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A Pd-doped FER zeolite sample is synthesized and investigated for potential use in the low-temperature NOx adsorption. The catalyst is characterized by BET, XRD, TEM, NaCl titration and by in-situ FT-IR spectroscopy following adsorption of CO, NH3 and NO for the characterization of the Pd sites. Over the calcined sample, Pd is present mainly as PdOx clusters/particles; near 13 % of Pd is present in the form of isolated ions. The NOx adsorption / desorption performances at low temperature are investigated by microreactor studies and operando FT-IR upon NO/O2 adsorption followed by TPD under relevant operating conditions, i.e. in the presence of water and carbon dioxide. It is found that NO adsorbs in the form of Pd nitrosyls (hydrated and anhydrous) and nitrates; near 59 μmol/gcat of NOx could be adsorbed, corresponding to a NO/Pd ratio near 0.6. Upon heating, nitrosyls decompose and transform into anhydrous species leading to NO desorption in the low-T region (below 200 °C) and at higher temperature, with maximum near 370 °C. The presence of CO negatively affects the NOx storage capacity, due to a competition of CO and NO over the Pd storage sites.

In this study the pathways involved in N2O formation over a commercial LNT catalyst are addressed, when using H2 as a reductant. For this purpose, flow microreactor experiments coupled with FT-IR under operando conditions are used. The results indicate that N2O formation occurs both at the lean-to-rich (primary N2O) and rich-to-lean (secondary N2O) transitions. Primary N2O originates at the reduction front due to the presence of partially reduced PGM sites that do not readily dissociates NO released from the stored NOx. Undissociated NO couples with N-adatoms leading to the formation of N2O. At variance, secondary N2O originates upon oxidation with NO/O2 of reducing species left adsorbed on the catalyst surface (adsorbed CO, isocyanates and possibly NH3) during the rich phase, as pointed out by FT-IR spectroscopy. The concentration of such adsorbed species is however limited and hence the formation of secondary N2O is much smaller than that of primary N2O, when using H2 as reducing agent. The emissions of N2O reduce upon increasing the temperature, and above 250 °C N2O formation is negligible. Finally the reactivity of N2O with adsorbed NOx species (nitrites) and with the actual reductants (H2 and NH3) is also investigated, to provide further indications concerning the pathways leading to N2O emissions. It is found that N2O does not react with NOx species stored downstream the reduction front; at variance both H2 and NH3 may reduce N2O to N2 and water at rather low temperatures. The role of this reaction on N2O emission is herein discussed.

In this paper mechanistic aspects involved in the formation of N2O over model Pt-Ba/Al2O3 and Rh-Ba/Al2O3 LNT catalysts are discussed. The reactivity of both gaseous NO and of stored NOx (nitrates) has been studied, with simultaneous surface characterization by operando FT-IR spectroscopy, using different reductants (i.e. H2, CO, CO + H2, CO + H2O) both under isothermal conditions and temperature programming. The results show that N2O formation may occur during both the lean/rich and rich/lean switches (primary and secondary N2O, respectively). In particular: i) primary N2O formation involves the presence of gas-phase NO and partially reduced metal sites; ii) N2O formation increases in the presence of CO because the reduction of the metal sites is slower, thus favoring N2O formation upon the lean/rich transition; iii) residual reducing species onto the surface (i.e. NCO−, CO) can react with NO giving the secondary N2O peak. A reaction pathway for N2O formation is suggested where metal sites (Pt or Rh) catalyse the NO dissociation reaction into N- and O-adatoms; N-species further interact with undissociated NO molecules leading to the formation of N2O (primary N2O). In additions, isocyanates formed during the NOx reduction in the presence of CO may participate in the N2O formation upon reaction with NO during the lean phase (secondary N2O). Pt- and Rh-based catalysts show similar behavior even if Rh-based catalyst is less reactive than Pt-based sample likely due to the lower dispersion of the noble metal.

