pCN heterojunction composite towards enhanced and continuous photocatalytic CO2 reduction to fuels

Applied Surface Science 485 (2019) 450–461

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Full length article

Cu-NPs embedded 1D/2D CNTs/pCN heterojunction composite towards enhanced and continuous photocatalytic CO2 reduction to fuels

T

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Muhammad Tahira, , Beenish Tahira, M.G.M. Nawawia, Murid Hussainb, Ayyaz Muhammadc a Department of Chemical Engineering, School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia b Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, Defence Road, Off Raiwind Road, Lahore 54000, Pakistan c Department of Chemical and Materials Engineering, King Abdulaziz University, 80204 Jeddah, Saudi Arabia

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 photo-reduction CO/CH4 1D/2D structure Cu/CNTs Protonation g-C3N4

Well-designed 1D/2D composite, consisting of carbon nanotubes modified protonated carbon nitrides (CNTs/ pCN) embedded with Cu-NPs were synthesized by a facile sonicated assisted chemical method. The performance of catalysts towards dynamic CO2 reduction to selective fuels was conducted under UV/visible light irradiations in a continuous flow photoreactor. Photo-activity of g-C3N4 for CO production was increased by 2.38 folds after surface charge modification due to faster charge carrier separation. Loading 3 wt% Cu and 0.5 wt% CNTs into pCN gave highest activity for the production of CO, CH4 and CH3OH under visible light. The highest CO yield as the main product over Cu-CNTs/pCN composite was 560 μmole g-cat−1 h−1, a 1.21, 1.36, 3.78 and 9.03 folds higher than using Cu/pCN, CNTs/pCN, pCN and g-C3N4 samples, respectively. This significantly enhanced photo-activity can be attributed to visible light absorption and multi-stage charges separation in the presence of Cu/CNTs. More interestingly, CO obtained under visible light was 4 times higher than using UV-light, while performance for CH4 production was 14 folds higher with visible light than using UV-light under the same operating conditions. Among the different catalyst loadings, 150 mg gave highest productivity, evidently due to limitations in exposed catalyst active surface area. Additionally, Cu-CNTs/pCN photocatalyst prevailed excellent stability in cyclic runs for continuous CO2 reduction. The findings of this work would be attractive for the development of CO2 conversion systems with renewable fuels production under solar energy.

1. Introduction

Recently, graphitic carbon nitrides (g-C3N4) as a metal free polymeric semiconductor material have received enormous attentions due to its activity under visible light (Ebg ~2.70 eV), non-toxic, high chemical/thermal stability, low-cost synthesis and higher reduction potential [17]. Many studies have focused on photocatalytic CO2 reduction and H2 production using g-C3N4 based photo-catalysts under UV and visible light irradiations [18,19]. However, drawbacks such as less surface area and faster charges recombination rate limits activity of bulk g-C3N4. Thus, cocatalyst is an important strategy to enhance photo-activity of g-C3N4, which can be achieved by loading with metals and coupling with other semiconductors [20,21]. Different metals such as Pd/g-C3N4 [22], Au-P/g-C3N4 [23], CeO2/g-C3N4 [24], AgCl/g-C3N4 [25] and Eu/g-C3N4 [26] have been reported with enhanced CO2 reduction efficiency. Among the metals, great consideration has been focused on less precious metals such as copper (Cu) due to its multioxidation state (Cuo, Cu+1, Cu2+), low cost and provides electron trapping sites over the semiconductor surface [27–30]. In addition,

The depletion of energy sources such as fossil fuels and global warming due to large scale CO2 emission in the atmosphere are the major problems facing by the mankind [1,2]. Among the different alternatives, utilization of CO2 as a carbon feed stock assisted CO2 reduction to useful chemicals and renewable fuels via photocatalysis is a potential pathway [3–6]. Among the semiconductors, TiO2 has been widely investigated because of its appreciable advantages for example relatively low-cost, non-toxic and high chemical/thermal stability [7–9]. However, CO2 is a most stable compound and is a challenging task for its direct conversion to chemicals. Besides, TiO2 is active only under UV-light irradiations due to large band gap (3.2 eV for anatase) and has lower photo-activity due to faster charge carrier recombination [10,11]. Therefore, visible light responsive and efficient photo-catalysts have gained intense considerations for selective CO2 reduction to fuels [12–16].

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Corresponding author. E-mail address: [email protected] (M. Tahir).

https://doi.org/10.1016/j.apsusc.2019.04.220 Received 31 January 2019; Received in revised form 13 April 2019; Accepted 24 April 2019 Available online 24 April 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Scheme for synthesis of protonated Cu-modified 1D/2D CNTs/pCN heterojunction composite catalysts.

