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Rsc.Li/Pccp Before Technical Editing, Formatting and Proof Reading View Article Online View Journal PCCP Physical Chemistry Chemical Physics Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: S. Tischer, M. Börnhorst, J. Amsler, G. Schoch and O. Deutschmann, Phys. Chem. Chem. Phys., 2019, DOI: 10.1039/C9CP01529A. Volume 19 Number 1 This is an Accepted Manuscript, which has been through the 7 January 2017 Pages 1-896 Royal Society of Chemistry peer review process and has been PCCP accepted for publication. Physical Chemistry Chemical Physics Accepted Manuscripts are published online shortly after acceptance, rsc.li/pccp before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard ISSN 1463-9076 Terms & Conditions and the Ethical guidelines still apply. In no event PAPER H.-P. Loock et al. shall the Royal Society of Chemistry be held responsible for any errors Determination of the thermal, oxidative and photochemical degradation rates of scintillator liquid by fluorescence EEM spectroscopy or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. rsc.li/pccp Page 1 of 14 Physical Chemistry Chemical Physics View Article Online DOI: 10.1039/C9CP01529A Thermodynamics and reaction mechanism of urea de- composition † Steffen Tischer,∗a Marion Börnhorst,b Jonas Amsler,b Günter Schoch,b and Olaf Deutschmanna;b Manuscript The selective catalytic reduction technique for automotive applications depends on ammonia pro- duction 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 calorime- try. 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. Accepted Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 1 Introduction from experimental results and literature data, 23 possible reac- Air pollution by nitrogen oxides from Diesel engines is a main tions including urea and its by-products biuret, cyanuric acid, am- problem concerning environment and society. Therefore, gov- melide, ammeline and melamine are presented. Further, cyanate ernments follow the need to regulate emissions by law (e.g. and cyanurate salts and cyanamide are proposed as possible in- 715/2007/EG, "Euro 5 and Euro 6"). 1 The favored method to termediates of high temperature urea decomposition. Triuret pro- Physics reduce nitrogen oxides is selective catalytic reduction (SCR) us- duction and decomposition is not accounted for in this study. ing ammonia as reducing agent. Due to security issues, passenger The authors classify urea decomposition into four temperature cars cannot be equipped with an ammonia tank. Consequently, regions. The first temperature regime from room temperature This article is licensed under a ammonia is provided in form of a urea-water solution, which de- to 190 ◦C comprises urea melting and vaporization starting from composes through thermolysis and hydrolysis when dispersively 133 ◦C. With increasing temperature, urea decomposes to am- dosed into the hot exhaust pipe. During the desired decomposi- monia and isocyanic acid, the latter leading to biuret, cyanuric Open Access Article. Published on 08 July 2019. Downloaded 7/8/2019 1:28:28 PM. ◦ tion of urea at temperatures above 130 C undesired intermedi- acid and ammelide formation. The second temperature region ◦ ates and by-products in liquid and solid form occur and stick to of 190–250 C is dedicated to biuret decomposition accompanied Chemical the wall of the exhaust pipe due to the inevitable spray-wall in- by several side reactions forming cyanuric acid and ammelide. At teraction. 2–4 These condensed deposits in the exhaust pipe bear 225 ◦C the melt is observed to be converted into a sticky, solid the risk of increased pressure drop and insufficient conversion matrix, which is assumed to originate from ionic formations of to ammonia. Thus there is a need to understand the decompo- different by-products without evidence. Besides small amounts sition kinetics of urea and its by-products, that has been exten- of ammelide, ammeline and melamine, cyanuric acid is the main sively studied by many authors. 5–11 Common experimental meth- component observed at 250 ◦C. The third temperature range from ods include Thermogravimetric Analysis (TG), Differential Scan- 250 to 360 ◦C represents the sublimation and decomposition of ning Calorimetry (DSC), High Performance Liquid Chromatogra- cyanuric acid. Ammelide, ammeline and melamine are proposed Chemistry phy (HPLC) and Fourier transform-infrared spectroscopy (FT-IR). to gradually decompose at temperatures above 360 ◦C marking A first detailed description of urea decomposition behavior was the fourth temperature region. The authors state elimination of proposed by Schaber et al. 8 based on TG, HPLC, FT-IR and am- ammelide at 600 ◦C and ammeline at 700 ◦C. High temperature monium ISE (ion-selective electrode) measurements. Concluding residues are not investigated. 