Promotion of formaldehyde oxidation over Ag catalyst by Fe doped MnOx support at room temperature

Article abstract of DOI:10.1016/j.cattod.2016.02.040

Catalytic oxidation at room temperature proves to be the most promising technology for HCHO abatement. The development of appropriate catalyst with high efficiency and low cost is crucial. In order to improve the catalytic activity of Ag/MnO_x catalyst for formaldehyde oxidation at room temperature, modification of MnO_x by Fe entailed highly improved catalytic activities due to the promoted interaction between Ag and the support. Ag/Fe-MnO_x with different content of doped Fe was prepared via one-pot synthesis method and characterized by means of ICP-AES, XRD, SEM, TEM, XPS, and H₂-TPR. The HCHO mineralization ratio at ambient temperature of Ag/Fe_0.1-MnO_x can reach 100% with an initial HCHO concentration of 230?ppm compared with 76% of Ag/MnO_x catalyst. The higher catalytic activity of Ag/Fe-MnO_x was resulted from the synergistic effect between Ag, Fe and MnO_x nanostructure, which changed the electronic density of Ag nanoparticles and promoted higher amount of surface adsorbed oxygen as well as easier activation of gaseous O₂. This work may provide a strategy for develop composite metal catalyst for controlling indoor air pollution.

Full text of DOI:10.1016/j.cattod.2016.02.040

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Promotion of formaldehyde oxidation over Ag catalyst by Fe doped
MnOx support at room temperature
Dandan Lia,b, Guoliang Yanga, Peilin Lia, Jinlong Wanga,b, Pengyi Zhanga,b,∗
a State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China
b Collaborative Innovation Center for Regional Environmental Quality, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic oxidation at room temperature proves to be the most promising technology for HCHO abate-
ment. The development of appropriate catalyst with high efficiency and low cost is crucial. In order to
improve the catalytic activity of Ag/MnOx catalyst for formaldehyde oxidation at room temperature,
modification of MnOx by Fe entailed highly improved catalytic activities due to the promoted interaction
between Ag and the support. Ag/Fe-MnOx with different content of doped Fe was prepared via one-pot
synthesis method and characterized by means of ICP-AES, XRD, SEM, TEM, XPS, and H2-TPR. The HCHO
mineralization ratio at ambient temperature of Ag/Fe0.1-MnOx can reach 100% with an initial HCHO
concentration of 230 ppm compared with 76% of Ag/MnOx catalyst. The higher catalytic activity of Ag/Fe-
MnOx was resulted from the synergistic effect between Ag, Fe and MnOx nanostructure, which changed
the electronic density of Ag nanoparticles and promoted higher amount of surface adsorbed oxygen as
well as easier activation of gaseous O2. This work may provide a strategy for develop composite metal
catalyst for controlling indoor air pollution.
Received 13 December 2015
Received in revised form 14 February 2016
Accepted 17 February 2016
Available online xxx
Keywords:
Ag/Fe-MnOx
Formaldehyde
Catalytic oxidation
Synergistic effect
Indoor air
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
harmless water and CO2 molecules [5]. Supported noble metals
possess high activity for HCHO oxidation at relatively low tem-
Formaldehyde (HCHO) is known as a major indoor air pollutant
in airtight buildings, which is continuously released from furni-
ture, building materials, and household products [1]. In some cases,
the atmospheric HCHO concentrations may also even reach indoor
levels typically in summer months, for example, in the polluted
urban areas caused by the increasing use of biofuels [2]. Hence,
formaldehyde is a typical pollutant in both indoor and outdoor air
and long-term exposure to the air even containing 0.5 ppm of HCHO
could cause adverse effects in human health [1,3]. Thus, the abate-
ment of HCHO at ambient temperature is a problem that should be
urgently solved.
For lowering the concentration of HCHO in the air, different
techniques have been developed. There are generally two methods
to reduce the HCHO concentration: physical adsorption method
and catalytic oxidation method [4]. Catalytic oxidation of HCHO
over heterogeneous catalysts is advantageous as compared to phys-
ical adsorption, because it can completely convert HCHO into
perature. For instance, Pt/TiO2, Au/Co3O4-CeO2 and Pd/TiO2 all
exhibited excellent activity at room temperature [6–8]. Chen et al.
[9] pointed out that the strong metal-support interaction, well dis-
persed Pt nanoparticles with small particle sizes and large surface
area of modified Pt/MnO2 catalyst were important for the enhanced
formaldehyde oxidation activity. Ag-supported catalysts, which are
much cheaper than the mentioned noble metals, were also proved
to have high low-temperature catalytic activities for HCHO oxi-
dation. Various Ag based catalysts were studied in recent years.
Among metal oxides supports, manganese oxides attracted spe-
cial attention for the high catalytic activity at low temperature
toward HCHO oxidation [10]. Huang et al. [11] reported that sil-
ver nanoparticles supported on Hollandite-type manganese oxide
nanorods exhibited good activity for HCHO oxidation. The con-
version was 100% at 383 K with an initial HCHO concentration of
400 ppm. Even though, the performance of Ag-supported catalyst
still needs improvement at low temperature compared to other
noble metal catalysts.
