Robust Photocatalytic H2O2 Production by Octahedral Cd3(C3N3S3)2 Coordination Polymer under Visible Light.

Herein, we reported a octahedral Cd3(C3N3S3)2 coordination polymer as a new noble metal-free photocatalyst for robust photocatalytic H2O2 production from methanol/water solution. The coordination polymer can give an unprecedented H2O2 yield of ca. 110.0 mmol • L(-1) • g(-1) at pH = 2.8 under visible light illumination. The characterization results clearly revealed that the photocatalytic H2O2 production proceeds by a pathway of two-electron reduction of O2 on the catalyst surface. This work showed the potential perspective of Mx(C3N3S3)y (M = transitional metals) coordination polymers as a series of new materials for solar energy storage and conversion.

Hydrogen peroxide (H2O2) is an environmentally benign oxidant widely applied in the areas of organic synthesis, the pulp and paper industry, and disinfection1. It is also a block building of post-fossil energy framework as a new solar fuel2345. However, the traditional anthraquinone method6, also referred as the indirect process, for H2O2 production is contrary to the concept of the modern green chemistry, because it not only involves the multistep reactions of high energy-consuming hydrogenation and oxidation, but also requires large production plants to minimize capital investment and to obtain highly concentrated H2O2 to reduce transportation costs. The direct synthesis of H2O2 by the noble metal-catalyzed reaction of molecular oxygen with hydrogen has proven to be feasible789101112, but the high production cost and the unsatisfactory efficiency limited its practical application on-large scale. Moreover, some cares have to be required for safe operation because of the potentially explosive danger of H2/O2 mixture. Recently, the photocatalyzed H2O2 synthesis has attracted more attention as an “ideal green” technique13141516. Without the presence of external H2, H2O2 can be produced on a micromolar order in O2-saturated water by ultraviolet irradiation of TiO217. By suppressing the back reaction, the photocatalytic H2O2 yield can be upgraded to a millimolar level over a surface-fluorinated TiO2, but along with a large amount of fluorine contaminant emitted into the aqueous solution18. Up to date, much work has devoted to chemically modifying TiO2 by noble metal loading or nonmetallic doping for H2O2 synthesis192021. Carbon nitride (g-C3N4) and its related composites modified by the electron-deficient aromatic diimide units were recently shown to be also photocatalytically active for H2O2 synthesis under visible light illumination222324, but it yielded H2O2 only on no more than a micromolar level. Thus, the development of visible-light-driven photocatalysts effective for safe H2O2 production on a millimolar and even molar scale remains a formidable challenge.

In natural systems, superoxide dismutases (SODs) are metalloprotein enzymes mildly catalyzed H2O2 production by the dismutation of superoxide into oxygen and hydrogen peroxide25. SODs including three major families of CuZn-SOD, Fe/Mn-SOD and Ni-SOD are essentially coordination compounds with late transition metal ions as central atoms and proteins as ligand. Inspired by these macromolecular metalloproteins, we have long looked for metal coordination polymers as robust photocatalysts to mimic the biocatalytic H2O2 production. Herein, a C3N3S3-based coordination polymer photocatalyst, Cd3(C3N3S3)2, was successfully developed to produce H2O2 on a millimolar level under visible-light irradiation. The yellowish coordination polymer was synthesized by a facile wet-chemical route under the ambient conditions according to the previous work reported by Chudy, J. C. et al. who carefully controlled the reaction condition to synthesize the coordination polymer with different stoichiometries26. The high stability and low solubility in aqueous solution provided an indicative of the coordination polymer as a catalyst or catalyst support27. The elemental analysis of as-synthesed Cd3(C3N3S3)2 was listed in Table S1, which clearly demonstrates that the molar ratio of C, N, S and Cd elements is 1: 0.98: 0.91: 0.55. The result confirms that the general molecular formula of the resultant product is Cd3(C3N3S3)2, also denoted as Cd3(TMT)2 where TMT is 2,4,6-trimercaptotriazine anion. The combination of XRD and FTIR characterizations (Fig. S2,S3, Supporting Information) proves the Cd2+-bridged structure of the as-synthesized coordination polymer as depicted in Fig. 1262728.

Figure 1

Structure of the Cd3(TMT)2 coordination polymer.

