ReviewPhotocatalytic reduction of CO2 with H2O on highly dispersed Ti-oxide catalysts as a model of artificial photosynthesis
Graphical abstract
Photocatalytic reduction of CO2 with H2O to produce CH3OH, CH4, and O2 as well as CO proceeds on highly dispersed tetrahedrally coordinated Ti-oxide single site catalyst constructed within zeolite framework structure (artificial photosynthesis).
Introduction
In recent years, there has been great concern over the many serious environmental problems and lack of natural energy resources that we face on a global scale. The increase in world population and industrial growth have all led to accelerated energy consumption and the unabated release of toxic agents and industrial wastes into the air and waterways, leading to pollution-related diseases, global warming and abnormal climatic changes. We, thus, hope to contribute to the development ecologically clean, safe and sustainable chemical technologies, materials and processes [1], [2], [3], [4], [5], [6].
Highly efficient titanium oxide-based photocatalytic materials have been applied to the development of various sustainable, non-hazardous and economic chemical processes under different reaction systems and conditions, as shown in Fig. 1. However, unlike natural photosynthesis in green plants, they make use of only 3–4% of the solar light that reaches Earth, necessitating the use of a UV light source. Significantly, investigations are, thus, underway to modify or sensitize TiO2 to operate under irradiation of much larger visible light regions of sunlight [1], [2], [3], [4], [5], [6].
In 1997, we successfully applied a physical method of metal-ion implantation to modify the electronic properties of various TiO2 photocatalysts, enabling them to absorb visible light and operate under visible light irradiation without any loss of their original photocatalytic reactivity under UV light irradiation. This modification method was completely different from the chemical doping method which leads to a dramatic decrease in their original photocatalytic reactivity under UV light irradiation [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. In 2000, we reported on a RF magnetron sputtering deposition method as a much simpler, less costly method as compared with metal ion-implantation [21], [22], [23], [24]. Unique and highly efficient Ti-oxide thin film photocatalytic materials which enable the absorption of visible light of longer than 550 nm were developed to operate as an effective environmentally-friendly photocatalyst, leading to the efficient use of solar energy for the separate production of clean H2 and O2 from water, as shown in Fig. 2 [4], [5], [6], [25], [26], [27], [28], [29], [30], [31], [32].
However, the design and development of photocatalytic systems that work effectively under sunlight irradiation to reduce CO2 with H2O to produce useful organic compounds and O2, i.e., an artificial system which resembles the photosynthesis in green plants is still one of the most important challenges facing chemical scientists.
In this review article, our work will focus on the possibility of developing an artificial photosynthesis system using highly dispersed and reactive Ti-oxide single site photocatalysts constructed within various zeolitic framework structures such as Ti-MCM-48 and Ti/Y-zeolite, etc.
Section snippets
Photocatalytic reduction of CO2 with H2O on highly dispersed Ti-oxides anchored on porous silica glass
In Fig. 3, the electronic properties, i.e., the energy levels of the conduction and valence band positions, and the photocatalytic activity of various types of titanium oxide photocatalysts are illustrated depending on their local structures from an extended-semiconducting structure to extremely small nano-particles and molecular-sized Ti-oxide single site catalysts. There are two research approaches, as shown by the arrows in Fig. 3, and one is to follow the right direction from normal TiO2
Photocatalytic reduction of CO2 with H2O on a Ti-oxide single site photocatalyst constructed within micro- and meso-porous framework structures
Catalytic systems incorporated or constructed within the zeolite cavities or framework structures have been shown to be effective and selective for various catalytic reactions due to their specific properties in molecular diffusion, high surface area, and shape selectivity, etc. We have investigated the construction of molecular-sized Ti-oxide single site catalysts within the framework structures of various porous materials such as micro-porous zeolites and meso-porous molecular sieves to apply
Design and development of Ti-oxide single site catalysts able to absorb and operate under visible light irradiation
The highly active tetrahedrally-coordinated Ti-oxide species absorb only very short UV light in the wavelength region of 220–280 nm and are unable to absorb and operate under longer UV, visible or natural sunlight [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. As has been mentioned, the modification of various commercially available powdered TiO2 photocatalysts was found to absorb and operate under visible light irradiation without any loss in their photocatalytic activity under UV
Hybridization of natural photosynthesis and artificial photosynthesis using thin film photocatalysts for the efficient use of solar energy in the evolution of clean H2 and O2 as well as in food production
Fig. 2 shows the yields of the evolved H2 and O2 under outdoor sunlight irradiation from 9:00 AM to 4:00 PM for one day. As can be seen, applying the newly developed and highly active visible light-responsive TiO2 thin film photocatalysts initiated the photocatalytic decomposition of H2O into H2 and O2 by separate evolution under sunlight irradiation, their yields increasing linearly with the sunlight irradiation time [25], [26], [27], [28], [29], [30], [31], [32], [74], [75], [76].
