Chemisorbed Oxygen on the Surface of Catalyst-Improved Cataluminescence Selectivity

Article abstract of DOI:10.1021/acs.analchem.6b01025

It is a critical scientific challenge to improve the selectivity of cataluminescence (CTL). Chemisorbed oxygen on the surface of catalysts is one of the essential factors for catalytic oxidization of gaseous reactant molecules during the CTL process. Therefore, it is necessary to investigate the influence of chemisorbed oxygen on the CTL. There exists different chemisorbed oxygen content on the surface of Y₂O₃ and its precursor, layered rare-earth yttrium hydroxides (Y-NO₃-LRHs). In this work, both of them were employed as catalyst models to catalytically oxidize common volatile organic compounds (VOCs) in order to explore the relationship between chemisorbed oxygen and CTL selectivity. It was found that LRHs demonstrated a superior selectivity toward ethyl ether in comparison with Y₂O₃. The mechanism study showed that only ethyl ether demonstrated the CTL behavior through the catalytical oxidation into CH₃CHO? intermediates on the surface of LRHs, while no CTL emissions occurred for the other VOCs because the insufficient chemisorbed oxygen of LRHs was incapable of oxidizing these VOCs into CO₂? intermediates. In addition, the luminescent rare-earth Eu³⁺ ions were doped in Y-NO₃-LRHs to further improve the CTL intensity of ethyl ether through the efficient energy transfer between CH₃CHO? intermediates and Eu³⁺ ions. Our work opens up a new route to improve CTL selectivity by tuning the chemisorbed oxygen on the surface of catalysts, different from the previous strategies of exploiting new solid catalysts or decreasing CTL reaction temperature.

Full text of DOI:10.1021/acs.analchem.6b01025

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Article
Chemisorbed Oxygen on the Surface of Catalyst–
Improved Cataluminescence Selectivity
Siming Wang, Wen Ying Shi, and Chao Lu
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b01025 • Publication Date (Web): 07 Apr 2016
Downloaded from http://pubs.acs.org on April 9, 2016
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Chemisorbed Oxygen on the Surface of Catalyst–Improved
Cataluminescence Selectivity
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Siming Wang, Wenying Shi and Chao Lu*
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical
Technology, Beijing 100029, China
Fax/Tel.: +86 10 64411957. E-mail: luchao@mail.buct.edu.cn
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ABSTRACT: It is a critically scientific challenge to improve the selectivity of
cataluminescence (CTL). Chemisorbed oxygen on the surface of catalysts is one of essential
factors for catalytic oxidization of gaseous reactant molecules during the CTL process.
Therefore, it is necessary to investigate the influence of chemisorbed oxygen on the CTL.
There exists different chemisorbed oxygen content on the surface of Y O and its precursor,
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layered rare-earth yttrium hydroxides (Y-NO
employed as catalyst models to catalytically oxidize common volatile organic compounds
VOCs) in order to explore the relationship between chemisorbed oxygen and CTL selectivity.
3
-LRHs). In this work, both of them were
(
It was found that LRHs demonstrated a superior selectivity towards ethyl ether in comparison
with Y O . The mechanism study showed that only ethyl ether demonstrated the CTL
2
3
behavior through the catalytical oxidation into CH
LRHs, while no CTL emissions occurred for the other VOCs because the insufficient
chemisorbed oxygen of LRHs was incapable of oxidizing these VOCs into CO
3
CHO* intermediates on the surface of
2
*
3
+
intermediates. In addition, the luminescent rare-earth Eu ions were doped in Y-NO -LRHs
3
to further improve the CTL intensity of ethyl ether through the efficient energy transfer
3+
3
between CH CHO* intermediates and Eu ions. Our work opens up a new route to improve
CTL selectivity by tuning the chemisorbed oxygen on the surface of catalysts, different from
the previous strategies of exploiting new solid catalysts or decreasing CTL reaction
temperature.
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INTRODUCTION
Cataluminescence (CTL) refers to a kind of chemiluminescence from the surface of solid
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,2
catalysts during heterogeneous catalytic oxidation reactions. In general, the occurrence of
CTL reaction needs three essential factors: i) suitable atmosphere environment containing
oxygen (i.e., chemisorbed oxygen on the surface of catalysts), ii) solid catalysts, and iii)
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proper reaction temperature. Over the past few decades, much attention has been paid to
improving the CTL selectivity, which mainly concentrated on exploiting new solid catalysts
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or decreasing CTL reaction temperature. However, chemisorbed oxygen on the surface of
catalysts in improving the CTL selectivity has been ignored. Therefore, it is absolutely critical
for controlling of chemisorbed oxygen on the surface of catalysts, so as to achieve CTL
sensors with excellent selectivity.