Results and Discussion

Figure 2 shows the low-magnification scanning electron microscopy (SEM) (Fig. 2A,B) and transmission electron microscopy (TEM) images (Fig. 2C) of the as-synthesized Cd3(TMT)2 coordination polymer. It well crystallizes as homogeneously dispersed nanocrystals with the perfect octahedron morphology characteristics. Additionally, it appears that the triangular surface of these octahedrons is sporadically covered by some irregular-shaped nanoparticles, which suggests that the formation of the octahedral nanocrystals maybe follow the “Oriented attachment” mechanism, just similar to the case of a previous work by Zeng and coworkers29. Namely, owing to the strong coordination capability of TMT with transition metal ions as well as the low Ksp value of Cd3(TMT)2 in water27, the addition of TMT ligand into the Cd2+aqueous solution leads first to the formation of Cd3(TMT)2 nanoparticular precipitate, and long reaction time endows the self-aggregation of these nanoparticles to construct the final 3D architectures. This hypothesis is confirmed by the SEM images of products at different reaction time as shown in Fig. S4 (Supporting Information). The TEM image in Fig. 2C further evidences that the product are structurally well-defined octahedrons with solid inner space. The selected area electron diffraction pattern (SAED) shown in Fig. 2D indicates the single crystal feature of the coordination polymer.

Figure 2

Typical (A, B) SEM, (C) TEM image and (D) the SAED pattern of the Cd3(TMT)2 coordination polymer.

The ultraviolet-visible diffuse reflectance spectrum of the Cd3(TMT)2 coordination polymer (Fig. S5, Supporting Information) displays a typical optical absorption of semiconductor. The corresponding to the optical band-gap energy of ca. 2.76 eV. To further investigate the band structure of the Cd3(TMT)2 coordination polymer, we also carried out electrochemical analysis. The typical Mott-Schottky plot of Cd3(TMT)2 in the dark (Fig. S6, Supporting Information) shows a positive slope of C−2-E plot, an indicative of n-type semiconductor30. The flat-band potential (Vfb) of about −0.78 V vs. NHE at pH 7.0 is determined from extrapolation to the X intercept in the Mott-Schottky plot. And by combining with the band-gap energy of ca. 2.76 eV estimated from the optical absorption, the valence band position of the Cd3(TMT)2 coordination polymer is calculated to be 1.98 V vs. NHE at pH 7.0. Thus, it is revealed from the band characteristics as illustrated by the insertion in Fig. S6 that, light-excited electrons in the conduction band of the coordination polymer possess a large thermodynamic driving force to reduce O2 (E°(O2/•O2−) = −0.16 V), and yet the potential of the photogenerated hole in the valence band is inadequate to oxidize OH- to hydroxyl radicals (E°(OH−/•OH) = 2.4 V). This result clealry indicates that the oxygen reduction reaction over the coordination polymer is feasible.

The activity results of photocatalytic H2O2 production from methanol aqueous solution confirm the conclusion above. As listed in Table 1, in pure water (Entry 1), the Cd3(TMT)2 semiconductor is reluctant to produce H2O2 under the indicated conditions. On the contrary, with the addition of methanol as hole scavenger and proton donor, which is beneficial for the separation of electron-hole pairs, 1.5 mmol•L−1 H2O2 is produced under visible-light irradiation (Entry 2). Notably, this yield is highly comparable to the system using Au-Ag/TiO2 under ultraviolet light irradiation as previously reported20. But in dark (Entry 3), no H2O2 is produced, confirming that the H2O2 production is driven by light absorption. According to the redox potentials of the electron/hole pairs of the coordination polymer, H2O2 can be stoichiometrically formed in an aerated aqueous solution via two different pathways as follows:

Table 1

Photocatalytic H2O2 evolution over Cd3(TMT)2 under different conditions.

Entry	pH	hν	Atmosphere	C(H2O2) [mmol • L−1]
1[a]	6.7	+	air	negligible
2[b]	6.7	+	air	1.5
3[b]	6.7	−	air	negligible
4[b]	6.7	+	N2	negligible
5[c]	5.8	+	air	1.75
6[c]	4.1	+	air	2.0
7[c]	2.8	+	air	8.75
8[d]	6.7	+	air	negligible

[a] Reaction conditions: 80 mg catalyst dispersed in 20 ml distilled water, visible light (λ ≥ 420 nm), room temperature, the reaction time is 4 hours; [b] Reaction conditions: 80 mg catalyst dispersed in 19 ml distilled water mixed with 1 ml methanol, other conditions as [a]; [c] pH of the solution was adjusted by con. HNO3, other conditions as [b]. [d] With the addition of AgNO3 as electron trapper, other conditions as [b]. The concentration of produced H2O2 was determined by KMnO4 titration20.