g-C3N4/Bi2O2CO3/CoFe2O4 [44] and g-C3N4/SnS2 [45] have been investigated in CO2 reduction and H2 production applications. Although, significant research has been conducted, however, lowcost materials as co-catalyst to design highly efficient g-C3N4 photocatalyst are demanding. Recently, the use of carbon nanotubes (CNTs) as a coupling material has been reporting an effective strategy to extend visible light absorption with faster charges separation [41,46]. The CNTs have electron accepting and transmitting properties with benefits such as high surface area and porous structure [47,48]. Special structure of CNTs makes its unique mechanical, electrical and thermal properties and facilitates charges transfer by functioning as an electron acceptor that inhibits the recombination of electron and hole pairs [49–51]. Therefore, construction of CNTs based g-C3N4 composite would be an effective strategy to enhanced photo-activity under visible light. For example, g-C3N4/TiO2/CNTs photocatalyst demonstrated significantly enhanced activity in degradation applications compared with pristine g-C3N4 [52]. Similarly, Z-scheme g-C3N4/CNTs/Bi2WO6 photo-catalyst showed higher performance for degradation of pollutants compared to pristine g-C3N4 [53]. Recently, we reported enhanced photoactivity of MMT/TiO2 for photocatalytic hydrogen production due to efficient charges separation by CNTs under visible light irradiations [54]. The efficiency of CNTs/g-C3N4 composite can be further improved

photo-catalytic CO2 reduction is a very complex process and CO2 reduction leads to variety of products during reduction process. However, Cu as a cocatalyst is most promising for CO2 reduction to hydrocarbon fuels. Although, Cu is considered as an efficient electro-catalyst to generate hydrocarbons during CO2 reduction [30], it is also useful for trapping and transporting charge carriers for selective photo-catalytic CO2 reduction to CO and hydrocarbons. Thus, Cu would be favorable to transfer charges from semiconductor to CO2 and would works as a sink to trap electrons and eventually promotes the production of fuels such as CH3OH, CO and CH4 [31–33]. In this perspective, Cu-doped TiO2 was investigated for simultaneous CO2 reduction and H2 production [34]. Similarly, Cu/In co-doped TiO2 was investigated for dynamic CO evolution during photocatalytic CO2 reduction [35]. In another work, enhanced CO2 reduction to CH3OH over Cu-C/TiO2 nanoparticles under UV and visible light has been reported [36]. Similarly, Cu-promoted g-C3N4 was employed and found enhanced CO2 to CO production due to faster charge carriers separation [37]. Cu/g-C3N4 was employed for photocatalytic CO2 and CH4 reduction in the presence of H2O with enhanced production of CO/ H2 under visible light [38]. In addition of metals loading, combining materials with g-C3N4 and forming composite is another alternative to increase g-C3N4 performance [39–41]. The different composite catalysts such as WO3/g-C3N4 [42], g-C3N4/ZnIn2S4 [19], CdIn2S4/g-C3N4 [43], 451

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(a)

heterojunction of CNTs/pCN loaded with Cu has been proposed.

3% Cu-0.5% CNTs/pCN 0.5 % CNTs/pCN 3% Cu/pCN g-C3N4

(002)

2. Experimental

Intensity (a.u)

2.1. Materials and chemicals

(100) Melamine (Sigma Aldrich AR ≥ 99%), carbon nanotubes (MWCNTs, > 98% carbon basis, Sigma-Aldrich), copper nitrate (Cu (NO3)2 3H2O, Molekula) and nitric acid (65%, Merck) chemicals were analytical grade and used without any further purification. 2.2. Synthesis of g-C3N4 and protonated g-C3N4 (pCN)

10

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The polymeric g-C3N4 was prepared by thermal decomposition of melamine in a muffle furnace according to previous work [62]. Typically, 5 g of melamine was placed in a ceramic crucible and heated at 550 °C for 2 h at a heating rate of 10 °C/min under air atmosphere. After cooled to room temperature, a yellow powder of g-C3N4 was collected and grinded into fine powder. The protonation process of g-C3N4 was conducted with aqueous nitric acid (0.1 M) solution to produce protonated carbon nitrides (pCN). Typically, 1 g of g-C3N4 powder dispersed in acid solution was sonicated for 30 min, afterwards stirred for 3 h for exfoliations to get nanosheets. The suspension was centrifuged and washed with deionized water to remove excess acid. Finally, the samples were oven dried at 80 °C for 12 h and grinded to get yellow colour 2D pCN nanosheets.

80

2-theta (degree)

(b)

3% Cu-0.5% CNTs/pCN 0.5 % CNTs/pCN 3% Cu/pCN g-C3N4

704 747

321 356

475 588

1232 1113 1307 1477 1617

Intensity/a.u

976

2.3. Synthesis of Cu-loaded 1D/2D CNTs/pCN composite catalyst

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A facile impregnation method was used for the preparation of Culoaded 1D/2D CNTs/pCN ternary composite samples. Initially, 1 g of pCN was dispersed in 50 mL DI water and ultra-sonicated for 30 min to get 2D pCN ultrathin sheets. CNTs were functionalized in a 100 mL solution of nitric acid and acetic acid (6 M each) with ratio 1:3 as reported in our previous work [51]. Next, 0.005 g (0.5 wt%) of CNTs were added into above solution and ultra-sonicated for 30 min to get uniform dispersion of CNTs with pCN sheets. Afterwards, the suspension was centrifuged and dried at 80 °C for 12 h to get 1D/2D CNTs/pCN composite samples. A series of Cu-loaded CNTs/pCN samples were prepared with different amounts of Cu-loading (1, 3 and 5 wt%) using the similar procedure. Typically, 0.5 g of CNTs/pCN was distributed in 50 mL of DI water under stirring and desired quantity of copper dissolved in DI water was added into to suspension. The suspension was evaporated by placing in a water bath and the resultant products were dried at 80 °C for 12 h and grinded into fine powder to get 1, 3, and 5 wt% Cu loaded CNTs/pCN samples. Cu/g-C3N4 samples were prepared using the similar procedure but without addition of CNTs. On the other hand, a similar approach was employed for synthesis of CNTs/g-C3N4 without adding Cu. The schematic for the preparation of Cu-loaded CNTs/pCN photo catalysts is depicted in Fig. 1.

