Ag-K/MnO2 nanorods as highly efficient catalysts for formaldehyde oxidation at low temperature

Article abstract of DOI:10.1039/c8ra01611a

A series of Ag-K/MnO₂ nanorods with various molar ratios of K/Ag were synthesized by a conventional wetness incipient impregnation method. The as-prepared catalysts were used for the catalytic oxidation of HCHO. The Ag-K/MnO₂ nanorods with an optimal K/Ag molar ratio of 0.9 demonstrated excellent HCHO conversion efficiency of 100% at a low temperature of 60 °C. The structures of the samples were investigated by BET, TEM, SEM, XRD, H₂-TPR, O₂-TPD and XPS. The results showed that Ag-0.9K/MnO₂-r exhibited more facile reducibility and greatly abundant surface active oxygen species, endowing it with the best catalytic activity of the studied catalysts. This work provides new insights into the development of low-cost and highly efficient catalysts for the removal of HCHO.

Full text of DOI:10.1039/c8ra01611a

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Ag–K/MnO nanorods as highly efficient catalysts
2
for formaldehyde oxidation at low temperature
Cite this: RSC Adv., 2018, 8, 14221
a
a
a
a
a
Suhong Lu, * Xue Wang, Qinyu Zhu, Canchang Chen, Xuefeng Zhou,
a
b
a
a
a
Fenglin Huang, Kelun Li, Lulu He, Yanxiong Liu and Fanjue Pang
2
A series of Ag–K/MnO nanorods with various molar ratios of K/Ag were synthesized by a conventional
wetness incipient impregnation method. The as-prepared catalysts were used for the catalytic oxidation
of HCHO. The Ag–K/MnO
2
nanorods with an optimal K/Ag molar ratio of 0.9 demonstrated excellent
ꢀ
HCHO conversion efficiency of 100% at a low temperature of 60 C. The structures of the samples were
Received 23rd February 2018
Accepted 26th March 2018
2 2 2
investigated by BET, TEM, SEM, XRD, H -TPR, O -TPD and XPS. The results showed that Ag–0.9K/MnO -
r exhibited more facile reducibility and greatly abundant surface active oxygen species, endowing it with
the best catalytic activity of the studied catalysts. This work provides new insights into the development
of low-cost and highly efficient catalysts for the removal of HCHO.
DOI: 10.1039/c8ra01611a
rsc.li/rsc-advances
metals, have been demonstrated to possess efficient low-
temperature catalytic activities for HCHO oxidation. For
1
Introduction
Formaldehyde (HCHO) is commonly released from building/ example, the price of Pt per gram for H
PtCl
on the Sigma-Aldrich
regarded as a major pollutant. It has been reported that long- website. It was reported that complete oxidation of HCHO was
$6H O is about 20
2
6 2
decorative materials and unregulated indoor smoking; it is times that of Ag per gram for AgNO
3
1,2
term exposure to indoor air containing even a few ppm of obtained over Ag/MnO
x
–CeO
2
catalysts at a temperature as low
ꢀ
HCHO threatens human health and causes health problems, as 100 C; the catalysts were synthesized by a deposition
2
1
22
such as nasal tumors, skin irritation, allergies, and decreased precipitation method. Ma et al. successfully prepared Ag/
3
,4
concentration.
investigated to eliminate indoor emission of HCHO to satisfy could achieve complete HCHO oxidation at temperatures above
2
Thus, numerous technologies have been CeO nanospheres by a one-step hydrothermal method which
ꢀ
23
stringent environmental regulations; these techniques include 110 C under relatively high space velocity. Huang et al.
adsorption, photo-catalysis, plasma technology, and thermal demonstrated that Ag nanoparticles-supported hollandite-type
catalytic oxidation.
