Visible-light-driven methane formation from CO2 with a molecular iron catalyst

Abstract

Converting CO2 into fuel or chemical feedstock compounds could in principle reduce fossil fuel consumption and climate-changing CO2 emissions1,2. One strategy aims for electrochemical conversions powered by electricity from renewable sources3,4,5, but photochemical approaches driven by sunlight are also conceivable6. A considerable challenge in both approaches is the development of efficient and selective catalysts, ideally based on cheap and Earth-abundant elements rather than expensive precious metals7. Of the molecular photo- and electrocatalysts reported, only a few catalysts are stable and selective for CO2 reduction; moreover, these catalysts produce primarily CO or HCOOH, and catalysts capable of generating even low to moderate yields of highly reduced hydrocarbons remain rare8,9,10,11,12,13,14,15,16,17. Here we show that an iron tetraphenylporphyrin complex functionalized with trimethylammonio groups, which is the most efficient and selective molecular electro- catalyst for converting CO2 to CO known18,19,20, can also catalyse the eight-electron reduction of CO2 to methane upon visible light irradiation at ambient temperature and pressure. We find that the catalytic system, operated in an acetonitrile solution containing a photosensitizer and sacrificial electron donor, operates stably over several days. CO is the main product of the direct CO2 photoreduction reaction, but a two-pot procedure that first reduces CO2 and then reduces CO generates methane with a selectivity of up to 82 per cent and a quantum yield (light-to-product efficiency) of 0.18 per cent. However, we anticipate that the operating principles of our system may aid the development of other molecular catalysts for the production of solar fuels from CO2 under mild conditions.

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Figure 1: Photochemical reduction of CO2 under visible light irradiation.
Figure 2: Methane detection.
Figure 3: Sketch of the proposed mechanism for CO2 reduction to CH4 by catalyst 1.

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Acknowledgements

This work was partially supported by the CNRS, Défi Transition énergétique “Emergence CO2” (the PERIODIC project). H.R. thanks the China Scholarship Council for a PhD fellowship (CSC student number 201507040033). We thank D. Clainquart (Université Paris Diderot) for assistance in gas chromatography/mass spectrometry analysis and I. Azcarate for porphyrin synthesis.

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M.R. conceived the research, J.B. and M.R. directed the project and co-wrote the paper. J.B. conceived the experimental setup. J.B., L.C.S. and H.R. conducted experiments. H.R., J.B. and M.R. analysed results. All the authors contributed to the scientific interpretation and reviewed the manuscript.

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Correspondence to Julien Bonin or Marc Robert.

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Extended data figures and tables

Extended Data Figure 1 Evolution of the absorption spectrum with time.

The absorption spectrum of a CO2-saturated acetonitrile solution containing 2 μM of 1, 0.2 mM of 4, 0.05 M of TEA upon visible (>420 nm) light irradiation remains stable over the course of experiments, highlighting the stability of the system. The inset shows the absorption spectrum of 2 μM of catalyst 1 in acetonitrile (no sensitizer 4), revealing that in the photocatalytic mix, >90% of photons above 420 nm are absorbed by 4.

Extended Data Figure 2 Sensitizer 4 emission quenching after excitation at 420 nm.

a, Upon increasing concentration of TEA in a 0.1 mM acetonitrile solution of 4, no emission quenching is observed, as confirmed by the Stern–Volmer analysis (inset). b, Upon increasing concentration of 1 in a 0.2 mM acetonitrile solution of 4, emission quenching is observed corresponding to a diffusion-controlled quenching rate of (1.7 ± 0.1) × 1010 M−1 s−1 as determined by Stern–Volmer analysis (inset). a.u., arbitrary units. I0/I is the emission intensity without quencher divided by the emission intensity with a known concentration of quencher.

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Rao, H., Schmidt, L., Bonin, J. et al. Visible-light-driven methane formation from CO2 with a molecular iron catalyst. Nature 548, 74–77 (2017). https://doi.org/10.1038/nature23016

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