Influence of alkali metals on Pd/TiO2 catalysts for catalytic oxidation of formaldehyde at room temperature

Article abstract of DOI:10.1039/c5cy01521a

We previously observed that sodium (Na) addition had a dramatic promotion effect on Pd/TiO₂ catalysts for formaldehyde (HCHO) oxidation. In this study, a series of alkali metal (Li, Na, K, Cs) doped Pd/TiO₂ catalysts were prepared and tested for ambient temperature HCHO oxidation. The results showed that the doped alkali metals have a common promotion effect on the performance of the Pd/TiO₂ catalysts for HCHO oxidation under ambient temperature, which followed the order K > Cs > Na > Li. X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), CO chemisorption, Transmission Electric Microscopy (TEM), temperature-programmed reduction by H₂ (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption by O₂ (O₂-TPD) methods were used to characterize the alkali metal doped catalysts to investigate the mechanism of the alkali metal promotion effect. The results showed that a negatively charged and well-dispersed Pd species was induced and stabilized by alkali metal addition, which facilitates the activation of chemisorbed oxygen, and then enhances the performance of alkali metal doped Pd/TiO₂ catalysts for room temperature HCHO destruction. The K-Pd/TiO₂ catalyst in particular possessed the highest Pd dispersion degree with active sites, attributing to the best activity in ambient temperature HCHO oxidation.

Full text of DOI:10.1039/c5cy01521a

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Influence of alkali metals on Pd/TiO catalysts for
2
catalytic oxidation of formaldehyde at room
Cite this: DOI: 10.1039/c5cy01521a
temperature†
Yaobin Li, Changbin Zhang, Hong He,* Jianghao Zhang and Min Chen
We previously observed that sodium (Na) addition had a dramatic promotion effect on Pd/TiO
for formaldehyde (HCHO) oxidation. In this study, a series of alkali metal (Li, Na, K, Cs) doped Pd/TiO
lysts were prepared and tested for ambient temperature HCHO oxidation. The results showed that the
doped alkali metals have a common promotion effect on the performance of the Pd/TiO catalysts for
HCHO oxidation under ambient temperature, which followed the order K > Cs > Na > Li. X-ray diffraction
XRD), Brunauer–Emmett–Teller (BET), CO chemisorption, Transmission Electric Microscopy (TEM),
2
catalysts
2
cata-
2
(
temperature-programmed reduction by H
2
2
(H -TPR), X-ray photoelectron spectroscopy (XPS) and
temperature-programmed desorption by O
2
2
(O -TPD) methods were used to characterize the alkali metal
doped catalysts to investigate the mechanism of the alkali metal promotion effect. The results showed that
a negatively charged and well-dispersed Pd species was induced and stabilized by alkali metal addition,
which facilitates the activation of chemisorbed oxygen, and then enhances the performance of alkali metal
Received 10th September 2015,
Accepted 5th November 2015
2 2
doped Pd/TiO catalysts for room temperature HCHO destruction. The K–Pd/TiO catalyst in particular
DOI: 10.1039/c5cy01521a
possessed the highest Pd dispersion degree with active sites, attributing to the best activity in ambient tem-
perature HCHO oxidation.
www.rsc.org/catalysis
decades, researchers paid attention to conventional catalysts
1
. Introduction
3
,12–20
including metal oxides (Co, Ni, Mn, Ag)
noble metal (Pt, Au, Pd, Rh) catalysts
and supported
for HCHO oxida-
2
1–33
Formaldehyde (HCHO) is recognized as a major indoor air
pollutant, which is mainly emitted from consumer products
and building/furnishing materials. Long-term exposure to
tion. A relatively high temperature is generally needed to
completely oxidize HCHO using the metal oxide catalysts. In
contrast, complete oxidation of HCHO can achieved on noble
metal catalysts, such as Pt-, Au- and Pd-based catalysts,
1
indoor air containing even very low concentrations of HCHO
may be detrimental to human health, leading to serious
health problems including nasal tumors, irritation of the
mucous membranes of the eyes and respiratory tract, and
skin irritation. Increasingly, humans are paying more atten-
tion to indoor air pollutants, therefore, the effective abate-
ment of indoor air HCHO is urgently needed to improve
indoor air quality and reduce public health risk.
Over the past decades, four technologies including adsorp-
tion, photo-catalysis, plasma technology and catalytic oxida-
tion were investigated for use in the removal of HCHO.
Among these methods, catalytic oxidation is known as the
most promising method for HCHO removal, which could
2
1–24,27,31
around room temperature.
Therefore, the supported
noble metal catalysts are more suitable for indoor air HCHO
purification.