In order to improve the activity of Ag-supported catalysts, Bai
et al. [12] pointed that the addition of K+ ions on Ag/Co3O4 cat-
alyst promoted the catalytic performance for HCHO oxidation by
providing more surface OH− species. Hu et al. [10] confined single
Ag atoms inside HMO support as catalytic centers via controllable
∗
Corresponding author at: State Key Joint Laboratory of Environment Simulation
and Pollution Control, School of Environment, Tsinghua University, Beijing 100084,
China.
E-mail address: zpy@tsinghua.edu.cn (P. Zhang).
http://dx.doi.org/10.1016/j.cattod.2016.02.040
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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metal-support interactions and the Ag sites with higher electronic
density of states facilitated the oxidation of HCHO through easier
activation of O2. Actually, the microstructures of active centers play
an important role in catalytic efficiency, such as the electronic and
geometric structures [13]. Thus, a feasible method to improve cat-
alytic activity is to perturb the electronic states of active sites by
adding another metal ion [10]. In previous reports, Fe was widely
used and showed good performance for catalytic reactions. Par-
maliana et al. [14] reported the introduction of Fe3+ ions to silica
samples promoted the catalytic activity by influencing the surface
redox properties for partial oxidation of methane to formaldehyde
at 923 K. Xu et al. [15] prepared Pt-Fe bicomponent catalysts and
the highly active and stable “FeO-on-Pt” system was efficient for CO
oxidation reactions. Furthermore, Otsuka et al. [16] claimed that
age pore size was calculated by applying BJH formula from the
desorption branch of N2 adsorption isotherms. Before the measure-
ment, catalysts were degassed at 423 K for 4 h. Elemental analysis
was performed by inductively coupled plasma-atomic emission
spectroscopy (ICP-AES). The morphologies of the samples were
observed with a higher solution field-emission scanning elec-
tron microscopy (Hitachi, Japan) operated at 30 kV. Transmission
electron microscopy (TEM) images, high-resolution transmission
electron microscopy (HR-TEM) images and elementary mapping
images were collected on a JEM-2010F microscope at an accelerat-
ing voltage of 200 kV. X-ray diffraction (Bruker, Germany) analysis
of the catalysts was performed using Cu-K␣ radiations at 40 kV
and 40 mA. The chemical state of catalysts surface elements were
investigated by X-ray photoelectron spectroscopy (Thermo Scien-
tific, USA) using a pass energy of 30 eV with Al-K␣ as the X-ray
excitation source. The spectra were calibrated with respect to the
C1s line of adventitious carbon at 248.8 eV. Hydrogen temperature-
programmed reduction (H2-TPR) measurements were carried out
on an AutoChem 2920 adsorption instrument (Micromeritics, USA)
equipped with a TCD detector. 50 mg samples were loaded and
pretreated with Ar at 473 K for 30 min to remove the adsorbed
hydrates. After cooling down to 313 K, 5% H2/Ar at a flow rate
of 60 mL/min was introduced, the temperature was then pro-
grammed to rise at a ramp of 5 K/min up to 773 K. The CO-TPD
was also conducted on the above instrument. 50 mg of samples
was first treated with He (20 mL/min) at 350 K for 30 min. Then
it was reduced under 5% H2-Ar at 350 K for 45 min. The sam-
ple was then purged with He (20 mL/min) until temperature was
decreased to 313 K. Subsequently, it was treated with 1% CO-He
(10 mL/min) and the peak recording started when the baseline was
stable.
Fe3+ sites in oxide system could enhance the formation of O2
2−
species, which was dominant for CH4 oxidation. Ferrihydrite (Fh)
supported Pt (Pt/Fh) catalyst prepared by Yan et al. [17] exhibited
excellent activity for room-temperature HCHO removal. It is corre-
lated with the abundance of surface hydroxyls, high Pt dispersion
and adsorption performance of Fh.
Therefore, in this study, we observed promoting effect on
formaldehyde oxidation over Ag/MnOx by doping Fe into texture
of MnOx support. It was found that improvement of the catalytic
activity was due to the enhancement of the activation oxygen
molecules capability on the catalyst surface caused by the change
of Ag species. Accordingly, insights into the connection between
synergistic effect and the catalytic activity for Ag/Fe-MnOx was
proposed.
2. Experimental
2.1. Catalyst preparation
The one-pot synthesis method in this article was based on our
previous report [18]. Ag/Fe-M−nOx was prepared by a redox reac-
tion between Mn2+ and MnO4 in the presence of ferric nitrate as
well as silver nitrate under alkaline condition. In a typical synthe-
sis, first, 3.92 g Mn(AcO)2·4H2O and 1.09 g AgNO3 were dissolved
in 30 mL deionized water. Fe(NO3)3·9H2O was also added into this
solution and its content was varied from 0.323 g Fe(NO3)3·9H2O
(Fe/Mn = 0.05, molar ratio) to 0.646 g Fe(NO3)3·9H2O (Fe/Mn = 0.1,
molar ratio). Then, the mixed solution was added dropwise to a
solution of 14 g KOH in 30 mL distilled deionized water under vig-
orous stirring, forming a light brown suspension. Subsequently, a
solution of 0.984 g KMnO4 in 100 mL distilled deionized water was
added slowly to the suspension under vigorous stirring, producing
a brownish black slurry. The obtained brownish black slurry was
shaken in a water baths shaker for 72 h at 313 K. Then, the obtained
precipitate was filtered and washed with distilled deionized water,
finally it was dried in an oven at 373 K for 12 h.
