Rq. Long et Rt. Yang, Characterization of Fe-ZSM-5 catalyst for selective catalytic reduction ofnitric oxide by ammonia, J CATALYSIS, 194(1), 2000, pp. 80-90
Fe-exchanged ZSM-5 (Fe/Al = 0.193), the most active catalyst known for the
selective catalytic reduction (SCR) of NO with ammonia, was characterized b
y X-ray photoelectron spectroscopy (XPS), H-2 temperature-programmed reduct
ion (H-2-TPR), electron spin resonance (ESR), and FT-IR spectroscopy. XPS a
nd ESR results indicated that iron cations were present mainly as Fe3+ ions
with tetrahedral coordination, along with a small amount of Fe2+ and aggre
gated Fe3+ ions. The Fe3+ ions could be partially reduced to Fe2+ ions by H
-2 at 300-600 degrees C, but the oxidation was reversible when O-2 was intr
oduced into the reduced catalyst at 500 degrees C. FTIR spectra showed that
NO molecules could be oxidized by O-2 to adsorbed N2O3, NO2, and nitrate.
The NOx adspecies were not stable at above 300 degrees C in He, but the ads
orbed NO and NO2 could be observed in flowing NO + O-2/He. NH3 molecules we
re adsorbed on Bronsted acid and Lewis acid sites of Fe-ZSM-5 to generate,
respectively, NH4+ ions (majority) and coordinated NH3 (minority). The NH4 ions with three hydrogen atoms (3H structure) bonded to AlO4 tetrahedra of
ZSM-5 were more stable at high temperatures (e.g., 300-400 degrees C) than
those with two bonds and the coordinated NH3. The pretreatment by SO2 + O-
2 at 400 degrees C increased the Bronsted acidity of the Fe-ZSM-5 due to fo
rmation of surface sulfate species of iron. This resulted in an enhancement
of SCR activities by the presence of H2O + SO2 at high temperatures (>350
degrees C). At 300 degrees C, the NH4+ ions with 3H structure were active i
n reacting with NO and NO + O-2, but the reaction rate with NO + O-2 was mu
ch higher than that with NO. The results indicate that NH4+ ions with 3H st
ructure as well as NO and NO2 adspecies play an important role in the SCR r
eaction on the Fe-ZSM-5 catalyst. The role of Fe3+ is to oxidize NO to NO2.
Overexchange decreased the activity; the optimal Fe/Al ratio was 0.19-0.43
. (C) 2000 Academic Press.