NOx Storage And Reduction Catalyst Research Articles

Comparison of Cu-CHA-Zeolites in the Hybrid NSR-SCR Catalytic System for NOx Abatement in Mobile Sources

DeNOx activity in a NSR–SCR hybrid system of two copper-containing chabazite-type zeolitic catalysts was addressed. A Pt-Ba-K/Al2O3 model catalyst was used as the NSR (NOx storage and reduction) catalyst. For the SCR (selective catalytic reduction) system, two Cu-CHA zeolites were synthesized employing a single hydrothermal synthesis method assisted with ultrasound and incorporating Cu in a 2 wt.%, 2Cu-SAPO-34 and 2Cu-SSZ-13. The prepared catalysts were characterized, and the crystallinity, surface area, pore size, HR-TEM and EDX mapping, coordination of Cu ions and acidity were compared. The NH3 storage capacity of the SCR catalysts was 1890 and 837 μmol NH3·gcat−1 for 2Cu-SAPO-34 and 2Cu-SSZ-13, respectively. DeNOx activity was evaluated for the single NSR system and the double-bed NSR–SCR by employing alternating lean (3%O2) and rich (1%H2) cycles, maintaining a concentration of 600 ppm NO, 1.5% H2O and 0.3% CO2 between 200 and 350 °C. The addition of the SCR system downstream of the NSR catalyst significantly improved NOx conversion mainly at low temperature, maintaining the selectivity to N2 above 80% and reaching values above 90% at 250 °C when the 2Cu-SSZ-13 catalyst was located. The total reduction in the production of NH3 and ~2% of N2O was observed when comparing the NSR–SCR configuration with the single NSR catalyst.

Advances in Designing Efficient La-Based Perovskites for the NOx Storage and Reduction Process

To overcome the inherent challenge of NOx reduction in the net oxidizing environment of diesel engine exhaust, the NOx storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNOx technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NOx abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NOx spillover and contributes to efficient NOx storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NOx storage, and NOx reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNOx efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application.

Hybrid Technology for DeNOxing by LNT-SCR System for Efficient Diesel Emission Control: Influence of Operation Parameters in H2O + CO2 Atmosphere

The behavior and operation parameters were analyzed for the hybrid LNT-SCR (Lean NOx-Trap–Selective Catalytic Reduction) system with advanced catalyst formulations. Pt-Ba-K/Al2O3 was used as an NSR (NOx Storage and Reduction) or LNT catalyst effective in NOx and soot simultaneous removal whereas Cu-SAPO-34 with 2 wt.% of copper inside the structure was the small pore zeolite employed as the SCR catalyst. Under alternating and cyclic wet conditions, feeding volumetric concentrations of 1000 ppm of NO, 3% of O2, 1.5% of water, 0.3% of CO2, and H2 as a reductant, the NOx-conversion values were above 95% and a complete mineralization to nitrogen was registered using θ ≤ 3 (20 s of regeneration) and a hydrogen content between 10,000 and 2000 ppm in the whole temperature range tested. An excess of hydrogen fed (above 1% v/v) during the rich phase is unnecessary. In addition, in the low temperature range below 250 °C, the effect is more noticeable due to the further ammonia production and its possible slip. These results open the way to the scale up of the coupled catalytic technologies for its use in real conditions while controlling the influence of the operation map.

Strontium doping and impregnation onto alumina improve the NOx storage and reduction capacity of LaCoO3 perovskites

