Catalytic activity and long-term stability of palladium oxide catalysts for natural gas combustion: Pd supported on LaMnO3-ZrO2

The catalytic activity and long-term stability of 2% Pd/LaMnO₃-ZrO₂ catalysts for natural gas combustion were deeply investigated. The catalyst, prepared via solution combustion synthesis, was completely characterized (XRD, BET, FESEM/EDS, TPC/TPD/TPR and FT-IR analysis) in the fresh status, and in the aged one, after prolonged treatment under hydro-thermal ageing and S-compounds poisoning (up to 3 weeks of hydro-thermal treatment at 800 °C under a flow of domestic boiler exhaust gases typical composition of 9% CO₂, 18% H₂O, 2% O₂ in N₂, including 200 ppmv of SO₂). An increased catalytic activity towards NG combustion with ageing was detected: the T₅₀, in fact, got lowered from 570 (fresh sample) to 465 °C (after 3 weeks ageing). Highly dispersed Pd centers were predominant on fresh catalyst. Upon ageing, oxygen covered Pd metal particles formed, at the expense of dispersed cationic and zerovalent Pd atoms. The increase in the catalytic activity was associated to the phase modification occurring in the bulk support, where Mn oxides, active towards CH₄ combustion, segregated. Moreover, bands due to sulfate species were detected in aged samples: IR analysis showed that Pd atoms did not interact significantly with these species. The bands of sulfate species decreased in intensity after 3 weeks ageing, likely mostly due to sintering of the catalyst, with the corresponding decrease in the surface area.     

Supported PdCl2CuCl2 catalysts for carbon monoxide oxidation II. XAFS characterization

A heterogenized Wacker catalyst system in which pores of a high surface area alumina were filled with an aqueous solution of PdCl₂CuCl₂ was active for the oxidation of CO near room temperature. The structure of the catalyst was studied by X-ray absorption fine structure (XAFS). The active phase of Pd was a Pd¹¹ species containing chlorine and, probably, carbonyl ligands. Direct interaction of PdPd or PdCu was not detected. The active phase of copper was found to be solid Cu₂Cl(OH)₃ particles in agreement with the X-ray diffraction (XRD) results. The presence of Cu was essential to keep the Pd in the Pd¹¹ state during the reaction. The rates of CO oxidation measured at temperatures of 30–70°C showed a minimum at 40°C, which was attributable partly to an unusual structure change of the active palladium species during the reaction at this temperature.     

Deactivation of V2O5-WO3-TiO2 SCR catalyst at biomass fired power plants: Elucidation of mechanisms by lab- and pilot-scale experiments

In this work, deactivation of a commercial type V₂O₅-WO₃-TiO₂ catalyst by aerosols of potassium compounds was investigated in two ways: (1) by exposing the catalyst in a lab-scale reactor to a layer of KCl particles or fly ash from biomass combustion; (2) by exposing full-length monolith catalysts to pure KCl or K₂SO₄ aerosols in a bench-scale reactor. Exposed samples were characterized by activity measurements, SEM-EDX, BET/Hg-porosimetry, and NH₃ chemisorption. The work was carried out to support the interpretation of observations of a previous study in which catalysts were exposed on a full-scale biomass fired power plant and to reveal the mechanisms of catalyst deactivation.Slight deactivation (about 10%) was observed for catalyst plates exposed to a layer of KCl particles at 350 °C for 2397 h. No deactivation was found for catalyst plates exposed for 2970 h to fly ash (consisting mainly of KCl and K₂SO₄) collected from an SCR pilot plant installed on a straw-fired power plant. A fast deactivation was observed for catalysts exposed to pure KCl or K₂SO₄ aerosols at 350 °C in the bench-scale reactor. The deactivation rates for KCl aerosol and K₂SO₄ aerosol exposed catalysts were about 1% per day and 0.4% per day, respectively.SEM analysis of potassium-containing aerosol exposed catalysts revealed that the potassium salt partly deposited on the catalyst outer wall which may decrease the diffusion rate of NO and NH₃ into the catalyst. However, potassium also penetrated into the catalyst wall and the average K/V ratios (0.5–0.75) in the catalyst structure are high enough to explain the level of deactivation observed. The catalyst capacity for NH₃ chemisorption decreased as a function of exposure time, which reveals that Brønsted acid sites had reacted with potassium compounds and thereby rendered inactive in the catalytic cycle. The conclusion is that chemical poisoning of active sites is the dominating deactivation mechanism, but physical blocking of the surface area may also contribute to the loss of activity in a practical application. The results support the observation and mechanisms of deactivation of SCR catalysts in biomass fired systems proposed in a previous study [Y. Zheng, A.D. Jensen, J.E. Johnsson, Appl. Catal. B 60 (2005) 253].     

