Search

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.

23-28 july 2006 Présentation : P. Granger

Zeolites partially exchanged with copper, in particular faujasites, in which the copper does not largely occupy the small cages (for example the sodalite cages of the faujasites), are used as non-nitrogen-protoxide-generating SCR catalysts. Particular attention is drawn to certain mixed rare-earth/copper zeolites. The selectivity of the reduction of NOx to N2 is correlated with the TPR diagram of the catalysts.

International audience; An Fe-Zeolite-beta (Fe-BEA) catalyst, composed only of Fe cations or oxocations in charge compensation sites of the zeolite, is active in the simultaneous removal of NO (1500 ppm) and N2O (1000 ppm) by NH3 (2500 ppm) in the presence of O2 (3 vol.%). In temperature-programmed surface reaction (TPSR) experiments (ramp: 10Kmin?1 from 423 to 823 K, space velocity: 200,000 h?1), the light-off temperature, at 50% conversion, is shifted to lower values viz. by 20K for NO (590 K) and 40K for N2O (670 K), compared to those found when processing NO and N2O alone. It is proposed that the removal of surface oxygen O, coming from the interaction of N2O with iron sites, is faster with NO than with NH3. NO2 which is then formed reacts in turn very fast with NO and NH3 through the classical selective catalytic reduction (SCR) of NOx by NH3 in the presence of O2.

International audience

International audience

In the context of decreasing the emissions of greenhouse gases, a Fe-exchanged zeolite-beta (Fe-BEA) catalyst is shown to be very active in the reduction of N 2 O by NH 3 in the presence of O 2 . The temperature at which 50% N 2 O conversion is obtained is lower by ca. 80 K compared to its catalytic decomposition in the absence of NH 3 . TPR, TPO, and TPD experiments after treatments in various atmospheres provide evidence that the reaction involves the redox cycle Fe III ↔Fe II where the Fe III active species are Fe oxocations of low nuclearity. N 2 O decomposes into O * surface species on specific reduced Fe sites with the concurrent release of N 2 ; these species do not compete with O * coming from O 2 for their removal by NH 3 . In the absence of O 2 , catalytic experiments with 14 N 2 O and 15 NH 3 show that: (1) N 2 is mainly formed from 14 N 14 N–O splitting to yield 14 N 2 , the O * species being in turn removed by 15 NH 3 to give 15 N 2 ; (2) some 14 N– 14 NO bond splitting occurs, which leads to 14 N 15 N after reaction of 14 NO * and 15 NH 3 through a classical SCR mechanism. The Fe active species in the N–NO splitting are inhibited in the presence of O 2 . The kinetics of N 2 O reduction by NH 3 obeys a Mars and van Krevelen oxido–reduction mechanism modified with an inhibiting term of NH 3 .

International audience; The reduction of N2O to N2 was carried out by temperature programmed surface reaction (TPSR) from 473 to 873 K in He, He + 3% O2, He + 0.02% NH3 and He + 3% O2 + 0.02% NH3 on H–FAU supported transition metal catalysts. In the absence of oxygen, the addition of NH3 considerably shifts the N2O conversion profile towards lower temperatures. This is particularly true for the Ru– and Rh–FAU which exhibit the onset of N2O conversion around 500 K. The addition of O2 inhibits this effect for most of the catalysts and NO formation due to NH3 oxidation is observed. However Fe–, Ni– and Co–FAU kept the best catalytic behaviour for N2O reduction to N2 in the presence of O2, when NH3 was added to the feed with respect to the decomposition reaction.

International audience; 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.

International audience; Entre 300 et 800 K, N2 est toujours le produit majoritairement formé en oxydation de NH3 sur des catalyseurs à base de zéolithe faujasite NaY et de cuivre (Cu(x)-NaY, x étant le taux d'échange théorique). Les vitesses de conversion de NH3 en réduction catalytique sélective (SCR) de NO par NH3 (NH3 + O2 + NO), et en oxydation de NH3 (NH3 + O2) sont similaires sur Cu(195)-NaY. Elles diffèrent sur Cu(76)-NaY à basse température (