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Highly Efficient Performance and Conversion Pathway of Photocatalytic NO Oxidation on SrO-Clusters@Amorphous Carbon Nitride Wen Cui, Jieyuan Li, Fan Dong, Yanjuan Sun, Guangming Jiang, Wanglai Cen, Shun Cheng Lee, and Zhongbiao Wu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00974 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017
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Highly Efficient Performance and Conversion Pathway of
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Photocatalytic NO Oxidation on SrO-Clusters@Amorphous
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Carbon Nitride
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Wen Cui a, Jieyuan Li b, Fan Dong a,*, Yanjuan Sun a, Guangming Jiang a,
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Wanglai Cen b, S. C. Lee c, Zhongbiao Wu d
6
a
7
of Environment and Resources, Chongqing Technology and Business University,
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Chongqing 400067, China.
9
b
Chongqing Key Laboratory of Catalysis and New Environmental Materials, College
College of Architecture and Environment, Institute of New Energy and Low Carbon
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Technology, Sichuan University, Sichuan 610065, China.
11
c
12
University, Hong Kong, China.
13
d
14
China.
Department of Civil and Environmental Engineering, The Hong Kong Polytechnic
Department of Environmental Engineering, Zhejiang University, Hangzhou 310027,
15 16 17 18
* To whom correspondence should be addressed. E-mail:
[email protected] (Fan Dong). Phone: +86 23 62769785 605. Fax:+86 23 62769785 605.
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ABSTRACT: This work demonstrates the first molecular-level conversion pathway
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of NO oxidation over a novel SrO-clusters@amorphous carbon nitride (SCO-ACN)
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photocatalyst, which is synthesized via co-pyrolysis of urea and SrCO3. The inclusion
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of SrCO3 is crucial in the formation of the amorphous carbon nitride (ACN) and SrO
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clusters by attacking the intralayer hydrogen bonds at the edge sites of graphitic
25
carbon nitride (CN). The amorphous nature of ACN can promote the transportation,
26
migration, and transformation of charge carriers on SCO-ACN. And the SrO clusters
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are identified as the newly formed active centers to facilitate the activation of NO via
28
the formation of Sr-NOδ(+), which essentially promotes the conversion of NO to the
29
final products. The combined effects of the amorphous structure and SrO clusters
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impart outstanding photocatalytic NO removal efficiency to the SCO-ACN under
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visible-light irradiation. To reveal the photocatalytic mechanism, the adsorption and
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photocatalytic oxidation of NO over CN and SCO-ACN are analyzed by in situ
33
DRIFTS, and the intermediates and conversion pathways are elucidated and compared.
34
This work presents a novel in situ DRIFTS-based strategy to explore the
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photocatalytic reaction pathway of NO oxidation, which is quite beneficial to
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understand the mechanism underlying the photocatalytic reaction and advance the
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development of photocatalytic technology for environmental remediation.
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1. INTRODUCTION
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With the improvement in the quality of life and growing environmental awareness
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among the public, strong emphasis has been placed on mitigating air pollution.1-3
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Nitric oxide (NO), which is one of the major contributors to photochemical smog,
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acid rain and ozone depletion and is primarily responsible for respiratory and
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cardiopulmonary diseases, has triggered much social concern.4,
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methods based on physical adsorption, biofiltration, and thermal catalysis were
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employed to remove NO from industrial emissions. However, these methods are not
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economically feasible for NO removal at parts per billion (ppb) levels.6, 7 As a green
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technology, photocatalysis has gained considerable attention in view of its feasibility
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and high efficiency for solar energy utilization and environmental remediation.8-10
5
Conventionally,
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Graphitic carbon nitride (CN), a metal-free layered conjugated semiconductor, was
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firstly reported as a visible-light photocatalyst by Wang et al.11 Owing to its appealing
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electronic band structure, physicochemical stability, and earth-abundant nature, CN
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has become a new research hotspot in the arena of environmental remediation and
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solar energy conversion.12-14 However, to improve the photocatalytic efficiency of CN
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and its adaptability in various application fields, further optimization on the CN
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performance is desirable and then the developed strategy including inner architecture
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modification and surface functionalization (elemental doping, copolymerization, and
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formation of heterojunctions) was proposed.15-19 For inner architecture modification,
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Kang et al. recently synthesized novel amorphous carbon nitride (ACN) by breaking
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the in-plane hydrogen bonds between strands of polymeric melon units via post-heat
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treatment of the partially crystalline CN at a high temperature of 620 °C for 2 h.20, 21
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With the as-prepared ACN, the high localization of photogenerated charge carriers
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within each melon strand was eliminated; the large potential barrier between the
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layers and across the hydrogen bonds located regions was reduced; the transfer of
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charge carriers was facilitated; and the light absorption range was broadened.
