Thermodynamics and reaction mechanism of urea decomposition

Selective catalytic reduction (SCR) for automotive applications depends on ammonia production from a urea-water solution by thermolysis and hydrolysis. In this process, undesired liquid and solid by-products are formed in the exhaust pipe. The formation and decomposition of these by-products have been studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Based on a previously published reaction mechanism by Brack et al. [1], a new reaction scheme is proposed that emphasizes the role of thermodynamic equilibrium of the reactants in liquid and solid phases [2]. The observed phenomenon of liquefaction and re-solidification of biuret in the temperature range 193–230 °C can be explained by formation of a eutectic mixture with urea. According to DSC data, the direct decomposition of urea to ammonia and isocyanic acid can be ruled out. The dominant route is a self-polymerisation of urea to biuret and triuret. Biuret and triuret decomposition are dominated by thermodynamic equilibria with gaseous isocyanic acid. For this, thermodynamic data of triuret have been refined. The apparent melting point of biuret at 193 °C is explained by the formation of a eutectic mixture within the urea-biuret-triuret-cyanuric acid ensemble. Furthermore, DSC data shows that cyanuric acid sublimates without decomposition at temperatures above 300 °C. Numerical simulations of the TGA and DSC experiments are performed by a multi-phase tank reactor model (DETCHEMMPTR [3]). The new reaction mechanism describes well the main features (decomposition steps and calorimetry) and dependencies (on heating rate and surface area) of the decompositions of urea, biuret, triuret and cyanuric acid. [1] W. Brack, B. Heine, F. Birkhold, M. Kruse, G. Schoch, S. Tischer and O. Deutschmann, “Kinetic modeling of urea decomposition based on systematic thermogravimetric analyses of urea and its most important by-products”, CES 106, 1–8 (2014). [2] S. Tischer, M. Börnhorst, J. Amsler, G. Schoch and O. Deutschmann, “Thermodynamics and reaction mechanism of urea decomposition”, PCCP, in press, DOI: 10.1039/C9CP01529A (2019). [3] www.detchem.co