The highly efficient removal of HCN over Cu8Mn2/CeO2catalytic material

Article abstract of DOI:10.1039/d0ra10177j

In this work, porous CeO₂flower-like spheres loaded with bimetal oxides were prepared to achieve effective removal of HCN in the lower temperature region of 30-150 °C. Among all samples, the CeO₂loaded with copper and manganese oxide

Full text of DOI:10.1039/d0ra10177j

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The highly efficient removal of HCN over Cu8Mn2/
CeO2 catalytic material†
Cite this: RSC Adv., 2021, 11, 8886
ab
b
Jie Sun, Jigang Li,b Yulin Yang,b Tian Zhou,b Shouping Weib
*
Zhihao Yi,
and Anna Zhua
In this work, porous CeO2 flower-like spheres loaded with bimetal oxides were prepared to achieve
effective removal of HCN in the lower temperature region of 30–150 ꢀC. Among all samples, the
CeO2 loaded with copper and manganese oxides at the mass ratio of 8/2 (Cu8Mn2/CeO2) exhibited
the highest catalytic activity: the HCN removal rate was nearly 100% at 90 ꢀC at the conditions of
120 000 hꢁ1 and 5 vol% H2O, the catalytic activity of which was higher than for other reported
catalysts. The introduction of MnOx could improve the dispersion of CuO particles and increase the
total acid sites of the prepared samples. It was proved that the synergy between CuO and MnOx, the
chemisorption oxygen, the oxygen vacancies, the Cu2+ and Mn4+ all played an important role in
determining the good catalytic activity of the prepared samples. NH3-TPD analysis indicated the
introduction of MnOx promoted the conversion of NH3 and N2 selectivity by increasing the acid sites
of the sample. According to the C, N balance data and FT-IR results, when the temperature was below
30 ꢀC, the removal of HCN over Cu8Mn2/CeO2 was mainly by chemisorption and the HCN
breakthrough behaviors corresponded to the Yoon and Nelson's model. When temperature was above
120 ꢀC, the HCN was totally removed by catalytic hydrolysis and catalytic oxidation.
Received 2nd December 2020
Accepted 1st February 2021
DOI: 10.1039/d0ra10177j
rsc.li/rsc-advances
Diverse strategies have been reported to remove HCN from
gas which involves absorption, adsorption, combustion, cata-
lytic hydrolysis and catalytic oxidation.8
1 Introduction
Hydrogen cyanide (HCN) is a kind of highly toxic and volatile
compound, with a boiling point of 26 ꢀC, which is widely used in
the laboratory and chemical industry.1 HCN could inhibit the
enzyme cytochrome oxidase, and interfere with aerobic respi-
ration at cellular level, which poses a threat to human health.2
Signicantly, as the precursor of nitrogen oxides such as NO,
NO2 and N2O, HCN could also lead to the greenhouse effect and
generation of photochemical fumes.3 In addition to laboratory
synthesis, there is a fair chance to produce HCN including the
processing of cyanide-containing chemicals,4 processing tail
gas of calcium and yellow phosphorus,5 fossil fuels and biomass
combustion,6 selective catalytic reduction of NOx as well as the
well-established three-way-catalyst process.7 To relieve or
reduce the negative effect on both the human body and the
environment caused by HCN, it is essential to develop highly
efficient dicyanide methods. Based on the above discussion, the
removal of HCN has been paid more and more attention and
the treatment of HCN has important prospects.
Liu et al.9 studied the catalytic combustion of HCN over ZSM-5
catalysts loaded with Cu, Co, Fe, Mn, Ni species. Among all
samples, Cu-ZSM-5 exhibited excellent HCN conversion activity
(T90 ¼ 350 ꢀC) and high N2 yield (>95%). Nickolov et al.10 reported
the adsorption characteristics of modied activate carbons (AC)
toward HCN. When loaded with Cu (5.6 wt%), Zn (5.24 wt%), Ag
(0.61 wt%), the AC material (VSZC) showed enhanced HCN
removal ability, which was proved to be comparable with the
standard ASC Whetlerite carbon. The Zn phase deposited near
the external surface of the sample was consumed rst to reduce
the production of cyanogen. It has also been proved that the
addition of negligible amount of Cr6+ (3.5 times lower than that
in ASC Whetlerite carbon) could enhance the conversion of
cyanogen. Hudson et al.11 synthesized an SBA-15 material modi-
ed with the BBOP and equilibrated with copper (II) nitrate.
Owing to the greater accessibility of the functional groups, the
doddered nature of the interconnected porous network and large
mesopores, The SBA-15 material showed an improved break-
through time for HCN (36 min) which was closed to the reference
AC adsorbents. Ma et al.12 investigated the Nb/La–TiOx catalyst's
activity for the hydrolysis of HCN, the conversion of HCN over
which could reach 100% accompanied with more than 90% NH3
selectivity. They proposed following mechanism of HCN hydro-
lysis over Nb/La–TiOx catalyst: HCN was rst converted to
aState Key Laboratory of NBC Protection for Civilian, Beijing, 102205, China
bDepartment of Chemistry Defense, Institute of NBC Defense, Beijing, 102205, China.
E-mail: magnsun@tsinghua.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra10177j
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intermediated CHONH2 under the function of catalyst, subse-
quently, the CHONH2 was continually hydrolyzed to NH4 and
2.1.2 Catalytic materials preparation. CeO2 loaded with
copper and manganese oxides were prepared by precipitation
method. The CeO2 sample was dissolved into deionized water with
magnetic stirring, followed by the addition of Cu(NO3)2$3H2O and
Mn(NO3)2$4H2O at the mass ratio of 8 : 2. The total content of Cu
and Mn oxides always accounted for 10 wt% of the CeO2 support.
