Pt dispersion effects during NOx storage and reduction on Pt/BaO/Al2O3 catalysts

This study provides insight into the effect of Pt dispersion on the overall rate and product distribution during NOx storage and reduction. The storage and reduction performance of Pt/BaO/A₂O₃ monoliths with varied Pt dispersion (3%, 8%, and 50%) and fixed Pt (2.48 wt.%) and BaO (13.0 wt.%) loadings is reported. At low temperature (<200 °C), the differences in storage and reduction activity were the largest between the three catalysts. The amount of NOx stored increased with increased dispersion, as did the amount of stored NOx that was reduced. These trends are attributed to larger Pt surface area and Pt–BaO interfacial perimeter, the latter of which enhances the spillover of surface species between the precious metal and storage components. At high temperature (370 °C), the stored NOx was almost completely regenerated for the three catalysts. However, the regeneration of the 3% dispersion catalyst was much slower, suggesting a rate limitation involving the reverse spillover of stored NOx to Pt and/or of adsorbed hydrogen from Pt to BaO. The results indicate that the catalyst dispersion and operating conditions may be tuned to achieve the desired ammonia selectivity. For the aerobic regeneration feed, the most (net) NH₃ was generated by the 50% dispersion catalyst at the lowest temperature (125 °C), by the 3% dispersion catalyst at the highest temperature (340 °C), and by the 8% dispersion catalyst at the intermediate temperatures (170–290 °C). Similar trends were observed for the net production of NH₃ with an anaerobic regeneration feed. A phenomenological picture is proposed that describes the effects of Pt dispersion consistent with the established spatio-temporal behavior of the lean NOx trap.     

Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N₂ conversion, minimal ammonia slip and null N₂O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al₂O₃ and Pt–Ce–Ba/Al₂O₃ were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N₂ adsorption–desorption isotherms at 77 K, X-ray diffraction, NH₃ temperature-programmed desorption, and H₂ chemisorption to relate textural and chemical characteristics with the DeNO_x performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH₃–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al₂O₃is beneficial for the NO-to-NOx oxidation and NO₂ storage, improving NO conversion at low temperature and reducing the NH₃ slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH₃ slip with superior N₂ selectivity for an extended temperature windows, including as low as 220 °C, and maintaining performance even after severe hydrothermal aging.     

Mean field modelling of NOx storage on Pt/BaO/Al2O3

A mean field model, for storage and desorption of NO_x in a Pt/BaO/Al₂O₃ catalyst is developed using data from flow reactor experiments. This relatively complex system is divided into five smaller sub-systems and the model is divided into the following steps: (i) NO oxidation on Pt/Al₂O₃; (ii) NO oxidation on Pt/BaO/Al₂O₃; (iii) NO_x storage on BaO/Al₂O₃; (iv) NO_x storage on Pt/BaO/Al₂O₃ with thermal regeneration and (v) NO_x storage on Pt/BaO/Al₂O₃ with regeneration using C₃H₆. In this paper, we focus on the last sub-system. The kinetic model for NO_x storage on Pt/BaO/Al₂O₃ was constructed with kinetic parameters obtained from the NO oxidation model together with a NO_x storage model on BaO/Al₂O₃. This model was not sufficient to describe the NO_x storage experiments for the Pt/BaO/Al₂O₃, because the NO_x desorption in TPD experiments was larger for Pt/BaO/Al₂O₃, compared to BaO/Al₂O₃. The model was therefore modified by adding a reversible spill-over step. Further, the model was validated with additional experiments, which showed that NO significantly promoted desorption of NO_x from Pt/BaO/Al₂O₃. To this NO_x storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO₂ and C₃H₆. The main reactions for continuous reduction of NO_x occurs on Pt by reactions between hydrocarbon species and NO in the model. The model is also able to describe the reduction phase, the storage and NO breakthrough peaks, observed in experiments.     

