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DISPLAYTITLE:NOx In atmospheric chemistry, is shorthand for nitric oxide (NO) and nitrogen dioxide (NO2), the nitrogen oxides that are most relevant for air pollution. These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone. gases are usually produced from the reaction between nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air; especially at high temperatures, such as in car engines. In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution. gases are also produced naturally by lightning. does not include nitrous oxide (N2O), a fairly inert oxide of nitrogen that contributes less severely to air pollution, notwithstanding its involvement in ozone depletion and high global warming potential. is defined as the sum of plus the compounds p

Estimation of the total VOC reactivity with OH by pump-and-probe OH lidar to determine a new indicator for ozone production

Many of our most severe problems of environmental pollution involve the Earth's atmosphere. The adverse effects of atmospheric pollution are particularly evident in the low troposphere, especially in urban/suburban areas where many sources contribute to the atmospheric burden. Ozone (O3) is one of the major constituents of the air pollution over cities commonly called photochemical smog. It is a secondary pollutant, produced by the reaction of primary pollutants, nitrogen oxides (NOx) and hydrocarbons (VOC), in the presence of sunlight. Although hydrocarbons and NOx have long been identified as the two key precursors to photochemical O3, the development of an effective strategy for reducing ozone production in photochemical smog by controlling the anthropogenic emissions of these precursors has proved to be problematic. The objective of the present study is to present and demonstrate the robustness of a new observable "indicator" for determining the sensitivity of ozone formation to VOC and NOx (VOC- and NOx-limited regime). This indicator is based on the reactivity of VOC and NOx with respect to OH. Simulation tests of the ability of this new indicator Θ = τOHVOC/τOHNOx to distinguish between a NOx and a VOC-limited regime were performed with a box model and a 3D model applied to a specific episode of photochemical smog (Athens) and of a less polluted area (the Swiss plateau). Whereas the NOx reactivity 1/τOHNOx can easily be measured by standard methods, the total VOC reactivity 1/τOHVOC will be estimated by measuring the inverse lifetime of OH with a new Pump-and-Probe Lidar instrument. The method consists of producing high OH concentrations by the flash photolysis of ozone and the subsequent O(1D) reaction with H2O and by following the OH relaxation by laser-induced fluorescence (LIF). Numerical simulation of the experiment will show that the total VOC reactivity with OH can be obtained by measuring the inverse lifetime of the OH and its absolute initial concentration with the Pump-and-Probe experiment as well as the NOx, CO and O3 concentrations with standard trace-gas detectors. This total in-situ VOC reactivity will be used to determine the new indicator Θ but will also help to test the accuracy of the VOC-lumped chemical mechanism in the numerical atmospheric simulation. In order to demonstrate the feasibility of the Pump-and-Probe experiment, laboratory investigations have been performed in controlled atmosphere: OH absorption and fluorescence emission spectra allowed the OH transition bands and the non-resonant fluorescence wavelength to be identified. High-resolution OH absorption spectra allowed the OH effective absorption cross section to be determined for assessing the experimental sensitivity and retrieving the OH concentration. The experimental retrieval of the rate constant for the OH+CH4 and OH+NO2 reactions reveals the ability of the pump-and-probe experiment to monitor the chemical kinetics of OH at atmospheric pressure and room temperature. OH relaxation measured in different VOC/NOx/O3 gas mixtures confirm the relevance of the model predictions of the OH relaxation approximated by a first-order chemical decay and the validity of the total VOC reactivity with respect to OH estimated by such an approximation. A first in-situ OH relaxation measurement realized directly in the air of the laboratory showed that the pump-and-probe method could be performed under typical PBL conditions. Preliminary range-resolved measurements yielding vertical profiles of the OH concentrations produced by flash photolysis of ambient ozone suggest a promising future for the Lidar pump-and-probe technique. The perspectives for the Pump-and-Probe Lidar experiment are expressed in the concluding section.

