Dennys Angove

Nitrous oxide formation during the reaction of simulated exhaust streams over rhodium, platinum and palladium catalysts

Abstract The formation of nitrous oxide during the reaction of a mixture of CO, NO, C3H6, C3H8, H2 and O2 over supported rhodium, platinum and palladium catalysts has been investigated under near-stoichiometric conditions. Rhodium gives the highest amount of N2O with a peak selectivity near 70% at 250°C followed by a steady decline to low levels by 400°C. With Pt N2O is seen in a narrow region just above 300°C and is removed at higher temperatures during oxidation of propane. Ammonia, probably produced via isocyanic acid, is the chief nitrogen-containing product of the reaction over Pt/Al2O3 below 300°C, but this route is much reduced when CeO2/Al2O3 is the support. Palladium-containing catalysts give rise to N2O in two regimes. It arises below 200°C by the reaction of NO with H2 and above 300°C by another process which unlike Rh and Pt, persists through to 500°C. The behaviour of a trimetal catalyst containing Pd, Pt and Rh can be interpreted as a composite of that of the individual metals. Pt/Rh-containing catalytic converters recovered from vehicles show pronounced differences in N2O production which correlate with a reduced ability to remove propane. The most probable cause of such changes is sintering of the platinum component as a result of exposure to high temperature. This may be the origin of the increased N2O production seen in driving cycle tests with high mileage vehicles.

The effects of residual chlorine on the behaviour of platinum group metals for oxidation of different hydrocarbons

Abstract The oxidation of a mixture of four hydrocarbons and carbon monoxide has been studied over alumina-supported palladium, platinum and rhodium catalysts made using chloride precursor salts. The order of the light-off temperatures for the hydrocarbons is the same for each metal (1-hexene, toluene, benzene and finally iso-octane) but the separation between each and their occurrence relative to carbon monoxide varies. The absolute values, especially for iso-octane, are strongly dependent on the pretreatment procedure. Exposure to H2/H2O in the place of O2 gives much higher activity. Activation can also be achieved by repeated runs with the reactant mixture. Chlorobenzene is produced in small amounts during experiments using partially activated catalysts especially with rhodium. Other reasons for believing that the development of activity is at least partly associated with the removal of chlorine are discussed.

Position dependent phenomena during deactivation of three-way catalytic converters on vehicles

Mapping procedures have been developed to characterise some of the position dependent phenomena occurring in three-way catalytic converters. The activity of small samples for the removal of CO, NO, propene and propane from a simulated mixture under slightly lean conditions was measured in a flow system and correlated with surface area. Contamination was determined by particle-induced X-ray emission (PIXE) with XRD used to follow structural changes in the washcoat. The procedures have been used to investigate a substantial set of converters which had seen extensive use on vehicles. Three of these converters, taken from vehicles which had failed a standard emission test, are discussed here. In one case, loss of surface area and CO/hydrocarbon/NO activity was greater at the front and is associated with phosphorus deposition. XRD measurements showed that operating temperatures were sufficiently high to result in the formation of cerium orthophosphate at the front and substantial growth in ceria particle size throughout which also contributed to activity loss. A second converter showed substantial loss of NO activity alone which was traced to high levels of lead, concentrated towards the front but significant throughout. A third converter of the same type had undergone a lesser loss of activity for NO removal at the front due to lead but the deterioration in CO and propene was greater towards the rear of the converter. This was associated with a loss of surface area caused by a period of overheating under net reducing conditions with XRD measurements showing the formation of cerium aluminate and a cerium barium magnesium hexaaluminate which are characteristic of such conditions.

The formation of benzene and chlorobenzene during the oxidation of toluene over rhodium-based catalysts

The oxidation of toluene and benzene in a four hydrocarbon mixture with and without carbon monoxide present has been investigated over Rh/Al2O3 and Rh/CeO2/Al2O3 catalysts containing residual chlorine. Up to 12% of toluene can be converted to benzene with a near stoichiometric feed containing carbon monoxide. Much less benzene is formed in the absence of carbon monoxide, especially with the ceria–alumina based catalyst which is more active for the oxidation of the aromatics. Formation of benzene probably proceeds within areas where toluene coverages are sufficiently high to provide the hydrogen atoms required for addition to a benzyl intermediate. With fresh catalysts chlorobenzene can be formed as a byproduct in concentrations up to 10% of that of the aromatic in the feed. Toluene is the largest source of chlorobenzene with the alumina-based catalyst when CO is present. Benzene is a larger source with Rh/CeO2/Al2O3 when the feed is dry but with water present the behaviour resembles that of Rh/Al2O3. The processes involved in chlorobenzene formation are uncertain but may include migration of chlorine from alumina to rhodium or the metal–support interface.

Development of a new smog chamber for studying the impact of different UV lamps on SAPRC chemical mechanism predictions and aerosol formation

Environmental context Chemical mechanisms are an important component of predictive air quality models that are developed using smog chambers. In smog chamber experiments, UV lamps are often used to simulate sunlight, and the choice of lamp can influence the obtained data, leading to differences in model predictions. We investigate the effect of various UV lamps on the prediction accuracy of a key mechanism in atmospheric chemistry. Abstract A new smog chamber was constructed at CSIRO following the decommissioning of the previous facility. The new chamber has updated instrumentation, is 35 % larger, and has been designed for chemical mechanism and aerosol formation studies. To validate its performance, characterisation experiments were conducted to determine wall loss and radical formation under irradiation by UV lamps. Two different types of blacklights commonly used in indoor chambers are used as light sources, and the results using these different lamps are investigated. Gas-phase results were compared against predictions from the latest version of the SAPRC chemical mechanism. The SAPRC mechanism gave accurate results for hydrocarbon reaction and oxidation formation for propene and o-xylene experiments, regardless of the light source used, with variations in ozone concentrations between experiment and modelled results typically less than 10 % over 6-h irradiation. The SAPRC predictions for p-xylene photooxidation showed overprediction in the rate of oxidation, although no major variations were determined in mechanism results for different blacklight sources. Additionally, no significant differences in the yields of aerosol arising from new particle formation were discernible regardless of the light source used under these conditions.