Simulated Diesel Exhaust Conditions Research Articles

Ag/ZSM-5 traps for C2H4 and C7H8 adsorption under cold-start conditions

A series of Ag ion-exchanged ZSM-5 (Si/Al = 11.5) zeolites with different Ag loadings (0.5, 0.8, 1.6, 3.0, 4.6 wt%) were investigated for their ability to adsorb and desorb C2H4 and C7H8 under simulated diesel exhaust conditions. Ion-exchanging ZSM-5 zeolites with Ag (Ag(0.8)/ZSM-5) led to an increase in the adsorption capacities of C2H4 and C7H8 compared to HZSM-5 in the absence of H2O. The C2H4 and C7H8 adsorption capacities of Ag(0.8)/ZSM-5 slightly decreased in the presence of H2O, indicating that H2O competes with both C2H4 and C7H8 for adsorption sites. Minor competitive adsorption between C2H4 and C7H8 was observed. Density functional theory calculations suggested that C2H4 is preferentially adsorbed in the sinusoidal channel of ZSM-5, while C2H4 and C7H8 are competing at the straight channels and the intersection of the sinusoidal and straight channels. With an increase in Ag loading from 0.5 to 4.6 wt% both C2H4 and C7H8 adsorption capacities increased from 38 to 163 and from 127 to 304 μmol/g, respectively. Temperature programmed desorption experiments indicated that an increase in Ag loading can lead to an increase in C2H4 and C7H8 desorption temperatures. During desorption the adsorbed C7H8 reacted to form CO, CH2O and C6H6, while C2H4 desorbed as is. Reductive pretreatments at 200 and 600 °C over Ag(4.6)/ZSM-5 resulted to a 33 and 58% decrease in C2H4 adsorption capacity, respectively, compared to its oxidized form. The C2H4 adsorption capacity can be mostly recovered after oxidation treatments at 600 °C, indicating that Ag+ reduction is reversible in ZSM-5 zeolites. Reductive pretreatments did not alter the C7H8 adsorption capacity.

Impact of Water Vapor on the Performance of a Cu-SSZ-13 Catalyst under Simulated Diesel Exhaust Conditions

<div class="section abstract"><div class="htmlview paragraph">Cu-SSZ-13 selective catalytic reduction (SCR) catalysts are broadly applied in diesel aftertreatment systems for the catalytic conversion of oxides of nitrogen (NO + NO<sub>2</sub>). Diesel exhaust contains a wide range of water vapor concentrations depending on the operating condition. In this study, we evaluate the impact of water vapor on the relevant SCR catalytic functions including NO<sub>x</sub> conversion, NO oxidation, NH<sub>3</sub> oxidation, and N<sub>2</sub>O formation under both standard and fast SCR conditions. Reactor-based experiments are conducted in the presence and absence of water vapor. Results indicate that water vapor can have both a positive and negative impact on low temperature NO<sub>x</sub> conversion for standard SCR reaction. At low inlet NO<sub>x</sub> concentrations, the presence of water vapor has a negative effect on NO<sub>x</sub> conversion, whereas, at high inlet NO concentrations, water vapor improves NO<sub>x</sub> conversion. At high temperatures, water vapor has a beneficial effect on NO<sub>x</sub> conversion at all NO<sub>x</sub> inlet concentrations evaluated. Experiments under fast SCR conditions reveal that water vapor improves low temperature NO<sub>x</sub> conversion mainly by minimizing the accumulation of ammonium nitrate, as evidenced with temperature programmed desorption (TPD) results. Furthermore, NO oxidation by ammonium nitrate formed under fast SCR conditions is shown to be significantly faster in the presence of water vapor.</div></div>

A comparative study of silver- and palladium-exchanged zeolites in propylene and nitrogen oxide adsorption and desorption for cold-start applications

