Hydrolysis and thermolysis of urea and its decomposition byproducts biuret, cyanuric acid and melamine over anatase TiO2

A major problem in the selective catalytic reduction of NOx with urea (urea-SCR) is the formation of urea decomposition byproducts, which can be suppressed by the use of TiO2 as a hydrolysis catalyst. Temperature programmed desorption and reaction (TPD/R) experiments with urea, biuret, triuret, cyanuric acid and melamine on TiO2-coated and inert cordierite monoliths were used to investigate the reaction network of urea byproduct formation and decomposition over anatase TiO2. All investigated compounds were found to be catalytically hydrolyzed over TiO2. Biuret was directly hydrolyzed to urea in one step, whereas melamine hydrolyzed step-wise via ammeline and ammelide to cyanuric acid. Finally, cyanuric acid completely hydrolyzed to ammonia and carbon dioxide. The formation of byproducts was strongly favored in the absence of water. A reaction network was developed for the uncatalyzed and catalytic decomposition of urea, showing the most important reactions of urea, isocyanic acid, biuret, triuret, cyanuric acid, ammelide, ammeline and melamine under low-temperature operating conditions in SCR systems. Our results support the approach of using a special hydrolysis catalyst for urea decomposition or of catalytic coatings on exhaust pipes to avoid byproduct formation.

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► Anatase TiO2 as hydrolysis catalyst for urea decomposition byproducts.
► Biuret is directly hydrolyzed in one step to urea over TiO2 at only 100 °C.
► Melamine is hydrolyzed in a multi-step reaction, starting at substituting the amine groups.
► Stability against hydrolysis over TiO2: cyanuric acid > ammeline > ammelide ≈ melamine.