2000

Raman shift (cm-1) Fig. 2. (a) XRD spectra of pure g-C3N4, 0.5% CNTs/pCN, 3% Cu/pCN and 3 wt % Cu-doped 0.5 wt% CNT/pCN samples; (b) Raman analysis of corresponding samples.

by protonation of g-C3N4 to get 2D structured pCN and developing 1D/ 2D heterojunction such as CNTs/pCN [55,56]. In this perspective, 1D metallic MoO2-C modified 2D g-C3N4 was tested for enhanced photocatalytic hydrogen production [57]. Similarly, 2D/2D SnS2/g-C3N4 has been reported for photocatalytic water splitting towards enhanced H2 evolution [58]. Moreover, C-dots modified g-C3N4/TiO2 composite with greatly improved H2 evolution was explored [59]. Recently, we reported surface charge modification of g-C3N4 with enhanced CO2 reduction to CO, CH4 and CH3OH [60,61]. According to literature, there is no work available on the use of CNTs/pCN heterojunction embedded with Cu-NPs for CO2 reduction applications. Therefore, it is envisaged that synergistic effect of Cu/CNTs in 1D/2D CNTs/pCN heterojunction would be promising for selective photo-catalytic CO2 reduction to fuels. Herein, fabrication of 1D/2D heterojunction of carbon nanotubes and protonated graphitic carbon nitride (CNTs/pCN) embedded with copper for selective and dynamic CO2 reduction has been investigated. The Cu-modified CNTs/pCN nanocomposites were characterized by XRD, RAMAN, TEM, N2 sorption, XPS, UV–visible and PL spectroscopic techniques. The performance of composite catalysts was investigated using UV and visible light irradiations in a continuous flow photoreactor system. The effects of irradiation time and catalyst loading were investigated to optimize photo-activity of catalyst and selectivity of carbon products. The stability of composite catalyst in cyclic runs was examined to determine reusability for continuous production of fuels. Finally, reaction mechanism for CO2 reduction with H2O over 1D/2D

2.4. Characterization In order to determine the phase and crystallinity of the synthesized catalyst, XRD analysis was conducted using Bruker Advance D8 operated at 40 kV and 40 mA with Cu/Kα radiation. High resolution transmission electron microscopy (HRTEM) analysis was further carried out for direct imaging of nanomaterials to obtain qualitative measures of the interaction of elements in the composite sample. Since, pure g-C3N4 was modified with Cu and CNTs, BET analysis was conducted to study the surface area, pore volume and pore width of the photocatalyst samples. N2 sorption isotherms were acquired at −196 °C employing ASAP 2020 Micrometric. The samples were degassed prior to analysis, at 250 °C for 4 h under vacuum to ensure removal of adsorbed moisture. X-ray photoelectron spectroscopy (XPS) data was obtained at room temperature using Omicron DAR-400 analyser. The photon energy was 452

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Fig. 3. TEM analysis of Cu-modified CNTs/pCN samples: (a) TEM image of g-C3N4 catalyst, (b) TEM images of 2D pCN sample; (c) Image of CNTs, (d-e) CNTsdistributed in pCN, (f) HRTEM image presenting Cu-loaded CNTs/pCN composite structure.

C2H4, C2H6, C3H6 andC3H8) and methanol (CH3OH).

calibrated using carbon peak (284.6 eV) as an internal standard. UV–visible spectrometer (Carry 100, Agilent) was used for the measurement of light absorbance response of the samples with the help of integrated spheres. Raman spectroscopy was used to observe vibrational, rotational, and other low-frequency modes of catalysts. Photoluminescence (PL) analysis was performed to understand the effects of sensitizers on the recombination of free electrons and holes pairs. Raman analysis was conducted on a HORIBA Scientific Raman Spectrophotometer. The wavelength of 532 nm was used for RAMAN analysis, while 325 nm laser emitting wavelength was employed to obtain PL spectra.

