The effect of CO2 and H2O on the NO/O-2 storage over a Pt-Ba/Al2O3 LNT catalyst was investigated in the temperature range 150-350 degrees C by combining FT-IR spectroscopy and microreactor flow experiments. It is found that the presence of water shows negligible effect on the NO storage at all the temperatures studied. Otherwise, the presence of CO2 inhibits the "nitrite route" (initial formation of surface nitrites and their subsequent evolution to nitrates), whereas the "nitrate route" (involving NO oxidation to NO2 over Pt and its subsequent adsorption on Ba in the form of nitrates) proceeds as in the absence of CO2. The inhibition effect of CO2 on nitrite formation increases on increasing the temperature. This behavior is likely related to the different thermal stability of nitrites and carbonates and, in particular, to the decreasing ability of nitrites to displace carbonates on increasing temperature. Simultaneously, the presence of CO2 in the feed significantly shortens or eliminates the dead time in the NO breakthrough at the reactor outlet. As a consequence, the presence of dead time in the absence of CO2 has been correlated to the occurrence of the nitrite route. (C) 2014 Elsevier B.V. All rights reserved.

The effect of water, in the temperature range 25-350 °C, and ammonia at RT on two different surface species formed on Pt-K/Al2O3 and Pt-Ba/Al2O3 NSR catalysts during NO(x) storage-reduction cycles was investigated. The surface species involved are nitrates, formed during the NO(x) storage step, and isocyanates, which are found to be intermediates in N2 production during reduction by CO. FT-IR experiments demonstrate that the dissociative chemisorption of water and ammonia causes the transformation of the bidentate nitrates and linearly bonded NCO(-) species into more symmetric species that we call ionic species. In the case of water, the effect on nitrates is observable at all the temperatures studied; however, the extent of the transformation decreases upon increasing temperature, consistent with the decreased extent of dissociatively adsorbed water. It was possible to hypothesize that the dissociative chemisorption of water and ammonia takes place in a competitive way on surface sites able to give bidentate nitrates and linearly bonded NCO(-) that are dislocated, remaining on the surface as ionic species.

Supported nickel catalysts were obtained by exchange of Mg/Al Layered Double Hydroxides (LDHs) compensated with NO3- ions with negatively charged Ni complexes, followed by thermal reduction. With this aim, suspensions of Ni complexes were prepared by controlled hydroxylation of Ni2+ cations in the presence of citrate (obtaining [Ni(C6H5O7)(OH)]y2y- species) or chloride (obtaining [NiCl4]2− species) complexing ions. For comparative purposes, other two supported Ni catalysts were prepared starting from Mg/Al LDHs compensated with Cl− ions and thereafter exchanged with [Ni(C6H5O7)(OH)]y2y- species (nominal degree of exchange: 100% and 20%).

Supported Ni catalysts (2 wt.% Ni) were investigated by FT-IR and UV-vis-NIR spectroscopy, using CO2, CH3CN and CO as probe molecules. The supports studied range from acidic (SiO2), via amphoteric (Al2O3, Mg(Al)O) to basic oxides (MgO, CaO).CO2 adsorption experiments allowed to obtain the following qualitative scale for the basic strength of O-2-sites and Mn+O2- pairs: Ni/CaO > Ni/MgO >= Ni/Mg(Al)O >> Ni/Al2O3. On Ni/SiO2 these sites are absent.CH3CN adsorption allowed to reveal acidic and strong basic Lewis sites with the following results: Ni/SiO2 does not contain basic or acidic Lewis sites. Ni/Al2O3 contains acidic Lewis sites but no strong basic Lewis sites. The ratio between the strong basic and the acidic Lewis sites decreases in the order: Ni/MgO > Ni/CaO > Ni/Mg(Al)O. The strength of Lewis acid sites decreases in the order Ni/Al2O3 >> Ni/Mg(Al)O > Ni/MgO > Ni/CaO.Interaction of CO at room temperature results into formation of Ni(CO)(4) and/or Ni(CO)(y < 4) subcarbonyls. In the case of Ni/SiO2 and Ni/Al2O3, the sub-carbonyls are weakly interacting with the support. For the other supports, Ni(CO)(4) and Ni(CO)(y < 4) sub-carbonyls are stabilized on the metal oxide and give rise to mono-and poly-nuclear complexes (containing 2 or 3 Ni atoms). On Ni/Mg(Al)O, even larger Ni clusters seem to be formed. The support capability of stabilizing mono- and poly-nuclear complexes can be ranked as follows: SiO2 << Al2O3 < Mg(Al)O < CaO < MgO. A reactivity scale of the Ni-0 particles with CO is also proposed: Ni/SiO2 << Ni/Al2O3 < Ni/Mg(Al)O approximate to Ni/CaO < Ni/MgO. (C) 2012 Elsevier B.V. All rights reserved.

