CHEMICAL ENGINEERING TRANSACTIONS VOL. 56, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Jiří Jaromír Klemeš, Peng Yen Liew, Wai Shin Ho, Jeng Shiun Lim Copyright © 2017, AIDIC Servizi S.r.l., ISBN 978-88-95608-47-1; ISSN 2283-9216 Photocatalytic Carbon Dioxide Reduction to Fuels Over Cu- Loaded g-C3N4 Nanocatalyst under Visible Light Beenish Tahir, Muhammad Tahir, Nor Aishah Saidina Amin* Chemical Reaction Engineering Group (CREG), Department of Chemical Engineering, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor Malaysia. *noraishah@cheme.utm.my Photocatalytic carbon dioxide (CO2) conversion to chemicals and fuels has gained significant consideration in industrial and scientific research. In this study, photocatalytic CO2 reduction to fuels over Cu-loaded graphitic carbon nitride (g-C3N4) under visible light irradiation has been investigated. The photocatalysts, synthesized by pyrolysis and impregnation method, were characterized by X-ray diffraction (XRD) Fourier transform infrared (FTIR) and Scanning electron microscopy (SEM). Interestingly, CO2 was efficiently converted to CH4 and CH3OH with smaller amounts of C2H4 and C2H6 hydrocarbons. The yield of CH4 evolution as the main product over 3 wt. % Cu/g-C3N4 was 217.8 µmole/g.cat under visible light irradiation, significantly higher than the amount of CH4 produced over the pure g-C3N4 catalyst (119 µmole/g.cat). The enhancement was attributed to charge transfer property and suppressed recombination rate by Cu-metal. The Cu-metal loaded into g-C3N4 enhanced CO3 reduction efficiency for CH4 production while the pure g-C3N4 was promising for both CH4 and CH3OH production. The single step conversion of CO2 to CH4 and CH3OH with appreciable amount of hydrocarbons under solar energy registered good photo-activity and selectivity of Cu/g-C3N4 catalyst. A photocatalytic reaction mechanism was proposed to corroborate with the experimental results over the Cu-loaded g-C3N4 photocatalyst. 1. Introduction Increasing levels of carbon dioxide (CO2) emissions in the atmosphere from fossil fuel combustion are widely recognized as one of the primary cause of greenhouse effect. Among the carbon capture and sequestration, development of an artificial photosynthesis system using solar energy is a promising strategy for the photocatalytic conversion of CO2 to solar fuels (Tahir et al. 2015b). Among the solar fuels, the production of CO (Tahir et al. 2016a), CH4 (Zhu et al. 2016) and CH3OH (Gusain et al. 2016) via a single step CO2 conversion, has sparked a new sustainable development in the field (He et al. 2016). Among the semiconductor materials, TiO2 is the most widely studied photocatalyst due to its numerous advantages such as low cost and excellent chemical and thermal stability (Tahir et al. 2016b). However, TiO2 is only active under UV-light irradiations and have poor photocatalytic activity due to the fast recombination of photo-generated charges (Tahir et al. 2015a). Considering the large portion of solar spectrum available, the demand for visible light responsive and low-cost photocatalysts has been regarded as an attractive area of research. Recently, the use of graphitic carbon nitride (g-C3N4) as a photocatalyst has been promising alternatives due to advantages such as visible light responsive, low-cost synthesis and high chemical/thermal stability (Ma et al. 2016). In the recent years, there have been number of studies focused on photocatalytic CO2 reduction by g- C3N4 based photocatalyst. However, the efficiency and selectivity of CO2 conversion over pure g-C3N4 photocatalyst is still quite limited. The photocatalytic activity of g-C3N4 could be enhanced by loading with metals or combining with other semiconductor materials. In this perspective, Pt-g-loaded C3N4/kNbO3 photocatalyst has been recently investigated for enhanced CO2 photoreduction under visible light irradiations (Shi et al. 2015). The amine-functionalized g-C3N4 has been reported to improve CO2 adsorption capacity with enhanced activity