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Article

Reactor Design for CO2 Photo-Hydrogenation toward Solar Fuels under Ambient Temperature and Pressure

1
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
2
Faculty of Chemical and Environmental Engineering, Lac Hong University, 812431, No. 10 Huynh Van Nghe, Buu Long, Bien Hoa, Dong Nai, Viet Nam
3
Chung-Shan Institute of Science and Technology, Tao Yuan 32599, Taiwan
4
Institute of Environmental Technology, VŠB-Technical University of Ostrava, 17. listopadu 15/2172, 708 33 Ostrava-Poruba, Czech Republic
*
Author to whom correspondence should be addressed.
Catalysts 2017, 7(2), 63; https://doi.org/10.3390/catal7020063
Submission received: 27 December 2016 / Revised: 6 February 2017 / Accepted: 8 February 2017 / Published: 16 February 2017
(This article belongs to the Special Issue Small Molecule Activation and Catalysis)

Abstract

:
Photo-hydrogenation of carbon dioxide (CO2) is a green and promising technology and has received much attention recently. This technique could convert solar energy under ambient temperature and pressure into desirable and sustainable solar fuels, such as methanol (CH3OH), methane (CH4), and formic acid (HCOOH). It is worthwhile to mention that this direction can not only potentially depress atmospheric CO2, but also weaken dependence on fossil fuel. Herein, 1 wt % Pt/CuAlGaO4 photocatalyst was successfully synthesized and fully characterized by ultraviolet-visible light (UV-vis) spectroscopy, X-ray diffraction (XRD), Field emission scanning electron microscopy using energy dispersive spectroscopy analysis (FE-SEM/EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET), respectively. Three kinds of experimental photo-hydrogenation of CO2 in the gas phase, liquid phase, and gas-liquid phase, correspondingly, were conducted under different H2 partial pressures. The remarkable result has been observed in the gas-liquid phase. Additionally, increasing the partial pressure of H2 would enhance the yield of product. However, when an extra amount of H2 is supplied, it might compete with CO2 for occupying the active sites, resulting in a negative effect on CO2 photo-hydrogenation. For liquid and gas-liquid phases, CH3OH is the major product. Maximum total hydrocarbons 8.302 µmol·g−1 is achieved in the gas-liquid phase.

Graphical Abstract

1. Introduction

Nowadays, it is important to develop an efficient and effective method for utilizing carbon dioxide (CO2) greenhouse gas. The CO2 captured from the atmosphere will be stored in the ground or the ocean [1]. Another option, which is a more attractive method for CO2 consumption, is to convert it into useful hydrocarbon fuels directly [2]. In industry, CO2 is widely used for Sabatier reaction [3,4] and reverse water-gas shift (RWGS) reaction [5,6]. Regarding the Sabatier reaction, CO2 reacts with H2 to form CH4 and H2O at 300–400 °C. As for RWGS reaction, CO2 firstly reacts with H2 to form CO and H2O; then CO can be easily converted into hydrocarbons by the Fischer-Tropsch reaction. However, both reactions require high temperature and high pressure to reduce CO2 into hydrocarbons. Thus, it is inevitable to develop another method to conduct CO2 reduction.
The possible CO2 conversion processes, including biological [7], catalytic [8,9,10,11], photocatalytic [12,13,14], and electrocatalytic [15,16] conversion are summarized in Table 1. Among these processes, photocatalytic CO2 reduction is one of the most promising technologies and has received much attention recently [14,17,18,19,20,21]. CO2 as a C1-feedstock could be reduced to desirable and sustainable solar fuels at ambient conditions under light irradiation.
Ideally, photocatalytic CO2 reduction could provide an alternative and sustainable pathway to producing desired hydrocarbon products from renewable energy and CO2. However, it is noted that their efficiency is still relatively low. To enhance the photocatalytic performances, several factors—such as light harvesting, loss of photons, product separation, and charge carrier recombination—are considered in our group to design and developed in a new photo-reactor. In 2007, a circulated photocatalytic reactor was developed, which could provide a large specific surface area and uniformity of gas concentrations in the reactor [22]. In 2008, the optical-fiber photo-reactor, which could deliver light efficiently and uniformly to the surface of a photocatalyst, was also successfully designed [23]. It is important to note that there are two main sources to conduct CO2 reduction, H2O(g) and H2(g). In a previous study, Abbott et al. calculated a series of changes of enthalpy (ΔH0) and changes of Gibbs free energy (ΔG0), respectively, for CO2 reduction to form hydrocarbons [24]. The result clearly shows that adding H2 largely decreases these values. That is, the CO2 reducing reaction in which the involved H2 requires lower energy and becomes more spontaneous. Therefore, in 2013, a novel twin reactor was successfully developed to hydrogenate CO2 into CH3OH [20]. However, it is noted that its efficiency is still relatively low.
In this study, 1 wt % Pt/CuAlGaO4 was prepared using a well-known solid-state fusion and photo-deposition method, respectively. Detailed characterization of photocatalyst was conducted to reveal its structure. Three kinds of reactors—including gas phase reactor, liquid phase reactor, and gas-liquid reactor—were designed and employed to study the influence of hydrogen (H2) and carbon dioxide (CO2) on the photo-hydrogenation of CO2. Additionally, a possible reaction pathway of CO2 photo-hydrogenation is also proposed based on the knowledge of products presented during the photocatalytic reaction.

