Significant enhancement in activity of Pd/TiO2 catalyst for formaldehyde oxidation by Na addition

Article abstract of DOI:10.1016/j.cattod.2016.05.037

Developing an effective material for indoor HCHO elimination at ambient temperature is of great significance. In this work, a series of Pd/TiO₂ catalysts with different Na loadings were prepared and tested for their activity in HCHO oxidation. The results indicated that Na addition had a remarkable promotion effect on Pd/TiO₂ catalysts for HCHO oxidation, and the optimal Na loading was 2 wt.%. Features of the catalysts including the reducibility, Pd dispersion and valence state of Pd and O were elucidated using a variety of characterization techniques. The results showed that 2.0Na-Pd/TiO₂ exhibited more exposure of active Pd sites and contained abundant surface OH groups, giving it the best HCHO catalytic oxidation performance of the studied catalysts. The 2.0Na-Pd/TiO₂ catalyst also had high resistance to effects of space velocity and relative humidity (RH), maintaining high HCHO conversion over a wide gas hourly space velocity range of 80,000–190,000 h^?1 and RH of 25%–65%.

Full text of DOI:10.1016/j.cattod.2016.05.037

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Significant enhancement in activity of Pd/TiO2 catalyst for
formaldehyde oxidation by Na addition
Yaobin Lia,b, Changbin Zhanga,b,∗, Hong Hea,b,c
a State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environment Sciences, Chinese Academy of Sciences,
Beijing 100085, China
b University of Chinese Academy of Sciences, Beijing 100049, China
c Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Developing an effective material for indoor HCHO elimination at ambient temperature is of great signif-
icance. In this work, a series of Pd/TiO2 catalysts with different Na loadings were prepared and tested
for their activity in HCHO oxidation. The results indicated that Na addition had a remarkable promotion
effect on Pd/TiO2 catalysts for HCHO oxidation, and the optimal Na loading was 2 wt.%. Features of the
catalysts including the reducibility, Pd dispersion and valence state of Pd and O were elucidated using a
variety of characterization techniques. The results showed that 2.0Na-Pd/TiO2 exhibited more exposure
of active Pd sites and contained abundant surface OH groups, giving it the best HCHO catalytic oxidation
performance of the studied catalysts. The 2.0Na-Pd/TiO2 catalyst also had high resistance to effects of
space velocity and relative humidity (RH), maintaining high HCHO conversion over a wide gas hourly
space velocity range of 80,000–190,000 h−1 and RH of 25%–65%.
Received 5 February 2016
Received in revised form 5 May 2016
Accepted 20 May 2016
Available online xxx
Keywords:
Sodium addition
Pd/TiO2
Formaldehyde
Room temperature
Promoter-metal interaction
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
method for indoor HCHO elimination due to its characteristics of
high efficiency and non-pollution [16,17]. Transition metal oxides,
With the advance of technology, people have gradually come
to prefer an indoor lifestyle, resuting in that increasing numbers of
people suffer from Sick Building Syndrome (SBS). Among indoor air
pollutants, formaldehyde (HCHO), which is mainly emitted from
consumer products and building/furnishing materials, is consid-
ered the most toxic [1]. It is well known that long-term exposure
to indoor air containing HCHO, even at very low ppm level, may lead
to serious health problems including nasal tumors, irritation of the
mucous membranes of the eyes and respiratory tract, skin irritation,
decreased concentration, and weakened immunity [2,3]. Therefore,
to improve indoor air quality and reduce public health risk, effec-
tive materials and technologies for indoor air HCHO removal are
urgently needed.
such as Co3O4 [12], MnO2 [13,18] and Ag2O [19], generally need
relatively high temperatures to completely oxidize HCHO, while
supported noble metal catalysts such as those with Pt [9–11], Au
[20–22], Rh [23] or Pd [15,24,25] have proved to be effective for
HCHO elimination at low or even room temperature, and therefore
they are more suitable for indoor air HCHO purification.
Alkali metals can serve as electronic or textural promoters for
catalysts in various catalytic processes [26] including oxidation of
NO [27] and CO [28,29], CO hydrogenation [30] and the water-gas
shift reaction [31–33]. Recently, we found that addition of alkali
metals (Li, Na and K) to Pt/TiO2 catalysts could lead to more atom-
ically dispersed Pt species and effectively enhance the O2 and H2O
activation, resulting in a remarkable promotion effect on the perfor-
mance of alkali-Pt/TiO2 catalysts in HCHO oxidation [10]. Moreover,
it was further found that Na addition also had an obvious promotion
effect on Pd/TiO2 catalysts for HCHO oxidation, due to the nega-
tively charged and well-dispersed Pd species induced and stabilized
by Na addition [25].
