NO Decomposition over Fe-ZSM-5: A Systematic Study in the Generation of Active Sites

We have carried out a systematic investigation of the critical activation parameters (i.e., final temperature (673-1273 K), atmosphere (He vs. O/He), and final isothermal hold (1 min-15 h) on the generation of "-sites", responsible for the direct NO decomposition over Fe-ZSM-5 (Fe content = 1200-2300 ppm). The concentration of -sites was determined by (ia) transient response of NO and (ib) CO at 523 K, and (ii) temperature programmed desorption (TPD) following nitrous oxide decomposition. Transient response analysis was consistent with decomposition of NO to generate (i) "active" -oxygen that participates in the low-temperature CO→CO oxidation and (ii) "non-active" oxygen strongly adsorbed that is not released during TPD. For the first time, we were able to quantify the formation of -sites, which requires a high temperature (>973) treatment of Fe-ZSM-5 in He over a short period of time (<1 h). In contrast, prolonged high temperature treatment (1273 K) and the presence of O in the feed irreversibly reduced the amount of active sites.