Chemisorbed Oxygen on the Surface of Catalyst ... - ACS Publications

Subscriber access provided by Weizmann Institute of Science

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

Chemisorbed

Oxygen

on

the

Surface

of

Catalyst–Improved

Cataluminescence Selectivity

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: [email protected]

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 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 towards 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.

2


Page 2 of 25

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Cataluminescence (CTL) refers to a kind of chemiluminescence from the surface of solid catalysts during heterogeneous catalytic oxidation reactions.1,2 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) proper reaction temperature.3-5 Over the past few decades, much attention has been paid to improving the CTL selectivity, which mainly concentrated on exploiting new solid catalysts or decreasing CTL reaction temperature.6-9 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 catalytic properties.10 Among them, Y2O3 has a C-type rare-earth oxide crystal structure, and oxygen vacancies are the major defects.11 With the abundant base catalytic active sites and oxygen vacancies to chemisorb oxygen on its surface,12,13 Y2O3 has been widely used as a common CTL catalyst in gas sensing.14-17 Y2O3 is mainly synthesized through calcining its precursors.18-20 Among them, layered rare-earth hydroxides (LRHs) are often used as Y2O3 precursors,19,20 which are layered metal hydroxides with a general formulae of R2(OH)5X·nH2O, wherein R is rare-earth ions on the layers and X substitutes for an extensive choice of intercalated anions.21,22 During the calcination process, abundant oxygen vacancies on the surface of Y2O3 are produced, facilitating the formation of more chemisorbed oxygen.23,24 Therefore, the variable content of chemisorbed oxygen between Y2O3 and LRHs will lead to the difference of catalytic oxidation activity.25 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

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 chemisorbed oxygen between Y2O3 and LRHs, it is reasonable to employ them as catalyst models to deeply investigate the influence of chemisorbed oxygen on the CTL selectivity. In this work, Y-NO3-LRHs showed a perfect selectivity towards ethyl ether among nine kinds of common volatile organic compounds (VOCs) in comparison with Y2O3. 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-NO3-LRHs than Y2O3 (Figure 1). In brief, for Y-NO3-LRH catalyst, only ethyl ether could generate the CTL emissions when its electronic excited state intermediates of CH3CHO (CH3CHO*) returned to their ground states. In contrast, many of the tested VOCs catalyzed by Y2O3 can generate the strong CTL signals. This can be explained that enough chemisorbed oxygen on Y2O3 can catalytically oxidize not only ethyl ether into CH3CHO* but also other VOCs into electronic excited state intermediates of CO2 (CO2*). Furthermore, the luminescent rare-earth Eu3+ ions were doped in Y-NO3-LRHs to further enhance the CTL intensity through the efficient energy transfer between CH3CHO* intermediates and Eu3+ ions. Therefore, Eu3+-doped Y-NO3-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.

4


Page 4 of 25

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 Y(NO3)3·6H2O, Eu(NO3)3·6H2O, NaOH, NaNO3, Na2CO3, 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 µL 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

5


equipped

with


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 environment chamber equipped with CaF2 windows (Thermo, USA). The scanning electron 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 N2 adsorption-desorption isotherms at 77 K. XPS measurements were performed on a photo electron spectrometer (VG ESCALAB MKII, Thermo ESCALAB 250, USA) at 2 × 10-9 Pa using Al Kα 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 µm). 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 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 3000 nm/min (the excitation lamp was off). The UV-visible spectra were measured on a USB 4000 miniature fiber optic spectrometer in absorbance mode with a DH-2000 deuterium and 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

