The SCR inlet temperature plays an important role in the choice of SCR catalysts. The upper temperature limit is determined by the aging behaviour of the catalyst, the lower temperature limit is determined by the ammonia sulphate formation temperature.

Ammonia sulphates can be formed as undesired byproducts of the SCR reaction from SO₃ and NH₃.

Background: The minimum temperature has to be selected so that no ammonium salts can deposit on the catalyst surface. The upper temperature limit prevents sintering, a process that destroys the pore structure of ceramic catalysts.

In addition to our “standard catalysts” for operating temperatures between 280°C and 450°C (535°F and 845°F), Johnson Matthey offers catalysts especially developed for low and high temperature applications. This results in High NO_x removal efficiency at a temperature range between 170°C and 510°C (335°F and 950°F).