Solvated Electrons for Photochemistry Syntheses Using Conjugated

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Solvated Electrons for photochemistry Syntheses Using Conjugated Carbon Nitride Polymers Honghui Ou, Chao Tang, Xinru Chen, Min Zhou, and Xinchen Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00314 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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ACS Catalysis

Solvated Electrons for Photochemistry Syntheses Using Conjugated Carbon Nitride Polymers Honghui Ou, Chao Tang, Xinru Chen, Min Zhou, and Xinchen Wang*

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350002, P. R. China

ABSTRACT: Solvated electron has attracted increasing research interest for its strong reducing ability properties. However, the generation and utilization of solvated electron in photocatalytic systems are rarely reported owing to the challenges in synthesis and its complex structures. Here, we present a photocatalytic system by accessing laboratory-scale concentrations of ammoniated electrons. Under visible light irradiation, ammoniated electrons are achieved by cyanamide-functionalized and potassium heptazine-based melon polymer (PC-HM) in the presence of an electron donor, which are stable for days. This PC-HM can produce ammoniated electrons at room

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temperature to reduce dioxygen, and thus enabling the production of H2O2 coupled to the selective oxidation of alcohols under visible light illumination. This work presents the possibility to take advantage of ammoniated electron for solar energy conversion in energy and advanced organic chemistry.

KEYWORDS: Carbon nitride • melon • solvated electrons • visible light • photocatalysis

INTRODUCTION

In natural photosynthesis, the excitons are efficiently dissociated in the photosystems II by in-situ electron storage in the form of NADPH or ATP, while holes transfer to the oxygen evolving complex (OEC) for the oxidation of water.1-2 Subsequently, the electrons are rapidly transferred to the photosystem I and used as driving energy for the hydrocarbon synthesis without the input of solar energy.3 The system is a natural selectivity to promise the dissociation of excitons, transfer and also to minimize electron-hole recombination.4-5 Therefore, developing artificial systems to emulate the


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ACS Catalysis

electron storage of NADPH or ATP in natural photosynthesis is ideal for improve efficiency for artificial system.

The solvated electron is an extremely attractive form of electron storage, which is stable for days. It has attracted worldwide attention in the fields of environmental remediation and organic synthesis.6-8 In particular, solvated electron has presented significant advantages in artificial photocatalysis. For instance, they may significantly advance exciton dissociation to minimize the light-induced electron-hole recombination, and open the prospect for light-independent reactions for solar fuel production to enable “dark” photocatalysis. Therefore, the study on solvated electrons and the development of the solvated-electron for photocatalysis could extend the use of the solar energy into broad aspects.

At present, common solvated electrons include ammoniated electrons and hydrated electrons. The most common method of producing hydrated electrons from water itself is radiolysis, but it can bring security concerns that the generation of ionizing radiation.9 Recently, it has been reported that hydrated electron is accessible in the photocatalytic


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system using ruthenium-trisbipyridyl ion [Ru(bpy)3]2+ and shows excellent catalytic performance.10 However, the hydrated electrons hardly be widely used because of their stability and very short existence. Therefore, various methods have emerged for the synthesis of ammoniated electrons. The most common method is by dissolving alkali metal (M) in liquid ammonia, and producing a characteristic deep blue solution that is conductive and paramagnetic. In addition, more systems have been explored for further study the synthesis of ammoniated electrons, such as the use of cryptands and crown ethers to further improve the solubility of the metal. However, it is a remarkable fact that these methods can only be realized at low-temperatures (-78 °C) (Figure 1a).11-12 Therefore, the development of a stable heterogeneous catalytic system at room temperature conditions with high efficiency for the synthesis of ammoniated electrons would be of significant synthetic utility.

