光触媒的二酸化炭素還元を高効率に促進する環境適合型分子システムの開発

概要

Due to the vast combustion of fossil fuels in the past century, humankind currently suffers from substantial climate changes due to the global warming by excessive atmospheric greenhouse gases. Among them, carbon dioxide (CO2) levels in the atmosphere have been steadily increasing. Recently, the International Panel on Climate Change (IPCC) predicted that CO2 concentrations could run up to 590 parts per million (ppm) by 2100 with the temperature rising at least 1.9 °C, which may contribute to unpredictable and serious non-natural disasters. Thus, greenhouse gas emissions must be substantially suppressed by replacing fossil fuels with sustainable energy sources, such as solar, hydroelectric, oceanic, geothermal, wind, hydrogen and biomass energy. Artificial synthesis of either C1 or C2 fuels from CO2 based on either photocatalytic or electrocatalytic process has also attracted much attention in order to develop technologies enabling renewable energy cycles.

Great efforts have been therefore devoted to developing artificial photosynthetic CO2 reduction systems. However, many of them have been studied in organic solvents. From a practical viewpoint, photocatalytic system using water as a solvent considerably benefits, since water is highly abundant and environmentally benign solvent and provides the protons and electrons required for the CO2 reduction. Nevertheless, it is highly challenging to use water as reaction media, since CO2 conversion may suffer from the competition with water reduction, which is kinetically and thermodynamically more favorable, leading to diminish the efficiency and selectivity of CO2 reduction vs. water reduction. In addition, environmentally friendly as well as earth-abundant systems should also be developed by limiting the use of precious metals towards the widespread implements in the future. In this regard, the aim of thesis here is to develop efficient homogeneous molecular photocatalytic systems based on the earth-abundant elements for CO2 reduction in fully aqueous media.

Cost effective Cu(I) complexes with highly luminescent, long-lived with tunable spectroscopic and redox properties are considered as appealing alternatives to replace traditional noble-metal (e.g., Ru, Ir) photosensitizers for artificial photolysis. Especially, the heteroleptic Cu(I) complexes of the type [Cu(P^P)(N^N)]+ (P^P = diphosphine; N^N = diimine) were targeted. By introducing sulfonate groups, a water-soluble Cu(I) photosensitizer (CuPS, see Figure 1) was firstly obtained and characterized by photophysical and electrochemical studies. This achievement gave a substantial impact in promoting the studies in this thesis.

Metalloporphyrins are known as the robust catalysts for CO2 reduction owing to the macrocyclic effect. To make the metalloporphyrin catalysts well soluble in aqueous media, the catalysts tethered to charged moieties, as those depicted in Figure 1, have been selected in this study. By employing the fully aqueous photocatalytic CO2 reduction systems consisting of CuPS as a photosensitizer and different cobalt porphyrin catalysts in combination with ascorbate (AscH– ) as a sacrificial electron donor, the high turnover number (TON) for CO formation and selectivity of CO2 reduction over H2 evolution (SelCO2) were obtained. The CuPS-driven CO2 reduction catalyzed by Co(TPPS) afforded TONCO of 1085 with SelCO2 of 90%. Co(pTMPyP) showed by outstanding catalytic performance in TONCO of 2680 and TOFCOmax of 2600 h-1 with slightly lower SelCO2 of 77%. Notably, the observed photocatalytic enhancement exhibited by Co(pTMPyP) was discussed in terms of the multi-electron chargeable character, where Co(pTMPyP) was confirmed to bind CO2 after at least four-electron reduction according to electrochemical studies. On the other hand, compared with Co(pTMPyP), substantial improvement in TONCO (4000) together with SelCO2 (90%) for Co(oTMPyP) could be achieved. Note that this TONCO by Co(oTMPyP) is among the highest values reported thus far within the group of fully earth abundant photocatalytic systems in water.

Finally, the mechanisms of two different type multi-electron chargeable cobalt porphyrin catalysts, Co(pTMPyP) and Co(oTMPyP), were successfully elucidated based on the electrochemical and density functional theory (DFT) studies, as shown in Figure 2. The H2 evolution path is strongly inhibited by the active intermediate of Co(oTMPyP) possessing a low-spin d7 CoII center due to its mismatch in forming an effective molecular orbital (MO) association with a 1s(H+) orbital. To the best of the knowledge, this is the first catalyst example avoiding the standard path relying on a filled dz2 orbital in binding CO2. Instead, both - and -type frontier MO associations are formed by two degenerated *(CO2) orbitals using a filled dxz and a half-filled dz2 orbital. However, in the case of Co(pTMPyP), highly electron charged intermediates show switching in configuration from (dxz) 2(dz2) 1 to (dxz) 1(dz2) 2 , permitting the standard -type interaction of the (dz2) 2 pair with CO2. Correlation between the multi-electron charging behaviors of cobalt porphyrins and the mechanism of photo- and electrocatalytic CO2 reduction is rationalized by using the DFT results. The achievement made in this thesis sheds new light on strategies to rationally control the reaction rates and/or pathways based on the frontier MO engineering.

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