3. Results and discussion 3.1. Characterization of catalysts The XRD patterns of the pure g-C3N4, Cu/pCN, CNTs/pCN and Cu/ CNTs modified pCN composite samples are shown in Fig. 2(a). XRD patterns of pure g-C3N4 show two distinct diffraction peaks appeared at 2θ values of 27.7° and 13.03°. The strong diffraction peak at 27.7° ascribes to (002) plane of carbon nitride, demonstrates typical graphitelike interlayer stacking of C3N4. Similarly, weak diffraction peak at 13.03° with plane (100) reflects in-plane structural packing. The XRD patterns of Cu and CNTs modified pCN samples show only g-C3N4 peaks, however, typical diffraction peaks relating to Cu and CNTs were not appeared. This may be due to the lower amounts of Cu and CNTs loading and uniform dispersion with g-C3N4 structure [52]. Therefore, Cu-modified CNTs/pCN did not alter polymeric construction during heterojunction formation. The furthermore information about the interaction of composite samples was obtained using Raman analysis. The Raman spectra for the pure pCN and Cu-embedded CNTs/pCN samples are presented in Fig. 2(b). The typical characteristic peaks of g-C3N4 at 321, 475, 704, 747, 866, 976, 1113, 1232, 1307, 1477 and 1617 cm−1 were appeared, corresponds to pure g-C3N4 structure [59]. No distinct variation in peaks of g-C3N4 for all the Cu/pCN, CNTs/pCN and CuCNTs/pCN samples was observed, employing that adding Cu and CNTs have trivial effects on the molecular skeleton and lattice structure of gC3N4. Furthermore, sharp peak of g-C3N4 appeared at 704 cm−1 reveals in-plane bending vibration of heptazine linkages while peak at 976 cm−1 can be ascribed to symmetric N-breathing mode of heptazine. In addition, broad and asymmetric peaks between 1300 and 1700 cm−1 attributing to stretching vibration of CeN and analogous to typical D and G bands in the typical carbon-based materials. Therefore, thermal decomposition of melamine at 550 °C is a preferable temperature for complete poly-condensation process to get pure g-C3N4 structure. In addition, Cu/CNTs loaded into pCN did not alter its structure and producing well dispersed Cu-loaded CNTs/pCN composites. The morphology and construction of heterojunction in Cu-loaded

2.5. Photoactivity test Photo-catalytic CO2 reduction experiments were conducted in a gas phase continuous flow photoreactor system as reported in our previous work [4]. The reactor was made of stainless chamber with a total volume 150 cm3 and equipped with temperature controller, quartz window for passing light irradiations, pressure sensor, cooling fans, water saturator and mass flow controllers. Typically, 150 mg powder sample was uniformly distributed at reactor bottom surface. A 200 W Hg reflector lamp (150 mW cm−2) was used as a source of UV-light and a solar simulator (100 mW cm−2) equipped with UV-light filters was used as source of visible light. Compressed CO2 (99.99%) was regulated by mass flow controller (MFC). Prior to reaction, CO2 was passed through the water saturator to carry moisture before entering into the reactor chamber. The CO2 was continuously flowing into the reactor to purge and to saturate the catalyst and this process was continued for 30 min. A CO2 gas flow rate of 5 mL min−1 was maintained in all the experiments. The experiments were initiated by turning on the lamp for a continuous operation of reaction. For the effect of catalyst loadings, 50 and 250 mg of catalyst were placed inside the reactor and repeated the same procedure as discussed above. The gaseous products were analyzed using an on-line gas chromatograph (GC-Agilent Technologies 6890N) equipped with a thermal conductivity detector (TCD) and a flame ionized detector (FID). The TCD detector was connected with Carboxen 1010 PLOT column while FID detector was connected with HP-PLOT Q column for the detection of C1–C3 hydrocarbons (CH4, 453

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Fig. 4. XPS analysis of 3% Cu-modified 0.5% CNTs/pCN composite sample: (a) Cu 2p; (b) N 1s; (c) C 1s, (d) O 1s.

shows C 1s spectra with four carbon peaks prominent at binding energies 284.60, 287.64 and 289.40 eV, resembles to CeC carbon species, N]C coordination in g-C3N4 framework, N-C-N functional group in gC3N4 and C]O and COO due to functionalized CNTs [63,64]. The O 1s spectra in Fig. 4(d) presents peaks at 532.87 and 538.30 eV due to adsorbed H2O and CeO species, respectively. N2 adsorption/desorption isotherm plots of pure g-C3N4 and Cu/ CNTs modified pCN samples are presented in Fig. 5. Evidently, isotherms reveals type IV and H3 hysteresis loop, corresponds to mesoporous structure of the samples [18]. The summary of specific surface area (SBET), BJH desorption pore volume (cm3/g) and pore size distribution is presented in Table 1. BET surface area (SBET) of pure g-C3N4 was 5 m2/g, increased to 10 m2/g due to 2D nanosheets of pCN. A further increased in BET surface area of 11 m2/g was obtained with 3% Cu-loading pCN sample. This reveals that protonation with exfoliation has an impact to improve the BET surface area of g-C3N4. However, SBET was significantly increased by adding 0.5 wt% CNTs into pCN. The SBET obtained for 0.5% CNTs/pCN was 19 m2/g, obviously due to higher surface area of CNTs. The addition of Cu into CNTs/pCN did not have significant effects, yet it was slightly decreased with 3% Cu-loading. Similar observations could be seen in pore volume of all the samples. In general, the pore volume obtained is very small, which reflects that pCN did not have porous structure. In addition, pore diameters were not

