Reductive removal of gaseous nitrous oxide by activated carbon with metal oxide catalysts

Article abstract of DOI:10.1039/c6ra26983d

The efficient reductive decomposition of gaseous nitrous oxide (N₂O) from industrial effluents is of practical importance in abating greenhouse gas emissions. In the present study, active carbon (AC) has been chosen as both a reductive agent and a catalyst support. Dozens of AC-based catalysts with different kinds and amounts of metal oxide have been prepared under various conditions and characterized. The performances of Cu-containing AC have been studied at varying gas flow rates, Cu contents, and calcination temperatures. N₂O in a gas mixture (42% N₂O, 58% N₂) was found to be completely removed by Cu-loaded AC (9 wt% Cu, calcined at 400 °C) at 325 °C with a GHSV of 2293 h^?1. This process is viable by virtue of the low cost of AC and easier manipulation and process control in comparison with alternative methods employing reductive gases such as hydrogen and ammonia.

Full text of DOI:10.1039/c6ra26983d

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Reductive removal of gaseous nitrous oxide by
activated carbon with metal oxide catalysts
Cite this: RSC Adv., 2017, 7, 10407
c
b
b
ab
Hong Meng,b Linpo Yuan,ab Jiajun Gao, Nannan Ren, Yingzhou Lu and Chunxi Li
*
The efficient reductive decomposition of gaseous nitrous oxide (N2O) from industrial effluents is of practical
importance in abating greenhouse gas emissions. In the present study, active carbon (AC) has been chosen
as both a reductive agent and a catalyst support. Dozens of AC-based catalysts with different kinds and
amounts of metal oxide have been prepared under various conditions and characterized. The
performances of Cu-containing AC have been studied at varying gas flow rates, Cu contents, and
calcination temperatures. N2O in a gas mixture (42% N2O, 58% N2) was found to be completely removed
by Cu-loaded AC (9 wt% Cu, calcined at 400 ꢀC) at 325 ꢀC with a GHSV of 2293 hꢁ1. This process is
viable by virtue of the low cost of AC and easier manipulation and process control in comparison with
alternative methods employing reductive gases such as hydrogen and ammonia.
Received 18th November 2016
Accepted 2nd February 2017
DOI: 10.1039/c6ra26983d
rsc.li/rsc-advances
according to the composition of the gas stream, which
complicates the process. Activated carbon (AC), as an inexpen-
1. Introduction
sive and active solid reducing agent, can overcome these
drawbacks, and is thus worthy of study for the reductive
removal of N2O. In fact, coal char is effective in reducing N2O
and NO to N2 at combustion temperature,17 and AC loaded with
alkali metal oxides shows better performance. Both Na and K
can catalyze the reaction between carbon and N2O, and K is
better than Na.18,19 The reaction between N2O and C may also be
catalyzed by Ni and Pt, and further promoted by K, as reported
by Gonçalves et al.20,21
To date, little research has been conducted to compare the
performances of different metal oxides supported on AC for the
removal of N2O under otherwise identical conditions. In this
paper we are focused on the treatment of the tail gas of adipic
acid production process through catalytic reduction of N2O
using AC as a solid reducing agent. For this purpose, we made
a comparative study on the catalytic performances of different
metal oxides for the reduction of N2O on AC, and CuO is found
to be the most efficient one. Based on catalyst characterization,
a structure–activity relationship has been elucidated, and
a reaction mechanism is proposed.
