Storage-reduction Catalyst Research Articles

Performance analysis, statistical modeling, and multiple response optimization of a novel fixed-bed quartz reactor packed with Ba-Pt@γ-AL2O3 using response surface methodology

In the present study, a novel fixed-bed continuous reactor with a preheating chamber was designed to be utilized for the practical application of removal studies of dangerous pollutants, especially NOX removal by NOX Storage Reduction (NSR) catalysts on a laboratory scale. The reactor's design and operational parameters, including outer wall temperature (50–600 °C), volumetric flow rate (0.3–3 L/min), wall temperature time (0.16–10 min), and granule surface area inside the preheating chamber (0–270 cm2), were statistically modeled and optimized using Response Surface Methodology (RSM). For more logical and effective parameter optimization, the ratio of gas and catalyst temperatures and pressure drop to the reactor outer wall temperature (GT/ROWT, CT/ROWT, and PD/ROWT) were also included in the optimization process. Experimental results showed that gas temperature, catalyst temperature, and pressure drop ranged from 31 to 177 °C, 51–585 °C, and 7–153 Pa, respectively. Optimal conditions were determined to be an outer wall temperature of 230 °C, a volumetric flow rate of 3 L/min, a wall temperature time of 0.16 min, and a granule surface area of 67.3 cm2. The results demonstrated that outer wall temperature, flow rate, time, and surface area of granules have significant and interaction effects on the responses and should be considered when researchers assess the removal efficiency of thermal catalysts.

Rh/CeO2+Pt/Ba/Mn/Al2O3 model NSR catalysts: Effect of Rh/Pt weight ratio

A series of model NOx storage-reduction (NSR) catalysts with different Rh/Pt weight ratios were prepared independently by wet impregnation and then mechanically mixed to investigate the effect of the Rh/Pt weight ratio on the overall NOx removal efficiency and nitrogen yield. XRD, EDS and H2-TPR studies indicated the coexistence of BaCO3, MnOx and BaMnO3 phases in the catalysts. Increasing Rh/Pt weight ratio enhanced NOx reduction under the rich phase, and improved overall N2-yield in the kinetic regime of NO conversion, but shows a negative effect in the equilibrium-limited regime above 400 °C.

Density functional theory study of NOx adsorption on alkaline earth metal oxide and transition metal surfaces

Since the emissions of nitrogen oxides (NOx) from automobiles cause air pollution, NOx storage-reduction (NSR) catalyst has been used to convert the NOx into harmless components such as N2 through the reduction of NOx. In this study, to provide fundamental understanding of key elementary steps of NSR, we established an extensive database for the adsorption properties of NO and NO2 on a wide range of metal and metal oxide surfaces. Our results show that the amount of charge transfer between NOx and surface is closely related to the molecular adsorption strength of NOx, and it changes the molecular stability of NOx on the surfaces by enlarging the inner bond length of N-O. Understanding the adsorption energy of the molecules or atoms that would participate in the reaction can be important to predict the ability of NOx storage and conversion in NSR. This study provides a useful insight for designing metals or metal oxides for NSR catalyst.

Al2O3-supported Pt/Rh catalysts for NOx removal under lean conditions

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.

New insights on the adsorption, thermal decomposition and reduction of NOx over Pt- and Ba-based catalysts

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.

Design of highly efficient NOx storage-reduction catalysts from layered double hydroxides for NOx emission control from naphtha cracker flue gases

Although there are many reports on the removal of NOx from vehicle and power plant emissions, little effort has been devoted to the removal of NOx from cracker flue gases. Here we report a systematic investigation on the design of highly efficient NOx storage-reduction (NSR) catalysts using highly dispersed aqueous miscible organic-layered double hydroxides (AMO-LDHs) derived mixed oxides (LDOs) for NOx emission control from naphtha cracker flue gases. For a series of binary M2+Al3+ LDOs, the influence of six divalent cations (Ca, Zn, Cu, Ni, Co, and Mg) on the NOx adsorption capacity, NO oxidation property, and the thermal stability of adsorbed NOx was systematically investigated. The optimal NOx storage temperature range for each binary LDO was also determined. Based on the fundamental findings of binary LDOs, a ternary LDH CoMgAl-CO3 derived LDO CoMgAlOx with a significantly improved NOx storage capacity was designed. By tuning the Co/Mg ratio, the NOx storage capacity was greatly improved from 0.14mmol/g for Mg3Al1Ox to 0.50mmol/g for Co1Mg2Al1Ox at 300°C. By further doping Pt and K2CO3, a new NSR catalyst composed of 1wt% Pt/15wt% K2CO3/Co1Mg2Al1Ox was obtained, which exhibited a very high NOx storage capacity of 1.2mmol/g. Finally, the NSR cycling performance, and the CO2, SO2 and H2O resistance of this catalyst were also investigated. Thanks for its superior NOx storage capacity, NSR cycling performance, and CO2, SO2 and H2O resistance, this new NSR catalyst showed great potential for the NOx control from cracker flue gases.