CNTs/g-C3N4 samples were examined using TEM and results are shown in Fig. 3. The morphology of pure g-C3N4 is presented in Fig. 3(a), which reveals compacts sheets attached together. However, exfoliation and protonation process gives thin sheets with 2D structure as presented in Fig. 3(b). Fig. 3(c) shows long and thin 1D structure of CNT having core and walls with uniform size. The interaction of CNTs with pCN is presented in Fig. 3(d–e). Evidently, there is good interaction between CNTs and pCN in the entire composite. The 1D CNTs are uniformly distributed over the 2D pCN sheets, thus successful development of 1D/2D heterojunction of CNTs/pCN. The further interaction of CNTs/pCN loaded with Cu was identified using HRTEM and the results are presented in Fig. 3(f). Obviously, there is a good interaction between CNTs and pCN, while Cu is evenly distributed over the composite structure. This 1D/2D heterojunction of CNTs/pCN with uniform distribution of Cu would be fruitful for charges migrations within the composite structure. The interaction between elements in Cu-modified CNTs/pCN composite can be obtained using XPS technique as presented in Fig. 4. Fig. 4(a) shows XPS bands of Cu with peaks appeared at 936.12 and 957.14 eV, corresponds to Cu 2p3/2 and Cu 2p1/2, respectively, attributed to Cu2+ oxidation state. The XPS peak of N 1s in Fig. 4(b) reveals peaks at 398.87 and 400.28 eV due to the existence of nitrogen as C=NC and C-NH2 from the g-C3N4 precursor, respectively [38]. Fig. 4(c) 454

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Fig. 5. N2 adsorption-desorption isotherms of CNTs, g-C3N4 and Cu/CNTs modified pCN samples.

to CNTs/pCN sample. This PL quenching was due to good heterojunction formation between CNTs/pCN and synergistic effects between Cu and CNTs [65].

Table 1 Summary of physiochemical properties of g-C3N4 and Cu/CNTs modified pCN samples. Catalyst

CNTs g-C3N4 pCN 3% Cu/pCN 0.5% CNTs/pCN 3% Cu-0.5%CNTs/pCN

BET surface area SBET (m2/g)

Pore volume (cm3/g)

Pore width (nm)

191 5 10 11 19 17

1.013 0.034 0.0689 0.0912 0.092 0.082

49.5 28.9 10.3 9.7 29.3 11.6

3.2. Photocatalytic CO2 reduction with H2O Initially, to ensure that the catalyst samples are clean and present process is a photocatalytic reduction process, blank experiments were performed in the presence of (UV and visible) light irradiation or photocatalyst and in the absence of CO2. In either case, carbon based products were not obtained. Therefore, CO, CH4 and CH3OH products were mainly obtained during photocatalytic CO2 reduction process in the presence of light irradiation and photo-catalyst. Fig. 7(a) shows performance of g-C3N4, pCN and Cu/CNTs modified pCN samples during photocatalytic CO2 reduction with H2O under visible light irradiations. Evidently, CO and CH4 were the main products with small quantity of CH3OH and C2H6. The CO of 62 μmole gcat−1 h−1 was produced over pristine g-C3N4, increased to 148 μmole g-cat−1 h−1 using pCN. The production of CH4 and CH3OH have similar trends over pure g-C3N4 and pCN under the same operating conditions. This reveals that protonation has an impact on the performance of gC3N4 which results in enhanced CO2 reduction efficiency. Therefore, acid treatment assisted ultra-sonication leads to exfoliation, forms ultrasmall pores, provides protons and improved the specific surface area. This aids in the higher separation and transfer efficiency of the photogenerated electrons and holes. Recently, we reported enhanced photoactivity of g-C3N4 after surface charge modification for photo-catalytic CO2 reduction and hydrogen production due to efficient trapping and transportation of charges [60,61,66]. The CNTs/pCN heterojunction composite reveals further improvement in CO2 reduction efficiency. Highest CO evolution of 410 μmole gcat−1 h−1 over CNTs/pCN was obtained, a 2.8 fold higher than pCN and 6.6 folds higher using pure g-C3N4. Similarly, amount of CH4 production was 1.15 and 1.52 folds higher than pCN and g-C3N4 samples, respectively. This was obviously due to larger BET surface area, excellent thermal and electrical characteristics of carbon nanotubes, effective acceptor to trap the electrons, thus prevents recombination of electron and holes, resulting in enhanced photo-activity. As for the

significantly altered in pure and Cu/CNTs-loaded pCN samples, yet it was somewhat reduced in Cu-loaded CNTs/pCN sample. Fig. 6(a) presents UV–visible diffuse reflectance spectra of pCN and Cu/CNTs modified pCN samples. The curve of pure pCN shows light absorbance wavelength at wavelength of 470 nm, attributed to band gap of 2.65 eV. Cu-modified pCN sample has much similar spectrum which exhibits band gap energy of 2.63 eV. However, a significant light absorption towards visible region was observed in Cu-modified CNTs/ pCN sample, with absorption wavelength of 483 nm, attributed to band gap of 2.57 eV. This reveals, CNTs not only improves conducive properties but also improves visible light absorption response of pCN. The photoluminescence (PL) was used to analyze the recombination rate of electrons and holes. The PL spectra of g-C3N4, CNTs/pCN, Cu/ pCN, Cu-CNTs/pCN were obtained under 325 nm excitation wavelength and the results are presented in Fig. 6(b). It can be seen that all the catalysts exhibit similar trends and g-C3N4 emission peak is the highest compared to the others, implies rapid recombination of the photoelectrons and holes in pure g-C3N4. However, an obvious quenching was observed in CNTs/pCN sample. This was obviously due to excellent thermal and electrical characteristics of carbon nanotubes, effective acceptor to trap the electrons, thus prevents recombination of electron and holes. Similar observations could be seen in Cu/pCN samples, whereas, PL intensity was significantly reduced due to copper as an efficient metal to trap the electrons and prevents their recombination rate. A further decreased in PL intensity was observed with Cu-loading 455