2
,3
Alkali metals can serve as electronic or textural promoters
3
4
35
for catalysts in various catalytic processes including NO
3
6,37
38
and CO oxidation,
CO hydrogenation and the water-gas
Recently, we found that the performance
catalysts for formaldehyde oxidation was dra-
matically improved by the addition of alkali metal ions (such
3
9–41
shift reaction.
of the Pt/TiO
2
4
–9
+
+
+
as Li , Na and K ), which due to an atomically dispersed Pt
2
4
species was induced and stabilized.
Then, we further
selectively decompose HCHO to harmless CO and water at a
reported that the performance of the Pd/TiO2 catalysts for
HCHO destruction under ambient conditions was also pro-
moted by Na addition, which resulted from the fact that a
negatively charged and well-dispersed Pd species was induced
and stabilized by Na species addition. Therefore, it is worth
exploring whether other alkali metals (such as Li, K and Cs)
2
10,11
low temperature without any secondary pollution.
For
State Key Joint Laboratory of Environment Simulation and Pollution Control,
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,
Beijing, 100085, China. E-mail: honghe@rcees.ac.cn; Fax: 86 10 62849123;
Tel: 86 10 62849123
31
also have a common promotion effect on the Pd/TiO catalysts
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
2
c5cy01521a
for HCHO oxidation.
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In the present work, a series of alkali metal (Li, Na, K, Cs) 3. Results
doped Pd/TiO catalysts were prepared and their HCHO per-
formance were tested at low temperature. It was found that
all the alkali metals enhanced the performance of the Pd/
2
3
.1. Activity test
The performance of Pd/TiO
Pd/TiO
results are shown in Fig. 1. When the concentration of HCHO
was 140 ppm, the Pd/TiO catalyst exhibited about 30%
HCHO conversion to CO at 25 °C and achieved 100% conver-
sion at around 125 °C. After doping with the alkali metals,
the performance of the Pd/TiO catalyst was dramatically
enhanced. The Li–Pd/TiO catalyst exhibited nearly 35%
2
and the Li-, Na-, K-, Cs-doped
catalysts for HCHO oxidation was tested and the
2
TiO catalysts and the promotion effect followed the order K
2
>
2
Cs > Na > Li. Among them, the K–Pd/TiO catalyst showed
2
the best performance in ambient temperature HCHO oxida-
tion. Based on the characterization results, the mechanism of
2
the influence of alkali metals on Pd/TiO
elucidated.
2
catalysts was further
2
2
HCHO conversion at room temperature and achieved 100%
2
. Experimental
conversion at around 100 °C, while the Na–, K– and Cs–Pd/
TiO catalysts achieved almost 100% HCHO conversion at 25
2
2
.1. Catalyst preparation
°
C. When tested under harsh reaction conditions of 550 ppm
1
wt% Pd/TiO and alkali metal doped 1 wt% Pd/TiO
2
2
cata-
−1
HCHO and a GHSV of 640 000 h , the HCHO conversion was
lysts with the same alkali metal/Pd mole ratio were prepared
by co-impregnation of TiO (Degussa P25, BET surface area
around 20% for the Na–Pd/TiO
2
catalyst, while it was
2
improved to around 43% and 60% for the Cs–Pd/TiO and K–
2
−1
2
5
3 2
9 m g ) with aqueous PdIJNO ) (Sigma-Aldrich) and alkali
2
Pd/TiO catalysts, respectively. According to the above results,
metal nitrate of Li, Na, K, or Cs (Sinopharm Chemical
Reagent Beijing Co., Ltd). After impregnation, the excess
water was removed in a rotary evaporator at 60 °C. The sam-
ples were dried at 110 °C for 12 h and then calcined at 400
the alkali metals all enhanced the performance of the Pd/
TiO2 catalyst for formaldehyde (HCHO) oxidation and the
promotion effects appreciably depended on the nature of the
doped alkali metals, following the order K > Cs > Na > Li.
°
C for 2 h. Before activity testing and characterization, the
samples were reduced with H at 350 °C for 30 min, and
denoted as TiO , Pd/TiO , Li–Pd/TiO , Na–Pd/TiO , K–Pd/TiO
2
2
3
.2. Characterization of catalysts
.2.1. Structural features of samples. The XRD patterns of
Pd/TiO and the alkali metal doped Pd/TiO catalysts together
with TiO -fresh are shown in Fig. 2. There was no Pd species
2
2
2
2
3
and Cs–Pd/TiO . The samples without pretreatment are
denoted as TiO
Pd/TiO -fresh, K–Pd/TiO -fresh and Cs–Pd/TiO -fresh.
