As a metal-free semiconductor photocatalyst, graphitic carbon nitride (g-C3N4) has been widely used in many photocatalytic applications, such as the removal of aqueous organic pollutants, evolution of hydrogen by water-splitting, reduction of carbon dioxide into hydrocarbon fuels, and synthesis of organic target compounds by selective oxidation [1-10]. However, its bulk layered structure and powder physical state severely limit the practical application of g-C3N4 in heterogeneous photocatalytic reactions [11-17]. On the one hand, the bulk layered structure of g-C3N4 limits the surface migration of photogenerated carriers and increases the mass transfer resistance in photocatalytic reactions. On the other hand, the powder physical state of g-C3N4 is inconvenient for practical application because of the additional solid-liquid separation step. Therefore, several studies have been conducted to develop a morphologically controlled synthesis and supporting material for g-C3N4 to overcome these disadvantages [18-24].
The construction of a porous microstructure is a promising strategy for the synthesis of g-C3N4 with enhanced photocatalytic activity [25]. Dong et al. [26, 27] synthesized porous g-C3N4 using hydrochloric acid-pretreated melamine as a precursor. In the present study we show that melamine pretreated with either hydrochloric acid or ethylene glycol (EG) can be used as a precursor for the preparation of porous g-C3N4. Interestingly, the present work also shows that porous g-C3N4 prepared using melamine co-pretreated with both of these reagents exhibits a more abundant porous microstructure than porous g-C3N4 prepared using melamine singly pretreated with just one of these reagents, which is attributed to the co-existence of different pore-fabricating units in the preparation system of the former.
In contrast to the preparation of supported titanium dioxide, supported g-C3N4 is difficult to fabricate because of the lack of a suitable sol-gel process [28-32]. Moreover, the practical application of g-C3N4supported on a substrate of fluorine-tin oxide (FTO) suffers from several drawbacks. First, the sheet-shaped physical structure of the FTO substrate is not conducive to the diffusion of reactants. Second, the absorption of light by the FTO substrate reduces the amount of light available for the photocatalyst. Therefore, g-C3N4 can only be coated on one side of the substrate. Third, no chemical bonds are formed between g-C3N4 and the FTO substrate. Therefore, the loss of g-C3N4 is inevitable in the course of the photocatalytic reaction.
To solve these drawbacks of g-C3N4 in practical applications, porous g-C3N4 and supported porous g-C3N4 were fabricated for the first time using melamine co-pretreated with hydrochloric acid and EG as a raw material and a quartz rod co-pretreated with hydrofluoric acid and (3-aminopropyl) trimethoxysilane as a substrate. The as-prepared porous g-C3N4 showed excellent photocatalytic activity because of its richly porous microstructure. The as-prepared supported porous g-C3N4 exhibited considerable stability because of the chemical interaction between porous g-C3N4 and the quartz rod substrate. In addition, the photocatalytic activity of the supported porous g-C3N4 was competitive with that of porous g-C3N4 in powder form because neither the surface migration of photogenerated carriers nor the diffusion of the target organic pollutant were affected by the construction of the quartz rod reactor.
The photocatalytic activities of the as-prepared porous g-C3N4 and supported porous g-C3N4 were preliminarily evaluated by the treatment of single-component organic wastewater under visible-light irradiation. Subsequently, the as-prepared porous g-C3N4 was further applied in conventional hydrogen evolution and a new system for simultaneous hydrogen evolution with organic-pollutant degradation. In the system for simultaneous hydrogen evolution with organic-pollutant degradation, both the hydrogen yield and the degradation efficiency were significantly decreased compared with conventional photocatalytic degradation and hydrogen evolution systems because of the difficulty of the redox reaction between water and the organic pollutant. However, both factors were found to increase with increasing photocatalytic activity of the as-prepared materials used in the system. The reason for this improvement was investigated.
