Martin Elsener

Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines

Abstract Urea-SCR, the selective catalytic reduction using urea as reducing agent, has been investigated for about 10 years in detail and today is a well established technique for DeNO x of stationary diesel engines. It is presently also considered as the most promising way to diminish NO x emissions originating from heavy duty vehicles, especially trucks. The paper discusses the fundamental problems and challenges if urea-SCR is extended to mobile applications. The major goal is the reduction of the required catalyst volume while still maintaining a high selectivity for the SCR reaction over a wide temperature range. The much shorter residence time of the exhaust gas in the catalyst will lead to higher secondary emissions of ammonia and isocyanic acid originating from the reducing agent. Additional problems include the control strategy for urea dosing, the high freezing point of urea, and the long term stability of the catalyst.

NOx-Reduction in Diesel Exhaust Gas with Urea and Selective Catalytic Reduction

Abstract High values of NOx reduction may be obtained with urea as a reducing agent and a standard SCR catalyst based on TiO2-WO3-V2O5. The process was carefully investigated for possible secondary emissions and it could be shown that urea-SCR does not lead to relevant emissions of nitrogen dioxide, nitrous oxide, hydrogen cyanide and isocyanic acid. Further investigations using a HPLC method have also proved that addition compounds of higher molecular mass than urea (and urea itself) are not emitted in appreciable amounts as long as the process is properly managed, i.e., as long as the emission of ammonia is kept low. The limiting secondary emission is ammonia slip, the major problem when ammonia is used directly as a reducing agent.

Control of a Urea SCR Catalytic Converter System for a Mobile Heavy Duty Diesel Engine

An advanced controller for a urea SCR (Selective Catalytic Reduction) catalytic converter system for a mobile heavy-duty diesel engine is presented. The after-treatment system is composed of the injecting device for urea solution and a single SCR catalytic converter. The control strategy consists of three parts: A primary feedforward controller, a surface coverage observer, and a feedback controller. A nitrogen oxide (NO x ) gas sensor with non-negligible cross-sensitivity to ammonia (NH 3 ) is used for a good feedback control performance. The control strategy is validated with ESC and ETC cycles: While the average NH 3 slip is kept below 10 ppm, the emission of NO x is reduced by 82%.

Adsorption and desorption of NO and NO2 on Cu-ZSM-5

Abstract The interaction of NOx from lean exhaust gases with copper-exchanged ZSM-5 has been studied by combined adsorption and temperature-programmed desorption. Experiments with NO at 200 °C show that this molecule cannot be stored on Cu-ZSM-5. On the other hand, NO2 may be stored over a wide temperature range from both dry and humid feeds. Therefore, in order to obtain storage of NO, NO must first be oxidized to NO2. However, the oxidation reaction is inhibited by water in the exhaust gas. The adsorption of nitrogen dioxide involves a disproportionation reaction yielding two stored nitrate species and one molecule of NO released to the gas phase: 3 NO 2 +[ CuO ]⇌ NO + Cu ( NO 3 ) 2 Water in the feed gas competes with the formation of nitrates, thus lowering significantly the amount of NO2 stored on Cu-ZSM-5. NO has also a negative effect on the storage of NO2. This effect is due to its reaction with the stored nitrates, causing a considerable shift in the equilibrium of the above reaction back to NO2.

Determination of urea and its thermal decomposition products by high-performance liquid chromatography

Abstract The thermal decomposition of urea can yield a wide variety of products; apart from ammonia and isocyanic acid, addition compounds of higher molecular mass may appear. In order to detect their presence in exhaust gases from a selective catalytic reduction (SCR) process using urea as a reducing agent, a chromatographic method was developed. The chromatographic separation is performed on an anion-exchange column using a phosphate buffer (pH 7) as eluent and UV detection at 190 nm. The method allows the simultaneous determination of neutral compounds (urea, biuret, melamine, ammeline) and of anions (cyanurate, isocyanate, acetate, formate, nitrite, nitrate, etc.). The value of the method for optimizing urea-SCR process design is illustrated.

Investigation of the ammonia adsorption on monolithic SCR catalysts by transient response analysis

Abstract The adsorption of ammonia on three different monolithic SCR catalysts was investigated experimentally. Two of the examined monoliths were coated cordierite honeycombs, containing either V2O5–WO3/TiO2 or V2O5/TiO2 as active components. The third sample was a commercially available extruded monolithic honeycomb consisting of V2O5–WO3/TiO2, and was tested for comparison. Different transient response methods and a modified temperature programmed desorption (TPD) method were used to investigate the ammonia adsorption in the presence of water (5%) and oxygen (10%). Under typical SCR conditions (stoichiometric ratio NH3:NO

Storage of NO2 on BaO/TiO2 and the influence of NO

Abstract The influence of NO on the adsorption and desorption of NO 2 on BaO/TiO 2 has been studied under lean conditions. The adsorption of NO 2 involves the disproportionation of NO 2 into an adsorbed nitrate species and NO released to the gas phase with a 3:1 ratio, BaO +3 NO 2 → NO + Ba ( NO 3 ) 2 . Three different nitrate species form on the catalyst: surface nitrates on the TiO 2 support, surface nitrates on BaO, and bulk barium nitrate. The stability of the three species in different gas feeds was investigated by temperature-programmed desorption (TPD). The reverse reaction of the NO 2 disproportionation has also been observed. If NO is added to the feed, nitrates previously formed on the sorbent will decompose into NO 2 . Therefore, the above chemical equation should be considered as an equilibrium reaction. Applying this finding to the NO x storage and reduction catalyst means that NO probably reacts with the previously formed nitrates yielding NO 2 as an intermediate product. This NO 2 is subsequently reduced by the reducing agents (hydrocarbons and CO) present during the regeneration period.

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.

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium-Based Catalysts

The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH3 ) on vanadium-based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time-resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH3 coordinated to Lewis sites on vanadia on tungsta-titania (V2 O5 -WO3 -TiO2 ), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono-oxo vanadyl group that reduces only in the presence of both nitric oxide and NH3 . We were also able to verify the formation of the nitrosamide (NH2 NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.