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Technical Paper

High Temperature Stability of Ceria-Zirconia Supported Pd Model Catalysts

A series of ceria and ceria-zirconia supported Pd model automotive catalysts were prepared and aged under air or redox conditions at 1050°C for 12 h. The supports were manufactured by different methods and represent a range of compositions. The samples were characterized before and after aging by BET, X-ray diffraction, mercury porosimetry, XPS, H2 temperature-programmed reduction, and oxygen storage capacity measurements. Oxygen storage measurements revealed that the behavior of the catalysts varied according to aging conditions and temperature of measurement. Pd/ceria-zirconia catalysts showed higher oxygen storage characteristics after 1050°C aging than Pd/ceria catalysts, and the phase purity of the ceria-zirconia was shown to positively affect the amount of oxygen storage. The initial rates of oxygen release from the model catalysts at 350°C were shown to depend on the preparation conditions of the supports.
Technical Paper

Laboratory Study to Determine Impact of Na and K Exposure on the Durability of DOC and SCR Catalyst Formulations

A laboratory flow reactor study was utilized to determine the durability impact of alkali metal (Na and K) exposure on three Pt/Pd-based diesel oxidation catalysts (DOC), two vanadium-based selective catalytic reduction (SCR) catalysts, and two Cu/zeolite-based SCR catalysts. All catalyst samples were contaminated by direct deposition of Na or K by an incipient wetness technique. The activity impact on the contaminated DOCs was accomplished by evaluating for changes in CO and HC light-off. The activity impact on the contaminated SCR catalysts was accomplished by evaluating for changes in the Standard SCR Reaction, the Fast SCR Reaction, the Ammonia Oxidation Reaction, and the Ammonia Storage Capacity. Contamination levels of 3.0 wt% Na was found to have a higher negative impact on Pt-based and zeolite containing DOCs for T-50 CO and HC light-off.
Technical Paper

Reductive Elimination as a Mechanism for Purging a Lean NOx Trap

The mechanism for the purging of a lean NOx trap has been investigated. For realistic purge times (e.g., 2 to 5 seconds), the stored NOx species do not decompose simply from equilibrium considerations (i.e., from the drop in O2 and NO concentrations during the rich purge). Instead, the decomposition of stored NOx is promoted by the reductants in the exhaust by a process referred to as reductive elimination. H2 is far more effective than CO or C3H6 for promoting this reductive elimination, particularly at low temperatures (e.g., 250°C). As long as H2 is available in the feedgas, H2O does not participate in the reductive elimination. However, if CO is the only reductant, H2O is needed to convert some of the CO to H2 through the water-gas-shift reaction. H2O is also important for the efficient storage of NOx during lean operation, possibly by enhancing the spillover of NO2 from a precious metal site to a NOx storage site.
Journal Article

A New Catalyzed HC Trap Technology that Enhances the Conversion of Gasoline Fuel Cold-Start Emissions

Passive in-line catalyzed hydrocarbon (HC) traps have been used by some manufacturers in the automotive industry to reduce regulated tailpipe (TP) emissions of non-methane organic gas (NMOG) during engine cold-start conditions. However, most NMOG molecules produced during gasoline combustion are only weakly adsorbed via physisorption onto the zeolites typically used in a HC trap. As a consequence, NMOG desorption occurs at low temperatures resulting in the use of very high platinum group metal (PGM) loadings in an effort to combust NMOG before it escapes from a HC trap. In the current study, a 2.0 L direct-injection (DI) Ford Focus running on gasoline fuel was evaluated with full useful life aftertreatment where the underbody converter was either a three-way catalyst (TWC) or a HC trap. A new HC trap technology developed by Ford and Umicore demonstrated reduced TP NMOG emissions of 50% over the TWC-only system without any increase in oxides of oxygen (NOx) emissions.
Journal Article

Enhanced Durability of a Cu/Zeolite Based SCR Catalyst

Passenger and light duty diesel vehicles will require up to 90% NOx conversion over the Federal Test Procedure (FTP) to meet future Tier 2 Bin 5 standards. This accomplishment is especially challenging for low exhaust temperature applications that mostly operate in the 200 - 350°C temperature regime. Selective catalytic reduction (SCR) catalysts formulated with Cu/zeolites have shown the potential to deliver this level of performance fresh, but their performance can easily deteriorate over time as a result of high temperature thermal deactivation. These high temperature SCR deactivation modes are unavoidable due to the requirements necessary to actively regenerate diesel particulate filters and purge SCRs from sulfur and hydrocarbon contamination. Careful vehicle temperature control of these events is necessary to prevent unintentional thermal damage but not always possible. As a result, there is a need to develop thermally robust SCR catalysts.
Journal Article

Detection, Origin and Effect of Ultra-Low Platinum Contamination on Diesel-SCR Catalysts

This paper discusses the poisoning of a selective catalytic reduction (SCR) catalyst by trace levels of platinum originating from an upstream diesel oxidation catalyst (DOC). A diesel aftertreatment system consisting of a DOC, urea based SCR Catalyst and a DPF was aged and evaluated on a 6.4 liter diesel engine dynamometer. The SCR catalyst system consisted of an Fe-zeolite catalyst followed by a Cu-zeolite catalyst. After approximately 400 hours of engine operation at varied exhaust flow rates and temperatures, deactivation of the SCR catalyst was observed. A subsequent detailed investigation revealed that the Cu catalyst was not deactivated and the front half of the Fe-based catalyst showed severe deactivation. The deactivated portion of the catalyst showed high activity of NH3 conversion to NOx and N2O formation. The cause of the deactivation was identified to be the presence of trace Pt contamination.
Journal Article

Impact and Prevention of Ultra-Low Contamination of Platinum Group Metals on SCR Catalysts Due to DOC Design

Diesel aftertreatment systems configured with a diesel oxidation catalyst (DOC) upstream of an urea selective catalytic reduction (SCR) catalyst run the risk of precious metal contamination. During active diesel particulate filter (DPF) regeneration events, the DOC bed temperature can reach up to 850°C. Under these conditions, precious metal (especially Pt) can be volatized and then deposited on a downstream SCR catalyst. In this paper, the impact of ultra-low contamination of platinum group metals (PGM) on the SCR catalyst was studied. A method based on precious metal volatilization of a Pt-rich DOC at 850°C and under lean gas conditions was employed to contaminate downstream FeSCR and CuSCR formulations. The contamination resulted in poor NOx conversion (via NOx remake) and excessive N2O formation. The precious metal volatilization method was employed to screen various Pt/Pd based DOCs to avoid contamination of the downstream FeSCR.