Our control experiments uncover some basic mechanism of the Cd3(TMT)2-catalyzed H2O2 evolution. When the system is bubbled with N2 to eliminate O2, no H2O2 is detected with KMnO4 titration method, indicating the participation of O2 in the photocatalytic H2O2 production. Thus, it can be concluded that the second potential pathway does not contribute to the H2O2 production in our system. This conclusion is also confirmed by our further control experiments (Entries 5–7). If water oxidation, as described by equation (2), is primarily responsible for the photocatalytic H2O2 production, and then increasing the concentration of H+will deteriorate the photocatalytic H2O2 production of the Cd3(TMT)2 semiconductor. However, it is shown that, the H2O2 concentration increases with the H+concentration in the solution, and the concentration of H2O2 reaches to about 8.75 mmol•L−1 at pH = 2.8. In addition, when AgNO3, which is an often-used electron trapper31, was added in the system (Entry 8), no detectable H2O2 is produced in the solution after 4 h of visible-light irradiation. It indicates the pivotal role of photo-electrons for H2O2 generation. Based on the activity results of the control experiments we can confirm that, in the case of Cd3(TMT)2-photocatalyzed H2O2 production, the overall reaction would be described by the equation (1), which features two-electron reduction of O2. It is also noteworthy that, in most cases as previously reported, the produced H2O2, a more reactive oxidation agent than O2, is presumably quick rebound and readily suffers from the reduction reaction by photo-generated electrons on the catalyst, which results in a very low efficiency of H2O2 evolution21. Interestingly, in the present study, the accumulation of H2O2 with concentration up to several millimoles per liter is achieved. It should be ascribed to the rapid desorption of H2O2 from the surface of the Cd3(TMT)2 photocatalyst, which suppresses the photocatalytic H2O2 decomposition.

5,5-dimethyl-1-pyrroline N-oxide (DMPO) trapping electron paramagnetic resonance (ESR) analysis was used to identify the intermediate oxygen species formed during the H2O2 evolution, as shown in Fig. 3. No ESR signal can be observed in dark. On the contrary, upon visible light irradiation (λ ≥ 420 nm), a set of ESR signals of DMPO-•O2−/•OOH adduct (•O2−, a product derived from oxygen reduction reaction: O2 + e− = •O2−) is discernable within 160 s, and no ESR signals of DMPO−•OH adduct occurs (Fig. S7, Supporting Information), indicating the absence of H2O2 decomposition induced by reduction reaction with electrons (e.g. H2O2 + e− = OH- + •OH)32. Furthermore, the intensity of the DMPO-•O2−/•OOH adduct signals increase gradually with irradiation time. All these facts are in good agreement with the activity results, indicating that the H2O2 evolution over the Cd3(TMT)2 coordination polymer proceeds via the O2 reduction process, as illustrated by equation (1).

Figure 3

ESR spectra of DMPO–•O2−/•OOH adduct in the Cd3(TMT)2/DMPO system before and after visible light irradiation.

The spectrum action of H2O2 production as shown in Fig. 4 further validates that the photoreaction proceeds through light-excitation of the coordination polymer. It appears that the H2O2 amount produced in the system decreases with increasing the incident light wavelength, matching well with the optical spectrum. This result clearly indicates that the H2O2 production is intrinsically a photocatalytic process driven by photoexcitation of the coordination polymer semiconductor. Therefore, we can propose a reasonable mechanism of Cd3(TMT)2-photocatalyzed oxygen activation for H2O2 production, as illustrated in Fig. 5. Under visible light irradiation, the electron-hole pairs are produced, and then methanol is oxidized by holes into formaldehyde and proton (eq. 3), which contributes to the separation of charge carriers and H+333435. The adsorbed oxygen molecules are spontaneously reduced by electrons to form superoxide radicals (eq. 4), which further react with protons to produce •OH2 radicals (eq. 5). The •OH2 radicals can readily undergo further reduction with e- (eq. 6), producing HO2- anions. Finally, just as demonstrated by equation (7), the negatively-charged HO2- reacts with H+, leading to the evolution of the final H2O2 product36.

Figure 4

Wavelength-dependent hydrogen peroxide evolution by Cd3(TMT)2 coordination polymer.

