Search

Abatement processes for the reduction of N 2 O emissions from acid nitric plants can be implemented in different positions. Among the different possibilities, a catalytic process set up between the ammonia converter and the absorber could be a practicable solution. In those running conditions, at high temperature, in the presence of NO, O 2 and water, the catalytic decomposition of N 2 O (in the absence of a reducing agent) can take place. However, catalysts usually suffer from a strong deactivation owing to the occurrence of thermal sintering which significantly lowers their specific surface area. Catalytic testing performed at laboratory scale showed that zirconia based catalysts stabilised by yttrium incorporation could be of potential interest. However, the mode of yttrium incorporation seems to be a key factor. According to the preparation procedure, surface yttrium enrichment may occur and then strongly inhibit the catalytic decomposition of N 2 O. Co-precipitation methods can be profitably used for the preparation of modified-ZrO 2 catalysts in order to obtain yttrium homogeneously distributed in the bulk material. According to this preparation method, a synergy effect on the catalytic activity and also on the stability has been observed on ZrO 2 containing low amount of yttrium whereas an inhibiting effect prevails on highly loaded yttrium based catalysts irrespective of the mode of yttrium incorporation.

International audience; The kinetics of the selective catalytic reduction (SCR) of NO by NH3 in the presence of O2 has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458 < TR < 513 K, 250 < NO (ppm) < 3000, 1000 < NH3 (ppm)< 4000, 1 < O2 (%) < 4. The kinetic orders were 0.8–1 with respect to NO, 0.5–1 with respect to O2, and essentially 0 with respect to NH3. Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH3 was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) CuI is oxidized by O2 to “CuII-oxo”, (ii) “CuII-oxo” reacts with NO to yield “CuII-NxOy”, and (iii) finally “CuII-NxOy” is reduced by NH3 to give N2, H2O, and the regeneration of CuI (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that CuI or “CuII-oxo” species constitute the rate-determining active center.

International audience; The effect of a co-cation (Cs, Ba, Ca, Sr, Ba, La, Ce, Sm, Dy, Yb) on the properties of Cu exchanged faujasite catalysts in the selective catalytic reduction (SCR) of NO by NH3 in an oxidising atmosphere has been studied. Temperature programmed reduction (TPR) by H2 of Cu-faujasite (Cu-FAU) was used as a tool to identify and quantify the nature and location of Cu species. The ‘blocking' of sodalites cages of Cu-FAU by alkali earth or lanthanide ions enhanced the SCR activity at low temperature (LT) and made the reaction fully selective towards N2 in the whole temperature range studied (up to 773 K). The formation of N2O in Cu-FAU zeolites results from the reaction between nitrogen monoxide and ammonia on next-nearest-neighbour (NNN) Cu ions located in sodalite cages.

International audience; The selective catalytic reduction (SCR) of NOx (NO + NO2) by NH3 in O2 rich atmosphere has been studied on Cu-FAU catalysts with Cu nominal exchange degree from 25 to 195%. NO2 promotes the NO conversion at NO/NO2 = 1 and low Cu content. This is in agreement with next-nearest-neighbor (NNN) Cu ions as the most active sites and with NxOy adsorbed species formed between NO and NO2 as a key intermediate. Special attention was paid to the origin of N2O formation. CuO aggregates form 40–50% of N2O at ca. 550 K and become inactive for the SCR above 650 K. NNN Cu ions located within the sodalite cages are active for N2O formation above 600 K. This formation is greatly enhanced when NO2 is present in the feed, and originated from the interaction between NO (or NO2) and NH3. The introduction of selected co-cations, e.g. Ba, reduces very significantly this N2O formation.

International audience; The selective catalytic reduction (SCR) of NO (2000 ppm) by NH3(2000 ppm) in the presence of oxygen (3%) was carried out on Cu(x)-FAU (x=theoretical exchange degree) catalysts prepared by ion exchange or impregnation and calcined at 773 K. The samples were characterized by UV–visible and IR spectroscopy, temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), and temperature-programmed desorption (TPD) of NH3. Ion-exchanged Cu(x)-FAU contains mainly Cu ions located in both supercages and sodalite cavities. In contrast, the impregnated sample contains mainly CuO. Ionic Cu is more active and selective to N2than CuO in the temperature range 450–750 K. In contrast, CuO aggregates lead to significant formation of N2O, with a bell-shaped dependency centered at ca. 540 K. IR spectroscopy and TPD of NH3show that the last NH3ligand was removed from Cu ions above 550 K. The SCR on Cu ions obeys a Cu2+?Cu+redox mechanism in which Cu2+is reduced to Cu+by NO+NH3and Cu+is oxidized to Cu2+by NO+O2, with evolution of N2and H2O. Both reduction and oxidation steps of Cu in the catalytic cycle encompass the reduction of NO in agreement with the SCR of14NO with15NH3. A new overall SCR reaction below 550 K was proposed: 10NH3+13NO+O2?15H2O+(23/2)N2. The active sites below 550 K are formed by several Cu neighbor ions, maybe [CuOCu]2+, probably located in the supercages. All Cu ions become active above 600 K. The partial reduction of NO to N2O occurs at high temperature (>650 K) on exchanged samples. This formation, up to 17% at full NO conversion, is likely to take place on Cu ions located within the sodalite cavities.

