Flexible Mg-Al layered double hydroxide supported Pt on Al foil for use in room-temperature catalytic decomposition of formaldehyde

Article abstract of DOI:10.1039/c6ra02569b

Room-temperature catalytic decomposition of formaldehyde (HCHO) is regarded as one of the best methods for indoor HCHO purification and removal. Herein, for the first time, a flexible and bendable Mg-Al layered double hydroxide (LDH) on aluminum (Al) foil was in situ prepared and further decorated with platinum (Pt) nanoparticles (NPs) for catalytic oxidation decomposition of HCHO at room temperature. Such a Pt-loaded large-area flexible LDH catalyst can be directly used and easily regenerated because of its flexibility and relatively low production cost. Moreover, it exhibited excellent catalytic performance toward oxidation decomposition of HCHO at room temperature due to its hierarchical macro-/mesoporous structure that facilitates the diffusion of reactants and products. Furthermore, abundant hydroxyl groups on the catalyst surface are also beneficial to HCHO oxidation decomposition. This work may provide some new insight into the design and fabrication of advanced catalysts for indoor HCHO removal and air purification.

Full text of DOI:10.1039/c6ra02569b

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Flexible Mg-Al Layered Double Hydroxide Supported Pt on the Al
foil for use in Room-Temperature catalytic decomposition of
formaldehyde
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
b
a,b,
DOI: 10.1039/x0xx00000x
Jiawei Ye, Bei Cheng, S. Wageh, Ahmed A. Al-Ghamdi and Jiaguo Yu
*
www.rsc.org/
Room-temperature catalytic decomposition of formaldehyde (HCHO) is regarded as one of the best methods for indoor
HCHO purification and removal. Herein, for the first time, flexible and bendable Mg-Al layered double hydroxide (LDH) on
aluminum (Al) foil was in-situ prepared and further decorated by platinum (Pt) nanoparticles (NPs) for catalytic oxidation
decomposition of HCHO at room temperature. Such a Pt-loaded large-area flexible LDH catalyst can be directly used and
easily regenerated because of its flexibility and relatively low production cost. Moreover, it exhibited an excellent catalytic
performance toward oxidation decomposition of HCHO at room temperature due to its hierarchical macro-/mesoporous
structure that facilitates the diffusion of reactants and products. Furthermore, abundant hydroxyl groups on the catalyst
surface are also beneficial to the HCHO oxidation decomposition. This work may provide some new insight into the design
and fabrication of advanced catalyst for indoor HCHO removal and air purification.
composed of NPs as the supports of noble metals. Practically, such
catalysts in the form of powders need to be adhered to other bigꢀ
1. Introduction
block substrates, requiring complicated processes and extra cost.
Moreover, it is of great difficulties in regenerating and recycling the
powder formed catalysts. To overcome the aforementioned
drawbacks of powder samples, flexible catalysts based on metal foils
will attract wider research interests due to their good flexibility, light
weight, and low cost.
HCHO is recognized as a major indoor air pollutant. Recently,
unprecedented use of building and furnishing materials results in a
higher emission of indoor HCHO than ever. Prolonged exposure to
even a few parts per million (ppm) of HCHO may cause serious
health problems including respiratory diseases, nasal tumors and
1
ꢀ3
skin irritation. Therefore, the development of efficient methods
and materials for the removal of indoor HCHO is essential to meet
human health needs.
Herein, flexible MgꢀAl LDH on Al foil (LDHꢀFoil) was first
prepared by in situ growth technique on the Al foil by means of
hydrothermal method in the presence of urea and magnesium nitrate.
The prepared flexible LDHꢀFoil sheet was then used as the
supporting materials of Pt NPs obtained by NaBH reduction method.