Conjugated polymer semiconductors, including conjugated triazine frameworks (CTFs),13 conjugated microporous polymers (CMPs),14-15 and conjugated carbon nitride polymers,16-19 hold great promise in solar-to-chemical conversion owing to their facile


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ACS Catalysis

processing and adjustable band gap.20-21 Among the known polymeric photocatalysts, the heptazine-based melon (HM; nominally, conjugated carbon nitride polymers) have motivated huge scientific activities because of its unusual electronic and optical properties and surprisingly high chemical and thermal stability.22-24 Recently, Lotsch et al. has reported that the color of the carbon nitride polymer solution can changes color from yellow to blue when irradiated using methanol as electron donors and under inert atmosphere.18, 25-26 This blue suspension can give off its trapped electrons in the dark to reduce water to H2.27 However, this blue suspension has not been further adequately analyzed and properly defined. In addition, the insightful understanding of the chemical structure, the band gap structure and the mechanism of color change is still missing.

Herein, by taking HM as an example, we present, characterize, and apply the heterogeneous catalytic system to liberate the solvated electrons. The photocatalyst is cyanamide-functionalized and potassium heptazine-based melon polymer (denoted henceforth as PC-HM) synthesized by thermocondensation of trichloromelamine, KSeCN and LiCl. This PC-HM catalyst has a higher valence band potential (+2.16 V vs.


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NHE). Therefore, it can give the corresponding polymeric excited electrons under visible light irradiation and in the presence of hole sacrificial reagents. In addition, the chemical structure of our catalyst also provides an environment similar to ammonia and alkali metals. As expected, the color of the as-prepared PC-HM solution changes from pristine yellow to blue when the system was subjected to light irradiation, under inert atmosphere and in the presence of hole sacrificial reagents such as methanol (MeOH) and benzyl alcohol. Remarkably, this blue solution is conductive and paramagnetic, similar to the dissolve of alkali metals in liquid ammonia, suggesting the formation of long-lived ammoniated electron. The extreme reducing power of this unique ammoniated electron in PC-HM and its long unquenched life as a ground-state species are synergistic. Furthermore, this study is also of relevance for photocatalytic water splitting field.

EXPERIMENTAL SECTION

Synthesis of samples. Trichloromelamine (TCMA) (2.29 g, 10 mmol), potassium thiocyanate (1.44 g, 10 mmol) without (DA-HM) and with (PC-HM) lithium chloride (0.61


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ACS Catalysis

g, 14 mmol) were ground together. Then the mixed precursor was transferred into a crucible, which was heated to 550 ºC with a ramp rate of 2.3 ºC/min in a muffle furnace with flowing nitrogen gas (2 L/min). The temperature was maintained for 4 hours, and then the samples were cooled down. The crude products were washed with deionized water in order to remove the salts. The final products were dried in a vacuum oven at 60 °C for 10 h.

For comparison, reference HM was prepared by heating melamine with a ramp rate of 2.3 ºC/min up to 550 °C for 4 hour under N2 atmosphere.

Characterization. Fourier transform infrared (FTIR) spectra, X-ray diffraction patterns (XRD) measurement, Raman spectra and X-ray photoelectron spectroscopy (XPS) data were recorded on a BioRad FTS 6000 spectrometer, Bruker D8 Advance diffractometer with CuKα1 radiation (λ = 1.5406 Å), Reflex Raman spectroscopy system under 325 nm excitation and Thermo ESCALAB250 instrument with a monochromatized Al Kα line source (200W) at 3.0×10-10 mba, respectively. UV-Vis diffuse reflectance spectra (UVVis DRS), Electron paramagnetic resonance (EPR) measurements, photoluminescence


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(PL) spectra, Mott-Schottky analysis were performed on Varian Cary 500 Scan UVvisible system, Bruker model A300 spectrometer, Edinburgh FI/FSTCSPC 920 spectrophotometer and BioLogicVSP-300 electrochemical system, respectively. Scanning electron microscopy (SEM) was operated by Nova Nano 230 microscope. Transmission electron microscopies (TEM) were operated by Zeis 912 microscope.

