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Letter
Efficient Visible Light Photocatalytic CO2 Reforming of CH4 Bing Han, Wei Wei, Liang Chang, Peifu Cheng, and Yun Hang Hu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02653 • Publication Date (Web): 17 Dec 2015 Downloaded from http://pubs.acs.org on December 19, 2015
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Efficient Visible Light Photocatalytic CO2 Reforming of CH4 Bing Han, Wei Wei, Liang Chang, Peifu Cheng, and Yun Hang Hu* Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931-1295, USA Supporting Information Placeholder demonstrated 16, 17, which stimulated an increasing interest in photocatalytic conversion of CO2 to useful compounds 18-23. So far, however, those photocatalytic processes have suffered from very low conversion efficiencies. Furthermore, there has been a very limited exploration for photocatalytic CO2 reforming of methane (PCRM) 24, and all reported PCRM processes exploited only UV light (which is about 4% of the total solar energy), leading to very low H2 and CO yields (at the level of µmol/h/gcat) 2426 . It is a long-time goal for photocatalytic processes to efficiently utilize visible light, since it counts about 45% solar energy. Herein, we report a novel approach, in which the combination of Pt/black TiO2 catalyst with a light-diffuse-reflection-surface of a SiO2 substrate created an efficient visible light photocatalytic CO2 reforming of methane.
ABSTRACT: Large energy requirement of CO2 reforming of methane (CRM) has obstructed its application. Solar energy is a solution for the issue. Different from efficient photocatalytic splitting of water, photocatalysis for CRM exhibits a very low efficiency and employs only UV light. This letter reports an efficient visible-light photocatalytic CRM by combining Pt/blackTiO2 catalyst with light-diffuse-reflection-surface. Under visible light illumination by filtering UV from AM 1.5G sunlight, H2 and CO yields reached 71 and 158 mmol/h/gcat with quantum efficiency of 32.3% at 550 oC and 129 and 370 mmol/h/gcat with quantum efficiency of 57.8% at 650 oC. Those yields are three orders of magnitude larger than reported values. Keywords: Photocatalysis • carbon dioxide • methane • visible light • titanium dioxide
Carbon dioxide (CO2) is the most important greenhouse gas. The increase of CO2 content in the atmosphere has been identified as a primary cause for the warming over the past century. Methane, which can absorb heat about 200 times more efficiently than carbon dioxide, is the second most prevalent greenhouse gas from human activities. Furthermore, methane is the main component of natural gas. The CO2 reforming of methane (CRM) (eq. 1), which was investigated as early as 1888 1, would be the most effective way of utilizing these two greenhouse gases. CH4+CO2=2CO+2H2,
∆Ho298=247kJ/mol
(1) Figure 1. Relationship between band structure of black TiO2 and redox potentials of CO2 reforming of CH4. The excitation of electrons from valence band (VB) of TiO2 to its conduction band (CB) requires the absorption of UV light, whereas the absorption of visible light is associated with the excitation of electrons from the donor (Ti3+) level to TiO2 CB.
CRM produces a synthesis gas (a mixture of H2/CO), which can be used as a feedstock for synthesis of liquid fuels and chemicals 2, 3 . Those economic and environmental incentives promoted intensive R&D efforts to develop effective catalysts for the heterogeneous catalytic CRM in the past century 2-6. Unfortunately, no industrial technology has been established for CRM because of its large energy cost caused by the ultrahigh stability of CO2 and methane. Such a thermodynamically unfavorable feature requires a high temperature (over 1000 K) for the reaction to proceed. A photocatalytic process for CRM would be a promising technology, which can not only lower reaction temperature, but also exploit solar energy. The photoelectrocatalytic splitting of water on TiO2 electrodes, which was discovered by Fujishima and Honda in 1972 7, created a new era in heterogeneous photocatalysis. Recently, numerous breakthroughs were reported for photocatalytic hydrogen production from water 8-15. On the other hand, the feasibility of photoelectrocatalytic reduction of CO2 to organic compounds was also
To design an efficient photocatalyst for CO2 reforming of methane, we must consider three requirements: (a) visible light can be absorbed, (b) the band structure of the catalyst matches the redox potentials of the reaction, and (c) the efficiency of light absorption is high. Although TiO2 is one of the most important semiconductors for photocatalysis, it can absorb only UV light due to its relatively large band gap energy (3.0~3.2 eV). Recently, however, Mao and his coworkers made a breakthrough, demonstrating that black TiO2 (which was synthesized by hydrogenating TiO2 powder) can remarkably absorb visible light 8. Priya et al. showed the visible light photocatalytic activity of doped black TiO2 for methylene blue degradation 27. Li et al. attributed the