Here we report the effects of strontium doping of perovskites (La1-xSrxCoO3) on NOx storage and reduction (NSR) as well as the effects of supporting the perovskite onto an alumina support. The NSR capacity of La0.7Sr0.3CoO3 improved with strontium doping: this material displayed a good balance between NO oxidation capacity and adsorption sites accessibility. Increased accessibility was mainly due to the strontium oxide segregates. In addition, different loadings of La0.7Sr0.3CoO3 perovskite (10, 20, 30, 40 and 50%) were impregnated onto alumina in order to increase the exposed surface area of the perovskite. This had the effect of increasing the NSR capacity of the perovskite. The results of X-Ray diffraction, UV–vis–NIR spectroscopy, N2 adsorption-desorption, electron microscopy, and temperature programmed techniques, demonstrated that the cobalt ions preferably formed cobalt aluminate (CoAl2O4) in the case of low perovskite loadings (<20%). Meanwhile, a well-developed perovskite phase was observed with the higher loadings (>30%). The specific NO oxidation rate per gram of perovskite increased dramatically with the incorporation onto an alumina support. 30% LSCO/Al2O3 sample had an oxidation rate of 138 μmol min-1 (g LSCO)-1 at 350 °C, more than double the rate of the bulk La0.7Sr0.3CoO3 (49 μmol min−1 (g LSCO)−1). Likewise, the 30% LSCO/Al2O3 sample had a higher NOx storage capacity than its bulk counterpart at 400 °C: 306 vs. 115 μmol (g LSCO)−1. The higher oxidation capacity of the alumina-supported samples also facilitated the diffusion of the intermediate compounds from oxidation to adsorption sites. Impregnating alumina with perovskite could be used to improve the efficiency of perovskite mediated NOx removal in automobile applications. Furthermore, adding palladium onto optimum alumina-supported perovskite sample, i.e. 1.5% Pd-30% LSCO/Al2O3, resulted in a clear improvement in NOx storage and reduction capacity. This last sample demonstrated a nitrogen yield as high as 65%, an improvement over the model Pt-based NSR catalyst.

The influence of xMnyCe/γ-Al2O3 on NOx catalysts on the properties of NOx storage and reduction over Pt-Ce-Ba/γ-Al2O3 catalysts

The aim of the study was to investigate the influence of Diesel Oxidation Catalysts (DOC) xMnyCe/γ-Al2O3catalysts on the properties of NOx storage and reduction (NSR) under the treatment of Pt-Ce-Ba/γ-Al2O3 catalysts. The physicochemical properties of DOC and NSR were determined with XRD, TEM and XPS. The results showed that Pt-CeO2 interaction had a great impact on the dispersion of the NSR catalyst. Furthermore, the performance of NO oxidation by DOC and NOx desorption by DOC/NSR combined system were evaluated based on the simulation experiments. Moreover, optimal operation of the single NSR catalyst and the DOC/NSR combined system were attained at 400°C, while the durations of the lean and rich periods were tL=240s and tR=30s/45s/60s, respectively. The single NSR catalyst showed that the highest NOx conversion and maximum NOx adsorption conversion were 58.0% and 74.8%. The DOC/NSR combined system allowed Pt10Ce15Ba/γ-Al2O3 catalyst storing the NO2 oxidized on the 6Mn10Ce/γ-Al2O3 catalyst, favoring the NOx storage process during the lean phase. And it also allowed the stored NOx species of NSR catalysts desorbing due to H2 oxidation by the 6Mn10Ce/γ-Al2O3 catalyst, favoring the NOx desorption process during the rich phase.

Effect of Calcination Heating Rate on Microstructure and Performances of NSR Catalyst

The Be/Ce/γ-Al2O3 compound catalysts were prepared by sol-gel process using γ-Al2O3, Ba (AC)2 and Ce (NO3)3·6H2O as raw materials, and the effect of calcination heating rate on microstructure and performance of the NOx storage and reduction (NSR) catalyst was investigated. The crystal structure, microstructure, absorption and reduction performances of the NSR catalyst were characterized and analyzed by X-Ray diffractometer (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Brunauer Emmett Teller (BET) and H2 temperature program reduction (H2-TPR). The experimental results show that the Ba and Ce elements mainly exist in the forms of BaCO3 and CeO2, respectively, and partial CeO2 is amorphous. The calcination heating rate can play a key role in the grain size and distribution of BaCO3 and CeO2, so that it can affect the surface area, pore volume and pore diameter of the NSR catalyst. Moreover, the reduction temperature of the NSR catalyst decrease first and then increase with the increase of calcination heating rate, and the reduction temperature of as-received NSR catalyst is the lowest as the reduction temperature is 6 °C/min, that is its reduction performance is the optimal.

