Toward Visible-Light Photochemical CO2‑to-CH4 Conversion in Aqueous Solutions Using Sensitized Molecular Catalysis

Abstract

Summary

A photochemical reduction of CO₂ was achieved using an iron porphyrin catalyst and an iridium complex as the photosensitizer. Carbon monoxide, produced with a TON of 178 under optimized conditions, was the main product, methane was produced in a substantial amount with the TON of 32, while hydrogen was formed as a minor byproduct. The experiments were performed under irradiation with λ > 420 nm in acetonitrile and in the presence of various amines as sacrificial electron donors.

Advances and special progress

The catalytic reduction of CO₂ could also be realized under aqueous conditions (acetonitrile/water 3:7 v:v), yielding carbon monoxide as the main product while methane was produced with selectivity up to 10% (TON of 3).

Additional remarks

Among the various amines employed as sacrificial electron donors, TEA appeared as the best choice to maximize CO₂ product formation.

Content of the published article in detail

The article contains results for the reduction of CO₂ to CO as the main product under visible-light irradiation using iron porphyrin complex Fe(pTMAPP)Cl₅ (Fe(pTMAPP)Cl5) as the catalysts, an iridium complex as the photosensitizer and various amines as the sacrificial electron donor. The catalytic system performs best (referring to the TON of CO production) with Ir(ppy)₃ (Ir(ppy)3) as the photosensitizer and TEA as the sacrificial electron donor.

Catalyst

Fe(pTMAPP)Cl5

Photosensitizer

[Ir(ppy)2(bpy)][PF6] Ir(ppy)3

Investigation

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	solvent A	.	additives	.	.	λexc [nm]	.	TON CO	TON CH4	TON H2	.
1.	Fe(pTMAPP)Cl5	0.002	[Ir(ppy)2(bpy)][PF6]	0.2	TEA	0.05	MeCN					> 420		178	32	103
2.	Fe(pTMAPP)Cl5	0.002	[Ir(ppy)2(bpy)][PF6]	0.2	TEOA	0.05	MeCN					> 420		134	23	67
3.	Fe(pTMAPP)Cl5	0.002	[Ir(ppy)2(bpy)][PF6]	0.2	DIPEA	0.05	MeCN					> 420		151	24	77
4.	Fe(pTMAPP)Cl5	0.002	[Ir(ppy)2(bpy)][PF6]	0.2	TEA	0.05	MeCN					> 420		24	3	5
5.	Fe(pTMAPP)Cl5	0.002	[Ir(ppy)2(bpy)][PF6]	0.2	TEOA	0.05	MeCN					> 420		19	3	4
6.	Fe(pTMAPP)Cl5	0.002	[Ir(ppy)2(bpy)][PF6]	0.2	DIPEA	0.05	MeCN					> 420		20	3	4
7.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN					> 420		198	31	24
8.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN					> 420			89	18
9.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN		TFE			> 420			100	19
10.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN		TFE			> 420			195	45

Experiment-Name: Photocatalytic reduction of CO2: conditions optimizationExport

	analyte	anl conc [mM]	reduction potential	solvent	electrolyte	.	.	gas	.	WE	.	.	RE	.
1.	[Ir(ppy)2(bpy)][PF6]	0.5	-2.0 , -1.4	MeCN	TBAPF6			argon		glassy carbon			SCE
2.	[Ir(ppy)2(bpy)][PF6]	0.5	-2.0, -1.4	MeCN	TBAPF6			CO		glassy carbon			SCE
3.	[Ir(ppy)2(bpy)][PF6]	0.5	-2.0, -1.4	MeCN	TBAPF6			CO		glassy carbon			SCE
4.	[Ir(ppy)2(bpy)][PF6]	0.5	-2.0, -1.4	MeCN	TBAPF6			CO		glassy carbon			SCE
5.	[Ir(ppy)2(bpy)][PF6]	0.5	-2.0	MeCN	TBAPF6			CO		glassy carbon			SCE

The results show an increase in current when the solution is irradiated and triethylamine (TEA) is added (see condition column).

Experiment-Name: Cyclic voltammetry in various conditionsExport

Sacrificial electron donor

In this study, the experiments were conducted with TEA (TEA), TEOA (TEOA), and DIPEA (DIPEA) as sacrificial electron donors.

Additives

In some experiments described in this study, TFE (TFE) was used as an additive.