Efficient decomposition of formaldehyde at room temperature over Pt/honeycomb ceramics with ultra-low Pt content

Article abstract of DOI:10.1039/c4dt01323a

Pt/honeycomb ceramic (Pt/HC) catalysts with ultra-low Pt content (0.005-0.055 wt%) were for the first time prepared by an impregnation of honeycomb ceramics with Pt precursor and NaBH₄-reduction combined method. The microstructures, morphologies and textural properties of the resulting samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The obtained Pt/HC catalysts were used for catalytic oxidative decomposition of formaldehyde (HCHO) at room temperature. It was found that the as-prepared Pt/HC catalysts can efficiently decompose HCHO in air into CO ₂ and H₂O at room temperature. The catalytic activity of the Pt/HC catalysts increases with increasing the Pt loading in the range of 0.005-0.013 wt%, and the further increase of the Pt loading does not obviously improve catalytic activity. From the viewpoint of cost and catalytic performance, 0.013 wt% Pt loading is the optimal Pt loading amount, and the Pt/HC catalyst with 0.013 wt% Pt loading also exhibited good catalytic stability. Considering practical applications, this work will provide new insights into the low-cost and large-scale fabrication of advanced catalytic materials for indoor air purification. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01323a

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Efficient decomposition of formaldehyde at room
temperature over Pt/honeycomb ceramics with
ultra-low Pt content
Cite this: DOI: 10.1039/c4dt01323a
a,b
a
a,c
Longhui Nie, Yingqiu Zheng and Jiaguo Yu*
Pt/honeycomb ceramic (Pt/HC) catalysts with ultra-low Pt content (0.005–0.055 wt%) were for the first
time prepared by an impregnation of honeycomb ceramics with Pt precursor and NaBH -reduction
4
combined method. The microstructures, morphologies and textural properties of the resulting samples
were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and
transmission electron microscopy (TEM). The obtained Pt/HC catalysts were used for catalytic oxidative
decomposition of formaldehyde (HCHO) at room temperature. It was found that the as-prepared Pt/HC
2 2
catalysts can efficiently decompose HCHO in air into CO and H O at room temperature. The catalytic
activity of the Pt/HC catalysts increases with increasing the Pt loading in the range of 0.005–0.013 wt%,
and the further increase of the Pt loading does not obviously improve catalytic activity. From the view-
point of cost and catalytic performance, 0.013 wt% Pt loading is the optimal Pt loading amount, and the
Pt/HC catalyst with 0.013 wt% Pt loading also exhibited good catalytic stability. Considering practical
applications, this work will provide new insights into the low-cost and large-scale fabrication of advanced
catalytic materials for indoor air purification.
Received 4th May 2014,
Accepted 2nd July 2014
DOI: 10.1039/c4dt01323a
www.rsc.org/dalton
However, room-temperature catalytic oxidative decomposition
of HCHO into CO and H O can overcome the above short-
2 2
Introduction
The quality of indoor air is very important for human comings and is considered the most promising strategy for
health because people often spend more than 80% of their removal of indoor HCHO, because it is environmentally
1
1,13–15
time in indoor space. Formaldehyde is a major indoor pollu- friendly and energy saving.
In the case of room-tempera-
tant and long-time exposure to HCHO-containing air may ture catalytic oxidation, various supported noble metal cata-
1
1,13–15,17–22
cause serious health problems including nasal tumors, eye lysts have been developed by some research groups.
1
,2
11
and skin irritation, nasopharyngeal cancer, etc. In order to For example, Zhang et al. firstly found that HCHO could be
clean the indoor air, many strategies have been developed to completely oxidized into CO and H O over various supported
2
2
3
–7
remove HCHO, for instance, adsorption,
photocatalytic oxi- noble metal (Pt, Rh, Pd and Au) catalysts with 1 wt% loading
8
,9
10
20
dation,
oxidation.
short lifetime due to its limited adsorption capacity. And Au/Co
photocatalytic oxidation, plasma technology and high-temp- could be completely oxidized into CO
erature thermal catalytic oxidation all need extra equipment with ≥0.1 wt% loading at room temperature. In our recent
such as a light source for photocatalytic oxidation, plasma study, our work indicated that HCHO was also completely oxi-
equipment for plasma technology, heating equipment for dized into CO and H O over Pt-supported catalysts with
high-temperature thermal catalytic oxidation), which brings ≥0.1 wt% loading.
