Search

Pd-promoted zeolites (Y, ZSM-5, FER, SSZ-13) were prepared and characterized to analyze their properties as low-temperature NOx adsorbers. The samples were investigated by BET and XRD and by in situ FT-IR spectroscopy of CO and NO adsorption to probe the Pd sites and the nature of the adsorbed NOx species. The NOx adsorption/desorption performances at low temperatures were examined by microreactor measurements upon NO/O2 adsorption followed by TPD in the presence of water and carbon dioxide. It was enlightened that: (i) the zeolite framework influences the Pd dispersion: the smaller the zeolite cage, the higher the Pd dispersion, irrespective of the Si/Al ratio. Accordingly, the following Pd dispersion order has been observed, inversely to the zeolite cage size: Pd/SSZ-13 > Pd/ZSM-5 ~ Pd/FER >> Pd/Y; (ii) Pd is present as isolated Pdn+ species and in PdOx particles; (iii) the Pd dispersion governs the NOx storage capacity: the smaller the zeolite cage, the higher the Pd dispersion and the storage capacity; (iv) NO adsorbs mainly in the form of Pd nitrosyls and nitrates; (v) NO desorption occurs both at a temperature below 200 °C and in a high-temperature range (near 350 °C).

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.

New C-supported bimetallic Ru–WOx catalysts, prepared by co-impregnation of RuCl3 and Na2WO4, proved highly efficient for the liquid-phase hydrogenolysis of aqueous glycerol into 1,2-propanediol (1,2-PDO). The tuning of the catalyst composition and major reaction parameters, specifically operating at 150 °C, 5 bar H2, and Ru : W = 4 : 1 mol/mol, allowed conversion of glycerol and 1,2-PDO selectivity of 73≥99% and 88–98%, respectively, with a carbon loss of

Ceria-based catalysts, with Cu in substitution of noble metals, were studied in a vertical microreactor system under isothermal conditions, where NOx was previously stored, followed by the reduction step conducted under H2. The possible remaining ad-NOx species after the reduction stage, were investigated by Temperature Programmed Desorption in He. In situ DRIFTS was used as a complementary technique for the analysis of the surface species formation/transformation on the catalysts’ surface. Catalysts containing both Ba and Cu were found to be selective in the NOx reduction, producing N2 and minor amounts of NH3 during the reduction step, as well as NO. The different ceria-based formulations (containing copper and/or barium) were prepared and tested at two different temperatures in the NOx reduction (NSR) processes. Their catalytic activities were analyzed in terms of their compositions and have been useful in the elucidation of the possible origin and relevant pathways for NOx reduction product formation, which seems to involve the oxygen vacancies of the ceria-based materials (whose generation seems to be promoted by copper) during the rich step. The scope of this work involves an interdisciplinary study of the impact that catalysts’ formulations (noble metal-free) have on their LNT performance under simulated conditions, thus covering aspects of Materials Science and Chemical Engineering in a highly applied context, related to the development of control strategies for hybrid powertrains and/or the reduction of the impact of cold-start emissions. This research was funded by the financial support from the Generalitat Valenciana (PROMETEO/2018/076 project) and the Spanish Ministry of Science and Innovation (PID2019-105542RB-I00 project) and the UE-FEDER funding. J.C.M.-M. also acknowledges Spanish Ministry of Science and Innovation for the financial support through an FPU grant (FPU17/00603).