Generally, rare-earth oxides are recognized as solid base catalysts owing to their strong
1
0
2 3
catalytic properties. Among them, Y O has a C-type rare-earth oxide crystal structure, and
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oxygen vacancies are the major defects. With the abundant base catalytic active sites and
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oxygen vacancies to chemisorb oxygen on its surface,
2 3
Y O has been widely used as a
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common CTL catalyst in gas sensing.
Y O is mainly synthesized through calcining its
2 3
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precursors.
precursors,
2 3
Among them, layered rare-earth hydroxides (LRHs) are often used as Y O
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which are layered metal hydroxides with a general formulae of
2 5 2
R (OH) X·nH O, wherein R is rare-earth ions on the layers and X substitutes for an extensive
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choice of intercalated anions.
During the calcination process, abundant oxygen vacancies
on the surface of Y O are produced, facilitating the formation of more chemisorbed
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3
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oxygen.
2 3
Therefore, the variable content of chemisorbed oxygen between Y O and LRHs
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will lead to the difference of catalytic oxidation activity. Generally, there are some important
causes from catalysts to affect the CTL process, mainly including chemisorbed oxygen,
surface area and basic sites. In this study, in order to investigate the effect of chemisorbed
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oxygen of catalysts on the CTL, we have to exclude the effect of the other causes of catalysts
on the CTL, such as the surface area and the basic sites. Inspired by the difference of
2 3
chemisorbed oxygen between Y O and LRHs, it is reasonable to employ them as catalyst
models to deeply investigate the influence of chemisorbed oxygen on the CTL selectivity.
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In this work, Y-NO -LRHs showed a perfect selectivity towards ethyl ether among nine
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2 3
kinds of common volatile organic compounds (VOCs) in comparison with Y O . X-ray
photoelectron spectroscopy (XPS) measurements, CTL spectra, in situ Fourier-transform
infrared (FT-IR) spectra, gas chromatography-mass spectrometry (GC-MS) experiments,
surface area and basic sites data demonstrated that the improved CTL selectivity was
attributed to less chemisorbed oxygen on Y-NO -LRHs than Y O (Figure 1). In brief, for
3
2
3
Y-NO
excited state intermediates of CH
contrast, many of the tested VOCs catalyzed by Y
This can be explained that enough chemisorbed oxygen on Y O can catalytically oxidize not
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-LRH catalyst, only ethyl ether could generate the CTL emissions when its electronic
CHO (CH CHO*) returned to their ground states. In
can generate the strong CTL signals.
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3
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O
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only ethyl ether into CH CHO* but also other VOCs into electronic excited state
3
3+
intermediates of CO
in Y-NO
between CH
2 2
(CO *). Furthermore, the luminescent rare-earth Eu ions were doped
3
-LRHs to further enhance the CTL intensity through the efficient energy transfer
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+
3+
3
3
CHO* intermediates and Eu ions. Therefore, Eu -doped Y-NO -LRHs can be
used to detect ethyl ether with selective and sensitive CTL performances. These inspiring
results demonstrated that our approach could open up a new route to improve the CTL
selectivity by tuning chemisorbed oxygen on the surface of catalysts.
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Figure 1. Schematic illustration of the improved CTL selectivity by tuning the chemisorbed
oxygen on the surface of catalysts.
EXPERIMENTAL SECTION
Chemicals and Materials. All reagents were obtained from commercial sources and used
without further purification. All solutions were freshly prepared with deionized water (18.2
MU cm, Milli Q, Millipore, Barnstead, CA, USA). Analytical grade chemicals, including
3 3 2 3 3 2 3 2 3
Y(NO ) ·6H O, Eu(NO ) ·6H O, NaOH, NaNO , Na CO , ethyl ether, acetaldehyde, acetone,
butanone, formaldehyde, alcohol, isopropyl ether, acetonitrile, acetic acid, phenol and
cyclohexane were purchased from Beijing Chemical Reagent Company (Beijing, China).
High performance liquid chromatography (HPLC) grade acetonitrile was supplied by Merck
KGaA (Darmstadt, Germany). A 100 mM stock solution of ethyl ether was freshly prepared
by diluting 52 ꢀL of ethyl ether into 5 mL of deionized water.
Apparatus. The powder X-ray diffraction (XRD) measurements were performed on a
Bruker
(Germany)
D8
ADVANCE
X-ray
diffractometer
equipped
with
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graphite-monochromatized Cu Kα radiation (λ=1.5406 Ǻ). The 2θ angle of the diffractometer
was stepped from 5° to 70° with a scan rate of 10°/min. The FT-IR spectra were obtained with
a Nicolet 6700 FT-IR spectrometer (Thermo, USA) using the KBr disk technique. In situ
FT-IR experiments were carried out on a Nicolet 380 FT-IR spectrometer with a controlled
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environment chamber equipped with CaF windows (Thermo, USA). The scanning electron
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microscopy (SEM) images were obtained on a Hitachi (Japan) S-4700 field-emission
scanning electron microscope equipped with energy dispersive X-ray (EDX) spectroscopy.