Photocatalytic hydrogen peroxide evolution. The photocatalytic reactions of hydrogen peroxide evolution were carried out in a Schlenk flask (100 mL). In the Schlenk flask, dispersing 50 mg catalyst powder in a MeOH/O2 (MeOH 3mL and H2O 27 mL) solution by ultrasonication for 5 min. Then, O2 was bubbled through the solution for 15 min. Flowing water was employed to control the reaction temperature at 298 K. After the reaction, the H2O2 amount was determined using a SHIMADZU UV-1780 UV-vis spectrophotometer by measuring its absorbance based on the formation of a yellow coloured complex TiIV- H2O2. When titanium oxysulfate (TiOSO4) reacted with H2O2, a yellow-colored complex (pertitanic acid) was formed and UV-Vis measurement was


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ACS Catalysis

done at 410 nm to colorimetrically determine the concentration of H2O2 (TiO2+ + H2O2 → [TiO(H2O2)]2+).28

Dark reaction. 50 mg catalyst powder, 27 mL of H2O, and 3 mL of benzyl alcohol were added into a Schlenk flask (100 mL). The system was evacuated to completely remove the residual oxygen and then filled with high purity Ar gas. The system was irradiated with a 300 W Xe lamp for 1 h to form a blue colored suspension. Then, O2 was injected into the system in dark for production of H2O2.

Photocatalytic activity for water splitting. 50 mg catalyst powder, 100 mL of H2O, and 10 mL of triethanolamine and 3 wt% Pt were added into a Pyrex top-irradiation reaction vessel connected to a glass closed gas system. The solution was evacuated several times to completely remove air. Then, the system was irradiated with a 300 W Xe lamp. In addition, the wavelength of the incident light photocatalytic activity was controlled by applying appropriate long-pass cut-off filters. After reaction, the evolved gases were analyzed by gas chromatography, using argon as the carrier gas.

The apparent quantum yield (AQY) was calculated as follow:


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Conditions: monochromatic LED lamps with band pass filter of 405. The irradiation area was controlled as 3×3 cm2. Where, Ne, M, Np and NA is the amount of reaction electrons, the amount of H2 molecules, the incident photons and Avogadro constant, respectively. In addition, c is the speed of light, h is the Planck constant, S is the irradiation area, t is the photoreaction time, P is the intensity of the irradiation and λ is the wavelength of the light.

The O2 evolution reaction was similar with H2 evolution, just without Pt cocatalyst and triethanolamine. In addition, using La2O3 and AgNO3 as pH buffer and sacrificial, respectively.

Photocatalytic activity for selective oxidation of benzyl alcohols. The conditions of reaction: benzyl alcohol (0.1mmol), benzotrifluoride (1.5 mL), catalyst (8 mg), O2 (1 atm), λ>420 nm, 40 °C, 4 h. The reactor is a 10 mL Pyrex glass reactor. After the reaction, the solution was centrifuged at 10000 rmp for 10 min to remove the catalyst. In


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ACS Catalysis

addition, the aldehydes were analyzed by Aglient Gas Chromatograph (GC-7820). The wavelength of the incident light photocatalytic activity for selective oxidation of benzyl alcohols to aldehydes was controlled by applying appropriate long-pass cut-off filters.

RESULTS AND DISCUSSION

To reveal why PC-HM was able to generate and utilize solvated electrons, we firstly investigated the structure of the as-prepared PC-HM. The samples were first characterized by Fourier transform infrared (FTIR) spectra. As shown in Figure 2b, the absorption band between 1100 and 1650 cm-1 are assigned to the feature distinctive stretching vibration modes of aromatic CN heterocycles, and the absorption peak at 800 cm-1 indicate the samples with tri-s-triazine or triazine as a main chemical structural unit.29 In addition, a broad band in the range of 3000-3700 cm-1 is assigned to the stretching mode of -NH2 or O-H group of polymer carbon nitride.30-31 For the PC-HM, the overall patterns of the spectra are the same as the HM. However, there exists an obviously signal at 2180 cm-1 compared with HM, which is attributed to the functional


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groups of C≡N.32-33 More chemical structural evidence for supporting these conclusions could be further obtained in the Raman spectrum (Figure S8a).

