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In selective catalytic reduction (SCR) systems for diesel vehicles the injected urea solution decomposes to ammonia and isocyanic acid (HNCO), which reacts with water to another ammonia molecule and carbon dioxide over the SCR catalyst or a special urea decomposition catalyst. The second reaction step, i.e. the catalytic hydrolysis of HNCO was studied on the anatase TiO2(101) surface and Al2O3(100) surface with ab initio density functional theory (DFT) calculations using a cluster model as well as with in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) investigations and kinetic experiments. The following mechanistic pathway has been identified to be most feasible: HNCO dissociatively adsorbs on the metal oxide surface as isocyanates, which are attacked by water, thereby forming carbamic acid at the surface. In a further step this intermediate is transformed to a carbamate complex, which leads to CO2 desorption and consequently NH3 formation. The comparison between the sum of the theoretical vibrational spectra of the reaction intermediates with the in situ DRIFT spectra also strongly supports the accuracy of the second reaction pathway. This mechanism holds also for the HNCO hydrolysis over γ-Al2O3 and the reactivity compared to TiO2 was found to be consistent with the heights of the barriers in the energy diagrams. Based on these promising preliminary results a computational screening has been started in order to predict the most active metal oxides and surfaces for this reaction. [ABSTRACT FROM AUTHOR]

In selective catalytic reduction (SCR) systems for diesel vehicles the injected urea solution decomposes to ammonia and isocyanic acid (HNCO), which reacts with water to another ammonia molecule and carbon dioxide over the SCR catalyst or a special urea decomposition catalyst. The second reaction step, i.e. the catalytic hydrolysis of HNCO was studied on the anatase TiO2(101) surface and Al2O3(100) surface with ab initio density functional theory (DFT) calculations using a cluster model as well as with in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) investigations and kinetic experiments. The following mechanistic pathway has been identified to be most feasible: HNCO dissociatively adsorbs on the metal oxide surface as isocyanates, which are attacked by water, thereby forming carbamic acid at the surface. In a further step this intermediate is transformed to a carbamate complex, which leads to CO2 desorption and consequently NH3 formation. The comparison between the sum of the theoretical vibrational spectra of the reaction intermediates with the in situ DRIFT spectra also strongly supports the accuracy of the second reaction pathway. This mechanism holds also for the HNCO hydrolysis over γ-Al2O3 and the reactivity compared to TiO2 was found to be consistent with the heights of the barriers in the energy diagrams. Based on these promising preliminary results a computational screening has been started in order to predict the most active metal oxides and surfaces for this reaction. [ABSTRACT FROM AUTHOR]

The influence of the combustion products of different lubrication oil additives and impurities in fuel or urea solution on the activity and selectivity of V2O5/WO3-TiO2 catalysts in the selective catalytic reduction (SCR) of nitrogen oxides by ammonia was investigated. Focusing on the deactivation by calcium, phosphate, and potassium, the DeNO x activity followed the order K ? Ca >PO4. This trend was investigated on the structural level of the catalyst by means of temperature programmed desorption of ammonia (NH3-TPD) and a DRIFT characterization of the adsorbed ammonia species. The results suggest that the studied elements strongly reduce the acidity of the SCR catalyst in the order K ? Ca >PO4 by mainly affecting the Brønsted acidity of the surface. [ABSTRACT FROM AUTHOR]

A DeNOx demonstration system for a diesel engine used in construction machineries and mobile cranes was setup. In preliminary experiments various extruded and coated SCR catalysts were evaluated with and without oxidizing pre-catalyst. The data from stationary tests with two selected catalysts were used to establish various model-based control algorithms for the optimum dosage of urea in the ESC and ETC. A NOx conversion of >93% at <10 ppm average ammonia slip could be achieved at a converter-to-swept volume ratio of <2.0. [ABSTRACT FROM AUTHOR]