To clearly illustrate the effect of doped Fe in MnOx on Ag species,
Fe-MnOx and Ag/MnOx were also synthesized for comparison. The
synthetic process for Fe-MnOx and Ag/MnOx were similar to the
method mentioned above. For the preparation of Fe-MnOx, 3.92 g
Mn(AcO)2·4H2O and 0.646 g Fe(NO3)3·9H2O (Fe/Mn = 0.1, molar
ratio) were dissolved in 30 mL deionized water. For the prepara-
tion of Ag/MnOx, 3.92 g Mn(AcO)2·4H2O and 1.09 g AgNO3 were
dissolved in 30 mL deionized water without adding ferric nitrate.
2.3. Activity measurement
The activity of catalysts for HCHO oxidation at ambient tem-
perature and the removal ratio at different temperatures were
performed respectively. The experiments at room temperature for
catalytic HCHO oxidation were carried out in an organic glass box
(volume = 4.8 L) with a layer of aluminum foil on its inner wall at
about 298 K. A glass culture dish (ꢀ = 9.5 cm) with 200 mg sample
dispersed on its bottom, was placed in the reactor and then covered
with a glass slide. A brushless fan was placed close to the culture
dish to accelerate HCHO volatilizing. After the reactor was sealed
and then 50 ␮L condensed HCHO solution (7.6%) was injected into
the reactor. The HCHO solution was volatilized completely in 1 h
and the concentration of HCHO approached to a stable state. The
analysis of HCHO was conducted by PN-2000HCHO online detec-
tor (PNLE, Shenzhen), and the CO2 was on-line conducted by a gas
chromatograph (GC-2014, Shimadzu) equipped with FID detector.
The initial concentration of HCHO was adjusted around 230 ppm.
After that, the cover on the culture dish was removed to make the
HCHO gas contact with catalysts. The concentration of HCHO and
CO2 were recorded on line.
To further evaluate the activity of different samples, a dynamic
test system was selected. The experiments were performed in
a fixed-bed reactor under atmospheric pressure with different
temperatures (298 K–473 K in this work). 200 mg catalyst (40–60
meshes) was placed in a quartz tube reactor (i.d. = 6 mm). 400 ppm
gaseous HCHO was generated by flowing synthetic air (21% O2 and
79% N2) over 25 g paraformaldehyde powder in a 400 mL glass bot-
tle which was putted in an incubator kept at 300 K. The flow rate
was 100 mL/min controlled by mass flow controller (MFC). The
concentration of CO2 were analyzed by the mentioned gas chro-
matograph before and after reaction. No other carbon-containing
2.2. Catalyst characterization
The structure parameters, pore characterization and specific
surface area of the samples were analyzed on Autosorb-1MP
instrument (Quantachrome, USA). Specific surface areas of the
samples were determined by applying BET method and the aver-
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dispersion were increased to 12.4% and 25.6% for Ag/Fe0.05-MnOx
and Ag/Fe0.1-MnOx, respectively. It is clear that the addition of Fe
improves the Ag dispersion.
The nitrogen physical adsorption and pore size distribution data
of the catalysts were displayed in Table 1. There were slight changes
of the specific surface area, pore volume and average pore diame-
ter among the samples due to the introduction of Ag and Fe. The
specific surface area decreased from 65.7 m2/g to 57.2 m2/g once
Ag particles were loaded on Fe0.1-MnOx compared with the pre-
vious one. The pore volume and pore size also decreased which
was consistent with specific surface area data. The decrease of spe-
cific surface area could be explained by the block of pores space
by Ag particles [21]. When the doping amount of Fe increase from
0 to 0.05 to 0.1 (Fe/Mn), the specific surface area increased from
47.6 m2/g to 52.3 m2/g to 57.2 m2/g. Higher specific surface area is
consistent with the better dispersion of Ag particles. The content
of Ag and Fe were also detected by ICP and XPS element analy-
sis. On one hand, the mass loading of Ag was calculated from the
XPS results were 3.58% for Ag/MnOx, 3.47% for Ag/Fe0.05-MnOx and
3.44% for Ag/Fe0.1-MnOx, which was consistent with the values of
3.12%, 2.96% and 2.88% from the ICP data, reflecting that lower con-
tent of Ag existed on the support surface due to the introduction of
doped Fe in MnOx. On the other hand, Fe species were also detected.
Fig. 1. XRD patterns of different samples.
compounds except CO2 in the products were detected. Thus, HCHO
conversion was calculated as follows:
There was not too much difference for the content of Fe in Fe0.1
-
MnOx and Ag/Fe0.1-MnOx. The content of Fe in Ag/Fe0.1-MnOx was
1.08% from ICP data which was almost 2 times to the content in
Ag/Fe0.05-MnOx (0.41%). In addition, the actual molar ratio of Fe/Mn
were both less than the designed value, indicating only a small
portion of ferric oxides or Fe atom loaded on the surface MnOx
or dispersed in the crystal lattice.