Specific surface areas were measured using an Autosorb-IQ-MP nitrogen adsorption
apparatus (Quantachrome, USA) based on the Brunauer-Emmett-Teller (BET) method from
N adsorption-desorption isotherms at 77 K. XPS measurements were performed on a photo
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electron spectrometer (VG ESCALAB MKII, Thermo ESCALAB 250, USA) at 2 × 10 Pa
using Al K X-ray as excitation source. The GC-MS experiments were carried out on a
Thermo Trace 1300-ISQ GC-MS system (Thermo, USA) equipped with a TR-5MS column
(length=30 m, inner diameter=0.25 mm, film thickness=0.25 m). Thermal gravity analysis
TGA) was measured on a TGA/DSC 1/1100 SF (Mettler, Toledo) in air atmosphere at a
heating rate of 10 °C/min. A biophysics chemiluminescence (BPCL) luminescence analyzer
Institute of Biophysics, Chinese Academy of Science, Beijing, China) was used to detect
α
ꢀ
(
(
CTL intensity. The CTL spectra of the CTL process were obtained using a Hitachi F-7000
fluorescence (FL) spectrophotometer (Tokyo, Japan) at a slit of 20 nm and a scanning rate of
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000 nm/min (the excitation lamp was off). The UV-visible spectra were measured on a USB
000 miniature fiber optic spectrometer in absorbance mode with a DH-2000 deuterium and
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tungsten halogen light source (Ocean Optics, Dunedin, USA). The heater controller
(Shenzhen Hengtai Electric Equipment Factory, China) was used to provide heater for the
ceramic rod. Volatile organic gases were transported by an air pump (Beijing Zhongxing
Huili Co. Ltd., Beijing, China). The values of pH were detected using a Five Easy pH meter
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(Mettler Toledo Instrument Company, Shanghai, China). The centrifugation was operated on a
Z-326K centrifugal machine (Hermle Labortechnik GmbH, Germany).
2 3
Synthesis of LRHs and Y O Catalysts. The LRH catalysts were synthesized by a
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hydrothermal route according to literature procedures with a little modification. The
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preparation was performed under a N atmosphere to exclude CO in aqueous solution.
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3 3 2 2
Briefly, solution A was prepared by dissolving 6.0 mmol Y(NO ) ·6H O in 30 mL of de-CO
and deionized water to avoid the formation of Y-CO
3
-LRHs. Solution B was prepared by
and deionized water.
dissolving 15.0 mmol NaOH and 4.3 mmol NaNO in 30 mL of de-CO
3
2
Then solution A and solution B were added dropwise at the same time to a 250 mL flask
under continuously stirring, maintaining pH 8.5 at room temperature in a N atmosphere. The
2
resulting white slurry was continuously stirred in the 250 mL flask for another 0.5 h under N
atmosphere. The obtained mixture was transferred into a 100 mL Teflon-lined stainless steel
autoclave and then was kept for 24 h at 120 °C. Finally, the obtained white Y-NO -LRH
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precipitates at room temperature were centrifuged, washed thoroughly with de-CO and
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deionized water for three times, and dried in a vacuum oven at 60 °C for 24 h. The resulting
Y-NO
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-LRH catalysts were ground into powder and calcined at 700 °C for 2 h with a heating
rate of 5 °C/min at the ramp stage to obtain Y
2
O
3
catalysts. In addition, for the preparation of
3
+
3+
Eu -doped Y-NO
3
-LRHs, a series of Y(1-x)Eu
x
3
-NO -LRHs with different Eu contents were
synthesized by adding Y(NO ) ·6H O (
a
M) and Eu(NO ) ·6H O (b M, in which a + b = 0.2
3
3
2
3 3
2
M;
CTL Measurements. The as-prepared 0.2 g catalyst powder (LRHs, Y
Y-NO -LRHs) was dispersed in deionized water to form a suspension and coated onto a
x = b/( a + b ) × 100 % ) to the solution A.
3+
2
3
O or Eu -doped
3
cylindrical ceramic heater (inner diameter=5 mm, length=8 cm, Shanghai Anting Factory,
Shanghai, China) to form a catalyst layer. The ceramic heater was then put into a quartz tube
(diameter=1 cm, length=10 cm, Institute of Chemistry, Chinese Academy of Science, Beijing,
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China). A certain concentration of ethyl ether solution (25 ꢀL) was injected into gasification
chamber and then reached a close reaction cell through the steady air flow from an air pump
at 200 mL/min. A BPCL ultraweak chemiluminescence analyzer was applied to monitor the
CTL signals with a working voltage of -1000 V and data integration time 1 s per spectrum.