2
2 2
-fresh, Pd/TiO -fresh, Li–Pd/TiO -fresh, Na–
2
2
2
2
2
2
2
0
(
Pd , PdO) observed for any samples, which indicated that
the Pd species were highly dispersed on the TiO sup-
2
2
.2. Catalyst characterization
port. However, Pd loading was identified by ICP-OES, as
listed in Table 1. The actual Pd loading on the catalysts is
close to the nominal ones. The results of the specific surface
X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), CO
chemisorption, inductively coupled plasma optical emission
spectrometry (ICP-OES), temperature-programmed reduction
area (SBET), average pore size (d
Pd/TiO and alkali metal-doped Pd/TiO
with fresh TiO are listed in Table 1. Compared with the bare
) and total pore volume of
p
by H
2
(H
2
-TPR), X-ray photoelectron spectroscopy (XPS) and
2
2
catalysts together
temperature-programmed desorption by O2 (O -TPD) were
2
2
carried out and the methods and instruments for the charac-
3
1
terization were the same as our previous work. Typically,
the C 1s peak (284.8 eV) was used to calibrate the binding
energy (B. E.) values and the spectra are shown in Fig. S1
(ESI†). Transmission electron microscopy (TEM) was carried
out on a Tecnai G2 F20 U-Twin field emission system oper-
ated at 200 kV.
2
.3. Catalyst activity testing
The method and equipment for catalyst activity testing were
3
1
the same as in our previous work. Typically, the relative
humidity was kept 30% balanced by helium. The total inlet
−
1
flow rate was 100 mL min , corresponding to a gas hourly
−
1
space velocity (GHSV) of 190 000 h . Measurements of the
turnover frequency (TOF) were obtained in a separate experi-
ment where the conversion of HCHO was kept below 30% by
varying the inlet HCHO concentration and GHSV, with negli-
gible heat and mass-transfer effects. The conditions of the
TOF measurement are shown in Table S1 (ESI†).
Fig. 1 HCHO conversion over Pd/TiO
doped Pd/TiO catalysts. Reaction conditions: 25 °C, 140 ppm HCHO,
0% 30% RH, He balance, GHSV 190 000
2
and alkali metal (Li, Na, K, Cs)
2
−
1
2
O
2
,
h
(inset: HCHO
conversion over Na–, K– or Cs–Pd/TiO
2
catalysts at 25 °C with 550
−1
ppm HCHO, GHSV 640 000 h and the other conditions are the
same).
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in Fig. S2† and Table 1. The TEM results indicated that the
Pd particle size was obviously decreased after the alkali metal
addition, which further confirmed that the alkali addition
had a promotion effect on the Pd dispersion.
3
.2.3. H -TPR. Fig. 3 shows the hydrogen temperature-
2
2
programmed reduction (H -TPR) profiles conducted over the
fresh TiO and Li-, Na-, K- or Cs-doped TiO (Fig. 3a) together
2
2
with Pd/TiO and the Li-, Na-, K- or Cs-doped Pd/TiO cata-
2
2
lysts (Fig. 3b) to study the reducibility of the catalysts. Pure
TiO exhibited no H consumption peak from the −50 to 600
2
2
°
°
C range, since TiO reduction normally occurs at T > 600
2
2
2,43
C.
2
There was a broad peak at 150–510 °C over Li/TiO , at
Fig. 2 XRD patterns of Pd/TiO
2
and alkali metal (Li, Na, K, Cs) doped
samples.
200–450 °C over Na/TiO , at 200–580 °C over K/TiO and at
2 2
Pd/TiO catalysts together with fresh TiO
2
2
3
10–590 °C over Cs/TiO , which may be ascribed to the reduc-
2
tion of residual Li, Na, K and Cs nitrates, respectively. After
Pd was loaded, a sharp PdO reduction peak appeared at
4
4
TiO , the S , pore volume and d of the Pd/TiO , Li–, Na–,
around 0 °C on the Pd/TiO catalysts. With the further addi-
2
BET
p
2
2
K–, and Cs–Pd/TiO
2
catalysts decreased to different degrees,
tion of alkali metals, the PdO reduction peak shifted to a
which may be ascribed to diffusion of alkali metal species
higher temperature at about 47 °C over the Li–Pd/TiO cata-
2
4
2
into the small pores of TiO
2
.
In order to eliminate the influ-
lyst, at about 58 °C over Na–Pd/TiO
and at about 70 °C over the K–Pd/TiO
reduction peak of alkali metal nitrates on the Li–Pd/TiO2,
Na–Pd/TiO , K–Pd/TiO and Cs–Pd/TiO catalysts shifted to
2
and Cs–Pd/TiO
2
catalysts
ence of the surface area, the normalized reaction rate per
2
catalyst. However, the
unit area (R ) at 25 °C with HCHO conversion kept below
s
6
0% was calculated and listed in Table 1. The R
s
results also
2
2
2
illustrated that all the alkali metal addition improved the per-
formance of the Pd/TiO2 catalyst for formaldehyde (HCHO)
oxidation, which followed the order K > Cs > Na > Li.