5
6
7
8
manganese oxide nanorods exhibited good activity for HCHO
In recent years, complete oxidation of HCHO over hetero- oxidation; complete conversion of HCHO was obtained at
ꢀ
geneous catalysts has been considered to be the most promising 110 C. In order to improve the activity of Ag-supported cata-
2
4
strategy for indoor HCHO removal due to its properties of high lysts, Li et al. indicated that Ag/Fe–MnO
efficiency and lack of pollution. Highly effective catalysts have catalytic activity than Ag/MnO due to the enhanced interaction
been prepared to reduce HCHO, such as transition metal between Ag and the support. The conversion of HCHO was
x
possesses higher
x
9
–11
12–15
16
17
ꢀ
25
oxides
and supported noble metals (Pt,
Au, and Pd ). 100% at 90 C. Qu et al. reported a bimetallic AgCo/
Generally, transition metal oxides can achieve complete oxida- APTES@MCM-41 catalyst that could achieve complete oxida-
ꢀ
ꢀ
tion of HCHO at very high temperatures (>100 C). In contrast, tion of HCHO at temperatures as low as 90 C because of the
18
supported noble metal catalysts such as Pt/TiO
2
,
Na-Pd/TiO
2
formation of strong metal–metal interactions between Ag and
ref. 19) and Au/Co O –CeO (ref. 20) have been proved to have Co. Bai et al. demonstrated that K–Ag/Co
excellent catalytic activity at relatively low temperatures, even complete conversion of HCHO at 70 C due to additional
room temperature. However, the high cost of the supported surface OH provided by K ions. However, the performance of
noble metals limits their widespread application. Therefore, it Ag-supported catalysts at low temperature still requires
26
(
3
O
4
displayed
3
4
2
ꢀ
ꢁ
+
is still a challenge to develop efficient and low cost catalysts.
improvement in comparison with other noble metal catalysts.
On the basis of current research, Ag-supported catalysts,
Due to its remarkable oxygen storage ability, manganese
which are much less expensive than the abovementioned noble oxide has attracted much attention and is regarded as an
effective catalyst for the catalytic oxidation of HCHO. For
27
x
example, Torres et al. fabricated mesoporous MnO catalysts
College of Chemistry and Chemical Engineering, Xi'an Shiyou University, Xi'an and observed their good catalytic activities for the complete
a
7
b
10065, China. E-mail: lusuhong@xsyu.edu.cn
oxidation of HCHO; using these catalysts, complete conversion
Shaanxi Coal and Chemical Technology Institute Co., Ltd, Xi'an 710070, China
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ꢀ
28
of HCHO was achieved at 145 C. Zhou et al. prepared porous stirring for 30 min, and then the appropriate amounts of AgNO
3
cellulose-supported MnO2 nanosheet catalysts and observed and KNO3 solution were added dropwise. Aer that, the
their excellent performance in the catalytic oxidation of HCHO resulting suspension was stirred at room temperature for 24 h
ꢀ
ꢀ
at a temperature of 140 C. Previous literature studies have re- and then evaporated at 80 C with continuous stirring. The
ꢀ
ported that manganese oxides with various morphologies obtained powder was dried at 80 C for 12 h. The amount of Ag
showed obvious differences in catalytic performance. For in the nal catalyst was 0.1 wt%, measured via ICP-AES. The as-
29
example, ower-like manganese oxide nanospheres (a-MnO
2
)
prepared samples were thus represented as Ag–xK/MnO
2
-r (x
ꢀ
outperformed other crystalline manganese oxides below 120 C indicates the molar ratio of K/Ag). Ag/MnO
2
-r was also synthe-
30
for the oxidation of HCHO. Studying the preparation and sized by a similar process to that described above.
catalysis of materials for rod-like tetragonal a-MnO , ower-like
2
hexagonal 3-MnO , and dumbbell-like tetragonal b-MnO , Shi
2
2
Characterization
31
et al. discovered that the catalytic activity decreased in the
order of a-MnO > 3-MnO > b-MnO . To the best of our
knowledge, the catalytic application of Ag/MnO nanorods for
The Ag contents of the prepared catalysts were determined by
inductively coupled plasma-atomic emission spectroscopy (ICP-
AES, Vista-MPX, Varian).
2
2
2
2
the catalytic oxidation of HCHO has not been reported. More-
over, it has been well acknowledged that alkali metals can serve
Nitrogen adsorption and desorption isotherms were
ꢀ
measured at ꢁ196 C using a Micromeritics apparatus
19,32
as electronic or textural promoters for catalysis.
The addi-
(ASAP2020HD88). The specic surface areas were computed by
tion of alkali metals (Li, Na and K) to Pt/TiO catalysts resulted
2
applying the Brunauer–Emmett–Teller (BET) method. The pore
size distribution was estimated from the desorption branch of
the isotherms using the BJH model. All samples were outgassed
in remarkable enhancements of their performance in HCHO
33
oxidation. It was further found that addition of K has an
obvious enhancing effect on Ag/Co catalysts for HCHO
3 4
O
ꢀ
under vacuum at 250 C for 4 h prior to analysis.