4
Particularly, this flexible and bendable macroꢀsized MgꢀAl LDHꢀAl
foil supported Pt catalyst (PtꢀLDHꢀFoil) with ~20 ꢁm thickness LDH
4
ꢀ10
Many methods such as adsorption (physical adsorption,
11ꢀ13
14ꢀ16
chemisorption
), plasma technical oxidation,
photocatalytic
1
7ꢀ19
2,20ꢀ32
degradation
and thermal catalytic oxidation decomposition
have been investigated and developed for the removal of indoor
formaldehyde in air. Among these methods, HCHO removal by
room temperature catalytic oxidation decomposition offers a feasible
strategy for solving this important environmental problem. By using layer exhibited good HCHO oxidation activity, which can be directly
roomꢀtemperature catalytic oxidation technology, the toxic used without further modification or remodeling of the catalyst. Thus,
formaldehyde can be completely decomposed into carbon dioxide
and water. Also, no additional apparatus and energy are needed
comparing with conventional highꢀtemperature thermal catalytic
oxidation and photocatalytic methods. To date, a lot of oxideꢀ
it is convenient to recycle the catalyst and to retrieve rare metals in
the catalyst after application. Considering the facile preparation
procedure of the PtꢀLDHꢀFoil catalyst, this work will provide some
new insight into the design and fabrication of new flexible and highꢀ
efficiency catalytic materials for indoor formaldehyde
2
,21,24,25,33ꢀ35
29
supported noble metal catalysts, such as Pt/TiO2,
3
6
20,27
22
23
Pt/Fe
Au/Co
2
O
3
,
Pd/TiO
2
,
Pt/SiO
2
,
Pt/Al
2
O
3
,
Au/CeO
2
,
and
3
7
3
O
4
ꢀCeO
2
have been studied for roomꢀtemperature catalytic decomposition.
oxidation decomposition of formaldehyde. However, most of the
above mentioned catalysts were mainly fabricated by using powders
2
. Experimental section
All chemicals used in this study were reagentꢀgrade without further
treatment. Distilled water was used in the whole experiment. Al foils
were cleaned in ethanol and deionized water before use.
a.
State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan, 430070,
China
Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah
b.
21589, Saudi Arabia.
Footnotes relating to the title and/or authors should appear here.
Preparation of MgꢀAl LDH-Foil
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
The flexible MgꢀAl LDHꢀFoil was fabricated by in situ growth
method onto commercial aluminium foil (with thicknesses of ca. 20
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µm in rectangular shapes with dimensions of 100 mm × 200 mm)
using hydrothermal method similar to the previously report by He et. was introduced into the DFTIR cell at room temperature via mass
2 2
O gas for 60 min. And then, the reactant gas mixture (HCHO+O )
3
8
al. In a typical preparation, 9.7 g urea and 6.9 g Mg(NO ) ꢂ6H O flow controllers at a flow rate of 30 mL/min. All spectra were
3
2
2
ꢀ
1
were dissolved in 500 mL deionized water in a glass vessel. The recorded with a resolution of 4 cm , and the background spectrum
cleaned Al foil with thickness of about 0.02 mm and a size of 10 × was subtracted from each spectrum, respectively.
2
2
0 cm was immersed into the solution and the glass vessel was
o
sealed, maintained at 80 C for 48 h. After hydrothermal reaction, Catalytic activity characterization
cooling to the room temperature, MgꢀAl LDH on the Al foil were
washed with deionized water and dried at 80 C for 6 h to obtain the
LDHꢀFoil sample.
Roomꢀtemperature catalytic oxidation decomposition of HCHO was
o
performed in a dark organic glass box covered by a layer of Al foil
o
paper on its inner wall at 25 C. A round piece of catalyst (10 cm in
diameter) was placed on the bottom of a glass Petri dish with a
diameter of 10 cm. After placing the sampleꢀcontained dish into the
Preparation of Pt loaded catalyst
In a typical preparation, a round piece of LDHꢀFoil (10 cm in bottom of reactor with a glass slider, 8 µL of condensed HCHO
diameter) was placed on the bottom of glass Petri dish with a solution (38 wt%) was injected into the reactor and a 5 W fan was
diameter of 10 cm. Then, 10 ml of NaBH ethanol solution (0.05 placed in the bottom of reactor during the whole reaction process.