6


Page 6 of 25

Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Mettler Toledo Instrument Company, Shanghai, China). The centrifugation was operated on a Z-326K centrifugal machine (Hermle Labortechnik GmbH, Germany). Synthesis of LRHs and Y2O3 Catalysts. The LRH catalysts were synthesized by a hydrothermal route according to literature procedures with a little modification.26 The preparation was performed under a N2 atmosphere to exclude CO2 in aqueous solution. Briefly, solution A was prepared by dissolving 6.0 mmol Y(NO3)3·6H2O in 30 mL of de-CO2 and deionized water to avoid the formation of Y-CO3-LRHs. Solution B was prepared by dissolving 15.0 mmol NaOH and 4.3 mmol NaNO3 in 30 mL of de-CO2 and deionized water. 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 N2 atmosphere. The resulting white slurry was continuously stirred in the 250 mL flask for another 0.5 h under N2 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-NO3-LRH precipitates at room temperature were centrifuged, washed thoroughly with de-CO2 and deionized water for three times, and dried in a vacuum oven at 60 °C for 24 h. The resulting Y-NO3-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 Y2O3 catalysts. In addition, for the preparation of Eu3+-doped Y-NO3-LRHs, a series of Y(1-x)Eux-NO3-LRHs with different Eu3+ contents were synthesized by adding Y(NO3)3·6H2O (a M) and Eu(NO3)3·6H2O (b M, in which a + b = 0.2 M; x = b/( a + b ) × 100 % ) to the solution A. CTL Measurements. The as-prepared 0.2 g catalyst powder (LRHs, Y2O3 or Eu3+-doped Y-NO3-LRHs) was dispersed in deionized water to form a suspension and coated onto a 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,

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

China). A certain concentration of ethyl ether solution (25 µL) 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. 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 measured at 8 cm-1 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 µL 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-NO3-LRHs and Y2O3. As shown in Figure 2A, the crystal 8


Page 8 of 25

Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


structure and phase composition of the Y-NO3-LRHs were characterized by XRD, the appearance of strong (002) and (004) characteristic reflections of the LRH-type materials at 10.8° and 19.6° indicated a well ordered structure within the layers.26 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 occurred when the Y-NO3-LRH powders were calcined at 700 ˚C, meaning the complete decomposition of Y-NO3-LRHs (Figure S1).27 As shown in Figure 2B, the XRD pattern indicated that the Y-NO3-LRHs after calcination were made of pure Y2O3 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-NO3-LRHs could not be seen, indicating that the lamellar structure of the precursor disappeared during the calcination process.19 And the SEM image of the obtained Y2O3 showed that the Y-NO3-LRHs layers collapsed into small pieces after calcination (inset of Figure 2B). The results of XRD and SEM indicated the forming of Y2O3. In addition, FT-IR spectra were also used to confirm the formation of Y2O3. Figure S2 indicated that the absorption peaks at 1361 and 3430 cm-1, assigned to interlayer nitrate groups and O-H stretching vibration, respectively, were sharply decreased when Y-NO3-LRHs were calcined to obtain Y2O3.20 Chemisorbed oxygen plays an important role in the CTL process.28,29 In this work, the O 1s XPS spectra were detected in order to compare the differences of the chemisorbed oxygen between Y-NO3-LRHs and Y2O3. Figure 2C showed three O 1s peaks of the Y-NO3-LRHs. They were attributed to H2O (533.2 eV), chemisorbed oxygen (Oad, 532.0 eV) and OH-/NO3groups (531.5 eV), respectively.30-33 However, for Y2O3 (Figure 2D), the O 1s peaks of H2O and OH-/NO3- groups were disappeared because of high-temperature calcination. A new peak at 529.2 eV was observed, attributed to the crystal lattice oxygen of Y-O.34 Table S1 showed that the percentage of the chemisorbed oxygen on the surface of Y-NO3-LRHs (10.07%) was

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

much less than Y2O3 (27.69%). Generally, the catalytic properties of catalysts depend on the surface area.35 The BET measurements indicated that there seems no big gap between Y-NO3-LRHs and Y2O3 (Table S2). In addition, the basic sites of catalysts play an important role on the intensity of CTL signals.36 Therefore, we increased the amount of basic sites of LRHs by changing intercalated anions from NO3- to CO32-.37 In principle, the basic character of catalysts can be evaluated by studying on its adsorbing capacity for phenol in cyclohexane solution, and higher phenol adsorption amount indicates stronger basic character of catalysts.38 Therefore, the surface basic sites between Y-CO3-LRHs and Y-NO3-LRHs were compared by studying the adsorption isotherms of phenol in cyclohexane solution in Figure S3. The adsorption equilibrium concentrations of phenol were measured by a UV-visible spectrometer (λmax = 272 nm). It can be seen that the adsorption capacity of phenol adsorbed on the surface of Y-CO3-LRHs (0.225 mmol/g) was more than that of Y-NO3-LRHs (0.181 mmol/g), meaning that the amount of basic sites on the surface of Y-CO3-LRHs was indeed more than that of Y-NO3-LRHs. Figure S4 showed that Y-CO3-LRHs still showed a good selectivity towards 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.