Une serie de catalyseurs Cu-NaY, prepares par echange et impregnation, a ete caracterisee par reduction (TPR) et oxydation (TPO) en temperature programmee et a ete testee en reduction catalytique selective (SCR) de NO par NH 3 et en oxydation de NH 3 . La TPR a permis d'evaluer les differentes especes cuivre presentes (Cu 2+ . [Cu-O-Cu] 2+ , CuO) dans les solides prepares. La TPO de Cu + -NaY, obtenu par reduction de Cu 2+ -NaY par NH 3 montre que NO seul ne reoxyde pas Cu + en Cu 2+ en dessous de 873 K, et que cette reaction est favorisee en presence de NO/02 par rapport a 02 seul. En SCR de NO par NH 3 en presence de 3% O 2 trois pics de conversion de NO sont observes si la reaction est realisee en temperature programmee. Le premier pic de conversion vers 420 K est un phenomene transitoire en raison de l'impossibilite de reoxyder Cu + en Cu 2+ dans ce domaine de temperature . La deuxieme vague vers 500 K apparait des que la reoxydation par le melange NO/O 2 devient possible et serait due a la presence de Cu en supercages. La formation de N 2 O dans cette zone de temperature se produit en presence d'agregats d'oxyde de cuivre dans le solide. Enfin des que la reoxydation de l'ensemble des ions Cu + est possible, la conversion en NO devient totale pour des temperatures superieures a 610 K. La formation de N20 dans ce dernier domaine de temperature provient principalement de la reaction entre NO et NH 3 dans les cages sodalites mais aussi de l'oxydation de NH3.

The influence of ammonia on the reduction of N2O in presence of oxygen over Fe‐zeolite has been studied. It is found that BEA zeolite is the most efficient host structure for iron ions to catalyse the reduction of N2O with NH3.

Resume Entre 300 et 800 K, N 2 est toujours le produit majoritairement forme en oxydation de NH 3 sur des catalyseurs a base de zeolithe faujasite NaY et de cuivre (Cu( x )-NaY, x etant le taux d'echange theorique). Les vitesses de conversion de NH 3 en reduction catalytique selective (SCR) de NO par NH 3 (NH 3 + O 2 + NO), et en oxydation de NH 3 (NH 3 + O 2 ) sont similaires sur Cu(195)-NaY. Elles different sur Cu(76)-NaY a basse temperature ( 3 en SCR est toujours tres superieure a celle de l'oxydation. En SCR de NO sur Cu(76)-NaY, deux vagues de formation de N 2 O sont observees vers 500 K et au-dela de 600 K, tandis que sur Cu(25)-NaY, aucune trace de N 2 O n'est detectee. La formation de N 2 O a basse temperature est attribuee a la decomposition de nitrate d'ammonium forme sur des agregats d'oxyde de cuivre. La formation de N 2 O a plus haute temperature resulterait de l'oxydation de NH 3 et serait reliee a la presence d'ions cuivre isoles proches voisins, ou de dimeres [CuOCu] 2+ .

Two tautomeric dihydropurine derivatives, C 14 H 15 N 5 O, namely 2,2-dimethyl-9-phenyl-1,2-dihydropurine-6-carboxamide, (A) (coloured orange-red), and 2,2-dimethyl-9-phenyl-2,3-dihydropurine-6-carboxamide, (B) (yellow), are confirmed to have hydrogen substituents at N1 and N3, respectively. A different pattern of observed bond lenghts in the purine rings of (A) and (B) illustrates differences in conjugation in accordance with the different bond alternation. The tautomerism also gives rise to differences in the hydrogen bonding, although both tautomers have an internal hydrogen bond from an amide N-H to an N atom of the five-membered ring, with N...N distances 2.84 (2) A in (A) and 2.836(6)A in (B).

Virtual worlds represent a new market with a distinct economy andmany individuals are trying to exploit this very new technology in thesearch of profitable opportunities. The current paper proposes to studyentrepreneurship in the Massively Multiplayer Online Role-PlayingGames (MMORPG) Second Life® and Entropia Universe® in whichmonetary trades are possible. A survey was proposed to the community of players of both games, and from a sample of 244 players, nineteenentrepreneurs were contacted for a second survey. The traits of theentrepreneurs were compared to those of the players andentrepreneurship was observed in Second Life® and Entropia Universe®. In fact, all the necessary conditions are present for entrepreneurship: a new technology giving new sources of revenues, an entrepreneur willing to invest money in order to increase his wealth, and a market with an economy well understood. The different entrepreneurs have developed successful ventures in several markets, and they had well defined the strategy they wanted to adopt. They have examined the different markets in which they have entered although they did not use all the tools known in the marketing fields. Further, some steps in the process of creation of the venture may not be important and some may be done relatively swiftly, thus the venture creation in MMORPG may be relatively easy. In conclusion, the venture creation may be relatively undemanding in virtual worlds, and this opens new possibilities for the future.

