Facile synthesis of Ag-modified manganese oxide for effective catalytic ozone decomposition

O3 decomposition catalysts with excellent performance still need to be developed. In this study, Ag-modified manganese oxides(AgMnOx) were synthesized by a simple co-precipitation method. The effect of calcination temperature on the activity of MnOx and AgMnOxcatalysts was investigated. The effect of the amount of Ag addition on the activity and structure of the catalysts was further studied by activity testing and characterization by a variety of techniques. The activity of 8%AgMnOxfor ozone decomposition was significantly enhanced due to the formation of the Ag1.8 Mn8 O16 structure, indicating that this phase has excellent performance for ozone decomposition.The weight content of Ag1.8 Mn8 O16 in the 8%AgMnOxcatalyst was only about 33.76%, which further indicates the excellent performance of the Ag1.8 Mn8 O16 phase for ozone decomposition. The H2 temperature programmed reduction(H2-TPR) results indicated that the reducibility of the catalysts increased due to the formation of the Ag1.8 Mn8 O16 structure.This study provides guidance for a follow-up study on Ag-modified manganese oxide catalysts for ozone decomposition.

Differences of the oxidation process and secondary organic aerosol formation at low and high precursor concentrations

Current atmospheric quality models usually underestimate the level of ambient secondary organic aerosol(SOA), one of the possible reasons is that the precursors at different concentrations may undergo different oxidation processes and further affect SOA formation. Therefore, there is a need to perform more chamber studies to disclose the influence. In this work, SOA formation over a wide range of initial precursor concentrations(tens of ppb to hundreds of ppb levels) was investigated in a 30 m3 indoor smog chamber,and mainly through the analysis of multiple generations of VOCs detected from HR-To FPTRMS to expound the difference in the oxidation process between low and high precursor concentrations. Compared to high initial concentrations, gas-phase intermediates formed at low concentrations had a higher intensity by about one order of magnitude, and the lowvolatility compounds also had a higher formation potential due to the competition between semi-volatile intermediates and precursors with oxidants. In addition, the formed SOA was more oxidized with higher f44 value(0.14 ± 0.02) and more relevant to real atmosphere than that formed at high concentrations. This work should help to deeply understand SOA formation and improve the performance of air quality models for SOA simulation.

Secondary aerosol formation and oxidation capacity in photooxidation in the presence of Al_2O_3 seed particles and SO_2

To investigate the sensitivity of secondary aerosol formation and oxidation capacity to NOx in homogeneous and heterogeneous reactions, a series of irradiated toluene/NOx/air and ?-pinene/NOx/air experiments were conducted in smog chambers in the absence or presence of Al2O3 seed particles. Various concentrations of NOx and volatile organic compounds(VOCs) were designed to simulate secondary aerosol formation under different scenarios for NOx. Under "VOC-limited" conditions, the increasing NOx concentration suppressed secondary aerosol formation, while the increasing toluene concentration not only contributed to the increase in secondary aerosol formation, but also led to the elevated oxidation degree for the organic aerosol. Sulfate formation was suppressed with the increasing NOx due to a decreased oxidation capacity of the photooxidation system. Secondary organic aerosol(SOA) formation also decreased with the presence of high concentration of NOx, because organo-peroxy radicals(RO2) react with NOx instead of with peroxy radicals(RO2 or HO2), resulting in the formation of volatile organic products. The increasing concentration of NOx enhanced the formation of sulfate, nitrate and SOA under "NOx-limited" conditions, in which the heterogeneous reactions played an important role. In the presence of Al2O3 seed particles, a synergetic promoting effect of mineral dust and NOx on secondary aerosol formation in heterogeneous reactions was observed in the photooxidation. This synergetic effect strengthened the positive relationship between NOx and secondary aerosol formation under "NOx-limited" conditions but weakened or even overturned the negative relationship between NOx and secondary aerosol formation under "VOC-limited" conditions. Sensitivity of secondary aerosol formation to NOx seemed different in homogeneous and heterogeneous reactions, and should be both taken into account in the sensitivity study. The sensitivity of secondary aerosol formation to NOx was further investigated under "winter-like" and NH3-rich conditions. No obvious difference for the sensitivity of secondary aerosol formation except nitrate to NOx was observed.