for CO2 photoreduction into CH4 and CH3OH (Huang et al. 2015). Similarly, selective photocatalytic CO2 reduction to CH3OH been reported using ZnO/g- C3N4 photocatalyst under visible light irradiations. The g-C3N4-N/TiO2 photocatalyst has been investigated for DOI: 10.3303/CET1756068 Please cite this article as: Tahir B., Tahir M., Amin N.A.S., 2017, Photocatalytic carbon dioxide reduction to fuels over cu-loaded g-c3n4 nanocatalyst under visible light, Chemical Engineering Transactions, 56, 403-408 DOI:10.3303/CET1756068 403 the selective CO2 photoreduction to CO (Zhou et al. 2014). Pt-loaded g-C3N4 with improved day-light induced photocatalytic CO2 reduction to CH4 was explored. Significantly improved g-C3N4 photoactivity was found with Pt-loading, perhaps, due to hindered charges recombination rate (Ong et al. 2015). On the other hand, copper based semiconductors are gaining large interest and are considered as efficient for selective CO2 photoreduction to CH4 and CH3OH (Liu et al. 2015). Therefore, it is anticipated that photocatalytic CO2 reduction over Cu-promoted g-C3N4 catalyst would be appreciable to stimulate photocatalytic CO2 reduction to selective fuels under visible light irradiations. In this study, highly active and visible light responsive Cu-promoted g-C3N4 Nanosheets were successfully synthesized by thermal treatment of melamine. The photoactivity of different Cu-loaded g-C3N4 catalysts were examined for selective CO2 photoreduction to fuels. In addition, the photocatalytic reaction mechanism for CO2 reduction were analysed based on the experimental results. 2. Experimental 2.1 Catalyst preparation and characterization The g-C3N4 was synthesized by the thermal treatment of Melamine (Sigma Aldrich AR ≥ 99%). In a typical process, 5 g of Melamine was put in a crucible with a cover then calcined in a Muffle Furnace and heated to 550 oC for 2 h. The sample was then washed with 0.1 mol/L nitric acid (Sigma Aldrich AR) and distilled water to remove any residual alkaline species (e.g. ammonia) adsorbed on the sample surface. After this, it was dried at 80 °C for 12 h in an oven. The Cu-loaded g-C3N4 samples were prepared by a wet-impregnation and sonication method. 0.5 g of g-C3N4 was dispersed in 20 mL water and specific amount of Cu (NO3)2.3H2O was added to it. The mixture was stirred for 4 h and then sonicated to get Cu-promoted g-C3N4 Nanosheets. The sample was oven dried at 80 °C for 12 h then calcined at 450 °C for 1 h. The crystalline structure of the catalysts were ascertained by X-ray diffraction (XRD) recorded on a powder diffractometer (Bruker Advance D8, 40 kV, 40 mA) using a Cu Kα radiation source in the range of 2θ = 5-80 ° with a step size of 0.05 ° and counting time of 5 s. The Fourier transform infrared (FTIR) spectrum of the sample was recorded on a Thermo Nicolet Avatar 360 FTIR spectrometer. The surface morphology was examined using field emission scanning electron microscopy (FESEM JEOL model JSM-6700F, Japan). 2.2 Photoactivity testing The reactor consists of stainless steel cylindrical vessel with a length of 5.5 cm and total volume of 150 cm3. 10 mg powder photocatalyst was evenly placed inside the cylindrical stainless steel chamber, equipped with a quartz window for passing light irradiations. The source of light irradiation was a simulated sunlight having the same spectra with real sunlight. Compressed CO2 (99.999 %) regulated by a mass flow controller (MFC) was bubbled through water saturator to carry moisture maintained at temperature of 30 oC. The reactor was purged and saturated for half an hour using a mixture of CO2 and water prior to the start of the experiment. The temperature inside the reactor was controlled using temperature controller. The products were analyzed using an on-line gas chromatograph (GC-Agilent Technologies 6890 N, USA) equipped with thermal conductivity detector (TCD) and flame ionized detector (FID). Furthermore, FID detector was connected with a HP-PLOT Q capillary column and TCD detector was connected to UCW982, DC-200, Porapak Q and Mol Sieve 5A columns. 