Mechanistic aspects involved in cyclic CO2 capture and methanation were analyzed by comparing the catalytic behavior of a Ru-BaO/Al2O3 (intimate mixture) with that of a mechanical mixture of BaO/Al2O3 + Ru/Al2O3. Both combinations showed similar performance in the storage of CO2 and in the thermal stability of adsorbed species: CO2 is strongly adsorbed over the BaO component, and cannot be completely released even at high temperatures. The H2 reactivity of the stored species is strongly enhanced for the intimate mixture, showing increased CH4 formation at lower temperatures thus indicating the synergistic effect between the storage and hydrogenation functions. Dual Function Material (DFM) experiments performed in the presence of O2 and steam during the CO2 capture step showed a decrease in the amounts of adsorbed CO2 and a lower methanation rate. When NOx is also present during the capture step, the adsorption of CO2 is hindered by strongly adsorbed NOx. However, while Ru-BaO/Al2O3 can be fully regenerated by H2, BaO/Al2O3 + Ru/Al2O3 suffers from deactivation due to the buildup of adsorbed NOx. It should be understood that in power plant applications the DFM will be located downstream from SCR and therefore the level of NOx will be in the ppm level. Furthermore, it is completely desorbed as NH3 and N2 upon the addition of H2 and thus will not have a significant impact of overall performance for CO2 capture and conversion.

In this study, a systematic investigation on Dual Functioning Materials (DFMs) for the capture and methanation of CO₂ is carried out. The attention is focused on the nature of the CO₂ adsorbent component (storage material, SM) varying between alkaline (Li, Na, K) and alkaline-earth (Mg, Ca, Ba) metal oxides in combination with Ru, both supported on an Al₂O₃ support. Combining gas phase reactivity analysis and FT-IR characterization, the samples are characterized in terms of CO₂ storage capacity. It is found that all the SM-containing samples adsorb significant amounts of CO₂ as carbonate species, with the higher amounts being adsorbed when the more thermally stable species are formed, i.e., when Ca, Ba, or K are employed as SMs. In all cases, the hydrogenation of the adsorbed carbonates to CH₄ occurs at lower temperature, if compared to their thermal desorption. However, in the case of Ca- and Ba-based DFMs, resilient carbonates are present on the material surface. It was found that the SMs able to form the more thermally stable carbonates upon CO₂ adsorption also showed the best performances in capture/methanation cycles at 350 °C, even if some residual carbonates were left on the DFM after the hydrogenation step. In particular, the following order of reactivity has in fact been observed in terms of CH₄ production: Ru–K ≥ Ru–Ba > Ru–Ca > Ru–Na ≫ Ru–Mg ≅ Ru–Li ≅ Ru. The presence of steam and O₂ during the capture step has a detrimental effect on the CO₂ adsorption for all samples and, as a result, on CH₄ production due to the competition of CO₂ and water for the same adsorption sites. Thus, only SMs able to form strongly bound carbonates species upon CO₂ exposure can retain significant CO₂ storage capacity also in the presence of water in the adsorption feed.

In this work, a fundamental and systematic study was conducted, leading to a better understanding of the phenomena occurring on the catalyst’s surface during the NOx reduction process in NSR systems. For this purpose, ceria-based catalysts, with Cu in substitution of noble metal, have been synthesized and deeply characterized by means of XRF, XPS, in situ (XRD, Raman spectroscopy and DRIFTS), temperature-programmed reduction under H2 (H2-TPR) and under NO reaction (NO isothermal reaction + NO-TPR). The whole results show the key role of copper to promote the reducibility and the creation of oxygen vacancies, allowing a high NO consumption and fast kinetics of N2O and N2 formation, until the oxygen vacancies consumption takes place. The study of the surface reactions taking place in the formation of adsorbed NOx species and the oxygen vacancies consumption with NO uptake is complex; however, a hydroxyl consumption route is found to be involved. The reduction of NO provided higher levels of N2 at higher temperatures; also, a very high efficiency of the previously created oxygen vacancies was found for this process. The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEO/2018/076 project) and the Spanish Ministry of Science and Innovation (PID2019-105542RB-I00 project) and the UE-FEDER funding. Martínez-Munuera also acknowledges Spanish Ministry of Science, Innovation and Universities for the financial support through a FPU grant (FPU17/00603).

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.

