Three-way Catalytic Performance Research Articles

Comparative catalytic performance of PGM-nanoparticle-doped alumina Xerogels and aerogels for potential application in automotive pollution mitigation

The structural (low density, high porosity), thermal (low thermal conductivity, high temperature stability), and chemical (high degree of tailorability) properties of aerogels make them attractive in a range of applications, including catalysis. The automotive industry relies on three-way-catalysts (TWCs) for automotive pollution mitigation, and TWCs in turn depend heavily on the use of platinum group metals (PGMs). Reduction of the use of PGMs in TWCs would be advantageous. To investigate the effect of aerogel structure and processing on three-way catalytic performance we prepared alumina wet gels doped with ca. 15 nm rhodium and palladium nanoparticles and dried them (1) supercritically to form aerogels and (2) under ambient conditions to form xerogels. Nanoparticle concentration was adjusted to ensure that both forms of the catalyst (aerogel and xerogel) had the same volumetric concentration of PGM after calcination to 800°C. Powder x-ray diffraction and transmission electron microscopy (TEM) confirmed the presence of PGMs. TEM images showed PGM particle agglomeration on the order of 40 to 200 nm in both the aerogel and xerogel materials. Gas adsorption analysis indicated that the aerogels have a surface area of 460 m2/g with most pores in the 2- to 100-nm range, whereas the xerogel surface area was 170 m2/g with a narrow pore distribution around 10 nm. Catalytic performance tests under a range of simulated automotive operating conditions show that, in all cases, the aerogels outperformed the xerogels. The aerogel light-off temperatures (the temperature at which a 50 % conversion is achieved) were 45 to 120 °C lower for the conversion of HCs, 25 to 55 °C lower for the conversion of CO and 40 to 145 °C lower for the conversion of NO. This study provides direct evidence that aerogel processing of catalysts provides catalytic performance benefits separate from the underlying chemistry of the catalysts.

Insights into the Reactivation Process of Thermal Aged Bimetallic Pt-Pd/CeO2-ZrO2-La2O3 Catalysts at Different Treating Temperatures and Their Structure–Activity Evolutions for Three-Way Catalytic Performance

CeO2-ZrO2-La2O3 supported Pt-Pd bimetallic three-way catalysts (0.6Pt-0.4Pd/CZL) were synthesized through the conventional impregnation method and then subjected to severe thermal aging. Reactivating treatments under different temperatures were then applied to the aged catalysts above. Three-way catalytic performance evaluations and dynamic operation window tests along with detailed physio-chemical characterizations were carried out to explore possible structure–activity evolutions during the reactivating process. Results show that the reactivating process conducted at proper temperatures (500~550 °C) could effectively restore the TWC catalytic performance and widen the operation window width. The suitable reactivating temperature ranges are mainly determined by the decomposing temperature of PMOx species, the thermal stability of PM-O-Ce species, and the encapsulation temperature of precious metals by CZL support. Reactivating under appropriate temperature helps to restore the interaction between Pt and CZL support to a certain extent and to re-expose part of the encapsulated precious metals. Therefore, the dynamic oxygen storage/release capacity, redox ability, as well as thermal stability of PtOx species, can be improved, thus benefiting the TWC catalytic performances. However, the excessively high reactivating temperature would cause further embedment of Pd by CZL support, thus leading to a further decrease in both dynamic oxygen storage/release capacity and the TWC catalytic performance after reactivating treatment.

