CO2 conversion to CO

Importance/relevance

Reduction of CO2 into energy-rich compounds is an important topic that can simultaneously tackle the shortage of fossil-fuel resources and global warming.36,40,42,288 Photocatalytic CO2 reduction utilizing solar light as an energy source has been widely investigated as a key technology for the conversion of abundant solar energy to chemical energy, so-called artificial photosynthesis.

Photosensitizers

Non-metal-containing photosensitizers

5,10-Di(2-naphthyl)-5,10-dihydrophenazine 3,7-Di((1,1'-biphenyl)-4-yl)-10-(naphthalen-1-yl)-10H-phenoxazine purpurin

Catalysts

Iron-based catalysts

Iron complexes: Known for the reduction of CO2 to H2 and CO [link to publication]

Rhenium-based catalysts

Rhenium complexes are known to act as photocatalysts and electrocatalysts for reducing CO2 to CO.

Here an example DOI as reference [TMo14][Pro21] [EtF21]

Combined Systems

Rhenium polypyridine complexes are known to act as photocatalysts and electrocatalysts for reducing CO2 to CO.289 A major problem with these photocatalysts is the lack of an extended absorption into the visible region. Therefore, they need to be supported by photosensitizers. The present approach is to fabricate supramolecular photocatalysts, similar to the photocatalytic H2 or O2 production systems. Supramolecular photocatalysts used in the reduction of CO2 act as both the light absorbing center and the catalytic center. Two distinct reaction mechanisms that use supramolecular photocatalysts for the reduction of CO2, are oxidative quenching (OQ) and reductive quenching (RQ).290 As shown in Fig. 56, for both mechanisms, the reduction of CO2 proceeds via the reduced form of the catalyst. Since the first supramolecular Ru–Ni complex used for the photochemical reduction of CO2 was reported by Kimura et al. in 1992,291 huge efforts had been devoted to the design of new bi- or multi-metallic analogues. Ishitani’s group have recently described the development of supramolecular photocatalysts for the photochemical reduction of CO2, 290 including Ru(II)–Re(I), Ru(II)–Ru(II), Ru(II)–Ni(II), Ru(II)–Co(III), Ir(III)–Re(I), Os(II)–Re(I), Pd(II)–Re(I), Zn(II)–Re(I), Fe(III)–Re(I), Co(II)–Re(I) and Cu(II)–Re(I) systems. Their chemical structures and photocatalytic CO2 reduction performance are presented in Fig. 57, 58 and Tables 9, 10. Therefore, in this section, we will mainly focus on the latest development of novel asymmetrical trinuclear supramolecular photocatalysts and their performance to supplement the recent review.290 Recently, Ishitani’s group synthesized two trinuclear supramolecular photocatalysts 319 and 320 (Fig. 59) containing three different metal centers Os(II)–Re(I)–Ru(II) for the first time316 via stepwise Mizoroki–Heck reactions. The energy transfer processes in these complexes were studied in detail. The key findings are: (1) highly efficient intramolecular energy transfer from an excited Re unit to an excited Ru unit was observed for both complexes, and the emission occurs mainly from excited Ru and Os units. (2) The faster energy transfer rate observed from the Re to the Ru unit, compared with from the Ru or Re to the Os unit, can be attributed to the different energy transfer mechanisms. For example, Furue et al. had reported that the intramolecular excited energy transfer in a Ru–Os dinuclear complex proceeds by a Fo¨rster mechanism,317 whereas the excited Re-Ru energy transfer proceeds via a Dexter mechanism.318 (3) In contrast to 319, the intramolecular excited Ru-Os energy transfer in 320 proceeded very slowly or did not occur, which can be attributed to the longer distance between the Ru and Os units in 320. Referring to supramolecular photocatalysts,290 dinuclear Ru–Re or Os–Re systems exhibit the best photocatalytic CO2 reduction performance reported to date for bimetallic systems. Inspired by this, two trinuclear complexes 319 and 320 were compared with their dinuclear Ru–Re and Os–Re analogues. As shown in Fig. 60a, both 319 and 320 can be used as supramolecular photocatalysts for the highly selective formation of CO. Importantly, both 319 and 320 exhibited superior catalytic ability and great durability in the CO2 reduction process. The TONs for 319 and 320. reached 3552 � 461 and 4347 � 421, respectively, after 35 h irradiation, which are higher than the summation of their parent dinuclear Ru–Re and Os–Re complexes. These two trinuclear complexes exhibit the highest TONCO for all reported photocatalytic reactions. The reason for this outstanding performance has been investigated by UV absorption measurements. As shown in Fig. 60b, upon irradiation at l 4 500 nm for 1 h, the spectrum of 319 showed a slight decrease in the band at ca. 420 to 500 nm. The spectral shape was maintained for 6 h. On the contrary, in the case of the Ru–Re analogue, the absorption band at 460 nm sharply decreased in intensity with continuous irradiation (Fig. 60c), which mainly reflects the decomposition of the Ru unit. The corresponding time courses of the absorbance changes at 460 nm for 319 and for dinuclear Ru–Re are shown in Fig. 60d. The change in the absorption intensity matched well with the time courses of photocatalytic CO formation for both catalysts (Fig. 60a). Therefore, this result indicated that the photocatalytic stability can be effectively improved by the introduction of the Os unit into the Ru–Re system. In summary, introduction of an additional photosensitising Os unit into Ru–Re systems to give trinuclear complexes 319 and 320 has two main advantages in comparison with the parent dinuclear complexes: (1) a wider range of visible light up to 730 nm can be strongly captured by the trinuclear complexes. (2) Photocatalytic durability of 319 and 320 is improved by 27% and 55%, respectively, compared with their parent dinuclear complexes.

Sacrificial electron donors

TEA TEOA BI(OH)H BIH BNAH 1-methylnaphthalene

Additional information/Literature

[ECo20], [CEP20]