1. Introduction
Many of the recent studies on catalysis for green and sustainable fuel production have been focused on CO₂ photoreduction for production of renewable Fuel [1, 2]. CO₂ photoreduction reaction has two simultaneous advantages: it reduces the pollutant gas CO₂ and provides a new source of fuel for the next generation power supplement. This investigation will continue to find an efficient, active, and selective photocatalyst under visible light irradiation. Among various phothocatalysts studied thus far, TiO₂ has attracted much more attention due to its numerous advantages including low cost, high photocorrosion resistance, low toxicity, powerful oxidation properties, and abundant availability [3].
CH₄ production yield in the presence of anatase TiO₂ is approximately 0.02 µmol gcat-1 h-1. This is significantly lower than what is needed for practical use. The large band gap (3.18 eV) of anatase TiO₂ [4] makes it active only under the ultra violet (UV) irradiation (wavelengths smaller than 387 nm) which is only 5% of the solar light spectrum that reaches the earth [5]. Another drawback of photocatalytic reduction of CO₂ in the presence of pure anatase TiO₂ is the rapid electron-hole recombination due to slower adsorption of CO₂ (10-8 to 10-3 s) compared with electron-hole recombination rate (10-9 s) [6-8]. Thus, recent research has focused on modifying the TiO₂ structure in order to overcome its shortcomings by doping it with various elements including transition metals, nobel metals, and non-metal elements [9-13].
It would be beneficial to transfer photogenerated electrons from TiO₂ to another material as electron acceptor in order to suppress the recombination of electron-hole for effective charge separation. The use of carbonaceous materials such as graphene oxide, carbon nitride, and multi-wall carbon nanotube as support for photocatalysts has been investigated recently due to their unique electron transfer properties through the heterojunction formed between carbon and TiO₂ where they function as electron acceptor thereby lowering the recombination rate of electrons and holes. Carbonaceous materials also possess high surface area as well as high mechanical and chemical resistance [14-17]. Further improvement in photocatalytic activity is possible, however, by doping noble metals into the carbon-TiO₂ composites [14-19]. These metals can act as electron sinks and slow down the recombination of electron-hole. In addition, noble metals may possess a localized surface plasmon resonance absorbance which in turn extend the light absorption range of TiO₂ into visible light region of the solar spectrum which is 47% of the solar light spectrum that reaches the earth [10]. It is also well-known that particle size and distribution and strength of the interaction between metal and semiconductor is of crucial importance since it causes smooth transfer of the photogenerated electrons from the conduction band of the semiconductor towards the metal. CO₂ conversion can be enhanced by smaller particle size and stronger interactions enhance [11] that could be achieved by employing a proper synthesis method [12]. It is well-known that the electrostatic self-assembly method results in strong electrostatic interactions and smaller particle size with higher dispersion compared with the impregnation method that results in weak interactions, larger particle size, and a lower dispersion [13]. There are, however, no or at least limited reports focusing on applying proper preparation methods to ensure the strength of the interactions between metal and semiconductor, particularly for organic semiconductors. With this end in view, employing electrostatic self-assembly preparation method may create highly active Pt-TiO₂/C catalysts for CO₂ photoreduction [24-27].
2. Experimental
2.1 Photocatalyst preparation
Carbonaceous supports were first functionalized with Poly(allylamine hydrochloride) (PAH) in the presence of NaBH4. Afterwards, Pt precursor (H2PtCl6) was adsorbed on the surface of the functionalized supports by the electrostatic interactions between the positively charged functional groups of PAH and negatively charged PtCl62- ions. Finally, PtCl62- ions were reduced in presence of the ethylene glycol and consequently Pt nanoparticles were deposited on the surface of the support.
2.1.1 Materials
Single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), activated carbon (AC), graphite powder, Poly(allylamine hydrochloride) [average MW of 56000], ethylene glycol (EG), Hexachloroplatinic acid (H2PtCl6·6H2O) and Anatase TiO2 were purchased from Sigma–Aldrich. Reduced graphene oxide (rGO) were synthesized from graphite powder by a modified Hummers method [28, 29].
2.1.2 Synthesis of PAH-Functionalized carbonaceous material
The C-PAH (C=SWCNT, MWCNT, AC and rGO) was prepared by simultaneous sonication and vigorous stirring of mixture of the 300 mg carbonaceous material and 500 ml of 0.5 wt% PAH aqueous solution at 70 ˚C. Afterwards, 25 ml of 1M NaBH4 solution was added and the solution was stirred for 24 h. Finally, C-PAH was collected by centrifugation and washed with deionized water to remove the excess of PAH and then dried at 90 ˚C in vacuum for 3 h. Surface functional groups (amine groups of PAH) were generated through this step.