Tuning the fill percentage in the hydrothermal synthesis process to increase catalyst performance for ozone decomposition

The performance of Ce-OMS-2 catalysts was improved by tuning the fill percentage in the hydrothermal synthesis process to increase the oxygen vacancy density.The Ce-OMS-2 samples were prepared with different fill percentages by means of a hydrothermal approach(i.e.80%,70%,50%and 30%).Ce-OMS-2 with 80%fill percentage(Ce-OMS-2-80%)showed ozone conversion of 97%,and a lifetime experiment carried out for more than20 days showed that the activity of the catalyst still remained satisfactorily high(91%).For Ce-OMS-2-80%,Mn ions in the framework as well as K ions in the tunnel sites were replaced by Ce^4+,while for the others only Mn ions were replaced.O2-TPD and H2-TPR measurements proved that the Ce-OMS-2-80%catalyst possessed the greatest number of mobile surface oxygen species.XPS and XAFS showed that increasing the fill percentage can reduce the AOS of Mn and augment the amount of oxygen vacancies.The active sites,which accelerate the elimination of O3,can be enriched by increasing the oxygen vacancies.These findings indicate that increasing ozone removal can be achieved by tuning the fill percentage in the hydrothermal synthesis process.

Distinct potential aerosol masses under different scenarios of transport at a suburban site of Beijing

In order to evaluate the secondary aerosol formation potential at a suburban site of Beijing,in situ perturbation experiments in a potential aerosol mass（PAM） reactor were carried out in the winter of 2014.The variations of secondary aerosol formation as a function of time,OH exposure,and the concentrations of gas phase pollutants and particles were reported in this study.Two periods with distinct secondary aerosol formation potentials,marked as Period Ⅰ and Period Ⅱ,were identified during the observation.In Period Ⅰ,the secondary aerosol formation potential was high,and correlated well to the air pollutants,i.e.,SO2,NO2,and CO.The maximal secondary aerosol formation was observed with an aging time equivalent to about 3 days of atmospheric oxidation.In period Ⅱ,the secondary aerosol formation potential was low,with no obvious correlation with the air pollutants.Meanwhile,the aerosol mass decreased,instead of showing a peak,with increasing aging time.Backward trajectory analysis during the two periods confirmed that the air mass in Period Ⅰwas mainly from local sources,while it was attributed mostly to long distance transport in Period Ⅱ.The air lost its reactivity during the long transport and the particles became highly aged,resulting in a low secondary aerosol formation potential.Our experimental results indicated that the in situ measurement of the secondary aerosol formation potential could provide important information for evaluating the contributions of local emission and long distance transport to the aerosol pollution.

A superior catalyst for ozone decomposition: NiFe layered double hydroxide

Ground-level ozone is harmful to human beings and ecosystems,while room-temperature catalytic decomposition is the most effective technology for ozone abatement.However,solving the deactivation of existing metal oxide catalysts was caused by oxygen-containing intermediates is challenging.Here,we successfully prepared a two-dimensional NiFe layered double hydroxide (NiFe-LDH) catalyst via a facile co-precipitation method,which exhibited stable and highly efficient performance of ozone decomposition under harsh operating conditions (high space velocity and humidity).The NiFe-LDH catalyst with Ni/Fe=3and crystallization time over 5 hr (named Ni3Fe-5) exhibited the best catalytic performance,which was well beyond that of most existing manganese-based oxide catalysts.Specifically,under relative humidity of 65%and space velocity of 840 L/(g·hr),Ni3Fe-5 showed ozone conversion of 89%and 76%for 40 ppmV of O3within 6 and 168 hr at room-temperature,respectively.We demonstrated that the layered structure of NiFe-LDH played a decisive role in its outstanding catalytic performance in terms of both activity and water resistance.The LDH catalysts fundamentally avoids the deactivation caused by the occupancy of oxygen vacancies by oxygen-containing species (H2O,O-,and O2-) in manganese-based oxide.This study indicated the promising application potential of LDHs than manganese-based oxide catalysts in removal of gaseous ozone.

Photocatalysis and ambient catalysis for environmental remediation

Environmental pollution is a major challenge faced by human beings, and is becoming more serious due to increasing urbanization and industrialization. Catalysis is the key technology for environmental remediation. As a “green” technology, photocatalysis and ambient catalysis could offer numerous opportunities to realize environmental remediation under mild reaction conditions with low energy input. Over the past two decades, great advances have been made on the design and synthesis of the photocatalysts and ambient catalysts, precise control of catalyst active sites, and the mechanistic understanding on the catalytic process for environmental remediation.