Then the Na2CO3 solution (0.5 mol Lꢁ1) was added at the speed of
1drop/s until the pH value of the solution reached 9. Aer being
stirred for 4 h, the suspension and precipitate were separated by
centrifugation and the precipitate was washed with deionized
water and ethanol for three times respectively. Finally, the samples
donated as Cu8Mn2/CeO2 were obtained aer undergoing such
steps: calcination, grinding, pelletizing and sieving. The total
metal mass percentage was calculated by the equation (M/(M +
CeO2), M ¼ Cu, Mn), in which wt% means the mass percentage.
The Cu8Ag2/CeO2, Cu8Ni2/CeO2, Cu8Co2/CeO2, Cu8Zn2/CeO2 and
Cu8Fe2/CeO2 samples were also prepared by the same method by
changing the nitrate solution of metal species.
+
HCOOH with the participation of acid sites of the catalyst.
+
Finally, the NH4 was converted into NH3$H2O, and simulta-
neous HCOOH was decomposed to CO and H2O or CO2 and H2O.
Hu13 reported the metal oxides (Fe, Cu) modied HZSM-5 cata-
lysts synthesized by incipient wetness method showed improved
performance toward HCN removal (100% conversion of HCN and
80% selectivity of N2), the high catalytic activity of which could be
attributed to the highly dispersed iron and copper composites on
the surface of the catalyst support, the excellent redox ability and
regulated acid properties. Wang et al.14 carried out a detail
investigation on coupling catalytic hydrolysis and oxidation of
HCN over a serious MnOx/TiO2–Al2O3 prepared via sol–gel
method. 15 wt% MnOx/TiO2–Al2O3 showed the best performance
on HCN oxidation, and nearly 100% HCN conversion and 70% N2
yield were obtained at 200 ꢀC.
From these above statements, it could be observed the
catalytic hydrolysis and oxidation based on these catalysts
ꢀ
require relatively higher temperature (>200 C) to achieve deep
2.2 Experimental setup
purication of HCN, which might cost a lot of energy. Adsorbent
materials showed excellent removal efficiency toward HCN at
room temperature while the incomplete conversion of HCN and
the secondary pollution caused by the desorption products limit
the further application of this method. Therefore, it is necessary
to develop novel catalytic materials to obtain high HCN removal
The catalytic activity test of the prepared samples was evaluated
in a xed-bed reactor, and the reaction scheme was shown in
Fig. S1.† The prepared sample (0.25 g) was placed in a quartz
tube (with an inner diameter of 5 mm) for each reaction. The
mixed gases containing 400 mg mꢁ3 HCN and 5 vol% H2O were
introduced into the xed-bed quart tubular reactor at a total
ow rate of 400 mL minꢁ1 (GHSV ¼ 120 000 hꢁ1). The exhaust
gas from the sample bed was rst analyzed (as described below)
and then absorbed and discharged.
ꢀ
rate at lower temperature range (30–150 C).
CeO2 has been widely used in various elds including low-
temperature water-gas shi reaction, fuel cells, phosphors,
catalysts, cocatalysts, catalyst supports as well as hydrogen
storage materials due to the high oxygen storage/release capacity
(OSC) and high oxygen mobility arising from facile redox cycle
between Ce4+ and Ce3+.15,16 In recent reports, CeO2 materials with
different structure and morphologies has been prepared and
evidenced to exhibit unique properties differing from common
CeO2.17,18 The porous CeO2 owerlike sphere has attracted lots of
interest for its larger pore volume, higher surface area and
marked hydrothermal stability when used as supports.19,20 Tan
et al.21 conducted the oxidation of styrene on the porous CeO2
owerlike sphere loaded with CuO, La2O3 and Fe2O3, and found
the CuO/CeO2 showed the highest activity. Sun et al.22 examined
the catalytic activities and hydrothermal stability of Cu/CeO2 for
ethanol stream reforming. At 300 ꢀC, the H2 selectivity could
reach 74.9% while the CO was below the detection line.
2.3 Detection and analysis of reacted products
The outlet gas from the sample bed was rst analyzed and then
absorbed by the NaOH solution (1 mol Lꢁ1). The HCN and NH3
were determined by iso-nicotinic-acid-3-methy-1-phenyl-5-
pyrazolone spectrophotometric method and sodium hypochlo-
rite–salicylic acid spectrophotometric method respectively, the
detection limits of which were 0.005 mg Lꢁ1 and 0.0025 mg Lꢁ1
respectively. The concentration of CO, CO2, NO and NO2 (mg
mꢁ3) were measured using a ue gas analyzed (DeTu, 350,
German) aer the gas stabilized for 80 min at the corresponding
temperature. The HCN removal rate and reaction products
selectivity were calculated according to the eqn (S1)–(S8).†
2.4 Characterization
In this work, the porous CeO2 owerlike spheres modied
with a series of bimetallic oxides (Cu–Ag, Cu–Mn, Cu–Zn, Cu–
Ni, Cu–Co, Cu–Fe) were prepared and their catalytic activity for
HCN removal were evaluated under lower temperature region
The related characterization information was detailed in the
ESI.†
ꢀ
(30–150 C). The reaction kinetics and the mechanism of HCN
3 Results and discussion
3.1 Catalytic activity of CeO2 loaded with copper and
removal over the synthesized samples were analyzed.
manganese oxides
2 Experimental
2.1 Catalytic materials preparation
In order to evaluate the catalytic activity of CeO2 and the CeO2
loaded with copper and manganese oxides, the four prepared
2.1.1 Porous CeO2 owerlike spheres preparation. The samples including CeO2, CuO/CeO2, MnOx/CeO2 as well as
synthesis of porous CeO2 owerlike spheres was according to Cu8Mn2/CeO2 were tested for the HCN catalytic removal and the
our previous method, which were reported in the literature.22
results were shown in Fig. 1. It could be observed the HCN
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Cu8Ni2/CeO2, Cu8Co2/CeO2, Cu8Ag2/CeO2 and Cu8Zn2/CeO2
samples. All samples exhibited the same HCN removal trend as
Cu8Mn2/CeO2 at the temperature region of 30–150 ꢀC: the HCN
removal rate increased as temperature improving. Cu8Ni2/CeO2
and Cu8Fe2/CeO2 showed the similar catalytic performance and
their activity were both stronger than CuO/CeO2. The results
indicated the introduction of iron and nickel species could
generated good synergy with copper oxide species. Signicantly,
the addition of iron species could strengthen the redox prop-
erties of catalysts and increase the dispersion of copper species
by inhabiting the particles agglomeration.27 For Cu8Co2/CeO2, it
exhibited the same catalytic activity toward HCN as CuO/CeO2.
