Influence of the Storage Material on the Storage of NOx at Low Temperatures

The NO _x storage performance at low temperature (100–200 °C) has been studied for model NO _x storage catalysts. The catalysts were prepared by sequentially depositing support, metal oxide and platinum on ceramic monoliths. The support material consisted of acidic aluminium silicate, alumina or basic aluminium magnesium oxide, and the added metal oxide was either ceria or barium oxide. The NO _x conversion was evaluated under net-oxidising conditions with transients between lean and rich gas composition and the NO _x storage performance was studied by isothermal adsorption of NO₂ followed by temperature programmed desorption of adsorbed species. The maximum in NO _x storage capacity was observed at 100 °C for all samples studied. The Pt/BaO/Al₂O₃ catalyst stored about twice the amount of NO _x compared with the Pt/Al₂O₃ and Pt/CeO₂/Al₂O₃ samples. The storage capacity increased with increasing basicity of the support material, i.e. Pt/Al₂O₃·SiO₂ < Pt/Al₂O₃ < Pt/Al₂O₃ · MgO. Water did not significantly affect the NO _x storage performance for Pt/Al₂O₃ or Pt/BaO/Al₂O₃.     

NO x uptake mechanism on Pt/BaO/Al2O3 catalysts

The NO _x adsorption mechanism on Pt/BaO/Al₂O₃ catalysts was investigated by performing NO _x storage/reduction cycles, NO₂ adsorption and NO + O₂ adsorption on 2%Pt/(x)BaO/Al₂O₃ (x = 2, 8, and 20 wt%) catalysts. NO _x uptake profiles on 2%\Pt/20%BaO/Al₂O₃ at 523 K show complete uptake behavior for almost 5 min, and then the NO _x level starts gradually increasing with time and it reaches 75% of the inlet NO _x concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO _x conversion at 523 K, only ~2.4 wt% out of 20 wt% BaO is converted to Ba(NO₃)₂. Adsorption studies by using NO₂ and NO + O₂ suggest two different NO _x adsorption mechanisms. The NO₂ uptake profile on 2%Pt/20%BaO/Al₂O₃ shows the absence of a complete NO _x uptake period at the beginning of adsorption and the overall NO _x uptake is controlled by the gas–solid equilibrium between NO₂ and BaO/Ba(NO₃)₂ phase. When we use NO + O₂, complete initial NO _x uptake occurs and the time it takes to convert ~4% of BaO to Ba(NO₃)₂ is independent of the NO concentration. These NO _x uptake characteristics suggest that the NO + O₂ reaction on the surface of Pt particles produces NO₂ that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO _x uptake, this spill-over process is very fast and so it is able to provide complete NO _x storage. However, the NO _x uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO₃)₂. The formation of Ba(NO₃)₂ around the Pt particles results in the development of a diffusion barrier for NO₂, and increases the probability of NO₂ desorption and consequently, the beginning of NO _x slip. As NO _x uptake by NO₂ spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO₂ uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO₂ concentration.     

Sulfur deactivation and regeneration of Pt/BaO/Al2O3 and Pt/SrO/Al2O3 NO x storage catalysts

Transient experiments were performed to study sulfur deactivation and regeneration of Pt/BaO/Al₂O₃ and Pt/SrO/Al₂O₃ NO _x storage catalysts. It was found that the strontium-based catalysts are more easily regenerated than the barium-based catalysts and that a higher fraction of the NO _x storage sites are regenerated when H₂ is used in combination with CO₂ compared to H₂ only.     

Effect of Co and Rh promoter on NOx storage and reduction over Pt/BaO/Al2O3 catalyst

Effect of cobalt and rhodium promoter on NO_x storage and reduction (NSR) kinetics was investigated over Pt/BaO/Al₂O₃. Kinetics of 2% cobalt loading over Pt/BaO/Al₂O₃ demonstrated highest NO_x uptake during lean cycle, while reduction efficiency during rich cycle appeared most poor. In contrast to this, rhodium showed suppressing effect of NO_x uptake during lean cycle and demonstrated an enhanced effect for the higher efficiency of NO_x reduction during rich cycle. DRIFT study for NO_x uptake and regeneration confirmed formation of surface BaNO_x from the band at 1300 cm⁻¹ and formation of bulk BaNO_x from the band at 1330 cm⁻¹.     

