Active and spectator adsorbates during NO decomposition over Cu-ZSM-5: Transient IR, site-poisoning, and site-promotion studies

A site-poisoning and -promotion technique was employed to investigate the r ole of adsorbates and to formulate a reaction pathway for the NO decomposit ion reaction over Cu-ZSM-5. Transient infrared and mass spectrometer studie s of pulse NO reaction on under- and over-exchanged Cu-ZSM-5 reveal Cu2+(NO ), Cu+(NO), bridging Cu2+(NO3-), and NO+ as the major adsorbates and N-2, N 2O, O-2, and NO2 as the products. SiH4 and H2O moderately inhibited Cu+(NO) and N-2 formation, but severely inhibited CU2+(NO3-) and O-2 formation. Ad dition of CO as a reducing agent led to the promotion of Cu2+ reduction to Cu+, depletion of Cu2+(NO3-), increase in NO conversion and O-2 formation a s well as formation of CO2. These results revealed that N-2 and O-2 formati on proceeds through separate rate-limiting steps; O-2 formation occurs via both autoreduction of CU2+ to Cu+ and decomposition of Cu2+(NO3-). The resu lts from pulse reaction studies on deactivated Cu-ZSIW-5 reveal that O-2 fo rmation is more sensitive to poisoning than NO dissociation and N-2 formati on. Although silanation may improve the hydrothermal stability of the catal yst, silanation severely inhibited O-2 and Cu2+(NO3-) formation through inh ibition of adsorbed O migration. Promotion of O-2 desorption by addition of a small amount of reducing agent such as CO in the presence of NO (NO/CO > 3.5) can greatly enhance NO decomposition activity. Further investigation is needed to study such an effect on the NO decomposition activity under th e O-2 environment. (C) 1999 Academic Press.