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Dual-monolith converters containing Pd-only catalysts followed by Pt/Rh three-way catalysts (TWCs) provide effective emission solutions for NLEV and Tier IIa close-coupled dual-bank V-8 applications due to optimal hydrocarbon and NOx light-off, transient NOx control, and balance of precious metal (PGM) usage. Dual-catalyst [Pd +Pt/Rh] systems on a 5.3L V-8 LEV light truck vehicle were characterized as a function of PGM loading, catalyst technology, and substrate cell density. NLEV hydrocarbon emission control of the 6500 lb vehicle was optimal using dual 1.2L converters with each containing front ceria-free Pd catalysts coupled with rear Pt/Rh TWCs. Advanced non-air prototype calibrations coupled with reduced catalyst washcoat mass on 600cpsi/4mil substrate resulted in minimal Pd usage of ∼0.02 toz/vehicle due to achieving catalyst inlet temperatures of 350-400°C in <10 sec on both banks of the V-8 engine.

Dual-brick catalyst systems containing Pd-only catalysts followed by Pt/Rh three-way catalysts (TWCs) are effective emission solutions for both close-coupled and underfloor LEV/ULEV applications due to optimal hydrocarbon light-off, NOx control, and balance of precious metal (PGM) usage. Dual-brick [Pd +Pt/Rh] systems on 3.8L V-6 LEV-calibrated vehicles were characterized as a function of PGM loading, catalyst technology, converter volumes, and substrate cell density. While hydrocarbon emissions improve with increasing Pd loading, decreasing the front catalyst volume at constant Pd content (resulting in higher Pd density) improved light-off emissions. Use of 600cpsi substrates improved underfloor NMHC emissions on a 3.8L vehicle by ∼ 6-10mg/mi compared to 400cpsi catalysts, and thus allowing reduction of catalyst volume while achieving ULEV emission levels without air addition.

Palladium-rhodium catalyst technologies have been investigated to establish the relationship between emission performance and their oxygen storage capacity (OSC) or other physical properties. Catalyst performance was evaluated using stand dynamometer and FTP testing after both oven air aging and engine aging. Monolith catalysts were characterized for aged surface area and precious metal dispersion. Various components of the washcoat supports were characterized by surface area and X-ray diffraction (XRD) analysis for phase composition and CeO2-ZrO2 solid solution crystallite size. The correlation between OSC delay times and tailpipe emissions for NMHC, CO and NOx was highly nonlinear in these studies. Addition of CeO2-ZrO2 solid solution components to the washcoat significantly improved steady state activity after aging, but did not significantly affect the correlation between emissions and OSC.

While often treated as separate entities, there is a significant interaction between engine operating parameters and the catalytic aftertreatment system in determining overall performance. The impressive gains in vehicle emissions and durability required for such marketplaces, as California and Europe provide excellent examples of this interrelationship. Similarly, the Indian marketplace can expect to follow these technology progressions in engine as well as catalyst application design. To progress from an unregulated emissions environment to the first level of catalyzed aftertreatment of relatively simply designed carbureted engines, and then to more sophisticated engines with fuel injection, India can take advantage of what has been learned in other marketplaces worldwide.

The beneficial effects of CeO2/ZrO2 solid solutions on the performance of fully formulated Pt, Rh TWC (three-way-conversion) catalysts were measured using both stand dynamometer and FTP testing after severe engine aging. The performance advantages were consistent with an enhancement of the chemical promotional effects of CeO2. These included increased effectiveness for CO and NOx conversion and to a lesser extent for HC compared to catalysts prepared with the same loading of Ce and Zr but no solid solution formation. Higher performance could be achieved with the CeO2/ZrO2 solid solution catalysts having half the Ce loading of conventional catalysts prepared with pure CeO2. The physico-chemical properties of the catalysts were characterized using both XRD and TPR. XRD was used to determine the degree of solid solution formation between CeO2 and ZrO2 and TPR was used to characterize the redox properties/oxygen storage of the catalysts before and after aging.

Both laboratory and engine dynamometer testing were used to characterize the relative activity of Pt, Pd and Rh supported on Ce and/or La stabilized supports. In the laboratory studies performance was measured after laboratory aging under conditions designed to simulate severe engine aging. The impact of Pt-Rh and Pd-Rh alloying on performance was examined as well as the cumulative effect of both metals on overall activity. The performance of laboratory aged non-alloyed Pt-Rh and Pd-Rh catalysts was dominated by the Rh function. For Pt-Rh the overall performance features for CO and NOx conversion were very similar over the Rh-only, Pt + Rh (separated metals) and alloyed Pt-Rh catalysts. Pt-Rh alloying was found to have a detrimental impact on high temperature HC performance.