Effect of support material Al2O3vs ZrO2-TiO2 on the Ba availability for NSR catalyst: An in situ and operando IR study

The evolution of Ba sites in two distinct NOx storage and reduction (NSR) catalysts that are based on alumina or zirconia-titania mixed oxide (ZrO2-TiO2 or ZT) during NOx adsorption/desorption was investigated by in situ and operando IR spectroscopy. Based on various evidences from the in situ study, medium sized Ba sites on alumina supported fresh catalysts are proposed to experience sintering under NOx adsorption to form bigger particles, while for ZT, initially possessing smaller sized Ba particles, the formation of a thin layer or very fine particles of Ba would proceed under the same condition. This evolution can also be affirmed by observations from the operando IR study showing that NOx adsorption on ZT supported catalyst is initially faster than on alumina supported catalyst (time on stream lower than 300s), but after long adsorption time (about 50min) the two catalysts show similar storage capacity. This new mechanistic insight suggests that NOx ad/desorption rate, which is critical for optimizing NSR performance, needs to be controlled by support materials whose interaction with the Ba particles not only determines their initial size (fresh catalyst) but also their resistance towards sintering during the NOx adsorption

On the Cu species in Cu/beta catalysts related to DeNOx performance of coupled NSR-SCR technology using sequential monoliths and dual-layer monolithic catalysts

Cu/beta catalysts are prepared starting from protonic or ammonic BEA zeolite following liquid ion exchange with copper. Alternatively, an intermediate ion exchange with Na ions is performed before copper ion exchange. Cu/beta catalysts are extensively characterized by XRF, N2 adsorption–desorption, FT-IR of NO and CO adsorption, EPR, H2-TPR, TEM and SEM in order to identify copper oxidation state (Cu+ or Cu2+) and copper species (agglomerated or Cu ions with different interaction with the support). Cu/beta catalyst prepared from ammonic BEA zeolite with intermediate Na ion exchange followed by copper ion exchange results in the most active catalyst in the NH3-SCR reaction, which is related to the coexistence of Cu+ and Cu2+ ions and to a more accessible location of such ions in the zeolite matrix. This catalyst is washcoated onto a monolithic substrate with an optimum loading of 0.32 g cm−3, in order to be placed downstream a NOx storage and reduction (NSR) monolithic catalyst, prepared by washcoating Pt-Ba/Al2O3 powder with an optimum loading of 0.25gcm−3. NSR-SCR catalyst in sequential beds results in a very active system with a NOx removal of 97% and a N2 selectivity of 96% at 275°C. NSR/SCR catalyst prepared in a unique dual layer device shows that top SCR layer loading can be tuned in order to improve DeNOx performance. Optimum SCR layer loading is observed for 0.02gcm−3, enhancing the NOx to N2 efficiency of single NSR catalyst but far away from the catalytic performance observed for the sequential NSR-SCR configuration.

Potentials for NOx and CO2 reduction of combined NSC + passive SCR system in Diesel passenger car application

The stringent pollutant and CO2 emission limitation in upcoming legislation norms for Diesel passenger car applications force the introduction of exhaust aftertreatment systems with maximum purification and energetic efficiency. The state-off-the-art technologies for NOx reduction, NOx storage and reduction catalyst (NSC) and selective catalytic reduction catalyst (SCR), show challenges for robust performance under strongly extended emission relevant operation conditions from cold start to high engine load and speed. One technical solution is the combination of NSC and passive SCR. This paper presents the experimental results obtained in the EU project “Powerful”. On engine test bench and demon-strator vehicle, an exhaust aftertreatment system combining NSC, passive SCR and fuel reformer technology was developed and experimentally investigated steady-state and on road. The engine independent NSC operation offers potentials for lowered fuel consumption penalty by avoiding engine rich operation. The intrinsic NH3 formation during the NSC regeneration is used for additional NOx conversion on the passive SCR and reduces NOx and CO2 emission.