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Consequently, much superior activity was observed for hydrogen generation in
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comparison to that of pristine CN.20, 21
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Besides, the conversion pathway for pollutant removal over a photocatalyst is key
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to understanding the underlying reaction mechanism, estimating the possible
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generation of toxic intermediates, and optimizing the photocatalyst performance.
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Although numerous efficient photocatalysts have been developed for NO removal,
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little attention has been paid to the conversion route of photocatalytic NO oxidation
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process.5, 10, 19, 22, 23 In situ DRIFTS is an effective tool for gas-phase reaction analysis
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because signals are observed even for slight changes at the molecular level; thus, it is
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well suited for investigating the related reaction pathway of photocatalytic NO
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oxidation.24-29
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Here, a facile method involving the co-pyrolysis of urea and SrCO3 was developed
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to prepare SrO clusters@amorphous carbon nitride (SCO-ACN), which exhibited
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substantially high visible-light photocatalytic NO removal efficiency. The high
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efficiency is attributed to (1) the amorphous nature of ACN that can promote the
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transportation, migration, and transformation of charge carriers and (2) the enhanced
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activation of NO via the formation of Sr-NOδ(+). To reveal the photocatalytic
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mechanism, the adsorption and photocatalytic oxidation of NO over CN and
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SCO-ACN were analyzed by in situ DRIFTS, and the intermediates and conversion
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pathways were elucidated and compared. Notably, SrO clusters were identified as the
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newly formed active centers to facilitate the activation of NO, which could effectively
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promote the conversion of NO to the final products. This work presents a novel in situ
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DRIFTS-based strategy to explore the photocatalytic reaction pathway of NO
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oxidation, which is quite beneficial to understand the mechanism of the photocatalytic
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reaction and advance the development of photocatalytic technology for environmental
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remediation.
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2. EXPERIMENTAL AND THEORETICAL SECTION
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2.1 Preparation of photocatalysts
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All chemicals employed in this study were analytical grade and were used without
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further treatment. In a typical synthesis procedure, 10 g of urea and a certain amount
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of SrCO3 were added in an alumina crucible (50 mL) with 20 mL distilled water. The
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obtained solution was transferred to an oven and dried at 60 °C. Then, the crucible
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with a cover was calcined at 550 °C for 2 h at a heating rate of 15 °C/min in static air.
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To investigate the effect of the CN-SrCO3 ratio, the SrCO3 content was controlled at
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0.06, 0.1, and 0.18 g, respectively, and the prepared samples were labeled as
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SCO-ACN-X (X represents the amount of SrCO3). For comparison, an ex situ
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mechanical mixture of CN and SrCO3 was prepared and named as SCO-CN.
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2.2 Characterization methods
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The crystal phase of the prepared samples was analyzed by X-ray diffraction (XRD)
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with Cu Kα radiation (model D/max RA, Rigaku Co., Japan). X-ray photoelectron
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spectroscopy (XPS) with Al Kα X-rays (hν = 1486.6 eV) radiation at 150 W (Thermo
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ESCALAB 250, USA) was used to investigate the surface properties. Fourier
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transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus spectrometer
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using samples embedded in KBr pellets. Scanning electron microscopy (SEM, model
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JSM-6490, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2010,
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Japan)
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adsorption-desorption isotherms were obtained on a N2 adsorption apparatus (ASAP
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2020, Micromeritics, USA). UV-vis diffuse-reflectance spectrometry (UV-vis DRS)
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measurements were performed on dry-pressed disk samples using a scanning UV-vis
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spectrophotometer (TU-1901, China) equipped with an integrating sphere assembly,
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with 100% BaSO4 as the reflectance sample. Photoluminescence (PL) studies (F-7000,
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HITACHI, Japan) were conducted to investigate the optical properties of the samples.