Vehicular pollution has become a major problem in urban areas due to the exponential increase in the number of automobiles. Typical exhaust emissions, which include nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO), soot, and particulate matter (PM), doubtless have important negative effects on the environment and human health, including cardiovascular effects such as cardiac arrhythmias and heart attacks, and respiratory effects such as asthma attacks and bronchitis. The mitigation measures comprise either the use of clean alternative fuels or the use of innovative technologies. Several existing emission control technologies have proven effective at controlling emissions individually, such as selective catalytic reduction (SCR) and lean NOx trap (LNT) to reduce NOx and diesel particulate filter (DPF) specifically for PM abatement. These after-treatment devices are the most profitable means to reduce exhaust emissions to acceptable limits (EURO VI norms) with very little or no impact on the engine performances. Additionally, the relative lack of physical space in which to install emissions-control equipment is a key challenge for cars, especially those of small size. For this reason, to reduce both volume and cost of the after-treatment devices integrated catalytic systems (e.g., a sort of a “single brick”) have been proposed, reducing both NOx and PM simultaneously. This review will summarize the currently reported materials for the simultaneous removal of NOx and soot, with particular attention to their nature, properties, and performances.

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.

In this study, Ce/Zr-based materials without noble metals in their formulation have been prepared, characterized and tested in NOx adsorption at different temperatures. In particular, the effect of the presence of Ba and Cu in the catalytic formulation has been investigated. The catalyst characterization shows that the redox properties of the catalysts are enhanced by Cu and inhibited by Ba. Upon interaction with NO/O2, it is found that NOx are stored at low temperatures (150 °C) in the form of nitrites, without any relevant NO oxidation activity. This suggests a direct NO uptake. At higher temperatures (300 °C) a significant NO oxidation activity is observed, that is enhanced by Cu and depressed by Ba. In this case, NOx are stored in the form of nitrates; the samples containing both Ba and Cu exhibits the better performances in terms of NOx adsorption capacity. The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEOII/2018/076 project), the Spanish Ministry of Science, Innovation and Universities (CTQ2015-64801-R project) and the UE-FEDER funding. JCMM wishes to thank Spanish Ministry of Education and Professional Training his PhD contract within the FPU programme (Grant No. FPU17/00603).

In this work the reactivity of Pt-Rh NOx storage-reduction (NSR) catalysts in the reduction of NOx under lean conditions is investigated. It is found that significant amounts of NOx are stored on both Rh- and Pt-based samples at all the investigated temperatures (in the range 150–350¿°C). Mostly chelating nitrites are adsorbed at the lowest investigated temperature (150¿°C), while nitrates (both bidentate and ionic) at higher temperatures. However, at all temperatures nitrites prevail at the beginning of the storage phase, while nitrates represent the most abundant adsorbed species after prolonged contact. Pt-containing catalysts (either monometallic Pt or bimetallic Pt/Rh) show higher NOx storage capacity than the Rh monometallic sample, possibly due to the higher dispersion of Pt vs. Rh and/or to the higher oxidizing capability of Pt vs. Rh. The stored NOx species show relevant thermal stability, and decompose to NOx and O2 upon heating. In particular, nitrites disproportionate to gaseous NO and nitrates; these latter then decompose to NOx and O2. On the Rh-Ba/Al2O3 catalyst the disproportionation reaction is observed with a higher temperature onset if compared to the Pt-based samples. The analysis of the reactivity of the stored NOx species (probed by isotopic labeling experiments and reduction with H2 and NH3) showed the lower reactivity of the Rh-Ba/Al2O3 sample; however Rh shows activity in the ammonia decomposition reaction to N2 and H2, unlike Pt. The lower reactivity of the Rh-Ba/Al2O3 sample is also pointed out by experiments under cyclic lean-rich conditions. However, the presence of Rh increases the reactivity of the catalyst in the steam reforming of hydrocarbons, especially at high temperature, and accordingly the reactivity of the bimetallic Pt/Rh sample at high temperatures is higher than that of the Pt and Rh monometallic catalysts.