Nitrous oxide (N2O) is a greenhouse gas with a global warming
potential value of about 310. As a by-product of the manufac-
ture of several chemical products, such as adipic acid and
nitric acid, its atmospheric concentration has increased
signicantly over the last decades, and continues to increase
by 0.2–0.3% per year.1 This inexorable trend has attracted
a great deal of attention for the development of an efficient
method for the removal of N2O from industrial exhausts,
especially for the tail gas of the adipic acid production process
using HNO3 as oxidizing agent, where the N2O content is as
high as 38% along with N2 48.1%, O2 4.4%, CO2 9.5%, and NO
0.03% (in wt%).2
A direct decomposition process is generally used for gas
effluents with low N2O concentrations,3 which operates at high
temperatures (>500 ꢀC) with various catalysts, e.g., hydrotalcite-
like materials,4 metal-containing zeolites,5–7 or supported metal
oxides,8–11 e.g. CaO–Co3O4,12 and NiLaOx.13 The decomposition
of N2O is composed of three steps, viz., adsorption, cleavage of
the N–O bond, and desorption of O2, of which the desorption of
O2 is considered as the rate-limiting step. To promote O2
desorption, some reductive substances, e.g., CH4, NH3,14 H2,15
and CO,16 have been used to instantly consume the in situ
generated O2. However, these reductive gases are expensive and/
or explosive, and their ow rate needs dynamic control
2. Experimental
2.1 Chemical materials
The AC used was a commercial coconut-shell-derived carbon
(Tangshan Hua Neng Technology Carbon Co., Ltd., Tangshan,
China). All metal nitrates, i.e. Cu(NO3)2$3H2O, Ce(NO3)3$6H2O,
Ni(NO3)2$6H2O, Zn(NO3)2$6H2O, Co(NO3)2$6H2O, Fe(NO3)3$9H2O,
and Mn(NO3)2, were of analytical reagent grade (Beijing Chem. Co.,
Ltd., Beijing, China, purity $ 99.0%) and were used as received. All
aqueous solutions were prepared with deionized water.
aState Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, P. R. China. E-mail: licx@mail.buct.edu.cn;
Fax: +86 10 64410308; Tel: +86 10 64410308
bCollege of Chemical Engineering, Beijing University of Chemical Technology, Beijing
100029, P. R. China
cSchool of Chemical Engineering and Pharmacy, Wuhan Institute of Technology,
Wuhan 430205, P. R. China
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reaction temperature was varied from 150 to 325 ꢀC in steps of
25 ꢀC to evaluate the activities of the catalysts calcined at
different temperatures. In all of the experiments, temperature
was increased at a rate of 5 ꢀC minꢁ1. At any given temperature,
the reaction duration was 1 h. The concentration of N2O in the
outlet gas was monitored online by means of a TM GC7700 gas
chromatograph equipped with a TCD and a Porapak Q (6 m ꢂ 3
mm) column.
2.2 Pre-treatment of AC
The AC was rstly ground into particles of 10–20 mesh, boiled
with deionized water for 15 min, and then washed ve times
with deionized water. The resultant particles were placed in an
ꢀ
oven and dried at 110 C for 24 h.
2.3 Preparation of AC-supported metal oxide catalysts
Certain amounts (0.1–11 wt%, mass fraction of metal atom in
AC) of metal nitrate were weighed and dissolved in water (300
mL), AC (30 g) was added to each metal nitrate solution, and the
mixtures were stirred magnetically for 24 h at room tempera-
3. Results and discussion
3.1 Characterization of the catalysts
ꢀ
ture. They were then placed in an oven and dried at 90 C for
Structural characteristics of AC. The AC support used here is
a coconut-shell-derived carbon material, which is commercially
available. Its structure has been characterized by many
researchers22–24 using N2 adsorption–desorption method, and
the structural parameters were determined as follows, i.e. BET
specic area 797 ꢃ 45 m2 gꢁ1, pore size 2.03 ꢃ 0.07 nm, and
total pore volume 0.402 ꢃ 0.02 m3 gꢁ1. Obviously, the AC has
a larger pore volume and specic area with rich porosity and
micropores, which is much higher than that of coal chars and
comparable with many commercial AC sorbents.25–27 The pore
size and surface area is known to be instrumental for the mass
transfer and reactivity for a heterogeneous reaction, and
a higher surface area and porosity is helpful for enhancing the
dispersion of the catalysts and accessibility of the AC and thus
the reduction rate of N2O. In short, higher surface area favors
the catalytic reactivity of N2O reduction on AC.
