Experimental parameters for the preparation of Mn/TiO2 catalysts by ultrasonic spray pyrolysis method for selective catalytic reduction of NOx at low temperature

Mn/TiO2 series selective catalytic reduction (SCR) catalysts with regular spherical shape could be obtained by ultrasonic spray pyrolysis method. The investigation of experimental parameters, such as decomposition temperature and the type and flow rate of carrier gas, showed that a high and stable NOx conversion at 70% could be achieved at 180 °C when the ultrasonic spray pyrolysis temperature was 500 °C and N2 was used as a carrier gas with a flow rate of 2 L/min. The physical and chemical properties of the catalysts were characterized by X-ray diffraction, SEM, and H2-TPR, revealing that a regular spherical structure, a small amount of Mn2O3 crystals, and high redox ability were the important factors affecting the activity of the catalyst. With these properties, Mn(0.5)/TiO2 (500 °C, N2, 2 L/min) exhibited more than 90% conversion at 240 °C with N2 selectivity above 96%

Analyses and Simulations of PM_2.5 Pollution Characteristics under the Influence of the New Year’s Day Effects in China

Regional haze often occurs after the New Year holiday. To explore the characteristics of PM2.5 pollutions under the influence of the New Year’s Day effect, this study analyzed the spatiotemporal changes relating to PM2.5 during and around the New Year’s Day holiday in China from 2015 to 2022, and used the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model to study the effects of human activities and meteorological factors on PM2.5 pollutions, as well as the differences in the contributions of different industries to PM2.5 pollutions. The results show that for the entire study period (i.e., before, during, and after the New Year’s Day holiday) from 2015 to 2022, the average concentrations of PM2.5 in China decreased by 41.9% overall. In 2019~2022, the New Year’s Day effect was significant, meaning that the average concentrations of PM2.5 increased by 18.9~46.8 μg/m3 from before to after the New Year’s Day holiday, with its peak occurring (64.3~74.9 μg/m3) after the holiday. In terms of spatial differences, the average concentrations of PM2.5 were higher in the Beijing–Tianjin–Hebei region, the Yangtze River Delta, and central China. Moreover, the Beijing–Tianjin–Hebei region and its surrounding areas, the Chengdu–Chongqing region, the Fenwei Plain, and the middle reaches of the Yangtze River region were greatly affected by the New Year’s Day effect. Human activities led to higher increases in PM2.5 in Henan, Hubei, Hebei, and Anhui on 3 and 4 January 2022. If the haze was accompanied by cloudy days or weak precipitation, the accumulation of surface water vapor and atmospheric aerosols further increased the possibility of heavy pollution. It was found that, for the entire study period, PM2.5 generated by residential sources contributed the vast majority (60~100 μg/m3) of PM2.5 concentrations, and that the main industry sources that caused changes in time distributions were industrial and transportation sources

One-pot template-free synthesis, growth mechanism and enhanced photocatalytic activity of monodisperse (BiO)(2)CO3 hierarchical hollow microspheres self-assembled with single-crystalline nanosheets

This work presents a one-pot template-free synthesis, detailed characterization, growth mechanism and application of well-defined uniform monodisperse (BiO)(2)CO3 hierarchical hollow microspheres self-assembled with single-crystalline nanosheets. The synthesis was conducted by hydrothermal treatment of bismuth citrate and sodium carbonate in water. Time-dependent evolutions of phase structure, composition, and morphology were investigated systematically and revealed that the growth mechanism of such novel structures involved a unique multistep pathway. First, near amorphous particles were produced through reaction, nucleation, crystallization, and aggregation processes. Then, stacked embryos of intermediate (BiO)(4)CO3(OH)(2) microspheres with attached particles were produced due to dissolution and recrystallization. Subsequently, stacked uniform solid microspheres with small particles attached on edges were generated by the consumption of particles through Ostwald ripening. The stacked microspheres further grew to form monodisperse hierarchical microspheres with a hole in the center, like flower buds. Finally, uniform monodisperse (BiO)(2)CO3 hierarchical hollow microspheres were produced through layers splitting. The aggregation of the self-assembled nanosheets contributed to the formation of 3D hierarchical architecture containing mesopores, which is favorable for efficient reactants transport and photo-energy harvesting. Furthermore, the band gap structure of (BiO)(2)CO3 was revealed by the experimental method combined with density functional theoretical calculation. As expected, the novel (BiO)(2)CO3 hierarchical hollow microspheres exhibited enhanced photocatalytic activity due to the special hierarchical morphology, exceeding that of (BiO)(2)CO3 particles and commercial P25. The as-prepared uniform (BiO)(2)CO3 microspheres with well-defined hierarchical hollow structures are also ideal candidates for investigating their architecture-dependent performances in other areas, such as solar energy conversion, catalysis, electronics and so on

Effect of Zr Addition on the Low-Temperature SCR Activity and SO₂ Tolerance of Fe–Mn/Ti Catalysts