4
mol/L) was added into the dish. After impregnation for 10 minutes, After 1 h, the HCHO solution was completely volatized and the
the solution was discarded and 2 ml of H PtCl ethanol solution
2
6
concentration of HCHO got stabilized. The concentrations of
HCHO, CO , CO and water vapour were online detected by a
(6.10 mmol/L) was added dropwise onto the LDHꢀFoil. After that,
2
an extra 2 ml of NaBH ethanol solution (0.05 mol/L) was added to
4
Photoacoustic IR Multigas Monitor (INNOVA air Tech Instruments
Model 1412). The HCHO vapour was allowed to reach
adsorption/desorption equilibrium with the reactor surface before the
catalytic test. The beginning concentration of HCHO after
equilibrium was controlled at ca. 200 ppm. The catalytic oxidation
reaction of HCHO started right after the glass slide cover over the
Petri dish was removed. The increase of CO concentration (△CO )
2 2
and the decrease of HCHO concentration were used to evaluate the
catalytic performance of the prepared samples.
o
the dish. Then, the sample was dried at 80 C for 1 h. Finally, the asꢀ
obtained products were washed with deionized water to remove
o
chloride anions and then dried at 80 C for 6 h. The real Pt loading in
the PtꢀLDHꢀFoil sample measured by inductively coupled plasma
atomic emission spectrometry (ICPꢀAES) was about 0.223 wt%. For
comparison, Pt loaded Al foil (PtꢀAlꢀFoil) was also prepared by the
above process.
Characterization
3
. Results and discussion
The phase structures of Al foil, LDHꢀFoil and PtꢀLDHꢀFoil were
analysed by a D/MaxꢀRB Xꢀray diffractometer (Rigaku, Japan) with
Phase structure and morphology
ꢀ1
Cu Kα radiation at a scan rate (2θ) of 0.05 s . The morphology
observation was performed by a JSMꢀ6510 scanning electron
microscopy (SEM, JEOL, Japan) at an accelerating voltage of 20
kV. Transmission electron microscopy (TEM) images were collected
on a JEMꢀ2100F microscopy at an accelerating voltage of 200 kV.
The elemental analysis of PtꢀLDHꢀFoil sample were performed by
using Field emission scanning electron microscope (FESEM, JEOL
7500F, Japan) equipped with an energy dispersive Xꢀray (EDX)
system operating at an accelerating voltage of 15 kV. Xꢀray
photoelectron spectra (XPS) measurements were carried out on VG
ESCALAB210 with Mg Kα source. All binding energies were
referenced to the C 1s peak at 285.0 eV of the surface adventitious
carbon. The BrunauerꢀEmmettꢀTeller (BET) specific surface area
(
S
BET) of the samples (the powder peeled off from the foil) was
analysed in a nitrogen adsorption apparatus (Micromeritics ASAP
020, USA). Prior to nitrogen adsorption measurements, all of the
2
o
samples were degassed at 180 C. The BET surface area was Figure 1. XRD patterns of LDHꢀFoil, PtꢀLDHꢀFoil and Al foil samples.
determined by a multipoint method using adsorption data in the
Xꢀray diffraction (XRD) was used to investigate the changes in
0
relative pressure (P/P ) range of 0.05−0.3. The pore size distribution
crystallographic structure of the asꢀprepared samples before and after
hydrothermal treatment and Pt loading. Figure 1 shows the
comparison of XRD patterns of LDHꢀFoil, PtꢀLDHꢀFoil and Al foil
samples. The diffraction peaks marked with ♦ and △ can be assigned
to MgꢀAl hydroxide hydrate (JCPDS No. 35ꢀ0964) and Al (JCPDS
No. 04ꢀ0787), respectively. The Al foil before hydrothermal
treatment has only metallic Al phase. After hydrothermal treatment,
curves were obtained via the BarretꢀJoynerꢀHalender (BJH) method
assuming a cylindrical pore model using the nitrogen adsorption
isotherm. The pore volume and the average pore size were calculated
using the nitrogen adsorption volume at a relative pressure (P/P
.97. In situ diffuse Fourier transform infrared spectroscopy
DFTIRS) was recorded on Thermo Fisher IS50. Before the
beginning of the DFTIR experiments, the catalyst was swept by pure
0
) of
0
(
2
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phase composition of the LDHꢀFoil sample has a significant change double hydroxide and the inner of Al foil still was metallic
from metallic Al to both MgꢀAl hydroxide hydrate and metallic Al. aluminium, which is consistent with the above XRD result.