Reaction conditions: 80 mg catalyst dispersed in 19 ml distilled water mixed with 1 ml methanol, room temperature. The reaction time is 4 hours.

Figure 5

Proposed mechanism of the H2O2 production on the visible-light-activated Cd3(TMT)2 under the ambient condition.

In order to clearly verify the photocatalytic reaction mechanism, the concentration of formic acid and formaldehyde has been further measured by the ion chromatography and acetylacetone spectrophotometry, as shown in Fig. S8. The concentration of HCHO and HCOOH gradually increases with the enhancement of reaction time under 24 h of visible-light irradiation, which is good consistent with the above reaction mechanism. Although the accumulated H2O2 can further oxidize HCHO into HCOOH, which will lead to some H2O2 loss, a stable H2O2 concentration in the aqueous solution can be achieved once a production-decomposition balance of H2O2 is reached, just as demonstrated by Fig. S9 (Supporting Information) representing a time curve of enzyme-biocatalyzed H2O2 evolution, over the coordination polymer from methanol aqueous solution. As we all known, the formation and decomposition of H2O2 follow zero- and first-kinetics toward H2O2 concentration, respectively1420. Therefore, the kinetic data can be modeled and explained by the equation: [H2O2] = (kf/kd){1-exp(1-kdt)}, where t is time, kf (mM h−1) and kd (h−1) are the formation and decomposition rate constants for H2O2, respectively. The kf and kd values of Cd3(TMT)2 polymer are 0.39 mM h−1and 0.04 h−1, respectively, indicating that the Cd3(TMT)2 polymer is a robust photocatalyst for H2O2 production. To check the photo-stability of the as-prepared photocatalyst, the photocatalytic evolution of H2O2 was repeated up to five cycles under the same conditions (Fig. 6). It can be clearly seen that after five successive operations, the coordination polymer still maintains the high photocatalytic activity for H2O2 production. In addition, its crystal structure does not change after photocatalytic reaction, as shown in Fig. S10 (Supporting Information). Those results indicate that the Cd3(TMT)2 polymer is able to serve as a stable, reusable photocatalyst for H2O2 generation from methanol/water solution.

Figure 6

Stability testing of photocatalytic activity of the Cd3(TMT)2 coordination polymer.

According to the characterization results above, we believe that the Cd3(TMT)2 coordination polymer can fulfill as a versatile visible-light photocatalyst. The activity results of photocatalytic degradation of Rhodamine B over Cd3(TMT)2 shown in Fig. S11 (Supporting Information) also confirms that it indeed enables the destruction of organic pollutants due to the •O2− generation. Importantly, a considerable amount of H2O2 is simultaneously produced along with the Rhodamine B photodegradation in the solution37 (Fig. S12, Supporting Information). This result suggests that the sacrificial agent, methanol, will be hopefully replaced by waste organic dyes for H2O2 production in the future, synchronously achievement of environmental remediation.

In summary, a bioinspired metal coordination polymer with a general molecular formula of Cd3(TMT)2 was reported for the first time to fulfill as a visible light photocatalyst effective for H2O2 evolution with the aid of methanol. The coordination polymer features a well-defined octahedral morphology and high crystallinity and shows robust photocatalytic H2O2 production on a millimolar level. The electrochemical analysis and ESR characterizations clearly reveal that the photocatalytic H2O2 evolution over the coordination polymer follows a mechanism of two-electron reduction of O2. This work shows the potential promising of the transitional metal coordination polymers in solar energy storage and conversion, especially organic photosynthesis.

Methods

Materials

Cd(NO3)2 • 4H2O and sodium hydroxide (NaOH) were supplied by Sinopharm chemical reagent Co., Ltd (Shanghai, China), trithiocyanuric acid (H3TMT) was purchased from Tokyo Chemical Industry Co., Ltd (Tokyo, Japan). All materials are analytical grade purity without further purification prior to use. Deionized (DI) water used in the synthesis was obtained from local sources.