NOx storage properties of Pt/Ba/Al model catalysts prepared by different methods: Beneficial effects of a N2 pre-treatment before hydrothermal aging

The influence of a pre-treatment at 700 °C, either under a O₂/N₂ mixture or only under N₂, and followed by a hydrothermal aging at 700 °C under wet air, was studied for Pt/Ba/Al NSR model catalysts prepared by different methods: (i) successive impregnation of Ba and Pt, (ii) co-addition of Pt and Ba and (iii) barium precipitation followed by Pt impregnation. The catalysts were evaluated by NOx storage capacity measurements and were characterized by N₂ adsorption, XRD, CO₂-TPD, H₂ chemisorption and H₂-TPR. The pre-treatment under N₂ largely improves the NOx storage performance in the whole studied temperature range (200–400 °C), with or without H₂O and CO₂ in the inlet gas. The better NOx storage properties of the catalysts treated under N₂ before aging are due to: (i) a higher NO oxidation activity (mainly linked to a higher platinum dispersion), (ii) a higher number of NOx storage sites resulting from a higher barium dispersion, and consequently to (iii) a higher Pt-Ba proximity.     

Revisiting the steady states of NO/O2/C3H6 on monolithic Pt/BaO/Al2O3 using bifurcation analysis

While NOx storage and reduction is periodically operated, steady‐state studies have been widely carried out to investigate the involved reaction mechanisms and effects of operating parameters. Due to the complex reaction chemistry and its coupling with transport phenomena, multiplicity may exist. A steady‐state monolith reactor model accounting for microkinetics and reaction heat effects was proposed in this study to avoid the evaluation of enthalpies of microreaction steps. Three simplified versions of the monolith model were developed based on various assumptions of the axial gradients. Steady‐state behaviors of NO/O₂/C₃H₆ system were investigated. A predictor‐corrector (PC) continuation method that does not require explicit evaluation of Jacobian Matrix was developed to solve the nonlinear system with a variable parameter of feed temperature. Model predictions were compared with experimental results. © 2013 American Institute of Chemical Engineers AIChE J 60: 623–634, 2014     

Rare earths based oxides as alternative materials to Ba in NOx-trap catalysts

The reduction of NOx from Diesel Engines is a major issue of automotive catalysis. In this paper, we investigate basic rare earths oxides (such as La₂O₃, Nd₂O₃ or Pr₆O₁₁) used as NOx-trap sites in replacement of Ba. These species are intimately mixed with (CeZr) solid solutions and the thus obtained mixed oxides are considered as Pt carrier. Investigations on powder model Pt-(CeZr)/RE₂O₃ (or Pr₆O₁₁) catalysts show promising results. These model catalysts present high NOx storage capacity, especially at low temperature, and improved resistance to sintering at 700 °C under lean hydrothermal conditions. Moreover, they show total desulfation at 600 °C i.e. 150 °C lower than current Pt-(BaO/Al₂O₃) technologies and are thus considered as more durable technologies.     

NOx storage properties of Pt/Ba/Al model catalysts prepared by different methods: Beneficial effects of a N2 pre-treatment before hydrothermal aging

The influence of a pre-treatment at 700 °C, either under a O₂/N₂ mixture or only under N₂, and followed by a hydrothermal aging at 700 °C under wet air, was studied for Pt/Ba/Al NSR model catalysts prepared by different methods: (i) successive impregnation of Ba and Pt, (ii) co-addition of Pt and Ba and (iii) barium precipitation followed by Pt impregnation. The catalysts were evaluated by NOx storage capacity measurements and were characterized by N₂ adsorption, XRD, CO₂-TPD, H₂ chemisorption and H₂-TPR. The pre-treatment under N₂ largely improves the NOx storage performance in the whole studied temperature range (200–400 °C), with or without H₂O and CO₂ in the inlet gas. The better NOx storage properties of the catalysts treated under N₂ before aging are due to: (i) a higher NO oxidation activity (mainly linked to a higher platinum dispersion), (ii) a higher number of NOx storage sites resulting from a higher barium dispersion, and consequently to (iii) a higher Pt-Ba proximity.     

