Photochemical Reduction of Carbon Dioxide to Formic Acid using Ruthenium(II)-Based Catalysts and Visible Light

Abstract

Summary

A photochemical reduction of CO₂ to formic acid was shown using a system containing an iridium-based photosensitizer, ruthenium photocatalyst and triethanolamine as the electron donor. Turnover numbers up to 526 and a selectivity of 80% towards formic acid were observed if the reaction was performed with [Ir(ppy)₂(bpy)]PF₆ ([Ir(ppy)2(bpy)][PF6]) as the photosensitizer and [Ru(bpy)₂(Cl)(CO)]PF₆ (100837) as the catalyst. The experiments were conducted under irradiation with Hg-lamp equipped with a λ=400–700 nm filter.

Advances and special progress

The employed photocatalytic system showed activity for the photoreduction of Na₂CO₃ to formic acid as well. A TON of 53 was observed after 5h reaction time.

Additional remarks

Experiments using different electron and proton donors such as triethylamine (TEA) and 1-benzyl-1,4-dihydronicotinamide (BNAH) were unsuccessful as the observed activities were lower than those obtained in the presence of TEOA.

Content of the published article in detail

The article contains results for the reduction of CO2 to formic under visible-light catalysis using ruthenium complexes as catalysts, the iridium complex [Ir(ppy)₂(bpy)]PF₆ ([Ir(ppy)2(bpy)][PF6]) as a photosensitizer and triethanolamine as the electron donor. The catalytic system performs best (referring to the TON of HCOOH production) with [Ru(bpy)₂(Cl)(CO)]PF₆ (100837) as photocatalyst.

Catalysts

Ru(bpy)2Cl2 Ru(bpy)2CO3 [Ru(bpy)2ClCO][PF6] [Ru(bpy)(H2O)(CO)][PF6] [Ru(bpy)(AcMe)2][PF6] [Ru(bpy)2HCO][PF6]

Photosensitizer

[Ir(ppy)2(bpy)][PF6]

Investigations

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	solvent A	.	λexc [nm]	TON CO	TON H2	TON HCOOH	.
1.	Ru(bpy)2Cl2	25	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	1	2	13
2.	Ru(bpy)2CO3	25	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	2	4	21
3.	[Ru(bpy)2ClCO][PF6]	25	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	13	8	62
4.	[Ru(bpy)2HCO][PF6]	25	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	8	8	34
5.	[Ru(bpy)(H2O)(CO)][PF6]	25	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	7	9	40
6.	[Ru(bpy)(AcMe)2][PF6]	25	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	3	4	27

Experiment-Name: CO2 reduction experimentsExport

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	solvent A	.	λexc [nm]	TON CO	TON H2	TON HCOOH	.
1.	Ru(bpy)2Cl2	6.2	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	3	9	26
2.	Ru(bpy)2CO3	6.2	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	3	11	39
3.	[Ru(bpy)2ClCO][PF6]	6.2	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	19	19	117
4.	[Ru(bpy)2HCO][PF6]	6.2	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	21	16	107
5.	[Ru(bpy)(H2O)(CO)][PF6]	6.2	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	21	14	75
6.	[Ru(bpy)(AcMe)2][PF6]	6.2	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	4	12	64
7.	[Ru(bpy)2ClCO][PF6]	3.1	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	36	33	296
8.	[Ru(bpy)2HCO][PF6]	3.1	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	16	29	145
9.	[Ru(bpy)(H2O)(CO)][PF6]	3.1	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	34	28	211
10.	[Ru(bpy)2ClCO][PF6]	1.6	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	40	67	419
11.	[Ru(bpy)2HCO][PF6]	1.6	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	25	62	225
12.	[Ru(bpy)(H2O)(CO)][PF6]	1.6	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	45	65	335
13.	[Ru(bpy)(AcMe)2][PF6]	1.6	[Ir(ppy)2(bpy)][PF6]	0.025	TEOA	NMP		400 - 700	4	44	75

Experiment-Name: Optimization of concentrationsExport

	analyte	reduction potential	solvent	electrolyte	.	.	WE	.	.	RE	.
1.	Ru(bpy)2Cl2	0,-2.11	MeCN	TBABF4			glassy carbon			Ag/AgNO3 in MeCN
2.	Ru(bpy)2CO3	0.45,-1.97	MeCN	TBABF4			glassy carbon			Ag/AgNO3 in MeCN
3.	[Ru(bpy)2ClCO][PF6]	1.22,-1.75	MeCN	TBABF4			glassy carbon			Ag/AgNO3 in MeCN
4.	[Ru(bpy)2HCO][PF6]	1.20,-1.61	MeCN	TBABF4			glassy carbon			Ag/AgNO3 in MeCN

Experiment-Name: Table 3 - CVExport

Sacrificial electron donor

In this study, the experiments were done with the sacrificial electron donor TEOA (100507).

Additives

No additives were used in the described experiments.