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Issue 4, 2013
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Catalytic urea hydrolysis in the selective catalytic reduction of NOx: catalyst screening and kinetics on anatase TiO2 and ZrO2

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Abstract

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.

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

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Publication details

The article was received on 28 Sep 2012, accepted on 29 Oct 2012 and first published on 30 Oct 2012


Article type: Paper
DOI: 10.1039/C2CY20668D
Citation: Catal. Sci. Technol., 2013,3, 942-951
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    Catalytic urea hydrolysis in the selective catalytic reduction of NOx: catalyst screening and kinetics on anatase TiO2 and ZrO2

    A. M. Bernhard, D. Peitz, M. Elsener, T. Schildhauer and O. Kröcher, Catal. Sci. Technol., 2013, 3, 942
    DOI: 10.1039/C2CY20668D

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