A kinetic model of the hydrogen assisted selective catalytic reduction of NO with ammonia over Ag/Al2O3

A global kinetic model which describes H₂‐assisted NH₃‐SCR over an Ag/Al₂O₃ monolith catalyst has been developed. The intention is that the model can be applied for dosing NH₃ and H₂ to an Ag/Al₂O₃ catalyst in a real automotive application as well as contribute to an increased understanding of the reaction mechanism for NH₃‐SCR. Therefore, the model needs to be simple and accurately predict the conversion of NO_x. The reduction of NO is described by a global reaction, with a molar stoichiometry between NO, NH₃ and H₂ of 1:1:2. Further reactions included in the model are the oxidation of NH₃ to N₂ and NO, oxidation of H₂, and the adsorption and desorption of NH₃. The model was fitted to the results of an NH₃‐TPD experiment, an NH₃ oxidation experiment, and a series of H₂‐assisted NH₃‐SCR steady‐state experiments. The model predicts the conversion of NO_x well even during transient experiments. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4325–4333, 2013     

TAP study of NOx storage and reduction on Pt/Al2O3 and Pt/Ba/Al2O3

A systematic mechanistic study of NO storage and reduction over Pt/Al₂O₃ and Pt/BaO/Al₂O₃ is carried out using Temporal Analysis of Products (TAP). NO pulse and NO/H₂ pump-probe experiments at 350 °C on pre-reduced, pre-oxidized, and pre-nitrated catalysts reveal the complex interplay between storage and reduction chemistries and the importance of the Pt/Ba coupling. NO pulsing experiments on both catalysts show that NO decomposes to major product N₂ on clean Pt but the rate declines as oxygen accumulates on the Pt. The storage of NO over Pt/BaO/Al₂O₃ is an order of magnitude higher than on Pt/Al₂O₃ showing participation of Ba in the storage even in the absence of gas phase O₂. Either oxygen spillover or transient NO oxidation to NO₂ is postulated as the first steps for NO storage on Pt/BaO/Al₂O₃. The storage on Pt/Ba/Al₂O₃ commences as soon as Pt–O species are formed. Post-storage H₂ reduction provides evidence that a fraction of NO is not stored in close proximity to Pt and is more difficult to reduce. A closely coupled Pt/Ba interfacial process is corroborated by NO/H₂ pump-probe experiments. NO conversion to N₂ by decomposition is sustained on clean Pt using excess H₂ pump-probe feeds. With excess NO pump-probe feeds NO is converted to N₂ and N₂O via the sequence of barium nitrate and NO decomposition. Pump-probe experiments with pre-oxidized or pre-nitrated catalyst show that N₂ production occurs by the decomposition of NO supplied in a NO pulse or from the decomposition of NOx stored on the Ba. The transient evolution of the two pathways depends on the extent of pre-nitration and the NO/H₂ feed ratio.     

NO + H2 reaction on Pt-Ru/SiO2 catalysts: correlation between nanostructure and catalytic activity and selectivity

Nitric oxide reduction by hydrogen has been studied on Pt-Ru/SiO₂ catalysts of various Pt/Ru atomic compositions in the temperature range 298–673 K. Physical characterization showed the presence of bimetallic particles which tend to be Pt-rich. The overall activity of the bimetallic catalysts suggests a dilution of the active component (Pt) in the range 373–523 K. The addition of Ru results in a general improvement of the N₂ selectivity and significant modifications in the product distribution are observed as a function of the catalyst composition. A bimetallic particle model is proposed in which various types of surfaces are exposed including those with pure Pt atoms and/or Pt-Ru mixture. This model allows to explain the overall activity and selectivity in the whole series of catalysts.     

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.     

