As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

A review on g-C3N4-based photocatalysts

[i] An inventory of air pollutant emissions in Asia in the year 2000 is developed to support atmospheric modeling and analysis of observations taken during the TRACE-P experiment funded by the National Aeronautics and Space Administration (NASA) and the ACE-Asia experiment funded by the National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA). Emissions are estimated for all major anthropogenic sources, including biomass burning, in 64 regions of Asia. We estimate total Asian emissions as follows: 34.3 Tg SO 2 , 26.8 Tg NO x , 9870 Tg CO 2 , 279 Tg CO, 107 Tg CH 4 , 52.2 Tg NMVOC, 2.54 Tg black carbon (BC), 10.4 Tg organic carbon (OC), and 27.5 Tg NH 3 . In addition, NMVOC are speciated into 19 subcategories according to functional groups and reactivity. Thus we are able to identify the major source regions and types for many of the significant gaseous and particle emissions that influence pollutant concentrations in the vicinity of the TRACE-P and ACE-Asia field measurements. Emissions in China dominate the signature of pollutant concentrations in this region, so special emphasis has been placed on the development of emission estimates for China. China's emissions are determined to be as follows: 20.4 Tg SO 2 , 11.4 Tg NO x , 3820 Tg CO 2 , 116 Tg CO, 38.4 Tg CH 4 , 17.4 Tg NMVOC, 1.05 Tg BC, 3.4 Tg OC, and 13.6 Tg NH 3 . Emissions are gridded at a variety of spatial resolutions from 1° × 1° to 30 s x 30 s, using the exact locations of large point sources and surrogate GIS distributions of urban and rural population, road networks, landcover, ship lanes, etc. The gridded emission estimates have been used as inputs to atmospheric simulation models and have proven to be generally robust in comparison with field observations, though there is reason to think that emissions of CO and possibly BC may be underestimated. Monthly emission estimates for China are developed for each species to aid TRACE-P and ACE-Asia data interpretation. During the observation period of March/ April, emissions are roughly at their average values (one twelfth of annual). Uncertainties in the emission estimates, measured as 95% confidence intervals, range from a low of ±16% for SO 2 to a high of ±450% for OC.

An inventory of gaseous and primary aerosol emissions in Asia in the year 2000 : NASA global tropospheric experiment transport and chemical evolution over the pacific (TRACE-P): Measurements and analysis (TRACEP1)

As the world’s second largest economy, China has experienced severe haze pollution, with fine particulate matter (PM) recently reaching unprecedentedly high levels across many cities, and an understanding of the PM formation mechanism is critical in the development of efficient mediation policies to minimize its regional to global impacts. We demonstrate a periodic cycle of PM episodes in Beijing that is governed by meteorological conditions and characterized by two distinct aerosol formation processes of nucleation and growth, but with a small contribution from primary emissions and regional transport of particles. Nucleation consistently precedes a polluted period, producing a high number concentration of nano-sized particles under clean conditions. Accumulation of the particle mass concentration exceeding several hundred micrograms per cubic meter is accompanied by a continuous size growth from the nucleation-mode particles over multiple days to yield numerous larger particles, distinctive from the aerosol formation typically observed in other regions worldwide. The particle compositions in Beijing, on the other hand, exhibit a similarity to those commonly measured in many global areas, consistent with the chemical constituents dominated by secondary aerosol formation. Our results highlight that regulatory controls of gaseous emissions for volatile organic compounds and nitrogen oxides from local transportation and sulfur dioxide from regional industrial sources represent the key steps to reduce the urban PM level in China.

https://www.pnas.org/content/pnas/111/49/17373.full.pdf

Elucidating severe urban haze formation in China

Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO2 by NO2 is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH3 neutralization or under cloud conditions. Under polluted environments, this SO2 oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH3 and NO2 control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

https://www.pnas.org/content/pnas/113/48/13630.full.pdf

Persistent sulfate formation from London Fog to Chinese haze

Haze in China has been increasing in frequency of occurrence as well as the area of the affected region. Here, we report on a new mechanism of haze formation, in which coexistence with NOx can reduce the environmental capacity for SO2, leading to rapid conversion of SO2 to sulfate because NO2 and SO2 have a synergistic effect when they react on the surface of mineral dust. Monitoring data from five severe haze episodes in January of 2013 in the Beijing-Tianjin-Hebei regions agreed very well with the laboratory simulation. The combined air pollution of motor vehicle exhaust and coal-fired flue gases greatly reduced the atmospheric environmental capacity for SO2, and the formation of sulfate was found to be a main reason for the growth of fine particles, which led to the occurrence of haze. These results indicate that the impact of motor vehicle exhaust on the atmospheric environment might be underestimated.