plasma technology,
and thermal catalytic at room temperature. Then, Ma et al. reported that HCHO
1
1–16
However, adsorption has a disadvantage of a could be partly oxidized into CO and H O on about 1 wt%
2
2
1
4
3
O
4
–CeO
2
at 25 °C. Huang et al. also found that HCHO
and H O on Pd/TiO
2
2
2
(
2
2
2
1,22
Among the above noble metal-sup-
more complicated processes and additional operating cost. ported catalysts, the Pt-supported catalyst exhibited a better
catalytic performance for decomposition of HCHO than other
noble metal catalysts at room temperature. However, most of
the Pt-supported catalysts were prepared in powder form,
which brings much trouble in actual applications because they
need to be pressed into tablets or attached to other big-block
supports. At the same time, it also opens the possibility of
high air resistance, and catalysts’ falling off from the tablets or
a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,
Wuhan University of Technology, Wuhan 430070, P. R. China.
E-mail: jiaguoyu@yahoo.com; Fax: +86-27-87879468; Tel: +86-27-87871029
b
School of Chemistry and Chemical Engineering, Hubei University of Technology,
Wuhan 430068, P. R. China
c
Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
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big-block supports. Furthermore, the high cost of noble metals The real content of Pt (the weight ratio of Pt to HC) in the
prevents their actual applications because most of the reported Pt/HC catalysts measured by inductively coupled plasma atomic
Pt-supported catalysts have high Pt content (most ≥0.5 wt%, emission spectrometry (ICP-AES) was 0.005, 0.008, 0.013, 0.032
the lowest Pt content is 0.1 wt%). So, there is an urgent need and 0.055 wt%, respectively. The Pt/HC samples with real Pt
to develop a high-performance HCHO room-temperature oxi- content of 0.005, 0.008, 0.013, 0.032 and 0.055 wt% were
dative catalyst with low noble metal loading content. Further, named as Pt5, Pt8, Pt13, Pt32 and Pt55, respectively.
to overcome the abovementioned drawbacks of powder cata-
Characterization
lysts, monolith-type catalytic materials can be considered.
2
3
Honeycomb ceramics (HC) are widely used as catalysts,
Pt/HC catalysts were analyzed using a D/Max-RB X-ray diffracto-
2
4
25
catalyst carriers and reactors due to their unique mechan- meter (Rigaku, Japan) with Cu K radiation at a scan rate (2θ)
α
ical and thermal properties. Also, the macropore structure of of 0.05° s− . A scanning electron microscopy (SEM) image was
HC will help to greatly reduce air resistance in gas phase reac- obtained on an S-4800 field emission SEM (FESEM, Hitachi,
tions. So, in this work, for the first time, we report the fabrica- Japan) at an accelerating voltage of 10 kV. The elemental
tion of novel and easily applied Pt/HC catalysts with an ultra- mapping over a desired region of the selected sample was
low Pt content (0.005–0.055 wt%) and their application to carried out by an energy-dispersive X-ray analyzer (EDX)
room temperature catalytic oxidation decomposition of HCHO attached to the SEM. Transmission electron microscopy (TEM)
from air. The prepared Pt/HC catalysts exhibit good catalytic and high-resolution transmission electron microscopy
1
activity and stability.
(HRTEM) images were collected on a JEM-2100F microscope at
an accelerating voltage of 200 kV. X-ray photoelectron spec-
troscopy (XPS) measurements were performed on a VG ESCA-
LAB250xi with X-ray monochromatisation. All binding energies
Experimental
Sample preparation
(
BE) were referenced to the C 1s peak at 284.8 eV of the surface
adventitious carbon. The dispersion of Pt was measured and
All reagents were of analytical grade and were used without calculated on the basis of CO chemisorption performed by a
further purification. The honeycomb ceramics (mainly com- BELCAT catalyst analyzer (BELCAT-B-293, BEL Japan). The
posed of α-Al O ) used were commercially available from sample (0.1117 g) was firstly pretreated in hydrogen (50 SCCM)
2
3
Wuhan University of Technology. The Pt/HC catalysts were pre- at 200 °C for 15 min and purged with helium (50 SCCM) for
pared by equivalent-volume impregnation of the honeycomb 15 min at the same temperature. Then, the catalyst was cooled
ceramic (see Fig. 1a, diameter: 4.75 cm, height: 1.2 cm, weight: to room temperature and CO pulses were injected from a
2
1.0 g, 300 mesh) with Pt precursor solution followed by calibrated on-line sampling valve. CO adsorption was assumed
2
1
reduction with NaBH4. In a typical preparation, one honey- to be complete after three successive peaks showed the
comb ceramic support was immersed in 12 mL of H PtCl
same peak areas. A CO/Pt stoichiometry of 1 was used for
aqueous solution with different concentrations (from 0.8, 1.6, calculations.