200–275, 113, 108 and 127 °C, respectively. The above results
indicated an interaction between the alkali metal species and
Pd species, which resulted in the alkali metal species stabiliz-
ing the PdO for the fresh catalysts and, after the PdO was
reduced, the existence of the Pd metal species facilitated the
3
.2.2. Pd dispersion and particle size. Pd dispersion was
measured by CO chemisorption and the results are listed in
Table 1. Without alkali metal addition, the Pd dispersion was
only 9.8%. With Li addition, the Pd dispersion increased
slightly to 13.1%. In contrast, the Pd dispersion was remark-
ably increased to 32.9%, 40.6% and 48.2% by Na, Cs and K
addition, respectively. It is clear that the alkali metals all
improved the Pd dispersion, depending on the nature of the
alkali metals. Based on the results of Pd dispersion, the TOFs
3
9
reduction of the residual alkali metal nitrates. Besides, also
observed in the HR-TEM results of the Na–Pd/TiO catalyst
(see Fig. S2f†) is a partially covered Pd particle by NaO on
2
x
2
the Na–Pd/TiO catalyst, which further indicated the exis-
3
9
tence of the promoter–metal interaction. In addition, the
H consumption of PdO on the samples was quantified and
2
further converted to the Pd content. The results are shown in
Table 1 which indicated that H consumption was in agree-
2
−
2
−1
were calculated at 25 °C. The value of TOF is 0.44 × 10
s
for the Pd/TiO2 catalyst. After the alkali metal addition, it
ment with expected Pd contents on the samples.
−
2
−1
−2 −1
−2 −1
improved to 0.54 × 10 s , 0.97 × 10 s , 4.05 × 10
s
3.2.4. XPS analysis. The states of Pd, Ti and O on the cata-
lyst surface were next identified with XPS measurements.
Fig. 4 showed the XPS spectra of the catalysts and Table 2
summarized the binding energies and the percentages of
−
2
−1
and 2.35 × 10
s
for the Li–, Na–, K– and Cs–Pd/TiO
2
cata-
lysts, respectively. To directly show the Pd particle size on the
catalysts, TEM was also carried out and the results are shown
Table 1 Pd composition, specific surface area (BET), average pore size (d
p
) and total pore volume of Pd/TiO
2
2
and alkali metal doped Pd/TiO catalysts
together with fresh TiO
TiO catalysts
2
, Pd mean particle size, Pd dispersion (DCO), TOF and normalized rate per unit area (R
s
) for Pd/TiO and alkali metal doped Pd/
2
2
a
b
c
d
e
Pd comp
(wt%)
Pd comp
(wt%)
BET
(m g )
d
(nm)
p
Pore volume
Pd particle
size (nm)
D
(%)
CO
TOF
R
s
2
-1
3
−1
−2 −1
−1
−2
Sample
TiO -fresh
(cm g
)
(×10
s
)
(nmol s
m )
2
58.3
53.1
44.4
56.9
53.6
48.7
28.2
28.7
38.4
31.7
31.5
30.1
0.43
0.38
0.42
0.40
0.42
0.37
Pd/TiO
Li–Pd/TiO
Na–Pd/TiO
K–Pd/TiO
2
0.95
0.98
0.98
0.97
0.96
1.11
1.06
1.07
1.08
1.02
6.1
5.7
4.7
3.4
4.2
9.8
13.1
32.9
48.2
40.6
0.44
0.54
0.97
4.05
2.35
2.0
2.8
5.2
19.3
17.7
2
2
2
Cs–Pd/TiO
2
a
b
c
2 2
Pd composition was measured by ICP-OES. Pd composition was calculated based on the H consumption of PdO in H -TPR. Dispersion of
d
Pd measured by CO chemisorption. TOF is calculated on the basis of DCO with the HCHO conversion kept below 30%. The conditions are
shown in Table S1 (ESI). The amount of HCHO reacted per unit time and per unit area.
e
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Fig. 3
H
2
-TPR profiles of fresh samples: (a) TiO
2
and alkali metal (Li,
Na, K, Cs) doped TiO
Pd/TiO samples.
