NSR Catalyst Research Articles

Labeled 15NO Study on N2 and N2O Formation Over Pt–Ba/Al2O3 NSR Catalysts

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.

The effect of CO on NO reduction over Pt/Pd-based NSR catalysts at low temperature

The present work investigates the effect of CO on NO reduction during the rich period over Pt/Pd-based NSR catalysts at low temperature. Three types of reducing agents (CO and/or H2) are used in this research. BET and XRD are used to determine physical properties, and in situ DRIFTS explore the competitive adsorption of NO–CO and the strength of CO adsorption on the noble metal component. It is found that NO is completely reduced over Pt/Pd-based catalysts at 150 °C when H2 is the reducing agent, while there is NO emission phenomenon when CO is the reducing agent. When H2 and CO are reductants together, catalysts with/without Pd exhibit different NO reduction activity. In situ DRIFTS results show that there is a competitive adsorption of NO and CO on Pt/Al2O3 and Pd/Al2O3, and Pt/Al2O3 presents stronger CO adsorption in comparison with Pd/Al2O3. It is probable that the NO reduction is related to CO adsorption on the noble metal component. The adsorption strength of CO on Pt/Pd-based catalysts affects the NO reduction performance during the rich period.

Effect of water and ammonia on surface species formed during NOx storage–reduction cycles over Pt–K/Al2O3 and Pt–Ba/Al2O3 catalysts

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.

Cu-zeolite NH3-SCR catalysts for NOx removal in the combined NSR–SCR technology

The challenge of efficient NOx removal from diesel and lean-burn engine exhaust gas by combining NSR and SCR catalyst is studied. Several Cu exchanged zeolites have been prepared, varying the preparation method (ion exchange and impregnation), the copper content (1–6%) and the zeolite (BETA and ZSM5). The prepared catalysts have been characterized, and acidity, surface area, crystallinity and metal reducibility have been compared. SCR experiments under 750ppm NO, 750ppm NH3 and 9.5% O2 (Ar to balance) discriminated low copper loading, prepared by ion exchange catalyst (Z-IE-1.4 and B-IE-2.1) as the most active for NOx conversion (>95%) in ample temperature range (280–450°C). These active SCR catalysts were placed downstream a monolith NSR Pt–BaO/Al2O3 catalyst, running under cycled lean–rich conditions, and the improvement on NOx removal and selectivity to only N2 were determined. In an ample range of temperature, from 200 to 400°C, NOx conversion was increased in more than 30%, also notably increasing the production of nitrogen, and reducing production of ammonia and N2O below 3% and 2%, respectively, when comparing the combined NSR–SCR configuration versus the single NSR catalyst.

Superior Performance of Mesoporous TiO2–Al2O3 Supported NSR Catalysts with the Support Synthesized Using Nonionic and Cationic Surfactants as Co-Templates

Mesoporous binary oxides TiO2–Al2O3 were prepared by citric acid complexation-organic template decomposition method using nonionic p-octyl polyethylene glycol phenyl ether (OP) and cationic cetyltrimethyl-ammonium bromide (CTAB) as co-templates; the corresponding NSR catalysts Pt/K/TiO2–Al2O3 were prepared by successive wetness impregnation. Multiple techniques including N2 physisorption, XRD, HR-TEM, NH3-TPD, H2-TPR and H2-chemisorption were employed for catalyst characterization. It is found that the support prepared using OP and CTAB as co-templates possesses much larger specific surface area (309 m2/g) than those prepared using CTAB as single template (275 m2/g) or using conventional co-precipitation (250 m2/g); meanwhile, this support exhibits the largest amount of surface acidic sites as indicated by NH3-TPD results, which makes its supported catalyst show the best sulfur-resistance performance among the catalysts with the support prepared by different methods. The results of H2-chemisorption and HR-TEM conformably indicate that this catalyst also possesses the highest dispersion of Pt, which determines its best NOx storage and reduction performance at lean/rich cycles, giving a mean NOx reduction percentage as high as 95 %.

Effect of carbon black combustion on NOx trap catalyst performances

The effect of carbon–NSR catalyst contact on the ability of the catalyst to store NOx is investigated. The catalysts used are model catalysts composed of Pt and/or Ba supported on Al2O3 (2%Pt/γ-Al2O3, 20%BaO/γ-Al2O3 and 2%Pt–20%BaO/γ-Al2O3). It is shown that the presence of carbon black in the catalytic bed involves a decrease of the NOx storage capacity (NSC) of the catalysts. Participation of a direct reaction between carbon particles and NOx storage sites to form carbonate adsorbed species is proposed. However, the loss of NSC measured in the presence of carbon varies as a function of the catalyst composition. It is shown that the presence of platinum limits the influence of carbon–catalyst contact. The influence of the proximity between the storage sites, Pt sites and their contact with carbon is discussed.

