Ammonia slip catalysts mechanism
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Ammonia slip catalysts (ASC) are used downstream of NH3-selective catalytic NOx reduction (SCR) systems to minimize ammonia slip from transport sector SCR units . ASCs can consist of an SCR active layer for NOx control, an oxidation layer for ammonia conversion, or a combination of both . Mathematical models of ASC systems involve flow and mass transfer in the monolithic channels as well as coupled reaction/diffusion processes in the catalytic layers . The kinetics of relevant reactions over different catalysts, including SCR, ammonia oxidation, and the formation of N2O, have been studied . The best ASC configurations feature fast diffusion in the catalytic layers, a thin top layer loading with some ammonia oxidation catalyst, and a mixed layer of ammonia oxidation catalyst and the SCR catalyst to limit the formation of N2O and NOx . Catalyst articles with ASCs comprising a blend of platinum on a support and a first SCR catalyst have been developed to improve NH3 conversion and reduce ammonia slip .
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