2. Results and Discussion

2.1. Characterization of Photocatalysts

All photocatalysts were fully characterized by several techniques. Brunauer-Emmett-Teller (BET), ultraviolet-visible light (UV-vis) spectroscopy; X-ray diffraction (XRD), transmission electron microscopy (TEM), and Field emission scanning electron microscopy (FE-SEM) using energy dispersive spectroscopy (EDS) analysis were used to reveal their structure and surface morphology of photocatalysts. X-ray photoelectron spectroscopy (XPS) was used to reveal the chemical state of the species.
Figure 1 displays the UV-vis light absorption spectra of CuAlGaO4 and 1 wt % Pt/CuAlGaO4 photocatalysts, respectively. Clearly, all the photocatalysts have the absorption band in the range of 250–670 nm, which is consistent with that reported previously [25]. It notes that loading of the Pt neither significantly improves the light absorption nor changes the band gap. A possible reason comes from the fact that Pt loading is very small and highly dispersed on the supporting surface.
Figure 2 shows the XRD patterns of CuAlGaO4, 1 wt % Pt/CuAlGaO4 photocatalysts and their starting materials of CuO, Al2O3, and Ga2O3. Clearly, no peaks can be assigned to either CuO, Al2O3, or Ga2O3, indicating that complete reaction of the precursors was achieved by solid-state fusion a mixture of Al2O3, Ga2O3, and ZnO at 1125 K for 12 h. The loading of Pt obviously retains the structural features of CuAlGaO4 photocatalyst, but its crystallites appear to be slightly decreased. Both photocatalysts have patterns similar to that of the single-phase spinel type structure of the CuAlGaO4 (JCPDS file, card No. 26-0514). However, there is no noticeable crystalline phase observed in the XRD pattern of Pt/CuAlGaO4 photocatalyst that can be attributed to Pt element. As discussed above, the Pt loading might be very small and highly dispersed on the supporting surface, which is consistent with the UV-vis spectrum.
Figure 3 shows the SEM images with corresponded elemental spectra of CuAlGaO4 and 1 wt % Pt/CuAlGaO4 photocatalysts, respectively. For CuAlGaO4 photocatalyst, it has cube-like particles and smooth surfaces. Most importantly, loading of Pt on CuAlGaO4 could not change the shape and morphology of the photocatalyst. Additionally, a uniform distribution of the Pt particles was found with a narrow size range of 4–20 nm.
Table 2 shows elemental analysis of CuAlGaO4 and 1 wt % Pt/CuAlGaO4, respectively. It clearly points out that only 1 wt % Pt/CuAlGaO4 showed the Pt signal (Figure 3). On the other hand, the other elemental signals (O, Al, Cu, Ga) were very similar between CuAlGaO4 and 1 wt % Pt/CuAlGaO4 photocatalysts.
Figure 4 reveals the TEM images, which were conducted to further understand the presenting of Pt loading on 1 wt % Pt/CuAlGaO4 photocatalyst. It has several well-dispersed Pt nanoparticles with corresponded size of near 4–20 nm are located on the surface of Pt/CuAlGaO4 photocatalyst.
XPS Pt4f spectra of CuAlGaO4 and 1 wt %Pt/CuAlGaO4 photocatalysts are shown in Figure 5. Most importantly, there are two possible peaks at 73.6–74.1 and 77.0–77.4 eV have been observed for 1 wt % Pt/CuAlGaO4 photocatalyst, which may be attributed to 4f7/2 and 4f5/2 of Pt oxide, respectively [26].
Lastly, the BET surface area of CuAlGaO4 and 1 wt % Pt/CuAlGaO4 was approximately 0.65 and 0.86 m2·g−1, respectively. Both values were not very high due to the solid-state fusion method which operates at a high temperature, causing the photocatalyst aggregation.

