Due to the global climate crisis brought by the continuous increase of CO2 emission in the atmosphere (BP statistical Review of World Energy June 2016), the utilization of CO2 as a carbon resource is an important subject in the science and industry fields. The reduction of CO2 to utilizable C1 resources is a major research target in chemical conversions of solar energy, together with the hydrogen evolution by water reduction, to provide a strategic way responding the energy and environmental problems (I. Willner et al., J. Am. Chem. Soc., 109:6080-6, 1987). Among visible-light induced multi-electron reductions of CO2, the two-electron reduction to CO is a kinetically favourable choice because of the relatively low reaction barrier compared to one-electron and other higher-reduction reactions (A. J. V. Underwood, Ind. Eng. Chem., 32:449-54. 1940). Moreover, CO is currently utilized as a carbon source for the production of chemicals in petroleum chemical industry, particularly for the methanol production from a mixture of CO and H2, so-called syngas (M. E. Dry, Catal. Today, 71:227-41, 2002; I. Wender, Fuel Process. Technol., 48:189-297, 1996).
Syngas has been also known as a key feedstock for the production of synthetic bulk chemicals via the Fischer-Tropsch (F-T) processing (A. J. V. Underwood, Ind. Eng. Chem., 32:449-54. 1940). For the production of chemicals from syngas, the tailoring of H2/CO ratio in syngas mixtures is critical, e.g. 2:1 H2/CO for methanol production and F-T hydrocarbon syntheses (M. E. Dry, Catal. Today, 71:227-41, 2002; K. C. Waugh, Catal. Today, 15:51-75, 1992) and 1:1 H2/CO for the production of aldehydes via hydroformylation of alkenes (M. Beller et al., J. Mol. Catal. A: Chem., 104:17-85, 1995). While the simultaneous formation of H2 and CO is known to occur by photochemical and electrochemical reductions of CO2 and water using Ni— (V. S. Thoi et al., J. Am. Chem. Soc., 135:14413-24, 2013), Re— (B. Kumar et al., Chem. Commun., 45:272-4, 2012), and Ru— (P. Kang et al., Energy Environ. Sci., 7:4007-12, 2014)-molecular catalysts, little has been referred to possible control of syngas compositions. Therefore, a new protocol that focuses on adjustability of H2/CO ratio in visible-light induced syngas production should be of scientific significance.
Conventional technologies for catalytic production of syngas require the production of hydrogen and carbon monoxide through independent catalytic conversion processes and mixing of the gases through additional processes. Further, most of the production technologies are limited to the utilization of by-product gases and related processes thereof. Recent technologies for direct production of syngas using molecular catalysts are based on electrochemical methods that still need external energy.
Thus, the present inventors have earnestly and intensively conducted research to develop a method for direct production of syngas without the need for external energy and a catalytic system for producing syngas in which the ratio of gases formed can be controlled, and as a result, found that when two types of transition-metal complex catalysts, i.e. a rhenium catalyst and a cobalt catalyst, are immobilized on TiO2 particles, the resulting catalytic system exhibits high catalytic activity, produces H2 and CO without external energy, and enables control over the ratio of H2/CO formed. The present invention has been accomplished based on this finding.