Structural properties and catalytic behaviour of iridium black in the selective reduction of NO by hydrocarbons

Several mechanical mixtures of Ir black or IrO2 with various mixing materia ls like silica xerogel, alumina, and H-ZSM-5 were tested for their activity in the selective reduction of NO by propene. Independent of the mixing mat erial used the unsupported Ir showed outstanding DeNOx activity compared to their supported counterparts. The conversion of NO to N-2 under lean condi tions as well as oxidizability and reducibility of the Ir-IrO2 system was f ound to depend strongly on the Ir crystallite size. With increasing crystal lite size, selectivity to N-2 is greatly enhanced mainly by the suppression of a reaction pathway leading to the production of NO2. Tailoring of Ir cr ystallite size by controlled sintering of Ir black afforded highly active I r catalysts without the need for further conditioning on stream. Among unsu pported platinum group metals iridium showed the best performance in NOx re moval, The catalytic behaviour of Ir black was compared to the behaviour of Pt black, Pd black, and Rh black, Unsupported IrO2 with relatively large c rystallite size (24 nm) was inactive for the selective reduction of NO with propene. Only IrO2 with small crystallite size (below ca. 11 nm) and high surface area could be activated by on-stream conditioning. Ir black and IrO 2 were characterized by XRD, TEM, SEM, thermal analysis, and krypton adsorp tion measurements. It was shown that it is unlikely that NO2 is an intermed iate in the reduction of NO. (C) 2001 Academic Press.