Oliver Kröcher

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

An overview is given of the selective catalytic reduction of NOx by ammonia (NH3‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH3‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH3‐SCR reaction over Fe‐ZSM‐5, with isolated species being the most active.

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.

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.

Subsecond and in Situ Chemical Speciation of Pt/Al2O3 during Oxidation Reduction Cycles Monitored by High-Energy Resolution Off-Resonant X-ray Spectroscopy

We report an in situ time-resolved high-energy resolution off-resonant spectroscopy study with subsecond resolution providing insight into the oxidation and reduction steps of a Pt catalyst during CO oxidation. The study shows that the slow oxidation step is composed of two characteristic stages, namely, dissociative adsorption of oxygen followed by partial oxidation of Pt subsurface. By comparing the experimental spectra with theoretical calculations, we found that the intermediate chemisorbed O on Pt is adsorbed on atop position, which suggests surface poisoning by CO or surface reconstruction.

Sol–gel derived hybrid materials as heterogeneous catalysts for the synthesis of N,N-dimethylformamide from supercritical carbon dioxide

Hybrid materials derived from group VIII metal–chloro complexes of the type MCl2X2(M = Pt, Pd), MClX3(M = Rh, Ir) and especially RuCl2X3[X = Ph2P(CH2)2Si(OEt)3, Me2P(CH2)2Si(OEt)3] by cocondensation with Si(OEt)4via a sol–gel process are highly active heterogeneous catalysts for the synthesis of N,N-dimethylformamide (dmf) from CO2, H2 and dimethylamine under supercritical conditions, affording turnover numbers up to 110 800 at 100% selectivity.

Silica hybrid gel catalysts containing ruthenium complexes: influence of reaction parameters on the catalytic behaviour in the synthesis of N,N-dimethylformamide from carbon dioxide

Abstract The synthesis of N,N-dimethylformamide from carbon dioxide, hydrogen and dimethylamine has been studied in an autoclave using a sol-gel derived heterogeneous catalyst made of RuCl2{PPh2(CH2)2Si(OEt)3}3 and Si(OEt)4 in a ratio of 1:50. The effect of the reaction variables on the activity and selectivity of the hybrid gel was examined by varying the initial concentrations of the catalyst and dimethylamine, the partial pressures of hydrogen and carbon dioxide, the temperature and the stirring frequency. Parametric investigations revealed that the measured reaction rates are not disguised by mass transfer phenomena under the conditions applied. The suitable temperature range of the reaction is between 370 and 400 K, with the upper temperature limit given by the thermal stability of the catalyst. Hydrogen appeared to be the limiting reactant since it significantly influenced the reaction rate. In contrast, the carbon dioxide partial pressure in the range 3–18 MPa and the dimethylamine concentration had only a negligible effect on the turnover frequency, indicating a zeroth order dependence. High concentrations of hydrogen and carbon dioxide in the liquid dimethylamine phase afford high concentrations of all reactants at the catalytic centres in an ideal reaction design.

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.

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.