10


Page 10 of 25

Page 11 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Powder XRD patterns and SEM images of (A) Y-NO3-LRHs and (B) Y2O3; XPS spectra of O 1s on the surface of (C) Y-NO3-LRHs and (D) Y2O3.

Mechanism of Improved CTL Selectivity of Y-NO3-LRHs. The synthesized Y-NO3-LRH and Y2O3 catalysts were coated onto a cylindrical ceramic heater for the CTL 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-NO3-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 Y2O3 (Figure 3B).

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. CTL intensities of nine kinds of VOCs on the surface of (A) Y-NO3-LRHs and (B) Y2O3, (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-NO3-LRH surface. The CTL spectra of VOCs were measured using the FL spectrophotometer without the use of the excitation lamp. The CTL spectrum of ethyl ether on the surface of Y-NO3-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 be CH3CHO*.39 However, the CTL spectra of the other tested VOCs can not be obtained owing to the low CTL signals. On the other hand, the intermediates of ethyl ether and acetone CTL reactions on the surface of Y-NO3-LRHs were confirmed by in situ FT-IR technique. Figure 5A showed that there was an obvious increase in the sharp feature at 1724 cm-1 when the reaction time was increased from 5 to 30 min at 175 °C for ethyl ether catalytic oxidation reaction on the surface of Y-NO3-LRHs. The band at 1724 cm-1 was assigned to the C=O stretch vibration in CH3CHO.40 However, the band at 1724 cm-1 did not change for the acetone catalytic oxidation reaction on the surface of Y-NO3-LRHs, meaning that no CH3CHO* intermediates were generated in the CTL reaction (Figure 5B). Finally, the 12


Page 12 of 25

Page 13 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


products of ethyl ether and acetone CTL reaction on the surface of Y-NO3-LRHs were further verified by GC-MS measurements. Figure S6A showed that there were two peaks at 1.59 and 1.73 min in the GC chromatogram from the CTL reaction of ethyl ether. The first one was 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, which were assigned to CH3CHO (inset of Figure S6A).39 In contrast, when acetone was catalyzed on the surface of Y-NO3-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-NO3-LRHs were CH3CHO*. We also explored the intermediates of CTL reactions of the tested VOCs on the Y2O3 surface. The CTL spectrum of ethyl ether showed a maximum emission wavelength at ~430 nm, indicating the formation of CH3CHO* intermediates (Figure 4B);39 but the CTL spectra of acetaldehyde, acetone and butanone on the surface of Y2O3 exhibited a maximum emission wavelength at ~500 nm, which normally was the electronic excited state of CO2 (CO2*) (Figure 4B).41 In situ FT-IR technique was also carried out to confirm the appearance of these intermediates. Figure 5C showed there were obvious increases at 2360, 2341 and 1731 cm-1 when ethyl ether was catalyzed on the surface of Y2O3 at different reaction time. The two bands at 2360 and 2341 cm-1 were attributed to the presence of CO2 gases, and the band at 1731 cm-1 was ascribed to the C=O stretch vibration in CH3CHO.40,42 In comparison, for the acetone catalytic oxidation reaction on the surface of Y2O3, there were only changes at 2360 and 2341 cm-1, indicating that only CO2 was produced in the whole CTL process (Figure 5D). The products of the ethyl ether and acetone CTL reaction on the Y2O3 were also detected by 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

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

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, which were assigned to CH3CHO (inset of Figure S6B).39 Therefore, the emitter of the ethyl ether CTL reaction on the surface of Y2O3 was CH3CHO*. In contrast, there were two peaks at 2.28 and 6.69 min in the GC chromatogram when acetone was catalyzed on the surface of Y2O3 (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 second peak were m/z 55, 83 and 98, assigned to mesityl oxide (inset of Figure S7B).43 The produced mesityl oxide was further decomposed into CO2, accompanying with the CTL emissions.36 Therefore, the emitter of the ethyl ether CTL reaction on the surface of Y2O3 was CH3CHO* and the luminescent intermediate of the other VOCs was CO2*. In summary, ethyl ether was catalytically oxidized on the surface of both Y-NO3-LRHs and Y2O3 to produce light as follows: CH3CH2OCH2CH3 + O2 → CH3CHO*