Electrical properties of V2O5–WO3/TiO2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNOx reaction

Electrical conductivity measurements on EUROCAT V₂O₅–WO₃/TiO₂ catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH₃ to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH₃ without O₂, and (ii) 2000 ppm NO + 2000 ppm NH₃ + 500 ppm O₂) to put in evidence redox processes in SCR deNO_x reaction.It was first demonstrated that titania support contains certain amounts of dissolved W⁶⁺ and V⁵⁺ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO₃/TiO₂ precursor of V₂O₅–WO₃/TiO₂ EUROCAT catalyst. It contained an average amount of 0.37 at.% W⁶⁺dissolved in titania, i.e. 1.07 × 10²⁰ W⁶⁺ cations dissolved/cm³ of titania. For the fresh catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved in titania were found to be equal to 1.07 × 10²⁰ and 4.47 × 10²⁰ cm⁻³, respectively. For the used catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved were found to be equal to 1.07 × 10²⁰ and 7.42 × 10²⁰ cm⁻³, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10²⁰ additional V⁵⁺ cations in titania.After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO⁻ and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH₃ reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N^o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to

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. In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.     

Adsorption and oxidation of NH3 over V2O5/AC surface

V₂O₅/AC has been reported to be active for selective catalytic reduction (SCR) of NO with NH₃ at around 200 °C and resistant to SO₂ deactivation. To elucidate its SCR mechanism, adsorption and oxidation of NH₃ over V₂O₅/AC are studied in this paper using TG, MS and DRIFTS techniques. It is found that the adsorption and oxidation of NH₃ take place mainly at VO bond of V₂O₅. A higher V₂O₅ loading results in more NH₃ adsorption on the catalyst. V₂O₅ contains both Brnsted and Lewis acid sites; NH₄⁺ on Brnsted acid sites is less stable and easier to be oxidized than NH₃ on Lewis acid sites. Gaseous O₂ promotes interaction of NH₃ with AC and oxidation of NH₃ over V₂O₅/AC. NH₃ is oxidized into NH₂ and acylamide structures and then to isocyanate species, which is an intermediate for N₂ formation.     

Transient TPSR, DRIFTS-MS and TGA studies of a Pd/ceria-zirconia catalyst in CH4 and NO2 atmospheres

In this study, the parameters governing the activity of Pd/ceria-zirconia catalysts in the selective catalytic reduction (SCR) of NO_x assisted by methane are investigated using a combination of temperature-programmed spectroscopic and thermogravimetric techniques and transient SCR conditions. By DRIFTS of adsorbed CO, it is established that Pd species on Ce_0.2Zr_0.8O₂ are mainly present in cationic form but exhibit high reducibility. As found by temperature-programmed surface reaction (TPSR) in CH₄ + NO₂ atmosphere, the CH₄-SCR reaction is initiated at 280 °C on Pd/Ce_0.2Zr_0.8O₂ and yields almost 100% N₂ above 500 °C. DRIFTS-MS and TGA experiments performed under transient SCR conditions show that DeNO_x activity is due to a surface reaction between some methane oxidation products on reduced Pd sites with ad-N_xO_y species presumably located on the support. The detrimental effect of O₂ on DeNO_x is explained by the promotion of the total combustion of methane assisted by the ceria-zirconia component at the expense of the SCR reaction above 320 °C.     

Influence of preparation methods of LaCoO3 on the catalytic performances in the decomposition of N2O

This study reports the potential interest of LaCoO₃ in the catalytic decomposition of N₂O from nitric acid plants. Typically, the exhaust gas contains NO, water and O₂ which usually induce strong inhibiting effects depending on the surface properties of the solids particularly the surface mobility of oxygen from LaCoO₃. Different preparation methods have been implemented, involving citrate route, reactive grinding and the use of templates, which lead to different structural and textural properties examined by X-ray diffraction, transmission electron microscopy and N₂ physisorption. EDX analysis and XPS measurements also revealed that different surface composition may alter subsequent interactions between the surface and the reactants and related catalytic performances. LaCoO₃ prepared by reactive grinding was found to be the most active catalyst due to a high specific surface area but the presence of Fe and Zn impurities inherent to the preparation method were suggested to interfere on the catalytic performances.     