2
6
3
1
4
4
.2, 6.5, to 12.9 mM) under shaking. After impregnation for
Catalytic activity test
0 min, the mixed solution (10 mL) of NaBH solution (from
4
.8, 9.6, 19.2, 39.0, to 77.4 mM) and NaOH solution (from 24, Formaldehyde is widely used by industry to manufacture
8, 96, 195, to 387 mM) was quickly added onto the immersed building materials and numerous household products. It is
support for 10 min (NaOH : NaBH : Pt = 25 : 5 : 1, molar ratio) also a by-product of combustion and certain other natural pro-
4
under shaking, and it should be noted that the mixed solution cesses. Thus, it is always present in substantial concentrations
can contact uniformly with the whole surface of the support. both indoors and outdoors. The room-temperature catalytic
After reduction, the samples were dried at 80 °C for 12 h. The oxidation of HCHO was performed in a dark organic glass box
nominal weight ratios of Pt to HC in obtained Pt/HC catalysts covered by a layer of aluminum foil on its inner wall at 25 °C
were 0.009, 0.018, 0.036, 0.072 and 0.144 wt%, respectively. and 50% relative humidity in the same way as our previously
2
1
reported work. The Pt/HC sample was placed on the bottom
of a glass Petri dish with a diameter of 14 cm. After placing the
sample dish on the bottom of the reactor with a glass slide
cover, 6 μL of condensed HCHO solution (38%) was injected
into the reactor and a 5 watt fan was placed on the bottom of
the reactor. After 2 h, the HCHO solution was volatilized com-
pletely and the concentration of HCHO was stabilized. The
2
analysis of HCHO and CO was on-line conducted by a photo-
acoustic IR multigas monitor (INNOVA AirTech Instruments
Model 1412). The HCHO vapor was allowed to reach adsorp-
tion–desorption equilibrium within the reactor prior to cata-
lytic activity experiments. The initial concentration of HCHO
after adsorption equilibrium was controlled at about 140 ppm,
Fig. 1 Photographs of the HC and Pt/HC (Pt13) samples.
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which remained constant until the glass slide cover on the the HC sample is composed of a large number of Al
Petri dish was removed to start the catalytic oxidation reaction with the size of about 0.5–5 μm. After the deposition of Pt, the
of HCHO. Each set of experiments was followed for about Al grain size of the Pt13 sample (see Fig. 3b) shows no
0 min. The CO concentration increase (ΔCO , which is the obvious change. In order to further analyse the element com-
2 3
O grains
2 3
O
6
2
2
difference between CO concentration at t reaction time and position of the Pt13 sample, the area element mappings of Al,
2
initial time, ppm) and the HCHO concentration decrease were O and Pt of Fig. 3b were obtained by an energy-dispersive X-ray
used to evaluate the catalytic performance.
analyzer attached to the SEM and the corresponding results
are shown in Fig. 3c, d and e, respectively. Fig. 3c, d and e
clearly show that a large number of uniform small dots in the
elemental area mappings of Al, O and Pt species are well dis-
tributed on the whole bulk catalyst and the density of dots for
Al and O elements is much bigger than that for the Pt
element. This is not surprising, because the Pt loading
amount of the Pt13 sample is 0.013%. The above elemental
analysis indicates that the deposited Pt is highly dispersed on
Results and discussion
Phase structures, morphology and element composition
The photographs of HC and Pt/HC (Pt13) were taken by a
digital camera. Fig. 1a shows the photograph of HC used in
this study. The diameter and height of HC are about 4.75 and
2 3
the surface of Al O grains in the Pt13 sample, which was
1
.2 cm, respectively. It can be seen that many triangular-
further confirmed by CO chemisorption with about 33.1% of
Pt dispersion.
Fig. 3f and g present TEM and HRTEM images of the Pt13
sample, respectively. It can be seen from Fig. 3f that some
small black dots marked in white circle, dispersed on the
shaped channels (300 meshes) were patterned in the HC,
which are beneficial to the deposition of the small Pt nano-
particles (NPs) on the interior surface of HC. HC not only provides
more active contact sites for HCHO but also reduces the air
resistance of the gaseous reaction. Further observation indi-
cates that the color of HC changed from white to grey black of
Pt/HC (see Fig. 1b) after Pt deposition, implying the presence
of Pt NPs.