2
; (b) Pd/TiO and alkali metal (Li, Na, K, Cs) doped
2
2
calculated XPS peak areas. Based on Fig. 5a, the peak at
45
3
36.4 eV on all catalysts should be attributed to PdO, which
is due to re-oxidation of metallic Pd by O and/or H O in air
2
2
3
9
during transfer of the sample to the XPS chamber. The
peak at 335.4 eV on Li–Pd/TiO and 335.1 eV on the other Pd/
TiO2 catalysts should be ascribed to metallic Pd.
alkali metal addition, a third peak appeared at 334.3, 334.1,
33.9 and 333.8 eV over the Li–, Na–, K–, Cs–Pd/TiO cata-
2
4
6,47
With
3
2
lysts, respectively. These results clearly showed that the
doped alkali metals, as electron donors, increased the
0
Fig. 4 XPS spectra of Pd/TiO
2
and alkali metal (Li, Na, K, Cs) doped Pd/
electron density of Pd by their interaction with metallic
TiO catalysts together with TiO sample: (a) Pd 3d, (b) Ti 2p and (c) O 1s.
2
2
4
8–50
Pd,
and this electron donation effect became stronger
with the increase in the atomic number of the alkali metal.
In addition, according to the relative ratio of Pd species of
the samples (Table 2), the negatively charged metallic Pd spe-
after Na, K, and Cs doping, respectively, which is attributed to
5
0
electron transfer from the alkali metals to Ti and O of TiO2
or may be partially attributed to further TiO reduction caused
cies were dominant on the alkali metal-doped Pd/TiO
2
cata-
2
5
2,53
lyst surfaces, therefore playing the key role in HCHO oxida-
tion under ambient temperature.
by the alkali metal addition
The O 1s XPS spectra of the catalysts are shown in Fig. 4c.
All samples exhibited two kinds of O species. The main peak
in the range of 529.7–529.0 eV was ascribed to the lattice oxy-
gen of bulk TiO (O ), and the shoulder peak at 531.5–530.9
The Ti 2p peaks of the series of Pd/TiO
with those of fresh TiO are shown in Fig. 4b. The peak at
58.8 eV for the fresh TiO catalyst was ascribed to Ti .
2
catalysts together
2
4
+ 29
4
A
2
2
I
5
4
negative shift of 0.3 eV for Pd/TiO
2
occurred after Pd was
may be assigned to the Ti–OH species (OII). Table 2 shows
the relative ratio of the Ti–OH species and lattice oxygen spe-
cies, denoted as O /O . The ratio over the TiO -fresh, Pd/TiO
loaded, which should be ascribed to partial reduction of TiO
2
5
1
during the H pretreatment. Compared with the Pd/TiO cat-
2
2
II
I
2
2
alyst, a further negative shift of 0.4, 0.5 and 0.7 eV occurred
and Li–Pd/TiO
2
samples was about 0.10, while it increased to
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Table 2 Pd 3d XPS data and the relative ratio of Pd species of Pd/TiO
2
which followed the order K > Cs > Na > Li. Among them,
2
and alkali metal doped Pd/TiO catalysts, and the relative ratio of oxygen
the K–Pd/TiO catalyst in particular possessed the best perfor-
2
species
mance in HCHO oxidation under ambient temperature.
Based on the characterization results, it is indicated that the
promotion effect should be closely related to several factors,
including Pd dispersion, the negatively charged metallic Pd,
surface oxygen and OH species.
The SBET results (Table 1) show that the SBET, pore volume
p 2 2
and d of the Pd/TiO , Li–, Na–, K–, and Cs–Pd/TiO catalysts
decreased to different degrees compared with the bare TiO2.
Pd 3d
Sample
B. E. (eV)
Ratio (%)
OII/O
I
Pd/TiO
2
336.4
34.0
66.0
12.3
16.0
71.7
6.1
14.4
79.5
7.3
13.6
79.1
9.8
13.9
76.3
0.10
0.11
3
35.1
Li–Pd/TiO
2
336.4
3
3
35.4
34.3
Na–Pd/TiO
2
336.4
0.14
0.16
0.15
3
3
35.1
34.0
However, the R results were in line with the activities shown
s
in Fig. 1, indicating that the reaction rates of the samples did
not depend on their SBET. As shown in Fig. 1 and Table 1,
there is a correlation between the performance of the cata-
lysts for HCHO oxidation and their Pd dispersion, indicating
that the Cs, Na and K addition led to a significant improve-
K–Pd/TiO
2
336.4
3
3
35.1
33.9
Cs–Pd/TiO
2
336.4
3
3
35.1
33.8
ment of Pd/TiO catalyst performance by inducing more dis-
2
persed Pd species on the catalyst surface and exposing more
Pd sites for HCHO oxidation. The improved Pd dispersion
may be ascribed to the strong metal–promoter interaction,
which can be observed from the results of H -TPR (Fig. 3). In
2
addition, the interaction also had an effect on the electron
density of metallic Pd. Based on Fig. 4a and Table 2, com-
2
pared with the Pd/TiO catalyst, negatively charged metallic
Pd species were formed and were dominant on all the alkali
metal doped samples. The alkali metal species, as electron
0
donors, increase the electron density of Pd through interac-
4
9
tion with metallic Pd. It has been reported that these nega-
tively charged metallic Pd species could enhance O adsorp-
tion through electron transfer from Pd metal to the anti-
2
2
9,56
Fig. 5
doped Pd/TiO
with 10% H /Ar for 1 h at 350 °C, followed by purging with He and
cooling down to 50 °C. Then the gas was switched to O for adsorp-
2
tion for 30 min and He for purging for 1 h.