Effective combination of non-thermal plasma and catalyst for removal of volatile organic compounds and NOx

A plasma/catalyst hybrid reactor was designed to overcome the limits of plasma and catalyst technologies. A two-plasma/catalyst hybrid system was used to decompose VOCs (toluene) and NOx at temperature lower than 150 °C. The single-stage type (Plasma-driven catalyst process) is the system in which catalysts are installed in a non-thermal plasma reactor. And the two-stage type (Plasma-enhanced process) is the system in which a plasma and a catalyst reactor are connected in series. The catalysts prepared in this experiment were Pt/TiO₂ and Pt/Al₂O₃ of powder type and Pd/ZrO₂, Pt/ZrO₂ and Pt/Al₂O₃ which were catalysts of honeycomb type. When a plasma-driven catalyst reactor with Pt/Al₂O₃ decomposed only toluene, it removed just more 20% than the only plasma reactor but the selectivity of CO₂ was remarkably elevated as compared with only the plasma reactor. In case of decomposing VOCs (toluene) and NOx using plasma-enhanced catalyst reactor with Pt/ZrO₂ or Pt/Al₂O₃, the conversion of toluene to CO₂ was nearly 100% and about 80% of NOx was removed. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

NO decomposition and reduction on Pt/Al2O3 powder and monolith catalysts using the TAP reactor

A systematic study over Pt/Al₂O₃ powder and monolith catalysts is carried out using temporal analysis of products (TAP) to elucidate the transient kinetics of NO decomposition and NO reduction with H₂. NO pulsing and NO–H₂ pump-probe experiments demonstrate the effect of catalyst temperature, NO–H₂ pulse delay time and H₂/NO ratio on N₂, N₂O and NH₃ selectivity. At lower temperature (150 °C) decomposition of NO is negligible in the absence of H₂, indicating that N–O bond scission is rate limiting. At higher temperature NO decomposition occurs readily on reduced Pt but the rate is inhibited by surface oxygen as reaction occurs. The reduction of NO by a limiting amount of H₂ at lower temperature indicates the reaction of surface NO with H adatoms to form N adatoms, which react with adsorbed NO to form N₂O or recombine to form N₂. In excess H₂, higher temperatures and longer delay times favor the production of N₂. The longer delay enables NO decomposition on reduced Pt with the role of H₂ being a scavenger of surface oxygen. Lower temperatures and shorter delay times are favorable for ammonia production. The sensitive dependence on delay time indicates that the fate of adsorbed NO depends on the concentration of vacant sites for NO bond scission, necessary for N₂ formation, and of surface hydrogen, necessary for hydrogenation to ammonia. A mechanistic-based microkinetic model is proposed that accounts for the experimental observations. The TAP experiments with the monolith catalyst show an improved signal due to the reduction of transport restrictions caused by the powder. The improved signal holds promise for quantitative TAP studies for kinetic parameters estimation and model discrimination.     

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.     

Influence of BaO in perovskite electrodes for the electrochemical reduction of NO x

Using the point electrode method, the effect of BaO on electrochemical reduction of NO _x was investigated using the perovskites La_0.85Sr_0.15MnO₃ (LSM15) and La_0.85Sr_0.15CoO₃ (LSCo15) as electrode materials. The experiments were carried out in the temperature range 400–600 °C in 1% NO and 10% O₂, respectively. For the LSM15 electrode the ability to reduce NO compared to O₂ was increased when applying 20 wt% BaO in the electrode while the current density of the electrode was decreased approximately a decade. For the pure LSM15 electrode the highest current density ratio of NO and O₂ was obtained at 400 °C while the optimal temperature in term of current density ratios was 500 °C for the LSM15 + BaO and LSM15 + BaO + Pt electrodes. The activity of the electrode decreases when applying BaO or BaO + Pt to the electrode except for the LSM15 + BaO + Pt electrode at 500 °C where the activity is approximately the same as for the LSM15 electrode. The formation of Ba(NO₃)₂ was clearly seen when applying 16.7 wt% BaO and 2 wt% Pt to the electrode. Generally it was observed that at 400 °C the activity of the electrodes were low, while at 600 °C the kinetics favored oxygen reduction compared to reduction of nitric oxide. The LSCO15 electrode containing BaO reacted to form a K₂NiF₄-structure and was not tested further.     