Over the past decades, researchers have focused their attention
on four technologies including adsorption [4,5], photo-catalysis [6],
plasma technology [7] and catalytic oxidation [8–15]. Among these
methods, catalytic oxidation is regarded as the most promising
Herein, a series of Pd/TiO2 catalysts with various amounts of
Na addition were prepared and tested to clarify the effect of the
amount of doped Na on their performance in HCHO oxidation. It was
found that the performance of the Pd/TiO2 catalyst was enhanced
by different degrees by 1.0–8.0 wt.% Na addition, that is, 2 wt.% Na
∗
Corresponding author at: State Key Joint Laboratory of Environment Simula-
tion and Pollution Control, Research Center for Eco-Environment Sciences, Chinese
Academy of Sciences, Beijing 100085, China.
E-mail address: cbzhang@rcees.ac.cn (C. Zhang).
http://dx.doi.org/10.1016/j.cattod.2016.05.037
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Li, et al., Significant enhancement in activity of Pd/TiO2 catalyst for formaldehyde oxidation by Na
addition, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.037

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Fig. 1. HCHO conversion over Pd/TiO2 catalysts. Reaction conditions: 140 ppm HCHO, 20% O2, 25% RH, He balance, GHSV 95,000 h−1
.
addition exhibited the best performance in HCHO oxidation, with
100% conversion of 140 ppm HCHO at GHSV of 95,000 h−1. Mean-
while, the activities of Na-Pd/TiO2 catalysts were also affected by
GHSV and relative humidity.
catalysts was measured by CO pulse chemisorption, which was also
conducted on the Micromeritics AutoChem II 2920 apparatus. The
samples (30 mg) were first pre-reduced at 350 ◦C for 30 min in 10%
H2/Ar, then purged with He for 30 min. Subsequently, the temper-
ature was lowered to room temperature in He flow. Pulses of 5%
CO/He were introduced to the catalyst until uptake saturation was
obtained. The CO consumption was monitored by TCD. The disper-
sion of Pd was calculated assuming a CO/Pd stoichiometric ratio of 1
[15]. X-ray photo-emission (XPS) measurements were recorded on
a Scanning X-ray Microprobe (AXIS Ultra, Kratos Analytical, Inc.).
The C 1 s peak (284.8 eV) was used to calibrate the binding energy
(BE) values.
2. Materials and methods
2.1. Materials preparation
1 wt.% Pd/TiO2 and Na-doped 1 wt.% Pd/TiO2 catalysts with 1, 2,
4 and 8 wt.% Na addition were prepared by co-impregnation of TiO2
(Degussa P25) with aqueous Pd(NO3)2 (Sigma-Aldrich) and NaNO3
(Sinopharm Chemical Reagent Beijng Co., Ltd). After impregnation,
the excess water was removed in a rotary evaporator at 60 ◦C.
The samples were dried at 110 ◦C overnight and then calcined at
400 ◦C for 2 h. Before activity testing and characterization, the sam-
ples were reduced with H2 at 350 ◦C for 30 min, and denoted as
Pd/TiO2, 1.0Na-, 2.0Na-, 4.0Na- and 8.0Na-Pd/TiO2. The samples
without pretreatment are denoted as TiO2-fresh, Pd/TiO2-fresh,
1.0Na-, 2.0Na-, 4.0Na- and 8.0Na-Pd/TiO2-fresh.
2.3. Catalytic activity testing
The activity tests of the catalysts (∼60 mg) were conducted in a
fixed-bed quartz flow reactor (i.d. = 6 mm) in an incubator kept at
25 ◦C. Gaseous HCHO generated by decomposition of paraformalde-
hyde was carried by helium. Water vapor was generated by flowing
helium through a water bubbler at 25 ◦C. The feed gas was com-
posed of 140 ppm HCHO, 20% O2 and 25% RH balanced by helium.
The total flow rate was 100 mL min−1, corresponding to a gas hourly
space velocity (GHSV) of 190,000 h−1. The inlet and outlet gases
were monitored by an FTIR spectrometer (Nicolet 380) equipped
with a 2 m gas cell and DTGS detector; resolution: 0.5 cm−1. More
details about quantitative analysis have been described in our pre-
vious work [25].