In this study Pd-promoted zeolites (BEA, FER, MFI and MOR) are synthesized and investigated for potential use in the low-temperature NOx adsorption. The catalysts are characterized by BET, XRD, UV–Vis and XRF, while the adsorption/desorption characteristics of the samples are investigated by NOx adsorption/TPD, with and without water in the feed. The nature of the adsorbed NOx species has been analyzed by operando FT-IR spectroscopy. Under dry conditions at 50 °C all the investigated zeolite frameworks are able to store significant amounts of NOx, up to 1 mmol/gcat for the MOR sample; the presence of Pd has not a significant impact on the amounts of the stored NOx. In fact, under dry conditions at 50 °C the zeolite frameworks oxidize NO to NO2, and store NOx from NO2 over the zeolitic support. FTIR data indicates that NOx are stored in the form of nitrosonium ion NO+ and nitrate ions; nitrosyl species formed on Al3+ and Pdn+ sites have also been observed, although to a lower extent. Evidences have also been provided for the existence of gaseous/weakly interacting species (mostly NO2) contained in the pores of the zeolites. Water inhibits NO oxidation over all investigated samples, resulting in a strong decrease in the NOx storage capacity. Still, appreciable amounts of NOx could be stored at 50 °C (up to 60 µmol/gcat for Pd/MFI) in the presence of water over the Pd-doped carriers.

The abatement of the pollutants deriving from diesel engines in the vehicle sector still represents an interesting scientific and technological challenge due to increasingly limiting regulations. Meeting the stringent limits of NOx and soot emissions requires a catalytic system with great complexity, size of units, and number of units, as well as increased fuel consumption. Thus, an after-treatment device for a diesel vehicle requires the use of an integrated catalyst technology for a reduction in the individual emissions of exhaust gas. The representative technologies devoted to the reduction of NOx under lean-burn operation conditions are selective catalytic reduction (SCR) and the lean NOx trap (LNT), while soot removal is mainly performed by filters (DPF). These devices are normally used in sequence, or a combination of them has been proposed to overcome the drawbacks of the individual devices. This review summarizes the current state of NOx and soot abatement strategies. The main focus of this review is on combined technologies for NOx removal (i.e., LNT–SCR) and for the simultaneous removal of NOx and soot, like SCR-on-Filter (SCRoF), in series LNT/DPF and SCR/DPF, and LNT/DPF and SCR/DPF hybrid systems.

In this paper mechanistic aspects on the formation of N2O over a model Pt–Ba/Al2O3 NOx storage and reduction catalyst are investigated. The data indicate that N2O 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 N2O formation is observed at temperatures where the reductant leads to a partial reduction of the Pt sites, at increasing temperatures in the order CO > NH3 > H2.

This chapter gives an overview on the combined lean NOx trap (LNT) and selective catalytic reduction (SCR) catalysts. In particular, the focus is on the reaction pathways involved in such systems when different reducing agents are being used to accomplish the rich phase in the LNT system. In combined LNT–SCR systems, NH3 emitted from the LNT catalyst during the rich phase is captured onto a SCR catalyst placed downstream the LNT brick. The stored ammonia is then used as reactant in the SCR catalyst upon reaction with NO slipped from the LNT catalyst during the lean phase. This increases the NOx removal efficiency and reduces the emissions of unwanted ammonia. When using hydrocarbons as reducing agents, a non-NH3 SCR reaction pathway is also operating: in fact, HCs slipped through the LNT catalyst react with NO over the SCR catalyst, thus contributing to the overall NOx abatement. The overall system efficiency depends on several factors including the catalyst formulation (LNT and SCR), the operating conditions and the reducing agents used during the rich phase. In addition, the system architecture (dual layer vs dual bed) plays a role in determining the system efficiency.

The effect of CO 2 and H 2 O on the NO/O 2 storage over a Pt-Ba/Al 2 O 3 LNT catalyst was investigated in the temperature range 150–350 °C by combining FT-IR spectroscopy and microreactor flow experiments. It is found that the presence of water shows negligible effect on the NO x storage at all the temperatures studied. Otherwise, the presence of CO 2 inhibits the “nitrite route” (initial formation of surface nitrites and their subsequent evolution to nitrates), whereas the “nitrate route” (involving NO oxidation to NO 2 over Pt and its subsequent adsorption on Ba in the form of nitrates) proceeds as in the absence of CO 2 . The inhibition effect of CO 2 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 CO 2 in the feed significantly shortens or eliminates the dead time in the NO x breakthrough at the reactor outlet. As a consequence, the presence of dead time in the absence of CO 2 has been correlated to the occurrence of the nitrite route.

