The selective catalytic reduction (SCR) of nitric oxide by propene under lean-burn conditions over Ir/Al2O3 and Ir/SiO2 was studied. Irrespective of the support, Ir supported catalysts were found to be active in the reduction of NO above 673K, Ir/SiO2 being the most active. For both catalysts, the activity in the reduction of NO into N2 was shown to be strongly enhanced after their exposure at 873K under the reaction mixture. In spite of varying initial Ir dispersions depending on the metal content and the support, Ir sintered after activation in the reaction mixture, which led to the same final Ir dispersion. An in situ FTIR study of both catalysts at various temperatures under the reaction mixture and under CO or NO used separately was carried out. It was shown that, under the reaction mixture, the surface of initially fully oxidised Ir particles progressively reduced with increasing temperatures into partially reduced Irδ+ surface species allowing the adsorption of both CO and NO (νCO at 2070–2050cm−1 and νNO at 1870cm−1). Additional species (formate, acetate, nitrate) formed only on the Al2O3 support surface but these species are thought not to participate into the NO reduction.

The interactions of CO with a high specific surface area tin dioxide was investigated by FTIR spectroscopy and thermogravimetric analysis. FTIR study of CO interactions have shown that CO can adsorb on cus (coordinatively unsaturated sites) Sn4+ cation sites (band at 2201 cm-1). In addition, CO reacts with surface oxygen atoms. This leads to the partial reduction of SnO2 surface and to the formation of ionised oxygen vacancies together with the release of free electrons, which are responsible for the loss of transmission. Formed CO2 can chemisorb on specific surface sites: on basic sites to form carbonates species and on acidic sites (Sn4+-CO2 species) which is in competition with the formation of Sn4+-CO species. TG experiment have shown that the reduction of SnO2 by CO at 400°C occurs in two steps. First, the reduction of SnO2 surface, which is a quick phenomenon. This has allowed to evaluate that more than 12% of reducible surface oxygens can react with CO, essentially because of the presence of a large amount of surface hydroxyl groups. The second step of the reduction of SnO2 would be the progressive reduction of SnO2 bulk by the slow diffusion of oxygen atoms from the bulk to the surface.

The adsorption of NO and its reaction with CO have been investigated on Pt/Al2O3 and Rh/Al2O3 by in situ infrared spectroscopy. Different spectroscopic features have been observed on Rh and Pt after NO exposure at various temperatures, mainly related to the build up of positively, neutral and negatively charged NO species on Rh, with neutral NO species predominating on Pt. The formation of Rh‐NO+ and gem‐dicarbonyl species RhI(CO)2 during the adsorption of NO and the CO + NO reaction has been associated mainly with surface Rh oxidation at low temperature. On the other hand, the reductive properties of CO prevail above 250°C leading to the disappearance of the gem‐dicarbonyl structure. These infrared observations have been discussed on the basis of previous kinetic observations. Copyright © 2002 John Wiley & Sons, Ltd.

The influence of ceria on the kinetic behavior of noble metals in the reduction of NO by CO has been investigated at 120 and 300°C on a freshly prepared Pt–Rh/Al2O3–CeO2 catalyst, and on an aged catalyst after reaction for 16 h at 500°C, within pressure ranges of 2.3–9×10−3 atm for CO and 1.5–8×10−3 atm for NO, using a differential fixed-bed flow reactor. These two temperatures have been selected from temperature-programmed experiments because they correspond to two very different regimes of activity of noble metals with ceria. It has been found that a rate equation derived from a bifunctional mechanism involving reaction paths either on metal or on ceria can correctly fit rate measurements performed at 120°C. In contrast a conventional mechanism earlier proposed for modeling the CO+NO reaction on Pt–Rh/Al2O3 at 300°C, where only noble metals are involved, enables modeling of rate measurements at 300°C on Pt–Rh/Al2O3–CeO2, which suggests that the interaction between ceria and noble metals is suppressed at that temperature. The temperature dependency of the rate constants and equilibrium constants for NO and CO adsorption has been quantified in order to explain such changes in the kinetic behavior of Pt–Rh/Al2O3–CeO2.

Nanosized cassiterite-type samples have been prepared following two different methods: nitric acid attack on metallic tin and neutralisation of SnCl4 solution by hydrazine. Samples have been characterised by different physico-chemical techniques (X-ray diffraction (XRD), isotherm measurements of N2 adsorption–desorption, scanning electronic microscopy (SEM), FTIR and UV–VIS spectroscopy). Structural, textural and semi-conductive properties have been thus investigated and their evolution as a function of the temperature of calcination in flowing oxygen up to 600°C has been studied. The progressive formation of oxygen vacancies, leading to non-stoichiometric SnO2, was observed during this treatment for both samples. An important feature is that, after this treatment, no oxygen vacancies could be observed at the crystallites surface, their formation requiring specific treatments like outgassing at high temperatures. Strong differences in textural properties, types of surface hydroxyl species and concentration of oxygen vacancies at the crystallites surface have been also evidenced between the two samples.