(1)

CH3CHO* → CH3CHO + hν

(2)

Other VOCs except for ethyl ether were catalytically oxidized on the surface of Y2O3 to emit light as follows: VOCs + O2 → CO2*

(3)

CO2* → CO2 + hν

(4)

In conclusions, Y2O3 had more chemisorbed oxygen on the surface, facilitating the catalytic oxidation of VOCs to generate CTL through the different reaction routes. Ethyl ether was catalytically oxidized to produce CH3CHO* on the surface of Y2O3, and the other VOCs were completely oxidized to generate CO2*. In contrast, Y-NO3-LRHs had less chemisorbed oxygen on the surface, only ethyl ether could be catalytically oxidized to produce the CTL by the formation of CH3CHO*, but it is hard to totally oxidize other VOCs to generate the CTL

14


Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


through CO2* as a result of insufficient chemisorbed oxygen on the Y-NO3-LRH surface. Therefore, the difference of CTL selectivity towards ether ethyl between Y-NO3-LRHs and Y2O3 was attributed to the chemisorbed oxygen on the surface of catalysts.

Figure 4. (A) CTL spectrum of ethyl ether on the surface of Y-NO3-LRHs; (B) CTL spectra of ethyl ether, acetaldehyde, acetone and butanone on the surface of Y2O3. Experimental conditions: VOC concentration, 300 mM; working temperature, 175 °C; air flow rate, 200 mL/min.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. In situ FT-IR spectra of (A) ethyl ether and (B) acetone CTL reaction on the surface of Y-NO3-LRHs, (C) ethyl ether and (D) acetone CTL reaction on the surface of Y2O3 at different reaction time (175 °C).

Eu3+-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 CTL signals through chemiluminescence energy transfer.44,45 Usually, Y3+ ions on the layers of Y-NO3-LRHs could be conveniently exchanged by other luminescent rare-earth ions.29 More importantly, the FL excitation spectrum of luminescent rare-earth ion of Eu3+ is from 350 to 500 nm,46 which is overlapped with the CTL spectrum of CH3CHO* intermediates (maximum at ~430 nm). Therefore, in order to further improve the CTL intensity of ethyl ether in the proposed system, the luminescent rare-earth ion of Eu3+ was chosen to be doped in Y-NO3-LRHs. The XRD patterns of a series of Y(1-x)Eux-NO3-LRHs were displayed in Figure S8, and the (220) basal spacings of these LRHs were tabulated in Table S3. It can be concluded that Eu3+ ions located on the layer with different concentrations have no effect on the crystal phase. Note that the (220) basal spacing increased as the Eu3+ content increased because of a slightly large ionic radius for Eu3+ in comparison to Y3+.26 In addition, the SEM image of Y0.9Eu0.1-NO3-LRHs showed that the plate-like morphology was still maintained (Figure S9A). The EDX spectrum of Y0.9Eu0.1-NO3-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 added in the synthesis process (Figure S9B and Table S4). These results revealed that the Eu3+ ions were successfully doped in the LRHs. Figure S10 indicated the CTL intensities were quite different under various Eu3+ doping

