NOx

About: NOx is a research topic. Over the lifetime, 26367 publications have been published within this topic receiving 496555 citations.

The Role of NO and NO2 in the Chemistry of the Troposphere and Stratosphere

Abstract: Oxides of nitrogen play an important role in the radical chemistry of the atmosphere and in the production and destruction of tropospheric and stratospheric ozone. Ozone is a principal agent in forming the OH radical which attacks inert gases in the troposphere. The acidity of precipitation is in part the result of HNO3 and H2SO4 formed by reactions involving the OH radical. Fast removal processes for oxides of nitrogen in the lower troposphere are described. Long-distance transport of oxides of nitrogen in peroxy-acetyl nitrate also receives attention. In the stratosphere, the participation of NO and NO2 in reactions that influence the equilibrium of photochemical systems may render the total ozone abundance insensitive to additions of oxides of nitrogen.

Anthropogenic drivers of 2013–2017 trends in summer surface ozone in China

Abstract: Observations of surface ozone available from ∼1,000 sites across China for the past 5 years (2013–2017) show severe summertime pollution and regionally variable trends. We resolve the effect of meteorological variability on the ozone trends by using a multiple linear regression model. The residual of this regression shows increasing ozone trends of 1–3 ppbv a−1 in megacity clusters of eastern China that we attribute to changes in anthropogenic emissions. By contrast, ozone decreased in some areas of southern China. Anthropogenic NOx emissions in China are estimated to have decreased by 21% during 2013–2017, whereas volatile organic compounds (VOCs) emissions changed little. Decreasing NOx would increase ozone under the VOC-limited conditions thought to prevail in urban China while decreasing ozone under rural NOx-limited conditions. However, simulations with the Goddard Earth Observing System Chemical Transport Model (GEOS-Chem) indicate that a more important factor for ozone trends in the North China Plain is the ∼40% decrease of fine particulate matter (PM2.5) over the 2013–2017 period, slowing down the aerosol sink of hydroperoxy (HO2) radicals and thus stimulating ozone production.

Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts

Abstract: Over the last several years, nitrogen oxide(s) (NOx) storage/reduction (NSR) catalysts, also referred to as NOx adsorbers or lean NOx traps, have been developed as an aftertreatment technology to reduce NOx emissions from lean‐burn power sources. NSR operation is cyclic: during the lean part of the cycle, NOx are trapped on the catalyst; intermittent rich excursions are used to reduce the NOx to N2 and restore the original catalyst surface; and lean operation then resumes. This review will describe the work carried out in characterizing, developing, and understanding this catalyst technology for application in mobile exhaust‐gas aftertreatment. The discussion will first encompass the reaction process fundamentals, which include five general steps involved in NOx reduction to N2 on NSR catalysts; NO oxidation, NO2 and NO sorption leading to nitrite and nitrate species, reductant evolution, NOx release, and finally NOx reduction to N2. Major unresolved issues and questions are listed at the end of ...

Nanogram nitrite and nitrate determination in environmental and biological materials by vanadium (III) reduction with chemiluminescence detection.

Abstract: Nitrite in environmental water samples is reduced at room temperature to nitric oxide in acidic medium containing vanadium (III). Nitrate is also rapidly reduced after heating to 80-90 degrees C. Nitric oxide is removed from the reaction solution by scrubbing with helium carrier gas and is detected by means of a chemiluminescence NOx analyzer. Nanogram detection limits are obtained. The method has the advantage of not requiring highly acidic solutions for nitrate reduction and has been applied to the analysis of a variety of environmental waters, sediment, plant materials, and human urine and blood serum.

A review of the selective reduction of NOx, with hydrocarbons under lean-burn conditions with non-zeolitic oxide and platinum group metal catalysts

Abstract: Research on the selective reduction of NOx with hydrocarbons under lean-burn conditions using non-zeolitic oxides and platinum group metal (PGM) catalysts has been critically reviewed. Alumina and silver-promoted alumina catalysts have been described in detail with particular emphasis on an analysis of the various reaction mechanisms that have been put forward in the literature. The influence of the nature of the reducing agent, and the preparation and structure of the catalysts have also been discussed and rationalised for several other oxide systems. It is concluded for non-zeolitic oxides that species that are strongly adsorbed on the surface, such as nitrates/nitrites and acetates, could be key intermediates in the formation of various reduced and oxidised species of nitrogen, the further reaction of which leads eventually to the formation of molecular nitrogen. For the platinum group metal catalysts, the different mechanisms that have been proposed in the literature have been critically assessed. It is concluded that although there is indirect, mainly spectroscopic, evidence for various reaction intermediates on the catalyst surface, it is difficult to confirm that any of these are involved in a critical mechanistic step because of a lack of a direct quantitative correlation between infrared and kinetic measurements. A simple mechanism which involves the dissociation of NO on a reduced metal surface to give N(ads) and O(ads), with subsequent desorption of N2 and N2O and removal of O(ads) by the reductant can explain many of the results with the platinum group metal catalysts, although an additional contribution from organo-nitro-type species may contribute to the overall NOx reduction activity with these catalysts. It is concluded, after the investigation of hundreds of catalyst formulations, that many of the fundamental questions relating to lean deNOx reactions have been addressed and the main boundary conditions have been established. It seems clear that catalysts with sufficient activity, selectivity or stability to satisfy the demanding conditions that appertain in automotive applications are still far away. The rapidly growing interest in NOx storage systems reflects this situation, and it now seems to be the case that acceptable direct NOx reduction catalysts may be very difficult to find for lean-burn applications.