Melamine (C3H6N6, CP grade) and chloroplatinic acid (H2PtCl6·6H2O, GR grade) were purchased from Sinopharm Chemical Reagent Co. Ltd. Quartz rods (2 mm × 10 cm, abbreviated QR) were purchased from Shenghui Quartz Products Co. Ltd. Hydrochloric acid (HCl, AR grade, 36%-38%), EG (C2H6O2, AR grade), and hydrofluoric acid (HF, AR grade, ≥40%) were purchased from Xilong Chemical Co. Ltd. (3-Aminopropyl) trimethoxysilane (C6H17NO3Si, 97%, abbreviated KH-540) and p-chlorophenol (C6H5OCl, GC grade, abbreviated PCP) were purchased from Aladdin Chemistry Co. Ltd. Rhodamine B (C28H31ClN2O3, AR grade, abbreviated RB), and triethylamine ((C2H5)3N, AR grade, abbreviated TEA) were purchased from Shanghai Fine Chemical Technology Co. Ltd. All chemicals were used without further purification. Double-distilled water was used in the catalyst preparation and subsequent catalytic tests.
In a typical synthesis, 3 g of melamine was placed into a 100-mL beaker, followed by the addition of 10 mL of HCl, 10 mL of EG, and 3 mL of water. After stirring the white viscous suspension for 1 h at room temperature, the HCl-and EG-co-pretreated melamine precursor was obtained by washing, centrifugation, and drying. Subsequently, the HCl-and EG-co-pretreated melamine precursor was transferred to a 10-mL alumina crucible with a cover. The crucible was heated to 250 ℃ from room temperature in a muffle furnace at a heating rate of 5 ℃/min, and then further heated to 550 ℃ at a rate of 10 ℃/min. After maintaining the temperature at 550 ℃ for 2 h, a sample of yellow porous g-C3N4 was obtained after natural cooling, which was denoted pg-C3N4-(HCl + EG). For comparison, samples denoted pg-C3N4-EG and pg-C3N4-HCl were also prepared by the same method but in the absence of HCl or EG, respectively, in the course of melamine pretreatment, and with the addition of 20 mL of EG or HCl (rather than 10 mL) during synthesis. Bulk g-C3N4 was also prepared by the same method but using non-pretreated melamine as a precursor.
In a typical synthesis, the quartz rods were soaked in dilute HF for 20 min at room temperature and then washed with water several times. The HF-pretreated quartz rods were placed into a 25 mL test tube containing 1 mL of KH-540 and 20 mL of ethanol, and then heated at 60 ℃ for 24 h. After washing the HF-and KH-540-co-pretreated quartz rods with ethanol and water several times, the above-prepared HCl-and EG-co-pretreated melamine precursor was uniformly coated onto the surface of the quartz rods. The coated quartz rods were transferred to a ash pan (6 cm × 12 cm) after drying at 60 ℃ for 24 h and heated at 550 ℃ for 2 h. Finally, the quartz rod-supported porous g-C3N4 samples were obtained after natural cooling, and denoted QR-pg-C3N4.
Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were recorded using a Quanta 200 environmental scanning electron microscope. Nitrogen gas porosimetry measurements were performed on a Quantachrome NOVA 2000e surface area and porosity analyzer after the samples were outgassed under a vacuum at 70 ℃ for 20 min and 150 ℃ for 6 h. X-ray diffraction (XRD) patterns were obtained using a D8 ADVANCE diffractometer via Cu-Kα radiation. Fourier transform infrared (FTIR) spectra were recorded on a Bruker VERTEX 70 FTIR apparatus.X-ray photoelectron spectroscopy (XPS) was performed using an Axis Ultra DLD instrument with a monochromated Al-Kα source at a residual gas pressure of less than 10-8 Pa. All the binding energies were referenced to the C 1s peak at 285 eV of the surface adventitious carbon. Ultraviolet-visible/diffuse reflectance spectroscopy (UV-Vis/DRS) was conducted using a Lambda 750S UV/VIS/NIR spectrometer. Photoluminescence (PL) measurements were carried out on a HITACHI F-7000 fluorescence spectrophotometer.
A PLS-SXE 300 Xe lamp (300-W, Beijing PerfectLight Co. Ltd., China) with an output wavelength λ > 320 nm served as the light source. The UV component of the irradiation from the lamp was removed using a 420-nm-cutoff filter, so that only visible light with an output wavelength λ > 420 nm was retained. 100 mg of powder photocatalyst and 100 mL of single-component organic wastewater (containing 10 ppm RB or 20 ppm PCP) were poured into a beaker with a quartz cover. Additionally, 100 mg of supported photocatalyst and 100 mL of single-component organic wastewater (20 ppm PCP) were poured into a self-designed quartz rod reactor (Scheme 2). For the powder photocatalyst, the suspension was ultrasonicated for 10 min and stirred in the dark until adsorption-desorption equilibrium. For the supported photocatalyst, the reactor was left to stand until adsorption-desorption equilibrium was reached. Subsequently, the light source was switched on, and fixed amounts of the reaction solution were extracted at pre-determined time intervals during irradiation. Changes in the RB concentrations were analyzed using a UNICO UV-2000 spectrophotometer at λ=554 nm. Changes in the PCP concentrations were analyzed using an Agilent 1100 series high-performance liquid chromatography (HPLC) C18 column and a UV detector (λ=277 nm), with acetonitrile/water (60/40 v/v) used as the mobile phase at a flow rate of 1.0 mL/min.
