Promoting Effect and Mechanism of Alkali Na on Pd/SBA-15 for Room Temperature Formaldehyde Catalytic Oxidation

Article abstract of DOI:10.1002/cctc.201901039

Room temperature HCHO catalytic oxidation over inert support loaded Pd catalysts remains a significant challenge. Herein, a series of alkali Na (1–4 wt.%) modified 1Pd/SBA-15 catalysts were prepared by a co-impregnation method and used for HCHO catalytic oxidation at room temperature. Results showed that alkali Na significantly improved the HCHO oxidation performance of 1Pd/SBA-15 and the optimal Na loading was 2 wt.%. BET, XRD, STEM, XPS, H₂-TPR, O₂-TPD and in situ DRIFTS were performed to reveal the promotion role of alkali Na. As a consequence, alkali Na addition could stabilize well dispersed Pd particles which then facilitated the O₂ activation and accelerated the decomposition of formate intermediates species, thus a superior HCHO catalytic oxidation performance was obtained. This study first clearly illustrated the promotion mechanism of alkali Na without the implication of active support and provides an efficient and facile way to achieve high dispersion of Pd particles on inert support and tremendously expands the selected range of supports for room temperature HCHO catalytic oxidation.

Full text of DOI:10.1002/cctc.201901039

Accepted Article
Title: Promoting effect and mechanism of alkali Na on Pd/SBA-15 for
room temperature formaldehyde catalytic oxidation
Authors: Ning Xiang, Yaqin Hou, Xiaojin Han, Yulin Li, Yaoping Guo,
Yongjin Liu, and Zhanggen Huang
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To be cited as: ChemCatChem 10.1002/cctc.201901039
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Promoting effect and mechanism of alkali Na on Pd/SBA-15 for room temperature
formaldehyde catalytic oxidation
Ning Xiang,[a, b] Yaqin Hou,[a, b] Xiaojin Han,[a] Yulin Li,[a, b] Yaoping Guo,[a, b] Yongjin Liu,[a, b] and
Zhanggen Huang*[a]
Abstract: Room temperature HCHO catalytic oxidation over inert
support loaded Pd catalysts remains a significant challenge. Herein,
a series of alkali Na (1-4 wt.%) modified 1Pd/SBA-15 catalysts were
prepared by a co-impregnation method and used for HCHO catalytic
oxidation at room temperature. Results showed that alkali Na
significantly improved the HCHO oxidation performance of 1Pd/SBA-
15 and the optimal Na loading was 2 wt.%. BET, XRD, STEM, XPS,
H2-TPR, O2-TPD and in situ DRIFTS were performed to reveal the
promotion role of alkali Na. As a consequence, alkali Na addition
could stabilize well dispersed Pd particles which then facilitated the
O2 activation and accelerated the decomposition of formate
intermediates species, thus a superior HCHO catalytic oxidation
performance was obtained. This study first clearly illustrated the
promotion mechanism of alkali Na without the implication of active
support and provides an efficient and facile way to achieve high
dispersion of Pd particles on inert support and tremendously expands
the selected range of supports for room temperature HCHO catalytic
oxidation.
because it could oxidize HCHO into harmless CO2 and H2O at
room temperature without additional heating equipment, light
source or plasma equipment, which is an environmentally friendly
and energy-saving technology.[6]
At present, catalysts for HCHO catalytic oxidation at room
temperature are mainly Pt based catalysts (such as Pt/TiO2,
Pt/AlOOH, and Pt/MnO2) deriving from their excellent ability for
activation of molecular oxygen.[7-14] Nevertheless, as another
important Pt group metal (PGM), Pd exhibited relatively inferior
HCHO oxidation activity at room temperature and thus increasing
efforts have been made to improve the HCHO oxidation
performance of Pd based catalysts in recently years.[15,16] For
example, Huang et al. reported Pd/TiO2 catalysts could achieve
HCHO complete elimination at room temperature.[17] Zhou et al.
synthesized a series of CeO2 with different exposed facets and
CeO2 nanocubes with exposed (100) facets supported Pd
catalysts showed excellent HCHO oxidation activity, which could
completely convert HCHO into CO2 at ambient temperature.[18]
Though some progress has been made, however, Pd based
catalysts for HCHO catalytic oxidation at room temperature are
mainly reducible oxides (i.e. active supports) supported Pd
catalysts and room temperature HCHO catalytic oxidation over
irreducible oxides (inert supports) supported Pd catalysts has
been rarely reported.[19]
On the other hand, alkali metal ions could improve the catalytic
performance in many heterogeneous catalysis reactions, such as
water gas shift reaction,[20-22] reverse water gas shift reaction,[23,24]
CO oxidation,[25] and propene oxidation.[26] Recently, Flytzani-
Stephanopoulos et al.[20-22] found that alkali metal ions can
stabilize an atomically dispersed Pt and Au species and
dramatically improve the catalytic activity for low-temperature
water-gas shift reaction. Zhang et al.[27] reported that alkali metal
ions (such as Li+, Na+, and K+) could stabilize an atomically
dispersed Pt species in the form of Pt-O(OH)x-Na, resulting in its
much better HCHO oxidation activity compared with Pt/TiO2.