Various noble metal compositions are used for three-way catalyst applications. The most typical composition contains platinum and rhodium at various loadings and ratios. Recently palladium and rhodium compositions have received considerable attention by automobile companies. The strengths and weaknesses of the various noble metal use strategies have been widely discussed. Unfortunately, the content for much of the discussion has been based on information generated in the early to mid-1970s with catalysts of relatively simple formulation when compared to today's higher technology products. The present study compares the relative durability performance of modern platinum/rhodium and palladium/rhodium catalysts of identical loading under a variety of aging and evaluation conditions. These conditions were chosen to simulate some of the operating conditions encountered in U.S. and European driving applications.

In studying H2S emissions, it is desirable to have an analytical technique which is rapid, continuous, accurate and easy to use in a laboratory or vehicle exhaust environment. Typically, H2S has been measured using the EPA impinger method with collection times on the order of 1 to 2 minutes. Other techniques have been developed with significantly shorter response times. However, it has been shown that the major release of H2S occurs in less than 20 seconds after a vehicle changes from rich to lean operation. Therefore, it is highly desirable to have an H2S analytical technique with a response time of less than 10 seconds. In this paper, the benefits of use of a chemical ionization mass spectrometer (CIMS) to continuously monitor H2S and SO2, emissions are reported. Using the CIMS technique, the effects of several operating parameters on the release of H2S and SO2 from automotive catalysts were studied.

Recent advances in three-way catalyst formulations have led to significant improvements in durability and performance. These advances for recent Pt/Rh catalyst formulations, for the most part/have been due to a reduction of thermal deactivation. Increased durability plays a critical role in the reducing noble metal usage/meeting tighter emission standards/and extending the durability warranty requirements. In reality, significant advances may be not be readily apparent because of the methods used to evaluate the technology. Some performance benefits may be transparent to particular durability and evaluation procedures, or certain vehicle emission systems. However, as part of an optimized vehicle/catalyst system, the performance benefits may be pronounced. This paper examines the benefits of improved three-way catalyst technologies in order to accelerate their application for tougher emission requirements.

The noble metal palladium (Pd) has the capability of simultaneously converting significant quantities of HC, CO and NOx in automotive exhaust. Primary interests in using palladium-containing TWC catalysts are overall noble metal cost reduction, reduction in rhodium usage and important performance advantages. Dynamometer aging experiments comparing palladium and platinum/rhodium catalysts were conducted under a variety of operating conditions. Vehicle evaluation of these aged catalysts under U.S. FTP-75, European ECE-15 and Japan 10-Mode conditions indicate that palladium-only TWC technology is viable for achieving high levels of three-way control. Vehicle aging studies (25K miles) were also conducted. They confirm the excellent durability results obtained from the dynamometer aging studies: the palladium-only TWC catalyst gave essentially equivalent U.S. FTP-75 and Japan 10-Mode performance to a high-tech platinum/rhodium catalyst.

A brief review of a variety of Inspection/Maintenance programs is provided. The costs and benefits of several of these programs are discussed. The effects of vehicle operating parameters on the performance of the catalyst in the exhaust system of the vehicle are discussed in detail. Key variables for catalyst performance are light-off performance and exhaust temperature and air/fuel ratio. The relative effects of common causes of catalyst deactivation on control of HC, CO and NOx are also discussed.

On a global basis there is a resurgence of interest in the use of palladium in automotive emission control catalysts because of cost, availability and performance advan­tages under certain operating con­ditions relative to more expensive noble metals. This paper reviews a variety of potential vehicle applic­ations for the use of palladium containing catalysts. Included in the study are for the replacement of platinum by palladium in conventional platinum/rhodium systems, palladium-only three-way catalysts, palladium-only dual bed catalysts and two-stroke and lean-burn engine applications.

Various catalytic control strategies must be carefully considered in order to make substantial progress towards meeting che more stringent NOx and hydrocarbon emission standards being proposed for the 1990s. In the development of newer catalyst technologies, this paper discusses the effects of noble metal loadings, catalyst volumes, improved washcoat technologies, base metal promoters/stabilizers, and air/fuel ratio operation on catalyst performance. The effect of various vehicle systems on FTP modal catalyst performance determined the factors influencing NOx control over selected vehicles. The results of these studies indicate that improvements in catalyst technologies will need to be systematically coupled with improvements in system control technology to simultaneously optimize the total emission system to achieve the more stringent standards being proposed.

An H2S odor problem has appeared for certain vehicles fitted with modern three-way catalysts. A sulfur storage/H2s release mechanism is proposed as a source of the odor problem. The effects of various operating parameters on the release of H2S are presented. Two methods of modifying three-way catalysts to minimize H2S release while maintaining good catalyst performance and high temperature durability are demonstrated.