100 mg of powder photocatalyst loaded with 3 wt% Pt co-catalyst and 100 mL of H2O containing 10 vol% TEA were poured into a quartz reactor. The above suspension was ultrasonicated for 10 min and stirred in the dark for 1 h. Subsequently, the light source was switched on, and further stirring was performed. The temperature of the suspension was maintained at 35±2 ℃ by circulation of water through an external cooling jacket. After irradiation under visible light with λ > 420 nm for 6 h, the generated hydrogen was analyzed in situ with a GC 7890-Ⅱ TCD gas chromatograph (TECHCOMP) using an MS-5 A column, which was connected to a circulating gas line with an argon carrier. For comparison, the photocatalytic tests were also performed in a pure TEA system.
The above-described hydrogen evolution system, i.e., 100 mL of H2O containing 10 vol% TEA, was replaced by 100 mL of an aqueous solution of an organic pollutant (50 ppm RB). After allowing the organic pollutant and the photocatalyst to reach adsorption-desorption equilibrium in preparation for the hydrogen evolution reaction, the saturated photocatalyst was separated and transferred to a new reaction solution. For comparison, the photocatalytic tests were also performed in a pure H2O system. The generated hydrogen was analyzed in situ with a GC 7890-Ⅱ TCD gas chromatograph. The change in the RB concentration was analyzed using a UNICO UV-2000 spectrophotometer at λ=554 nm.
The morphologies of the as-prepared bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) were characterized by TEM observation (Fig. 1(a)-(d)). As shown in Fig. 1(a), the bulk g-C3N4 exhibited a non-porous graphite-like layered microstructure. It can be clearly seen from Fig. 1(b) and (c) that samples of porous g-C3N4 with different pore geometries were successfully fabricated using melamine pretreated with EG or HCl, respectively. The different pore geometries of pg-C3N4-EG and pg-C3N4-HCl suggest the presence of different pore-fabricating units in the catalyst preparation systems. Compared with pg-C3N4-EG and pg-C3N4-HCl, the formation of a more richly porous microstructure for pg-C3N4-(HCl + EG) can be attributed to the co-existence of different pore-fabricating units in the combined catalyst preparation system (Fig. 1(d)). Fig. 1(e) and (f) show an SEM image and a randomly sampled photograph of QR-pg-C3N4, respectively, demonstrating that porous g-C3N4 was successfully coated onto the quartz rod substrate with a uniform and stable morphology. The porous g-C3N4 coating had a thickness of about 20 μm.
The textural properties of the as-prepared bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) were characterized by measuring their nitrogen gas porosity (Fig. 2). As shown in Fig. 2(a), the type-Ⅱ isotherm of bulk g-C3N4implies its non-porosity. The type-Ⅳ isotherms with H3-type hysteresis loops for pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) are evidence of their porous microstructures. As shown in Fig. 2(b), the Barrett-Joyner-Halenda (BJH) pore-size distribution curves reveal that all of the tested materials exhibited a weak narrow peak in the range of 3-5 nm, which can be attributed to the released NH3, which acted as a soft template in the course of melamine polycondensation. The strong, broad pore-size distribution peaks in the range of 5-120 nm for pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) originate from the formation of porous microstructures. The Brunauer-Emmett-Teller (BET) surface areas of pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) were larger than that of bulk g-C3N4 because their porous microstructures resulted in more exposed geometrical surfaces compared with the bulk layered structure of the latter. The BET surface area of pg-C3N4-(HCl + EG) was larger than those of pg-C3N4-EG and pg-C3N4-HCl because of its more richly porous microstructure.
