NOx

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

The effect of zeolitic protons on NOx reduction over Pd/ZSM-5 catalysts

Abstract: Formation of PdO particles is observed in ion-exchanged Pd/HZSM-5 andPd/NaZSM-5 after reduction and reoxidation at 500°C. Formation of Pd2+ ions is not observed inPd/HZSM-5 in the absence of NOx, but in an atmorphere of NO2 + O2 the PdO particles are transformed into Pd2+ ions. Even inPd/NaZSM-5, roughly 20% of the PdO particles are converted to ions in NOx; the protons needed for this process are mainly formed during reduction of Pd2+ to Pd0. When a reduced and reoxidizedPd/HZSM-5 catalyst is exposed to CH4 + NO2 + O2, the rate of catalytic NO2 reduction to N2 slowly increases over 24 h from 20 to 50% while PdO is converted to Pd2+, but the rate of CH4 oxidation remains constant with time on-stream. Apparently, Pd2+ ions are needed for SCR of NOx, but for methane oxidation with O2, Pd2+ ions and PdO particles have similar activity.Pd/HZSM-5 is active even when Pd is initially present as PdO particles, butPd/NaZSM-5 remains inactive also when part of the Pd is present as Pd2+ ions. This shows that protons are not only required to convert PdO into Pd2+, but also thatPd/HZSM-5 acts as a bifunctional catalyst. Possible routes for CH4 and C3H8 activation are discussed.

Engine exhaust gas purification device

Abstract: An exhaust gas purification device comprising a three way catalyst, an oxidizing catalyst and a NOx absorbent which are arranged in this order in the exhaust passage. A rich air-fuel mixture is burned in the combustion chamber, and secondary air is supplied upstream of the three way catalyst so as to make the air-fuel ratio of the exhaust gas flowing into the three way catalyst a rich air-fuel ratio greater than the air-fuel ratio of the air-fuel mixture burned in the combustion chamber. Further, secondary air is supplied upstream of the oxidizing catalyst to make the air-fuel ratio of the exhaust gas flowing into the oxidizing catalyst and the NOx absorbent slightly leaner than the stoichiometric air-fuel ratio.

The formation of nitro-aromatic compounds under high NO x and anthropogenic VOC conditions in urban Beijing, China

Abstract: . Nitro-aromatic compounds (NACs), as important contributors to the light absorption by brown carbon, have been widely observed in various ambient atmospheres; however, their formation in the urban atmosphere was little studied. In this work, we report an intensive field study of NACs in summer 2016 at an urban Beijing site, characterized by both high- NOx and anthropogenic VOC dominated conditions. We investigated the factors that influence NAC formation (e.g., NO2 , VOC precursors, RH and photolysis) through quantification of eight NACs, along with major components in fine particulate matter, selected volatile organic compounds, and gases. The average total concentration of the quantified NACs was 6.63 ng m −3 , higher than those reported in other summertime studies (0.14–6.44 ng m −3 ). 4-Nitrophenol (4NP, 32.4 %) and 4-nitrocatechol (4NC, 28.5 %) were the top two most abundant NACs, followed by methyl-nitrocatechol (MNC), methyl-nitrophenol (MNP), and dimethyl-nitrophenol (DMNP). The oxidation of toluene and benzene in the presence of NOx was found to be a more dominant source of NACs than primary biomass burning emissions. The NO2 concentration level was found to be an important factor influencing the secondary formation of NACs. A transition from low- to high- NOx regimes coincided with a shift from organic- to inorganic-dominated oxidation products. The transition thresholds were NO2 ∼ 20 ppb for daytime and NO2∼25 ppb for nighttime conditions. Under low- NOx conditions, NACs increased with NO2 , while the NO 3 - concentrations and ( NO 3 - ) / NACs ratios were lower, implying organic-dominated products. Under high- NOx conditions, NAC concentrations did not further increase with NO2 , while the NO 3 - concentrations and ( NO 3 - ) / NACs ratios showed increasing trends, signaling a shift from organic- to inorganic-dominated products. Nighttime enhancements were observed for 3M4NC and 4M5NC, while daytime enhancements were noted for 4NP, 2M4NP, and DMNP, indicating different formation pathways for these two groups of NACs. Our analysis suggested that the aqueous-phase oxidation was likely the major formation pathway of 4M5NC and 3M5NC, while photo-oxidation of toluene and benzene in the presence of NO2 could be more important for the formation of nitrophenol and its derivatives. Using the (3M4NC + 4M5NC) ∕ 4NP ratios as an indicator of the relative contribution of aqueous-phase and gas-phase oxidation pathways to NAC formation, we observed that the relative contribution of aqueous-phase pathways increased at elevated ambient RH and remained constant at RH > 30 %. We also found that the concentrations of VOC precursors (e.g., toluene and benzene) and aerosol surface area acted as important factors in promoting NAC formation, and photolysis as an important loss pathway for nitrophenols.

Impact of the isoprene photochemical cascade on tropical ozone

Abstract: . Tropical tropospheric ozone affects Earth's radiative forcing and the oxidative capacity of the atmosphere. Considerable work has been devoted to the study of the processes controlling its budget. Yet, large discrepancies between simulated and observed tropical tropospheric ozone remain. Here, we characterize some of the mechanisms by which the photochemistry of isoprene impacts the budget of tropical ozone. At the regional scale, we use forward sensitivity simulation to explore the sensitivity to the representation of isoprene nitrates. We find that isoprene nitrates can account for up to 70% of the local NOx = NO+NO2 sink. The resulting modulation of ozone can be well characterized by their net modulation of NOx. We use adjoint sensitivity simulations to demonstrate that the oxidation of isoprene can affect ozone outside of continental regions through the transport of NOx over near-shore regions (e.g., South Atlantic) and the oxidation of isoprene outside of the boundary layer far from its emissions regions. The latter mechanism is promoted by the simulated low boundary-layer oxidative conditions. In our simulation, ~20% of the isoprene is oxidized above the boundary layer in the tropics. Changes in the interplay between regional and global effect are discussed in light of the forecasted increase in anthropogenic emissions in tropical regions.