Enhanced Low Temperature NO x Reduction Performance Over Bimetallic Pt/Rh–BaO Lean NO x Trap Catalysts

The overall NSR operation was tested over a bimetallic Pt/Rh–BaO lean NO _x trap (LNT) catalyst in the range of 473–673 K with simulated diesel exhausts and compared to monometallic 1 wt% Pt/BaO/γ-Al₂O₃ and 0.5 wt% Rh/BaO/γ-Al₂O₃ samples. The results showed the beneficial effect of the simultaneous presence of 0.5 wt% Pt and 0.25 wt% Rh on the catalytic performance under lean-burn conditions at low temperatures. It was observed that both Pt/BaO/γ-Al₂O₃ and Rh/BaO/γ-Al₂O₃, which both were mildly aged, have limited NO _x reduction capacity at 473 K. However, combining Pt and Rh in the NO _x storage catalyst assisted the NO _x reduction process to occur at lower temperatures (473 K). One possible reason could be that the combined Pt and Rh sample was more resistant to aging. In addition, the NO₂-TPD data showed that the presence of Rh into the Pt/BaO/γ-Al₂O₃ system has a considerable effect on the spill-over process of NO _x , accelerating the release of NO _x at lower temperatures. These results were in a good agreement with the observed higher rate of oxygen release of the bimetallic Pt/Rh catalyst, leaving a significant number of noble metal sites available for adsorption at lower temperatures than that of the monometallic Pt sample. The superior NSR performance of the bimetallic Pt/Rh/BaO/γ-Al₂O₃ catalyst under lean-burn conditions suggested the existence of synergetic promotion effect between the Pt and Rh components, increasing the NO _x reduction efficiency in comparison with that of the monometallic Pt and Rh–BaO LNT catalysts.     

Simultaneous Removal of NO x and Soot Over Pt–Ba/Al2O3 and Pt–K/Al2O3 DPNR Catalysts

The effect of NO _x storage on the soot combustion activity when alkaline- and alkaline/earth-containing model DPNR catalysts are used is investigated in this work. The influence of different experimental conditions (NO concentration, temperature, and particulate loading) is addressed and discussed in relation to the NO _x storage efficiency and soot oxidation activity as well.     

Spectrokinetic Analysis of the NOx Storage Over a Pt–Ba/Al2O3 Lean NOx Trap Catalyst

In this work the kinetics of the (reactive) lean accumulation phase of the NO_x storage-reduction process is described through a detailed kinetic model, involving both the gas-phase molecules and the adsorbed species. Kinetic data have been collected following a novel approach based on simultaneous operando spectroscopic measurements and on-line pulse reactor effluent analysis. To our knowledge, this is the first time the temporal evolutions of the concentration of both the surface and the gas species are used jointly to describe the kinetic of a transient catalytic process.     

The NO x Reduction Mechanism by H2 under near Isothermal Conditions over Pt–K/Al2O3 Lean NO x Trap Systems

The reduction of NO _x stored over a Pt–K/Al₂O₃ Lean NO _x Trap is analyzed when H₂ is used as reductant. An in series 2-steps process is herein proposed, involving at first the formation of NH₃ upon reaction of nitrates with H₂ (step 1), followed by the reaction of NH₃ with residual nitrates to give N₂ (step 2).     

Enhanced High Temperature Performance of MgAl2O4-Supported Pt–BaO Lean NOx Trap Catalysts

The structural and chemical characteristics of Pt/BaO lean NO_x trap (LNT) catalysts supported on ??-Al₂O₃ and MgAl₂O₄ are compared in this study. The Pt?CBaO/MgAl₂O₄ sample shows relatively low NO_x uptake at temperatures below 300?°C, and the temperature of maximum NO_x uptake (T_max) is shifted to 350?°C in comparison to that of Pt?CBaO/Al₂O₃ (T_max?~?250?°C). More importantly, the NO_x uptake over the MgAl₂O₄-supported catalyst at 350?°C is twice that of the alumina-based one. The shift toward the higher temperature NO_x uptake is explained by the larger interfacial area between Pt and BaO, due to smaller Pt clusters as evidenced by TEM and Pt L3 EXAFS. In situ TR-XRD results demonstrate that the formation of a BaAl₂O₄ phase in the BaO/MgAl₂O₄ LNT catalyst occurs at a temperature about 100?°C higher than on BaO/Al₂O₃, which may also represent a beneficial attribute of the BaO/MgAl₂O₄ LNT with respect to catalyst stability.     