2.2. Characterization of catalysts
Powder X-ray diffraction (XRD) was used to identify the crys-
talline phases present in the catalysts. Their XRD patterns were
collected with an X’Pert PRO MPD X-ray powder diffractometer
with Cu K␣ radiation operated at 40 kV and 40 mA. A Quantachrome
Quadrasorb SI-MP analyzer was used to analyze the specific surface
area and pore characterization of the catalysts, operated at −196 ◦C
over the whole range of relative pressures. H2 temperature-
programmed reduction (H2-TPR) experiments were performed in
a Micromeritics AutoChem II 2920 apparatus, equipped with a
computer-controlled CryoCooler and a thermal conductivity detec-
tor (TCD). The samples were placed in a U-type tube and pretreated
in air (50 mL min−1) at 400 ◦C for 30 min and then cooled down to
25 ◦C. After purging with Ar for 30 min, the reduction profiles were
collected by passing a flow of 10% H2/Ar through the sample at a rate
of 50 mL min−1. The temperature was increased from −50 to 600 ◦C
at a rate of 10 ◦C min−1, and the H2 consumption was monitored by
TCD after removal of the H2O produced. The Pd dispersion of the
3. Results and discussion
3.1. Catalytic performance
The performance of Pd/TiO2 and 1.0Na-, 2.0Na-, 4.0Na- and
8.0Na-Pd/TiO2 catalysts for HCHO oxidation was tested and the
results are shown in Fig. 1. For Pd/TiO2, 20% conversion of
HCHO was achieved at 25 ◦C and 100% at 100 ◦C. Its performance
was improved by Na addition, and 70%, 75% and 80% of HCHO
could be converted to H2O and CO2 with no any by-products
on 8.0Na-Pd/TiO2, 4.0Na-Pd/TiO2 and 1.0Na-Pd/TiO2 respectively
at room temperature, and 100% HCHO conversions could be
Please cite this article in press as: Y. Li, et al., Significant enhancement in activity of Pd/TiO2 catalyst for formaldehyde oxidation by Na
addition, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.037

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Fig. 2. XRD patterns of fresh Pd/TiO2 and Na doped Pd/TiO2 catalysts together with fresh TiO2 samples and reduced 8.0Na-Pd/TiO2.
Fig. 3. H2-TPR profiles of fresh samples including fresh Pd/TiO2 and Na doped Pd/TiO2 samples together with fresh TiO2 and 2.0Na/TiO2.
obtained on all three catalysts at about 80 ◦C. By comparison,
almost 100% HCHO conversion was achieved on 2.0Na-Pd/TiO2
at room temperature. The above results showed that the perfor-
mance of Pd/TiO2 catalysts for HCHO catalytic oxidation followed
the order 2.0Na-Pd/TiO2 > 1.0Na-Pd/TiO2 > 4.0Na-Pd/TiO2 > 8.0Na-
Pd/TiO2 > Pd/TiO2. It was clear that Na addition had a promotion
effect on Pd/TiO2 catalysts for HCHO catalytic oxidation, and that
the optimal Na loading was 2 wt.%.
Pd/TiO2-fresh samples, indicating that Pd species had a very high
dispersion degree on the samples. A peak at 2␪ = 29.4◦ appeared
when the amount of doped Na was higher than 2.0 wt.% and its
intensity was gradually increased with further increase in the
amount of Na addition. In addition, other peaks at 2␪ = 31.9, 38.9,
42.5 and 56.5◦ were also observed on 8.0Na-Pd/TiO2-fresh, which
are all attributed to residual NaNO3 (PDF #00-001-0840). After
reduction, the peaks associated with residual NaNO3 on the 8.0Na-
Pd/TiO2 catalyst disappeared, indicating that it was removed during
H2 reduction.
3.2. Characterization of catalysts
The specific surface areas (SBET) of the catalysts were measured.
