Photochemical reduction of carbon dioxide to carbon monoxide in water using a nickel(II) tetra-azamacrocycle complex as catalyst

Experiments with the titanium dioxide-ruthenium tris-bipyridine-nickel cyclam system for the photocatalytic reduction of CO2

“…Formation of formic acid is largely favored in the process, with a HCOOH/CO ratio of about 5 -6, and a significant amount of H 2 . The selectivity and the course of the process is qualitatively and quantitatively comparable with that obtained using a similar system in water, with a higher powered lamp (1000 W Xenon arc with a continuous emission) [13], while in this case, low-consumption LEDs are used. As mentioned above, introducing a carboxyl or phosphonyl group in the bipyridine ring of the Ru(II) complexes does not hinder their activity as electron transfer photosensitizers, at least in the present case.…”

“…However, when present at Notice that the photolyzed solution was diluted by a factor ten. [12,13] (judging from the spectrum in Figure 3), through two subsequent steps. By contrast, adding 5 × 10 -3 m 2 caused a smaller shift and a much reduced decrease, demonstrating that 1a was partially or totally preserved.…”

“…The latter process is obviously pH dependent and occurs significantly at low pH (below pH 4) [8]. Photochemistry offers an alternative approach, as first reported by Hawecker et al [11] A key advancement in this direction was obtained by the Calvin group in the 1990s [12,13], with the demonstration that both CO and H 2 were generated by visible light irradiation (1000 W lamp) of a CO 2 saturated aqueous solution containing tris -(2,2 ′ -bipyridyl)ruthenium(II) chloride as the photosensitizer, ascorbic acid as the electron donor, and 1,4,8,11-tetraazacyclotetradecane nickel(II) chloride as the catalyst. A related system was reported by Mochizuki shortly afterwards [14] and further studies have been carried out in the frame of homogeneous photocatalyzed reduction of CO 2 [15,16].…”

Activation of Carbon Dioxide

“…These reactions can proceed at reasonable reduction potentials between À0.24 and À0.61 V (NHE) (Equations (6.12-6.16); the reduction potentials, E , refer to pH 7 in aqueous solutions versus NHE), while the formation of the CO 2 . À radical anion is estimated to take place at À2.1 V. 104 Reduction of CO (in the presence of H þ ) supplies CH 2 . radicals that may yield methane directly or leads to higher hydrocarbons (e.g., ethene or ethane) by recombination.…”

Conversion of CO2 via Visible Light Promoted Homogeneous Redox Catalysis

“…In alternative catalyst systems such as RhCl 3 , NiCl 2 , CuCl 2 or K 2 PtCl 4, the selectivity towards CO 2 reduction was found to be further reduced. Cobalt macrocycles such as CoHMD as photosensitizer ( Figure 5) [72,73]. These Ni-cyclam catalysts were modified by Kimura et al for the photocatalytic CO 2 reduction by synthesis of supramolecular Ru-Ni catalysts with covalent linkage of the Ru-based chromophores to the reduction site ( Figure 6).…”