Platinum Oxidation and Sulphur Deactivation in NOx Storage Catalysts

Flow reactor experiments and X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the importance of platinum oxide formation on Pt/BaO/Al₂O₃ NO _x storage catalysts during reactions conditions. The reaction studied was NO(g) + 1/2 O₂(g) NO₂(g). During NO₂ exposure of the catalyst the NO₂ dissociation rate decreased during the reaction. This activity decrease with time was also studied with XPS and it was found to be due to platinum oxide formation. The influence of sulphur exposure conditions on the performance of the NO _x storage catalysts was studied by exposing the samples to lean and/or rich gas mixtures, simulating the conditions in a mixed lean application, containing SO₂. The main results show that all samples are sensitive to sulphur and that the deactivation proceeds faster when SO₂ is present in the feed under rich conditions than under lean or continuous SO₂ exposure. Additionally, the influence of the noble metals present in the catalysts was investigated regarding sulphur sensitivity and it was found that a combination of platinum and rhodium seems to be preferable to retain high performance of the catalyst under SO₂ exposure and subsequent regeneration. Finally, the behaviour of micro-fabricated model NO _x storage catalysts was studied as a function of temperature and gas composition with area-resolved XPS. These model catalysts consisted of a thin film of Pt deposited on one-half of a BaCO₃ pellet. It was found that the combination of SO₂ and O₂ resulted in migration of Pt on the BaCO₃ support up to one mm away from the Pt/BaCO₃ interface.     

Role of Support in Lean DeNOx Catalysis

FTIR and pulse thermal analysis were applied to investigate catalysts containing Pt (1 wt%)/Ba (17 wt%) supported on -Al₂O₃, SiO₂ and ZrO₂. The aim was to learn how the support material affects the thermal stability of barium carbonate and its activity in the reaction to bulk Ba(NO₃)₂. The lower thermal stability of BaCO₃ in alumina supported samples was found to influence the formation of barium nitrate during the NO _x storage process. Quantification of Ba(NO₃)₂ formed during NO _x storage indicated that for alumina supported catalysts only ca. 30% of barium present in the sample is involved in the storage process. The low thermal stability found for alumina supported barium nitrite excludes its role in the formation of barium nitrate during interaction of NO _x with the catalyst at 300 °C. The studies indicate that -Al₂O₃ plays a major role in influencing the thermal stability of BaCO₃ and Ba(NO₃)₂. This finding seems to be relevant for the higher activity of -Al₂O₃-supported catalysts in NO _x storage reduction reactions.     

NOx Storage and Reduction Catalysts for Automotive Lean-Burn Engines: Effect of Parameters and Storage Materials on NOx Conversion

The effect of various parameters on the NO _x conversion over NO _x storage and reduction catalysts supported on alumina was investigated. The Pt/BaO/Al₂O₃ catalyst exhibited a higher NO _x reduction activity than the Pt/Al₂O₃ catalyst under the static and cycling conditions. The activity of Pt/BaO/Al₂O₃ catalyst was improved in the cycled feedstream. The Pt/SrO/Al₂O₃ was found to have as high activity as Pt/BaO/Al₂O₃ for NO _x reduction. In order to achieve effective reduction of NO _x , NO _x storage in the form of Me(NO₃)₂ (Me = Ba or Sr) is more favorable than other nitrates and the rich condition should be chosen in such a way that the sorption capacity can be fully regenerated at a fast rate and the inhibition effect by strongly adsorbed molecules derived from C₃H₆ and CO can be minimized.     