The formation of N2O has been studied by means of isothermal lean-rich experiments at 150, 180 and 250 °C over Pt–Ba/Al2O3 and Pt/Al2O3 catalysts with H2 and/or C3H6 as reductants. This allows to provide further insights on the mechanistic aspects of N2O formation and on the influence of the storage component. Both gas phase analysis and surface species studies by operando FT-IR spectroscopy were performed. N2O evolution is observed at both lean-to-rich (primary N2O) and rich-to-lean (secondary N2O) transitions. The production of both primary and secondary N2O decreases by increasing the temperature. The presence of Ba markedly decreases secondary N2O formation. FT-IR analysis shows the presence of adsorbed ammonia at the end of the rich phase only for Pt/Al2O3 catalyst. These results suggest that: (i) primary N2O is formed when undissociated NO in the gas phase and partially reduced metal sites are present; (ii) secondary N2O originates from reaction between adsorbed NH3 and residual NOx at the beginning of the lean phase. Moreover, N2O reduction was studied performing temperature programming temperature experiments with H2, NH3 and C3H6 as reducing agents. The reduction is completely selective to nitrogen and occurs at temperature higher than 250 °C in the case of Pt–Ba/Al2O3 catalyst, while lower temperatures are detected for Pt/Al2O3 catalyst. The reactivity order of the reductants is the same for the two catalysts, being hydrogen the more efficient and propylene the less one. Having H2 a high reactivity in the reduction of N2O, it could react with N2O when the regeneration front is developing. Moreover, also ammonia present downstream to the H2 front could react with N2O, even if the reaction with stored NOx seems more efficient.

In this study mechanistic aspects related to the adsorption of NOx over alumina-supported Pt, Ba and PtBa catalysts are deepened, that are of interest for NOx Storage-Reduction (NSR) catalysts and other applications involving the adsorption of NOx (e.g. Passive NOx Adsorbers, PNAs). NOx adsorption is investigated at low and high temperatures (150 °C and 350 °C, respectively); the thermal decomposition and the reactivity with H2 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 NOx are derived. In particular it is found that at 150 °C nitrites are formed on all surfaces when starting from NO/O2; 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 NO2 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 NO2 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 NOx 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 NO2 is also formed in significant amounts that may participate to the formation of nitrites/nitrates adspecies following a NO2 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 NOx at much lower temperatures than the adsorption; the role of the interaction between Pt and the storage sites on this step is herein discussed.

This chapter refers to the DPNR (Diesel Particulate–NOx Reduction) technology which aims at the simultaneous removal of soot and NOx from diesel exhausts by coupling the LNT technology for NOx reduction with the combustion of soot over the same LNT catalysts. First, the behavior of conventional Pt-based LNT catalyst in the soot combustion is analyzed. Then, the capability of Pt-based LNT catalysts to accomplish the removal of NOx in the presence of soot under cycling conditions (i.e. alternating lean–rich phases according to the typical DPNR strategy) is addressed. Particular emphasis is given to the specific role of each catalyst component and on the complex interaction between the de-NOx and de-soot functions. Finally, the potentialities of novel Pt-free DPNR formulations as well as alternative catalytic approaches are considered.

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 chapter is focused on the reduction process operating on a LNT model catalyst, with the aim of shedding light on fundamental aspects related to the mechanisms of stored NOx release, of released NOx reduction and of the formation of the various reduction products. This overview is the result of fundamental systematic studies obtained by means of catalytic tests under nearly isothermal conditions over a model Pt–Ba/Al2O3 catalyst where the attention is focused on the reactivity of the active sites. The gas phase analysis was combined with the surface species characterization performed by operando FT-IR spectroscopy. The results of dedicated experiments in which labeled 15NO species have been used are also reported, aiming at a better understanding of the reaction mechanisms leading to the formation of N2.