The appearance of the MgꢀAl hydroxide hydrate phase is ascribed to
TEM image (Figure 2e) of the PtꢀLDHꢀFoil sample displays that
the growth of MgꢀAl LDH layer on the surface of Al foil. The XRD many small Pt NPs with size of ca. 5ꢀ10 nm are uniformly deposited
pattern of the PtꢀLDHꢀFoil sample is similar to that of the LDHꢀFoil on the surface of the sample. These highly dispersed Pt NPs are
sample, indicating that the Pt loading has no obvious effect on the beneficial for enhancing the rate of catalytic oxidation reaction of
phase structure of LDHꢀFoil sample. No any diffraction peaks of Pt formaldehyde in air because they can provide more active centres for
are observed in the XRD pattern of PtꢀLDHꢀFoil sample mainly due HCHO oxidation than the aggregated Pt NPs. EDX analyses were
to Pt with low amount, small particle size and weak further performed for analysing the composition of PtꢀLDH foil
2
0,25
crystallization.
sample. Mg, Al, O and Pt elements were detected in the EDX
spectrum (Figure 2f). Mg, Al and O are from MgꢀAl LDHs and Pt is
from the loading and deposition of Pt NPs, suggesting Pt NPs were
successfully deposited on the surface of MgꢀAl LDH nanoplates.
Figure 2. Macroscopic photograph (a) of PtꢀLDHꢀFoil sample, SEM images
b), high magnification SEM image (c), crossꢀsection SEM image (d), TEM
(
image (e) and EDX spectrum (f) of PtꢀLDHꢀFoil sample.
The prepared PtꢀLDHꢀFoil sample exhibits good flexibility and
deformability, which can be bent to different shapes without
damaging it just as the Al foil can. Figure 2a presents the camera
photograph of PtꢀLDHꢀFoil sample, indicating that the surface
appears grey and black colour due to the formation of LDH on the
Al foil and deposition of Pt NPs. The morphology and surface
microstructure of the samples were further investigated by SEM and
TEM (see Figure 2bꢀd). SEM images (Figure 2b and c) show that Ptꢀ
LDHꢀFoil samples have hierarchical porous structures, which
consists of closely connected nanoplates with size of ca. 3 to 5 ꢁm
and the thickness ranging from 100 to 200 nm. Comparing the SEM
image of PtꢀLDHꢀFoil sample with that of LDHꢀFoil sample (not
shown here), no obvious morphology change was found, indicating
that the deposition of Pt NPs did not obviously change the
morphology and microstructure of the LDHꢀFoil sample. SEM
image (see Figure 2d) of crossꢀsection of PtꢀLDHꢀFoil sample
indicates that the whole thickness of PtꢀLDHꢀFoil sample is ca. 18
ꢁ
m. Among them the thickness of LDH layer on Al foil surface is
about 4 ꢁm and the thickness of inner unꢀreacted Al layer within the
sample is about 10 ꢁm. The SEM observation indicates that only
surface aluminium of Al foil was reacted to form MgꢀAl layered
Figure 3. XPS survey spectra (a) of LDHꢀFoil and PtꢀLDHꢀFoil samples,
highꢀresolution XPS spectra for O 1s (b), Al 2p and Pt 4f (c) of LDHꢀFoil
and PtꢀLDHꢀFoil samples.
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XPS analysis
JoynerꢀHalenda) method present a wide range of 10ꢀ100 nm. No
great change in the shape of isotherms and pore size distribution
curves between two samples is observed. This further suggests that
the porous structure of LDHꢀFoil sample was well preserved after
The surface chemical composition and element chemical status
of the prepared samples were investigated by Xꢀray photoelectron
spectroscopy (XPS). The XPS survey spectra (Figure 3a) of LDHꢀ
Foil and PtꢀLDHꢀFoil samples indicate the presence of Mg, Al, O deposition of Pt NPs. These hierarchical mesopores and macropores
and elements for both samples, and their corresponding can effectively act as gasꢀexchange channels for the gaseous
photoelectron peaks appear at binding energies of ca. 1303 (Mg 1s),
5 (Al 2p), 532 (O 1s), and 285 eV (C 1s). The peak of Na 1s is only
observed in PtꢀLDHꢀFoil sample because the Na species were
introduced in the Pt loading process due to the use of NaBH
reactants. The C 1s XPS peak at the binding energy of 285 eV is due
to the adventitious hydrocarbon, which originates from the XPS the LDHꢀFoil sample, thus leading to the formation of numerous
C
formaldehyde and CO
2
products. The specific surface area of Ptꢀ
7
2
LDHꢀFoil (67 m /g) sample is higher than that of LDHꢀFoil sample
2
(
41 m /g). This is due to the generation of NaOH during the NaBH
4
ꢀ
4
reduction process, and the produced NaOH can etch the surface of
instrument itself.