Noteworthy, when cobalt and sliver oxides were introduced, the
activity of two samples were lower than CuO/CeO2, this might be
ascribed to the weaker synergy and lower CuO active sites on the
support surface. Among all samples, the Cu8Mn2/CeO2 still
showed the most remarkable catalytic performance toward
HCN, implying the synergy between copper and manganese was
stronger than other bimetal oxides. Table S1† showed the
comparison of HCN catalytic removal activity over various
catalysts, indicating the Cu8Mn2/CeO2 exhibited the lowest
T100% temperature, at which HCN could be totally removed.
In order to investigate the thermal stability of Cu8Mn2/CeO2,
the Cu8Mn2/CeO2 pretreated at 200 ꢀC, 400 ꢀC and 600 ꢀC were
tested for the HCN removal and the results shown in Fig. S3†
indicated the catalytic activity of Cu8Mn2/CeO2 were not inu-
enced when the pretreated temperature varied. Further TG-DCS
analysis shown in Fig. S4† also proved the Cu8Mn2/CeO2
showed high thermal stability.
Fig. 1 The HCN removal rate over CeO2, CuO/CeO2, MnOx/CeO2 and
Cu8Mn2/CeO2 samples. Reaction condition: 120 000 hꢁ1, 5 vol% H2O.
removal rate of all samples increased with increasing the reac-
tion temperature from 30 ꢀC to 150 ꢀC. The CeO2 support
showed the lowest activity toward HCN, even at 150 ꢀC, the HCN
removal rate was still below 70%. The HCN removal rate over
CuO/CeO2 were higher than MnOx/CeO2 throughout the reac-
tion temperature region, indicating the CuO phase on the CeO2
played an important role in the HCN catalytic removal. The
reason why CuO exhibited good catalytic performance could be
explained by the interaction between CuO and HCN which was
demonstrated in following equations.23,24
2CuO + 4HCN / 2CuCN + (CN)2 + 2H2O
(1)
(2)
Cu2þ
It was necessary to know whether the Cu8Mn2/CeO2 could be
recycled. The regeneration of the Cu8Mn2/ꢀCeO2 were performed
by the calcination of the samples at 450 C for 4 h. Four recy-
cling runs for the Cu8Mn2/CeO2 were carried out, the results
shown in Fig. S5† indicated the Cu8Mn2/CeO2 could still achieve
nearly 80% removal rate toward HCN even aer 4 runs.
2ðCNÞ þ 2H2O þ O2
4HOCN
ƒƒƒƒ!
2
Cu2þ
2HCN þ O2
2HOCN
(3)
ƒƒƒƒ!
The highest HCN removal rate was obtained for Cu8Mn2/
CeO2: it was equal to 73% at 30 ꢀC and reached nearly 100% at
90 ꢀC. To determine the optimal Cu/Mn mass ratio, a serious of
CuxMny/CeO2 materials including Cu8Mn2/CeO2, Cu6Mn4/
CeO2, Cu4Mn6/CeO2 and Cu2Mn8/CeO2 were prepared and the
catalytic activity of these samples were compared with CuO/
CeO2 and MnOx/CeO2. As presented in Fig. S2,† the Cu8Mn2/
CeO2 still exhibited the highest activity and the optimal Cu/Mn
ratio was 8 : 2. The addition of a small amount of MnOx could
enhanced the catalytic activity of CuO/CeO2 and the results
might be attributed to the synergy between copper and
manganese oxides, which was in accordance with the study of Li
and Njagi et al. who studied the catalytic activity of Cu–Mn–O
catalysts toward HCN and CO respectively.25,26
3.3 Analysis of HCN reaction products over Cu8Mn2/CeO2
The HCN removal rate and the reaction products over Cu8Mn2/
CeO2 were investigated at the temperature region of 30–150 C
ꢀ
Cu2+ + Mn3+ 4 Cu+ + Mn4+
(4)
3.2 Catalytic activity of CeO2 loaded with different bimetal
oxides
Fig. 2 investigated the HCN catalytic removal rate over CeO2
Fig. 2 The HCN removal rate over CeO2 loaded with different bimetal
loaded with different bimetal oxides including Cu8Fe2/CeO2, oxides. Reaction condition: 120 000 hꢁ1, 5 vol% H2O.
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and the results were shown in Fig. 3. The products detected in
the outlet gas involved CO, CO2, NH3, NO and NO2. The selec-
tivity of CNꢁ and N2 were calculated according to the C-balance
N-balance data and the concentration of all reaction products
were listed in Table S2.†
At 30 ꢀC, the selectivity of CO and CO2 were not in line with
the HCN removed, which implied most HCN were converted to
CNꢁ due to the chemical interaction between CuO and MnOx
phase on the CeO2 support (eqn (1)) and the HCN was mainly
removed by chemisorption. The small amount of CO and NH3
might be attributed to the catalytic hydrolysis reaction of HCN
(shown in eqn (5)), while the CO2 were assigned to the catalytic
oxidation (shown in eqn (6)). In addition, the oxidation of CO
could also generate CO2.
