The Formation and Reactions of Hydrogen Cyanide under the Conditions of the Selective Catalytic Reduction of NO by Isobutane on Cu-MFI

The possible involvement of cyanide species in the isobutane/NO/O2 selective catalytic reduction (SCR) reaction over Cu-MFI has been investigated. The reaction undergoes gradual deactivation at 275°C with the emergence of cyanogen, not HCN, as a significant product. Loss of activity is accompanied by the formation of a deposit which yields HCN when heated in the absence of oxygen. At 300°C HCN itself undergoes nearly complete oxidation to CO2 and N2 for some hours when reacted with O2 over Cu-MFI. However, deactivation then sets in with the emergence of cyanogen as a product, followed by unreacted HCN and then NH3. A similar sequence of events occurs more rapidly at lower temperatures and is accompanied by the accumulation of carbon- and nitrogen-containing species, especially adsorbed ammonia, which probably arises through hydrolysis of HCN. Addition of NO then leads to a staged recovery in activity as cyanogen production passes through a maximum, while N2 is produced initially from adsorbed NH3 by the NH3-SCR reaction and subsequently from cyanide species. At 270°C HCN is completely converted to CO2 and N2 when reacted in NO/O2 mixtures but at 200°C the major steady-state product is cyanogen with a yield over 70%. The higher reactivity of HCN in the presence of NO/O2 compared to that in O2 alone is attributable to the formation of NO2. Deactivation during the isobutane/NO/O2 reaction has similarities to that occurring during the HCN/O2 one since NO2 is nearly absent during the SCR reaction under partial conversion conditions as a result of its back conversion to NO through reaction with carbonaceous deposits. As a consequence, while HCN formed during SCR reacts further, some is converted only as far as cyanogen.