Andreas M. Bernhard

Catalytic urea hydrolysis in the selective catalytic reduction of NOx: catalyst screening and kinetics on anatase TiO2 and ZrO2

The catalytic hydrolysis of urea was investigated under conditions relevant for the selective catalytic reduction of NOx (urea-SCR). The hydrolysis activities of the tested catalysts coated on cordierite monoliths were in the order ZrO2 > TiO2 > Al2O3 > H-ZSM-5 > SiO2. A comparison with isocyanic acid (HNCO) hydrolysis on the same catalysts showed that urea decomposition was much slower than HNCO hydrolysis; hence, catalytic urea thermolysis into NH3 and HNCO is likely to be the rate-determining step in urea decomposition. Interestingly, a different order of catalyst activities was found in water-free experiments on urea thermolysis: TiO2 > H-ZSM-5 ≈ Al2O3 > ZrO2 > SiO2. The widely accepted reaction pathway for urea decomposition, namely urea thermolysis followed by HNCO hydrolysis, seems to be valid on all the tested catalysts except ZrO2: The high urea hydrolysis activity of the ZrO2 catalyst compared to its low urea thermolysis activity suggested a different reaction pathway, in which water directly attacks adsorbed urea rather than adsorbed HNCO.

Evaporation of urea at atmospheric pressure.

Aqueous urea solution is widely used as reducing agent in the selective catalytic reduction of NO(x) (SCR). Because reports of urea vapor at atmospheric pressure are rare, gaseous urea is usually neglected in computational models used for designing SCR systems. In this study, urea evaporation was investigated under flow reactor conditions, and a Fourier transform infrared (FTIR) spectrum of gaseous urea was recorded at atmospheric pressure for the first time. The spectrum was compared to literature data under vacuum conditions and with theoretical spectra of monomolecular and dimeric urea in the gas phase calculated with the density functional theory (DFT) method. Comparison of the spectra indicates that urea vapor is in the monomolecular form at atmospheric pressure. The measured vapor pressure of urea agrees with the thermodynamic data obtained under vacuum reported in the literature. Our results indicate that considering gaseous urea will improve the computational modeling of urea SCR systems.

Laboratory test reactor for the investigation of liquid reducing agents in the selective catalytic reduction of NOx.

A test reactor was designed and built for investigating liquid reducing agents in the selective catalytic reduction (SCR) process in the laboratory. The design of the experimental setup is described in detail and its performance was evaluated. Using a glass nebulizer, liquid reducing agents were sprayed directly onto a catalyst positioned in a heated glass reactor with a length of 250 mm and an internal diameter of 20.4 mm or 40 mm. Model exhaust gases were mixed from individual gas components and were heated up to 450 °C in a heat exchanger before entering the reactor. The off-gas was analyzed using two complimentary techniques, a multi-component online FTIR gas analysis and a liquid quench gas absorption setup, to detect higher molecular compounds and aerosols. Due to the versatility of construction, processes not related to SCR, but involving three-phase reactions with gases, liquids and a catalyst, can also be investigated.

Quantification of Gaseous Urea by FT-IR Spectroscopy and Its Application in Catalytic Urea Thermolysis

An analysis method was developed for the quantification of gaseous urea in model exhaust gases by FT-IR spectroscopy. The method was applied for the investigation of the catalytic thermolysis of urea, which is used as ammonia storage compound in the selective catalytic reduction of NOx in diesel engines.

Ammonia Storage and Release in SCR Systems for Mobile Applications

In mobile SCR applications, the most widespread reducing agent is ammonia. However, due to its toxicity it is not stored directly as pressurized or liquefied gas. Instead, an aqueous solution of 32.5 wt % urea is commonly used as ammonia precursor. The urea solution can be dosed into the main exhaust pipe, where it decomposes in the hot exhaust gas and on the SCR catalyst to yield ammonia and carbon dioxide. Urea as ammonia precursor and its decomposition will be discussed in the first part of this chapter, with a focus on catalytic decomposition and byproduct formation. In the second part, alternative ammonia precursor compounds, including solid ammonia precursors will be presented. These exhibit a higher ammonia storage density, a lower melting point and/or a higher stability when stored at elevated temperatures compared to urea solution.