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 potentiality of ceria/zirconia based catalysts in the simultaneous removal of particulate matter (soot) and NOx is investigated in this work, and compared with that of a model LNT Pt-K/Al2O3 sample. Ceria-zirconia (molar ratio 75/25) catalysts doped with Pt, Au, Ru or Fe (2% by weight) and containing K (7% by weight) were prepared by a modified citrate method and characterized by X-ray diffraction, surface area and pore volume measurements. The behavior of the catalysts in the soot combustion and NOx removal was separately analyzed by means of temperature programmed oxidation (TPO), isothermal combustion and isothermal NOx adsorption experiments. The results showed that all the ceria/zirconia based catalysts are more active than Pt-K/Al2O3 in soot combustion; the Ru-containing system also showed NOx storage performances comparable to Pt-K/Al2O3. Accordingly the capability of the Ru-based catalyst to accomplish the removal of NOx in the absence and in the presence of soot was further investigated by reactivity experiments and FT-IR spectroscopy to analyze both the gas phase and the catalyst surface species. The data indicate that the Ru-based system is able to simultaneously remove soot and adsorb NOx pointing out higher performances in the soot combustion as compared to the Pt-K/Al2O3 catalyst, and similar behavior in the NOx storage capacity. However the NOx reduction activity results lower than the traditional LNT Pt-based catalyst. Conversely, when the Ru-based catalyst is mixed with the LNT sample (physical mixture) a NOx reduction efficiency similar to Pt-K/Al2O3 is found.

In this work, we have investigated the release and reduction of NOx (nitrates) stored on Rh–Ba/Al2O3 and Pt–Rh–Ba/Al2O3 catalysts and compared with Pt–Ba/Al2O3. 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.

The combustion of model soot (i.e. Printex U) by NO 2 and O 2 in the presence of water was investigated in a fixed bed micro reactor under isothermal conditions. A wide range of experimental conditions (e.g. 250–350 °C, 0–5 vol% O 2 , 0–500 ppmv NO 2 , 0–5 vol% H 2 O) were considered in order to investigate the role of NO 2 , O 2 and H 2 O in the combustion process. It was found that the oxidation of soot is initiated by NO 2 at temperatures where oxygen is unreactive. In addition, a synergistic effect between NO 2 and O 2 on soot combustion was observed in the presence of the NO 2 -O 2 mixture. Both the direct reaction between soot and NO 2 and also the contribution of gaseous oxygen on the combustion process appeared to rise with the increasing temperature and in the presence of water. Finally, the kinetic analysis of the experimental results showed a non-negligible dependence of the kinetic parameters on carbon conversion and in particular the poor accuracy of the shrinking-core model at low conversion degree.

Silver- and ruthenium-based materials are herein investigated for the simultaneous removal of particulate matter (soot) and NOx and their behaviour is compared with that of a model Pt–Ba/Al2O3 catalyst. The catalytic activity for diesel soot combustion, in loose contact conditions, is studied by means of TPO (temperature programmed oxidation) while NOx removal is investigated through NOx Storage-Reduction cycles (SRC), performed both in the presence and in the absence of soot. Both the Ag- and Ru-based formulations result active in the soot oxidation, more than the traditional Pt-containing DPNR catalyst. Also, the Ru-based sample shows remarkable performances in the DeNOx-DeSoot activity, although its activity in the reduction step of the stored NOx still needs further improvements.

The influence of the noble metals (Pt vs. Rh) on the NOx storage reduction performances of lean NOx trap catalysts is here investigated by transient micro-reactor flow experiments. The study indicates a different behavior during the storage in that the Rh-based catalyst showed higher storage capacity at high temperature as compared to the Pt-containing sample, while the opposite is seen at low temperatures. It is suggested that the higher storage capacity of the Rh-containing sample at high temperature is related to the higher dispersion of Rh as compared to Pt, while the lower storage capacity of Rh-Ba/Al2O3 at low temperature is related to its poor oxidizing properties. The noble metals also affect the catalyst behavior upon reduction of the stored NOx, by decreasing the threshold temperature for the reduction of the stored NOx. The Pt-based catalyst promotes the reduction of the adsorbed NOx at lower temperatures if compared to the Rh-containing sample, due to its superior reducibility. However, Rh-based material shows higher reactivity in the NH3 decomposition significantly enhancing N2 selectivity. Moreover, formation of small amounts of N2O is observed on both Pt- and Rh-based catalyst samples only during the reduction of highly reactive NOx stored at 150 °C, where NOx is likely in the form of nitrites.

