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In this work, we have investigated the release and reduction of NO (nitrates) stored on Rh-Ba/AlO and Pt-Rh-Ba/AlO catalysts and compared with Pt-Ba/AlO. It is shown that the release occurs at the interface between the PGM sites and the storage sites, can be probed by isotopic exchange experiments and is governed by the rate of oxygen removal from the metallic PGM sites. Accordingly the presence of a reductant, that keeps the PGM sites in a reduced state by scavenging oxygen adatoms formed upon nitrate decomposition, greatly favors the release of NO and its subsequent reduction. In these steps, not much differences have been observed considering either Pt- or Rh-based samples. [ABSTRACT FROM AUTHOR]

In this paper mechanistic aspects on the formation of NO over a model Pt-Ba/AlO NO storage and reduction catalyst are investigated. The data indicate that NO formation involves the coupling of N-adspecies with undissociated NO molecules: accordingly this requires the presence of partially reduced Pt sites, to catalyse the NO dissociation into N- and O-adspecies on one side and to prevent complete NO dissociation on the other side. In fact NO formation is observed at temperatures where the reductant leads to a partial reduction of the Pt sites, at increasing temperatures in the order CO > NH > H. [ABSTRACT FROM AUTHOR]

The release and reduction of NO x stored on Pt–Ba/Al 2 O 3 is here investigated under nearly isothermal conditions. It is shown that the release driven by adsorption–desorption equilibrium of stored NO x is negligible. It is also shown that NO isotopic exchange between stored 14 NO x and 15 NO in absence of O 2 , which implies the release of stored NO x and has the potential to decouple NO x release from its further reduction, is catalyzed by Pt and is observed from temperatures well below that of the NO x thermal desorption. It is suggested that reduced Pt sites activate stored NO x that are destabilized and eventually released as NO x in the gas phase or adsorbed onto Pt. Therefore, under nearly isothermal conditions, the NO x release is a chemical reaction catalyzed by Pt occurring at the Pt/Ba interface. In the presence of reductants, the release of stored NO x and its further reduction cannot be decoupled. The higher is the reducing power the more effective is the reduction of Pt and activation of stored NO x by metal Pt that represent the first step of release and reduction of stored NO x . [ABSTRACT FROM AUTHOR]

Mechanistic aspects involved in the formation of N and of NO during the reduction of NO, stored nitrites and stored nitrates in the presence of NO are investigated in this work by means of isotopic labeling experiments over a model PtBa/AlO NSR catalyst. The reduction of gaseous labeled NO with unlabelled NH leads to the formation of NO at low temperature (below 180 °C), and of N at high temperature. All N possible isotopes are observed, whereas only labeled molecules have been detected in the case of NO. Hence the formation of nitrous oxide involves undissociated NO molecules, whereas that of N can be explained on the basis of the statistical coupling of N- and N-adatoms on Pt. However, due to a slight excess of the mixed NN isotope, a SCR-like pathway likely operates as well. The reduction of the stored labelled nitrates is very selective to N and all isotopes are observed, confirming the occurrence of the recombination pathway. However also in this case a SCR-like pathway likely occurs and this explains the abundance of the NN species. When the reduction of the stored nitrates is carried out in the presence of NO, this species is preferentially reduced pointing out the higher reactivity of gaseous NO if compared to the nitrates. [ABSTRACT FROM AUTHOR]

Abstract: The effect of cerium addition to a model Pt-Ba/Al2O3 lean NO x trap catalyst (LNT) is investigated in this paper by a combination of characterization techniques and reactivity experiments. It is found that the presence of Ce does not alter the morphological and structural characteristics of the catalyst, although slight changes in the Ba-carbonate phase are apparent. On the other hand, the presence of Ce does not seem to affect significantly the characteristics of Pt, in particular its redox state. CO adsorption experiments pointed out the presence of Pt° particles presenting different coordination degree, whose characteristics are however not significantly affected by the presence of Ce. The storage capacity of the PtBa/Al sample is hardly affected by the presence of Ce; however Ce favours the thermal decomposition of the stored NO x species. The Ce-doped sample also shows a slightly higher reactivity in the reduction of the stored NO x by H2: in fact the reaction is monitored at lower temperatures. A slightly higher selectivity towards the formation of N2 is also observed. [Copyright &y& Elsevier]

