Photo-generated dinuclear {Eu(II)}2 active sites for selective CO2 reduction in a photosensitizing metal-organic framework

Abstract

Photocatalytic reduction of CO₂ is a promising approach to achieve solar-to-chemical energy conversion. However, traditional catalysts usually suffer from low efficiency, poor stability, and selectivity. Here we demonstrate that a large porous and stable metal-organic framework featuring dinuclear Eu(III)₂ clusters as connecting nodes and Ru(phen)₃-derived ligands as linkers is constructed to catalyze visible-light-driven CO₂ reduction. Photo-excitation of the metalloligands initiates electron injection into the nodes to generate dinuclear {Eu(II)}₂ active sites, which can selectively reduce CO₂ to formate in a two-electron process with a remarkable rate of 321.9 μmol h^-1 mmol_MOF^-1. The electron transfer from Ru metalloligands to Eu(III)₂ catalytic centers are studied via transient absorption and theoretical calculations, shedding light on the photocatalytic mechanism. This work highlights opportunities in photo-generation of highly active lanthanide clusters stabilized in MOFs, which not only enables efficient photocatalysis but also facilitates mechanistic investigation of photo-driven charge separation processes.

Fig. 1

Synthesis of H₃L. Chemical structure of the tricarboxylate metalloligand used in the synthesis of Eu-Ru(phen)₃-MOF. (i) HNO₃, H₂SO₄, KBr, NaOH, 90 °C, 96% yield; (ii) 4-carboxybenzaldehyde, HAc, 100 °C, NH₄Ac, 120 °C, 88% yield; (iii) RuCl₃·3H₂O, EG, 180 °C, KFP₆(aq), NaOH(aq), THF, EtOH, 80 °C, 87% yield

Fig. 2

X-ray crystal structure of Eu-Ru(phen)₃-MOF. a Stick/polyhedra model structure of the metalloligand. b Stick model representation of a single 3D framework viewed along the [010] direction showing the 1D channels c with window dimensions of 31 × 16 Å. d Ball-and-stick model of [Eu₂(μ₂-H₂O)(H₂O)₃(-COO⁻)₆] building unit in Eu-Ru(phen)₃-MOF. e Stick model showing the interpenetrated frameworks in Eu-Ru(phen)₃-MOF and f the two neighboring networks stabilized by the π–π stacking interactions

Fig. 3

Photocatalytic CO₂ reduction performance. a Time profiles of HCOO⁻ produced catalyzed by Eu-Ru(phen)₃-MOF or H₃L or without catalyst under irradiation with a Xe lamp (420–800 nm). b The ¹³C NMR spectrum of products in liquid phase after reacting with ¹³CO₂ and ¹²CO₂, respectively. c The amount of HCOO⁻ produced for reusing three times. Samples were recovered after each cycle and reused under identical reaction conditions. d PXRD patterns for as-synthesized Eu-Ru(phen)₃-MOF and after photocatalytic reaction, showing its well-retained structure during the catalysis

Fig. 4

Spectroscopic evidence for effective electron transfer process. a Normalized UV–vis of Eu-Ru(phen)₃-MOF and H₃L in DMF. Inset: Emission spectra of Eu-Ru(phen)₃-MOF and H₃L (λ_ex = 465 nm). b Normalized luminescence decay traces of Eu-Ru(phen)₃-MOF and H₃L over the first 50 ns (λ_ex = 377 nm). Inset: Decay transients measured at 630 nm (λ_ex = 465 nm). c Transient absorption spectra of Eu-Ru(phen)₃-MOF and H₃L at various time delays, and d corresponding kinetic traces at 604 nm

Fig. 5

Photocatalytic in situ EPR characterization. a Schematic light-induced dynamics of Eu-Ru(phen)₃-MOF based on the initial excitation of the Ru photocenter and the pathways of electron transfer from Ru to catalytic Eu₂ oxo-cluster center. b In situ EPR spectra of Eu-Ru(phen)₃-MOF under different conditions

Fig. 6

Density functional theory calculation. a The calculated CO₂ adsorption structure. b Charge difference density of CO₂ adsorption structure of Eu(II)₂

Fig. 7

Fluorescence quenching. a Emission spectra of H₃L after the addition of different amounts of [Eu₂(MMA)₆(H₂O)₄] and b TEOA in DMF with 465 nm excitation

Fig. 8

Proposed catalytic mechanism of photocatalytic CO₂ reduction to HCOOH. The photo-initiated electron transfers from Ru photocenters to dinuclear Eu₂ oxo-clusters in Eu-Ru(phen)₃-MOF lead to the photo-reduction of CO₂. ΔE₁ = 2.07 eV, ΔE₂ = −0.88 eV, ΔE₃ = −1.19 eV, and ΔE’ = −0.69 eV