As shown in
Summary
In the present article, the results of investigations on the photocatalytic reduction of CO2 with H2O, artificial photosynthesis, highly active tetrahadrally-coordinated Ti-oxide species with TiO4 units within micro- and meso-porous framework structures, and how to modify these catalysts to enable the absorption of and operation under visible light were covered. Advanced metal ion-implantation was shown to open the way to more innovative possibilities in artificial photosynthesis as well as in
Acknowledgements
The author (M. A.) would like to thank his graduate students, colleagues and collaborators for their hard work and efforts in realizing the photocatalytic reduction of CO2 with H2O. They are acknowledged as follows: Assoc. Prof. K. Ikeue (Tokyo Uni. Sci. Technol., Yamaguchi), Prof. H. Yamashita (Osaka Uni.), Prof. M. Ogawa (Waseda Uni.), Dr. M. Harada (Tokyo Electro. Ltd.), Assoc. Prof. Y. Hu (South China Uni. of Technol.), Assoc. Prof. M. Kitano (Tokyo Institute Technol.), Assoc. Prof. Y.
Masakazu Anpo is presently a Vice President and Executive Director of Osaka Prefecture University. He is a pioneer in the study of photochemical reactions on solid surfaces including catalysts and has published the first Japanese book in this field, “Photocatalysis”, with Profs. Y. Kubokawa, K. Honda, Y. Saito in 1988. An English book, “Photochemistry on Solid Surfaces”, was published with Prof. T. Matsuura in 1989 from Elsevier. He has published more than 100 books. Dr. Anpo has received
References (78)
Catal. Surv. Jpn.
(1997)Pure Appl. Chem.
(2000)- et al.
J. Catal.
(2003) Bull. Chem. Soc. Jpn.
(2004)- et al.
Annu. Rev. Mater. Res.
(2005) - et al.
Environmentally Benign Photocatalysts, Applications of Titanium Oxide-based Materials
(2010) - et al.
Res. Chem. Intermed.
(1998) - et al.
J. Phys. Chem. B
(1998) - et al.
J. Phys. Chem. B
(1998) - et al.
J. Synchrotron Rad.
Stud. Surf. Sci. Catal.
J. Synchrotron Rad.
J. Synchrotron Rad.
Res. Chem. Intermed.
Res. Chem. Intermed.
Nucl. Instrum. Methods B
Stud. Surf. Sci. Catal.
Catal. Surv. Asia
Catal. Lett.
Catal. Today
Surf. Sci. Jpn.
J. Phys. Chem. B
Chem. Lett.
Top. Catal.
J. Phys. Chem. B
Appl. Catal. A: Gen.
J. Phys. Chem. B
Appl. Catal. A: Gen.
Catal. Today
Mater. Sci. Forum
Surface Photochemistry
Adv. Catal.
Photofunctional Zeolites
Chem. Mater.
J. Phys. Chem. B
J. Photochem. Photobiol. A: Chem.
J. Photochem. Photobiol. C: Photochem. Rev.
Cited by (0)
Masakazu Anpo is presently a Vice President and Executive Director of Osaka Prefecture University. He is a pioneer in the study of photochemical reactions on solid surfaces including catalysts and has published the first Japanese book in this field, “Photocatalysis”, with Profs. Y. Kubokawa, K. Honda, Y. Saito in 1988. An English book, “Photochemistry on Solid Surfaces”, was published with Prof. T. Matsuura in 1989 from Elsevier. He has published more than 100 books. Dr. Anpo has received awards from the Japan Photochemical Society (1994), Chemical Society of Japan and Ministry of Education of Japan (2009) as well as other awards and honors. He has published over 475 original papers, serves on the editorial boards of numerous international journals and is Editor-in-Chief of the international journal, “Research on Chemical Intermediates” (Springer, USA). Dr. Anpo served as an active member of the International Association of Catalysis Societies (IACS), Catalysis Society of Japan and as a founding member of the Asia-Pacific Association of Catalysis Societies (APACS) established in 2004. Dr. Anpo has currently elected as one of only a few Members of the Academia Europaea from Asia. Since April 1st, 2009, Dr. Anpo plays an important role in organizing university as Vice-President & Executive Director of Osaka Prefecture University. Dr. Anpo's current research topics deal with the design and development of highly efficient visible-light-responsive TiO2 photocatalysts and their application to the decomposition of water into H2 and O2 with a separate evolution as well as the design of highly active single-site transition metal oxide catalysts within the framework of zeolites and meso-porous materials as photocatalysts for environmental remediation and preservation processes and the production of clean energies. His dream is the establishment of solar chemistry as a new environmentally-friendly science and technology.