The phase structures of the as-prepared bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) were characterized by XRD analysis (Fig. 3). The bulk g-C3N4 exhibited a typical (002) interlayer-stacking peak at 27.5°, corresponding to an interlayer distance of d=0.33 nm, and a (100) peak at 12.9° corresponding to an in-plane structural packing motif with a period of 0.675 nm. Compared with bulk g-C3N4, the weaker diffraction intensity of the (002) peaks for pg-C3N4-EG and pg-C3N4-HCl can be attributed to the reduced content of layered morphologies in the porous microstructures of the latter materials. For pg-C3N4-(HCl + EG), the diffraction intensity of the (002) peak was weaker still than those of pg-C3N4-EG and pg-C3N4-HCl because of its even more richly porous microstructure.
The changes in the chemical structure of the pretreated melamine were confirmed by FTIR spectral characterization. As shown in Fig. 4(a), the characteristic FTIR peaks of melamine and EG-pretreated melamine were very similar, indicating that the chemical structure of melamine remained intact after EG pretreatment. However, the characteristic peaks of HCl-pretreated and HCl/EG-co-pretreated melamine were shifted with respect to melamine but did not correspond to those of pure cyanuric acid. This result indicates the formation of hydrogen-bonded aggregates of melamine-cyanuric acid during the pretreatment of melamine by HCl [33]. The FTIR spectra of bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) are shown in Fig. 4(b). For all the tested materials, the sharp peak at 813.5 cm-1 is the typical bending vibration of s-triazine units, the series of peaks in the range of 1100-1700 cm-1 is attributed to stretching modes, including C-N and C=N, in the CN heterocycles, and the broad absorption peaks in the range of 2900-3400 cm-1 originate from the stretching vibrational modes of primary (-NH2) and secondary (-NH) amines. Evidently, the FTIR spectra of pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) were similar to that of bulk g-C3N4, indicating that these species retained the same chemical structure as bulk g-C3N4 after the formation of their porous microstructures. However, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) exhibited stronger FTIR modes compared with g-C3N4 because they contained greater amounts of exposed surface functional groups.
The surface composition and chemical state of the constituent elements of bulk g-C3N4 and pg-C3N4-(HCl + EG) were characterized by a high-resolution XPS probe technique (Fig. 5). As shown in Fig. 5(a), bulk g-C3N4 exhibited a peak in the C 1s binding-energy region centered at 285.0 eV, which is typically assigned to C-C and/or C=C, and originates from the adventitious reference carbon on the surface. The peak centered at 288.3 eV originates from sp2-hybridized C atoms bonded to N in an aromatic ring (N=C-(N)2), while the peak centered at 289.1 eV is assigned to sp2 C atoms in an aromatic ring attached to primary and secondary amines (N=C (N)-NH2, N=C (N)-NH). Compared with bulk g-C3N4, the XPS peaks of sp2 C atoms for pg-C3N4-(HCl + EG) were more intense and shifted to higher binding energies because the formation of a porous microstructure increased the number of exposed surface functional groups, so that the original electronic environment of the sp2 C atoms was perturbed by the increased presence of surface-NH2 and-NH groups. The high-resolution XPS results for the N 1s binding-energy regions (Fig. 5(b)) are consistent with the C 1s results just described. For bulk g-C3N4, the peak centered at 398.8 eV is assigned to sp2-hybridized aromatic N atoms bonded to C atoms (C=N-C). The peak centered at 400.2 eV is related to either tertiary N groups ((C)3-N) linking structural motifs (C6N7) or amino groups carrying hydrogen ((C)2-NH, C-NH2) in connection with structural defects and incomplete condensation. The peak at 400.8 eV corresponds to N atoms bonded to three C atoms in an aromatic ring (N-(C)3). The weak peak at 404.2 eV is attributed to charging effects or positive-charge localization in heterocycles [34]. For pg-C3N4-(HCl + EG), the XPS peaks of the N 1s binding-energy regions were more intense and shifted to higher binding energies compared with those of bulk g-C3N4 because of the increased number of exposed surface functional groups and the changes in the original electronic environment of the N atoms.
The light absorption properties of the as-prepared bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) were studied by UV-Vis/DRS. As shown in Fig. 6(a), bulk g-C3N4 displayed typical semiconductor absorption within the region of 200-465 nm, originating from an induced electronic transition from the valence band (VB), populated by N 2p orbitals, to the conduction band (CB), formed by C 2p orbitals. Compared with bulk g-C3N4, the enhanced light absorption in the region of 200-465 nm for pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) can be attributed to the increased efficiency of the electronic transition from VB to CB, because their porous microstructures are more favorable than the bulk layered structure for this transition.
