Decomposition of Urea in the SCR Process: Combination of DFT Calculations and Experimental Results on the Catalytic Hydrolysis of Isocyanic Acid on TiO2 and Al2O3

Abstract

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 TiO 2 (101) surface and Al 2 O 3 (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 CO 2 desorption and consequently NH 3 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 c-Al 2 O 3 and the reactivity compared to TiO 2 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.

Modelling Catalyst Surfaces Using DFT Cluster Calculations

“…As a next step the adsorption of isocyanic acid TiO 2 and Al 2 O 3 has been investigated. The calculations revealed that dissociative as well as molecular adsorption of HNCO is possible and energetically feasible on both the TiO 2 (101) [ 6 , 7 ] and the Al 2 O 3 (100) surface [ 8 , 32 , 33 ]. Figure 5 shows adsorption energies as well as geometric structures of HNCO interacting with different sites of TiO 2 (101) and Al 2 O 3 (100).…”

“…A variety of different catalysts can be used for SCR processes, such as TiO 2 or Al 2 O 3 for the urea decomposition and more complex systems, such as V 2 O 5 /WO 3 -TiO 2 or metal-exchanged zeolites, for the actual SCR reaction. We will exemplarily show the differences in the reaction mechanisms of the hydrolysis of isocyanic acid (HNCO) on anatase TiO 2 (101) and γ-Al 2 O 3 [6–8], which were revealed combining ab-initio DFT calculations using a cluster model with in situ DRIFTS investigations. Furthermore, a characterization study on the deactivation of V 2 O 5 /WO 3 -TiO 2 SCR catalysts [9] by alkali metals originating from additives or impurities from fuels and lubrication oils has been performed.…”

A Review of Urea Pyrolysis to Produce NH3 Used for NOx Removal

“…Czekaj and Kröcher studied the catalytic effect of TiO 2 and c-Al 2 O 3 on HNCO hydrolysis by Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS) [38]. e experimental results proved that both TiO 2 and c-Al 2 O 3 had significant catalytic effects on HNCO hydrolysis under the same reaction conditions and TiO 2 had better catalytic performance than c-Al 2 O 3 .…”

“…According to their experimental data and calculation results, the activation energy of the HNCO hydrolysis over the two catalysts changed with the temperature range. When the temperature was higher than 200°C, the activation energy of HNCO hydrolysis over c-Al 2 It could be concluded from the analysis of the above literature [33][34][35][36][37][38] that the HNCO hydrolysis over metal oxides catalysts could be basically determined as a reaction process under mass transfer control. However, the determination of whether external or internal mass transfer is still controversial now.…”

Ammonia Storage and Release in SCR Systems for Mobile Applications

“…Since HNCO, originating from urea thermolysis, plays a key role in byproduct formation, a catalyst can largely reduce byproduct formation by HNCO hydrolysis [10,11,37]. The best hydrolysis catalyst known for the urea-SCR application is anatase TiO 2 [10,37,[47][48][49]. Also, V 2 O 5 /WO 3 -TiO 2 [13,50] and zeolite-based [10,11,51] SCR catalysts provide high hydrolysis activities.…”