大气“霾化学”:概念提出和研究展望

大气污染是人类面临的重大环境挑战。我国大气污染具有高度的复合污染特征,其形成过程既有高强度的颗粒物均相成核现象,又有多介质非均相致霾过程,同时耦合了强的大气氧化性以及O3污染,是不同于洛杉矶光化学烟雾和伦敦烟雾的新型“霾化学”烟雾污染。“霾化学”区别于并突破现有的理论认识,是解析我国典型多介质复合污染环境下PM2.5成因以及PM2.5与O3污染间非线性复杂关系,综合研究气、液、固多介质非均相过程的大气污染化学。研究“霾化学”过程对精准控制我国乃至其他国家大气复合污染意义重大。本文提出和总结了大气“霾化学”概念,并对“霾化学”理论的完善和发展进行了展望。

Recent advances in catalytic decomposition of ozone

Ozone(O3),as a harmful air pollutant,has been of wide concern.Safe,efficient,and economical O3 removal methods urgently need to be developed.Catalytic decomposition is the most promising method for O3 removal,especially at room temperature or even subzero temperatures.Great efforts have been made to develop high-efficiency catalysts for O3 decomposition that can operate at low temperatures,high space velocity and high humidity.First,this review describes the general reaction mechanism of O3 decomposition on noble metal and transition metal oxide catalysts.Then,progress on the O3 decomposition performance of various catalysts in the past 30 years is summarized in detail.The main focus is the O3 decomposition performance of manganese oxides,which are divided into supported manganese oxides and non-supported manganese oxides.Methods to improve the activity,stability,and humidity resistance of manganese oxide catalysts for O3 decomposition are also summarized.The deactivation mechanisms of manganese oxides under dry and humid conditions are discussed.The O3 decomposition performance of monolithic catalysts is also summarized from the perspective of industrial applications.Finally,the future development directions and prospects of O3 catalytic decomposition technology are put forward.

A case study of Asian dust storm particles:Chemical composition,reactivity to SO_2 and hygroscopic properties

Mineral dust comprises a great fraction of the global aerosol loading,but remains the largest uncertainty in predictions of the future climate due to its complexity in composition and physico-chemical properties.In this work,a case study characterizing Asian dust storm particles was conducted by multiple analysis methods,including SEM-EDS,XPS,FT-IR,BET,TPD/mass and Knudsen cell/mass.The morphology,elemental fraction,source distribution,true uptake coefficient for SO 2,and hygroscopic behavior were studied.The major components of Asian dust storm particles are aluminosilicate,SiO 2 and CaCO 3,with organic compounds and inorganic nitrate coated on the surface.It has a low reactivity towards SO 2 with a true uptake coefficient,5.767×10-6,which limits the conversion of SO 2 to sulfate during dust storm periods.The low reactivity also means that the heterogeneous reactions of SO 2 in both dry and humid air conditions have little effect on the hygroscopic behavior of the dust particles.

Detrimental role of residual surface acid ions on ozone decomposition over Ce-modified γ-MnO2under humid conditions

In the study,the catalyst precursors of Ce-modifiedγ-MnO2 were washed with deionized water until the pH value of the supernatant was 1,2,4 and 7,and the obtained catalysts were named accordingly.Under space velocity of 300,000 hr-1,the ozone conversion over the pH=7 catalyst under dry conditions and relative humidity of 65%over a period of 6 hr was 100%and 96%,respectively.However,the ozone decomposition activity of the pH=2 and 4 catalysts distinctly decreased under relative humidity of 65%compared to that under dry conditions.Detailed physical and chemical characterization demonstrated that the residual sulfate ions on the pH=2 and 4 catalysts decreased their hydrophobicity and then restrained humid ozone decomposition activity.The pH=2 and 4 catalysts had inferior resistance to high space velocity under dry conditions,because the residual sulfate ion on their surface reduced their adsorption capacity for ozone molecules and increased their apparent activation energies,which was proved by temperature programmed desorption of O2 and kinetic experiments.Long-term activity testing,X-ray photoelectron spectroscopy and density functional theory calculations revealed that there were two kinds of oxygen vacancies on the manganese dioxide catalysts,one of which more easily adsorbed oxygen species and then became deactivated.This study revealed the detrimental effect of surface acid ions on the activity of catalysts under humid and dry atmospheres,and provided guidance for the development of highly efficient catalysts for ozone decomposition.