EPFL2000

Study of N2O decomposition over Fe-ZSM-5 with transient methods

Petra Prechtl

Nitrous oxide is the third most important gas contributing to global warming and its concentration in the atmosphere is still on the rise. Nitric acid production represents the largest source of N2O in the chemical industry, with a total annual emission of about 400 kt N2O. An efficient way to reduce the N2O is the catalytic decomposition into N2 and O2. ZSM-5 zeolites containing iron are known to be effective in this reaction involving Fe(II)-extraframework species as active sites. But the nature of these sites and the reaction network is still under debate. Often the N2O is in the exhaust together with other gases like NOx, COx, oxygen, and water vapor, which may influence the catalyst efficiency towards N2O decomposition. The effect of these gases on the reaction mechanism is still not understood yet. The aim of this work is to study the decomposition mechanism of N2O over isomorphously substituted Fe-ZSM-5 (0.02-0.55% Fe) using transient response methods. Kinetic studies under steady-state conditions give overall information about the reaction, whereas transient experiments lead to information on the individual steps and the possibility to suggest a reaction model. Transient response methods are applied in vacuum using a Temporal Analysis of Products (TAP) setup and at ambient pressure using a Micromeritics AutoChem 2910 analyzer. The influence of different gases such as NO, H2O and NO2 on the decomposition of N2O over Fe-ZSM-5 is investigated to gain insight into their effect on this reaction. The TAP multifunctional reactor system is used to perform transient pulse experiments for micro-kinetic analysis of complex heterogeneous catalytic reactions. This setup allows to dose precisely reactants and to monitor product formation with submillisecond time resolution implying the absence of external mass or heat transfer. A reproducible low hydroxylation level of the catalyst can be attained in vacuum. The results from the transient responses with the Micromeritics analyzer are obtained under conditions more relevant to "operando". The mechanism of N2O decomposition over Fe containing ZSM-5 was studied by transient response methods at ambient pressure with the Micromeritics analyzer and under vacuum with the TAP setup. Both approaches showed that the mechanism strongly depends on the reaction temperature, but in all cases involves atomic oxygen loading from N2O with N2 evolution as the first reaction step. The surface oxygen, (O)Fe, formed from N2O possesses very high reactivity and oxidizes CO to CO2 already at 373 K. A complete saturation of the active sites can be reached by the catalyst exposure to N2O at low temperatures (T < 600 K in vacuum, T < 523 K at ambient pressure). Under these conditions oxygen is stored and does not desorb. On that basis, the concentration of sites active for the surface atomic oxygen loading from N2O is determined by pulse experiments using the TAP setup. The concentration of active sites is found to be the same as in a flow experiment with the Micromeritics analyzer. The mechanism of surface atomic oxygen recombination and desorption as O2 depends on the temperature. At temperatures higher than 773 K, the mechanism probably involves direct (O)Fe recombination. The investigation of the response profiles upon N2O pulses at 803 K evidences the O2 formation is the rate-determining step since the oxygen desorption is found to be slow compared to N2O and N2. Furthermore, a fit of the oxygen desorption curve corresponded to a first order dependence on the oxygen surface coverage. In the range of 523 to 773 K, the O2 formation is facilitated by NO, which is formed on the catalyst surface from N2O. The addition of NO to the gas phase confirmed the accelerating effect of NO on the oxygen desorption. NO and (O)Fe are adsorbed on adjacent, but different sites and are not competing for the same ones. Adsorbed NO accommodates oxygen from N2O forming higher oxidized nitrogen oxide species NO2/NO3 which decompose back to NOads and molecular oxygen. Thus, the recombination of surface oxygen loaded from N2O is accelerated by a NO/NOx redox cycle whereas adsorbed NOx acts as co-catalyst. Adsorbed NO is oxidized by N2O easily as shown also by temperature-programmed desorption (TPD) experiments. Therefore, the interaction of gaseous NO2 with Fe-ZSM-5 at 523 K in vacuum and at ambient pressure is investigated and demonstrates the formation of two different NOx surface species accompanied by the evolution of gaseous NO. These surface species block the sites for oxygen loading from N2O whereas the amount of reversible adsorbed N2O stays constant compared to the standard pre-treated catalyst. The formation of reactive surface oxygen from NO2 could neither be evidenced in TPD nor with CO oxidation experiments. Adsorbed water strongly influences the N2O decomposition inhibiting it completely, at least at temperatures ≤ 673 K. Water pulses at 523 and 593 K result in partial desorption of the loaded atomic surface oxygen as O2, indicating the competition of water for the same adsorption sites. The residual adsorbed oxygen desorbs during TPD experiments at 849 K instead of 715 K for the dry catalyst and it can not oxidize CO to CO2. It means that the active sites are transformed into an inactive form in the presence of water. The oxygen which stays on the catalyst seems to participate in the Fe(II) oxidation to the hydroxylated Fe(III) species. These Fe(III) species are inactive in N2O decomposition and could be activated again by high temperature treatment in He or in vacuum. During TAP experiments only small amounts of water are dosed and the water desorption is facilitated due to vacuum. Therefore, the effect of water could be shown to be reversible at temperatures > 700 K. At 803 K oxygen desorbs significantly faster with increasing water amount suggesting a water assisted O2 desorption. Simultaneously, the catalyst activity decreases to about a half of the initial activity of the dry zeolite. To conclude, from the TAP reactor data obtained under vacuum conditions and the results from the transient flow experiments at ambient pressure it follows that the mechanism of the N2O decomposition over Fe-containing ZSM-5 depends strongly on the reaction temperature and on the presence of NO, H2O and NO2. In general, below 523 K only surface atomic oxygen loading from N2O takes place with simultaneous evolution of N2 in the gas phase. The O2 formation proceeds via direct (O)Fe recombination at temperatures above 773 K. In the range of 523 to 773 K, the recombination of the atomic oxygen involves NOx adsorbed on the catalyst surface.