XPS was performed to analyze the surface chemical compositions and chemical states of PC-HM. As shown in Figure 3, the peak of C 1s binding energies at 286.4 and 288.0 eV are the major carbon species in the HM framework, could be related to sp2-bonded carbon centers in aromatic rings (N-C≡N). In addition, the peak with binding energy of 284.6 eV was identified as carbon impurities from carbon contamination.34-36 In addition, the XPS of PC-HM in the N 1s binding energy regions was further obtained in the Figure 3b. The peak at 398.6 eV and 399.9 eV are attributed to C-N-C groups and the tertiary nitrogen nitrogen N-(C)3 groups, respectively.37-38 in addition, the peak at 400.9 eV and 404.1 eV could be ascribed to the presence of N-H groups and the charging effects or positive charge localization in aromatic CN heterocycles, respectively.39 High-resolution XPS investigations further confirmed the absence of Cl, Li and Se and revealed the presence of K in the PC-HM. The structure difference between PC-HM and HM may lie in the introduction of cyano groups (C≡N) and K+ ions.


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ACS Catalysis

Figure 2a shows the XRD pattern of the as-prepared PC-HM. Two domain peaks at 13.0° and 27.4° in the HM, attributing to the in-plane repeating unit and interlayer stacking peak of conjugated aromatic rings, can be indexed to the (100) and (002) peaks for graphitic materials, respectively.40-41 In comparison with HM, it is obvious that the (002) peak of the PC-HM at 27.7° significantly decreased and a slight shift of the (002) peak from 27.4° to 28.0° (Figure S1). In addition, the peak at 13.0° of the PC-HM shifted to a lower diffraction degree of 8.0°. The change in the peak (002) of the PC-HM clearly indicating that the layer distance decreases significantly, which was most likely attribute to the interaction between conjugated aromatic layers are enhanced.

The photoluminescence spectra (PL) and time-resolved PL spectra of pristine HM and PC-HM were shown in Figures S12a and S12b, respectively. The pristine HM and PCHM shows a similar emission peak at 460 nm, which originated from the band-to-band recombination of the electrons and holes.42 The wavelength of emission peak of the samples also corresponds to its wavelength of absorption edge (Figure S2), which further proves that they with the similar energy gap (approximately 2.70 eV). In addition,


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it is clear that the samples present totally different the PL intensity. A decrease in the PL intensity and the mean radiative lifetimes of PC-HM, possibly originated from the PCHM is equipped with the electron donor-acceptor units (the -C≡N groups are the electron acceptor, and the NH/NH2 groups are the electron donor).43 The information on the π-conjugation system of PC-HM can be further investigated by solid-state ESR characterizations (Figure S6). The EPR intensity for PC-HM is lower than that of HM, which indicates the effective extension of the π-conjugation system of heterocycles in PC-HM. In addition, the photocurrent of PC-HM is around 3 times higher than that of HM (Figure S13), which indicating faster interface charge transfer.44

In our case, the as-obtained PC-HM not only shows a higher efficiency in the exciton splitting and charge separation, but also has abundant amino groups and K+ in its chemical structure. Remarkably, the pale yellow colloidal suspension (MeOH 3 mL and H2O 27 mL) prepared with PC-HM under Ar atmosphere changed almost instantaneously to blue upon under visible light irradiation. This blue color persisted during 1 h with no light source (Figure S4). In addition, this blue suspension has a


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higher conductivity than that of yellow suspension (Figure 4b). The high electrical conductivity is attributed to ammoniated electrons that are released in the blue solution. As a control experiment, neither color change nor higher conductivity was observed for HM after light irradiation (Figure S4). The color change in this system is attributed to ammoniated electrons which absorb energy in the visible region of light (Figure 4e and Figure S5), just like the solution that dissolves alkali metals in liquid ammonia. After light irradiation, this blue suspension without significant color changes for days under Ar atmosphere (Figure S17), indicating the higher stability of this ammoniated electrons under inert atmosphere.