26
oxidation.
Therefore, in this work, MnO
a hydrothermal preparation method and Ag–K/MnO
The morphologies of the products were determined by
scanning electron microscopy (SEM, America FET Quanta 600
FEG) and transmission electron microscopy (TEM, Tecnai G2
F20).
2
nanorods were synthesized by
nanorods
2
were prepared by a conventional wetness incipient impregna-
tion method. These catalysts were applied for the catalytic
oxidation of HCHO. The catalysts were characterized by BET,
TEM, SEM, XRD, H -TPR and O -TPD. It is proposed that the
Powder X-ray diffraction (XRD) measurements of the cata-
lysts were carried out using a Panalytical Empyrean X-ray
diffractometer with Cu-Ka radiation (l ¼ 0.154 056 nm) at 40
2
2
abundant surface oxygen species and low-temperature reduc-
ibility enhanced the catalytic oxidation activity of the Ag–K/
MnO nanorods.
2
ꢀ
ꢁ1
ꢀ
kV and 30 mA with a scanning speed of 2 min from 10.0 to
ꢀ
8
0.0 .
The temperature programmed reduction (H -TPR) equip-
2
ment consisted of a thermal conductivity detector (TCD) con-
nected to a ow-control system and a programmed heating unit.
About 50 mg of catalyst was used in each measurement. The
2
Experimental
Materials
reduction agent of 10% H
2
/N
2
was introduced with a ow rate of
KMnO , Mn
4
S
O
4
, AgNO
3
and KNO
3
(analytically pure reagents)
ꢁ1
ꢀ
ꢀ
6
0 mL min ; each sample was heated to 650 C from 40 C at
ꢁ1
were obtained from Sinopharm Chemical Reagent Co., Ltd. The
reagents were used as received without further purication.
ꢀ
a rate of 10 C min
.
Oxygen temperature-programmed desorption (O
2
-TPD) tests
were conducted using the same equipment as the H -TPR tests.
2
Prior to each run, 60 mg of sample was loaded and pretreated
Preparation of catalysts
ꢁ1
ꢀ
with 21 vol% O /N (30 mL min ) at 300 C for 1 h, then pre-
2
2
The MnO
method similar to previous studies.
mmol of MnSO precursor were dissolved in 160 mL of
2
nanorods were synthesized by a hydrothermal
ꢀ
ꢀ
reduced in a stream of 10 vol% H
2
/N
2
ow at 200 C for 1 h with
3
1,34
4
16 mmol of KMnO and
ꢀ
ꢁ1
a heating rate of 10 C min . Aer cooling to 65 C in the same
6
4

ow, the adsorption of O
2
was operated in a ow of 21% O
2
/N
2
deionized water and then stirred for 1 h to form a homogeneous
solution. Subsequently, the mixture was transferred into
a stainless steel autoclave for thermal treatment at 160 C for
ꢁ
1
ꢀ
at a rate of 20 mL min for 1 h at 65 C. Finally, the catalyst was
heated from 65 C to 500 C at a constant heating rate of
ꢀ
ꢀ
ꢀ
ꢀ
ꢁ1
ꢁ1
10 C min in a ow of He (60 mL min ). The desorption of
12 h. Aer cooling, the resulting precipitate was collected by
oxygen was calculated from the signal of a TCD detector.
X-ray photoelectron spectroscopy (XPS) was performed on
a Thermo Scientic K-Alpha instrument using MgKa radiation

ltration, washed with deionized water to remove any possible
ꢀ
residual reactants and dried at 60 C for 24 h. The acquired
ꢀ
powder was calcined in a muffle oven at 300 C for 4 h with
(1653.6 eV) at a beam power of 250 W. The binding energy of C
ꢀ
ꢁ1
a heating rate of 1 C min , thus affording the nal MnO
2
1s (284.6 eV) was used as the internal standard.
nanorods (designed as MnO -r).
