The removal of nitrogen oxides present in the gases emitted from sources such as electric power generation boilers, stationary internal combustion engines and gas turbine engines— all of which are likely to use excess oxygen to achieve maximum fuel efficiency—can be achieved by a selective reduction process using ammonia as the reducing agent, the reaction being carried out over a base metal oxide catalyst. It had been considered that hydrocarbons were ineffective for this reaction but recent work has indicated that with suitable catalysts it may be possible to use them in a process which removes nitrogen oxides from the exhaust gases of both diesel and lean-burn gasoline-fuelled engines. For practical application, high activity under high space velocity and also high selectivity would be required.

The activity and durability of a variety of catalysts have been investigated, but to-date there has been only little interest in using the plat-inum metals as catalysts for this purpose. Now, however, a team at the National Institute for Resources and Environment, Tsukuba, Japan, have investigated the performance of platinum, palladium, rhodium, iridium and ruthenium supported on γ-alumina as catalysts for this application (A. Obuchi, A. Ohi, M. Nakamura, A. Ogata, K. Mizuno and H. Ohuchi, “Performance of Platinum-Group Metal Catalysts for the Selective Reduction of Nitrogen Oxides by Hydrocarbons”, Appl. Catal. B: Environ., 1993, 2, (1), 71–80).

The addition of some hydrocarbons to the exhaust is necessary to compensate for the greater amount of nitrogen oxides generally emitted from combustors operating under netoxidising conditions, and the properties of propene favour its use as the reducing agent. With both model mixtures and real diesel exhaust gases it was found that platinumrhodium/γ-alumina displayed high activity for nitrogen oxides conversion, at 200 to 350°C. These catalysts are similar to the three-way catalysts used for controlling emissions from gasoline fuelled engines. It is concluded that platinum metals catalysts will find practical usage for this purpose, especially if their selectivity to nitrogen is improved.