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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 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.

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 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.

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.