The XRD patterns of the HC and Pt/HC (Pt13) samples are
presented in Fig. 2. The XRD results indicate that all the diffr-
action peaks of the HC and Pt/HC samples can be indexed to
alpha-alumina, which are consistent with the JCPDS file of
surface of Al
ence of Pt NPs. Their size is about 4–5 nm. The HRTEM image
Fig. 3g) of Pt NPs shows that the lattice spacing in white circle
is ca. 0.224 nm, which is consistent with the lattice spacing of
2 3
O grains, are clearly observed, implying the pres-
(
2
7
the (111) planes of metallic Pt, also further confirming that
the black NPs are Pt NPs. Combining the above results of the
TEM image (Fig. 3f) and area element mapping of Pt (Fig. 3e)
2
8
as well as the results reported in the literature, it can be
inferred that Pt may have two existence forms: one is in the
form of Pt NPs, and the other is some metallic Pt atoms uni-
Al
mainly composed of alpha-alumina. Also, no obvious change
can be observed in the height and position of Al diffraction
2 3
O (JCPDS no. 43-1484), indicating that the HC sample is
2 3
O
formly deposited on the surface of Al O grains.
peaks before and after the deposition of Pt NPs (for the HC
and Pt/HC samples). Further observation shows that the diffr-
action peaks of Pt cannot be observed in the XRD pattern of
the Pt/HC sample, mainly due to its extremely low loading
2 3
XPS analysis
The composition and element chemical state of the prepared
samples were investigated by XPS and the results are shown in
Fig. 4. It is difficult to detect a Pt signal by XPS due to the
extremely low Pt content of 0.005–0.055 wt%. Therefore, in
order to detect a Pt XPS signal more easily, a 1 wt% Pt/HC
sample was prepared by the above Pt deposition method using
the ground HC powder as the support. XPS survey spectra (see
Fig. 4a) of HC and Pt/HC show that the elements Al, O, Sm,
Na, Si, and C exist in the HC sample and the XPS peaks of Pt
also appear in the Pt/HC sample after deposition of Pt. It can
be observed that the peak of Na 1s for Pt/HC is much stronger
than that of HC. This is because the deposition of Pt intro-
duces some Na element into the Pt/HC sample because of
2
1,26
(0.005–0.055 wt%).
SEM, TEM and HRTEM results of the HC and Pt13 samples
are shown in Fig. 3. The SEM image (Fig. 3a) of HC shows that
4
using NaOH and NaBH as precursors. Because the Al 2p line
of the support overlaps with the Pt 4f line of the active com-
ponent usually used for the spectroscopic analysis of plati-
num, the Pt 4d line was chosen to identify Pt species in this
study. The high-resolution XPS spectra of Pt 4d, O 1s and C 1s
regions are shown in Fig. 4b, c and d, respectively. The peak
observed at 314.4 eV for the Pt/HC sample is assigned to
2
9,30
Pt 4d5/2 of metal Pt,
further confirming the presence of
Fig. 2 XRD patterns of the HC and Pt/HC (Pt13) samples.
Pt(0). A high-resolution XPS spectrum of O 1s is shown in
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Fig. 3 SEM image (a) of the HC sample, SEM image (b) and EDX element mappings (c: Al Kα, d: O Kα, and e: Pt Mα) of the prepared Pt13 sample,
and TEM (f) and HRTEM (g) images of the Pt13 sample.
Fig. 4c. Two fitted peaks located at 530.8 and 532.2 eV are assigned to C of Na CO , mainly resulting from the reactions
2
3
3
2
2
observed for both HC and Pt/HC, which can be assigned to the between NaOH and CO .
lattice oxygen (O 1s) of Al
hydroxyl (OH) groups, respectively. An extra peak at 536.0 eV
is also observed for Pt/HC, which can be attributed to Na Fig. 5 shows a comparison of the catalytic performance of the
2 3
O and the oxygen of the surface
2
2
Catalytic activity
3
1
KLL. The high-resolution C 1s spectra of HC and Pt/HC are HC, Pt5, Pt8, Pt13, Pt32 and Pt55 samples towards HCHO oxi-
shown in Fig. 4d. The peak located at 284.8 eV (C–C bond) can dation at room temperature. As can be seen in this figure, the
be assigned to the adventitious carbon. Compared to HC, a Pt loading content exhibits a significant influence on the
new peak at 289.0 eV appears in the spectra of Pt/HC, which is HCHO oxidation catalytic activity of the prepared samples.
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Fig. 4 Typical survey spectra (a) of the HC and Pt/HC samples, high-resolution XPS spectra for Pt 4d (b) of the Pt/HC sample and O 1s (c), C 1s (d)
of the HC and Pt/HC samples.