EPR spectroscopy studies were performed to further determine the nature of this blue suspension. As shown in Figure 4d, there are many peaks in the magnetic field from 3516 to 3522 G, indicating there are s several types of radicals with different electronic structure.45 The strongest peak (1) is attributed to unpaired electrons in carbon atom in the aromatic rings of PC-HM.46 According to the literature, another peak (2) is corresponds to ammoniated electrons.47 These data further confirming that this system


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can produce ammoniated electrons at room temperature under irradiation. In addition, the blue crystalline electrides may only form by adding a cosolvent such as diethyl ether or trimethylamine in the alkali metal solution. Interestingly, in our case, the blue polymeric carbon nitride electrides formation does not require any additives.

However, not all samples containing the same chemical structure (NH/NH2, -C≡N and K+ ions) can generate ammoniated electrons.27 For example, no obvious color change can be observed on CN aerogels48 and DA-HM when they are illuminated in methanol solution (Figure S16). Therefore, the chemical structure of PC-HM only provides an environment similar to ammonia and alkali metals. Even more remarkable is the band structure of PC-HM. According to the bandgap energies obtained from the Tauc plot (Figure S2) and the Mott-Schottky analysis (Figure S3), we can obtain the bandgap structures for PC-HM and HM as displayed in Figure 5. Obviously, PC-HM possesses a band structure with a deep VB level (1.17 V) of high photooxidation capability. On the other hand, compared with HM, PC-HM also has a greatly lowered CB level (by 0.5 V), downshift from -1.02 to -0.54 V vs. NHE. Therefore, it is reasonable to detemine that


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PC-HM radical anion, formed from PC-HM upon its single electron reduction, would be stable and have longer unquenched lifetime.49-50

However, when inert gases was replaced with an air (O2), this blue suspension became yellow (Figure 4a). But the substitution of MeOH with sacrificial electron donor benzyl alcohol, the color remained blue. A significant decay in the color was observed, when O2 was added to this blue pre-irradiated suspension, indicating the transfer of the ammoniated electrons from PC-HM to O2 (Figure 4a). Therefore, these unique ultralonglived ammoniated electrons can be stored for a long time in PC-HM and transferred to O2 when available.

After an hour of visible light irradiation, the system (PC-HM/ benzyl alcohol) was subjected to dark. As shown in Figure 3a, after O2 was injected into the blue suspension for a half an hour in the dark the amount of generated H2O2 reached maximum of near 5.0 μmol. Meanwhile, this blue suspension came back to pristine yellow, indicating that the ammoniated electrons were consumed by O2. In addition, the selectivity and production rate of benzaldehyde (BZH) is calculated as > 95 % and 5.5 μmol for PC-HM,


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respectively. The same reaction conditions were additionally examined for four cycles (Figure 6b). No noticeable decay of H2O2 and BZH evolution rate was observed, suggesting the good stability of PC-HM.

Based on the above results, a mechanistic proposal for H2O2 and BZH evolution is given in Figure 7b. Conduction band electrons and valence band holes are produced by the irradiation of the PC-HM semiconductor.51 Because the chemical structure of PCHM has an environment similar to ammonia and alkali metals, the excited electrons are stored on its surface to form ammoniated electrons. When O2 was injected into the blue suspension system, the molecular oxygen is reduced by the ammoniated electrons to the superoxide radical anion (•O2-) (Figure 7a). In addition, the hole oxidizes the alcohol to the corresponding cation radical, finally results in the formal liberation of H2O2 and the observed ketone.52

To further investigate the strong photoredox capability of PC-HM, we then confirmed that the PC-HM system allowed for H2O2 production and simultaneous benzyl alcohol oxidation under visible-light. As shown in Table 1 and Figure S22, the selectivity and


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conversion rate of oxidizing benzyl alcohol to benzaldehyde is calculated as > 95 % and 45 % for PC-HM, respectively. However, the HM catalysts only show a very low activity. In the absence of irradiation (entry 3), a negligible benzaldehyde (420 nm OH

O

+ O2

+ H2O2

Conversion.

BZH[b]

H2O2

[%]

[μmol]

[μmol]

HM

2

7

n.d.

2

PC-HM

45

43

40

3

PC-HM [c]

Solvated Electrons for Photochemistry Syntheses Using Conjugated

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