Analogously, the promotion role of alkali metal ions was also
observed on the Pd/TiO2, Ag/Co3O4, and Pt/MnO2 catalysts for
HCHO catalytic oxidation.[28-34] Nevertheless, researches mainly
focused on the reducible oxides (such as TiO2, Co3O4, and MnO2).
It is inevitably that there will be a strong interaction between alkali
ions and reducible oxides.[35] In contrast, irreducible oxides (inert
supports) are merely as carriers to disperse active sites and the
implication of active support could be excluded.[23] Therefore, we
suppose that inert support (such as zeolites, SiO2) supported
noble metal catalysts could be more suitable as model catalysts
to elucidate the essential promotion role of alkali ions in HCHO
oxidation reaction. As far as we know, the promoting mechanism
Introduction
Formaldehyde (HCHO) mainly emitted from building/furnishing
materials and numerous household products is a dominant indoor
air pollutant, especially in airtight indoor spaces.[1] It is reported
that acute exposure to HCHO irritates the mucous membranes of
the eyes and respiratory tract; long-term exposure to HCHO even
at very low concentrations increases the risk of nasopharyngeal
cancer and leukemia.[2] Because of the huge harm of HCHO to
human health, in the past several decades, significant efforts have
been devoted for indoor HCHO removal. Currently, adsorption,
photocatalysis, plasma technology and catalytic oxidation are the
common technologies to remove HCHO from indoor air.[3-5]
Among these technologies, room temperature HCHO catalytic
oxidation is recognized as one of the most promising approaches
[a]
Dr. N. Xiang, Dr. Y. Hou, Prof. X. Han, Dr. Y. Li, Dr. Y. Guo, Dr. Y.
Liu, Prof. Z. Huang
State Key Laboratory of Coal Conversion
Institute of Coal Chemistry
Chinese Academy of Sciences
Taiyuan 030001 (P.R.China)
E-mail: zghuang@sxicc.ac.cn.
[b]
Dr. N. Xiang, Dr. Y. Hou, Dr. Y. Li, Dr. Y. Guo, Dr. Y. Liu
University of Chinese Academy of Sciences
Beijing 100049 (P.R.China)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201901039
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of alkali metal ions for inert support loaded Pd catalysts has not
been reported.
1Pd/SBA-15 exhibited the best catalytic activity, it was used for
HCHO oxidation at room temperature in the following experiments.
In addition, compared with active support, inert support
commonly possessed a higher specific surface area, being
favorable for the dispersion of active sites and the adsorption of
HCHO.[36] Furthermore, higher thermal and chemical stability of
inert support are also beneficial for preparation and long-term use
of catalyst. SBA-15 as a common inert support has been widely
applied as adsorbent and catalyst support due to its high surface
area and pore volume, controllable pore diameter, superb thermal
and chemical stability.[37] For instance, Nomura et al.[38]
synthesized a novel amine-functionalized SBA-15 that exhibited
outstanding HCHO adsorption capacity owing to its high surface
area and the incorporation of primary amine groups. An et al.[39]
prepared a Pt/SBA-15 catalyst which could reach 100% HCHO
conversion at 50 °C. Moreover, the mesoporous structure of SBA-
15 can also facilitate HCHO transport and reaction, which is of
vital significance for ppm-level HCHO abatement.[36] Therefore,
SBA-15 could be a promising support to prepare high efficient
HCHO oxidation catalyst and it is especially important that it can
exclude its interaction with alkali Na. Thereby, essential
promotion mechanism of alkali Na could be clearly illustrated.
Herein, we prepared a series of alkali Na (1-4 wt.%) modified
1Pd/SBA-15 catalysts and found that Na addition dramatically
improved HCHO oxidation performance of 1Pd/SBA-15 at room
temperature. Different characterization was performed to reveal
the essential promotion mechanism of alkali Na on Pd. Based on
the characterization results, structure-activity relationships were
investigated and the possible HCHO oxidation mechanism was
proposed.
Figure 1. HCHO oxidation activity over xNa-yPd/SBA-15 catalysts. Reaction
conditions: 170 ppm HCHO, 60% RH, 20% O2, N2 balance, temperature = 20 °C,
WHSV = 160000 mL gcat-1 h-1 (A), and 480000 mL gcat-1 h-1 (B).