Pathways for N2 and N2O Formation during the Reduction of NOx over Pt–Ba/Al2O3 LNT Catalysts Investigated by Labeling Isotopic Experiments

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.

Preparation and Characterization of the Typical Pt-NSR Catalyst

A series of Ba-Al-O NSR supports and Pt/Ba-Al-O NSR catalysts are prepared by co-precipitation and impregnation method in this work. The catalyst and the support are characterized by XRD, SEM, SBET performance testing. The structure and texture of the supports is observed and discussed. The results of SBET indicate that the supports possess relative high specific surface area (94~110 m2/g). Temperature programmed reduction is characterized by means of H2-TPR.

NOx storage and reduction on a SrTiCuO3 perovskite catalyst studied by operando DRIFTS

The behaviour of a perovskite SrTiCuO3 as NSR catalyst has been studied by simultaneous DRIFTS and activity measurement. The amount of NOx stored on the perovskite SrTiCuO3 (41μmol/m2 at 300°C) is much higher, when compared on a surface area basis, than on noble metal-containing reference catalysts (0.3–23μmol/m2). The perovskite SrTiCuO3 is able to perform both the NO oxidation to NO2 and the NOx storage, and copper enhances both the NO oxidation and NOx chemisorption capacities of copper-free SrTiO3 perovskite. During NOx chemisorption on SrTiCuO3, the following process have been identified (i) NO chemisorption and nitrite formation or NO oxidation to gas phase NO2 and further chemisorption of NO2 with nitrites or nitrates formation, (ii) nitrite oxidation to nitro and/or nitrates, and (iii) nitro and/or nitrate decomposition with release of NO2. NOx chemisorption at 300°C is more effective than chemisorption at 350°C, while regeneration at 350°C is more effective than regeneration at 300°C because: (i) decreased NOx slip (only for CO regeneration) and (ii) maintains the same chemisorption capacity of the perovskite after regeneration. H2 is a better reductant than CO, and the differences between both reducing gases are much higher at 300°C than at 350°C.

Control of NOx storage and reduction in NSR bed for designing combined NSR–SCR systems

A detailed analysis of the catalytic behavior in the storage and reduction of NOx of a homemade Pt–BaO/Al2O3 NSR monolith catalyst determining the N2/NH3 production surface in response to operational variables, namely temperature and hydrogen concentration during rich conditions was obtained. The production response curves are used to design the optimal operation, with high NOx removal efficiency and maximum production of nitrogen, when running the single NSR catalyst and the combined NSR–SCR configuration with Fe-beta zeolite catalyst placed downstream the Pt–BaO/Al2O3 monolith. Optimal operation of the single NSR monolith was attained at 300°C and 1% H2, which showed NOx removal of 74% with 64% of nitrogen production (both related to the total amount of NO in the feedstream, difference was mainly ammonia). The double NSR–SCR configuration allowed the Fe-beta catalyst storing ammonia to react with NOx leaving the NSR during the subsequent lean phase. This has a benefit on both the NOx removal efficiency and the N2 production. The amount of ammonia needed for total NOx removal efficiency and N2 production can be supplied by tuning temperature and H2 concentration. Temperature of 300°C and 3% H2 during rich conditions, produced 23% of ammonia at the exit of NSR. The SCR reaction between this amount of ammonia adsorbed on Fe-beta and the untrapped NO in NSR, resulted in practical total NOx removal efficiency (98%) and N2 production of 97% when the double NSR–SCR system is operated.

Influence of Ba precursor on structural and catalytic properties of Pt–Ba/alumina NOx storage-reduction catalyst

The effect of the choice of barium precursor (nitrate or acetate), in the preparation of Pt–Ba/γ-Al2O3 NSR catalysts, on their structure and catalytic behavior has been investigated by means of X-ray diffraction (XRD), temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and catalytic tests. XRD patterns showed that the type of Ba precursor and repeated NSR cycles influence the crystallinity of Ba species. Depending on the Ba precursor, remarkable differences in Ba(NO3)2 crystallite sizes as well as availability of the Pt for hydrogen chemisorption were observed. TPD and TPR studies showed that the catalyst prepared using the Ba acetate-precursor route is more efficient in the decomposition and reduction of Ba nitrates at lower temperatures. The catalyst prepared by the Ba acetate-precursor route showed both enhanced NOx storage ability and selectivity toward nitrogen in the reduction process, which is attributed to the intimate contact of the small crystallites of the Ba-component and the Pt particles. Environmental transmission electron microscopy (ETEM) measurements revealed that introduction of H2O vapor induces morphological changes of the Ba phase leading to partial liquefaction as a result of the formation of hygroscopic barium hydroxides. The interplay of this liquid-like behavior of the Ba phase together with the chemical transformations occurring during NOx storage and reduction can lead to remarkable structural changes affecting the NSR performance.

