Close-coupled Catalysts Research Articles

Effect of the loading sequence of CeO2 and Pd over Al2O3 on the catalytic performance of Pd-only close-coupled catalysts

The Pd-only close-coupled catalysts of the different loading sequence of CeO2 and Pd over Al2O3 were prepared using consecutive wetness impregnation method. Differences in structural, surface properties and catalytic performance, together with the thermal aging behaviors of catalyst at 1000 °C were investigated systematically by XRD, TPR, XPS, ICP, CO adsorption and TEM. Consequently, the Ce/Pd/Al catalysts loaded Ce after Pd over the alumina show superior structural properties before and after aging treatment, suggesting the Ce/Pd/Al catalysts possess better thermal resistance than Pd/Ce/Al catalysts loaded Pd after Ce. Moreover, the aged Ce/Pd/Al catalysts possess stronger reducible probed by TPR, and HRTEM and CO adsorption show smaller particles size of Pd and higher palladium dispersion. As a result, for Ce/Pd/Al catalysts, more palladium sites play a role as active site, which promotes low temperature conversion of pollutants effectively. The catalytic performance of the catalysts is forcefully subject to palladium dispersion and reduction property, especially for NO conversion, and the light-off temperature of NO over aged Ce/Pd/Al is 27 °C lower than aged Pd/Ce/Al. The results have demonstrated that novel loading sequence is beneficial to catalytic activity which is maintained even after aging treatment.

Catalytic Activity of C 3 H 8 over Fresh and Aged Pd/Al 2 O 3 Close-Coupled Catalysts

The Pd/Al 2 O 3 close-coupled catalysts supported on modified alumina were prepared by an impregnation method, and then characterized by H 2 -temperature programmed reduction (H 2 -TPR), CO chemisorption, and X-ray photoelectron spectroscopy (XPS). Both total catalytic activity and single reactions in related to C 3 H 8 conversion were tested in simulated exhaust gas. The results show that the amount of active PdO x decreases and negative metallic Pd 0 appears during aging treatment. The light-off temperature ( T 50 ) and complete conversion temperature ( T 90 ) increase by 76 °C and 64 °C, respectively. After aging treatment, signifying that the low-temperature activity decreases more obviously than high temperature property. The results of comparison among single reactions indicate that the light-off activity of reactions which occur in atmosphere with the participation of NO but without H 2 O injection decreases dramatically after the aging treatment. The inhibitive effect of NO on light-off activity and the positive effect of H 2 O on high temperature property become obvious during the aging treatment.

Effect of high temperature pretreatment on the thermal resistance properties of Pd/CeO2/Al2O3 close-coupled catalysts

Fresh Pd/CeO2/Al2O3 close coupled catalyst was prepared by the stepwise impregnation method and calcined at 550 °C for 3 h, which was then pretreated at 700, 800, and 900 °C for 3 h, respectively. Finally, these pretreated catalysts were aged at 1000 °C for 3 h to study their anti-aging properties. The catalytic activities of the catalysts were investigated detailedly, and the results showed that the catalyst pretreated at 800 °C before aging treatment possessed the best anti-aging performance for C3H8 oxidation. XRD and XPS results indicated that well-crystallized CeO2 particles were formed during calcinations at 800 °C, which made CeO2 an effective promoter. HRTEM revealed that Pd particles found on the edge of CeO2 over the aged catalyst pretreated at 800 °C were relatively smaller than those over the catalysts without pretreatment. H2-TPR and XPS results also implied that the interaction between well-crystallized CeO2 and Pd suppressed the deactivation of PdO sites and further enhanced the catalytic performance.

A highly efficient Rh-modified Pd/Al2O3 close-coupled catalyst

The close-coupled catalysts Pd/Al2O3 and Rh-Pd/Al2O3 were prepared by the impregnation method and characterized by H2 temperature-programmed reduction, CO chemisorption, and X-ray photoelectron spectroscopy. Both overall catalytic activity and specific reactions associated with C3H8 elimination were assessed. The light-off temperature and complete conversion temperature decreased by 23 and 18 °C, respectively, upon addition of Rh to the Pd/Al2O3 catalyst. The addition of Rh promotes the catalytic activity during C3H8 reactions, particularly in the presence of NO. The introduction of Rh not only inhibits sintering of PdOx and increases the dispersion of these same species, but also changes the electronic state of the PdOx in the catalyst.