Moreover, the HCHO oxidation activity of 2Na-1Pd/SBA-15 was
compared with several reported supported noble metal catalysts
for HCHO catalytic oxidation at room temperature and the results
are summarized in Table S1. Clearly, it can be seen that 100%
HCHO oxidation can be obtained over 2Na-1Pd/SBA-15 in a
higher inlet HCHO concentration and WHSV compared with the
listed catalysts, further demonstrating the excellent catalytic
activity of as-prepared 2Na-1Pd/SBA-15.
Water vapor is inevitable in the indoor air and it is also produced
during the HCHO catalytic oxidation.[30] It is vital to study the effect
of RH on HCHO catalytic oxidation activity. Figure 2 shows the
effect of RH (0%, 30%, 60%, and 90%) on HCHO conversion of
2Na-1Pd/SBA-15. As a result, in the absence of H2O, the HCHO
conversion of 2Na-1Pd/SBA-15 was only 46%. With increasing
the RH, it was improved to 89% at 30% RH and reached to 100%
at 60% RH, possibly deriving from enhanced O2 activation ability
in the presence of moderate H2O.[40] However, the HCHO
conversion sharply decreased to 79% when the RH further
increased to 90%, which could be due to the competitive
adsorption between water vapor and HCHO on the active sites.[15]
Aside from catalytic activity, the stability of a novel catalyst for
total oxidation of HCHO is also very important from a practical and
commercial point of view. The results of stability tests of HCHO
catalytic oxidation over 1Pd/SBA-15 and 2Na-1Pd/SBA-15 are
shown in Figure 3. It is obviously seen that the HCHO conversion
Results and Discussion
The activity of xNa-yPd/SBA-15 for HCHO oxidation at room
temperature is depicted in Figure 1. For the bare SBA-15 and
2Na/SBA-15, almost no HCHO conversion was observed at room
temperature. While the HCHO conversion notably increased to
73% over 1Pd/SBA-15, indicating that Pd species were the active
sites for HCHO catalytic oxidation at room temperature. Moreover,
it was noted that all of the Na promoted 1Pd/SBA-15 catalysts (Na
loading=1%, 2%, and 4%) achieved 100% HCHO conversion
under same reaction conditions, which demonstrated that alkali
Na had a remarkable promotion effect on 1Pd/SBA-15 catalyst for
room temperature HCHO oxidation. Meanwhile, it was found that
the HCHO oxidation activity of xNa-1Pd/SBA-15 (x = 0, 1, 2, and
4) was dramatically influenced by Na loading when WHSV further
-1
increased to 480000 mL gcat h-1 and the result is shown in the
Figure 1B. There existed a volcanic shaped relationship between
catalytic activity and Na loading of xNa-1Pd/SBA-15 for room
temperature HCHO oxidation. For the 1Pd/SBA-15 catalyst, the
HCHO conversion was only 17% when WHSV increased to
-1
480000 mL gcat h-1. With the introduction of alkali Na, the
conversion was improved to 78% at 1% Na loading and reached
to the maximum of 97% when Na loading was 2%. Whereas,
further increasing the Na loading to 4% would dramatically
decrease the HCHO conversion to 74%. Considering that 2Na-
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Figure 2. HCHO oxidation activity of 2Na-1Pd/SBA-15 as a function of relative
humidity. Reaction conditions: 170 ppm HCHO, 0-90% RH, 20% O2, N2 balance,
temperature = 20 °C, WHSV = 160000 mL gcat-1 h-1.
of 1Pd/SBA-15 rapidly decreased over time and almost
completely deactivated after 12 h. Comparatively, 2Na-1Pd/SBA-
15 catalyst achieved nearly 100% HCHO conversion and no
appreciable deactivation could be observed over a long test of 40
h. The results indicate that Na addition not only increases the
HCHO oxidation activity of 1Pd/SBA-15, but also dramatically
improves its stability for HCHO catalytic oxidation at room
temperature. In summary, the as-prepared 2Na-1Pd/SBA-15 in
this work is a promising catalyst for room temperature HCHO
oxidation due to its superior activity, excellent stability, and no
secondary pollution.
Figure 4. (A) Small-angle XRD patterns, and (B) wide-angle XRD patterns of
samples.
blockage of the smaller micropores and secondary mesopores by
Na and Pd species. Interestingly, it was noted that 1Pd/SBA-15
with a higher specific surface area but showed a lower HCHO
oxidation activity compared with the Na promoted 1Pd/SBA-15
(Figure 1), revealing that the specific surface area could be not a
key factor to determine HCHO catalytic activity but some other
physicochemical properties.
The small-angle XRD patterns of the samples are shown in
Figure 4A. All of the samples showed three well-defined diffraction
peaks 100 (2θ = 1°), 110 (2θ = 1.6°), and 200 (2θ = 1.9°) planes
which were characteristic peaks of well-ordered 2D hexagonal
mesostructured material with p6mm symmetry.[41] With increasing
the Na and Pd contents, the diffraction peaks intensity of (100),
(110), and (200) planes decreased gradually, indicating that Na
and Pd addition decreased the pore regularity of SBA-15.