Fundamental Investigations of Thermal Aging Phenomena of Model NOx Storage Systems

Investigations of the aging behavior induced by high temperatures coupled with oxidizing atmosphere of model NO _x storage systems Ba/Al₂O₃ and Ba/CeO₂ are reported in this paper. The samples were prepared, calcined and exposed to temperatures between 500 and 1000 °C in air for 12 h for thermal aging. Samples were characterized with XRD, HRSEM, DSC-TGA-MS and BET analyses. In XRD investigations of all model systems calcined at 500 °C for 2 h, the NO _x storage component was present in form of BaCO₃. The release of CO₂ as a result of the decarbonization of the NO _x storage component at increased temperatures was verified by thermogravimetric investigations. In the case of Ba/Al₂O₃, already during calcination a partial reaction of the NO _x storage component with Al₂O₃ resulting in the formation of barium aluminate was observed. In the model system Ba/CeO₂ the decomposition of the barium carbonate started above 780 °C and the formation of a barium cerium mixed oxide was observed. The presence of the barium containing NO _x storage component has a strong influence on the specific surface area of the model NO _x storage systems. The morphology and crystallite size of CeO₂ modified with the barium containing NO _x storage component exhibited distinct changes compared to the unmodified oxide. The NO _x storage efficiency determined by model gas tests of freshly prepared and engine aged model NO _x storage catalysts correlates well with the above described observations.     

Dynamics and selectivity of NOx reduction in NOx storage catalytic monolith

Several nitrogen compounds can be produced during the regeneration phase in periodically operated NO_x storage and reduction catalyst (NSRC) for conversion of automobile exhaust gases. Besides the main product N₂, also NO, N₂O, and NH₃ can be formed, depending on the regeneration phase length, temperature, and gas composition. This contribution focuses on experimental evaluation of the NO_x reduction dynamics and selectivity towards the main products (NO, N₂ and NH₃) within the short rich phase, and consequent development of the corresponding global reaction-kinetic model. An industrial NSRC monolith sample of PtRh/Ba/CeO₂/ -Al₂O₃ type is employed in nearly isothermal laboratory micro-reactor. The oxygen and NO_x storage/reduction experiments are performed in the temperature range 100–500 °C in the presence of CO₂ and H₂O, using H₂, CO and C₃H₆ as the reducing agents.The spatially distributed NSRC model developed earlier is extended by the following reactions: NH₃ is formed by the reaction of H₂ with NO_x and it can further react with oxygen and NO_x deposited on the catalyst surface, producing N₂. Considering this scheme with ammonia as an active intermediate of the NO_x reduction, a good agreement with experiments is obtained in terms of the NO_x reduction dynamics and selectivity. A reduction front travelling in the flow direction along the reactor is predicted, with the NH₃ maximum on the moving boundary. When the front reaches the reactor outlet, the NH₃ peak is observed in the exhaust gas. It is assumed that the ammonia formation during the NO_x reduction by CO and HCs at higher temperatures proceed via the water gas shift and steam reforming reactions producing hydrogen. It is further demonstrated that oxygen storage effects influence the dynamics of the stored NO_x reduction. The temperature dependences of the outlet ammonia peak delay and the selectivity towards NH₃ are correlated with the effective oxygen and NO_x storage capacity.     

Influence of the Type of Reducing Agent (H2, CO, C3H6 and C3H8) on the Reduction of Stored NOX in a Pt/BaO/Al2O3 Model Catalyst

In this investigation, a comparative study for a NO _X storage catalytic system was performed focusing on the parameters that affect the reduction by using different reductants (H₂, CO, C₃H₆ and C₃H₈) and different temperatures (350, 250 and 150 °C), for a Pt/BaO/Al₂O₃ catalyst. Transient experiments show that H₂ and CO are highly efficient reductants compared to C₃H₆ which is somewhat less efficient. H₂ shows a significant reduction effect at relatively low temperature (150 °C) but with a low storage capacity. We find that C₃H₈does not show any NO _X reduction ability for NO _X stored in Pt/BaO/Al₂O₃ at any of the temperatures. The formation of ammonia and nitrous oxide is also discussed.     

Comparative NOx Reduction Behavior of Pt, Pd, and Rh Supported Catalysts in Simulated Exhaust Gases as a Function of Oxygen Concentration

NO _x reduction activity on Pt and Pd catalysts had a maximum for S value as stoichiometry number at a fixed temperature, and the S value at the maximum NO _x conversion increased with decreasing temperature. NO _x conversion on Rh catalyst increased with decreasing S value, but independent of temperature. As for the effect of HC on NO _x reduction behavior, it was concluded that, for Pt and Pd catalysts, HC adsorbs strongly on the catalysts surface to cause the self-inhibition. Increasing O₂ concentration lead to oxidation of HC, but decreased the value of NO/O₂ ratio. The balance point of the two factors generated a maximum NO _x conversion. For Rh catalyst, the strongly adsorbed oxygen is more reactive with decreasing S value, and thus NO _x conversion is increased.     