The chemical state and Cu+ stability for three-way catalytic performance of Cu-added Al2O3 catalysts

Cu-added γ-Al2O3 (CuAl) catalysts having varying Cu loadings (3, 5, 8, and 10 mol%) were prepared at 900 °C. The effects of the copper oxidation states and the stability of these states under oxidizing and reducing atmospheres were examined. The use of these materials as three-way catalysts (TWCs) was also assessed. Analyses by X-ray diffraction and transmission electron microscopy indicated that copper oxide species were well dispersed and formed a solid solution in a surface layer of the γ-Al2O3. The TWC activity was found to increase with the addition of copper, and the 8 mol% CuAl catalyst showed very high performance despite being heat-treated at high temperatures. In the case of the 10 mol% CuAl catalyst, the excess Cu loading resulted in the precipitations of copper species on the γ-Al2O3 surface, which decreased catalytic activity. The oxidation states also differed between these catalysts. X-ray absorption fine structure and temperature-programmed reduction data demonstrated variations in copper oxidation states among the catalysts before and after oxidation and reduction treatments. The 8 mol% CuAl catalyst had a high Cu+/Cu2+ ratio and contained stable Cu+ species during both oxidation and reduction trials. The stability of the copper oxidation states is evidently a factor affecting catalytic activity.

Nanometric Metal Overlayer Catalysts: Fundamental Aspects and Applications

Nanometric metal overlayer catalysts have been developed as novel catalyst structures with high three-way catalytic performance for practical applications. The metal overlayer is prepared via a dry process using pulsed arc plasma deposition, unlike wet coating processes for conventional powder catalysts containing nanoparticles. The key point to achieving high performance is the extremely high turnover frequencies for specific chemical reactions catalyzed by a large two-dimensional surface compared with metal nanoparticles. This overview study presents the preparation, structure, performance, reaction mechanism, and thermal stability of metal overlayer catalysts, focusing on rhodium (Rh) and catalytic conversion of nitric oxide (NO). Rh plays a pivotal role in the reduction of NO to N2 in automotive three-way catalytic converters. The potential benefit of the overlayer structure is the minimum loading of precious metal, such as Rh, which is limited and expensive.

The influence of κ-Ce2Zr2O8 content on three-way catalytic performance

The CeO2-ZrO2 based oxide (CZ) with κappa-phase CZ (κ-Ce2Zr2O8) has been examined as a promising alternative to the traditional CZ used in the three-way catalysts (TWCs). However, forming pure κ-Ce2Zr2O8 requires huge energy, and it remains obscure whether more κ-Ce2Zr2O8 is better. Herein we synthesized a series of CZ with increasing κ-Ce2Zr2O8 content by increasing reduction time, as well as the supported Pd-only catalysts, for investigating the influence of κ-Ce2Zr2O8 content on TWC performance. The results evidenced that κ-Ce2Zr2O8 started to form at surface and gradually covered the whole surface. The crystal interface between κ-Ce2Zr2O8 and the cubic-phase matrix, was thus first increased but then decreased as the κ-Ce2Zr2O8 content increased from 0 % to 34.3 %. Interestingly, the presence of 19.8 % κ-Ce2Zr2O8 could dramatically improve the reducibility of CZ, making reduction temperature decrease from 598 ℃ to 446 ℃, however, further improvement was not achieved by increasing κ-Ce2Zr2O8. More importantly, the Pd-CZ interaction strength was found to be positively corelated with the amount of the mentioned crystal interface. The catalytic activity followed the similar variation tendency with the Pd-CZ interaction. Among them, Pd/CZ1 with 19.8 % κ-Ce2Zr2O8 showed the optimal catalytic performance. Based on the analyses, the improved TWC performance by κ-Ce2Zr2O8 was ascribed to the strengthened Pd-CZ interaction. Therefore, instead of increasing the κ-Ce2Zr2O8 content by consuming huge energy, constructing larger content of the mentioned crystal interface is more efficient for improving TWC performance.