The mechanism involved in the formation of N2 and of N2O during the reduction of nitrates stored onto a Pt–Ba/Al2O3 LNT catalyst is investigated using labeled NO and unlabeled ammonia, in the presence and in the absence of NO in the gas phase. The reduction of the stored NO x species (labeled nitrates) with NH3 leads to the selective formation of N2. Based on the isotopic distribution, it appears that N2 formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NO and NH3 at metal Pt sites. When the reduction of the stored nitrates is carried out in the presence of NO in the gas phase, NO is preferentially reduced. This implies that the rate determining step of the reduction of nitrates by ammonia is likely associated with the release of stored NO x . Negligible amounts of nitrous oxide have been observed during the NH3-TPSR with adsorbed nitrates, whereas relevant quantities of N2O have been detected at low temperatures (below 180 °C) in the runs performed in the presence of NO in the gas phase. The data converge to indicate that N2O formation involves the presence of gaseous NO and this suggests that the formation of nitrous oxide occurs either through the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NH x species.

The effect of soot on the NO x storage-reduction performances of a Pt–K/Al2O3 catalyst is analysed in this work by performing lean-rich cycles at constant temperature. The interaction between soot and the stored NO x has been also investigated by temperature programmed methods. It has been found that the presence of soot reduces the NO x storage capacity of the catalyst; besides the presence of soot favours the decomposition of the stored nitrates. A direct reaction between the stored nitrates and soot is suggested, that has been explained on the basis of the surface mobility of the adsorbed nitrates.

The interaction of soot with nitrates stored on a model PtBa/Al2O3 LNT catalyst sample is here investigated under realistic conditions, i.e. upon lean/rich cycling in the presence of water and CO2. The presence of soot inhibits the NOx storage capacity of the catalyst during the lean phase at different temperatures, in the range 200–350 °C, but does not affect significantly the regeneration process of the stored nitrates. Simultaneously with the NOx storage, soot is converted to CO2 at temperatures above 300 °C. Soot is oxidized by NO2 formed by NO oxidation on Pt sites, but the participation of the stored nitrates in the soot oxidation is also likely, as pointed out by TPD/TPO experiments showing the occurrence of a surface reaction involving soot and the stored nitrates.

Mechanistic aspects involved in the formation of N2 and of N2O during the reduction of gas-phase NO and of NOx stored over a model PtBa/Al2O3 NSR catalyst are investigated using unlabeled ammonia and labeled NO. The reduction of the stored NOx species (labeled nitrites and nitrates) with NH3 leads to the selective formation of N2 as a major product and of small amounts of nitrous oxide. Based on the nitrogen isotopic distribution, it appears that N2 formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NOx- and NH3-related surface intermediates, although an SCR-pathway (involving the coupling of NH3- and NO-derived ad-species) is also likely to occur. It appears as well that the formation of nitrous oxide involves either the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NHx fragment.

A detailed analysis of the catalytic behaviour in the storage/reduction of NO x of single Pt–Ba/Al 2 O 3 LNT and FeZSM-5 SCR systems and of combined LNT + SCR configurations is analyzed and compared in this work under clean conditions (i.e. in the absence of CO 2 and H 2 O in the feed stream). By working under dilute conditions (i.e. with low reductant concentrations) and with an inert purge between the lean and the rich phases, nearly ideal isothermal conditions could be attained and the chemical pathways operating in the each step of the NSR cycle could be analyzed for the investigated catalysts configurations. It is found that when the SCR catalyst is placed downstream the NSR catalyst bed (double-bed configuration), ammonia released from the LNT sample during the rich phase is stored on the SCR catalyst placed downstream and is then converted to N 2 in the subsequent lean phase according to the occurrence of a SCR reaction. This has a benefit on both the NO x removal efficiency and the N 2 selectivity. If the zeolite SCR catalyst is mixed with the LNT sample (physical mixture), during the rich phase the SCR catalyst traps ammonia which being intermediate in N 2 formation leads to a decrease in the evolution of nitrogen at the reactor outlet. Ammonia stored on the SCR catalyst then reacts with NO x during the subsequent lean phase, leading to a significant N 2 evolution: this increases the NO x removal efficiency and the N 2 selectivity if compared to the single Pt–Ba/Al 2 O 3 catalyst sample.