3. Results and discussion 3.1 Characterization analysis of catalysts Figure 1 (a) presents the spectra of g-C3N4 and Cu/g-C3N4 samples and can be identified the presence of two peaks. The peak at 27.3 ° located at plane (002) presents interlayer stacked conjugated aromatic system, while the peak at ~ 13 ° with plane (100) reveals the intra-planar structural packing of the aromatic system. The signals corresponding to Cu in metal or oxides states were not detected in all the XRD patterns. This was possibly due to the lower amounts, which were below the detection limit of XRD or the Cu-species were highly dispersed over the g-C3N4 structure. Figure 1 (b) shows Infrared spectra of g-C3N4 and Cu-loaded g-C3N4 samples. All the spectra bands are attributed to the samples containing g-C3N4. An absorption band at 805 cm-1 is associated with the bending nodes of the parent structure of g-C3N4 in both the samples. The N-H stretch of the heterocyclic amines present in the g-C3N4 structure are identified by the bands at 1248-1571. The band located at 1,640 cm-1 can be attributed to the stretching vibration band of the C-N of amines in g-C3N4 (He et al., 2015). 404 5 10 15 20 25 30 35 40 45 50 55 60 In te n s it y ( a .u ) 2-theta (degree) (002) (100) g-C 3 N 4 3% Cu/g-C 3 N 4 (a) 4000 3500 3000 2500 2000 1500 1000 500 T ra n s m it ta n c e ( a .u .) Wavenumber (cm -1 ) g-C 3 N 4 3% Cu/g-C 3 N 4 (b) Figure 1: (a) XRD analysis of Cu/g-C3N4 samples; (b) FTIR spectra of the corresponding samples. Figure 2 exhibits the morphology of pure and modified g-C3N4 samples. From Figure 2 (a), it can be seen that g-C3N4 has obvious lamellar and irregular folding structures like wrinkled sheets, in which layers are stacked together. Meanwhile, 3 % Cu/g-C3N4 in Figure 2 (b) depicts porous g-C3N4 layers of sheet-like structure. The slight disparity between the two samples is possibly due to effective stirring and sonication during metal impregnation of Cu/g-C3N4 sample. Figure 2: SEM images of Cu/g-C3N4 samples; (a) SEM image of g-C3N4, (b) SEM image of Cu/g-C3N4. 3.2 Photocatalytic CO2 reduction with H2O Initially, blank experiments were conducted to confirm products formed were due to photoreduction of CO2 only. Experiments were conducted in a gas phase system at 100 oC and irradiation time 2 h. In all types of catalysts, carbon containing compounds were not detected in the reaction system without reactants or light irradiations. Thus, any carbon containing compounds produced were derived from CO2 photo-reduction with CH4 and CH3OH were found to be major CO2 photo-reduction product in all the experiments. The effects of Cu loading on the activity of g-C3N4 for CO2 reduction to CH4 and CH3OH under visible light irradiations are presented in Figure 3.With pure g-C3N4, the yield of CH4 is lower ,but gradually increases `with Cu loading until it attained an optimum yield at 3 wt. % Cu. With increasing Cu loading the yield decreases. This can be attributed to higher rates of charge recombination centres, resulting in reduced photoactivity (Liu et al. 2015).The effectiveness of Cu-loading was much appreciable for CH4 production than the CH3OH. On the other hand, yield of CH3OH production gradually reduced with Cu-loading. The result indicates that the loading of the 405 pure catalyst (g-C3N4) with Cu metal become more efficient for CH4 production, evidently due to reducing the band gap with hindered recombination rate of electron and hole pairs and ultimately increased in the product yield. However, pure g-C3N4 was more efficient for both CH4 and CH3OH production due to reason as discussed in reaction mechanism. The effects of Cu onto g-C3N4 performance for CO2 photo-reduction to hydrocarbons (C2H4 and C2H6) is presented in Figure 3 (b). Evidently, hydrocarbons were detected over all type of photocatalysts. However, Cu- loaded g-C3H4 have much appreciable effect of hydrocarbons production due to the more trapping and transport of