2
Ag–K/MnO2 nanorods with different molar ratios (K/Ag ¼
Catalytic activity tests
0
.5, 0.9, 1.3) were prepared by a conventional wetness incipient
impregnation method, and the general steps were as follows: 1 g The catalytic activity tests of the as-prepared catalysts for HCHO
MnO -r was dispersed in 40 mL deionized water under vigorous oxidation were performed in a xed-bed reactor under
2
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2 2 2
Table 1 Physical parameters of Ag–xK/MnO -r with Ag/MnO -r and MnO -r
2
ꢁ1
3
ꢁ1
a
Catalyst
S
BET (m g
)
Pore diameter (nm)
Pore volume (cm g
)
Ag mass loading (wt%)
MnO
Ag/MnO
Ag–0.5K/MnO
Ag–0.9K/MnO
Ag–1.3K/MnO
2
-r
102.1
71.2
68.1
60.8
45.9
12.2
15.2
15.9
18.1
19.2
0.37
0.33
0.32
0.30
0.25
—
2
-r
0.11
0.13
0.10
0.12
2
2
2
-r
-r
-r
a
Calculated from the ICP data.
atmospheric pressure. Before the catalytic activity tests, the Ag– diameters of about 80 nm and lengths ranging from 2 to 5 mm.
K/MnO
2
ꢀ
nanorods (50 mg) were pretreated with 21 vol% O
2
/N
2
2
Moreover, the Ag–0.9K/MnO -r catalyst also demonstrated
ꢀ
at 300 C for 1 h and then cooled to 200 C. The gas was a rod-like morphology similar to that of MnO
switched to 10 vol% H
Aer that, the temperature was cooled to 25 C. Gaseous HCHO
2
-r.
ꢀ
2
/N
2
for 1 h at a temperature of 200 C.
ꢀ
XRD patterns
Fig. 2 displays the XRD patterns of the as-prepared Ag–xK/
MnO -r catalysts, Ag/MnO -r and MnO -r. It can be seen that all
the samples consisted of single-phase a-MnO (JCPDS PDF 44-
was generated by owing 21% O
a thermostatic water bath. The standard feed gas containing
00 ppm of HCHO, 21% O and N comprising the balance was
2 2
/N over paraformaldehyde in
2
2
2
3
2
2
ꢁ1
2
introduced into the reactor. The total ow rate was 30 mL min
in a mass space velocity of 36 000 mL gcat
the products, such as CO and CO , were executed using a gas
chromatograph equipped with a hydrogen ame ionization
detector (FID) and Ni catalyst convertor. No carbon-containing
2
compounds other than CO in the products were detected for
ꢁ
1
ꢁ1
0141). However, no feature peaks corresponding to the Ag and K
species could be found in the XRD patterns; the absence of
peaks related to Ag and K species probably resulted from the low
loading content and high dispersion of Ag and K with small
h . The analyses of
2
22,35
diameters.
the tested catalysts. Thus, HCHO conversion was calculated as
follows:
H
-TPR
Fig. 3 illustrates the H
MnO -r and Ag–xK/MnO
2
2
½
CO
2
ꢂ
2
-TPR proles of the pure MnO
-r samples. For the sample of pure
-r, Ag/
2
out
HCHO conversionð%Þ ¼
ꢃ 100
½
HCHOꢂ
2
in
MnO -r, a strong reduction peak was observed in the tempera-
ture range from 149–450 C with an overlap peak centered at
2
where [CO
CO concentration in the products and the concentration of
HCHO in the gas ow, respectively.
2
]
out and [HCHO]in in the formula correspond to the
ꢀ
2
ꢀ
327 C; this can be assigned to the combined reduction of
MnO
2
/Mn
2
O
3
to Mn
3
O
4
and of Mn
3
O
4
to MnO. This is similar to
-r, a shoulder peak was
3
5
the observation of Li et al. For Ag/MnO
centered at 306 C and a strong broad reduction peak appeared
between 324 C and 432 C. Obviously, the addition of Ag to
MnO -r dramatically shied the reduction temperatures to
lower regions, indicating the occurrence of metal-support
2
interactions between Ag and MnO -r. It has been reported
that the reducibility of a material can be signicantly improved
by noble metal loading, which is oen interpreted in terms of
the activation and spillover of hydrogen from Ag atoms to
MnO -r. The H spillover phenomenon showed an ability to
2 2
adsorb, activate, and migrate hydrogen. Activated hydrogen on
the Ag surface can readily migrate to the surface of the MnO -r
support and participate in the reduction reaction.