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Photocurrent measurements were carried out using an electrochemical system
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(CHI-660B, Chenhua, China), wherein the working electrode was irradiated by a 300
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W Xe lamp with a 420 nm cut-off filter. Steady and time-resolved fluorescence
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emission spectra were recorded at room temperature with a fluorescence
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spectrophotometer (Edinburgh Instruments, FLSP-920). Electron spin resonance
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(ESR) of radicals spin-trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was
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recorded on a JES FA200 spectrometer. Samples for ESR measurements were
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prepared by mixing them in a 50 mM DMPO solution tank (aqueous dispersion for
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DMPO-•OH and methanol dispersion for DMPO-•O2−) and irradiated by visible light.
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2.3 Evaluation of photocatalytic activity
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were
used
to
characterize
the
morphology
and
structure.
N2
The photocatalytic activity was evaluated based on the removal efficiency of NO at
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ppb levels in a continuous flow reactor with 0.2 g prepared samples. The
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concentration of NO was continuously detected by a NOx analyzer (Thermo
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Environmental Instruments Inc., model 42c-TL). A 150 W commercial tungsten
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halogen lamp (the average light intensity was 0.16 W/cm2) that was vertically placed
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above the reactor glowed when the adsorption-desorption equilibrium was achieved.
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A detailed description of the photocatalytic apparatus is available in Supporting
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Information.
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2.4 In situ DRIFTS investigation
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In situ DRIFTS measurements were conducted using a TENSOR II FT-IR
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spectrometer (Bruker) equipped with an in situ diffuse-reflectance cell (Harrick) and a
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high-temperature reaction chamber (HVC), as shown in Scheme 1. The reaction
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chamber was equipped with three gas ports and two coolant ports. High-purity He,
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high-purity O2, and 100 ppm of NO (in He) mixture could be fed into the reaction
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system, and a three-way ball valve was used to switch between the target gas (NO)
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and purge gas (He). The total gas flow rate was 100 mL/min, and the concentration of
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NO was adjusted to 50 ppm by dilution with O2. The chamber was enclosed with a
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dome having three windows, two for IR light entrance and detection, and one for
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illuminating the photocatalyst. The observation window was made of UV quartz and
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the other two windows were made of ZnSe. A Xe lamp (MVL-210, Optpe, Japan) was
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used as the irradiation light source. Before measurements, the prepared samples were
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placed in a vacuum tube and pretreated 1h at 300 oC.
150 151 152
Scheme 1. The designed reaction system for the in situ DRIFTS signal recording.
2.5 DFT calculations
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All the spin-polarized DFT-D2 calculations were performed by applying the code
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VASP5.3.5,30,
31
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exchange and correlation functional.32 The projector-augmented wave (PAW) method
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was employed, with a cut-off energy of 400 eV.33 The Brillouin zone was set using 5×
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5 × 1 K-points. All atoms were allowed to be relaxed and converged to 0.02 eV/Å.
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The nudged elastic band (NEB) method 34, 35 was used to search the reaction pathways
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from the initial state (IS) to the respective final state (FS). The transition state (TS)
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was determined using the climbing image method and verified with a single
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imaginary frequency (f/i).
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3. RESULTS AND DISCUSSION
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3.1 Chemical composition and phase structure
utilizing the generalized gradient approximation with the PBE
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The chemical structure and composition of CN and SCO-ACN-0.1 are examined by
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XPS measurements. In XPS survey spectra (Figure S1a and S1b in Supporting
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Information), C1s, N1s, Sr3d and O1s signals can be observed for the SCO-ACN-0.1
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sample. As shown in Figure S1c, the corresponding binding energies of C1s at 284.6
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and 288.1 eV are ascribed to the sp2 C−C bonds and sp2-bonded carbon in the
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N-containing aromatic rings (N–C=N), respectively.36,
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deconvoluted into four peaks at 398.8 eV, 400.4 eV, 401.6 eV and 404.5 eV (Figure
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1a). The main peak centered at 398.8 eV originates from the sp2-bonded N involved in
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the triazine rings (C–N=C), and the weak peak at 400.4 eV is due to the tertiary
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nitrogen N–(C)3 groups in CN.36, 37 The C–N=C, N–(C)3, and N–C=N groups make up
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the basic substructure units of CN polymers and construct the heptazine heterocyclic
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ring (C6N7) units. Notably, the peak (at 401.6 eV) due to amino functions (C–N–H) is
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clearly be observed in CN sample, but the intensity of this peak is low in
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SCO-ACN-0.1, indicating that some of the hydrogen bonds in the intralayer
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framework of CN have been eliminated. Furthermore, the atomic ratio of carbon to
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nitrogen (C : N) gradually increases from 3:4.34 for the pristine CN to 3:3.60 for
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SCO-ACN-0.1 (Table S1 in Supporting Information), demonstrating that some of the
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amine groups (NH2/NH) from CN are lost along with the breaking of hydrogen
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bonds.20 Hence the microstructure of CN would be changed when the hydrogen bonds
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are broken via the reaction between CN and SrCO3 during co-pyrolysis process. The
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related high-resolution spectra of Sr3d and O1s are shown in Figure 1b and S1d,
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indicating that strontium oxide is formed on the CN surface.