The Lewis and Bronsted acidic properties of the surface sites of a high surface area tin(IV) oxide sample were investigated by the CO adsorption at low temperature using IR spectroscopy. Two distinct cus (coordinatively unsaturated sites) Sn4+ cation sites having different Lewis acidic strength have been evidenced: (i) Sn4+ I strongly bonding CO (nu(co) = 2210-2196 cm(-1)), (ii) Sn4+ (II) (nu(CO) = 2183=2177 cm(-1)), being less acidic. Several types of OH groups have been also evidenced: (i) OH occluded in the bulk and inaccessible to CO (nu(OH) = 3445- 3435 cm(-1)), (ii) non-acidic OH groups (nu(OH) = 3740-3725 and 3660 cm(-1)) which give rise to a nu(CO) band at 2155 cm(-1) and nu(OH) bands respectively at 3640 and 3570 cm(-1) (Deltanu(OH) = 90 cm(-1)) as CO adsorption occurs, (iii) slightly Bronsted acidic OH groups (nu(OH) = 3625 cm(-1)) which lead to a nu(CO) band at 2164 cm(-1) and a n band at 3740 cm(-1) (Deltanu(OH) = 155 cm(-1)). Depending on the treatment in vacuum (at room temperature or at 773 K) prior to CO adsorption, the populations of these different sites may vary. An outgassing in vacuum at 773 K leads to the partial dehydroxylation of the SnO2 surface, increasing the number of Sn4+ Lewis acidic sites.

Platinum-based catalysts can perform the selective reduction of NOx under lean-burn conditions, though they function only over a narrow activity–temperature window and form undesirably high quantities of N2O. With these stationary and transient experiments conducted using a Pt/SiO2 catalyst, we attempted to better understand the mechanism of SCR NO when C3H6 is used as reductant. We propose a redox-type reaction pathway featuring NO adsorption as well as surface oxidation and reduction. To this end, a precise balance in hydrocarbon and oxygen surface coverage is required.

The selective catalytic reduction (SCR) of nitric oxide by propene over Ir/Al2O3 under lean-burn conditions (1000 vpm NO, 2000 vpm C3H6, 500 vpm CO, 10 vol.% O2) was studied. The activity was shown to be strongly enhanced after exposure of the catalyst at 600°C under the reaction mixture, irrespective of the oxidising or reducing pre-treatment. Simultaneously, the Ir dispersion decreased from 78 to 10%. The influence of each component of the reaction mixture on the activation process was examined. The presence of both CO and O2 was found to be necessary to activate Ir/Al2O3 while NO would not be. In situ FT-IR results revealed that initially fully oxidised Ir particles partially reduced in the feed to form Ir0 reduced surface sites (νCO at 2060 cm−1) which adsorbed CO up to 350–400°C. The activation under reactants was related to the formation of these sites. The presence of reduced (or partially reduced) Ir sites, possibly siting at the surface of IrO2 particles and stabilised by CO adsorption, was proposed to be responsible for the SCR activity.

The surface composition of alumina-supported Pd, Rh, and Pd–Rh catalysts fresh or after high temperature treatments under oxidizing (O2 in N2) or reducing (H2 in N2) mixture has been characterized using various complementary techniques: chemisorption and titration of H2 and O2 probe molecules, 18O2/16O2 isotopic equilibration (OIE), FTIR spectroscopy of adsorbed CO and NO, and electron microscopy with X analysis. From both techniques it is concluded that the surface composition is close to the bulk composition for all the fresh bimetallics, whatever the Rh content. The surface state of the bimetallics treated at 1173 K is strongly dependent on the nature of the gas mixture. There is a relative increase in surface Pd after treatment in an oxidative medium, because of a Rh3+ migration into alumina, and in surface Rh after treatment in a reducing medium. The specific role of alumina can explain these results, in contradiction with thermodynamic models predicting the reverse situation (enrichment in Rh in O2 and in Pd in H2). Activity for propane oxidation at 573 K and for propane steam reforming at 673 K was also determined over fresh and sintered catalysts. Because Pd is more active than Rh in oxidation and Rh is more active than Pd in steam reforming, activity changes with the Rh content, before and after sintering, may reflect modifications of surface composition. The results obtained with the series of Pd–Rh catalysts are in agreement with the measurements made by OIE and FTIR spectroscopy. The hypothesis and the conditions of applications of the methods are discussed.

