STUDIES OF THE SELECTIVE REDUCTION OF NITRIC-OXIDE BY CARBON-MONOXIDEIN THE PRESENCE AND ABSENCE OF HYDROGEN OVER AU NAY CATALYSTS/

The selective reduction of NO with CO in the presence and absence of h ydrogen over Au/NaY catalysts has been studied by in situ FTIR spectro scopy and under steady-state conditions in a flow mode iii the tempera ture range 473-723 K. The NCO intermediates found by FTIR absorption a t 2280-2240 cm(-1) after contacting, at 423-573 K, an Au/NaY catalyst with an NO-CO-H-2 mixture shows a dependence on the presence of H-2 wh ich functions in N-O dissociation in this temperature region. Removal of NCO groups from the catalyst with time at 473 K, proposed to be lim ited by the prerequisite reaction between adsorbed NCO and NO in the g as phase, to form N-2 and CO2 is accelerated with increasing temperatu res. The effect of adding H-2 to an NO-CO-Hestream on the conversions of both NO and CO to, respectively, N-2 and CO2 were found to be consi stent with a temperature-dependent mechanism. The yields of N-2 and CO 2 were increased in the presence of hydrogen, when NCO complexes were present on the gold catalyst, up to 573 K. Above this temperature, whe re direct NO + CO is the only competing reaction, the presence of hydr ogen reduced conversion. The activities of the gold catalysts were mai ntained even at temperatures as high as 723 K, suggesting that a large fraction of partially charged Au-I cations were stabilized by the fra mework of NaY zeolite. This species, after pumping off the reacting mi xture gases at 473 K and collecting the spectra on cooling the gold ca talyst, gave a characteristic carbonyl IR absorption band at 2188 cm(- 1), reasonably assigned to a CO vibration of carbonyl coordinated to A u-I.