2.2. Photocatalytic CO2 Reduction with Gas, Liquid, Gas-Liquid Phase Reactors

In this study, several blank tests in all type of reactors (gas phase, liquid phase, and gas-liquid phase reactors) were conducted to answer the question whether the formation of the reaction products come from photo-hydrogenation and not from CO2 contaminations or the photocatalyst itself. The result of tests was shown in Table 3. The result clearly shows that if CO2 was not introduced to the reacting system, no product was generated even with the presence of the hydrogen, photocatalyst, and light irradiation. It is important to remember that the photocatalyst synthesis process did not use any carbon-containing precursor. This blank test result is further confirmation that the photocatalyst itself contains nearly no carbon residue. Table 3 also clearly shows that, without any of three essential elements in performing the photo-hydrogenation reaction, including (a) CO2, (b) photocatalyst and (c) light source, no product could be detected.
For CO2 reduction, there are several possible C1 products, such as formic acid, formaldehyde, methanol, and methane. Because of the high detection limit of formaldehyde by HPLC (3.3 ppm), we used the Nash reagent [27] to react with formaldehyde first, then analyzed the colored product diacetyldihydrolutidine (DDL) by UV-vis at 414 nm to quantify the formaldehyde. The detail was described in the previous literature [27,28]. Even the detection limit of this method was 0.17 µmol·L−1, which was about three orders lower than HPLC, formaldehyde was not observed under all investigated conditions. However, it is noted that although we did not detect formaldehyde, some of the formaldehyde might also be formed on the surface of the catalyst [29,30]. For the gas phase reactor, there is also no formic acid and methanol could be detected, and only methane evolution would be discussed in this case. The CH4 yield under gas phase conditions was shown in Figure 6.
Most interestingly, H2 plays an active role on CH4 yield.
CO2(g) + 4H2(g) → CH4 (g) + 2H2O(l)
H0 = −259.9 kJ/mol; ∆G0 = −132.4 kJ/mol
It is evident that the conversion of CO2 into CH4 has negative ∆H0 and ∆G values, meaning that the process is spontaneous, equilibrium favorable, and exothermic. In this study, the enthalpy (∆H0) and Gibbs free energy (∆G0) values at 298 K for several interesting reactions were calculated by Aspen [31]. At first 2 h, the initial rates of CH4 yield were 0.01, 0.57, 0.73, and 0.40 µmol·g−1·h−1 that correspond to H2 partial pressure = 0, 0.01, 0.1, 0.2 atm, respectively. It is worth noting that increasing the partial pressure of H2 would enhance the yield of the product. Among the conditions of H2 partial pressure, 0.1 atm shows the best performance. However, an extra supply of H2 might compete with CO2 for occupying the active sites, bringing a negative effect on CO2 photo-hydrogenation. As shown in Figure 6, CH4 evolution became stable after two hours in reaction, implying that the reactions in the system reached a dynamic equilibrium.
A comparative experiment was also conducted in the liquid phase. The correlation between the amount of CH4, CH3OH, and HCOOH formed and the reaction time for the liquid phase are shown in Figure 7.
The main products of the liquid phase reactor were CH4, CH3OH, and HCOOH, respectively.
CO2(g) + 3H2(g) → CH3OH(l) + H2O(l)
H0 = −137.8 kJ/mol; ∆G0 = −10.7 kJ/mol
CO2(g) + H2(g) → HCOOH(l)
H0 = −31.0 kJ/mol; ∆G0 = +34.3 kJ/mol