Silver and palladium ion-exchanged BEA zeolites (Si/Al = 12.5) and silver ion-exchanged ZSM-5 zeolites (Si/Al = 15) were studied for their ability to adsorb and desorb propylene and NO under simulated diesel exhaust conditions. The adsorption experiment results demonstrated the excellent ability of bare BEA zeolites to adsorb propylene, but only a small amount of NO. The presence of H2O inhibited the adsorption of both C3H6 and NO. Ion-exchanging BEA zeolites with Ag (1.2 and 5.1 wt.% Ag/BEA) attenuated the inhibiting effect of H2O on C3H6 adsorption, while NO storage remained inhibited. Adsorption experiments indicated that C3H6 and NO competed with each other for Pd sites with C3H6 showing stronger adsorption compared to NO, whereas Ag sites preferentially adsorbed C3H6. DRIFTS data indicated the formation of nitrate, formate and acetate species on Ag and Pd, while C3H6 and NO adsorption was also observed on the zeolite hydroxyl groups in the absence of H2O. However, the C3H6 and NO adsorption on the zeolite hydroxyl groups was significantly inhibited in the presence of H2O. Additionally, nitrosyl, acrolein and carbonate species were formed over 1.0 wt.% Pd/BEA. The effluent analysis during the temperature-programmed desorption in the DRIFTS reactor revealed that adsorbed C3H6 and NO reacted during the release to form oxidation reaction byproducts. The 1.0 wt.% Pd/BEA zeolite showed the greatest oxidation ability during desorption with the majority of stored C3H6 converted to CO2.

Raspberry colloid-templated approach for the synthesis of palladium-based oxidation catalysts with enhanced hydrothermal stability and low-temperature activity

It is becoming increasingly urgent to develop and utilize novel, more efficient and stable materials for mobile and stationary emission control applications as the deleterious consequences of anthropogenic air pollution are becoming more evident and pressing. Tightening regulations, particularly related to automotive exhaust treatment, together with continued improvements in engine design, that result in lowering the engine operating temperatures and inadvertently lead to the release of an overwhelming proportion of pollutants during the cold start, present new challenges for materials design, specifically for oxidation catalysts. In particular, improvements in the low-temperature activity while maintaining catalyst stability at high temperatures are required from the next-generation catalyst. Typical catalysts for removal of pollutants from automotive exhaust streams incorporate platinum group metals (PGMs). They tend to be inefficient at low temperatures (below 250 °C), thus accounting for the cold start problem, yet sinter and lose their activity at high temperatures that are frequently encountered during catalyst operation. High PGM loadings are often employed to compensate for catalyst inefficiencies and fast degradation, ultimately resulting in high-cost catalytic converters. We have developed a new approach for the design and formation of catalytic materials that allows for both significantly more efficient PGM incorporation and improved overall catalyst performance at reduced PGM loadings. The method provides control over the composition and geometry of the support through self-assembly of sacrificial composite template — “raspberry” polymeric colloids decorated with catalytic particles — accompanied by infiltration with metal-oxide precursor and subsequent removal of the colloids. This method simultaneously structures the porous network and organizes the catalytic particles within it. Uniquely, the resulting catalytic particles are partially embedded in the support matrix and partially exposed to the pore interior, producing catalytic sites that are both stable and accessible. Herein, the feasibility of this novel and versatile approach for automotive catalytic conversion is demonstrated: the studies include testing alumina-based raspberry-colloid-templated (RCT) catalysts containing Pd nanoparticles (RCT Pd/Al2O3) for oxidation of propane and carbon monoxide under simulated diesel exhaust conditions and hydrothermal aging at 800 °C for 50 h in the simulated stream. The RCT Pd/Al2O3 catalysts exhibit exceptional activity toward CO oxidation, reduced reaction onset temperature, and high stability to elevated temperatures (demonstrated through prolonged exposure to temperatures up to 950 °C) and reactive gas streams, without migration, sintering or loss of the precious metal NPs. Notably, the novel catalyst shows the same or slightly better performance than the commercial catalysts even when the PGM load is reduced by ∼80 % compared to the commercial counterparts. These results provide confidence for the utilization of the RCT approach for the fabrication of robust nanostructured catalysts for next-generation, energy-efficient catalytic converters with improved performance at low and high temperatures and reduced costs. The RCT methodology is, in addition, highly generalizable, and can thus be applied for the design of a wide range of catalytic systems in the automotive sector and beyond.