Catalyst Preparation

The monodisperse Cd3(TMT)2 octahedrons are prepared by a facile template-free wet-chemical synthesis at room temperature. Typically, 0.015 mol cadmium nitrate, Cd(NO3)2 • 4H2O, was dissolved in 200 mL DI water under mechanically stirring to form a transparent solution (denoted as solution A). Trithiocyanuric acid (0.01 mol) was dissolved in 200 mL 0.15 mol • L−1 NaOH aqueous solution to form yellowish homogenous solution, which is denoted as solution B. In order to avoid the formation of cadmium hydroxide precipitation, solution B was slowly added into solution A drop-by-drop under vigorously stirring. Afterwards, the system was aged for 24 h with mildly stirring. The products were then separated by filtration, washed by DI water, and fully dried at 333 K in oven to get the final resultants, namely Cd3(TMT)2 octahedrons.

Characterizations

The phase composition of the as-prepared samples was determined on a Bruker D8 Advance X-ray diffractometer (XRD) using Ni-filtered Cu Kα radiation at 40 kV and 40 mA in the 2θ ranging from 20° to 80° with a scan rate of 0.02° per second. Field-emission scanning electron microscopy (FE-SEM) was used to characterize the morphology and elemental distribution of the as-prepared samples on a FEI Nova NANOSEM 230 spectrophotometer. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) images and energy-dispersive X-ray spectroscopy (EDX) were obtained using a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The optical properties of the as-prepared samples were analyzed by UV–vis diffuse reflectance spectroscopy (DRS) using a UV–vis spectrophotometer (Cary 500, Varian Co.), in which BaSO4 was employed as the internal reflectance standard. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a Thermo Scientific ESCA Lab 250 spectrometer which consists of a monochromatic Al Kα as the X-ray source, a hemispherical analyzer and sample stage with multi-axial adjustability to obtain the surface composition of the sample. All of the binding energies were calibrated by the C 1 s peak at 284.6 eV. The concentration of formic acid was measured by the ion chromatography (Dionex, ICS-1100). Electron spin resonance (ESR) signal of the radicals spin-trapped by 5,5-dimethyl-l-pyrroline-N-oxide (DMPO) was recorded on a Bruker EPR A300 spectrometer. The irradiation source (λ ≥ 420 nm) was a 300 W Xe arc lamp system and the whole ESR experiment was measured under room temperature. The settings for the ESR spectrometer were as follows: center field = 3507 G, microwave frequency = 9.84 GHz and power = 6.36 mW. The Mott-Schottky experiments were obtained on a Precision PARC workstation. The electrochemical analysis was carried out in a conventional three-electrode cell using a Pt plate and an Ag/AgCl electrode as the counter electrode and reference electrode, respectively. For electrode preparation, indium-tin oxide (ITO) glass was firstly cleaned by sonication in ethanol for 30 min and dried at 353 K. The boundary of ITO glass was protected using scotch tape. 5 mg of sample was dispersed in 0.5 mL ethanol by sonication to get a slurry. The slurry was spread onto the pretreated ITO glass. After air drying, the working electrode was further dried at 393 K for 2 h to improve adhesion. Then the scotch tape was unstuck and the uncoated part of the electrode was isolated with epoxy resin. The exposed area of the working electrode was 0.25 cm2. Mott-Schottky experiments were measured in a sodium sulfate electrolyte solution (0.2 M) (pH = 6.8), the potential ranged from −0.2 V to 0.8 V, and the perturbation signal were 10 mV with the frequency at 1 K Hz.

Photocatalytic activity test

In a typical photocatalytic reaction, a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight Co., Ltd.) with a UV−CUT filter to cut off light of wavelength <420 nm was used as the irradiation source. 80 mg of photocatalyst was added into 20 mL of the methanol aqueous solution (19 mL H2O with 1 mL methanol). Before visible light illumination, the above suspension was stirred in the dark for 1 h to ensure the establishment of adsorption-desorption equilibrium between the sample and reactant. During the process of the reaction, 5 mL of sample solution was collected after 4 hours of visible light irradiation and centrifuged to remove the catalyst completely at 12000 rpm. Afterward, the concentration of H2O2 was determined by KMnO4 titration (cKMnO4 = 1 mmol • L−1) with the addition of 5 mL 1 M H2SO4 solution. When the solution becomes the pink after the addition of KMnO4 solution and keeps the color of solution for 30 s, the concentration of KMnO4 solution is equivalent to the concentration of H2O2. All of the experimental processes were conducted under ambient temperature.

Additional Information

How to cite this article: Zhuang, H. et al. Robust Photocatalytic H2O2 Production by Octahedral n class="Chemical">Cd3(C3N3S3)2 Coordination Polymer under Visible Light. Sci. Rep. 5, 16947; doi: 10.1038/srep16947 (2015).