O
2
-TPD profiles of Pd/TiO
2
and alkali metal (Li, Na, K, Cs)
bonding π* orbital of O2.
The results of Ti XPS (Fig. 4b) show that the alkali metal
addition may facilitate the reduction of TiO . It has been
reported that water could readily dissociate on TiO defects
2
samples. Conditions: the samples were pre-reduced
2
2
2
5
7
to form OH groups, which can be demonstrated by the
results of O 1s XPS (Fig. 4c). The O 1s XPS results indicated
that the concentrations of OH groups for the Na–, K–, and
0
.14, 0.16 and 0.15 with Na-, K- and Cs-doped Pd/TiO2,
respectively, indicating that Na, K and Cs addition may
increase the concentration of surface OH groups.
Cs–Pd/TiO
2
catalysts were enhanced after Na, K, and Cs addi-
5
5
tion. It has been reported that surface OH, on the one hand,
2
4,26,28
3
.2.5. O -TPD. In order to investigate O activation and
can accelerate partial oxidation of HCHO to formate
and then facilely oxidize formate under ambient tempera-
2
2
mobility over the Pd/TiO
alysts, O
are shown in Fig. 5. Only one broad O2 desorption peak
between 50 °C and 400 °C appeared on all the Pd/TiO sam-
ples. For the Pd/TiO catalyst, the O desorption peak was
2
2
and alkali metal doped Pd/TiO cat-
2
4,26
-TPD experiments were carried out and the results
ture;
facilitate O
increase the diffusion of oxygen along the surface Ti (5c) to
on the other hand, the OH species can not only
2
adsorption on the defects on TiO (110) but also
2
2
2
5
8,59
the alkali metal–Pd–TiO
2
interface.
The adsorbed oxygen
2
2
located at 221 °C. After alkali metal addition, the O desorp-
tion peak shifted to lower temperatures, located at 200 °C for
was then activated by the negatively charged metallic Pd spe-
cies and subsequently oxidized the adsorbed HCHO. There
was little difference observed in the concentration of the sur-
2
2 2
Li–Pd/TiO , 180 °C for Na–Pd/TiO and about 175 °C for K–
Pd/TiO and 173 °C for Cs–Pd/TiO catalysts, which indicated
face OH groups and the mobility of chemisorbed O
Na-, K- and Cs-doped catalysts; however, the K–Pd/TiO
lyst exhibited much higher TOFs (see Table 1) than the Na–
and Cs–Pd/TiO2 catalysts, which may be attributed to the
2
for the
cata-
2
2
that alkali metal addition enhanced the mobility of the
chemisorbed oxygen over the doped Pd/TiO catalysts.
2
2
smaller Pd particle size over K–Pd/TiO
metal–support interface. On the one hand, the adsorbed H
can easily dissociate into a surface OH group on oxygen
2
catalyst with more
4
. Discussion
2
O
As shown in Fig. 1, the alkali metals had a common promo-
tion effect on the Pd/TiO2 catalysts for HCHO oxidation,
5
7
vacancies, especially in the proximity of the Pd particles at
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6
0
the metal–support interface. On the other hand, the dissoci-
14 Y. R. Wen, X. Tang, J. H. Li, J. M. Hao, L. S. Wei and X. F.
Tang, Catal. Commun., 2009, 10, 1157–1160.
15 X. F. Tang, Y. G. Li, X. M. Huang, Y. D. Xu, H. Q. Zhu, J. G.
Wang and W. J. Shen, Appl. Catal., B, 2006, 62, 265–273.
16 Z. P. Qu, S. J. Shen, D. Chen and Y. Wang, J. Mol. Catal. A:
Chem., 2012, 356, 171–177.
ation of the molecularly adsorbed mobile oxygen may also
take place at the metal–support interface. The activated sur-
face OH and O species play important roles in direct HCHO
6
0
2
4
oxidation.
5
. Conclusions
17 S. Imamura, D. Uchihori, K. Utani and T. Ito, Catal. Lett.,
1
994, 24, 377–384.