The morphology of different samples were observed by SEM in
Fig. 2. As reported in the literature [22], Mn3O4 usually displays
a well-defined octahedral shape. In can be concluded that Mn3O4
with octahedral structure existed in all four samples. Besides, par-
ticles without special morphology were also observed. It is another
proof that amorphous MnOx coexisted during the synthetic pro-
cess. No obvious Ag particles were observed due the restricted
resolution of SEM.
To see the Ag particles and their dispersion on the catalysts, TEM
analysis was performed and the results are shown in Fig. 3. The
addition of Ag causes black spots in Fig. 3 which are Ag nanopar-
ticles to be uniformly distributed on the surface of support. The
mean diameters of Ag particles over Ag/MnOx, Ag/Fe0.05-MnOx and
Ag/Fe0.1-MnOx are 13.8 nm, 8.5 nm and 4.1 nm, respectively. The
TEM results illustrate that the mean size of Ag particles decreased
with Fe addition, which confirms that the addition of Fe improves
the dispersion of Ag particles.
[CO2]out vol%
HCHO conversion% =
× 100
HCHO in vol%
[
]
where [CO2]out is the concentration in the products (ppm),
and the [HCHO]in is the HCHO concentration of the feed gas
(ppm).
3. Results and discussion
3.1. Phase structure and morphology
Fig. 1 gives the XRD patterns for the catalysts. All the samples
showed strong peaks between 25◦ and 40◦. The peaks located at
28.9◦, 32.4◦ and 36.1◦ were the characteristic diffraction peaks of
Mn3O4 structure (JCPDS, No. 80-0382), which could be assigned
to (112), (103) and (211) crystal facet, respectively. Besides typi-
cal peaks of Mn3O4, characteristic diffraction peaks of amorphous
manganese oxides located at 12.5◦, 25.2◦ and 51.0◦ were also
observed in Fe0.1-MnOx and Ag/MnOx [19]. However, the diffrac-
tion peaks of amorphous manganese oxides became not obvious
when Fe(NO3)3·9H2O and AgNO3 were both added during synthetic
process. No diffraction peaks of FeOx species were observed for
Fe0.1-MnOx as well as Ag/Fe0.05-MnOx and Ag/Fe0.1-MnOx, imply-
ing that FeOx nanoparticles were dispersed very well or atomic Fe
was probably incorporated into the manganese lattice with replac-
ing the manganese cations since the ionic radius of Fe3+ (0.055 nm)
is smaller than that of Mn2+ (0.067 nm) and Mn3+ (0.058 nm), and
almost the same with that of Mn4+ (0.054 nm) [20]. To further
investigate the effect of higher Fe content, Ag/Fe0.5-MnOx was also
synthesized, the XRD result in Fig. S1 show the appearance of Fe3O4
phase in Ag/Fe0.5-MnOx. Thus, when the higher dosage of Fe was
adopted during synthesis process, Fe may exist in a separate phase
rather than entering in the lattice of MnOx. As for Ag/MnOx, the
diffraction peaks at 38.1◦, 44.3◦, 64.4◦ and 77.4◦ can be ascribed to
the presence of metal silver (JCPDS, No.87-0597). The characteristic
peak located at 38.1◦ of silver in Ag/Fe0.05-MnOx and Ag/Fe0.1-MnOx
were not intensive compared with that of Ag/MnOx, implying that
the addition of Fe may enhance the dispersion degree of silver
species. To prove the above speculation, Ag dispersion was mea-
sured by CO chemisorption. Without Fe addition, the Ag dispersion
for Ag/MnOx sample was only 7.3%. After the addition of Fe, the Ag
Fig. 4 further shows the dispersion of Ag, Fe and Mn in Ag/Fe0.1
-
MnOx sample. When the magnification was further amplified,
Ag(111) faces with lattice spacing of 0.236 nm can be observed and
the clusters were observed with the maximum diameter of 10 nm
showed in Fig. 4e, confirming the presence of Ag nanoparticles on
the catalyst surface. In order to further investigate the distribu-
tion of Ag L␣, Fe K␣ and Mn K␣ atom on the catalyst, elemental
mapping were displayed in Fig. 4b–d. It was demonstrated that Ag
could be found in clusters and Fe was in a pretty high dispersion
degree as well as Mn, confirming the possibility of incorporation for
Fe into manganese lattice structure. In addition, the phenomenon
that the dots of Mn in Fig. 4c were much denser than that of Fe in
Fig. 4d, was related with the low loading content of Fe as showed
in Table 1. It can be concluded that Fe3+ incorporated into the man-
ganese lattice and substituted for Mnx+ (x could be 2, 3 or 4). Thus,
a proper model for iron distribution was proposed as showed in
Fig. 4f. The Ag nanoparticles loaded on the support surface and were
not showed in the model.
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Table 1
Physical parameters of different samples.