Remarkably, the diffraction peak of (100) plane shifted toward
larger 2θ degree after the loading of Pd. It can be attributed to the
shrinkage of SBA-15 framework accompanied by a decrease in
the cell parameter a0 due to the formation of reduced Pd
species.[42] Figure 4B shows the wide-angle XRD patterns of
samples. A broad diffraction peak at 2θ ≈ 22° was observed in all
samples, which was assigned to amorphous silica of SBA-15. The
weak diffraction peaks appearing at 2θ ≈ 40° and 46° were
ascribed to metallic Pd (111) and (200) faces, respectively.[41] It
could be clearly seen that the full width of half maximum (FWHM)
of Pd (111) face significantly increased after alkali Na addition,
indicating the decrease of Pd particle size. As a consequence,
alkali Na promoted 1Pd/SBA-15 catalysts exhibited much better
Figure 3. Stability tests of 1Pd/SBA-15 and 2Na-1Pd/SBA-15. Reaction
conditions: 170 ppm HCHO, 60% RH, 20% O2, N2 balance, temperature = 20 °C,
WHSV = 160000 mL gcat-1 h-1.
N2 adsorption-desorption isotherms and the corresponding
pore size distributions of samples are displayed in Figure S1. All
samples exhibited type IV isotherms with a hysteresis loop of H1
type in the P/P0 range of 0.6-0.8, suggesting the existence of
mesoporous structure.[41] As displayed in Table 1, BET specific
surface area, average pore diameter and pore volume of
xNa−yPd/SBA-15 decreased gradually as the increase of Na and
Pd contents. It can be attributed to the coverage of Na and Pd
species. Abnormally, an increasing of average pore diameter was
observed on 4Na−1Pd/SBA-15 (7.75 nm) compared with other
xNa−yPd/SBA-15 (6.55-6.59 nm). It might be due to the further
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Table 1. Textural properties, average Pd particle size, dispersion, and TOF of samples.
BET
Pore volume
(mL/g)
0.96
Pore diameter
(nm)
Pd particle
Dispersion[a]
TOF
(×10-2 s-1)
-
Sample
(m2/g)
696.95
382.87
555.34
371.33
267.21
155.05
size (nm)
(%)
-
SBA-15
7.78
-
2Na/SBA-15
0.70
6.55
-
-
-
1Pd/SBA-15
0.79
6.56
8.43
6.48
4.45
7.01
13.3
17.3
25.2
16.0
0.72
4.21
5.12
4.08
1Na-1Pd/SBA-15
2Na-1Pd/SBA-15
4Na-1Pd/SBA-15
0.65
6.59
0.52
6.55
0.45
7.75
[a] Dispersion of Pd determined by HAADF-STEM images, Dispersion (%) = 1.12/dPd (nm) × 100%.
Figure 5. HAADF-STEM images and the corresponding Pd particle size distribution histograms of samples: (A) 1Pd/SBA-15; (B) 1Na-1Pd/SBA-15; (C) 2Na-
1Pd/SBA-15; (D) 4Na-1Pd/SBA-15. (E) EDS elemental mapping of 2Na-1Pd/SBA-15.
HCHO oxidation activity. Furthermore, no diffraction peaks of Na
species were detected in 2Na/SBA-15 and all Na promoted
1Pd/SBA-15, suggesting that Na species were highly dispersed.
As displayed in Figure S2, the as-prepared SBA-15 support
showed well-ordered 2D hexagonal mesostructured, in accord
with the small-angle XRD results. In order to get insight into the
effect of Na on the Pd dispersion, HAADF-STEM characterization
was performed to measure the Pd particle size of the xNa-
1Pd/SBA-15. Representative HAADF-STEM images and
corresponding Pd particle size distribution histograms of xNa-
1Pd/SBA-15 are shown in Figure 5. For the 1Pd/SBA-15 catalyst,
the average Pd particle size was ca. 8.43 nm. As the increasing
of Na loading, the mean Pd particle size of xNa-1Pd/SBA-15
gradually decreased to 6.48, and 4.45 nm, and then increased to
7.01 nm upon the addition of 4% Na deriving from the coverage
of Pd by the excess Na species.[30] The above results suggested
that moderate Na addition was favorable to stabilize well
dispersed Pd particles. More interestingly, as displayed in Table1,
the average pore diameter of SBA-15 (7.78 nm) is smaller than
average Pd particle size of 1Pd/SBA-15 (8.43 nm) but larger than
that of Na promoted 1Pd/SBA-15 catalysts (4.45-7.01 nm). It
means that Pd particles of 1Pd/SBA-15 might mainly locate
outside of SBA-15 pores and lack of well anchoring, so these Pd
particles tend to aggregate during HCHO oxidation. In contrast,
most of Pd particles of Na promoted 1Pd/SBA-15 catalysts were
confined in the pore channels of SBA-15, which were in
accordance with HAADF-STEM images. Benefiting from well Pd
dispersion of 2Na-1Pd/SBA-15, thereby it exhibited remarkable
HCHO oxidation activity at room temperature. Additionally, as can
see from the EDS elemental mapping of 2Na-1Pd/SBA-15 (Figure
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5E), Pd and Na species were uniformly distributed on the SBA-15,
demonstrating the successful deposition of Pd and Na.
