Mats Laurell

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 oscillation on the catalyst conversion performance was made, with particular emphasis on the ageing effects. The oxidation of hydrocarbons and CO, and the reduction of NO was significantly higher during oscillations at 753 K than at steady state, while this effect was not found at 653 K. The spatially resolved activity profile revealed large differences between the oxidation and NO reduction activities, the latter being the most significantly deteriorated catalyst function. For the covering abstract see IRRD 858138.

Hydrothermal Aging-Induced Changes in Washcoats of Commercial Three-Way Catalysts

In order to quantify hydrothermal aging effects on the mono- and bi-metallic Pd and Rh supported on cerium-zirconium promoted alumina commercial three-way catalysts (TWCs), the catalysts were tested both fresh and after accelerated hydrothermal aging. The catalytic tests showed clear deactivation of the aged samples and influence on TWC’s property, such as: light-off temperature, specific surface area (BET), dispersion of noble metals, oxygen storage capacity, oxygen storage capacity complete and labile oxygen storage capacity. Water inhibition have been investigated and confirmed for the performance of NOx reduction of the fresh catalyst. X-ray photoelectron spectroscopy was used to study changes in the oxidation state of the Pd and Rh present in the washcoat of the catalyst, before and after hydrothermal aging. Element maps by SEM-EDX analysis was perform in order to characterize the catalysts morphology, the surface’s composition and element distribution.

A Metal Substrate with Integrated Oxygen Sensor; Functionality and Influence on Air/Fuel Ratio Control

In order to achieve ultra low emission levels with three-way catalysts, an early accurate air/fuel ratio control is essential. Positioning the oxygen sensor in the first part of the substrate helps to protect the oxygen sensor from being splashed by wat er during cold start, so that early heating and activation becomes a less limiting factor. For emission control purpose, a position of a rear sensor in the warm part of the catalyst gives improved possibilities for oxygen buffer control during catalyst warming up conditions. This enhances balancing HC and NOx in an early phase. In addition, for OBD reasons it is possible to locate the sensor in any axial position in the catalyst, which improves design possibilities for cold start detection, even for single brick catalyst systems. The paper describes the construction of the catalyst with an integrated oxygen sensor. In addition, it details flow distribution measurements with and without internal mixing at the position of the oxygen sensor. Test results of the influence of an integrated oxygen sensor on lambda control of a SULEV concept engine demonstrate its function.

A Comparison of Fuel-Cut Ageing during Retardation and Fuel-Cut during Acceleration

The effect of various fuel-cut agings, on a Volvo Cars 4-cylinder gasoline engine, with bimetallic three-way catalysts (TWCs) was examined. Deactivation during retardation fuel-cut (low load) and acceleration fuel-cut (high load, e.g. gearshift or traction control) was compared to aging at =1. Three-way catalysts were aged on an engine bench comparing two fuel-cut strategies and their impact on of the life and performance of the catalysts. In greater detail, the catalytic activity, stability and selectivity were studied. Furthermore, the catalysts were thoroughly analyzed using light-off and oxygen storage capacity measurements. The emission conversion as a function of various lambda values and loads was also determined. Fresh and 40-hour aged samples showed that the acceleration fuel-cut was the strategy that had the highest contribution towards the total deactivation of the catalyst system. Also, the retardation fuel-cut was found to be detrimental to the catalyst system but not to the same extent as an acceleration fuel-cut. During the aging procedure, exotherms were observed at the fuel-cut and the intensity of these exotherms was increasing with the length of aging time. The growing exotherms could be explained by the decomposition of HC into C and H2 and their subsequent oxidation at lean conditions. Also, the fuel-cut-off temperature measurements demonstrated that the magnitude of those exotherms was related to the total number as opposed to the total length of the fuel-cut. Copyright

Comparison of Lab Versus Engine Tests In the Development of a Highly Efficient Ammonia Formation Catalyst for a Passive SCR System

Commercial three way catalysts have limited capacity towards reducing NOx in the presence of excessive oxygen. This prevents lean-burn combustion concepts from meeting legislative emission standards. A solution towards decreasing NOx emissions in the presence of excess air is the use of a passive-SCR system. Under rich conditions ammonia is formed over an ammonia formation catalyst, the ammonia is stored in the SCR and in its turn reacts with the NOx under lean engine conditions. Here up-scaled Pt/Al2O3 and Pd/Al2O3 catalysts as well as a commercially Pd-Rh based three-way catalyst (TWC) are evaluated using both engine and further lab-scale tests. The purpose of these tests is to compare the ammonia production for the various catalysts under various lambda values and temperatures by means of engine and lab scale tests. The Pd/Al2O3 showed little sensitivity to temperature both under engine and lab scale experiments. The Pt/Al2O3 was affected to a large extend by temperature for both test methods. The TWC showed stable production during the engine measurements while under lab tests an increased temperature resulted in a lower ammonia yield. Differences between the engine and lab scale tests are mainly due to catalyst temperatures, space velocity, CO poisoning and uncertainties in the composition of the engines exhaust gas. Both Pt/Al2O3 and Pd/Al2O3 form ammonia although the former generate higher amounts at high temperature but are believed to suffer from CO poisoning at low temperatures.

Fuel-cut based rapid aging of commercial three way catalysts - Influence of fuel-cut frequency, duration and temperature on catalyst activity

In order to quantify fuel-cut aging effects on commercial bimetallic Pd/Rh three-way catalysts (TWCs), supported on cerium-zirconium promoted alumina, full-size automotive catalysts were exposed to accelerated fuel-cut aging on an engine test bench, with a variation in temperature, fuel-cut frequency and fuel-cut duration. After aging, samples of the catalysts were tested in a laboratory environment for Light-off temperature (T50), Specific surface area (BET), Dispersion of noble metals and changes in the oxidation state of Pd and Rh. The catalytic tests showed clear deactivation of the aged samples and influence on the TWCs properties. The light off temperature and noble metal dispersion were found to be a clear function of oxygen exposure to the catalysts, i.e. fuel-cut frequency and duration, while the specific surface area was found to be a function of fuel-cut frequency. No changes in oxidation states of Pd and Rh could be detected. The variation in temperature had the lowest influence on the TWC properties among the varied parameters.