Novel synthesis and characterization of Fe-ZSM-5 nanocrystals in hot compressed water for selective catalytic reduction of NO with NH₃

This paper describes a new synthetic method to prepare a composite of metal oxide nanoparticles and zeolite (Fe-ZSM-5) in hot compressed water. Two approaches, one-pot immobilization of iron oxide on ZSM-5 and post-ZSM-5-synthesis iron oxide impregnation are studied. The products show various morphologies, which lead to different activities during the selective catalytic reduction of NO with NH₃. In the first approach, Fe-ZSM-5 is synthesized in hot compressed water from precursors with various concentrations of Fe(NO₃)₃, while in the second route, firstly, ZSM-5 powder is prepared and then impregnated with iron oxide. The state of Fe in Fe-ZSM-5 was studied through TEM, XPS, XRD, NH₃-TPD, and H₂-TPR. Both methods result in the production of highly dispersed iron species, however, Fe-ZSM-5 prepared from the one-pot synthesis method contains more acidic sites and more reducible iron species because of the presence of FeO_x nanoparticles mainly in the intra-framework of the zeolite. Furthermore, it is observed that the increase in iron concentration in the precursor has an adverse effect on the surface area and acidity of the final Fe-ZSM-5 product. On the other hand, Fe-ZSM-5 obtained from the impregnation process contains higher quantities of iron in the extra-framework position in the form of bulky aggregates with lower activity. Consequently, Fe-ZSM-5 synthesized through the one-pot route showed higher activity because of the well dispersive iron oxide in the zeolite support. The highly dispersive state is attributed to the predominant presence of iron bound to the framework of aluminum in the zeolite. The results also show that the protonation of the catalyst has no crucial effect on the activity of the catalyst, while the oxidation state of iron oxide is greatly responsible for the SCR activity of the catalyst.