FTIR characterization. FTIR spectra of the pristine AC and
metal-oxide-loaded AC samples are shown in Fig. 1. All samples
showed an absorption at n z 3435 cmꢁ1, which could be ascribed
to the stretching vibration of O–H28,29 due to the presence of
water. The water contents of all samples were similar, except for
AC-supported Cu(NO3)2, as evidenced by the similar intensities of
their O–H peaks. Furthermore, all of the samples showed two
bands centered at n ¼ 2921 and 2852 cmꢁ1 due to asymmetric
and symmetric C–H stretching vibrations in aliphatic CH, CH2,
and CH3 groups.29,30 A scissor vibration of a CH2 group is
responsible for an absorption peak at n ¼ 1455 cmꢁ1, which was
no longer seen for the metal-oxide-loaded AC samples. For AC-
supported Cu(NO3)2, two bands were seen at n ¼ 1383 and
1340 cmꢁ1 attributable to nitrate (n ¼ 1410–1340 cmꢁ1),30 which
disappeared in the calcined ACs. From the above results, we can
deduce that: (1) all nitrates were decomposed to the corre-
72 h until the water had completely evaporated. The resultant
AC was transferred to a muffle furnace and calcined at 200, 300,
or 400 ꢀC for 2 h. Hereinaer, the obtained catalysts are
designated as xM-AC-T, where M represents the supported
metal, x is the mass fraction of metal atoms in the AC, and T is
the calcination temperature.
2.4 Catalyst characterization
Fourier-transform infrared (FTIR) spectra in the range n ¼ 4000–
400 cmꢁ1 were recorded at room temperature on a Nicolet 6700
spectrometer. Spectra were collected aer 32 scans at 4 cmꢁ1
resolution. The spectra were acquired from KBr pellets
composed of 1 mg of the samples and 100 mg of KBr dried at
200 ꢀC for 24 h. X-ray diffraction (XRD) patterns of the calcined
samples were recorded at room temperature on a D/Max 2500
(VB2+ PC) diffractometer using Cu-Ka radiation, operated at 40
kV and 200 mA. The patterns were recorded over a 2q range of
10–90ꢀ with a 0.02ꢀ step size. Simultaneous thermogravimetric
analysis (TGA) and derivative thermogravimetric analysis curves
were recorded on a TGA/DSC1/1100 SF apparatus at a heating
rate of 10 ꢀC minꢁ1 over the range 25–700 ꢀC in air. X-ray
photoelectron spectroscopy (XPS) spectra were recorded on an
ESCALAB 250 apparatus with a scanning electron spectroscopy
for chemical analysis (ESCA) microscope equipped with an Al
monochromatic X-ray source (300 W), employing a beam
diameter of 2 mm in CAE analyzer mode. All binding energies
were referenced to the C 1s peak at 285 eV. The individual
components were obtained by curve tting.
2.5 N2O removal experiments
A stainless steel tubular reactor of F5 (i.d.) ꢂ 650 mm (effective sponding metal oxides upon calcination at 400 ꢀC; (2) the
length 200 mm) was used to investigate the activity of metal structural change of AC is virtually irrelevant to the metal species.
oxides supported on AC for N2O removal at atmospheric pres-
Fig. 2 shows the FTIR spectra of Cu-loaded AC samples
ꢀ
sure. Catalyst (2.0 g) was loaded into the reactor each time, and calcined at 200, 300, and 400 C, respectively. It is evident that
the gas mixture (42% N2O, 58% N2) was fed at rates varying from the loaded Cu(NO3)2 was completely converted to CuO in all of
50 to 150 mL minꢁ1 by means of a V10FA mass ow controller. the calcined samples.
The reaction temperature was controlled through a proportion
XRD characterization. The XRD patterns of the pristine and
integral derivative (PID)-regulated oven. To compare the cata- metal-oxide-loaded AC samples are shown in Fig. 3. The pristine
lytic performances of different transition metal oxides, the AC showed two broad peaks, viz. at 2q z 23ꢀ and 43ꢀ, respec-
reaction temperature was varied from 300 to 550 ꢀC in steps of tively, which are characteristic of amorphous carbon.31 These two
50 ꢀC. To study the effect of the Cu-loading on the performance peaks were also present in the patterns of all of the metal-oxide-
of the AC-supported Cu catalysts, the reaction temperature was loaded AC samples, indicating retention of the amorphous
ꢀ
ꢀ
varied from 300 to 550 C in steps of 25 C. Furthermore, the structure of AC in the modied AC catalysts. The pattern for
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Fig. 3 XRD patterns of pristine AC and metal-oxide-loaded samples.