2
ꢀ
3
Results and discussion
ꢀ
ꢀ
N adsorption and desorption
2
2
The specic surface areas, pore diameters and pore volumes of
the Ag–xK/MnO -r samples, along with those of Ag/MnO -r and
2
2
MnO -r, are listed in Table 1. Among all the samples, pure
2
2
ꢁ1
MnO -r possessed the highest surface area (102.1 m g ) and
2
3
ꢁ1
pore volume (0.37 cm g ). However, the surface area and pore
volume showed remarkable decreases aer the addition of Ag
and K; especially, when the molar ratio of K/Ag was 1.3, these
values were 45.9 m g and 0.25 cm g , respectively, for Ag–
.3K/MnO -r. Meanwhile, the average pore diameters of these
24
2
ꢁ1
3
ꢁ1
2
1
2
samples increased from 12.2 to 19.2 nm, which indicates that
Ag and K nanoparticles formed on the surface and blocked the
Compared with the reduction features of Ag/MnO
reduction peaks of the Ag–xK/MnO -r samples shied towards
lower temperatures; this shi was most noticeable for the Ag–
.9K/MnO -r catalyst. Moreover, without changes in its peak
pattern, the reduction temperature of Ag–0.9K/MnO
2
-r, the
2
22,24
small pores.
0
2
SEM and TEM images
2
ꢀ
-r showed
ꢀ
The morphologies of the MnO
2
-r and Ag–0.9K/MnO
2
-r samples the best low-temperature reducibility, which was 14 C and 3 C
were investigated using SEM (Fig. 1). It was observed that the lower than those of Ag–0.5K/MnO -r and Ag–1.3K/MnO -r,
2
2
ꢀ
MnO -r sample derived hydrothermally at 160 C displayed respectively. A similar enhancing effect of K addition on cata-
2
26
a rod-like morphology, in good agreement with previous reports lytic reducibility was studied previously. Bai et al. reported that
31,34
+
for MnO
2
in the literature.
2 3 4
The MnO -r samples had the presence of K enhanced the reducibility of K–Ag/Co O due
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2 2 2
Fig. 1 SEM images of (a and b) MnO -r and (c and d) Ag–0.9K/MnO -r. TEM image of (e) MnO -r.
to the stronger metal-support interaction. They also found that
the presence of K ions increased the number of Co cations. ibility is related to oxygen vacancies. Catalysts with better
In addition, previous studies reported that catalyst reduc-
+
3+
3
6
24,38
Pan et al. indicated that acid-treated MnO samples showed reducibility can yield more oxygen vacancies.
Thus, Ag–0.9K/
-r could produce more oxygen vacancies with the assis-
the manganese ions, which appeared to be responsible for their tance of K than Ag/MnO -r. Gas-phase oxygen molecules could
enhanced catalytic properties in HCHO oxidation. Hence, it is be readily adsorbed in the oxygen vacancies, forming active
x
the best redox properties due to the higher oxidation states of MnO
2
2
+
20,39
reasonable to speculate that the addition of K improved the oxygen species.
2
This result is in agreement with the O -TPD
low-temperature reducibility and higher oxidation states of results (see Fig. 4).
manganese ions of the Ag–0.9K/MnO -r sample in this study.
2
The sufficient manganese cations implied that the oxygen
O -TPD
2
species in the Ag–0.9K/MnO -r sample could be more readily
2
O -TPD experiments over the pure MnO -r, Ag/MnO -r, Ag–0.5K/
MnO -r, Ag–0.9K/MnO -r and Ag–1.3K/MnO -r catalysts were
2 2 2
carried out to determine the mobility of oxygen species, as
activated and that they migrated and improved the catalytic
performance of HCHO oxidation (see Fig. 5).
2
2
2
37
Fig. 2 XRD patterns of (a) MnO
2
-r, (b) Ag/MnO
2
-r, (c) Ag–0.5K/MnO
2
-
Fig. 3
MnO
H
2
-TPR profiles of (a) MnO -r, (c) Ag–0.5K/
2
-r, (b) Ag/MnO
2
r, (d) Ag–0.9K/MnO -r and (e) Ag–1.3K/MnO
2
2
-r.
2
-r, (d) Ag–0.9K/MnO -r and (e) Ag–1.3K/MnO -r.
