Pd 및 Rh 계 TWC 촉매의 내구성
- Pd 및 Rh 계 TWC 촉매의 내구성
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- Simultaneous removal of the three major air pollutants such as CO, HC and NOx emitted from gasoline engines using a three-way catalyst (TWC) converter is a well-established, commercially proven aftertreatment technology. The TWC contains noble metals such as rhodium (Rh), palladium (Pd) and/or platinum (Pt) as active catalytic ingredients for TWC reactions. In addition, a variety of promoters and stabilizers are also included in the modern TWC system to improve the catalyst activity and stability. However, the TWC is prone to deactivation due to the thermal degradation of catalyst components and chemical poisoning by various contaminants present in the exhaust stream. Current and upcoming emission standards such as ULEV and SULEV require not only a high catalytic performance but also catalyst durability up to 120,000 miles. Thus, it is important to develop a thermally and chemically durable TWC through better understanding of the catalyst deactivation mechanism.
In the present study, the deactivation of TWCs in the warm-up catalytic converters (WCC) installed in a passenger vehicle has been investigated with respect to the catalyst field-mileages. Three test modes have been employed to examine the alteration of the TWC performance upon aging
sweep test (ST), steady-state sweep test (st-ST) and light-off test (LOT). In the ST with the air/fuel ratio (A/F) oscillation in the amplitude of 0.5 at a frequency of 1.0 Hz to simulate the actual operation condition of the gasoline engine, a gradual deactivation of CO oxidation activity of the customer-aged Pd TWCs was observed under the rich feed condition at 400 oC, while the C3H6 oxidation activity was hardly affected. No deactivation of the Pd TWC for the C3H6 oxidation in the ST, regardless of the catalyst mileages, may be attributed to the strong intrinsic catalytic activity of Pd for hydrocarbon oxidation. The decreases of CO and C3H6 oxidation activities of TWCs were not proportional to the catalyst mileages in both the steady-state sweep test (st-ST) and the light-off test (LOT) without A/F perturbation, although the initial deactivation of TWCs for those reactions was clearly observed in both test modes. These observations indicate that the commercial TWCs lost their catalytic activities due to the sintering of Pd in the initial period of the catalyst aging, followed by a gradual decrease of oxygen storage capacity (OSC) with the catalyst aging.
A variety of characterization studies have been conducted for better understanding on the deactivation kinetics of the field-aged TWCs. The CO chemisorption study has revealed that the sintering of Pd has occurred mostly in the initial 21,000 mile of the catalyst mileage. However, the sintering of Pd itself is not the only cause for the gradual deactivation of TWC with respect to the catalyst mileage. The OSCs of the TWCs decreased gradually as a function of the catalyst mileage. Both the degradation of cerium zirconium mixed oxide (CZ) and the weakening of Pd-Ce interaction were the most plausible causes for the decrease of OSC, resulting in the decrease of the CO oxidation activity of the aged TWCs under the rich feed condition of the ST. The degradation of CZ was caused by the phase separation of CeO2 from CexZr(1-x)O2 in the initial period of the catalyst aging due to the exposure of the TWC to the high-temperature exhaust gas. Ce+4 in the separated CeO2 was then gradually transformed to Ce3+, forming stable CePO4 due to the phosphorus poisoning as the catalyst mileage increased The interaction of Pd with CeO2 became weaker by the sintering and encapsulation of Pd upon aging.
The 2nd part of the thesis is on the development of a thermally stable TWC system comprised of Rh and Pd catalysts. Prior to the incorporation of Rh and Pd catalysts into a monolith reactor, the catalytic activity and thermal durability of Rh supported on a variety of metal oxides have been investigated in the light-off test (LOT) mode. Among the Rh/metal oxide catalysts employed, the Rh/ZrO2 catalyst showed the highest thermal stability. The deactivation behavior of the Rh/metal oxides was dependent strongly on the types of the supports employed. Upon aging of the Rh/Al2O3 and /SiO2 catalytic systems, Rh was transformed into inactive phases such as Rh aluminate and Rh silicide, respectively. In addition, Rh was buried into the sublattice region of the ceria-based supports by aging without forming the chemically inactive Rh phase.
Also demonstrated in the second part of the thesis are the superior thermal stabilities of Rh/ZrO2s, which strongly depended on the structure of the ZrO2 supports. ZrO2 were prepared by the sol-gel (SG) and precipitation (P) methods. For a comparative study, commercial ZrO2 (com) and ZrO2 (P1000) were also employed. ZrO2 (P1000) was prepared by the extra calcination of ZrO2 (P) at 1000 oC for 20 hrs in static air, which is an equivalent condition to the catalyst aging. The ZrO2 (P and SG) supports in both tetragonal and monoclinic phases revealed higher TWC performance and stronger thermal stability than ZrO2 (com and P1000) in the monoclinic phase only. The two primary causes for the deactivation of the Rh/ZrO2 catalysts upon thermal aging are the sintering of Rh and the burial of Rh species into the sublattice region of the ZrO2 support. Results have indicated that the strong interaction of Rh with the tetragonal ZrO2 is the key factor preventing the burial of Rh into the sublattice of ZrO2 of the Rh/ZrO2s (P and SG). Due to the lack of the strong interaction between Rh and monoclinic ZrO2, the number of surface Rh sites decreased from the surface of the Rh/ZrO2 (com and P1000) catalysts upon oxidative thermal aging by the burial mechanism.
Since the NH3 formed over Rh catalysts can be reoxidized to NOx over the downstream oxidation catalyst such as Pd/Al2O3, it is important to determine the activity of the Rh catalysts for NH3 formation during the course of the TWC reaction, prior to the fabrication of the monolithic TWC containing both Rh and Pd catalysts. Comparative kinetic studies of TWC reactions revealed that the Rh/ZrO2 catalysts (P and SG) produced the least amount of NH3 during the course of the TWC reaction among the Rh/metal oxides examined. These observations have indicated that the Rh/ZrO2 is the most promising deNOx catalyst for the advanced TWC system for the gasoline engine due to its superior thermal stability and low activity for harmful NH3 formation during the TWC reactions.
Among the monolith reactor configurations comprised of the Rh/ZrO2 (P) and Pd/Al2O3, the dual-brick monolith reactor has shown much stronger thermal durability than the double-layered counterpart. Particularly, the dual-brick monolith has revealed a stronger thermal stability than the benchmarked commercial TWC containing a number of promoters and stabilizers. Since the washcoat layer of the Rh/ZrO2 was completely separated from that of the Pd/Al2O3 in the dual-brick configuration, the formation of a Pd-Rh complex decreasing the deNOx performance of the TWC can be prevented during the catalyst aging of the dual-brick monolith. On the other hand, the double-layered monoliths including the commercial TWC monolith have shown a significant decrease of the deNOx performance upon aging, mainly due to the formation of a Pd-Rh complex as determined by the H2 TPR study.
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