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catalyst yielded highest amount of CO production which is 560 μmole g-cat−1 h−1 compared to 1% Cu/0.5% CNTs/pCN (506 μmole gcat−1 h−1) and 5% Cu-0.5% CNTs/pCN (536 μmole g-cat−1 h−1). It can be clearly seen that optimized amount of Cu-loading is 3 wt%, where the highest amount of photo-activity of CNTs/pCN for CO production was achieved which is 1.2, 1.22, 1.37, 3.78 and 9.03 folds higher than Cu/pCN, CNTs/pCN, pCN and pure g-C3N4 samples, respectively. Similar observations could be seen for the production of CH4, yet there was no significant effect on the production of CH3OH and C2H6. In general, enhanced performance of CNTs/pCN loaded with Cu was due to 1D/2D heterojunction formation and synergistic effects between Cu/ CNTs, which results efficient photo-generated charges separation for selective photocatalytic CO2 conversion to fuels. In order to investigate performance of CNTs/Cu loaded pCN samples, a correlation between BET surface area and CO/CH4 products was established to understand prominent contributor factor in CO2 reduction process as presented in Fig. 7(b). The BET surface area was higher in order of CNTs/pCN (19 m2/g) > Cu-CNTs/pCN (19 m2/g) > Cu/ pCN (11 m2/g) > pCN (10 m2/g) > g-C3N4 (5 m2/g). However, CO yield as the main products was higher in the order of Cu-CNTs/pCN (560 μmole g-cat−1 h−1) > Cu/pCN (460 μmole g−1 −1 cat h ) > CNTs/pCN (410 μmole g-cat−1 h−1) > pCN (148 μmole g-cat−1 h−1) > g-C3N4 (62 μmole g-cat−1 h−1). Evidently, the BET surface area of Cu/pCN is closer to pCN but its CO productivity efficiency is 3.10 fold higher than using pure pCN. Similarly, BET surface area of CNTs/pCN is higher than Cu/pCN but CO production was lower than Cu/pCN. Furthermore, the amount of CO produced over Cu-CNTs/ pCN was 3.78 folds higher than pCN but the surface area was only 1.54 folds higher. These results could be explained based on distinguished characteristics of Cu for evolution of C1 products in addition of trapping and transportation of charge carriers. Thus, prominent factor to promote photo-activity was due to 1D/2D heterojunction formation between CNTs/pCN, higher visible light absorption, reducible characteristics of Cu for CO2 reduction and synergistic effects between Cu/CNTs for trapping and transportation of charge carriers. The optimized 3% Cu-0.5% CNTs/g-C3N4 photo-catalyst was further investigated at different irradiations times under visible light in a continuous flow fixed bed reactor and the results are presented in Fig. 8. Evidently, CO was detected as the main product with appreciable amount of CH4 and smaller amount of CH3OH over the entire irradiation time. The highest yield of 1326 μ-mole g-cat−1 was obtained for CO, 5.18 fold higher than CH4 production and 64.68 times the amount of CH3OH while employing the same operating environments. Noticeably, a continuous CO production can be seen with irradiation time due to higher efficiency and productivity of Cu-CNTs/g-C3N4 composite photo-catalyst functional under visible light. Furthermore, CO production as the main products can be explained based on reduction potential which is obviously lower CO2/CO (−0.48 V) than the CB of g-C3N4 (−1.23 V). Therefore, continuous CO2 reduction to CO was obviously due to visible light absorption, efficient charges separation due to welldesigned 1D/2D heterojunction formation and synergistic effects between Cu/CNTs. Fig. 9 presents production of CO and CH4 over different types of photo-catalysts in a continuous flow fixed bed photoreactor under UVlight irradiation. Using pure g-C3N4, CO production was in smaller amount, which was greatly improved in Cu and CNTs modified pCN samples as shown in Fig. 9(a). This was obviously due to hindered charges recombination rate by the presence of Cu and CNTs. A further enhancement in pCN performance towards CO evolution was obtained by co-loading Cu and CNTs. The highest amount of CO of 343 μ-mole gcat−1 over 3% Cu-0.5% CNTs/pCN was obtained. This amount of CO production was 1.27 folds the amount produced over 0.5% CNTs/pCN, 1.73 folds than 3% Cu/pCN and 3.10 times higher than CO obtained using pure g-C3N4. It could be seen that under visible light irradiations, Cu/pCN was more favorable for CO production than CNTs/PCN, while under UV-light, production of CO was somewhat increased with CNTs

Fig. 6. (a) UV–visible analysis of pCN and Cu/CNTs modified pCN samples; (b) PL analysis of g-C3N4, 3% Cu/pCN, 0.5% CNTs/pCN samples and 3% Cu-0.5% CNTs/pCN samples.