ꢀ
ꢁ
C0 ꢁ C
Fig. 4 Plot of ln
–t for Cu8Mn2/CeO2 bed at temperature
C
HCN + H2O / NH3 + CO
(5)
(6)
region of 10–30 ꢀC. Reaction condition: 120 000 hꢁ1, 5 vol% H2O.
4HCN + 5O2 / 4CO2 + 2N2 + 2H2O
The catalytic hydrolysis of HCN over Cu8Mn2/CꢀeO2 showed
ꢀ
low activity at the temperature region of 30 C–60 C, the NH3
yield changed little with temperature increasing. However, the
high CO2 selectivity indicated the catalytic oxidation of HCN
was enhanced, the CNꢁ was oxidized to CO2 and released more
active sites, which could strengthen the HCN removal rate.
The selectivity of NH3 and CO2 increased signicantly from
ꢀ
60–150 C as results of catalytic hydrolysis and catalytic oxida-
tion of HCN showing high activity. The generated NO and NO2
might be attributed to the byproducts of catalytic oxidation of
HCN. Compared with NH3 generated by catalytic hydrolysis, the
amount of CO disappeared with temperature increasing due to
the oxidization of CO to CO2. When the temperature reached
120 C and increased further, the CO2 selectivity stayed 100%,
implying the HCN was completely removed by catalytic hydro-
lysis and oxidation.
Fig. 5 The SEM images of the CeO2. (1) and (2) Low and high
magnification SEM images of CeO2.
ꢀ
be attributed to the increase of acid sites on the Cu8Mn2/CeO2
aer MnOx introduced and the generated NH3 could be absor-
bed on the Cu8Mn2/CeO2 samples and converted into N2.
More signicantly, the NH3 selectivity generated by catalytic
hydrolysis was below than 20% at the whole reaction tempera-
ture, which were lower than the reported literature.12,13 It might
3.4 Breakthrough behaviors of HCN over Cu8Mn2/CeO2
By analyzing the reaction products of HCN, it was concluded
that the HCN was mainly removed by chemisorption at 30 C.
ꢀ
To study the catalytic materials' adsorption performance, the
breakthrough behaviour of HCN over Cu8Mn2/CeO2 was inves-
tigated and the breakthrough curves were shown in Fig. S6.†
The Yoon and Nelson's equation was introduced to describe
the breakthrough behaviour of HCN over Cu8Mn2/CeO2.28
ꢀ
ꢁ
C0 ꢁ C
ln
¼ kðs ꢁ tÞ
(7)
C
where C0 was the inlet concentration of HCN, mg mꢁ3; C
referred to the breakthrough concentration of HCN, mg mꢁ3; t
was the breakthrough time, min; s was the time required to
obtain 50% breakthrough, minꢁ1; k was the lumped shape
factor of breakthrough, minꢁ1
.
Fig.
4
demonstrated the experimental results of the
ꢀ
ꢁ
C0 ꢁ C
ln
and t curves, the values of k and s at different
C
Fig. 3 The removal rate of HCN and selectivity of reacted products
over Cu8Mn2/CeO2 at different temperature. Reaction condition:
120 000 hꢁ1, 5 vol% H2O.
temperature were calculated and the results were listed in Table
S3.† With temperature increased, the thermal motion of HCN
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Fig. 7 The HR-TEM images of the Cu8Mn2/CeO2 at different magni-
fication. (1) The overall Cu8Mn2/CeO2 morphology; (2) and (3) the
polycrystalline structure of Cu8Mn2/CeO2; (4) the CuO phases; (5) the
crystal planes of Cu8Mn2/CeO2; (6) the crystal planes of CeO2.
Fig. 6 The SEM images of the Cu8Mn2/CeO2 and the corresponding
elements (O, Cu, Ce, Mn) mapping. (1) and (2) Low and high magnifi-
cation SEM images of Cu8Mn2/CeO2.
Table 1 The structural parameters of the CeO2 loaded with different
bimetal oxides
Samples
CuO (wt%)
MxOy (wt%)
Crystal sizea
Cu8Mn2/CeO2
Cu8Zn2/CeO2
Cu8Ni2/CeO2
Cu8Co2/CeO2
Cu8Fe2/CeO2
Cu8Ag2/CeO2
9.7
9.3
9.5
9.3
10.7
9.4
1.8
2.1
2.3
2.3
2.0
1.7
—
30
17
22
—
41
a
Crystallite sizes calculated from the line broadening of the (002) plane
of CuO.
became intense, and the number of activated molecules
increased, which accelerated the saturation of activated sites on
the Cu8Mn2/CeO2. HCN was absorbed on the samples' surface
in the form of CNꢁ and could not be converted further. The
activated sites could not be released, as a result, the break-
through curve became steeper and breakthrough time became
shorter as temperature increased.
Fig. 8 The XRD patterns of CeO2 loaded with different bimetal oxides.
demonstrated the actual metal oxides loading were in accor-
dance with theoretical value.
In order to analyze the microstructure and crystallinity of the
Cu8Mn2/CeO2 samples, the HR-TEM characterization was
carried out and the results were shown in Fig. 7. As shown in
Fig. 7(1), it could be seen the Cu8Mn2/CeO2 exhibited a sphere
structure with diameter of 2–3 mm, which was consistent with
SEM characterization. Fig. 7(2) indicated the Cu8Mn2/CeO2 was
polycrystalline structure composed of many nanocrystals at
different orientations.30 The nanocrystals marked in Fig. 7(4)
was assigned to CuO species loaded on CeO2 support, the crystal
size of which were around 10–15 nm. Three interplanar spac-
ings of the ordered stripes marked in Fig. 7(5) were 0.27 nm,
0.31 nm and 0.23 nm, corresponding to the (200) and (111)
lattice planes of CeO2 and (111) lattice planes of CuO.31
However, there no MnOx phase observed in Fig. 7(5), this could
be explained by the high dispersion of MnOx due to the low
loading. Fig. 7(6) displayed the TEM image for CeO2, it could be
concluded the structure of CeO2 was not changed aer metal
oxides loaded and still exposed (200) and (111) lattice planes.
