Renewable Catalysts for the Reduction of CO to Methanol

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X-ray absorption spectroscopy at NSLS-II was used to study the evolution of Ru complexes during the catalytic reduction of CO to methanolCredit: ACS Energy Lett. 9, 3815-3817 (2024).

The Science         

The reduction of carbon monoxide (CO) to methanol was achieved using renewable organic hydride donors, ruthenium complexes, and visible light.

The Impact

Developing solar-driven catalysts that can convert greenhouse gasses such as CO and CO2 into liquid fuels will benefit the environment and boost the economy.

Summary

Many strategies are being pursued to capture greenhouse gasses such as CO and CO2 from the atmosphere and convert them to economically-important liquid fuels. Ideally, these reactions would be carbon-neutral where solar energy is the only energy input employed, and the small molecules found in the air are the only reactants. However, the underlying chemistry required to generate solar liquid fuels selectively and efficiently is underdeveloped, and key mechanistic steps remain poorly understood.

Despite significant progress in the generation of liquid fuels from CO2 through heterogeneous electrocatalysis, achieving high selectivity and efficiency remains a critical challenge. Homogeneous molecular catalysts are a promising option, but their development for efficiently generating a single liquid fuel from CO2 or from CO remains elusive. A viable strategy to generate liquid fuels from CO2 is to employ two distinct catalysts in a cascade fashion, in which the first catalyst reduces CO2 to CO, and the second catalyst upgrades the CO product into a liquid fuel.

In this work, the mechanism for the selective conversion of a transition metal (ruthenium) carbonyl complex to a hydroxymethyl complex that releases methanol upon illumination with visible light was studied. Each step was successfully quantified with organic hydride (dihydrobenzimidazole) reductants. X-ray absorption spectroscopy (XAS) at National Synchrotron Light Source II beamlines Inner-Shell Spectroscopy, Tender Energy X-ray Absorption Spectroscopy, and Spectroscopy Soft and Tender was used to study the ruthenium K- and L3-edges along with the oxygen K-edge, where each step of the CO reduction was studied separately. By studying the Ru(II) metal center and ligand absorption edges separately, the oxidation state of the Ru(II) was determined at each step.

XAS results showed that methanol generation by renewable organic hydrides occurs by multielectron, multiproton transfer to the CO ligand while the metal center remained in the Ru(II) formal oxidation state. The Ru L3 and O K edge features indicate a strong Ru–C bond in in the initial catalytic step that is consistent with the resilience of this complex in solution and at high temperatures. Identification and quantification of methanol as the sole CO reduction product was confirmed by NMR spectroscopy and gas chromatography.

The high selectivity and mild reaction conditions suggest a viable approach for methanol production from CO, and from CO2 through cascade catalysis, with renewable organic hydrides that bear similarities to Nature’s NADPH/NADP+.

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Contact

Javier J. Concepcion
Brookhaven National Laboratory
jconcepc@bnl.gov

José A. Rodriguez
Brookhaven National Laboratory
rodrigez@bnl.gov

Publications

Irene Barba-Nieto, Andressa V. Müller, Charles J. Titus, Dominik Wierzbicki, Cherno Jaye, Mehmed Z. Ertem, Gerald J. Meyer, Javier J. Concepcion, José A. Rodriguez Formal Oxidation States and Coordination Environments in the Catalytic Reduction of CO to MethanolACS Energy Lett. 9 (8), 3815-3817 (2024).

Funding

This work is supported as part of the Center for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0021173. The XAS measurements were done at the National Synchrotron Light Source II (8-ID, 8-BM, and 7-ID-1 beamlines), a user Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract No. DE-SC0012704. The 7-ID-1 beamline is supported by NIST.

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