1c:I[419806,["/_next/static/chunks/bb923814e1f4d2e9.js","/_next/static/chunks/dd3ee9f5b7716ca6.js","/_next/static/chunks/4204a0ae82542ca8.js","/_next/static/chunks/1270dead247da766.js","/_next/static/chunks/6e1aa89a642f40be.js","/_next/static/chunks/2d7aa753e81324a8.js","/_next/static/chunks/8ffd0b71567920d8.js","/_next/static/chunks/67ea6dfd21f16c5f.js","/_next/static/chunks/24eb4379f68013fd.js","/_next/static/chunks/b8cd775ffdf935bd.js","/_next/static/chunks/983103cd9eb7b428.js","/_next/static/chunks/263f2cdcb9aa2341.js","/_next/static/chunks/294b9525248c8e5f.js"],"default"] 1d:T43f,This review covers the literature through 1991 on nitric oxide catalysis as applied to automobile exhaust systems. Attention is given to the threeway catalyst system which simultaneously promotes the reduction of nitrogen oxides and the oxidation of carbon monoxide and hydrocarbons. These systems have been used on most passenger cars in the United States since 1982. Prior to 1980, emission control catalysts were oxidation catalysts, and reduction in exhaust nitric oxide was achieved using engine modifications (i.e., exhaust gas recirculation). This review focuses on catalytic control of NO, for gasoline-fueled vehicles (not diesels and alternate fuels) and primarily on developments reported since 1982. The term NO, refers to both NO and NOz. The reader is referred to an earlier publication by the author for a general review of automobile catalytic converters [1] and to a review by Egelhoff [2] on the nitric oxide literature through 1980. The recent literature on NO, reduction in lean exhaust is cited although such catalyst systems have not reached commercial application.1e:T53e,The low-temperature behavior of the selective catalytic reduction (SCR) process with feed gases containing both NO and NO₂ was investigated. The two main reactions are 4NH₃ + 2NO + 2NO₂ → 4N₂ + 6H₂O and 2NH₃ + 2NO₂ → NH₄NO₃ + N₂ + H₂O. The "fast SCR reaction" exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO and dominates at temperatures above 200 °C. At lower temperatures, the "ammonium nitrate route" becomes increasingly important. Under extreme conditions, e.g., a powder catalyst at T ≈ 140 °C, the ammonium nitrate route may be responsible for the whole NO_x conversion observed. This reaction leads to the formation of ammonium nitrate within the pores of the catalyst and a temporary deactivation. For a typical monolithic sample, the lower threshold temperature at which no degradation of catalyst activity with time is observed is around 180 °C. The ammonium nitrate route is interesting from a standpoint of general DeNO_x mechanisms: This reaction combines the features typical to the SCR catalyst with the features of the NO_x storage-reduction catalyst, i.e., NO_x adsorption to a basic site.1f:T6f5,

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