Finally, the CTL signals were imported to a computer for data acquisition. The relative CTL
signals were obtained by subtracting thermal noise.
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In Situ FT-IR Measurements. In situ FT-IR measurements were employed to investigate
the CTL reaction intermediates of VOCs during the CTL process. The synthesized catalysts in
the form of compressed self-supporting pellets were placed into the thermostatic reactor
mounted in the cell compartment of FT-IR spectrometer. The background spectra were
collected at room temperature and 175 °C by a temperature programmed process. After
cooling to room temperature, the catalyst was exposed in air flow containing VOCs for 1 h
until adsorption saturation, and the physisorbed VOCs on the surface of catalyst were
removed by blowing with air for 30 min. Then the temperature was raised from room
temperature to 175 °C at a heating rate of 10 °C/min and remained the temperature of 175 °C
for 30 min. The FT-IR spectra were obtained at certain intervals. Each spectrum was
-1
measured at 8 cm spectral resolution and 32 scanning numbers.
GC-MS Measurements. GC-MS measurements were carried out to identify the CTL
reaction products of VOCs on the surface of catalysts. During the GC-MS experiments, 25 ꢀL
of 100 mM VOCs was repeatedly injected into the CTL system for 5 h and 10 mL HPLC
grade acetonitrile was used to collect the reaction products during the CTL reaction. The flow
rate of air carrier was kept as low as 50 mL/min to prevent the overflow of reaction products.
RESULTS AND DISCUSSION
Characterization of Y-NO
3
2 3
-LRHs and Y O . As shown in Figure 2A, the crystal
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structure and phase composition of the Y-NO
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-LRHs were characterized by XRD, the
appearance of strong (002) and (004) characteristic reflections of the LRH-type materials
26
at 10.8° and 19.6° indicated a well ordered structure within the layers. The typical SEM
image showed that LRHs were composed of microcrystalline particles with a plate-like
morphology (insert of Figure 2A). The TGA curve was steady and no mass variation
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occurred when the Y-NO
decomposition of Y-NO
indicated that the Y-NO
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-LRH powders were calcined at 700 ˚C, meaning the complete
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-LRHs (Figure S1). As shown in Figure 2B, the XRD pattern
3
2 3
-LRHs after calcination were made of pure Y O with a series of
diffraction peaks at 29.3°, 33.9°, 48.7° and 57.8° (JCPDS No. 89-5591), and the peak at 10.8°
from the Y-NO -LRHs could not be seen, indicating that the lamellar structure of the
3
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precursor disappeared during the calcination process. And the SEM image of the obtained
showed that the Y-NO -LRHs layers collapsed into small pieces after calcination (inset
of Figure 2B). The results of XRD and SEM indicated the forming of Y . In addition,
FT-IR spectra were also used to confirm the formation of Y O . Figure S2 indicated that the
Y
2
O
3
3
2
O
3
2
3
-1
absorption peaks at 1361 and 3430 cm , assigned to interlayer nitrate groups and O-H
stretching vibration, respectively, were sharply decreased when Y-NO -LRHs were calcined
3
2
0
2 3
to obtain Y O .
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Chemisorbed oxygen plays an important role in the CTL process.
In this work, the O 1s
XPS spectra were detected in order to compare the differences of the chemisorbed oxygen
between Y-NO -LRHs and Y O . Figure 2C showed three O 1s peaks of the Y-NO -LRHs.
3
2
3
3
-
-
2 3
They were attributed to H O (533.2 eV), chemisorbed oxygen (Oad, 532.0 eV) and OH /NO
3
0-33
groups (531.5 eV), respectively.
However, for Y
2 3 2
O (Figure 2D), the O 1s peaks of H O
-
-
and OH /NO
3
groups were disappeared because of high-temperature calcination. A new peak
34
at 529.2 eV was observed, attributed to the crystal lattice oxygen of Y-O. Table S1 showed
that the percentage of the chemisorbed oxygen on the surface of Y-NO -LRHs (10.07%) was
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much less than Y
Generally, the catalytic properties of catalysts depend on the surface area.35 The BET
measurements indicated that there seems no big gap between Y-NO -LRHs and Y
Table S2). In addition, the basic sites of catalysts play an important role on the intensity of
2 3
O (27.69%).
