STUDIES OF THE SURFACE SPECIES FORMED FROM NO ON COPPER ZEOLITES

The surface chemistries of CuZSM-5 and of CuY preparations have been compared, using spectra of adsorbed NO taken under dynamic flow conditions at various temperatures between 173 and 723 K. Flow microbalance experiments were made with NO/He mixtures. At NO decomposition temperatures, a curious time dependence was observed where adsorption on the fresh catalyst increased rapidly, passed through a maximum, and then decreased again as it reached a steady state. In the steady state, the weight of CuZSM-5 was greater than its initial weight. Under favorable circumstances this increment was sufficiently large to permit observation of surface species present by IR. It was also shown that these species could be fully desorbed by flushing with pure He. Similar behavior was observed with CO, but in this instance the final weight was less than the initial weight. The weight loss with CO at 773 K (after flushing) corresponded almost exactly to a one-electron reduction of the copper ions (Cu2+ to Cu+). The spectra obtained after various pretreatments emphasized the differences between the CuY and CuZSM-5. At 173 K only the bands corresponding to chemisorption on Cu2+ appeared on the fully oxidized CuY, and only the bands corresponding to chemisorption on Cu+ appeared following reduction. With CuZSM-5, both sets of bands appeared in both instances, including those for the dinitrosyl species. The same bands appeared at higher temperatures, where oxidation occurred at rates that were temperature and pressure dependent. At 373 K and above, only species bound to Cu2+ were detected on CuY, whereas with CuZSM-5 bands for the mononitrosyls on both Cu+ and Cu2+ appeared, as well as those for adsorbed NO2, nitrito, nitrato, and nitro species. The superior catalytic behavior of CuZSM-5 may be the result of its ability to stabilize these species. To aid in interpretation, a brief spectroscopic study of the chemisorption of NO2 was carried out. The relevance of these findings to the mechanism of NO decomposition is discussed.