Urea decomposition and HNCO hydrolysis studied over titanium dioxide, Fe-Beta and γ-Alumina

The catalytic effect of titanium dioxide, Fe-Beta, γ-Alumina, on the thermal decomposition of urea and hydrolysis of HNCO, was investigated using differential scanning calorimetry (DSC) and mass spectrometry (MS). The catalytic materials were coated on cordierite substrates and a pure cordierite sample was also used for comparison. The urea was administered by impregnating the monoliths with an urea/water solution. The experiments were performed using a constant heating rate of 10 K/min and over a temperature range of 25–500 °C. A sweep gas flow rate of 80 mL/min of either dry or humid Ar was used. The results show that TiO2 is the best hydrolysis catalyst. Fe-Beta showed a very large ammonia production, due to selective adsorption of urea during the impregnation of the monolith in the urea solution. One experiment with lower flow, higher urea loading and increased ramp speed conducted in dry Ar over TiO2 showed a large formation of biproducts. This experiment was repeated in the presence of water and this decreased the formation of CYA and biuret significantly. The reason for this is the effective hydrolysis of the HNCO over titania, which hindered the bi-product formation.

Thermogram and evolved NH3, HNCO and CO2 for the TiO2 monolith impregnated with a 30% solution of urea and water with a sweep flow rate of 80 mL/min of humid Ar. The first event on the DSC curve is the evaporation of water between approximately 25 and 100 ̊C. The second event is the melting of urea which occurs at 130–140 ̊C, this is followed closely by the decomposition and hydrolysis of the urea between 150 and 210 ̊C. The hydrolyses is enhanced by the titania coating.Figure optionsDownload as PowerPoint slideHighlights
► TiO2 exhibited the best hydrolysis performance compared to inert cordierite, alumina and Fe-Beta
► All samples performed the hydrolysis better under humid conditions, as was expected.
► Fe-Beta produced huge amounts of ammonia and CO2 (higher than accounted for by the weighing procedure). This was attributed to selective accumulation of urea within the zeolite during impregnation
► One experiment with lower flow, higher urea loading and increased ramp speed conducted in dry Ar over TiO2 showed a large formation of biproducts.
► In the presence of water titania negated the formation of biuret and CYA. This was attributed to the effective hydrolysis of HNCO preventing it to act as a pre-cursor in the formation of biuret and CYA.