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8
9
0
3
6
CTL signals. Therefore, we increased the amount of basic sites of LRHs by changing
-
2- 37
intercalated anions from NO
3 3
to CO . In principle, the basic character of catalysts can be
evaluated by studying on its adsorbing capacity for phenol in cyclohexane solution, and
38
higher phenol adsorption amount indicates stronger basic character of catalysts. Therefore,
the surface basic sites between Y-CO -LRHs and Y-NO -LRHs were compared by studying
3
3
the adsorption isotherms of phenol in cyclohexane solution in Figure S3. The adsorption
equilibrium concentrations of phenol were measured by a UV-visible spectrometer (
72 nm). It can be seen that the adsorption capacity of phenol adsorbed on the surface of
Y-CO -LRHs (0.225 mmol/g) was more than that of Y-NO -LRHs (0.181 mmol/g), meaning
λ
max
=
2
3
3
that the amount of basic sites on the surface of Y-CO -LRHs was indeed more than that of
3
Y-NO -LRHs. Figure S4 showed that Y-CO -LRHs still showed a good selectivity towards
3
3
ethyl ether. In conclusion, the high selectivity of the proposed CTL reaction was mainly
ascribed to the chemisorbed oxygen on the surface of LRHs, independent of the surface area
and the basic sites of catalysts.
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3 2 3
Figure 2. Powder XRD patterns and SEM images of (A) Y-NO -LRHs and (B) Y O ; XPS
spectra of O 1s on the surface of (C) Y-NO -LRHs and (D) Y O .
3
2
3
Mechanism of Improved CTL Selectivity of Y-NO
3
-LRHs. The synthesized
Y-NO -LRH and Y catalysts were coated onto a cylindrical ceramic heater for the CTL
3
2 3
O
measurements, respectively. Nine kinds of VOCs, including ethyl ether, acetaldehyde,
acetone, butanone, formaldehyde, alcohol, isopropyl ether, acetonitrile and acetic acid (each
20.0 mM), were separately injected into the proposed flow system in Figure S5. The CTL
signals in Figure 3 indicated that Y-NO
3
-LRHs showed an excellent selectivity towards ethyl
ether among the tested VOCs (Figure 3A); however, some of the tested VOCs exhibited the
strong CTL signals on the surface of Y
2
O
3
(Figure 3B).
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0
3
Figure 3. CTL intensities of nine kinds of VOCs on the surface of (A) Y-NO -LRHs and (B)
2 3
Y O , (a) ethyl ether, (b) acetone, (c) butanone, (d) acetaldehyde, (e) formaldehyde, (f)
alcohol, (g) isopropyl ether, (h) acetonitrile, (i) acetic acid. Experimental conditions: VOC
concentration, 20.0 mM; working temperature, 175 °C; air flow rate, 200 mL/min.
We studied the luminescence intermediates of CTL reactions of the tested VOCs on the
Y-NO
spectrophotometer without the use of the excitation lamp. The CTL spectrum of ethyl ether on
the surface of Y-NO -LRHs showed only one emission band in the range of 400-500 nm with
a maximum emission wavelength located at ~430 nm (Figure 4A), indicating the emitter may
3
-LRH surface. The CTL spectra of VOCs were measured using the FL
3
3
9
be CH
owing to the low CTL signals. On the other hand, the intermediates of ethyl ether and acetone
CTL reactions on the surface of Y-NO -LRHs were confirmed by in situ FT-IR technique.
3
CHO*. However, the CTL spectra of the other tested VOCs can not be obtained
3
-1
Figure 5A showed that there was an obvious increase in the sharp feature at 1724 cm when
the reaction time was increased from 5 to 30 min at 175 °C for ethyl ether catalytic oxidation
-1
reaction on the surface of Y-NO -LRHs. The band at 1724 cm was assigned to the C=O
3
40
-1
stretch vibration in CH
acetone catalytic oxidation reaction on the surface of Y-NO
CH CHO* intermediates were generated in the CTL reaction (Figure 5B). Finally, the
3
CHO. However, the band at 1724 cm did not change for the
3
-LRHs, meaning that no
3
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products of ethyl ether and acetone CTL reaction on the surface of Y-NO -LRHs were further
verified by GC-MS measurements. Figure S6A showed that there were two peaks at 1.59 and
.73 min in the GC chromatogram from the CTL reaction of ethyl ether. The first one was
1
attributed to unreacted ethyl ether, and the second one was ascribed to the CTL product. In the
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full scan mass spectra, the main fragment ions for the second peak were m/z 44, 29 and 15,
3
9
which were assigned to CH
3
CHO (inset of Figure S6A). In contrast, when acetone was
catalyzed on the surface of Y-NO
3
-LRHs, there was only one peak at 2.28 min in the GC
chromatogram attributed to unreacted acetone (Figure S7A). In addition, no other CTL
products were generated. Therefore, the luminescent intermediates of the ethyl ether CTL
reaction on the surface of Y-NO -LRHs were CH CHO*.