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Subsequently, X-ray diffraction is employed to elucidate the crystal structures of
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the as-prepared samples. The formation of CN polymer is indicated by the two
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characteristic diffraction peaks at 13.1° and 27.2°, which arise from the in-plane
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structural repeating motifs of the aromatic systems and the interlayer reflection of a
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graphite-like structure, respectively.38, 39 As shown in Figure 1c, the two peaks in the
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case of the SCO-ACN-X samples gradually disappear or are diminished with the
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addition of SrCO3. The disappearance of the peak at 13.1° intuitively reflects that the
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in-plane periodicity of the aromatic systems has been destroyed. Correspondingly, the
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formed irregular intralayer structure induce fluctuations in the interlayer structure and
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disturb the periodic stacking of the layers. Hence, with the introduction of SrCO3, no
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characteristic diffraction peaks of CN are observed for the SCO-ACN-X samples. And
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characteristic diffraction peaks of SrCO3 also have not been detected in SCO-ACN-X
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samples. However, the XRD pattern of SCO-CN developed by the ex situ method
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displays both the characteristic diffraction peaks of SrCO3 and CN. Combining the
200
XPS results, we can conclude that the synergic interactions between CN and SrCO3
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during the in situ thermal processes would break the intralayer hydrogen bonds,
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resulting in the formation of ACN.
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FT-IR spectra are measured to verify whether the basic atomic structures of CN
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would be destroyed via the construction of amorphous structure by breaking intralayer
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hydrogen bonds of CN (Figure 1d). A strong adsorption band of the heptazine
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heterocyclic ring (C6N7) units at 1700-1200 cm-1 is detected.40 A sharp peak at 810
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cm-1 corresponding to the breathing mode of the heptazine ring system can also be
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observed, which indicates that the SCO-ACN-X samples maintain the basic CN
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atomic structures.41 The broad peak located at 3500-3100 cm-1 can be attributed to the
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residual N-H components and the O-H bands, associated with the uncondensed amino
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groups and the absorbed H2O molecules, respectively. Notably, the absorption
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intensity of the N-H components at 3500-3100 cm-1 decrease gradually with the
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addition of SrCO3. Also, the absorption band at 890 cm-1 assigned to the deformation
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mode of N-H gradually diminishes. Furthermore, a newly generated absorption band
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at 2166 cm-1, which is due to the stretching vibration of N=C=N, can be observed in
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the spectrum of SCO-ACN-X. The change in the IR bands indicates that the
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introduction of SrCO3 in the urea polymerization process only destroys the periodic
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arrangement of the interlayers melon strands but maintains the basic atomic structures
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of the strands to afford unique amorphous arrangements of short-range order and
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long-range disorder. Therefore, a facile co-pyrolysis method can be employed to
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synthesize ACN by breaking the hydrogen bonds to destroy the intralayer long-range
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atomic order arrangements.
223 224
Figure 1. The N1s XPS spectra of CN and SCO-ACN-0.1 (a), high-resolution Sr3d XPS spectra
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of SCO-ACN-0.1 (b), XRD pattern (c) and FT-IR spectra (d) of CN and SCO-ACN-X.
