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

摘要

The influence of the reducing agent on the selective reduction of NO_x over Ir black was investigated with special respect to the by-products formed. Propene, propane, CO, and H_2 were used as reducing agents in the presence and absence of oxygen and with either NO or NO_2 as NO_x component. Though all reducing compounds applied were able to reduce IrO_2 in a similar way, it was found that only propene exhibits appreciable activity for the reduction of NO under oxidizing conditions. In oxygen-free atmospheres under large excess of the reducing agent, with propene considerable amounts of HCN were formed and with H_2 the major product was NH_3. Furthermore the decomposition of propene, propene, butene, and 2-methyl-propane over Ir black and the activity of the produced carbonaceous deposits for NO reduction was investigated. In the sequence of propane > butene > 2-methyl-propene > propane the amount of carbonaceous deposits decreased in line with the ability of NO reduction. This corroborates the crucial role of carbonaceous deposits in the NO reduction process and supports an adsorbate-assisted mechanism of NO reduction. The presence of NO_x shift propene oxidation light-off to higher temperatures irrespective of the NO_x component used. Experiments using NO_2 as NO_x component usually did not change the reaction pattern as compared to the situation with NO. This adds further evidence that NO_2 is no necessary reaction intermediate in the reduction of NO over Ir black.