Synthesis of Hexagonal YbMnO3 and Its Three-Way Catalytic Performance

Synthesis of Hexagonal YbMnO3 and Its Three-Way Catalytic Performance

Metal–organic frameworks derived Ce0.3Ni0.7Ox carrier improve Pd dispersion and three-way catalytic performance

Metal–organic frameworks derived Ce0.3Ni0.7Ox carrier improve Pd dispersion and three-way catalytic performance

Construction of bimetallic Pt–Pd/CeO2–ZrO2–La2O3 catalysts with different Pt/Pd ratios and its structure–activity correlations for three-way catalytic performance

A series of Pt–Pd bimetallic catalysts supported on CeO2–ZrO2–La2O3 mixed oxides were synthesized through the conventional impregnation method. Three-way catalytic performance evaluations along with detailed physio-chemical characterizations were carried out to establish possible structure–activity correlations. Results show that on the one hand, different Pt/Pd ratios can strongly affect the TWC behaviors of Pt–Pd/CZL catalysts by modulating the synergistic effect between Pt and Pd. On the other hand, higher Pt/Pd ratio also favors better dispersion of precious metals. Such improved precious metals (PM) dispersion can promote the metal-support interaction and increase the surface oxygen vacancies concentration, thereby raising the dynamic oxygen storage/release capacity, improving the redox ability as well as enhancing the thermal stability of the Pt–Pd/CZL catalyst. Moreover, the strong metal-support interaction can augment surface oxygen vacancy concentration, thereby benefiting low temperature CO and NO reaction via augmented NOx adsorption and nitrate conversion.

The strengthened Pd-(CeO2-ZrO2) interaction by κ-Ce2Zr2O8 and the improved three-way catalytic performance

The migration metallic species between Al2O3 and CeO2-ZrO2 (CZ) is one of the main causes for the deactiviation of the three-way catalysts (TWCs). In order to address this issue, κappa-phase κ-Ce2Zr2O8 was constructed to strengthen Pd-CZ interaction and thus stabilize palladium species. For comparison, tetragonal-phase Ce0.5Zr0.5O2 with κ-Ce2Zr2O8 phase (κCZ) and without (tCZ) were prepared. The (κCZ+Al2O3) supported catalyst (Pd/κCZA) exhibited a higher activity than Pd/tCZA even after aging at 900 °C or 1000 °C for 4 h. The larger reaction rate and TOF values of Pd/κCZA-900 evidenced its higher intrinsic activity, especially for C3H8 conversion. The analyses proved that κ-Ce2Zr2O8 could strengthen Pd-CZ interaction, subsequently induced Pd to selectively interact with CZ rather than Al2O3 and inhibited palladium from migrating from CZ to Al2O3. Additionally, in situ DRIFTS analysis proved that the strengthened Pd-CZ interaction facilitated to regenerate active palladium and NO transforming to reactive monodentate nitrate/nitrite.

Comparative study of three-way catalytic performance over Pd/CeO2-ZrO2-Al2O3 and Pd/La-Al2O3 catalysts: New insights into microstructure and thermal stability

Series of Pd/CeO2-ZrO2-Al2O3 (Pd/CZA) with different pore structures and lanthanum modified Pd/La-Al2O3 (Pd/La-A) were prepared via coprecipitation or hydrothermal method. The obtained catalysts were subjected to TWC activity evaluation, thermal stability tests as well as systematic characterizations including TEM, BET, Rietveld XRD, XPS, H2-TPR, O2-TPSR, and in situ DRIFTS. Results show that the TWC catalytic performance of Pd/Al2O3 catalyst can be obviously improved by CeO2-ZrO2 modification compared to La2O3 modification, due to the strong interaction between PdOx species and CeO2-ZrO2 support. The excellent thermal stability of Pd/CZA-2 catalyst is related to its different microstructure compared to Pd/CZA-1, because the abundant dissociated Zr4+ due to phase separation in Pd/CZA-2 could hinder the θ-Al2O3 to α-Al2O3 phase transition and retain small-sized PdOx species with excellent reoxidation ability. Besides, DRIFTS studies reveal clear structure-activity correlations between the adsorbed reaction intermediates and TWC catalytic performance, i.e. The formation of NO-Pd0 species on fresh Pd/CZA-1 is beneficial for NO conversion, while the rapid adsorption/decomposition of HC and carbonate species on Pd/CZA also contributes to better HC/CO oxidation. Nevertheless, the active sites poisoning by HC strong adsorption along with temperature-stable nitrate species on Pd/La-A cost its inferior catalytic behaviour regardless of thermal ageing.