EPFL2007

SWISS MOTOR. Modification d’un moteur diesel pour le fonctionnement au gaz naturel en cogénération. Partie 1: fonctionnement avec mélange stoechiométrique (l = 1).

Daniel Favrat, Roger Röthlisberger

In the frame of the decentralised electrical power and heat cogeneration with stationary I. C. engines operating on natural gas, the Swiss limit for exhaust gas emissions, with 80 mg/mN3 (5% O2), is the most severe in force on an international level. On the other hand, the only technology able to meet this requirement, with the use of a stoechiometric mixture and a three way catalyst, allows a relatively low global engine efficiency and is characterised by a limited reliability of the exhaust gas treatment system. In this context, the Laboratory of Industrial Energy Systems of the EPFL has undertaken, on the basis of a converted diesel engine for natural gas operation, to develop new technologies (mixture, ignition, ...) and to evaluate their potential to reduce exhaust gas emission, particilarly NOx, and reliability. The objective of the first part of this project is to study the behaviour of a naturally aspirated version of the engine “Liebherr G926”, fuelled with a stoechiometric mixture. The main engine characteristics and perfomances have been used to reduce noxious emissions. The main results obtained are: A variation of the spark timing at wide open throttle has allowed to determine the best operating condition with a mixture of l=1.001. The maximal mechanical power output of 106.3 ± 0.5 kW at 1502 ± 5 rpm and the maximal global efficiency of 35.9 + 1,7/-0.6% have been achieved at a spark timing of 20 °CA BTDC. The lowest cycle to cycle variability, with a coefficitent of variability of the mean effective pressure smaller than 1.5%, is reached at the same spark timing (Figure 5.11). No occurrence of knock has been noticed on the spark timing range between 5 and 30 °CA BTDC. An increase of the spark gap from 0.2 up to 1.2 mm reduced the combustion cycle by cycle variability (Figure 5.21) without modifying the engine performances and emissions (Figures 5.15 and 5.17), at full as well as at half load. A increase, respectively a decrease of two units of the spark thermal index from a basis value of 5 has no significant influence on engine performances and emission (Figures 5.23 to 5.26). Similarly, an increase of the central electrode length from 1 up to 3 mm has no detectable effect on the engine behaviour (Figures 5.23 5o 5.26). The use of a three way catalyst for the exhaust gas treatment allows to reduce the NOx and CO emissions well under the Swiss limits (Figure 5.35).

1996