Effects of the addition of small quantities of ceria on the catalytic behavior of Pd-only close-coupled catalysts during automobile exhaust elimination

A series of La-Al2O3 supported Pd-Ce close-coupled catalysts were synthesized by a co-adsorption impregnation method and subsequently investigated. In the case of fresh catalysts, the presence of an optimal concentration of ceria obviously promoted the catalytic activity during HC and NOx eliminations, owing to the interaction between the palladium oxide species (PdOx) and CeO2 that improves the oxidation of Pd0 to PdO and enhances the adsorption of nitrite/nitrates and isocyanate intermediate species on the support under reaction conditions. Pd-Ce(2.0)/La-Al2O3 catalyst (where 2.0 is the wt% of CeO2) exhibited the highest catalytic activity for HC and NO eliminations. Following aging at 1100 °C, the operational window for HC and NOx conversions was broadened and the thermal stability of the catalysts was also improved as a result of the presence of an appropriate quantity of ceria (2.0–4.0 wt%). The enhanced interactions between PdOx and CeO2 evidently led to the formation of a Pd-O-Ce system, stabilizing PdOx species with smaller particle sizes at high temperatures. This result is significant since it suggests a means of improving the catalytic performance of Pd-only close-coupled catalysts during automobile exhaust elimination.

(CeO<SUB>2</SUB>-ZrO<SUB>2</SUB>-Al<SUB>2</SUB>O<SUB>3</SUB>)-(La<SUB>2</SUB>O<SUB>3</SUB>-Al<SUB>2</SUB>O<SUB>3</SUB>) Complex Oxide Supported Pd Close-Coupled Catalysts: Effects of Support Calcination Temperature

(CeO<SUB>2</SUB>-ZrO<SUB>2</SUB>-Al<SUB>2</SUB>O<SUB>3</SUB>)-(La<SUB>2</SUB>O<SUB>3</SUB>-Al<SUB>2</SUB>O<SUB>3</SUB>) Complex Oxide Supported Pd Close-Coupled Catalysts: Effects of Support Calcination Temperature

Influences of Pd precursors and preparation method on the catalytic performances of Pd-only close-coupled catalysts

The properties and stabilities of Pd/Al2O3 close-coupled catalysts prepared with different methods and Pd precursors (PdCl2, Pd(NO3)2, and Pd(NH3)2(NO2)2) have been investigated, and characterized by N2 adsorption at low temperature, H2 chemical adsorption, X-ray diffraction (XRD), H2 temperature programmed reduction (H2-TPR), transmission electron microscopy (TEM) and three-way catalytic activity testing. The results show that the catalyst prepared by a prefabricate method with Pd(NH3)2(NO2)2 precursor behaves higher catalytic activity and thermal stability for removing HC, CO and NOx, due to its higher BET surface area, higher Pd dispersion, smaller Pd particle sizes. And the prefabricate method can lead to stronger interaction between the Pd active species and the support by means of a strong electrostatic attraction and ion/ligand-exchange between the Pd complexes and the support surface, which restrict the remotion and sintering of Pd active particles at high temperature. Therefore, a prefabricate method is more suited to preparing close-coupled catalysts than a traditional impregnation method.

Effects of BaO Content on Properties of Ba-Modified Alumina and Catalytic Performance of Its Supported Pt-Rh Close-Coupled Catalysts

Effects of BaO Content on Properties of Ba-Modified Alumina and Catalytic Performance of Its Supported Pt-Rh Close-Coupled Catalysts