0.01Cu-AC-400 features two distinct peaks at 35ꢀ and 38ꢀ, which
can be ascribed to CuO (JCPDS).32 Except for 0.01Cu-AC-400, the
diffraction peaks of other metal oxides on the AC samples were
virtually unobservable. This may indicate amorphous structures
of these metal oxides due to their poor crystallinity at a calcina-
tion temperature of 400 ꢀC. Chmielarz et al. studied cobalt-
containing hydrotalcite-like materials, and found that an amor-
phous structure of the metal oxide was formed when the calci-
nation temperature was 600 ꢀC, but that the crystallinity
increased signicantly with temperature.33 Therefore, the poor
crystallinity of some of the present metal oxides on AC may have
resulted from the lower calcination temperature.
Fig. 1 FTIR spectra of pristine AC and metal-oxide-loaded samples.
XRD patterns of the Cu-loaded AC samples calcined at 200,
ꢀ
300, and 400 C are presented in Fig. 4. Several peaks attribut-
able to CuO and Cu2O were observed,32,34 and the intensities of
these peaks increased steadily with increasing calcination
temperature, implying increasing crystallinity of the copper
oxides. Additionally, the content of Cu2O increased with
increasing calcination temperature, suggesting that a certain
ꢀ
amount of CuO was reduced by AC at 400 C.
Thermal analysis. TGA results pertaining to three 0.09Cu-AC
samples calcined at 200, 300, and 400 ꢀC are presented in
Fig. 5. It can be seen that each of the samples showed a small
Fig. 2 FTIR spectra of AC with CuO calcined at different temperatures.
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Fig. 4 XRD patterns of AC-supported CuO calcined at different
temperatures (Cu2+1O is a crystal mixture of CuO and Cu2O).
peak before 100 ꢀC due to the evaporation of water. Based on the
derivative weight curves, the initiating temperatures of carbon
oxidation by air are about 280, 335, and 375 ꢀC for 0.09Cu-AC-200,
0.09Cu-AC-300, and 0.09Cu-AC-400, respectively, and the corre-
sponding temperatures for complete carbon oxidation are about
522, 587, and 607 ꢀC. The reactivity of the metal-oxide-loaded
carbon samples decreases with increasing calcination tempera-
ture for the Cu-loaded AC catalysts, which may be attributed to
the decreasing accessibility of carbon to oxygen due to the
blanketing effect of the well-developed metal oxide crystalline
layer on the surface of the AC. The results also indicate that AC
can also be oxidized by the coexistent O2 in the gas stream at
about 375 ꢀC, indicating that the coexistent O2 may impose
a negative inuence on the reduction of N2O, as discussed latter.
3.2 Comparison of catalytic activities of different metal
oxides
The catalytic activities of different metal oxides were evaluated in
the tubular reactor using 2 g of each catalyst at a gas ow rate of
100 mL minꢁ1, and the results are shown in Table 1. It can be seen
that N2O was not decomposed over the pristine AC below 350 ꢀC,
but its decomposition increased signicantly with increasing
temperature. For example, the conversion of N2O increased
drastically from 6% at 400 ꢀC to 100% at 550 ꢀC. Moreover,
different metal oxides exhibited different activities for the
conversion of N2O. Specically, 0.01Cu-AC-400 and 0.01Ni-AC-400
completely removed N2O at 400 ꢀC and 450 ꢀC, respectively. The
products were exclusively N2 and CO2, as bets the occurrence of
the redox reaction between AC and N2O according to eqn (1):
Fig. 5 TGA curves of AC-supported CuO calcined at 200 ꢀC, 300 ꢀC,
and 400 ꢀC.
to the different activities of the supported metal oxides for the
redox reaction of AC and N2O. The catalytic activities decreased in
the order Cu > Co > Ni > Fe > Mn > Ce > Zn. This order conforms to
that reported by Campa et al. for the reductive removal of N2O
with CH4, where the order was Cu > Co > Mn.35 Therefore, 0.01Cu-
AC-400 proved to be the most active catalyst for N2O removal
among all those studied, and was deemed worthy of further study.