As shown in Table 1, the SBET of pure TiO2 was 58.9 m2/g and the
1.0Na-Pd/TiO2 and 2.0Na-Pd/TiO2 catalysts exhibited SBET of about
3.2.1. Structural features of samples
Catalyst structures were characterized by XRD. As shown in
Fig. 2, there were no Pd species (Pd and PdO) observed on the
54.0 m2 −1, which was similar with that of the Pd/TiO2 catalyst.
g
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Table 1
The specific surface area (SBET) and Pd dispersion of the catalysts.
a
b
Sample
BET (m2 g−1
)
DCO (%)
TOF (x10−2, s−1
)
TiO2
Pd/TiO2
1.0Na-Pd/TiO2
2.0Na-Pd/TiO2
4.0Na-Pd/TiO2
8.0Na-Pd/TiO2
58.9
53.1
54.0
53.9
43.8
36.6
–
9.8
25.8
32.9
20.5
16.0
–
0.44
0.88
0.97
1.00
1.01
a
dispersion of Pd measured by CO chemisorption.
TOF is calculated based on DCO with the HCHO conversion kept below 30%.
b
However, it remarkably decreased when the amount of Na addi-
tion was more than 4 wt.%, that is, 43.8 m2 g−1 for 4.0Na-Pd/TiO2
and 36.6 m2 g−1 for 8.0Na-Pd/TiO2, which may be due to excess
Na species depositing in and blocking the small pores, so that the
reactants have difficulty diffusing to the active sites in the pores.
This is possibly one of the reasons for the poorer performance
of 4.0Na-Pd/TiO2 and 8.0Na-Pd/TiO2 catalysts, compared to the
2.0Na-Pd/TiO2 catalyst.
3.2.2. Reducibility of catalysts
The reducibility of the catalysts was studied by H2-TPR, and their
profiles are shown in Fig. 3. No H2 consumption was observed on
pure TiO2 in the temperature range since the reduction of TiO2
normally occurs at T > 600 ◦C [34]. With Na addition on pure TiO2,
a wide H2 consumption peak at 200–450 ◦C appeared on fresh
2.0Na/TiO2, which may be ascribed to the reduction of residual
NaNO3 on the fresh samples. With Pd loaded on pure TiO2, a sharp
PdO reduction peak appeared at around 0 ◦C on fresh Pd/TiO2 cata-
lyst [35]. With Na addition on Pd/TiO2, two H2 consumption peaks
were obtained on the fresh 1.0Na-Pd/TiO2 and 2.0Na-Pd/TiO2. The
peaks at low temperature (30 ◦C for fresh 1.0Na-Pd/TiO2 and 57 ◦C
for fresh 2.0Na-Pd/TiO2) are associated with the reduction of PdO,
and the ones at high temperature (79 ◦C for fresh 1.0Na-Pd/TiO2
and 72 ◦C for fresh 2.0Na-Pd/TiO2) may be attributed to the reduc-
tion of residual NaNO3. With further increases in Na loading, the
two peaks completely merged into one sharp and strong peak at
76 ◦C for both fresh 4.0Na-Pd/TiO2 and 8.0Na-Pd/TiO2. Based on
the above results, there may be an interaction between Na and Pd
species (promoter-metal interaction), which stabilizes the PdO for
the fresh catalysts and facilitates the reduction of the residual alkali
metal nitrates [31].
3.2.3. Pd dispersion
Pulse CO chemisorption was used to measure the Pd disper-
sion of the catalysts, and the results are listed in Table 1. Without
Na addition, the Pd/TiO2 catalyst showed a low Pd dispersion of
9.8%. With 1.0 wt.% Na addition, the Pd dispersion on 1.0Na-Pd/TiO2
remarkably increased to 25.8%. It reached its maximum of 32.9% on
the 2.0Na-Pd/TiO2 catalyst. However, with more than 2.0 wt.% Na
addition, it slightly decreased to 20.5% and 16.0% on 4.0Na-Pd/TiO2
and 8.0Na-Pd/TiO2 catalysts, which may be attributed to the cov-
erage of Pd by the excess Na species. Based on the results of Pd
dispersion, TOFs of Na-doped Pd/TiO2 catalysts for HCHO oxida-
tion were calculated at 25 ◦C and the results were added in Table 1.
The value of TOF was 0.44 × 10−2 s−1 for the Pd/TiO2 catalyst. After
Na addition, it was improved to 0.88 × 10−2 s−1, 0.97 × 10−2 s−1
,
1.00 × 10−2 s−1 and 1.01 × 10−2 s−1 for 1Na-, 2Na-, 4Na- and 8Na-
Pd/TiO2 catalysts, respectively. It is clear that the catalytic activities
were not only related to the Pd dispersion, but also affected by some
other factors, such as the concentration of surface hydroxyl. More-
over, it is clear that a proper amount of Na addition has a promoting
effect on the Pd dispersion because of the promoter-metal interac-
tion, while excess Na addition will result in a negative effect on the
performance of Pd/TiO2 catalysts for HCHO oxidation.