Sample
Surfacearea(m2/g)
PoreVolume(cm3/g)
Averagepore diameter(nm)
Mass loading (wt.%)
Aga
Agb
Fea
Feb
Mnb
Fe0.1-MnOx
Ag/MnOx
Ag/Fe0.05-MnOx
Ag/Fe0.1-MnOx
65.7
47.6
52.3
57.2
0.496
0.253
0.308
0.372
27.0
17.2
20.7
23.5
–
–
0.88
–
0.38
0.71
1.43
–
0.41
1.08
52
47
44
51
3.58
3.47
3.44
3.12
2.96
2.88
a
Calculated from the XPS data.
Calculated from the ICP data.
b
Fig. 2. SEM images of (a) Fe0.1-MnOx, (b) Ag/MnOx, (c) Ag/Fe0.05-MnOx, (d) Ag/Fe0.1-MnOx.
3.2. Chemical characterization
deconvolution. The peak at lower binding energy (529.3–529.8 eV)
corresponded to the lattice oxygen (Olatt), in a typical format of Mn-
O-Mn [24]. Furthermore, the doping of Fe caused negative shift for
binding energies of O1s of lattice oxygen (Mn-O-Mn) from 529.8 eV
XPS patterns of the four samples are displayed in Fig. 5. The
Mn2p spectra in Fig. 5a showed two peaks at 642.0 eV and 653.7 eV,
which could be assigned to Mn2p3/2 and Mn2p1/2. After the curve-
fitting analysis of Mn2p3/2, the species of Mn4+ (644.0 0.1 eV),
Mn3+ (642.6 0.1 eV) and Mn2+ (641.4 0.1 eV) all existed. The
ratios of Mnx+/Mntotal for each sample were calculated and showed
in Table 2. According to the results, the average oxidation state of
Mn were 3.03, 3.08, 3.10 and 3.12 respectively. Ag3d photoelec-
tron spectra of Ag/MnOx, Ag/Fe0.05-MnOxand Ag/Fe0.1-MnOx were
showed in Fig. 5b. The two peaks of the binding energy around
368 eV and 374 eV were assigned to the escape of electrons from
the Ag 3d3/2 and 3d5/2 core levels [23]. The Ag 3d3/2 was selected to
fit the curve. The peak located at 368.0 eV was assigned to metal-
lic silver (Ag◦) while the peak located at 367.4 eV corresponded to
cationic silver (Ag+) [23]. The proportion of Ag+ in the total silver
species was 61% in Ag/Fe0.1-MnOx, which is much higher compared
with that of Ag/Fe0.05-MnOx (55%) and Ag/MnOx (45%), as calcu-
lated in Table 2. The addition of Fe in MnOx support was benefitial to
the presence of Ag+ due to some interfacial interaction between Ag
and Fe doped MnOx. Besides, the O1s spectra were also presented
in Fig. 5c. The peaks could be assigned into two oxygen species by
to 529.6 eV to 529.5 eV for Ag/MnOx, Ag/Fe0.05-MnOx and Ag/Fe0.1
-
MnOx. The possible explanation is that electron transfer occurred
from Ag to Olatt after Fe doping. Electron would transfer from Ag
to Fe to Olatt, inducing the negative shift of Olatt and the increas-
ing ratio of Ag+ to Ag◦. The other peak at higher binding energy
(530.9–531.4 eV) represented surface adsorbed oxygen (Osurf). The
content of Osurf was higher in Fe/Ag0.1-MnOx catalyst (25%) com-
pared with other samples. Osurf can bond with charge compensating
H by covalently bond to form hydroxyl group (-OH). The increase of
O
surf/Osurf + Olatt and Ag+/Ag◦ + Ag+ occurred simultaneously after
Fe doping MnOx compared with pure MnOx over Ag catalyst. It has
been reported in a lot of literatures that −OH played an impor-
tant role in the oxidation of formate species to CO2 and H2O [5,25].
Hence, the high ratio of Osurf/Osurf + Olatt of Ag/Fe0.1-MnOx may be
responsible for its high catalytic activity.
The reducibility of samples were tested by H2-TPR experiments
and the results were presented in Fig. 6.The profiles of different
samples showed the reduction process in the range of 350–750 K.
Fe0.1-MnOx showed one broad peak with a maximum at 600 K, rep-
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Fig. 3. TEM images and size distribution of Ag particles of different samples: (a) Ag/MnOx, (b) Ag/Fe0.05-MnOx, (c) Ag/Fe0.1-MnOx.
Table 2
XPS results of different samples.
Sample
BE (eV)
Ag+/Ag0 + Ag+(%)
BE (eV)
Osurf/Osurf + Olatt(%)
Mnx+/Mntotal (%)
Averageoxidation state of Mn
Ag+
Ag0
Olatt
Osurf
Mn4+
Mn3+
Mn2+
Fe0.1-MnOx
Ag/MnOx
Ag/Fe0.05-MnOx
Ag/Fe0.1-MnOx
–
–
–
529.3
529.8
529.6
529.5
530.9
531.4
531.2
531.1
19
22
24
25
19.1
18.6
21.3
23.4
34.8
40.3
36.1
34.3
46.1
41.1
42.6
42.3
3.03
3.08
3.10
3.12
367.4
367.3
367.4
368.0
368.0
368.1
45
55
61
resenting the reduction process of MnOx (Mn3O4 and MnO2) to
MnO [26,27]. A small shoulder peak at 640 K could also be found. As
for Ag/MnOx, there was a significant decrease of reduction peaks
due to Ag loading. Two peaks were centered at 470 K and 488 K,
which were similar to previous reports [27]. Peak at 470 K may be
attributed to the first reduction step of amorphous MnO2 to Mn3O4.