2.1.3 Synthesis of 5wt% Pt-25wt% TiO₂ Functionalized carbonaceous material
Typically, 1980 mg of C-PAH and 79 mg of TiO2 were added to a solution of 0.1 mmol H2PtCl6 in EG. 1 M NaOH solution in EG was added to keep the pH of the solution above 12. At high pH conditions, oxidization of EG resulted in glycolate anions release which strongly stabilize Pt nanoparticles [30, 31]. After 1 h of stirring, solution reflux conditions were established at 130 ˚C for 4 h. After cooling down, the pH of the solution was decreased and kept at lower than 3 by adding HNO3 solution. After 48 h of stirring the resulting material was washed with deionized water until complete removal of Cl¯ ions and subsequently dried at 90 ˚C in vacuum for 24 h.
2.2 Photocatalyst characterization
Transmission electron microscopy (TEM) micrographs were obtained using a Zeiss EM900 transmission electron microscope with accelerating voltage of 80 keV. TEM images were analyzed using the Digital Micrograph software package (Gatan Inc.) with at least 100 particles imaged and analyzed for each sample to determine the average diameter. X-ray diffraction (XRD) patterns were obtained using a PW3040/60 X’Pert PRO MPD (PANalytical) diffractometer equipped with a Cu Kα radiation source (λ= 1.5406 Å) and operating at 40 kV and 40 mA. A continuous mode was used for collecting data in the 2θ range of 20˚ to 100˚ at a scanning speed of 10˚ min-1. Brunauer-Emmett-Teller (BET) surface areas of the catalysts were evaluated from nitrogen adsorption-desorption isotherms at -196˚C using a Belsorp mini II apparatus. Samples were gassed out at 120˚C for 15 h prior to analysis. Ultra violet-visible (UV-vis) diffuse reflectance spectra (DRS) spectra were obtained using an Agilent Cary 100 UV-vis spectrophotometer under ambient temperature in the wavelength range of 200–800 nm. The Kubelka-Munk function [F(R)] was employed in order to determine the band gap energies of the photocatalysts, i.e. the plot of [F(R)·hν]0.5 versus hν (Tauc plot) was extrapolated.
2.3 CO₂ photoreduction evaluation
CO₂ photoreduction experiments were carried out in a continuous gas-phase fixed-bed photoreactor setup at 1 bar and 25 °C. First, 100 mg of catalyst powder was placed in the quartz reactor. Afterwards, high-purity (99.99 %) CO₂ gas was bubbled through a water bubbler in order to produce a mixture of CO₂ and a sacrificial reagent (water vapor). Prior to irradiation, CO₂ was purged into the reactor (50 ml min-1 for 1 h) in order to guarantee the complete elimination of air from reactor system while ensuring the complete adsorption of CO₂ molecules on the surface of the catalyst. Subsequently, CO₂ flow rate was set to 50 ml min-1 and a 15 W energy-saving daylight lamp (Philips) was turned on as the visible light irradiation source for 12 h. The product gas was swept by He as a carrier gas and analyzed continuously using an online Varian CP-3800 gas chromatograph that was equipped with a Hayesep Q and a Molecular Sieve column. Thermal conductivity and flame ionization detectors (TCD and FID, respectively) were used to analyze the product gases.
In order to ensure the sole formation of CH₄ from photocatalytic reduction of CO₂ with water vapor under visible light irradiation, control experiments were carried out under conditions of eliminating (a) visible light irradiation, (b) photocatalyst, (c) CO₂ (substituting N₂ for CO₂) and (d) water vapor. For all control experiments, there was no evidence of CH₄ formation. The photocatalyst performance was evaluated based on CH₄ yield defined as:
All data are mean value of 3 experiments and error bars indicate one standard deviation.
3. Results and discussion
3.1 TEM
TEM images and particle size distribution of Pt are presented in Figure 1. All Pt nanoparticles on PAH-functionalized carbon supports are in truncated cubic form with highly uniform dispersion as well as narrow particle size distribution due to the electrostatic interaction between PtCl62- and surface of the carbonaceous material. Thus, Pt nanoparticles had smaller particle size compared with those reported for Pt/C catalysts prepared by other methods [32-36]. Moreover, the Pt particle size depended on the type of carbonaceous material used as catalyst support. Pt particles supported on MWCNT and AC were smallest (1.35 nm) and largest (4.38 nm), respectively.
3.2 XRD
XRD patterns of catalysts and corresponding pure supports are shown in Figure 2. Compared with the pure supports, Pt containing samples had four additional peaks. Dashed lines at 2θ˚=39, 46, 67, and 81 corresponding to planes with (1 1 1), (2 0 0), (2 2 0) and (3 1 1) miller indices, respectively, are consistent with face-centered cubic phase Pt0 [ICDD card No. 04-0802]. The peak observed at the dotted line at 2θ˚=26 corresponds to TiO2.
