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Technical Paper

Nitrous Oxide Formation Over Three-Way Catalyst

The formation of Nitrous Oxide (N2O) over an aged three way catalyst was analysed in a laboratory reactor for a variety of simulated Otto engine exhaust gas conditions. Nitrous Oxide formation was further analysed during FTP75 dynamometer test with a car. The car was equipped with either an aged catalyst or a fresh one. A fast response diode laser system was modified to enable detection of Nitrous Oxide and Carbon Monoxide simultaneously. From laboratory data the kinetics of Nitrous Oxide formation were evaluated with mathematical simulations and a mechanism was suggested. The results were compared to data from vehicle tests and the results were discussed in the light of the laboratory study. Two general trends were confirmed, i) N2O formation increases at slightly lean conditions: ii) catalysts with a low degree of deterioration gave lower N2O emissions, iii) the extent of N2O formation goes though a maximum with respect to dissociation rate of NO.
Technical Paper

Deterioration of Three-Way Automotive Catalysts, Part II - Oxygen Storage Capacity at Exhaust Conditions

Catalysts aged under different on-road conditions were analysed with respect to their conversion of CO and HC at step changes of the synthetic exhaust gas composition. Time resolved diode laser spectroscopy and fast response FID analysis were used to characterise the catalyst response to transient changes of CO and hydrocarbons in the exhaust gas. The oxygen storage capacity was monitored at various conditions; flow rate, catalyst temperature, previous exposure to oxidizing or reducing atmosphere and amplitude of the perturbation. The technique appeared to provide a sensitive probe for analysis of the dynamic oxygen storage capacity of new and aged catalysts at exhaust like conditions. The results correlate well with the transient emission performance during vehicle tests. Further, surface characterization using SEM/EDS and XPS techniques indicated that phosphate formation was the most probable cause of deactivation.
Technical Paper

Deterioration of Three-Way Automotive Catalysts, Part I - Steady State and Transient Emission of Aged Catalyst

Five field-aged catalysts with different mileages were analysed with respect to emission performance and structural changes. The FTP-75 emission results were compared to synthetic exhaust gas tests including: i) light-off, ii) lambda screening at stationary and oscillating stoichiometry, iii) space velocity variation. Several samples from different positions of one catalyst were used to achieve the spatially resolved activity profile for that catalyst. Surface characterisation was used to characterise accumulated catalyst poison. Laboratory space velocity test was concluded to be a sensitive probe for catalyst performance: good correlation to vehicle emission data was found. An analysis of the influence of temperature and λ oscillation on the catalyst conversion performance was made, with particular emphasis on the ageing effects.