Catalytic hydrolysis of HCN over H-ferrierite

Article abstract of DOI:10.1246/cl.2000.986

The decomposition of HCN over H-ferrierite below 670 K proceeded mainly by hydrolysis to NH₃ and CO. In the presence of NO and NO₂, NH₃ formed further reacted to form N₂ and N₂O.

Full text of DOI:10.1246/cl.2000.986

986
Chemistry Letters 2000
Catalytic Hydrolysis of HCN over H-Ferrierite
Tetsuya Nanba,* Akira Obuchi, Somchai Akaratiwa, Shetian Liu, Junko Uchisawa, and Satoshi Kushiyama
Atmospheric Environmental Protection Department, National Institute for Resources and Environment
16-3 Onogawa, Tsukuba, Ibaraki, 305-8569
(Received May 24, 2000; CL-000503)
The decomposition of HCN over H-ferrierite below 670 K
About 30 mg of H-ferrierite (Tosoh; HSZ-720HOA,
SiO2/Al2O3= 17.0) was wash-coated onto a small piece of
cylindrical cordierite honeycomb (8 mmφ, 9 mm length; 400
cell inch–2), and the catalytic reactions were carried out under
conventional flow reactor conditions with a flow rate of 160
mL min–1.7 HCN was continuously generated by the reaction
between KCN and H2SO4 and was added to the reactant gas.
HCN concentration was 220–300 ppm. The N2, N2O, CO, and
CO2 produced were analyzed with GC, and other gaseous prod-
ucts were analyzed with an FTIR (Nicolet; Magna 560, resolu-
tion set at 0.5 cm–1) equipped with a multi-reflection gas cell
(Gemini Specialty Optics; Mercury Series, optical path length =
2 m).
proceeded mainly by hydrolysis to NH3 and CO. In the pres-
ence of NO and NO2, NH3 formed further reacted to form N2
and N2O.
The selective catalytic reduction of NOx with hydrocar-
bons (HC-SCR) has attracted the attention of many catalyst
researchers over the past decade.1 Although many studies have
investigated the reaction steps and the intermediates thought to
be involved in this process, the reaction mechanisms are still
not fully understood. Several research groups have observed
nitrogen-containing by-products such as HCN,2,3 HNCO,4 and
acrylonitrile5 and suggested that they may be intermediates in
the HC-SCR process. In this study, we focused our interest on
HCN and investigated its decomposition behavior over H-fer-
rierite, which was reported to be a good catalyst for HC-SCR,6
and the role for N2 and N2O formation in HC-SCR.
Figure 1 shows the decomposition behavior of HCN in the
presence of various co-existing gases. NOx (= NO + NO2) con-
version in figures 1(c) and 1(d) was defined as (NOx inlet
concn(ppm) – NOx outlet concn (ppm))/(HCN inlet concn
(ppm)) × 100. Product yield was defined as (product concn
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
987
(ppm))/(HCN inlet concn (ppm)) × 100. As shown in Figure
1(a), HCN decomposed into equivalent amounts of NH3 and
CO in the presence of H2O vapor. Evidently, hydrolysis
occurred according to the reaction:
HCN conversion increased with increasing temperature. Since
equivalent amounts of NH3 and CO were also produced under
O2 + H2O flow below 670 K (Figure 1(b)), it is clear that HCN
was preferentially hydrolyzed under these conditions. Above
670 K, small amounts of NO and CO2 were detected. Since the
decomposition of NH3 or CO was negligible under similar con-
ditions, we speculate the NO and CO2 to be formed by the oxi-
dation of HCN. Figures 1(c) and (d) show HCN decomposition
behavior in the presence of NO + O2 + H2O and NO2 + O2 +
H2O, respectively. HCN conversions increased, compared to
the results obtained in the absence of NO or NO2. However,
CO yields were almost equal to HCN conversions below 670 K.
On the other hand, NH3 was not formed at all; instead, N2 was
formed in amounts equal to the amount of HCN converted.
Moreover, NOx conversion in figure 1(c) was very close to N2
yield. These results strongly suggest that one NO molecule
reacts with one NH3 molecule produced by HCN hydrolysis to
give one N2 molecule. In figure 1(d), NOx conversion was
apparently higher than HCN conversion and N2 yield but it was
due to the formation of HNO3 and HNO2. In the presence of
NO2, HNCO was formed at high temperatures, probably due to
the oxidation of HCN by NO2, and N2O was formed with the
maximum yield at 623 K, presumably as a product of the reac-
tion between NO2 and NH3. The increase in the HCN conver-
sion in the presence of NOx is discussed below.
tion of NO2 (1000 ppm) with C2H4 (1000 ppm) in the presence
of O2 (5%) over the same H-ferrierite, and observed the produc-
tion of 76, 106, and 51 ppm HCN at 523, 623, and 723 K,
respectively. Taking account of the high reactivity toward
hydrolysis, HCN observed in the HC-SCR process may not be a
simple by-product but may be one of the intermediates toward
N2 and N2O. The mechanism of HCN formation in HC-SCR is
under investigation and will be reported elsewhere.
Figure 2 shows the selectivities of N2 and N2O in the NH3
+ NO + NO2 + O2 reaction at 623 K with a varying ratio of NO
and NO2 in the reactant gas (total NOx = 1000 ppm). Under all
conditions, NH3 was completely converted into N2 or N2O.
N2O formation was observed particularly in the presence of
much more NO2 than NO. It is evident from these results that
NH3 reacts rapidly on H-ferrierite with both NO and NO2 to
produce N2 and N2O. Also, it is strongly suggested that self-
poisoning by the NH3 formed was suppressed due to the reac-
tion between NH3 and NOx, so that HCN hydrolysis, occurring
most probably on acid sites, was enhanced in the presence of
NOx.
In conclusion, below 670 K the decomposition of HCN
over H-ferrierite proceeds preferentially through hydrolysis,
which results in the formation of NH3 and CO. In the presence
of NO or NO2, the NH3 formed reacts further with NOx to form
N2 and N2O, with N2O produced only in the presence of much
more NO2 than NO. We separately performed selective reduc-
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