The main reduction peak at 488 K was due to the reduction process
of Mn3O4 to MnO [28]. It has been reported the material reducibil-
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Fig. 4. (a) TEM image of Ag/Fe0.1-MnOxcatalyst, elemental mapping of (b) Ag, (c) Mn, (d) Fe images of Ag/Fe0.1-MnOx catalyst, (e) High resolution TEM images of Ag/Fe0.1-MnOx
catalyst, (f) proposed model for the distribution of Fe in MnOx (the red ball-O atom, the purple ball-Mn atom, the yellow ball-Fe atom). (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this article.)
ity could be enhanced by noble metal loading significantly [29].
In this study, the enhancement of reducibility for Ag/MnOx was
related with the interaction between Ag and MnOx, which favored
the spillover of hydrogen from silver atoms to manganese oxides
[30].Without changing in peak pattern, the reduction tempera-
ture of Ag/Fe0.05-MnOx was 6 K lower compared with Ag/MnOx.
When the doping content of Fe increased (Ag/Fe0.1-MnOx), the
starting reduction peak shifted to much lower temperature. This
phenomenon implied the existence of interaction between Ag-
Fe (Fe in MnOx lattice) and MnOx support. In addition, previous
studies reported that catalyst reducibility was correlated with oxy-
gen vacancies. Catalysts with better reducibility (lower reduction
temperature in H2-TPR experiment) could produce more oxygen
vacancies [25,31]. Thus, Ag/Fe-MnOx could produce more oxy-
gen vacancies with the assistance of Fe than Ag/MnOx. Gas-phase
oxygen molecules were easy to adsorb in the oxygen vacancies,
resulting the filling of oxygen vacancies and the forming of active
oxygen species (Osurf) [7,32].
Ag dispersion in Fig. 3 and the activity in Fig. 7a, there is a correla-
tion between the catalytic performance for HCHO oxidation and the
Ag dispersion. The Fe addition increased the catalytic performance
by inducing more dispersed Ag particles on the catalyst surface for
HCHO oxidation.
To further confirm the activity of catalyst, dynamic tests were
also performed. As shown in Fig. 7b, with the increase in the
temperature, the complete conversion of Fe0.1-MnOx catalyst is bal-
anced at 473 K. Ag/MnOx catalyst showed better low temperature
conversion than Fe0.1-MnOx catalyst because the addition of Ag
can provide sufficient active sites for the HCHO oxidation reaction.
Complete conversion of Ag/MnOx was 393 K. When Fe was doped
in the crystal lattice in MnOx, there was an obvious promotion
of the catalytic activity. Complete conversion of Ag/Fe0.05-MnOx
was 373 K. With the increase content of doped Fe in MnOx,
Ag/Fe0.1-MnOx showed higher performance with the complete con-
version about 363 K. The results revealed the catalytic performance
of the four samples in the order of Ag/Fe0.1-MnOx > Ag/Fe0.05
-
MnOx > Ag/MnOx > Fe-MnOx. Thus, the introduction of doped Fe in
crystal lattice of MnOx over Ag catalyst obviously promoted the
catalytic performance for HCHO oxidation.
3.3. Catalytic activity
The stability of catalyst is an important for practical application.
In order to investigate the stability of Ag/Fe0.1-MnOx, its activity
for HCHO oxidation was repeated 8 times at room temperature.
Initial HCHO concentration for each test was around 230 ppm. As
shown in Fig. 7c, the generation of CO2 over Ag/Fe0.1-MnOx for the
8th test did not changed compared to the 1st test, demonstrating
that Ag/Fe0.1-MnOx catalyst could be efficient and stable over a long
period of time.
Here, a possible reaction mechanism for HCHO oxidation on
Ag/Fe-MnOx catalyst was proposed, as shown in Fig. 8. Ferric irons,
which doped in MnOx lattice with replacing manganese, had a
fine distribution. Silver species, on the catalyst surface, could be
close to Mn and Fe atoms. The connection between Ag and Fe was
the main factor that differentiated Ag/Fe-MnOx from Ag/MnOx.
As can be seen from Fig. 7a, for Fe0.1-MnOx, the value of ꢁCO2
was 60 ppm after 1 h reaction, less than the decrease amount of
HCHO (140 ppm), indicating that it was physical adsorption rather
than catalytic degradation of HCHO occurred on Fe0.1-MnOx. The
ꢁCO2 of Ag/MnOx was 170 ppm after 1 h reaction, almost three
times to that of Fe0.1-MnOx. In addition, the activities of Ag/Fe-
MnOx were further enhanced. The ꢁCO2 reached 230 ppm for
Ag/Fe0.05-MnOx and 260 ppm for Ag/Fe0.1-MnOx. Besides, Ag/Fe0.1
-
MnOx possessed potential for continuous HCHO oxidation, as the
uptrend of ꢁCO2 curve was evident compared with Ag/MnOx. The
phenomenon that the yield of CO2 (260 ppm) for Ag/Fe0.1-MnOx
was slightly higher than the initial HCHO concentration (230 ppm)
was due to desorption of residual HCHO molecules from the reac-
tor inner wall and subsequent conversion to CO2. Comparing the
Please cite this article in press as: D. Li, et al., Promotion of formaldehyde oxidation over Ag catalyst by Fe doped MnOx support at room
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Fig. 6. H2-TPR profiles of Fe0.1-MnOx, Ag/MnOx, Ag/Fe0.05-MnOxand Ag/Fe0.1-MnOx
catalysts.