micropores and small mesopores, and the increase of specific surface
Highꢀresolution O 1s spectra of LDHꢀFoil and PtꢀLDHꢀFoil
area after Pt loading.
(
Figure 3b) show two peaks appearing at 535.5 and 532.0ꢀ532.2 eV,
which can be assigned to adsorbed water (H O) and surface hydroxyl
groups (Mg, AlꢀOH) in the MgꢀAl double hydroxide, respectively.
2
3
9ꢀ
41
Highꢀresolution Al 2p spectra of LDHꢀFoil shows a peak at 74.8
eV, which can be assigned to Al 2p of MgꢀAl layered double
4
2
hydroxide. The highꢀresolution Pt 4f and Al 2p spectra of PtꢀLDHꢀ
Foil show five peaks at 71.2, 74.5, 74.8, 74.2 and 77.5 eV, which
correspond to Pt 4f7/2, Pt 4f5/2, Al 2p, Pt 4f7/2 and Pt 4f5/2
0
0
4+
4+
,
1
8,39,43,44
respectively.
The XPS results further demonstrate that Pt
NPs were deposited on the surface of MgꢀAl LDH.
Figure 5. In situ DFTIR spectra of PtꢀLDHꢀFoil catalyst as a function of
2
time under a flow of HCHO/O at room temperature.
In situ DFTIRS
In order to detect the intermediates in catalytic oxidation
decomposition of formaldehyde and catalytic reaction mechanism, in
situ DFTIR spectra were recorded. Figure 5 presents in situ DFTIR
spectra of PtꢀLDHꢀFoil catalyst upon exposure to HCHO/O
2
at room
temperature at the different time. The sample was only exposed in
ꢀ
1
HCHO/O mixed gases for 2 min, the bands located at 1370 cm and
2
Figure 4. N
2
adsorptionꢀdesorption isotherms and the corresponding pore
ꢀ1
1
540 cm are observed, which can be attributed to symmetric and
size distribution curves (inset) for the LDHꢀFoil and PtꢀLDHꢀFoil samples.
asymmetric stretching of COO, respectively. This suggests that
formate species are the intermediate products of HCHO oxidative
decomposition and HCHO can be immediately oxidized into formate
Nitrogen adsorption
Nitrogen adsorptionꢀdesorption isotherms were measured to over the surface of PtꢀLDHꢀFoil sample. There bands at 2745, 2802
ꢀ
1
determine the specific surface areas and pore size distribution of the and 2901 cm can be assigned to the stretching vibration of CꢀH,
LDHꢀFoil and PtꢀLDHꢀFoil samples (see Figure 4). Before nitrogen suggesting that HCHO molecules are adsorbed on the surface of the
adsorption measurement, the LDHs and PtꢀLDHs were scraped off PtꢀLDHꢀFoil samples. In addition, the bands at 1655, 3252 and 3520
ꢀ
1
from corresponding LDHꢀFoil and PtꢀLDHꢀFoil samples and then cm are due to the produced water during the oxidation
measured. The isotherms corresponding to the above two samples decomposition of formaldehede.4
are of type IV according to BrunauerꢀDemingꢀDemingꢀTeller
7,48
(BDDT) classification and show high adsorption values at high
relative pressure range of 0.9ꢀ1.0, indicating the existence of large
4
5
mesopores and macropores. The hysteresis loops are of type H3,
4
6
indicating the presence of plateꢀlike particles and slitꢀlike pores.