It is clear that all exothermic reactions exhibit negative ∆H0 values. Moreover, many reactions are also associated with negative ∆G0 values and, as a consequence, the reaction will be thermodynamically favorable. Most importantly, CH3OH was found to be the major product under current condition. This fact is that CH3OH requires only six photoelectrons for the photo-hydrogenation of CO2 while CH4 needs to have eight photoelectrons for reaction. Last but not least, an extra amount of H2 provided may also suppress the yield of products, which is consistent with the observation in the gas phase.
The correlations between the amount of CH4, CH3OH, and HCOOH formed and the reaction time for the combined gas-liquid phase are shown in Figure 8.
Similar to the result of the liquid phase, CH3OH still is the main product for gas-liquid phase. On the other hand, the CH4 yield has a tendency to be suppressed after conducting the experiment for two hours. It is possible that an intermediate product of CO2 photo-reduction, CO, could be generated as the reaction proceeds (Equation (4)). Following is the competing reaction pathways of photo-reduction of CO to CH3OH and HCOOH via H2 and H2O, respectively (Equations (5) and (6)). This result implies that the presence of photocatalyst in the gas phase will promote the converting of CH4 into another compound, such as CH3OH or HCOOH, resulting in a decrease of CH4 yield.
CO2(g) + CH4(g) → 2CO(g) + 2H2(g)
H0 = −247.5 kJ/mol; ∆G0 = +170.8 kJ/mol
CO(g) + 2H2(g) → CH3OH(l)
H0 = −131.6 kJ/mol; ∆G0 = −29.9 kJ/mol
CO(g) + H2O(g) → HCOOH(l)
H0 = −24.8 kJ/mol; ∆G0 = +15.1 kJ/mol
In brief summary, the possible reaction pathways of CO2 photo-hydrogenation is illustrated in Scheme 1. This mechanism is proposed based on the knowledge of products presented during the photocatalytic reaction. Our observation indicates that CH4 is the only product under gas phase condition. On the other hand, CH4, CH3OH, and HCOOH are found in the liquid and gas-liquid phase reactions. Although we could not measure an intermediate CO product quantitatively, we expect that CO might be generated during the reaction. In the gas-liquid phase reactor, the photocatalyst is well packed and dispersed on the quartz plate in the gas phase. Hence, it might promote the transformation of CO into CH3OH and HCOOH products. We do believe that conducting the experiment in different phases (such as gas, liquid, and gas-liquid phases) could not change the mechanism of the photo-hydrogenation of CO2. However, the presence of photocatalyst in different phases might accelerate different pathways of the reaction.
Table 4 summarizes the product yields at two hours in three kinds of reactors under different H2 partial pressures. We see that H2 plays an active role in the photocatalytic reduction of CO2. Additionally, a remarkable synergetic activity was clearly observed when the experiment was conducted in gas-liquid phase under 0.01 atm of H2. In more detail, the total products in the gas and liquid phase are 0.400 and 0.243 µmol·g−1, respectively. Interestingly, about 8.302 µmol·g−1 is achieved in the gas-liquid phase.
Lastly, the quantum efficiencies in three kinds of reactors under 0.01 atm of H2 are also calculated. The highest quantum efficiency was in the gas-liquid phase reactor for 0.0011%, while the gas and liquid phases were about 0.0001% and 0.0005%, respectively. The possible reason for higher quantum efficiency in the gas-liquid phase reactor is that both H2 in the gas phase and the proton in liquid phase could be utilized simultaneously, affording more chances to conduct CO2 photo-reduction.