Gain-scheduled output-feedback control of uncertain input-delay LPV systems with saturation via polytopic differential inclusion

This paper examines the control design for parameter-dependent input-delay linear parameter-varying (LPV) systems with saturation constraints. A linear differential inclusion approach is implemented to formulate the saturation nonlinearity. Subsequently, a gain-scheduled dynamic output-feedback controller is structured to conform to the saturation representation. By means of a Lyapunov–Krasovskii functional, sufficient delay-dependent conditions are derived for the analysis and feedback control synthesis of the closed-loop system in the presence of uncertainties and exogenous disturbances. Disturbance rejection objectives following an induced -norm and an norm formulation are examined. Estimation of the domain of attraction with the parametrisation of the admissible LPV controllers is presented. Three different optimisation objectives with respect to disturbance rejection, disturbance tolerance, and domain of attraction expansion are formulated. The proposed design methodology is examined in the context of the air-fuel ratio (AFR) regulation in a spark ignition (SI) engine with a speed-dependent delay and model parameters in the presence of canister purge disturbances and AFR dynamics uncertainty. The injector is restricted in the amount of fuel it can deliver to the cylinder. The three-way catalytic converter's performance in terms of oxygen storage and emission reduction is taken into consideration. Closed-loop simulation results are provided to validate the methodology.

Thermal stability and SO2 resistance of Pd/Rh-perovskite based three-way catalyst wash-coated on cordierite monoliths

The effect of hydrothermal aging and SO2 poisoning on the three-way catalytic performance of dispersed Pd/Rh-based and Pd/Rh perovskite-based catalysts were studied by using a dynamic activity test system that simulates redox fluctuations of gasoline engine exhaust conditions. Both catalysts were supported on ceria-zirconia-alumina and wash-coated on cordierite monoliths and were characterized by ICP-OES, BET, XRD, TEM-EDX, DRIFTS, and CO chemisorption. Their performance was very close when the catalysts were fresh but, after 3 h of hydrothermal aging at 1000 °C, the dispersed Pd/Rh-based catalyst lost its performance especially with respect to C3H8 and NO conversions; however, the Pd/Rh perovskite-based catalyst kept its activity. Parallel to that, CO uptake capacity losses after hydrothermal aging were 85% and 30% for the dispersed metallic catalyst and the perovskite-based catalyst, respectively. Moreover, 20 ppm SO2 exposure decreased CO uptake capacities by 61% and 48% for the two catalysts, respectively.

The Strengthened Pd-(Ceo2-Zro2) Interaction by Κ-Ce2zr2o8 and the Improved Three-Way Catalytic Performance

The Strengthened Pd-(Ceo2-Zro2) Interaction by Κ-Ce2zr2o8 and the Improved Three-Way Catalytic Performance

A Noble-Metal-Free Ce0.90Co0.10O2 – δ Catalyst with Enhanced Three-Way Catalytic Performance

A Noble-Metal-Free Ce0.90Co0.10O2 – δ Catalyst with Enhanced Three-Way Catalytic Performance

担持ロジウム触媒の三元触媒活性に及ぼす熱処理ガス条件の影響

担持ロジウム触媒の三元触媒活性に及ぼす熱処理ガス条件の影響

Morphology controllable synthesis of Pd/CeO2–ZrO2 catalysts and its structure-activity relationship in three-way catalytic performance