metal loading, CO yield of 460 μmole g-cat−1 h−1 was achieved over 3 wt% Cu-loading, slightly higher than using CNTs/pCN. However, the amount of CH4 production of 105 μmole g-cat−1 h−1 was obtained over 3% Cu/pCN, 1.4, 1.7 and 2.2 folds higher than using CNTs/pCN, pCN and g-C3N4 samples, respectively. Among the hydrocarbons, traces production of C2H6 was detected and it was highest in Cu-modified composite samples. During CO2 reduction process, Cu has a potential to generate C1 products (for example CO, CH4 and CH3OH). This is because intermediates produced during CO2 reduction can be promoted by metallic Cu for selective production of C1 products and other organic compounds. Recently, Chen et al. [30] investigated photo-catalytic CO2 reduction with benzyl alcohol and reported enhanced benzyl acetate production in the presence of Cu2O/Cu. This reveals that Cu is more favorable for the production of CH4 in addition of CO, while CNTs were promising for enhancing CO production. However, in electro-catalytic process, Cu is very promising for CO2 reduction to liquid fuels such as HCOOH, CH3OH and C2H6O [29]. Therefore, Cu is promising for photocatalytic CO2 reduction to C1 products in a gas phase process, while it favorable for production of liquid organic compounds in electro-catalytic process. The effects of co-loading of Cu and CNTs was further explored using different amounts of Cu-loading (1%, 3% and 5%) at one fixed amount of CNTs loading (0.5%) and results are presented in Fig. 7(a). Among the Cu and CNTs co-loadings, 3 wt% Cu with 0.5 wt% CNTs/pCN 456

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Fig. 7. (a) Photocatalytic CO2 reduction with H2O over different catalysts under visible light irradiations (#1) g-C3N4; (#2) pCN, (#3) 0.5% CNTs/pCN, (#4) 3% Cu/ pCN; (#5) 1% Cu-0.5% CNTs/pCN; (#6) 3% Cu-0.5% CNTs/pCN; (#7) 5% Cu-0.5% CNTs/pCN; (b) Plots of BET surface area and production rates over different catalysts (#1) g-C3N4; (#2) pCN, (#3) 0.5% CNTs/pCN, (#4) 3% Cu/pCN; (#6) 3% Cu-0.5% CNTs/pCN.

Using pure g-C3N4, highest amount of CH4 was produced while its productions trends were different in Cu and CNTs loaded pCN samples. The production of CH4 was higher over the Cu-loaded pCN than using CNTs under the same operating conditions. This reveals Cu is favorable for CH4 production; while CO can be produced in significant amount in the presence of CNTs. This was obviously due to unique characteristics of Cu for the production of hydrocarbons as discussed previously. The co-loading Cu and CNTs into pCN did not have significant effect on the yield of CH4 production. More importantly, a continuous CH4 production was observed over the time on stream, evidently, CO2 was first converted to CO, which was further responsible for CH4 production through multiple reactions under UV-light irradiations. The performance of Cu-CNTs/pCN for the production of CO, CH4 and CH3OH during CO2 reduction with H2O under both types of irradiation (UV and visible light) has been discussed in Fig. 10. Evidently, photo-catalyst has significantly higher productivity under solar energy than using UV-light irradiations. In this perspective, the amount of CO

loading. This could be explain based on Cu characteristics for visible light absorption and it also works as a cocatalyst for CO2 reduction due to its lower band gap and higher reduction potential for CO2 to CO conversion. In general, superior charges separation in composite was due to combined effects of Cu/CNTs such as provides faster separation of charges with their hindered charges recombination rate. Furthermore, initially, highest amount of CO production was detected at the start of the reaction, then declined over the time on stream and these trends were obvious in all types of photo-catalysts. This reveals that using UV-light irradiations, catalyst was not stable over the irradiation time compared to visible light irradiations. This was due to very strong light intensity and possibly it would promote reversed oxidation process as similar observations have been reported in literature [60]. Thus, higher photo-activity and stability could be obtained under visible light irradiations under continuous flow reaction system. The performance of Cu-modified CNTs/pCN composite catalyst for CH4 production under UV-light irradiations is presented in Fig. 9(b).

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Fig. 8. Effect of irradiation time on the photo-activity of 3% Cu-0.5% CNTs/ pCN catalyst for photocatalytic CO2 reduction with H2O under visible light in a fixed bed reactor.

Fig. 10. Performance comparison of products over 3% Cu-0.5 CNTs/pCN under UV and visible light irradiations.