The series of alkali metals show a common promotion effect
on the Pd/TiO catalyst for ambient temperature HCHO oxi-
1
8 J. Quiroz, J.-M. Giraudon, A. Gervasini, C. Dujardin, C.
2
Lancelot, M. Trentesaux and J.-F. Lamonier, ACS Catal.,
dation. The addition of Li, Na, K and Cs all could induce and
stabilize a negatively charged metallic Pd species on the TiO2
surface through the promoter–metal interaction, and then
enhanced the chemisorption of surface oxygen species. Fur-
thermore, the Na, K, and Cs addition could also induce more
reaction sites by improving the Pd dispersion and number of
surface OH groups, and enhance the mobility of chemisorbed
oxygen. The K–Pd/TiO2 catalyst in particular possessed the
highest Pd dispersion and smaller Pd particles with more
metal–support interfaces which facilitated chemisorption and
dissociation of oxygen and formation of surface OH groups,
contributing to its highest activity among the alkali metal
doped catalysts.
2
015, 5, 2260–2269.
1
2
9 B. Y. Bai and J. H. Li, ACS Catal., 2014, 4, 2753–2762.
0 X. F. Tang, J. L. Chen, Y. G. Li, Y. Li, Y. D. Xu and W. J.
Shen, Chem. Eng. J., 2006, 118, 119–125.
2
2
1 C. B. Zhang, H. He and K.-i. Tanaka, Catal. Commun.,
2
005, 6, 211–214.
2 C. B. Zhang, H. He and K.-i. Tanaka, Appl. Catal., B,
006, 65, 37–43.
2
2
2
3 C. B. Zhang and H. He, Catal. Today, 2007, 126, 345–350.
4 C. B. Zhang, F. D. Liu, Y. P. Zhai, H. Ariga, N. Yi, Y. C. Liu,
K. Asakura, M. Flytzani-Stephanopoulos and H. He, Angew.
Chem., Int. Ed., 2012, 51, 9628–9632.
2
2
2
2
5 S. S. Kim, K. H. Park and S. C. Hong, Appl. Catal., A,
2
011, 398, 96–103.
Acknowledgements
6 D. W. Kwon, P. W. Seo, G. J. Kim and S. C. Hong, Appl.
Catal., B, 2015, 163, 436–443.
7 H. F. Li, N. Zhang, P. Chen, M. F. Luo and J. Q. Lu, Appl.
Catal., B, 2011, 110, 279–285.
8 B. B. Chen, C. A. Shi, M. Crocker, Y. Wang and A. M. Zhu,
Appl. Catal., B, 2013, 132, 245–255.
This work was financially supported by the National Natural
Science Foundation of China (51221892, 21422706, 21577159)
and the Program of the Ministry of Science and Technology
of China (2012AA062702).
2
3
9 H. B. Huang and D. Y. C. Leung, ACS Catal., 2011, 1, 348–354.
0 E. Jeroro and J. M. Vohs, J. Am. Chem. Soc., 2008, 130,
10199–10207.
Notes and references
1
2
3
4
5
6
J. Gunschera, S. Mentese, T. Salthammer and J. R. Andersen,
Build. Environ., 2013, 64, 138–145.
G. D. Nielsen, S. T. Larsen and P. Wolkoff, Arch. Toxicol.,
31 C. B. Zhang, Y. B. Li, Y. F. Wang and H. He, Environ. Sci.
Technol., 2014, 48, 5816–5822.
32 S. Imamura, Y. Uematsu, K. Utani and T. Ito, Ind. Eng.
Chem. Res., 1991, 30, 18–21.
33 Q. L. Xu, W. Y. Lei, X. Y. Li, X. Y. Qi, J. G. Yu, G. Liu, J. L. Wang
and P. Y. Zhang, Environ. Sci. Technol., 2014, 48, 9702–9708.
34 W. D. Mross, Catal. Rev.: Sci. Eng., 1983, 25, 591–637.
35 S. S. Mulla, N. Chen, L. Cumaranatunge, W. N. Delgass, W. S.
Epling and F. H. Ribeiro, Catal. Today, 2006, 114, 57–63.
36 I. V. Yentekakis, G. Moggridge, C. G. Vayenas and R. M.
Lambert, J. Catal., 1994, 146, 292–305.
2
013, 87, 73–98.
L. Ma, D. S. Wang, J. H. Li, B. Y. Bai, L. X. Fu and Y. D. Li,
Appl. Catal., B, 2014, 148–149, 36–43.
H. Q. Rong, Z. Y. Liu, Q. L. Wu, D. Pan and J. T. Zheng,
Cellulose, 2009, 17, 205–214.
J. C. Moreno-Pirajan, J. Tirano, B. Salamanca and L. Giraldo,
Int. J. Mol. Sci., 2010, 11, 927–942.
F. Shiraishi, D. Ohkubo, K. Toyoda and S. Yamaguchi, Chem.
Eng. J., 2005, 114, 153–159.