226 227
3.2 Morphology and formation mechanism SEM images are presented to investigate the morphology differences between the
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original CN and the ACN. As shown in Figure S2a-2d, the pristine CN is formed by
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the stacking of silk-like nanosheets, and the SCO-ACN-X samples generally maintain
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a similar morphology. After careful observation, a number of pores in the
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SCO-ACN-X samples can be found, as opposed to the pristine CN. The formation of
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this special porous structure should be associated with the interaction between CN and
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SrCO3. Concretely, during co-pyrolysis, CO32- in SrCO3 will attack the hydrogen
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bonds of CN to release H2 and CO2 gas, probably in the form of bubbles, thus
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breaking the intralayer hydrogen bonds of CN and yielding ACN. Bursting of the
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bubbles leads to the formation of a porous structure. Utilizing the DFT method, NEB
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calculations are thus carried out to further confirm this deduction. As shown in Figure
238
S2f, a lower energy barrier and less energy adsorption are observed in the reaction at
239
the edge sites of CN. Specifically, HCO3− generation at edge site manifests lower
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energy barrier ( Eb, 1.03 eV) to reach the transitional state (TS), compared with that of
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bridge site (1.44 eV). Besides, less reaction energy (Er) is observed in the reaction at
242
edge site. This result indicates that CO32− in SrCO3 is preferable to attack hydrogen
243
bonds at the edge sites of CN, which is beneficial to the formation of ACN (Figure
244
S3). This result indicates that the special porous structure dominantly originates from
245
H2 and CO2 gas generation at the edge sites of CN. Therefore, the increased exposure
246
of the bare edge in SCO-ACN-X is certified, which contributes to the construction of
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ACN. Besides, the EDX elemental mapping of SCO-ACN-0.1 (Figure S2e) suggests
248
that the C, N, Sr, and O elements are distributed uniformly. However, the actual form
249
of residual SrO remains debatable.
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Next, TEM observations are carried out to further investigate the microstructure, as
251
shown in Figure 2. As opposed to the primary layered CN nanosheets, the
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SCO-ACN-0.1 sample shows clear lattice fringes with a lattice spacing of 2.581 Å
253
(circled by the red dashed line), which match the spacing of the (200) crystal planes of
254
the SrO clusters (2-5 nm) formed by the thermal decomposition of SrCO3 during
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co-pyrolysis. Hence a facile co-pyrolysis method has been developed to prepare
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amorphous CN decorated with SrO clusters.
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Figure 2. TEM and HRTEM images of CN (a) and SCO-ACN-0.1 (b).
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The N2 adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH)
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pore-size distribution curves (Figure S4) also reflect the formation of mesopores. The
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specific surface area and pore volume of SCO-ACN-X decrease, as shown in Table S1.
262
According to the comparison, the specific surface area and porous structures are not
263
the key factors responsible for the enhanced photocatalytic activity of SCO-ACN-X.
264
3.3 Optical properties, charge separation, and charge transfer
d200 = 2.581 Å
265
PL spectra and ns-level time-resolved fluorescence decay spectra are recorded
266
(Figure 3) to investigate the transfer of photogenerated carriers. In contrast to CN
267
which shows a strong band-to-band emission peak at 442 nm, the SCO-ACN-0.1
268
sample exhibits a much diminished PL peak (Figure 3a). The quenching of the PL
269
peaks can be ascribed to the inhibition of radiative recombination pathways, which
270
are associated with the unique short-range order but long-range disorder amorphous
271
arrangements of ACN that can eliminate the high localization of charge carriers within
272
each melon strand and decrease the large potential barrier in the regions between the
273
layers and across the hydrogen bonds to boost the separation of charge carriers.20, 21
274
The SrO has a band gap of 6.1 eV, with conduction band (CB) and valence band (VB)
275
positions at -3.08 and 3.02 eV, repectively.42, 43 Considering the band structure of CN,
276
the visible light-induced carriers from ACN could not be transferred to SrO. Thus, the
277
enhanced charge separation and transfer characteristics of SCO-ACN are irrelevant to
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the SrO clusters. Correspondingly, the radiative lifetime of SCO-ACN-0.1 is longer
279
than that of CN (Figure 3b), further confirming the effective transfer of carriers to
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inhibit the recombination of electron-hole pairs directly originating from the special
281
amorphous structure. Besides, the photocurrent density-time response plot via on-off
282
cycles of the samples under visible-light irradiation is employed to evaluate the
283
interfacial charge separation dynamics (Figure S5a). Owing to the enhanced
284
electron-hole separation and transfer, the photocurrent response intensity of
285
SCO-ACN-0.1 is higher than primary CN. As shown in Figure S5b, a typical
286
semiconductor absorption in the blue light range is observed for all samples. Owing to
287
the construction of unique amorphous arrangements of short-range order and
288
long-range disorder, a red-shift of the optical absorption band edge can be observed.