Performance and Stability of Wet-Milled CoAl2O4, Ni/CoAl2O4, and Pt,Ni/CoAl2O4 for Soot Combustion

Low-energy wet milling was employed to activate commercial CoAl2O4 spinel and disperse mono- and multimetallic nanoparticles on its surface. This method yielded efficient Pt,Ni catalysts for soot oxidation in simulated diesel exhaust conditions. The characterization and activity results indicated that although Ni/CoAl2O4 was highly active, the presence of Pt was required to obtain a stable Ni(0.25 wt. %),Pt(0.75 wt. %)/CoAl2O4 catalyst under the operating conditions of diesel particulate filters, and that hot spots formation must be controlled to avoid the deactivation of the cobalt aluminate. Our work provides important insight for new design strategies to develop high-efficiency low-cost catalysts. Platinum-containing multimetallic nanostructures could efficiently reduce the amount of the costly, but to date non-replaceable, Pt noble metal for a large number of industrially important catalytic processes.

Effect of preparation methods on the catalytic activity of La0.9Sr0.1CoO3 perovskite for CO and C3H6 oxidation

La0.9Sr0.1CoO3 perovskite oxides were prepared by a hard template and a sol-gel method, respectively. The catalytic activities for oxidation of CO and C3H6 were examined. XRD, BET, TEM, TPR, and TGA techniques have been used to characterize structural properties, reducibility, mobility of lattice oxygen, and oxygen release rate. La0.9Sr0.1CoO3 synthesized by the hard template (SBA-15) method had a higher surface area and smaller particle size, providing more surface adsorbed oxygen. TPR and TGA results showed that oxygen in La0.9Sr0.1CoO3 prepared by the hard template method can be more easily extracted in H2 atmosphere than oxygen in the sol-gel prepared sample. These improved properties of La0.9Sr0.1CoO3 synthesized by the hard template (SBA-15) method facilitate catalytic oxidation reactions resulting in enhanced light-off performance for oxidation of CO or C3H6 compared to the sample prepared by the sol-gel method. Furthermore, the La0.9Sr0.1CoO3 catalyst prepared by the hard template method showed improved activity for the simultaneous oxidation of CO and C3H6 under simulated diesel exhaust conditions. Both catalysts showed almost no deactivation after 48 h on stream.

Activating low-temperature diesel oxidation by single-atom Pt on TiO2 nanowire array

Supported metal single atom catalysts (SACs) present an emerging class of low-temperature catalysts with high reactivity and selectivity, which, however, face challenges on both durability and practicality. Herein, we report a single-atom Pt catalyst that is strongly anchored on a robust nanowire forest of mesoporous rutile titania grown on the channeled walls of full-size cordierite honeycombs. This Pt SAC exhibits remarkable activity for oxidation of CO and hydrocarbons with 90% conversion at temperatures as low as ~160 oC under simulated diesel exhaust conditions while using 5 times less Pt-group metals than a commercial oxidation catalyst. Such an excellent low-temperature performance is sustained over hydrothermal aging and sulfation as a result of highly dispersed and isolated active single Pt ions bonded at the Ti vacancy sites with 5 or 6 oxygen ions on titania nanowire surfaces.

Doped barium cerate perovskite catalysts for simultaneous NOx storage and soot oxidation

A series of doped barium cerate perovskite catalysts (BaCeO3-δ, BaCe0.8Zr0.2O3-δ and BaCe0.8Zr0.1Co0.1O3-δ) were prepared by a two-step process consisting of solid-state synthesis followed by high-energy ball milling. X-ray diffraction studies showed that the dopants (Zr and Co) were incorporated into the perovskite lattice. TPR studies revealed that the high-energy ball milling process significantly enhanced the reducibility of the catalysts. XPS data indicated a shift in electron density away from the metal centres and an increase in defect oxides, for the high-energy ball milled catalysts, which led to enhanced activity of the catalysts. The catalytic performance was evaluated in a fixed bed reactor system under simulated diesel exhaust conditions, including water. The high-energy ball milled barium cerate sample co-doped with Zr and Co exhibited NOx storage of ∼228 μmol/g at 380 °C and a Tmax, at which the rate of soot oxidation reaction was maximum, of 436 °C.

Improved thermal stability of a copper-containing ceria-based catalyst for low temperature CO oxidation under simulated diesel exhaust conditions

CuO supported on a commercial mixed cerium–zirconium oxide shows remarkable improvement in CO oxidation after high temperature hydrothermal aging.