Nitrites dominate! Nitrites and nitrates are formed on the catalyst surface upon storage of NOx over a model Pt–Ba/Al2O3 NOx storage–reduction catalyst. Nitrites are formed through the oxidation of NO at the Pt–Ba interface (nitrite route), while nitrates are formed upon nitrite oxidation and/or NO2 adsorption following NO oxidation at Pt sites. The nitrite route is the unique pathway responsible for the storage of NOx at low temperatures, but it also dominates at high temperatures.

The reduction by CO under dry condition of NOx species stored at 350 C onto a Pt-Ba/Al2O3 Lean NOx Trap catalyst is investigated by means of transient response methods (CO-TPSR and CO-ISC experiments) and complementary FTIR spectroscopy. It is shown that the reduction by CO under nearly isothermal conditions of nitrates stored onto Pt Ba/Al2O3 occurs through a Pt-catalyzed pathway which leads mainly to nitrogen and does not proceeds via the thermal decomposition of stored nitrates with release of NOx in the gas phase. The CO-TPSR and CO-ISC experiments and the combined FTIR study show that (i) the reduction of nitrates by CO occurs via a consecutive reaction scheme with formation of nitrites and then of surface isocyanate/cyanate species followed by the reaction of these species with residual nitrites to give nitrogen; (ii) the reaction of NCO species with nitrites to give nitrogen is slower than the reduction of nitrates to give nitrites and then NCO species; (iii) upon contact with oxygen at increasing temperature. NCO species can be re-oxidized to surface nitrites at first and then to surface nitrates; (iv) the formation of nitrogen is observed during the reduction of stored NOx by CO, and during the oxidation of surface NCO species upon oxygen addition and upon admission of NOx in the presence of excess O-2. In all cases, the formation of nitrogen can be explained by the reaction between surface NCO species and nitrites. The mechanism for the reduction of stored NOx by CO under dry and near isothermal conditions is discussed. (C) 2010 Elsevier Inc. All rights reserved.

Morphological, textural, and surface properties of a NSR (NOx storage reduction) Pt-K/Al2O3 model catalyst (Pt 1 wt %; K 5.4 wt %) were characterized by means of XRD, HRTEM, and FT-IR spectroscopy. Thin crystalline K-containing layers, in the form of cubic K2O and monoclinic K2CO3 and very small roundish Pt particles with a mean diameter of 1.5 nm, have been observed. Monoclinic K2CO3 disappears, and a certain degree of Pt sintering occurs (d(Pt) approximate to 3.4 nm) after use. However, the presence of potassium limits the Pt sintering which occurs on the Pt/Al2O3 reference sample (Pt 1 wt %). FT-IR spectra of CO adsorbed at RT, compared with those recorded for Pt/Al2O3, revealed a marked interaction between the Pt and K phases that is much higher than the interaction between the Pt and Ba phases observed for the classic Pt-Ba/Al2O3 catalyst. CO2 adsorption at RT indicated a high heterogeneity of the K phase, evidenced by the formation of a variety of surface-carbonate-like species (mainly bridging carbonates on K sites). Minor amounts of nitrites and nitrates were formed at RT under NO admission, while the uptake was sensibly higher under NO/O-2 or NO2 admission; nitrites (mono- and bidentate) and nitrates (ionic and bidentates) were formed in different amounts, both relative and absolute, and the nitrate to nitrite ratio increased in parallel with the NO/O-2 ratio. Also, at each contact time, the amount of the stored NOx species increased upon increasing the NO/O-2 ratio.