16


Page 16 of 25

Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


concentrations in Y-NO3-LRHs. It can be seen that the Y-NO3-LRHs with 10% Eu3+ doping concentration had the maximal CTL intensity. Furthermore, a further increase of Eu3+ concentration led to a decrease in the CTL intensity, which may originate from the energy transfer between the excited and unexcited Eu3+ ions.47 Finally, a good CTL selectivity of Y0.9Eu0.1-NO3-LRHs towards ethyl ether was illustrated in Figure S11. In order to clarify the origin of the improved CTL intensity of ethyl ether on the surface of Eu3+-doped Y-NO3-LRHs, we compared the CTL spectra of the Y-NO3-LRHs in the presence and absence of Eu3+. After doping with 10% Eu3+, the CTL spectrum of ethyl ether on the surface of Y0.9Eu0.1-NO3-LRHs showed that the CTL peak at ~430 nm attributed to CH3CHO* disappearred, and an new CTL peak at ~620 nm was observed (Figure 6A). These resutls indicated the formation of a new emitter. Next, we investigated the FL spectra of Y0.9Eu0.1-NO3-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→7F1 magnetic-dipole transitions and 5D0→7F2 forced electric-dipole transitions, respectively.48,49 The CTL emission at ~620 nm (Figure 6A) almostly overlapped the FL emission of Y0.9Eu0.1-NO3-LRHs at 619 nm (Figure 6B), indicating that the Eu3+ ions in Y0.9Eu0.1-NO3-LRHs could act as efficient energy acceptors with 58.1% transfer efficiency for CH3CHO* intermediates. Therefore, the Eu3+-doped Y-NO3-LRHs could further improve the CTL signals via the efficient energy transfer, accompanying with high selectivity towards ethyl ether as follows: CH3CH2OCH2CH3 + O2 → CH3CHO*

(5)

CH3CHO* + Eu3+ → CH3CHO + Eu3+*

(6)

Eu3+* → Eu3+ + hν

(7)

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. (A) CTL spectrum of ethyl ether on the surface of Y0.9Eu0.1-NO3-LRHs; (B) FL excitation spectrum obtained under emission at 619 nm and emission spectrum obtained under excitation at 396 nm of Y0.9Eu0.1-NO3-LRHs.

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

18


Page 18 of 25

Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the CTL intensity versus ethyl ether concentration was liner in the range of 1.0-100 mM. The linear regression equation was described as I=994.1C – 240.42 (R2=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%.

CONCLUSIONS In conclusion, we demonstrated that the CTL selectivity can be improved by tunning chemisorbed oxygen of catalysts. The CTL selectivity of Y2O3 towards VOCs was remarkably unsatisfied because Y2O3 possessed more chemisorbed oxygen. However, only 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 further improved when the luminescent rare-earth Eu3+ ions were doped in Y-NO3-LRHs. The improved CTL intensity was attributed to the energy transfer from CH3CHO* intermediates to Eu3+ ions in Y-NO3-LRHs and the CTL signal was discovered to be directly proportional to 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.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

ASSOCIATED CONTENT Supporting Information Available TGA curve of Y-NO3-LRHs; FT-IR spectra of Y-NO3-LRHs and Y2O3; adsorption isotherms for phenol from cyclohexane solution on Y-CO3-LRHs and Y-NO3-LRHs; CTL selectivity of Y-CO3-LRHs; schematic diagram of the CTL configuration system; GC-MS chromatograms from the CTL reaction of ethyl ether on the surface of Y-NO3-LRHs and Y2O3; GC-MS chromatogram from the CTL reaction of acetone on the surface of Y-NO3-LRHs and Y2O3; powder XRD patterns of Y(1-x)Eux-NO3-LRHs; SEM image and EDX spectrum of Y0.9Eu0.1-NO3-LRHs; effect of Eu3+-doping concentration on the CTL intensity; CTL selectivity of Y0.9Eu0.1-NO3-LRHs; effect of working temperature and air flow rate on the CTL

intensity;

CTL

intensity

of

different

concentrations

of

ethyl

ether

on

Y0.9Eu0.1-NO3-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-NO3-LRHs at 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 Y0.9Eu0.1-NO3-LRHs; binding energy and percentage of different chemical states of O 1s on the surface of Y-NO3-LRHs and Y2O3; BET of Y-NO3-LRHs and Y2O3; the (220) basal spacings of Y(1-x)Eux-NO3-LRH powder; EDX elemental analysis of Y0.9Eu0.1-NO3-LRHs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax/Tel.: +86 10 64411957. Notes 20


Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program, 2014CB932103), the National Natural Science Foundation of China (21375006 and 21575010), and Innovation and Promotion Project of Beijing University of Chemical Technology.