3
3
We also explored the intermediates of CTL reactions of the tested VOCs on the Y
2 3
O
surface. The CTL spectrum of ethyl ether showed a maximum emission wavelength at ~430
3
9
nm, indicating the formation of CH
3
CHO* intermediates (Figure 4B); but the CTL spectra
of acetaldehyde, acetone and butanone on the surface of Y O exhibited a maximum emission
2
3
wavelength at ~500 nm, which normally was the electronic excited state of CO (CO *)
2
2
41
(
Figure 4B). In situ FT-IR technique was also carried out to confirm the appearance of these
-1
intermediates. Figure 5C showed there were obvious increases at 2360, 2341 and 1731 cm
when ethyl ether was catalyzed on the surface of Y
2
O
3
at different reaction time. The two
-1
bands at 2360 and 2341 cm were attributed to the presence of CO gases, and the band at
2
-
1
40,42
1
731 cm was ascribed to the C=O stretch vibration in CH CHO.
In comparison, for the
, there were only changes at 2360
was produced in the whole CTL process (Figure 5D).
The products of the ethyl ether and acetone CTL reaction on the Y were also detected by
3
2 3
acetone catalytic oxidation reaction on the surface of Y O
-1
and 2341 cm , indicating that only CO
2
2
O
3
GC-MS. Figure S6B showed that there were two peaks at 1.59 and 1.73 min in the GC
chromatogram when ethyl ether was injected into the present system. The first one was
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attributed to unreacted ethyl ether, and the second one was ascribed to the CTL product. In the
full scan mass spectra, the main fragment ions for the second peak were m/z 44, 29 and 15,
39
3
which were assigned to CH CHO (inset of Figure S6B). Therefore, the emitter of the ethyl
ether CTL reaction on the surface of Y O was CH CHO*. In contrast, there were two peaks
2
3
3
0
1
2
3
4
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0
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0
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0
at 2.28 and 6.69 min in the GC chromatogram when acetone was catalyzed on the surface of
(Figure S7B). The first one was attributed to remanent acetone, and the second one was
ascribed to the CTL product. In the full scan mass spectra, the main fragment ions for the
2 3
Y O
43
second peak were m/z 55, 83 and 98, assigned to mesityl oxide (inset of Figure S7B). The
produced mesityl oxide was further decomposed into CO , accompanying with the CTL
2
3
6
emissions. Therefore, the emitter of the ethyl ether CTL reaction on the surface of Y O was
2
3
CH
In summary, ethyl ether was catalytically oxidized on the surface of both Y-NO
to produce light as follows:
CH CH OCH CH + O CH CHO*
3 2
CHO* and the luminescent intermediate of the other VOCs was CO *.
3
-LRHs and
2 3
Y O
→
(1)
(2)
3
2
2
3
2
3
CH CHO*
→ CH CHO + hν
3
3
Other VOCs except for ethyl ether were catalytically oxidized on the surface of Y
emit light as follows:
VOCs + O CO
CO2* CO + hν
2 3
O to
2
→
2
*
(3)
(4)
→
2
In conclusions, Y O had more chemisorbed oxygen on the surface, facilitating the catalytic
2
3
oxidation of VOCs to generate CTL through the different reaction routes. Ethyl ether was
catalytically oxidized to produce CH CHO* on the surface of Y , and the other VOCs were
completely oxidized to generate CO *. In contrast, Y-NO -LRHs had less chemisorbed
3
2 3
O
2
3
oxygen on the surface, only ethyl ether could be catalytically oxidized to produce the CTL by
the formation of CH CHO*, but it is hard to totally oxidize other VOCs to generate the CTL
3
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through CO * as a result of insufficient chemisorbed oxygen on the Y-NO -LRH surface.
2
3
Therefore, the difference of CTL selectivity towards ether ethyl between Y-NO
was attributed to the chemisorbed oxygen on the surface of catalysts.
3
-LRHs and
2 3
Y O
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Figure 4. (A) CTL spectrum of ethyl ether on the surface of Y-NO -LRHs; (B) CTL spectra
3
of ethyl ether, acetaldehyde, acetone and butanone on the surface of Y O . Experimental
2
3
conditions: VOC concentration, 300 mM; working temperature, 175 °C; air flow rate, 200
mL/min.
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Figure 5. In situ FT-IR spectra of (A) ethyl ether and (B) acetone CTL reaction on the surface
of Y-NO
3 2 3
-LRHs, (C) ethyl ether and (D) acetone CTL reaction on the surface of Y O at
different reaction time (175 °C).
0
1
2
3
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0
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0
3
+
Eu -Doped-LRH-Enhanced CTL. In general, doping suitable luminescent rare-earth
ions into catalysts have been demonstrated to be an effective way to further improve the
4
4,45
3+
CTL signals through chemiluminescence energy transfer.