/pdf/mineral-dust-and-nox-promote-the-conversion-of-so2-to-3vo8thrwj2.pdf

Mineral dust and NOx promote the conversion of SO2 to sulfate in heavy pollution days

In this work, graphitic carbon nitride-titanium dioxide (g-C3N4-TiO2) was successfully prepared by a facile calcination route utilizing commercial P25 and melamine as the precursors. The as-prepared g-C3N4/TiO2 photocatalysts were characterized systematically to elucidate their morphological structure and physico-chemical properties. The photocatalytic performance of g-C3N4-TiO2 composites was investigated for the removal of NOx in air. At the optimal g-C3N4 content (∼15 wt%, labeled as M400), the conversion of NOx was 27%, which is higher than that of pure P25 (17%) and g-C3N4 (7%) under visible light. The activity of M400 was also enhanced under UV light. However, a mechanically mixed g-C3N4 and TiO2 sample (with the content of g-C3N4 the same as M400, labeled as M0 + g-C3N4) did not improve the conversion of NOx. Therefore, the interaction of g-C3N4 and P25 is important for the activity. EPR results indicated that O2− is the main active species for NO oxidation to NO3− under visible and UV light, which is responsible for the difference in activity between M400 and M0 + g-C3N4. The present study can improve our understanding of NO removal on the photocatalyst surface and the mechanism for the activity enhancement by the formation of g-C3N4-TiO2.

Enhanced photocatalytic oxidation of NO over g-C3N4-TiO2 under UV and visible light

Transition metal (Cerium, Cobalt, and Iron) doped cryptomelane-type manganese oxide (M-OMS-2) catalysts have been successfully synthesized and characterized. The different metal-ion-doped K-OMS-2 catalysts showed distinct differences in their ozone decomposition activity. Cerium-doped OMS-2 materials showed ozone conversion of 90% under RH = 90% and space velocity of 600,000 h−1. X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) data suggested that the as-synthesized M-OMS-2 materials were all crystalline with no segregated metal oxide impurities. ICP-OES and XAFS results confirmed that Co3+ and Fe3+ replace Mn3+ in the cryptomelane structure and Ce4+mainly replaces the K+ in the tunnel and partially replaces the Mn4+ in the framework of the cryptomelane structure. Because of the differences in the substitution sites and the ionic radius of dopants, the morphologies of the catalysts were different. The Mn3+content and number of surface defects play a key role during the decomposition of ozone. Ce-OMS-2 is a promising catalyst for purifying waste gases containing ozone under high-humidity conditions.

Transition metal doped cryptomelane-type manganese oxide catalysts for ozone decomposition

Transition metal (cerium and cobalt) doped γ-MnO2 (M-γ-MnO2, where M represents Ce, Co) catalysts were successfully synthesized and characterized. Cerium-doped γ-MnO2 materials showed ozone (O3) conversion of 96% for 40 ppm of O3 under relative humidity (RH) of 65% and space velocity of 840 L g–1 h–1 after 6 h at room temperature, which is far superior to the performance of the Co-γ-MnO2 (55%) and γ-MnO2 (38%) catalysts. Under space velocity of 840 L g–1 h–1, the conversion of ozone over the Ce-γ-MnO2 catalyst under RH = 65% and dry conditions within 96 h was 60% and 100%, respectively, indicating that it is a promising material for ozone decomposition. XRD and HRTEM data suggested that Ce-γ-MnO2 formed mixed crystals consisting of α-MnO2 and γ-MnO2 with specific surface area increased from 74 m2/g to 120 m2/g compared to undoped γ-MnO2, thus more surface defects were introduced. H2-TPR, O2-TPD, XPS, Raman, and EXAFS confirmed that Ce-γ-MnO2 exhibited more surface oxygen vacancies and surface defects, whi...

Oxygen Vacancies Induced by Transition Metal Doping in γ-MnO2 for Highly Efficient Ozone Decomposition.

High temperature reduction generally induces the sintering of supported noble metals, therefore resulting in a negative effect on their performance. Here, we show that high temperature reduction was able to dramatically increase the activity of Pd/TiO 2 for ambient HCHO oxidation. We prepared a Pd/TiO 2 catalyst and pre-reduced it with H 2 at low temperature (300 °C) and high temperature (450 °C), respectively, and then tested the activity for HCHO oxidation at ambient temperature. The Pd/TiO 2 -450R catalyst showed a much better performance than Pd/TiO 2 -300R at room temperature. 100% HCHO conversion could be obtained on the Pd/TiO 2 -450R catalyst at a GHSV of 95000 h −1 and 140 ppm inlet HCHO. The catalysts were then characterized by using Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and other methods. The results indicate that high temperature reduction could induce the strong metal-support interaction (SMSI), decreasing the surface Pd particle size by partially encapsulating and trapping Pd clusters with TiO 2 , and also could produce more oxygen vacancies, beneficial to the activation of O 2 and formation of surface OH groups, opening a more effective pathway for ambient HCHO oxidation. Therefore, the Pd/TiO 2 -450R catalyst demonstrated high activity for ambient HCHO oxidation.

Jinzhu Ma

Papers

Mineral dust and NOx promote the conversion of SO2 to sulfate in heavy pollution days

Enhanced photocatalytic oxidation of NO over g-C3N4-TiO2 under UV and visible light

Transition metal doped cryptomelane-type manganese oxide catalysts for ozone decomposition

Oxygen Vacancies Induced by Transition Metal Doping in γ-MnO2 for Highly Efficient Ozone Decomposition.

High temperature reduction dramatically promotes Pd/TiO2 catalyst for ambient formaldehyde oxidation