Based on the Pd dispersion determined by HAADF-STEM, the
turnover frequencies (TOF) of the samples were calculated at
20 °C and the results are summarized in Table 1. The TOF of
1Pd/SBA-15 was only 0.72×10-2 s-1. With the introduction of Na,
the TOF dramatically increased to 4.21×10-2, and 5.12×10-2 s-1,
and then slightly decreased to 4.08×10-2 s-1 for the 1Na, 2Na, and
4Na modified 1Pd/SBA-15 catalysts, respectively. It further
confirms the promotion effect of alkali Na on 1Pd/SBA-15 in
HCHO catalytic oxidation. However, the promotion effect would
decrease in the presence of excessive Na due to the coverage of
active sites.
Figure 7. XPS spectra of 1Pd/SBA-15 and Na promoted 1Pd/SBA-15 catalysts:
(A) Pd 3d, (B) Na 1s.
TEM images of spent 1Pd/SBA-15 (12 h) and 2Na-1Pd/SBA-15
(40 h) are displayed in Figure S3. Obviously, heavy aggregation
of Pd particles was observed over 1Pd/SBA-15 after a long test of
12 h which could be due to that Pd particles located outside of
SBA-15 pores lacked of well stabilization, thus tending to
aggregate with increasing reaction time. And it could in turn lead
to the fast deactivation of 1Pd/SBA-15 owing to the reduction of
active sites. Moreover, similar phenomenon was also observed in
Au/SiO2 and Au/HZSM-5 during room temperature HCHO
oxidation process.[46] While the Pd particles of 2Na-1Pd/SBA-15
were still well dispersed and mainly confined in the pore channels
of SBA-15 and no appreciable aggregation could be detected over
a 40 h long test. Thus, an enhanced HCHO catalytic oxidation
activity and stability are obtained over 2Na-1Pd/SBA-15.
XPS was performed to illustrate the effect of Na addition on the
chemical state of elements and the results are shown in Figure 7.
Figure 7A shows the XPS spectrum of Pd 3d. The binding
energies of Pd 3d5/2 at 335.3-335.5 eV and 336.5-336.6 eV were
assigned to metallic Pd (Pd0) and ionic Pd (Pd2+), respectively,
indicating the coexistence of metallic and ionic Pd in xNa-
1Pd/SBA-15. Figure 7B displays the XPS spectrum of Na 1s. The
binding energy of Na 1s1/2 at 1071.6-1072.0 eV was attributed to
Na+ and it could be observed that the peak intensity of Na
increased with the increase of Na loading. In addition, the molar
ratio of Pd2+/(Pd0+Pd2+) and binding energy of Na+ were
summarized in Table 2. It is worth noting that the proportion of
Pd2+ increased from 36.96% to 46.45% as the increase of Na
loading from 1% to 4%. At the same time, the binding energy of
Na+ gradually shifted to lower binding energy with the increase of
Figure 6. (A): The relationship between HCHO oxidation activity and Pd particle
size; (B): The relationship between TOF and Pd particle size.
Numerous studies demonstrate that the HCHO catalytic activity
of supported noble metal catalysts depends strongly on the noble
metal particles size.[17,43] Generally speaking, the smaller noble
metal particle size means the more active sites and lower
coordination number and consequently higher catalytic
performance will be obtained under the same noble metal
loading.[43-45] To reveal the effect of Pd particle size on catalytic
performance, the relationship between HCHO oxidation activity
(and TOF) and average Pd particle size is shown in Figure 6. Of
particular note is there existed a linear correlation between Pd
particle size and the HCHO oxidation activity (and TOF), i.e. the
xNa-1Pd/SBA-15 with a smaller Pd particle size showed a higher
HCHO oxidation activity (and TOF). Therefore, Pd particle size
plays an important role in determining the HCHO catalytic activity.
In addition, TEM characterization was also performed to study the
fast deactivation reason of 1Pd/SBA-15 (Figure 3) and typical
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Na loading. It can be attributed to the electron transfer that the
partial 3d electrons of Pd transferred into the 1s orbit of Na,[21,22,47]
demonstrating a strong metal-promoter interaction between Pd
and Na species which is favorable to stabilize highly dispersed Pd
particles, as confirmed by HAADF-STEM (Figure 5).