Durability performance characteristics of copper-containing base metal catalysts and base metal/low noble metal catalysts have been determined in laboratory and engine aging conditions under well controlled stoichiometric closed-loop A/F operation. Cu-Cr base metal formulations yield significant HC and CO conversions under stoichiometric operation after aging at 620°C, but deteriorate rapidly at high-temperature (750°C inlet) stoichiometric operation. Incorporation of Rh into the base metal formulation substantially improved NOx performance, a major weakness of base metal catalysts. The addition of Cu-Cr base metals substantially improves CO oxidation over Pt, Pd, and Rh catalysts but was accompanied by some loss of HC conversion over Pt and Rh. A Cu-Cr/Pd catalyst, however, also had better HC conversions as well as significantly improved light-off performance when compared to a Pd-only catalyst.

On occasions automotive fuels have been contaminated by adventitious admixtures of silicon (Si)-containing compounds which have deleterious effects on automotive catalysts and oxygen sensors. The deactivation of monolithic automotive catalysts by fuel-derived silicon is due to deposition of crystalline silica (∝-SiO2) on the catalyst surface which causes mass transfer limitations and may ultimately result in plugging of the monolith. Stoichiometric conversions efficiency of three-way catalysts (TWCs) from various low-mileage vehicles were significantly deteriorated; e.g., from typical three-way efficiencies of −95% conversion to <50% conversion at 550°C after only 1500 mi of vehicle use. Laboratory aging of a TWC exposed to combustion products of isooctane fuel containing 20 ppm Si resulted in a continual decline in three-way conversions to <40% after 15,000 simulated miles.

The durability of automotive three-way catalysts (TWCs) for European applications were investigated as a function of higher temperatures encountered in autobahn driving modes over extended periods of time, potentially higher residual lead (Pb) levels anticipated in European marketed unleaded fuels, and occasional misfueling with leaded fuels. In laboratory durability and dynamometer aging studies, platinum-rhodium (Pt-Rh) TWCs at higher loadings than currently used in US applications maintained substantial three-way conversions when aged under rich conditions (λ ∼ 0.9) at maximum temperatures of ∼ 900 to 1000°C with 3 mg Pb/L fuel levels. Increasing maximum catalyst aging temperatures from 730°C to 1000°C resulted in ∼50% reduction in BET surface area which increased stoichiometric hydrocarbon light-off temperatures, but improved net NO and HC conversions after light-off due to lower Pb retention on the TWC.

The deactivation of automotive catalysts by engine oil-derived components of phosphorus and zinc can occur by the formation of an amorphous zinc pyrophosphate (Zn2P2O7) that is impervious to gas diffusion. The catalyst poison, derived from antiwear oil additive zinc dialkyl dithiophosphate (ZDP) in low-temperature exhaust environments, appears as glassy, amorphous deposits on catalysts as shown by scanning electron microscopy (SEM). Laboratory studies were performed to understand the effects of exhaust stoichiometry, temperature, rate of oil burn, and chemical form of P and Zn compounds on glaze formation. The formation of the amorphous deposits using a laboratory pulsator apparatus showed that noncombusted ZDP causes the glaze formation. Electron microprobe studies indicated the association of P with Zn on precious metal films exposed to ZDP combustion products. Secondary ion mass spectrometry (SIMS) confirmed a similar P to Zn correspondence on the vehicle-aged catalysts.

Post-mortem analyses of 50,000 mi (80,000 km) vehicle-aged catalysts revealed that the use of 0.125g Mn/gal (33 mg/L) as MMT (methyl-cyclopentadienyl manganese tricarbonyl) significantly reduces phosphorus and zinc retention levels at the catalyst inlets by ~20-fold and ~5-fold, respectively. In subsequent laboratory pulsator experiments the presence of 0.016 to 0.157g Mn (as MMT)/gal (4 to 41 mg/L) isooctane fuel containing a 10-fold excess of ZDP (zinc dialkyldithiophosphate, source of oil P and Zn) similarly reduced the retention of P and Zn on TWCs by proportional amounts, while the TWCs maintained significantly higher 3-way conversions than in the absence of MMT. The combustion of Mn from MMT to very stable Mn3O4 probably serves as a scavenger in the exhaust for transporting away fuel- and oil-derived catalyst poisons such as P, Zn, and Pb. The utility of the laboratory results will require verification in vehicle studies.

Rhodium supported catalysts capable of withstanding temperatures above 600°C under oxidizing conditions while maintaining a resistance to chemical poisons have been developed by reducing the undesirable interaction of Rh2O3, with γ-alumina support material. The impregnation of Rh on a zirconia (ZrO2) washcoat provides a well dispersed, thermally-stable active phase. When the Rh/ZrO2 phase is in turn supported on a high surface area γ-Al2O3 washcoated monolith, the resulting (Rh/ZrO2)/γ-Al2O3 catalyst also has sufficient surface area for dispersion of other active metals, as well as to provide a sink for fuel-and oil-derived contaminants. Upon heating at 850°C in air, the Rh area is decreased by 95% when supported on γ-Al2O3 but is lowered only by 15% when ZrO2 is used to separate Rh from γ-Al2O3.