2N2O + C ¼ 2N2 + CO2
(1)
3.3 Reactivity of Cu-based ACs under varying conditions
Effect of Cu loading on the catalytic activity. As shown in
As evidenced by the FTIR and XRD results, the major differ- Fig. 6, the Cu loading had a signicant impact on the removal
ence between the catalysts lay in the loaded metal species, while of N2O. The complete decomposition temperature of N2O
the structure of the AC remained essentially the same. Therefore, decreased greatly with increasing Cu loading, e.g., from 450 ꢀC
ꢀ
the reactivity differences of the catalysts can be mainly attributed to 325 C as the Cu loading was increased from 0.1 wt% to 9
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Table 1 Dependence of conversion of N2O on temperature
N2O conversion (%)
Pristine/metal-loaded ACs
300 ꢀC
350 ꢀ
C
400 ꢀ
C
450 ꢀ
C
500 ꢀ
C
550 ꢀ
C
AC
0
2
1
3
3
4
0
3
4
0
3
2
13
5
18
1
4
6
16
49
81
66
100
—
28
30
61
39
100
94
100
—
—
100
100
100
100
—
100
—
—
—
—
—
—
0.01Mn-AC-400
0.01Fe-AC-400
0.01Co-AC-400
0.01Ni-AC-400
0.01Cu-AC-400
0.01Zn-AC-400
0.01Ce-AC-400
0.01CoCe-AC-400
13
17
35
29
100
5
8
35
17
wt%. At any given temperature, e.g. 300 ꢀC, the conversion N2O and effective volume of reactor) values of 764.4, 1528.8, and
increased with increasing Cu loading up to 9 wt%, beyond 2293.2 hꢁ1. The results are plotted in Fig. 8. At a lower reaction
which the Cu loading had little further inuence on the activity. temperature of 300 ꢀC, the conversion rate of N2O decreased with
Therefore, the optimal Cu loading was determined as 9 wt%.
increasing gas ow rate, being restricted by the reaction kinetics.
Effect of calcination temperature. Three Cu-loaded AC The conversion rates of N2O at the above ow rates were 22%,
samples with different calcination temperatures (200, 300, and 21%, and 12%, respectively. However, when the temperature was
ꢀ
400 C) were prepared, namely 0.09Cu-AC-200, 0.09Cu-AC-300, increased to 325 ꢀC, the conversion rate of N2O surged to 100% at
and 0.09Cu-AC-400, and their catalytic performances are gas ow rates of 100 and 150 mL minꢁ1, but the N2O conversion
compared in Fig. 7. Evidently, the calcination temperature had was only 51% at a gas ow rate of 50 mL minꢁ1. This phenom-
a signicant effect on the catalytic reduction of N2O, and enon could be attributed to the exothermic nature of the reaction
a higher calcination temperature was unfavorable for the cata- and the fast reaction kinetics at a higher temperature. As the ow
lytic reaction of AC and N2O. Specically, the complete removal rate of N2O is increased, more reaction heat is released due to the
ꢀ
temperature of N2O increased steadily from 275 C for 0.09Cu- efficient oxidation of carbon by N2O, which raises the tempera-
AC-200, to 300 ꢀC for 0.09Cu-AC-300, and to 325 ꢀC for ture of the AC, and thereby accelerates the reaction dramatically.
0.09Cu-AC-400. This phenomenon has also been observed by On the other hand, increasing gas ow rate can shorten the
Chmielarz et al.,33 who ascribed it to the changing crystal form retention time, and thus result in incomplete reduction of N2O.
and the decreasing surface area with increasing calcination Therefore, the reaction temperature and retention time syner-
temperature. This explanation is consistent with the increasing gistically determine the effect of ow rate on N2O conversion. The
crystallinity of copper oxides for 0.09Cu-AC-200, 0.09Cu-AC-300, appropriate GHSV was identied as 2293.2 hꢁ1
and 0.09Cu-AC-400, as evidenced by the XRD patterns in Fig. 4.
.
Effect of gas ow rate. The inuence of gas ow on the N2O
3.4 Catalytic mechanism
removal efficiency was studied for 0.09Cu-AC-400 at three
different ow rates, namely 50, 100, and 150 mL minꢁ1, corre-
sponding to GHSV (gas hourly space velocity; ratio of ow rate of
The decomposition process of N2O is believed to involve its
redox reaction with carbon under catalysis by the loaded metal
Fig. 6 Effect of Cu loading on N2O conversion.
Fig. 7 Dependence of N2O conversion on calcination temperature.
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(UR, obtained by heating the NTR at 400 ꢀC in N2O for 2 h).