Methanol interaction with NO2: An attempt to identify intermediate compounds in CH4-SCR of NO with Co/Pd-HFER catalyst

The objective of this work is the study of fundamental common aspects of NO_x catalytic reduction over a Co/Pd-HFER zeolite catalyst, using methanol or methane as reducing agent. Temperature Programmed Surface Reaction (TPSR) studies were performed with reactant mixtures comprising NO₂ and one of the reducing agents.The formation of formaldehyde was detected in both studied reactions (NO₂–CH₄ and NO₂–CH₃OH) in the temperature range between 100 and 220 °C. At higher temperature, when the NO_x reduction process effectively begins, formaldehyde starts to be consumed.Using methanol as reducing agent, nitromethane and nitrosomethane, are detected. At 300 °C these species are consumed and cyanides and iso-cyanides formation occurs. On the contrary, with methane, these last species were not detected; however, there are strong evidences for CH₃NO and CH₃NO₂ formation.Thus, using methanol or methane, similar phenomena were detected. In both cases, common intermediary species seem to play an important role in the NO_x reduction process to N₂.These results suggest that methanol can be considered as a reaction intermediate species in the mechanism of the reduction of NO₂ with methane, over cobalt/palladium-based ferrierite catalysts.     

Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd

The present work has been undertaken to tailor Pt/Al₂O₃ catalysts active for NO oxidation even after severe heat treatments in air. For this purpose, the addition of Pd has been attempted, which is less active for this reaction but can effectively suppress thermal sintering of the active metal Pt. Various Pd-modified Pt/Al₂O₃ catalysts were prepared, subjected to heat treatments in air at 800 and 830 °C, and then applied for NO oxidation at 300 °C. The total NO oxidation activity was shown to be significantly enhanced by the addition of Pd, depending on the amount of Pd added. The Pd-modified catalysts are active even after the severe heat treatment at 830 °C for a long time of 60 h. The optimized Pd-modified Pt/Al₂O₃ catalyst can show a maximum activity limited by chemical equilibrium under the conditions used. The bulk structures of supported noble metal particles were examined by XRD and their surface properties by CO chemisorption and EDX-TEM. From these characterization results as well as the reaction ones, the size of individual metal particles, the chemical composition of their surfaces, and the overall TOF value were determined for discussing possible reasons for the improvement of the thermal stability and the enhanced catalytic activity of Pt/Al₂O₃ catalysts by the Pd addition. The Pd-modified Pt/Al₂O₃ catalysts should be a promising one for NO oxidation of practical interest.     

Pd-doped LaCoO3 regenerative catalyst for automotive emissions control

The effect of partial substitution of Co by Pd in LaCoO₃ perovskite structure (i.e., LaCo_0.95Pd_0.05O₃) and the reductive diffusion of Pd from the bulk of perovskite to its surface, thus forming Pd nanoparticles, on CO and C₃H₈ oxidation present in air (simulated exhaust gas) are reported. X-ray powder diffraction (XRD) analyses confirm the perovskite structure for the catalysts. Scanning electron microscopy (SEM) and BET surface area measurements show that partial substitution of Co by Pd decreases the crystallite size of the perovskite and therefore increases its surface area. H₂-temperature programmed reduction (TPR) experiments reveal that Pd reduces at 135 °C and facilitates the reduction of Co in the perovskite structure. By partial reduction of the Pd containing catalyst at 180 °C for 30 min, the complete oxidation temperatures of CO and C₃H₈ decrease by about 70 and 50 °C, respectively.The reduction duration of the Pd containing catalyst strongly affects the T₅₀ and T₉₀ temperatures (temperatures at which 50 and 90% conversion occurs, respectively) and has an optimum, where it decreases by increasing the reduction temperature of the catalyst.     

N2O decomposition over K-promoted Co-Al catalysts prepared from hydrotalcite-like precursors

N₂O decomposition was investigated over a series of K-promoted Co-Al catalysts. The activity tests showed that doping with K greatly enhanced the catalytic activity of the Co-Al catalyst, and the enhancement was critically dependent on the amount of K and the calcination temperature. When the catalyst had a K/Co atomic ratio of 0.04 and was calcined at 700–800 °C, a full N₂O conversion could be reached at a reaction temperature of 300 °C. Moreover, even under the simultaneous presence of 4% O₂ and 2.6% water vapor, such high-temperature treated K/Co-Al catalyst exhibited high reactivity and stability, with the N₂O conversion remaining at a constant value of 92% over 40 h run at 360 °C. In contrast, non-doped Co-Al catalyst showed a severe activity loss under such reaction conditions. A combination of characterization techniques was employed to reveal the promoting role of K and the effect of calcination temperature. The results suggest that doping with K increases the electron density of Co and weakens the Co–O bond, thus promoting the activation of N₂O on the Co sites and facilitating the desorption of oxygen from the catalyst surface. High-temperature calcinations made the desorption of O₂ proceed more readily.     