The photocatalytic quantum efficiencies of the as-prepared bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) were studied by PL measurements. As shown in Fig. 6(b), with an excitation wavelength of 330 nm and an operating voltage of 400 V, bulk g-C3N4 exhibited a broad fluorescence emission peak in the range of 400-600 nm. This finding suggests that the photoinduced e--h+ pairs generated within the bulk g-C3N4had a tendency to recombine. Compared with bulk g-C3N4, the successively decreasing PL intensities of pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) indicate that their porous microstructures promoted the efficient separation and transportation of the photogenerated carriers.
To date, there are few reports of the fabrication of porous g-C3N4 by the use of a pretreated precursor. Dong et al. [27] suggested that pretreatment with HCl would alter the condensation behavior of melamine, and thus induced the formation of porous g-C3N4, through the protective neutralization of the amino groups by reaction with HCl, thus limiting the involvement of the amino groups in thermal condensation. From the above characterization results we conclude that the successful construction of the porous microstructures is attributed to the presence of pore-fabricating units in the preparation systems for porous g-C3N4. These pore-fabricating units effectively hindered the continuous polycondensation of melamine during bulk g-C3N4 formation. Therefore, a porous microstructure was formed in the regions where melamine polycondensation was terminated.
As shown in Scheme 1, for the EG-pretreated melamine system, the pore-fabricating units were formed by hydrogen bonding interactions between melamine and EG. The EG-protected melamine was unable to self-react with the original melamine species, thus terminating melamine polycondensation. Eventually, a porous microstructure was formed in the terminal region of melamine polycondensation. In the case of HCl pretreatment, melamine and cyanuric acid co-existed in the preparation system because of the formation of the latter by the acid-base reaction between melamine and HCl. Typically, cyanuric acid exists in two isomeric forms, s-triazine-2, 4, 6-trione and s-triazine-2, 4, 6-triol. The trione form is unable to react with melamine by high-temperature polycondensation to produce pores. However, the triol form can react with melamine by intermolecular dehydration. Therefore, a porous microstructure was formed in the terminal region of the polycondensation of melamine and the s-triazine-2, 4, 6-triol form of cyanuric acid. For the HCl-and EG-co-pretreated melamine system, a more richly porous microstructure was fabricated because of the co-existence of different pore-fabricating units. Specifically, the three types of pore-fabricating units in this catalyst preparation system were EG-protected melamine, cyanuric acid in the s-triazine-2, 4, 6-trione form, and EG-protected cyanuric acid in the s-triazine-2, 4, 6-triol form. Compared with the singly EG-or HCl-pretreated melamine systems, the HCl-and EG-co-pretreated melamine system contained not only more types of pore-fabricating units but a greater absolute amount of them. Therefore, a richly porous microstructure was fabricated in the terminal region of melamine polycondensation.
Photocatalytic degradation tests were conducted in an aqueous solution containing oxygen from dissolved air. The photocatalytic activity of the as-prepared bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) was preliminarily evaluated by the degradation of aqueous RB. As shown in Fig. 7(a), the direct photodegradation experiment (i.e., in the absence of a catalyst) found that the RB concentration in the reaction system showed negligible changes under visible-light irradiation for 90 min. For the catalytic experiments, adsorption tests were performed to confirm that the adsorption-desorption equilibria had been reached prior to irradiation with the Xe lamp. The percentages of RB adsorbed on the bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) were 1.9%, 12.4%, 15.5%, and 18.6%, respectively. The photocatalytic activities of the as-prepared photocatalysts for the degradation of aqueous RB followed the order of bulk g-C3N4 < pg-C3N4-EG < pg-C3N4-HCl < pg-C3N4-(HCl+EG). Compared with bulk g-C3N4, the increased photocatalytic activity of pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) can be attributed to their porous microstructures, which enlarged the BET surface areas, improved the light-absorption capacities, and increased the photocatalytic quantum efficiencies of these as-prepared materials. The photocatalytic activity of pg-C3N4-(HCl + EG) was higher than those of pg-C3N4-EG and pg-C3N4-HCl owing to its more richly porous microstructure.