The interaction between Ag-Fe-Olatt was formed through an elec-
tron transfer. A similar promotion effect of Fe addition on catalytic
reactions was studied before. Kobayashi et al. [33] reported that
the highly dispersed Fe3+ in silica network promoted oxygen acti-
vation for methane oxidation. Alptekin et al. [34] claimed that
interaction of FePO4 with SiO2 apparently favored the formation
of surface hydroxyl groups. Hence, it is reasonable to speculate
that interaction of Ag-Fe with MnOx in this study is responsible for
the good performance of HCHO oxidation over Ag/Fe-MnOx cata-
lyst. Electron first transferred from Ag◦ to Fe3+and then from Fe2+
to Olatt, resulting in the forming of Ag+ and O*. The nucleophilic
attack to C-H of formaldehyde molecule by O* then occurred [7,35],
producing surface formate species and an oxygen vacancy. The for-
mate species adsorbed on the support surface and formed HCOO-M
group [7]. Gas-phase molecular oxygen would adsorb on the oxy-
gen vacancy and resulted in vacancy healing [36]. Surface formate
species were then oxidized by the surface HO-M group directly into
CO2 and H2O [37]. Briefly, surface hydroxyls were enhanced by the
introduction of Fe and Fe also promoted the activation of gas-phase
oxygen molecules through electron transfer, which subsequently
enhanced the catalyst activity for HCHO oxidation. Furthermore,
the Fe addition induced more dispersed Ag particles on catalyst
surface which promoted this process.
4. Conclusion
In this study, Ag/Fe-MnOx was prepared by doping Fe into
texture of MnOx support over Ag catalyst. Ag/Fe-MnOx exhibited
higher catalytic HCHO activity with the almost same silver loading
compared with Ag/MnOx. Fe was well dispersed in MnOx sup-
port by incorporating into the manganese lattice. The interaction
between Ag, Fe and MnOx support existed, which may resulted in
the increase of Ag+ and Osurf species. Electrons transfer from Ag
to Fe to Olatt promoted the activation of gas-phase O2, and sub-
sequently enhanced the catalytic HCHO oxidation. In addition, the
introduction of Fe brought more dispersed Ag particles with smaller
size on the catalyst surface, which facilitated the above reaction
process. This work provided a possibility for the preparation and
use of composite metal catalyst in HCHO oxidation. More accurate
controls about catalyst nanostructure are undergoing.
Fig. 5. XPS spectra of (a) Mn2p, (b) Ag3d, (c) O1s.
Please cite this article in press as: D. Li, et al., Promotion of formaldehyde oxidation over Ag catalyst by Fe doped MnOx support at room
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Fig. 8. Possible mechanism of HCHO oxidation process over Ag/Fe-MnOx catalyst.
Acknowledgment
This work was funded by the National High Technology Research
and Development Program of China (2012AA062701), National
Nature Science Foundation of China (21221004, 21411140032)
and Tsinghua University Initiative Scientific Research Program
(20131089251).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.02.
040.
References
[1] T. Salthammer, Angew. Chem. Int. Ed. 52 (2013) 3320–3327.
[2] J. Duan, J. Tan, L. Liu, S. Wu, J. Hao, Atmos. Res. 88 (2008) 25–35.
[3] K. Toda, S. Yunoki, A. Yanaga, M. Takeuchi, S. Ohira, P. Dasgupta, Environ. Sci.
Technol. 48 (2014) 6636–6643.
[4] J. Wang, R. Yunus, J. Li, P. Li, P. Zhang, J. Kim, Appl. Surf. Sci. 357 (2015)
787–794.
[5] J. Wang, P. Zhang, J. Li, C. Jiang, R. Yunus, J. Kim, Environ. Sci. Technol. 49
(2015) 12372–12379.
[6] L. Nie, J. Yu, X. Li, B. Cheng, G. Liu, M. Jaroniec, Environ. Sci. Technol. 47 (2013)
2777–2783.
[7] C. Ma, D. Wang, W. Xue, B. Dou, H. Wang, Z. Hao, Environ. Sci. Technol. 45
(2011) 3628–3634.
[8] C. Zhang, Y. Li, Y. Wang, H. He, Environ. Sci. Technol. 48 (2014) 5816–5822.
[9] Y. Chen, J. He, H. Tian, D. Wang, Q. Yang, J. Colloid Interface Sci. 428 (2014) 1–7.
[10] P. Hu, Z. Amghouz, Z. Huang, F. Xu, Y. Chen, X. Tang, Environ. Sci. Technol. 49
(2015) 2384–2390.