Improvement in sulfur desorption of NOX storage and reduction catalysts using a Ba–Ti composite oxide

A Ba–Ti composite oxide was formed on a NOx storage and reduction catalyst via impregnation of a Ba–Ti precursor solution composed of H2O2 added to a complex prepared using the citric acid method. The structure of the Ba–Ti composite in solution was analyzed by chemical composition analysis and FT-Raman and UV–vis spectroscopy. MM2 calculations were performed to propose its chemical structure. Both Ba and Ti together were found to form a composite molecule in the solution. Furthermore, TEM-EDX and XRD analyses of the Ba–Ti composite oxide on the catalyst prepared by impregnation with the Ba–Ti composite aqueous solution revealed that Ba and Ti in the catalyst were highly dispersed at the nm scale. The formation of the Ba–Ti composite oxide on the NSR catalyst enhanced sulfur desorption efficiency and led to high-performance NOX conversion as a NOX storage and reduction activity catalyst after desulfation treatment. It was assumed that the existence of nano-scaled Ba compounds combined with Ti was efficient for the inhibition of the sintering of barium sulfate and its facile decomposition. It was found that dispersion of Ba compounds for NOX storage materials using a Ba–Ti complex solution is an efficient way to improve the durability of NSR catalysts.

Controlling the selectivity to N 2O over Pt/Ba/Al 2O 3 NO X storage/reduction catalysts

N 2O formation during the reduction process over NSR catalysts is analysed in this paper. The effect of catalyst characteristics and different operation parameters (chemical nature of the reductant, temperature and cycle timing) are addressed and discussed in terms of N 2O selectivity.

Modified textures and redox activities in Pt/Al2O3 + BaO/CexZr1−xO2 model NSR catalysts

The present work investigates the catalytic performances of 20wt.% Ba supported CexZr1−xO2 (x=0, 0.3, 0.5, 0.7, 1) NOx storage catalysts, where 2wt.% Pt/Al2O3 part was blended in as physical mixtures. The interests were focused on the effects of Ce–Zr compositions as support oxides to barium species in NOx storage and release cycles. Higher Ce content favors rapid NOx up-taking and stable adsorption in adequate amount of small BaCO3 particles. The increasing amount of Zr in CexZr1−xO2 may benefit the formation of porous texture, but it brings the higher risk of BaZrO3 formation during the ageing. It not only consumes active Ba sites for NOx storage, but induces even severe sintering to these materials. Additionally, the introduced ZrO2 may increase oxygen mobility to improve redox activities of the catalysts. This improvement could lower the NH3 release in rich period. However, to another extreme, a too strong oxidative activity is going to inhibit the NOx reduction due to its stabilization to intermediate NO2δ+ species.

New insights into the promoting effect of H2O on a model Pt/Ba/Al2O3 NSR catalyst

The effects of H2O on the performance of a model Pt/Ba/Al2O3 catalyst and regeneration of the catalyst surface were investigated, both in the absence and presence of CO2. In the absence of CO2, an unexpected promotional effect of H2O was observed at low temperature. For example, in the presence of H2O, 90% NOX conversion was obtained at 150°C, which is the same as that at 350°C under otherwise identical conditions. The results demonstrate that regeneration, the rate-limiting step at low temperature, occurs through hydrogen spillover to the nitrates, and not through reverse NOX species migration to the Pt sites, and the presence of H2O greatly promotes the hydrogen spillover rate by providing and stabilizing surface hydroxyl groups. The promotional effect of H2O gradually decreases with increasing temperature, and shows no positive effect above 350°C. In the presence of CO2, however, the presence of H2O always results in improved performance in the entire temperature range. Besides enhanced hydrogen spillover, another contribution of H2O when CO2 is present, is to weaken the detrimental effect of CO. CO is formed via the reverse water–gas shift reaction and poisons Pt sites as well as forms barium isocyanates. The H2O decreases the amount of CO formed as well as hydrolyzes the –NCO species.