3. Materials and Methods

3.1. Preparation of Photocatalysts

CuAlGaO4 photocatalyst was firstly prepared by solid-state fusion method, which is mentioned in previous studies [19,25]. Firstly, copper oxide (CuO, Showa, Tokyo, Japan), aluminum oxide (Al2O3, Type A-5, Sigma-Aldrich, St. Louis, MO, USA), and β-gallium trioxide (β-Ga2O3, ≥99.9%, Sigma-Aldrich) powders were mixed in the molar ratio of Cu/Al/Ga = 1:1:1 and pulverized in a mortar. Subsequently, the resulting mixture was calcined at 1150 °C for 12 h, and then cooled to room temperature and further pulverized to obtain a CuAlGaO4 powder.
1 wt % Pt/CuAlGaO4 photocatalyst was prepared by the photo-deposition method as described in the same reference. The required amount of chloroplatinic acid hydrate (H2PtCl6·xH2O, ≥99.9%, Sigma-Aldrich) solution was mixed with the as-prepared CuAlGaO4 powder. Herein, the loading of Pt on CuAlGaO4 is 1 wt %. The mixed solution was irradiated by a UV source (320–500 nm, EXFO S1500, EXFO Inc., Quebec City, QC, Canada) for 90 min to perform the photo-deposition process. After that, the solid product was centrifuged and washed with deionized water several times. Finally, the washed material was dried at 80 °C for 8 h to obtain 1 wt % Pt/CuAlGaO4 powder.

3.2. Characterization of Photocatalysts

X-ray diffractometer (XRD, Ultima IV, Rigaku, Tokyo, Japan) equipped with Cu Kα (1.5418 Å) was used to verify the crystalline structure of photocatalysts. The UV-vis diffuse reflectance spectrum of the photocatalyst was fully recorded over the range 300–800 nm by a Cary 100 UV-visible spectrometer (UV-vis, Varian Cary 100, Agilent Technologies, Santa Clara, CA, USA). BaSO4 was used as the reflectance standard. A field emission scanning electron microscope (FE-SEM, Nano SEM 230, FEI, Hillsboro, OR, USA) equipped with energy dispersive spectroscopy (EDS) was used to directly reveal the presence of the atomic elements in the photocatalysts. Transmission electron microscopy (TEM, Hitachi H-7100, Hitachi Inc., Tokyo, Japan) was performed to check the surface morphology of CuAlGaO4 and 1 wt % Pt/CuAlGaO4. The state of Pt loaded on CuAlGaO4 was determined by X-ray photoelectron spectroscopy (XPS, Thermo Theta Probe, Thermo Fisher Scientific Inc., East Grinstead, UK) equipped with a Mg Kα (1253.6 eV) X-ray source. The Brunauer-Emmett-Teller (BET) surface area of photocatalysts was measured by a specific surface area analyzer (ASAP 2020, Micromeritics, Norcross, GA, USA).