REFERENCES (1) Wang, X.; Na, N.; Zhang, S. C.; Wu, Y. Y.; Zhang, X. R. J. Am. Chem. Soc. 2007, 129, 6062-6063. (2) Na, N.; Zhang, S. C.; Wang, S.; Zhang, X. R. J. Am. Chem. Soc. 2006, 128, 14420-14421. (3) Yang, P.; Ye, X. N.; Lau, C.; Li, Z. X.; Liu, X.; Lu, J. Z. Anal. Chem. 2007, 79, 1425-1432. (4) Zhang, L. C.; Song, H. J.; Su, Y. Y.; Lv, Y. TrAC Trends Anal. Chem. 2015, 67, 107-127. (5) Weng, Y. Y.; Zhang, L. C.; Zhu, W.; Lv, Y. J. Mater. Chem. A 2015, 3, 7132-7138. (6) Zhang, L. C.; Zhou, Q.; Liu, Z. H.; Hou, X. D.; Li, Y. B.; Lv, Y. Chem. Mater. 2009, 21, 5066-5071. (7) Song, H. J.; Zhang, L. C.; He, C. L.; Qu, Y.; Tian, Y. F.; Lv, Y. J. Mater. Chem. 2011, 21, 5972-5977. (8) Li, Z. H.; Xi, W.; Lu, C. Sens. Actuators B 2015, 219, 354-360. (9) Almasian, M. R.; Na, N.; Wen, F.; Zhang, S. C.; Zhang, X. R. Anal. Chem. 2010, 82, 3457-3459. (10) Hattori, H. Chem. Rev. 1995, 95, 537-558. (11) Jollet, F.; Noguera, C.; Gautier, M.; Thromat, N.; Duraud, J. P. J. Am. Ceram. Soc. 1991, 74, 358-364. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(12) Kogler, M.; Köck, E. M.; Perfler, L.; Bielz, T.; Stöger-Pollach, M.; Hetaba, W.; Willinger, M.; Huang, X.; Schuster, M.; Klötzer, B.; Penner, S. Chem. Mater. 2014, 26, 1690-1701. (13) Burbano, M.; Norberg, S. T.; Hull, S.; Eriksson, S. G.; Marrocchelli, D.; Madden, P. A.; Watson, G. W. Chem. Mater. 2012, 24, 222-229. (14) Cao, X. A,; Tao, Y.; Li, L. L.; Liu, Y. H.; Peng, Y.; Li, J. W. Luminescence 2011, 26, 5-9. (15) Rao, Z. M.; Liu, L. J.; Xie, J. Y.; Zeng, Y. Y. Luminescence 2008, 23, 163-168. (16) Wu, Y. Y.; Zhang, S. C.; Wang, X.; Na, N.; Zhang, Z. X. Luminescence 2008, 23, 376-380. (17) Zhang, Z. Y.; Xu, K.; Xing, Z.; Zhang, X. R. Talanta 2005, 65, 913-917. (18) Devaraju, M. K.; Yin, S.; Sato, T. Inorg. Chem. 2011, 50, 4698-4704. (19) Zhu, Q.; Li, J. G.; Zhi, C. Y.; Li, X. D.; Sun, X. D.; Sakka, Y.; Golberg, D.; Bando, Y. Chem. Mater. 2010, 22, 4204-4213. (20) Zhong, H. X.; Ma, Y. L.; Cao, X. F.; Chen, X. T.; Xue, Z. L. J. Phys. Chem. C 2009, 113, 3461-3466. (21) Geng, F. X.; Matsushita, Y.; Ma, R. Z.; Xin, H,; Tanaka, M.; Lzumi, F.; Lyi, N.; Sasaki, T. J. Am. Chem. Soc. 2008, 130, 16344-16350. (22) Gándara, F.; Perles, J.; Snejko, N.; Iglesias, M.; Gómez-Lor, B.; Gutiérrez-Puebla, E.; Monge, M. Á. Angew. Chem. 2006, 118, 8166-8169. (23) Garcia-Barriocanal, J.; Rivera-Calzada, A.; Varela, M.; Sefrioui, Z.; Iborra, E.; Leon, C.; Pennycook, S. J.; Santamaria, J. Science 2008, 321, 676-680. (24) Ding, B. F.; Qian, H. J.; Han, C.; Zhang, J. Y.; Lindquist, S. E.; Wei, B.; Tang, Z. L. J. Phys. Chem. C 2014, 118, 25633-25642. (25) Shi, J. L. Chem. Rev. 2013, 113, 2139-2181. (26) Wang, L.; Yan, D. P.; Qin, S. H.; Li, S. D.; Lu, J.; Evans, D. G.; Duan, X. Dalton Trans. 2011, 40, 11781-11787.

