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

Spatially Resolved Measurements of HNCO Hydrolysis over SCR Catalysts

In order to understand deposit formation during urea selective catalytic reduction (SCR) resulting from isocyanic acid (HNCO) formation, the present study investigates the potential of HNCO hydrolysis by spatially resolved gas phase concentration profiles along a single catalyst channel of commercial Cu-zeolite and V-based SCR catalysts. The spatially resolved profiles, obtained in a special hot gas test rig via capillary technique, provide information on reaction rates of HNCO hydrolysis, NH$_3$ adsorption and NO conversion, hereby revealing a better performance of the standard V-based catalyst regarding the HNCO hydrolysis, which is attributed to the TiO$_2$ support

Thermodynamics and reaction mechanism of urea decomposition

The selective catalytic reduction technique 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 and differential scanning calorimetry. A new reaction scheme is proposed that emphasizes the role of thermodynamic equilibrium of the reactants in liquid and solid phases. Thermodynamic data for triuret have been refined. The observed phenomenon of liquefaction and re-solidification of biuret in the temperature range 193–230 °C is explained by formation of a eutectic mixture with urea

NaWuReT Colloquium: From PhD Student to Assistant Professor – Early Career Chemical Engineers in Academia

The Nachwuchs Reaktionstechnik (NaWuReT) are early-career scientists from the ProcessNet Division Reaction Engineering. In autumn 2021, they organized an online colloquium with international early-career scientists from the chemical engineering community. Five guests were invited to give a scientific talk and provide insights into their career paths. The guests gave advice and emphasized the main challenges and opportunities during their early careers. Crucial points are networking, guidance, mentoring, as well as funding acquisition and the personal work-life balance