CATALYTIC ACTIVITY OF ZSM-5 ZEOLITE LOADED WITH TRANSITION METALS FOR THE SELECTIVE CATALYTIC REDUCTION OF NOX

Catalytic activity measurement were carried out on catalysts formulated from ZSM-5 zeolite separately loaded with transition metals (Cu, Co, Ni, Fe) for the removal of NOx in simulated diesel exhaust conditions. Various factors were investigated to compare the performance of these catalysts including catalyst preparation method, Si/Al ratio of the parent zeolite support, and durability in the exhaust stream containing water vapor or SO2. In many experimental conditions except in the presence of water vapor or SO2, Cu-ZSM5 remained the most active catalyst with 100% NOX conversion at around 350°C. C0-ZSM5 and Ni-ZSM5 could also achieve complete NOx conversion but at a much higher temperature of around 450°C. Fe-ZSM5 catalyst was the least active catalyst probably due to the formation of iron oxide particle, which accelerated formation of carbon monoxide.

Effect of preparation method on the performance of silver-zirconia catalysts for soot oxidation in diesel engine exhaust

This paper presents a study on the effect of the preparation method on silver-zirconia (Ag/ZrO2) catalysts’ physicochemical characteristics and their activity for soot oxidation in diesel engine exhaust. Two series of Ag/ZrO2 catalysts with different molar fractions of Ag (5–30 mol%) were prepared by incipient wetness impregnation (IM) and citric acid-assisted sol-gel (SG) methods The catalysts were characterized using Brunauer-Emmett-Teller (BET) surface area analysis, X-ray diffraction (XRD), O2 chemisorption, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and O2 temperature-programmed desorption (O2-TPD) techniques. The characterization results showed that silver was present mainly in the metallic state demonstrating heterogeneity in the Ag particle size distribution. XRD experiments indicated the presence of a tetragonal zirconia phase in Ag/ZrO2-SG catalysts. Soot oxidation activity of the silver-zirconia catalysts was evaluated by temperature-programmed oxidation (TPO) in presence of 10 vol% oxygen using carbon black (CB) as model soot. Catalytic activity was shown to be dependent on the method of preparation and silver loading. The synergetic effect of metal and support on the reaction was higher for the SG catalysts. The SG catalyst containing 30 mol% Ag exhibited the best catalytic performance for CB oxidation during TPO experiments. Adsorption properties of catalysts were evaluated with the O2-TPD. The amounts of desorbed oxygen correlated very well with the values of Tmax obtained in TPO runs indicating that adsorbed atomic oxygen played a major role in the mechanism of CB oxidation. Testing of the best catalyst under simulated diesel exhaust conditions revealed improved CB oxidation activity in the presence of water and no detrimental effect from SO2. These experiments showed that the new Ag/ZrO2-SG catalyst enabled effective soot combustion under conditions that do not require NOx. Soot removal over silver-zirconia catalysts can be an alternative method to NO-based technologies, which uses expensive Pt catalyst.

Indium-doped Co3O4 nanorods for catalytic oxidation of CO and C3H6 towards diesel exhaust

Platinum-group metals are widely used as diesel oxidation catalysts (DOCs) for exhaust control. It is a challenge to improve performance and reduce the cost of DOCs, while also to avoid interference of hydrocarbons and sintering of platinum metals at high temperatures. We present here an indium-doped Co3O4 uniform nanorod catalyst whose catalytic performance in CO, C3H6, and even NO oxidation is comparable to platinum-group DOCs in diesel exhaust. No obvious deactivation was observed in long-term stability tests under simulated diesel exhaust conditions. These indium-doped Co3O4 nanorods might open a promising pathway towards low-cost efficient diesel exhaust control systems. Characterization results indicated that lattice oxygen could be much more easily abstracted by hydrogen or carbon monoxide from indium-doped Co3O4 than from Co3O4 and the physical mixture Co3O4 and In2O3. The presence of indium with its large cation radius could influence the chemical status of surface/chemisorbed oxygen in Co3O4-In2O3 nanorods, thereby increasing the mobility of lattice oxygen involved in the catalytic oxidation reaction. The reaction mechanism of catalytic oxidation of CO and C3H6 were evaluated based on kinetic and FTIR studies. For CO oxidation, activated CO3* reduced by adsorbed CO* in an irreversible step to generate the final product of CO2 could be considered as the kinetically-relevant step. DRIFT spectroscopy confirmed that only stable carbonate species were observed over Co3O4-In2O3 nanorods that might be further reduced by CO to form CO2. For C3H6 oxidation, the incorporation of activated oxygen (O*) into anion vacancy of catalyst surface was the kinetically-relevant step, while the active sites on catalyst surface should be totally covered by the intermediates of C3H6 or its generated species, which actually acted as the most abundant surface intermediates (MASI). DRIFT spectroscopy confirmed that C3H6 and its related intermediates like formate, acetate, and acetone species would be formed over Co3O4-In2O3 nanorods.