Soot combustion activity of NOx-sorbing Cs–MnOx–CeO2 catalysts

Activities of Cs-loaded MnO_x–CeO₂ for combustion of model diesel soot (carbon black) and sorptive NO uptake have been studied. MnO_x–CeO₂ is a pseudo-solid solution having redox properties favorable for soot oxidation. The addition of Cs not only lowered the temperature of soot ignition (T_i), but also increased oxidative NO_x adsorption to form nitrate on the surface. Soot ignition over Cs–MnO_x–CeO₂ was further promoted in a stream of NO/O₂, presumably because nitrate on the surface plays a role of an oxidizing agent. Soot ignition started just before sharp desorption of NO_x, suggesting that adsorbed nitrate species would directly interact with soot.     

Low-temperature catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

MnO_x–CeO₂ mixed oxide catalysts prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of chlorinated aromatic volatile organic compounds (CVOCs). MnO_x–CeO₂ catalysts with the different ratio of Mn/Ce + Mn were found to possess high catalytic activity for catalytic combustion of CB, and MnO_x(0.86)–CeO₂ was the most active catalyst, on which the complete combustion temperature (T_90%) of chlorobenzene was 236 °C. The stability of MnO_x–CeO₂ catalysts in the CB combustion was investigated. MnO_x–CeO₂ catalysts with high Mn/Ce + Mn ratios present high stable activity, which is related to their high ability to remove Cl species adsorbed and a large amount of active surface oxygen.     

The Effect of a Changing Lean Gas Composition on the Ability of NO2 Formation and NOx Reduction over Supported Pt Catalysts

Three model catalysts (Pt/Al₂O₃, Pt/TiO₂, Pt/V₂O₅/TiO₂) were examined in regard to their NO₂ formation ability under a changing lean gas composition. The results show that the NO to NO₂ oxidation function as well as the NO _x reduction under lean gas conditions is affected by a change in the lean gas atmosphere. The NO oxidation activity also decreased with time, for Pt/Al₂O₃ and Pt/TiO₂, and a possible explanation may be platinum oxide formation. This deactivation was not observed for Pt/V₂O₅/TiO₂.     

Activity of oxygen reduction reaction on small amount of amorphous CeOx promoted Pt cathode for fuel cell application

Pt on ceria (CeO_x) particles supported on carbon black (CB) were synthesized using the combined process of hot precipitation and impregnation methods. During 30 cycles of cyclic voltammetry pre-treatment in the potential ranging from −0.2 to 1.3 V (V vs. Ag/AgCl), it was observed that a small amount of CeO_x, which consisted of the interface region between Pt and CeO_x, remained on Pt particles. Other free CeO_x particles were dissolved into H₂SO₄ aqueous solution. To develop the Pt-CeO_x/CB catalyst, the surface chemical states, the net chemical composition, morphology and electrochemical behavior in H₂SO₄ aqueous solution were characterized. Our microanalysis and electrochemical analysis indicate that the active CeO₂ with high specific surface area provides the continuous amorphous cerium oxide (Ce³⁺, Ce⁴⁺) layer with pores on the surface of Pt particles. It is concluded that the amorphous cerium oxide layer on Pt inhibits the oxidation of Pt surface and contributes to enhancement of the activity on Pt cathode. The single cell performance was also improved using the Pt-CeO_x/CB cathode. Based on all data, it is expected that the design based on characterization of the interface between Pt and small amount of amorphous cerium oxide layer could help in preparation of more active Pt catalyst.     