2
2
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XPS spectra
The XPS spectra of the MnO -r, Ag/MnO -r and Ag–0.9K/MnO -r
2
2
2
samples are displayed in Fig. 5. As shown in Fig. 5A, the two
peaks of binding energy around 368.0 eV and 375.0 eV can be
assigned to the escape of electrons from the 3d5/2 and 3d3/2 core
levels. The Ag 3d5/2 binding energies of Ag/MnO
2
-r and Ag–0.9K/
MnO -r are 368.3 eV, reecting the metallic state. The Ag atoms
2
of Ag–0.9K/MnO -r have a relatively higher electronic density of
2
states than those of Ag/MnO -r, originating from the different
2
46
density of states of the Ag d orbitals. In the Mn 2p spectra
Fig. 5B), two obvious peaks were found. The splitting energy of
(
4
+
the Mn 2p peak (11.8 eV) approached that of Mn , in accord
Fig. 4
O
2
-TPD profiles of (a) MnO
2
-r, (b) Ag/MnO
2
-r, (c) Ag–0.5K/ with the energy separation between Mn 2p3/2 and Mn 2p1/2 re-
MnO
2
-r, (d) Ag–0.9K/MnO -r and (e) Ag–1.3K/MnO
2
2
-r.
37
2
ported previously. In comparison with MnO -r, apparent
upshis of the Mn 2p3/2 and Mn 2p1/2 peaks occurred for Ag/
MnO -r and Ag–0.9K/MnO -r, which indicates that the addition
2
2
4
+
40,41
of K increased the number of Mn ions. Fig. 5C displays that K
shown in Fig. 4. According to the literature,
the desorption
2
p has two components at BE ¼ 291.8 and 295.0 eV, which
oxygen species of the oxides can be generally ranked in the
+
26
demonstrates the presence of K ions. The BE of O 1s in
Fig. 5D shows two surface oxygen species by deconvolution of
the O 1s spectra. A shoulder on a higher BE (532.3 eV) can be
assigned to adsorbed oxygen or surface hydroxyl species, while
the main peak at about 529.9 eV can be attributed to surface
lattice oxygen atoms. Moreover, discernible shis to a lower
binding energy for Ag/MnO
observed in comparison to MnO
2
in their negative charge. It has been reported that increased
surface adsorbed oxygen and hydroxyl species play crucial roles
sequence of oxygen molecule (O
2
) > oxygen molecule anion
ꢁ
ꢁ
2ꢁ
(O2 ) > oxygen anion (O ) > lattice oxygen (O ). The desorption
ꢀ
of O2 can occur at a very low temperature (<200 C), while
surface active oxygen species such as O and O can desorb
ꢁ
ꢁ
2
ꢀ
ꢀ
between 200 C and 400 C and the lattice oxygen from MnO
2
ꢀ
20,42
can desorb above 400 C.
r, the large peak between 150 C and 350 C can be assigned to
2
As shown in Fig. 4, for pure MnO -
ꢀ
ꢀ
2
-r and Ag–0.9K/MnO
-r were
2
ꢁ
ꢁ
-r, accounting for an increase
O
2
, O
2
and O desorption. Aer the introduction of Ag,
46
signicant increases of the desorption peaks in the range of
ꢀ
1
75–400 C were noted for Ag/MnO -r. Moreover, the desorption
2
26,46
to promote catalytic activity for HCHO oxidation.
Fig. 5D, it can be observed that the intensity of the higher BE
531.8 eV) of the Ag–0.9K/MnO -r catalyst is the strongest,
From
peak positions shied to a lower temperature range. These data
suggest that the presence of Ag is benecial to produce surface
(
2
2
active oxygen in the Ag/MnO -r catalyst. Similar phenomena
40
22
40
implying that it contains more surface active oxygen. The result
is in good agreement with the O -TPD analysis (Fig. 4).
were observed by Huang et al. and Ma et al. Huang et al.
observed that the formation of Ru–O–Ce in Ru/CeO can
remarkably increase the amount of oxygen vacancies (a reec-
2
2
22
tion of the surface active oxygen). Ma et al. observed that the Catalytic activity
presence of Ag in Ag/CeO nanospheres facilitates the genera-
tion of much more surface active oxygen desorption during the
2
Formaldehyde conversion was measured as a function of reac-
tion temperature over the Ag–xK/MnO -r catalysts together with
2
O -TPD process.