Morphological, textural, and surface properties of a NSR (NOx storage reduction) Pt−K/Al2O3 model catalyst (Pt 1 wt %; K 5.4 wt %) were characterized by means of XRD, HRTEM, and FT-IR spectroscopy. Thin crystalline K-containing layers, in the form of cubic K2O and monoclinic K2CO3 and very small roundish Pt particles with a mean diameter of 1.5 nm, have been observed. Monoclinic K2CO3 disappears, and a certain degree of Pt sintering occurs (dPt ≈ 3.4 nm) after use. However, the presence of potassium limits the Pt sintering which occurs on the Pt/Al2O3 reference sample (Pt 1 wt %). FT-IR spectra of CO adsorbed at RT, compared with those recorded for Pt/Al2O3, revealed a marked interaction between the Pt and K phases that is much higher than the interaction between the Pt and Ba phases observed for the classic Pt−Ba/Al2O3 catalyst. CO2 adsorption at RT indicated a high heterogeneity of the K phase, evidenced by the formation of a variety of surface-carbonate-like species (mainly bridging carbonates on K sit...

In this paper the reactivity of NOx stored at 350 degrees C onto Pt-Ba/Al2O3 LNT catalyst in the reduction by H-2, CO and CO/H2O mixture is investigated by means of transient experiments and complementary FT-IR analyses.Hydrogen is found to be the best reductant, being characterized by the highest NO removal efficiency in the range 150-350 degrees C. It is proposed that the reduction by hydrogen proceeds according to a dual-steps mechanism in which NOx are first reduced to ammonia, which in turn further reacts with other nitrate species leading selectively to N-2.Conversely, when CO is used as reducing agent under dry conditions only a part of the initially stored NOx is removed as N-2, the other fraction being reduced to adsorbed cyanates/isocyanates species. It is proposed that the cyanate/isocyanate ad-species lead to the formation of nitrogen by oxidation involving other nitrate species; this reaction is self-poisoned by CO. In the presence of water, CO reduced nitrates into cyanates/isocyanates ad-species that were readily hydrolyzed to ammonia; this species is hence precursor in N-2 formation upon reaction with residual nitrate species. Alternatively, stored nitrates are reduced by H-2 formed via the water gas shift (WGS) reaction. (C) 2010 Elsevier B.V. All rights reserved.

The introduction of lean-burn engine technology has prompted the development of NOx storage-reduction (NSR or Lean NOx Traps, LNT) catalysts, which are currently based on a PtBa/Al2O3 system. Potassium is another element that has shown potentials as a storage component. This work explores the role of K during the NOx storage phase and the subsequent reduction using transient activity data and FT-IR analyses over a PtK/Al2O3 powdered catalyst. The catalytic behaviour of a PtK/Al2O3 system has been compared with that of a PtBa/Al2O3 catalyst. NOx storage experiments indicate similar storage efficiency in the two cases.The storage pathways are similar on both the storage components: nitrites and nitrates are formed, the first ones only at the beginning of the storage, quickly transformed in nitrates.The reduction of NOx stored over both systems is also analyzed when H2 is used as reductant. An in series 2-steps process is herein reported, involving at first the formation of NH3 upon reaction of nitrates with H2 (step 1), followed by the reaction of NH3 with residual nitrates to give N2 (step 2). The influence of the storage component on the Pt sites modifies the reactivity of H2 towards stored nitrates.