Development of a NOx Storage-Reduction Catalyst Based Min-NOx Strategy for Small-Scale NG-Fueled Gas Engines

Development of a NO_x Storage-Reduction Catalyst Based Min-NO_x Strategy for Small-Scale NG-Fueled Gas Engines

Clarification of NH3 Formation Mechanism on a Diesel Engine NOx Storage Reduction Catalyst Under Rich Conditions and Evaluation of the SCR Benefit at WLTC

Recent studies have shown that the combination of a NOx storage reduction catalyst (NSR) and a passive selective catalytic reduction catalyst (pSCR) would be a technical solution to meet post EU6 emission legislation for small passenger cars. In this case, NH3 formed during rich NSR regeneration is used to reduce remaining NOx on a SCR catalyst which is located downstream of the NSR. In this study, first, the NH3 formation mechanism of NSR under different emission λs, rich durations, and stored NOx values are clarified using a diesel engine operating under steady-state operation. Finally, NOx conversion and the associated fuel consumption penalty are clarified under the Worldwide harmonized light vehicle test cycle (WLTC). It was identified that NH3 starts to be produced by the NSR after oxygen storage capacity depletion. Next, a NH3 peak occurs as a result of the converted pre-stored and engine out NOx. If the rich operation is long enough, the pre-stored NOx contribution fades and NH3 concentration lowers to an equivalent level as the engine out NOx. From this understanding, several WLTC patterns were measured varying the number of rich operations and their duration in order to identify best regeneration strategy with a minimum fuel consumption penalty. The optimal pSCR benefit, in terms of fuel penalty, is achieved when the rich operation duration is (1) longer than the breakthrough time, (2) includes the initial NH3 peak resulting from the conversion of the pre-stored NOx.

Sulfation and Desulfation Behavior of Pt-BaO/MgO-Al2O3 NOx Storage Reduction Catalyst.

The comparative study between Pt-BaO/Al2O3 and Pt-BaO/MgO-Al2O3 gives the information about the effect of MgO addition to Al2O3 support on the sulfation and desulfation behavior of Pt-BaO/MgO-Al2O3 NOx storage reduction catalyst. The sulfated two samples were analyzed by using element analysis (EA), X-ray diffraction (XRD), H2 temperature programmed reaction (H2 TPRX) and NOx uptake measurement. The amount of sulfur uptake on 2 wt% Pt-20 wt% BaO/Al2O3 and 2 wt% Pt-20 wt% BaO/MgO-Al2O3 are almost identical as 0.45 and 0.40 of S/Ba, respectively, which yields the drastic decrease in NOx uptake for both sulfated samples. However, after desulfa- tion with H2 at 600 degrees C, the residual sulfur amount on MgO-Al2O3 supported catalyst is three times larger than that on Al2O3 supported one, indicating that sulfur species formed on the former are more stable than those on the latter. It is also well corresponding to the H2 TPRX results where the main H2S peak from MgO-Al2O3 supported sample is observed at higher temperature than Al2O3 supported one, resulting in the lower NOx uptake activity of former sample than the latter one. Meanwhile, after desulfation of MgO-Al2O3 supported sample at 700 degrees C and 800 degrees C, the activity is recovered more significantly due to the removal of the large amount of sulfur while Al2O3 supported one decreases monotonically due to the sintering of Pt crystallite and the formation of BaAl2O4 phase. It is summarized that MgO-Al2O3 supported catalyst enhances the thermal stability of the catalyst, however, forms the stable sulfate species, which needs to be improved to develop the more sulfur resistant NSR catalyst system.

Study of N2O Formation over Rh- and Pt-Based LNT Catalysts

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.