Flame-Made Pt/K/Al2O3 for NO x Storage–Reduction (NSR) Catalysts

High surface area Pt/K/Al₂O₃ catalysts were prepared with a 2-nozzle flame spray method resulting in Pt clusters on γ-Al₂O₃ and amorphous K storage material as evidenced by Raman spectroscopy. The powders had a high NO _x storage capacity and were regenerated fast in a model exhaust gas environment. From 300 to 400 °C no excess NO _x was detected in the off gas during transition from fuel lean to fuel rich conditions, resulting in a highly effective NO _x removal performance. Above 500 °C, the NSR activity was lost and not recovered at lower temperatures as K-compounds were partially crystallized on the catalyst.     

Influence of Ageing, Silver Loading and Type of Reducing Agent on the Lean NO x Reduction Over Ag–Al2O3 Catalysts

The influence of ageing temperature, silver loading and type of reducing agent on the lean NO _x reduction over silver–alumina catalysts was investigated with n-octane and bio-diesel (NExBTL) as reducing agent. The catalysts (2 and 6 wt% Ag–Al₂O₃) were prepared with a sol–gel method including freeze drying and the evaluation of NO _x reduction and aging were performed using a synthetic gas-flow reactor. The results indicate a relatively high NO _x reduction for both reducing agents. The hydrothermally treated 6 wt% Ag–Al₂O₃ sample displays a maximum NO _x reduction of 78 % at 350 °C for n-octane as reductant and the corresponding value for NExBTL is 60 %. Furthermore, the catalysts show high durability and an increase in activity for NO _x reduction after ageing at temperatures up to 650 °C, with n-octane as reducing agent.     

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.     

Reaction Pathways in the Reduction of NO x Species by CO over Pt–Ba/Al2O3: Lean NO x Trap Catalytic Systems

The reduction by CO of NO _x species stored over Pt–Ba/Al₂O₃ Lean NO _x Trap systems is analysed in this work. The reaction mechanisms and pathways leading to N₂ formation both under dry and wet conditions are investigated by complementary transient dynamic experiments and FTIR analyses.     

Role of Pt Oxidation State on the Activity and Selectivity for Crotonaldehyde Hydrogenation Over Pt–Sn/Al2O3–La and Pt–Pb/Al2O3–La Catalysts

Pt–Sn/ALa10 and Pt–Pb/ALa10 catalysts (10 wt% La₂O₃) were studied in the selective hydrogenation of crotonaldehyde. Oxidized Pt²⁺ and reduced Pt⁰ species were identified by XPS on the bimetallic catalysts. High selectivity to crotylalcohol was obtained on the Pt–Pb/ALa10 catalyst where an electron transfer effect from Pb to Pt was proposed. For the Pt–Sn/ALa10 catalyst the formation of Pt–SnO _x –La₂O₃ complexes showing low activity and low selectivity was inferred.     

State of Pt/SiO2 Catalysts After Reaction with a Lean NO–C3H6–O2 Mixture

During the selective reduction of NO_x under lean-burn conditions with Pt/SiO₂ solids, a particle size dependency has previously been observed. Furthermore, under the reactant mixture up to 773 K, the Pt particles sinter at the same time as the solids are activated. In fact, the size dependency is linked to the strength of the Pt–O bond and to the ease of NO dissociation.     

Influence of Mn and Fe Addition on the NO x Storage–Reduction Properties and SO2 Poisoning of a Pt/Ba/Al2O3 Model Catalyst

This work deals with the effect of Mn or Fe addition on the NO _x storage–reduction properties of a Pt/Ba/Al₂O₃ model catalyst. NO _x storage capacity, SO₂ poisoning and regeneration and NO _x removal efficiency under rich/lean cycling conditions are studied. Fe addition to Pt/Ba/Al₂O₃ leads only to a small increase of NO _x storage capacity, and more interestingly, to a better sulfur removal due to the inhibition of bulk barium sulfate formation. Unfortunately, the NO _x storage property cannot be fully recovered. Moreover, Fe addition results in a decrease in the NO _x removal efficiency. Mn addition also improves the NO _x storage capacity, but no significant influence on the sulfur elimination is observed. Mn-doped catalyst does not improve the NO _x removal efficiency, but NH₃ selectivity is found to drastically decrease at 400 °C, from 20 to 3%. In addition, the NO _x conversion can be improved at higher H₂ concentration in the rich pulse, always keeping NH₃ selectivity at low level.     

