Chemistry Letters Vol.33, No.4 (2004)
419
1
0
8
6
4
2
0
0
0
0
0
0
0
0
0
0
.09
.08
.07
.06
.05
.04
.03
.02
.01
0
µ
0
60
120
180
240
300
360
Time [min]
Figure 3. The time course of the reaction products formation
during denitration process. CO2 ( , ), NO + NO2 ( , ),
and N2O ( , ) were measured. The closed symbols represent
the experiments with AC whereas the open symbols represent
the blank experiments.
0
60 120 180 240 300 360
Time [min]
Figure 2. The time course of HNO2 formation as a function of
AC amount suspended in 200 mL of reacting solution. 2.0 g ( ),
.020 g ( ), and 0.0 g (blank experiment) ( ).
dition of AC (i.e. 10 g/L) suppresses practically the induction
period.
0
The relationship between the decrease of the HCOOH con-
centration in the reacting solution and CO2 formation is not af-
fected by the presence of AC. The amount of formed CO2 is sig-
nificantly smaller than HCOOH consumption. It can be also
observed that the amounts of reacted HCOOH and correspond-
ing CO2 are not related to the presence of AC. In other words,
the carbon balance is not affected by the presence of AC. From
here it is clear that the AC itself is not involved as a reactant in
the formation of CO2. At present, it is not clear the reason for the
difference observed between carbon consumption and release.
As a conclusion, AC was found to be an effective catalyst
for denitration of concentrated HNO3 by HCOOH at low temper-
ature (325 K). This catalytic effect was attributed to rapid forma-
tion of HNO2 on the adsorption site of AC. The delay in HNO2
formation was found to be dependent on the concentration of the
AC adsorption sites. AC promotes the fast formation of HNO2.
The catalytic reduction on the adsorption sites on the surface
of AC is significantly different from the redox-type mechanism
the formation of HNO2 is presented in Figure 2. It can be ob-
served that the induction period for HNO2 formation decreased
gradually with increasing the amount of AC in reacting suspen-
sion. After the induction period, which was related to the AC
amount, the rate of HNO2 formation and the maximum concen-
ꢂ2
tration of HNO2 (ca. 7.2ꢁ10 mol/L) were independent of the
AC concentration. The HNO2 formation rate measured for 2.0,
0
.020, and 0.0 g of AC were 1.3ꢁ10ꢂ3, 0.7ꢁ10 , and 0.7ꢁ10
ꢂ3
ꢂ3
[
mol/L/s], respectively. The experimental data show that the ad-
sorption sites of AC promote the rapid formation of HNO2, up
.005 mol/L. Once the reaction is triggered by the fast formation
0
of HNO2, the reaction develops autocatalytically and AC does
not behave as catalyst any more. Thus, the function of the AC
is to enhance the formation of HNO2 up to a threshold value.
The effect of AC observed here is exactly same to what has been
2
,3
reported for precious metals.
The formation of gaseous reaction products (NO, N2O, NO2,
and CO2) in the presence or absence (blank experiment) of AC is
comparatively presented in Figure 3. The formation of N2 and
CO during denitration reaction was not evidenced. From Figure
3
proposed for the surface of Pt. However, our experimental re-
sults cannot rule out completely the possibility that the redox-
type mechanism becomes a predominant reaction route for long
reacting time (i.e. >240 min). It is obvious that using AC has
several advantages as a catalyst when compared to supported
precious metal catalysts. It is inexpensive and the problem of
metal dissolution is solved. The denitration system based on
AC should be further improved to decrease the maximum con-
centration level of HNO2, and thus to completely exclude the
hazard of explosions.
3
it comes out that the reaction mechanism is independent on the
presence or absence of AC, because the profile of reaction prod-
ucts is similar in both cases. From analysis of the experimental
data depicted in Figures 2 and 3, three stages of the chemical de-
nitration by formic acid can be identified. In the first stage, called
induction period, the concentration of HNO2 increase up to ca.
0.005 mol/L. In the second stage the concentration of HNO2 in-
creases fast as a result of an autocatalytic generation of HNO2,
ꢂ3
with a rate of around 0.9ꢁ10 mol/L/s, until a threshold value
References
of ca. 0.07 mol/L is reached. Then, in the third stage, the effec-
tive denitration starts by the formation of gaseous reaction prod-
ucts (NO, N2O, NO2, and CO2). The surface of AC has a cata-
lytic effect only in the first stage of denitration process by
increasing the initial rate of HNO2 formation. In the absence
of AC, the low temperature denitration (i.e. 325 K) requires an
induction period as long as 240 min. On the other hand, the ad-
1
E. R. Merz, ‘‘Denitration of Radioactive Liquid Waste,’’
Graham & Helaszovich, London (1986), p 1.
J. C. Fanning, Coord. Chem. Rev., 199, 159 (2000).
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4
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Published on the web (Advance View) March 6, 2004; DOI 10.1246/cl.2004.418