DeNOx Performance and Reaction Mechanism of the Di-Air System

The detailed reaction mechanism of Di-Air, which showed the unprecedented high deNOx performance at high temperature conditions, was studied in this work. Since the Di-Air phenomenon occurs with continuous short pulse injections of hydrocarbons over NSR (NOx storage and reduction) catalyst, this study focused on the specific function of HC as an effective reductant for NOx reduction reaction. As a first step, the deNOx performance was compared with three different reductant gases including CO, H2 and C3H6 in a modified synthetic gas bench equipped with a gas injector which enables continuous small amount of injections. At inlet gas temperature of 450 °C C3H6 showed the best deNOx performance whereas H2 was the best at 150 °C. Moreover the result of temperature programmed desorption showed that intermediate species represented by –NCO (isocyanates) produced from C3H6 was thermally more stable than that produced from CO. These results confirmed that the injected HC reacts with adsorbed NOx on NSR catalyst generating thermally stable intermediates, which could contribute the high deNOx performance at high temperature conditions.

Studies on the regeneration of sulfur-poisoned NOX storage and reduction catalysts, including a Ba composite oxide

The analysis of sulfur deterioration and regeneration of a NOX storage and reduction (NSR) catalyst containing Ba as the storage material and TiO2 as the support was conducted. It was found that the inhibition of sulfur re-adsorption during the sulfur desorption procedure under a rich atmosphere is important for improving sulfur desorption from a sulfur poisoned NSR catalyst, and that the combination of a Ba compound and a TiO2 support is effective for the inhibition of sulfur poisoning under both lean and rich conditions. On the basis of these results, a highly dispersed Ba-Ti composite oxide catalyst was prepared using a Ba-Ti complex precursor in order to realize the nm-scale distribution of Ba and Ti. Furthermore, it was concluded that the principal factor contributing to the high performance of the Ba-Ti catalyst in terms of the sulfur desorption rate was its ability to inhibit repetitive adsorption during rich treatment for sulfur desorption.

Modeling the transient cyclic operation of a commercial NSR catalyst

A pseudo 1-D model of an industrial NOx storage and reduction (NSR) catalyst has been developed under isothermal conditions. The Pt/Ba proximity or the presence of barium in different chemical forms such as BaO and BaCO3 can result in a wide range of storage rates. To represent these multiple storage sites, an idealized storage particle with a spherical configuration is proposed where the outer layer is assumed to represent the sites with the highest storage capacities/rates while subsequently deeper layers represent the sites with relatively lower storage capacities. It is further assumed that the NOx species first react with the particles in the outer layer, followed by the particles in subsequent deeper layers. To model the progressively decreasing storage rates, a shrinking core approach is used with a diffusivity coefficient, decreasing nonlinearly along the particle depth. The degree of nonlinearity depends on the complexity of the catalyst and therefore, depending on the catalyst, being studied, one can calibrate this function instead of quantifying different types of storage sites.During cyclic operation starting with a completely regenerated catalyst, model predictions for the NOx concentration profiles are in a good agreement with the experimental data during both the transient and steady cycle-to-cycle regimes. It is also shown that with limited amount of reductant, the regeneration of storage sites takes place only in the front portion of the catalyst up to a certain length and the remaining part of the catalyst is only involved in the storage reactions. After a certain number of cycles, the back of the catalyst reaches its saturation limit for storing NOx and from then on, only the amount of catalyst being regenerated in the front part of the catalyst during the rich phase participates in storage.

Investigation of combined DOC and NSRC diesel car exhaust catalysts

The system of two monolithic catalytic reactors – diesel oxidation catalyst (DOC) and NO x storage and reduction catalyst (NSRC) – is studied. The catalysts are used in series for the conversion of exhaust gases from automobiles with Diesel engines (or other lean-burn engines). Several tens of catalytic reactions are considered in the description by a model based on a system of non-linear partial differential equations (mass and enthalpy ballances). A simulation study is performed to examine the effects of parameters determining the performance of the upstream-located DOC (size and effective heat capacity of the monolith, catalytic activity in individual reaction steps) on the performance of the NSRC in the European driving cycle (NEDC). Effective numerical methods used for the solution of the resulting non-linear PDEs are described. Results of the maximization of the overall NO x conversion over the NEDC for several DOC and NSRC reactor configurations and different regeneration strategies are presented. It is shown, that the DOC may have a positive effect on the overall NO x conversion over the NEDC. Regarding NO x conversion over the NEDC, the effect of the NO x storage capacity of the NSRC is significant.