electrons over Cu/g-C3N4 structure. Among the hydrocarbons, the yield of C2H6 was much higher in comparative to C2H4 and similar observation could be seen in all type of samples. This development has confirmed Cu-loaded g-C3N4 Nanosheets as a favourable visible light responsive photocatalyst for CH4, CH3OH and hydrocarbon production under solar energy. g-C3N4 1% Cu 3% Cu 5% Cu 0 40 80 120 160 200 240 Y ie ld o f C H 4 ( µ m o le g -c a t. - 1 ) CH 4 CH 3 OH Cu-loading (wt. %) 0 20 40 60 80 100 Y ie ld o f C H 3 O H ( µ m o le g -c a t. - 1 ) Figure 3: Effect of Cu loading onto the photoactivity of g-C3N4 Nanosheets for CO2 reduction with H2O to CH4 and CH3OH at 100 oC and irradiation time 2 h. g-C3N4 1% Cu 3% Cu 5% Cu 0.0 0.6 1.2 1.8 2.4 3.0 3.6 Y ie ld o f H C s ( µ m o le g -c a t. - 1 ) Cu-loading C 2 H 4 C 2 H 6 Figure 4: Photoreduction of CO2 to hydrocarbons using H2O reductant over Cu-loaded g-C3N4 samples under visible light irradiation and irradiation time 2 h. 406 3.3 Reaction mechanism The multi-step reaction pathway of the process splits into three parts: the formation of the CO2 radical, water splitting and formation of CH4, CH3OH and hydrocarbons. The feasible reaction pathway for this process is given by reactions in Eqs (1) – (8). visible light 3 4 -g C N e h    (1) 2 2 2 / 2Cu Cue e     (2) o 2 2 CO e CO     (3) 2 H O h OH H      (4) 2 3 2 6 6CO H e CH OH H O       (5) 2 4 2 8 8 2CO H e CH H O       (6) 2 2 4 2 2 12 12 4CO H e C H H O       (7) 2 2 6 2 2 14 14 4CO H e C H H O       (8) In the case of visible light irradiations, the photo-generated electrons and holes are produced over g-C3N4 photocatalyst. Eq (1) and (2) reveals photo-excited electron-hole pair production and their trapping by Cu-metal, resulting in prolonged lifetime of charges to precede oxidation and reduction process. The holes are used for oxidation of H2O while electrons are consumed by CO2 for its reduction as explained in Eq (3) and (4). Reactions in Eqs (5) to (8) divulged production of CH4, CH3OH and hydrocarbons through utilization of H+ ions and electrons in multi-step process. The photocatalytic activity is related to the band structure and the mechanism for the production of these products over Cu/g-C3N4 photocatalyst is explained in Figure 5. Under the light irradiations, VB electrons of g- C3N4 can transfer to Cu-metal which cause electron-hole pair separation. This is due to the possibility of electron trapping by the copper ions as a result of the difference in reduction potential of Cu2+ which is more positive than the conduction band edge of g-C3N4 (-1.23 V vs. NHE). Since the reduction potential of CO2/CH3OH (-0.38 V) and CO2/CH4 (-0.24 V) are less than the conductance band of g-C3N4 (-1.23 V), thus production of these products are feasible. As discussed previously, g-C3N4 was favourable for CH3OH production but copper has proven to be the preferred metal in the photoreduction of CO2 to CH4 as it demonstrates some level of selectivity for CH4 production. Therefore, both reductant and metals are relatively important in photocatalytic CO2 reduction applications for selective fuels. Figure 5: Schematic presentation of photocatalytic CO2 reduction with H2O to CH4 and CH3OH over Cu-loaded g-C3N4 photocatalyst under solar energy irradiations. 407 4. Conclusions Cu-loaded g-C3N4 nanosheets were developed for gas phase photocatalytic CO2 reduction by H2O under visible light irradiation. The yield rate of CO2 reduction increased significantly by introducing Cu into g-C3N4 catalyst. The yield rate of CH4 as the key product over Cu/g-C3N4 was 217.8 µmole-g-cat.-1, 1.83 fold higher when compared with pure g-C3N4 photocatalyst. Besides, significant amount of CH3OH with appreciable amounts of C2 hydrocarbons were also detected in the product mixture. The experimental results conferred that g-C3N4 is an efficient material functional under solar energy while Cu-promoted enhanced the photocatalytic CO2 reduction to solar fuels. 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