8 Eichelbaum et al. 10 propose a reaction network for urea de- a Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technol- composition consisting of nine major reactions based on simul- ogy (KIT), Karlsruhe, Germany. Fax: +49 721 608 44805; Tel: +49 721 608 42114; taneous TG/DTA (Differential Thermal Analysis) coupled with Physical E-mail: [email protected] GC/MS (Gas Chromatography/Mass Spectrometry) gas analyses. b Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute Decomposition reactions of ammelide, ammeline and melamine of Technology (KIT), Karlsruhe, Germany. † Electronic Supplementary Information (ESI) available: [details of any supplemen- are defined and total decomposition is observed for temperatures ◦ tary information available should be included here]. See DOI: 00.0000/00000000. above 625 C. However, the proposed reaction scheme lacks in 1–13 | 1 Physical Chemistry Chemical Physics Page 2 of 14 View Article Online DOI: 10.1039/C9CP01529A description of relevant parallel and equilibrium reactions of urea Table 1 Experiments used for this study by-products. Acceleration of urea pyrolysis by different metal 10 Type Crucible Substance Initial Ramp exchanged zeolites is demonstrated. A more detailed reaction weight (mg) (K/min) scheme derived by flow reactor experiments using FTIR spec- TG plate urea 6.23 2 troscopy for gaseous and HPLC for solid reaction product analysis TG cylinder urea 6.18 2 11 TG cylinder urea 60.3 2 covers 15 decomposition reactions. For the first time, triuret TG cylinder urea 10.8 10 production and decomposition is included and several reactions TG cylinder 32.5 wt-% UWS 27.5 10 are proposed to be equilibrium reactions. TG plate biuret 5.24 2 TG cylinder biuret 5.18 2 Based on the proposed reaction schemes, a first kinetic model TG cylinder biuret 9.8 10 TG cylinder biuret 95.8 2 (to 195 ◦C) for evaporation and decomposition of urea water solution was de- ◦ 12 TG cylinder biuret 94.6 2 (to 200 C) veloped by Ebrahimian et al. The model describes urea decom- TG cylinder biuret 94.4 2 (to 210 ◦C) position to ammonia and isocyanic acid and the equilibrium re- TG cylinder triuret 6.98 2 action forming biuret. Reactions from biuret to cyanuric acid and TG cylinder triuret 10.31 10 TG plate cyanuric acid 5.87 2 from cyanuric acid to ammelide and isocyanic acid are included. TG cylinder cyanuric acid 5.37 2 Manuscript Ammelide is assumed to decompose to gaseous by-products. TG cylinder cyanuric acid 10.1 10 Current kinetic models are based on the reaction network pro- TG plate ammelide 5.58 2 TG cylinder ammelide 8.72 2 11 posed by Bernhard et al. and validated against TG and HPLC TG cylinder ammelide 9.34 10 experimental data. The model includes formation and decompo- DSC cylinder urea 14.2 2 sition reactions of the most relevant by-products and reproduces DSC cylinder biuret 12.8 2 DSC cylinder triuret 5.9 2 the characteristic decomposition stages of urea adequately. 13 DSC cylinder cyanuric acid 16.4 2 However, important physical and chemical processes are not ac- counted for. A biuret matrix species is defined to cover the effect Accepted ◦ of solidification at 220 C. Ammelide decomposition is modeled gravimetrical analysis, a Netzsch STA 409 C was equipped with Creative Commons Attribution-NonCommercial 3.0 Unported Licence. as sublimation while further high molecular by-products are not the thermal controller TASC 414/2. The following standard pro- included in the model. Further, recent investigations have shown cedure is conducted for each deposit sample and for reference that the current kinetic model lacks in reproduction of mass trans- measurements. Representative samples are ground and placed in fer effects at the sample surface and in the prediction of reactions a corundum crucible with an initial sample mass of 5–100 mg. including gas phase species. The samples are heated from 40 to 700 ◦C at a constant heating Physics Thermogravimetric measurements have shown a strong influ- rate of 2 or 10 K/min. TG analysis is performed in synthetic air ence of the experimental boundary conditions on urea decompo- (20.5% O2 in N2) using a purge gas flow rate of 100 mL/min.
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