[11] Z. Huang, X. Gu, Q. Cao, P. Hu, J. Hao, J. Li, X. Tang, Angew. Chem. 124 (2012)
4274–4279.
[12] B. Bai, J. Li, ACS Catal. 4 (2014) 2753–2762.
[13] J.K. Nørskov, T. Bligaard, B. Hvolbæk, F. Abild-Pedersen, I. Chorkendorffc, C.H.
Christensen, Chem. Soc. Rev. 37 (2008) 2163–2171.
[14] A. Parmaliana, F. Arena, F. Frusteri, A. Martınez-Arias, M.L. Granados, J.L.G.
Fierro, Appl. Catal. A: Gen. 226 (2002) 163–174.
[15] H. Xu, Q. Fu, Y. Yao, X. Bao, Energy Environ. Sci. 5 (2012) 6313–6320.
[16] K. Otsuka, I. Yamanaka, Y. Wang, Stud. Surf. Sci. Catal. 119 (1998) 15–24.
[17] Z. Yan, Z. Xu, J. Yu, M. Jaroniec, Environ. Sci. Technol. 49 (2015) 6637–6644.
[18] J. Wang, D. Li, P. Zhang, Q. Xu, J. Yu, Rsc. Adv. 5 (2015) 100434–100442.
[19] X. Li, D. Li, Z. Wei, X. Shang, D. He, Electrochim. Acta 121 (2014) 415–420.
[20] M. Polverejan, J.C. Villegas, S.L. Suib, J. Am. Chem. Soc. 126 (2004) 7774–7775.
[21] J.-Y. Luo, M. Meng, X. Li, X.-G. Li, Y.-Q. Zhu, T.-D. Hu, Y.-N. Xie, J. Zhang, J. Catal.
254 (2008) 310–324.
Fig. 7. (a) The concentration changes of HCHO and ꢁCO2 as a function of time
at room-temperature, (b) HCHO removal efficiency for Fe0.1-MnOx, Ag/MnOx,
Ag/Fe0.05-MnOx and Ag/Fe0.1-MnOx at different temperature, (c) Production of CO2
with reaction time over Ag/Fe0.1-MnOx catalyst during 8 repeated tests.
[22] W. Xiao, P. Zhou, X. Mao, D. Wang, J. Mater. Chem. A 3 (2015) 8676–8682.
[23] G.B. Hoflund, Z.F. Hazos, G.N. Salaita, Phys. Rev. B 62 (2000) 11126–11133.
[24] M. Wang, P. Zhang, J. Li, C. Jiang, Chin. J. Catal. 35 (2014) 335–341.
[25] C. Zhang, Y. Li, Y. Wang, H. He, Environ. Sci. Technol. 48 (2014) 5816–5822.
[26] Y.F. Han, F. Chen, Z. Zhong, K. Ramesh, L. Chen, E. Widjaja, J. Phys. Chem. B 110
(2006) 24450–24456.
[27] S. Liang, F. Teng, G. Bulgan, R. Zong, Y. Zhu, J. Phys. Chem. C 112 (2008)
5307–5315.
[28] L. Christel, A. Pierre, D.A.-M.R. Abel, Thermochim. Acta 306 (1997) 51–59.
[29] S.A.C. Carabineiro, S.S.T. Bastos, J.J.M. Orfao, M.F.R. Pereira, J.J. Delgado, J.L.
Fifueiredo, Catal. Lett. 134 (2010) 217–227.
[30] R. Xu, X. Wang, D. Wang, K. Zhou, Y. Li, J. Catal. 237 (2006) 426–430.
Please cite this article in press as: D. Li, et al., Promotion of formaldehyde oxidation over Ag catalyst by Fe doped MnOx support at room
temperature, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.040

G Model
CATTOD-10072; No. of Pages9
ARTICLE IN PRESS
D. Li et al. / Catalysis Today xxx (2016) xxx–xxx
9
[31] Q. Liu, Y. Bie, S. Qiu, Q. Zhang, J. Sainio, T. Wang, J. Lehtonen, Appl. Catal. B:
Environ. 147 (2014) 236–245.
[35] Q. Xu, W. Lei, X. Li, X. Qi, J. Yu, G. Liu, P. Zhang, Environ. Sci. Technol. 48 (2014)
9702–9708.
[32] B. Liu, Y. Liu, C. Li, W. Hu, P. Jing, Q. Wang, J. Zhang, Appl. Catal. B: Environ. 127
(2012) 47–58.
[33] T. Kobayashi, N. Guilhaume, J. Mike, N. Kitamura, M. Haruta, Catal. Today 32
(1996) 171–175.
[36] H.Y. Kim, H.M. Lee, G. Henkelman, J. Am. Chem. Soc. 134 (2012) 1560–1570.
[37] C. Zhang, F. Liu, Y. Zhai, H. Ariga, N. Yi, Y. Liu, H. He, Angew. Chem. Int. Ed. 51
(2012) 9628–9632.
[34] G.O. Alptekin, A.M. Herring, D.L. Williamson, T.R. Ohno, R.L. McCormick, J.
Catal. 181 (1999) 104–112.
Please cite this article in press as: D. Li, et al., Promotion of formaldehyde oxidation over Ag catalyst by Fe doped MnOx support at room
temperature, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.040