The Case for Selective Ammonia Oxidation Catalysts in On Road SCR Systems

Joseph  Patchett, Engelhard Corporation

Unreacted ammonia leaving an SCR reactor must be limited to prevent particulate formation and the irritating odor associated with ammonia. Changes in the NOx conversion with aging time of SCR catalysts is usually manifested by a decrease in NOx conversion with a corresponding increase in unreacted ammonia. The decline in NOx conversion can be recovered by increasing the ammonia to NOx ratio, but often with an unacceptable ammonia slip. Compact SCR systems therefore must include provisions to control ammonia slip.

A catalyst effective in removing ammonia without producing additional NOx or N2O is one means to control unreacted ammonia. The effectiveness of an ammonia oxidation (AMOX) catalyst is evaluated not only by its ability to remove ammonia but also to control the selectivity of the reaction products such as NOx, N2O and nitrogen. The selectivity of an AMOX catalyst depends on many variables such as the formulation, the precious metal loading and the operating conditions. This presentation demonstrates the effectiveness of incorporating an AMOX catalyst into a SCR system and some of the operating characteristics of these catalysts.

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