NO + CO interaction and NCO formation on PdY zeolite studied by infrared spectroscopy

Abstract

Infrared measurements have been made to examine what surface species exist during the NO + CO reaction on PdY zeolite catalysts treated in different ways, with special emphasis on the formation of a surface NCO complex. Results for NO adsorption suggest that Pd crystallites (produced by high-temperature reduction of the sample) are at least partially oxidized by NO, even at 193 K. In the presence of NO and CO the formation of a mixed complex [graphic omitted] can be detected at the same temperature. Although the complex is very unstable, in the presence of gaseous NO and CO it exists on the surface up to 423 K. When NO and CO were reacted at 473–673 K, a new band appeared at 2280 cm^–1 in the i.r. spectra and was attributed to the asymmetric stretching vibration of a surface NCO (isocyanate) species. Since HNCO (isocyanic acid) adsorption on HY zeolite causes the appearance of the same band and no NCO complex was detected on HY during the NO + CO reaction, it is assumed that NCO is formed on the Pd and migrates onto the uncoordinated Al³⁺ ions in the zeolite framework.