3.5 SEM and TEM analysis
The morphological properties of samples were characterized by
SEM analysis. As presented in Fig. 5, the CeO2 support exhibited
owerlike mesoporous microspheres with average diameter of
2–4 mm and dispersed evenly. There were many petal-like
nanosheets on the microsphere surface and these nanosheets
interweaved together to form the porous structure.29 Fig. 6
showed the morphology and size of Cu8Mn2/CeO2 materials, it
could be observed the metal oxide particles were loaded
dispersively on the CeO2 supports and the porous structure were
covered aer metal oxide species loaded. EDS analysis shown in
Fig. S6† implied the Cu and Mn elements had uniform distri-
bution on the CeO2 surface. The SEM images for Cu8Zn2/CeO2,
Cu8Ni2/CeO2, Cu8Co2/CeO2, Cu8Fe2/CeO2 and Cu8Ag2/CeO2
were shown in Fig. S7.† The actual metal oxides loading of all
samples were calculated according to EDS analysis shown in
Fig. S8† and the results were listed in Table 1, which
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Table 2 The textural data and crystallite size of the CeO2, CuO/CeO2,
MnOx/CeO2, Cu8Mn2/CeO2
Surface area/
Pore volume/
Pore size/
nm
Samples
(m2 gꢁ1
)
(cm3 gꢁ1
)
CeO2
104.4
59.1
96.3
40.1
0.12
0.23
0.16
0.22
4.6
14.3
6.5
CuO/CeO2
MnOx/CeO2
Cu8Mn2/CeO2
24
3.6 XRD analysis
Fig. 8 showed the XRD patterns of CeO2 loaded with different
bimetal oxides. The main diffraction peaks at 28.8ꢀ, 33.1ꢀ, 47.8ꢀ,
56.5ꢀ, 59.5ꢀ, 69.3ꢀ, 77.1ꢀ and 79.2ꢀ were ascribed to the cubic
uorite cerium oxide and the peaks at 35.4ꢀ and 38.7ꢀ were
attributed to the CuO (JCPDS 34-0394).32 Except Cu8Ag2/CeO2
exhibited the Ag2O diffraction peak at 38.2ꢀ, no diffraction
peaks derived from other metal or metal oxides (Mn, Zn. Ni, Co,
Fe) were observed in the XRD patterns.33 The results implied
these metal oxides were well dispersed on the CeO2 support due
to theirs low loading. The crystal size of CuO loaded on CeO2 in
all samples was calculated according to the Scherrer equation
and the results were listed in Table 1. It could be seen the
introduction of ZnO or Ag2O promoted the agglomeration of
CuO on the surface of supports and increased the crystal size.
For NiO or Co3O4, they showed little effect toward the crystal
size of CuO. When introduced MnOx or Fe2O3, the diffraction
peaks of CuO disappeared, indicating the CuO species were
dispersed well as amorphous oxides or some Cu species doped
into the lattice of CeO2 and formed Cu–Ce–O solid solution.31
CuO and MnOx species could strengthen the synergy between
CuO and CeO2 support to restrain the growth of CuO particles.
Combined with catalytic activity test results, it could be deduced
the bimetal synergistic effect could improve samples' activity by
strengthening the interaction between metal oxides and
supports. The XRD patterns of CeO2, CuO/CeO2 and MnOx/CeO2
were shown in Fig. S9,† and the crystal size of metal oxide
particles calculated were also listed in Table 2.
Fig. 9 The H2-TPR profiles of the prepared CeO2, CuO/CeO2, MnOx/
CeO2 and Cu8Mn2/CeO2 samples.
Fig. 10 The NH3-TPD profiles of CeO2, CuO/CeO2, MnOx/CeO2 and
Cu8Mn2/CeO2 samples.
assigned to the presence of hydrogen spillover process from
metal Cu surface and the interaction between CuO and CeO2.34
For MnOx/CeO2, the rst reduction peak at 250 ꢀC was
assigned to the reduction of MnO2 or Mn2O3 to Mn3O4, the
second peak at 410 ꢀC was related to the reduction of Mn3O4 to
MnO and surface Ce4+ to Ce3+
.
