6
2
NANBA ET AL.
H C = CH2
scope of the present study. Further work adopting in situ
studies will throw more light on this aspect.
2
?
NO2
5
. CONCLUSIONS
H C = CH
2
ThereactionpathsfortheselectivereductionofNO with
2
C2H4 (C2H4-SCR-NO2) over H–ferrierite were studied by
investigating the formation and reaction of the by-products.
The results are summarized as follows:
NO2
HCN
1
. Nitroethylene (NE), HNCO, and HCN were detected
as nitrogen-containing by-products in C2H4-SCR-NO2.
. NE was converted to HNCO, HCN, NH3, HCHO, CO,
NO2
NO2
2
and CO2 in the absence of NOx . In the presence of NO2,
the NE may directly react with NO2 to form N2 and N2O.
3. HNCO had a high reactivity for hydrolysis and is
completely decomposed into NH3 and CO2 above 200 C.
HNCO hydrolysis followed by the reaction of NH3 with
HNCO
H O
H O
2
2
◦
>
350°C
CO2
CO
NOx to form N2 is a very feasible pathway.
4
. HCN was mostly hydrolyzed to NH3 and CO at high
NH3
temperatures and partly oxidized to HNCO in the pres-
ence of NO2. The main pathway for HCN formation in the
SCR is still unknown, since the HCN concentrations were
much higher than the value predicted by the decomposition
of NE.
NO, NO2
N + N O
2
2
5. N2 and N2O were formed by the reactions of NH3 with
FIG. 8. Speculated mechanism for selective catalytic reduction of NO, and NH with NO , the latter reaction being much
3
2
NOx by C2H4 over H–ferrierite.
faster.
. A reaction network was proposed for C2H4-SCR-NO2,
6
in which NO2 is converted to N2 and N2O, through the
formation of NE followed by its reaction with NO2 or its
decomposition into mainly HNCO, subsequent hydrolysis
to NH3, and finally its reaction with NOx .
temporarily changed to an excessively reduced state if the
NE is decomposed into HNCO and then is converted to
NH3. N2 is formed from these species by the combination
with oxidative NO2 and NO. We think that a step in which
a nitro compound is formed, or, in other words, a step in
which a C–N bond is formed, is essential in these SCR pro-
cesses, because after the establishment of this bond, the N
atom involved seems to be readily converted to excessively
reduced states and then to N2.
REFERENCES
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(
1990).
In this study we have focused our attention on under-
standing the nitrogen chemistry of a model SCR reaction
and the elucidation of promising pathways connecting the
reactants to the products through gaseous intermediates.
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an inactive by-product. However, we suppose that if a by-
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reaction and is converted to the same products under sim-
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1