Isocyanic acid hydrolysis over Fe-ZSM5 in urea-SCR

Two different iron exchanged zeolites were studied for the hydrolysis of isocyanic acid (HNCO) to ammonia, which is an important intermediate step in the selective catalytic reduction (SCR) of NO with urea. The hydrolysis reaction proceeded with very high activity and selectivity on over-exchanged as well as low-exchanged Fe-ZSM5 catalyst samples. The hydrolysis activity of the catalysts was inversely correlated with their Brønsted acidity, which decreased with the degree of iron exchange and the degree of ageing. Thus, over-exchanged Fe-ZSM5 had the highest activity, which even increased when the catalyst was aged.