The results indicated the
35
interaction of MnOx and CeO2 were weaker than CuO and CeO2,
which might help explain why CuO/CeO2 showed high catalytic
activity than MnOx/CeO2. When loaded with MnOx, the Cu8Mn2/
CeO2 not only exhibited the three reduction peaks corre-
sponding to CuO/CeO2 but also showed the reduction peak of
MnO2 or Mn2O3 to Mn3O4, while the reduction peak of Mn3O4 to
MnO was not observed, implying the Mn4+ were more actively
than Mn3+ or Mn4+. Combing with the XRD analysis, the MnOx
had a high dispersion on the CeO2 support due its low content
and the interaction between MnOx and CeO2 prevented the
reduction of Mn3O4 to MnO. Compared with CuO/CeO2, the
peak intensity of Cu8Mn2/CeO2 was improved and the H2
consumption also increased, which implied the Cu8Mn2/CeO2
showed high catalytic activity. The TPR peaks and H2
consumption of CuO/CeO2 and Cu8Mn2/CeO2 were summarized
3.7 H2-TPR
The reduction properties of CeO2, CuO/CeO2, MnOx/CeO2 and
Cu8Mn2/CeO2 were investigated by H2-TPR analysis and the
results were shown in Fig. 9. The reduction peaks of CeO2 have
been detailed discussed in our previous work.8
Aer peak tting process, three reduction peaks were
observed in the CuO/CeO2 spectra. The rst peak at 158 ꢀC were
attributed to the reduction of nely disperseꢀd CuO species on
the CeO2 supports, the second peaks at 181 C came from the
reduction of CuO with moderate crystal size which interacted
with the oxygen vacancies of CeO2 and the third peak at 212 ꢀC
was related to the reduction of a part of the surface ceria, the
copper oxide incorporated into the ceria lattice and the large
copper oxide phase. Compared with CeO2, the surface ceria
reduction peak shied to lower temperature, which could be
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Table 3 The surface atom types and atomic ratio among Cu8Mn2/
CeO2 samples
in Table S4.† In addition, the reduction peaks of Cu8Mn2/CeO2
were shied to higher temperature region, which might be
attributed to the formation of polymeride.36
O (100%)
Cu (100%) Ce (100%) Mn (100%)
Samples Oa Ob Og Cu2+ Cu+ Ce4+ Ce3+ Mn4+ Mn3+ Mn2+
3.8 NH3-TPD
Fresh
54 35 11 31
47 31 22 25
44
52
58
73
68
65
27
32
35
21
18
14
54
33
48
0
19
11
In order to probe the acidity of CeO2, CuO/CeO2, MnOx/CeO2
and Cu8Mn2/CeO2 samples, the NH3-TPD analysis were carried
out. As presented in Fig. 10, the NH3 desorption peaks could be
divided into three parts: peaks at 100–200 ꢀC were corresponded
to the desorption of physically absorbed NH3 and NH3 absorbed
30 ꢀ
90 ꢀ
C
C
65 26
9
19
ꢀ
on the weak Brønsted acid sites, the peaks at 200–500 C were
+
attributed to the desorption of NH4 absorbed on the strong
Brønsted acid sites and the peaks at above 500 ꢀC were related to
the desorption of NH3 absorbed on the weak and strong Lewis
acid sites.37,38 The area of the NH3 desorption peaks directly
related to the amount of acidic sites and their acidic strength,
respectively. Quantitative analysis of NH3 desorption at corre-
sponding temperature over all samples were calculated and the
results were listed in Table S5.† It could be observed the total
amount of acid sites of Cu8Mn2/CeO2 at lower temperature were
improved and a desorption peak at above 500 ꢀC region was
observed, indicating the acidic strength of samples was
enhanced when MnOx was introduced. Signicantly, the NH3
desorption amount of Cu8Mn2/CeO2 was higher than CuO/CeO2
at 100–200 ꢀC region, which implied the Cu8Mn2/CeO2 could
bind more NH3 at lower temperature than CuO/CeO2. The
stronger NH3 adsorption capacity of samples could enhance the
absorption ability toward NH3 generated by catalytic hydrolysis
and promote the conversion of NH3 to N2 which was consistent
with the results of Fig. 3.
Fig. 12 The O 1s spectra of Cu8Mn2/CeO2 samples.
metal oxides loaded.29 The pore size distribution of three cata-
lytic materials were calculated and the results were shown as the
insert part in Fig. 11. The textural properties including the
specic surface area, pore volumes, and pore size were listed in
Table 2. The specic surface area of three catalytic materials
decreased while the pore volumes and pore size increased and
Cu8Mn2/CeO2 showed the smallest specic surface area 40.1 m2
gꢁ1 and largest pore size 24 nm respectively. These might be
assigned to the fact that copper and manganese oxides covered
the external surface of CeO2, and block the micropores struc-
ture, which was consistent with SEM analysis.13
3.9 BET analysis
The N2 desorption–desorption isotherms of CeO2, CuO/CeO2,
MnOx/CeO2 as well as Cu8Mn2/CeO2 were displayed in Fig. 11.
The adsorption–desorption isotherms of all samples showed
a type IV trend with a H2-type hysteresis loop, implying the
typical mesoporous structure of CeO2 was not inuenced aer
3.10 XPS analysis
By analyzing the reaction products, it could be obtained the
HCN was mainly removed by catalytic hydrolysis and catalytic
oxidation when the reaction temperature was higher than 90 ꢀC.
To further investigate the reaction mechanism between HCN
and Cu8Mn2/CeO2, the XPS characterization was carried out to
analyze the chemical state, surface atomic concentration and
the dispersion of main elements of the samples before and aer
reaction and the results were summarized in the Table 3.
Fig. 12 showed the O 1s XPS spectra of the fresh samples and
samples reacted at 30 ꢀC and 90 ꢀC. It could be observed an
asymmetrical peak at the bonding energy range of 528–536 eV,
which could be resolved into three distinct peaks: peak located
at 529.5 eV (denoted as Oa) was ascribed to the lattice oxygen,
peak located at 531.1 eV (denoted as Ob) was corresponded to
Fig. 11 The nitrogen adsorption–desorption isotherms and pore size
distributions of CeO2, CuO/CeO2, MnOx/CeO2 and Cu8Mn2/CeO2
samples.
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Fig. 14 The Cu 2p spectra of Cu8Mn2/CeO2 samples.
Fig. 13 The Ce 3d spectra of Cu8Mn2/CeO2 samples.
the surface chemisorption oxygen and the peak at 532.2 eV
(denoted as Og) was assigned to the adsorbed water or surface
OH species on the samples.39 The content of the Oa, Ob and Og
were calculated and listed in the Table 3. As presented in Table
ꢀ
3, aer reacted with HCN at 30 C, the Ob content of Cu8Mn2/
CeO2 decreased from 35% to 31% which indicated the Ob took
part in the HCN catalytic removal over the samples due to its
higher mobility. The increase of Og content was attributed to
the H2O in the reaction system absorbed on the surface of
samples. With temperature increased further, the Ob content
decreased continuously and Og content also decreased,
implying the catalytic hydrolysis activity enhanced, the Ob and
H2O absorbed worked together to contribute to the HCN cata-
lytic removal over Cu8Mn2/CeO2.