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.

Ruthenium-based materials are investigated for the simultaneous removal of soot and NOx and their behavior is compared with that of model Pt-Ba/Al2O3 and Pt-K/Al2O3 catalysts.; The materials, 1%wt Ru-10%wt AM/MO (AM = Ba or K; MO= Ce0.8Zr0.2O2, O-2, Al2O3), have been prepared by incipient wetness impregnation and characterized by BET, XRD, HRTEM and TPR experiments. The catalytic activity for diesel soot combustion is studied under loose contact conditions by means of TPO (Temperature Programmed Oxidation) while the reactivity in the NO, removal is investigated by Isothermal Concentration Step Change experiments.; All the ruthenium-based formulations are active in the combustion of diesel soot, particularly potassium-doped materials that show low onset temperatures, near 230 degrees C. The catalytic activity in the simultaneous removal of soot and NOx shows that Ru-based catalysts are very efficient in the oxidation of soot also under isothermal conditions and that their NOx removal capacity (i.e. storage capacity) is almost comparable to that of traditional Pt-based LNT systems. However, the NOx reduction efficiency of the Ru-containing catalysts is lower than that of traditional LNT Pt-based catalysts. (C) 2016 Elsevier B.V. All rights reserved.

The effect of potassium on the oxidation of a model carbonaceous material (Printex U, namely, soot for brevity) has been investigated under isothermal conditions. For this purpose, Raman spectroscopy, Transmission Electron Microscopy (TEM), and Brunaure, Emmet, Teller surface area characterization have been applied to investigate structural changes occurring during soot oxidation both in the presence and in the absence of potassium. The Raman spectra of the model soot during combustion showed that oxidation preferentially involves the amorphous carbon fraction of the soot and only subsequently it affects the more ordered sp2 domains. However, in the K-doped Printex U the oxidation of both the amorphous and more ordered sp2 structures occurs concurrently. These findings have been confirmed by TEM analysis and explain the observed higher combustion activity of K-containing sample.Copyright © 2016 American Association for Aerosol Research

In this paper, mechanistic aspects involved in the formation of N2O over Pt-BaO/Al2O3 and Rh-BaO/Al2O3 model NOx Storage-Reduction (NSR) catalysts are discussed. The reactivity of both gas-phase NO and stored nitrates was investigated by using H2 and NH3 as reductants. It was found that N2O formation involves the presence of gas-phase NO, since no N2O is observed upon the reduction of nitrates stored over both Pt- and Rh-BaO/Al2O3 catalyst samples. In particular, N2O formation involves the coupling of undissociated NO molecules with N-adspecies formed upon NO dissociation onto reduced Platinum-Group-Metal (PGM) sites. Accordingly, N2O formation is observed at low temperatures, when PGM sites start to be reduced, and disappears at high temperatures where PGM sites are fully reduced and complete NO dissociation takes place. Besides, N2O formation is observed at lower temperatures with H2 than with NH3 in view of the higher reactivity of hydrogen in the reduction of the PGM sites and onto Pt-containing catalyst due to the higher reducibility of Pt vs. Rh.