Fig. 9 shows the Cu 2p XPS spectra.
As can be seen from the XPS spectra, the primary and
shoulder peaks appear at binding energies (B.E.) of around 933
and 943 eV, respectively, for Cu 2p3/2. As shown in Table 2, the
B.E. values of the primary peaks for NTR and UR were slightly
lower than that for OR, implying a lower valence of Cu in NTR
and UR. This might be attributed to the reduction of CuO by
carbon at a high temperature of 400 ꢀC. All of the samples
showed a shake-up satellite peak at around 943 eV, which is
characteristic of Cu2+. Meanwhile, the weakened satellite peaks
for NTR and UR imply decreased amounts of Cu2+ and
increased amounts of Cu+ or Cu0 therein. The intensity ratios of
the satellite and primary peaks for Cu 2p3/2 are shown in Table
2. This ratio is about 0.55 for standard CuO and 0 for a Cu0
phase.36 As can be seen in Table 2, NTR showed the lowest
valence of Cu, UR showed a slightly higher valence, and OR
showed the highest valence.
Fig. 8 Effect of gas flow rate on N2O conversion.
In addition to the XPS results, the above samples were also
analyzed by XRD. As is evident from Fig. 10, CuO peaks were
only seen for OR; they were no longer seen for NTR due to
complete conversion to Cu2O and Cu. Supposedly, at a high
oxide, as is manifested by the consumption of AC and the
presence of CO2 in the gas effluent. However, this process may
be achieved in two different ways, i.e., catalytic decomposition
of N2O, forming O2 and in situ oxidation therewith of the
carbon support, or reduction of copper oxides by carbon to
Cu metal, followed by the in situ oxidation of Cu by N2O. To
identify the reaction mechanism, we studied the XPS patterns of
three different reagents, namely, the original reactant (OR,
0.03Cu-AC-200), the nꢀitrogen-treated reactant (NTR, obtained
by heating OR at 400 C in N2 for 5 h), and the used reactant
ꢀ
temperature of 400 C under nitrogen, the CuO component in
the OR sample was successively reduced by AC to Cu2O and Cu,
such that the amount of Cu became dominant. In contrast, only
CuO and Cu2O were found in UR, and no elemental Cu could be
detected. This may be the net result of the oxidation of Cu by
N2O and the reduction of CuO by carbon when NTR was heated
ꢀ
to 400 C in N2O atmosphere. This phenomenon is consistent
with earlier investigations by Carabineiro et al.,37,38 and Zhu
et al.19
Based on our experimental evidence and above analysis, the
catalytic mechanism of CuO for the reductive removal of N2O by
AC is assumed to proceed as follows:
4CuO + C / 2Cu2O + CO2
2Cu2O + C / 4Cu + CO2
2Cu + N2O / Cu2O + N2
Cu2O + N2O / 2CuO + N2
(2)
(3)
(4)
(5)
In this process, CuO is involved in transfer of the O atom
from N2O to carbon, accelerating the overall redox reaction
between N2O and AC. In short, AC in the present process serves
Table 2 Cu 2p high-resolution XPS spectra
Cu 2p3/2
mpa B.E.
Cu 2p3/2
spb B.E.
Splitting energyc
(eV)
d
Sample
Isp/Imp
OR
NTR
UR
933.1
932.9
932.7
942.7
944.0
943.3
9.6
11.1
10.6
0.21
0.03
0.04
a
b
“mp” denotes the main peak. “sp” denotes the shake-up satellite
c
peak. “splitting energy” is the energy difference between mp and sp.
d
“Isp/Imp” is the intensity ratio of sp and mp.
Fig. 9 Cu 2p XPS patterns of OR, NTR, and UR.
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ambient temperature, and nally, the ultrane ashes can be
further removed to reach the environmental standard by wet or
electrostatic dust collector.
4. Conclusions
N2O can be completely decomposed by AC at 550 ꢀC through
a redox reaction between AC as reductant and N2O as oxidant.
