Adsorption and hydrolysis of isocyanic acid on TiO2

The interaction of isocyanic acid (HNCO) with titania in the anatase modification has been investigated by means of DRIFT spectroscopy and TPD experiments. HNCO adsorbs dissociatively on TiO2 to yield isocyanate (NCO) groups, bound to Ti4+ sites, and hydroxyl (OH) groups. The stability of these NCO groups decreases with increasing temperatures, and at T > 200 °C they are very easily removed from the surface. In the presence of water, the hydrolysis of the NCO species to ammonia is also fostered by elevating the temperature. Moreover, HNCO adsorption on a sample previously exposed to ammonia revealed that competitive adsorption occurs on the surface. The catalytic activity of TiO2 for the hydrolyis of HNCO was investigated in a micro plug-flow reactor and correlated with the stability and reactivity of the isocyanate species found by DRIFT spectroscopy. The results are discussed in view of the role of the HNCO hydrolysis in the selective catalytic reduction (SCR) of diesel NOx emissions with urea (urea-SCR process).