2 2
Fig. 5 Changes in formaldehyde concentration (a) and ΔCO (the difference between CO concentration at t reaction time and initial time, ppm) (b)
as a function of reaction time for the HC and Pt/HC samples with different Pt contents.
In the absence of Pt, for the HC sample, the HCHO concen- oxidation and HCHO is mainly adsorbed on the surface of HC.
tration decreases in the initial 10 min and then remains Therefore, it can be reasonably inferred that the presence of Pt
almost unchanged during the subsequent 50 min. Accordingly, is indispensable for room temperature HCHO oxidation
the CO concentration has no obvious increase in the whole decomposition. However, in the presence of a small amount
2
process, implying that the HC sample is not active for HCHO of Pt, the activity of the Pt5 sample was obviously enhanced.
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The HCHO concentration (shown in Fig. 5a) decreases with and H
2
O. The catalytic activity of the Pt/HC samples shows a
increasing reaction time; meanwhile, the CO2 concentration tendency to increase with increasing Pt loading content.
shown in Fig. 5b) increases, indicating that the Pt5 sample is However, when the Pt loading content is higher than 0.013 wt%,
(
active for HCHO oxidation and HCHO is oxidized into CO
2
a further increase in the content of Pt loading does not
cause a great increase in the catalytic activity. This is because a
further increase in the Pt loading would lead to the aggrega-
tion of Pt NPs and the growth of Pt particles (the results are
not shown here). Thus, there is no obvious increase in the
number of reactive sites for the Pt32 and Pt55 samples. So,
considering Pt cost and catalytic activity, the optimal Pt
loading was determined to be 0.013 wt% for Pt/HC catalysts.
The catalytic oxidation activity of Pt/HC toward HCHO can be
understood by the following suggested mechanism (see Fig. 6)
1
1,21,33
based on the previously reported results.
In the case of
are
catalytic oxidation of HCHO over Pt/HC, HCHO and O
2
firstly adsorbed onto the HC and Pt surface (step I), respect-
ively. The O2 molecule was dissociated into two adsorbed
3
3
active O atom species on the surface of Pt NPs. After that,
HCHO is oxidized to formate species by the adsorbed active O
atom at the Pt NPs surface (step II). Then, the surface formate
Fig. 6 Proposed mechanism for the catalytic oxidation of HCHO over
Pt/HC.
species are further oxidized into carbonic acid species (H
step III), and finally, carbonic acid will decompose into CO
and H O (step IV).
2 3
CO )
(
2
2
Why can the Pt/HC catalysts with ultra-low Pt content
(
0.005–0.013 wt%) exhibit high catalytic performance for oxi-
dation decomposition of HCHO at room temperature? One
possible explanation is when the HC is used as the support to
deposit the Pt NPs. The Pt NPs are mainly deposited on the
outer surface of HC due to its big Al O grain size and
2
3
compact structure. Therefore, the HCHO molecules are much
easier to contact with the Pt NPs on the surface of HC than
those in powder-like catalysts. Because most Pt NPs in powder-
like catalysts are deposited on the inner surface of porous cata-
lysts, the inner Pt NPs are not easily approached by HCHO
molecules. The difference between Pt/HC and Pt/particle cata-
lysts in HCHO oxidation is illustrated in Fig. 7. It is notable
2
that the increase of CO concentration is larger than the
decrease of HCHO concentration for all Pt/HC catalysts. This is
because some adsorbed HCHO molecules on the reactor
Fig. 7 Illustration of the Pt/HC (1) and Pt/particles (2) catalysts for
HCHO oxidation.
2
Fig. 8 Changes in formaldehyde concentration (a) and ΔCO (b) as a function of reaction time for the Pt13 sample in five recycle tests.
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surface will gradually desorb with decreasing the concen-
tration of HCHO in the reactor, and these desorbed HCHO
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2
, thus resulting in the
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The stability of catalysts is also very important for their
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further investigated by performing the recycle experiments five
times (see Fig. 8). After five recycles, the oxidation rate of
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HCHO shown in Fig. 8a and the generation rate of CO shown
in Fig. 8b over Pt13 do not show any significant changes as
compared with those obtained in the first-cycle, suggesting
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indoor HCHO and the purification of indoor environments.
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Acknowledgements
(
This work was supported by the 863 Program (2012AA062701),
NSFC (21177100, 51272199 and 51320105001), Fundamental
Research Funds for the Central Universities (WUT: 2014-
VII-010), Self-determined and Innovative Research Funds of
SKLWUT (2013-ZD-1 and 2013-KF-10) and Young and Middle-
aged Project in Hubei Province Department of Education
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Notes and references
1
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