3.3. Photo-Hydrogenation of CO2 Reaction

In this study, we perform the experiments in three kinds of batch reactor systems: gas phase reactor, liquid phase reactor, and the gas-liquid phase reactor (Scheme 2).
For the gas phase reactor, the cylindrical tube reactor was made of Pyrex with a volume of 260 mL (Scheme 2a). A quartz plate, 92 × 40 × 2 mm, was placed inside the reactor and inclined at an angle of 20° to face the light source, 300 W Xenon lamp (Model 66902, Newport, Irvine, CA, USA). During the photoreaction, the light source was placed at an equal-distance from the reactor to the lamp so that the catalyst could receive the same amount of light intensity (mostly visible light), which was around 270 mW·cm−2. Powder photocatalyst (0.30 g) was evenly packed on the quartz plate. For the liquid phase reactor, the reactor was also made by Pyrex with a volume of 385 mL (Scheme 2b). 0.30 g photocatalyst was well dispersed in the liquid phase with stirring. A 300 W Xenon lamp was also provided as the light source. The liquid phase solution, which contained 180 mL 0.1 M NaOH (Alfa Aesar, Ward Hill, MA, USA), was used to increase the solubility of CO2. For the gas-liquid phase reactor (Scheme 2c), it was almost the same with the liquid phase reactor that combined with a Teflon plate. 0.10 g and 0.20 g 1 wt % Pt/CuAlGaO4 photocatalysts were well packed and dispersed on the Teflon plate and in the solution, respectively. Before introducing the CO2 (99.999%, Air Products, Taipei, Taiwan) and H2 (99.99%, Air Products) into the reactor, Argon (99.9995%, Air Products) was purged to bring out any impurity contained within.
In this study, all the samples were taken and analyzed every two hours. Both methane (CH4, gas phase) and methanol (CH3OH, liquid phase) were detected by individual GC-FID (China GC 2000, China Chromatography Co., Taipei, Taiwan) equipped with a packed column (molecular sieve 5A, 3.5 m) with a carrier gas of Ar (99.9995%, Air Products). Formic acid (HCOOH, liquid phase) was detected by High Performance Liquid Chromatography (HPLC) (Shodex RI-2000, Shoko Co., Tokyo, Japan) equipped with a column (ICSep ICE-ORH-801, 300 mm, Interchim, Montluçon, France).

4. Conclusions

A potentially clean, low-cost CO2 photo-hydrogenation method to convert the solar energy into desirable and sustainable energy—such as methanol, methane, and formic acid—has been demonstrated. In this study, CO2 as a C1-feedstock was photo-reduced in the gas phase, liquid phase, and gas-liquid phase, respectively, over 1 wt % Pt/CuAlGaO4 photocatalyst under different H2 partial pressures. Interestingly, a remarkable result has been observed in the gas-liquid phase. The result also shows that increasing the partial pressure of H2 could improve the yield of products. However, it notes that H2 might compete with CO2 for occupying the active sites when an extra amount of H2 is provided, bringing about the adverse effect on CO2 photo-hydrogenation. For liquid and gas-liquid phases, CH3OH is the major product while CH4 is the only product for the gas phase. The quantum efficiencies in gas, liquid, and gas-liquid phase under 0.01 atm of H2 are about 0.0001%, 0.0005%, and 0.0011%, respectively. It is worthwhile to mention that this direction not only can potentially depress atmospheric CO2, but also weaken dependence on fossil fuel.

Acknowledgments

The authors gratefully acknowledge the Chung-Shan Institute of Science & Technology, Taiwan (Grant No. CSIST-808-V309(103)), and the Ministry of Science and Technology, Taiwan (Grant No. NSC 103-2923-E-002-009-MY3), and Grant Agency of the Czech Republic (project No. 14-35327J) for financial support.