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 potential of silver-based catalysts in the simultaneous removal of particulate matter (soot) and NO x is investigated in this work and compared with that of a model Pt–Ba/Al 2 O 3 catalyst. The Ag (5 wt%) – Ba (10 wt%)/MO (MO = CeO 2 , ZrO 2 , Al 2 O 3 ) and Ag (5 wt%) – Sr (10 wt%)/CeO 2 catalysts have been prepared by incipient wetness impregnation and characterized by BET, XRD, HRTEM, XPS and temperature-programmed reduction (TPR) experiments. The behavior of the catalyst in the soot combustion (under loose conditions) and NO x removal has been separately analyzed by means of temperature programmed oxidation (TPO) and isothermal concentration step change (ICSC) experiments, respectively. The results show that all the catalysts are active in soot combustion with a significant decrease of oxidation onset temperature compared to uncatalyzed soot oxidation. The removal of NO x in the absence and in the presence of soot was investigated under cycling conditions, i.e. alternating lean–rich phases according to the LNT strategy. It has been found that the Ag-based samples are able to simultaneously remove soot and NO x . In particular, comparing the behavior of the prepared catalysts, the Ba-containing systems showed higher NO x storage capacity than Sr-catalyst; also, the nitrogen selectivity increased even if resulted lower than the traditional LNT Pt-based catalyst. A detrimental effect of soot on the NO x storage activity has been also observed.

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 .

In this paper, fundamental issues of the NSR technology are addressed. The reaction paths and the mechanism involved in the storage of NO x and the relevance of the proximity between the noble metal and the alkali or alkaline-earth component of the catalyst are discussed first. Then the release of NO x , the reaction pathways of NO x reduction by different reducing agents, and the mechanisms involved in the formation of N2, NH3 and N2O are illustrated. The use of the combined technologies LNT + SCR and DPNR is addressed next and key aspects that highlight the synergies of the individual technologies are discussed. Finally, the perspectives of the NSR technology are outlined.

The reduction with n-heptane of gaseous NO and NOx stored over a model PtBa/Al2O3 catalyst is investigated in this paper by transient microreactor flow experiments coupled with in situ FT-IR spectroscopy for the analysis of the surface species. It is found that n-heptane is an effective reductant of both gaseous NO and stored NOx at temperatures above 200 and 250 °C, respectively. The reduction leads to the selective formation of N2 above 300 °C, whereas significant amounts of other species (N2O) are also formed at lower temperatures. A reaction pathway for the reduction of stored NOx is suggested where n-heptane initially reduces the oxygen-covered Pt sites. This leads to the activation/destabilization of the stored NOx, leading to the NOx release. Released NOx dissociate over Pt sites into N- and O-adatoms: the O-adatoms are scavenged by the hydrocarbon leading to COx and H2O, whereas N-adatoms may recombine with undissociated NO molecules, with other N-adatoms, or with H-ad-species to give N2O, N2, and...

DPNR Pt-K/Al 2 O 3 catalyst has been studied for the simultaneous removal of soot and NO x , in the presence of both water and carbon dioxide. In particular, the effect of soot on the NO x storage activity and on the stability of Pt-K/Al 2 O 3 catalyst has been addressed. It has been found that: (i) the presence of soot decreases the NO x storage activity at any temperature in the range 250–350 °C; (ii) the stability of the adsorbed species is lower in the presence of soot; (iii) nitrates ad-species directly interact with soot and participate to the soot combustion; (iv) aged catalyst shows a lower storage capacity and a lower soot oxidation activity attributed to loss of K-active species.

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 reduction of NO x stored over a Pt–Ba/Al2O3 Lean NO x Trap is analysed when H2, 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 H2 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.

Mechanistic aspects involved in the formation of N2 and of N2O 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/Al2O3 NSR catalyst. The reduction of gaseous labeled NO with unlabelled NH3 leads to the formation of N2O at low temperature (below 180 °C), and of N2 at high temperature. All N2 possible isotopes are observed, whereas only labeled molecules have been detected in the case of N2O. Hence the formation of nitrous oxide involves undissociated NO molecules, whereas that of N2 can be explained on the basis of the statistical coupling of 15N- and 14N-adatoms on Pt. However, due to a slight excess of the mixed 15N14N isotope, a SCR-like pathway likely operates as well. The reduction of the stored labelled nitrates is very selective to N2 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 14N15N 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.