Fig. 15 The Mn 2p spectra of Cu8Mn2/CeO2 samples.
Ce 3d XPS spectra of Cu8Mn2/CeO2 were displayed in Fig. 13.
Aer peak tting process, it could be observed eight binding
energy peaks at the bonding energy region of 875–925 eV. The
binding energy peaks of u0 and v0 were assigned to the Ce3+
species while others were related to Ce4+ ions and the content of
Ce3+ and Ce4+ were listed in the Table 3.40 With reaction
temperature increased from 30 to 90 ꢀC, the Ce3+ content
increased from 27% to 35%, while the Ce4+ content decreased
from 73% to 65%, owing to the synergy between Cu2+ and Cu3+
shown as following equation.41
bonded with Cu2+ and generated CuCN due to chemisorption,
and promote the reduction of Cu2+to Cu+, which implied the
Cu2+ played an important role in the HCN catalytic removal over
Cu8Mn2/CeO2.
Fig. 15 demonstrated the Mn 2p spectra of all samples, there
were two main peaks at 642 eV and 653 eV, which were ascribed
to the Mn 2p3/2 and Mn 2p1/2 respectively. The Mn 2p3/2 spectra
could be decomposed into four bonding energy peaks: 640.3,
(8) 641.6, 643.2 and 646.2 eV, corresponding to Mn2+, Mn3+, Mn4+
and manganese nitrate.43 For the fresh Cu8Mn2/CeO2 samples,
Mn3+ and Mn4+ peaks were observed while Mn2+ peak was not
Ce4+ + Cu+ # Ce3+ + Cu2+
2+
ꢀ
Oxygen vacancies could be generated as a function of surface
Ce3+ on the samples, which could avoid the CuO particles
agglomeration and improve its dispersion on CeO2 support by
enhancing the synergy between CuO and CeO2.40 This could
help explain why no diffraction peak of CuO was observed in the
XRD patterns of Cu8Mn2/CeO2.
Cu 2p spectra were tted with four energy peaks and the
results were shown in Fig. 14, the bonding energy peaks m0, n0
represented Cu+ while m0, n0 peaks were related to Cu2+. Aer
reacted with HCN, the Cu2+ decreased from 31% to 19% while
Cu+ increased from 44% to 58%.42 It might be ascribed to the
chemical interaction between CuO phase and HCN: HCN
founded. When reacted with HCN at 30 C, the Mn content
increased to 19% while Mn3+ and Mn4+ content all decreased,
indicating the Mn3+ and Mn4+ participated in the HCN catalytic
removal over Cu8Mn2/CeO2, and the reaction system referred
the reduction of Mn4+ to Mn3+ and Mn3+ to Mn2+. With reaction
temperature increased to 90 ꢀC, the Mn3+ content still
increased, while Mn3+ and Mn2+ content decꢀreased, which
demonstrated the HCN catalytic removal at 90 C was mainly
attributed to the reduction of Mn4+ to Mn3+. It could be deduced
that Mn4+ played an important role in the investigated reac-
tions, and the results were consistent with the H2-TPR analysis.
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Combing the SEM and TEM analysis, the CeO2 exhibited a 3D
porous structure which was benecial for gas transport.29 The
owerlike sphere structure of CeO2 could also improve the
dispersion of metal species.21 Besides, the addition of MnOx
could restrain the growth of CuO particles and improve their
dispersion on the CeO2 by the synergistic effect according to the
XRD results, which promoted the removal of HCN over Cu8Mn2/
CeO2 sample. H2-TPR analysis indicated the redox ability of
CeO2 was enhanced aer CuO and MnOx loaded, as reported,
the excellent activity of CeO2 based catalysts derived from the
revisable Ce3+–Ce4+ transition in CeO2 support associated with
oxygen vacancy formation and migration which might
strengthen the interaction between CuO, MnOx and CeO2
support and promoted its redox ability.47
The CuO phase on the CeO2 played an important role in the
HCN removal process, the HCN was absorbed on the Cu8Mn2/
CeO2 to generate CuCN by the reduction of Cu2+ to Cu+. The
reaction products of HCN over Cu8Mn2/CeO2 at the temperature
region of 30–150 ꢀC indicated the catalytic hydrolysis and
catalytic oxidation improved as temperature increasing.
Combing the XPS analysis, the chemisorption oxygen, the
oxygen vacancies, the Cu2+ and Mn4+ all played an important
role in determining the excellent HCN removal performance of
Cu8Mn2/CeO2. NH3-TPD characterization indicated the addi-
Fig. 16 The FTIR spectra of Cu8Mn2/CeO2 samples.
Besides, the redox couple shown in following equation could
also strengthen the samples' activity toward HCN.44
Cu2+ + Mn3+ ¼ Cu+ + Mn4+
(9)
3.11 The results of FT-IR
The FT-IR spectra of fresh and used Cu8Mn2/CeO2 which reac- tion of MnOx could increase the acid sites of Cu8Mn2/CeO2,
ted with HCN at 30 ꢀC, 90 ꢀC and 120 ꢀC were analyzed to which could strengthen the absorption ability of NH3 and
investigate the reaction mechanism of HCN over prepared promoted its conversion.
samples. As presented in Fig. 16, all samples exhibited two
Based the FT-IR results, the CNꢁ absorbed on the samples
characteristic bonds at 3416.3 cmꢁ1 and 1650 cmꢁ1 corre- disappeared when the reaction temperature was above 120 C,
sponding to the stretching vibration of surface OH groups and this could be explained by the totally conversion of CNꢁ. At this
the bending vibration of H2O adsorbed, which were benecial moment, the HCN removal was completely in the form of
to the HCN catalytic hydrolysis and oxidation.45 The absorption catalytic hydrolysis and catalytic oxidation. When the temper-
peak at 774.2 cmꢁ1 was assigned to the Ce–O asymmetric ature was lower than 30 ꢀC, the removal of HCN was mainly
vibrational stretch and peak at 1319.2 cmꢁ1 might be attributed attributed to the sample's chemisorption. The HCN break-
ꢀ
2ꢁ
to the CO3 vibrational stretch.29 Aer reacted at 30 C, a new through behavior on the catalytic materials bed was corre-
ꢀ
peak appeared at 2148.5 cmꢁ1 which was ascribed to the CNꢁ sponded to the Yoon and Nelson's model.