Fast Colorimetric Assay for Screening NSR Catalyst

The NSR (NOx storage and reduction) technology is one of the most promising methods for removing NOx from next-generation lean-burn and diesel engines. Three major drawbacks, including the low temperature activity, the catalyst sulfur tolerance and the use of the large amount of the noble metals for the commercial application of NSR catalytic system, may be resolved by the development of a better catalytic system through a fast assay method in line with the combinatorial chemistry technique. The colorimetric method is the most effective way for screening a large number of the catalyst samples at once, although an appropriate dye for revealing the color specific to NOx should be examined in advance. A knowledge-based colorimetric methodology for screening a better NSR catalyst by combinatorial approach has been developed and validated by the experimental results determined by means of traditional, time-consuming techniques such as TPD.

Global kinetic model for the regeneration of NO x storage catalyst with CO, H 2 and C 3H 6 in the presence of CO 2 and H 2O

Heterogeneous 1D model with global kinetics is proposed for industrial NO x storage and reduction catalyst (NSRC) on the basis of lab experiments in a mini-reactor in the temperature range 100–500 ° C. The NO x reduction dynamics and selectivity towards N 2 or NH 3 are modelled for three main reducing components present in the rich exhaust gas: CO, H 2, and unburned hydrocarbons (HCs). The following reactions are considered: CO, H 2 and HC oxidation, NO x reduction, NO/NO 2 transformation, NO and NO 2 storage, oxygen storage effects, water gas shift and steam reforming, and reduction of the stored NO x by H 2, CO and HC. Ammonia is formed mainly by the reaction of H 2 with NO x , but also by the water-assisted reaction of CO with NO x (formation and consequent hydrolysis of isocyanates). Inclusion of the latter route is necessary to explain the NH 3 formation in CO-rich mixtures without H 2 at lower temperatures. At higher temperatures, water gas shift and steam reforming reactions enable in situ H 2 production from CO, HC and H 2O. The formed ammonia subsequently reacts with oxygen and NO x deposited on the catalyst surface downstream, which results in NH 3 wave travelling along the catalytic monolith. The highest NH 3 yield is obtained around 200 ° C, when the NH 3 formation from the accumulated NO x is already ignited, while the ammonia consumption reactions are still relatively slow. At lower temperatures CO inhibits the NO x reduction by H 2 (i.e., the NH 3 production). At higher temperatures the ammonia oxidation reactions become fast enough to eliminate most of the NH 3 produced locally from the stored NO x . However, ammonia can be still observed after the completion of the regeneration, when it is formed steadily from the rich inlet gas containing NO x . Model results are confronted with the lab data, and calculated evolution of concentration profiles inside the monolith is discussed. The developed model is validated by engine test driving cycle data.

Deactivation of barium oxide-based NO x storage and reduction catalyst by hydrothermal treatment

The deactivation of a barium oxide-based NOx storage and reduction (NSR) catalyst with hydrothermal treatment was studied by treating it with 10 vol% water vapor diluted in nitrogen at 850°C. XRD, XPS, SEM, IR of CO adsorption, and the N2 adsorption was used to investigate the physical and chemical changes of the NSR catalyst caused by the hydrothermal treatment. The 12 h hydrothermal treatment decreased its NO2 storage capacity by 20%. However, the hydrothermal treatment significantly decreased its ability to reduce the stored NO2. The formation of an inactive phase consisting of platinum and aluminum is believed to be the cause of the severe deactivation of the NSR catalyst.