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absorption of visible light to the formation of oxygen vacancies (generating Ti3+) in hydrogenated rutile TiO2 28. Furthermore, we experimentally obtained the band structure of the black TiO2, which contains a generated donor level (Ti3+) of 1.30 eV below TiO2 CB (Figures S1-S3 in Supporting Information). Those allow us to generate a relationship between the band energy structure of black TiO2 and the redox potentials of CO2 reforming of methane. As shown in Figure 1, one can see that those energies meet the thermodynamic requirement for visible light photocatalytic CO2 reforming of methane: (I) Although CO2/CO redox potential is slightly more negative than TiO2 CB edge at room temperature, it changes to more positive than the CB edge at 150 oC or higher. This indicates that the reduction of CO2 to CO prefers a temperature above 150 oC. The redox potential of CO/CH4 is always more negative than the energy level of the Ti3+ at room temperature or above, satisfying the energy requirement for the oxidation of CH4 to CO. (II) The energy gap (1.3 eV) between the Ti3+ level and TiO2 CB ensures the absorption of visible light (and even near IR). This encouraged us to select the black TiO2 as a semiconductor for our catalyst. It is well-known that Pt possesses a high conductivity to transfer electrons and an excellent activity to break CH bonds of methane 29. It is reasonable for us to select Pt as an active component for the catalyst. Furthermore, our previous work (for photocatalytic conversion of water to H2) demonstrated that the light-diffuse-reflection-surface of a SiO2 substrate could increase the light absorption of a photocatalyst by 100 times 30. Therefore, the dispersion of Pt/black TiO2 catalyst on the lightdiffuse-reflection-surface of a SiO2 substrate is designed to meet the three requirements for an efficient visible light photocatalyst of CO2 reforming of methane. This was confirmed by the following experiments.
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shown in Figure 2A and 2B, the reaction without light illumination hardly took place at 550 oC or below. When reaction temperature increased to 650 oC, the yields (9 and 99 mmol/h/gcat) of H2 and CO were obtained. This is a typical thermocatalytic CO2 reforming of methane, which needs a high temperature. In contrast, when the catalyst was irradiated by AM 1.5 global sunlight, the reaction started at 350 oC, which is 200 oC lower than that without light illumination. When temperature increased to 550 oC, the yields of H2 and CO reached 96 and 191 mmol/ h/gcat, respectively. Those impressive yields are 150 times larger than those without light illumination. Since an in-situ thermocouple (contacted to the catalyst surface) for accurate temperature-control was employed to eliminate the effect of light illumination on the temperature of the catalyst, the large enhancement of H2 and CO yields by light illumination is due to the photocatalytic reaction. This was confirmed using Pt/Al2O3 catalyst. Because Al2O3 is an insulator with a large energy gap (about 8.8 eV), Pt/Al2O3 may not be a photocatalyst. In other words, the reaction over Pt/Al2O3 is only a thermocatalytic process. As shown in Figures 2C and 2D, one can see that H2 and CO yields over Pt/Al2O3 catalyst (dispersed on the light-diffuse-reflection-surface of a SiO2 substrate) is almost the same for the processes with and without light illumination. This clearly demonstrates that the light illumination could not affect the thermocatalytic process. Therefore, for the Pt/black TiO2 catalyst, the difference of H2 (or CO) yields with and without light illumination would be photo H2 (or CO) yield, which was shown with the apparent quantum efficiency in Figure 3. Under the illumination of AM 1.5 global sunlight, the photo H2 and CO yields reached 95 and 191 mmol/h/gcat at 550 oC, 135 and 299 mmol/h/gcat at 650 oC, and 208 and 258 mmol/h/gcat at 700 oC. Those yields are at least three orders of magnitude larger than reported results (that were at the level of µmol/gcat/h) 24-26. The apparent quantum efficiencies (QE) for the entire 1.5 global sunlight also reached high values, namely, 40.7, 64.9, and 48.3% at 550, 650, and 700 oC, respectively (Figure 3C). It should be noted that QE increased to the maximum value at 650 oC and then decreased with increasing temperature. Such a QE increase-thendecrease feature can be explained as follows: As temperature increased up to 650 oC, the reduction potential of CO2/CO became more positive and oxidation potential of CO/CH4 became more negative (Figure 1). As a result, the over potentials of the half reactions increased with increasing temperature, leading to an increase in quantum efficiency (Figure 3C). In contrast, when temperature was above 650 oC, thermocatalytic products jumped, causing a decrease in photocatalytic contribution and thus in quantum efficiency. Furthermore, the larger CO yield than H2 one indicates that the reverse water gas shift reaction (CO2+H2↔CO+H2O) took place.