The above results are in good agreement with the SEM images. The
pore size distribution curves (inset) of two samples fitted from the
adsorption branch of the nitrogen isotherm using the BJH (Barettꢀ
4
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Figure 6. Suggested catalytic decomposition mechanism of formaldehyde at
room temperature.
On the basis of the above results and discussion, the reaction
mechanism for room temperature catalytic HCHO oxidation on the
PtꢀLDHꢀFoil is proposed and presented in Figure 6. Firstly, HCHO
molecules are adsorbed on the surface hydroxyl via hydrogen bond Figure 7. Changes in the decrease of formaldehyde concentration (a) and the
increase of CO concentration △CO (the difference in CO concentration
2
2
2
2
and gaseous O molecules are adsorbed and activated on the surface
between t time and initial time) (b) for PtꢀAlꢀFoil, LDHꢀFoil and PtꢀLDHꢀ
Foil samples used in the room temperature catalytic decomposition of
43,49,50
of Pt NPs.
After that, adsorbed HCHO molecules are first
oxidized into formate intermediates by activated oxygen atoms and formaldehyde.
the consumed O molecules can be supplied from the air (step I).
2
Then, the surface formate species are further oxidized into adsorbed Evaluation of catalytic activity
carbonic acid (step II). The produced carbonic acid is very unstable
and will rapidly decompose into CO and H O, which will finally
2 2
The catalytic activity of the LDHꢀFoil, PtꢀAlꢀFoil and PtꢀLDHꢀ
Foil samples toward HCHO catalytic decomposition at room
temperature are shown in Figure 7. As can be seen in this figure, the
concentration of HCHO only slightly decreases, and the
desorb from the sample surface, and the catalyst is regenerated (step
III). Obviously, hydroxyl groups of PtꢀLDHꢀFoil surface are
beneficial to the above catalytic reaction because they can efficiently
capture the gaseous HCHO molecules and intermediate products,
thus resulting in the increase of oxidation reaction rate of HCHO.
Meanwhile, hierarchical macroꢀ/mesopores of PtꢀMgꢀAl LDH
samples is also favourable to formaldehyde oxidation
2
concentration of CO nearly keep almost unchanged during the
whole HCHO catalytic oxidation for LDHꢀFoil and PtꢀAlꢀFoil
samples, indicating that these two samples have no catalytic
activities toward HCHO oxidation. Contrarily, for PtꢀLDHꢀFoil
sample, not only the HCHO concentration greatly decrease, but also
the CO2 concentration increase simultaneously with increasing
reaction time, suggesting that formaldehyde was completely
2
3,51
decomposition.
most of the Pt NPs more exposed to gas reactants, but also reduces
the diffusion resistance for the exchange of reactants (O and HCHO)
and products (CO and H O), thus accelerating the reaction and
The hierarchical porous structure not only allows
2
decomposed into CO
indicates that the increase of CO
2
and H
2
O. Further observation from Figure 7
concentration is slightly higher
2
2
2
decomposition rate of gaseous formaldehyde molecules. As a result,
not surprising, the prepared flexible PtꢀLDHꢀFoil sample exhibited a
good room temperature catalytic HCHO oxidation activity.
than the decrease of HCHO concentration. Not surprising, because
some HCHO molecules were desorbed from the reactor surface with
decreasing the HCHO concentration in the reactor. These desorbed
HCHO molecules were further oxidized into CO
increase of CO concentration.
2
, which result in the
2
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5
1372190 and 51272199), Deanship of Scientific Research (DSR) of
King Abdulaziz University (90ꢀ130ꢀ35ꢀHiCi), the Fundamental
Research Funds for the Central Universities (2015ꢀIIIꢀ034), Selfꢀ
determined and Innovative Research Funds of SKLWUT (2015ꢀZDꢀ
1) and the Natural Science Foundation of Hubei Province of China
(
No. 2015CFA001)
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Figure 8. Changes in formaldehyde concentration and the produced CO
concentration as a function of reaction time for PtꢀLDHꢀFoil sample in
recycling tests.
2
4
.
The stability and repeatability of an indoor air purification
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2
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This study was partially supported by the 973 program
(2013CB632402), NSFC (21433007, 51320105001, 21573170,
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