The reduction of NO stored over a Pt-Ba/AlO Lean NO Trap is analysed when H, CO or heptane are used as reductants. In all cases, the reduction proceeds via a Pt-catalyzed process involving the formation of intermediate species like ammonia and isocyanates in the case of H and CO, respectively. No specific intermediates have been observed when heptane is used as reductant. It is claimed that the role of the reductant is to keep Pt in a reduced state; this favours nitrate decomposition and reduction over the Pt sites. The effect of water on the reaction is also investigated. [ABSTRACT FROM AUTHOR]

Mechanistic aspects involved in the formation of N2andof N2O during the reduction of gas-phase NO and of NOxstored over a model PtBa/Al2O3NSR catalyst are investigated using unlabeled ammonia andlabeled NO. The reduction of the stored NOxspecies (labeled nitrites and nitrates) with NH3leadsto the selective formation of N2as a major product andof small amounts of nitrous oxide. Based on the nitrogen isotopicdistribution, it appears that N2formation occurs primarilythrough the statistical coupling of N-atoms formed by dissociationof NOx- and NH3-related surfaceintermediates, although an SCR-pathway (involving the coupling ofNH3- and NO-derived ad-species) is also likely to occur.It appears as well that the formation of nitrous oxide involves eitherthe coupling of two adsorbed NO molecules or the recombination ofan adsorbed NO molecule with an adsorbed NHxfragment. [ABSTRACT FROM AUTHOR]

In this study mechanistic aspects related to the adsorption of NO x over alumina-supported Pt, Ba and PtBa catalysts are deepened, that are of interest for NO x Storage-Reduction (NSR) catalysts and other applications involving the adsorption of NO x (e.g. Passive NO x Adsorbers, PNAs). NO x adsorption is investigated at low and high temperatures (150 °C and 350 °C, respectively); the thermal decomposition and the reactivity with H 2 of the adsorbed species is also addressed. By coupling FT-IR spectroscopy and microreactor studies, new insights on the adsorption, decomposition and reactivity of the stored NO x are derived. In particular it is found that at 150 °C nitrites are formed on all surfaces when starting from NO/O 2 ; different species are formed onto the different storage sites (Ba vs. Al) that can be distinguished spectroscopically. Different routes for the storage of nitrites are highlighted, i.e. direct oxidative NO uptake and nitrite formation involving NO 2 produced by oxidation of NO. Clear and novel evidence is herein provided that the direct NO oxidative uptake is much faster than the route involving the NO to NO 2 oxidation. Both routes are catalyzed by Pt, although Ba is also able to store nitrites; the role of the interaction between Pt and the storage sites is discussed. When the NO x storage is carried out at higher temperatures (350 °C), both routes are greatly favored although nitrites can hardly be observed being readily transformed into nitrates. Besides NO 2 is also formed in significant amounts that may participate to the formation of nitrites/nitrates adspecies following a NO 2 disproportion pathway for which clear and new spectroscopic evidences are herein provided. The adsorbed species (nitrites or nitrates) start to decompose above the adsorption temperature, i.e. 150 °C and 350 °C, respectively. In the presence of Pt, the decomposition of the stored nitrites leads to the formation of nitrates and NO due to the occurrence of a nitrite disproportion reaction. The formed nitrates show high thermal stability and decompose only at high temperatures, thus making these systems unappropriated for PNA applications. In the presence of a reductant, Pt catalyzes the reduction of the stored NO x at much lower temperatures than the adsorption; the role of the interaction between Pt and the storage sites on this step is herein discussed. [ABSTRACT FROM AUTHOR]