289
To be specific, the gradual destruction of the intralayer long-range atomic order by
290
breaking hydrogen bonds increases both the density and distribution of localized
291
states, which is responsible for the increased visible light absorption.20, 21
292 293 294
Figure 3. PL spectra (a), ns-level time-resolved fluorescence spectra (b) for as-prepared samples.
3.4 Photocatalytic activity and conversion pathway of NO oxidation
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As shown in Figure 4a, all the SCO-ACN-X samples exhibit superior activity
296
compared to pristine CN. In particular, the SCO-ACN-0.1 sample reaches an
297
unprecedented high NO removal ratio of 50.0%. Thus, a facile co-pyrolysis method
298
has been developed to prepare highly efficient SCO-ACN and the optimized
299
preparation conditions are confirmed. To further demonstrate the interaction between
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CN and SrCO3 during co-pyrolysis, the mechanically mixed SCO-CN sample is also
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tested, and a slight enhancement of photocatalytic activity is observed. Therefore, the
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combined disruption of the intralayer long-rang atomic order structure of CN and
303
coupling with the SrO clusters are beneficial to enhance the photocatalytic activity.
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And the reaction rate constant k of CN and SCO-ACN are determined to be 0.0915
305
and 0.1367 min−1, respectively (Fig. S6). Correspondingly, the apparent quantum
306
efficiency was estimated to be 19.12 and 28.57% for CN and SCO-ACN-0.1,
307
respectively (see the details about the apparent quantum efficiency calculations in the
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Supporting Information), which indicates the SCO-ACN samples exhibit higher
309
apparent quantum efficiency than pristine CN. And the slight reduction in
310
photocatalytic activity after several minutes can be ascribed to the accumulation of
311
generated intermediates and final products occupying the active sites. The final
312
products (NO3−) can be easily removed by water washing.
313
The photocatalytic efficiency is strongly related to the number of the electron-hole
314
pairs generated under light irradiation, as well as their evolution route. The
315
photogenerated electrons and holes can migrate to the surface of the photocatalyst and
316
then be trapped, generally by the oxygen and surface hydroxyls, to ultimately form
317
superoxide radicals (•O2−) and hydroxyl radicals (•OH) that react with the adsorbed
318
pollutant. For further investigating the reactive species responsible for the
319
photocatalytic removal of NO, the ESR spin-trap with DMPO technique is employed
320
to detect the DMPO-•O2− and DMPO-•OH signals in the CN and SCO-ACN-0.1
321
suspension (Figure 4b). As expected, a much stronger DMPO-•O2− signal is observed
322
for SCO-ACN-0.1 than for CN. This improvement is associated with the improved
323
electron excitation properties and better charge transfer characteristics, which
324
facilitate the generation of photogenerated electrons to trap molecular oxygen and
325
produce more •O2−. Interestingly, a strong DMPO-•OH adduct signal generated by
326
SCO-ACN-0.1 is detected. The potential energy of the VB holes (1.40 eV) from CN is
327
more negative than the OH−/•OH and H2O/•OH potentials (1.99 and 2.37 eV) and
328
cannot directly oxidize OH−/H2O into •OH radicals. However, the observed •OH
329
radicals in Figure 4b can be formed through the reduction of •O2− via the following
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route: •O2− → H2O2 → •OH (the detection of H2O2 has been demonstrated in
331
Figure S7).44 Therefore, we conclude that the SCO-ACN significantly promotes the
332
transportation, migration, and transformation of charge carriers and then facilitates the
333
generation of reactive radicals for NO oxidation.
334 335
Figure 4. Visible-light photocatalytic activities of as-prepared samples for NO removal (a), and
336
DMPO spin-trapping ESR spectra of samples (b).