Steam effect on NO x reduction over lean NO x trap Pt–BaO/Al2O3 model catalyst

The effect of steam on NO _x reduction over lean NO _x trap (LNT) Pt–Ba/Al₂O₃ and Pt/Al₂O₃ model catalysts was investigated with reaction protocols of rich steady-state followed by lean–rich cyclic operations using CO and C₃H₈ as reductants, respectively. Compared to dry atmosphere, steam promoted NO _x reduction; however, under rich conditions the primary reduction product was NH₃. The results of NO _x reduction and NH₃ selectivity versus temperature, combined with temperature programmed reduction of stored NO _x over Pt–BaO/Al₂O₃ suggest that steam causes NH₃ formation over Pt sites via reduction of NO _x by hydrogen that is generated via water gas shift for CO/steam, or via steam reforming for C₃H₈/steam. During the rich mode of lean–rich cyclic operation with lean–rich duration ratio of 60 /20 s, not only the feed NO, but also the stored NO _x contributed to NH₃ formation. The NH₃ formed under these conditions could be effectively trapped by a downstream bed of Co²⁺ exchanged Beta zeolite. When the cyclic operation was switched into lean mode at T < 450 °C, the trapped ammonia in turn participated in additional NO _x reduction, leading to improved NO _x storage efficiency.     

A comparative study of the selective oxidation of NH3 to N2 over gold, silver and copper catalysts and the effect of addition of Li2O and CeOx

This paper describes the selective oxidation of ammonia into nitrogen over copper, silver and gold catalysts between room temperature and 400 °C using different NH₃/O₂ ratios. The effect of addition of CeO_x and Li₂O on the activity and selectivity is also discussed. The results show that copper and silver are very active and selective toward N₂. However the multicomponent catalysts: M/Li₂O/CeO_x/Al₂O₃ (M: Au, Ag, Cu) perform the best. On all three metal containing catalysts the activity and selectivity is influenced by the particle size and the interaction between metal particles and support.     

SOx storage and release kinetics for ceria-supported platinum

The SO_x storage and release kinetics on CeO₂ have been studied by lean SO_x adsorption and temperature programmed desorption for different pairwise configurations of individual monolith samples, i.e., Pt/CeO₂ + SiO₂, Pt/SiO₂ + CeO₂, CeO₂ + Pt/SiO₂ and CeO₂ + SiO₂. In the case of sole ceria, SO_x adsorption proceeds both via SO₂ and SO₃ adsorption although the latter channel is kinetically favored. Hence, the rate of SO₂ oxidation is crucial for the overall SO_x storage kinetics. It is also found that physical contact between Pt and ceria is important for the storage process. This is attributed to efficient transport routes for SO_x (surface diffusion and spill-over processes) and/or specific adsorption sites at the platinum–ceria interface. The main route for SO_x release is found to be thermal decomposition where the effect of platinum is minor, although an indirect effect cannot be ruled out. Different mechanistic scenarios for SO_x adsorption are discussed, which may serve as a guide for future experiments.     

Characterisation by TPR, XRD and NOx Storage Capacity Measurements of the Ageing by Thermal Treatment and SO2 Poisoning of a Pt/Ba/Al NOx-Trap Model Catalyst

The deactivation of a Pt/Ba/Al₂O₃ NO _x -trap model catalyst submitted to SO₂ treatment and/or thermal ageing at 800 °C was studied by H₂ temperature programmed reduction (TPR), X-ray diffraction (XRD) and NO _x storage capacity measurements.The X-ray diffractogram of the fresh sample exhibits peaks characteristic for barium carbonate. Thermal ageing leads to the decomposition of barium carbonate and to the formation of BaAl₂O₄. The TPR profile of the sulphated sample shows the presence of (i) surface aluminium sulphates, (ii) surface barium sulphates, (iii) bulk barium sulphates. The exposure to SO₂ after ageing leads to a small decrease of the surface barium-based sulphates, expected mainly as aluminate barium sulphates. This evolution can be attributed to a sintering of the storage material. TPR experiments also show that thermal treatment at 800 °C after the exposure to SO₂ involves the decomposition of aluminium surface sulphates to give mainly bulk barium sulphates, also pointed out by XRD. Thus, the thermal treatment at 800 °C leads to a stabilization of the sulphates.These results are in accordance with the NO _x storage capacity measurements. On non-sulphated catalysts, the treatment at 800 °C induces to a decrease of the NO _x storage capacity, showing that barium aluminate presents a lower NO _x storage capacity than barium carbonate. Sulphation strongly decreases the NO _x storage capacity of catalysts, whatever the initial thermal treatment, showing that barium sulphates inhibit the NO₂ adsorption. Moreover, the platinum activity for the NO to NO₂ oxidation is lowered by thermal treatments.