vibrational stretch, indicating the HCN bonded with Cu2+ and
Based above analysis, the HCN reaction mechanism over
generated CuCN due to chemisorption.46 When the reaction Cu8Mn2/CeO2 at 30–150 C was described as follows: HCN was
temperature increased to 90 ꢀC, the peak at 2148.5 cmꢁ1 could rst absorbed and then decomposed into CNꢁ on the surface of
still be observed, which implied the chemisorption were still Cu8Mn2/CeO2 due to the chemisorption of bimetal oxides on
involved in the reaction system for removal of HCN. With the CeO2. With reaction temperature increased, the CNꢁ could
temperature increased further, the peak corresponding to C]N be oxidized into –NCO with the participation of Ob, Cu2+, Mn4+
vibrational stretch disappeared while other peaks were not and oxygen vacancies.10,12 The formed –NCO were further con-
changed, indicating the Cu8Mn2/CeO2 exhibited high stability verted into NH3 and CO by catalytic hydrolysis. On the other
in the reaction system, and the results were consistent with the hand, the generated –NCO species were oxidized to form CO2,
XRD and ICP analysis shown in Fig. S10 and Table S6† respec- N2 and byproducts such as NO and NO2 by catalytic oxidation.
tively. The CNꢁ absorbed might be converted into N2, NH3 and The schematical diagram of the HCN reaction mechanism over
CO2 with the participation of O2, H2O and OH species. As Cu8Mn2/CeO2 was shown in Fig. S11.†
ꢀ
a result, the active site on the Cu8Mn2/CeO2 was supplemented,
which could capture more HCN molecular to strengthen the
catalytic activity of the samples.
5 Conclusions
The CuxMny/CeO2 catalytic material was synthesized by
precipitation method to achieve deep purication of HCN at the
4 Discussion
lower temperature region of 30–150 ꢀC. By investigating the
Based activity test results, the Cu8Mn2/CeO2 was chosen as the effect of different metal oxide species and the Cu/Mn mass ratio
ideal catalytic material. It was believed the CeO2 still played an on the sample's catalytic activity, the Cu8Mn2/CeO2 sample was
important role in the HCN catalytic removal over Cu8Mn2/CeO2. chosen as the ideal catalytic material, which could obtain
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almost 100% removal rate toward 400 mg mꢁ3 HCN at 90 C.
The introduction of MnOx could improve the dispersion of CuO
particles and increase the total acid sites of the prepared
samples. It was proved the synergy between CuO and MnOx, the
9 N. Liu, X. N. Yuan, B. H. Chen, Y. X. Li and R. D. Zhang,
Selective catalytic combustion of hydrogen cyanide over
metal modied zeolite catalysts: from experiment to
theory, Catal. Today, 2017, 297, 201–210.
ꢀ
chemisorption oxygen, the oxygen vacancies, the Cu2+ and Mn4+ 10 R. N. Nickolov and D. R. Mehandjiev, Comparative study on
all played an important role in determining the good catalytic
activity of the prepared samples.
removal efficiency of impregnated carbons for hydrogen
cyanide vapors in air depending on their phase
composition and porous textures, J. Colloid Interface Sci.,
2004, 273, 87–94.
Author contributions
11 M. J. Hudson, J. P. Knowles and P. J. F. Harris, The trapping
and decomposition of toxic gases such as hydrogen cyanide
using modied mesoporous silicates, Microporous
Mesoporous Mater., 2004, 75, 121–128.
Z. H. Yi: conceptualization, formal analysis, investigation,
writing original dra. J. Sun: funding acquisition, formal anal-
ysis, Rresources, writing review and editing. Y. L. Yang: data
curation. J. G. Li: investigation. T. Zhou: investigation. S. P. Wei: 12 Y. X. Ma, F. Wang, X. Q. Wang, P. Ning, X. Jing and
investigation. A. N. Zhu: investigation.
J. H. Cheng, The hydrolysis of hydrogen cyanide over Nb/
La-TiOx catalyst, J. Taiwan Inst. Chem. Eng., 2017, 70, 141–
149.
Conflicts of interest
13 Y. N. Hu, J. P. Liu, J. H. Cheng, L. L. Wang, L. Tao, Q. Wang,
X. Q. Wang and P. Ning, Coupling catalytic hydrolysis and
oxidation of HCN over HZSM-5 modied by metal (Fe,Cu)
oxides, Appl. Surf. Sci., 2017, 427, 843–850.
14 X. Q. Wang, J. H. Cheng, X. Y. Wang, Y. Z. Shi, F. Y. Chen,
X. L. Jing, F. Wang, Y. X. Ma, L. L. Wang and P. Ning, Mn
based catalysts for driving high performance of HCN
catalytic oxidation to N2 under micro-oxygen and low
temperature conditions, Chem. Eng. J., 2018, 333, 402–413.
15 Y. S. She, Q. Zheng and L. Li, Rare earth oxide modied CuO/
CeO2 catalysts for the water-gas shi reaction, Int. J.
Hydrogen Energy, 2009, 34, 8929–8936.
There are no conicts to declare.
Acknowledgements
The authors acknowledge the Scientic Research Program
(Zhuangzong [2018] No. 635) foundation and the Chinese Civil
Air Defense Office [2014] No. 251-61 for the nancial support of
the current study.
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