7
8
C. H. Ao and S. C. Lee, Appl. Catal., B, 2003, 44, 191–205.
F. Holzer, U. Roland and F. D. Kopinke, Appl. Catal., B,
37 B. Mirkelamoglu and G. Karakas, Appl. Catal., A, 2006, 299,
84–94.
2
002, 38, 163–181.
M. B. Chang and C. C. Lee, Environ. Sci. Technol., 1995, 29,
81–186.
38 A. M. Kazi, B. Chen, J. G. Goodwin, G. Marcelin, N.
Rodriguez and T. K. Baker, J. Catal., 1995, 157, 1–13.
39 X. L. Zhu, M. Shen, L. L. Lobban and R. G. Mallinson,
J. Catal., 2011, 278, 123–132.
40 Y. P. Zhai, D. Pierre, R. Si, W. Deng, P. Ferrin, A. U. Nilekar,
G. W. Peng, J. A. Herron, D. C. Bell, H. Saltsburg, M. Mavrikakis
and M. Flytzani-Stephanopoulos, Science, 2010, 329, 1633–1636.
41 M. Yang, S. Li, Y. Wang, J. A. Herron, Y. Xu, L. F. Allard, S. Lee,
J. Huang, M. Mavrikakis and M. Flytzani-Stephanopoulos,
Science, 2014, 346, 1498–1501.
9
1
1
0 J. Quiroz Torres, S. Royer, J. P. Bellat, J. M. Giraudon and
J. F. Lamonier, ChemSusChem, 2013, 6, 578–592.
1 J. J. Pei and J. S. S. Zhang, HVACR Res., 2011, 17, 476–503.
2 C. Egawa, I. Doi, S. Naito and K. Tamaru, Surf. Sci.,
1
1
1
986, 176, 491–504.
3 B. Y. Bai, H. Arandiyan and J. H. Li, Appl. Catal., B,
013, 142, 677–683.
1
2
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
4
4
4
4
4
4
4
4
5
5
2 L. Xue, H. He, C. Liu, C. B. Zhang and B. Zhang, Environ.
Sci. Technol., 2009, 43, 890–895.
3 N. S. de Resende, J.-G. Eon and M. Schmal, J. Catal.,
52 H. Onishi, T. Aruga, C. Egawa and Y. Iwasawa, Surf. Sci.,
1988, 199, 54–66.
53 P. Panagiotopoulou and D. I. Kondarides, J. Catal.,
2009, 267, 57–66.
1
999, 183, 6–13.
4 H. Q. Zhu, Z. F. Qin, W. J. Shan, W. J. Shen and J. G. Wang,
J. Catal., 2004, 225, 267–277.
5 K. Otto, L. P. Haack and J. E. deVries, Appl. Catal., B,
54 L. H. Nie, J. G. Yu, X. Y. Li, B. Cheng, G. Liu and M.
Jaroniec, Environ. Sci. Technol., 2013, 47, 2777–2783.
55 I. M. Brookes, C. A. Muryn and G. Thornton, Phys. Rev. Lett.,
2001, 87, 2661031–2661034.
56 C. G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda and D.
Tsiplakides, Electrochemical activation of catalysis: promotion,
electrochemical promotion, and metal–support interactions, Kluwer
Academic Publishers/Plenum Press, New York, 2001, pp. 35–86.
57 Z. Zhang, O. Bondarchuk, B. D. Kay, J. M. White and Z.
Dohnalek, J. Phys. Chem. B, 2006, 110, 21840–21845.
58 H. B. Huang, X. G. Ye, H. L. Huang, L. Zhang and D. Y. C.
Leung, Chem. Eng. J., 2013, 230, 73–79.
1
992, 1, 1–12.
6 H. Q. Yang, G. Y. Zhang, X. L. Hong and Y. Y. Zhu, J. Mol.
Catal. A: Chem., 2004, 210, 143–148.
7 Q. Lin, Y. Ji, Z. D. Jiang and W. D. Xiao, Ind. Eng. Chem.
Res., 2007, 46, 7950–7954.
8 L. F. Liotta, G. Deganello, P. Delichere, C. Leclercq and G. A.
Martin, J. Catal., 1996, 164, 334–340.
9 L. F. Liotta, G. A. Martin and G. Deganello, J. Catal.,
1
996, 164, 322–333.
0 V. Pitchon, M. Guenin and H. Praliaud, Appl. Catal.,
990, 63, 333–343.
1 V. V. Rozanov and O. V. Krylov, Russ. Chem. Rev., 1997, 66,
17–130.
59 L. M. Liu, B. McAllister, H. Q. Ye and P. Hu, J. Am. Chem.
Soc., 2006, 128, 4017–4022.
60 M. M. Schubert, S. Hackenberg, A. C. van Veen, M. Muhler,
V. Plzak and R. J. Behm, J. Catal., 2001, 197, 113–122.
1
1
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