FTIR and Transient Reactivity Experiments of the Reduction by H2, CO and HCs of NO x Stored Over Pt–Ba/Al2O3 LNTs

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.     

Cyclic Lean Reduction of NO by CO in Excess H2O on Pt–Rh/Ba/Al2O3: Elucidating Mechanistic Features and Catalyst Performance

This study provides insight into the mechanistic and performance features of the cyclic reduction of NO_x by CO in the presence and absence of excess water on a Pt–Rh/Ba/Al₂O₃ NO_x storage and reduction catalyst. At low temperatures (150–200 °C), CO is ineffective in reducing NO_x due to self-inhibition while at temperatures exceeding 200 °C, CO effectively reduces NO_x to main product N₂ (selectivity >70 %) and byproduct N₂O. The addition of H₂O at these temperatures has a significant promoting effect on NO_x conversion while leading to a slight drop in the CO conversion, indicating a more efficient and selective lean reduction process. The appearance of NH₃ as a product is attributed either to isocyanate (NCO) hydrolysis and/or reduction of NO_x by H₂ formed by the water gas shift chemistry. After the switch from the rich to lean phase, second maxima are observed in the N₂O and CO₂ concentrations versus time, in addition to the maxima observed during the rich phase. These and other product evolution trends provide evidence for the involvement of NCOs as important intermediates, formed during the CO reduction of NO on the precious metal components, followed by their spillover to the storage component. The reversible storage of the NCOs on the Al₂O₃ and BaO and their reactivity appears to be an important pathway during cyclic operation on Pt–Rh/Ba/Al₂O₃ catalyst. In the absence of water the NCOs are not completely reacted away during the rich phase, which leads to their reaction with NO and O₂ upon switching to the subsequent lean phase, as evidenced by the evolution of N₂, N₂O and CO₂. In contrast, negligible product evolution is observed during the lean phase in the presence of water. This is consistent with a rapid hydrolysis of NCOs to NH₃, which results in a deeper regeneration of the catalyst due in part to the reaction of the NH₃ with stored NO_x. The data reveal more efficient utilization of CO for reducing NO_x in the presence of water which further underscores the NCO mechanism. Phenomenological pathways based on the data are proposed that describes the cyclic reduction of NO_x by CO under dry and wet conditions.     

Effect of the Proximity of Pt to Ce or Ba in Pt/Ba/CeO2 Catalysts on NO x Storage–Reduction Performance

The effect of Pt location in Pt/Ba/CeO₂ catalysts for NO _x storage–reduction (NSR) was analyzed. The Pt location on BaCO₃ or CeO₂ support was controlled by changing the angle (φ) between the two flame sprays producing these two components. As-prepared flame-made catalysts contain PtO _x which must be reduced during the fuel rich phase to become active for NO _x storage and reduction of NO _x . For Pt on BaCO₃ this process was significantly faster than for Pt on CeO₂. The increased reduction ability of Pt on Ba is reflected in the light off temperatures: for Pt on CeO₂ temperatures around 330 °C were needed to combust 20% of C₃H₆ in air while for Pt on BaCO₃ only 250 °C were required for the same conversion. The ability to control the location of Pt or other noble metals is, therefore, essential to optimize the catalysts for a given Pt/Ba/CeO₂ weight ratio. The best performance was observed when most of the Pt constituent was located near Ba-containing sites.     

Oxygen Storage Capacity of Pt–CeO2 and Pt–Ce0.5Zr0.5O2 Catalysts

Cerium dioxide containing materials have involved much interest in recent years owing to their broad range of applications in various fields. The most important property that makes ceria as an excellent catalytic material is its oxygen storage and release capacity via the ability of ceria to rapidly switch from Ce³⁺ to Ce⁴⁺ under reducing and oxidizing conditions, respectively. Zirconia is incorporated into the ceria matrix as it greatly improves thermal stability and oxygen diffusion through the bulk of the crystallites.     

NO x Storage and High Temperature Soot Oxidation on Pt–Sr/ZrO2 Catalyst

NO _x storage–release and soot oxidation have been studied using strontium promoted zirconia with different surface areas. It was found that the amount of the adsorbed NO _x increases with the increase of the support surface and Sr concentration up to 20 wt% Sr. Introduction of platinum has no effect on the amount of NO _x stored, but improve soot oxidation due to recycling of NO to NO₂. A combination of oxidation catalyst with NO _x storage materials enables to lower the temperature of 20% soot conversion up to 100 °C in comparison with un-catalyzed soot oxidation.