Zr is added to the Fe–Mn/Ti catalyst to increase NO conversion and improve SO₂ tolerance. It is found that 0.03 is the optimal ratio for Zr/(Ti + Zr). With this ratio, the NO conversion below 150 °C increases, and the SO₂ poisoning is alleviated, while the further increase of Zr does not have a positive effect on NO conversion and SO₂ tolerance. With Zr additive, more manganese oxides are reduced in the form of MnO₂ and Mn₂O₃ at lower temperature in H₂-TPR, and the total amount of H₂ consumption rises, indicating better redox properties. It leads to the increase of NO complexes on the catalysts. Despite the small decrease of NH₃ adsorption, the reaction via the L–H way is promoted and NO conversion increases. Furthermore, more nitrates and NO₂ are formed in the reaction with SO₂ on Fe–Mn/Ti–Zr(0.03) compared to Fe–Mn/Ti, so the L–H reaction way is less inhibited by SO₂ with Zr additive, and the SO₂ tolerance of this catalyst is also improved

Mechanism of NH₃ Selective Catalytic Reduction Reaction for NO_<i>x</i> Removal from Diesel Engine Exhaust and Hydrothermal Stability of Cu–Mn/Zeolite Catalysts

Zeolite-supported catalysts are effective in selective catalytic reduction (SCR) of NO_<i>x</i> from the diesel engine exhaust, whereas the reaction mechanism remains unclear. In this work, in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) spectroscopy was used to investigate the SCR reaction mechanism on Cu–Mn/ZSM-5 and Cu–Mn/SAPO-34. Langmuir–Hinshelwood (L-H) and Eley–Rideal (E-R) reaction mechanisms were both involved in the SCR reaction. In the L-H reaction mechanism, NO₂ and NH₄⁺ were two important intermediates. The transformation of bidentate nitrates to monodentate nitrates, and further to NO₂, played an important role. The amount of bidentate nitrates and monodentate nitrates on Cu–Mn/ZSM-5 was much less than that on Cu–Mn/SAPO-34. It led to less NO removal through the L-H reaction mechanism on Cu–Mn/ZSM-5. The formation and consumption of coordinated NH₃, as the intermediate in the E-R reaction mechanism, was similar on both catalysts. Therefore, the different catalytic activities between the two catalysts should be mainly due to the L-H reaction mechanism, the main mechanism at low temperature. The hydrothermal aging treatment significantly reduced the amount of NH₄⁺ and NO_<i>x</i> complexes on Cu–Mn/ZSM-5, and the variation of the SCR reaction through the L-H mechanism was also the main reason for different hydrothermal stabilities of the two catalysts

A Novel Four-Way Plasma-Catalytic Approach for The After-Treatment of Diesel Engine Exhausts

This study proposes a novel four-way plasma-catalytic removal of particulate matter, nitrogen oxides, hydrocarbons, and carbon monoxide without external heating using a pulsed dielectric barrier discharge reactor combined with Au/CaSO₄/γ-Al₂O₃ catalyst balls. With the optimized amount of Au and CaSO₄, over 90% of particulate matter, 40% of nitrogen oxides, and nearly 100% of hydrocarbons were removed from the diesel engine exhaust, while the increase of carbon monoxide concentration due to the oxidation of particulate matter and hydrocarbons and the decomposition of carbon dioxide were minimized. Importantly, the performance of the plasma-catalytic reactor was maintained without decline for the duration of the experiment (7 h) with an attractive cost/performance ratio. These findings offer a new approach to the simultaneous treatment of diesel exhausts and provide a novel concept for the design of a practical and compact after-treatment device for the diesel engine exhaust gases

Decrease of VOC emissions from vehicular emissions in Hong Kong from2003 to 2015: Results from a tunnel study

Vehicular emissions are one of major anthropogenic sources of ambient volatile organic compounds (VOCs) in Hong Kong. During the past twelve years, the government of the Hong Kong Special Administrative Region has undertaken a series of air pollution control measures to reduce vehicular emissions in Hong Kong. Vehicular emissions were characterized by repeated measurement in the same roadway tunnel in 2003 and 2015. The total net concentration of measured VOCs decreased by 44.7% from 2003 to 2015. The fleet-average VOC emission factor decreased from 107.1 &plusmn; 44.8 mg veh&minus;1 km&minus;1 in 2003 to 58.8 &plusmn; 50.7 mg veh&minus;1 km&minus;1 in 2015, and the total ozone (O3) formation potential of measured VOCs decreased from 474.1 mg O3 veh&minus;1 km&minus;1 to 190.8 mg O3 veh&minus;1 km&minus;1. The emission factor of ethene, which is one of the key tracers for diesel vehicular emissions, decreased by 67.3% from 2003 to 2015 as a result of the strict control measures on diesel vehicular emissions. Total road transport VOC emissions is estimated to be reduced by 40% as compared with 2010 by 2020, which will be an important contributor to achieve the goal of total VOC emission reduction in the Pearl River Delta region. The large decrease of VOC emissions from on-road vehicles demonstrates the effectiveness of past multi-vehicular emission control strategy in Hong Kong