22


Page 22 of 25

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Lee, S. S.; Lee, B. I.; Kim, S. J.; Byeon, S. H.; Kang, J. K. Inorg. Chem. 2012, 51, 10222-10232. (28) Nakagawa, M.; Okabayashi, T.; Fujimoto, T.; Utsunomiya, K.; Yamamoto, I.; Wada, T.; Yamashita, Y.; Yamashita, N. Sens. Actuators B 1998, 51, 159-162. (29) Na, N.; Zhang, S. C.; Wang, X.; Zhang, X. R. Anal. Chem. 2009, 81, 2092-2097. (30) Rosenthal, D.; Ruta, M.; Schlögl, R.; Kiwi-Minsker, L. Carbon 2010, 48, 1835-1843. (31) Stoev, M.; Katerski, A. J. Mater. Chem. 1996, 6, 377-380. (32) Goh, K. H.; Lim, T. T.; Dong, Z. L. Environ. Sci. Technol. 2009, 43, 2537-2543. (33) Wei, M.; Xu, X. Y.; Wang, X. R.; Li, F.; Zhang, H.; Lu, Y. L.; Pu, M.; Evans, D. G.; Duan, X. Eur. J. Inorg. Chem. 2006, 14, 2831-2838. (34) Rouffignac, P.; Park, J. S.; Gordon, R. G. Chem. Mater. 2005, 17, 4808-4814. (35) Kim, J. Y.; Park, J. C.; Kang, H.; Song, H.; Park, K. H. Chem. Commun. 2010, 46, 439-441. (36) Zhang, L. J.; Chen, Y. C.; He, N.; Lu, C. Anal. Chem. 2014, 86, 870-875. (37) Constantino, V. R. L.; Pinnavaia, T. J. Inorg. Chem. 1995, 34, 883-892. (38) Aramendía, M. Á.; Borau, V.; Jiménez, C.; Marinas, J. M.; Romero, F. J. Appl. Catal. A: Gen. 1999, 183, 73-80. (39) Zhang, L. J.; Wang, S.; Lu, C. Anal. Chem. 2015, 87, 7313-7320. (40) Idriss, H.; Diagne, C.; Hindermann, J. P.; Kiennemann, A.; Barteau, M. A. J. Catal. 1995, 155, 219-237. (41) Li, Z. H.; Xi, W.; Lu, C. Sens. Actuators B 2015, 207, 498-503. (42) Neatu, S.; Maciá-Agulló, J. A.; Concepción, P.; Garcia, H. J. Am. Chem. Soc. 2014, 136, 15969-15976. (43) Zhang, L. J.; Rong, W. Q.; Chen, Y. C.; Lu, C.; Zhao, L. X. Sens. Actuators B 2014, 205, 82-87.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(44) Zhou, Q.; Zhang, L. C.; Fan, H. Y.; Wu, L.; Lv, Y. Sens. Actuators B 2010, 144, 192-197. (45) Zhang, Z. Y.; Xu, K.; Baeyens, W. R. G.; Zhang, X. R. Anal. Chim. Acta 2005, 535, 145-152. (46) Gaedtke, C.; Williams, G. V. M.; Janssens, S.; Raymond, S. G.; Clarke, D. J. Phys. Status Solidi C 2012, 12, 2247-2250. (47) Frindell, K. L.; Bartl, M. H.; Popitsch, A.; Stucky, G. D. Angew. Chem. 2002, 114, 1001-1004. (48) Mhlongo, G. H.; Dhlamini, M. S.; Swart, H. C.; Ntwaeaborwa, O. M.; Hillie, K. T. Opt. Mater. 2011, 33, 1495-1499. (49) Pacold, J. I.; Tatum, D. S.; Seidler, G. T.; Raymond, K. N.; Zhang, X. Y.; Stickrath, A. B.; Mortensen, D. R. J. Am. Chem. Soc. 2014, 136, 4186-4191.

24


Page 24 of 25

Page 25 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For TOC only:

25


Chemisorbed Oxygen on the Surface of Catalyst ... - ACS Publications

Recommend Documents