337
To understand the mechanism of photocatalytic NO oxidation, in situ DRIFTS is
338
carried out to monitor time-dependent evolution of the reaction intermediates and
339
products over the photocatalyst surface, as shown in Figure 5. The background
340
spectrum is recorded before injecting NO into the reaction chamber. The NO
341
absorption bands appear once NO comes in contact with the photocatalyst at 25 oC
342
under dark conditions. Absorption bands of N2O at 2282 and 2244 cm−1 are detected,
343
indicating the adsorption of NO over CN.24, 25 In the OH stretching region, a negative
344
band at around 3550 cm−1 is observed, along with adsorption IR bands at 1193-1142
345
cm−1 due to NO−/NOH, and at 2087 and 934 cm−1 due to NO2.24-26 This result
346
indicates the disproportionation of NO on the surface of CN. The following reaction
347
can be proposed: 3NO + OH− = NO2 + NO− + NOH.24-27 The other absorption bands
348
developed progressively can be assigned to the stretching vibration of monodentate
349
(1060-1010 cm−1) and bidentate (at 1125 and 1109 cm−1) nitrites, or to bidentate
350
(1227 and 1060-1010 cm−1), bridging (1001cm−1), and chelating bidentate (986 cm−1)
351
nitrates.24-26,28,29,45 The formation of nitrites and nitrates over CN during NO
352
adsorption are mainly due to the active two-coordinated N atoms of CN that facilitate
353
the formation of the activated oxygen species and then enhance the oxidation capacity
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of the surface oxygen species for NOx oxidation.46, 47
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In the case of SCO-ACN-0.1, the adsorption of nitro compounds can be observed
356
before the visible-light irradiation, as shown in Figure 5b, similar to NO adsorption
357
over the CN sample. However, significant differences between the two cases can be
358
identified. An outstanding band appears at 2126 cm−1 and is associated with the
359
nitrosyl (Sr-NOδ(+)) species, an intermediate formed during NO oxidation.48, 49 NO
360
molecules tend to interact with SrO to form the more stable nitrosyl intermediate.27, 49
361
Partial charge transfer from the 5s orbital of NO to Sr2+ results in the formation of
362
Sr-NOδ(+).25, 49 This is consistent with the observation that the vibration frequencies of
363
nitrosyls are higher than those of NO molecules (1876 cm−1) but lower than those of
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NO+ free ions (around 2200 cm−1). The adsorbed nitrosyls, the main reaction
365
intermediates, would be preferentially oxidized to nitro compounds by reactive
366
oxygen species.49 Hence SrO clusters can be identified as the newly formed active
367
centers to facilitate the activation of NO via the formation of Sr-NOδ(+), which
368
effectively promotes the conversion of NO to the final products.
369
Once the adsorption equilibrium is achieved, a visible-light source is applied to
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initiate the photocatalytic reaction. Figure 5b shows the IR spectra for CN under
371
visible-light irradiation in time sequence. The “baseline” spectrum is the same as that
372
of “NO + O2 20 min” in Figure 5a. In the range 2300-2050 cm−1, the absorption bands
373
at 2282 and 2244 cm−1 disappear, indicating the consumption of N2O/NO
374
accumulated during NO adsorption. Correspondingly, the peak intensities of some
375
intermediates (nitrito, NO−/NOH) and final products (nitrites, nitrates) significantly
376
increase. The ESR results demonstrate that the surface superoxide radicals are the
377
major radical species (Figure 4b). Thus, the superoxide radicals should be responsible
378
for the conversion of the intermediates into the final products under visible-light
379
irradiation. The adsorption-photocatalysis mechanism on the pristine CN is illustrated
380
in Scheme 2a.
381
The time-dependent IR spectra for the photocatalytic NO oxidation over
382
SCO-ACN-0.1 are recorded and shown in Figure 5d. In the range 1250-900cm−1, the
383
IR absorption bands show a similar pattern as that observed for CN. The increased
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negative peak intensity of the OH groups at 3700-3350 cm−1 can attribute to the
385
consumption of OH groups for the generation of hydroxyl radicals, which is one of
386
the oxidation mediators for NO removal. A slightly intensified Sr-NOδ(+) band at 2126
387
cm−1 can be observed. Notably, a new absorption band at 2215 cm−1 is detected,
388
which can be ascribed to the fact that the electrons from the adsorbed NO are trapped
389
by the photogenerated holes and then converted into NO+ (free ions). Sr-NOδ(+) and
390
NO+ are the dominant products of NO activation on SCO-ACN.
391
Also, according to IR spectra in time sequence, the temporal evolution of
392
normalized absorbance of adsorbed Sr-NOδ(+) and NO3− species on the photocatalysts
393
surface during NO adsorption process and photocatalytic NO oxidation process can be
394
provided. For the concerned species, the normalized absorbance is calculated by
395
considering their individual maximum absorbance as 1. The normalized absorbance of
396
intermediates (Sr-NOδ(+)) and final product (NO3−) are illiustrated in Figure 5c and 5f.