Haze insights and mitigation in China: An overview

The present article provides an overview of the chemical and physical features of haze in China, focusing on the relationship between haze and atmospheric fine particles, and the formation mechanism of haze. It also summarizes several of control technologies and strategies to mitigate the occurrence of haze. The development of instruments and the analysis of measurements of ambient particles and precursor concentrations have provided important information about haze formation. Indeed, the use of new instruments has greatly facilitated current haze research in China. Examples of insightful results include the relationship between fine particles and haze, the chemical compositions and sources of particles, the impacts of the aging process on haze formation, and the application of technologies that control the formation of haze. Based on these results, two relevant issues need to be addressed： understanding the relationship between haze and fine particles and understanding how to control PM2.5.

Review of heterogeneous photochemical reactions of NOy on aerosol-A possible daytime source of nitrous acid (HONO) in the atmosphere

As an important precursor of hydroxyl radical, nitrous acid （HONO） plays a key role in the chemistry of the lower atmosphere. Recent atmospheric measurements and model calculations show strong enhancement for HONO formation during daytime, while they are inconsistent with the known sources in the atmosphere, suggesting that current models are lacking important sources for HONO. In this article, heterogeneous photochemical reactions of nitric acid/nitrate anion and nitrogen oxide on various aerosols were reviewed and their potential contribution to HONO formation was also discussed. It is demonstrated that HONO can be formed by photochemical reaction on surfaces with deposited HNO3 , by photocatalytic reaction of NO2 on TiO2 or TiO2 -containing materials, and by photochemical reaction of NO2 on soot, humic acids or other photosensitized organic surfaces. Although significant uncertainties still exist in the exact mechanisms and the yield of HONO, these additional sources might explain daytime observations in the atmosphere.

Effect of TiO_2 calcination temperature on the photocatalytic oxidation of gaseous NH_3

Carbon-modified titanium dioxide （TiO2） was prepared by a sol-gel method using tetrabutyl titanate as precursor, with calcination at various temperatures, and tested for the photocatalytic oxidation （PCO） of gaseous NH3 under visible and UV light. The test results showed that no samples had visible light activity, while the TiO2 calcined at 400℃ had the best UV light activity among the series of catalysts, and was even much better than the commercial catalyst P25. The catalysts were then characterized by X-ray diffractometry, Brunauer-Emmett-Teller adsorption analysis, Raman spectroscopy, thermogravimetry/differential scanning calorimetry coupled with mass spectrometry, ultraviolet-visible diffuse reflectance spectra, photoluminescence spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy. It was shown that the carbon species residuals on the catalyst surfaces induced the visible light adsorption of the samples calcined in the low temperature range （〈 300℃）. However, the surface acid sites played a determining role in the PCO of NH3 under visible and UV light over the series of catalysts. Although the samples calcined at low temperatures had very high SSA, good crystallinity, strong visible light absorption and also low PL emission intensity, they showed very low PCO activity due to their very low number of acid sites for NH3 adsorption and activation. The TiO2 sample calcined at 400℃ contained the highest number of acid sites among the series of catalysts, therefore showing the highest performance for the PCO of NH3 under UV light.

Aggregation-induced emission of copper nanoclusters

Copper nanoclusters(Cu NCs)have recently emerged as promising luminophores,featuring ultra-small size,reasonable photostability,large Stokes shift,and long emission lifetimes.Aggregation-induced emission(AIE)has been often used to further improve both the emission intensity and stability of these clusters,with plenty of potential applications in the fields of chemical sensing and bioimaging.This review starts with a summary of the current understanding of emission mechanisms of Cu NCs and proceeds with the analysis of contributions from the Cu metal core and the organic ligands.We summarize the recent research progress on the design of ligands,and the ways on how to induce aggregation of the Cu NCs through electrostatic charge neutralization,host-guest interactions,and the use of templates.We also discuss the current understanding of emission mechanisms of Cu NCs experiencing AIE,such as the often-cited restriction of intramolecular motion and contributions from Cu(I)molecular complexes.We finish this review by providing concluding remarks and offering our own perspective on the active field of AIE of Cu NCs,with a hope to further promote the research on the fundamental aspects of this useful phenomenon.