Study of DPNR catalysts for combined soot oxidation and NO x reduction

The behavior of a model PtBa/Al 2O 3 model NSR catalyst in both the NO x storage/reduction and soot oxidation is investigated in this work. It is found that the presence of soot negatively influences the NO x storage capacity of the catalyst, evaluated at 623 K in presence of water and CO 2 in the feed stream: in fact the amounts of NO x stored in presence of soot decrease by nearly 30% in the presence of roughly 10% (w/w) of soot. The presence of soot has a destabilizing effect on the NO x adsorbed species, which decompose to a large extent in the absence of gas phase NO and oxygen. This has also been confirmed by dedicated TPD and TPO experiments. However the presence of soot does not appreciably affect the behavior of the PtBa/Al 2O 3 catalyst in the reduction by H 2 of the stored nitrates, being in all cases N 2 the major reaction product along with minor amounts of ammonia. During the storage of NO x , soot oxidation takes place. Notably, the stored NO x participate in the soot oxidation upon release of NO 2 and O 2 which actively oxidize soot. However a direct participation of the adsorbed NO x species in the oxidation of soot cannot be excluded.

High-Temperature NO x Storage and Sulfur-Resistance of the Lithium-Based Lean-Burn NO x Trap Catalyst Pt/Li/TiO2–Al2O3

A series of lithium-based lean-burn NO x trap catalysts Pt/Li/TiO2–Al2O3 were prepared by sequential impregnation. The doping of TiO2 into the support Al2O3 significantly enhances the sulfur-resistance performance of the catalyst Pt/Li/Al2O3. On TiO2–Al2O3 mixed oxides, the Pt and lithium species are more highly dispersed, giving higher NO x storage capacity, as compared with those on Al2O3 and TiO2. In situ DRIFTS reveals that the NO x storage on Pt/Li/TiO2–Al2O3 mainly proceeds on –OLi sites forming bidentate nitrate species at 500 °C. The catalyst Pt/Li/TiO2–Al2O3 with 40% TiO2 in the support is the most promising one applicable to the lean-burn NO x abatement. The sulfated NSR catalyst Pt/Li/TA(40) is easier to reduce than the sulfated Pt/Ba/TA(40), showing better regeneration ability.

NO x removal over a double-bed NSR-SCR reactor configuration

A detailed analysis of the catalytic behaviour in the storage/reduction of NO x of single Pt–Ba/Al 2O 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 2O 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 2O 3 catalyst sample.

In situ DRIFT–TRM study of simultaneous NO x and soot removal over Pt–Ba and Pt–K NSR catalysts

The Pt–Ba/γ-Al 2O 3 and Pt–K/γ-Al 2O 3 catalysts have been studied in simultaneous NO x and soot removal under cyclic lean-rich conditions. The DRIFT–TRM activity studies were accompanied by a detailed TEM/HRTEM, XRD, and XPS characterization of the catalysts samples. It was found that the alkali component stabilizes the Pt 0/Pt ox ratio and better catalyst performance during simultaneous removal of NO x and soot was obtained for Pt–Ba, where Pt 0/Pt ox ratio was close to 0.70. Moreover, the NO to NO 2 oxidation rate was higher for the Pt–Ba catalyst than that for the Pt–K catalyst. The study reveals that soot has an influence both on the catalyst accumulation capacity and on the catalyst regeneration. As it is shown in the present work, the amount of NO x stored during the lean phase of TRM increases gradually during soot removal from the catalytic system. The regeneration process depends on alkali element type and is limited for the Pt–K catalyst.

Isotopic studies of NO X storage and reduction on Pt/BaO/Al 2O 3 catalyst using temporal analysis of products

A systematic isotopic study over Pt/BaO/Al 2O 3 powder catalyst was carried out using temporal analysis of products (TAP) to elucidate the role of the Pt/BaO interface and spillover processes during NO X storage and reduction. The sequential pre-nitration of Pt/BaO/Al 2O 3 using NO and 15NO followed by reduction with H 2 results in the preferential evolution of 15N-containing species during initial H 2 pulses. The evolution shifts toward unlabeled N-containing species in later H 2 pulses. The data suggest that NO X storage proceeds radially outward from the Pt crystallites and that some mobility of the stored NO X species exists. The evolution of N 2 and 15NN during 15NO–H 2 pump–probe on a pre-nitrated (using unlabeled NO) catalyst confirms the involvement of spillover processes at the Pt/BaO interface. The evolution of nitrogen takes place by NO decomposition as well as by the reaction of stored NO X with H 2 to form adsorbed N and eventually N 2. A significant fraction of N 2 is also produced via NH 3 serving as an intermediate. The results suggest that the local gradients at the Pt/BaO interface in the stored NO X are important and that these should be taken into account in NSR catalyst design and modeling.