Tham khảo tài liệu 'internal combustion engines fundamentals episode 2 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 650 INTERNAL COMBUSTION ENGINE FUNDAMENTALS Ế FIGURE 11-53 Catalytic converters for spark-ignition engine emission control a monolith design b pelletized POLLUTANT FORMATION AND CONTROL 651 running lean of stoichiometric or with a pump that introduces air into the exhaust ports just downstream of the valve. Venturi air addition into the exhaust port using the pressure pulsations generated by the exhaust process can also be used to add the required air. Because of their high intrinsic activity noble metals are most suitable as the catalytic material. They show higher specific activity for HC oxidation are more thermally resistant to loss of low-temperature activity and are much less deactivated by the sulfur in the fuel than base metal oxides. A mixture of platinum Pt and palladium Pd is most commonly used. For the oxidation of co olefins and methane the specific activity of Pd is higher than that of Pt. For the oxidation of aromatic compounds Pt and Pd have similar activity. For oxidation of paraffin hydrocarbons with molecular size greater than C3 Pt is more active than Pd. Pure noble metals sinter rapidly in the 500 to 900 C temperature range experienced by exhaust catalysts. Since catalytic behavior is manifested exclusively by surface atoms the noble metals are dispersed as finely as possible on an inert support such as y-A2O3 which prevents particle-to-particle metal contact and suppresses sintering. The particle size of the noble metal particles in a fresh catalyst is less than 50 nm. This can increase to 100 nm when the catalyst is exposed to the high temperatures of the exhaust in vehicle operation. Typical noble metal concentrations in a commercial honeycomb catalyst are between 1 and 2 g drn3 of honeycomb volume with Pt Pd 2 on a weight basis. As a rough rule of thumb the ceramic honeycomb volume required is about half the engine displaced volume. This gives a space velocity through the converter volume flow rate of exhaust divided by converter
