4.2. Ageing procedures
Catalysts were aged to simulate the high temperature conditions in the exhaust gas streams. The aged catalysts were prepared from the fresh catalyst by using different thermal ageing procedures, as described in Table 7. In addition to those presented in Table 7, the effect of some other ageing atmospheres, such as 5 % O2/N2 were also tested. Thermal and hydrothermal ageings were accomplished in a tubular furnace (inlet diameter 40 mm and length 1500 mm) in the temperature range of 800° to 1200°C, and varying the ageing times (3, 24, or 42 hours). Figure 14 presents the experimental set-up for the ageing furnace. The temperature of the ageing furnace was monitored by a thermocouple that was in close contact with the catalysts, and several catalysts were aged simultaneously. The total gas volumetric flow rate during the ageing was 1 dm3/min (25°C, 1 atm). The standard size of the cylindrical catalyst in the laboratory tests was 1.4 cm3 by volume (length 28 mm and diameter 8 mm). Therefore, GHSV was approximately 43 000 h-1 during the ageing procedure.
Table 7. Ageing procedures.
| Gas feed | Temperatures | Duration |
|---|---|---|
| 1. Oxidative ageing (air) | 800°–1200°C | 3, 24 or 42 hours |
| 2. Reductive ageing (5 vol-% H2 and N2 balance) | 800°–1200°C | 3, 24 or 42 hours |
| 3. Inert ageing (nitrogen) | 800°–1200°C | 3 or 24 hours |
| 4. Hydrothermal oxidative ageing (air and 10 vol-% H2O) | 800°–1200°C | 3 hours |
| 5. Hydrothermal reductive ageing (5 vol-% H2, 10 vol-% H2O and N2 balance) | 800°–1200°C | 3 hours |
| 6. Engine ageing (exhaust gas) | 1030°–1060°C | 40 hours |
| 7. Vehicle ageing (exhaust gas) | 100 000 km |
In the thermal ageing procedure, catalysts were treated in oxidative (air), inert (nitrogen), and reductive (5% H2/N2) ageing atmospheres. Hydrothermal ageings were accomplished in oxidative (H2O/air) and reductive (H2O/H2) ageing atmospheres in the presence of 10 vol-% water vapour, which corresponded to the water flow of 0.1 cm3/min into the furnace. Ageing temperatures higher than 1200°C were not used in this study because those temperatures are not present in normal operating conditions. Engine ageing was carried out in the exhaust gas stream of a V8 engine for 40 hours. The ageing procedure was composed of rich (50 min, 1030°C, λ =0.98-0.99) and stoichiometric (10 min, 1050°–1060°C, λ =1.00) air-to-fuel ratios carried out consecutively. The vehicle ageing was accomplished under European driving conditions (100 000 kilometres). Kemira Metalkat Oy (Finland) provided the industrial reference catalysts (fresh, engine-aged and vehicle-aged). All the catalysts were identical in chemical composition and thus only the changes induced by ageings were considered. The engine-aged and vehicle-aged catalysts were also divided into different test zones (as presented in Fig. 15) in order to evaluate the effect of the position-dependent phenomena. In particular, the characterization results of different test zones were used when evaluating the role of poisons on the catalytic activity.