2
Ag/MnO -r and pure MnO -r. As shown in Fig. 6, pure MnO -r
2
2
2
With the addition of K, the original desorption tempera-
ture of the surface active oxygen species shied to lower
afforded HCHO conversion of less than 89% in the temperature
range investigated. With 0.1% Ag addition, 85.4% HCHO
temperatures for Ag–xK/MnO
Ag/MnO -r. For example, the desorption of oxygen from the
surface active oxygen species appeared at 218 C on Ag–0.5K/
2
-r in comparison with those of
ꢀ
conversion was tested at 60 C over Ag/MnO
2
-r. Obviously, the
-r was promoted by the pres-
2
-r achieved 92.7% HCHO
2
catalytic performance of Ag/MnO
ence of K ions. Ag–0.5K/MnO
conversion at 60 C. With increasing K ion content, Ag–0.9K/
2
ꢀ
+
ꢀ
ꢀ
MnO -r, 197 C on Ag–0.9K/MnO -r and 207 C on Ag–1.3K/
ꢀ
+
2
2
MnO -r. This indicates that the Ag–0.9K/MnO -r catalyst
2
2
MnO -r showed the highest performance, with 100% HCHO
2
most readily desorbed the surface active oxygen species. This
is in good accordance with the TPR measurements (see
ꢀ
conversion at 60 C. The conversion of HCHO slightly decreased
+
with further increase of the K ion loading amount. 99.6%
4
3,44
Fig. 3). As reported,
the oxygen desorption temperature of
ꢀ
conversion of HCHO was obtained at 60 C over Ag–1.3K/MnO
2
-
catalytic materials is related to their catalytic oxidation
activity. Lower starting oxygen desorption temperatures may
be signicantly favorable for high activity in HCHO oxidation
due to the abundance of active surface oxygen species that
r. The results revealed that the catalytic activity followed the
decreasing order of Ag–0.9K/MnO -r > Ag–1.3K/MnO -r > Ag–
.5K/MnO -r > Ag/MnO -r > MnO -r. The optimal molar ratio of
K/Ag was 0.9. Thus, the addition of K enhanced the catalytic
activity for HCHO oxidation, which is similar to Na or K-
2
2
0
2
2
2
4
5
can readily participate in the catalytic oxidation reaction.
Here, it can be expected that Ag–0.9K/MnO -r will possess
19,26
2
promoted catalysts.
K
It is well known that the presence of
excellent catalytic performance for HCHO oxidation at low
temperature.
+
ꢁ
ions provides surface OH species, which may play an
important role in the catalytic oxidation of HCHO. Nie et al.
47
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ꢁ
interactions between HCHO and OH present on TiO
2
. Bai
26
et al. prepared 3D K-Ag/Co O catalyst and discovered that the
addition of K ions obviously promoted the catalytic perfor-
mance for HCHO oxidation because of the surface OH species
provided by K ions. Yang et al. reported that the superior
performance of PtNi(OH) /g-Al could be attributed to the
3
4
+
ꢁ
+
48
x
2 3
O
preferred hydroxyl-facilitated HCHO oxidation pathway through
formate oxidation by the abundant associated hydroxyl groups
near the Pt active sites.
The Ag-based catalysts currently being applied for HCHO
oxidation are summarized in Table 2. It can be clearly seen that
the Ag–0.9K/MnO -r catalysts show the best catalytic activities at
2
the lowest temperature.
2
Catalytic stability of the 0.9K/MnO -r samples is very
important for HCHO decomposition; therefore, cycle and
ꢀ
stability tests were carried out at 60 C for 60 h with a WHSV of
ꢁ1
ꢁ1
36 000 mL gcat
h , as displayed in Fig. 7 and 8. No noticeable
decrease in the catalytic activity was found during ve on/off
cycles. As shown in Fig. 7, the catalytic activity of Ag–0.9K/
MnO -r showed no obvious change up to the end of the exper-
2
iments, indicating the outstanding stability and efficiency of
this catalyst for HCHO removal.
Generally, it is well acknowledged that the surface area,
surface active oxygen species, and reducibility are crucial
49
parameters inuencing the catalytic performance of a catalyst.
Notably, the Ag–xK/MnO -r samples, which have slightly
2
smaller specic surface areas, exhibited higher catalytic activity
50
than Ag/MnO -r; this was also mentioned in previous reports.