Simultaneous catalytic oxidation of carbon monoxide, hydrocarbons and soot with Ce–Zr–Nd mixed oxides in simulated diesel exhaust conditions

Ce0.73−xZr0.27NdxO2 mixed oxides (x≤0.3) were prepared, characterized by XRD, Raman spectroscopy, N2 adsorption isotherms and H2-TPR, and tested for simultaneous CO, propylene, benzene and soot oxidation in a gas mixture containing O2, NOx, H2O, CO2, CO, propylene (model aliphatic hydrocarbon) and benzene (model aromatic hydrocarbon) that simulates a diesel exhaust. Ce–Zr mixed oxide doping with a low atomic fraction of neodymium (0.01≤x≤0.09) promotes the creation of oxygen vacancies, has a minor effect in the BET specific surface areas of the oxides, increases the surface ceria reducibility and has a positive effect in the catalytic activity. On the contrary, higher neodymium atomic fractions (x=0.2 and 0.3) promote sintering, with a drastic decrease of the BET specific surface area, surface reducibility and catalytic activity. The Ce0.73−xZr0.27NdxO2 catalysts with x≤0.09 are able to accelerate simultaneously soot, propylene and benzene combustion, and as a general trend, the catalytic behavior of Ce0.73Zr0.27O2 is improved by low atomic fraction neodymium doping (0.01≤x≤0.09). These Ce0.73−xZr0.27NdxO2 mixed oxides with 0.01≤x≤0.09 are also able to accelerate CO oxidation in a certain extent, but there is a net production of CO during soot combustion because the oxidation capacity of these oxides is not high enough to oxidize all CO released as soot combustion product.

Effect of CO2, H2O and SO2 in the ceria-catalyzed combustion of soot under simulated diesel exhaust conditions

The effect of CO2, H2O and SO2 in the Ce0.73Zr0.27O2 and Ce0.64Zr0.27Nd0.09O2 catalyzed combustion of soot with NOx+O2 has been studied. Combustion experiments performed in a fix-bed reactor with soot-catalyst mixtures prepared in loose contact mode showed that CO2, H2O and SO2 lower the activity of both catalysts, and the inhibiting effect follows the trend SO2>H2O>CO2. Regardless the gas mixture composition, the catalytic activity for soot combustion of Ce0.64Zr0.27Nd0.09O2 is equal or higher to that of Ce0.73Zr0.27O2 because Nd3+ doping seems to promote the participation of the active oxygen mechanism together with the NO2-assisted mechanism in the catalytic combustion of soot. The maximum soot combustion rate achieved during a Ce0.64Zr0.27Nd0.09O2-catalyzed reaction in NOx/O2/CO2/H2O/N2 is about three times higher than that of the uncatalyzed combustion, and this catalyst also improves the CO2 selectivity.In situ DRIFTS experiments showed that CO2, H2O and SO2 compete with NOx for the adsorption sites on the catalysts’ surface. CO2 partially impedes the catalytic oxidation of NO to NO2, affecting much more to the Nd3+-containing catalyst; however, the contribution of the active oxygen mechanism seems to remain relevant in this case. H2O also hinders the catalytic oxidation of NO to NO2 on both catalysts, and therefore the catalytic combustion of soot, because delays the formation of nitrogen reaction intermediates on the catalysts’ surface and favors the formation of more stable nitrogen surface species than in a H2O-free gas stream. For both catalysts, SO2 chemisorption (with sulfate formation) is even able to remove nitrogen surface groups previously formed by NOx chemisorption, which significantly inhibits the catalytic oxidation of NO to NO2 and the catalytic combustion of soot.