Figure 2. H2 and CO yields from CO2 reforming of methane (reactant composition=1:1 CH4/CO2 and GHSV=40000 ml/gcat/h): (A) H2 and (B) CO over Pt/black TiO2 catalyst dispersed on the light-diffuse-reflection-surface of a SiO2 substrate, and (C) H2 and (D) CO over Pt/Al2O3 catalyst dispersed on the light-diffusereflection-surface of a SiO2 substrate. The Pt/black TiO2 catalyst (15 mg), which was synthesized with an impregnation method and then hydrogenated, was dispersed on the light-diffuse-reflection-surface of a 4 cm2 SiO2 substrate (that was synthesized by mixing silicon dioxide and quartz wool with water, followed by heating to 1100 °C) (Figures S4-S5 in Supporting Information). Then, the catalyst/substrate was located in a quartz tube reactor and heated to a selected temperature by an electrical furnace (Figure S6 in Supporting Information). CH4/CO2 mixture gas (1:1 ratio and GHSV=40000 ml/gcat/h) flow was introduced into the reactor. All products were analyzed by Gas Chromatograph (GC) and Mass Spectrometer (MS). As
Figure 3. Photocatalytic CO2 reforming of methane over Pt/ black TiO2 catalyst dispersed on the light-diffuse-reflection-surface of a
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SiO2 substrate: (A) Photo H2, (B) Photo CO, (C)Quantum efficiency vs. temperature, and (D) Quantum efficiency vs. wavelength. To reveal the efficiency of visible light for the photocatalytic CO2 reforming of methane over Pt/black TiO2 catalyst dispersed on the light-diffuse-reflection-surface of a SiO2 substrate, UV light was completely filtered from AM 1.5 global sunlight. Figure 2A and 2B show that the visible light illumination resulted in excellent H2 and CO yields of 71 and 158 mmol/h/gcat at 550 oC, 129 and 370 mmol/h/gcat at 650 oC, and 237 and 480 mmol/h/gcat at 700 oC. Higher CO yield than H2 may be due to the reaction between CO2 and H2, which consumes H2 with production of CO. By subtracting thermocatalytic contribution, we obtained visible light photo H2 and CO yields, which are 70 and 158 mmol/h/gcat at 550 oC, 120 and 271 mmol/h/gcat at 650 oC, and 188 and 220 mmol/h/gcat at 700 oC (Figures 3A and 3B). Furthermore, Figure 3C shows impressive apparent quantum efficiencies for the entire visible light range (32.3, 57.8, and 38.6% at 550, 650, and 700 oC, respectively). This would be the first time to obtain efficient visible light photocatalytic CO2 reforming of methane. The relationship between quantum efficiencies and wavelengths further demonstrated that all wavelengths (from 395 to 950 nm) can contribute to photo CO2 reforming of methane (Figure 3D). Furthermore, the quantum efficiency decreased with increasing photon wavelength are due to two factors: (1) Light adsorption decreases with increasing its wavelength (Figure 3D) and (2) Photons with a shorter wavelength can excite electrons to higher energy levels in conduction band, which can enlarge the over potential for charge carriers and thus increase their reactivity. The deactivation of Pt/black TiO2 for visible light photocatalytic CRM was observed and due to the phase transformation of TiO2 (Figures S10-S12 in Supporting Information). In summary, the combination of Pt/black TiO2 catalyst with the light-diffuse-reflection-surface of a SiO2 substrate created an efficient visible light photocatalytic process for CO2 reforming of methane. The obtained photo CO and H2 yields are 1000 times larger than reported values. Furthermore, the process exhibited high apparent quantum efficiencies (for the entire visible light range) of 32.3% at 550 oC and 57.8% at 650 oC. This provides a novel photocatalytic approach for efficient conversions of the two greenhouse gases using solar energy.
ASSOCIATED CONTENT Supporting Information Experimental details, materials, methods, and characterizations Figures S1 to S12. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
*E-mail:
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the U.S. National Science Foundation (NSF-CBET-0931587). Hu also thanks Charles and Carroll McArthur for their great support.
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