Usually, Y ions on the
layers of Y-NO -LRHs could be conveniently exchanged by other luminescent rare-earth
3
2
9
3+
ions. More importantly, the FL excitation spectrum of luminescent rare-earth ion of Eu is
4
6
from 350 to 500 nm, which is overlapped with the CTL spectrum of CH
3
CHO*
intermediates (maximum at ~430 nm). Therefore, in order to further improve the CTL
3
+
intensity of ethyl ether in the proposed system, the luminescent rare-earth ion of Eu was
chosen to be doped in Y-NO -LRHs.
3
The XRD patterns of a series of Y(1-x)Eu -NO -LRHs were displayed in Figure S8, and the
x
3
3+
(220) basal spacings of these LRHs were tabulated in Table S3. It can be concluded that Eu
ions located on the layer with different concentrations have no effect on the crystal phase.
3
+
Note that the (220) basal spacing increased as the Eu content increased because of a slightly
3
+
3+ 26
large ionic radius for Eu in comparison to Y . In addition, the SEM image of
Y Eu -NO -LRHs showed that the plate-like morphology was still maintained (Figure
0
.9
0.1
3
S9A). The EDX spectrum of Y0.9Eu0.1-NO
3
-LRHs clearly indicated the existence of Y and Eu
elements, and the average atomic ratio of Eu to Y was about 1.1:10, which was close to that
3+
added in the synthesis process (Figure S9B and Table S4). These results revealed that the Eu
ions were successfully doped in the LRHs.
3+
Figure S10 indicated the CTL intensities were quite different under various Eu doping
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+
concentrations in Y-NO
3
3
-LRHs. It can be seen that the Y-NO -LRHs with 10% Eu doping
3+
concentration had the maximal CTL intensity. Furthermore, a further increase of Eu
concentration led to a decrease in the CTL intensity, which may originate from the energy
3+
47
transfer between the excited and unexcited Eu ions. Finally, a good CTL selectivity of
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Y Eu -NO -LRHs towards ethyl ether was illustrated in Figure S11.
0
.9
0.1
3
In order to clarify the origin of the improved CTL intensity of ethyl ether on the surface of
3+
3 3
Eu -doped Y-NO -LRHs, we compared the CTL spectra of the Y-NO -LRHs in the presence
3
+
3+
and absence of Eu . After doping with 10% Eu , the CTL spectrum of ethyl ether on the
surface of Y Eu -NO -LRHs showed that the CTL peak at ~430 nm attributed to
0.9
0.1
3
CH CHO* disappearred, and an new CTL peak at ~620 nm was observed (Figure 6A). These
3
resutls indicated the formation of a new emitter. Next, we investigated the FL spectra of
Y0.9Eu0.1-NO
3
-LRHs (Figure 6B). Under 396 nm excitation, the two strong FL emission peaks
appeared at around 595 and 619 nm. They were the characteristic peaks of the emission bands
arising from 5D0 7F magnetic-dipole transitions and 5D0 7F forced electric-dipole
transitions, respectively.
→
→
1
2
48,49
The CTL emission at ~620 nm (Figure 6A) almostly overlapped
3+
the FL emission of Y0.9Eu0.1-NO
in Y0.9Eu0.1-NO
3
-LRHs at 619 nm (Figure 6B), indicating that the Eu ions
3
-LRHs could act as efficient energy acceptors with 58.1% transfer efficiency
3+
3 3
for CH CHO* intermediates. Therefore, the Eu -doped Y-NO -LRHs could further improve
the CTL signals via the efficient energy transfer, accompanying with high selectivity towards
ethyl ether as follows:
CH
3
3
CH
2
OCH
2
CH
3
+ O
2
→
CH
3
CHO*
(5)
(6)
(7)
3+
3+
CH
CHO* + Eu
→
3
CH CHO + Eu *
3
+
3+
Eu *
→ Eu + hν
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Figure 6. (A) CTL spectrum of ethyl ether on the surface of Y0.9Eu0.1-NO -LRHs; (B) FL
excitation spectrum obtained under emission at 619 nm and emission spectrum obtained under
excitation at 396 nm of Y Eu -NO -LRHs.
0.9
0.1
3
Analytical Performances. Working temperature and air flow rate were optimized to
further improve the CTL intensity. The effect of working temperature on the CTL intensity
was studied in the range of 145-205 °C (Figure S12A). The CTL intensity of ethyl ether
increased gradually when the temperature increased from 145 to 175 °C and then decreased
when the temperature was higher than 175 °C. The influence of carrier gas flow rate on the
CTL intensity was examined in the range from 50 to 350 mL/min (Figure S12B). The CTL
intensity of ethyl ether increased gradually when the air flow rate increased from 50 to 200
mL/min and then decreased when the air flow rate was higher than 200 mL/min. Therefore,
175 °C and 200 mL/min were chosen as the optimal working temperature and air flow rate,
respectively.