Selective oxidation of methanol to dimethoxymethane over acid-modified V2O5/TiO2 catalysts

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V₂O₅/TiO₂ catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W() = 15%) and V₂O₅ (W(V₂O₅) = 15%) components loaded in the acid-modified V₂O₅/TiO₂ catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H₂SO₄-modified V₂O₅/TiO₂ catalyst gives a much higher DMM yield at 150 °C than the unmodified one.     

Selective Decomposition of NO in the Presence of Excess O2 in Electrochemical Cells

NO decomposition in solid electrolyte cells was investigated in the presence of excess O₂. The results show that NO is decomposed via an electrocatalytic mechanism rather than electrolysis in the range of 1–4 V of applied voltage. The NO is catalytically decomposed to N₂ on the cathode surface and O^2– produced in situ is transferred through the yttria-stabilized zirconia (YSZ) to the anode by direct current (d.c.) and then is evolved in the form of O₂, which helps to maintain the active state of the cathode. In a Pd/YSZ/Pd cell, the palladium metal surface is the active site for NO decomposition, while in the RuO₂/Pd/YSZ/Pd cell, the partially reduced RuO _x (0 < x < 2) is the main active site for NO decomposition. At 600 °C, the rate-determining step for the overall transportation of O^2– from cathode to anode in the RuO₂/Pd/YSZ/Pd cell is the transportation of O^2– at the cathode Pd/YSZ interface. The transportation rate of O^2– at the cathode M/YSZ interface decreases in the order of Ag > Au > Pd > Pt. Substitution of the Pd cathode by Ag leads to an increase in current density by a factor of 3.5. A higher NO decomposition parameter (=13.4) is also achieved at a lower temperature of 500 °C.     

Catalytic behaviour and characterisation of CuO1−z(La2O3)z/2 based catalysts deposited on γ-Al2O3 and prepared in situ with 0.0 ⩽z⩽1.00, without and with addition of Pd

(CuO)_1–z(La₂O₃)_z/2 based catalysts with 0.0z1.0 supported on -Al₂O₃ have been prepared in situ and the phases formed have been identified by XRD, SEM and TEM/EDS studies. The catalyst with z=0.5 exhibited the best catalytic activity for oxidation of CO (T ₅₀=295 and 390C with degrees of conversions of 93 and 92% at 450C under rich and lean conditions, respectively) and C₃H₆ (291 and 414C; 93 and 83%) and reduction of NO (405C; 60 and 0%). This catalyst contained appreciable amounts of the perovskite phase LaAl_1–xCu_xO₃ and the enhanced catalytic properties are ascribed to the presence of this phase. Addition of Pd to this catalyst implied that the degree of conversion of NO increased and that the light-off temperatures for all involved gas species decreased. Ageing experiments revealed that LaAl_1–xCu_xO₃ decomposed and that Cu containing Pd particles were formed during this procedure which in turn deteriorated the catalytic properties of the catalyst.     

iso-Octane partial oxidation over Ni-Sn/Ce0.75Zr0.25O2 catalysts

In this study, Ni/Ce_0.75Zr_0.25O₂ catalyst was doped with different amounts of Sn by co-impregnation method. The catalysts were characterized by BET, H₂ chemisorption, XRD, TPR, TEM, XPS and tested for iso-octane partial oxidation (iC8POX) to H₂ in the temperature range of 400–800 °C at atmospheric pressure. The results showed that most of Sn species were present on the surface of Ni particles and did not modify the reducibility of the support. Addition of a small amount of Sn (<0.5 wt.%) lowered the catalytic activity for iso-octane partial oxidation by less than 5% while the extent of carbon deposition was decreased by more than 50%. However, Sn loadings higher than 1 wt.% caused a massive drop in catalytic activity. This indicates that as long as the Ni surface is only partially covered with Sn species, the active sites for the partial oxidation of iso-octane remain intact, while the surface site ensembles required for carbon formation are blocked.     

NO reduction with H2 in the presence of excess O2 over Pd/MFI catalyst

The NO SCR (selective catalytic reduction) activity with H₂ in the presence of excess O₂ was investigated over Pd/MFI catalyst prepared by sublimation method. With GHSV=90?000 h⁻¹, a very high steady-state conversion of NO to N₂ (70%) is achieved at 100 °C. Significant reorganizations take place inside the catalyst upon its first contact with all reactants and products at the reaction temperature. Pd⁰, which has a significant role in the NO-H₂-O₂ reaction, is possibly the active site for NO reduction. The formation of Pd-β hydride deactivates the catalyst for NO reduction. Throughout the entire NO-H₂-O₂ reaction, no N₂O or NO₂ is formed; N₂ is the only N-containing product. The presence of O₂ inhibits the formation of undesirable NH₃. The rate of the NO+H₂ reaction is fast or comparable to that of the H₂+O₂ reaction. The oxidation of Pd⁰ and subsequent agglomeration of PdO are responsible for the decreased NO reduction activity at high temperature.     