The decomposition reaction is greatly enhanced by some tran-
sition metal oxide catalysts, among which CuO has proved to be
the best one. Complete removal of N2O was achieved by 0.09Cu-
AC-200 at a much lower temperature of 275 ꢀC. The catalytic
mechanism of CuO has been studied by XPS and XRD, which
would seem to involve the reduction of CuO to Cu2O and Cu by
AC, and oxidation of Cu to Cu2O and CuO by N2O. In this
process, CuO is involved in transfer of the O atom from N2O
to carbon, accelerating the overall redox reaction between N2O
and AC.
Fig. 10 XRD patterns of loaded ACs in different statuses.
as a consumptive solid reductant for reducing N2O to nitrogen,
while CuO functions as a catalyst for the reaction between N2O
and carbon.
References
1 M. A. Zamudio, S. Bensaid, D. Fino and N. Russo, Ind. Eng.
Chem. Res., 2011, 50, 2622–2627.
3.5 Applicability and challenge of the present process
Catalytic reduction of N2O may be achieved at mild conditions
by using AC as solid reducing agent and CuO as catalyst. This
process is especially suitable for the treatment of tail gas of the
adipic acid production process with high concentration of N2O,
and is advantageous due to its safety, ease of control, low
operating temperature (300–400 ꢀC), and lower cost of AC. And
even the industrial waste AC may be also applicable, which
further reduces the raw material cost and makes a resource use
of another industrial waste. However, some challenges should
be considered and tackled properly, e.g. competitive oxidation
of O2 with N2O and emission of toxic metal oxide nanoparticles
from the exhaust gas. In fact, such competitive oxidation is
inevitable in all reductive removal processes of N2O with varying
2 N. Li, Chem. Ind. Eng. Prog., 2007, 26, 1659–1661.
3 Y. Shen, C. Li, Y. Tang and S. Zhu, RSC Adv., 2015, 5, 13212–
13219.
4 S. Kannan, Appl. Clay Sci., 1998, 13, 347–362.
5 J. K. Lee, Y. J. Kim, H. J. Lee, S. H. Kim, S. J. Cho, I. S. Nam
and S. B. Hong, J. Catal., 2011, 284, 23–33.
6 B. M. Abu-Zied, Microporous Mesoporous Mater., 2011, 139,
59–66.
7 B. H. Chen, N. Liu, X. Y. Liu, R. D. Zhang, Y. P. Li, Y. X. Li and
X. L. Sun, Catal. Today, 2011, 175, 245–255.
8 T. P. Gaidey, Russ. J. Appl. Chem., 2009, 82, 1689–1705.
9 V. G. Komvokis, M. Marti, A. Delimitis, I. A. Vasalos and
K. S. Triantafyllidis, Appl. Catal., B, 2011, 103, 62–71.
reducing agents, such as H2, CO, and CH4. However, the reac- 10 S. S. Kim, S. J. Lee and S. C. Hong, Chem. Eng. J., 2011, 169,
tivity of N2O with AC is higher than that of O2, as manifested by 173–179.
its lower reaction temperature on 0.09Cu-AC-400, i.e. 300 ꢀC for 11 M. Hussain, D. Fino and N. Russo, J. Hazard. Mater., 2012,
N2O (Fig. 6) and about 375 ꢀC for air (O2) (Fig. 5). Therefore, the
211, 255–265.
inhibition effect of O2 for the reduction of N2O may be quite low 12 Q. Zhang, X. Tang, P. Ning, Y. Duan, Z. Song and Y. Shi, RSC
at appropriate conditions, for example, the presence of O2 was Adv., 2015, 5, 51263–51270.
found to have no inuence on the N2O-char reactivity at higher 13 C. Li, Y. Shen, S. Zhu and S. Shen, RSC Adv., 2014, 4, 29107–
temperature.39,40 In contrast, at lower temperature of 320 ꢀC and
29119.
lower N2O content (0.3% N2O in helium gas), the coexistent 14 T. Xu, C. Wang, X. Wu, B. Zhao, Z. Chen and D. Weng, RSC
1.5% O2 may lead to a 20% lowering of reactivity of N2O on Adv., 2016, 6, 97004–97011.