Author Contributions

Jeffrey Chi-Sheng Wu, Chun-Ying Chen, and Wei-Hon Wang conceived and designed the experiments; Chun-Ying Chen performed the experiments; Chun-Ying Chen, Van-Huy Nguyen, and Joseph Che-Chin Yu analyzed the data; Van-Huy Nguyen and Joseph Che-Chin Yu wrote the paper. Wei-Hon Wang and Kamila Kočí participated in research discussion.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Ultraviolet-visible light (UV-vis) spectra for CuAlGaO4 and 1 wt % Pt/CuAlGaO4 photocatalysts.
Figure 1. Ultraviolet-visible light (UV-vis) spectra for CuAlGaO4 and 1 wt % Pt/CuAlGaO4 photocatalysts.
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Figure 2. X-ray diffraction (XRD) patterns of photocatalysts: CuAlGaO4 and 1 wt % Pt/CuAlGaO4, in compared with starting materials of CuO, Al2O3, and Ga2O3.
Figure 2. X-ray diffraction (XRD) patterns of photocatalysts: CuAlGaO4 and 1 wt % Pt/CuAlGaO4, in compared with starting materials of CuO, Al2O3, and Ga2O3.
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Figure 3. Scanning electron microscopy (SEM) images with corresponded elemental spectra of (a) CuAlGaO4 and (b) 1 wt % Pt/CuAlGaO4 photocatalysts.
Figure 3. Scanning electron microscopy (SEM) images with corresponded elemental spectra of (a) CuAlGaO4 and (b) 1 wt % Pt/CuAlGaO4 photocatalysts.
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Figure 4. Transmission electron microscopy (TEM) images of photocatalysts: (a) CuAlGaO4 and (b) 1 wt % Pt/CuAlGaO4 photocatalysts.
Figure 4. Transmission electron microscopy (TEM) images of photocatalysts: (a) CuAlGaO4 and (b) 1 wt % Pt/CuAlGaO4 photocatalysts.
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Figure 5. The Pt 4f X-ray photoelectron spectroscopy (XPS) spectra for (a) CuAlGaO4 and (b) 1 wt % Pt/CuAlGaO4 photocatalysts.
Figure 5. The Pt 4f X-ray photoelectron spectroscopy (XPS) spectra for (a) CuAlGaO4 and (b) 1 wt % Pt/CuAlGaO4 photocatalysts.
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Figure 6. The time-dependent yield of CH4 evolution over 1 wt % Pt/CuAlGaO4 photocatalyst under different H2(g) partial pressures in a gas phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only.
Figure 6. The time-dependent yield of CH4 evolution over 1 wt % Pt/CuAlGaO4 photocatalyst under different H2(g) partial pressures in a gas phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only.
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Figure 7. The time-dependent yield of (a) CH4 evolution, (b) CH3OH formation, and (c) HCOOH formation over 1 wt % Pt/CuAlGaO4 photocatalyst under different H2(g) partial pressures in a liquid phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only.
Figure 7. The time-dependent yield of (a) CH4 evolution, (b) CH3OH formation, and (c) HCOOH formation over 1 wt % Pt/CuAlGaO4 photocatalyst under different H2(g) partial pressures in a liquid phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only.
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Figure 8. The time-dependent yield of (a) CH4 evolution, (b) CH3OH formation, and (c) HCOOH formation over 1 wt % Pt/CuAlGaO4 photocatalyst under different H2(g) partial pressures in gas-liquid phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only.
Figure 8. The time-dependent yield of (a) CH4 evolution, (b) CH3OH formation, and (c) HCOOH formation over 1 wt % Pt/CuAlGaO4 photocatalyst under different H2(g) partial pressures in gas-liquid phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only.
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Scheme 1. The reaction pathways during the photo-hydrogenation of CO2.
Scheme 1. The reaction pathways during the photo-hydrogenation of CO2.
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Scheme 2. Three kinds of reactors: (a) Gas phase reactor; (b) liquid phase reactor; (c) gas-liquid phase reactor.