2
This observation implied that the specic surface area was not
the decisive factor for HCHO oxidation. In addition to the
specic surface area, better low-temperature reducibility and
abundant surface active oxygen species are signicantly bene-

cial for enhanced catalytic activity. In our case, by comparing
the activity data and characterization results, it can be seen that
there are obvious relationships of the reducibility (Fig. 3) and
surface active oxygen species (Fig. 4) with the catalytic activity
(
Fig. 5). The decreasing order of reducibility and oxygen
desorption is Ag–0.9K/MnO -r > Ag–1.3K/MnO -r > Ag–0.5K/
MnO -r > Ag/MnO -r > MnO -r, which is the same as the
2
2
2
2
2
2 2
Fig. 5 XPS spectra of (a) MnO -r, (b) Ag/MnO -r and (c) Ag–0.9K/
MnO -r. (A) Ag 3d, (B) Mn 2p, (C) K 2p and (D) O 1s.
2
investigated the reactivity of Pt/TiO
2
catalysts with the assis-
tance of NaOH toward HCHO oxidation; they found that the Fig. 6 HCHO conversion over (:) MnO
2
-r, (C) Ag/MnO
-r and (-) Ag–1.3K/MnO
lysts. Reaction conditions: 300 ppm HCHO, 21 vol% O
2
-r, (;) Ag–
0
.5K/MnO
2
-r, (+) Ag–0.9K/MnO
2
2
-r cata-
catalyst showed higher activity than that without NaOH due to
the introduction of additional surface hydroxyl groups (OH ),
which can favor HCHO adsorption via strong hydrogen-bond
ꢁ
2
and N
2
ꢁ
1
(
balance). The total flow rate and WHSV are 30 mL min
and
ꢁ1 ꢁ1
36 000 mL gcat
h , respectively.
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Table 2 Catalytic activities of Ag-based catalysts in the references and in this work
ꢀ
Catalyst
Ag/MnO
Reaction conditions
T
100 ( C)
Ref.
1
ꢁ1
ꢁ1
x
–CeO
nanosphere
Ag/Fe0.1–MnO
AgCo/APTES@MCM-41
D K–Ag/Co
Ag–0.9K/MnO
2
580 ppm HCHO, 18% O
810 ppm HCHO, 20% O
230 ppm HCHO, 21% O
500 ppm HCHO, 20% O
100 ppm HCHO, 20% O
300 ppm HCHO, 21% O
2
2
2
2
2
2
, 30 000 mL g
, 84 000 h
, 30 000 mL gcat
catꢁ
h
h
100
110
90
90
70
21
22
24
25
26
Herein
ꢁ1
Ag/CeO
2
ꢁ1
x
ꢁ1
ꢁ1
, 9000 mL gcat
h
ꢁ1
3
3
O
4
, 30 000 h
, 36 000 mL g
ꢁ1
2
-r
catꢁ
1
h
60
oxidation of HCHO. The molar ratio of K/Ag greatly inuenced
the catalytic activity of the Ag–K/MnO nanorods, and the
optimal K/Ag ratio was found to be 0.9. It was observed that Ag–
.9K/MnO -r presented outstanding catalytic activity; this cata-
2
0
2
lyst achieved 100% HCHO conversion at a low temperature of
ꢀ
6
0 C. The excellent performance of Ag–0.9K/MnO -r resulted
2
from its facile reducibility and highly abundant surface active
oxygen species. This work provides new insight into the devel-
opment of low-cost and high-efficiency catalysts for low-
temperature catalytic oxidation of HCHO.
ꢀ
2
-r for HCHO oxidation at 60 C
Conflicts of interest
Fig. 7 Cycle test of Ag–0.9K/MnO
under the reaction conditions of 300 ppm HCHO, 21 vol% O
balance).
2
and N
2
There are no conicts to declare.
(
Acknowledgements
This work was sponsored nancially by the Science & Tech-
nology Plan Project of Xi'an City (No. 2017081CG/RC044
(XASY006)), the Special Scientic Research of Shaanxi Educa-
tional Committee (No. 17JK0608), the College Students' Inno-
vative Entrepreneurial Training Program of Xi'an Shiyou
University (No. 201610705027 and YCS17221013) and Shaanxi
Province (No. 2017107051486), and the National Nature Science
Foundation of China (No. 21606177).
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