Identification of urea decomposition from an SCR perspective; A combination of experimental work and molecular modeling

•First DFT study of urea byproduct formation without the influence of a catalyst.•Optimized structures, coordination states and transition states found by DFT.•Detailed reaction pathways presented.•Energy level diagrams for comparison of competing reactions.•New FTIR method for urea and its byproducts has been developed.

For the automotive industry selective catalytic reduction (SCR) is the most effective way to get rid of poisonous nitrogen oxides (NOX) in the exhaust. Urea is used as precursor for the reducing agent ammonia. The stricter legislation on NOX emissions for heavy duty vehicles in Euro 6 makes it even more important to optimize the conversion of urea to ammonia.Competing with ammonia formation is the formation of two byproducts commonly observed in the SCR system, biuret and cyanuric acid. These byproducts are formed before the SCR catalyst in the trucks. In the literature different possibilities on the reaction pathway to biuret and cyanuric acid are described. The aim of this study is to understand which of these pathways that is more likely to occur.In this study, the decomposition and synthesis pathways from urea to biuret and cyanuric acid were identified by a combination of laboratory experiments and Density Functional Theory (DFT) calculations. This is the first DFT study on these reactions performed without the influence of a catalyst. Accordingly, this is a study of the reactions where they do occur, before and not over the catalyst. A new gas phase FTIR method for urea and its byproducts was developed.

Graphical abstractIn this study, the decomposition and synthesis pathways from urea to biuret and cyanuric acid were identified by a combination of laboratory experiments and DFT (Density Functional Theory) calculations.Figure optionsDownload full-size imageDownload as PowerPoint slide