The Effect of La2O3 on Pd Close-Coupled Catalysts

Palladium close-coupled catalysts were prepared by impregnation method with different amounts of La 2 O 3 as a promoter. Low temperature N 2 adsorption-desorption, X-ray diffraction, H 2 -temperature programmed reduction (H 2 -TPR), CO-chemisorption, and X-ray photoelectron spectroscopy (XPS) were used to characterize the effect of La 2 O 3 on the physicochemical properties of the prepared catalysts. Their catalytic activities toward C 3 H 8 conversion were evaluated using a gas mixture that simulates the emissions from a gasoline engine. The results of catalytic activity indicate that the presence of La 2 O 3 significantly improves catalytic performance and enhances the thermal resistance of the catalysts. H 2 -TPR results suggest that the addition of La 2 O 3 increases the reductive ability of the catalysts and shifts the reduction temperature to a lower value. The XPS and CO-chemisorption results indicate that doping by La 2 O 3 also improves the dispersion of the Pd species and decreases the mean particle size of palladium. The optimal loading amount of La 2 O 3 is 10%. ：采用浸渍法制备了不同 La 2 O 3 含量的 Pd 密偶催化剂. 使用低温 N 2 吸附-脱附、X 射线衍射、H 2 程序升温还原、CO 化学吸附和 X 射线光电子能谱等技术考察了 La 2 O 3 对催化剂性能的影响. 模拟汽车尾气条件考察了催化剂上丙烷的转化活性. 活性测试结果表明, La 2 O 3 的添加能显著提高催化剂对丙烷的转化活性和催化剂的热稳定性. H 2 -TPR 结果表明 La 2 O 3 的添加降低了催化剂的还原温度, 提高了催化剂的氧化还原性能. CO 化学吸附和 XPS 结果表明, La 2 O 3 的添加提高了 Pd 物种的分散, 降低了 Pd 的平均颗粒度. La 2 O 3 的最佳添加量为 10%. Adding La 2 O 3 to Pd close-coupled catalysts as a promoter can improve their catalytic performance toward C 3 H 8 conversion and the optimum La 2 O 3 content is 10%.

Durability of three-way and close-coupled catalysts for Euro IV regulation

The durability of three-way catalyst (TWC) and corresponding close-coupled catalyst (CCC) for Euro IV stage regulation was investigated through Vehicle Road Running Mode tests, whereas emissions of regulated pollutants of three car fleet were investigated at every 100,000 km miles. The results showed that HC, NO x , and CO emission values could meet Euro IV regulation limits at every point. The redox properties of TWC and CCC were measured by CO reduction during each isothermal. It was obvious that both aged TWC and aged CCC behaved a good redox property at 673 and 773 K. Based on XRD and BET measurement results, TWC and CCC washcoat were characterized with good thermal stability.

Impact of different configurations of a Diesel oxidation catalyst on the CO and HC tail-pipe emissions of a Euro4 passenger car

Different configurations were used to study the impact of a Diesel oxidation catalyst position on the HC and CO oxidation efficiency of a Euro4 Diesel passenger car. An oxidation catalyst of 0.8 L downstream EGR port is the original configuration. Another two configurations are studied in this work: the same catalytic volume put upstream EGR port and four small close-coupled catalysts of 0.06 L each put in the exhaust manifold very close to the engine exhaust valve. The results show that the configuration upstream EGR port leads to a significant increase on HC conversion efficiency; however, CO tail-pipe emissions remain practically unchanged. The four small close-coupled catalysts put in the exhaust manifold, despite the higher exhaust temperature, show lower oxidation efficiency due to the high space velocity, however, they can be used in the case of little available space for a bigger catalyst. In all configurations studied, there is a significant difference on CO and HC conversion efficiency and thus on tail-pipe emissions in the case of aged oxidation catalysts compared to fresh ones.

Predicting the flow field inside a close-coupled catalyst—the effect of entrance losses

A methodology is described for improving the prediction of the flow distribution in automotive close-coupled catalysts using computational fluid dynamics (CFD). Steady and pulsating flow simulations have been performed and compared with measurements obtained on an isothermal flow rig. By incorporating an extra pressure loss due to oblique entry at the monolith improved flow predictions are obtained. This work extends the methodology that has previously been developed for axisymmetric systems.

Low Emission Vehicle Technology for Ultra Low Emissions Vehicle and European Stage 3 Emissions Standards

Technology to meet the California Air Resources Board's (CARB) ultra low emissions vehicle (ULEV) legislation is being developed by most major vehicle manufacturers. To achieve the required reduction in cold start emissions, mainly HC and CO, compared to current three-way catalyst (TWC) technology, very early catalyst light-off has to be achieved. Electrically heated catalysts (EHC) have shown good potential, together with exhaust gas igniters (EGI) and close-coupled catalysts. A ‘total system’ approach has been demonstrated to achieve significant Federal test procedure (FTP) emissions reductions based on EHC, reduced thermal inertia manifolds and advanced engine control technology. This paper evaluates the suitability of ULEV technology to the European Commission (EC) stage 3 emissions standards. Proposed changes to the European test cycle are discussed and their effect on emissions quantified.