the co-oxidation of formaldehyde (HCHO) and carbon monoxide (CO) were evaluated at room temperature. The results show that the Pt–Au/TiO2 catalyst prepared via introducing Pt by impregnation and subsequently introducing Au by deposition–precipitation, exhibits excellent catalytic performance for the co-oxidation of HCHO and CO. The characterizations of the catalyst by means of Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Temperature Programmed Reduction (TPR) and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS) revealed that the isolated Pt and Au sites are essential to the co-oxidation of HCHO and CO, because the HCHO oxidation occurring over Pt active sites while CO oxidation occurring over Au active sites can be conducted without mutual interference, thus simultaneously achieving both the high oxidation activity of HCHO and CO.