On the quantitative aspects of hydrolysis of isocyanic acid on TiO2

The selective catalytic reduction with aqueous solutions of urea is currently seen having the highest potential to reduce NOx and particulate emissions for commercial diesel powered vehicles. Ammonia as the actual reduction medium is formed from urea in two consecutive reactions, i.e. via the thermolysis of urea to isocyanic acid and NH3 and the catalyzed hydrolysis of HNCO over TiO2 to NH3 and CO2. A kinetic model for the hydrolysis reaction was derived for a reaction scheme comprising a set of elementary steps. To minimize the number of unknown variables in the kinetic model for the overall rate, the equilibrium constants for both reactants (HNCO and H2O) and products (NH3 and CO2) were determined from adsorption isotherms using Langmuir and multilayer adsorption models. A data set consisting of 49 data points for the rate determined at varying reactant concentrations was fitted with the kinetic model using a non-linear least mean squares regression analysis.