Publisher Summary This chapter presents the effect of the reductant nature on the catalytic removal of nitrous oxide (N2O) on an iron (Fe)-zeolite-beta catalyst. A Fe(97)-BEA catalyst, prepared by conventional ion-exchange procedure and calcined at 773 K, almost contains iron-binuclear-oxo-species in charge compensation of the BEA structure. The re-oxidation of iron (II) species by N2O leads to new oxo-species reducible at lower temperature than the dimer species. Among hydrogen (H2), carbon monoxide (CO), propene, and ammonia (NH3) in the reduction of N2O, CO, propene, and NH3 are selective reductants. For these three reductants, a similar light-off temperature of ca 638 K is obtained in the selective catalytic reduction of N2O. CO is efficient from 473 K, but its activity is limited by the reoxidation of Fe II to Fe III species by N2O.

The selective catalytic reduction (SCR) by NH3 is the most important technology to control the emissions of NOx from stationary sources. A new mechanism is proposed for the SCR of NO on Cu-exchanged FAU catalysts which markedly differs from that occurring on the V2O5-TiO2 catalysts; NO takes part in the reoxidation of Cu+ to Cu2+, but not of V4+ to V5+. This conclusion was achieved by decomposing the catalytic cycle into the oxidation and reduction steps of Cu species by reactant mixtures composed of NO, O2, and NH3 and the comparison with the global SCR reaction. A quantum chemical (QM) modeling of the catalytic site has been untertaken, leading to a better knowledge of the Cu electronic structure.

A Fe-exchanged zeolite beta is shown to be very active in the reduction of N20 to N2 by NH3 in presence of O2. The temperature of 50% N20 conversion is lower by ca 80 K than that of its catalytic decomposition in absence of NH3. Kinetics with 14N20 and 15NH3 has shown that N20 is converted at ca 86% through N-0 splitting and 14% N-N splitting. The former reaction involves the redox cycle Fe3+ ↔ Fe2+ where the active species are likely binuclear Fe oxocations. This conclusion was reached from TPR, TPO and TPD experiments. The intimate mechanism and the sites responsible for N-N splitting were not yet identified.

Activity in n-butane isomerization reaction of various alkaline salts of H 3 PW 12 O 40 and H 4 SiW 12 O 40 was shown to be strongly dependent on the strength and number of accessible protons whereas the stability with time on stream was correlated to the presence of mesoporosity. For the liquid iC 4 /C 4 =continuous alkylation reaction, the strength and the number of acid sites appeared less important than the existence of mesoporosity indicating that the diffusion of the reactants and of the products plays an important role in this reaction.

Several retrospective community surveys have provoked speculation concerning the safety of carotid endarterectomy (CEA) throughout the United States. To address this serious issue, surgical outcome was calculated for 8535 CEAs entered prospectively into a computer registry by 51 trained vascular surgeons in a large metropolitan area from 1973 to 1985. A total of 7480 procedures (88%) were performed for symptomatic lesions and 1055 (12%) were performed for asymptomatic stenosis or ulceration. There were 178 operative strokes (2.1%) and 135 early deaths (1.6%), for a combined morbidity-mortality rate of 3.2%. Fatal events were attributed to cardiac disease in 0.7%, neurologic complications in 0.6%, and other causes in 0.3%. The stroke rate (n = 148; 2.0%) in symptomatic patients was better than that in asymptomatic patients (n = 30; 2.8%) (chi 2 = 5.2; p less than 0.025), but the combined morbidity-mortality rates (2.9% and 3.7%) were not statistically different. The incidence of stroke reported by surgeons who performed more than 5 CEAs annually (1.7%) was statistically superior to the stroke rate (3.4%) among those with less experience (chi 2 = 37.1; p less than 0.0001). Nevertheless, both groups had acceptable results that were consistent with their training and continued interest in vascular surgery.

This paper discusses the development of a simulation package for investigating ramp-freeway accidents. Ramp-freeway accidents often occur due to close car following, lack of time for ramp drivers to look for freeway gaps and/or inadequate acceleration lane length. Rear end collisions can occur on ramps because of close car following and differential gap acceptance criterion of ramp vehicle. This software enables the user to vary the characteristics of the ramp vehicles and permits him to conduct a simulation run on conditions which may lead to accidents in the ramp-freeway