Fig. 4. XPS spectra of Pd/TiO2 and Na doped Pd/TiO2 catalysts: (a) Pd 3d, (b) Ti 2p
and (c) O 1s.
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Fig. 5. Effect of gas hourly space velocity (GHSV) on HCHO conversion over the 2.0Na-Pd/TiO2 catalyst (reaction conditions: HCHO 140 ppm, RH = 25%).
Fig. 6. Effect of relative humidity (RH) on HCHO conversion over the 2.0Na-Pd/TiO2 catalyst (reaction conditions: HCHO 140 ppm, GHSV = 95,000 h−1).
3.2.4. XPS analysis
species. According to Fig. 4b, the binding energy of Ti 2p shifted
from 458.5 eV (for Pd/TiO2 catalyst) to 458.0 eV (for 1.0Na-, 2.0Na-,
4.0Na- and 8.0Na-Pd/TiO2 catalysts) after Na addition, which may
be attributed to Na addition facilitating the reduction of TiO2 to
form oxygen vacancies. It is well known that oxygen vacancies
not only could facilitate oxygen adsorption [40] but also dissoci-
ation of adsorbed water to form surface OH groups [41]. O 1 s XPS
spectra of the catalysts are shown in Fig. 4c. Two oxygen species
were observed on all the catalysts. The one at 529.7 eV for Pd/TiO2
and 529.4 eV for the Na doped Pd/TiO2 catalysts is ascribed to lat-
tice oxygen of TiO2 while the other one is attributed to surface
OH groups [42]. It is clear that the Na addition increased the con-
centration of surface OH groups. Our recent work has clarified the
important role of surface OH groups in HCHO catalytic oxidation
on alkali metal doped Pt/TiO2 catalysts, that is, the intermediate
formate can be directly oxidized to H2O and CO2 by the surface OH
groups, resulting in a more effective pathway for HCHO oxidation
The states of Pd, Ti and O elements on the catalyst surface
were identified by XPS measurements. As shown in Fig. 4a, two
peaks were observed on the Pd/TiO2 catalyst. Specifically, the main
peak at 335.1 eV is attributed to metallic Pd [36,37] and the one at
336.4 eV is ascribed to PdO [38] formed by re-oxidation of metal-
lic Pd by O2 and/or H2O before transfer of the samples to the XPS
chamber. With Na addition, a third peak was obtained at lower
binding energy, 334.0 eV, which can be attributed to negatively
charged metallic Pd, formed by electron transfer from Na species
to the metallic Pd because of the promoter-metal interaction [39].
The negatively charged metallic Pd species could facilitate oxygen
chemisorption [15]. With increasing amounts of Na addition, the
PdO species gradually decreased on the 1.0Na- and 2.0Na-Pd/TiO2
catalysts and finally disappeared on the 4.0Na- and 8.0Na-Pd/TiO2
catalysts, which together with the gradual decrease of Pd XPS
intensity may due to the coverage of Pd species by excess Na
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[10]. Though both the 4.0Na- and 8.0Na-Pd/TiO2 catalyst possessed
abundant surface OH groups, the coverage of their active sites by
the excess Na species resulted in a decrease in their performance.
In contrast, abundant surface OH groups and the exposure of more
active sites led to 2.0Na-Pd/TiO2 having the best HCHO oxidation
performance among the catalysts in the present work.
Acknowledgment
This work was financially supported by the National Natural Sci-
ence Foundation of China (51221892, 21422706, 21577159) and
the Program of the Ministry of Science and Technology of China
(2012AA062702).
3.3. Effect of GHSV and RH
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Na addition had a dramatic promoting effect on Pd/TiO2 cata-
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closely depended on the amount of Na addition, and the optimal
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charged metallic Pd, which would facilitate oxygen adsorption and
also increase the concentration of surface OH groups, therefore
leading to the high performance of the 2.0Na-Pd/TiO2 catalyst in
HCHO catalytic oxidation. However, excess Na addition may cover
some of active Pd sites and then result in the decrease of catalytic
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Please cite this article in press as: Y. Li, et al., Significant enhancement in activity of Pd/TiO2 catalyst for formaldehyde oxidation by Na
addition, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.037