100-300 °C, which was ascribed to the release of chemisorbed
oxygen.[34] For the 1Pd/SBA-15, the desorption peak of
chemisorbed oxygen was located at 230 °C.[15] Comparatively,
with the introduction of Na, the desorption peak of chemisorbed
oxygen shifted to lower temperature (168, 141, and 206 °C for the
1Na, 2Na, and 4Na promoted 1Pd/SBA-15 catalysts,
respectively), suggesting a remarkable improvement in O2
activation and mobility of chemisorbed oxygen. Moreover, the
addition of Na also increased the uptake of chemisorbed oxygen
of 1Pd/SBA-15 catalyst. According to literatures, O2 activation
was considered as a key step for HCHO oxidation at room
temperature and O2 activation commonly occurs on the surface of
noble metal particles for supported noble metal catalysts.[8,48,49] It
is widely accepted that highly dispersed noble metal catalysts
exhibited much superior O2 activation ability, not just because of
their abundant active sites, but also because highly dispersed
noble metal particles with low-coordination number possessed
much superb O2 activation ability.[50,51] Therefore, supported noble
metal catalysts with higher noble metal dispersion could exhibit
better O2 activation ability and similar phenomenon was also
observed in literatures.[28,29] And in fact, all Na promoted
1Pd/SBA-15 catalysts with higher Pd dispersion exhibited better
O2 activation ability and possessed much more abundant
chemisorbed oxygen species compared to 1Pd/SBA-15,
consequently leading to superb HCHO oxidation activity.
Table 2. XPS data of 1Pd/SBA-15 and Na promoted 1Pd/SBA-15 catalysts.
Sample
Pd2+/(Pd0+Pd2+
36.96%
)
Na BE (eV)
-
1Pd/SBA-15
1Na-1Pd/SBA-15
2Na-1Pd/SBA-15
4Na-1Pd/SBA-15
41.46%
1072.0
1071.9
1071.6
43.48%
46.45%
To elucidate the effect of Na addition on the reducibility of
catalysts, H2-TPR measurements over xNa-yPd/SBA-15 were
carried out and the results are shown in Figure 8. For the
2Na/SBA-15, a broad reduction peak in the temperature range of
350-550 °C was observed, which could be assigned to the
reduction of Na species. For the 1Pd/SBA-15, the sharp peak at -
3 °C was ascribed to the reduction of PdO and the negative peak
at 56 °C could be due to the decomposition of β-PdH.[15,28] With
the increase of Na loading, the decomposition temperature of β-
PdH remained constant and the reduction peak of PdO gradually
shifted to higher temperature (from -3 °C, to 25, 31 and 36 °C).
On the other hand, the reduction peak of Na species over the
4Na-1Pd/SBA-15 shifted to lower temperature (385 °C) compared
with 4Na/SBA-15 (507 °C, Figure S4). The results further confirm
the interaction between Na and Pd in the Na promoted 1Pd/SBA-
15 catalysts, which hinders the reduction of Pd species and
facilitates the reduction of Na species. Based on the above
characterization results, the addition of Na could stabilize well
dispersed Pd particles through
a strong metal-promoter
interaction, thus enhanced room temperature HCHO oxidation
performance was obtained.
Figure 9. O2-TPD profiles of 1Pd/SBA-15 and Na promoted 1Pd/SBA-15
catalysts.
In order to detect the reaction intermediates and determine the
reaction mechanism of HCHO catalytic oxidation, in situ DRIFTS
experiments were conducted over 1Pd/SBA-15 and 2Na-
1Pd/SBA-15. Figure 10A and B show the spectra over 1Pd/SBA-
15 and 2Na-1Pd/SBA-15 exposed to the flow of HCHO + H2O +
O2 + N2 gas mixture for 60 min at room temperature, respectively.
The sharp peaks at 3740 and 1645 cm-1 and the broad band
ranged from 3600 cm-1 to 3000 cm-1 belonged to adsorbed
water.[45,52] The peaks at 2960, 2920, and 2850 cm-1 were
assigned to υas(CH) and υs(CH) of formate species. The peaks
located at 1556 and 1356 cm-1 were attributed to υas(COO) and
υs(COO) of formate species, respectively.[7,53] The peaks at 1270
and 1120 cm-1 were attributed to υs(CH2) and υ(CO) of
dioxymethylene (DOM) species, which were formed owing to
nucleophilic attack of surface oxygen.[52,54] After exposing to
Figure 8. H2-TPR profiles of 2Na/SBA-15, 1Pd/SBA-15, and Na promoted
1Pd/SBA-15 catalysts.
O2-TPD profiles of xNa-1Pd/SBA-15 are displayed in Figure 9.
