Health & Environmental Research Online (HERO)


Print Feedback Export to File
939792 
Journal Article 
Hydrolysis and oxidation of gaseous HCN over heterogeneous catalysts 
Kröcher, O; Elsener, M 
2009 
Yes 
Applied Catalysis B: Environmental
ISSN: 0926-3373 
92 
1-2 
75-89 
WOS:000271372200011 
The hydrolysis and oxidation of HCN, which is a potential toxic emission of automotive catalysts, were systematically examined with model gas experiments on typical hydrolysis, SCR and oxidation catalysts. TiO(2)-anatase showed the highest HCN hydrolysis activity among the hydrolysis catalysts, with approximately two times more activity than Al(2)O(3), On Fe-ZSM-5, HCN was converted to NH(3) to the same degree as on TiO(2). In the presence of NO(x), the NH(3) formed from HCN reacted in the SCR reaction to form nitrogen.



On Pd- and Pt-containing oxidation catalysts, which are used in SCR systems as ammonia slip catalysts, HCN is converted with very high activity above 250-300 degrees C. The same reaction products are formed as in the oxidation of NH(3), i.e., aside from nitrogen N(2)O and NO(x) appear as unwanted reaction products depending on the temperature and gas composition. Similarly high HCN conversions, but clearly better N(2) selectivities, were reached on Cu-ZSM-5 and MnO(x)-Nb(2)O(5)-CeO(x).



The precise measurement of all relevant gas components allowed us to develop a reaction scheme for the HCN decomposition chemistry over a variety of heterogeneous catalysts. Over hydrolyzing catalysts water interacts with HCN, forming methanamide and then ammonium formate, which decomposes to ammonia and formic acid. The formic acid finally thermolyzes to water and CO. Catalysts with oxidizing properties oxidize HCN to HNCO in the first reaction step, which then hydrolyzes to unstable carbannic acid. Carbamic acid decomposes to CO(2) and NH(3), Which can be further oxidized to N(2), N(2)O or NO(x). The oxidation of HCN to HNCO may also proceed with (CN)(2) as an intermediate over Pd-, Pt- and Cu-containing catalysts. (C) 2009 Elsevier B.V. All rights reserved. 
HCN hydrolysis; HCN oxidation; Heterogeneous catalysts; SCR; TWC