The effect of regeneration conditions on the selectivity of NO x reduction in a fully formulated lean NO x trap catalyst

The effect of regeneration conditions on NH 3 formation in a fully formulated Pt–Rh/BaO/Al 2O 3 lean NO x trap catalyst was investigated. Experiments were performed on a bench flow reactor under simulated diesel exhaust conditions, employing NO x storage/reduction cycles. Using CO/H 2 as the reductant, the selectivity of NO x reduction to NH 3 increased with increasing regeneration time, reductant concentration and space velocity, and decreased with increasing amount of stored NO x and increasing temperature. At a given temperature the effect of these parameters on NH 3 selectivity can be interpreted in terms of the local H 2:NO x ratio at the precious metal sites and the extent to which NH 3 is consumed in the reductant front as it propagates through the catalyst. However, selectivity to NH 3 increased with increasing temperature (>300 °C) during rich purging using C 3H 6 as the reducing agent. It was shown that NH 3 selectivity was governed by the steam reforming activity of the catalyst, selectivity to NH 3 increasing with increasing H 2 generation. Experiments using a second catalyst to which ceria had been added as an OSC material confirmed these trends, although the presence of the ceria resulted in lower selectivity to NH 3 when using H 2 and/or CO as the reductant. After aging, the catalysts displayed increased selectivity to NH 3; this is attributed in part to lengthening of the NO x storage-reduction zone, as demonstrated by SpaciMS data, and decreased OSC, resulting in decreased NH 3 consumption by NO x and O 2 downstream of the reductant front.

Analysis of large oxygenated and nitrated polycyclic aromatic hydrocarbons formed under simulated diesel engine exhaust conditions (by compound fingerprints with SPE/LC-API-MS).

The analysis of organic compounds in combustion exhaust particles and the chemical transformation of soot by nitrogen oxides are key aspects of assessment and mitigation of the climate and health effects of aerosol emissions from fossil fuel combustion and biomass burning. In this study we present experimental and analytical techniques for efficient investigation of oxygenated and nitrated derivatives of large polycyclic aromatic hydrocarbons (PAHs), which can be regarded as well-defined soot model substances. For coronene and hexabenzocoronene exposed to nitrogen dioxide under simulated diesel exhaust conditions, several reaction products with high molecular mass could be characterized by liquid chromatography-atmospheric pressure chemical (and photo) ionization-mass spectrometry (LC-APCI-MS and LC-APPI-MS). The main products of coronene contained odd numbers of nitrogen atoms (m/z 282, 256, 338), whereas one of the main products of hexabenzocoronene exhibited an even number of nitrogen atoms (m/z 391). Various reaction products containing carbonyl and nitro groups could be tentatively identified by combining chromatographic and mass spectrometric information, and changes of their relative abundance were observed to depend on the reaction conditions. This analytical strategy should highlight a relatively young technique for the characterization of various soot-contained, semi-volatile, and semi-polar reaction products of large PAHs. FigureLC-APCI-MS analysis of nitrated coronene (and HBC): Total-Ion-Chromatogram (TIC), Extracted Ion Chromatograms (EICs) and corresponding mass spectrum (top).

Influence of ceria on the NO x storage/reduction behavior of lean NO x trap catalysts

The effect of La 2O 3-stabilized ceria incorporation on the functioning of fully formulated lean NO x trap catalysts was investigated. Monolithic catalysts were prepared, corresponding to loadings of 0, 50 and 100 g CeO 2/L, together with a catalyst containing 100 g/L of ceria–zirconia (Ce 0.7Zr 0.3O 2). Loadings of the other main components (Pt, Rh and BaO) were held constant. Catalyst evaluation was performed on a bench flow reactor under simulated diesel exhaust conditions, employing NO x storage/reduction cycles. NO x storage efficiency in the temperature range 150–350 °C was observed to increase with ceria loading, resulting in higher NO x conversion levels. At 150 °C, high rich phase NO x slip was observed for all of the catalysts, resulting from an imbalance in the rates of nitrate decomposition and NO x reduction. Optimal NO x conversion was obtained in the range 250–350 °C for all the catalysts, while at 450 °C high rich phase NO x slip from the most highly loaded ceria-containing catalyst resulted in lower NO x conversion than for the ceria-free formulation. N 2O was the major NO x reduction product at 150 °C over all of the catalysts, although low NO x conversion levels limited the N 2O yield. At higher temperatures N 2 was the main product of NO x reduction, although NH 3 formation was also observed. Selectivity to NH 3 decreased with increasing ceria loading, indicating that NH 3 is consumed by reaction with stored oxygen in the rear of the catalyst.