3.3 N₂-physisorption
BET surface area and pore volume of catalysts and pure supports are reported in Table 1. There was only a little or no decrease observed in the BET surface area of Pt-TiO₂ loaded samples compared with pure supports. No significant decrease was also observed in the pore volume of catalysts compared to the pure supports. Pt-TiO₂/AC had the highest BET surface area and pore volume.
3.4 Optical properties
UV-vis DRS of samples over the wavelength range of 200–800 nm are shown in Figure 3 in order to determine the optical properties of the prepared samples. TiO₂ exhibited no absorption above its sharp absorption edge of 388 nm since it is not photocatalytically active in the visible region (400–800 nm). A slight red shift of the peak towards the visible light region was observed for Pt-TiO₂/rGO and Pt-TiO₂/AC samples while Pt-TiO₂/MWCNT and Pt-TiO₂/SWCNT samples showed significant red shift of the peak towards the visible light region as well as visible light absorbance at a great intensity in the visible region. A red shift in absorption edge of Pt-TiO₂/C samples is attributed to narrowing of band gap which in turn is ascribed to the chemical bond formation between carbonaceous material and TiO₂ [6] and a consequent modification of electron–hole formation during visible light irradiation [18]. The carbon-based composites are known to be able to promote a rapid photoinduced charge separation as well as a slow charge recombination since they could accept photogenerated electrons from TiO₂ [19]. The stronger absorption in the visible region could also be related to the surface plasmon resonance due to the presence of Pt [39, 40] or low-energy transitions (at longer wavelengths) between the valence band of TiO₂ and localized energy levels introduced to the bandgap of TiO₂ (below the conduction band) by Pt [14, 41, 42]. In the case of Pt-TiO₂/AC, absorption decreases with increasing the wavelength. A similar phenomena was observed and reported elsewhere [43, 44].
In order to evaluate the band gaps, modified Kubelka-Munk function was employed (Table 2). All absorption band gaps are smaller than that of TiO₂ which confirms a red shift in the absorption edge of composites as compared with anatase TiO₂ in Figure 3. A mixed energy level is formed between the valence band and the conduction band due to chemical bonding formed between materials. This will result in the promotion of the visible light absorption and consequently in the improvement of the photocatalytic performance that would be further confirmed by conducting photocatalytic reduction of CO₂ in the presence of the prepared composites.
4. CO₂ photoreduction
The photocatalytic reduction of CO₂ was carried out in a continuous gas phase photoreactor under visible light irradiation. Photocatalytic performances were evaluated based on the yield of CH₄, the only product gas that was formed. As reported in Figure 4, CH₄ was first detected 0.5 h after the start of the irradiation and progressively reached its maximum value after 2 h of reaction. In comparison with the anatase TiO₂, the photocatalytic activity of the composites were significantly higher and varying in the order of Pt-TiO₂/MWCNT>Pt-TiO₂/SWCNT>Pt-TiO₂/rGO>Pt-TiO₂/AC. Higher photocatalytic activity of composites can be attributed to their lower band gaps with respect to that of anatase TiO₂ that would enable them to absorb visible light. Moreover, synergistic effect of addition of Pt and carbonaceous material resulted in extension of the life time of charge carriers that consequently enhanced the photocatalytic activity of the composites. In addition, the composites were photostable since after 12 h of reaction, the yield of CH₄ decreased by less than 5 % of its value after 2 h of reaction when it had reached its maximum.
The total CH₄ yield over 12 h of reaction is presented in Figure 5. The highest and the lowest CH₄ yields were observed for Pt-TiO₂/MWCNT and Pt-TiO₂/AC, respectively. There was a strong correlation between the Pt-TiO₂ particle size and the photoactivity of the catalysts with smaller particles having improved photocatalytic performance.
For comparison, a summary of investigations published in the open literature that have reported the CO₂ photoreduction under similar reaction conditions using composites of carbon, TiO₂ and Pt is presented in Table 3. TiO₂ employed in this study had a similar CH₄ yield as compared with other investigations [14, 45]. Pt-TiO₂/MWCNT had the highest yield of 1.9 μmol gCat-1 h-1 among all reported photocatalysts that was at least 3 times higher than those reported for composites of MWCNT and TiO₂ in the absence of Pt [19, 46]. CH₄ yield for Pt-TiO₂/rGO was approximately 2 and 3.5 times higher than those for the same reported composite [9] and rGO/TiO₂ composite [6], respectively.
5. Conclusion
Pt-TiO₂ photocatalysts supported on different carbonaceous material (MWCNT, SWCNT, rGO and AC) were prepared by the electrostatic self-assembly method and were used in CO₂ photoreduction under visible-light irradiation for CH₄ production. The activities of the photocatalysts were in the order of MWCNT>SWCNT>rGO>activated carbon with the highest yield of CH₄ (1.9 μmol gCat-1 h-1) observed for Pt-TiO₂/MWCNT catalyst. Composites are highly photostable with the loss of less than 5 % of their activity in terms of CH₄ yield after 12 h of reaction.