Under the optimal conditions, the CTL intensity of ethyl ether improved with an increase in
the concentration and was saturated at about 300 mM (Figure S13A). The calibration curve of
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the CTL intensity versus ethyl ether concentration was liner in the range of 1.0-100 mM. The
2
linear regression equation was described as I=994.1C – 240.42 (R =0.9994), where I was the
CTL intensity and C was the concentration of ethyl ether, respectively (Figure S13B). And the
detection limit of ethyl ether was as low as 0.5 mM (S/N=3, Figure S14A). The dynamic
response analysis of the present CTL system was investigated by injecting 5.0 mM ethyl ether
into the present system. It can be found that the CTL intensity rapidly increased from the
baseline to the maximum value within two seconds after the sample injection, indicating a
rapid ethyl ether CTL reaction (Figure S14B). In addition, the operational reproducibility was
investigated by continual injections of 20.0 mM ethyl ether into the CTL system for 20 times
(Figure S15). The relative standard deviation (RSD) was 2.3%.
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CONCLUSIONS
In conclusion, we demonstrated that the CTL selectivity can be improved by tunning
2 3
chemisorbed oxygen of catalysts. The CTL selectivity of Y O towards VOCs was
remarkably unsatisfied because Y possessed more chemisorbed oxygen. However, only
2
O
3
ethyl ether exhibited a strong CTL signal on the surface of LRHs with less chemisorbed
oxygen among all the tested VOCs. In addition, the CTL intensity of ethyl ether could be
3
+
further improved when the luminescent rare-earth Eu ions were doped in Y-NO
improved CTL intensity was attributed to the energy transfer from CH CHO* intermediates to
-LRHs and the CTL signal was discovered to be directly proportional to
3
-LRHs. The
3
3
+
Eu ions in Y-NO
3
the concentration of ethyl ether. The excellent CTL performances made the LRHs promising
for potential applications in sensing ethyl ether. More importantly, we believe that this work
is not only of importance for a better understanding how chemisorbed oxygen affects the CTL
selectivity but also of great potential to be extended to selectively detect other VOCs by
changing the chemisorbed oxygen of catalysts through suitable methods.
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ASSOCIATED CONTENT
Supporting Information Available
0
1
2
3
4
5
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9
0
1
2
3
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9
0
TGA curve of Y-NO
for phenol from cyclohexane solution on Y-CO
Y-CO -LRHs; schematic diagram of the CTL configuration system; GC-MS chromatograms
-LRHs; FT-IR spectra of Y-NO
-LRHs and Y
2 3
O ; adsorption isotherms
3
3
3
-LRHs and Y-NO
3
-LRHs; CTL selectivity of
3
from the CTL reaction of ethyl ether on the surface of Y-NO
chromatogram from the CTL reaction of acetone on the surface of Y-NO
powder XRD patterns of Y(1-x)Eu -NO -LRHs; SEM image and EDX spectrum of
-LRHs; effect of Eu -doping concentration on the CTL intensity; CTL
selectivity of Y Eu -NO -LRHs; effect of working temperature and air flow rate on the
3
-LRHs and Y
2 3
O ; GC-MS
3
2 3
-LRHs and Y O ;
x
3
3+
Y0.9Eu0.1-NO
3
0
.9
0.1
3
CTL intensity; CTL intensity of different concentrations of ethyl ether on
-LRHs surface and calibration curve between CTL intensity and ethyl ether
concentration; CTL response profile of ethyl ether on the surface of Y0.9Eu0.1-NO -LRHs at
Y0.9Eu0.1-NO
3
3
the limit of detection (0.5 mM) and kinetic CTL intensity-time profile of 5.0 mM ethyl ether;
relative CTL intensity of the repeated injections of 20.0 mM ethyl ether on the surface of
Y
0.9Eu0.1-NO
3
-LRHs; binding energy and percentage of different chemical states of O 1s on
the surface of Y-NO -LRHs and Y ; BET of Y-NO -LRHs and Y ; the (220) basal
-NO -LRH powder; EDX elemental analysis of Y0.9Eu0.1-NO -LRHs.
3
2
O
3
3
2 3
O
spacings of Y(1-x)Eu
x
3
3
This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: luchao@mail.buct.edu.cn. Fax/Tel.: +86 10 64411957.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by the National Basic Research Program of China (973 Program,
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014CB932103), the National Natural Science Foundation of China (21375006 and
1575010), and Innovation and Promotion Project of Beijing University of Chemical
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Technology.
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