Hydrolysis and oxidation of gaseous HCN over heterogeneous catalysts

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.TiO2-anatase showed the highest HCN hydrolysis activity among the hydrolysis catalysts, with approximately two times more activity than Al2O3. On Fe-ZSM-5, HCN was converted to NH3 to the same degree as on TiO2. In the presence of NOx, the NH3 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 °C. The same reaction products are formed as in the oxidation of NH3, i.e., aside from nitrogen N2O and NOx appear as unwanted reaction products depending on the temperature and gas composition. Similarly high HCN conversions, but clearly better N2 selectivities, were reached on Cu-ZSM-5 and MnOx-Nb2O5-CeOx.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 carbamic acid. Carbamic acid decomposes to CO2 and NH3, which can be further oxidized to N2, N2O or NOx. The oxidation of HCN to HNCO may also proceed with (CN)2 as an intermediate over Pd-, Pt- and Cu-containing catalysts.