Cu2O-AC catalyst25 due to the blocking effect of O2 for the active 15 M. Mihet, V.-M. Cristea, P.-S. Agachi, A.-M. Cormos and
sites of AC.26 As a reducing agent, AC will be oxidized and
M. D. Lazar, RSC Adv., 2016, 6, 89259–89273.
consumed gradually, and the residue toxic metal oxides may be 16 P. Nematollahia and M. D. Esrali, RSC Adv., 2016, 6, 59091–
emitted from the exhaust gas to the atmosphere. However, this 59099.
problem may be solved through conventional technical 17 J. Rodriguez-Mirasol, A. C. Ooms, J. R. Pels, F. Kapteijn and
approaches. For example, by using a moving-bed reactor, most J. A. Moulijn, Combust. Flame, 1994, 99, 499–507.
of the ultrane metal oxide particles will be retained in the 18 Z. H. Zhu and G. Q. Lu, J. Catal., 1999, 187, 262–274.
reactor due to the ltering effect of the granular AC catalyst, 19 Z. H. Zhu, G. Q. Lu and R. T. Yang, J. Catal., 2000, 192, 77–87.
some of the particles can be collected as sediment in the baffled 20 F. Gonçalves and J. L. Figueiredo, Appl. Catal., B, 2004, 50,
heat exchanger for cooling the high temperature exhaust gas to
271–278.
This journal is © The Royal Society of Chemistry 2017
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RSC Advances
Paper
21 F. Gonçalves and J. L. Figueiredo, Appl. Catal., B, 2006, 62, 32 J. Co, P. D. Standards, Powder Diffraction File, ASTM,
181–192. Newtown Square, PA, USA, 2004, pp. 48–1548.
22 Y. Chen, Z. Hu, X. Wang, G. Zhao, Y. Liu and W. Liu, Acta 33 L. Chmielarz, M. Rutkowska, P. Kustrowski, M. Drozdek,
´
Phys.-Chim. Sin., 2008, 249, 1589–1596.
23 J. Sun, H. Wang and B. Sun, Chin. J. Environ. Eng., 2011, 5,
261–266.
24 C. Wang, Y. Liu, X. Zheng, Z. Wen, A. Liu, X. Xu and Z. Hu,
Gongneng Cailiao, 2010, 9, 1520–1528.
25 Z. H. Zhu, L. R. Radovic and G. Q. Lu, Carbon, 2000, 38, 451–
464.
26 Y. H. Li, G. Q. Lu and V. Rudolph, Chem. Eng. Sci., 1998, 53,
1–26.
27 S. A. Carabineiro, F. B. Fernandes, A. M. Ramos, J. Vital and
I. F. Silva, Catal. Today, 2000, 57, 305–312.
Z. Piwowarska, B. Dudek, R. Dziembaj and M. Michalik, J.
Therm. Anal. Calorim., 2011, 105, 161–170.
34 J. Co, P. D. Standards, Powder Diffraction File, ASTM,
Newtown Square, PA, USA, 2004, pp. 05–0667.
35 M. C. Campa, V. Indovina and D. Pietrogiacomi, Appl. Catal.,
B, 2012, 111, 90–95.
36 A. Patel, P. Shukla, T. Rufford, S. Wang, J. Chen, V. Rudolph
and Z. H. Zhu, Appl. Catal., A, 2011, 409, 55–65.
37 S. A. Carabineiro, F. B. Fernandes, R. J. C. Silva, J. S. Vital,
A. M. Ramos and I. M. Fonseca, Catal. Today, 2008, 133,
441–447.
28 A. N. Zhou, X. L. Ma and C. S. Song, Appl. Catal., B, 2009, 87, 38 S. A. Carabineiro, F. B. Fernandes, J. S. Vital, A. M. Ramos
190–199. and I. M. Fonseca, Appl. Catal., B, 2005, 59, 181–186.
29 Y. F. Jia and K. M. Thomas, Langmuir, 2000, 16, 1114–1122. 39 I. Gulyurtlu, H. Esparteiro and I. Cabrita, Fuel, 1994, 73,
30 B. H. Stuart, Infrared Spectroscopy: Fundamentals and
Applications, Wiley, 2004.
31 W. N. R. W. Isahak, M. W. M. Hisham and M. A. Yarmo, J.
Chem., 2013, 2013, 1–6.
1098–1102.
40 J. Rodriguez-Mirasol, A. C. Ooms, J. R. Pels, F. Kapteijn and
J. A. Moulijn, Combust. Flame, 1994, 99, 499–507.
10414 | RSC Adv., 2017, 7, 10407–10414
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