All of the samples exhibited a broad desorption peak located at
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Figure 10. In situ DRIFTS spectra over (A1 and A2) 1Pd/SBA-15, and (B1 and B2) 2Na-1Pd/SBA-15 catalysts after the flow of HCHO + H2O + O2 + N2 gas mixture
for 60 min at room temperature.
reactant gas containing HCHO, no peaks of adsorbed HCHO
could be detected on the 1Pd/SBA-15 and 2Na-1Pd/SBA-15,
suggesting that HCHO could be easily oxidized into DOM and
formate species by the surface oxygen on both of the catalysts.
Therefore, DOM and formate species were the main reaction
intermediates of HCHO catalytic oxidation over the 1Pd/SBA-15
and 2Na-1Pd/SBA-15. It should be noted that the peak located at
1070 cm-1 over the 1Pd/SBA-15 was ascribed to the υ(CO) stretch
of polyoxymethylene (POM)[55] and its peak intensity gradually
increased over time, suggesting the accumulation of
polyoxymethylene (POM) on the 1Pd/SBA-15 during the HCHO
literatures, the decomposition of formate species is regarded as
the rate-determining step for HCHO oxidation which strongly
depends on surface oxygen species.[7,48] Therefore, 2Na-
1Pd/SBA-15 with better O2 activation ability and richer surface
oxygen species exhibited a faster formate species decomposition
rate, and thus an enhanced HCHO catalytic oxidation
performance was obtained.
In the end, a possible mechanism of HCHO oxidation over
1Pd/SBA-15 and 2Na-1Pd/SBA-15 is proposed and shown in
Figure 11. Firstly, gaseous O2 is adsorbed and dissociated into
surface oxygen over Pd nano-particles (step (1)). Benefiting from
oxidation. Therefore, in addition to heavy Pd particles aggregation, its higher Pd dispersion, 2Na-1Pd/SBA-15 exhibited much better
coverage of active sites could be also another reason leading to
fast deactivation of 1Pd/SBA-15 (Figure 3). And the trends of
outlet HCHO and CO2 concentrations in the chamber (insets of
Fig 10A2 and B2) also verified the speculation. The HCHO
catalytic activity of 1Pd/SBA-15 increased in initial stage and then
gradually declined with increasing reaction time, whereas 2Na-
1Pd/SBA-15 exhibited higher HCHO conversion throughout the
reaction process, which further confirmed that alkali Na could
improve the HCHO oxidation activity and stability of 1Pd/SBA-15.
On the other hand, it is worth noting that the peak intensity of
formate species (2960, 2920, 2850, and 1556 cm-1) on the 2Na-
1Pd/SBA-15 was much weaker than that of 1Pd/SBA-15 whereas
the outlet CO2 concentration of 2Na-1Pd/SBA-15 (inset of B2)
was higher than that of 1Pd/SBA-15 (inset of A2) at the same time,
which unambiguously verified that 2Na-1Pd/SBA-15 possessed a
faster formate species decomposition rate. Based on earlier
O2 activation ability and possessed richer surface oxygen species
compared with 1Pd/SBA-15. Afterwards, HCHO is oxidized into
DOM and formate species by nucleophilic attack of surface
oxygen (step (2) and step (3)). On the 1Pd/SBA-15, formate
species is further oxidized into CO2 and H2O by surface oxygen
(step (4)). Comparatively, for the 2Na-1Pd/SBA-15, a faster
decomposition rate of formate species was obtained due to its
much abundant surface oxygen species, consequently rendering
an enhanced room temperature HCHO oxidation performance.
O2 → 2[O]s
HCHO + [O]s → [H2CO2]s
[H2CO2]s → [HCOO]s + [H]s
(1)
(2)
(3)
[HCOO]s + [O]s + [H]s → CO2 + H2O (4)
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The Brunauer-Emmett-Teller (BET) surface area of the
samples was determined by N2 adsorption-desorption isotherms
at -196 °C using an Autosorb-iQ-2 instrument (Quantachrome
Instrument Crop.).
Wide-angle X-ray diffraction (XRD) and small-angle XRD
patterns were performed on a Bruker D8-Advanced diffractometer
system with Cu Kα radiation (λ = 1.5418 Å) operated at 40 kV and
40 mA.
Transmission electron microscopy (TEM) and High-angle
annular dark-field scanning transmission electron microscopy
(HAADF-STEM) were performed on a JEM-2100F and Tecnai G2
F20 S-Twin, respectively. The the accelerating voltage of
instrument was 200 kV. The distribution of elements of 2Na-
1Pd/SBA-15 was assessed by energy dispersive X-ray
spectroscopy (EDS) performed in the scanning transmission
electron microscopy (STEM) mode.
Figure 11. Possible mechanism of HCHO oxidation over 1Pd/SBA-15 and 2Na-
1Pd/SBA-15.
XPS measurements were performed on an AXIS ULTRA DLD
photoelectron spectroscopy with an Al Kα X-ray source and the
binding energy (B.E.) was calibrated by the C 1s peak (284.8 eV).