Introduction Lean NOx Traps (LNTs) represent a viable solution for the abatement of NOx emissions from lean–burn engines [1]. Several studies were published on the mechanisms of both the NOx storage [1] and reduction [2], but a complete understanding of these processes has not yet been achieved. We have previously shown that on Pt-Ba/Al2O3 LNT systems the reduction of nitrates by H2 involves a catalytic pathway involving Pt, and is not initiated by the thermal decomposition of the stored NOx ad-species [2]. We also showed that this process involves a 2-steps in series molecular pathway in which NH3 is formed as intermediate [3]. In this study the reaction mechanism involved in the reduction of stored NOx when CO is used as reducing agent are analyzed, and the pathways originating the main reduction product (N2) are addressed.

The reduction by CO of NO x species stored over Pt–Ba/Al2O3 Lean NO x Trap systems is analysed in this work. The reaction mechanisms and pathways leading to N2 formation both under dry and wet conditions are investigated by complementary transient dynamic experiments and FTIR analyses.

Morphological, textural, and surface properties of several Pt-Ba/Al2O3 NOx storage reduction (NSR) catalysts at different Ba loading in the range 0-30 wt % were characterized by means of X-ray diffraction, high-resolution transmission electron microscopy, and Fourier-transform infrared (FT-IR) spectroscopy using CO, CO2, and CH3CN as probe molecules. Upon increasing the Ba loading, the Pt exposure progressively decreased, accounting for sintering and masking of the Pt particles by Ba, whereas the interaction between Pt and the barium oxide phase increased. After a few cycles of heating in NO2 and subsequent evacuation (conditioning treatment), BaCO3 initially present evolved to Ba(NO3)(2) and then decomposed into a well-dispersed nanosized BaO phase. Upon conditioning, a slight sintering of Pt is observed. However, the presence of Ba avoided a more stressed sintering, as it occurred for the Pt/Al2O3 sample. Investigation by CO2 and CH3CN adsorption followed by FT-IR spectroscopy revealed a high heterogeneity at the BaO surface. In particular, upon CO2 adsorption a variety of surface carbonate-like species were formed (mainly bridging and chelating carbonates on Ba sites). The most relevant features upon CH3CN adsorption were the fort-nation of anionic species on strongly basic oxygen ions of Ba2+ O2- pairs and the presence of acetonitrile molecules polarized by highly uncoordinated Ba2+ ions, stable under evacuation at room temperature. The FT-IR characterization with the three test molecules suggests that the best spread of the Ba phase is obtained for a loading between 16 and 23 wt %.

NOx storage-reduction catalysts based on Pt and Pt/Cu supported on Mg(Al)O mixed oxides obtained from hydrotalcite-type compounds were studied. In particular, NOx storage capability upon admission of NO and NO2 in the absence or in the presence of excess oxygen at temperatures up to 623K was investigated by in situ FT-IR spectroscopy. For comparative purposes, the bare mixed oxide support was also investigated. Pure NO2 is adsorbed according to a dismutation reaction with the simultaneous formation of nitrates and nitrites that progressively evolve to nitrates (“dismutation route”), the nitrite evolution to nitrates is promoted by the metal phase. When metal phase is present, the nitrites oxidation is further accelerated by the oxygen presence. Pure NO is not stored. When O2 is present, NO is stored according to two different pathways, both promoted by the metal phases: (i) the NO oxidation to nitrites followed by their oxidation to nitrates (“nitrite route”); (ii) the NO oxidation to NO2, followed by the “dismutation route”.