The stability and photocatalytic activity of the as-prepared QR-pg-C3N4 were evaluated by applying it for the degradation of aqueous PCP in three consecutive cycles. As shown in Fig. 7(b), the capacity of the as-prepared QR-pg-C3N4 for aqueous PCP degradation proved highly stable. We attribute this to the chemical interaction between the quartz rod substrate and porous g-C3N4 (Scheme 2). The photocatalytic activity of QR-pg-C3N4 remained at a similar level to that of powder pg-C3N4-(HCl + EG), indicating that the surface migration of photogenerated carriers and the diffusion of the target organic pollutant were not affected after the construction of the quartz rod reactor (Scheme 2). The stability and photocatalytic activity of the as-prepared QR-pg-C3N4 suggest its potential utility in the practical treatment of industrial wastewater.
TEA is commonly used as an electron donor in traditional photocatalytic hydrogen evolution systems. Therefore, the photocatalytic activities of bulk g-C3N4, pg-C3N4-EG, pg-C3N4-HCl, and pg-C3N4-(HCl + EG) for hydrogen evolution by water-splitting were evaluated in a TEA aqueous solution (10 vol%) under visible-light (λ > 420 nm) irradiation for 6 h in the presence of 3 wt% Pt co-catalyst. As shown in Fig. 8(a), the photocatalytic activities of the tested photocatalysts for hydrogen evolution by water-splitting followed the order of bulk g-C3N4 < pg-C3N4-EG < pg-C3N4-HCl < pg-C3N4-(HCl + EG). Thus, pg-C3N4-(HCl + EG) showed the highest hydrogen evolution activity among the tested samples, presumably because of its richly porous microstructure. However, when tested in a pure TEA system, pg-C3N4-(HCl + EG) showed a much lower hydrogen evolution activity, presumably because of the absence of water. This verifies that the production of hydrogen originated from water-splitting rather than TEA decomposition.
In the system for simultaneous hydrogen evolution with organic-pollutant degradation, the hydrogen yield and degradation efficiency were both reduced compared with the respective conventional systems (Fig. 8(b)). In previous work [23] we showed that both photocatalytic degradation and hydrogen evolution were redox reactions and that the photocatalyst acted as an electron-transfer medium in the redox process. However, in the system for simultaneous hydrogen evolution with organic-pollutant degradation, the redox reaction between the organic pollutants and water was heavily disfavored because of the poor electron-donating ability of the pollutants, hence the decreased hydrogen yield and degradation efficiency. Interestingly, however, in the simultaneous system, the hydrogen yield and degradation efficiency both increased as the photocatalytic activity of the as-prepared materials used in the system increased. This indicates that enhancing the electron-transfer capability of the photocatalyst facilitated the redox reaction between organic pollutants and water. Additionally, because hydrogen evolution is a redox reaction, it was heavily disfavored in the pure H2O or pure TEA systems because of the absence of a reducing agent (TEA) or oxidizing agent (H2O), respectively. Therefore, the hydrogen yield was low in these cases.
Porous g-C3N4 and supported porous g-C3N4 were successfully fabricated by using HCl-and EG-co-pretreated melamine as a raw material and quartz rod as a substrate. The formation of a richly porous microstructure can be attributed to the co-existence of different pore-fabricating units in the preparation system for porous g-C3N4. The as-prepared pg-C3N4-(HCl + EG) showed excellent photocatalytic activity because of the enlarged BET surface area, improved light-absorption capacity, and increased photocatalytic quantum efficiency resulting from its richly porous microstructure. The considerable stability of the as-prepared QR-pg-C3N4 can be attributed to the chemical interaction between the porous g-C3N4 and quartz rod substrate. The QR-pg-C3N4 and pg-C3N4-(HCl + EG) had similar photocatalytic activities, indicating that the surface migration of photogenerated carriers and the diffusion of the target organic pollutant were not affected after the construction of the quartz rod reactor. In a system for simultaneous hydrogen evolution with organic-pollutant degradation, the finding that both the hydrogen yield and the degradation efficiency increased with increasing photocatalytic activity of the as-prepared materials indicates that enhancing the electron-transfer capability of the photocatalyst facilitated the redox reaction between organic pollutants and water.
2017,
Vol. 38 Issue (3): 498-508 
PDF
2017,
Vol. 38 Issue (3): 498-508 PDF