The generation of N 2 and N 2 0 during NO decomposition and reduction on Pt/SiO 2 was investigated by means of a TAP reactor. With rich mixtures the hydrocarbon serves to create free surface sites for NO decomposition and/or to induce a direct reaction of NO with carbonaceous residues, especially for C 3 H 6 . Furthermore, results of both transient and steady-state experiments indicate that adsorbed N 2 0 is an intermediate leading to N2.

Platinum-based catalysts can be used for the selective reduction of NOx in lean burn conditions, but they form undesirably high quantities of N2O. In this study conducted under stationary and transient conditions, we attempted to better understand the mechanism of SCR (NO) over Pt. Me found that N-2 selectivity increased with contact time, that significant quantities of N-2 were formed when N2O was used as the reactant, and that N2O was formed more quickly than N-2 when NO was used as the reactant. These results led us to propose a kinetic model in which adsorbed N2O is an intermediate for NO reduction into N-2. (C) 2000 Academie des sciences Editions scientifiques et medicales Elsevier SAS.

Various Pd based catalysts supported on silica and containing manganese have been prepared and characterized. Two Mn species have been detected, i) reduced Mn in direct interaction with Pd on the metallic particles ii) oxidised Mn layed on SiO2 and showing "reactive oxygens". An increase in activity for reduction of NO is observed on the catalysts containing Mn with an optimum for a Pd/Mn atomic ratio of about one. This enhancement of activity is due to either the presence of Pd-Mn dual sites or to a bifunctional mechanism between reduced Pd and oxidised Mn at the vicinity of Pd.

This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H2O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into Nz increases with the number of isolated Cu2+/Cu+ ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H2O) mixture, because of nitric acid formation linked to NO2 which is either formed in the pipes of the apparatus or on the catalyst itself.

This paper deals with the redox properties of Cu ions implanted in ZSM-5 and supported on Al2O3, catalysts active in the selective reduction of NO by hydrocarbons such as propane. Data on the reducibility of the Cu systems in various atmospheres (vacuum, CO, H2(,) O-2) and on their DeNO(x) activity are presented. The methods used to obtain informations on the surface and bulk transformations (and their link with catalytic behaviour) are complementary: UV-visible diffuse reflectance spectroscopy being useful to detect the presence of Cu2+ and Cu-0, while Cu+ is detected indirectly by the analysis of the IR spectrum of CO bound selectively to this cation.The main contributions to the previous knowledge are the following: it is possible to distinguish CO bound to isolated and non-isolated Cu+ ions; the isolated Cu2+ ions are reducible under vacuum without participation of organic impurities; the more active solids for the NO reduction into N-2 are characterized by the presence of isolated Cun+ ions beside the additional influence of the zeolitic framework; after the formation of Cu+ ions the redox cycles are reversible but, after the formation of Cu-0, the reversibility or irreversibility of the redox cycles and the restoration of the SCR activity are function of the copper content; the activity decreases after agglomeration into bulk oxides; there is no formation of bulk CuO during the reaction and, with reducing and moderate oxidizing mixtures, part of the copper remains as cuprous ions. (C) 1998 Elsevier Science B.V.

Catalysts with Pd deposited on various supports such as ZrO2 or mixed supports such as Al2O3-ZrO2-BaO have been used for the elimination of NOx. Their activity in the presence of complex mixtures like CO-NO-O-2-C3H6-CO2-H2O is better than that of Pd/Al2O3. This increase in activity is due not to a modification of the electronic properties of Pd but to a direct participation of the support in the process. From the analysis of the experimental results, a bifunctional mechanism is proposed and discussed.

The reduction of NO by CO is performed over palladium supported on activated carbons and on a graphite of lower surface area. The presence of very low amounts of Pd gives active and stable catalysts towards the NOCO reaction and the contribution of the surface carbon atoms to the process is negligible, the presence of CO lowering the consumption of the carbon support by NO. In the absence of any other reductant in the gas phase, NO is reduced by reaction with the carbon atoms. In the presence of an excess of O2 (net-oxidizing NOCOO2 mixtures), the consumption of the carbon supports by O2 is important. The reactions of NO with the C atoms are also inhibited, NO reacting with CO but O2 reacts with both reductants, CO and C.

Cu/Al2O3 solids with various Cu loadings, between 0.3 and 6.4 wt %, are used for the reduction of NO by propane in the absence and in the presence of oxygen (up to 10 vol. %) in the 423 - 773 K temperature range. At a given temperature and for high Cu loadings, the introduction of oxygen induces a decrease in the activity in nitrogen formation. For low Cu loadings the activity increases with the oxygen content in the 1-2 vol. % range, then slightly decreases for higher oxygen amounts. The nature of the Cu species accessible to CO and NO is determined by infrared spectroscopy. High Cu loadings favor the formation of bulk oxides at the surface of the support whereas low Cu loadings favor the formation of isolated Cu species. The selective reduction of NO is thus related to the presence of these isolated copper species easily reduced and reoxidized.