Investigation into Low-Temperature Urea-Water Solution Decomposition by Addition of Titanium-Based Isocyanic Acid Hydrolysis Catalyst and Surfactant

Mitigation of urea deposit formation and improved ammonia production at low exhaust temperatures continues to be one of the most significant challenges for current generation selective catalytic reduction (SCR) aftertreatment systems. Various technologies have been devised to alleviate these issues including: use of alternative reductant sources, and thermal treatment of the urea-water solution (UWS) pre-injection. The objective of this work was to expand the knowledge base of a potential third option, which entails chemical modification of UWS by addition of a titanium-based urea/isocyanic acid (HNCO) decomposition catalysts and/or surfactant to the fluid. Physical solid mixtures of urea with varying concentrations of ammonium titanyl oxalate (ATO), oxalic acid, and titanium dioxide (TiO₂) were generated, and the differences in NH₃ and CO₂ produced upon thermal decomposition were quantified. It was found that addition of 2.0 mol % ATO to urea increased CO₂ production by 821 % and NH₃ production by 96 % at temperatures ≤ 215 °C, indicating significantly enhanced hydrolysis of HNCO. Conversely, it was demonstrated that addition of oxalic acid or TiO₂ to urea exhibited little effect on NH₃ and CO₂ production, indicating both the importance of titanium in the mixture and adequate catalyst-substrate contact. Previous work by the authors demonstrated that addition of ATO to UWS did indeed result in decreased deposit formation in the exhaust system when compared to conventional UWS, and the extent of deposit reduction could be further enhanced by addition of surfactant to the solution. In this work, six distinct surfactants were added to UWS both with and without ATO, and the effect on deposit formation was quantified. In all cases, addition of both surfactant and ATO resulted in superior deposit mitigation when compared to UWS treated with ATO or surfactant individually. Finally, various ammonium peroxo-hydroxo titanium coordination complexes were synthesized and evaluated for catalytic urea decomposition using TGA-FTIR and compared against the observed activity of ATO.