Pt-containing Mg/Al layered double hydroxides (LDHs) at different Pt loadings in the range 0-3.2 wt.% were prepared by coprecipitation, impregnation and sol-gel methods. After activation and reduction treatments, Pt nanoclusters interacting with Mg(Al)O supports are obtained. The nature of the Pt phase and the acid-base properties were investigated by FT-IR spectroscopy upon adsorption of CO and CH3CN, respectively. In particular, the ratio between the number of basic and acid sites, I-b/I-a, was found to be in good agreement with the Mg/Al ratio in the solid, the basic character of the mixed oxides increasing with the Mg content. Moreover, for each Pt loading, the I-b/I-a ratio found after oxidizing treatment was markedly lower than after reduction. This feature was ascribed to the formation of a well dispersed surface Pt delta+Ox, phase. CO adsorption reveals, in the coprecipitated and impregnated samples, the presence of Pt sites, isolated or belonging to very small clusters, strongly interacting with the basic oxygen anions of the support.Finally, the behaviour of Pt/Mg(Al)O systems as multifunctional catalysts were checked in the cascade reaction between benzaldehyde (BAL) and propanal (PAL). (C) 2007 Elsevier Inc. All rights reserved.

Three types of multifunctional supported I'd catalysts (0.2 wt.% Pd) have been obtained from layered double hydroxides (LDH) precursors by: (i) impregnation with Pd(acac)(2) of a Mg(Al)O support, (ii) coprecipitation of a multicationic Pd/Mg/Al LDH, and (iii) synthesis of a Pd colloid Mg/Al LDH nanocomposite. The precursors have then been calcined at 773 K and reduced at 523 K to obtain the catalysts, which exhibit very different basic properties and Pd particle size distributions. Their potential was evaluated in the one-pot synthesis of 2-methyl-3-phenyl-propanal (MPPAL) from benzaldehyde and propanal. The full one-pot synthesis of MPPAL was performed sequentially operating in two stages, i.e. for 20 h in N-2, then for 7 h in H-2 atmospheres. The activities of the catalysts in the first stage of the reaction (condensation and dehydration steps) are related to their reconstruction ability in the propanol/water solvent to give Bronsted basic sites. The hydrogenation ability of the catalysts decreases when the average Pd-0 particle size decreases. The specific behavior of the nanosized particles accounts for a decrease of the adsorption strength of C=C bond due to the enhancement of the metal-support interaction. The catalyst prepared from a Pd nanocomposite precursor ensures the best balance between the different functions. It leads to a MPPAL selectivity of 77% at 64% benzaldehyde conversion. Benzyl alcohol formed by hydrogenation of benzaldehyde on Pd and hydrogen transfer from propanol is the main secondary product. However, the process is negatively affected by the high consumption of propanal from self-condensation. (c) 2006 Elsevier B.V. All rights reserved.

The semi-hydrogenation of 2-butyne-1,4-diol has been performed with supported Pd catalysts (0.2–0.3wt% Pd) obtained from layered double hydroxides (LDH) precursors. The catalysts have been prepared by three different routes: (i) impregnation with Pd(acac)2 of a Mg(Al)O support (Pd/Mg(Al)O-imp), (ii) coprecipitation of a multicationic Pd/Mg/Al LDH (Pd/Mg(Al)O-cop), and (iii) synthesis of a Pd colloid Mg/Al LDH nanocomposite (Pd/Mg(Al)O-nc). After calcination at 723K and reduction at 483K the catalysts exhibit specific surface areas ranging from 180 to 280m2g−1 and a predominant basic character. The Pd particles have a good dispersion in the three catalysts, especially for Pd/Mg(Al)O-imp with a narrow distribution in size centered at ca. 2nm. Both activities and selectivities to 2-butene-1,4-diol range as follows: Pd/Mg(Al)O-imp>Pd/Mg(Al)O-nc>Pd/Mg(Al)O-cop in agreement with the dispersion of the samples. Rehydration of the catalysts increases the activity but decreases the selectivity to 2-butene-1,4-diol. Formation of butane-1,4-diol, occurs by consecutive hydrogenation of 2-butene-1,4-diol as well as direct hydrogenation of 2-butyne-1,4-diol through a carbyne species.