H2-TPR was tested on an AutoChem II 2920 instrument
(Micromeritics Instrument Crop.) equipped with a TCD detector.
Typically, 60 mg of samples were pretreated in Ar at 300 °C for
60 min followed by cooling down to -50 °C. Then, samples were
reduced in a flow of H2/Ar mixture (5 vol.% H2) from -50 °C to
600 °C with a heating rate of 10 °C/min.
Conclusions
In summary, this work first clearly illustrated the promotion
mechanism of alkali Na on 1Pd/SBA-15 for HCHO catalytic
oxidation. The characterization results demonstrated that alkali
Na could stabilize well dispersed Pd particles which then
facilitated O2 activation and accelerated formate species
decomposition rate. Eventually, all of them resulted in the best
HCHO catalytic oxidation performance of 2Na-1Pd/SBA-15,
which could completely transform 170 ppm HCHO into CO2 at
O2-TPD was tested on
a
ChemStar TPx instrument
(Quantachrome Instrument Crop.) equipped with a TCD detector.
120 mg of samples were firstly pretreated in H2/Ar mixture (10
vol.% H2) at 300 °C for 30 min followed by cooling down to 35 °C.
Then, samples were treated in a flow of O2/He mixture (3 vol.%
O2) at 35 °C for 90 min and the weakly adsorbed O2 was removed
by purging with pure He at 35 °C for 60 min. Finally, TPD was
conducted by heating samples in a flow of pure He from 35 °C to
350 °C with a heating rate of 10 °C/min.
ambient temperature at a WHSV of 160000 mL gcat h-1. This
-1
study provides an efficient and facile way to achieve high
dispersion of Pd particles on inert support and significantly
expands the selected range of supports for room temperature
HCHO catalytic oxidation.
In situ Diffuse Reflectance Infrared Fourier Transform
Spectroscopy (DRIFTS) was recorded at ambient temperature on
a Bruker Tensor 27 spectrometer. The scan range was from 4000-
1000 cm−1 with 100 scans at a resolution of 4 cm−1. Before
measurements, the samples were pretreated in a pure N2 flow at
150 °C for 30 min to remove adsorbed water and impurities
followed by cooling down to room temperature to collect the
background spectrum. Then, the samples were treated with the
reactant gas (170 ppm HCHO, 60% relative humidity, 20% O2 and
balanced by N2) for 60 min and each DRIFT spectrum was
recorded by subtracting the background spectrum. Meanwhile,
the in situ chamber was combined with a portable FTIR gas
analyzer (Gasmet Instruments DX-4000) to analyze the outlet
HCHO and CO2 concentrations, simultaneously.
Experimental Section
Preparation of SBA-15 support
The SBA-15 mesoporous silica was synthesized according to
Zhao’s method[56] and the specific steps were listed in supporting
information.
Preparation of catalysts
The xNa-yPd/SBA-15 catalysts (x and y represent the weight
percentage of Na and Pd to the support, respectively. x = 0, 1, 2,
4; y = 0, 1) were prepared by co-impregnation of SBA-15 with
aqueous Pd(NO3)2 (Aladdin) and NaNO3 (Tianjin Tianli Chemical
Reagent Ltd). The resulting suspension was stirred at 50 °C until
the water was completely evaporated. The obtained samples
were dried at 100 °C for 8 h and then calcined at 400 °C for 4 h.
Finally, the catalyst samples (40-60 mesh) were reduced in 10
vol.% H2/N2 atmosphere at 300 °C for 30 min before test.
Catalytic tests
HCHO catalytic oxidation activity and stability measurements
were carried out in a continuous-flow fixed-bed reactor. Typically,
75 mg (or 25 mg) catalyst was packed in a U-shaped quartz
reactor (i.d. = 4 mm), which was placed in a thermostatic water
bath and the reaction temperature was set as 20 °C. The home-
made HCHO generator system in this work was similar to the
apparatus of He Hong’s group.[7] The HCHO feed gas was
Catalyst characterization
The actual Na and Pd contents of samples were measured by
Inductively Coupled Plasma Optical Emission Spectrometer (ICP-
OES, Thermo iCAP 6300) and the results are shown in Table S2.
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Ning Xiang, Yaqin Hou, Xiaojin Han,
Yulin Li, Yaoping Guo, Yongjin Liu, and
Zhanggen Huang*
Page No. – Page No.
Promoting effect and mechanism of
alkali Na on Pd/SBA-15 for room
temperature formaldehyde catalytic
oxidation
A serious of alkali Na promoted 1Pd/SBA15 catalysts were prepared by a co-impregnation method and tested for HCHO catalytic
oxidation at room temperature in this work. As a result, the addition of alkali Na could stabilize well dispersed Pd particles which then
facilitated O2 activation and accelerated formate species decomposition rate. All of them contributed to the superior HCHO catalytic
oxidation performance of 2Na-1Pd/SBA-15.
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