Catalytic behaviour of La4PdO7 based catalysts when exposed to car exhaust conditions

The catalytic properties of ex situ prepared La₄PdO₇ were studied under automotive exhaust conditions. Monophasic La₄PdO₇ was deposited onto-Al₂O₃ coated cordierite monoliths and light-off curves were recorded under oxidising and reducing conditions. Initially the catalytic activity for oxidation of CO and C₃H₆ and reduction of NO was low but after the catalyst had been exposed to a reducing atmosphere of simulated car exhaust composition at 600°C a three-way catalytic behaviour was obtained with aT ₅₀ value of about 340°C. Using oxidising conditions and a similar activation procedure, a catalyst with CO and C₃H₆ oxidation activity (T ₅₀=405° C) but minor activity for reduction of NO was obtained. This behaviour is interpreted in terms of La₄PdO₇ being decomposed and nanosized PdO particles being formed; these in turn are reduced to Pd and Pd is the catalytically active component in the catalyst. These conclusions are supported by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy studies combined with element analysis of freshly prepared and thermally activated catalysts.     

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Ammonium nitrate is thermally stable below 250 °C and could potentially deactivate low temperature NO_x reduction catalysts by blocking active sites. It is shown that NO reduces neat NH₄NO₃ above its 170 °C melting point, while acidic solids catalyze this reaction even at temperatures below 100 °C. NO₂, a product of the reduction, can dimerize and then dissociate in molten NH₄NO₃ to NO⁺ + NO₃⁻, and may be stabilized within the melt as either an adduct or as HNO₂ formed from the hydrolysis of NO⁺ or N₂O₄. The other product of reduction, NH₄NO₂, readily decomposes at ≤100 °C to N₂ and H₂O, the desired end products of DeNO_x catalysis. A mechanism for the acid catalyzed reduction of NH₄NO₃ by NO is proposed, with HNO₃ as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNO_x systems could help mitigate catalyst deactivation at low operating temperatures (<150 °C).     

Catalytic NO–H2–CO–O2 reactions over Pt-supported mesoporous yttrium oxide

Catalytic light-off of a stream of NO, H₂, CO in an excess O₂ has been studied over various metal oxides loading 1 wt% Pt. Because a low-surface area Y₂O₃ (<5 m² g⁻¹) was found to exhibit the highest de-NO_x activity, a mesoporous Y₂O₃ was then synthesized from an yttrium-based surfactant mesophase templated by dodecyl sulfate , which was anion-exchanged by acetate (AcO = CH₃COO⁻). The product showed a 3-D mesoporosity with a large surface area (396 m² g⁻¹) and the Pt-supported catalyst achieved much improved light-off characteristics suitable for the low-temperature de-NO_x in the presence of CO and excess O₂.     

Excellent sulfur resistance of Pt/BaO/CeO2 lean NOx trap catalysts

In this work, we investigated the NO_x storage behavior of Pt/BaO/CeO₂ catalysts, especially in the presence of SO₂. High surface area CeO₂ (110 m²/g) with a rod like morphology was synthesized and used as a support. The Pt/BaO/CeO₂ sample demonstrated slightly higher NO_x uptake in the entire temperature range studied compared with Pt/BaO/γ-Al₂O₃. More importantly, this ceria-based catalyst showed higher sulfur tolerance than the alumina-based one. The time of complete NO_x uptake was maintained even after exposing the sample to 3 g/L of SO₂. The same sulfur exposure, on the other hand, eliminated the complete NO_x uptake time on the alumina-based NO_x storage catalysts. TEM images show no evidence of either Pt sintering or BaS phase formation during reductive de-sulfation up to 600 °C on the ceria-based catalyst, while the same process over the alumina-based catalyst resulted in both a significant increase in the average Pt cluster size and the agglomeration of a newly formed BaS phase into large crystallites. XPS results revealed the presence of about five times more residual sulfur after reductive de-sulfation at 600 °C on the alumina-based catalysts in comparison with the ceria-based ones. All of these results strongly support that, besides their superior intrinsic NO_x uptake properties, ceria-based catalysts have (a) much higher sulfur tolerance and (b) excellent resistance against Pt sintering when they are compared to the widely used alumina-based catalysts.