NSR Catalysis studied using Scanning Tunnelling Microscopy

In this paper we report the fabrication of model catalysts prepared to understand the structure of the BaO surface. This utilises the ‘inverse’ catalyst method, that is, the oxide layer is fabricated onto the top of a metal single crystal surface. We show that we can atomically resolve the surface structure of BaO(111) and that it presents a (2×2) reconstruction at its surface. Under other dosing conditions we can produce a layer which is metastable at 573K, which we believe to be the peroxide, BaO₂. We have shown that the BaO layer can store NO_x from a mix of NO and oxygen, even under the extremely low exposure conditions of UHV, proving that the NOx storage process is a facile one. The results indicate that it is not necessary to have NO2 in the gas phase in order to store NO_x.     

Dual‐bed Catalytic System for the Selective Reduction of NOx with Propene

For the removal of NO_x from exhausts containing excess oxygen, the selective catalytic reduction of NO_x using hydrocarbons (HC‐SCR) is highly interesting, especially for car applications (lean deNO_x) [1]. Two types of HC‐SCR catalysts can be distinguished. High‐temperature catalysts operate at temperatures above 573 K, showing moderate activity, and (mostly) low heat and poison resistance, but high N₂ selectivity. Low‐temperature catalysts, usually based on Pt, operate between 473 and 573 K, showing opposite features: very high activity, as well as heat and poison resistance, and a low N₂ selectivity by forming N₂O. Our strategy for developing an active and stable deNO_x system without N₂O emission is the implementation of a separate catalytic function for N₂O removal as a second stage.     

Enhanced Sulfur Tolerance of Ceria-Promoted NO x Storage Reduction (NSR) Catalysts: Sulfur Uptake, Thermal Regeneration and Reduction with H2(g)

SO _x uptake, thermal regeneration and the reduction of SO _x via H₂(g) over ceria-promoted NSR catalysts were investigated. Sulfur poisoning and desulfation pathways of the complex BaO/Pt/CeO₂/Al₂O₃ NSR system was investigated using a systematic approach where the functional sub-components such as Al₂O₃, CeO₂/Al₂O₃, BaO/Al₂O₃, BaO/CeO₂/Al₂O₃, and BaO/Pt/Al₂O₃ were studied in a comparative fashion. Incorporation of ceria significantly increases the S-uptake of Al₂O₃ and BaO/Al₂O₃ under both moderate and extreme S-poisoning conditions. Under moderate S-poisoning conditions, Pt sites seem to be the critical species for SO _x oxidation and SO _x storage, where BaO/Pt/Al₂O₃ and BaO/Pt/CeO₂/Al₂O₃ catalysts reveal a comparable extent of sulfation. After extreme S-poisoning due to the deactivation of most of the Pt sites, ceria domains are the main SO _x storage sites on the BaO/Pt/CeO₂/Al₂O₃ surface. Thus, under these conditions, BaO/Pt/CeO₂/Al₂O₃ surface stores more sulfur than that of BaO/Pt/Al₂O₃. BaO/Pt/CeO₂/Al₂O₃ reveals a significantly improved thermal regeneration behavior in vacuum with respect to the conventional BaO/Pt/Al₂O₃ catalyst. Ceria promotion remarkably enhances the SO _x reduction with H₂(g).     

NH3-Slip Catalysts: Experiments Versus Mechanistic Modelling

The oxidation of ammonia on a Pt/Al₂O₃ coated monolith has been studied under automotive NH₃-slip catalyst conditions. Ammonia conversion as well as the selectivities towards the products N₂, N₂O and NO are well described by a mechanistic model that is based on reaction mechanisms originally developed for NH₃ oxidation in nitric acid production plants.