The activity in the soot combustion of K- and Ba-based catalysts in “loose” and “full” contact is investigated in this study. Under loose contact systems (obtained by mechanically mixing the soot and catalysts), the K-containing samples are very active in the oxidation of soot, as opposite to the Ba-containing samples. Also, a synergistic effect in this reaction is seen between K and Pt for the case of the K-containing samples. Under full contact conditions (obtained by impregnation of the soot with aqueous metal solutions), the reactivity of soot increases; in these conditions the performance of the poorly reactive Ba-containing samples becomes similar to the very active K-based system. This result points out the key-role played by both the selected alkaline or alkaline earth metal oxides in the soot combustion, and suggests that the poor activity of Ba under loose conditions is possibly due to the low mobility of the Ba surface species. Finally, it is found that the presence of NO favors the soot oxidation under loose contact, specifically over noble metal-containing samples, due to the oxidation of NO to NO2. However the presence of the alkaline and alkaline earth oxide also affects the soot oxidation activity in the presence of NO. This has been ascribed to the formation of Ba- and K-nitrite/nitrate species which participate in the soot combustion process by giving rise to NOx upon nitrite/nitrate decomposition and/or by directly reacting with soot.

A series of Pt–Ba/Al2O3 NOx storage-reduction (NSR) catalysts having different Ba loadings (in the range 0–30%, w/w) have been prepared, characterized and tested in the NOx adsorption-reduction under realistic operating conditions. Accordingly, the role of the Ba loading on the storage of NOx and on their subsequent reduction has been addressed. The results indicated that the presence of Ba negatively affects both the catalyst morphological properties and the Pt dispersion in the samples with Ba loading higher than 10%, while a strong increase of the NOx adsorption capacity is observed. This effect has been associated to the involvement in the NOx storage process of Pt–Ba neighboring species. The analysis of the storage phase also showed that the highest the Ba content, the lowest the NO2 concentration at catalyst saturation: this effect has been related to the decrease in the Pt content of the catalyst following Ba addition to the samples and/or to a possible masking of Pt sites when the Ba coverage approaches and exceeds the monolayer capacity of the support. Concerning the reduction phase, the data pointed out that for all the investigated catalysts the stored NOx species are readily reduced upon H2 admission: nevertheless, the reduction process resulted very complex and it can lead to nitrogen and/or ammonia depending on the formulation and properties of the catalysts.

Transient NOx storage and TPD runs were performed for a feed of NO or NO2 in the presence or absence of O2 over Ba/Al2O3 and Pt−Ba/Al2O3. A kinetic model for NOx storage previously developed in the literature was used to simulate the majority of the results, after only the Pt and Ba dispersions had been adapted to a set of experimental data. The model describes well the NO2 storage over Ba/Al2O3 through a two-step disproportionation reaction and the oxidizing capacity of barium nitrates toward NO. However, the model cannot account at the same time for the oxidation of NO and for the storage of NOx during the adsorption of NO in the presence of oxygen over Pt−Ba/Al2O3. The lack of an appropriate balance between the NO oxidation and NOx storage routes is critical because these two characteristics are interconnected and are both involved in NOx storage. This finding ultimately motivates further modeling efforts.

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.

The reaction network of the NO x storage and reduction over Pt−Ba/Al 2 0 3 ‘lean NO x trap’ catalysts is presented in this paper. The NO x storage was investigated at first. The collected results showed that a dual pathway is operating when starting from NO/O 2 mixtures: the first route implies the well-known oxidation of NO to NO 2 , and its subsequent adsorption via disproportionation to form nitrates (nitrate route), whereas the second novel route consists of a stepwise oxidation of NO in the presence of oxygen to form nitrite ad-species, which are progressively oxidized to nitrates (nitrite route). Then the reduction of stored NO x with hydrogen is addressed. The bulk of data points out that the reduction of stored nitrates occurs under near isothermal conditions through a Pt-catalysed surface reaction that does not involve the thermal desorption of the stored nitrates as a preliminary step. A specific role of a Pt−Ba interaction was suggested, which plays a role in the NO x storage phase as well. The effect of the operating conditions (temperature, presence of CO 2 and water) on both adsorption-reduction phases has been also analysed.