produced under solar energy was 1325 μ-mole g-cat−1, 5.71 folds higher than using UV-light while keeping all operating conditions consistent. Similarly, CH4 production of 251 μ-mole g-cat−1 was obtained under visible light, 5.70 times higher than its evolution under UV-light. However, trends for CH3OH production was only 3.17 fold higher under solar energy than using UV-light irradiations. Although, UV-light has higher light intensity (150 mW cm−2) than visible light (100 mW cm−2) but production of both CO and CH4 were higher under visible light irradiations. This reveals that performance of photo-catalysts could not be enhanced by increasing light intensity but absorption of light spectrum is also important for producing charge carriers during CO2 reduction process. Therefore, enhanced photo-activity in CO2 reduction would probably be due to high visible light absorption, more production of electrons with their hindered charges recombination rate and synergistic effects between Cu and CNTs loaded on pCN photocatalyst. The effect of different amounts of Cu-CNTs/pCN catalyst loadings on the CO2 reduction activity under visible light is presented in Fig. 11. Fig. 11(a) shows CO production with different catalyst loading, whereas, different loadings have different trends with the irradiation time. Initially, highest CO production was observed using 50 mg catalyst loading. This was obviously due to higher light absorption and more exposure of catalyst for the absorption of light irradiation for CO2 reduction. However, after 3 h of irradiation time, photo-activity of 50 g was declined, while 150 mg gave continuous CO production. This was probably due to adsorbed CO2 was converted to products, resulting in declined CO production. On the other hand, more catalyst loading e.g., 250 mg was not favorable to promote photo-activity. This was evidently due to lesser exposed catalyst surface available for the photo-chemical reaction. The production of CH4 with different catalyst loading under visible light irradiations is presented in Fig. 11(b). Interestingly, CH4 production trends were similar to CO evolution in which, initially highest amount of CH4 was produced using 50 g of catalyst loading but gradually declined with irradiation time. Conversely, production of CH3OH was highest using 150 g of catalyst loading (Fig. 11c). This shows, using higher loading, there was more CO2 adsorption with more production of electrons which were favorable for CH3OH production. However, much higher catalyst loading was not helpful to further promote the catalyst efficiency for CH3OH production. These results divulge that catalyst loading is somewhat, important to get higher yield and selectivity under solar energy irradiations. The stability analysis of Cu-CNTs/pCN photo-catalyst was evaluated via cycling experiments to understand the life of catalyst. For the

Fig. 9. Photocatalytic CO2 reduction with H2O over different photo-catalysts in a gas phase fixed bed photoreactor system under UV-light irradiation: (a) CO production; (b) CH4 production.

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Fig. 12. Stability analysis of photocatalytic CO2 reduction with H2O over 3% Cu-0.5% CNTs/pCN photo-catalyst under visible light irradiation in a continuous flow fixed bed reactor.

The schematic for photocatalytic CO2 reduction with H2O under visible light irradiation is presented in Fig. 13. When light strikes over the photo-catalyst surface, electrons (e−) and holes (h+) are produced. Using pure g-C3N4, photo-recombination occurs in nanoseconds where the photo-excited electrons tend to recombine with the holes which eventually reduce the yield of desired products. However, with the presence of CNTs and Cu dopants in the pCN, the photo-excited electrons would be transported towards CO2 for its reduction. Thermodynamically, electrons trapped by metal within the photo-catalyst are more feasible if the reduction potential of the metal is more positive than the g-C3N4 conductance band. The production of CH3OH over gC3N4 was possible due to its lower reduction potential CO2/CH3OH (−0.38) than the conductance band of g-C3N4 (−1.23 V). Similarly, CO production as the main product can be explained based on thermodynamics reduction potentials. Evidently, conductance band of g-C3N4 CB (−1.23 V) is much higher than reduction potential of CO2/CO (−0.48 V), which confirms, thermodynamically favorable process for its production [67]. Thus, significantly improved CO production would be due to reduction potential difference of CO2/CO with CB of g-C3N4. Furthermore, reduction potential of CO2/CH4 (−0.24 V) is much lower than CB of g-C3N4 (−1.23 V) which favors its production in the existing photocatalysis system. However, lower CH4 production compared to CO would probably be due to eight electrons requires for CH4 production compared to two electrons only for CO production. In general, significantly enhanced performance of composite photo-catalyst was due to high visible light absorption, more production of electrons and hindered charges recombination rate. In conclusion, enhanced CO productions was obviously due to protonation, 1D/2D CNTs/pCN heterojunction and synergistic effects between Cu/CNTs, all these factors leads to hindered charges recombination rate for selective CO2 reduction to solar fuels.

Fig. 11. Photocatalytic CO2 reduction with H2O in a gas phase photoreactor system over 3% Cu-0.5% CNTs/pCN with different catalyst ladings under visible light: (a) CO production, (b) CH4 production, (c) CH3OH production.

4. Conclusions

stability analysis, after completing first run, catalyst was flushed with N2 flow to remove the products and to clean the reactor before starting the next run. The results of CO and CH4 productions during CO2 reduction under visible light in cyclic runs has been discussed in Fig. 12. Evidently, CO and CH4 production were consistent in all the cyclic runs, which confirm catalyst sustained photo-activity in cyclic runs. This further indicates a favorable process for CO2 reduction to fuels under visible light irradiations in a continuous flow photo-reactor system.

Cu-loaded 1D/2D CNTs/pCN heterojunction catalysts were successfully fabricated using thermal decomposition and wet impregnation method. The protonation of g-C3N4 has improved the efficiency due to faster charges separation. The resultant effects of Cu and CNTs loadings to pCN on the photo-reduction of CO2 with H2O into CH3OH, CO and CH4 were investigated using fixed bed photo-reactors under UV and visible light irradiations. In comparison with different percentage of Cu loadings, 3 wt% Cu-loaded into 0.5% CNTs/pCN was found to be the optimum which gave the highest yield of CO in both types of light 459

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Fig. 13. Proposed schematic of charges and reactants transformation over Cu-CNTs/pCN under visible light irradiation.

irradiation. The enhanced CO production was due to 1D/2D CNTs/pCN heterojunction formation and synergistic effects between Cu/CNTs which hindered charges recombination rate. The prolonged stability was obtained in cyclic runs. Therefore, Cu-loaded CNTs/pCN has potential to be employed as an efficient and stable photo-catalyst for photocatalytic CO2 reductions to solar fuels.

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