CeO2–ZrO2 mixed oxides with controlled morphologies (i.e., nanorod, nano-polyhedral and disordered shape) were synthesized through either hydrothermal or coprecipitation method, and the corresponding supported Pd/CeO2–ZrO2 catalysts were also prepared by conventional impregnation. The obtained catalysts were subjected to systematic structural/physio-chemical characterizations as well as three-way catalytic performance evaluations. The results showed that CeO2–ZrO2 mixed oxides presented different exposed crystal planes: namely disordered CZ-di mainly exposed (111) crystal planes, nanorod CZ-ro mainly exposed (111) crystal planes and few (100) crystal planes, while nano-polyhedral CZ-po exposed both (111) and (100) crystal planes. Clear structure-activity relationship could be demonstrated between the TWC (Three-way catalysts) performance of Pd/CeO2–ZrO2 and such different microstructures, as (100) and (111) crystal planes exposed by CZ-ro and CZ-po were beneficial for the surface oxygen vacancies formation, thus increasing the Pd-support interaction and effectively decreasing the light-off temperature of HC and NO elimination. On the other hand, the disordered CZ-di retained more lattice oxygen content, leading to better catalytic activity of CO elimination for the corresponding Pd/CZ-di catalyst.

Pd-Based Catalyst on Alumina with Perovskite (La0.67Fe0.83Cu0.17O3) to Reduce Ammonia Content in Natural Gas Exhaust

La0.67Fe0.83Cu0.17O3 with perovskite structure was prepared by sol–gel method, and 1.5 wt% Pd/Al2O3 was prepared by initial wet impregnation method. Pd/Al2O3 and perovskite prepared above were mixed by mechanical ball-milling with mass ratios of 10:1, 5:1 and 3:1, respectively, and represented by 10Pd/Al-La, 5Pd/Al-La and 3Pd/Al-La. Pd/Al refers to Pd/Al2O3 catalyst. In theoretical air–fuel ratio conditions ( $$\lambda$$ =1), the simulation of chemical metering of natural gas vehicle exhaust gas mixture on the catalysts were tested. The results showed that the perovskite could significantly reduce the ammonia formation in the exhaust gas. Moreover, the three-way catalytic performance of Pd/Al and 3Pd/Al-La were also tested in the feeds without water, CH4 or CO. The results demonstrated that the perovskite greatly reduced the ammonia content in the three exhaust gases. However, the ammonia content over bare Pd/Al was much higher in the presence of CO, and almost no ammonia was generated in the absence of CH4. The catalysts were also investigated by N2-physisorption, CO-chemisorption, XRD, EDX mapping and XPS techniques. The results showed that the perovskite may promote the reaction of NO and CO towards high N2 selection in the presence of CH4 and H2O, resulting in lower ammonia formation. Our work provided a new method for reducing ammonia formation over TWC catalysts for stoichiometric natural gas vehicles.

Influence of crystal structure of Y-doped ZrO<sub>2</sub> as support oxide on the three-way catalytic performance of supported Rh catalyst

Influence of crystal structure of Y-doped ZrO<sub>2</sub> as support oxide on the three-way catalytic performance of supported Rh catalyst

Development of a thermally stable Pt catalyst by redispersion between CeO2 and Al2O3.

For catalytic systems consisting of Pt as the active component and CeO2–Al2O3 as the support material, the metal–support interaction between the Pt and CeO2 components is widely applied to inhibit aggregation of Pt species and thus enhance the thermal stability of the catalyst. In this work, a highly thermostable Pt catalyst was prepared by modifying the synthesis procedure for conventional Pt/CeO2/Al2O3 (Pt/Ce/Al) catalyst, that is, the CeO2 component was introduced after deposition of Pt on Al2O3. The obtained CeO2/Pt/Al2O3 (Ce/Pt/Al) catalyst exhibits significantly different aging behavior. During the hydrothermal aging process, redispersion of Pt species from the surface of Al2O3 to the surface of CeO2 occurs, resulting in a stronger metal–support interaction between Pt and CeO2. Thus, the formed Pt–O–Ce bond could act as an anchor to retard aggregation of Pt species and help Pt species stay at a more oxidative state. Consequently, excellent reduction capability and superior three-way catalytic performance are acquired by Ce/Pt/Al-a after hydrothermal aging treatment.

Correction to: Effect of Copper Loading in Copper-Alumina Aerogels on Three-Way Catalytic Performance

Correction to: Effect of Copper Loading in Copper-Alumina Aerogels on Three-Way Catalytic Performance