The impact of urea on the performance of metal exchanged zeolites for the selective catalytic reduction of NOx: Part I. Pyrolysis and hydrolysis of urea over zeolite catalysts

Urea-SCR over metal exchanged zeolites is one of the leading catalytic technologies to abate NOx emissions in diesel exhaust. Ideally, urea injected into the diesel exhaust upstream of the SCR catalyst decomposes only to the gaseous products CO2 and NH3, where the latter gas can react with NOx emissions to form harmless N2 and H2O. However, solid by-products can be formed as well, and if deposited on the catalyst harm the long-term catalytic performance. In order to identify the impact of various urea decomposition products on the catalytic activity, we studied the pyrolysis and hydrolysis of neat urea and of urea over different zeolites (H-Y, Cu-Y, H-Beta, Na-Beta, and Fe-Beta). The experiments were run in dry and steam-containing N2 between 20 and 750 °° C by using simultaneous thermogravimetric analysis (TGA), differential thermoanalysis (DTA), and online GC/MS evolved gas analysis. Solid intermediate products at different decomposition temperatures were identified by means of ATR-FTIR and luminescence spectroscopy. As for neat urea, CO2, NH3 and HNCO could be detected as major gaseous products. At 270 °° C significant amounts of cyanuric acid and ammelide and at 500 °° C of melem and melon were identified as solid intermediates. Above 625 °°C, all solid residues decomposed to cyanogen and isocyanic acid. Furthermore, it could be shown clearly that the investigated zeolites significantly accelerate the pyrolysis of urea and cyanuric acid, and the hydrolysis of HNCO, by shifting the decomposition processes to lower temperatures and by inhibiting the formation of solid by-products. In addition, the presence of steam in the feed gas can prevent even further the formation of solid residues and the high temperature adsorption of gaseous products.