397
According to the tendency of species evolution, it can be clearly observed that the
398
adsorption and transformation of Sr-NOδ(+) and NO3− both in NO adsorption process
399
and photocatalytic NO oxidation process are greatly boosted on SCO-ACN, indicating
400
the construction of SrO clusters@amorphous carbon nitride could promote the ability
401
for NO activation. Consequently, the enhanced NO activation accelerates the
402
formation of intermediates and then facilitates the conversion from original pollutant
403
or intermediates to final products.
404 405
The conversion pathways for the NO adsorption and photocatalytic NO oxidation processes on SCO-ACN are proposed for the first time, as depicted in Scheme 2b.
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Figure 5. In situ IR spectra of NO adsorption (a, b) and visible-light reaction processes (c, d) over
408
CN and SCO-ACN-0.1, temporal evolution of normalized absorbance of adsorbed Sr-NO
409
NO3 species on photocatalysts surface during NO adsorption process (c) and photocatalytic NO
410
oxidation process (f).
δ(+)
and
−
411
And there are some differences exist between the adsorption and photocatalysis
412
processes for CN and SCO-ACN-0.1. Firstly, owing to the introduction of SrCO3, the
413
NO molecules tend to be adsorbed on the SrO clusters to form Sr-NOδ(+), and the
414
reaction intermediates would be preferentially oxidized by reactive oxygen species.
415
Secondly, the increased consumption of OH groups in SCO-U-0.1 during the reaction
416
is not only beneficial for the conversion of intermediates but also contributes to the
417
generation of hydroxyl radicals, in accordance with the ESR results. Last, although
418
the adsorption and photocatalytic NO oxidation with SCO-ACN are slightly different
419
from those with the pristine CN, the SrO clusters do not change the overall conversion
420
pathway of photocatalytic NO oxidation. Significantly, the SrO clusters as newly
421
formed active centers facilitate the activation of NO via the formation of
422
Sr-NOδ(+)/NO+ and promote the conversion of NO to the final products (Scheme 2c).
423
Therefore, both the conversion pathway of photocatalytic NO oxidation and the
424
reasons for the enhanced photocatalytic activity are directly reflected in the in situ
425
DRIFTS spectra.
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Scheme 2. The conversion pathways of adsorption and photocatalytic oxidation of NO over CN (a)
428
and SCO-ACN-0.1 (b), the illustration of the catalyst structure and key role of SrO clusters (c).
429 430
ASSOCIATED CONTENT
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Supporting Information.
432
Evaluation of photocatalytic activity. The XPS survey spectra of CN (a) and
433
SCO-ACN-0.1 (b), high-resolution C1s (c) and O1s (b) XPS spectra of SCO-U-0.1
434
sample. SEM images of as-prepared samples (a-d), EDX elemental mapping of N, C,
435
Sr and O in image for SCO-ACN-0.1 sample (e) and relative energy comparison for
436
CO32− → HCO3− reaction at bridge and edge sites of carbon nitride (f). Optimized
437
structures of pristine g-C3N4, CO32− and HCO3− ions (a). Reaction pathways of HCO3−
438
generation at edge (b) and bridge (c) sites in CN. N2 adsorption-desorption isotherms
439
curves (a) and pore-size distribution (b) of as-prepared samples. Table showing SBET,
440
pore volume, formula, and NO removal ratio of the samples. Transient photocurrent
441
densities (a) and UV-vis DRS spectra (b) for as-prepared samples. Reaction rate
442
constants k of the as-prepared samples. Visible-light-driven H2O2 formation over CN
443
in 60 minutes. Table listing assignments of the FT-IR bands observed during NO
444
adsorption over the photocatalysts. Table assignments of the FT-IR bands observed
445
during photocatalytic NO oxidation over photocatalysts. This material is available
446
free of charge via the Internet at http://pubs.acs.org.
447 448
ACKNOWLEDGMENTS
449
This research was financially supported by National Natural Science Foundation of
450
China (51478070, 21501016 and 51108487), the National Key R&D project
451
(2016YFC0204702),
452
(CXTDG201602014) and the Key Natural Science Foundation of Chongqing
453
(cstc2017jcyjBX0052).
the
Innovative
Research
Team
of
Chongqing
454 455
Author Contributions
456
The manuscript was written through contributions of all authors. All authors have
457
given approval to the final version of the manuscript.
458 459
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