Impact of soot‐NSR catalyst contact depending on reactive gas composition on NOx storage

Interaction between a model soot (carbon black) and a model NOx storage reduction catalyst (Pt‐Ba/Al2O3) is investigated as a function of the strength of the carbon–catalyst contact and the presence or not of H2O in the reactive gas flow. Experiments performed at laboratory scale, using a fixed bed reactor, revealed that the inhibiting effect of carbon on the NOx storage capacity (NSC) of the catalyst increases with the strength of carbon–catalyst contact and affects NOx adsorption on both Al sites and Ba sites, whereas the oxidative function of the catalyst (Pt sites) is not affected. In the presence of water in the reactive gas flow, it was observed that carbon prevents the inhibition of the oxidative function of the catalyst by H2O. On the other hand, the inhibiting effect of carbon on NSC decreases. Hence, a noncumulative effect of carbon and H2O impacts on NSC is observed. © 2015 American Institute of Chemical Engineers Environ Prog, 35: 14–19, 2016

Synthesis of Pt doped Mg–Al layered double oxide/graphene oxide hybrid as novel NOx storage–reduction catalyst

We report the synthesis of Pt doped Mg–Al layered double oxide/graphene oxide (Pt–LDO/GO) hybrid as novel NOx storage and reduction (NSR) catalyst.

Pt/CexPr1−xO2 (x = 1 or 0.9) NOx storage–reduction (NSR) catalysts

Model Pt/Ce0.9Pr0.1O2 and Pt/CeO2 NOx storage–reduction catalysts were prepared via nitrate calcination, co-precipitation and carbon-templating routes. Raman spectroscopic data obtained on the catalysts indicated that the introduction of praseodymium into the ceria lattice increased the concentration of defect sites (vacancies), arising from the higher reducibility of the Pr4+ cation compared to Ce4+. For the Pr-promoted samples, H2-TPR profiles contained high temperature bulk reduction peaks which were less pronounced compared with their ceria analogs, indicating that the presence of praseodymium enhances oxygen mobility due to the creation of lattice defects. Under lean-rich cycling conditions, the cycle-averaged NOx conversion of the Pt/Ce0.9Pr0.1O2 samples was in each case substantially higher than that of the Pt/CeO2 analog, amounting to a difference of 10–15% in the absolute NOx conversion in some cases. According to DRIFTS data, a double role can be assigned to Pr doping; on the one hand, Pr accelerates the oxidation of adsorbed NOx species during the lean periods. On the other hand, Pr doping destabilizes the adsorbed NOx species during the rich periods, and the kinetics of nitrate decomposition are faster on Pt/Ce0.9Pr0.1O2, leading to improved catalyst regeneration. These results suggest that ceria-based mixed oxides incorporating Pr are promising materials for NOx storage–reduction catalysts intended for low temperature operation.

Origin of double dinitrogen release feature during fast switching between lean and rich cycles for NOx storage reduction catalysts

The performance of NOx storage and reduction over 1.5wt% Pt/20wt% KNO3/K2Ti8O17 and 1.5wt% Pt/K2Ti8O17 catalysts has been investigated using combined fast transient kinetic switching and isotopically labelled 15NO at 350°C. The evolution of product N2 has revealed two significant peaks during 60s lean/1.3s rich switches. It also found that the presence of CO2 in the feed affects the release of N2 in the second peak. Regardless of the presence/absence of water in the feed, only one peak of N2 was observed in the absence of CO2. Gas-phase NH3 was not observed in any of the experiments. However, in the presence of CO2 the results obtained from in situ DRIFTS-MS analysis showed that isocyanate species are formed and stored during the rich cycles, probably from the reaction between NOx and CO, in which CO was formed via the reverse water–gas shift reaction.

Structured Perovskite-Based Catalysts and Their Application as Three-Way Catalytic Converters—A Review

Automotive Three-Way Catalysts (TWC) were introduced more than 40 years ago. Despite that, the development of a sustainable TWC still remains a critical research topic owing to the increasingly stringent emission regulations together with the price and scarcity of precious metals. Among other material classes, perovskite-type oxides are known to be valuable alternatives to conventionally used TWC compositions and have demonstrated to be suitable for a wide range of automotive applications, ranging from TWC to Diesel Oxidation Catalysts (DOC), from NOx Storage Reduction catalysts (NSR) to soot combustion catalysts. The interest in these catalysts has been revitalized in the past ten years by the introduction of the concept of catalyst regenerability of perovskite-based TWC, which is in principle well applicable to other catalytic processes as well, and by the possibility to reduce the amounts of critical elements, such as precious metals without seriously lowering the catalytic performance. The aim of this review is to show that perovskite-type oxides have the potential to fulfil the requirements (high activity, stability, and possibility to be included into structured catalysts) for implementation in TWC.

