Verfahren und Reaktor zur Entfernung von Stickoxiden aus Sauerstoff enthaltenden Abgasen von Verbrennungskraftmaschinen

1384248 Treating exhaust gas IMPERIAL CHEMICAL INDUSTRIES Ltd 25 April 1972 [7 May 1971] 13722/71 Heading F1B A process of treating I.C. engine exhaust gas to remove nitrogen oxides, the gases containing oxygen as a result of incomplete combustion in the engine and also oxidizable constituents in excess of what will react with the oxygen, which comprises passing the gases in a first stage over a catalyst having primarily an oxidation activity until a substantial proportion of the oxygen has been reacted and then in a second stage over a catalyst having primarily a reduction activity until nitrogen oxides are substantially decomposed. The first catalyst is a platinium gauge or palladium or platinium on a support. The second catalyst is a base metal, i.e. nickel or cobalt and may also contain copper. Alternatively the second catalyst may be ruthenium. The volume of the first catalyst is 0.02 to 0.2 times the volume of the second catalyst when both are in the same physical form. The space velocity over the first catalyst is 0.15 to 3.0 x 106 hour -1 and the velocity over the second catalyst is 0.15 to 3.0 x 105 hour -1. A third treatment stage involving the addition of air and catalytic oxidation may be provided. The reaction temperature in the second stage is over 700‹C e.g. 800-950‹C when a base metal catalyst is used and is 300-700‹C e.g. 300- 500‹C when a precious metal catalyst is used. The first stage catalyst is operated at 400-1000‹C depending on the temperature required at the inlet to the second stage. The oxygen content of the gas entering the first stage is typically 0.5% and this is preferably reduced to less than 0.15%: Catalysts The oxidation catalyst can also be a metallic oxide or oxide mixture selected from the transition metals such as copper, titanium, tin, lead, phosphorus, bismuth, vanadium, niobium, tantalum, chromium, tungsten, manganese and rhenium. Preferably it contains a metal or alloy from Group VIII of the Periodic Table e.g. rhodium, palladium or platinum. The support may be highly calcined magnesia, alumina, mullite. spinel, zirconia, zirconium silicate, hafnia or thoria and may be combined with a low-silica hydraulic cement. The catalytic material is suitably supported on calcined clay/cement granules.