Scheme 2. Three kinds of reactors: (a) Gas phase reactor; (b) liquid phase reactor; (c) gas-liquid phase reactor.
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Table 1. Summary of different processes of CO2 reduction.
Table 1. Summary of different processes of CO2 reduction.
Conversion ProcessMain ProductsAdvantagesDisadvantagesRef.
BiologicalHCOOH: conversion efficiency of (22 ± 9) × 10−2%Biological capability to synthesize liquid fuelsComplicated and cumbersome biological processes[7]
Catalytic (Heterogeneous catalysis)CH4: 3.8 × 104 μmol·h−1·g−1 catalystHigh efficiencyHigh temperature requirement[8]
CH3OH: 8.8% of CO2 conversion with the corresponded selectivity of 63%[9]
CH3OH: 193.9 g·kgcat−1·h−1[10]
Catalytic (Homogeneous catalysis)CH3OH: 79% yield[11]
Photocatalytic (Heterogeneous catalysis)CH4: 0.56 μmol h−1·g−1 catalystStorage of solar energyLow efficiency[12]
CH3OH: 4.6 μmol h−1·g−1 catalyst[13]
Photocatalytic (Homogeneous catalysis)CO: 12.66 h−1 of TOF[14]
ElectrocatalyticLiquid fuelsConverting CO2 directly to liquid fuels (long-chain molecule)High energy barrier needs overcoming[15,16]
Table 2. Energy dispersive spectroscopy (EDS) analysis for element compositions of CuAlGaO4 and 1 wt % Pt/CuAlGaO4 photocatalysts.
Table 2. Energy dispersive spectroscopy (EDS) analysis for element compositions of CuAlGaO4 and 1 wt % Pt/CuAlGaO4 photocatalysts.
ElementAtomic Ratio (%)
CuAlGaO4Pt/CuAlGaO4
O61.961.6
Al23.022.0
Cu12.813.0
Ga2.33.2
PtN/D 10.2
1 N/D—Not detection.
Table 3. Summary of blank tests for photo-hydrogenation of CO2.
Table 3. Summary of blank tests for photo-hydrogenation of CO2.
No.Type of ReactorsExperimental ConditionsYield of Products (µmol·g−1)
CO2H2 (0.01 atm)Photocatalyst (1 wt % Pt/CuAlGaO4)Light Source
1Gas phase reactorX 1O 2OOBDL 3
2OOXOBDL
3OOOXBDL
4Liquid phase reactorXOOOBDL
5OOXOBDL
6OOOXBDL
7Gas-liquid phase reactorXOOOBDL
8OOXOBDL
9OOOXBDL
1 X—absent in the photoreactor; 2 O—present in the photoreactor; 3 BDL—below the detection limit of gas chromatography.
Table 4. Summary of the product yields at 2 h in three kinds of reactors over 1 wt % Pt/CuAlGaO4 under different H2 partial pressures.
Table 4. Summary of the product yields at 2 h in three kinds of reactors over 1 wt % Pt/CuAlGaO4 under different H2 partial pressures.
EntryPhaseH2 Partial Pressure (atm)Product Yields (µmol·g−1)
CH4CH3OHHCOOHTotal HCs 1
1Gas0.000.012 ± 0.010BDL 2BDL0.012
20.010.400 ± 0.100BDLBDL0.400
30.200.780 ± 0.300BDLBDL0.780
4Liquid0.000.010 ± 0.0100.235 ± 0.100BDL0.245
50.010.149 ± 0.050BDL0.094 ± 0.0450.243
60.200.112 ± 0.0300.340 ± 0.200BDL0.452
7Gas-Liquid0.000.014 ± 0.0100.285 ± 0.100BDL0.299
80.010.480 ± 0.2007.352 ± 2.1000.470 ± 0.1008.302
90.200.666 ± 0.1200.445 ± 0.1100.145 ± 0.0101.255
1 Total hydrocarbons yield = CH4 yield + CH3OH yield + HCOOH yield); 2 BDL—below the detection limit of gas chromatography.

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Chen, C.-Y.; Yu, J.C.-C.; Nguyen, V.-H.; Wu, J.C.-S.; Wang, W.-H.; Kočí, K. Reactor Design for CO2 Photo-Hydrogenation toward Solar Fuels under Ambient Temperature and Pressure. Catalysts 2017, 7, 63. https://doi.org/10.3390/catal7020063

AMA Style

Chen C-Y, Yu JC-C, Nguyen V-H, Wu JC-S, Wang W-H, Kočí K. Reactor Design for CO2 Photo-Hydrogenation toward Solar Fuels under Ambient Temperature and Pressure. Catalysts. 2017; 7(2):63. https://doi.org/10.3390/catal7020063

Chicago/Turabian Style

Chen, Chun-Ying, Joseph Che-Chin Yu, Van-Huy Nguyen, Jeffrey Chi-Sheng Wu, Wei-Hon Wang, and Kamila Kočí. 2017. "Reactor Design for CO2 Photo-Hydrogenation toward Solar Fuels under Ambient Temperature and Pressure" Catalysts 7, no. 2: 63. https://doi.org/10.3390/catal7020063

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