AN XPS study of the interaction of model Ba/TiO2 AND Ba/ZrO2 NSR catalysts with NO2

X-ray photoelectron spectroscopy is used to study the interaction of model NO2 storage-reduction catalysts (NSR catalysts) Ba/TiO2 and Ba/ZrO2 with NO2. The catalysts are prepared on the surface of ultrathin Al2O3 film substrates obtained by the FeCrAl alloy oxidation. It is shown that at room temperature the model catalysts react with NO2 with the successive formation of surface barium nitrite and nitrate. The NO2 reduction with the formation of barium nitrite at the initial step of the interaction is assumed to be accompanied by the oxidation of residual metallic barium and amorphous carbon impurity. It is found that the formation of barium nitrate proceeds more efficiently on Ba/ZrO2 rather than on Ba/TiO2.

Electrochemically-assisted NOx storage–reduction catalysts

Supported noble-metal catalysts are extensively used in industry, particularly for car exhaust treatments. The challenge is to maximize the catalytic efficiency while minimizing the noble metal loading. Recent advances from the literature propose the use of electropromoted nanodispersed catalysts in order to assist catalytic reactions with electrochemical mechanisms: this type of catalyst design, inspired from fuel cells, combines electrochemical promotion of catalysis and high metallic active site availability. This paper explores whether electrochemical effects can improve the catalytic performances of nanodispersed NOx storage and reduction (NSR) catalysts deposited in the porosity of SiC mini-DPFs. NSR catalysts were elaborated employing Pt and Rh nanoparticles as electrodes, supported on yttria stabilized zirconia (YSZ) or gadolinium doped cerium oxide (GDC) nanoparticles (electrolyte), and used for NOx abatement in diesel exhaust gas conditions. Improved NOx removal efficiency was obtained. Comparison of YSZ (ionic conductor) and GDC (mixed ionic and electronic conductor) electrolytes showed that GDC-based catalytic nano-cells present the highest catalytic activities. A detailed analysis of the reaction products showed that electrochemical reactions can take place on nano-electrodes, in addition to catalytic reactions.

Sulfation and lean/rich desulfation of a NOx storage reduction catalyst: Experimental and simulation study

A detailed study of the sulfation and desulfation phenomena of a NOx storage reduction catalyst using synthetic gas bench is presented. The experimental observations and results were put together, in order to develop a model, which can simulate the NOx storage reduction catalyst behavior during the sulfation and desulfation conditions. The target is to use the model for simulation of the lean/rich desulfation strategies, in order to comprehend the complicated phenomena and ultimately predict the sulfur speciation at the outlet of the NOx storage reduction catalyst. Sulfation tests in the presence and in the absence of O2 at different temperatures were conducted, in order to identify the different sulfation sites. Desulfation tests with constantly rich conditions and lean/rich oscillation, using H2 as the reducing gas, were performed in order to investigate the reaction pathways of the decomposition of the sulfates. Additionally, some preliminary investigation was performed using CO and decane in the reducing gas mixture, in order to evaluate the NOx storage reduction behavior under more realistic gas composition for diesel exhaust gas. The important role of the accurate prediction of sulfur axial distribution at the desulfation onset was identified. It was found that cerium oxides play a critical role in the sulfation process by increasing the sulfur capacity of the NOx storage reduction catalyst. Finally, it was clarified that the oxygen stored on cerium oxides strongly affects the selectivity of the desulfation products by oxidizing H2S and controlling H2 availability.

Effect of H2O on the Morphological Changes of KNO3 Formed on K2O/Al2O3 NOx Storage Materials: Fourier Transform Infrared and Time-Resolved X-ray Diffraction Studies

Based on the combined FTIR and XRD studies, we report here that H2O induces a morphological change of KNO3 species formed on model K2O/Al2O3 NOx storage-reduction catalysts. Specifically as evidenced by FTIR, the contact of H2O with NO2 preadsorbed on K2O/Al2O3 promotes the transformation from bidentate (surface-like) KNO3 species to ionic (bulk-like) ones irrespective of K loadings. Once H2O is removed from the sample, a reversible transformation into bidentate KNO3 is observed, demonstrating a significant dependence of H2O on such morphological change. TR-XRD results show the formation of two different types of bulk KNO3 phases (orthorhomobic and rhombohedral) in an as-impregnated sample. Once H2O begins to desorb above 400 K, the former is transformed into the latter, resulting in the existence of rhombohedral KNO3 phase only. On the basis of consistent FTIR and TR-XRD results, we propose a model for the morphological changes of KNO3 species with respect to NO2 adsorption/desorption, H2O and/or heat tr...

NOx storage-reduction (NSR) catalysts for diesel emissions control: Toward the understanding of sulfur poisoning mechanism

NOx storage-reduction (NSR) catalysts for diesel emissions control: Toward the understanding of sulfur poisoning mechanism