Dynamics and selectivity of NO x reduction in NO x 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 N 2, also NO, N 2O, and NH 3 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, N 2 and NH 3) within the short rich phase, and consequent development of the corresponding global reaction-kinetic model. An industrial NSRC monolith sample of PtRh/Ba/CeO 2/ γ -Al 2O 3 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 CO 2 and H 2O, using H 2, CO and C 3H 6 as the reducing agents. The spatially distributed NSRC model developed earlier is extended by the following reactions: NH 3 is formed by the reaction of H 2 with NO x and it can further react with oxygen and NO x deposited on the catalyst surface, producing N 2. 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 NH 3 maximum on the moving boundary. When the front reaches the reactor outlet, the NH 3 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 NH 3 are correlated with the effective oxygen and NO x storage capacity.

New approach to enhance the NOx storage performance at high temperature using basic MgAl2O4 spinel support

With the purpose of improving the NOx storage performance of NOx storage and reduction (NSR) catalysts above 673K, a new approach using a support with a higher basicity than γ-Al2O3 was proposed and exemplified by using the MgAl2O4 spinel as the support for an NSR catalyst containing the potassium NOx storage material. The NOx storage amounts on the fresh and thermally-aged K/Pt/MgAl2O4 catalysts at 873K were apparently higher than those on the K/Pt/Al2O3 catalysts. The higher potassium basicity resulting from the interaction between the basic MgAl2O4 support and the potassium NOx storage material enhanced the stability of the stored NOx over the fresh and thermally-aged K/Pt/MgAl2O4 catalysts, which leads to their competitive NOx storage amounts versus those of the K/Pt/Al2O3 catalysts. These results indicated a breakthrough that improved the performance of the NSR catalysts at high temperature.

Identifying critical factors in the regeneration of NOx-trap materials under realistic conditions using fast transient techniques

NOx storage and reduction (NSR) catalysts are considered to be one of the most promising solutions to meet the upcoming NOx exhaust emission regulations. However, despite the high level of activity in this area, it is surprising that most studies have been conducted under conditions that are far from realistic conditions. In the present work a fast transient apparatus has been developed to study NSR catalysts under realistic temporal conditions, i.e. where the storage time was 1–2min and the regenerative rich time was of the order of 1s. To exemplify the value of this methodology, the performance of a 1%Pt/17.5%Ba/γ-Al2O3 NSR catalyst was studied as a function of the temperature, rich phase duration, reductant concentration, the type of reductant and the effect of an inert gas purge. From these results it is clearly found that excess reductant to regenerate the trap at short regeneration times is advantageous and that hydrogen is a better reductant than CO. This is especially true at low temperatures where the rate of reduction of released NOx is not sufficiently fast to allow for complete regeneration of the trap. For a given quantity of reductant, a long, low concentration rich pulse regenerates the catalyst more effectively than a short, high concentration pulse. Readsorption of NOx released during the rich phase but not reduced quickly enough can be a significant cause of loss of trap performance. This study underlines the importance of the regeneration step in the NSR process and highlights the need to investigate these systems using fast transient techniques capable of reproducing the short regeneration times used in real systems.

Performance of Co-containing NO x storage and reduction catalysts as a function of cycling condition

Co-containing NO x storage and reduction (NSR) catalysts were studied for their NO x storage and NO x conversion behavior. It was found that addition of 5% Co increased the NO x storage capacity of a 1% Rh/15% Ba catalyst by 50% and that of a 1% Pt/15% Ba catalyst by 100%. This promotional effect was attributed to the high oxidizing ability of Co, present in the form of Co 3O 4, providing extra oxidation sites for NO to NO 2 conversion and more contact area for NO 2 spillover to Ba storage sites. Empirical models were developed to predict the NO x conversion capacity of the catalyst as a function of total cycle time and the fraction spent in the lean phase. Using these models, it was found that 5% Co also improved the performance of 1% Pt/15% Ba and 1% Rh/15% Ba catalysts at higher lean fractions, allowing substantial improvement in overall fuel efficiency. The reduction capacity of Co was much less than that of noble metals, such that noble metals were still necessary (although at a reduced loading) for regenerating the catalyst in the rich phase. Along these lines, it was found that a 0.25% Pt/5% Co/15% Ba-containing catalyst showed better performance than a 1% Pt/15% Ba-containing catalyst. This implies that the cost of NSR catalysts can be substantially reduced by replacing some of the Pt with Co as the active oxidizing metal in NSR catalyst formulations.