Reduction of nitrogen oxides over unsupported iridium: effect of reducing agent

The influence of the reducing agent on the selective reduction of NOx over Ir black was investigated with special respect to the by-products formed. P ropene, propane, CO, and H-2 were used as reducing agents in the presence a nd absence of oxygen and with either NO or NO2 as NOx component. Though all reducing compounds applied were able to reduce IrO2 in a similar way, it w as found that only propene exhibits appreciable activity for the reduction of NO under oxidizing conditions. In oxygen-free atmospheres under large ex cess of the reducing agent, with propene considerable amounts of HCN were f ormed and with H-2 the major product was NH3. Furthermore the decomposition of propene, propane, butene, and 2-methyl-propene over Ir black and the ac tivity of the produced carbonaceous deposits for NO reduction was investiga ted. In the sequence of propene > butene > 2-methyl-propene > propane the a mount of carbonaceous deposits decreased in line with the ability of NO red uction. This corroborates the crucial role of carbonaceous deposits in the NO reduction process and supports an adsorbate-assisted mechanism of NO red uction. The presence of NOx shifts propene oxidation light-off to higher te mperatures irrespective of the NO, component used. Experiments using NO2 as NOx component usually did not change the reaction pattern as compared to t he situation with NO. This adds further evidence that NO2 is no necessary r eaction intermediate in the reduction of NO over Ir black. (C) 2001 Elsevie r Science B.V. All rights reserved.