Studies of the selective reduction of nitric oxide by carbon monoxide in the presence and absence of hydrogen over Au/NaY catalysts

Abstract

The selective reduction of NO with CO in the presence and absence of hydrogen over Au/NaY catalysts has been studied by in situ FTIR spectroscopy and under steady-state conditions in a flow mode in the temperature range 473–723 K. The NCO intermediates found by FTIR absorption at 2280–2240 cm^–1 after contacting, at 423–573 K, an Au/NaY catalyst with an NO–CO–H₂ mixture shows a dependence on the presence of H₂ which functions in N—O dissociation in this temperature region. Removal of NCO groups from the catalyst with time at 473 K, proposed to be limited by the prerequisite reaction between adsorbed NCO and NO in the gas phase, to form N₂ and CO₂ is accelerated with increasing temperatures. The effect of adding H₂ to an NO–CO–He stream on the conversions of both NO and CO to, respectively, N₂ and CO₂ were found to be consistent with a temperature-dependent mechanism. The yields of N₂ and CO₂ were increased in the presence of hydrogen, when NCO complexes were present on the gold catalyst, up to 573 K. Above this temperature, where direct NO + CO is the only competing reaction, the presence of hydrogen reduced conversion. The activities of the gold catalysts were maintained even at temperatures as high as 723 K, suggesting that a large fraction of partially charged Au^I cations were stabilized by the framework of NaY zeolite. This species, after pumping off the reacting mixture gases at 473 K and collecting the spectra on cooling the gold catalyst, gave a characteristic carbonyl IR absorption band at 2188 cm^–1, reasonably assigned to a CO vibration of carbonyl coordinated to Au^I.